C Cathode Catalyst Degradation in a Polymer Electrolyte Fuel

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Pt−Co/C Cathode Catalyst Degradation in a Polymer Electrolyte Fuel Cell Investigated by an Infographic Approach Combining ThreeDimensional Spectroimaging and Unsupervised Learning Yuanyuan Tan,† Hirosuke Matsui,†,‡ Nozomu Ishiguro,‡ Tomoya Uruga,§,∥ Duong-Nguyen Nguyen,⊥ Oki Sekizawa,§,∥ Tomohiro Sakata,§ Naoyuki Maejima,† Kotaro Higashi,§ Hieu Chi Dam,*,⊥ and Mizuki Tada*,†,‡ Downloaded via KEAN UNIV on July 24, 2019 at 04:50:31 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Department of Chemistry, Graduate School of Science & Research Center for Materials Science (RCMS), Nagoya University, Furo-cho, Chikusa, Nagoya, Aichi 464-8602, Japan ‡ RIKEN SPring-8 Center, Koto, Sayo, Hyogo 679-5148, Japan § Innovation Research Center for Fuel Cells, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan ∥ Japan Synchrotron Radiation Center, SPring-8, Koto, Sayo, Hyogo 679-5198, Japan ⊥ Japan Advanced Institute of Science and Technology, Asahidai, Nomi, Ishikawa 923-1292, Japan S Supporting Information *

ABSTRACT: Catalyst degradation at the cathode of a membrane electrode assembly (MEA) remains a critical issue for practical polymer electrolyte fuel cell (PEFC) operation, but such wet systems impede detailed visualization of degradation events in the cell during its operation. In this work, for the first time, operando spectroimaging (X-ray absorption near-edge structure−computed tomography) was used to produce clear three-dimensional (3D) images of the morphology, Pt and Co distributions, Co/Pt atomic ratio, and Pt valence state of a Pt−Co/C cathode catalyst in a PEFC MEA before and after performing a PEFC-accelerated degradation test. The infographic approach combining the operando spectroimaging and unsupervised learning of the 3D images revealed a catalyst degradation mechanism with different degradation behaviors for Pt and Co in the bimetallic catalyst and negligible migration of the Pt catalyst in local parts of the MEA.

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analysis,9,13−17 theoretical investigation,18,19 microscopy,20,21 and spectroscopy.22−27 Pt3Co (Pt−Co) is the most typical composition of Pt−Co alloy structures, and commercialized PEFCs include Pt−Co/C catalysts at the cathode. The alloying of Co with Pt has been proposed to reduce Pt d-band center and weaken surface Pt−O bonds, resulting in the enhancement of the ORR activity. Under acidic reaction conditions in a PEFC membrane electrode assembly (MEA), Pt−Co alloy particles are stabilized in a core−shell structure with a Pt−Co alloy core and Pt-rich surface shell.16,22,28−30 Pt3Co/C catalysts are more durable than Pt/C catalysts, but still degrade at cathode to an unacceptable degree under PEFC operating conditions.31,32 Although the dissolution and aggregation of Pt in Pt−Co/C catalysts are more suppressed than they are in Pt/C catalysts, Co gradually dissolves from the bimetallic catalyst, and hence the alloy benefit gradually

INTRODUCTION The development of sustainable and fossil-free pathways to energy production has attracted much attention for achieving an environmentally friendly society. Polymer electrolyte fuel cells (PEFCs) offer much promise for replacing fossil fuels with clean energy that is free of carbon byproducts. However, current PEFC systems suffer from critical problems, in particular low oxygen reduction reaction (ORR) activity and poor durability of cathode electrocatalysts such as Pt nanoparticles during practical PEFC operation.1−3 In the search for suitable ORR catalysts, numerous forms of catalysts such as nanoparticles, nanoplates, and nanowires have been developed to reduce Pt loading and improve the ORR performance at the cathode,4−8 and one of the most promising ORR catalysts is Pt-M (M: Co, Ni, etc.) bimetallic nanoparticles. In particular, Pt−Co catalysts9−12 show better activity and durability than standard Pt/C catalysts and are useful as PEFC cathode catalysts. The origin of the ORR activity and durability of Pt−Co bimetallic catalysts has been investigated by electrochemical © XXXX American Chemical Society

Received: May 27, 2019 Revised: July 5, 2019 Published: July 10, 2019 A

DOI: 10.1021/acs.jpcc.9b05005 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Scheme 1. Schematic of Operando XANES−CT Imaging under PEFC Operating Conditions and Reconstructed 3D Images of the Cathode Catalyst Layer in an MEA

layers (Toray Ind., Inc., TGP-H-090, ≈300 μm) and gaskets (Teflon sheets, NICHIAS, ≈300 μm) were also used. PEFC Operations. The PEFC cell was operated at 353 K and its relative humidity was controlled by supplying humidified gases [anode: H2 (99.99999%), cathode: N2 (99.99999%) or air (80% N2 and 20% O2; 99.9999%)] using a commercial gas supply kit (CNF52742, NF Co., Ltd.). The relative humidity and flow rates are summarized in Table S1. The MEA was mounted in an operando XANES−CT cell (Figure S1). A potentiostat/galvanostat (VSP, BioLogic Science Instruments Co., Ltd.) was used for PEFC operations. Initially, 150 aging cycles were conducted in 14 current steps (0 → 0.02 → 0.045 → 0.068 → 0.090 → 0.112 → 0.180 → 0.270 → 0.360 → 0.450 → 0.540 → 0.720 → 0.900→ 1.350 → 1.800 A with a duration of 6 s for each step). Cyclic voltammetry (CV) data were recorded between 0.05 and 1.0 V (vs RHE) at a scanning rate of 10 mV s−1 (anode: H2 flow, cathode: N2 flow). ADT cycles were performed by repeating rectangular voltage cycles between 0.6 and 1.0 V and holding each voltage for 3 s (anode: H2 flow, cathode: N2 flow). Scanning Electron Microscopy−Energy-dispersive Xray Spectroscopy Analysis. The MEA used for the XANES−CT measurement after 34 000 ADT cycles was observed by scanning electron microscopy (SEM) (5.9 kV, SU6600, Hitachi High-Technologies Co.). The MEA was carefully cut (around the center of the X-ray-irradiated position) and taped onto a vertical sample stage using a carbon-coated tape. The thicknesses of the cathode catalyst layer, Nafion membrane, and anode catalyst layer were about 60, 50, and 15 μm, respectively (Figure S2). Transmission Electron Microscopy Analysis. Cathode catalyst particles in a fresh MEA after the similar aging and in the degraded MEA after 34 000 ADT cycles and the XANES− CT measurements were scratched off carefully and dispersed in ethanol. This dispersion was subjected to sonication for 20 min, and then, one drop of the resulting solution was administered onto the surface of the carbon-coated copper grids. Subsequently, the sample was dried thoroughly overnight for transmission electron microscopy (TEM) observations. For each sample, 10 TEM images (×100 000) were recorded using a JEOL 2100F HK operating at 200 kV with a current of 105 μA. More than 200 particles were counted for the calculation of particle size distributions. Operando XANES−CT Measurements. Operando XANES−CT measurements were taken at the BL36XU beamline, SPring-8 (8 GeV, 100 mA, Japan). The PEFC cell was irradiated with X-rays monochromatized by Si(111) channel-cut crystals and passed through a paper rotation diffuser (Scheme 1). X-ray transmission images of the sample

disappears. There are many reports on the preparation of durable cathode catalysts with bimetallic structures, but to our knowledge, there are no reports yet on PEFC cathode catalysts without degradation at cathode in MEAs. The elucidation of the process by which a Pt−Co/C catalyst degrades in an MEA would be expected to provide key clues to improving cathode catalyst durability. To figure out the process by which PEFC cathode catalysts degrade under PEFC working conditions in the presence of abundant fuel and water, we have developed an operando three-dimensional (3D) spectroimaging technique that combines computed tomography (CT) imaging and hard X-ray absorption fine structure (XAFS) spectroscopy. 33 The development of imaging techniques using X-rays as the probe is cutting-edge,34 and X-ray tomography,35−37 X-ray laminography,38 X-ray ptychography,39−43 and X-ray microscopy (X-ray photoemission electron microscopy,44 scanning transmission X-ray microscopy,45 etc.) have been recently applied to solid catalysts. In particular, the combination of XAFS and CT (XAFS−CT) provides not only a 3D image of a sample obtained by the simple reconstruction of transmission images but also 3D images of element distribution and chemical states (valence and local structures,46 etc.). In 2017, we reported a demonstration study of an operando XAFS−CT analysis of a Pt/C cathode catalyst in a PEFC MEA and visualized serious dissolution of Pt in the MEA under PEFC operating conditions.33 In this paper, we report an infographic approach combining operando 3D spectroimaging and unsupervised learning of imaging data to elucidate the event of the degradation of the Pt−Co/C cathode catalyst in a PEFC. Pt LIII-edge and Co Kedge X-ray absorption near-edge structure (XANES)−CT was performed under PEFC operating conditions to produce, for the first time, 3D images of the morphology, Pt density, Co density, Co/Pt atomic ratio, and Pt valence states of the Pt− Co catalyst in an MEA before and after a PEFC-accelerated degradation test (ADT). The unsupervised learning of the large amount of imaging data provided the mechanism of the degradation of the Pt−Co catalyst and showed negligible migration of Pt occurring in local regions of the MEA.

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EXPERIMENTAL SECTION MEA and Materials. A commercial 1 × 1 cm2 MEA (prepared by EIWA Co. Ltd., Japan) consisting of a cathode catalyst of 46.7 wt % Pt3Co/C [3.0 mg cm−2, TEC36E52E, Tanaka Kikinzoku Kogyo (TKK)], anode catalyst of 46.3 wt % Pd/C (0.5 mg cm−2, TECPdE50, TKK), and Nafion membrane (NR-212, Sigma-Aldrich) was used. Gas diffusion B


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Figure 1. (a) Experimental sequence of the operando XANES−CT imaging and PEFC operation. (b) TEM images of the cathode catalysts scratched from MEAs (1) before the ADT and (2) after 34 000 ADT cycles. (c) Particle size distributions of the cathode catalysts analyzed from the TEM images.

were recorded using a high-resolution X-ray image unit (AA50, Hamamatsu Photonics, K.K.) coupled with a low-noise sCMOS camera (Orca-Flash 4.0, Hamamatsu Photonics, K.K.). The effective viewing area of the image was 666 × 666 μm2 with a pixel resolution of 325 nm. However, the actual spatial resolution was around 1 μm because of blurring on the scintillator crystal. The first XANES−CT data were recorded after the MEA was subjected to aging (Figure 1a). H2 (anode) and N2 (cathode) gases were flowed through the cell, and its voltage was kept at 0.4 or 1.0 V during the XANES−CT measurements. The PEFC cell was mounted on a rotation stage and rotated perpendicularly with respect to the incident X-ray beam (Scheme 1). At each particular angle, a quick scan of the Pt LIII-edge (11.386−11.697 keV, merged to 375 points) or Co K-edge XANES (7.517−7.849 keV, merged to 374 points) spectrum was recorded using an X-ray image unit for 25 and 18 s, respectively, and then, the rotation angle θ was changed in steps of 1° (Figure S3). The quick XANES scanning was repeated over a range of θ from −80° to 80° to obtain 161 data sets of quick XANES spectra. Incident X-rays images (I0(E)) were acquired at the beginning and end of the quick XANES measurements by removing the PEFC cell from the X-ray beams. The dark signal of the sCMOS camera (Idark) was also measured 30 times to obtain an averaged background image. The total measurement time was 1.9 h for the series of Pt LIIIedge XANES−CT data and 1.6 h for the Co K-edge XANES− CT data. Analysis of the XANES−CT Data. The recorded XANES−CT data were systematically analyzed as follows (Scheme 2). Step 1: Observed I(X′, Y′, θ, E) images were converted into the absorption coefficient μt(X′, Y′, θ, E) by using Beer’s law with I0(X′, Y′, E) and Idark(X′, Y′) images and Eq. 1. Step 2: Position drifts in the X′ Y′ directions of μt(X′, Y′) were calibrated in the image matrix at each energy. During the calibration, an image at the 10th energy point was used as a reference. Then, transmission XANES spectra, one at each pixel, were obtained. Step 3: These XANES spectra were each independently fitted using Eq. 2 to obtain parameters of a1 (∼intensity of background), b1 (edge jump = element density), and c1 (whiteline height ≈ valence) at each pixel. a1, a2, b1, b2, c1, and c2 were free parameters. In order to reduce free-fitting parameters for curve-fitting analysis, b3 and c3 were fixed to values estimated by free fitting of representative 2D-XANES spectra with good

Scheme 2. Protocol of the XANES−CT Data Analysis and the Coordinates for the CT Reconstruction

S/N ratios [b3 (Pt: 1.9, Co: 2.3), and c3 (Pt: 3.2, Co: 5.5)]. b3 and c3 were found to be constant regardless of θ, the position in the MEA (X′, Y′), and PEFC cell voltage, and thus, we fixed b3 and c3 in subsequent fitting analyses. Step 4: The 3D matrix (X′, Y′, θ) of each parameter (a1, b1, c1) in the X′Y′-project coordinates were converted into a sinogram of X′−θ cross sections with the Y′ axis. An image filter was used to correct transmission data of the flat membrane sample by considering thickness in different projection angles. Then, the converted sinograms were reconstructed into a 3D (X, Y, Z) matrix in real space using the angle-limited CT calculation of the ordered-subset C


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Figure 2. 3D images and cross-sectional images of the MEA with the Pt−Co/C cathode catalyst reconstructed from the operando Pt LIII-edge and Co K-edge XANES−CT data. Shown are images corresponding to morphology (μt at 11.497 keV before the Pt LIII-edge), Pt density (Pt LIII-edge jump), Co density (Co K-edge jump), and Co/Pt ratio (1.17 × Co density/Pt density, calculated on the 3D images). Field of view: X = 550, Y = 555, and Z = 60 μm. Cross-sectional images: Z = 60 μm (interface between the cathode catalyst layer and the Nafion membrane) and 30 μm (center of the cathode catalyst layer).

expectation maximization (OS-EM) method.47−49 The coordinates are shown in Scheme 2. The OS-EM method analysis was performed on the data in subsets within iterations, which reduced the calculation cost while maintaining precise results. In addition, binning to 3 × 3 × 3 pixel3 (975 × 975 × 975 nm3), which is almost consistent to the actual spatial resolution, was selected for the calculation. Step 5: We calculated the Co/Pt atomic ratio image (X, Y, Z) using Eq. 3 from the reconstructed images of b1(Pt LIIIedge) (X, Y, Z) and b1(Co K-edge) (X, Y, Z). The absorption coefficient ratio of the Co K-edge to the Pt LIII-edge was determined to be 1.17 (see the Supporting Information). The Co/Pt atomic ratio color image was made in the ratio range of 0 (only Pt) to 1 (Co/Pt = 1). The parts with a Co/Pt ratio >1 were colored in white and corresponded to the parts with overly low Pt densities such as cracks. We calculated a Pt valence (formal charge) image (X, Y, Z) using eq 4 from the reconstructed images of c1(Pt LIII-edge) (X, Y, Z) and b1(Pt LIII-edge) (X, Y, Z), as c1/b1. Pt foil (Pt0), Pt(acac)2 (Pt2+), and PtO2 (Pt4+) were used as standard samples for the calibration curve of Pt formal charge and the calculated c1/b1 values were converted to Pt valence using the calibration curve. Unsupervised Data Mining of the Reconstructed Images. From the viewpoint of the data mining approach, the study of the process by which the Pt−Co cathode catalyst degraded in the MEA was equivalent to the problem of modeling the distribution of descriptive variablesthat is, unsupervised learning50 of this phenomenon. First, we collected a dataset + containing p = 550 × 555 × 61 data instances that corresponded to the voxels in the real device. Each data instance was described by m = 15 descriptive variables x that stored information of the corresponding voxel. For each observation, t is the state of the sample, fresh, ADT 21 000 cycles, ADT 34 000 cycles. The descriptive variables x for a given data instance corresponding to a voxel at (x, y, z) included (1) Pt density (ρPtt), (2) Co density (ρCot), (3) Pt valence state at 1.0 V (valPt‑1.0t), (4) Pt valence state at 0.4 V (valPt‑0.4t), and (5) distance from the closest surface (distance

f rom surfacet). In this study, distance from surfacet for a given voxel was measured by calculating the Euclidean distance from the voxel to the closest voxel that has a morphological value lower than 10−5. We investigated the distribution of data instances of + in the representation space x by using a mixture of Gaussian models.51 The model had the form K

p(xi|θ ) =

∑ πk 5(μk , Σk) k=1

For a given number of mixture components K, the estimation of the value of the parameter was made by using an expectation-maximization algorithm. To determine the number of mixture components, a maximizing Bayesian information criterion (BIC)52 score process with different randomized initial trial states was utilized.

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RESULTS AND DISCUSSION Operando XANES−CT Measurements and Analysis. For the 3D XANES−CT analysis, it was important to record the very weak XANES signals from a tiny area (325 × 325 × 325 nm3). As described in the Experimental Section, we performed quick scanning of Pt LIII-edge or Co K-edge XANES spectrum at each rotation angle (θ), and the transmission images of the quick XANES scan were recorded on a high-resolution X-ray image unit. A series of Pt LIII-edge or Co K-edge transmission XANES data of the MEA (1 × 1 cm2) with the commercial Pt−Co (Pt/Co = 3/1)/C cathode catalyst and the Pd/C anode catalyst mounted in the PEFC cell on a stage for rotation perpendicular to the incident X-ray beam was obtained at every 1° in the θ range from −80° to 80° (Figure S3). The operando XANES−CT measurements of the fresh MEA were taken at 0.4 or 1.0 V, and the ADT, which involved a repetition of voltage cycles between 0.6 and 1.0 V, was started in the PEFC cell to produce the typical degraded state in the cell. After 21 000 cycles of the ADT, a second set of XANES−CT measurements was taken and the ADT was D


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the cathode catalyst layer and the gas diffusion layer at the cathode side (Scheme 2). Pt in the fresh MEA was found as tiny spots. After the 21 000 cycles of the ADT, the color contrast in the Pt density images was broad, suggesting the occurrence of significant aggregation of Pt in most domains in the catalyst layer. This broadening was also observed after the 34 000 cycles of ADT. We previously observed, using CT−XANES, the dissolution and degradation of a Pt/C catalyst without Co.33 In the case of the Pt/C catalyst, the dissolution, migration, and aggregation of Pt catalyst particles were much more conspicuous than they were for the Pt−Co/C catalyst, in particular in the depth direction of the MEA. Significant degradation around the edges of the carbon support was observed in the Pt/C cathode catalyst layer.19,55 The present XANES−CT analysis of the Pt−Co/C catalyst suggested that the alloying of Co with Pt significantly suppressed the degradation of the cathode catalyst layer, specifically suppressing corrosion of the carbon support and dissolution of Pt in the cathode catalyst layer. Dissolution and Migration of Co of the Pt−Co/C Cathode Catalyst in the MEA. In addition to the Pt density images, we successfully visualized the 3D Co density images in the MEA by carrying out a Co K-edge XANES−CT analysis (Figure 2). As the number of ADT cycles was increased, the Co intensity in the cathode catalyst layer decreased, indicative of dissolution of the Co species from the cathode catalyst layer in the MEA. Dissolution of Co proceeded across the entire cathode catalyst layer, and large aggregation of dissolved Co species was not observed in the cross-sectional Co density images (Figure 2). The loss of Co in the cathode catalyst layer was estimated to be about 20% after the 21 000 cycles of the ADT and about 50% after the 34 000 cycles of the ADT, whereas the loss of Pt was less than 6% after the ADT. Accompanying the ADT-induced dissolution of Co, the atomic ratio of Co/Pt significantly decreased across the entire cathode catalyst layer (Figure 2). A similar decrease in the Co/ Pt ratio in the cathode catalyst layer was also indicated by the SEM−EDX results (0.3 → 0.2). The dissolution, segregation, and dealloying of Co from Pt3Co catalysts have been reported as the results of electrochemical analysis, ex situ TEM, theoretical calculations, and so forth.21,31,56−59 The operando XANES−CT imaging carried out in the current work provided actual 3D images of the Pt−Co/C catalyst degradation in the MEA under practical PEFC operating conditions for the first time. Pt Valence State Image of the Pt−Co/C Cathode Catalyst in the MEA. The Pt LIII-edge XANES−CT analysis provided Pt valence state images at 1.0 and 0.4 V (Figure 3a,b), which revealed differences in the Pt valence state in the cathode catalyst layer under PEFC operating conditions. Before the ADT, Pt around the edges of the domains was positively charged at 1.0 V, and very few domains showed oxidized species at 0.4 V. After the 21 000 cycles of the ADT, a similar contrast was observed at 1.0 V, but the brightness of the Pt valence image at 0.4 V was reduced. ΔPt-val, defined as difference between the Pt valence at 1.0 V and that at 0.4 V, was greater after the 21 000 cycles of the ADT than before (Figure 3c). The reduction of the Pt valence at 0.4 V was attributed to the increase in the catalyst particle size, resulting in the decrease in the number of surface sites in the Pt−Co catalyst. Note that marked differences in the Pt valence states at both 1.0 and 0.4 V were observed after the 34 000 cycles of the

repeated up to a total of 34 000 cycles (Figure 1a). We successfully recorded the XANES−CT data under fresh conditions, after 21 000 ADT cycles and after 34 000 ADT cycles for the same part of the MEA, which allowed us to investigate the process by which the Pt−Co/C cathode catalyst degraded during the ADT. The obtained transmission XANES−CT data were first converted to absorption coefficients (Scheme 2). Although the Co K-edge jump was too small to estimate the Co valence state, the recorded Pt LIII-edge XANES spectra could provide valuable information about the amount (density) of Pt and about the Pt valence state. We conducted a curve-fitting analysis of the transmission XANES spectra before performing CT reconstruction in order to extract the following parameters: intensity at 11.497 keV before the Pt LIII-edge, which reflected the morphology of the sample; Pt LIII-edge jump, which was relative to the amount of Pt (Pt density); Pt LIII-edge white-line height, which was relative to the Pt valence state; and the Co K-edge jump, which was relative to the amount of Co (Co density). The extracted parameters in the 2D projection images (X′, Y′, θ) were reconstructed into 3D matrices (X, Y, Z). Then, the images of the Co/Pt atomic ratio (X, Y, Z) and Pt valence (X, Y, Z) were calculated by using the reconstructed images. The Pt LII-edge has been shown to also be useful for the analysis of Pt L-edge XANES spectra,53,54 but it was impossible to record operando XANES−CT data at both Pt LII-edge and Pt LIII-edge XANES at the same time. In this study, we used Pt LIII-edge XANES for the operando CT imaging. Ex Situ SEM and TEM Analysis. After typical MEA aging, the average particle size of the Pt−Co cathode catalyst was estimated using TEM analysis to be 5.3 nm (Figure 1b1,c1). The electrochemical active surface area (ECSA) of the fresh MEA was determined using CV to be 20.5 m2 g_Pt−1 (Figure S5). After the 34 000 ADT cycles and all operando XANES− CT measurements, we conducted scanning electron microscopy−energy-dispersive X-ray spectroscopy (SEM−EDX) and TEM analyses of the catalyst scratched from the used MEA. The average particle size of the cathode catalyst increased to 6.5 nm and aggregation of the catalyst particles was observed (Figure 1b2,c2). Each component in the MEA showed negligible change in thickness. ECSA values determined before and after the XANES−CT measurements were also similar (Figure S5b). Dissolution and Migration of Pt of the Pt−Co/C Cathode Catalyst in the MEA. Figure 2 shows the reconstructed 3D images and cross-sectional images of the fresh MEA and those after the 21 000 and 34 000 cycles of the ADT. The CT imaging and the reconstruction provided clear images of the morphology of the MEA, and these images showed domain structures and cracks in the cathode catalyst layer, which were similar to that observed when using SEM. Similar patterns of crack structures of the cathode catalyst layer before and after the ADT were observed (Figure 2), suggesting that the same regions were successfully visualized as a result of the operando CT imaging. The acquired cross-sectional images of the Pt and Co densities around the center of the cathode catalyst layer (Z = 30 μm) and the interface of the cathode catalyst layer and Nafion membrane (Z = 60 μm) showed changes in the distribution of Pt and Co in the MEA resulting from the ADT (Figure 2). The Z direction was defined as the depth direction of the MEA, with the origin defined as the interface between E


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catalyst has not yet been clarified, the present study clearly suggested an irreversible degradation of the Pt−Co catalyst resulting from the dissolution of Co. Unsupervised Learning of the Visualized 3D Images of the Pt−Co Catalyst in the MEA. The 3D XANES−CT imaging suggested the occurrence of intrinsic heterogeneous degradation of the Pt−Co/C catalyst in the MEA. Each 3D image reconstructed from the XANES−CT analysis contained 18 620 250 structural parameter data points (morphology, Pt density, Co density, Co/Pt atomic ratio, and Pt valence states at 1.0 and 0.4 V, etc.). Therefore, we conducted data mining of the large amount of imaging data to extract correlations between the visualized parameters, which were related to the degradation of the catalyst in the MEA. Unsupervised learning was the process used to find correlations between the structural parameters in the observed 3D imaging data. Two-dimensional Pearson plots of Pt density versus Pt valence state at 1.0 V, displayed in Figure 4, were obtained to investigate the correlations between these two parameters. The plot at Z = 60 μm for the fresh MEA, displayed in Figure 4a1′, showed a negative correlation between Pt density and Pt valence state. We found that there were several peaks in the plot of the intensity of the Pt density, and we applied a Gaussian mixture model, using a maximizing BIC score process, to estimate the number of components of the plot. This process indicated the presence of three components, corresponding to low Pt density (denoted as G1 and shown in blue), medium Pt density (G2, green), and high Pt density (G3, red). The magnitudes of the components are presented at the top of the Gaussian plots (a1′). The images of the distributions of the three components are shown in Figure 4a). Similar results were obtained for a different depths, namely, Z = 30 μm, as shown in Figure 4a2′,a2. This analysis at 1.0 V was also applied to both depths of the sample after 21 000 and 34 000 ADT cycles, and also indicated the presence (in each of these four cases) of three components (G1, G2, and G3) (Figure 4b′,c′). However, the shapes of the Pearson plots of Pt density versus Pt valence state changed markedly as a result of the ADT. Specifically, in contrast to the above-described negative correlation between Pt density and Pt valence state observed for the fresh state, which indicated

Figure 3. Cross-sectional images of the Pt valence states in the MEA with the Pt−Co/C cathode catalyst at Z = 30 μm (center of the cathode catalyst layer). Pt valence was calculated as c1/b1 on the reconstructed images at (a) 1.0 and (b) 0.4 V. The parts with Pt valence outside of each scale bar are shown in white. (c) ΔPt-val is defined as the difference between the Pt valence at 1.0 V and that at 0.4 V.

ADT (Figure 3). Specifically, at 1.0 V, much more of the MEA sample showed higher Pt valence after the 34 000 cycles than before the ADT or even after only 21 000 ADT cycles (Figure 3a). Similar changes were observed for 0.4 V (Figure 3b). These results suggested that the Pt species in the catalyst was significantly oxidized during the last ADT process from 21 000 to 34 000 cycles. After the 34 000 ADT cycles, Pt species positively charged were found at 0.4 V in small cracks and near the interface of large cracks, whereas these were observed at 1.0 V in any places of the cathode catalyst layer. The oxidation was probably caused by the dissolution of Co from the Pt−Co catalyst in the cathode catalyst layer observed using Co K-edge XANES−CT, as shown in Figure 2. ΔPt-val was lower after 34 000 cycles than in the fresh MEA and after 21 000 ADT cycles (Figure 3c). This result suggested a decrease in the redox activity of the Pt catalyst during the last 13 000 ADT cycles. Although the threshold of such degradation of the Pt−Co

Figure 4. (Bottom) Pearson plots of Pt density versus Pt valence state at 1.0 V at Z = 60 μm (1: interface of Nafion and the cathode catalyst layer) and 30 μm (2: center of the cathode catalyst layer). Each plot could be classified into three Gaussian curves, denoted as G1, G2, and G3. (Top) Cross-sectional images of the distributions of these components (blue: G1, green: G2, and red: G3). (a) Fresh state. (b) After 21 000 ADT cycles. (c) After 34 000 ADT cycles. F


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Figure 5. (a) Pearson plots of the calculated distance from the surface vs ΔPt (difference in Pt density between two states) or ΔCo (difference in Co density between two states) at Z = 30 μm (center of the cathode catalyst layer). Top: Fresh state → state of 21 000 ADT cycles, bottom: state of 21 000 ADT cycles → state of 34 000 ADT cycles. Each ΔPt or ΔCo plot could be classified into four Gaussian curves, denoted as G1, G2, G3, and G4. (b) Cross-sectional images of their distributions overlaid on the morphology images (dark blue: G1, sky: G2, yellow: G3, and red: G4). (b-All) All groups on the morphology images.

experiment the two states were fresh → 21 000 ADT cycles, and in the other experiment they were 21 000 ADT cycles → 34 000 ADT cycles. Other Pearson plots of the distance from the surface versus Pt density, Co density, and Pt valence state at 1.0 V are shown in the Supporting Information (Figures S7−S9). Here, the presence of four components (G1, G2, G3, and G4) was suggested by a similar unsupervised learning using the Gaussian mixture models applied to the Pearson plots of the distance from the surface versus ΔPt or ΔCo (Figure 5a). G1 and G2 had negative values of ΔPt or ΔCo, showing parts of the sample whose amount of Pt or Co was decreased as a result of the ADT process. These shows parts whose Pt or Co catalyst dissolved and migrated to other parts by the ADT process. On the other hand, G4 showed parts with increasing amounts of Pt or Co species resulting from the ADT process. The zero value for G3 meant parts whose quantity of Pt or Co were almost constant by the ADT. The distributions of the four components (G1, G2, G3, and G4) are shown in Figure 5b. Note that components G1, G2, and G4 of ΔPt were distributed from the surface to the bulk, but G3 (yellow) of ΔPt, which showed a negligible effect of the ADT on the Pt density, was remarkably localized at the surface (distance from

that the dispersed part (G1 with low Pt density) was more oxidized than the aggregated part (G3 with high Pt density), hardly any correlation was observed in the Pearson plots after 21 000 ADT cycles (Figure 4b′), and positive correlations were observed after 34 000 ADT cycles (Figure 4c′), the latter result indicating that G1 with low Pt density was more reduced than G3 with high Pt density. In addition to the changes in the plot shape, the average Pt valence was observed to be shifted upward after 34 000 ADT cycles, as seen in Figure 4c′. These trends were attributed to the loss of the Co alloying effect to reduce Pt d-band center19,31,54−57 caused by the ADT-induced dissolution of Co from the Pt−Co catalyst. A similar analysis of the Co density is presented in Figure S6. We calculated the geometric distance from the surface (depth of cracks in the cathode catalyst layer) by using the visualized morphological image of the cathode catalyst layer in Figure 2. The distance from a given voxel to the surface was defined by the Euclidean distance from the voxel to the closest voxel having a morphological value lower than 10−5 in the morphology image in Figure 2. Figure 5 shows the Pearson plots of the calculated distances from the surface (horizontal axis) versus ΔPt or ΔCo, defined as the difference in Pt or Co density between two ADT states (vertical axis), where in one G



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surface ≈ 0), as shown in Figure 5(a-ΔPt1). Most of G3 was found at the parts whose distance from surface was less than 3 μm. These results suggested that there were durable parts of the sample, especially around the cracks in the cathode catalyst layer, which showed negligible loss of Pt during the first ADT process. In contrast to ΔPt, all components (G1−G4) of ΔCo were randomly dispersed independently to the distance from the surface (Figure 5(a-ΔCo1)). The data mining of the differences in the Pt or Co density between the two ADT states suggested that the regulation of 3D morphology inside the cathode catalyst layer is one of the key parameters to consider in order to control the degradation of the Pt−Co catalyst in the cathode of the MEA.

ACKNOWLEDGMENTS This work was supported by the NEDO program, KAKENHI Kiban (B) (26288005 and 18H01940), the Kao Foundation for Arts and Sciences, and the R-ing (Reaction Infography) World Research Unit (B-1) at Nagoya University and was partly supported by PRESTO and the “Materials Research by Information Integration” initiative (MI2I) project of the Support Program for Starting Up Innovation Hub, by the Japan Science and Technology Agency (JST). Operando XANES−CT measurements were taken at SPring-8 (nos. 2016A7821, 2016B7821, 2017A7820, 2017A7821, and 2018A7820). We thank A. Kodaira (Nagoya University) and Dr. S. Nakao (Institute for Molecular Science) for TEM and SEM measurements. TEM data were collected at the High Voltage Electron Microscope Laboratory, Institute of Materials and Systems for Sustainability, Nagoya University, supported by Advanced Characterization Nanotechnology Platform of the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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CONCLUSIONS The operando 3D XANES−CT imaging of the MEA under PEFC operating conditions and its unsupervised data mining successfully visualized the degradation of the Pt−Co/C catalyst under practical conditions in the MEA for the first time. The CT reconstruction of the structural parameters extracted from the Pt LIII-edge and Co K-edge XANES spectra provided clear 3D images of the locations and valence states of the metals of the cathode catalyst. The different manners by which Pt and Co degraded resulted in heterogeneous degradation of the cathode catalyst in the MEA, highly dependent on the structure (crack) of the carbon support. The infographic approach combining the 3D chemical imaging and unsupervised learning appears to be a promising way to reveal intrinsic events of practical materials and devices.

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REFERENCES

(1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining theory and experiment in electrocatalysis: Insights into materials design. Science 2017, 355, No. eaad4998. (2) Stephens, I. E. L.; Rossmeisl, J.; Chorkendorff, I. Toward sustainable fuel cells. Science 2016, 354, 1378−1379. (3) Debe, M. K. Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012, 486, 43−51. (4) Xia, W.; Mahmood, A.; Liang, Z.; Zou, R.; Guo, S. Earthabundant nanomaterials for oxygen reduction. Angew. Chem., Int. Ed. 2016, 55, 2650−2676. (5) Jiang, K.; Wang, P.; Guo, S.; Zhang, X.; Shen, X.; Lu, G.; Su, D.; Huang, X. Ordered PdCu-based nanoparticles as bifunctional oxygenreduction and ethanol-oxidation electrocatalysts. Angew. Chem., Int. Ed. 2016, 55, 9030−9035. (6) Bu, L.; Zhang, N.; Guo, S.; Zhang, X.; Li, J.; Yao, J.; Wu, T.; Lu, G.; Ma, J.-Y.; Su, D.; Huang, X. Biaxially strained PtPb/Pt core/shell nanoplate boosts oxygen reduction catalysis. Science 2016, 354, 1410− 1414. (7) Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C.-Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B. V.; Lin, Z.; et al. Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction. Science 2016, 354, 1414−1419. (8) Jiang, K.; Zhao, D.; Guo, S.; Zhang, X.; Zhu, X.; Guo, J.; Lu, G.; Huang, X. Efficient oxygen reduction catalysis by subnanometer Pt alloy nanowires. Sci. Adv. 2017, 3, No. e1601705. (9) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends in electrocatalysis on extended and nanoscale Pt-bimetallic alloy surfaces. Nat. Mater. 2007, 6, 241−247. (10) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of platinum and early transition metals as oxygen reduction electrocatalysts. Nat. Chem. 2009, 1, 552−556. (11) Saedy, S.; Palagin, D.; Safonova, O.; van Bokhoven, J. A.; Khodadadi, A. A.; Mortazavi, Y. Understanding the mechanism of synthesis of Pt3Co intermetallic nanoparticles via preferential chemical vapor deposition. J. Mater. Chem. A 2017, 5, 24396−24406. (12) Dai, S.; Hou, Y.; Onoue, M.; Zhang, S.; Gao, W.; Yan, X.; Graham, G. W.; Wu, R.; Pan, X. Revealing surface elemental composition and dynamic processes involved in facet-dependent oxidation of Pt3Co nanoparticles via in situ transmission electron microscopy. Nano Lett. 2017, 17, 4683−4688. (13) Koh, S.; Toney, M. F.; Strasser, P. Activity-stability relationships of ordered and disordered alloy phases of Pt3Co electrocatalysts

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b05005. Details of the PEFC operating conditions, XANES−CT cell, SEM images, XANES−CT measurement sequence, XANES spectra, CV curves, and data mining results (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.C.D.). *E-mail: [email protected] (M.T.). ORCID

Yuanyuan Tan: 0000-0001-5307-2938 Hirosuke Matsui: 0000-0001-5483-3487 Nozomu Ishiguro: 0000-0003-3330-6236 Author Contributions

The manuscript was written as a result of contributions from all of the authors. All of the authors have given approval to the final version of the manuscript. Funding

This work was supported by the NEDO program, KAKENHI Kiban (B) (26288005 and 18H01940), and the Kao Foundation for Arts and Sciences. This work was partly supported by PRESTO and the “Materials Research by Information Integration” initiative (MI2I) project of the Support Program for Starting Up Innovation Hub, by the Japan Science and Technology Agency (JST). Notes

The authors declare no competing financial interest. H


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The Journal of Physical Chemistry C for the oxygen reduction reaction (ORR). Electrochim. Acta 2007, 52, 2765−2774. (14) Dubau, L.; Maillard, F.; Chatenet, M.; Guetaz, L.; André, J.; Rossinot, E. Durability of Pt3Co/C cathodes in a 16 cell PEMFC stack: macro/microstructural changes and degradation mechanisms. J. Electrochem. Soc. 2010, 157, B1887−B1895. (15) Paulus, U. A.; Wokaun, A.; Scherer, G. G.; Schmidt, T. J.; Stamenkovic, V.; Radmilovic, V.; Markovic, N. M.; Ross, P. N. Oxygen Reduction on Carbon-Supported Pt−Ni and Pt−Co Alloy Catalysts. J. Phys. Chem. B 2002, 106, 4181−4191. (16) Colón-Mercado, H. R.; Popov, B. N. Stability of platinum based alloy cathode catalysts in PEM fuel cells. J. Power Sources 2006, 155, 253−263. (17) Dai, S.; You, Y.; Zhang, S.; Cai, W.; Xu, M.; Xie, L.; Wu, R.; Graham, G. W.; Pan, X. In situ atomic-scale observation of oxygendriven core-shell formation in Pt3Co nanoparticles. Nat. Commun. 2017, 8, 204. (18) Anderson, A. B.; Roques, J.; Mukerjee, S.; Murthi, V. S.; Markovic, N. M.; Stamenkovic, V. Activation Energies for Oxygen Reduction on Platinum Alloys: Theory and Experiment. J. Phys. Chem. B 2005, 109, 1198−1203. (19) Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Nørskov, J. K. Changing the activity of electrocatalysts for oxygen reduction by tuning the surface electronic structure. Angew. Chem., Int. Ed. 2006, 45, 2897− 2901. (20) Zhao, X.; Hayashi, A.; Noda, Z.; Kimijima, K. i.; Yagi, I.; Sasaki, K. Evaluation of change in nanostructure through the heat treatment of carbon materials and their durability for the start/stop operation of polymer electrolyte fuel cells. Electrochim. Acta 2013, 97, 33−41. (21) Takao, S.; Sekizawa, O.; Samjeské, G.; Nagamatsu, S.-i.; Kaneko, T.; Yamamoto, T.; Higashi, K.; Nagasawa, K.; Uruga, T.; Iwasawa, Y. Same-View Nano-XAFS/STEM-EDS imagings of Pt chemical species in Pt/C cathode catalyst layers of a polymer electrolyte fuel cell. J. Phys. Chem. Lett. 2015, 6, 2121−2126. (22) Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D. Structurally ordered intermetallic platinum-cobalt core-shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts. Nat. Mater. 2013, 12, 81−87. (23) Hanawa, H.; Kunimatsu, K.; Watanabe, M.; Uchida, H. In situ ATR-FTIR analysis of the structure of Nafion-Pt/C and NafionPt3Co/C interfaces in fuel cell. J. Phys. Chem. C 2012, 116, 21401− 21406. (24) Takagi, Y.; Nakamura, T.; Yu, L.; Chaveanghong, S.; Sekizawa, O.; Sakata, T.; Uruga, T.; Tada, M.; Iwasawa, Y.; Yokoyama, T. X-ray photoelectron spectroscopy under real ambient pressure conditions. Appl. Phys. Express 2017, 10, 076603. (25) Cui, Y.; Harada, Y.; Ikenaga, E.; Li, R.; Nakamura, N.; Hatanaka, T.; Ando, M.; Yoshida, T.; Li, G.-L.; Oshima, M. In situ hard X-ray photoelectron study of O2 and H2O adsorption on Pt nanoparticles. J. Phys. Chem. C 2016, 120, 10936−10940. (26) Kaneko, T.; Samjeské, G.; Nagamatsu, S.-i.; Higashi, K.; Sekizawa, O.; Takao, S.; Yamamoto, T.; Zhao, X.; Sakata, T.; Uruga, T.; Iwasawa, Y. Key structural kinetics for carbon effects on the performance and durability of Pt/carbon cathode catalysts in polymer electrolyte fuel cells characterized by in situ time-resolved X-ray absorption fine structure. J. Phys. Chem. C 2016, 120, 24250−24264. (27) Wanjala, B. N.; Fang, B.; Luo, J.; Chen, Y.; Yin, J.; Engelhard, M. H.; Loukrakpam, R.; Zhong, C.-J. Correlation between atomic coordination structure and enhanced electrocatalytic activity for trimetallic alloy catalysts. J. Am. Chem. Soc. 2011, 133, 12714−12727. (28) Chen, S.; Sheng, W.; Yabuuchi, N.; Ferreira, P. J.; Allard, L. F.; Shao-Horn, Y. Origin of oxygen reduction reaction activity on “Pt3Co” nanoparticles: atomically resolved chemical compositions and structures. J. Phys. Chem. C 2009, 113, 1109−1125. (29) Stamenković, V.; Schmidt, T. J.; Ross, P. N.; Marković, N. M. Surface Composition Effects in Electrocatalysis:Kinetics of oxygen

reduction on well-defined Pt3Ni and Pt3Co alloy surfaces. J. Phys. Chem. B 2002, 106, 11970−11979. (30) Hwang, S. J.; Kim, S.-K.; Lee, J.-G.; Lee, S.-C.; Jang, J. H.; Kim, P.; Lim, T.-H.; Sung, Y.-E.; Yoo, S. J. Role of electronic perturbation in stability and activity of Pt-based alloy nanocatalysts for oxygen reduction. J. Am. Chem. Soc. 2012, 134, 19508−19511. (31) Ishiguro, N.; Kityakarn, S.; Sekizawa, O.; Uruga, T.; Matsui, H.; Taguchi, M.; Nagasawa, K.; Yokoyama, T.; Tada, M. Kinetics and mechanism of redox processes of Pt/C and Pt3Co/C cathode electrocatalysts in a polymer electrolyte fuel cell during an accelerated durability test. J. Phys. Chem. C 2016, 120, 19642−19651. (32) Nikkuni, F. R.; Dubau, L.; Ticianelli, E. A.; Chatenet, M. Accelerated degradation of Pt3Co/C and Pt/C electrocatalysts studied by identical-location transmission electron microscopy in polymer electrolyte environment. Appl. Catal., B 2015, 176−177, 486−499. (33) Matsui, H.; Ishiguro, N.; Uruga, T.; Sekizawa, O.; Higashi, K.; Maejima, N.; Tada, M. Operando 3D visualization of migration and degradation of a platinum cathode catalyst in a polymer electrolyte fuel cell. Angew. Chem., Int. Ed. 2017, 56, 9371−9375. (34) Grunwaldt, J.-D.; Wagner, J. B.; Dunin-Borkowski, R. E. Imaging Catalysts at Work: A Hierarchical Approach from the Macroto the Meso- and Nano-scale. ChemCatChem 2013, 5, 62−80. (35) Ebner, M.; Marone, F.; Stampanoni, M.; Wood, V. Visualization and quantification of electrochemical and mechanical degradation in Li ion batteries. Science 2013, 342, 716−720. (36) Meirer, F.; Morris, D. T.; Kalirai, S.; Liu, Y.; Andrews, J. C.; Weckhuysen, B. M. Mapping metals incorporation of a whole single catalyst particle using element specific X-ray nanotomography. J. Am. Chem. Soc. 2015, 137, 102−105. (37) Liu, Y.; Meirer, F.; Krest, C. M.; Web, S.; Weckhuysen, B. M. Relating structure and composition with accessibility of a single catalyst particle using correlative 3-dimensional micro-spectroscopy. Nat. Commun. 2016, 7, 12634. (38) Saida, T.; Sekizawa, O.; Ishiguro, N.; Hoshino, M.; Uesugi, K.; Uruga, T.; Ohkoshi, S.-i.; Yokoyama, T.; Tada, M. 4D visualization of a cathode catalyst layer in a polymer electrolyte fuel cell by 3D laminography-XAFS. Angew. Chem., Int. Ed. 2012, 51, 10311−10314. (39) Wise, A. M.; Weker, J. N.; Kalirai, S.; Farmand, M.; Shapiro, D. A.; Meirer, F.; Weckhuysen, B. M. Nanoscale chemical imaging of an individual catalyst particle with soft X-ray ptychography. ACS Catal. 2016, 6, 2178−2181. (40) Ihli, J.; Jacob, R. R.; Holler, M.; Guizar-Sicairos, M.; Diaz, A.; da Silva, J. C.; Sanchez, D. F.; Krumeich, F.; Grolimund, D.; Taddei, M.; et al. A three-dimensional view of structural changes caused by deactivation of fluid catalytic cracking catalysts. Nat. Commun. 2017, 8, 809. (41) Hirose, M.; Ishiguro, N.; Shimomura, K.; Burdet, N.; Matsui, H.; Tada, M.; Takahashi, Y. Visualization of heterogeneous oxygen storage behavior in platinum-supported cerium-zirconium oxide three-way catalyst particles by hard X-ray spectro-ptychography. Angew. Chem., Int. Ed. 2018, 57, 1474−1479. (42) Wu, J.; Zhu, X.; West, M. M.; Tyliszczak, T.; Shiu, H.-W.; Shapiro, D.; Berejnov, V.; Susac, D.; Stumper, J.; Hitchcock, A. P. High-resolution imaging of polymer electrolyte membrane fuel cell cathode layers by soft X-ray spectro-ptychography. J. Phys. Chem. C 2018, 122, 11709−11719. (43) Hirose, M.; Ishiguro, N.; Shimomura, K.; Nguyen, D.-N.; Matsui, H.; Dam, H.-C.; Tada, M.; Takahashi, Y. Oxygen-diffusiondriven oxidation behavior and tracking areas visualized by X-ray spectro-ptychography with unsupervised learning. Commun. Chem. 2019, 2. DOI: 10.1038/s42004-019-0147-y (44) Karim, W.; Spreafico, C.; Kleibert, A.; Gobrecht, J.; VandeVondele, J.; Ekinci, Y.; van Bokhoven, J. A. Catalyst support effects on hydrogen spillover. Nature 2017, 541, 68−71. (45) Wu, J.; Melo, L. G. A.; Zhu, X.; West, M. M.; Berejnov, V.; Susac, D.; Stumper, J.; Hitchcock, A. P. 4D imaging of polymer electrolyte membrane fuel cell catalyst layers by soft X-ray spectrotomography. J. Power Sources 2018, 381, 72−83. I


Article

The Journal of Physical Chemistry C (46) Matsui, H.; Maejima, N.; Ishiguro, N.; Tan, Y.; Uruga, T.; Sekizawa, O.; Sakata, T.; Tada, M. Operando XAFS Imaging of Distribution of Pt Cathode Catalysts in PEFC MEA. Chem. Rec. 2018, 18, 1−14. (47) Shepp, L. A.; Vardi, Y. Maximum likelihood reconstruction for emission tomography. IEEE Trans. Med. Imaging 1982, 1, 113−122. (48) Lange, K.; Carson, R. J. EM reconstruction algorithms for emission and transmission tomography. J. Comput. Assist. Tomogr. 1984, 8, 306−316. (49) Hudson, H. M.; Larkin, R. S. Accelerated image reconstruction using ordered subsets of projection data. IEEE Trans. Med. Imaging 1994, 13, 601−609. (50) Ghahramani, Z. Unsupervised Learning. In Advanced Lectures on Machine Learning; Bousquet, O.; von Luxburg, U.; Ratsch, G., Eds.; Springer Berlin/Heidelberg, 2004; Vol. 3176, pp 72−112. (51) Kenvin, P. M. Machine Learning: a Probabilistic Perspective; MIT Press, 2012. (52) Schwarz, G. Estimating the dimension of a model. Ann. Stat. 1978, 6, 461−464. (53) Lei, Y.; Jelic, J.; Nitsche, L. C.; Meyer, R.; Miller, J. Effect of particle size and adsorbates on the L3, L2 and L1 X-ray absorption near edge structure of supported Pt nanoparticles. Top. Catal. 2011, 54, 334−348. (54) Schweitzer, N.; Xin, H.; Nikolla, E.; Miller, J. T.; Linic, S. Establishing relationships between the geometric structure and chemical reactivity of alloy catalysts based on their measured electronic structure. Top. Catal. 2010, 53, 348−356. (55) Takao, S.; Sekizawa, O.; Nagamatsu, S.-i.; Kaneko, T.; Yamamoto, T.; Samjeské, G.; Higashi, K.; Nagasawa, K.; Tsuji, T.; Suzuki, M.; et al. Mapping platinum species in polymer electrolyte fuel cells by spatially resolved XAFS techniques. Angew. Chem., Int. Ed. 2014, 53, 14110−14114. (56) van der Vliet, D. F.; Wang, C.; Li, D.; Paulikas, A. P.; Greeley, J.; Rankin, R. B.; Strmcnik, D.; Tripkovic, D.; Markovic, N. M.; Stamenkovic, V. R. Unique electrochemical adsorption properties of Pt-skin surfaces. Angew. Chem., Int. Ed. 2012, 51, 3139−3142. (57) Rasouli, S.; Ortiz Godoy, R. A.; Yang, Z.; Gummalla, M.; Ball, S. C.; Myers, D.; Ferreira, P. J. Surface area loss mechanisms of Pt3Co nanocatalysts in proton exchange membrane fuel cells. J. Power Sources 2017, 343, 571−579. (58) Ahluwalia, R. K.; Papadias, D. D.; Kariuki, N. N.; Peng, J.-K.; Wang, X.; Tsai, Y. F.; Graczyk, D. G.; Myers, D. J. Potential dependence of Pt and Co dissolution from platinum-cobalt alloy PEFC catalysts using time-resolved measurements. J. Electrochem. Soc. 2018, 165, F3024−F3035. (59) Escaño, M. C. S.; Kasai, H. First-principles study on surface structure, thickness and composition dependence of the stability of Pt-skin/Pt3Co oxygen-reduction-reaction catalysts. J. Power Sources 2014, 247, 562−571.

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C Cathode Catalyst Degradation in a Polymer Electrolyte Fuel

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