Degradation Kinetics during Oxygen Electrocatalysis on Perovskite

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Degradation Kinetics during Oxygen Electrocatalysis on Perovskite-based Surfaces in Alkaline Media Daniel Bick, Tobias B. Krebs, Dominik Kleimaier, Alexander F. Zurhelle, Georgi T. Staikov, Rainer Waser, and Ilia Valov Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03733 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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Degradation Kinetics during Oxygen Electrocatalysis on Perovskite-based Surfaces in Alkaline Media

D. S. Bick1,2, T. B. Krebs1, D. Kleimaier1, A. F. Zurhelle1,2, G. Staikov1,2, R. Waser1,2,3 and I. Valov1,2,3

1

Institute for Materials in Electrical Engineering and Information Technology (IWE2), RWTH Aachen University of Technology, D-52074 Aachen, Germany 2

JARA – Fundamentals of Future Information Technology, FZ Jülich, D-52425 Jülich, Germany 3

Peter Grünberg Institute, FZ Jülich, D-52425 Jülich, Germany

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Abstract Oxygen evolution reaction (OER) during alkaline water electrolysis is the bottleneck of water splitting. Perovskite materials have been particularly proposed as good and economically reasonable electrocatalysts for the OER, showing promises and advantages with respect to classic metallic electrodes. However, the degradation of perovskites during catalysis is limiting their service lifetime. Recently, the material BaCo0.98Ti0.02O3-δ:Co3O4 was shown to be electrocatalytically and chemically stable during water electrolysis even at industrially relevant conditions. The lifetime of this perovskite-based system is prolonged by a factor of 10 in comparison to e.g. Pr0.2Ba0.8CoO3-δ and is comparable to the industrially applied electrodes. Here we report on the degradation kinetics of several OER catalysts at room temperature, comparatively studied by monitoring the oxygen evolution at microelectrodes. A decrease of the reaction rate within max. 60 seconds is observed, that is related to chemical and/or structural changes at the oxide surface.


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1 Introduction The international power economy is facing big challenges to lower the intensive use of fossil fuels and nuclear power sources while covering the rising power demand 1-3. The upcoming change to renewable energy sources makes buffer storage systems a key factor for the compensation of periodically generating power sources (wind & solar)

4, 5

. Alkaline water

electrolysis is a reliable technology for hydrogen production as a buffer medium in industrial scales

6, 7

. However, efficient water splitting requires affordable electrocatalysts, especially

for the oxygen evolution reaction (OER), with simultaneously high lifetimes and good electrocatalytic performance, high electronic conductivity and low usage of noble or highcost metals. Various perovskite electrocatalysts are known to date8 that were found promising in respect to OER9-12. Recently, we published a detailed analysis on BaCoO3-based perovskite catalysts, discussing the electrocatalytic properties and the degradation mechanism

13-15

. The catalysts with the optimal chemical composition Pr0.2Ba0.8CoO3-δ (0.2

PBCO) was found to be stable throughout its lifetime, but its lifetime and electrocatalytic performance are inferior to those of BaCo0.98Ti0.02O3-δ:Co3O4 (BCT:CB 2). It was shown, that perovskites are easily transformed into an amorphous phase during OER at industrially relevant conditions, a process also leading to chemical instability and leaching. This process is kinetically limited at room temperatures, but still persists at the interfaces16. We suggested an improvement of material properties by introducing BCT:CB 2, which exhibits a selfassembling effect during the reported phase transition, showing a short-range order comparable to tetragonal tungsten bronzes (TTB) which hinders further Ba leaching and thus retains high electrocatalytic activity and stability of chemical composition. 15 In general, most perovskite OER catalysts will become amorphous during OER8,

16-17

,

especially at industry conditions (80°C)14, which is mostly accompanied by A-site cation 3 ACS Paragon Plus Environment

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leaching. However, kinetics of this phase transition, as well as the direct influence on the OER rate at the catalyst–electrolyte interface, are still poorly explained. It was suggested that the catalyst surface leaches A-site cations, due to the gas evolution from lattice oxygen (Lattice Oxygen Evolution (Reaction) - LOE(R))18, 19, which is related to the oxygen mobility in the lattice and consequently a function of oxygen vacancy concentration. According to the model of Fabbri et al.20, dissolution and re-deposition of A-site cations onto the metal–(oxy)hydroxide interface is strongly influencing the long-term stability of the catalyst and directly linked with LOE. Earlier, Bergmann et al.21 observed a reversible crystalline to amorphous phase transition in Co3O4 in OER conditions. In general, amorphous OER catalyst materials are often found to be more active than crystalline systems22, 23. Thus, chemical and/or structural changes at the catalyst–electrolyte interface should directly influence the OER rate. In this case, optical microscopy provides a direct access to the OER rate by monitoring and evaluation of the oxygen bubble evolution24, being a measure for the change in catalytic properties of the oxide material. In contrast to methods, which monitor catalytic properties of gas evolving electrodes by observing periodic bubble detachment25, 26, microscopy of the bubble growth rate is not dependent on the detachment event. In this work, we report on an effect of decrease of the oxygen evolution rate within the first 60 s during alkaline water electrolysis and relate this phenomenon to structural and/or chemical changes at the electrode–liquid interface. We discuss on the degradation kinetics behind the interfacial reconstruction by evaluating the OER rate in these first 60 seconds, where oxygen bubble formation is monitored in situ by microscopy methods at microelectrodes. We succeeded to resolve changes in the electrode reaction rate on perovskite catalysts, related to interfacial modifications, highlighting the dependence of the reaction rate on the surface composition and structure. The change in reaction rate can be 4 ACS Paragon Plus Environment

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suppressed by defect-chemical engineering using BCT:CB 2, which was reported to be a stable oxide OER catalyst15.

2 Experimental 2.1 Synthesis All samples were synthesized by a chemical solution deposition27 route on platinized Si substrates as previously described13, 14, resulting in single-phase, polycrystalline perovskite electrocatalyst thin films as confirmed by XRD. All prepared thin films are binder-free. We used polycrystalline Pt (111) films on Si (100) plain substrates as reference samples. The working microelectrode was prepared by photolithography, using the resist AZ 5214 E (MicroChemicals) and a mask for a 50 µm disk. Spinning for 30 s at 1000 rpm results in a resist thickness of around 3 µm. After drying the resist for 7 min at 90 °C, the sample was exposed for 16.5 s. The developer AZ826MEF (MicroChemicals) was used for a period of 90 s afterwards. As the last step, the resist is crosslinked for 10 min at 200°C in air to use the resist as an electrical insulating layer in the following experiments. 2.2 Electrochemical Measurements All electrochemical measurements were performed in a membrane-free electrochemical glass-cell with three electrode geometry and 0.1 M Kaliumhydroxide (KOH) electrolyte (N2-saturated), using a Pt counter electrode and a Hg/HgO reference electrode in 0.1 M KOH (Radiometer Analytical). The cell is equipped with a microscope camera system (Lenses: Opto - 7:1 Modulare C-Mount Zoom; Camera: Allied Vision Manta) with the optical axis orthogonal to the sample surface. 5 ACS Paragon Plus Environment

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Chronocoulometric measurements were conducted with a resolution of 100 points/s for the current at different potentials for 60 s. This time scale was selected, to avoid complete blocking of the electrode and interaction of bigger bubbles with the resist edge at the microelectrodes, leading to uncertainties in the experimental procedure. All video recordings were analyzed manually, generating 50 points/60s for the bubble diameter. Measurements were only considered valid, when no leakage current was detected. Bubble formation anywhere else, except within the microelectrode was excluded by monitoring the complete surface area. Oxygen bubble evolution at microelectrodes is not practically influenced by diffusion limitations of the reactant OH- because of its high mobility, D ≈ 10-5 cm2s-1 @RT28.

2.3 Data processing The gas volume V of the bubble is calculated from the frame-wise manually measured horizontal (d1) and vertical (d2) bubble diameters (measuring ± 2.5 %) in pixel counts. The pixel counts to real size conversion as well as a correction to the real as prepared electrode diameter, which is measured with a scale microscope, are included in factor k (Eq. 1). ଵ

ܸ = × ߨ( ଺

ௗభ ାௗమ ଶ

× ݇)ଷ

(Eq. 1)

The volume V is then frame-wise corrected by the volume of the spherical calotte, which is the difference between a perfect sphere and the real sphere sticking at the electrode– electrolyte interface, presuming a contact diameter of 60 % of the bubble diameter. The bubble volume is also frame-wise corrected to oxygen-diffusion into the electrolyte. For better data handling, the diffusion is measured for bubble sizes from 8.4×10-4 mm3 to 6 ACS Paragon Plus Environment

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1.1×10-3 mm3 and assumed to be linear over the whole experiment in first approximation. However, the diffusion will be a function of interface area in reality. The molar amount of oxygen n is then calculated within the ideal gas law, assuming an ideal gas at standard ambient temperature and pressure.

3 Results and Discussion Basic requirement for the conducted experiments and analysis is the growth and nucleation of a single bubble at the microelectrode. To confirm this condition, we continuously used a second camera to monitor and ensure that the isolating resist is dense in the area exposed to electrolyte and no additional bubbles are formed. Within the area of the microelectrode, several nucleation sites are capable to initiate bubble formation. However, within 0.07 s, only one nucleation site remains dominant on the whole electrode area. Throughout the measurement, the position of the oxygen bubble remains constant. Figure 1 shows a single bubble growing over time at 50 µm disc-shaped PBCO microelectrode. With increasing bubble diameter (volume), the area of the microelectrode becomes shielded by the bubble and the current decreases (Figure 2). In case of full shielding, the current completely drops to zero (only for times exceeding 60 s). In our analysis we account for the decreasing electrode area over time, and all bubble-growth measurements are represented using the electrical charge (instead of time) as x-coordinate. The reaction rate of the oxygen evolution reaction at the catalyst surface is then estimated from the slope of measured oxygen volume over flown electrical charge. The steady-state OER reaction rate is expected to be constant during application of a constant potential, excluding wear or degradation effects. Indeed for the case of Pt 7 ACS Paragon Plus Environment

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microelectrodes, a constant reaction rate is observed for all potentials as shown in Figure 3 (here: at 2 V vs. Hg/HgO). Pt is chosen because of its known chemical stability. In contrast for BSCF and PBCO perovskite microelectrodes, we found a different behavior: We observed a change of the OER rate during the reaction as shown in Figure 4 (for PBCO). During our chronocoulometric experiments the reaction rate changes and we can define two periods, characterized by different reaction rates (further on denoted as period 1 and 2). The reaction rate at a potential of 1.1 V (all potentials vs. Hg/HgO) during period 1 is 50.6 ± 1.3 mm3C-1, which is higher than during period 2 with 32.3 ± 0.8 mm3C-1 (Figure 4a.). Between these two periods, a transition from slope 1 to slope 2 can be observed. The width of this transition region depends on the magnitude of the applied potential. The extrapolation of the two reaction rates in both periods shows that the width of the transition region between slope 1 and 2 decreases with increasing potential (from 1.1 V to 1.15 V and 1.25 V). The transition region completely disappears at a potential of 1.3 V and higher (Figure

4d.). Figure 5 illustrates the trend observed in the bubble formation experiments with increasing applied potentials in the range from 1.1 V to 1.3 V. With the decrease of the width (duration) transition region, the reaction rate for period 1 decreases by 4 % to 48.6 ± 1.3 mm3C-1. In contrast, the reaction rate in period 2 increases significantly by 24 % to 40.1 ± 1.0 mm3C-1 (Figure 5a.). Period 2 is extended at higher potential values, while the transition region is diminished (Figure 5b) and the midpoint position of the transition region is shifted to earlier times (Figure 5c.). The narrowing of the transition region is a function of both time and electric charge flow (Figure 5d). We correlate the change in OER rate to changes at electrode–electrolyte interface and more specifically to chemical and/or structural changes at the oxide surface. This conclusion is supported by comparing the chronocoulometric responses of both perovskites 0.2 PBCO and 8 ACS Paragon Plus Environment

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BCT:CB 2, as shown in Figure 6c. It has been demonstrated that BCT:CB 2 is more stable than 0.2 PBCO15, and this trend is also observed in this experiment. BCT:CB 2 is characterized by an almost constant reaction rate (Figure 6b), whereas the rate changes for PBCO. The maximal difference between slopes 1 and 2 measured for BCT:CB 2 is as low as 10 mm3C-1 @ 1 V (Figure 6a), which is much lower than for 0.2 PBCO (18.3 mm3C-1 @ 1 V). Hence, the degradation in BCT:CB 2 is strongly suppressed, although not stopped completely. As a result, region 1 and 2 merged and the transition region cannot be resolved properly anymore (similar to Pt electrode). The overall reaction rate of BCT:CB 2 lies between 38.5 and 43 mm3C-1 between 1-1.3 V vs. Hg/HgO. Conclusively, we found a change in OER rate, which occurs in the first seconds of the reaction. This change is characterized by two distinctive slopes observed in the chronocoulometric dependence, and an intermediate transition region, which becomes shorter for higher applied potentials. For noble metals, e.g. Pt, and more stable oxide catalysts, the dependence is apparently linear (constant OER rate). From the fact that the duration of period 1 is not changing with the magnitude of the potential, while the transition region and region 2 are changing, we conclude that the mechanism behind the OER rate change is determined by an interface degradation process. We speculate that at increased potentials, the higher driving force will lead to faster cation depletion within the surface region. The initially observed higher OER rate can then be attributed to lattice oxygen evolution, which enhances the OER catalysis additionally, as long as lattice oxygen is available for the reaction. Accordingly, the OER catalysis is enhanced during perovskite decomposition, which fits the concept of LOER based on surface reconstruction proposed by Fabbri et al.20 In the case of BCT:CB 2, the crystal decomposition is hindered by the stabilization of a short range self-assembly effect15, which inhibits LOER in the first place and 9 ACS Paragon Plus Environment

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preserves the oxides’ initial catalytic properties. Talking about LOER, the amount of lattice oxygen bound in BCT:CB 2 is lower than in PBCO (considering δ ≈ 0.5), which could impact the surface reconstruction as well.


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Conclusion We report for first time on a phenomenon showing that the oxygen evolution rate on perovskites (in contrast to metal electrodes) can be non-constant and shows a decrease within the first 60 seconds. The correlated change in reaction rate can be attributed to chemical and/or structural degradation of the catalyst–electrolyte interface. The transition between this two reaction rates is potential dependent and the initially high reaction rate drops and stabilizes at a lower OER reaction rate. Particularly, these measurements have demonstrated that the surface degradation of perovskite surfaces occurs much faster at room temperature than the bulk degradation. We suggest LOER as a reason for initially higher OER rates in degrading perovskite catalysts. The inherent relation between catalyst degradation and catalytic activity and performance regarding the OER illustrates the importance of monitoring and controlling structural changes at the electrode/liquid interface.


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Figure 1. Oxygen bubbles evolving from a 50 µm PBCO microelectrode in 0.1 M KOH. a. after 10 s b. after 20 s c. after 40 s d. after 60 s applied potential (end of measurement).


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Figure 2. Bubble shielding: current over time and simultaneous oxygen evolution. The potential is applied for 60 s.


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Figure 3. Oxygen evolution on a Pt microelectrode (here: at 2V vs. Hg/HgO).


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Figure 4. Determination of the oxygen evolution reaction rate at periods 1 and 2 at a. 1.1 V b. 1.15 V c. 1.25 V d. 1.3 V applied potential vs. Hg/HgO for PBCO thin film microelectrodes. The linear fits are performed using the first 10 data points for each period and are extrapolated over the whole data set. The extent in which the linear fits match data points of their period and the resulting transition region duration are plotted in Figure 5b.


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Figure 5. Characterizing plots as a function of potential of a. the development of the oxygen evolution reaction rate. b. period duration c. position of the transition region and d. duration/width of the transition region.


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Figure 6. Determination of the oxygen evolution reaction rate of BCT:CB 2 at a. 1.0 V b. 1.3 V c. Overall reaction rate for BCT:CB 2 and 0.2 PBCO at potentials of 1 – 1.3 V vs. Hg/HgO in 0.1 M KOH at RT. BCT:CB 2 exhibits a more linear behavior than 0.2 PBCO.


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Degradation Kinetics during Oxygen Electrocatalysis on Perovskite

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