Benchmarking the Performance of Thin-Film Oxide Electrocatalysts for

Research Article pubs.acs.org/acscatalysis

Benchmarking the Performance of Thin-Film Oxide Electrocatalysts for Gas Evolution Reactions at High Current Densities Alberto Ganassin,† Artjom Maljusch,† Viktor Colic,‡ Lukas Spanier,‡ Kurt Brandl,‡ Wolfgang Schuhmann,*,† and Aliaksandr Bandarenka*,†,‡,§ †

Analytical Chemistry - Center for Electrochemical Sciences (CES), Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany ‡ Energy Conversion and Storage − ECS, Physik-Department, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany § Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany

ABSTRACT: Oxide materials are among the state-of-the-art heterogeneous electrocatalysts for many important large-scale industrial processes, including O2 and Cl2 evolution reactions. However, benchmarking their performance is challenging in many cases, especially at high current densities, which are relevant for industrial applications. Serious complications arise particularly due to (i) the formation of a nonconducting gas phase which blocks the surface during the reactions, (ii) problems in determination of the real electroactive electrode area, and (iii) the large influence of surface morphology alterations (stability issues) under reaction conditions, among others. In this work, an approach overcoming many of these challenges is presented, with a focus on electrochemically formed thin-film oxide electrocatalysts. The approach is based on benefits provided by the use of microelectrodes, and it gives comprehensive information about the surface roughness, catalyst activity, and stability. The key advantages of the proposed method are the possibility of characterization of the whole microelectrode surface by means of atomic force microscopy and an accurate assessment of the specific activity (and subsequently stability) of the catalyst, even at very high current densities. Electrochemically deposited CoOx thin films have been used in this study as model catalysts. KEYWORDS: electrocatalysis, oxygen evolution reaction, microelectrodes, cobalt oxide, electrolysis

1. INTRODUCTION Efficient electrocatalysis for future sustainable provision of renewable energy and large-scale industrial processes is dependent on the ability to develop substantially improved functional electrocatalytic surfaces capable of generating gaseous products. For instance, water splitting, which is central for the hydrogen economy, results in the formation of H2 and O2, which are both forming a nonconducting phase at the electrode surface. However, the formation of a nonconducting gas-phase does not only make the design and operation of electrochemical devices challenging, it further complicates catalyst activity assessment, thus impeding understanding of material properties.1−4 Oxide materials represent a class of electrocatalysts vital for numerous applications related to the catalysis of gas-evolving reactions.5−7 However, a number of serious difficulties arise © 2016 American Chemical Society

when trying to investigate their performance under reaction conditions. First of all, accurate quantification of the specific activity is not straightforwardly feasible.8−10 Methodologically, it is difficult to immobilize oxide materials at the surface so that the geometric area of common electrodes is reasonably close to the real electroactive surface. Interestingly, common methods used in classical heterogeneous catalysis, for example, the Brunauer−Emmett−Teller (BET) method, are often not reliable in this case because there is unfortunately no direct correlation between the BET-determined surface area and the electrochemically active surface area.11 Furthermore, errors are involved in determining the values for the real area by the BET method caused by blocking of pores by adsorbed H2O and Received: February 14, 2016 Published: March 18, 2016 3017

DOI: 10.1021/acscatal.6b00455 ACS Catal. 2016, 6, 3017−3024

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Figure 1. Schematic representation of the rotating microelectrode setup. (A) Adaptor, (B) microelectrode assembly with (D) the microelectrode, (C) the complete setup (without the cell), and (E) a picture of the workstation part (circled in C).

OH−.12 Their removal, even under vacuum conditions, requires temperatures high enough to provoke sintering of oxides.13 Alternatives (e.g., electrochemical impedance spectroscopy) are often difficult to apply primarily due to the poorly understood double layer frequency dispersion phenomenon, where multiple hardly predictable parameters can lead to unacceptable errors in the determination of the surface area based on the “interfacial capacitance”.14−18 Another distinctive feature of gas-evolving electrodes is that the active sites at the surface are periodically or stochastically blocked by gas bubbles, complicating the assessment of intrinsic catalytic properties. The normally used “macro” (i.e., from several millimeters to several centimeters in diameter) rotating disc electrodes give unacceptably large noise due to the gas evolution process even at relatively small current densities of ∼20 mA cm−2 and even using high rotation velocities.19−22 Therefore, the comparison of oxide electrocatalysts is often performed at a current density of 10 mA cm−2,23−26 which is more than 1 order of magnitude less than those used in industry. Consequently, catalysts’ activity beyond this current density limit is rarely addressed in research laboratories. In the case of a partially covered and nonplanar geometry, misinterpretation of data resulting from hydrodynamic experiments performed via rotating disc electrodes (RDE) is also a common source of errors.27 Additionally, the above-mentioned current densities are too small to be seriously considered in real large-scale applications, which require at least 1 order of magnitude higher values.28,29 In this work, we propose an approach which minimizes the influence of the most serious issues complicating the performance assessment of thin-film oxide electrocatalysts for the gas evolution reactions. The proposed approach consists of several steps. The first step involves the electrochemical deposition of the thinnest possible oxide film which demonstrates the highest catalytic activity without displaying any noticeable influence of the substrate. Roughness and porosity of the catalyst layer are therefore minimized. The film is deposited on microelectrodes with a typical diameter of ca. 25 μm, which is opening the possibility to visualize the complete electrode surface by means of atomic force microscopy (AFM) allowing for a reasonable (it is assumed that the dimension of a catalytic center is not smaller than ∼0.3 nm) assessment of the roughness and for

further comparison of the surface morphology after stability measurements. The following steps involve activity/stability measurements using a rotating microelectrode (RME) setup, which minimizes the influence of the formed gaseous product. The combination of the small surface area and the hydrodynamic method performed with RME allow reaching high current densities relevant for industrial applications that are normally not achievable by using the standard RDEs. This approach allows further reduction of the surface blockage by gas bubbles and diminishes mass transport limitations. One of the state-of-the-art industrially relevant oxygen evolution reaction electrocatalysts, namely, CoOx film, is used as a model object.

2. EXPERIMENTAL SECTION Polycrystalline Au-electrodes (3 mm in diameter, CH Instruments), polycrystalline Pt-electrodes polished down to 30 nm roughness (5 mm in diameter, Mateck), glassy carbon (GC) electrodes (5 mm in diameter, Pine Instruments), and Pt microelectrodes were used. The procedures for the preparation of microelectrodes (diameter 25 μm) have been described in detail in ref 30. The potential was controlled using a VSP-300 potentiostat (Bio-Logic) in all experiments. All potentials in this work are referred to the RHE scale. A Hg/HgSO4 electrode (SE Analytics or Schott) and a Pt wire (Goodfellow) were used as reference and counter electrodes, respectively. Prior to the CoOx film deposition, Au-electrodes were cycled in the potential range from 0.25 to 1.85 V vs RHE in Arsaturated (Ar 5.0, Air Liquide) 0.05 M H2SO4 (Merck, Suprapur) electrolytes. Pt microelectrodes, Pt “macro” electrodes, and GC-electrodes were cycled in 0.5 M H2SO4 from 0.05 to 1.4 V vs RHE until reproducible voltammograms were obtained. The working electrolytes were prepared from KOH (≥99% K+, Merck or 85% KOH Grüssing) by dilution with ultrapure water from Evoqua Ultra Clear 10 TWF 30 UV (Evoqua) or Siemens ultrapure (Siemens) water purification systems. Cobalt oxide films were deposited from a solution prepared using Na2SO4·10 H2O (≥99%, AppliChem, and ≥99% SigmaAldrich), CH3COONa (≥99%, J.T. Baker, and ≥99% SigmaAldrich), and CoSO4·7 H2O (≥99%, Sigma-Aldrich) via cyclic 3018

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ACS Catalysis voltammetry by cycling the potential between 1.05 to 1.55 V vs RHE. The OER activity of CoOx films deposited on “macro” electrodes was assessed (without iR-correction) at 1.83 V vs RHE by determining the activity as a function of the charge associated with the redox transformations in the deposited film in 0.1 M KOH solution at 50 mV/s sweep rate. The OER activities of the CoOx films deposited on the microelectrodes were measured in oxygen-saturated (O2, 5.0, Air Liquide) 0.1 M KOH solutions at rotation speeds of 0, 1000, and 3000 r.p.m. and at a potential scan rate of 1 mV/s. Electrochemical deposition experiments were performed in a conventional two-compartment cell with a three-electrode setup and a single-compartment three-electrode cell. Activity measurements were performed in a special two-compartment, three-electrode cell in order to permit the use of the RME (Figure 1). AFM experiments were performed in the tapping mode using a “Nanowizard 3” (JPK Instruments) and a multimode EC-STM/AFM instrument (Veeco) with a Nanoscope IIID controller.

Figure 2. Electrodeposition of CoOx on a Au-electrode in subsequently performed cyclic voltammograms (scan rate 20 mV/s). Different colors are employed to visualize the variation in anodic and cathodic current related to the oxidation/reduction of Co(II)/Co(III).

3. RESULTS AND DISCUSSION 3.1. CoOx Film Formation. CoOx thin films were chosen as model catalysts as they are known to be active and relatively stable electrocatalysts for the OER.31,32 Their availability, low cost, relatively good thermodynamic stability, and low electrical resistance are the key advantages.33 In order to maximize the homogeneity of the deposited film and reduce its surface roughness, a film having the minimum thickness exhibiting bulk-like catalytic properties of the oxide was deposited. It should be, however, taken into account that bubble nucleation and bubble departure are strongly correlated to the film morphology34 which, in turn, also affects the electrocatalytic properties of the oxide. CoOx electrodeposition occurs according to the following reaction sequence:35 Co2 + + (n + 2)H 2O → CoOOH·nH 2O + 3H+ + e− (1)

2Co2 + + (n + 3)H 2O → Co2O3·nH 2O + 6H+ + 2e− (2)

The resulting Co-deposit is nonstoichiometric. Therefore, we refer to the deposited cobalt oxide film as CoOx. The solution used for deposition contained 0.1 M CoSO4, 0.1 M CH3COONa, and 0.1 M Na2SO4 (as described in ref 36), and in order to have a more precise control over the CoOx layer thickness, electrodeposition via cyclic voltammetry was used. Figure 2 shows a pair of anodic/cathodic waves growing from cycle to cycle, thus revealing the CoOx film formation. By integrating the anodic parts of the voltammograms, the voltammetric charge can be derived. The voltammetric charge due to the redox transformations in the deposited films gives a rough estimation of their nominal thickness. As shown in Figure 3A for the case of “macro” Au-, Pt-, and GC-electrodes, the amount of CoOx deposited on the electrode surface and the nature of the electrode material influence the overall OER activity of the obtained catalysts deposits. After a rapid increase for small deposited amounts of CoOx, the activity reaches a plateau at ∼15 mC cm−2 and does not change considerably after that charge is reached. At deposition charges of > ∼ 15 mC cm−2 of CoOx, the support material basically does not

Figure 3. (A) Dependence of the OER activity (measured in 0.1 M KOH) of CoOx thin films deposited on Au, GC, and Pt electrodes on the amount of the deposited catalyst (derived from the voltammetric charge transferred during the electrodeposition process in the redox transformations of the films). Activities are obtained at 1.83 V vs RHE and are not corrected for the iR drop to avoid additional sources of possible errors in determination of the minimal film thickness exhibiting the bulk-like catalytic activities. Each dot represents a unique activity measurement for a freshly deposited film. Lines in (A) are given as a guide for eyes. (B) AFM image representing the morphology of the films (deposited on Pt) at the initial stage of the film deposition (∼1 mC cm−2). (C) AFM image of the deposited film at the stage when the nature of the underlying electrode surface does not influence the overall activity (∼15 mC cm−2).

influence the catalytic activity. Hence, we suppose that in this case, the catalytic activity depends solely on the properties of the CoOx film. This is an important issue as Bell et al.37 have recently shown how the turnover frequency for the OER depends on the substrate for very thin Co-oxide catalyst layers and how it decreases to that of bulk CoOx as the thickness of 3019

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Figure 4. AFM characterization of a deposited CoOx layer: (A) 2D image of the modified microelectrode surface. (B) A photographic image illustrating the ratio between the inert glass housing and the Pt surface of a 25 μm diameter Pt microelectrode. (C) A typical AFM line profile through the center of the CoOx modified microelectrode. The axes are plotted in two different scales (μm on the x-axis and nm on the y-axis) to stress the height difference in the line profile, so that the observed “concavity” corresponds to only ∼2% changes as compared to the whole electrode surface.

problems with the current density, for example, being effectively uniform across the entire surface. As the observed “concavity” in Figure 4C corresponds to ∼2% changes as compared with the whole electrode surface, one-dimensional diffusion models are fully applicable in this case. 3.3. Activity Measurements and Stability Assessment. The electrocatalytic properties of the CoOx electrocatalyst were evaluated using the CoOx modified microelectrodes under different experimental conditions in the RME setup. The overpotential for the OER was determined by the activation overpotential combined with the diffusional one caused by mass transport limitations. Therefore, the measured current depends on both intrinsic kinetic properties and mass transport (eq 3):

the cobalt oxide deposit increases. Figures 3B,C show AFM images of the deposited films in the case of a very small nominal thickness (corresponding to the situation when the influence of the underlying electrode material cannot be neglected) and relatively thick films (corresponding to the situation with supposedly no influence from the underlying electrode material). The voltammetric charges corresponding to the minimum thickness exhibiting bulk-like catalytic properties were then chosen for the catalyst film deposition on the microelectrodes. 3.2. Microscopic Characterization of CoOx Modified Microelectrodes. The morphology of the CoOx films deposited on Pt microelectrodes has been characterized by means of AFM. The small diameter of the microelectrodes (25 μm) enables the visualization of the whole electrode surface, as well as estimation of the “real” roughness (Figure 4A,C). The precise evaluation of the morphology permits, in turn, a very rigorous assessment of the specific activity for the OER by normalizing the activity to the surface area, which is very close to the real one. It should be noted that the state of the electrode surface is important because bubbles are formed preferentially at certain nucleation sites. The activity of the catalyst is additionally determined by the amount and properties of these surface sites. Consequently, with the complete characterization of the surface it is in principle possible to link the observed activity changes to the morphology of the layer. Note that line profiles similar to those shown in Figure 4C can easily be used to calculate the real (electrochemically active) surface area of microelectrodes with high accuracy and to ensure that the deposited electrocatalyst layer is quasi-uniform. Additionally, it should be emphasized that in the case of a micro-RDE (RME), the ratio between the inert housing diameter and the diameter of the electroactive CoOx surface is much higher as compared with conventional macroscopic RDEs (Figure 4B), which further reduces unwanted possible

1/J = 1/Jk + 1/Jd

(3)

where J is the measured current, Jk is the kinetic current, and Jd is the diffusion current. The precise evaluation of the impact of the mass transport on the overall kinetics can be performed by using two well-defined models of mass transport. The first is the “linear diffusion” model applicable for common RDE measurements. The second is based on hemispherical diffusion obtained from microelectrode experiments. For the OER, for reasons discussed above, none of these regimes can be achieved; therefore, the estimation of the intrinsic catalytic properties of the catalysts is challenging.38,39 The electrocatalytic activity in case of a “macro”-electrode is determined by the most active spots on the surface,40 that is, where the morphology of the surface allows a faster nucleation and release of gas bubbles. However, in the assessment of the electrocatalytic properties of the catalyst, the activity is averaged over the whole surface, including areas on which bubble departure is significantly hindered due to morphology (i.e., the surface is almost permanently blocked by the insulating gas phase). By performing the proposed experiments using a RME configuration the linear and hemispherical diffusion are combined, thus greatly enhancing the mass transport. 3020

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Figure 5. (A) Examples of RME-voltammograms for CoOx films in O2-saturated 0.1 M KOH solution with and without microelectrode rotations (potential scan rate: 1 mV/s). (B) A typical impedance spectrum (open circles) of a Pt microelectrode (∼25 μm in diameter) in 0.1 M KOH electrolyte together with the fitting according to ref 19 (solid line).

Consequently, enhanced diffusion of the gaseous products significantly reduces the blockage of the surface, enabling a more precise assessment of the intrinsic activity of the catalyst as well as the effect of the rotation rate on the linear sweep voltammogram (LSV) (Figure 5). Additionally, the use of microelectrodes allows almost perfect iR-drop free measurements as the measured electrode potential is close to the real one with maximum 1−2 mV difference (Figure 5B). If we compare the voltammogram taken at 3000 rpm (Figure 5) with the one obtained at 1000 rpm or under “static” conditions, it can be seen that the hydrodynamic conditions indeed remove O2-related effects likely originating from the formation of the product and possible changes in the local pH value due to the high current densities. Hence, a more accurate characterization of the catalyst film properties at industrially relevant current densities (i.e., ≥ ∼ 0.2 A cm−2, ref 41) becomes possible. The absence of significant mass-transfer limitations on the current allows the observation of quasi-Tafel relationships at higher current densities. This, in turn, permits a more comprehensive analysis of the electrode kinetics. At a rotation speed of 0 rpm, a noticeable negative deviation from linearity appears before industrially relevant current densities are reached, whereas under rotation, the deviation from linearity is observed only at larger overpotentials. All polarization curves show deviations from a “Tafel line” at high current densities (see Figure 6). This can be attributed to uncompensated Ohmic resistance, but it can also reveal a change in the Tafel slope itself. With the increase in current density, the Ohmic resistance increases due to the presence of gas bubbles in the electrolyte and the reduction of the effective surface area upon blockage of the surface by bubbles. Both effects converge to an increased deviation from the actual Tafel line with increasing overpotential. Thus, it is necessary to perform an appropriate correction for the obtained polarization curves in order to extract meaningful data for the Tafel plot analysis. With the aim of distinguishing between Ohmic effects and changes in the Tafel slope, the experimental data can be treated by assuming that the electrode potential E at each current value is approximately given by the equation: E = a − b ln I + RI

Figure 6. Logarithm of the current density j as a function of overpotential η for the OER calculated from LSV at 1 mV/s and at a rotation speed of 3000 rpm. Orange, magenta, blue, and green squares represent the relevant current densities at 0.4 A cm−2, 0.3 A cm−2, 0.2 A cm−2, 0.1 A cm−2, respectively. The linear Tafel fit of the experimental values is represented by a red line.

where a and b are the Tafel constant and slope, respectively, R is the uncompensated resistance, and I is the measured current. However, taking into account the resulting values of R estimated using electrochemical impedance spectroscopy and the measured current densities, the correction in the electrode potential is close to 1−2 mV in the case of a microelectrode (see Figure 5B). From the intercept a, the exchange current density and hence the standard heterogeneous rate constant k0 can be calculated. However, due to the irreversibility of OER, it has earlier been concluded that the principle of reversibility does not apply in this case.42 As a consequence, the steady-state kinetic data on the OER can only yield mechanistic information concerning the rate-determining step (RDS) described by the Tafel slope (b-coefficient). For OER, it has generally a value between 30 and 120 mV dec−1,43 and it indicates whether the RDS is a first or a second electron transfer step and if a chemical reaction is involved. In eq 4, R is assumed to be a constant independent from the current. The implication of such an assumption is that usually the uncompensated resistance is underestimated for gas evolution reactions. Consequently, the calculated R deviates

(4) 3021

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results of galvanostatic experiments, which were then performed at 3000 r.p.m. using CoOx thin films on Pt microelectrodes and on an unmodified Pt microelectrode. The galvanostatic experiments were performed at industrially relevant current densities. The difference in activity between CoOx and PtOx is ∼0.7 V at the same current density and rotation speed. CoOx exhibits an overpotential of ∼0.66 V at a current density 0.4 A cm−2 and of ∼0.51 V at a current density of 0.17 A cm−2, respectively. Separate galvanostatic experiments at 0.17 A cm−2 and 0.4 A cm−2 (Figure 8A) were also performed. At these current densities, the CoOx catalyst layer was completely degraded after about 8−9 min, and the overpotential increased until reaching the value close to that for an unmodified Pt microelectrode. After the complete degradation of the CoOx film, galvanostatic experiments were performed on the Pt microelectrode at the same rotation rates to visualize the difference in overpotential for OER at the same current density for the two different materials. The overpotential for the Pt electrode increases with time due to the formation of platinum oxide on the surface.44 3.4. Degradation Assessment. Following the galvanostatic experiments, morphological characterization was performed using AFM. The galvanostatic measurements at high current densities destroyed the catalyst layer (Figure 8). AFM characterization enables an alternative method for the quantification of the corrosion rate of the oxide electrocatalyst as compared with techniques which are used nowadays such as inductively coupled plasma mass spectrometry,45−48 quartz crystal microbalance,49,50 and rotating ring disk electrodes.51,52 Profilometry of the microelectrode (Figure 8D) visualizes the depletion of the catalyst after the galvanostatic experiments. The thickness of the corroded catalyst after voltammetric and

significantly from the correct value and leads to an incorrect estimation of the reaction kinetics. In fact, a continuous increase in the Tafel slope with applied potential may simply be the result of a reduction in the effective electrode surface area with increasing gas evolution speed. Therefore, caution should be taken when interpreting multiple Tafel regions in terms of a possible mechanistic pathway. After the fitting of the quasi-linear part of the voltammogram, the Tafel slope was estimated to be ∼84.4 mV dec−1 for a rotation rate of 3000 r.p.m. (Figure 6). Figure 7 shows the

Figure 7. Galvanostatic measurements in O2-saturated 0.1 KOH at 3000 r.p.m. Measurements were performed on CoOx-microelectrodes at 0.17 A cm−2 (red dots), at 0.4 A cm−2 (black dots), and at an unmodified Pt microelectrode at 0.4 A cm−2 (blue triangles) for 5 min.

Figure 8. (A) Dependence of the electrode potential on time during the galvanostatic polarization at an effective current density of 0.17 A cm−2 at 3000 r.p.m. and (B) 0.4 A cm−2 at 2000 r.p.m. using CoOx thin film deposited on Pt-microelectrodes. Note that the electrode potential suddenly increases when the CoOx thin-film electrocatalyst is completely removed from the surface due to degradation. (C) AFM characterization of an initially CoOx-modified Pt-microelectrode after electrochemical activity and stability measurements. (D) Profilometry of the microelectrode when the catalyst (and probably also some of the Pt) has been removed due to high anodic current densities. The axes are plotted in two different scales (μm on the x-axis and nm on the y-axis) to stress the height difference in the line profile. The sudden increases in potential after ∼160 s, ∼ 350 s in panel B are due to the partial blockage of the surface by gas bubbles. Properties of different electrodes are shown in Figure 7 and panels A,B to demonstrate reproducibility and different situations causing final degradation of the catalyst. 3022

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Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft. V.C., L.S., K.B, A.B. acknowledge financial support by the GDCh (Gesellschaft Deutscher Chemiker), the Deutsche Forschungsgemeinschaft as well as the Cluster of Excellence Nanosystems Initiative Munich (NIM).

galvanostatic experiments is 200 nm, as derived from the AFM images, the corrosion rate at industrial current densities can be calculated to be up to ∼10 mm per year. It can be noted that higher pH values of solutions used in industry might reduce the local pH-variations at the surface during OER, thus possibly reducing the degradation rate of the material. Investigation of many aspects of catalyst aging, however, remains controversial. The degradation of a catalyst is influenced by many parameters (cell potential, potential dynamics, temperature, pH value, impurities, etc.)53 which impede a standardized investigation protocol. In the case of high-surface-area nanoparticles, several degradation mechanisms are involved, such as particle detachment from the conductive support, particle agglomeration, and dissolution, or Ostwald ripening. Moreover, the estimation of parameters describing a nanoparticle-based catalyst (catalyst loading, particle size, interparticle distance) is challenging. Bulk or thin-film catalysts allow for morphological characterization of the electrocatalyst surface, thus avoiding the aforementioned problems for nanoparticles. However, when bulk or thin-film catalysts are employed, the current densities normally used for stability quantification are 1 order of magnitude lower than industrially used current densities.45,48,50 Nevertheless, the proposed benchmarking method prevents challenges of high-surface-area catalysts and at the same time enables quantification of the corrosion rate of the catalyst at industrial current densities.

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4. CONCLUSIONS A relatively simple approach based on benefits provided by the use of microelectrodes is proposed to accurately evaluate the performance of oxide electrocatalysts at high current densities. The approach involves electrochemical deposition of the thinnest possible oxide films, which demonstrate the highest bulk-like catalytic activity, without displaying any influence of the underlying electrode material. The films are deposited on microelectrodes that simultaneously solve two issues. It is possible to visualize the whole electrode surface with AFM for a detailed assessment of the surface roughness and to minimize the influence of the formed nonconducting gaseous products. In addition, the combination of the small surface area and hydrodynamic conditions caused by the high rotation speed allows reaching high current densities at values that are relevant for industrial applications, which are normally not achievable by using the standard state-of-the-art protocols. In summary, the proposed relatively simple methodology permits an exhaustive evaluation of the catalyst activity and stability at virtually any industrially relevant current densities.

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REFERENCES

(1) Zeradjanin, A. R.; Ventosa, E.; Bondarenko, A. S.; Schuhmann, W. ChemSusChem 2012, 5, 1905−1911. (2) Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. ChemCatChem 2010, 2, 724−761. (3) Vayenas, C. G. J. Solid State Electrochem. 2011, 15, 1425−1435. (4) Zeradjanin, R.; Topalov, A. A.; Van Overmeere, Q.; Cherevko, S.; Chen, X.; Ventosa, E.; Schuhmann, W.; Mayrhofer, K. J. J. RSC Adv. 2014, 4, 9579−9587. (5) Minh, N. Q. J. Am. Ceram. Soc. 1993, 76, 563−588. (6) Manthiram, A.; Vadivel Murugan, A.; Sarkar, A.; Muraliganth, T. Energy Environ. Sci. 2008, 1, 621−638. (7) Hamdani, M.; Singh, R. N.; Chartier, P. Int. J. Electrochem. Sci. 2010, 5, 556−577. (8) Strmcnik, D.; Hodnik, N.; Hocevar, S. B.; Van der Vliet, D.; Zorko, M.; Stamenkovic, V. R.; Pihlar, B.; Markovic, N. M. J. Phys. Chem. C 2010, 114, 2640−2644. (9) Masa, J.; Batchelor-McAuley, C.; Schuhmann, W.; Compton, R. G. Nano Res. 2014, 7, 71−78. (10) Lyons, M. E. G.; Brandon, M. P. J. Electroanal. Chem. 2010, 641, 119−130. (11) Trasatti, S.; Petrii, O. A. J. Electroanal. Chem. 1992, 327, 353− 376. (12) Burke, L. D.; Murphy, O. J. J. Electroanal. Chem. Interfacial Electrochem. 1980, 112, 39−50. (13) Burke, L. D.; Murphy, O. J. J. Electroanal. Chem. Interfacial Electrochem. 1979, 96, 19−27. (14) Lasia, A. Electrochemical Impedance Spectroscopy and its Applications; Springer-Verlag: New York, 2014; p 367. (15) Ehrlich, S. G.; Moran, P. J. Corrosion 1989, 45, 689−696. (16) Roy, S. K.; Orazem, M. E. J. Electrochem. Soc. 2009, 156, B203− B209. (17) Kerner, Z.; Pajkossy, T. Electrochim. Acta 2000, 46, 207−211. (18) Pajkossy, T. Solid State Ionics 2005, 176, 1997−2003. (19) Colic, V.; Tymoczko, J.; Maljusch, A.; Ganassin, A.; Schuhmann, W.; Bandarenka, A. S. ChemElectroChem 2015, 2, 143−149. (20) Rincon, R. A.; Battistel, A.; Ventosa, E.; Chen, X.; Nebel, M.; Schuhmann, W. ChemSusChem 2015, 8, 560−566. (21) Eigeldinger, J.; Vogt, H. Electrochim. Acta 2000, 45, 4449−4456. (22) Janssen, L. J. J.; Barendrecht, E. Electrochim. Acta 1979, 24 (6), 693−699. (23) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2013, 135, 16977−16987. (24) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2015, 137, 4347−4357. (25) Matsumoto, Y.; Sato, E. Mater. Chem. Phys. 1986, 14, 397−426. (26) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110, 6446−6473. (27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 2001. (28) Manabe, A.; Kashiwase, M.; Hashimoto, T.; Hayashida, T.; Kato, A.; Hirao, K.; Shimomura, K.; Nagashima, I. Electrochim. Acta 2013, 100, 249−256. (29) Holladay, J. D.; Hu, J.; King, D. L.; Wang, Y. Catal. Today 2009, 139, 244−260. (30) Katemann, B. B.; Schuhmann, W. Electroanalysis 2002, 14, 22− 28. (31) Schultze, J. W.; Trasatti, S. Electrodes of Conductive Metallic Oxides, Part A; Elsevier Scientific Publishing Company: Amsterdam /New York, 1980.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +49 (0) 234 3226200. *E-mail: [email protected]. Tel.: +49 (0) 89 289 12531. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to Dr. Volodymyr Kuznetsov (RUB), Martin Trautmann (RUB), Yunchang Liang (TUM), and Jonas Pfisterer (TUM) for AFM measurements. A.G., A.M, W.S., A.B. acknowledge financial support from the Cluster of 3023

DOI: 10.1021/acscatal.6b00455 ACS Catal. 2016, 6, 3017−3024

Research Article

ACS Catalysis (32) Schultze, J. W.; Trasatti, S. Electrodes of Conductive Metallic Oxides, Part B; Elsevier Scientific Publishing Company: Amsterdam/ New York, 1981. (33) Kinoshita, K. Electrochemical Oxygen Technology; WileyInterscience: New York, 1992. (34) Zeradjanin, A. R.; Menzel, N.; Schuhmann, W.; Strasser, P. Phys. Chem. Chem. Phys. 2014, 16, 13741−13747. (35) Matsumoto, Y.; Bando, N.; Hombo, J.; Sasaki, T. J. Electroanal. Chem. 1995, 395, 45−49. (36) Tench, D.; Warren, L. F. J. Electrochem. Soc. 1983, 130 (4), 869−872. (37) Yeo, B. S.; Bell, A. T. J. Am. Chem. Soc. 2011, 133, 5587−5593. (38) Vogt, H.; Balzer, R. J. Electrochim. Acta 2005, 50, 2073−2079. (39) Eigeldinger, J.; Vogt, H. Electrochim. Acta 2000, 45, 4449−4456. (40) Chen, X.; Maljusch, A.; Rincon, R. A.; Battistel, A.; Bandarenka, A. S.; Schuhmann, W. Chem. Commun. 2014, 50, 13250−13253. (41) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. Int. J. Hydrogen Energy 2013, 38, 4901−4934. (42) Hoare, J. P. The Electrochemistry of Oxygen; Wiley-Interscience: New York, 1968. (43) Damjanovic, A.; Dey, A.; Bockris, J. O’M. Electrochim. Acta 1966, 11, 791−814. (44) Iwakura, C.; Fukuda, K.; Tamura, H. Electrochim. Acta 1976, 21, 501−508. (45) Topalov, A. A.; Katsounaros, I.; Auinger, M.; Cherevko, S.; Meier, J. C.; Klemm, S. O.; Mayrhofer, K. J. J. Angew. Chem., Int. Ed. 2012, 51, 12613−12615. (46) Cherevko, S.; Topalov, A.; Zeradjanin, A. R.; Katsounaros, I.; Mayrhofer, K. J. J. RSC Adv. 2013, 3, 16516−16527. (47) Xing, L.; Hossain, M. A.; Tian, M.; Beauchemin, D.; Adjemian, K. T.; Jerkiewicz, G. Electrocatalysis 2014, 5, 96−112. (48) Reier, T.; Oezaslan, M.; Strasser, P. ACS Catal. 2012, 2, 1765− 1772. (49) Wickman, B.; Grçnbeck, H.; Hanarp, P.; Kasemo, B. J. Electrochem. Soc. 2010, 157, B592−B598. (50) Frydendal, R.; Paoli, E. A.; Knudsen, B. P.; Wickman, B.; Malacrida, P.; Stephens, I. E. L.; Chorkendorff, I. ChemElectroChem 2014, 1, 2075−2081. (51) Vukovic, M. J. Chem. Soc., Faraday Trans. 1990, 86, 3743−3746. (52) Danilovic, N.; Subbaraman, R.; Chang, K.-C.; Chang, S. H.; Kang, Y.; Snyder, J. D.; Paulikas, A. P.; Strmcnik, D.; Kim, Y.-T.; Myers, D. J.; Stamenkovic, V. R.; Markovic, N. M. J. Phys. Chem. Lett. 2014, 5, 2474−2478. (53) Mench, M. M.; Kumbar, E. C.; Veziroglu, T. N. Polymer Electrolyte Fuel Cell Degradation; Elsevier Scientific Publishing Company: Amsterdam/New York, 2012; pp 90−91.

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DOI: 10.1021/acscatal.6b00455 ACS Catal. 2016, 6, 3017−3024