The effects of redox-cycling and oxidation kinetics on anode-supported microtubular SOFCs

H. Monzón and M. A. Laguna-Bercero

Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC- Universidad de Zaragoza, C/ María

de Luna 3 E-50018, Spain

Abstract

Accidental reoxidation is one of the most critical issues of SOFC anodes, leading to

performance degradation or even producing catastrophic failure of the cell. In this work

several redox cycling experiments over microtubular SOFC cells are reported. Oxidation kinetic

parameters have been determined for this geometry by Thermogravimetric Analysis

(TGA).Oxidation kinetics is known to be diffusion limited at typical operation conditions and,

for this reason, strictly dependant on geometry. A linear relationship was found between

expansion kinetics and damage on SOFC anodes in a series of dilatometric experiments. A

microtubular Ni-YSZ/YSZ/LSM-YSZ cell was redox cycled several times at partial oxidation

conditions (10% oxidation) and performance degradation was measured by j-V

characterization and the loss was found to be approximately 2% in performance per cycle.

1- Introduction

Fuel cells represent an efficient and clean approach to transforming fuel into electric energy.

By eliminating the chemical-to-heat and heat-to-mechanical energy transformation steps a

high electric energy yield can be achieved. Solid oxide fuel cells (SOFC) are a type of fuel cell

where the electrolyte is an ion conducting ceramic oxide, commonly, yttria stabilized zirconia

(YSZ). These materials present an acceptable value of ion conductivity at working

temperatures between 800 and 1000°C.

Anode materials for SOFC are porous cermets composed of an electric conducting phase

interpenetrating an ionic conducting phase, commonly nickel-YSZ. This electrode operates in

contact with the fuel stream and thus nickel remains in the reduced state. Either fuel supply

interruption, seal leakage or system shutdown can produce the anode oxidation (Ni transforms

into NiO), which has considerably fast kinetics at high temperatures, being this one of the

major issues in SOFC anodes [1,2]. This oxidation will cause de Ni phase to expand over 60%

while the YSZ phase and electrolyte will oppose to this expansion. This situation will generate a

large amount of strained zones on both phases. Some of this strain will be accommodated by a

combination of elastic and pseudoplastic deformation, and some will be relaxed through

microcracking [3]. As the zirconia phase will act as a rigid backbone, its mechanical properties

are closely related to the redox stability of the material. When using an anode supported

configuration, this process is even more critical, as anode expansion can affect the thin

electrolyte layer. If cracking occurs in the electrolyte layer, the single cell will cease to work. If

the oxidation proceeds without electrolyte cracking, restoration of reducing atmosphere will

reduce again the anode cermet. Nevertheless, due to irreversibilities during the redox cycles,

damage accumulation can be expected in this electrode, as well as performance degradation

[4,5,6,7,8].

In order to minimize the impact of the reoxidation on this material some strategies have been

recently reported in the literature, such as softer operation conditions (i.e. lower

temperatures), increased porosity, modified microstructure or lower nickel contents

[9,10,11,12]. A very high degree of redox stability has been reported on anode materials

fabricated by nickel impregnation over porous zirconia. On the other hand these materials

present nickel coarsening problems on the long term operation [13].

Microtubular SOFC configuration has been widely studied and also present some advantages in

comparison with other SOFC designs [14,15]. One is their higher resistance to thermal cycles.

These properties could be linked to redox-cycling resistance. For example, any sudden fuel

supply interruption or seal leakage could be counteracted by rapidly cooling the system to

avoid a fast nickel reoxidation. Up to date little work has been done on redox-cycling using

anode-supported microtubular SOFCs. For example, Pusz et al. [16] studied Ni/8YSZ anodes

fabricated using fine and coarse particle size powders, then exposed them to several redox

cycles and finally compared them. They observed that anodes using fine-structured powders

showed better mechanical strength after exposure to three redox cycles than the coarse-

structured material. They believed that a better adhesion between the anode material and the

Ni current collector was achieved for the fine-structured powder. The redox tolerance of

Ni/YSZ anode supported microtubular SOFC was also studied by Dikwal et al. [17]. They

performed experiments at 800 °C in partial oxidation and reduction conditions and a total of

52 redox cycles and found an average degradation rate of 0.3% per cycle. They identified two

modes of degradation: (a) anode degradation due to micro-crack formation; and (b)

degradation due to delamination at the electrode–electrolyte interfaces.

In the present paper we have performed similar redox cycling studies on anode supported

microtubular cells fabricated in our laboratory. Different findings concerning the relationship

between damage suffered by the samples and the kinetics of the deformation that takes place

will be discussed.

2- Materials and methods

The experiments were carried out in either bulk anode samples in the form of bars or in

tubular cells, depending on the type of experiment.

2.1- Anode sample preparation

Anode materials were fabricated using the powder composition as described elsewhere [18].

This powder composition consists of a mixture of commercial NiO (Alfa Aesar 99%), which was

previously ball-milled down to a final particle size of about 1 micron, 8YSZ (TZ-8YS: 8 mol%

Y2O3-stabilized ZrO2, 99.9% Tosoh, Japan) and corn starch as pore former. This composition

leads to a 50% porous anode with a 50% nickel and 50% zirconia phases on a solid volume

basis after sintering and the subsequent reduction.

Several cylindrical samples with this anode composition were fabricated via cold isostatic

pressing (CIP). Polyvinyl alcohol (PVA) was added as binder followed by pressing at 200 MPa

for 5 min using a latex mould. The resulting bar was then fired at 1350ºC for 2h. The bars were

then cut into 5 mm long and 4 mm in diameter cylindrical samples using a diamond saw. The

density of all samples was calculated geometrically and was found to be very close to the

theoretical values. This calculation is consistent with mercury intrusion porosimetry

measurements reported for similar samples [19]. All NiO-YSZ composite samples had a 65%

relative density, with deviations under 2% in all cases. This density leads to a 50% relative

density Ni-YSZ cermet.

2.2- Microtubular cell fabrication

Same composition was used to fabricate tubular green bodies by CIP, using latex mold and a

steel rod as die. These green tubes were pre-sintered at 950ºC during 4 hours. Electrolyte layer

was then deposited by wet basis spray coating, using 8YSZ (TZ-8Y: 8 mol% Y2O3-stabilized ZrO2,

99.9% Tosoh, Japan) in 2-propanol as coating agent; electrolyte thickness was controlled by

weight and was estimated to be around 20μm. Anode and electrolyte were co-sintered at

1350ºC for 2 hours. Two different materials were used as the cathode. For the first cell, porous

platinum ink (6082A, Metalor) was used, applied by brush coating, and sintered at 800 ºC for 1

hour. For the second cell a LSM-YSZ (Fuel cell materials) cathode layer was deposited by dip

coating and fired for 1.5 hours at 1250 ºC. Both cells were 12 cm long and had 3.5 mm outer

diameter; both cathodes were applied over one square centimeter surface due to measuring

equipment limitations.

2.3- Dilatometric measurements

The cylindrical samples were heated up to 800°C (typical operation temperature) in a

thermomechanical analyzer (Setaram, SETSYS 200, France). Performing redox cycles using this

setup allows the measurement of the sample elongation in real time while oxidation/reduction

proceeds. 5% hydrogen in Argon (5%H2/Ar) was initially used as reducing agent and thus

during the first heating, the initial NiO-YSZ precursor was reduced to the Ni-YSZ cermet. Once

the temperature was reached and a stable measurement was obtained, the inlet gas was

switched to synthetic air. The oxidation-reduction process was repeated for a total of three full

cycles, considering that the oxidation/reduction was completed when there was no sample

length variation.

2.4- Thermo-Gravimetric Analysis (TGA) of a microtubular sample

As oxidation kinetics of the Ni-YSZ cermet was found to be diffusion limited, as it will be

discussed in the results section, a TGA experiment was carried out to determine the kinetics on

a microtubular sample, as geometrical dependence is expected in the kinetics of a diffusion

limited process. For the present experiment, a 2 mm long tube was used. Similarly to the

dilatometric studies, the sample was heated up to 800°C in 5% H2/Ar atmosphere, then the

sample was purged under N2 atmosphere for 20 minutes and finally oxidized in air. During this

time sample weight was recorded as a measurement of the degree of oxidation (DoO) using a

TGA (TA instruments STD 29/60 simultaneous DTA-TGA).

2.5- Electrochemical measurements

j-V measurements for the microtubular cell were recorded using a VSP

Potentiostat/Galvanostat (Princeton Applied Research, Oak Ridge, US). Electrical connections

were made using Pt wires in a 4 tip measurement setup. A microtubular Ni-YSZ/YSZ/Pt cell was

employed for this experiment. The cell was heated up to the working temperature in a small

tubular furnace. 800°C was selected as the furnace set point temperature. The air temperature

close to the cell cathode was measured using an external thermocouple and was around

750°C. Although at these temperatures the performance of the cell is relatively poor due to

the low conductivity of the electrolyte and also to the low electronic conductivity of the LSM,

we have selected this temperature in order to compare with other existing results in the

literature and also to avoid a fast reoxidation of the Ni cermet. The performance at cell

temperature of 900°C for similar cells fabricated in our laboratory is about 800 mW cm-2

[18,19]. Pure hydrogen was humidified by bubbling through RT deionized water (97%H2-

3%H2O) and then fed to the anode side of the cell using a flow rate of 30 ml/min. The cathode

side was open to air atmosphere, as for low current regimes this has been proven enough, and

introduces a minor temperature fall compared to forcing an air flow towards the cathode.

2.6- Microstructural characterization

Scanning Electron Microscopy (SEM) experiments were performed under an accelerating

voltage of 20 kV using a JEOL 6400 SEM (JEOL, USA). Elemental

X-ray analysis were fitted with Oxford Instruments INCA energy dispersive

X-ray spectroscopy (EDS) analytical system. Some of the experiments were also performed

using a Merlin FE-SEM (Carl Zeiss).

3. Results

3.1- In situ Redox experiments in a dilatometer

The present experiment was performed over four different analogue samples, and each one

was cycled to a total of three redox cycles. As nickel oxidation takes place, bulk sample

expansion is recorded, as shown in figure 1. As previously mentioned, this bulk expansion

corresponds partially to elastic and pseudoplastic deformation and microcracking. Once the

steady state was reached, the carrier gas was switched back to 5%H2/Ar. At this time, NiO to Ni

reduction in the cermet leads to a small bulk contraction while some elongation is not

recovered. This non reversible cumulative redox deformation is an indicator of the redox

damage suffered by the material and is named in the literature as cumulative redox strain

(CRS) [20]. The oxidation-reduction process was repeated a total of three full cycles. As

observed in figure 1, CRS deviations for the different analyzed samples are important, and

there will be further discussed. The humps that can be observed after expansion finishes and

before reduction correspond to slight temperature changes caused by the gas change.

The present experiment also gives us useful information about the kinetics that rules the

oxidation/reduction process. As the length change is recorded in real time as

oxidation/reduction takes place, the elongation vs. time curve could be obtained. Assuming

that there is a relationship between degree of oxidation (DoO) and elongation, we can obtain

some information from the reaction kinetics. CRS can be calculated as shown in eq. 1 where L

is the instant sample length and L0, the initial sample length.

(1)

For each oxidation/reduction, normalized CRS can be calculated by dividing each CRS at each

moment by the maximum CRS on that cycle. The representation of this normalized CRS value

vs. time gives us useful information about deformation kinetics during oxidation/reduction.

3.2- Electrochemical measurements

3.2.1-Full redox cycle

A microtubular Ni-YSZ/YSZ/Pt cell was measured at 800°C (furnace temperature). An open

circuit voltage of around 1.084 V was measured, which is very close to the theoretical one of

1.132 V predicted by the Nernst equation. Once stable conditions were reached, the anode

input gas was switched to laboratory air using the same flow rates. The OCV was now recorded

while oxidation is taking place: a fast fall in the OCV was first recorded followed by a plateau at

about 650 mV, which corresponds to the coexistence of Ni and NiO phases [21]. After this

linear regime, the OCV fell down to zero, indicating that all Ni has been oxidized to NiO. After

this, hydrogen flow was restored but an important hydrogen leakage was detected and the

initial OCV was not further recovered. Post-mortem analysis of the cell revealed severe

electrolyte cracking all over the cell, as observed in figure 2, indicating that one full redox cycle

is catastrophic for this type of anode supported cells with a very thin electrolyte. Micro-

cracking is clearly observed in figures 2b and 2c, especially near the YSZ electrolyte, as

previously reported by other authors [2].

Similar experiments where catastrophic failure was not reached have been also reported [22],

but their cells presented severe performance degradation (over 50% fall in power output).

3.2.2-Partial oxidation cycles

After the full redox cycle experiment, partial oxidation cycles up to a certain extent (about

22%) were proposed. For this test, a Ni-YSZ/YSZ/LSM-YSZ microtubular cell was employed. This

experiment was designed to study the structural degradation caused in the Ni-YSZ anode. For

this reason, operation conditions of this test were chosen to avoid catastrophic failure and the

cell performance was not optimized, which would have required a higher operation

temperature. The oxidation extent was controlled by time, using the kinetic data obtained

from the TGA experiment. Figure 3 shows oxidation vs. time recorded in the TGA experiment

for a tubular sample. As described in the corresponding section, a purge was introduced in the

system before the oxidation in air, during this purge some oxidation occurred due to some

content in oxygen in the purging gas used ( about 10-13 bar of O2). For this reason the oxidation

vs. time curve showed in figure 3 starts at time 0 at approximately 6% oxidation. During 100

seconds the oxidation occurred up to a 28% extent, subtracting the initial extent, this yields an

oxidation increase of approximately 22%.

Similar working conditions were used for the present experiment. A single j-V measurement

was recorded, and the presence of hysteresis indicates an electrode activation phenomenon

[23]. Prior to any further j-V measurements, the cell was held at a constant current of 70 mA

cm-2 during 30 min in order to fully activate the LSM-YSZ cathode. Partial oxidation

experiments were finally carried out using the following procedure. Firstly, the inlet chamber

(anode site) was purged using Argon. Purge is complete when a constant OCV measurement

was obtained. For this working conditions this measurement is about 734 mV and corresponds

to a state were no hydrogen is left in the anode side and Ni acts as electron donor while

oxidizing to NiO. Once this state is reached, anode gas input is switched to synthetic air (20%

O2, 80% N2) at a flow rate of 30 ml/min. This flow is kept for 100 seconds, which corresponds

to a 22% oxidation according to kinetic data obtained from previous experiments. After this

oxidation period, the anode site was again purged using Ar for 30 seconds, and H2 atmosphere

was finally restored. Initial OCV values were recovered after a period of about 30 seconds. This

partial redox cycle was repeated a total of nine times, performing polarization measurements

after each single cycle. A thermal cycle was also performed for inspection between cycles 5

and 6. j-V measurement after cycle 6 is not shown, as the LSM-YSZ electrode was not fully

activated and the polarization values do not follow the trend. Prior to cycle 7, the cell

electrodes were fully activated and the subsequent cycles followed the initial trend as

observed in figure 4. The presence of increased scattering data for cycles 8 and 9 could be an

indication of small damage in the cell (by the presence of microcracks in the anode and/or the

electrolyte). Another possibility could be a defect in an electrical contact, as the data seems to

be noisier when increasing the current density. For that reason we decided to conclude the

experiment at this stage.

4- Discussion

4.1- Ni-YSZ oxidation kinetics

Reduction and oxidation kinetics for Ni-YSZ cermets have been widely studied in the literature

[20,24,25 and references there in]. It is believed that high temperature nickel oxidation follows

a two-step mechanism. First, a nickel oxide layer is formed on the outer surface of the particle;

this process follows a logarithmic law, as specific volume of NiO is greater than that of metallic

Ni cracks often appear on this step. Once the NiO layer reaches a thickness of about 0.1 μm,

the second step takes place. This step consists of two competing processes: the inner diffusion

of O2- through the NiO and cracks, and the outer diffusion of Ni2+ ions. This second process is

faster at 800°C and tends to dominate at these temperatures, though the balance between

these two processes controls the microstructural modification that the cermet undergoes [27].

As bulk deformation is caused by nickel oxidation, it is expected to find a relationship between

deformation kinetics and oxidation kinetics. It was found that deformation kinetics follow a

parabolic law, as oxidation does in the mechanism proposed formerly. From the four different

experiments carried out in the dilatometer, the parabolic kinetic constant kp can be obtained

as the squared slope of the lineal part of the normalized CRS vs time1/2 plot, as seen in figure

5.a, for all analyzed samples the parabolic interval corresponds to a normalized CRS of

between 20 and 75%. Significant dispersion is observed for the different analyzed samples; this

effect could be associated to the diffusion limitation of the process, as samples were

fabricated via CIP using calibrated latex molds, but their geometries could present either small

deviations from perfect cylindrical bodies or fabrication defects. Another remarkable result

from these experiments is the evolution observed in the kp values of each sample after the

different redox cycles. Figure 6 shows this trend of the acceleration of the deformation kinetics

as the different cycles take place. This effect could be again explained by the diffusion

limitation, as redox cycling modifies the microstructure, generating additional porosity and

probably more specific surface. Additionally, by plotting the total CRS reached by each sample

as a function of kp in the first oxidation, a lineal relationship was found (figure 7). This

relationship apparently is indicating that the faster the oxidation process takes place the more

damage is resulting in the cermets, and it prevents pseudoplastic deformation from relaxing

the generated stresses. It is well known that one of the factors which have a major influence

on the redox damage on these materials is the temperature at which oxidation takes place. In

these experiments we have tested several samples at the same temperature and obtained

different deformation kinetics, which were associated to differences in the microstructure, and

we have seen that these kinetics lead to different amounts of accumulated redox damage. We

have also performed SEM studies of all the samples in order to see any microstructural

differences between samples. No apparent differences were observed between the four

analyzed samples, although careful image analysis is under way in order to confirm this. In

addition, small differences could be observed between the sample at the initial stage and the

sample after 3 redox cycles, as observed in figure 8. The presence of bigger cracks and also the

bigger pores which seem to have been formed during the cycles will contribute to faster

oxidation kinetics. We can conclude that damage taken by Ni-YSZ cermets is proportional to

the oxidation kinetics, and probably most of the influence of temperature over damage can be

explained through faster kinetics.

4.2 - NiO-YSZ reduction kinetics

For NiO reduction kinetics, a close relationship was found between the measured contraction

kinetics and the reduction mechanism. NiO reduction follows linear kinetics, which means that

it is limited by a superficial adsorption step. Two additional stages can be seen in this reaction

kinetics, the first takes place at low conversion and corresponds to an induction period, and

the second one is observed at high conversion rates where kinetics are parabolic-like, probably

because water vapor removal is inhibited by the cermet densification and thus a diffusion

limited regime is taking place. The deformation kinetic constant for the lineal part was

obtained from the slope of the normalized CRS vs. time plot as shown in figure 5.b. A lineal

behavior was observed for values of CRS between 0.3 and 0.8. Values for the kinetic constant

obtained in these experiments (average obtained value of 3,68 min-1) show no evolution

through the redox cycling and are in good agreement with reduction kinetic values present in

the literature [28]. This result indicates that contraction is lineal with reduction, and that this

process is reaction rate limited and not diffusion limited as it was observed for the oxidation

process, so microstructural modification has a minor effect on it.

4.3- Damage and degradation in microtubular cells

As exposed in the experimental methods section, these devices were not found to be able to

withstand a single full redox cycle, as bulk anode expansion causes irreversible electrolyte

cracking. On the other hand, these cells present a very good thermal shock resistance [18],

suggesting that some damage can be prevented by rapid cooling in the event of an accidental

oxidation.

Some more quantitative information can be obtained from the partial oxidation experiments

performed. From the polarization curves measured after each cycle, two parameters were

used for comparative purposes: the maximum power output generated and the internal

resistance of the cell. Both parameters show a small progressive degradation as cycles

accumulate. The data represented in figure 9 shows a power output decay of about 2% for

each cycle. This degradation could be associated with a loss of

electrical contact inside the anode as the microstructure is modified during

the redox cycling, which could increase the electrical resistance.

The results exposed in this paper show that permanent performance degradation occurs in

these devices even when exposed to short oxidations. These results also validate the

assumption of damage accumulation, as the degradation measured after each cycle was found

to be approximately constant.

The microstructure of this cell was also examined by SEM. Figure 10.a shows the three layers

present in the cell, from left to right: cathode, electrolyte and anode. The observation

conditions used allow compositional contrast between YSZ and nickel, YSZ is seen dark gray,

while nickel is almost white [23]. No damage was found on the electrolyte and no

delamination was observed between electrode-electrolyte interfaces. This supports the idea of

the observed performance degradation being caused by anode microstructural degradation.

Figure 10.b shows the anode microstructure, were porosity can be observed in black in two

different geometries: 10 μm pores generated by pore former and submicron porosity

generated during reduction. This degradation can be qualitatively observed as a slight

modification in the geometry of the submicron porosity as well as an overall volume increase

of this phase.

5- Conclusions

The present experiments showed that anode supported microtubular geometry presents a

poor resistance towards redox cycling, as oxidation kinetics are relatively fast due to the small

anode thickness in a diffusion limited regime. Additionally the thin electrolyte layer will crack if

anode expansion reaches a critical level, as seen in the full redox cycle experiment. As

microtubular configuration allows rapid thermal cycling, coupled redox and thermal

experiments are suggested in order to reduce the damage. Additionally, it has been proven

that even partial oxidation cycles cause irreversible performance degradation, which is

accumulated from cycle to cycle. For these particular operation conditions and cell setup

tested, this performance degradation was around 2% power output lost after every 10%

oxidation cycle.

5 – Acknowledgements

The authors thank grants MAT2009-14324-C02-01, financed by the Spanish Government

(Ministerio de Ciencia e Innovacion) and Feder program of the European Community, and

GALC-2009/09, financed by the Aragon Government and La Caixa Foundation, for funding the

project. The use of Servicio de Microscopia Electronica (University of Zaragoza) is also

acknowledged. M.A.L.-B. finally thanks the JAE-program (CSIC) for financial support.

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Figure captions

Figure 1: Elongation recorded during redox cycling for samples: 1 (a), 2(b), 3(c), 4(d).

Figure 2: Post mortem inspection of the full reoxidized cell. (a): Overal view of severe

electrolyte cracking, (b): SEM micrograph showing how electrolyte cracks are penetrating

through the whole electrolyte, (c): Wide cracking cause the platinum cathode layer to crack

and delaminate.

Figure 3: Oxidation vs. time for a tubular sample recorded in a TGA experiment.

Figure 4: Electrochemical measurements of the cell after each partial redox cycle.

(a):Polarization curves, (b): Power output curves. For clarity means only some cycles have been

represented, though we have observed that all of them follow the same trend.

Figure 5: Fit of dilatometry measurements for (a) oxidation of sample 1, (b) reduction of

sample 2.

Figure 6: Evolution of kinetic constants of the four samples as a function

of the number of oxidation.

Figure 7: Oxidation kinetics - Cumulative redox strain (CRS) relationship.

Figure 8: Observed anode samples at initial state (a,b) and after 3 redox cycles (c,d).

Figure 9: Evolution of internal resistance and maximum power in redox cycled cell.

Figure 10: FE-SEM micrographs of partial redox cycled sample showing an overview of the

cathode, electrolyte and anode (a) and detail of anode microstructure (b).

Figure 1.a

Figure 1.b

Figure 1.c

Figure 1.d

Figure 2.a

Figure 2.b

Figure 2.c

Figure 3

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

0 100 200 300 400 500 600

Do

O [

%]

Time [s]

Figure 4.a

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 20 40 60 80 100 120

E (V

)

j(mAcm-2)

1

3

5

9

Figure 4.b

0

10

20

30

40

50

60

0 20 40 60 80 100 120

Po

we

r d

en

sity

(m

Wcm

-2)

j (mAcm-2)

1

3

5

9

Figure 5.a

y = 0.2904x - 0.609 R² = 0.9993

y = 0.4571x - 0.6931 R² = 0.9992

y = 0.552x - 0.8453 R² = 0.9948

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 1 2 3 4 5

no

rmal

ized

CR

S

time1/2[min1/2]

1st oxidation

2nd oxidation

3rd oxidation

Figure 5.b

y = -0.0391x + 0.8107 R² = 0.9994

y = -0.0421x + 0.805 R² = 0.9988

y = -0.0415x + 0.7978 R² = 0.9962

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 2 4 6 8 10 12 14

no

rmal

ized

CR

S

time [minutes]

2nd reduction

3rd reduction

1st reduction

Figure 6

0.0000

0.0500

0.1000

0.1500

0.2000

0.2500

0.3000

0.3500

1 2 3

kp [

min

-1]

Oxidation number

1

2

3

4

Figure 7

y = 0.1337x - 0.014 R² = 0.9727

0

0.005

0.01

0.015

0.02

0.025

0.03

0.15 0.17 0.19 0.21 0.23 0.25 0.27 0.29 0.31

CR

S

Kp 1st oxidation

Figure 8.a

Figure 8.b

Figure 8.c

Figure 8.d

Figure 9

40.00

42.00

44.00

46.00

48.00

50.00

52.00

54.00

56.00

58.00

4

4.5

5

5.5

6

6.5

7

0 2 4 6 8 10

max

po

we

r o

utp

ut

(mW

cm-2

)

Inte

rnal

re

sist

ance

[Ωcm2]

Redox cycles

Figure 10.a

Figure 10.b