Master Thesis

Improving Stability and Efficiency of

Earth-abundant Electrocatalysts for

Water Oxidation

Clemens Wunder

Master thesis, 60 hp

Examiner:

Passed:

I

Abstract

The idea to convert the energy of the sunlight into a conveniently usable form exists for

many years, but how can this be achieved? Solar cells are a rapidly advancing

technology, and their broad application increases the conversion of solar energy into

electricity. However, the electrochemical potential created by solar cells cannot be

stored at large enough scale. Thus, new technology is required for storing and using the

produced energy. One promising approach is to employ solar electricity for the

electrochemical splitting of water into hydrogen and oxygen, thus allowing the storage

of solar energy in form of chemical fuels. In this master thesis, cheap and scalable

catalysts made from different combinations of iron and cobalt were studied for their

suitability as catalysts for electrochemical water oxidation at various pHs (0.3 - 6),

electrolytes (H2SO4 and H3PO4) and minerals (oxides and phosphates), aiming for

application in proton-exchange membrane-based water electrolyzes. A catalyst

composition of 20% Co and 80% Fe oxide prepared at a spraying temperature of 450 °C

and measured in 0.5 M sodium phosphate resulted in the highest activity at pH 2

(overpotential of 560 mV to produce a current density of 10 mA/cm2), while the highest

stability (stable overpotential to maintain a current density of 10 mA/cm2), more than

7 days, was observed at pH 6.

II

III

List of abbreviations

A Ampere, Unit for the current

CP Chrono potentiometry (Δt > 1 ms), Measures the stability

CV Cyclic voltammogram, Measures the IV-Curve and the Activity

EDS Energy-dispersive X-ray spectroscopy, Elemental analysis, or chemical

characterization

EIS Electrochemical impedance spectroscopy, Investigates the surface

FRA Frequency Response Analyzer, Measures the internal resistance

(impedance)

HER Hydrogen Evolution Reaction, Production of H2

IR Internal Resistance, Resistance between working electrode and counter

electrode

NRR Nitrogen Reduction Reaction, Production of NH3

OER Oxygen Evolution Reaction, Production of O2

RHE Reversible Hydrogen Electrode, Relates the potential to a standard

hydrogen electrode

SEM Scanning Electron Microscopy, High quality picture of the surface

V Volt, Unit for the potential

XRD X-ray Diffraction (Crystallography), Determines the atomic structure of

a crystal

XPS X-ray photoelectron spectroscopy, Investigates the first surface layers

Author contribution

The synthesis and optimization of the catalysts and the measurements on the

galvanostat/potentiostat were carried out by the author. Cheng Choo Lee (Umeå Electron Microscopy Core Facility) measured the SEM data and Andrey Shchukarev (Umeå X-ray Photoelectron Spectroscopy Platform) measured and analyzed the XPS

data. Wai Ling Kwong guided the lab work and measured the XRD spectra. Johannes Messinger was supervising the whole project.

IV

V

Table of contents

Abstract ............................................................................................................................I

Author contribution ........................................................................................................ III

Table of contents ............................................................................................................. V

1. Introduction .................................................................................................................. 1

Aim of the Master Thesis .............................................................................................. 4

2. Popular Scientific Summary including Social and Ethical Aspects ................................... 4 2.1 Popular Scientific Summary..................................................................................... 4

2.2 Social and Ethical Aspects ....................................................................................... 5

3. Experimental ................................................................................................................ 5

3.1 The Synthesis of the Catalysts.................................................................................. 5

3.2 The Electrochemical Measurements ......................................................................... 5

3.3 Characterizations of the Catalysts............................................................................. 6

4. Results ......................................................................................................................... 7

4.1 The best Iron and Cobalt Combination for OER ........................................................ 7 4.2 The Mineralogical Properties ................................................................................... 9

4.3 Elemental Distribution........................................................................................... 10

4.4 The Influence of the Electrolyte pH ........................................................................ 11

4.5 The Catalytic Stability ........................................................................................... 13

4.6 Changing the Spraying Temperature and the Electrolyte .......................................... 14

4.7 The Surface Chemical Properties............................................................................ 16

4.8 Introducing/Replacing Metal Oxide with Metal Phosphate ....................................... 17 4.9 The Stability of the Metal Phosphates ..................................................................... 17

4.10 The Chemical Composition of the Phosphate Samples ........................................... 18

4.11 Phosphate Samples – The Impact of the pH........................................................... 19

5. Discussion .................................................................................................................. 21

6. Conclusions and Outlook............................................................................................. 22

Acknowledgement .......................................................................................................... 23

References ..................................................................................................................... 23

Appendix ........................................................................................................................ A A The Sample Preparation............................................................................................ A

B SEM Pictures of the Sample...................................................................................... B

C The Overpotential of C2F8-250, C4F6-250 and C6F4-250 .......................................... D

D Different Ionic Strengths for the Phosphate and Oxide Samples at 450 °C.................... D

E The XPS Measurements .............................................................................................E

F Stability Curves of C4F6-450 at pH 2,4 and 6............................................................. H

G All CV Curves of the 250°C Samples at pH 0.3.......................................................... H H All CV Curves of C4F6-250 at Different pHs.............................................................. J

I The CV Curves of C4F6-250 and C4F6-450 at pH 2 .................................................... K

J The Maghemite and Hematite Structure ......................................................................M

K The XRD and SEM Spectra of C4F6-250 and C4F6-450 ............................................M

L The SEM Spectra of C4F6-450/02, C4F6-450/03 and C4F6-450/04 ............................. O

M The Atomic Concentrations (XPS) of C4F6-250, C4F6-450 and C4F6-450/03 ............ R

N The Activity of C4F6-250, C10-250 and F10-250 at pH 2............................................ S

O The CV Curves of C4F6-450 at pH 2, 4 and 6 ............................................................. S P The CV Curves of C4F6-450/02, C4F6-450/03 and C4F6-450/04 at pH 2 .....................T

Q The CV Curves of C4F6-450/03 at pH 1 to 6 ............................................................. U

R The Nyquist Plot of C4F6-450/03 at pH 1 to 6 ........................................................... V

S The pH of Co and Fe Chloride in Solution ............................................................ V

T The Nickel Doped Catalysts ................................................................................. W U The Solubility of Fe and the Eh-pH Diagram ...................................................... W V The Achieved Co Content by SEM and XPS ....................................................... X

VI

1

1. Introduction

Efficient electrochemical water oxidation is considered to be a key technology for allowing the storage of renewable electricity from sunlight or wind in fuels.[1][2] In

this electrolytic process water is split into hydrogen and oxygen (Reaction 1).

𝐻2𝑂(𝑙) → 𝐻2(𝑔) + 12⁄ 𝑂2 (𝑔) (1)

This reaction can be separated into the cathodic half-cell reaction (hydrogen evolution reaction, HER) and the anodic half-cell reaction (oxygen evolution reaction, OER).

Both half-cell reactions are shown for acidic conditions in Figure 1. The protons can travel through the electrolyte and the membrane separator, while the electrons that are

generated on the anode via the OER travel to the cathode through an external circuit, where they are employed to reduce protons to molecular hydrogen, H2. An external potential is applied to drive the overall reaction since it is not spontaneous. The applied

potential has to be at least 1.23 V in order to split the O-H bond. [3]

Figure 1: Schematic depiction of the electrochemical water splitting reaction under acidic conditions. The left part

visualizes the oxygen evolution reaction leading to the production of molecular oxygen and protons, while the right

part shows the reduction of protons to molecular hydrogen. The reaction is driven by an external potential.[3] The

two compartments are separated by a proton exchange membrane to avoid mixing of the gases.

The best electrocatalysts for the acidic HER is platinum and the best catalyst for the acidic OER are IrO2 or RuO2. As these catalysts are precious and expansive, new materials for OER are currently under investigation. [4]

The investigated catalyst must fulfill two requirements. First, it should have a high activity. This means that it should split the water bond of 1.23 V with a minimal need

of excessive energy (overpotential). One way to investigate the activity of a catalyst is to connect the cell to a potentiostat and measure a cyclic voltammogram (CV).[5] The applied potential vs Ag/AgCl is converted to the potential vs the reversible hydrogen

electrode (RHE) according to Equation 2: 𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑣𝑠 𝑅𝐻𝐸 = 𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑣𝑠 𝐴𝑔/𝐴𝑔𝐶𝑙 − 𝐼 ∗ 𝑅𝑠 + 0.222 + 0.059 ∗ 𝑝𝐻 (2)

2

where I is the current, Rs the internal resistance and 0.222 V the standard potential of Ag/AgCl vs the standard hydrogen electrode (SHE). The factor 0.059 is calculated from

R*T/F where R is the gas constant, T the absolute temperature (T=298K) and F the Faraday constant. The reference to the RHE is made because the hydrogen electrode is

set as a zero-potential electrode in the literature. All electrode potentials are noted as the potential with reference to RHE, in order to allow cross-comparisons.

The energy loss (caused by various factors including the sluggish electrocatalytic performance, electrolyte resistance, electrical resistance in electrodes etc.) associated

with practical electrocatalytic systems often results in the need to apply potentials greater than the water binding strength of 1.23V for water splitting reaction to occur. This excessive applied potential is denoted as overpotential needed to compensate the

energy loss and will not result in the splitting of water, but e.g. lead to the transformation of part of the electric energy into thermal energy.[6] In this thesis, the

overpotential is quoted with respect to the current density of 10 mA/cm2 (horizontal line, Figure 2) and should be as low as possible for a high activity. This specific current density is commonly regarded as a benchmarking figure of merit for solar-fuel device

commercialization, because it corresponds to a solar-to-H2 conversion efficiency of roughly 10% when the electrochemical cell is coupled to a solar cell under 1 sun

illumination.[7]

Figure 2: Change of current density with potential. The dashed red line marks the current density of 10 mA/cm2

used for benchmarking the catalysts.

The second requirement which must be fulfilled is that the catalyst should have a high stability. The stability describes how long the catalyst will remain stable within the

given electrochemical surroundings (regarding the electrolyte pH and applied potential). The current density of 10 mA/cm2 will be consistently upheld for hours

while adjusting the potential. An ideal sample would require the same potential for hours/days/years, but an actual catalyst will degrade or decay with time. This will in turn result in a higher resistance, therefore necessitating an even higher potential than

before as expressed by Equation 3.

𝑈 = 𝑅 ∗ 𝐼 (3)

The degradation is occurring in a complex manner (Figure 3), where initially the slope increases only gradually (between the grey lines) and thereafter exponentially (above

the grey lines). In agreement with literature practice, we have selected a voltage increase of 0.5 V to define the point at which the catalyst turns unstable (red point).

3

Figure 3: A schematic stability curve (CP) as a function of time vs the potential. While the sample degrades

progressively over time, the point of instability is defined for benchmarking purposes as 0.5 V increase over the

initial potential required to sustain water oxidation at a current density of 10 mA/cm2 (red point).

The impedances of the samples were investigated by electrochemical impedance spectroscopy (EIS). The resulting Nyquist plots can be used to determine the electrode

resistance (Rs, uncompensated resistance between the working and the reference electrode), the catalyst-electrolyte charge transfer resistance Rct and the diffusion layer resistance R2 (Figure 4)[8]. The electrochemical equivalent circuit model used to fit the

Nyquist plot was [Rs(RctQct)(R2Q2)], see insert in Figure 4.

Figure 4: The Nyquist plot shows the Cartesian coordinates, the real part of the impedance measurements on the X

axis, and the imaginary part on the Y axis and is used to investigate the electrode resistance (Rs), the charge

transfer resistance (Rct) and the diffuse layer resistance (R2). Rct is specific for the catalyst, while Rs display the

electrolyte resistance and the geometric setup of the cell, and R2 shows the electrical conductivity of the catalyst

and its conductivity to the substrate. The parameters are extracted by fits (black line) to the data points (blue dots)

using the equivalent circuit diagram shown in the inset. The fit consists of two semi-circles, where the respective

width signifies the electrode resistance of the catalyst (Rct; orange) and of the diffuse layer resistance (R2; green).

The displacement from the Y-axis is due to the electrolyte resistance, marked with a blue double-sided arrow.

Additionally to the electronical characterization of the catalyst, a chemical characterization of the samples was carried out by XRD, SEM, EDS and SEM. First,

the mineral, which catalyzes the water splitting, was determined by powder X-ray diffractometer (XRD). Next, the homogeneity of the catalysts was investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy

(EDS). Last, the surface of the catalyst was investigated by X-ray photoelectron spectroscopy (XPS).

4

Aim of the Master Thesis

The aim of this thesis was to optimize the synthesis, composition and operation conditions of a catalysts for electrochemical water oxidation. The catalysts,

combinations of Fe and Co, were evaluated in terms of activity and stability, and characterized regarding their composition and minerology. The performance of the best catalyst was compared to data reported in the literature.

2. Popular Scientific Summary including Social and Ethical

Aspects

2.1 Popular Scientific Summary

The idea of collecting energy from the sun and using it for our daily needs has been a dream of mankind for some time, but how can this be accomplished? Solar cells as well

as wind and water turbines are already getting more and more common. However, for eventually replacing all fossil fuels one also needs to be able to store the renewable

electricity they produce, because mankind has a need for an uninterrupted supply with energy and cannot wait for the sun to shine to be able to use electrically powered devises. Additionally, solar cells are most efficient near the equator due to the direct

sun irradiation. Thus it would be advantages if solar energy could be stored and transported to where it is needed.[9]

One possible solution for this task might be electrochemical water splitting. The oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) can be used to produce molecular hydrogen (H2) and oxygen (O2) from water by using a

catalyst and the energy of the sun.[10] When the energy is needed, the recombination of hydrogen and oxygen can produce the necessary energy to power up for example

cars or airplanes.[11] The storage of light or electrical energy in form of H2 is a well-known idea, but demands a good catalyst to produce the required amounts of hydrogen. This technology has the potential to replace fossil fuels, which would make, for

example, the transportation sector more environmental friendly as CO2 emissions would be drastically reduced.[12][13] For this to happen, the catalyst would be needed

on an industrial scale and it would be connected to a solar cell to split water while hydrogen and oxygen are collected.[14] The hydrogen can then be compressed and filled into a car where it can be oxidized in contact with air. While the beginning has

already been made as some H2 powered cars are already on the market, this dream will not be achieved easily, since the infrastructure and technology for a large scaled

renewable (‘green’) hydrogen production and transportation are not fully developed yet. For example, the catalyst and electrolysis conditions still need to be developed and optimized. The desired catalyst must fulfill two conditions: first, it must be efficient

and stable over a long period of time, since the catalyst cannot be exchanged after 5 min of production as this would drastically increase the costs. Secondly, as the energy

demand is tremendous, the catalyst must be cheap to produce and its constituents abundant enough to allow large scale implementation. This motivates research that will allow replacing presently used noble metal catalysts, which do have a high activity and

stability, but are expensive and rare. The investigated catalysts in this work were made of the more abundant elements iron and cobalt. The goal of my project was to optimize

their performance and to compare it to the requirements stated above.

5

2.2 Social and Ethical Aspects

The research done for this thesis was use-inspired and application-oriented research

(Pasteur). The best possible outcome will contribute to improve our living conditions [15]. As a long term project, the OER will provide possibilities that allow the production of hydrogen and oxygen from water and solar energy (received through

a solar cell), which can be used to power cars or any other electronic device, thus reducing the carbon emission on the Earth. Not only does this goal justify the research

for this specific topic, but some adjustments were made in order to minimize the environmental burden. First, the experiments were all carried out using only small amounts of material. This attempt was used so that excessive waste formation was

prevented. The not-avoidable waste was disposed following the rules and regulations. The risk for the researchers and other persons was kept as low as possible by working

under a fume hood. One potential downside, however, is the general application of cobalt, the mining of which currently appears in parts of the world to be heavily reliant on child labor.[16][17]

3. Experimental

3.1 The Synthesis of the Catalysts

The catalysts were synthesized using spray-pyrolysis deposition on a hotplate. The

precursors used were aqueous solutions of iron (III) chloride hexahydrate and/or cobalt (II) chloride hexahydrate. The total metal concentration was kept at 0.05 M (see Table

2 for precursor details). The spray nozzle was placed at 30 cm distance and tilted 45° from a titanium foil (substrate), which was heated on a hot plate at 250 °C or 450 °C. Compressed air at 0.5 bar was used as a carrier gas to spray the precursor solution onto

the Ti substrate.

The sprayed samples were then placed in a sealed round-bottom flask and flushed with argon for 30 min before annealing for 30 min at 450°C under a static Ar environment. For P doping, the samples were annealed in the presence of sodium hypophosphite

powder. A copper wire was glued to the back of the annealed sample using graphite conductive adhesive to establish an electrical contact for electrochemical measurement. 3.2 The Electrochemical Measurements

The electrochemical cell consisted of the catalyst, platinum coil and Ag/AgCl/1 M KCl

as the working electrode, counter electrode and reference electrode, respectively. These electrodes were placed in contact with an aqueous electrolyte consisting of sulfuric acid or phosphate buffer solution, with pHs (pH 0.3 – pH 6) adjusted with sodium hydroxide

or phosphoric acid. The electrochemical cell is connected to a potentiostat/galvanostat (Metrohm

AUTOLAB AUT85125) for CP, CV and electrochemical impedance spectroscopy (EIS) measurements. All electrochemical data shown in Section 4 are iRu-corrected (i is the measured current while Ru is the uncompensated internal resistance measured by

EIS). All reported potentials hereafter were measured against the reversible hydrogen electrode: VRHE = VAg/AgCl + 0.222V + 0.059*pH. The overpotential η needed to drive

the OER is calculated as η = VRHE – 1.23V.

6

The electrocatalytic current at the working electrode was measured at positively applied potential. In the used configuration (Figure 5), the OER occurs at the catalyst

surface while the HER is catalyzed at the platinum surface. The geometric area of the catalyst exposed to the electrolyte was 0.2 cm2.

Figure 5: The oxygen evolution reaction and the hydrogen evolution reaction in an acidic, wet chemical cell using

platinum as the counter electrode and silver/silver chloride as the reference electrode.

3.3 Characterizations of the Catalysts

The samples were investigated by SEM, EDS, XPS and XRD. The mineralogical properties of the samples were examined using powder X-ray diffractometer (XRD)

(PANalytical Xpert3; Cu Kα radiation; 45kV; 40mA). The morphology was investigated by Scanning electron microscopy (SEM). The

instrument was a Carl Zeiss Merlin field-emission scanning electron microscope FESEM (operating at 5 kV 120 pA), while the SEM images were taken with SmartSEM software.

The elemental mapping was carried out by energy-dispersive X-ray spectroscopy (EDS) on an Oxford Instruments energy dispersive X-ray spectrometer EDS (operating

at 15 kV, 300 pA, dwell time for area analysis 30s, and map analysis 100us). The EDS data were taken with Aztec software. The samples were mounted onto an aluminum stub with carbon adhesive tape.

The XPS (X-Ray Photoelectron Spectroscopy) spectra were collected with a Kratos Axis Ultra DLD electron spectrometer using monochromated Al Kα source operated at

150 W. Analyzer pass energy of 160 eV for acquiring wide spectra and a pass energy of 20 eV for individual photoelectron lines were used. For non-conductive samples, the surface potential was stabilized by the spectrometer charge neutralization system. The

binding energy (BE) scale was referenced to the C 1s line of aliphatic carbon, set at 285.0 eV. No BE scale correction was done for conductive samples. Processing of the

spectra was accomplished with the Kratos software.

7

4. Results

4.1 The best Iron and Cobalt Combination for OER

The activities of different cobalt and iron compositions were investigated by measuring CV curves showing the generated current density plotted against the applied potential.

The standard electrical potential for water oxidation is 1.23 V vs. RHE, but all known catalysts require a higher applied potential (i.e. overpotential) to overcome energy

losses connected to, for example, catalytic charge transfer resistance and the charge transport across the catalyst-current collector interface.[3] Therefore, the best performing sample regarding the activity is the one closest to 1.23 V vs. RHE at bench-

marking current density of 10 mA/cm2. Figure 6a shows the best polarization curves of the samples of two measurements

prepared at an annealing temperature of 250°C. These samples will be denoted as F[y]C[10-y]-250, where F, C and y represent Fe, Co and the relative Fe content in 20% steps, respectively, while 250 specifies the annealing temperature. Both single-metal

oxides of 100% FeOX (F10-250) (see Table 2 for synthesis conditions) and 100% CoOX (C10-250) produced similar levels of current densities within the applied potential

region, which indicates a similar level of catalytic activity. Bimetallic oxides F[y]C[10-y]-250, on the other hand, generated higher current densities at given applied potentials, thus showing an improved catalytic activity as compared to F10-250 and C10-250 (the

spraying temperature is further explained in the discussion section). Each catalyst was measured two times, while each measurement contains six CV curves. (3 from low

potential to high potential and 3 the other way). All CV curves regarding the composition are shown in appendix G: Figure 36-Figure 38. The CV curves shifted to higher potentials over the 6 scans (Example of C4F6-250 in Figure 36) as the catalysts

reached equilibrium with their electrolyte surrounding. Thus, the last scan was used for characterizing the activity, because it represents best the behavior of the catalyst at

equilibrium. The same metal composition of the spraying solution did not in all cases produce the same overpotential for the two measurements, since the spaying synthesis is prone to

variation with unexperienced operators as many factors contribute to the final result . This variability thus dominates the error bars shown, which also include the much

smaller error range of the potentiostat (0.1 mV). The best average activities were observed with C4F6-250, C6F4-250 and C8F2-250 (see Figure 6b), with average overpotentials between 570 mV and 630 mV. These overpotentials can be considered

to be similar. For comparison, F10-250 and C10-250 generated a current density of 10 mA/cm2 at an overpotentials of 750 mV and 780 mV, respectively.

8

Figure 6: (a) Best CV scans of different iron-cobalt catalysts at pH 0.3 (0.5 M H2SO4). The CV’s are displaying the

current density in mA/cm2 as a function of the potential vs RHE, only the sixed scans (from low to high potential)

for each sample are shown (all data are given in Appendix G (Figures 38-40). The scan rate was 10 mV/s. All

catalysts were prepared and measured 2 times. (b) Average overpotentials with error bars derived from the two

independently prepared (different days) catalyst samples.

The Tafel slope can be used to investigate the intrinsic catalytic activity of catalysts.

The overpotential is plotted versus the current density J and shows the catalytic

response with overpotential. The plot of log J vs η has a linear range (Figure 7), which

follows the equation:

log 𝐽 = 𝑎 +1

𝑏𝜂

The constant a displays the exchange current density, while the constant b displays the

Tafel slope and can be used to interpret the rates of the mechanism for the whole reaction. A lower slope can be interpreted as a faster mechanism and a better charge transport. A good catalyst only needs a small increase of the overpotential in order to

increase the catalytic current by a factor of 10. [7][18] The Tafel slope of C10-250 and F10-250 are 112 mV/dec and 115 mV/dec, while the slope of C4F6-250 is only

72 mV/dec.

Figure 7: The Tafel plots as a function of overpotential per current density of F10-250 (purple), C10-250 (red) and

C4F6-250 (blue) in 0.5 M H2SO4 are shown. The measured data (dots) were fitted with a linear function (Tafel

slope) that is displayed in the corresponding color.

Electrochemical impedance measurements were performed to investigate the interfacial charge transfer resistances of the samples. Figure 8 shows the results

presented in the form of a Nyquist plot, where the diameter of the semicircle of the Nyquist plot gives the Rct (catalyst-electrolyte interfacial charge transfer resistance)

9

for the OER. The Rct is 18 Ω for C4F6-250, 20 Ω for C10-250 and 71 Ω for F10-250. Furthermore, F10-250 has another consecutive semicircle (R2) that is due to a poor

charge transport (resistance of 40 Ω) in the bulk of the film.[8] C4F6-250 has the lowest resistance which agreed to the result from the Tafel plot. The Tafel slope and the

Nyquist plot confirm that the combination of Fe and Co does produce an improved catalyst in comparison to both metals alone.

Figure 8: The Nyquist plots (Z´(Ω) vs -Z´´(Ω)) of C10-250 (blue), F10-250 (red) and C4F6-250 (orange) at 1.7 V

at pH 2 are shown. The points are the measured data while the lines in the same color show the fit. The Rct is 18 Ω

for C4F6-250, 20 Ω for C10-250 and 71 Ω for F10-250. F10-250 has a second semi-circle of 40 Ω.

4.2 The Mineralogical Properties

The mineralogical compositions of F10-250, C2F8-250, C4F6-250, C6F4-250, C8F2-

250 and C10-250 were investigated by XRD (Figure 9) with freshly prepared and washed samples. All samples show characteristic peaks of the titanium substrate around 38°, 40° and 53°. The first two samples of 0% cobalt (F10-250) and 20% cobalt (C2F8-

250) show the characteristic peaks of the hematite structure (24°, 33°, 35°, 41°, 50°, 53°, 63° and 64°)[19], while the samples of 40-80% cobalt (C4F6-250, C6F4-250 and

C8F2-250) show the characteristic peaks of the maghemite structure (30°, 35°, 44°, 57° and 63°).[20] The 100% cobalt sample (C10-250) shows the peaks from Co(II) oxide.[21] No peaks of cobalt-compounds were found for samples other than the one

with 100% cobalt, thus suggesting that Co is either present as an amorphous Co(II) phase or is included in the vacancies of the Fe(III) oxide crystal structure as a

substitutional aliovalent Co(II) doping.[22] Although they do share the same composition, maghemite and hematite differ in terms of their crystal structure, since hematite is hexagonally close-packed, while maghemite

is cubic close-packed (Appendix J: Figure 42). The cubic close-packed structure results in vacancies in the maghemite structure that allow the doping of cobalt and increase

catalytic performance. It can be assumed that the Co-content changes the dominant iron oxide phase from hematite to maghemite (for 40% to 80% Co in spraying solution) by increasing the stability of maghemite. This Co-induced phase change from the less

OER-active hematite to the more OER-active Co-doped maghemite structure leads to the observed increase in OER activity.[23] Both maghemite samples C4F6-250 and

C6F4-250 had similar OER activities (between 570 mV and 630 mV), with C4F6-250 showing a marginally higher activity. The mineral assignment was confirmed by the XPS scan (Appendix M: Table 5). A full analysis is pending, but the characteristic

10

peaks for maghemite [24] were detected and only small deviations were seen due to the Co doping.

Figure 9: The XRD spectra of F10-250, C2F8-250, C4F6-250, C6F4-250, C8F2-250 and C10-250 are shown and

the peaks are matched with hematite (H), maghemite (M), cobalt oxide (C) and the blank titanium substrate (*).

4.3 Elemental Distribution

The XRD suggested that C4F6-250 builds a maghemite Fe3+ oxide crystal structure

with a Co substitutional aliovalent Co(II) doping. A SEM/EDS scan (Figure 10) was used to investigate the elemental distribution of C4F6-250 to check if a second

amorphous Co-oxide region that was not involved in the doping could be present. The freshly sprayed samples F10-250, C2F8-250, C4F6-250, C6F4-250, C8F2-250 and C10-250 were investigated by EDS/SEM (Appendix B, Figure 21-Figure 26) after

rinsing them with water. The Fe ion concentration was equally distributed over most of the substrate, but Figure 10a shows two spots that contain nearly no Fe. At the same

time, these spots had the highest Co concentration (Figure 10b). The O concentration is mainly homogenous all over the substrate, but also visibly reduced in the Co-rich areas (Figure 10c). The same trend can be seen for C2F8-250 and C6F4-250 (shown in

appendix C: Figure 27). The SEM investigation (Figure 10d) showed that the “spots” were three dimensional on top of the substrate in form of drops. The XPS analyzes

(Appendix M: Table 5) of a second set of sprayed, but unwashed samples confirmed the Co-Cl on top of the surface (atomic concentration (AC) of 14%). The difference between these two is only that the Co-Fe ratio was nearly 1:1, which is closer to the

sprayed ratio of 40 to 60%. This can be used to verify that the washing step removes most, but not all Co chloride from the surface.

The Fe source was FeCl3·6H2O, which is present as octahedral complex. The dominant species in the solid state is the [Fe(H2O)4Cl2]·Cl(H2O)2

salt, while the dominant species in a dilute acidic solution is [Fe(H2O)6]3+. The water evaporated during the spraying

synthesize, so that the diluted solution of 0.05 M changes into a concentrated solution where the dominant species is trans-[FeCl2(H2O)4]+. The solubility and the species of

iron oxides are pH-dependent [25] (Appendix U: Figure 54), but the pH of the solution of Fe chloride in water was not adjusted and was measured as 1.7 for the pure Fe solution, resulting in Fe hydroxide formation [Fe(OH)x(H2O)6-x](3-x)+ (i.e.

[Fe(OH)(H2O)5]2+ or [Fe(OH)2(H2O)4]+). A more detailed explanation can be found in the study of I. Persson 2018.[26][27] The source of Co was CoCl2·6H2O which is

mainly present as trans-[Co(H2O)4Cl2]·2 H2O salt in the solid form, where the additional two water molecules are linked to the two chloride ions via hydrogen bonds. The Co-species in a diluted solution has been reported to be [Co(H2O)6]2+. The

11

concentrated solution contains different Co complexes like [Co(H2O)6]2+, [CoCl(H2O)5]+, [CoCl2(H2O)4], [CoCl3(H2O)]- and [CoCl4]2-.[28] The pH of the

solution of the Fe and/or Co chloride salt in water was not adjusted and resulted in an acidic pH between 5.6 (100% Co) and 1.7 (100% Fe) (Appendix S: Table 6), since the

positive charged Fe3+ can bind OH- ions. These ions are generated from water, which results in the release of protons into the solution and the resulting change in pH. The Fe and Co chloride precursor mixture was sprayed onto the heated Ti surface to

form the oxide film. As discussed, due to dehydration of the water drops during the deposition process we do not know which exact species reaches the Ti-surface. For the

further discussion we thus use the known properties of the starting salts as approximation. FeCl3 ·6H2O begins the oxidation process around 250 °C, while CoCl2·6H2O begins the oxidation process above 400 °C.[29][30] The required high

temperature for CoCl2·6H2O implies that the Co precursor tends to stay as the chloride form when the precursor aerosol dehydrates on the surface of the substrate, while the

Fe complex oxidizes. The thermal treatment at 250 °C was done before the measurement in the acidic electrolyte or the measurement by SEM/EDS, so that there may be some Co chloride which did not form oxides and thus is easily rinsed off

thereafter with water. This is in line with the result that the atomic ratio of 40% Co and 60% Fe for this sample was not achieved but only 20% Co were measured during the

elemental analysis (SEM/EDS) of the whole sample (Appendix V: Figure 55). The samples of the XPS scan were not washed before the measurement and contained the sprayed ratio (Table 5), revealing that the washing procedure removed Co chloride

from the surface. The same trend could be seen for 60 and 80% Co samples.

Figure 10: The C4F6-250 sample was investigated by EDS/SEM and the mapping of the a) detected Fe atoms, b)

detected Co atoms and c) detected O atoms are shown. d) The SEM image of the C4F6-250 is shown.

4.4 The Influence of the Electrolyte pH

The optimal OER conditions for C4F6-250 were investigated by determining the effect of the electrolyte pH on the catalytic activity. The polarization curves in Figure 11a

show that the catalytic activity decreases with increasing electrolyte pH. Each catalyst was measured two times with six CV curves each, but for reasons explained above only

the sixed scans of the best performing sample are shown here. By contrast, Figure 11b displays the average overpotential that both samples required to generate a current density of 10 mA/cm2, which increased from 600 mV to 1000 mV as the pH increases

12

from 0.3 to 6. The pH dependencies of C2F8-250 and C6F4-250 (20% and 60% cobalt samples) are shown in Figure 27 (Appendix C), which confirms the trend that the

overpotential is increasing with pH. In order to investigate this behavior, a more detailed investigation of the surface of the catalyst in the electrolyte was required. The

layer between the bulk aquatic electrolyte and the solid Fe phase can be assumed to be a double layer with O or OH groups on the surface. The characteristics of this layer can be used to describe the transfer of ions (charge carriers, e.g. protons) between the

electrolyte and the surface. The characteristics of this layer is strongly pH dependent [31] and can be displayed by looking at the Nyquist plot of C4F6-250 (Figure 11c) and

the resulting uncompensated resistance (Rs) (Figure 11d). The uncompensated resistance displays the resistance between the working electrode and the reference electrode (the resistance of the electrolyte). A lower pH implies an increase in charge

carries, which increases the layer capacitances while lowering the resistance. Each resistance was measured in two attempts with 4 curves each. The uncompensated

resistance increased with pH from 15 Ω at pH 0.3 up to 60 Ω at pH 6. This increase in the electrochemical/electrolyte resistance (Figure 11d) leads to a decrease in activity (Figure 11b) since a higher resistance in the electrolyte means that a higher potential is

required to achieve a current density of 10 mA/cm2. This can be explained by the relation U(Potential)=R(Resistance)*I(Current), which shows that the potential must

increase if the resistance increases (as long as the current remains constant).

Figure 11: (a) CV curves (6th scan) and (b) overpotentials of C4F6-250 (2 measurements each) at different pHs (0.3

– 6). (c) Nyquist plot of C4F6-250 (2 measurements each with 4 scans) at different pHs between 0.3 and 6 in an

H2SO4 electrolyte. (d) Uncompensated resistance (Rs; x-axis intercept) of C4F6-250 as a function of pH (0.3 – 6).

13

4.5 The Catalytic Stability

The best activity was achieved using C4F6-250 (similar to C6F4-250 and C8F2-250) at pH 0.3 with 0.5 M H2SO4, but a good catalyst must as well offer a high stability. The

stability was investigated by measuring a chrono potentiometry (CP) curve of C4F6-250 at different pH values between 0.3 and 6 (Figure 12a). This measurement was only performed once due to the long measurement times. The stability at pH 0.3 was 2 h

50 min increasing toward 5 h 10 min at pH 1, reaching a peak at pH 2 with an overall stability of 14 h and 30 min. The stability of 5 h 30 min at pH 3 was decreasing slightly

towards pH 6 (2 h 30 min). Before we look at the reason why pH 2 was the best condition for the stability of C4F6-250, the highest stability of C4F6-250 at pH 2 was compared with the stability of C10-250 and F10-250 at pH 2 (Figure 12b). The stability

of C10-250 was only 3 h 30 min, while the stability of F10-250 (15 h 50 min) and C4F6-250 (14 h 30 min) were on a similar, but slightly higher level. The difference

between C4F6-250/F10-250 and C10-250 can be explained by the varying Co content since the stability of F10-250 confirms that a higher Fe content is correlating to a higher stability. This can be used to explain why higher Co contents resulted in a lower

stability, i.e. in a reduced time where 10 mA/cm2 can be achieved. The activity of C4F6-250, C10-250 and F10-250 at pH 2 are shown in Appendix N: Figure 48c and

the overpotential is shown in Appendix N: Figure 48d. The overpotential of C4F6-250 was the lowest one with 670 mV, while F10-250 and C10-250 (750-780 eV) were less active, following the same trend as at pH 0.3 (Figure 6).

Figure 12: The stability curves of a) C4F6-250 at pH 0.3 to 6 and b) of C4F6-250, C10-250 and F10-250 at pH 2

measured in H2SO4 and NaOH.

The stability of C4F6-250 was the highest at pH 2, which can be explained by looking

at the Pourbaix diagrams of Co (Figure 13a) and Fe (Figure 13b). The displayed Fe Pourbaix diagram was the one from hematite, while the sample was maghemite. The one for maghemite was not available and it was assumed that they are similar. The

Pourbaix diagram of Co is an approximation as well, since it displays pure Co oxide and not a Co oxide substitution into maghemite. The Pourbaix diagrams contain two

dashed orange lines, whereof the higher one displays the required potential for OER and will be analyzed. The diagrams of Fe and Co show that both elements are stabilized in their ionic forms (blue boxes) at the oxidative potential (dotted, orange line) at a

pH<2. The stabilization of the ionic form in turn leads to the detachment of ions from the catalytic surface, thus leading them to enter the electrolyte solution. This explains

why the stability decreased below pH 2 (dotted, blue line). The Pourbaix diagram showed that Co is stabilized as Co3O4 above pH 2 and at increasing oxidative potentials (green Co3O4 box). The stabilization of Fe2O3 (green Fe2O3 box) at pH 2 improves the

14

stability at first, but it reduces the stability with increasing pH. This behavior can be explained by looking at the phase transition at a higher oxidative potential (red line).

The amount of excess oxidative potential (the overpotential) for the switch to the unstable FeO4

2- phase decreases with increasing pH, thus reducing the overall stability.

This behavior is in line with the observation, which shows a generally decreasing stability and increasing solubility associated with increasing pH and applied potential. It can be concluded that Co doping improves the activity, but is ineffective in improving

the stability.[32][33][34]

Figure 13: The Pourbaix diagram of (a) cobalt and (b) Fe showing the stable phases in dependence of the applied

E(V) and the pH. It is noted that the Pourbaix diagram of Fe does not distinguish between the mineral forms of

Fe2O3, hematite and maghemite, and thus is a guide only. The diagrams were constructed for a low metal

concentration, i.e., 10 nM, which is chosen owing to the low catalyst loading and a likely gradual dissolution of the

catalyst.[35] They are used as an indicator of equilibrium of the catalyst in aqueous system. Diagrams obtained from

references[32][33][34].

4.6 Changing the Spraying Temperature and the Electrolyte

The conditions for C4F6-250 were changed with the aim to improve the activity and stability. The sulfuric acid was exchanged by a phosphate electrolyte and the spraying

temperature was raised to 450 °C to achieve an oxidation of cobalt chloride to cobalt oxide. The used electrolyte was a combination of monosodium phosphate and disodium phosphate (Details in appendix A, Table 2 and Table 3). The best ionic strength of the

electrolyte was investigated by using Ir as catalyst (Appendix D: Figure 28) and was observed to be 1 M. Furthermore, the metals were dissolved in water which evaporates

faster due to the higher temperature resulting in a lower amount of surface oxide formation per sprayed volume of solution. Thus, the sprayed volume was raised from 100 mL H2O to 400 mL H2O to achieve the same catalyst deposition on the Ti substrate.

The activity (Figure 14a) and the overpotential (Figure 14b) of C4F6-450 at pH 2, 4 and 6 were measured. The activity of C4F6-450 favors a low pH since the activity

decreases from pH 2 (average overpotential of 560 mV) towards pH 4 (680 mV) and pH 6 (730 mV). All CV curves are shown in appendix O: Figure 49. On the other hand, the stability of C4F6-450 (Figure 14c and d) increased by increasing pH, since C4F6-

450 was only stable for half a day at pH 2, while the stability improves towards pH 4 (2.1 days) and pH 6 (more than 7 days). The fluctuation in the stability curve of C4F6-

450 is due to the building of gaseous hydrogen and oxygen, which sticks as bubbles on the surface until it detaches. These gas bubbles are temporarily reducing the contact area of the catalyst with the electrolyte and therefore reducing the activity.

15

Figure 14: (a) Activity, (b) overpotential, (c) CP curve and (d) stability of C4F6-450 at pH 2, 4 and 6 adjusted with

0.5 M NaOH in 1M monosodium /disodium phosphate.

The stability of C4F6-450 at pH 6 in 1M monosodium/disodium phosphate lasted for more than 7 days, but it was not monitored to the point of collapse. It is possible that

its total stability may exceed even longer periods of time, which would be of great interest for future investigations. In the literature, a long-term stable γ-manganese oxide

was found by Ailong Li et al.[36], which was stable for more than 8000 h at pH 2 in case of a constant potential (±50 mV), but the stability decreased under a varying potential. The attempt behind this idea was reasonable because the idea behind the OER

project is to connect the catalyst to a solar cell in the future, so that the catalyst must be stable even though the sun irradiation, and therefore the potential, may be

fluctuating. The stability of C4F6-450 at pH 6 with a varying potential between 1.2 V and 2.4 V was decaying within a day instead of lasting for over a week (Figure 15). The required potential vs RHE to achieve a current density of 10 mA/cm2 was

increasing by roughly 100 mV/ 3 h.

16

Figure 15: The stability of C4F6-450 under fluctuating potential displayed as different CV curves (1.2 to 2.4 V

uncompensated potential) at pH 6 in 3 h steps over a time period of 24 h.

4.7 The Surface Chemical Properties

The surface chemical properties of freshly prepared, unwashed C4F6-250 and C4F6-450 were investigated using XPS. The atomic concentration (AC) for different

elements at their specific binding energy is shown in Table 5. The XPS analysis showed that the ratio of Fe to Co was roughly 1:1 for both samples. The ratio of the SEM study showed only 20% cobalt which can be explained by the different preparations. The

XPS samples were only sprayed and annealed before measuring them, while the SEM samples were sprayed, annealed, and washed before the measurement. The Fe phase in

C4F6-250 and C4F6-450 was maghemite (XRD of C4F6-250 and C4F6-450 in Appendix K: Figure 43) with the specific binding energies of 710 eV and 724 eV[37], but the Co phase did change with the spraying temperature. Cobalt was present as Co

chloride in C4F6-250, while C4F6-450 contained either metallic Co or Co(II) oxide. The oxidation temperature of Cobalt is 400 °C, so that an increase of the spraying

temperature to 450 °C naturally explains the oxidation of Co2+. This oxidation results in the decrease of Co chloride and increase in Co oxide with increasing temperature above 400 °C. This can be seen in the Cl 2p3/2 atomic concentration too, since the

atomic concentration of 14% for C4F6-250 decreased to 2% for C4F6-450. Both samples contained bonded oxygens, whereof most of it were bound as metal oxides

(64-71%) or metal hydroxides (19-24%). Furthermore, both samples contained carbon in different forms. The source of this carbon is mainly contamination from the atmosphere which is higher for C4F6-450 than for C4F6-250, since the higher

temperature favors carbon from the air to settle down. Additionally, the increase in the spraying temperature resulted in an increase in the spraying time and volume, to

achieve the same catalyst loading on the substrate. This increase results in more carbon which came in contact with the surface and bound to it. Last, the S 2p3/2 analysis showed that some sulfur is bound to the surface of C4F6-450. The atomic concentration

of sulfur was low and came from contamination. The catalyst C4F6-250 was investigated by XRD (Appendix K: Figure 43) and showed only the peaks of

maghemite and therefore it can be assumed that Co doped maghemite is responsible for OER for the C4F6-450 sample as well. The intensity of the titanium substrate peaks decreased which indicates a higher catalyst thickness. The increase of the temperature

thus reduced the amount of cobalt chloride and increased the amount of cobalt oxide. The SEM scan revealed that Cobalt oxide forms an aerosol with the iron oxide, which

results in a homogenic distribution of cobalt over the surface (Appendix K: Figure 44).

17

4.8 Introducing/Replacing Metal Oxide with Metal Phosphate

The synthetic conditions for C4F6-450 were further modified by changing the Fe/Co oxides to Fe/Co phosphate with the aim to further improve the activity and/or stability

in the phosphoric electrolyte[38][39][40]. The oxide samples were transformed into phosphate samples via phosphidation using PH3 gas, which was produced by heating up sodium hypophosphite to 450 °C while annealing. The impact of different amounts

of sodium hypophosphite on the activity of C4F6-450 were investigated. Therefore, the samples are denoted as C4F6-450/XX to indicate the amount of added sodium

hypophosphite to the flask before annealing, ensuring that C4F6-450/03 contains 40% Co, 60% Fe and 0.3 g of sodium hypophosphite. The CV curves of C4F6-450 doped with different amounts of hypophosphite are shown in Appendix P: Figure 50. The 6th

scans are shown in Figure 16a and the calculated overpotential is shown in Figure 16b. The sample C4F6-450/03 had the lowest overpotential of the hypophosphite samples

with 600 mV, while both, the sample with more hypophosphite C4F6-450/04 (overpotential of 620 mV) and the sample with less hypophosphite (overpotential of 690 mV), resulted in a higher overpotential (no repeat measurements). However, all

these samples showed an equal or if at all slightly worse result as compared to C4F6-450, which was not treated with hypophosphite. C4F6-450 had an average

overpotential of 590 mV with the better measurement (pH 2).

Figure 16: (a) CV scans and (b) overpotential of C4F6-450, C4F6-450/02, C4F6-450/03 and C4F6-450/04. The

spraying was performed at 450 °C and the electrolyte pH during the measurement was adjusted to pH 2.0 using 0.5

M H3PO4 and 0.5 M NaOH. The differently treated phosphate samples were only measured once.

4.9 The Stability of the Metal Phosphates

The phosphate samples, which were treated with 0.3 g of sodium hypophosphite

(C4F6-450/03) were comparable in their activity to C4F6-450. Next, the stability of C4F6-450 with different treatments of hypophosphite were measured at pH 2 (Figure

17a). The stability of C4F6-450/03 (8 h 10 min) was higher than that of C4F6-450/02 (5 h 10 min) and C4F6-450/04 (4 h 40 min), following the same trend as in the activity (Figure 16). The stability of C4F6-450 at pH 2 without a hypophosphite treatment on

the other hand, was significantly higher (13 h 10 min) than the treated ones. The Tafel plots of C4F6-450 and C4F6-450/03 (Figure 17b) were investigated. The Tafel slope

increased from 110 mV/dec to 127 mv/dec after the treatment with 0.3 g of hypophosphite, which is in line with the above observation that the phosphite treatment did not improve the catalytic activity of C4F6-450 at pH 2.

18

Figure 17: (a) Stability as a function of the amount of hypophosphite in the flask before annealing (pH 2, 450 °C).

(b) The Tafel plot of C4F6-450 (110 mV/dec) and C4F6-450/03 (127 mV/dec) at pH 2.

4.10 The Chemical Composition of the Phosphate Samples

The chemical composition of C4F6-450/02, C4F6-450/03 and C4F6-450/04 were

investigated by XRD (Figure 18). The titanium substrate shows two specific peaks around 38° and 40°, which were visible on C4F6-450/02 and C4F6-450/04, but not on C4F6-450 and 4F6-450/03. The absence or different intensities of these peaks find their

explanation in the different thicknesses of the catalyst layers, which are a consequence of incomplete control of the synthetic conditions. The dominant phase for all samples

was maghemite, which was visible at 30°, 36° and 43°. The samples C4F6-450/03 showed additional FeP peaks at 33°, 37°, 46°, 47° and 49°.[41] The sample treated with 0.3 g sodium hypophosphite (C4F6-450/03, light blue line, Figure 18) resulted in the

highest activity (Figure 16) and stability (Figure 17) and is the only one displaying these FeP peaks. The SEM investigation of C4F6-450/02, C4F6-450/03 and C4F6-

450/04 (Appendix L: Figure 45Figure 47) showed that Co, Fe and P are distributed homogeneously over the surface, but only C4F6-450/03 showed the FeP peaks in XRD. The SEM scan (Appendix L: Table 4) showed that C4F6-450/03 contains a higher

atomic ratio of 23.8% P and 56.5% O compared to C4F6-450/02 and C4F6-450/04. It can be asssumed that only C4F6-450/03 contained phosphate on the surface while

C4F6-450/02 and C4F6-450/04 had amorphous phosphate on the surface.

Figure 18: The XRD peaks of the 40% Co sample (450 °C) treated with different amounts of hypophosphite,

matched with the titanium substrate (*), the maghemite peaks (M) or the FeP peaks (P) .

19

The XPS scan of the surface (Appendix M: Table 5) confirmed the SEM result, which suggested that the surface of C4F6-450/03 is covered with bonded phosphate. The P 2p3/2 scan resulted in an atomic concentration of 8.35% at a binding energy of 133.1 eV, which is typical for phosphate[42]. Nearly half of the surface (46.3%) was covered with metal oxides/hydroxides and another 7.72% of the atomic concentration on the surface were phosphate bounded oxygen. This can be seen at a binding energy of 533.1 eV, which could also fit C-OH bonds but these would also be visible in the C 1s spectra at 287.1 eV, where nothing was detected for C4F6-450/03. The carbon content from contaminations was present with a total of 26.25%. This value is lower than for C4F6-450, since the atomic concentrations are relative and the surface contains mainly phosphate. Furthermore, the carbon contamination came from the surrounding during the spraying process while the catalyst was heated. Comparing C4F6-450 and C4F6-450/03, the core Fe3+ 2p3/2 peak of phosphate and/or maghemite is located at a higher binding energy, which indicates that the Fe sites of the phosphate sample exhibit a more positive partial charge. It was assumed that this appears to adversely affect the OER activity by binding with the OH- (dissociated from water molecules) too strongly, so that it hinders the desorption of O2. The FePO4 peak of C4F6-450/03 comes from the phosphidation process where phosphate is produced, while C4F6-450 could contain these peaks due to contamination or contact to the phosphate electrolyte.[43] The XPS analyzes of O 1s revealed that the Fe-O on the surface is not completely gone after transferring the oxide sample into the phosphate sample, but the amount of Fe-O connection on the surface (around 530 eV) was reduced. Mainly OH/FePO4 (531.5 eV)[44] and H2O (533 eV) were found on the surface. These findings are in accordance with the previous result that the surface is mainly build out of Fe-P and FePO4 compounds. The comparison of the O 1s spectra of C4F6-450 and C4F6-450/03 showed that the Fe-O peak of the phosphate sample shifted to a lower binding energy. The partial charge at the O site of the phosphate sample is more negative compared to that of the oxide sample. This adversely affects the OER activity by acting as an electrically repulsive shield to the O of water molecules, thus hindering the dissociative water adsorption process (primary step for OER) on the sample surface.[31] The more negative partial charge at the O site of the phosphate sample attracts and reacts with the free H+ ions (from the electrolyte). This leads to more severe corrosion as compared to the oxide sample, which explains the decrease in the stability for the phosphate samples compared to the oxide samples. The P 2p spectrum of C4F6-450/03 (Appendix E: Figure 29) showed that P2p1/2 and P2p3/2 are visible between 131 and 135 eV, confirming the phosphate on the surface. The combination of XPS and XRD showed that the catalyst surface is mainly covered with phosphate, which is most likely amorphous as planned, since the peaks in the XRD are rather small and broad than intense and sharp. 4.11 Phosphate Samples – The Impact of the pH

Next, the impact of different pHs on the activity and stability of C4F6-450/03 was investigated. The 6th (Figure 19a) CV scans (Appendix Q: Figure 51) were converted

into the overpotential (Figure 19b), where they show the lowest overpotential, about 600 mV at pH 2. This trend in the activity can be explained by looking at the Nyquist

plot (Appendix R: Figure 52), which showed that the electrolyte resistance (Figure 19d) shows the same behavior as the activity. The Pourbaix diagram (Figure 19c) showed that pH 1 stabilizes aquatic H3PO4, while pH 2 and above stabilize

the singly deprotonated H2PO4-. The stabilization of the charged H2PO4

- instead of

H3PO4 stabilizes the binding of phosphate to the catalyst instead of dissolving into the

electrolyte. Additionally, the stabilization of the ionic H2PO4- and H+ at pH 2 and above

results in more free protons for the charge transfer, thus resulting in a higher activity. The increase in pH above 2 reduces the number of available protons for the charge

transfer, thus reducing the activity of the catalysts. These samples were only measured once, so that the electrolyte resistance at pH 6 can be assumed to be an error and is

most likely based on an error in the pH 6 electrolyte preparation.

20

Figure 19a: (a) Activity, (b) overpotential and (d) stability of C4F6-450/03 as a function of pH adjusted with 0.5 M

NaOH in 1M monosodium /disodium phosphate. (c) The Pourbaix diagram of phosphate at a low metal

concentration, i.e., 10 nM to display the behavior in solution.[32][33][34].

The stability of C4F6-450/03 was investigated (Figure 20) and is displayed in Figure

20b. The stability was increasing with increasing pH from 50 min (pH 1) to 31 h and 50 min (pH 6). The increase in pH increases the stability, since less protons are in the

electrolyte which can attack the phosphate on the surface. A lower pH means that more protons can bind with phosphate, which then detaches from the surface and dissolves into the electrolyte.[35][36]

Figure 20: (a) CP curves, (b) stability of C4F6-450/03 at different pHs. The electrolyte was 1 M

monosodium/disodium phosphate adjusted with 0.5 M NaOH.

21

5. Discussion

The original idea of this work was to investigate the production of ammonia from

gaseous nitrogen and a liquid proton source. The mechanism to produce ammonia consists of the nitrogen reduction reaction (NRR) and the oxygen evolution reaction (OER). Unfortunately, the study of the NRR could not be adapted to a work of this

scale, which is why the focus was shifted on the OER side instead. In this work, different combination of iron and cobalt were investigated for their ability to catalyze

the OER. They were tested at two different spraying conditions and at different pHs between 0.3 and 6. The synthesis was carried out at two different temperatures. The substrate temperature

influences the physical properties of the deposited catalysts, including the catalyst loading, mineralogy, and overall quality (e.g. catalyst adhesion to the Ti foil). In this

study, a minimal substrate temperature of 250 °C was chosen to be above the temperature for the decomposition of FeCl3. 6H2O (120-180 °C for first-stage dehydration) [29] and CoCl2.6H2O (130 °C for first-stage dehydration) [30] and to

compensate for heat loss to the surrounding. This temperature was above the temperature of oxide formation for Fe chloride but under the one from Co chloride

(400 °C). In later part of the work, a higher substrate temperature of 450 °C was chosen leading to an improved film quality (film adhesion to substrate, homogeneity) and the resultant enhanced catalytic performance (Figure 14, Appendix I, Figure 41).

The best working composition in terms of stability and activity was achieved with a spraying content of 40 to 80% cobalt and 60 to 20% iron for all three samples because

Co-doping induces the mineralogical phase transformation from hematite in the pure and lightly-doped Fe oxide samples, to maghemite in the highly doped (40-80% Co) Fe oxide. The actual composition for these samples was 20% Co and 80% Fe, since

most of Co was present as chloride, which was washed away before the measurement (SEM). The enhanced intrinsic OER activity of Co-doped Fe oxide samples as

compared to that of the pure Fe oxide is due to (1) the crystal polymorph effect where previous studies have provided evidence that maghemite shows increased OER activity compared to hematite[18] and (2) because of the co-doping effect where Co improves

(i) the intrinsic catalytic activity and (ii) the electron transport within the film (as seen in EIS data)[31]. While Co-doping improves the activity, it neither deteriorates nor

improves the stability as compared to pure Fe oxide. The catalysts sprayed at 250 °C had the highest average activity (average overpotentials between 570 mV and 630 mV) at pH 0.3 and the highest stability (14 h 20 min) at pH 2.

Next, two catalysts were synthesized at the spraying temperature of 450 °C resulted in: The phosphate and the oxide samples. The XRD and XPS results show that the main

product of the phosphidation of Co-Fe oxide is Co-Fe phosphate as planned. Transforming the oxide into phosphate shows adverse effect for both catalytic stability and activity. The XPS shows that the Co site is more negatively charged and the Fe site

is more positively charged for the Co-Fe phosphate as compared to Co-Fe oxide. This indicates that for Co-Fe phosphate, (1) there may be an electrical repulsion that hinders

the oxygen of the water molecules to absorb dissociative water at the catalyst surface; (2) the binding of OH- (dissociated from water molecules) to Fe site may be too strong for subsequent desorption of O2 product; (3) free H+ (from acidic electrolyte) may be

favorably attracted to the O site, thus leading to corrosion from proton attacks. The phosphate samples (0.3 g of sodium hypophosphite) of 40% Co and 60% Fe reached

the highest activity at pH 2 (overpotential of 600 mV), and the highest stability at pH 6 (33 h 10 min). The oxide samples on the other hand, which were synthesized at 450 °C as well, were more active at pH 2 (average overpotential of 560 mV). They were stable

for more than 7 days at pH 6. Furthermore, the impact of Ni as a third metal besides Fe

22

and Co was investigated, but did not result in any improvement on neither the activity nor the stability (Appendix T: Figure 53), which is why the data were not analyzed in

this work. The catalysts C4F6-250 and C4F6-450 will be set into context to check how good they

are, therefore the overpotentials of C4F6-250 at pH 0.3 (570 mV) and C4F6-450 at pH 2 (560 mV) were compared with the literature. First, Mondschein et al [47] investigated a cheap Co based catalyst which will be compared with the catalysts from

this work. Mondschein investigated a nanostructured film of Co3O4 on fluorine-doped tin oxide in 0.5 M H2SO4 which had an overpotential of 570 mV under acidic conditions

and was stable for over 12h. The nanostructured Co3O4 had a lower activity than C4F6-450 (560 mV) but was better than C4F6-250 (670 mV) and the C10-250 sample (780 mV, same electrolyte and pH). The impact of a nanostructure was not investigated

during this work, but five recently published OER catalysts (Table 1) are nanostructured in form of nanowires, nanosheets or other nanostructures, which makes

them of interest for future studies. The pros and cons of nanostructures are that they are not as fast and as easy to synthesize as the spraying method used in this work, but the surface area is increased which results in a higher activity. The Ir nanowire and the

nitrogen-doped carbon nanosheet are more active than C4F6-450, but Ir is a noble metal and therefore more expansive and not as suitable for mass production as C4F6-450.

The nitrogen-doped carbon nanosheet has the advantage of the high surface area of the nanosheet, which C4F6-450 does not have. A future attempt could be to investigate if the synthesis of C4F6-450 on a nanosheet could lead to an increase in the activity. The

investigated electrolyte in this thesis was either H2SO4 or H3PO4, the impact of HClO4 as a potential electrolyte was not part of these studies.

Table 1: Literature comparison of recently published OER catalysts under acidic conditions

Catalyst Electrolyte Method Overpotential Stability Source

Ir nanowires

(ultrathin)

0.1 M

HClO4

Wet chemical

method

390 mV Stable

for 11+ h

[48]

Maghemite/Hematite 0.5 M H2SO4 Spray pyrolysis 650 mV 24 h [18]

Nitrogen-doped

carbon nanostructures

0.1 M

HClO4

Wet impregnation

technique

390 mV 100

cycles

[49]

Co-MoS2 nanosheet 0.5 M H2SO4 Hydrothermal

method

540 mV Stable

for 11+ h

[50]

Ag-doped Co3O4

nanowires

0.5 M H2SO4 Hydrothermal

method

680 mV 10 h [51]

6. Conclusions and Outlook

The topic of this thesis was to investigate the OER activity and stability of different Fe and Co compositions under various conditions. The investigation of the composition

revealed that 40% to 80% Co and 60% to 20% Fe was the best performing spraying composition in terms of activity and stability, since it created a Co-doped maghemite

Fe(III) phase with a ratio of 20 % Co and 80% Fe on the catalyst. The activity (570-630 mV at pH 0.3) was improved by changing the electrolyte from H2SO4 to H3PO4 and the synthesis temperature from 250 °C to 450 °C, thus resulting in a higher activity

(560 mV at pH 2). The increase in the spraying temperature also increased the stability from 14 h and 30 min to more than seven days. The catalyst made from 40% Co and

60% Fe in H3PO4 (synthesized at 450 °C) were doped with sodium hypophosphite to increase their OER potential but the activity (650 mV) and the stability (33 h and 10 min at pH 6) could not be improved any further with these methods.

23

The investigation of non-noble metals for the oxygen evolution reaction is a promising path that can lead to a combination of metals which are cheap, stable and suitable for

mass production with a high activity in order to use and store the energy of the sun or other renewable electricity from other sources. The combination of Fe and Co is

promising, but must be investigated in the alkaline region as well to see if the activity and stability could be refined any further. Another possibility to increase the activity and stability would be to investigate the impact of the substrate or alternatively the

fabrication of a nanostructure to increase the surface. Lastly, the addition of yet another metal to the composition may lead to further improvements regarding catalyst stability

and activity.

Acknowledgement

I am grateful that I was able to study the topic of the oxygen evolution reaction in the research group of Johannes Messinger at the university of Umeå. A special thanks to

Wai Ling for supporting and helping me with all the practical work in the lab. I want to thank Léon and Lisa for proofreading my thesis and their support. It was a nice year

investigating the topic of the ammonia production and the water oxidation. Even though I had to drop the topic of the ammonia production out of gaseous nitrogen due to the time shortage, it was interesting and helpful to see how a new research topic is

set up from scratch and then experiencing the subsequent development of the project. Finally, I want to thank my friends and family for supporting me during all this time.

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A

Appendix

A The Sample Preparation

The weight at 250 °C was limited to 0.05 M in total. The input and the volume were quadrupled for the 450 °C samples to achieve the same loading.

Table 2: The sample preparation for 250°C and 450 °C

Sample Temperature

[°C]

Volume

[mL]

Iron

[g]

Cobalt

[g]

0% Cobalt

100% Iron

250 100 1.351 0

20%

Cobalt

80%

iron

250 100 1.081 0.238

40% Cobalt

60% Iron

250 100 0.811 0.476

60% Cobalt

40% Iron

250 100 0.540 0.713

80% Cobalt

20% Iron

250 100 0.270 0.951

100% Cobalt

0% Iron

250 100 0 1.189

0% Cobalt

100% Iron

450 400 5.404 0

40% Cobalt

60% Iron

450 400 3.242 1.903

100%

Cobalt

0%

Iron

450 400 0 4.757

The electrolytes for the phosphate samples were prepared as shown in Table 3. The pH

was adjusted with 0.5 M H3PO4 and 0.5 M NaOH if needed.

Table 3: Weight in for 50 mL electrolyte for different pHs

pH Monosodium phosphate, monohydrate [g]

Disodium phosphate, heptahydrate [g]

0.3 7.000 0

1 7.000 0

2 7.000 0

3 6.899 0.002

4 6.889 0.021

5 6.791 0.211

6 5.947 1.851

B

B SEM Pictures of the Sample

Figure 21: SEM picture of F10-250.

Figure 22: SEM mapping of C2F8-250.

B

Figure 23: SEM mapping of C4F6-250.

Figure 24: SEM mapping of C6F4-250.

C

Figure 25: SEM mapping of C8F2-250.

Figure 26: SEM picture of C10-250.

D

C The Overpotential of C2F8-250, C4F6-250 and C6F4-250

Figure 27: The overpotential of C2F8-250, C4F6-250 and C6F4-250. The trend that the overpotential was increasing

with pH was seen through all samples. The local minima at pH 0.3 using C4F6-250 could be confirmed.

D Different Ionic Strengths for the Phosphate and Oxide Samples at 450 °C

Figure 28: Three different electrolytes with different ionic strengths are shown using iridium at pH 2.

E

E The XPS Measurements

The atomic concentrations are shown in Table 5.

Figure 29: The XPS a) P 2p3/2 spectrum of C4F6-450/03 and b) S 2p3/2 spectrum of C4F6-450.

Figure 30: The XPS Cl 2p3/2 spectrum of a) C4F6-250 and b) C4F6-450.

F

Figure 31: The XPS C 1s spectrum of a) C4F6-250, b) C4F6-450 and c) C4F6-450/03.

Figure 32: The XPS O 1s spectrum of a) C4F6-250, b) C4F6-450 and c) C4F6-450/03.

G

Figure 33: The XPS Fe 2p3/2 spectrum of a) C4F6-250, b) C4F6-450 and c) C4F6-450/03.

Figure 34: The XPS Co 2p3/2 spectrum of a) C4F6-250 and b) C4F6-450.

H

F Stability Curves of C4F6-450 at pH 2,4 and 6

Figure 35: The CP curves of C4F6-450 at pH 2 in a 1M Monosodium/disodium phosphate electrolyte.

G All CV Curves of the 250°C Samples at pH 0.3

Figure 36: a) All 6 CV curves and b) the resulting overpotential of C4F6-250 at pH 0.3. The last curve of the 6.

scan was used. The same concept was used for all CV curves shown in this thesis.

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

0 1 2 3 4 5 6 7

Pote

nti

al vs

RH

E [

V]

time [days]

C4F6-450/03, pH 2

C4F6-450/03, pH 4

C4F6-450/03, pH 6

0

2

4

6

8

10

12

14

16

18

20

1.55 1.65 1.75 1.85

Cu

rren

t den

sity

[m

A/c

m2]

Potential vs RHE [V]

1. Scan

2. Scan

3. Scan

4. Scan

5. Scan

6. Scan

C4F6-250

537542

549552

559

569

520

530

540

550

560

570

580

1.

Scan

2.

Scan

3.

Scan

4.

Scan

5.

Scan

6.

Scan

Ove

rpote

nti

al

[mV

]

a b

I

Figure 37: The measured CV curves of a) F10-250, b) C2F8-250, c) C4F6-250, d) C6F4-250, e) C8F2-250 and f)

C10-250 at pH 0.3. The two measurements of C10-250 were measured from the same sample to see the degradation

and do not represent a second set of measurements.

0

2

4

6

8

10

12

14

16

18

20

1.6 1.8 2.0 2.2

Cu

rren

t den

sity

[m

A/c

m2]

Potential vs RHE [V]

25.02.2020

06.01.2020

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1

Cu

rren

t den

sity

[m

A/c

m2]

Potential vs RHE [V]

25.02.2020

09.01.2020

17.12.2019

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1

Curr

ent

dens

ity

[mA

/cm

2]

Potential vs RHE [V]

17.02.2020

16.12.2019

0

2

4

6

8

10

12

14

16

18

20

1.5 1.6 1.7 1.8 1.9

Cu

rren

t den

sity

[m

A/c

m2]

Potential vs RHE [V]

17.12.2019

09.01.2020

25.02.2020

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1

Cu

rren

t den

sity

[m

A/c

m2]

Potential vs RHE [V]

25.02.2020

17.12.2019

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1 2.3

Cu

rren

t den

sity

[m

A/c

m2]

Potential vs RHE [V]

06.01.2020

06.01.2020

a

e f

d c

b

J

Figure 38: The overpotential of the 6th CV curves of F10-250, C2F8-250, C4F6-250, C6F4-250, C8F2-250 and C10-

250. All 6th curves are shown in a), while b) shows the average between the curves for the same catalyst together with

the error range.

H All CV Curves of C4F6-250 at Different pHs

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1

Curr

ent

dens

ity

[mA

/cm

2]

Potential vs RHE [V]

17.02.2020

16.12.2019

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1

Curr

ent

dens

ity

[mA

/cm

2]

Potential vs RHE [V]

07.01.2020

05.02.2020

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1 2.3

Cu

rre

nt d

en

sity

[m

A/c

m2

]

Potential vs RHE [V]

07.01.2020

05.02.2020

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1 2.3

Curr

ent

dens

ity

[mA

/cm

2]

Potential vs RHE [V]

07.01.2020

07.01.2020

05.02.2020

c d

b a

K

Figure 39: The measured CV curves of C4F6-250 in H2SO4 at a) pH 0.3, b) pH 1, c) pH 2, d) pH 3, e) pH 4, f) Ph 5

and g) pH 6.

Figure 40: The overpotential of the 6th CV curves of C4F6-250 at different pHs. All 6th curves are shown in a),

while b) shows the average between the curves for the same pH together with the error range.

I The CV Curves of C4F6-250 and C4F6-450 at pH 2

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1 2.3

Curr

ent

dens

ity

[mA

/cm

2]

Potential vs RHE [V]

07.01.2020

05.02.2020

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1 2.3 2.5

Curr

ent

dens

ity

[mA

/cm

2]

Potential vs RHE [V]

07.01.2020

07.01.2020

05.02.2020

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1 2.3 2.5

Curr

ent

dens

ity

[mA

/cm

2]

Potential vs RHE [V]

07.01.2020

05.02.2020

g

f e

L

Figure 41: The CV curve of C4F6-250 and C4F6-450 at pH 2.

0

5

10

15

20

25

30

1.5 1.7 1.9 2.1 2.3

Cu

rren

t den

sity

[m

A/c

m2]

Potential vs RHE [V]

C4F6-250

C4F6-450

M

J The Maghemite and Hematite Structure

Figure 42: The a) hexagonally close-packed structure of hematite and the b) cubic close-packed structure of

maghemite.[52]

K The XRD and SEM Spectra of C4F6-250 and C4F6-450

Figure 43: The XRD spectra of C4F6-250 and C4F6-450 together with the titanium substrate (*) and the maghemite

peaks (M).

30 32 34 36 38 40 42 44 46 48 50

Inte

nsi

ty [a.u

.]

2θ (degree)

C4F6-250

C4F6-450

Ti plate

Maghemite

M

M

M

M

M

M

**

*

*

M

M

M

M

M

M

a b

N

Figure 44: The SEM mapping of C4F6-450 of a) Fe, b) Co, c) O, d) Cl, e) C and f) the SEM picture.

e f

c d

a b

O

L The SEM Spectra of C4F6-450/02, C4F6-450/03 and C4F6-450/04

Figure 45: The SEM mapping of C4F6-450/02 of a) Fe, b) Co, c) O, d) Cl, e) P and f) the SEM picture.

e f

c d

a b

P

Figure 46: The SEM mapping of C4F6-450/0 of a) Fe, b) Co, c) O, d) Cl, e) P and f) the SEM picture.

c

e f

A#

f

d

a b

Q

Figure 47: The SEM mapping of C4F6-450/04 of a) Fe, b) Co, c) O, d) Cl, e) P and f) the SEM picture.

Table 4: The atomic concentration (SEM) of C4F6-450/02, C4F6-450/03 and C4F6-450/04

C4F6-450/02 C4F6-450/03 C4F6-450/04

Element Atomic concentration [%]

Fe 21.3 12.5 24.1

Co 7.1 6.6 5.7

P 21.9 23.8 10.6

O 48.2 56.5 54.8

Cl 0.5 0.4 0.6

e f

c d

a b

R

M The Atomic Concentrations (XPS) of C4F6-250, C4F6-450 and C4F6-450/03

The XPS graphs are shown in Appendix E.

Table 5: The XPS analyzes of the different atomic concentrations of C4F6-250 and C4F6-450.

Binding energy [eV] Name

C4F6-250 AC [%]

C4F6-450 AC [%]

C4F6-450/03 AC [%]

P 2p 3/2

133.1 Phophate 8.35

S 2p 3/2

161.3 S 2- inorganic 1.47

162.5 S2 2- inorganic 1.56

163.5 Thiol, -SH 0.77

Cl 2p 3/2

197.9 Cl- inorganic 1.87 1.83

199.3 Co-Cl 11.72 0.29

C 1s

284.1 sp2 carbon 15.17 54.13 15.01

285.8 C-(C,H) 4.55 5.75 4.68

287.1 C-OH 2.28 2.49

288.4 C=O, COOH 1.82 2.26 6.56

290.8 π-π* exitation 1.07 1.61

O 1s

529.7 M oxide 14.3 (64%) 12.45 (71%) 1.1 (2%)

530.9 M-OH 5.38 (24%) 3.41 (19%) 45.2 (84%)

531.9 C=O organic 1.47 (7%) 0.83 (5%)

533.1 C-OH, organic 1.03 (5%) 0.96 (5%) 7.72 (14%)

Fe 2p 3/2

710.5 Fe(III) 14.06 5.31 8.21

Co 2p 3/2

778.1 Co metal 1.3

780.3 Co (II), CoO 3.59

781.7 & 784.1 CoCl 13.95

Fe/Co Ratio 1:1 1:1

S

N The Activity of C4F6-250, C10-250 and F10-250 at pH 2

Figure 48: The a) CV curve and b) overpotential of C4F6-250, C10-250 and F10-250 at pH 2 measured in H2SO4 and

NaOH.

O The CV Curves of C4F6-450 at pH 2, 4 and 6

Figure 49: The CV curve of C4F6-450 at a) pH 2,b) pH 4 and c) pH6 in a phosphoric electrolyte. The 6th curve and

the error of the measurement are shown in d. C4F6-450 was measured only once at pH 4, since the second sample

was damaged.

T

P The CV Curves of C4F6-450/02, C4F6-450/03 and C4F6-450/04 at pH 2

Figure 50: The CV curves of a) C4F6-450/02, b) C4F6-450/03 and c) C4F6-450/04. All of this samples were measured

one time. The CV curves of e) the 6th scans were converted into e) the overpotential together with C4F6-450. The

curves were detected in a phosphoric electrolyte at pH 2.

U

Q The CV Curves of C4F6-450/03 at pH 1 to 6

Figure 51: The CV curves of C4F6-450/03 at a) pH 1, b) pH 2, c) pH 3, d) pH 4, e) pH 5 and f) pH 6 in 0.5 M H3PO4.

V

R The Nyquist Plot of C4F6-450/03 at pH 1 to 6

Figure 52: The a) Nyquist plot of C4F6-450/03 at pH 1 to 6 and b) the resulting electrolyte resistance.

S The pH of Co and Fe Chloride in Solution

Table 6: The pH of different Co and Fe chloride salt compositions in water.

Sample Test 1 Test 2 Test 3 Average

C10 5.55 5.63 5.61 5.58

C8F2 2.27 2.27 2.26 2.27

C6F4 2.02 2.03 2.03 2.03

C4F6 1.88 1.89 1.90 1.89

C2F8 1.81 1.81 1.80 1.81

F10 1.73 1.71 1.72 1.72

W

T The Nickel Doped Catalysts

Figure 53: (a) CV curves, (b) activity of C2F8-250, C4F6-250 and C6F4-250 with 10% nickel substitution.(c) CP

curve and (d) stability of C4F6-250 and C4F6-250 -10% Nickel substitution.

U The Solubility of Fe and the Eh-pH Diagram

Figure 54: (a) Solubilities of goethite, hematite and lepidocrocite as a function of pH. (b) The Eh-pH diagram of Fe.

The stability field of maghemite can be assumed to be at higher Eh values, likely overlapping with the one of hematite

at circumneutral pH.

X

V The Achieved Co Content by SEM and XPS

Figure 55: The sprayed (grey) and measured Co content of CXFX-250. The SEM measurement (blue) detects the

whole sample, while the XPS measurement (orange) only scans the surface. Furthermore, the SEM samples were

washed after the synthesis, while the XPS samples were measured as synthesized.