Stabilization of Pt Nanoparticles Due to Electrochemical Transistor

Article pubs.acs.org/cm

Stabilization of Pt Nanoparticles Due to Electrochemical Transistor Switching of Oxide Support Conductivity Tobias Binninger,† Rhiyaad Mohamed,‡ Alexandra Patru,† Kay Waltar,†,⊥ Eike Gericke,¶ Xenia Tuaev,§ Emiliana Fabbri,† Pieter Levecque,‡ Armin Hoell,§ and Thomas J. Schmidt*,†,∥ †

Paul Scherrer Institut, Electrochemistry Laboratory, CH-5232 Villigen PSI, Switzerland HySA/Catalysis, University of Cape Town, Rondebosch 7701, South Africa ¶ Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, D-12489 Berlin, Germany § Institut für Nanospektroskopie, Helmholtz-Zentrum für Materialien und Energie, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany ∥ Laboratory of Physical Chemistry, ETH Zürich, CH-8093 Zürich, Switzerland ‡

S Supporting Information *

ABSTRACT: Polymer electrolyte fuel cells (PEFCs) offer an efficient way of chemical-to-electrical energy conversion that could drastically reduce the environmental footprint of the mobility and stationary energy supply sectors, respectively. However, PEFCs can suffer from severe degradation during start/ stop events, when the cathode catalyst is transiently exposed to very high potentials. In an attempt to mitigate corrosion of conventional carbon support materials for Pt catalyst nanoparticles under these conditions, conductive metal oxides like antimony-doped tin oxide (ATO) are considered alternative support materials with improved corrosion resistance. A combined in situ anomalous small-angle X-ray scattering and post mortem transmission electron microscopy study reveals PEFC-relevant degradation properties of ATO-supported Pt in comparison to carbon-supported Pt catalysts. Against expectation, the superior stability of ATO-supported Pt nanoparticles cannot be merely explained by improved support corrosion resistance. Instead, the dominant loss mechanism of electrochemical Ostwald ripening is strongly suppressed on ATO support, which can be explained with a potential-dependent switching of support oxide surface conductivity. This electrochemical transistor effect represents an important design principle for the development of optimized metal oxide support materials that protect supported Pt nanoparticles at high potentials, where careful consideration of the metal oxide flatband potential is required in order to maintain high catalyst performance at normal PEFC cathode operation conditions at the same time.

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INTRODUCTION Polymer electrolyte fuel cells (PEFCs) are efficient chemical-toelectrical energy converters that produce electricity from a controlled, spatially separated reaction of a fuel, e.g. hydrogen, with oxygen. They represent an attractive alternative to Li-ion batteries for powering electric vehicles where their major advantage can be seen in the short refueling times of hydrogen1 (comparable to gasoline) in comparison with long battery charging times. Further promising applications of PEFC technology are foreseen for a reduction of environmental impact in the aviation sector2 and for domestic combined heat and power systems.3 In all cases, the critical challenges to be met by PEFC technology are cost, performance, and durability. PEFCs rely on active catalyst materials especially for the oxygen reduction reaction (ORR) at the cathode side. Standard ORR catalysts are Pt nanoparticles supported on high surface area carbon materials. However, the electrochemical conditions at PEFC cathodes are challenging for the stability of such catalyst materials. This is most severe during PEFC start and stop events, when the cathode potential can reach values © 2017 American Chemical Society

equivalent to 1.4−1.5 VRHE (vs reversible hydrogen electrode, RHE).4,5 At such high potentials, the carbon oxidation reaction6 proceeds at fast rate leading to corrosion of the carbon support for Pt nanoparticles.5,7 As a consequence of support corrosion, entire Pt nanoparticles get detached from their initial anchoring points8 and form agglomerates with other Pt particles.9 Furthermore, electrochemical dissolution of Pt occurs at potentials above 0.8 VRHE10,11 which either results in loss of Pt into the electrolyte phase or in a growth of larger Pt particles due to redeposition at lower potentials (electrochemical Ostwald ripening). The overall effect of the various degradation mechanisms is a loss of the electrochemically active Pt surface area (ECSA) which, in turn, leads to a decrease of the absolute electrocatalytic ORR activity and PEFC performance. Whereas a precise quantification of the individual contributions of different mechanisms to the overall ECSA Received: November 15, 2016 Revised: March 1, 2017 Published: March 6, 2017 2831

DOI: 10.1021/acs.chemmater.6b04851 Chem. Mater. 2017, 29, 2831−2843

Article

Chemistry of Materials

metal oxide support has a stabilizing effect on the Pt nanoparticles under transient high-potential conditions that goes beyond the expected improvement of the support corrosion resistance. These findings are explained with an electrochemical switching of the metal oxide conductivity that protects Pt nanoparticles from dissolution at high potentials. Whereas potential-dependent conductivity properties of Pt/ metal oxide catalysts have been recently proposed based on an interaction between Pt and metal oxide support,37 the switching of conductivity observed in the present work is shown to originate from the fundamental electrochemical behavior of semiconducting metal oxides alone.

degradation is difficult, it clearly depends on various catalyst properties like the Pt loading9 and the initial Pt particle size distribution.12 Also, influence of the type of carbon support on the Pt degradation mechanism has been established.13−15 In an attempt to improve the start/stop stability of Pt-based PEFC cathode catalysts, metal oxide materials have attracted research interest as alternative to conventional carbon supports for Pt nanoparticles.16−19 In spite of the variety of metal oxides that are thermodynamically stable at PEFC cathode conditions,20 the electronic conductivity requirements for electrocatalyst supports are difficult to be met by these materials. Some metal oxides, such as RuO2, possess high intrinsic conductivity, and the Pt/TiO2−RuO2 cathode catalyst was shown to enable remarkable PEFC performance and stability.21 On the other hand, electronic conductivity can be induced in metal oxides by incorporation of suitable doping elements into the host oxide structure. Sn-doped indium oxide, e.g., was used as Pt support to yield a catalyst with high ORR activity and stability properties.22 Also antimony-doped tin oxide (ATO, Sb-SnO2) provides enough conductivity to be used as support for the Pt ORR catalyst, and Pt/ATO catalysts proved good stability properties in fundamental electrochemical tests and PEFC tests that mimicked the corrosive PEFC start/stop conditions.23−26 Whereas numerous studies have focused on benchmarking ORR activity and stability of Pt/metal oxide PEFC cathode catalysts versus standard Pt/C catalysts, more fundamental investigations in order to understand the mechanisms behind improved stability properties of Pt/metal oxide catalysts appear to be scarce. For this purpose, additional characterization of the catalyst nanostructure is required. Transmission electron microscopy (TEM) has been commonly applied for the study of Pt/C degradation mechanisms.8−11,13,27 However, this technique is difficult to be applied in situ under potential control, and the analysis of individual spots can be nonrepresentative for the properties of the entire catalyst sample. Furthermore, whereas Pt nanoparticles can be easily identified on a carbon support, the distinction between catalyst particles and support materials becomes difficult on metal oxides that contain heavier elements. These drawbacks of TEM analysis can be overcome by small-angle X-ray scattering (SAXS). TEM and SAXS can be seen as complementary techniques where the former allows for an analysis of individual particles in direct space, and the latter yields nanostructure information in reciprocal space averaged over the entire volume of the catalyst sample irradiated by the X-ray beam. SAXS can be conveniently applied in situ during electrochemical characterization with the use of an appropriate cell.28 Furthermore, anomalous SAXS (ASAXS) is element sensitive and can be used at synchrotron X-ray sources in order to separate different scattering contributions from Pt nanoparticles, support material, and cell components.29−32 In situ (A)SAXS has been applied previously for the study of degradation properties of carbonsupported Pt nanoparticles.14,33−36 We report on a combined in situ ASAXS and post mortem TEM study of support-dependent degradation mechanisms of Pt nanoparticles. The stability properties of Pt nanoparticles supported on metal oxides (Sb-SnO2) under high-potential cycling are compared with the ones of carbon-supported Pt catalysts. The Pt ECSA is used as the central descriptor of the catalyst state at different stages of the degradation protocol, and the contribution of different degradation mechanisms to the observed ECSA loss is discussed. Our results show how the

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EXPERIMENTAL SECTION

Catalyst Materials. Two different Pt/ATO catalysts were synthesized. The ATO support material was a commercial rutile phase Sb0.1Sn0.9O2 nanopowder (supplied by nanograde AG, Switzerland) with an average particle size of approximately 12 nm that was produced by flame spray synthesis. Pt nanoparticles were deposited with a nominal loading of 10 wt % once on the as-received ATO powder (Pt/ATO) and once on the same ATO powder after a prereduction treatment (Pt/ATO reduced) where the ATO powder was kept for 2 h at 450 °C in a 5% H2/Ar gas flow. The rationale behind the reduction treatment was a possibly enhanced anchoring of Pt nanoparticles on the reduced ATO support surface. In addition, catalytic interactions between Pt and a reduced SnO2 surface leading to improved ORR activity have been reported previously.38 Pt nanoparticle deposition was done by a modified organometallic chemical deposition method:39 The accurately weighed Pt(acac)2 (Sigma-Aldrich, 97 atom %) and ATO support powders were physically mixed before placing in a stainless steel reactor. The reactor was then placed in a tubular furnace attached to an Ar gas line. The furnace was heated to 100 °C in Ar flow over a 30 min period and held at that temperature for another 30 min. Subsequently, Ar flow was stopped, and the furnace, still kept under inert conditions in Ar atmosphere, was heated to an operating temperature of 350 °C over a 60 min period and held at that temperature for another 120 min. Finally, the reactor was removed from the furnace and left to cool to room temperature overnight. With the same method, Pt nanoparticles were deposited with a nominal loading of 20 wt % on a commercial Vulcan XC-72R (Cabot Corporation) carbon support (Pt/VC). Another carbon-supported Pt catalyst with smaller Pt particle size, cf. Results section, was prepared with a nominal loading of 20 wt % Pt on onionlike carbon spheres (Pt/OLC) using a polyol method for the Pt nanoparticle synthesis as described in ref 40. Onionlike carbons (OLC) with a specific surface area of 390 m2/g were supplied from the Leibniz Institute for New Materials, Saarbrücken, Germany, and they consist of multiple spherical graphitic carbon shells.41,42 Physical Characterization. Ex situ electronic conductivity of the ATO support was determined. For this purpose, dry ATO powder was compressed at a pressure of 625 kPa between two metal disk contacts. Linearity of the direct current−voltage curve was confirmed, and an ATO conductivity of 6.4 × 10−4 S/cm was found classifying the material as a semiconductor. ATO support materials were characterized by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with a Co Kα radiation source. XRD patterns of ATO and ATO reduced are shown in Supporting Information Figure S1. All peak positions of the asreceived ATO could be ascribed to the tetragonal rutile SnO2 lattice (PDF No. 01-070-6995). After reduction treatment, the ATO reduced showed largely the same structure. However, a small additional crystalline side phase became visible that could be associated with pure rhombohedral Sb (PDF No. 00-035-0732). Rietveld refinement revealed that approximately 24% of the overall Sb content segregated into this phase with a large Sb crystallite size of approximately 170 nm. It should be noted that the reduction treatment of the ATO was performed with the purpose of an enhanced interaction between Pt 2832


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Table 1. Characterization Results: Pt Loading, Initial Area-Weighted Average Diameter ⟨D⟩A,0 of Pt Nanoparticles, MassSpecific ECSA (ms-ECSA), Mass-Specific Pt Surface Area (ms-A) Determined from ASAXS, ECSA-Specific ORR Current ORR (jORR ECSA), and Mass-Specific ORR Current (jms ), Both Evaluated at 0.9 VRHE catalyst nominal 10 10 20 20

wt wt wt wt

%Pt/ATO %Pt/ATO red. %Pt/VC %Pt/OLC

Pt loading experiment 9.1 10.7 19.5 16.0

wt wt wt wt

% % % %

⟨D⟩A,0 (nm)

ms-ECSA (m2Pt/gPt)

ms-A (m2Pt/gPt)

2 jORR ECSA (μA/cmPt)

jORR ms (A/gPt)

3.1 2.4 4.0 2.0

65.6 100.9 59.6 81.8

90.0 117.9 67.9 112.4

100.1 67.9 235.3 169.4

65.2 68.6 142.8 138.8

yield an ATO loading of 1.6 mgATO/cm2. EIS was performed in a N2purged electrolyte at different electrode potentials in the range 0−1 VRHE with a modulation potential amplitude of 25 mV. At each potential, the electrode was kept for 1 min prior to EIS measurements. For Mott−Schottky analysis, the potential-dependent capacitance was extracted from the imaginary part of the impedance at a frequency of 500 Hz. In order to take into account the shunt capacitance of the glassy carbon substrate, the latter was first determined in a separate EIS measurement with the clean glassy carbon electrode and then subtracted from the capacitance measured for the porous ATO electrode. Anomalous SAXS Experiments. In situ ASAXS experiments were performed at the 7T MPW SAXS beamline at BESSY II synchrotron at Helmholtz-Zentrum Berlin, Germany. SAXS patterns were recorded with a CCD detector (MAR165), which was placed at the end of an evacuated scattering flight tube with flexible length based on an edge welded bellow system.44 The two-dimensional raw data pictures were corrected for dark current of the CCD itself as well as for the incoming photon-flux, sample transmission, and geometrical effects like the projection of the detector plane on the sphere with radius equal to the sample−detector distance. The scattering curves were transformed to differential cross sections per area with the help of a glassy carbon standard. ASAXS analysis was used in order to extract the Pt scattering contribution from the overall scattering signal containing background scattering from the support material and cell components (Kapton windows). For this purpose, SAXS curves were recorded at two different X-ray energies (E1 ≈ 11550 eV, E2 ≈ 11400 eV) below the Pt L3-absorption edge (EPt,L3 = 11564 eV). Subtraction of the differential SAXS cross sections at E1 and E2 yielded the net Pt scattering curve after normalization with the scattering contrast (|f Pt(E 1)|2−| f Pt(E2)|2).30 The complex-valued atomic scattering factors f Pt(E) were taken according to Cromer-Liberman calculations from the Hephaestus X-ray spectroscopy analysis software45 after correction for chemical shifts of the Pt L3-absorption edge. However, the net Pt scattering curve extracted by this procedure not only consists of the Pt partial structure factor (PSF), but it yields a sum of Pt PSF and a Pt particle−support interference term.32 Although this interference contribution is weak for weakly scattering carbon support materials, it becomes dominant for metal oxide supports that contain heavier elements. For this reason, the resulting net Pt scattering curves were fitted with a standard model of spherical Pt particles which was extended by a model for the scattering interference between spherical Pt particles and spherical support particles introduced previously.32 The corresponding fitting function Spp(q) + αSps(q) with the Pt partial structure factor (PSF) Spp(q), the Pt−support interference PSF Sps(q), and the scattering ratio α is explained in detail in ref 32. Commonly used log-normal Pt particle size distributions of the form

and the reduced ATO surface. Indirect evidence for the successful reduction of the ATO surface can be seen in the drastically smaller Pt particle size achieved on the ATO reduced, as discussed below. XRD analysis of the OLC support material was performed using a PANalytical Empyrean diffractometer with a Cu Kα radiation source. The XRD pattern of OLC is also shown in Supporting Information Figure S1, where the Cu Kα 2θ scale of the OLC measurement was converted to the Co Kα 2θ scale. Two different phases were observed: one major peak resulting from the onionlike graphitic layer structure (graphite PDF No. 00-001-0640) which is clearly visible in the TEM image in Supporting Information Figure S2 and two additional peaks that could be associated with the residual diamond structure (PDF No. 00-006-0675) of the precursor diamonds used for OLC synthesis. Pt loadings of the ATO-supported and of the carbon-supported catalysts were confirmed experimentally by inductively coupled plasma optical emission spectrometry (ICP-OES) and thermogravimetric analysis (TGA), respectively. For ICP-OES analysis, catalyst samples were digested in concentrated 3:1:1 hydrochloric acid (HCl), hydrofluoric acid (HF), and nitric acid (HNO3) at 180 °C using a MARS-5 microwave system. The subsequent ICP-OES analysis was done using a Varian ICP 730-ES Spectrophotometer. TGA was carried out with a heating rate of 5 °C/min up to a temperature of 800 °C in air using a Mettler Toldedo TGA/SDTA 851e instrument. Experimentally determined Pt loadings are summarized in Table 1. The three catalysts synthesized by organometallic chemical deposition revealed Pt loadings in good agreement with the targeted nominal loadings. The Pt/OLC catalyst showed slightly lower Pt loading compared to the nominal value indicating minor loss of Pt during the polyol Pt deposition procedure. Electrochemical Characterization. Fundamental electrochemical characterization was performed in a standard three-electrode glass cell in 0.1 M HClO4 electrolyte using a Biologic VSP 300 potentiostat with a Au counter electrode and a Hg/Hg2SO4 reference electrode (ALS Co., Ltd.) which was calibrated vs RHE. Porous thin-film electrodes43 of the catalyst materials were prepared on glassy carbon disks (5 mm diameter) by drop-coating an ink with a composition of 5 mg of catalyst powder, 1 mL of water (Milli-Q), 4 mL of isopropyl alcohol (Sigma-Aldrich, 99.9% for HPLC), and 20 μL of Nafion (Aldrich, 5 wt % solution) to yield an electrode Pt loading of approximately 18 μgPt/ cm2. Oxygen reduction reaction (ORR) activities were determined by rotating disk electrode (RDE) experiments in O2-saturated electrolyte. The electrode was rotated at 1600 rpm, and the potential was cathodically scanned at 5 mV/s. The recorded ORR polarization curves, presented in Supporting Information Figure S3a, were first IRcorrected with an ohmic resistance of the electrolyte determined by electrochemical impedance spectroscopy and then corrected for oxygen mass transport losses using the Koutecky−Levich equation. The resulting kinetic ORR currents, shown in Supporting Information Figure S3b, were normalized with respect to the ECSA and with respect to the Pt mass to obtain the ECSA-specific kinetic currents ORR jORR ECSA and the mass-specific kinetic currents jms , respectively. Potential-dependent space charge properties of the nanopowder ATO support material were studied by electrochemical impedance spectroscopy (EIS). For this purpose, porous thin-film ATO electrodes were prepared on glassy carbon disks (5 mm diameter) by drop-coating an ink with a composition of 80 mg of ATO powder, 1 mL of water (Milli-Q), 4 mL of isopropyl alcohol (Sigma-Aldrich, 99.9% for HPLC), and 50 μL of Nafion (Aldrich, 5 wt % solution) to

2

P(D) =

(log(D /(2μ))) 1 − 2σ 2 e 2π σ(D/2)

(1)

where D denotes the spherical Pt particle diameter, resulting in a very good fit quality (cf. Supporting Information Figures S5−S8). The use of the spherical particle model is furthermore justified by the apparent spherical shape of Pt nanoparticles in high-magnification TEM images presented in Supporting Information Figures S9−S12. The numberweighted size distribution P(D) is very sensitive to the small particle 2833


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Figure 1. Cyclic voltammograms for ECSA determination at a sweep rate of 20 mV/s at the beginning, at maximum ECSA activation, and at the end of the protocol recorded during in situ ASAXS degradation experiments for Pt/VC catalyst (a), Pt/OLC catalyst (b), Pt/ATO catalyst (c), and Pt/ ATO reduced catalyst (d). [*During the in situ ASAXS degradation experiment of the Pt/ATO reduced catalyst (d), an electronic artifact developed due to a limitation of the counter electrode capacitance in combination with a limited voltage range of the potentiostat. This limitation resulted in a delay of the oxidation charge from hydrogen underpotential deposition (Hupd) in the anodic sweep direction, which explains the unusual shift of the Hupd oxidation peak to higher potentials in the CV after 1000 degradation cycles.] quantities are difficult to estimate. The size distribution P(D) itself is comparably robust against small variations of analysis parameters, so that derived quantities such as ⟨D⟩A and ms-A are associated with a relative error of approximately 10%. In contrast, the total number of Pt particles N resulting from the fitted absolute scaling constant must be seen with a large absolute systematic error precluding a direct analysis of the absolute Pt surface area A and Pt mass m. However, relative variations of A and m in the course of in situ degradation experiments are largely independent of such systematic errors, and, therefore, also are associated with an error of approximately 10%. Electrochemical Degradation Protocol. An electrochemical three-electrode flow-cell setup described in ref 28 was used for in situ ASAXS experiments. Nitrogen equilibrated 0.1 M HClO4 electrolyte (prepared from Kanto Chemical Co., Inc., 60% HClO4) was flowed through the cell at room temperature at a flow rate of 0.05 mL/min. Ag|AgCl reference electrodes (Harvard Apparatus Low-Leakage with NaCl filling solution) were used (calibrated vs RHE). Two cells were operated in parallel for efficient usage of beamtime using a BioLogic SP-300 and a Metrohm μAutolabIII/FRA2 potentiostat, respectively. Catalyst materials were preactivated with cyclic voltammetry (CV) performing 50 potential cycles between 0.03 and 1 VRHE at a sweep rate of 50 mV/s. This step was necessary since catalysts initially showed poorly defined Pt-related CV features and CV curves quickly

fraction which, in terms of numbers, can be dominating. For catalytic applications, however, the distribution of the (catalytically active) surface area is of fundamental importance. Therefore, for further analysis, the surface area-weighted size distribution

PA(D) =

πD 2 P(D) ⟨A⟩

(2)

was used, where πD2 is the surface area of a spherical particle with diameter D, and ⟨A⟩ = ∫ πD2P(D)dD is the average surface area per Pt particle according to the spherical particle model with size distribution (1). From the distribution (2), the surface area-weighted average diameter of Pt nanoparticles ⟨D⟩A = ∫ DPA(D)dD was calculated. The total number N of Pt particles per electrode area was extracted from the absolute scaling constant of the fit of the net Pt scattering curves.32 With the use of N, the total surface area of Pt nanoparticles A = N⟨A⟩ and the total mass of Pt m = Nρ∫ (π/6)D3P(D)dD, with the density of Pt ρ, were calculated (both quantities normalized per electrode area). Finally, the mass-specific Pt surface area ms-A = A/m was determined. In fact, ms-A is independent of N, because both A and m are proportional to N, so that N cancels in the fraction. Due to the complex process of ASAXS analysis and data fitting with an influence of many different parameters, errors on the resulting 2834


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Figure 2. Area-weighted Pt particle size distributions (normalized to the total Pt surface area) and net Pt scattering curves for Pt/VC catalyst (a), Pt/ OLC catalyst (b), Pt/ATO catalyst (c), and Pt/ATO reduced catalyst (d). The scattering curve of Pt/VC after 1000 degradation cycles revealed no feature of individual Pt particles in the respective q-range so that no size distribution could be determined in this case. changed during the first cycles. PEFC-relevant stability properties of the catalyst materials were tested with an electrochemical degradation protocol that mimicked the corrosive conditions at PEFC cathodes during start and stop events. For this purpose, 1000 CV cycles (degradation cycles) between 0.5 and 1.5 VRHE were performed at a sweep rate of 50 mV/s. At different stages during the degradation protocol, 2 CV cycles at 20 mV/s were performed in the lower potential range (0.03−1 VRHE) that allowed to determine the electrochemically active Pt surface area (ECSA) by hydrogen underpotential deposition (Hupd) analysis assuming a specific Hupd-charge of 210 μC/cm2Pt.46 At the same stages of the degradation protocol in situ ASAXS experiments were performed on the catalyst materials with the electrode potential kept constant at 0.5 VRHE (for Pt/C catalysts) and at 0.75 VRHE (for Pt/ ATO catalysts) during SAXS acquisition in order to keep the Pt nanoparticles in a well-defined reduced state. The higher holding potential for the Pt/ATO catalysts was chosen to avoid possible Sb adsorption on the Pt surface at potentials below approximately 0.6−0.7 VRHE due to Sb species in the electrolyte that may result from dissolution/leaching from the ATO support material. Elemental analysis of the end-of-test 0.1 M HClO4 electrolyte used during in situ ASAXS experiments was performed by ICP-OES with a Varian VISTA Pro AX spectrometer. Electrode Fabrication. Flow-cell electrodes for in situ ASAXS experiments were prepared from the catalyst powders with a spraying technique on conductive Kapton substrate.28 Catalyst inks were

prepared by mixing a given amount of catalyst powder with Nafion solution (5 wt % Sigma-Aldrich) dissolved in 80 wt % water and 20 wt % isopropyl alcohol. The Nafion content of the electrodes was fixed at 30 wt % with respect to the catalyst weight for Pt/VC and Pt/OLC catalysts and at 10 wt % for Pt/ATO catalysts, respectively, to account for different support mass densities.19 Catalyst inks were sprayed on conductive Kapton substrate until an electrode thickness of 25−40 μm with a Pt loading of 200−400 μgPt/cm2 was reached. TEM. Transmission electron microscopy (TEM) analysis was performed on pristine and end-of-test flow-cell electrodes, respectively, using a Technai F20 electron microscope operated at 200 kV in bright field acquisition mode. The porous electrode layer was scraped off the conductive Kapton substrate using a scalpel. The yield was suspended in 0.5 mL ethanol and sonicated for about 30 s. Then, 20 μL was deposited onto a copper grid for TEM analysis.

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RESULTS Electrochemical Characterization. Table 1 presents initial average Pt particle diameters ⟨D⟩A,0 determined from ASAXS along with results of fundamental electrochemical characterization of the different catalysts. Mass-specific ECSA (ms-ECSA) values of the catalysts were obtained from thin-film RDE experiments in a standard glass cell by normalization with the precise value of the thin-film electrode Pt mass loading. 2835


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Figure 3. Evolution of Pt ECSA, Pt surface area A determined from ASAXS, and Pt mass m determined from ASAXS as a function of the number of degradation cycles. Values are given relative to the point of maximum catalyst activation (in terms of ECSA). Scattering curves of the Pt/VC catalyst after 500 and 1000 degradation cycles showed no signature of individual Pt particles in the respective q-range (cf. Figure 2a) so that no fit was possible and quantitative ASAXS analysis is missing for this catalyst.

Also given in the table are the values of the mass-specific Pt surface area (ms-A) determined from ASAXS (cf. Experimental Section). For all catalysts, the ms-ECSA values are 12−27% smaller than the corresponding values of ms-A, which could be expected due to the contact area between Pt and support that is not accessible for the electrolyte. A larger value of the ECSA-specific ORR activity jORR ECSA was found for the Pt/VC catalyst in comparison to Pt/OLC, which could be expected from the larger Pt particle size of the former, cf. Table 1. However, the reduced intrinsic activity of the Pt/ OLC catalyst is compensated by the larger ms-ECSA so that mass-specific ORR activities of both carbon-supported catalysts are comparable. Strong Pt particle size effects can also be established by comparing the values of jORR ECSA between the two ATO-supported catalysts with strongly decreased intrinsic ORR activity for the smaller Pt particles. Also in this case, the reduced intrinsic activity of the Pt/ATO reduced catalyst is compensated by the larger ms-ECSA so that mass-specific ORR activities of both ATO-supported catalysts are comparable. However, the overall reduction of ORR activity of the ATOsupported catalysts in comparison to the carbon-supported catalysts cannot be explained by Pt particle size effects, because

the latter represent both maximal and minimal particle sizes. This result is further discussed below. In Situ ASAXS Degradation Results. Cyclic voltammograms for ECSA determination recorded during in situ ASAXS experiments are shown in Figure 1. Typical Pt CV features of Hupd below approximately 0.4 VRHE and Pt oxidation/reduction above approximately 0.6 VRHE are observed for both carbonsupported catalysts. For ATO-supported catalysts extended activation behavior was observed with maximum ECSA reached after 100 high-potential cycles. A shift of Pt oxidation/ reduction features toward lower potentials was observed in comparison to the carbon-supported catalysts, which could be an indication of metal−support interactions in the Pt/ATO system. The increased oxide coverage of the Pt surface could be one explanation for the reduced ORR activities of ATOsupported Pt, cf. Table 1. Additional CV features in the potential range 0.4−0.6 VRHE, especially in the initial CVs of the Pt/ATO catalysts, could be associated with adsorption of Sb species on the Pt surface. Sb species in the electrolyte could result from dissolution of Sb dopant out of the ATO support surface. The oxidative removal of adsorbed Sb from the Pt surface at high potentials could explain the extended activation 2836


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Figure 4. TEM images of the Pt/VC catalyst (a) and Pt/OLC catalyst (b) in the initial state (left) and after 1000 degradation cycles (right). For each catalyst, initial and final images are sized in a way to achieve the same apparent TEM magnification scale. Due to the small initial Pt particle size of the Pt/OLC catalyst, cf. Table 1, Pt particles are difficult to be distinguished in the initial state at the chosen magnification. However, individual Pt particles are clearly discernible in high-magnification TEM images presented in Supporting Information Figure S10. Different TEM magnifications in (a) and (b).

behavior of ATO-supported catalysts. This hypothesis was furthermore supported by potential holding experiments that revealed ECSA-deactivation of Pt/ATO catalysts after holding for 1 h at potentials below 0.6−0.7 VRHE. Accuracy of ECSA determination by Hupd analysis for ATO-supported Pt was confirmed by comparison of CVs recorded in N2-saturated and in CO-saturated electrolyte during glass cell experiments with thin-film RDE electrodes, cf. Supporting Information Figure S4. No relevant CV features related to ATO were observed in the Hupd potential region after blocking the Pt surface with CO. Figure 2 shows the net Pt scattering curves extracted by ASAXS analysis (insets) and the Pt nanoparticle size distributions resulting from fits of the respective Pt scattering curves as a function of degradation potential cycling. The areaweighted size distributions PA(D) have been furthermore multiplied with the total surface area A of Pt nanoparticles (cf. Experimental Section). With this scaling, a loss of Pt surface area is directly reflected in a loss of the integral area below the distribution curve. Figure 3 presents the relative losses of Pt ECSA, Pt surface area A determined from ASAXS, and Pt mass m determined from ASAXS as a function of the number of degradation cycles. Values are given relative to the point of maximum ECSA (fully activated state of the catalyst), which was reached after the first

10−100 degradation cycles. Figures 4 and 5 present TEM images of the fresh catalysts in the initial state (left) and the catalysts after 1000 degradation cycles (right). Results presented in Figures 2−5 will be discussed for individual catalysts in terms of the following degradation mechanisms that occur during high-potential cycling: •Pt mass loss due to dissolution in electrolyte: At potentials above 0.8 VRHE, Pt can dissolve electrochemically.10,11 The resulting Ptx+ ions in the electrolyte can diffuse through the electrode pore structure and reach the bulk electrolyte flow to be carried away. This process results in a loss of Pt surface area and Pt mass. •Electrochemical Ostwald ripening: Dissolved Ptx+ ions can redeposit on existing Pt nanoparticles at low potentials. Since smaller Pt particles are thermodynamically more prone to dissolution than larger ones,11 this process leads to an overall growth of larger Pt particles at the expense of the smaller ones (electrochemical Ostwald ripening). Pt size distributions shift to larger diameters, and the Pt surface area decreases. However, the Pt mass in the electrode remains preserved. The shape of Pt particles can be expected to remain approximately spherical during electrochemical Ostwald ripening because of the minimum surface-to-mass ratio of the spherical geometry. 2837


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Figure 5. TEM images of the Pt/ATO catalyst (a) and Pt/ATO reduced catalyst (b) in the initial state (left) and after 1000 degradation cycles (right). For each catalyst, initial and final images are sized in a way to achieve the same apparent TEM magnification scale. Different TEM magnifications in (a) and (b).

•Support corrosion and Pt agglomeration: Support corrosion can lead to a detachment of entire Pt nanoparticles from their initial anchoring points,8 which, as a consequence, can float to different locations of the electrode, either resettling at another spot of the support material surface or forming agglomerates with other Pt nanoparticles.9 The increased particle−particle contact in such agglomerate structures reduces the electrochemically active Pt surface area (ECSA). Due to the low mobility of Pt atoms at room temperature, it can be assumed that the primary Pt particles within such agglomerates cannot coalesce by diffusion of individual Pt atoms. Therefore, Pt agglomerates resulting from support corrosion can be expected to consist of a conglomerate of primary Pt particles with preserved size and shape. However, the agglomerate structure can introduce an additional structure factor in the Pt SAXS scattering curves that weakens the SAXS signature of individual, primary Pt nanoparticles. It is important to emphasize that the fit of the net Pt SAXS scattering curves with the spherical particle model yields the size distribution of compact primary Pt nanoparticles. The superimposed structure of loose agglomerates of such primary particles possibly resulting from support corrosion is not reflected within this size distribution. Therefore, a shift of the size distribution toward larger particle diameters in the course of the degradation protocol can be directly attributed to the process of electrochemical Ostwald ripening. Pt Supported on Vulcan Carbon (Pt/VC). Pt scattering curves (inset in Figure 2a) of the Pt/VC catalyst reveal a shoulder around q = 1 nm−1 that is characteristic of an

ensemble of Pt nanoparticles. The light slope in the plateau region (q ≈ 0.5 nm −1 ) indicates a slight degree of agglomeration of the primary Pt nanoparticles. This picture is confirmed by the TEM image of the fresh catalyst in Figure 4a (left). The initial size distribution of primary Pt particles is centered around an area-weighted average diameter of ⟨D⟩A,0 = 4.0 nm. Pt particles of that size are reported to be relatively stable.12 As a consequence, the size distribution reveals only a minor shift to larger diameters during the first 100 degradation cycles that, according to the discussion above, can be attributed to electrochemical Ostwald ripening. However, scattering curves after 1000 degradation cycles show no signature of individual Pt particles in the respective q-range so that no size distribution could be determined. At the same time, a significant Pt ECSA loss of 86% was established (cf. Figure 3a). The TEM image of the end-of-test electrode in Figure 4a (right) reveals the presence of very large Pt particles (up to approximately 50 nm diameter) and large areas of bare Vulcan support surface. The support structure appears largely intact after 1000 degradation cycles. However, a certain amount of surface corrosion of the Vulcan support cannot be excluded. The large Pt particles have a compact form and do not reveal a loose agglomerate structure with preserved primary particle constituents which would be expected as a result from support corrosion, cf. discussion of basic degradation mechanisms above. Therefore, electrochemical Ostwald ripening can be seen to be strongly involved in the formation of these large, compact Pt particles. 2838


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Chemistry of Materials Pt Supported on Graphitic Onionlike Carbon Spheres (Pt/OLC). Initial Pt particle sizes of the Pt/OLC catalyst are drastically smaller than for the Pt/VC catalyst, which can be explained with different Pt nanoparticle synthesis methods (cf. Experimental Section). The Pt size distribution with an initial average diameter of ⟨D⟩A,0 = 2.0 nm quickly shifts to larger diameters in the course of the first 100 degradation cycles. This fast growth of Pt nanoparticles can be explained with electrochemical Ostwald ripening which is stronger in comparison to the Pt/VC, because of the reduced dissolution stability of the smaller initial Pt particles. Pt scattering curves after 1000 degradation cycles still reveal an ensemble of distinct Pt nanoparticles with a very broad size distribution around a final average diameter of ⟨D⟩A,1000 = 8.4 nm. Furthermore, the compact, spherical shape of the large final Pt nanoparticles revealed in the TEM image in Figure 4b (right) indicates electrochemical Ostwald ripening to be the dominant mechanism responsible for particle growth, cf. discussion above. From TEM images, no clear evidence of onion carbon support corrosion is provided, and the support structure after degradation testing appears largely intact. However, also for the OLC support, surface corrosion cannot be excluded based on TEM analysis. Pt mass loss due to dissolution into the bulk electrolyte flow is negligible as shown in Figure 3b. Similar to the Pt/VC catalyst, a dramatic ECSA loss of 85% in the course of degradation cycling was found, and a slightly smaller loss of Pt surface area was determined from ASAXS (71%). Although ASAXS-fitted values must be considered with a relative error of approximately 10%, cf. Experimental Section, this discrepancy could indicate a certain amount of Pt particle agglomeration due to OLC support corrosion in the final state leading to larger fraction of contact area and thus to larger ECSA loss, cf. discussion of different degradation mechanisms above. Indeed, the TEM image of the catalyst in the final state in Figure 4b (right) reveals a loose agglomeration of spherical primary Pt particles that could be a result of Pt particle detachment due to carbon support corrosion. Pt Supported on Antimony-Doped Tin Oxide (Pt/ ATO). Fitting the Pt scattering curves of the Pt/ATO catalyst (cf. Figure 2c) reveals an initial average diameter of ⟨D⟩A,0 = 3.1 nm. During the first 100 degradation cycles the size distribution and Pt surface area remain almost constant. Only after 1000 degradation cycles, a certain amount of electrochemical Ostwald ripening can be established from a moderate shift of the size distribution to a final average diameter of ⟨D⟩A,1000 = 3.8 nm. The concomitant loss of Pt surface area (25−30%) is significantly less than for both carbon-supported catalysts (cf. Figure 3). Pt ECSA loss and loss of Pt surface area determined from ASAXS are in very good agreement. Taking into account a relative error of approximately 10% on ASAXS-derived values, cf. Experimental Section, no significant Pt mass loss could be established during the degradation protocol. This finding is supported by ICP-OES analysis of the end-of-test electrolyte which did not reveal the presence of dissolved Pt ions. TEM images (Figure 5a) neither indicate ATO support corrosion nor Pt nanoparticle agglomeration in the final state. Pt particles in the final state appear spherical and slightly larger than in the initial state, although it must be emphasized that the distinction between Pt particles and support by TEM becomes difficult for this catalyst. Pt Supported on Reduced Antimony-Doped Tin Oxide (Pt/ATO Reduced). The same deposition method for Pt nanoparticles on the prereduced ATO powder (Pt/ATO

reduced) results in smaller particle sizes in the initial catalyst compared to the deposition on the as-received ATO powder (Pt/ATO). This could be seen as a consequence of an enhanced binding of Pt on the reduced ATO surface leading to a larger number of Pt nuclei during Pt nanoparticle synthesis and thus to smaller particle size for the same Pt loading. The initial average diameter of ⟨D⟩A,0 = 2.4 nm is close to the one of the Pt/OLC catalyst. However, whereas the latter shows drastic Pt particle growth due to electrochemical Ostwald ripening, the Pt/ATO reduced catalyst reveals a similar behavior as the Pt/ ATO catalyst despite the smaller initial particle size: The size distributions and Pt surface area remain almost constant during the first 100 degradation cycles and indicate only minor particle growth after 1000 degradation cycles to a final average diameter of ⟨D⟩A,1000 = 2.7 nm. Also for this catalyst, no significant Pt mass loss could be established (cf. Figure 3d). Loss values of 34% and 24% for ECSA and for the Pt surface area determined from ASAXS, respectively, were found. In order to explain this minor discrepancy, an error of approximately 10% on the value of ASAXS-derived Pt surface area should be taken into account. Furthermore, a possible underestimation of ECSA at the later stages of the Pt/ATO reduced degradation protocol must be considered resulting from a limitation of the counter electrode capacitance that developed in the course of this experiment, cf. Figure 1d.

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DISCUSSION In all cases, fit of the ASAXS data revealed that the total mass of Pt nanoparticles was conserved throughout the degradation protocol in good approximation. This result indicates that Pt ions dissolved in the electrolyte within the porous electrode at high potentials were reduced and redeposited on existing Pt nanoparticles at low potentials before reaching the bulk electrolyte flow. This interpretation is supported by analysis of the end-of-test electrolyte using ICP-OES which did not reveal the presence of dissolved Pt ions. Because of the conservation of total Pt mass, the loss of total Pt surface area determined from the fit of ASAXS data results almost entirely from the shift of the size distribution toward larger primary particle diameters and, therefore, can be attributed to electrochemical Ostwald ripening, cf. discussion above. Thus, the good agreement between the loss of Pt surface area determined from ASAXS and the loss of Pt ECSA strongly suggests that electrochemical Ostwald ripening was largely responsible for the Pt ECSA degradation during potential cycling between 0.5−1.5 VRHE under the chosen experimental conditions at room temperature. This was not only the case for both ATO supported catalysts but also for the carbonsupported Pt/OLC (cf. Figure 3b). The lack of fits of the ASAXS data at later stages of the degradation protocol for the Pt/VC catalyst precludes a quantification of the observed Pt ECSA loss in terms of different degradation mechanisms in this case. However, also for this catalyst, the shift of the Pt size distribution already during the first 100 degradation cycles (cf. Figure 2a) indicates a strong involvement of electrochemical Ostwald ripening in the significant Pt ECSA degradation. In addition, a recent study also has revealed indication for a strong contribution of associative mechanisms, such as electrochemical Ostwald ripening, to the degradation of Vulcan carbon supported Pt nanoparticles during high-potential cycling.47 The surprising result of the present work lies in the fact that this process of electrochemical Ostwald ripening and the corresponding Pt ECSA loss were strongly suppressed for both 2839


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Figure 6. Schematic of electrochemical transistor switching of support oxide conductivity: At potentials below the flat-band potential, E < Efb, support oxide conductivity is high and Pt nanoparticles are well connected (a). At high potentials, E > Efb, the conductivity of the support oxide surface layer switches off and protects the Pt nanoparticles from dissolution (b).

effectively electrically disconnecting and protecting Pt nanoparticles from high dissolution potentials, respectively. Flat-band potentials of antimony-doped tin oxide Efb ≤ 0.5 VRHE have been reported.51−55 These values appear too low for application of ATO as PEFC cathode catalyst support, because PEFC cathode operation potentials are larger than 0.5 VRHE. However, it has been pointed out that nanostructured ATO electrodes are electronically applicable up to potentials significantly larger than Efb,55 which is confirmed by the Mott−Schottky plot of the ATO used in the present study shown in Figure 7: The typical linear Mott−Schottky

ATO-supported Pt catalysts in comparison to the carbonsupported Pt nanoparticles. Pt particle size effects can be excluded as origin of this enhanced stability of the Pt/ATO catalysts, because of the similarity of initial Pt particle sizes of the Pt/OLC and the Pt/ATO reduced catalysts. Furthermore, within the first 100 degradation cycles, Pt/VC with the largest initial Pt particle size already revealed electrochemical Ostwald ripening (cf. Figure 2a), whereas both Pt/ATO catalysts with smaller initial particle size had stable size distributions. Since Pt dissolution is the initial step of electrochemical Ostwald ripening, the observed slowdown of the latter for ATO-supported Pt can be concluded to be a result of a suppression of electrochemical Pt dissolution on ATO support at high potentials. This effect cannot be simply explained by the general electronic resistance of the ATO support: Although not particularly high, the ex situ ATO conductivity of 6.4 × 10−4 S/ cm appears sufficient to sustain reasonable ORR performance of Pt/ATO catalysts at potentials Efb, the semiconductor surface gets depleted of mobile electrons resulting in a decrease of in-plane surface conductivity. This behavior of surface conductivity switching as a function of electrode potential can be regarded as direct electrochemical analogue of a field-effect transistor. Figure 6 schematically shows how this ‘electrochemical transistor effect’ of ATO support explains the observed stabilization of Pt nanoparticles: At potentials below the flatband potential, E < Efb (Figure 6a), the conductivity of the surface layer of support oxide particles is high, and supported Pt nanoparticles are electronically well connected. This is the ideal situation at PEFC operation conditions (cathode potentials Efb (Figure 6b), e.g. during PEFC start/stop, the conductivity of the support oxide surface layer is decreased due to depletion of mobile electrons. As a consequence, the overall resistance along the current paths to the supported Pt nanoparticles is strongly increased, thus

Figure 7. Mott−Schottky plot of the inverse square of ATO capacitance vs applied potential. Capacitance values were determined by electrochemical impedance spectroscopy at a frequency of 500 Hz in 0.1 M HClO4 electrolyte.

relationship between C−2 and the applied potential E known for planar semiconductor electrodes50 is not reflected in this case of a porous, nanoparticulate ATO electrode. However, the observed continuous curvature is in good agreement with Mott−Schottky plots calculated for high surface area semiconductor electrodes in ref 55 based on a space charge model for an assembly of spherical semiconductor particles. Although, in this case, Efb cannot be easily determined from the plot, the onset of the strong increase of C−2 observed in Figure 7 indicates that electronic depletion of the nanoparticulate ATO support occurs at potentials above 0.6−0.8 VRHE. Thus, the conductivity switching potential of the present ATO support lies within the potential range of PEFC cathode operation. The reduced operando support conductivity could contribute to the decrease of the ORR activity established for the Pt/ATO and Pt/ATO reduced catalysts in comparison to the carbonsupported catalysts. From these results, an ideal value of the support flat-band potential of approximately 1 VRHE could be postulated which marks the border between the PEFC cathode 2840


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Figure 8. Cyclic voltammograms of Pt/VC and Pt/ATO electrodes in the low potential range at 20 mV/s (a) and high potential range at 50 mV/s (b). The current axes in (a) and (b) are scaled in the same proportion as the respective sweep rates, so that the magnitude of currents can be visually compared between (a) and (b).

mechanism could explain the overproportional loss of CV currents above the ATO conductivity switching potential of approximately 0.6 VRHE compared to the loss of Pt ECSA in the course of the degradation protocol observed for Pt/ATO and Pt/ATO reduced catalysts as shown in Figure 1c,d. The operando conductivity switching was shown in this work for the case of ATO support material. Whereas strongly decreased ATO conductivity at high potentials protected supported Pt nanoparticles from electrochemical degradation, it was also found that the low position of the ATO flat-band potential could limit the performance of Pt/ATO as ORR catalyst at PEFC cathodes. However, the potential-dependent formation of a space-charge accumulation/depletion layer, and thus the electrochemical transistor effect, is the general behavior of semiconducting electrodes in contact with an electrolyte, which opens the possibility to discover other semiconducting metal oxides with a more suitable position of the flat-band potential for application as PEFC cathode catalyst support. Furthermore, from a more general perspective, the flatband potential Efb appears to be a fundamental descriptor of semiconducting metal oxides used as electrocatalyst supports: The value of Efb determines in what potential range the support can provide sufficient conductivity to enable good electrocatalyst performance and in what potential range decreased support conductivity could have beneficial effects on electrocatalyst stability.

potential range of operation (1 VRHE), respectively. The effect of potential-dependent support conductivity switching can be directly seen by comparing the CVs of Pt/ VC and Pt/ATO electrodes in the low potential range (CVs used for ECSA determination, Figure 8a) and high potential range (degradation CVs, Figure 8b), respectively. Note that the CV sweep rates are different in Figure 8a and 8b, but the current axes are scaled in the same proportion as the sweep rates so that the magnitude of currents can be directly visually compared. Whereas CV currents of both electrodes are very similar in magnitude in the low potential range (similar Pt ECSA), the degradation CV of the Pt/ATO catalyst is strongly suppressed in comparison to the Pt/VC catalyst, which can be regarded as a direct consequence of a strongly decreased ATO support conductivity in the high potential range. In contrast, Vulcan carbon support retains high conductivity also at high potentials, leading to strong Pt oxidation/dissolution currents (anodic scan) and reduction/redeposition currents (cathodic scan), respectively. Furthermore, all CVs presented in Figure 8 are recorded at the same stage of the degradation protocol with the CVs in 8a recorded af ter the CVs in 8b. For this reason, degradation effects can be excluded as origin of the different current magnitude for the Pt/ATO catalyst in 8a and 8b. Nevertheless, the possible influence of ATO support degradation must be discussed. ICP-OES analysis of the endof-test electrolyte showed the presence of dissolved Sb species corresponding to approximately 30% of the initial Sb content of the corresponding Pt/ATO and Pt/ATO reduced electrodes. No dissolved Sn was detected. Whereas these results prove the stability of the SnO2 host metal oxide, Sb dopant appears unstable due to leaching from the host oxide structure and dissolution in the electrolyte. This finding is in agreement with previous reports.25,26 Although dopant leaching is an additional degradation effect that must be taken into account in Pt/metal oxide electrocatalyst systems, the observation of Sb leaching during in situ ASAXS experiments does not alter the conclusions regarding the electrochemical transistor effect. In fact, a partial loss of Sb dopant could enhance the transistor effect because of an increasing thickness of the support space charge layer with decreasing dopant concentration. This

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CONCLUSIONS Support-dependent degradation of Pt nanoparticles under PEFC-relevant conditions was investigated for different Pt/C and Pt/ATO catalysts in a combined in situ ASAXS and postmortem TEM study. The electrochemical degradation protocol mimicked the corrosive environment encountered by the PEFC cathode catalyst during start/stop events. Electrochemical Ostwald ripening was found largely responsible for Pt surface area degradation at room temperature in all cases. As a consequence, the superior stability that was established for the Pt/ATO catalysts could be explained by a potential-dependent switching of support oxide surface conductivity in analogy to the principle of a field-effect transistor. This electrochemical transistor ef fect of the ATO leads to a strongly reduced support 2841


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Research Foundation (NRF) Innovation Postdoctoral Fellowship towards this research is hereby acknowledged by R.M.

conductivity at high potentials and thus protects Pt nanoparticles from dissolution, which is the initial step of electrochemical Ostwald ripening. The critical property that controls the switching of support surface conductivity is the flat-band potential Efb of the metal oxide. Our findings reveal a fundamentally important design principle for conductive metal oxide support materials in electrocatalysis: The potentialdependent operando conductivity properties of the metal oxide support and, thus, its flat-band potential must be suitable for the potential range of application. This insight motivates further research for conductive metal oxide support materials for the PEFC cathode catalyst with an optimized flat-band potential value of Efb ≈ 1.0 VRHE. Such a specifically designed metal oxide support could combine the Pt nanoparticle protection mechanism at transient high potentials >1.0 VRHE with a high performance at PEFC cathode operation potentials

Stabilization of Pt Nanoparticles Due to Electrochemical Transistor

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