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Theses and Dissertations
2007
Sputter deposition of iron oxide and tin oxide based
films and the fabrication of metal alloy based
electrodes for solar hydrogen production
Daniel Sporar
The University of Toledo
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Recommended Citation
Sporar, Daniel, "Sputter deposition of iron oxide and tin oxide based films and the fabrication of metal alloy based electrodes for solar
hydrogen production" (2007). Theses and Dissertations. 1327.
http://utdr.utoledo.edu/theses-dissertations/1327
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A Thesis
Entitled
Sputter Deposition of Iron Oxide and Tin Oxide Based Films and the Fabrication of
Metal Alloy Based Electrodes for Solar Hydrogen Production
By
Daniel Sporar
Submitted as partial fulfillment of the requirements for
The Master of Science degree in Chemical Engineering
____________________________________
Advisor: Dr. Xunming Deng
____________________________________
College of Graduate Studies
The University of Toledo
May 2007
The University of Toledo
College of Engineering
I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY
SUPERVISION BY Daniel Sporar
ENTITLED
Sputter Deposition of Iron Oxide and Tin Oxide Based Films and the
Fabrication of Metal Alloy Based Electrodes for Solar Hydrogen
Production
BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF
Master of Science in Chemical Engineering
Thesis Advisor: Dr. Xunming Deng
Recommendation concurred by
Committee
Dr. G. Glenn Lipscomb
Dr. Steven E. LeBlanc
On
Final Examination
Dean, College of Engineering
An Abstract of
Sputter Deposition of Iron Oxide and Tin Oxide Based Films and the Fabrication of
Metal Alloy Based Electrodes for Solar Hydrogen Production
Daniel Sporar
Submitted as Partial Fulfillment of the Requirement for
The Master of Science in Chemical Engineering
The University of Toledo
May 2007
This M.S. Thesis describes the fabrication and characterization of n-type iron (III)
oxide thin film semiconductors as well as n-type fluorine doped tin dioxide thin film
semiconductors for use as the top oxide layer of hybrid multijunction PEC electrodes and
as a transparent conductive corrosion resistant layer, respectively. Also described is the
fabrication and characterization of various anode and cathode materials in an attempt to
devise high quality, cost-effective electrocatalysts for the electrolytic evolution of
hydrogen and oxygen gases. Iron (III) oxide thin films were radio frequency sputterdeposited under variable conditions. Dopants were incorporated via co-sputtering in an
attempt to enhance photocurrent response and overall film stability in basic media, 33 %
potassium hydroxide. Iron (III) oxide thin films deposited with a chamber atmosphere
iii
containing 2 % oxygen in argon at 100 W at 400 °C for 110 min demonstrated the highest
observed photocurrents of 0.34 mA/cm2 under 0.75 sun illumination; efficiency was
0.56 % at a potential of 0.38 V. The films were also stable. Fluorine doped tin dioxide
thin films were fabricated in the same fashion as the iron (III) oxide thin films; samples
deposited at 50 W with the chamber atmosphere containing 5 % oxygen in argon at
250 °C for 135 min demonstrated photocurrents of 0.2 mA/cm2, although they lacked
stability. Iron (III) oxide was deposited onto the top of a triple-junction amorphous
silicon solar cell to investigate its usefulness as a protective oxide layer.
Anodes and cathodes that were investigated for enhanced electrocatalytic
properties consisted of various materials produced by various methods. Current densities
and hydrogen evolution rates were measured. Electrodes demonstrating the greatest
performance were made by mixing nickel, aluminum, and molybdenum powders in
nickel trays at a ratio of 88:5:7, and then sintering them for four hours in a furnace at
900 °C. The electrodes were then soaked in 33 % potassium hydroxide in order to leach
out the aluminum, thus creating porous structures of high surface area. Current densities
near 40 mA/cm2 measured at 1.8 V have been demonstrated after 1000 hours of
accelerated continuous long-term testing at a potential of 2.2 V.
iv
Foreword
I would like to give my sincere thanks to my thesis advisor, Dr. Xunming Deng,
for his support during both my undergraduate and graduate careers at The University of
Toledo. Due to his generosity, I have been able to expand my education beyond the
scope of a degree based solely in chemical engineering, allowing me to develop a more
complete understanding of the link between science, engineering, and technology. I
would also like to thank the other members of my thesis committee, Dr. Glenn G.
Lipscomb and Dr. Steven E. LeBlanc, for their patience as I completed the requirements
for my Master of Science in Chemical Engineering degree.
I would like to give my sincere thanks to my co-worker, mentor, and friend, Dr.
William B. Ingler, whose competence and guidance have allowed me to become a more
successful graduate student.
I found his experience working with thin film
semiconductors to be invaluable as I developed this body of work. I would also like to
thank my co-worker and friend Dr. Mahabala Adiga for his knowledge and support
concerning my electrocatalyst work. It has been an honor to have had the privilege of
working with both individuals.
I would like to thank the faculty and staff of the Department of Chemical and
Environmental Engineering for providing an excellent academic and professional
education and overall positive experience during both my undergraduate and graduate
careers at The University of Toledo; especially Dr. Arunan Nadarajah who personally
recruited me into the graduate studies program. I would also like to thank the faculty and
staff of the Department of Physics and Astronomy for their support throughout my
graduate career.
I would like to thank Dr. Pannee Burckel, the Chemical Instrumentation Specialist
at the College of Arts and Sciences Instrumentation Center, for her training on and
assistance with x-ray diffraction measurements and SEM imaging. I would also like to
thank the graduate students Xinmin Cao for his assistance with thin film thickness and
band gap calculations, Xiesen Yang for his assistance with AFM imaging, and Dinesh
Attygalle for his assistance with work done on both the thin film semiconductor research
and the electrocatalyst research. I would like to acknowledge the efforts of Anupam
Dighe, Amrutha Asthana, Madhu Kondapi, and Puneeta Bhadsavle, all part-time graduate
students, for their assistance with work done on sintered electrodes.
v
Table of Contents
Abstract
iii
Foreword
v
Table of Contents
vi
List of Figures
viii
List of Tables
xiii
List of Equations
xiv
Part 1: Thin Film Semiconductors
Section 1-1
Introduction
1
Section 1-2
Experimental
12
A.
Preparation for the Deposition of n-type Fe2O3 and
F-SnO2 Thin Films by RF Sputter Deposition
12
B.
Fabrication of n-type Fe2O3 Thin Films With and
Without Incorporation of Metal Dopants
15
C.
Fabrication of n-type F-SnO2 Thin Films
16
Thin Film Characterization
18
A.
Photocurrent Measurements
18
B.
Film Stability Measurements
19
C.
UV-vis Spectroscopic Measurements
19
D.
X-ray Diffraction Spectra Measurements
20
E.
Film Thickness Measurements
21
F.
Atomic Force Microscopy Measurements
21
G.
Annealing
21
Section 1-3
vi
Section 1-4
Results and Discussion
22
A.
n-type Fe2O3
22
B.
Tantalum Doped n-type Fe2O3
33
C.
Zirconium Doped n-type Fe2O3
33
D.
Indium Doped n-type Fe2O3
34
E.
Antimony Doped n-type Fe2O3
39
F.
n-type F-SnO2
41
Summary
45
Section 1-5
Part 2: Electrodes Exhibiting Enhanced Electrocatalytic Performance
Section 2-1
Introduction
47
Section 2-2
Experimental
49
A.
Basic Electrode Materials
49
B.
Sputter-Deposited Electrodes
49
C.
Electroplated Electrodes
51
D.
Raney Nickel
53
E.
Sintered Electrodes
53
Section 2-3
Electrode Characterization
57
Section 2-4
Results and Discussion
60
Section 2-5
Summary
70
Future Work
71
References
72
vii
List of Figures
1.1
Water electrolysis by conventional means. When a potential is applied to the
electrodes, the electrolyte completes the circuit and allows current to flow. If the
applied potential is high enough to overcome the water splitting potential (1.23 V)
and the electrode overpotentials, then water molecules dissociate into hydronium
ions (H+) and hydroxyl ions (OH-) (1). The hydroxyl ions are oxidized at the
anode and form oxygen molecules (2). The hydrogen ions are reduced at the
cathode and form hydrogen molecules (3). Because there are two hydrogen atoms
to every oxygen atom in the water molecule, twice as much hydrogen gas is
produced with respect to oxygen………………………………………………….3
1.2
Standard, AM 0 and AM 1.5, solar spectrum. Ultraviolet range is from 115 to
400 nm; visible range is from about 400 to 800 nm. The area under the UV
portion of the curve is much less than the area under the visible portion of the
curve……………………………………………………………………………….4
1.3
PEC system utilizing a TCO protective layer deposited upon a multijunction solar
cell.14 From left to right, the H2 catalyst could be platinum islets or a
molybdenum compound, the multijunction is a-Si, the transparent protective film
could be ITO or TiO2, and the O2 catalyst could be a cobalt compound.20…….....5
1.4
General design for a hybrid multijunction PEC.21 α-Fe2O3 would act as the
photoactive semiconductor, replacing the top cell of the solid-state multijunction
(a-Si). The interface layer is a very thin layer of ITO and is used to reduce the
series resistance between the solid-state multijunction and the photoactive
semiconductor. The metal substrate is generally stainless steel and the hydrogen
evolution reaction (HER) catalyst is usually platinum islets, or a molybdenum
compound………………………………….………………………………………6
1.5
When light with energy hν hits the semiconductor electrode, electrons may
become excited up to the conduction band (EC) from the valence band (EV). The
electrons (e-) then move to the back of the electrode while holes (+) accumulate at
the front surface. EF, the Fermi level, is the highest energy state which electrons
may occupy at absolute zero. In p-type semiconductors it is located closer to the
valence band, and in n-type semiconductors it is located closer to the conduction
band…………………………………………………………………………….….8
1.6
General reaction mechanism for the evolution of hydrogen and oxygen using a
self-driven PEC system utilizing thin film semiconductors. Water adsorbs onto
the photoanode and dissociates into hydronium (H+) and hydroxyl ions (OH-).
Formation of electron and hole pairs occurs at the photoanode where O2 is formed
and H2 is formed at the cathode…………………………………………………...8
viii
1.7
Argon ions (Ar+) from the plasma cloud, confined just above the target by a
magnetic field generated by magnets located within the sputter gun, bombard the
target physically removing small amounts of the target material upon impact. The
particles, which are neutrally charged, are ejected ballistically and deposit on a
substrate. Over time a very thin film of target material builds up on the surface of
the substrate. The substrate is rotated in order to ensure a uniform substrate
temperature and film deposition…………………………………………………11
1.8
Diagram of standard substrate orientation in the substrate holder, viewed from the
deposition side. One piece of Tec-15 glass and one piece of ITO coated glass
were placed in the center of the substrate holder and plain glass microscope slides
were used to fill in the rest of the spaces. Thin pieces of stainless steel were used
to block part of the thin film deposition in order to leave bare electrical contacts
used for measuring photocurrent and film stability. The deposition on the plain
glass was used for transmission measurements………………………………….13
1.9
Temperature calibrations for the vacuum deposition chamber. After several runs,
spot checks were done to make sure the bulbs were maintaining the same power
density. Full calibrations were done after every change of the halogen
bulbs…………………………………………………………………………...…15
1.10
All of the thin films were fabricated in this sputter chamber. The flange / door is
located on the left-hand side of the chamber (1) near the RF generators (2). The
sample rotator is located at the top of the chamber (3) along with the power input
for the lamps (4) and the thermocouple (5). The vacuum gauges are located on
the right side of the chamber (6), above the pressure and gas flow control panel
(7)………………………………………………………………………………...17
1.11
Photocurrent density (jP, µA/cm2) as a function of oxygen concentration (%) in
argon ambient of n-type α-Fe2O3 thin films deposited at 400 °C for 120 min with
100 W deposition power. Adding oxygen to the chamber atmosphere allowed any
free iron to oxidize (reactive sputtering) resulting in higher quality films that were
more stable in solution during electrochemical testing…………………………..23
1.12
Photocurrent density (jP, µA/cm2) versus substrate temperature (°C) for n-type αFe2O3 thin films deposited on (a) Tec-15 glass and (b) ITO. All samples
represented were sputtered with a target power of 100 W with 2 % oxygen in
argon ambient for 110 min………….……………………………………..……..24
1.13
Stability scans for n-type α-Fe2O3 thin films deposited with a power of 100 W at
400 °C for various amounts of time. Films deposited for 90 min were more stable,
but not very conductive. As the deposition time was increased to 135 min the
film stability decreased by a small amount, and the film conductivity
increased.………………………………………..………………………….........25
ix
1.14
Photocurrent density (jP, µA/cm2) as a function of film deposition time (min) for
α-Fe2O3 thin films deposited on (a) T-15 glass and (b) ITO. The optimum
deposition time was found to be 110 min with the Fe2O3 deposition power set to
100 W. UV-vis transmission spectra were used to calculate an average film
thickness of 280 nm……………………………………………………………...26
1.15
Chopped scan of n-type α-Fe2O3 under alternating 0.75 sun illumination and
ambient room lighting (dark). When the film surface was illuminated,
photocurrent was generated, demonstrated by a sharp increase in current density
(mA/cm2). When the light source was then blocked, the current density sharply
droped back to the dark current density value..…………..………………...…....27
1.16
XRD spectra for n-type α-Fe2O3 thin films measured from 25° to 65° (2θ). As
film thickness increases, the peaks become more intense and defined indicating
increased crystallinity (a). As deposition temperature increases, the peaks become
more intense and defined indicating increased crystallinity (b). All peaks
correspond to only α-Fe2O3.…………………………………………...…..…….29
1.17
Film thickness as a function of deposition time. All films were deposited under
similar conditions. The error bars are one standard deviation…………………..30
1.18
UV-vis transmission spectra of n-type α-Fe2O3 thin film deposited at 400 °C for
110 min. Due to the film being very thin (285 nm) there are very few interference
fringes.
A tauc plot was used to determine a band gap of 2.04
eV.…………………………………………………………………………….….31
1.19
Tauc plot for an n-type α-Fe2O3 thin film deposited for 110 min at 400 °C with a
Fe2O3 r.f. deposition power of 100 W in a chamber atmosphere containing 2 %
oxygen in argon ambient at a pressure of 6 mTorr. The band gap was determined
to be about 2.04 eV.…………………………………………………………..….32
1.20
AFM images of n-type α-Fe2O3 thin films deposited at, from left to right, 300,
350, and 400 °C. The dimensions of each image are 5000 ¯ 5000 nm. Films
deposited with higher substrate temperatures have rougher surfaces and
demonstrate greater photoactivity……………………..……………………....…32
1.21
Photocurrent density (jP, µA/cm2) versus applied potential (mV) of an In-Fe2O3
thin film electrode. Light and dark currents were measured using a light chopping
method by manually blocking the light source and then illuminating the electrode
in 5 s intervals……………………………………………………………………35
1.22
Photocurrent density (jP, µA/cm2) versus applied potential (mV) of an In-Fe2O3
thin film electrode annealed up to 6 hours in an inert argon atmosphere at
550 °C.……………………………………………………………………….…..35
x
1.23
UV-vis spectroscopic measurement of an In-Fe2O3 thin film electrode deposited
with an indium power of 20 W at 200 °C having a thickness of 980 nm by Tauc
calculations. A Dektak3ST surface profiler was also used to measure film
thickness and similar values were obtained……………………………………...36
1.24
Tauc plot for In-Fe2O3 sample deposited for 120 min at 200 °C with an indium r.f.
deposition power of 20 W and a Fe2O3 rf deposition power of 100 W. The
chamber atmosphere contained 5 % oxygen in argon ambient at a pressure of 6
mTorr. The band gap was determined to be 2.6 eV……………………………..37
1.25
X-ray diffraction measurements of In-Fe2O3 thin film electrodes deposited at
200 °C and with varying indium target deposition powers (5 to 20 W). Peak
intensities are greater for higher indium target powers. Peaks indicate the
presence of (a) α-Fe2O3 and indium and iron oxide compounds such as (b) InFeO3
and (c) InFe2O4.……………………………………………………………...…..37
1.26
X-ray diffraction measurements of In-Fe2O3 thin film electrodes deposited at
varying substrate temperatures with an indium target deposition power of 10 W.
Peak intensities are greater for higher temperatures, indicating greater crystallinity.
Film composition includes (a) Fe2O3, (b) InFeO3, and (c) InFe2O4……………...38
1.27
AFM images of indium doped α-Fe2O3 deposited with an indium target power of
5 W (a, b) and 20 W (c, d). Both films were deposited at 200 °C with 5 % oxygen
in argon ambient for 120 min. Films deposited at higher temperatures
demonstrated greater photocurrents due to greater active surface areas. The scale
for both images is 5000 ¯ 5000 nm……………………………………………..40
1.28
Photocurrent density (jP, mA/cm2) versus applied potential (mV) of a Sb-Fe2O3
thin film electrode. Light and dark currents were measured using a light chopping
method by manually blocking the light source and then illuminating the electrode
in 5 s intervals…………………………………………………………….…..….41
1.29
Photocurrent density (jP, µA/cm2) versus applied potential (mV) for a F-doped
SnO2 thin film electrode deposited on standard ITO. Light and dark currents were
measured using a light chopping method by manually blocking the light source
and then illuminating the electrode in 5 s intervals……………………………...42
1.30
UV-vis transmission spectra for F-SnO2 thin film deposited at 50 W for 135 min
at 250 °C. The film transparency was found to be near 90% in the visible portion
of the spectrum, and a band gap of 3.38 was determined from tauc plot
calculations………………………………………………………………………42
1.31
Tauc plot for a F-SnO2 thin film deposited at 50 W for 135 min at 250 °C. The
value of the band gap, Eg (eV), was found to be 3.37 eV………………………..43
xi
1.32
XRD spectra of a F-SnO2 thin film. All peaks correspond to SnO2. The amount
of fluorine present is too small to be detected using the available
instrumentation…………………………………………………………………..43
2.1
Concentration calibration curve for the platinum electroplating solution……….52
2.2
The stainless steel box used for sintering the metal powders. There were four
levels labeled from A to D from the bottom to the top. On each level, up to six
electrodes could be placed for sintering, each labeled from a to f. The box was
open on the front and back sides so that the levels could be accessed…………..56
2.3
8-chamber electrolyzer with H2 and O2 gas collection capabilities. Each chamber
was divided into two compartments separated by a nylon membrane. The
displacement of water was used to measure gas flow rates……………………...56
2.4
H2 generation rates (mL/min) measured over time (min) for selected samples;
sputtered CrN (▲), and sputtered Ni-Co-Mo from (●)…………………………..62
2.5
Accelerated long-term testing of various nickel cathodes. Current densities (j,
mA/cm2) were measured at 1.8 V over a period of several hundred hours of
continuous operation at 2.2 V. Degradation of performance occurred over time
until equilibrium was obtained.
The sintered electrode 123005D (▲)
demonstrated the greatest performance due to its porosity and large surface area.
The electroplated nickel electrode (○) performed well initially, but after 200 hours
its performance had degraded considerably. The platinum coated nickel sponge
(□) demonstrated the lowest performance due to the platinum coating being too
thin, as well as having less active surface area……………………………….….63
2.6
XRD spectra of nickel powders of various purities. No difference in composition
had been observed. All peaks correspond to nickel……………………………..65
2.7
Current density (j, mA/cm2), measured at a potential of 1.8 V, as a function of
electrode size. Three electrodes fabricated under identical conditions were tested.
It was observed that as electrode size was increased, values of current density
dropped……………………………………………………………………….….66
2.8
Current density (j, mA/cm2) as a function of time (h) for various sintered
electrodes, measured at a potential of 1.8 V. Electrodes run continuously over an
extended period of time suffered from degradation of performance…………….67
2.9
Pourbaix diagrams for the nickel – water system at 25 °C. At high pH and high
potential, a passive oxide layer forms at the electrode surface…………………..69
xii
List of Tables
1.1
Crystal size (nm) based on α-Fe2O3 deposition conditions. Crystal size increases
as film thickness is increased and as deposition temperature increases………....28
1.2
Crystal size based on In-Fe2O3 deposition conditions. Crystal sizes increase as In
power is increased (with the Fe2O3 power held at 100 W) and as deposition
temperature is increased……………………………………….……………...….38
2.1
List of materials studied for use as cathodes and anodes for water electrolysis.
Most materials that were studied were nickel based………………………..……50
2.2
List of various aluminum and nickel powders investigated for use in producing
sintered electrodes………………………………………………………………..55
2.3
Sample heating sequence program for sintering. The first input variable c01 is the
starting temperature inside the furnace. Input t01 is the time it takes to reach
temperature c02, and so on. Input -121 stops the program at the end of the
sequence………………………………………………………………………….55
2.4
Current densities (j, mA/cm2) for various anode and cathode combinations
measured at an applied potential of 1.8V. Initially, the combination of platinum
coated nickel sheet as an anode and sputter coated porous nickel as the cathode
demonstrated the greatest performance, determined by comparing current density
values………………………………………………………………………...…..61
2.5
H2 and O2 evolution rates (ml/min) measured at an applied potential of 1.8 V for
various cathodes with a 3N nickel anode. The volume of H2 generated should be
exactly double the value of the volume of O2 generated based on water
electrolysis reaction stoichiometry…………………………………………...….61
2.6
List of sintered electrodes, from Figure 2.8, subjected to accelerated long-term
testing in 5.9 M potassium hydroxide at a potential of 1.8 V……………..……..67
xiii
List of Equations
Equation 1:
4H2O → 4H+ + 4OH-……………………………………………………...3
Equation 2:
4OH- → 2H2O + O2……………………………………………………….3
Equation 3:
4H+ + 4e- → 2H2…………………………………………………………..3
Equation 4:
2H2O → 2H2 + O2 .......................................................................................3
Equation 5:
Si + 2OH- + 2H2O → SiO2(OH22-) + 2H2………………………………....5
Equation 6:
hν ≥ Eg ……………………………………………………………….……6
Equation 7:
c = λν……………………………………………………………………...6
Equation 8:
photoanode (α-Fe2O3, etc.) + sunlight → 4h+ + 4e-……………………….8
Equation 9:
4OH- + 4h+ → O2 + 2H2O………………………………………………...8
Equation 10: H2O + photocatalyst + sunlight → H2 + ½O2……………………………..8
Equation 11: ε = [(jP ¯ E°rev) / Io] ¯ 100 %...................................................................19
Equation 12: D = 0.9λ / (βcosθ)………………………………………………………..20
Equation 13: Fe + 4Fe2O3 → 3Fe3O4…………………………………………………..23
Equation 14: η = 1.23 / V…………………………………………………………..…..58
Equation 15: χ = P / H2………………………………………………………………...58
xiv
1-1
Introduction
Hydrogen (H2) as an alternative fuel with respect to fossil fuels is of great interest
due to the fact that it is very attractive as an environmentally friendly energy carrier and
source; assuming that it is produced in an environmentally friendly way. Hydrogen may
be utilized either by combustion, producing work, or by running it through a fuel cell to
generate electricity. Both systems produce only water (H2O) as a byproduct and minimal
pollution in the form of nitrogen oxides.1-5
It is very expensive and time consuming to harvest hydrogen in any appreciable
amount from air because it makes up only 0.00005 % of the earth’s atmosphere by
volume.6 Currently steam reformation of natural gas is the primary means of producing
hydrogen, but unfortunately this process is heavily dependant upon the use of fossil fuels
which generate pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOx) and carbon
dioxide (CO2), as well as others.1-7 These pollutants are responsible for undesirable
phenomena such as acid rain and smog, and some would argue that the emission of
carbon dioxide as well as other greenhouse gases contributes to global warming. All of
these phenomena affect a negative impact upon the environment and human health.3-9
Fossil fuels are also non-renewable resources and as supplies diminish energy prices
associated with their use will continue to rise, especially with world-wide energy
consumption continually increasing.4-5
1
Another method utilized for producing hydrogen is water electrolysis, which is
the process of breaking a water molecule into its constituent parts by applying a voltage
across two electrodes immersed in an aqueous electrolyte. The electrolyte completes the
circuit allowing current to flow through the solution, and in so doing, if the applied
potential is of a sufficient magnitude, the water chemically breaks down and forms
hydrogen (H2) at the cathode and oxygen (O2) at the anode (Figure 1.1). Theoretically,
the minimum voltage required to split water is 1.23 V at 25 °C, based on the Nernst
equation, although realistically a voltage greater than 1.5 V is necessary due to electrode
overpotentials.4-5, 10
Overpotential is a potential barrier that must be overcome before
current can flow through the electrode / solution interface. Water electrolysis is ideal due
to the great abundance of water available on this planet.
However, the electricity
required to operate this type of system is still generated using conventional methods such
as the burning of fossil fuels.4
Ultimately, current processes of generating such a clean fuel as hydrogen require
methods which are not environmentally friendly. However, clean energy sources such as
hydroelectric, wind, and solar may be applied to water electrolysis technology.1-5, 7, 10-14
Hydroelectric and wind power are limited in that there are only so many places where
one may build a dam or wind farm, but solar energy has shown great promise in
providing solutions to the world’s energy problems and for producing clean sustainable
energy in the near future.
2
Vapp
O2
4H2O → 4H+ + 4OH-
(1)
4OH- → 2H2O + O2
(2)
4H+ + 4e- → 2H2
(3)
___________________________
H2
2H2O → 2H2 + O2
Anode (+)
(4)
Cathode (-)
Figure 1.1: Water electrolysis by conventional means. When a potential is applied to
the electrodes, the electrolyte completes the circuit and allows current to flow. If the
applied potential is high enough to overcome the water splitting potential (1.23 V) and
the electrode overpotentials, then water molecules dissociate into hydronium ions (H+)
and hydroxyl ions (OH-) (1). The hydroxyl ions are oxidized at the anode and form
oxygen molecules (2). The hydrogen ions are reduced at the cathode and form hydrogen
molecules (3). Because there are two hydrogen atoms to every oxygen atom in the water
molecule, twice as much hydrogen gas is produced with respect to oxygen (4).
The process of photoelectrochemical decomposition of water using semiconductor
photoelectrodes was first reported in 1972 by Fujishima and Honda.15 Titanium dioxide
(TiO2) was used to absorb ultraviolet (UV) radiation and internally generate an electric
potential due to the separation of charges within the material. The potential was not very
large, but when it was enhanced by applying a small outside bias it was observed that
water could be electrochemically split into hydrogen and oxygen gases.
Titanium dioxide may absorb UV light up to 414 nm, which makes up only a
small portion of the total solar spectrum (Figure 1.2), because its band gap is relatively
high at 3.2 eV.16 Due to titanium dioxide not being able to absorb enough solar energy, it
3
is unable to produce sufficient voltage required to split water; this is the reason why
Fujishima and Honda had to apply an outside potential bias to their electrode in order for
their system to work. This limitation indicates that titanium dioxide cannot itself stand
up to the task of photoelectrochemically generating hydrogen.
Figure 1.2: Standard, AM 0 (upper) and AM 1.5 (lower), solar spectrum. UV range is
from 115 to 400 nm; visible range is from about 400 to 800 nm. The area under the UV
portion of the curve is much less than the area under the visible portion of the curve.
A great deal of effort has been invested into devising photoelectrochemical cell
(PEC) systems that will generate hydrogen efficiently and at low cost.1-2, 5, 10-13, 15, 18-23
One proposed system involves submersing a multijunction PEC electrode, coated with a
transparent conductive oxide (TCO) layer, into an electrolyte which would then
spontaneously generate the gas upon illumination with sunlight (Figure 1.3).1, 20-21 An
alternative approach utilizes hybrid multijunction PEC electrodes having photoactive
semiconductor (PAS)-electrolyte junctions.21-23 The photoelectrodes utilized are triple
junction amorphous silicon (a-Si) solar cells.20-25 The advantage of using these devices is
4
that they may be fabricated at low cost and they absorb in the visible portion of the solar
spectrum which contains the greatest amount of radiation with sufficient energy for water
splitting that is able to reach the surface of the earth, allowing for the most efficient use
of the solar spectrum.11 Unfortunately, the silicon alone will deteriorate in the electrolyte
(Equation 5), so a protective layer is necessary.
Si + 2OH- + 2H2O → SiO2(OH22-) + 2H2
(5)
Figure 1.3: PEC system utilizing a TCO protective layer deposited upon a multijunction
solar cell.14 From left to right, the H2 catalyst could be platinum islets or a molybdenum
compound, the multijunction is a-Si, the transparent protective film could be ITO or TiO2,
and the O2 catalyst could be a cobalt compound.20
The first PEC system described requires the TCO as corrosion protection.
Examples of possible TCO materials for use in basic media include indium-tin oxide
(ITO), fluorine doped tin dioxide (F-SnO2, FTO), and titanium dioxide.1, 5, 13, 20, 26 The
5
alternative PEC system does not need to be coated with a TCO. Instead the topmost
junction of the multijunction is replaced with a photoactive semiconductor material
which must be chemically stable (Figure 1.4). An example of a possible photoactive
semiconductor material for use in basic media is iron (III) oxide (hematite, α-Fe2O3).21-23
Figure 1.4: General design for a hybrid multijunction PEC.21 α-Fe2O3 would act as the
photoactive semiconductor, replacing the top cell of the solid-state multijunction (a-Si).
The interface layer is a very thin layer of ITO and is used to reduce the series resistance
between the solid-state multijunction and the photoactive semiconductor. The metal
substrate is generally stainless steel and the hydrogen evolution reaction (HER) catalyst is
usually platinum islets, or a molybdenum compound.
When a semicounductor electrode becomes illuminated, it absorbs photons with
energies (hν) greater than its band gap energy (Eg),
hν ≥ Eg
(6)
where h is Planck’s constant (4.14 x 10-15 eV·s) and ν is the frequency of the light. The
frequency may be obtained from the wavelength (λ) by the relationship,
c = λν
6
(7)
where c is the speed of light (3 x 108 m/s).
The band gap is the energy difference between the material’s valence and
conduction bands. Materials with band gaps greater than 4 eV are considered insulators;
those with little or no band gap are considered metals. Semiconductors lie in between.
When photon energy is absorbed, electrons from the valence band are excited up into the
conduction band and a positively charged hole is left in the valence band.
Semiconductors possessing n-type characteristics (α-Fe2O3, F-SnO2) have upward band
bending; the excited electrons move to the back of the electrode and the holes move to
the surface (Figure 1.5). The reaction that takes place in solution (Figure 1.6) is similar
to that for standard electrolysis. However, with a self-driven PEC no outside voltage bias
needs to be applied. All of the voltage required to run the reaction is generated within the
device itself and the oxygen and hydrogen gases are generated on the front and back
surfaces, respectively.
FTO is a good candidate as a TCO for self-driven PECs because it acts as an
optical window allowing photons of interest to pass through to be absorbed within the
multijunction on which it is deposited.28-36
The band gap of FTO is near 3.3 eV, and as
with titanium dioxide it will only absorb photons in the UV region of the spectrum.28
FTO is sufficiently conductive, and it has been found to be stable in aqueous alkaline
solutions. FTO thin films have been fabricated by various methods including chemical
vapor deposition (CVD), evaporation, spray pyrolysis, sol-gel, and sputtering.28-36
7
eEC
Surface
EF
Eg
hν
EV
+
Figure 1.5: When light with energy hν hits the semiconductor electrode, electrons may
become excited up to the conduction band (EC) from the valence band (EV). The
electrons (e-) then move to the back of the electrode while holes (+) accumulate at the
front surface. EF, the Fermi level, is the highest energy state which electrons may occupy
at absolute zero. In p-type semiconductors it is located closer to the valence band, and in
n-type semiconductors it is located closer to the conduction band.
4H2O → 4H+ + 4OH-
(1)
photoanode (α-Fe2O3, etc.) + sunlight → 4h+ + 4e-
(8)
4OH- + 4h+ → O2 + 2H2O
(9)
4H+ + 4e- → 2H2
(3)
The overall reaction is given as:
H2O + photocatalyst + sunlight → H2 + ½O2
(10)
Figure 1.6: General reaction mechanism for the evolution of hydrogen and oxygen using
a self-driven PEC system utilizing thin film semiconductors. Water adsorbs onto the
photoanode and dissociates into hydronium (H+) and hydroxyl ions (OH-). Formation of
electron and hole pairs occurs at the photoanode where O2 is formed and H2 is formed at
the cathode.
8
Iron (III) oxide, among other iron oxides, is commonly found as rust which is
formed from iron (Fe) in the presence of water. However, it may be produced as the
anhydrous form which has been found to be a cheap and useful semiconductor material.
α-Fe2O3 has a band gap of 2.0 to 2.2 eV and may absorb electromagnetic radiation with
wavelengths up to 621 nm which is in the visible portion of the solar spectrum. Its band
gap makes it a perfect candidate as the top junction for hybrid multijunction PECs
because it will absorb in the same range as the portion of the multijunction it replaces.
Also, α-Fe2O has been found to demonstrate photoactivity which is necessary for this
application; a photocurrent of at least 7.5 mA/cm2 is required in order to effectively
current match with the middle and bottom component solar cells of the multijunction.
Current matching is important because the cell with the lowest current will limit the
performance of the entire device. A protective TCO layer is not necessary for this system
due to the stability of α-Fe2O3 in aqueous alkaline solutions. As with FTO films,
hematite thin films may be fabricated by a number of methods including CVD, sol-gel,
spray pyrolysis, laser ablation deposition, wet chemical deposition, and sputtering.37-53
The problem with pure α-Fe2O3 is that it is a resistive semiconductor, and therefore the
introduction of dopants of higher valence to make it more n-type, or lower valence to
make it more p-type, has also been investigated to enhance its electrical characteristics.5459
Generally, optimized α-Fe2O3 and FTO thin films have been deposited or
annealed at temperatures exceeding 400 °C. However, this is undesirable because if the
amorphous silicon multijunctions are subjected to temperatures greater than 250 °C then
9
they significantly degrade.
Therefore methods of producing these materials at low
temperatures are being investigated.50, 59
The thin films described in this work were fabricated using a process called radio
frequency (r.f.) sputter deposition. In the process, the substrates onto which the films are
to be deposited are placed into a vacuum chamber, also known as the sputter chamber,
and all of the air is pumped out of the chamber until sufficient vacuum (10-5 torr or less)
is obtained. Argon (Ar) gas is then passed into the chamber. Within the chamber there
may be one or multiple sputter guns onto which targets are placed; targets are disks made
out of the materials that are to be deposited, or their metal counterparts that will be
oxidized during the deposition. A r.f. power supply is connected to the sputter gun and
when the power is turned on an electric field is generated between the target and the
substrate. The current is alternating, so free electrons are alternately attracted to and
repelled from the target. As the electrons move between the target and the substrate they
collide with argon atoms and knock off more electrons.
This process continues
exponentially until a cloud of argon ions (Ar+) and electrons, called plasma, is generated.
The argon ions bombard the surface of the target and physically knock off pieces which
are ballistically ejected and deposited onto the substrate (Figure 1.7). The same process
would occur at the surface of the substrate, however magnets placed within the sputter
gun confine the plasma to a region immediately above the target. In this way, small
pieces of target material are sputtered onto the substrate and over time a thin film is
developed.
10
α-Fe2O3, SnF2/SnO2, etc.
substrate holder
substrate
Ar+
Ar+
target
cap (anode)
sputter gun (cathode)
Figure 1.7: Argon ions (Ar+) from the plasma cloud, confined just above the target by a
magnetic field generated by magnets located within the sputter gun, bombard the target
physically removing small amounts of the target material upon impact. The particles,
which are neutrally charged, are ejected ballistically and deposit on a substrate. Over
time a very thin film of target material builds up on the surface of the substrate. The
substrate is rotated in order to ensure a uniform substrate temperature and film deposition.
11
1-2
A.
Experimental
Preparation for the Deposition of n-type α-Fe2O3 and F-SnO2 Thin Films by
R.F. Sputter Deposition
The substrates used for the thin film depositions were borosilicate microscope
slides (1 ¯ 25 ¯ 75 mm, Erie Scientific Company). The films were deposited on both
plain slides and on slides which had been r.f. sputter-coated with ITO films (70 nm thick).
The ITO was deposited from an indium oxide (In2O3) / tin dioxide (SnO2) target (90 wt%
and 10 wt% respectively, K. J. Lesker) in a custom-built sputter deposition chamber with
a gun power of 60 W at a temperature of 200 °C for 9.25 min. The glass slides were
cleaned in a de-ionized water / acetone bath placed in a sonicator (Branson 8510), and
then dried under a stream of nitrogen (N2) gas (99.999 %, Linde). Thin films were also
deposited on Tec-15 3.2, FTO coated glass (3.2 ¯ 25 ¯ 75 mm, Pilkington). The Tec-15
glass was cut by hand from a 1 ft2 sheet, and the sized pieces were cleaned in the same
manner as the microscope slides. Four to five glass substrates were placed in a 4 ¯ 4
inch stainless steel substrate holder (Figure 1.8). Strips of stainless steel were used to
cover parts of the substrates allowing for bare electrical contacts (ITO, FTO) necessary
for photocurrent and stability measurements.
The substrate holder was placed into the deposition chamber through a removable
flange on the side of the chamber. Steel tracks held the substrates over the target(s); up to
12
three targets (2 inch diameter) placed on three individual sputter guns could be utilized
simultaneously. Once the substrates were centered over the target(s), the flange was
replaced and a rotary vane vacuum (roughing) pump (DUO 10C, PK D62 727 B, Pfeiffer)
was used to pump the chamber down to about 100 mTorr. The roughing pump was then
isolated from the chamber and a turbomolecular (turbo) pump (TMU 261P DN 100 CF-F
3P, PM P02 826H, Pfeiffer) was used to pump down to less than 10-5 torr. Chamber
pressure was monitored with a vacuum gauge measurement and control system (type
146C, MKS).
A convection gauge (MKS) was used to measure pressure from atmospheric to
about 100 mTorr, a Baratron capacitance manometer (MKS) was used to measure
pressures in an operating range of 1 to 100 mTorr, and an ion gauge (MKS) was used to
glass
Tec-15
metal cover
ITO on glass
glass
m
e
t
a
l
c
o
v
e
r
g
l
a
s
s
Figure 1.8: Diagram of standard substrate orientation in the substrate holder, viewed
from the deposition side. One piece of Tec-15 glass and one piece of ITO coated glass
were placed in the center of the substrate holder and plain glass microscope slides were
used to fill in the rest of the spaces. Thin pieces of stainless steel were used to block part
of the thin film deposition in order to leave bare electrical contacts used for measuring
photocurrent and film stability. The deposition on the plain glass was used for
transmission measurements.
13
measure pressure in ultra high vacuum (UHV). Once the chamber pressure was below
10-5 torr, gas flow, substrate heating, and substrate rotation was initiated.
The gases utilized in the deposition of the thin films were argon (99.998 %,
Linde) and an oxygen / argon mixture (20 % O2 and Ar balance, Linde). Gas flow was
controlled with nitrogen mass flow controllers (MKS) connected to a four-channel
readout (type 247, MKS). Two transformers, an isolation transformer (115 V, 8.7 A,
Stancor GIS 1000) in line with a variable transformer (140 V, 10 A, Staco Energy
Products Company), were connected to two halogen bulbs (J120V-500W/FCL, USHIO)
wired in parallel. The bulbs were housed above the substrate holder; reflectors directed
the radiation down onto the substrate holder allowing for substrate heating up to 450 °C.
Temperatures were set via the variable transformer, but the units were of an arbitrary
scale. Therefore a multimeter (OmegaetteTM, Omega) was connected to the transformer
and a temperature versus voltage calibration was done prior to depositions (Figure 1.9).
During the calibration the thermocouple (K-type, Omega) was attached directly onto the
substrate holder, but during deposition the thermocouple could not be directly attached to
the substrate holder due to its rotation; it was attached to part of the lamp housing in a
location that would not obstruct the deposition process. Heat transfer in vacuum is slow
and therefore the temperature gauge (Omega) reading was lower than the actual
temperature of the substrate; during depositions the temperature gauge reading was taken
only as a relative value of ancillary importance. A sample rotator (K. J. Lesker) ensured
uniform heating and deposition.
14
450
400
Temperature (°C)
350
300
250
200
150
100
50
0
0
10
20
30
40
50
60
70
80
90 100 110
Potential (V)
Figure 1.9: Temperature calibrations for the vacuum deposition chamber. After several
runs, spot checks were done to make sure the bulbs were maintaining the same power
density. Full calibrations were done after every change of the halogen bulbs.
B.
Fabrication of n-type α-Fe2O3 With and Without Incorporation of Metal
Dopants
When the substrate temperature had reached the desired value, the r.f. generator
(ACG-3B, ENI) was turned on and set to the desired value. Oxide targets (e.g. α-Fe2O3)
are brittle and the power had to be increased at a slow rate (1 W per second) to prevent
the target from cracking from an increase of vibrations and temperature within the target.
After the plasma was ignited it was allowed to sit for 10 to 15 minutes in order to reach a
steady state for deposition. A shield located over the target prevented the substrate from
getting coated during the pre-sputtering. After pre-sputtering, the shield was raised and
the sputter deposition was initiated.
Pure α-Fe2O3 thin films were directly deposited from a 0.25 inch thick hematite
target (99.9 wt%, K. J. Lesker) under varying conditions. The parameters that were
15
varied were rf power (50 to 100 W), substrate temperature (200 to 475 °C), oxygen
concentration in the chamber atmosphere (0 to 10 % by volume in Ar ambient), and
deposition time (90 to 150 min). The chamber pressure, controlled by a gate valve
located just above the turbo pump inlet, was held at 6 mTorr during all depositions.
Doped α-Fe2O3 was done by co-sputtering. The process was the same as that for
the deposition of pure α-Fe2O3, however two targets (hematite and the dopant material)
were sputtered simultaneously. For the deposition of tantalum (Ta) doped α-Fe2O3, the
α-Fe2O3 power was held at 80 W while a 0.25 inch thick tantalum (99.9 wt%, K. J.
Lesker) target power was varied from 5 to 30 W; temperature was varied from 200 to
300 °C. Indium (In) doping was done with a 0.25 inch thick indium target (99.995 wt%,
K. J. Lesker). The α-Fe2O3 power was held at 100 W while the indium power was varied
from 5 to 20 W, and the temperature was varied from 150 to 250 °C. The oxygen
concentration in the chamber atmosphere was also varied from 0 to 10 % by volume in
argon ambient.
The chamber pressure was held at 6 mTorr for all doped sample
depositions.
C.
Fabrication of n-type F-SnO2 (FTO)
FTO thin films were deposited in the same manner as the α-Fe2O3 thin film
depositions. A 0.25 inch thick tin fluoride (SnF2) / tin dioxide (SnO2) target (25 wt% and
75 wt% respectively, Cerac, Inc.) was used as the sputter source. Films were deposited at
temperatures of 250 and 300 °C. Power was varied from 40 to 60 W, the oxygen
concentration was varied from 10 to 20 % by volume in argon ambient, chamber pressure
16
was varied from 6 to 15 mTorr, and deposition time was varied from 60 to 165 min.
Figure 1.10 shows the actual sputter chamber used for all thin film depositions.
3
4
5
6
1
2
7
Figure 1.10: All of the thin films were fabricated in this sputter chamber. The flange /
door is located on the left-hand side of the chamber (1) near the r.f. generators (2). The
sample rotator is located at the top of the chamber (3) along with the power input for the
lamps (4) and the thermocouple (5). The vacuum gauges are located on the right side of
the chamber (6), above the pressure and gas flow control panel (7).
17
1-3
A.
Thin Film Characterization
Photocurrent Measurements
Photoelectrochemical measurements were made in a 250 ml round bottom flask
with three ground glass fittings (NDS Technologies, Inc.). The electrode set-up consisted
of a working electrode (n-type α-Fe2O3 and F-SnO2 thin films), which had an average
area of 1.0 cm2, a platinum gauze counter electrode (99.9 wt%, 52 mesh, Alfa Aesar), and
a reference electrode (Radiometer analytical saturated calomel electrode, SCE). The
surfaces of the working electrodes were illuminated with a 100 W xenon lamp (Oriel
Instruments) with a light intensity of 0.75 suns. An Oriel Research arc lamp system (100
W) was used. The intensity of the light was measured by placing a silicon thermopile
(UDT Sensors, Inc.) in the path of the light beam in the same location as where the
working electrodes were illuminated, and a digital multimeter (Radioshack, model
number 22-813), to which the thermopile was connected, represented the intensity as
current. The value obtained was compared to a standard current of 25 mA for 1 sun light
intensity. The electrolyte solution for the n-type electrodes was 33 % (5.9 M) potassium
hydroxide (KOH). Photocurrent (jP, mA/cm2) as a function of applied potential (V) vs.
SCE were measured using a Voltalab PGZ 301 potentiostat and plotted using
VoltaMaster 4 Electrochemical Software version 5.10. An applied potential range of
18
–0.5 to +1 V was used at a scan rate of 20 mV/s. The thin film electrodes were
illuminated with 5 s chopped light.
The total photoconversion efficiency for a self-driven PEC (ε, %), may be
calculated by using the following equation given as,57-58
ε = [(jP ¯ E°rev) / Io] ¯ 100 %
(11)
where jP is the measured photocurrent, E°rev is the standard state reversible potential (1.23
V for splitting water) and Io is the light intensity.
B.
Stability Measurements
Film stability measurements were made using the same set-up used for the
photocurrent measurements. Under constant ambient lighting, an applied potential cycle
of –1 to +3 V was applied at a scan rate of 60 mV/s. Films were determined to be stable
when the forward and reverse scans were linear (beyond the onset potential) and no redox
reaction was observed.
C.
UV-vis Spectroscopic Measurements
UV-vis spectra of n-type α-Fe2O3 and FTO thin films were recorded using a
UV/VIS/NR-Cary 5 Diode Array (HP8452A).
19
Samples deposited on plain glass
microscope slides were used, with a pure glass substrate as a baseline. The spectra of all
samples were measured in the wavelength range between 300 and 2000 nm.
Film thickness was calculated using an interference method, a procedure
developed for calculating the thickness of thin films.60 Band gap values were determined
from tauc plot calculations.61
D.
X-ray Diffraction Spectra Measurements
X-ray diffraction (XRD) spectra were collected on a X-ray powder difractometer
(X’Pert Pro, PANalytical) with a Dell Optiplex PC utilizing X’Pert Data Collector
software. The scans were collected via a glazing angle in the range from 25 to 75 ° (2θ)
using copper Kα radiation with a wavelength of 0.15405 nm. Scans were analyzed with
X’Pert High Score Plus software (PANalytical) and matched against the International
Center for Diffraction Data (ICDD) database.
The crystal size of the thin film may be determined by applying the XRD data to
Scherer’s equation,62
D = 0.9λ / (βcosθ)
(12)
where D is the average crystal size, λ is the wavelength, β is the peak width at half
maximum, and θ is the diffraction peak angle.
20
E.
Thin Film Thickness Measurements
The thickness of the n-type α-Fe2O3 and the FTO thin films was measured using a
Dektak3ST surface profiler, as well as by the interference method using UV-vis
spectroscopy.60-61
F.
Atomic Force Microscopy Measurements
Atomic Force Microscopy (AFM) measurements were done with a PicoSPMII
system (Molecular Imaging). The AC-AFM non-contact mode was utilized.
G.
Annealing
Selected films were annealed in a high temperature box furnace (ST-1700-666,
Sentro Tech Corporation). A flow of industrial grade argon gas (99+ %, Linde) was
directed down into the furnace through a feed-through located at the top of the furnace to
create an inert atmosphere. Samples that were annealed were cut from larger samples so
that trends could be observed as annealing conditions were changed.
21
1-4
A.
Results and Discussion
n-type α-Fe2O3
The results of photocurrent-potential dependence were optimized with respect to
several parameters including r.f. power, substrate temperature, oxygen concentration in
the argon ambient and deposition time. α-Fe2O3 films were deposited on both ITO
coated glass and on Tec-15. Depositions on the Tec-15 produced films demonstrating
generally higher photocurrents following clearer trends due to consistent film quality.
The Fe2O3 targets used for the thin film depositions contained small amounts of
pure iron (Fe) which the manufacturer added in order for the targets to be pressed more
easily during fabrication. Also, during the sputtering process an oxygen deficiency may
occur in an inert atmosphere contributing to the reduction of the Fe2O3 target to Fe3O4,
then FeO, and then to pure Fe.42, 63 Film deposition in an inert argon atmosphere resulted
in poor quality films that were often times magnetite, not hematite. Magnetite is black in
color, conductive and magnetic, though not photoactive. Addition of oxygen to the
chamber atmosphere during deposition alleviated the problem and was found to enhance
the stability and photoactivity of the films because the free iron from the target was
oxidized to Fe2O3 and reduction of the target was minimized (Figure 1.11).
The
optimum oxygen concentration was determined to be 2 % in argon ambient.
The
reduction of hematite to magnetite is expressed in Equation 13.
22
2
Photocurrent Density (µA/cm )
160
140
120
100
80
60
40
20
0
0
2
4
6
8
10
O2 Concentration (%)
Figure 1.11: Photocurrent density (jP, µA/cm2) as a function of oxygen concentration
(%) in argon ambient of n-type α-Fe2O3 thin films deposited at 400 °C for 120 min with
100 W deposition power. Adding oxygen to the chamber atmosphere allowed any free
iron to oxidize (reactive sputtering) and prevented the reduction of the target, resulting in
higher quality films that were more stable in solution during electrochemical testing.
Fe + 4Fe2O3 → 3Fe3O4
(13)
Over a range of deposition temperatures from 250 to 475 °C, n-type α-Fe2O3 thin
films were found to exhibit greater photoactivity as substrate temperature was increased
(Figure 1.12). However, when depositions were carried out at the higher end of the
temperature range, the halogen bulbs tended to degrade quickly because the power
required to attain those temperatures was near the bulbs maximum power rating. After
running depositions at high temperatures, and therefore high voltages, the actual
temperature would be lower the next time a deposition would be done at a given voltage.
As the bulbs degraded, more power was required to reach the same temperature over
23
subsequent runs; this was not recognized initially. All temperature data was normalized
to correct values using calibrations.
a
2
Photocurrent Density (µA/cm )
250
200
150
100
50
0
225
250
275
300
325
350
375
400
425
375
400
425
Temperature (°C)
2
Photocurrent Density (µA/cm )
300
b
250
200
150
100
50
0
225
250
275
300
325
350
Temperature (°C)
Figure 1.12: Photocurrent density (jP, µA/cm2) versus substrate temperature (°C) for ntype α-Fe2O3 thin films deposited on (a) Tec-15 glass and (b) ITO. All samples
represented were sputtered with a target power of 100 W with 2 % oxygen in argon
ambient for 110 min.
24
The α-Fe2O3 films were found to be stable in 5.9 M potassium hydroxide (KOH)
up to 3 V versus SCE at all deposition temperatures investigated once oxygen had been
added to the chamber atmosphere. Films that were deposited for 90 min exhibited the
greatest degree of stability and as deposition time and therefore film thickness increased,
some stability was lost (Figure 1.13), although conductivity increased as the films
became thicker. Photocurrent density versus film deposition time was plotted (Figure
1.14) and an optimum deposition time of 110 min was obtained.
135 min
250
2
Current Density (mA/cm )
300
200
150
110 min
100
105 min
50
90 min
0
0
1
2
3
Potential (V)
Figure 1.13: Stability scans for n-type α-Fe2O3 thin films deposited with a power of 100
W at 400 °C for various amounts of time. Films deposited for 90 min were more stable,
but not very conductive. As the deposition time was increased to 135 min the film
stability decreased by a small amount, and the film conductivity increased.
25
a
2
Photocurrent Density (µA/cm )
250
200
150
100
50
0
80
100
120
140
160
Deposition Time (min)
b
2
Photocurrent Density (µA/cm )
300
250
200
150
100
50
0
80
100
120
140
160
Deposition Time (min)
Figure 1.14: Photocurrent density (jP, µA/cm2) as a function of film deposition time
(min) for α-Fe2O3 thin films deposited on (a) T-15 glass and (b) ITO. The optimum
deposition time was found to be 110 min with the Fe2O3 deposition power set to 100 W.
UV-vis transmission spectra were used to calculate an average film thickness of 280 nm.
Most n-type α-Fe2O3 thin films demonstrated photoactivity upon illumination, but
only to a small degree. The greatest photocurrent observed, ~ 0.34 mA/cm2 under 0.75
sun illumination, occurred for a sample deposited with an r.f. power of 100 W, at 400 °C
26
with 2 % oxygen in argon for 110 min. The resulting efficiency was 0.56 % at a potential
of 0.38 V (with an open circuit potential of 0.382 V). Films demonstrating the highest
photocurrents always showed a sharp response to chopped light illumination (Figure
1.15). However, immediately after the sharp response a drop in the photocurrent density
was observed until a steady state was achieved. The drop in performance is attributed to
the poor conductivity of the films.
The gas flow rate was initially set at 8 sccm for the pure argon feed, and 2 sccm
for the 10 % oxygen in argon feed, resulting in the 2 % mixture in the chamber.
Eventually, the 10 % oxygen in argon gas cylinder was replaced with a 20 % oxygen in
argon gas cylinder. The optimized flow rates were then changed to 18 sccm for the argon
feed and 2 sccm for the mixture feed. There was no observable difference in the films
after the change of cylinders. The chamber pressure was held at 6 mTorr.
2
Photocurrent Density (mA/cm )
0.5
0.4
light
0.3
0.2
0.1
0.0
dark
-0.1
-0.2
-500
-250
0
250
500
Potential (mV)
Figure 1.15: Chopped scan of n-type α-Fe2O3 under alternating 0.75 sun illumination
and ambient room lighting (dark). When the film surface was illuminated, photocurrent
was generated, demonstrated by a sharp increase in current density (mA/cm2). When the
light source was then blocked, the current density sharply droped back to the dark current
density value.
27
X-ray diffraction was used to verify the composition of the thin films as α-Fe2O3,
and to determine if there was any correlation between a films performance and its
crystallinity. Glazing angle scans, where the x-ray beam contacts the film surfaces at a
constant low angle, were used instead of standard scans because of the thinness of the
films. Using a glazing angle scan allows for the x-ray beam to contact as much of the
film material as possible by entering at a very shallow angle of incidence.
Film crystallinity appears to be a function of both film thickness and deposition
temperature (Table 1.1). Thicker films deposited at the same temperature tended to
demonstrate increased crystallinity and crystal size.
Films deposited at higher
temperatures also demonstrated greater crystallinity and crystal size (Figure 1.16). More
samples should be tested to see if the observed trends in crystal size are statistically
significant.
Table 1.1: Crystal size (nm) based on α-Fe2O3 deposition conditions. Crystal size
increases as deposition temperature increases, and increases as film thickness increases.
SAMPLE
TEMP. (°C)
TIME (min)
110
110
THICKNESS
(nm)
394
336
CRYSTAL
SIZE (nm)
85
316
ST491
ST601
250
375
ST587
ST592
ST496
400
400
400
110
110
110
208
236
288
108
133
191
28
a
20000
18000
288 nm
Counts
16000
14000
12000
10000
236 nm
8000
208 nm
6000
4000
25
30
35
40
45
50
55
60
65
2θ (deg)
21000
b
18000
Counts
15000
375°C
12000
9000
250°C
6000
25
30
35
40
45
50
55
60
65
2θ (deg)
Figure 1.16: XRD spectra for n-type α-Fe2O3 thin films measured from 25° to 65° (2θ).
As film thickness increases, the peaks become more intense and defined indicating
increased crystallinity (a). As deposition temperature increases, the peaks become more
intense and defined indicating increased crystallinity (b). All peaks correspond to only
α-Fe2O3.
Film thicknesses were compared to deposition times in order to better understand
deposition rates (Figure 1.17). A Dektak3ST surface profiler was used to obtain the film
thickness measurements. Fe2O3 films deposited at 400 °C with 2 % oxygen in argon, but
29
with deposition times varying from 90 to 150 min, were analyzed. The deposition rate
for a Fe2O3 target with an r.f. power of 100 W appears to be non-linear overall, though a
linear fit may work well for depositions between 90 and 120 minutes. Film thickness
decreases after 135 minutes. It is hypothesized that the film, sitting in a hot environment
for an extended period of time, was effectively annealed. Enough energy was available
for the Fe2O3 particles to reorganize themselves, filling in any voids created during the
sputtering process thus resulting in a thinner film. Further study must be conducted in
order to confirm the observed trend.
280
260
Thickness (nm)
240
220
200
180
160
140
120
100
80
80
90
100
110
120
130
140
150
160
Deposition Time (min)
Figure 1.17: Film thickness as a function of deposition time. All films were deposited
under similar conditions. The error bars are one standard deviation.
UV-vis transmission spectra (Figure 1.18) were used to determine that the αFe2O3 film transparencies were near 70 %. Tauc plots were generated from the spectra
and used to determine film band gaps in the expected range of 2 to 2.2 eV (Figure 1.19).
UV-vis transmission spectra were also utilized in film thickness calculations using the
interference method.60 However, the thickness values obtained from the interference
30
method may not be accurate due to the lack of enough interference fringes necessary for
accurate calculations. A Dektak3ST surface profiler was used to make multiple thickness
measurements at various points at the edge of the films; they were averaged and used as
the scientifically correct values.
Atomic force microscopy (AFM) was used to generate images of the surfaces of
selected α-Fe2O3 thin films (Figure 1.20). Films deposited at higher temperatures appear
to have greater surface roughness indicating greater surface area. Films deposited at
higher temperatures demonstrate greater photoactivity due to the increased active surface
area of the films.
Transmission (%)
100
80
60
40
20
0
300
600
900
1200
1500
1800
2100
Wavelength (nm)
Figure 1.18: UV-vis transmission spectra of n-type α-Fe2O3 thin film deposited at
400 °C for 110 min. Due to the film being very thin (285 nm) there are very few
interference fringes. A tauc plot was used to determine a band gap of 2.04 eV.
31
0.09
0.08
0.07
sqrt(αhν)
0.06
0.05
0.04
0.03
0.02
0.01
0.00
2.00
2.05
2.10
2.15
2.20
2.25
hν
Figure 1.19: Tauc plot for an n-type α-Fe2O3 thin film deposited for 110 min at 400 °C
with a Fe2O3 r.f. deposition power of 100 W in a chamber atmosphere containing 2 %
oxygen in argon ambient at a pressure of 6 mTorr. The band gap was determined to be
about 2.04 eV.
Figure 1.20: AFM images of n-type α-Fe2O3 thin films deposited at, from left to right,
300, 350, and 400 °C. The dimensions of each image are 5000 ¯ 5000 nm. Films
deposited with higher substrate temperatures have rougher surfaces and demonstrate
greater photoactivity.
An n-type α-Fe2O3 thin film was sputter deposited onto the top of an a-Si triple
junction solar cell in an effort to determine if the film would protect the solar cell under
operating conditions in 5.9 M potassium hydroxide. Epoxy was used to seal the edges of
the cell to prevent the electrolyte from contacting the solar cell / α-Fe2O3 interface.
32
However, after a couple of minutes of testing it was found that the electrolyte was able to
find its way under the epoxy and to the interface region where it began to react with the
silicon. The α-Fe2O3 was able to protect the solar cell from the top, but the cell broke
down from the inside out. A better method for sealing the edges of the solar cell is being
investigated.
B.
Tantalum Doped n-type α-Fe2O3
Tantalum (Ta) doped Fe2O3 (Ta-Fe2O3) showed some stability in 5.9 M potassium
hydroxide at the lower tantalum deposition powers, but as the amount of tantalum in the
films increased the stability was reduced. There was no appreciable photoactivity in any
of the tantalum doped α-Fe2O3 samples, most values were near 1 µA/cm2.
Focus was shifted away from studying the tantalum doped thin films due to their
poor performance. No solid correlations between deposition conditions and performance
were made.
C.
Zirconium Doped n-type α-Fe2O3
Zirconium (Zr) was co-deposited with Fe2O3 at 200 °C with Zr r.f powers
ranging from 5 to 20 W and the Fe2O3 power held at 100 W.
No appreciable
photocurrents were observed and investigations into this material were suspended.
33
D.
Indium Doped n-type α-Fe2O3
The results of photocurrent-potential dependence were optimized with respect to
several parameters, including r.f. power and substrate temperature. The indium (In)
doped Fe2O3 (In-Fe2O3) films were found to be stable in 5.9 M potassium hydroxide.
Most samples showed evidence of photoactivity from 1 – 5 µA/cm2.
The greatest
photocurrent observed, ~ 33 µA/cm2 under 0.75 sun illumination, occurred for a sample
deposited on ITO at 200 °C with a Fe2O3 r.f. power of 100 W and an In power of 20 W
with 5 % oxygen in argon (Figure 1.21). The gas flow rate was 15 sccm for the pure
argon feed and 5 sccm for the 20 % oxygen in argon mixture. The chamber pressure was
held at 6 mTorr, and the deposition time was 120 min.
In doped films were annealed at 550 °C in an argon atmosphere for 2, 4, and 6
hours. Onset potential and photoresponse both were improved as the annealing time was
increased to 4 hours. An improvement of photoresponse from 33 µA/cm2 to 110 µA/cm2
was observed. However, films annealed for more than 4 hours demonstrated a drop in
performance (Figure 1.22).
As the power for the indium target was increased the film thickness increased and
more interference fringes were developed on the UV-vis spectra. Transmission was
found to be up to near 80 % in the visible portion of the spectrum for the films with
higher indium doping (Figure 1.23). The band gap of the material increased from about
2 to 2.62 eV (Figure 1.24) as the indium deposition power was increased from 5 to 20 W
respectively.
34
2
Photocurrent Density (µA/cm )
80
60
40
20
0
-20
-40
-500
-250
0
250
500
750
1000
Potential (mV)
Figure 1.21: Photocurrent density (jP, µA/cm2) versus applied potential (mV) of an InFe2O3 thin film electrode. Light and dark currents were measured using a light chopping
method by manually blocking the light source and then illuminating the electrode in 5 s
intervals.
2
Photocurrent Density (µA/cm )
120
100
4h
80
6h
2h
60
0h
40
20
0
-20
-500
-250
0
250
500
750
1000
Potential (mV)
Figure 1.22: Photocurrent density (jP, µA/cm2) versus applied potential (mV) of a InFe2O3 thin film electrode annealed up to 6 hours in an inert argon atmosphere at 550 °C.
35
X-ray diffraction techniques were used to determine that the thin films contained
α-Fe2O3 as well as indium and iron oxide compounds such as InFeO3 and InFe2O4. Pure
indium and iron metal were not found to be present within the films due to the addition of
oxygen to the deposition chamber atmosphere during fabrication. As indium deposition
power was increased from 5 to 20 W, the amount of indium compounds present within
the films increased (Figure 1.25) and crystal size also increased (Table 1.2).
As
substrate temperature was increased from 150 to 250 °C the film crystallinity increased
(Figure 1.26) and crystal size increased as well. Shifting of some peaks was observed
because the sample being measured may not have been set completely horizontal and if it
is not set perpendicular to the plane of the x-ray beam then the signals may drift.
Transmission (%)
100
80
60
40
20
0
300
600
900
1200
1500
1800
2100
Wavelength (nm)
Figure 1.23: UV-vis spectroscopic measurement of a In-Fe2O3 thin film electrode
deposited with an indium power of 20 W at 200 °C having a thickness of 980 nm by Tauc
calculations. A Dektak3ST surface profiler was also used to measure film thickness and
similar values were obtained.
36
0.09
0.08
0.07
sqrt(αhν)
0.06
0.05
0.04
0.03
0.02
0.01
0.00
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
hν
Figure 1.24: Tauc plot for a In-Fe2O3 sample deposited for 120 min at 200 °C with an
indium r.f. deposition power of 20 W and a Fe2O3 rf deposition power of 100 W. The
chamber atmosphere contained 5 % oxygen in argon ambient at a pressure of 6 mTorr.
The band gap was determined to be 2.6 eV.
a
50000
Counts
40000
b
a
a
b
a
c
a a 20 W
30000
20000
15 W
10000
10 W
5W
0
25
30
35
40
45
50
55
60
65
70
2θ (deg)
Figure 1.25: X-ray diffraction measurements of In-Fe2O3 thin film electrodes deposited
at 200 °C and with varying indium target deposition powers (5 to 20 W). Peak intensities
are greater for higher indium target powers. Peaks indicate the presence of (a) α-Fe2O3
and indium and iron oxide compounds such as (b) InFeO3 and (c) InFe2O4.
37
b
21000
18000
a
Counts
15000
a
a
b
a
c
12000
250 °C
9000
200 °C
6000
3000
150 °C
0
25
30
35
40
45
50
55
60
65
2θ (deg)
Figure 1.26: X-ray diffraction measurements of In-Fe2O3 thin film electrodes deposited
at varying substrate temperatures with an indium target deposition power of 10 W. Peak
intensities are greater for higher temperatures, indicating greater crystallinity. Film
composition includes (a) Fe2O3, (b) InFeO3, and (c) InFe2O4.
Table 1.2: Crystal size based on In-Fe2O3 deposition conditions. Crystal sizes increase
as In power is increased (with the Fe2O3 power held at 100 W) and as deposition
temperature is increased.
SAMPLE
TEMP. (°C)
TIME (min)
200
200
200
200
INDIUM
POWER (W)
5
10
15
20
120
120
120
120
CRYSTAL
SIZE (nm)
36
59
64
75
ST188
ST186
ST189
ST191
ST209
ST186
ST193
150
200
250
10
10
10
120
120
120
27
59
83
Atomic force microscopy (AFM) was used to compare the surfaces of films
deposited under varying conditions. Films that were deposited with higher indium target
powers showed greater surface roughness, and therefore greater effective surface area
38
(Figure 1.27).
Films demonstrating greater surface area tend to generate higher
photocurrents under illumination.
In-Fe2O3 exhibits stability in 33% KOH and demonstrates photoactivity under
illumination. The films are 80% transparent in the visible portion of the spectrum, and
were fabricated at low temperatures. However, the band gap for the film currently
demonstrating the greatest performance is 2.65 eV which is too high for use as the top
junction absorber layer in a a-Si hybrid photoelectrode. More work must be done to
optimize the material with respect to photocurrent and band gap.
E.
Antimony Doped n-type α-Fe2O3
Iron (III) oxide films were doped with antimony (Sb-Fe2O3) in order to enhance
the films optoelectronic properties. All depositions were done at 200 °C with the Fe2O3
r.f. power held at 100 W and varying the Sb power from 5 to 20 W. None of the samples
demonstrated any appreciable photocurrent under 0.75 sun illumination.
Annealing a Sb-Fe2O3 film at 550 °C for two hours resulted in improvement in
photoresponse from almost nothing to 0.37 mA/cm2 at a potential of 0.76 V (Figure
1.29). The film was deposited with a Sb r.f power of 20 W. No further studies were
performed on Sb-Fe2O3 samples because annealing the films defeats the purpose of
developing high quality films at low temperatures.
39
a
c
b
d
Figure 1.27: AFM images of indium doped α-Fe2O3 deposited with an indium target
power of 5 W (a, b) and 20 W (c, d). Both films were deposited at 200 °C with 5 %
oxygen in argon ambient for 120 min. Films deposited at higher temperatures
demonstrated greater photocurrents due to greater active surface areas. The scale for both
images is 5000 ¯ 5000 nm.
40
2
Photocurrent Density (mA/cm )
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
-400
-200
0
200
400
600
800
1000
Potential (mV)
Figure 1.28: Photocurrent density (jP, mA/cm2) versus applied potential (mV) of a SbFe2O3 thin film electrode. Light and dark currents were measured using a light chopping
method by manually blocking the light source and then illuminating the electrode in 5 s
intervals.
F.
n-type F-SnO2
Fluorine (F2) doped tin oxide thin films (F-SnO2) were deposited primarily at
250 °C at varying deposition times. Almost all of the FTO samples demonstrated poor
stability in 5.9 M potassium hydroxide, although some films demonstrated appreciable
photoresponse upon illumination. The highest quality films demonstrated photocurrents
of ~ 0.2 mA/cm2 (Figure 1.29) and transparencies near 90 % in the visible range (Figure
1.30). Band gaps in the range of 3.3 to 3.4 eV were calculated using tauc plots generated
from the transmission spectra (Figure 1.31); these values agree with literature values for
pure SnO2. Unfortunately, the FTO films lacked sufficient conductivity for their intended
41
purpose as a TCO layer for self-driven PECs. X-ray diffraction spectra were obtained
and showed strong peaks for SnO2 (Figure 1.32). There was no evidence of fluorine.
2
Photocurrent Density (µA/cm )
200
150
100
50
0
-50
-500
-250
0
250
500
750
1000
Potential (mV)
Figure 1.29: Photocurrent density (jP, µA/cm2) versus applied potential (mV) for a Fdoped SnO2 thin film electrode deposited on standard ITO. Light and dark currents were
measured using a light chopping method by manually blocking the light source and then
illuminating the electrode in 5 s intervals.
Transmission (%)
100
80
60
40
20
0
300
600
900
1200
1500
1800
2100
Wavelength (nm)
Figure 1.30: UV-vis transmission spectra for F-SnO2 thin film deposited at 50 W for 135
min at 250 °C. The film transparency was found to be near 90% in the visible portion of
the spectrum, and a band gap of 3.38 was determined from tauc plot calculations.
42
0.125
sqrt(αhν)
0.100
0.075
0.050
0.025
0.000
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
4.1
hν
Figure 1.31: Tauc plot for a F-SnO2 thin film deposited at 50 W for 135 min at 250 °C.
The value of the band gap, Eg (eV), was found to be 3.37 eV.
12000
10000
Counts
8000
6000
4000
2000
0
30
35
40
45
50
55
60
65
2θ (deg)
Figure 1.32: XRD spectra of a F-SnO2 thin film. All peaks correspond to SnO2. The
amount of fluorine present is too small to be detected using the available instrumentation.
Tec 15, FTO coated glass from Pilkington, was tested alone and demonstrated
poor stability in 5.9 M potassium hydroxide as well. The Tec 15 glass generated no
43
photocurrent under illumination although it was more conductive than the sputtered films
reported.
The studies conducted on this material were limited due to breakdown of the SnF2
/ SnO2 target, and therefore no solid correlations could be made regarding deposition
conditions and performance. Over time the target became bonded to the steel cap of the
sputter gun and during the removal process it shattered into many pieces. It is believed
that this occurred because the target contained a high amount of SnF2 (25 wt%). The
fluorine is likely to have contributed to a reaction with the steel, causing the bonding of
the cap to the gun.
The calculated band gap of 3.3 – 3.4 eV for this material corresponds to literature
values of 3.35 eV for undoped SnO2.28 Also, there was no evidence of fluorine on the
XRD spectra. Therefore the films are believed to be doped in such a small amount, if at
all, that the physical and optical properties are unchanged from those of pure SnO2.
44
1-5
Summary
The major contributions of the thin film semiconductors work is summarized
below:
1)
Fabrication of r.f. sputter deposited n-type iron (III) oxide (α-Fe2O3) thin film
semiconductors exhibiting a high level of transparency in the visible portion of the
spectrum (70 %), chemical stability in electrolyte (5.9 M potassium hydroxide), and
appreciable photocurrent generation (~ 0.34 mA/cm2 under 0.75 sun illumination) has
been achieved. A total photoconversion efficiency of 0.56 % was demonstrated at a
potential of 0.38 V (with an open circuit potential of -0.382 V). An optimum substrate
temperature of 400 °C was found to be the primary factor in producing high quality αFe2O3 thin films.
2)
Fabrication of low-temperature (< 250 °C) r.f. sputter deposited indium doped n-
type iron (III) oxide (In-Fe2O3) thin films semiconductors exhibiting a high level of
transparency in the visible portion of the spectrum (> 80 %) and chemical stability in
electrolyte (5.9 M potassium hydroxide) has been achieved.
The In-Fe2O3 films
demonstrated photocurrents of about 33 µA/cm2 under 0.75 sun illumination. X-ray
diffraction (XRD) spectra indicate that the In-Fe2O3 films are of a mixed structure of αFe2O3 and indium and iron oxide compounds such as InFeO3 and InFe2O4. Annealing the
45
films for 4 hours at 550 °C in an argon atmosphere has been shown to enhance
performance of the In-Fe2O3 films; an increase of photocurrent density from 33 to 110
µA/cm2 has been observed. Annealing beyond 4 hours provided no additional benefit
and resulted in a slight decrease in performance.
3)
Fabrication of r.f sputtered tantalum, antimony, and zirconium doped iron (III)
oxide thin films, under varying deposition conditions, has been achieved. None of the
films demonstrated appreciable photoactivity. However, annealing the Sb-Fe2O3 films at
550 °C in an argon atmosphere enhanced the films performance. Investigation into these
materials was suspended due to their poor performance.
4)
Fabrication of n-type fluorine doped tin oxide (F-SnO2) thin film semiconductors
exhibiting a high level of transparency in the visible portion of the spectrum (90 %) has
been achieved. The F-SnO2 thin films demonstrated appreciable photocurrent (~ 0.2
mA/cm2 under 0.75 sun illumination). However, the films were found to have poor
conductivity and to be unstable in electrolyte (5.9 M potassium hydroxide). The SnO2 /
SnF2 target reacted with the stainless steel cap of the sputter gun (due to the presence of
fluorine) and shattered upon removal. Continued research on sputter deposited F-SnO2
thin films has been postponed until a method of deposition without the target reacting
with the components of the chamber is devised.
46
2-1
Introduction
Water electrolysis was first demonstrated in 1800 by Nicholson and Carlisle; they
ran an electric current through water and were able to generate hydrogen and oxygen
gases.4 Having studied their work, a British judge named Sir William Grove deduced
that if one were to do the opposite by combining hydrogen and oxygen, then electricity
might be produced; the concept was proven and published in 1839.4
After the
development of this first fuel cell, there was very little interest or advancement in the
technology until the 1950’s.4
There are many types of fuel cells, and one of the most promising for consumer
use is the proton exchange membrane (PEM) fuel cell which operates on hydrogen and
oxygen. It operates at a relatively low temperature (80 °C) compared to other types of
fuel cells such as molten carbonate fuel cells (MCFC’s, 600 - 700 °C) or solid oxide fuel
cells (SOFC’s, 1000 °C), and it generates only water as a byproduct so pollution is of no
concern (minimal amounts of NOx may be produced if air is used as the cathode gas
rather than pure oxygen).2,
4
However, hydrogen gas is not readily available in the
atmosphere and must be produced by steam reformation of natural gas or by water
electrolysis.
Generally, for both electrolysis and fuel cell applications, porous electrodes allow
for the development of greater current densities, and therefore greater gas evolution in the
case of electrolyzers, than planar electrodes due to the enhanced contact area with the
47
electrolyte.4, 64 A catalyst material within the pores, usually platinum, helps to enhance
the electrode performance by facilitating charge transfer because of its very low
overpotential.4 Nickel and nickel alloys are often used as inexpensive substitutes for
platinum electrocatalysts due to their lower cost and relatively low overpotentials.65-68
Electroplating is a process in which metal ions, from either the anode (+) or the
solution, are deposited on the cathode (-) where the metal ions are reduced to metal.
Sintering is a process of forming a coherent mass of material, usually from metal powder,
by heating without melting.
At a sufficient temperature, the powder particles gain
enough energy to move around, or diffuse, and get closer to each other. If they get close
enough then they may stick without actually melting, fusing to the other particles.
Sintering may be done by the loose method or by the slurry method. Loose
sintering involves sintering only the dry powder, resulting in a free packing arrangement
of the powder particles and therefore greater porosity. However, due to the looser
packing, electrode strength is an issue. Slurry sintering requires adding an agent to the
powder mixture (e.g. water) and results in more closely packed structures than with loose
sintering. Electrodes made by the slurry method tend to be more consistent in quality and
demonstrate greater strength.69
Other factors that affect sintered electrode quality are sintering temperature, time,
powder characteristics, and sintering atmosphere which may be reactive (H2) or inert
(Ar).64, 69-71
48
2-2
A.
Experimental
Basic Electrode Materials
Many materials were investigated concerning their electrocatalytic properties with
respect to the evolution of hydrogen gas via water electrolysis (Table 2.1). 3N (99.9
wt%) nickel (Ni) sheet (ESPI) was used as a constant anode and as a baseline for the
performance of all of the electrodes that were tested. Nickel sponge was obtained from
Marketech International, and it was flattened using a hydraulic press. Monel 400, a
nickel (60 %) and copper (40 %) alloy, was obtained from MSC Industrial Supply Co.,
and SAF 2304 stainless steel was obtained from Sandmeyer Steel Company. A mixed
metal oxide (MMO) catalyst mesh was obtained from Eltech Systems Corporation.
B.
Sputter-Deposited Electrodes
Nickel alloys were prepared in a custom-made sputter chamber described in part
one of this work. A nickel target (99.995 wt%, Lesker) was co-sputtered in an argon
atmosphere with aluminum (Al), cobalt (Co), and molybdenum (Mo) (99.99 wt%, 99.95
wt%, and 99.95 wt%, respectively, K.J. Lesker) onto cleaned 3N nickel substrates under
various conditions such as target power, substrate temperature, and deposition time.
Chromium nitride (CrN) electrodes were also deposited in the sputter chamber; a Cr
49
target (99.95 wt%, Lesker) was sputtered onto cleaned 3N nickel substrates in a mixed
chamber atmosphere of nitrogen and argon.
Table 2.1: List of materials studied for use as cathodes and anodes for water electrolysis.
Most materials that were studied were nickel based.
BASIC MATERIAL
SPECIFIC MATERIAL
nickel
3N Ni sheet
Ni sponge
flattened Ni sponge
sintered Ni
nickel alloy
Ni-Al
Ni-Co
Ni-Co-Al
Ni-Co-Mo
Ni-Cu (Monel 400)
stainless steel
SAF 2304
platinum coatings
Ni (Pt)
Ni sponge (Pt)
flattened Ni sponge (Pt)
SAF 2304 (Pt)
other
CrN
MMO catalyst mesh
Raney Ni
50
Prior to the depositions, the nickel substrates were cleaned with detergent
(Powdered Precision Cleaner, Alconox) and de-ionized water, rinsed with acetone and
ethanol, and dried under a stream of nitrogen. After deposition, sputtered nickel and
aluminum alloys were soaked in 6 M potassium hydroxide at a temperature of about
70 °C prior to testing in order to leach out any aluminum, leaving behind a porous nickel
structure.
3N nickel and SAF 2304 substrates were prepared for electroplating by a process
of scratching the surfaces with a file to roughen them up, and then cleaned using the
method outlined in the sputtering process description. The substrates were roughened up
to promote better platinum adhesion. Nickel sponge was soaked in ammonium hydroxide
(NH3OH) to remove any oxides, and then rinsed in de-ionized water and dried under a
stream of compressed air.
C.
Electroplated Electrodes
Platinum was electroplated from a hexachloroplatinic acid solution.
5g of
dihydrogen hexachloroplatinate (IV) hexahydrate, H2PtCl6·6H2O (Alfa Aesar), was
dissolved in 200 ml of de-ionized water producing a solution with a platinum
concentration of 9.42 mg/ml. The cleaned substrates, acting as cathodes, were wired to
the negative terminal of a dc power supply and submersed in the aqueous solution in a
shallow plastic container. A piece of platinum foil was wired to the positive terminal of
the power supply, acting as the anode. The power supply was set to approximately 1V
and the platinum foil was submersed into the solution just above the substrate. As the
51
anode was moved over the substrate, platinum was electrochemically deposited onto its
surface from solution.
A concentration calibration curve was generated by serial dilution of a sample of
the stock electroplating solution which allowed for a rough calculation of how much
platinum was being deposited. A small amount of the stock solution and the diluted
samples were each placed in a watch glass and set under the light of a solar simulator
(Oriel). A silicon detector was used to convert the light passing through the samples into
current density readings which were plotted versus the platinum concentration in solution
(Figure 2.1).
10
9
8
CPt (mg/ml)
7
6
5
4
3
2
1
0
22.4
22.8
23.2
23.6
24.0
24.4
2
JSC (mA/cm )
Figure 2.1: Concentration calibration curve for the platinum electroplating solution.
52
D.
Raney Nickel
Raney 2400 nickel slurry catalyst (Aldrich) was pressed into the nickel mesh
structure by hand under an argon atmosphere inside of a glove bag. It was then kept
under water until testing.
The nickel catalyst is pyrophoric, whereas it could
spontaneously ignite in air; therefore its contact with air was kept to a minimum.
E.
Sintered Electrodes
Sintered electrodes were fabricated by mixing various combinations of nickel and
aluminum (Al) powders, sometimes with the addition of cobalt oxide (CoO) or
molybdenum (Mo) powders, into various types of metal trays and heating them up to 800
to 1465 °C in a high temperature box furnace (ST-1700-666, Sentro Tech Corporation).
Heating rates and hold times were also investigated. Finished samples were soaked in
33 % potassium hydroxide to leach out any aluminum in order to enhance the active
surface areas of the electrodes.
The various metal tray materials that were investigated were nickel, stainless steel,
and nickel sponge. 1.5 ¯ 1.5 inch pieces of metal were cut from larger sheets using a
metal shearer (Di-Acro). They were shaped into trays by hand and then washed with
detergent (Powdered Precision Cleaner, Alconox) and de-ionized water, rinsed with
acetone and ethanol, and then dried under a stream of nitrogen.
The powders that were investigated were primarily nickel and aluminum of
varying purity and particle size (Table 2.2). Addition of cobalt oxide and molybdenum
53
powders (Aldrich) was also investigated. The cobalt oxide and molybdenum powder
sizes were less than 10 and 2 µm respectively. The powders were weighed out in plastic
weighing trays and then poured into glass vials. The vials were then capped and the
powders were mixed by vigorous shaking for a couple of minutes. The powders were
then poured into the metal trays, leveled off, and then placed inside a 3.5 ¯ 4 inch
stainless steel box containing four vertical shelves (Figure 2.2). Six electrodes could be
placed on each level, allowing for a total of twenty four electrodes to be sintered during
each run. The leveled box was placed inside another stainless steel box that was open on
the top and then placed inside the furnace. A flow of argon was directed down into the
box to purge it of any oxygen by displacement of atmosphere by the heavier argon gas.
After allowing the box to purge for about 15 min, the furnace was turned on and a heating
program was entered into a control panel on the front of the furnace. Table 2.3 shows a
typical heating routine used for making sintered electrodes. The first input variable c01
was the starting temperature inside the furnace (25 °C). Input t01 was the time it took to
reach the next temperature c02, and t02 was the time it took to reach temperature c03.
Input t04 was the time the temperature was held at 900°C. Input -121 stopped the
program at the end of the sequence. Once the program had been set, the run button was
pressed and the program proceeded automatically. At the end of the sequence the furnace
was turned off and allowed to cool down to room temperature. The argon was used to
keep the electrodes from oxidizing as the furnace cooled.
54
Table 2.2: List of various aluminum and nickel powders investigated for use in
producing sintered electrodes.
POWDER
VENDOR
SIZE (µm)
PURITY (wt%)
Aluminum
Alfa Aesar
44 - 420
99.8
Aluminum
Alfa Aesar
149 - 420
99.5
Aluminum
ESPI
< 44
99.5
Nickel
Accumet
44 - 74
99.9
Nickel
Alfa Aesar
< 149
99 +
Nickel
Alfa Aesar
149 - 297
99.7
Nickel
Alfa Aesar
44 - 420
99.8
Nickel
Alfa Aesar
3-7
99.9
Nickel
ESPI
< 44
99.8
Nickel
ESPI
< 149
99.8
Nickel
Sigma Aldrich
< 149
99.99
Nickel
Sigma Aldrich
< 149
99.999
Table 2.3: Sample heating sequence program for sintering. The first input variable c01
is the starting temperature inside the furnace. Input t01 is the time it takes to reach
temperature c02, and so on. Input -121 stops the program at the end of the sequence.
c01
t01
c02
t02
c03
t03
c04
t04
c05
t05
25°C
60 min
800°C
30 min
850°C
30 min
900°C
240 min
900°C
-121
55
a
b
c
d
e
f
D
C
B
A
Figure 2.2: The stainless steel box used for sintering the metal powders. There were
four levels labeled from A to D from the bottom to the top. On each level, up to six
electrodes could be placed for sintering, each labeled from a to f. The box was open on
the front and back sides so that the levels could be accessed.
Figure 2.3: 8-chamber electrolyzer with H2 and O2 gas collection capabilities. Each
chamber was divided into two compartments separated by a nylon membrane. The
displacement of water was used to measure gas flow rates.
56
2-3
Electrode Characterization
Each electrode, with the exception of the sintered electrodes, was cut to the
dimensions of 3.25 ¯ 3.25 inches, and a hole was drilled or punched through the top so
that wires could be connected. 14 gauge insulated copper wire was threaded through the
holes and then sealed with high strength 5-minute epoxy (Devcon). Sputtered alloy
electrodes were coated on the back side with epoxy if the sputtered coating covered only
one side. All electrical connections were checked prior to electrochemical testing.
Most of the electrodes were tested in a custom-built 8-chamber electrolyzer with
hydrogen and oxygen gas collection capabilities (Figure 2.3). The electrolyzer was
constructed from acrylic with individual chamber dimensions of 1.5 ¯ 3.5 inches. Each
chamber was separated into two compartments, one for the anode and the other for the
cathode, by a hydrophilic nylon membrane with a 0.2 to 0.5 µm pore size. An acrylic cap
was set down into place over the electrolyzer, with a rubber gasket in between, and
secured with stainless steel screws thus sealing each chamber from the others. Holes
were drilled through the gasket and the cap to allow for the electrode connections, and for
rubber tube connections allowing for gas collection.
Variable DC power supplies
(MPJA) were used to apply potentials ranging from 1.6 to 2.0 V to the electrodes, and
currents were recorded. The power supplies had digital displays for the voltage and
current values, but they were not very accurate so multimeters were connected to the
terminals to obtain more accurate numbers.
57
The hydrogen and oxygen gases were individually collected in graduated
cylinders that were filled with water and turned upside-down in a custom built trough
filled with water. The rubber gas collection tubes from each compartment of each
chamber were inserted into the graduated cylinders from the bottom, under water, and
pushed to the top of the cylinders. As gas collected inside the cylinders the pressure
displaced an equal volume of water down and out into the trough. The volumetric rate of
gas evolution could be measured by observing the level of the water in each cylinder over
time using a stop-watch.
The maximum overall electrolyzer efficiency (η) is the ratio of the minimum
water splitting potential and the actual operating potential (V) of the system,
η = 1.23 / V
(14)
However, industry reports their system performance as the amount of power
consumed per volume of hydrogen produced,
χ = P / H2
(15)
The units utilized by commercial manufacturers are kWh for power and m3 for the
volume of hydrogen gas.
It was observed that the level of electrolyte on each side of the membrane would
fluctuate over time indicating that pressure was alternately building up on each side of the
electrolyzer cell. The hydrogen and oxygen collection rates were therefore not constant
58
over time. Measurements were made when the electrolyte level on each side of the
membrane was equal, and rates were calculated and then averaged over time.
The sintered electrodes were tested by bending part of the metal tray up and
attaching an alligator clip directly to it. They were measured in a glass beaker, but the
testing procedure was the same as with the other electrodes that were tested in the
electrolyzer. Hydrogen generation rates were not measured for the sintered electrodes
because the 8-chamber electrolyzer developed leaks.
Selected electrodes were subjected to accelerated long-term testing by holding the
potential at 2.2 V continuously for hundreds of hours. Multiple times a day, over the
course of many days, current measurements were made from 1.6 to 2.0 V. The electrodes
were periodically removed from solution and cleaned to regenerate surface conditions
and potentially improve performance.
59
2-4
Results and Discussion
Initially, a number of electrodes were fabricated and tested against each other, and
hydrogen evolution rates were measured.
Of the combinations tested (Table 2.4),
platinum coated nickel as an anode and sputter deposited nickel as a cathode
demonstrated the greatest current density at an applied potential of 1.8 V. Most of the
sputtered electrodes, with the exception of the Ni-Al, did not survive the testing process;
the coatings fell apart in solution. The Raney nickel that was incorporated into the nickel
sponge did not adhere well and the electrode performed poorly. The MMO catalyst mesh
was initially stable in solution, but it slowly reacted in the potassium hydroxide after it
was cut to a smaller size and eventually fell apart. The sintered nickel electrodes were
not being investigated initially.
The gas collection data (Table 2.5) shows that sputtered porous nickel as a
cathode demonstrated the highest average hydrogen generation rate of 2.3 ml/min at an
applied potential 1.8 V.
The ratio of oxygen to hydrogen gas evolution was
approximately one-half, which is what one would expect based on water electrolysis
reaction stoichiometry.
Average rates were determined from multiple data points
collected over time (Figure 2.4). The hydrogen evolution rates were used to calculate
total electrolyzer performances ranging from 3.8 to 7.3 compared to commercial
efficiencies ranging from 4.0 to 6.3.
60
Table 2.4: Current densities (mA/cm2) for various anode (A) and cathode (C)
combinations measured at an applied potential of 1.8 V. Initially, the combination of
platinum coated nickel sheet as an anode and sputter coated porous nickel as the cathode
demonstrated the greatest performance, determined by comparing current density values.
Ni
ST254
sputtered
Ni-Al
Ni
1.98
ST254
Ni-Al
17.4
Pt coated
Ni
4.39
19.33
SAF2304
0.53
1.24
Pt coated
SAF2304
3.23
Ni (Pt)
SAF 2304
SAF 2304
(Pt)
1.82
22.93
MMO
mesh
2.93
15.95
13.22
9.5
13.17
10.69
16
0.5
1.46
1.2
3
5.12
Mesh
Table 2.5: H2 and O2 evolution rates (mL/min) measured at an applied potential of 1.8 V
for various cathodes with a 3N nickel anode. The volume of H2 generated should be
exactly double the value of the volume of O2 generated based on water electrolysis
reaction stoichiometry.
CATHODE
H2 EVOLUTION
RATE (mL/min)
O2 EVOLUTION
RATE (mL/min)
O2/H2
RATIO
Sputtered Ni
2.29
1.25
0.55
η
(kWh/m3)
4.2
Ni-Co-Mo
0.69
0.34
0.49
3.9
SAF 2304
0.64
0.39
0.61
3.8
SAF 2304 (Pt)
1.20
0.73
0.61
7.3
Sintered nickel electrodes were tested as cathodes with 3N nickel sheets as anodes.
One electrode, sample 123005D, composed of 95 wt% 325 mesh nickel and 5 wt% 325
mesh aluminum, sintered at 900°C for one hour, demonstrated the greatest initial current
density of ~ 120 mA/cm2 at a potential of 1.8 V. It was subjected to accelerated long61
term testing by running electrolysis at a potential of 2.2 V (Figure 2.5). After over 700
hours of continuous operation the electrode demonstrated an average current density of
approximately 33 mA/cm2. An electroplated nickel electrode and a platinum coated
nickel sponge electrode were also tested as cathodes under identical conditions and their
performance was compared to sample 123005D. The sintered electrode demonstrated
greater performance over the other two types of electrodes based on current density
measurements and long-term stability.
The electroplated electrode performed well
initially, but quickly degraded because of poor adhesion of the electroplated nickel
particles to the nickel substrate. It is hypothesized that the platinum coated nickel sponge
did not perform well due to its low active surface area compared to the other two
electrodes.
2.00
1.75
H2 Rate (mL/min)
1.50
1.25
1.00
0.75
0.50
0.25
0.00
0
30
60
90
120
150
180
210
Time (min)
Figure 2.4: H2 generation rates (mL/min) measured over time (min) for selected
samples; sputtered CrN (▲), and sputtered Ni-Co-Mo from (●).
62
2
Current Density (mA/cm )
130
120
110
100
90
80
70
60
50
40
30
20
10
0
0
100
200
300
400
500
600
700
800
Time (h)
Figure 2.5: Accelerated long-term testing of various nickel cathodes. Current densities
(j, mA/cm2) were measured at 1.8 V over a period of several hundred hours of continuous
operation at 2.2 V. Degradation of performance occurred over time until equilibrium was
obtained. The sintered electrode 123005D (▲) demonstrated the greatest performance
due to its porosity and large surface area. The electroplated nickel electrode (○)
performed well initially, but after 200 hours its performance had degraded considerably.
The platinum coated nickel sponge (□) demonstrated the lowest performance due to the
platinum coating being too thin, as well as having less active surface area.
Sintered electrode 123005D was not reproducible. At the time when it was
fabricated, the powder trays were not being placed inside of a stainless steel box. The
trays filled with powder were placed directly onto the floor of the furnace with the argon
purge blowing directly down on top of them. Nickel oxide formed over the top of the
samples and had to be removed by hand before testing. When the oxide layer of sample
123005D was removed, it was done in such a way as to create a very gravelly surface
underneath.
It is hypothesized that the random structure was responsible for the
electrodes high performance.
After the electrodes were sintered in the stainless steel box, results could be
reproduced and trends were observed. A sintering temperature between 850 and 900 °C
63
was found to be optimal. If the temperature was too low then the diffusion bonding
between the nickel particles was not strong enough and the electrodes would fall apart. If
the temperature was too high then the diffusion bonding was too great and the pore sizes
were too small. The reduced surface area resulted in lower current density values.
Nickel-aluminum electrodes sintered within the optimal range consistently demonstrated
initial current densities near 25 mA/cm2.
Electrodes sintered with a one hour hold time tended to not be very durable, and
they could not be removed from the trays in which they were placed. Extending the hold
time to four hours resulted in hard, durable bricks that could be removed from the trays.
However, even with extended hold times, in order for the powders to hold together well it
was observed that the nickel powder purity had to be 99.99% or greater.
X-ray
diffraction measurements were performed on samples of nickel powder of varying
purities and no difference was observed in the spectra (Figure 2.6). The amount of
impurities in the different powders was too small to be observed. It was also observed
that the various powders that were investigated were composed of differing particle sizes.
The dependence of electrode quality on sintering temperature and powder particle size
and shape needs to be more thoroughly investigated.
Addition of molybdenum enhanced the performance of the sintered nickel
electrodes (cathodes). The optimal ratio of nickel, aluminum, and molybdenum was
found to be 88:5:7 based on accelerated continuous long-term testing. After over 1000
hours of continuous testing, current densities near 40 mA/cm2 have been demonstrated.
The addition of cobalt oxide has enhanced nickel electrode (anode) performance.
64
Electrodes demonstrating current densities exceeding 20 mA/cm2 have been consistently
produced.
30000
Counts
25000
20000
99+ %
15000
99.7 %
10000
99.9 %
5000
99.99 %
0
40
60
80
100
120
2θ (deg)
Figure 2.6: XRD spectra of nickel powders of various purities. No difference in
composition had been observed. All peaks correspond to nickel.
Three Ni-Al-Mo electrodes fabricated under identical conditions were submersed
in 5.9 M potassium hydroxide with varying two-dimensional areas exposed, and values of
current density were measured at a potential of 1.8 V (Figure 2.7). The composition of
all three electrodes, by weight, was 80:10:10 of Ni, Al, and Mo respectively. Each was
sintered at 900 °C for four hours, and the nickel powder purity of each electrode was
99.99 wt%. Two of the electrodes were tested by submersing each at one half of its full
area, three quarters of its full area, and then completely. The third sintered electrode,
which had an initial two dimensional surface area much smaller than the other two, was
completely submersed in electrolyte and tested at the same potential.
Electrode
performance, based on current density measurements, tended to be greater for smaller
65
electrodes. The reason for the observed trend has not been conclusively identified and is
being investigated.
2
Current Density (j, mA/cm )
40
35
30
25
20
15
10
5
10
15
20
25
30
35
40
45
50
2
Area (cm )
Figure 2.7: Current density (j, mA/cm2), measured at a potential of 1.8 V, as a function
of electrode size. Three electrodes fabricated under identical conditions were tested. It
was observed that as electrode size was increased, values of current density dropped.
All of the electrodes that were tested long-term suffered degradation of
performance over time (Figure 2.8 and Table 2.6).
The sintered electrodes were
periodically removed from the potassium hydroxide and rinsed off in de-ionized water
and then immediately re-tested.
The cleaned electrodes showed enhanced initial
performance, and then the current density values dropped over several hours of
continuous operation. The electrodes were also agitated in solution during electrolysis in
order to remove any gas bubbles that may have adsorbed onto the electrode surfaces,
resulting in enhanced performance and then degradation over time. When performance
was recovered, it was never recovered to the initial testing value.
66
45
70
40
60
35
30
50
2
j (mA/cm )
2
j (mA/cm )
80
40
30
20
10
0
20
15
10
5
a
0
25
200
400
600
800
1000
0
1200
b
0
200
30
50
25
40
20
j (mA/cm2)
60
30
20
10
0
600
800
1000
1200
Time (h)
2
j (mA/cm )
Time (h)
400
15
10
5
c
0
200
400
600
800
1000
0
1200
d
0
100
Time (h)
200
300
400
500
Time (h)
Figure 2.8: Current density (j, mA/cm2) as a function of time (h) for various sintered
cathodes tested against sintered Ni-Al-CoO anodes, measured at a potential of 1.8 V.
Electrodes run continuously over an extended period of time suffered from degradation of
performance.
Table 2.6: List of sintered electrodes, from Figure 2.8, subjected to accelerated longterm testing in 5.9 M potassium hydroxide at a potential of 1.8 V.
ELECTRODE
COMPOSITION
Ni PURITY (wt%)
AREA (cm2)
a
88:5:7
99.99
7.02
b
80:10:10
99
7.56
c
80:10:10
99
8.1
d
80:10:10
99.99
28.8
67
Temporary degradation of electrode performance is believed to be due to
adsorption of hydrogen gas or hydroxyl ions at the surface of the electrodes, blocking
potential reaction sites. The adsorbed species reduced the active surface area of the
electrodes resulting in lower current densities.
Permanent degradation of electrode
performance is believed to be due to electrochemical passivation of the anode under
operating conditions. At the pH and potential of the system, the nickel theoretically will
develop a thin protective oxide coating which decreases the electrodes conductivity in
solution (Figure 2.9).5 However, no evidence supporting the passivation theory has been
observed from x-ray diffraction scans due to the oxide layer being very thin. Also,
physical degredation of the electrode surfaces is believed to contribute to a drop in
performance. Over time the gas evolution physically wore down the surface of the
electrodes reducing the active surface area. Evidence of this was observed as black
particles settled at the bottom of the electrolyzer during operation.
Overall, sintered nickel-molybdenum electrodes demonstrated the greatest
performance based on current density measurements, electrochemical stability, and
physical durability.
An optimum metal powder mixing ratio of 88:5:7 for nickel,
aluminum, and molybdenum respectively produced the best electrodes (cathodes) based
on accelerated long-term testing data. A sintering temperature of 900 °C, held for a
period of four hours, produced hard bricks that handled well. Addition of cobalt to the
nickel electrodes, instead of molybdenum, was investigated for the production of high
quality anodes. Electroplated nickel electrodes, fabricated by another individual within
the lab, have shown promise, but their physical durability is an issue. Both types of
68
nickel electrodes, sintered and electroplated, were porous in structure, having very large
active surface areas.
Figure 2.9: Pourbaix diagrams for the nickel – water system at 25 °C. At high pH and
high potential, a passive oxide layer forms at the electrode surface.
69
2-5
Summary
The major contributions of the electrocatalyst work are summarized below:
1)
Sintered nickel based cathodes exhibiting a high degree of electrocatalytic activity,
electrochemical stability, and physical durability have been produced. The optimum
mixed metal powder weight ratio was found to be 88:5:7 for the nickel, aluminum, and
molybdenum powders respectively. Sintering at a temperature of 900 °C for a period of
4 hours produced electrodes that were hard and durable. After accelerated long-term
continuous testing for over 1000 hours, current densities near 40 mA/cm2 have been
demonstrated at a potential of 1.8 V.
2)
Sintered nickel based anodes exhibiting a high degree of electrocatalytic activity
and physical durability have been produced. The optimum mixed metal powder weight
ratio was found to be 78:10:12 for the nickel, aluminum, and cobalt powders respectively.
Sintering at a temperature of 900 °C for a period of 4 hours produced electrodes that were
hard and durable.
70
Future Work
In the future more work needs to be done on doping n-type α-Fe2O3 with
materials that will allow high quality films demonstrating photocurrent densities of at
least 7 mA/cm2 to be deposited at low temperatures (< 250 °C). Sputtering with a Fe2O3
target doesn’t seem to be an effective route to achieving the stated goals so reactive
sputtering with an iron target in an oxygenated atmosphere is being considered as an
alternative. Possible new doping materials include strontium (Sr) and zinc oxide (ZnO).
When films demonstrating the desired performance are producible on a consistent basis
then a-Si solar cells will have to be coated in order to gauge hybrid PEC system
performance.
In order to gain a better understanding of the r.f. sputter deposition of iron (III)
oxide thin films, more samples should be fabricated under conditions that will allow for
more definite conclusions concerning deposition time, temperature, film thickness, and
crystal size.
Many more sintered nickel electrodes need to be tested long term, with hydrogen
generation rates measured as well, so that a better understanding of long-term
performance can be developed. Also, more work needs to be done on determining
exactly what parameters affect the sintered nickel electrode performance (i.e. powder
particle shape, size, and purity). Anode performance was not the main focus of the
research up to this point; most focus was directed towards cathode performance.
Therefore the fabrication of high quality anodes with enhanced catalytic activity with
respect to oxygen evolution should be investigated.
71
References
(1)
Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C.C. Photo-electrochemical hydrogen
generation from water using solar energy. Materials-related aspects.
International Journal of Hydrogen Energy (2002), 27, 991-1022
(2)
Nowotny, J.; Sorrell, C. C.; Sheppard, L. R.; Bak, T. Solar-hydrogen:
Environmentally safe fuel for the future. International Journal of Hydrogen
Energy (2005), 30 (5), 521-544
(3)
Koroneos, C.; Dompros, A.; Roumbas, G.; Moussiopoulos, N. Life cycle
assessment of hydrogen fuel production processes. International Journal of
Hydrogen Energy (2006), 31, 939-944
(4)
Bockris, J. O’M.; Reddy, A. K.N. Modern electrochemistry 2B. Electronics in
chemistry, engineering, biology, and environmental science.
Kluwer
Academic / Plenum Publishers (2000)
(5)
Pourbaix, M. Atlas of electrochemical equilibrium in aqueous solutions.
Pergamon Press (1966)
(6)
Tromp, T. K.; Shia, R.; Allen, M.; Eiler, J. M.; Yung, Y. L. Potential
environmental impact of a hydrogen economy on the stratosphere. Science
(2003), 300, 1740-1742
(7)
Elam, C. C.; Evans, R. J. Overview of hydrogen production. Prepared PaperAmerican Chemical Society, Division of Fuel Chemistry (2004), 49(2), 481-482
(8)
Chakraborty, J. The geographic distribution of potential risks posed by
industrial toxic emissions in the U.S. Journal of Environmental Science and
Health (2004), A39 (3), 559-575
(9)
Walters, S.; Ayres, J. The health effects of air pollution. Pollution: Causes,
Effects and Control (4th Edition), Cambridge: Royal Society of Chemistry
(2001), 268-295
(10)
Licht, Stuart. Solar water splitting to generate hydrogen fuel: photothermal
electrochemical analysis. Journal of Physical Chemistry B (2003), 107 (18),
4253-4260
(11)
Rocheleau, Richard E.; Miller, Eric L. Photoelectrochemical production of
hydrogen: engineering loss analysis. International Journal of Hydrogen Energy
(1997), 22 (8), 771-782.
(12)
Gratzel, M. Photoelectrochemical cells. Nature (2001), 414, 338-344
72
(13)
Rocheleau, Richard E.; Miller, Eric L. Photoelectrochemical hydrogen
production. Proceedings of the 2001 DOE Hydrogen Program Review (2001)
(14)
Koroneos, C.; Spachos, T.; Moussiopoulos, N. Exergy analysis of renewable
energy sources. Renewable Energy (2003), 30, 295-310
(15)
Fujishima, A.; Honda, K.
Electrochemical photolysis of water at a
semiconductor electrode. Nature (1972), 238, 37-38
(16)
Zhang, W; Li, Y.; Shenglong, Z.; Wang, F. Fe doped photocatalytic TiO2 film
prepared by pulsed dc reactive magnetron sputtering. Journal of Vacuum
Science and Technology A (2003), 21 (6), 1877-1882
(17)
Bolton, J.R. Solar production of hydrogen. IEA Technical Report (1996)
(18)
Kocha, S,; Montgomery, D.; Peterson, M.; Turner, J. Photoelectrochemical
decomposition of water utilizing monolithic tandem cells. Solar Energy
Materials and Solar Cells (1998) 52, 389-397
(19)
Kainthla, R. C.; Zelenay, B.; Bockris, J. O’M. Significant efficiency increase in
self-driven photoelectrochemical cell for water photoelectrolysis. Journal of
the electrochemical Society (1987), 134 (4), 841-845
(20)
Rocheleau, R. E.; Miller, E. L.; Misra, A. High-efficiency photoelectrochemical
hydrogen production using multijunction amorphous silicon
photoelectrodes. Energy & Fuels (1998), 12 (1), 3-10
(21)
Miller, E. L.; Rocheleau, R. E.; Deng, X. M. Design considerations for a
hybrid amorphous silicon/photoelectrochemical multijunction cell for
hydrogen production. International Journal of Hydrogen Energy (2003), 28 (6),
615-623
(22)
Miller, E. L.; Paluselli, D.; Marsen, B.; Rocheleau, R. E. Development of
reactively sputtered metal oxide films for hydrogen-producing hybrid
multijunction photoelectrodes. Solar Energy Materials & Solar Cells (2005),
88 (2), 131-144
(23)
Miller, E. L.; Rocheleau, R. E.; Khan, S.
A hybrid multijunction
photoelectrode for hydrogen production fabricated with amorphous
silicon/germanium and iron oxide thin films. International Journal of
Hydrogen Energy (2004), 29(9), 907-914
(24)
Deng, X.; Schiff, E. A. Amorphous silicon based solar cells. Handbook of
Photovoltaic Engineering, Wiley (2002)
73
(25)
Deng, X.; Cao, X.; Ishikawa, Y.; Du, W.; Yang, X.; Das, C.; Vijh, A.
Fabrication and characterization of triple-junction amorphous silicon based
solar cell with nanocrystalline silicon bottom cell. IEE WCPEC (2006)
(26)
Minami, T. New n-type transparent conducting oxides. MRS Bullitan (2000),
38-44
(27)
Besser, R. Bulk micromachining of silicon: wet etching. Short Course on
MEMS and Nanotechnology, Stevens Institute of Technology (2002)
(28)
Reddy, S. R.; Mallik, A. K. UV absorption studies of undoped and fluorinedoped tin oxide films. Thin Solid Films (1986), 143, 113-118
(29)
Martel, A.; Caballero-Briones, F.; Fandino, J.; Castro-Rodriguez, R.; BartoloPerez, P.; Zapata-Navarro, A.; Zapata-Torres, M.; Pena, J. L. Discharge
diagnosis and controlled deposition of SnOx:F films by DC-reactive
sputtering from a metallic tin target. Surface and Coatings Technology (1999),
122 (2-3), 136-142
(30)
Gamard, A.; Babot, O.; Jousseaume, B.; Rascle, M.; Toupance, T.; Campet, G.
Conductive F-doped tin dioxide sol-gel materials from fluorinated β diketonate tin(IV) complexes. Characterization and thermolytic behavior.
Chemistry of Materials (2000), 12 (11), 3419-3426.
(31)
Franc, C.; Jousseaume, B.; Linker, M.; Toupance, T. Thermally induced
elimination reactions in xerosols derived from (fluoroorgano)tin compounds:
A new efficient way to prepare F-doped tin dioxide materials. Chemistry of
Materials (2000), 12, 3100-3107
(32)
Martel, A.; Caballero-Briones, F.; Bartolo-Perez, P.; Iribarren, A.; CastroRodriguez, R.; Zapata-Navarro, A.; Pena, J. L.
Chemical and phase
composition of SnOx:F films grown by dc reactive sputtering. Surface and
Coatings Technology (2001), 148 (2-3), 103-109
(33)
Chaudhuri, T.; De, A.; Biswas, P. Kr. Development of sol-gel fluorine doped
tin oxide film on glass. Transactions of the Indian Ceramic Society (2003), 62
(4), 208-212
(34)
Mahon, M.F.; Molloy, K.C.; Stanley, J.E.; Rankin, D.W.H.; Robertson, H.E.;
Johnston, B.F. Atmospheric pressure deposition of fluorine-doped SnO2 thin
films from organotin fluorocarboxylate precursors. Applied Organometallic
Chemistry (2005), 19 (5), 658-671
(35)
Aukkaravittayapun, S.; Wongtida, N.; Kasecwatin, T.; Charojrochkul, S.;
Unnanon, K.; Chindaudom, P. Large scale F-doped SnO2 coating on glass by
spray pyrolysis. Thin Solid Films (2006), 496, 117-120
74
(36)
Kaneko, S.; Nakajima, K.; Kosugi, T.; Murakami, K. Spray pyrolysis
deposition of oriented, transparent and conductive tin(IV) oxide thin films.
Ceramic Transactions (1999), 100(Dielectric Ceramic Materials), 165-174
(37)
McGregor, K. G.; Calvin, M.; Otvos, J. W. Photoeffects in iron(III) oxide
sintered semiconductors. Journal of Applied Physics (1979), 50 (1), 369-73
(38)
Sharon, M.; Prasad, B. M. Preparation and characterization of iron oxide thin
film electrodes. Solar Energy Materials (1983), 8, 457-469
(39)
Itoh, K.; Bockris, J. O’M. Thin film photoelectrochemistry: Iron oxide.
Journal of the Electrochemical Society (1984), 131 (6), 1266-1271
(40)
Khan, S. U. M.; Zhou, Z. Y. Photoresponse of undoped and iodine-doped iron
oxide thin film electrodes. Journal of Electroanalytical Chemistry (1993),
357 (1-2), 407-420
(41)
Majumder, S. A.; Khan, S. U. M. Photoelectrolysis of water at bare and
electrocatalyst covered thin film iron oxide electrode. International Journal of
Hydrogen Energy (1994), 19 (11), 881-887
(42)
Stenberg, T.; Vuoristo, P.; Keranen, J.; Mantyla, T.; Buchler, M.; Virtanen, S.;
Schmuki, P.; Bohni, H. Characterization of r.f.-sputtered iron oxide films for
modeling passive films. Thin Solid Films (1998), 312, 46-60
(43)
Khan, S. U. M.; Akikusa, J. Photoelectrochemical splitting of water at
nanocrystalline n-Fe2O3 thin-film electrodes. Journal of Physical Chemistry B
(1999), 103, 7184-7189
(44)
Yubero, F.; Ocana, M.; Caballero, A.; Gonzalez-Elipe, A. R.
Structural
modifications produced by the incorporation of Ar within the lattice of Fe2O3
thin films prepared by ion beam induced chemical vapor deposition. Acta
Materialia (2000), 48 (18/19), 4555-4561
(45)
Pal, B.; Sharon, M. Preparation of iron oxide thin film by metal organic
deposition from Fe(III)-acetylacetonate: a study of photocatalytic properties.
Thin Solid Films (2000), 379 (1, 2), 83-88
(46)
Qian, X.; Zhang, X.; Bai, Y.; Li, T.; Tang, X.; Wang, E.; Dong, S.
Photoelectrochemical
characteristics
of
α-Fe2O3
nanocrystalline
semiconductor thin film. Journal of Nanoparticle Research (2000), 2, 191-198
(47)
Leighton, C.; Hoffmann, A.; Fitzsimmons, M. R.; Nogues, J.; Schuller, I. K.
Deposition of epitaxial a-Fe2O3 layers for exchange bias studies by reactive
dc magnetron sputtering. Philisophical magazine B (2001), 81 (12), 1927-1934
75
(48)
Kulkarni, S. S.; Lokhande, C. D. Structural, optical, electrical and dielectrical
properties of electrosynthesized nanocrystalline iron oxide thin films.
Materials Chemistry and Physics (2003), 82 (1), 151-156
(49)
Peng, Y.; Park, C.; Laughlin, D. E. Fe3O4 thin film sputter deposited from iron
oxide targets. Journal of Applied Physics (2003), 93 (10), 7957-7959
(50)
Miller, E. L.; Paluselli, D.; Marsen, B.; Rocheleau, R. E. Low-temperature
reactively sputtered iron oxide for thin film devices. Thin Solid Films (2004),
466 (1-2), 307-313
(51)
Akl, A. A. Optical properties of crystalline and non-crystalline iron oxide
thin films deposited by spray pyrolysis. Applied Surface Science (2004),
233 (1-4), 307-319
(52)
Akl, A. A. Microstructure and electrical properties of iron oxide thin films
deposited by spray pyrolysis. Applied Surface Science (2004), 221 (1-4), 319329
(53)
Petitjean, C.; Rousselot, C.; Pierson, J. F.; Billard, A. Reactive sputtering of
iron in Ar-N2 and Ar-O2 mixtures. Surface and Coatings Technology (2005),
200 (1-4), 431-434
(54)
Schumacher, L. C.; McIntyre, N. S.; Mamiche-Afara, S.; Dignam, M. J.
Photoelectrochemical properties of indium doped iron oxide. Journal of
Electroanalytical Chemistry (1990), 277, 121-138
(55)
Khader, M. M.; Saleh, M. M. Photoelectrochemical characteristics of ferric
tungstate. Journal of Solid State Electrochemistry (1998), 2, 170-175
(56)
Aroutiounian, V. M.; Arakelyan, V. M.; Shahnazaryan, G. E.; Stepanyan, G. M.;
Turner, J. A.; Khaselev, O.
Investigation of ceramic Fe2O3{Ta}
photoelectrodes for solar energy photoelectrochemical converters.
International Journal of Hydrogen Energy (2002), 27 (1), 33-38
(57)
Ingler, W. B. Jr.; Khan, S. U. M. Photoresponse of spray pyrolytically
synthesized magnesium-doped iron (III) oxide (p-Fe2O3) thin films under
solar simulated light illumination. Thin Solid Films (2004), 461, 301-308
(58)
Ingler, W. B. Jr.; Khan, S. U. M. Photoresponse of spray pyrolytically
synthesized copper-doped p-Fe2O3 thin films electrodes in water splitting.
Thin Solid Films (2005), 30, 821-827
(59)
Ingler, W. B. Jr.; Sporar, D.; Deng, X. Sputter deposition of In-Fe2O3 films for
photoelectrochemical hydrogen production. ECS Transactions (2006), 3 (5),
253-259
76
(60)
Swanepoel, R. Determination of the thickness and optical constants of
amorphous silicon. Journal of Physics E: Scientific Instruments (1984), 17, 896903
(61)
Cisneros, J. I. Optical characterization of dielectric and semiconductor thin
films by use of transmission data. Applied Optics (1998), 37 (22), 5262
(62)
Cullity, B. D. Elements of X-Ray Diffraction. 2nd Ed. Addison-Wesley (1978),
555
(63)
Edstrom, J. O.
Solid state diffusion in the reduction of hematite.
Communication from the Research Organization of Jernkontoret (1957), 141,
809-836
(64)
Tracey, V. A. Sintering of porous nickel - theoretical and practical
considerations. Modern Developments in Powder Metallurgy (1981), 12, 423-38
(65)
Suffredini, H. B.; Cerne, J. L.; Crnkovic, F. C.; Machado, S. A. S.; Avaca, L. A.
Recent developments in electrode materials for water electrolysis.
International Journal of Hydrogen Energy (2000), 25 (5), 415-423
(66)
De la Torre, S. D.; Oleszak, D.; Kakitsuji, A.; Miyamoto, K.; Miyamoto, H.;
Martinez-S., R.; Almeraya-C, F.; Martinez-V., A.; Rios-J., D. Nickel molybdenum catalysts fabricated by mechanical alloying and spark plasma
sintering. Materials Science & Engineering, A: Structural Materials: Properties,
Microstructure and Processing (2000), A276 (1-2), 226-235
(67)
Huber, G. W.; Shabaker, J. W.; Dumesic, J. A. Raney Ni-Sn catalyst for H2
production from biomass-derived hydrocarbons. Science (2003), 300, 20752077
(68)
Sanches, L. S.; Domingues, S. H.; Carubelli, A.; Mascaro, L. H.
Electrodeposition of Ni-Mo and Fe-Mo alloys from sulfate-citrate acid
solutions. Journal of the Brazilian Chemical Society (2003), 14 (4), 556-563
(69)
Tracey, V. A. Nickel powders into sintered structures for the alkaline
battery:
porosity studies. Industrial & Engineering Chemistry Product
Research and Development (1986), 25, 582-585
(70)
Tracey, V. A. Effect of sintering conditions on structure and strength of
porous nickel. Powder Metallurgy (1979), 22 (2), 45-8
(71)
Inco Limited Nickel powder, battery plaques and other sintered powder
products. Research Disclosure (1989), 25
77