Visualization and understanding of the degradation behaviors of a PEFC Pt/C cathode electrocatalyst using a multi-analysis system combining time-resolved quick XAFS, three-dimensional XAFS-CT, and same-view nano-XAFS/STEM-EDS techniques†

Kotaro Higashi ^a, Shinobu Takao ^a, Gabor Samjeské ^ab, Hirosuke Matsui ^b, Mizuki Tada ^bc, Tomoya Uruga ^ad and Yasuhiro Iwasawa *^ae
aInnovation Research Center for Fuel Cells, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan. E-mail: iwasawa@pc.uec.ac.jp; Tel: +81-42-443-5921
^bDepartment of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya, Aichi 464-8602, Japan
^cResearch Center for Materials Science, Nagoya University, Chikusa, Nagoya, Aichi 464-8602, Japan
^dJASRI/SPring-8, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
^eGraduate School of Informatics and Engineering, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan

First published on 28th May 2020

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Introduction

The performance of polymer electrolyte fuel cells (PEFCs), which can realize clean and highly efficient power generation at low temperatures, is being promoted towards full-scale commercialization as an energy source for automobiles.^1–10 The major challenges in improving the performance of PEFCs are lowering the cost by improving the activity of the Pt electrode catalyst in the oxygen reduction reaction (ORR) of a fuel cell Pt/C cathode,^11–24 particularly improving long-term durability,^25,26 and reducing the amount of Pt.^27–30 However, many issues such as active/deactivated structures and states, their spatial distributions and degradation factors have not been elucidated at the atomic/molecular level, and the time-space information of each factor and state is poor, which hinders the development and rational design of a highly efficient and durable fuel cell electrode catalyst. To elucidate the key factors and mechanisms for ORR promotion and degradation suppression, analytical studies using various methods have been performed.^31–35 Among these analytical methods, the measurement method using high-intensity synchrotron radiation hard X-rays utilizes the excellent transmission performance of hard X-rays. This enables the nondestructive in situ/operando measurements of the membrane-electrode assembly (MEA), which is the power-generating site in actual PEFCs. As such, it is one of the most powerful analytical tools for time-resolved (real-time) and spatially-resolved (imaging) studies on PEFC degradation.^36–39

We built a BL36XU beamline at SPring-8 dedicated to the analysis of practical PEFCs^39,40 and constructed in situ/operando analytical methods and measurement systems at the BL36XU level for understanding the degradation phenomenon and mechanism of multi-layered PEFCs composed of spatially inhomogeneous and multiple constructional elements using time-resolved quick X-ray absorption fine structure (QXAFS) analysis,^41–50 a same-time measurement technique of time-resolved QXAFS and X-ray diffraction (XRD),^51,52 three-dimensional (3D) XAFS-computed tomography (XAFS-CT) imaging,^38,53–55 a same-view combination technique of two-dimensional (2D) nano-XAFS and scanning transmission electron microscopy/energy dispersive spectroscopy (STEM/EDS),^56–59 ambient pressure hard X-ray photoemission spectroscopy (AP-HAXPES),^60–62 high-energy resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy, and so on.^39,40

Until now, we have been analyzing the reaction and degradation processes of the electrocatalyst in PEFCs using different MEA samples using the above-mentioned measurement methods. However, the PEFC reaction/degradation processes proceed spatially and non-uniformly, and there are individual differences between the MEA samples. In addition, there are electrochemical performance differences derived from PEFC XAFS cells. Thus, the results obtained from the independent measurements made at varying beam times using the respective measurement methods are not always consistent and do not often lead to clear conclusions. Therefore, measurements are performed simultaneously or for the same time-series using multiple analysis methods at the same beamline for the same region in the same sample. Then, by performing an integrated evaluation from the analysis of the experimental data, highly detailed and reliable analytical information is obtained, which is important. To achieve this, we developed a time-resolved QXAFS/3D XAFS-CT/2D nano XAFS/STEM-EDS multi-measurement system with the same field of view and time series at the BL36XU beamline. In this study, the multi-measurement system was applied for the first time to visualize and elucidate the degradation place and mechanism of PEFCs.

Among the various degradation processes, the serious performance degradation due to the shut-down/start-up operation of PEFCs is one of the major issues that must be solved. Various studies on MEA degradation by the shut-down/start-up cycles have been performed and reviewed by Yu et al.³⁴ The anode-gas exchange (AGEX) cycles (start-up/shut-down simulation) bring about serious irreversible degradation of both Pt nanoparticles and carbon support. The MEA degradation by the shut-down/start-up cycles is mainly due to a spike-like large voltage increase to about 1.4–1.6 V_RHE (V vs. RHE),^63–65 whose spike-like voltage increase is explained by the Reverse-Current Decay Mechanism.⁶⁶ Our studies to date have shown that the AGEX treatment (start-up/shut-down simulation) causes the transformation of initial active Pt species to less active and inactive Pt species as evidenced from the voltage-transient response kinetics by time-resolved QXAFS^48,63 and the corrosion of the carbon support and the associated Pt²⁺ elution and shedding of Pt as visualized by nano-XAFS/STEM-EDS same-view imaging.^57,58 Moreover, the catalyst degradation due to the AGEX cycles is severe near the anode gas inlet in the MEA and occurs non-uniformly in the direction of thickness in the cathode catalyst layer.⁴⁸

In this study, we evaluated the activity of the Pt/C catalyst against the voltage transient response (evaluation of the rate constant of the elementary reaction process) by in situ time-resolved QXAFS measurements at each stage of degradation during the AGEX cycles. In situ 3D XAFS-CT imaging of the observation region with a 2 μm spatial resolution revealed the 3D spatial distribution of the catalytically active Pt species in the cathode catalyst layer. The XAFS-CT technique has the advantage that an in situ observation of a large area (several hundred μm²) is possible. However, due to the lack of the measurement angle region, the highly spatially-resolved imaging of Pt in the direction of MEA film thickness is difficult. Therefore, we also measured and evaluated the same visual field by the 2D nano-XAFS with a spatial XAFS resolution of 100 nm scale and STEM-EDS with a sub-nanometer resolution for the micrometer-sized MEA fragment sample that was cut out from the XAFS-CT measurement region of the AGEX-degraded MEA. This article elucidates the AGEX degradation process of the Pt/C cathode catalyst layer of an MEA based on the results of the in situ/ex situ simultaneous multi-series measurements in the same visual field.

Experimental methods

MEA

Pt/C (TKK, TEC10E50E) was used as a cathode catalyst for the PEFC MEA (0.6 mg-Pt cm⁻²; Pt: 46.1 wt%). Ru/C (TKK, TECRu(ONLY)E50; Ru: 30 wt%; 0.6 mg-Ru cm⁻²) was used as an anode catalyst to avoid interference with the XAFS measurements of the Pt/C cathode. MEAs (3 × 3.3 cm²) used in this study were provided by EIWA FC Development Co., Ltd and were mounted into a homemade PEFC single cell for SR X-ray-based multi-measurements (Fig. S1, ESI†).³⁹

AGEX treatment and electrochemical measurements

The electrochemical conditioning of the PEFC and electrochemical measurements such as cyclic voltammetry and current–voltage (I–V) measurements are described in the caption of Fig. S2 (ESI†). Electrochemical parameters describing the MEA Pt/C performance, such as electrochemical active surface area (ECSA), maximum power density, mass activity (MA), and surface specific activity (SA), before and after the AGEX cycles simulating the start-up/shut-down conditions of FC vehicles were measured using a PEFC setup, as shown in Fig. S1 (ESI†).^48,63 The valve allows a continuous flow of both H₂ gas and air, and a short switching time in the millisecond time range was achieved by a four-way solenoid valve (Swagelok). The cathode was connected as a working electrode, and the anode served as a combined counter electrode.

During the in situ SR-based X-ray experiments, the gas flows of H₂ (99.99999%; 150 sccm) for the anode and N₂ (99.99995%; 300 sccm) or air (atmospheric composition; 900 sccm) for the cathode were regulated by mass-flow controllers and the gases were bubbled through humidifiers at 351 K. The humidified gases were supplied to a PEFC heated at 353 K, which resulted in ∼93% relative humidity (RH). High-pressure gases with grade 1 purity were purchased from Taiyo Nippon Sanso Corp. The PEFC MEA was conditioned by 150 conditioning (aging) cycles with a sequence of stepwise galvanostatic current steps every 6 s from the open circuit voltage (OCV) to a potential of approximately 0.3 V_RHE (V vs. RHE) under H₂ (anode) and air (cathode) operating atmospheres.

An in situ time-resolved QXAFS/XAFS-CT imaging multi-measurement system

Fig. 1 The equipment layout of the in situ time-resolved QXAFS/XAFS-CT imaging multi-measurement system installed at the BL36XU beamline of SPring-8. The PEFC cell is mounted on a precision rotary stage and can be rotated precisely in an angle range of −80° to 80°, allowing the reaction gas to flow through a spiral tube.^48,54 The X-ray transmission window of the PEFC cell is located slightly closer to the gas inlet than the middle position of the MEA (Fig. S1, ESI†). The transmission method, QXAFS and 3D XAFS-CT imaging were performed using an ion chamber and a 2D X-ray detector. By moving the 2D X-ray detector in the direction perpendicular to the incident X-ray using an automatic translation stage, both measurements can be switched automatically. QXAFS and XAFS-CT measurements were performed repeatedly on the same sample in the same observation area for the aged MEA sample and the degraded MEA sample after 100, 200, and 300 AGEX cycles.

In situ time-resolved QXAFS measurements under transient response conditions and data analysis

A series of in situ time-resolved QXAFS spectra at the Pt L_III-edge for Pt/C in the PEFC MEA under transient potential operations were recorded in the transmission mode using two ion chambers (I₀: Ar 15%/N₂ 85%; I_t: N₂ 100%) for incident and transmitted X-rays, respectively. The energy of the X-rays from a tapered undulator light source was quickly scanned using a Si(111) direct servomotor-driven compact channel-cut crystal monochromator while measuring the current/charge of the PEFC during the transient response processes.³⁹ The cell voltage was changed from the OCV to 0.4 V_RHE and maintained for 40 s, and then increased from 0.4 V_RHE to 1.0 V_RHE (at time = zero, for the transient voltage-up process) and maintained for 40 s. Then, the process was reversed, that is, the cell voltage was decreased from 1.0 V_RHE to 0.4 V_RHE (at time = zero, for the transient voltage-down process). The transient responses of the MEA cathode electrocatalyst under the voltage cycling operations were measured by using QXAFS analysis at a time resolution of 100 ms for 30 s from 10 s before each voltage jump. The series of the obtained QXAFS spectra was analyzed in a similar way to that mentioned in previous reports^45,47,48 using the Larch code containing the IFEFFIT package ver.2 (Athena and Artemis).^67–69 The white line peaks of the normalized QXANES spectra were analyzed by a linear combination of a Lorentzian function and an arctangent function. The error ranges of the QXANES curve fitting were estimated as 95% confidence intervals.

The white line peak intensities in the Pt L_III-edge QXANES spectra were plotted against the response time in the transient-response processes, 0.4 → 1.0 V_RHE and 1.0 → 0.4 V_RHE. The rate constants for the white line peak intensity changes were determined by data fitting using a single exponential function or a linear combination of two exponential functions for the QXAFS analysis data for 0–15 s after the voltage jump (0.4 → 1.0 V_RHE and 1.0 → 0.4 V_RHE).^44,47,48

Background subtraction in the QEXAFS analysis was performed using Autobk. The extracted k²-weighted QEXAFS oscillations were Fourier-transformed to R-space over k = 30–120 nm⁻¹, and the curve fittings were performed in the R-space (0.14–0.30 nm), which covers Pt–O and Pt–Pt contributions. We conducted the curve-fitting analysis systematically in the identical k and R ranges using the all time-resolved QXAFS data. The fitting parameters of each shell were coordination number (CN), interatomic distance (R), correction-of-edge energy (ΔE₀) (ΔE₀(Pt–Pt) = ΔE₀(Pt–O)), and Debye–Waller factors (σ²) for Pt–Pt and Pt–O. The Debye–Waller factors (σ²) for Pt–Pt and Pt–O determined by the curve-fitting processes for the series of QEXAFS data were averaged and fixed as 0.007 and 0.004 Ų, respectively, to obtain convincing data for the series of AGEX experiments. The phase shifts and amplitude functions for Pt–Pt and Pt–O were calculated from the FEFF 8.4 code using structural parameters obtained from the crystal structures of Pt and PtO. The amplitude reduction factor (S₀²) for the Pt–Pt bonds in this study was estimated to be 0.836 by analyzing a Pt foil. The error ranges of the curve-fitting analysis of QEXAFS Fourier transforms were based on the definition of the Larch code. The quality of the observed 100 ms time-resolved QEXAFS data was good enough to achieve a curve-fitting analysis similar to that reported previously.^47,48,50

In situ 3D XAFS-CT imaging measurements and data analysis

In situ 3D XAFS imaging was performed using the full-field XAFS-CT method at the PEFC cell potentials of 0.4 V_RHE and 1.0 V_RHE.^38,54 At each PEFC rotation angle, 2D Pt L_III edge QXAFS imaging was performed by energy scanning the Si(111) channel-cut monochromator using a 2D X-ray image detector (Fig. 1). The measurement was performed every 1° in the PEFC rotation angle range of −80° to 80°. The background, Pt-edge jump quantity, and Pt XANES white line peak intensity were calculated by fitting the XANES spectrum of each pixel of the 2D projected XAFS image obtained with a straight line, and a linear combination of an Arctangent function and a Lorentzian function. By reconstructing these three-dimensional images, the 3D spatial distribution of the absorption amount, Pt amount, and Pt valence of the entire sample was obtained.^38,54 The spatial resolution of the reconstructed image was 2 μm and the reconstructed area was 670 μm × 670 μm.

Ex situ same-view measurements of 2D nano-XAFS/STEM-EDS

The sample for the same visual field nano-XAFS/STEM-EDS measurement was obtained by taking out the MEA from the PEFC after subjecting it to 300 AGEX cycles in a glove bag filled with N₂ gas at ambient pressure saturated with water vapor and preparing a sliced MEA sample that was then sealed in a self-fabricated SiN membrane cell.^56–59

The Pt L_III-edge nano-XAFS spectra were recorded in the fluorescence mode using a 25-element Ge detector by a scanning focused X-ray beam (167 nm × 156 nm) via Kirkpatrick-Baez (KB) mirrors. The STEM-EDS and TEM images were obtained on a JEM-2100F system equipped with an energy-dispersive spectrometer at 200 kV. We used a 0.7 nm electron beam for STEM-EDS observation. STEM-EDS analysis was conducted on the same area of the sliced MEA sample as that used for the nano-XAFS analysis. The sample temperature was controlled at 300.5 K using a cryo-holder.

Results and discussion

Electrochemical performances of MEA Pt/C cathode electrocatalysts after AGEX cycles

Fig. S2 (ESI†) shows the CVs and I–V curves as well as the changes in the electrochemical parameters (ECSA, MA, and SA). After aging, the ECSA decreased with an increase in the number of AGEX cycles and decreased by approximately 70% after 300 AGEX cycles. In addition, the MA decreased unambiguously to approximately 70% after 300 AGEX cycles. As a result, the SA reduced only a little after 300 AGEX cycles.

In situ time-resolved Pt L_III-edge QXAFS under transient voltage operations

Fig. 2 shows the temporal changes in the white line peak height (corresponding to Pt valence) obtained from the voltage transient response time-resolved in situ QXAFS measurements during the AGEX degradation process. Fig. S3 (ESI†) shows the time variation of the coordination number of the Pt–Pt bond (CN(Pt–Pt)) and the coordination number of the Pt–O bond (CN(Pt–O)) obtained from the in situ QEXAFS analysis. The change in the transient-response kinetics of the white line peak height became smaller on increasing the AGEX cycles, which corresponds to the degradation of the Pt/C cathode catalyst. The temporal changes in the white line peak height were approximated by fitting with a two-component exponential function (yellow lines in Fig. 2), and the corresponding rate constants, k₁ and k₂, for active and less active Pt nanoparticles, respectively, were obtained as listed in Table 1. The activity of the less active Pt species was only 5.7% of that of the active Pt species. The amounts of active and less active Pt nanoparticles (A₁ and A₂) and their ratios were also calculated (Table 1). In addition, inactive Pt nanoparticles (A₃), which did not show a transient response, were formed by the AGEX operation (Table 1). Fig. 3 shows the change in the ratio of each component (A₁, A₂ and A₃) and the change in ECSA due to AGEX degradation. The AGEX cycles greatly increased the proportion of inactive Pt species. The presence of three differently reactive Pt species and their behaviors due to the AGEX cycles are consistent with our previous findings.⁴⁸ As shown in Fig. 3, the ratio of the active species (A₁) obtained from QXAFS is notably reduced compared with the ECSA. Moreover, even the ratio (A₁ + A₂) including the less active Pt species is smaller than the ECSA. This difference is because the ECSA is the average of the electrochemically active surface area of the Pt catalyst in the entire MEA, whereas QXAFS measures the activity of the Pt catalyst in the X-ray irradiation area in the MEA. In our previous report,⁴⁸ the in situ time-resolved QXAFS measurements of a PEFC with three X-ray irradiation windows showed that the closer the X-ray irradiation area to the gas inlet, the greater the degradation due to the generation of inactive Pt species. In this measurement, since the X-ray irradiation area is provided near the gas inlet (Fig. S1, ESI†), the ratio of the degradation species obtained from QXAFS is higher than the ECSA degradation ratio. However, regardless of the extent of the problem, the decrease in the active Pt species by time-resolved QXAFS and the decrease in the sum of the fractions of the active Pt species and less-active Pt species were greater than the decrease in the electrochemical measurement even when the X-ray irradiation area was closer to the middle position of the MEA or closer to the outlet. Therefore, it has been pointed out that the MEA Pt/C cathode catalyst layer in the traditionally designed PEFC stack structure may not electrochemically show the intrinsic catalytic activity of the original Pt nanoparticles on carbon.

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In situ three-dimensional (3D) XAFS-CT imaging

Fig. 4 shows the 3D images of the distribution of Pt amount and Pt valence in the aged MEA Pt/C cathode catalyst layer before the AGEX degradation treatment, as obtained by 3D XAFS-CT measurements. The actual thickness of the catalyst layer is approximately 12 μm, while in the CT reconstruction image, it increased to approximately 60 μm. This is because the XAFS-CT measurement area is limited to the range of −80° to 80°, and data loss occurs in the 80° to 90° area. Therefore, the spatial resolution in the direction of the film thickness particularly decreased, while the image elongated.⁵⁴ Fortunately, this does not affect the current discussion in this study.

Fig. 5 shows the cross-sectional images exhibiting the Pt amount distributed at each depth in the film thickness direction of the MEA Pt/C cathode catalyst layer before AGEX degradation and after 300 AGEX cycles at a cell voltage of 1.0 V_RHE, as obtained by 3D XAFS-CT measurements. The cross-sectional images after 100 and 200 AGEX cycles are shown in Fig. S4 (ESI†). As observed from the in situ 3D XAFS-CT imaging with a spatial resolution of 2 μm, there are almost no differences in the Pt distributions and morphologies, such as cracks, in the MEA film surface at any depth in the film thickness direction. On the other hand, in our previous 3D XAFS-CT measurement of the degraded MEA Pt/C samples after an accelerated durability test (ADT) based on the rectangular potential (0.6–1.0 V_RHE) cycle load, Pt elution and aggregation were observed in the Pt/C cathode catalyst. In addition, movement was observed both in the MEA plane and in the depth direction.^38,54 The difference between the two indicates that the AGEX degradation mechanism is different from the rectangular potential ADT degradation mechanism.

Fig. 6 shows the cross-sectional in situ 3D XAFS-CT images showing the Pt valence (Pt oxidation degree) distribution at each depth of the cathode catalyst layer at cell voltages of 0.4 V_RHE (reducing state) and 1.0 V_RHE (oxidative state) before AGEX degradation and after 300 AGEX cycles. Fig. S5 (ESI†) shows the results at 0.4 V_RHE and 1.0 V_RHE after 100 and 200 AGEX cycles. The areas with a higher Pt valence (degree of oxidation) when the voltage is increased from 0.4 to 1.0 V_RHE correspond to the areas with a higher activity of the Pt catalyst particles. With degradation due to the AGEX treatment, the amount of change in Pt valence from 0.4 to 1.0 V_RHE decreased, which indicates that the amount of distributed active Pt particles decreased. Fig. 7 shows the average amount of the change in Pt valence for each thickness layer due to the AGEX degradation, expressed as a normalized ratio using the value before AGEX degradation. After 100 AGEX cycles, the amount of change in Pt valence decreased both around the GDL boundary of the catalyst layer and around the electrolyte membrane boundary. On the other hand, beyond 200 AGEX cycles, the dependence on the depth direction decreased and the amount of the change in Pt valence decreased over the entire cathode catalyst layer.

Fig. 8 shows the ECSA obtained from the electrochemical measurements as well as the changes in the amount of Pt valence weight-averaged by Pt amount due to AGEX degradation for the amount of the change in Pt valence obtained from the voltage transient responses in in situ time-resolved QXAFS, and X-ray irradiation area determined from the in situ 3D XAFS-CT measurement results. The results of ECSA, MA, QXAFS, and XAFS-CT also decreased monotonically with increasing AGEX degradation cycles. The amount of decrease increased in the order of ECSA ≈ MA < QXAFS < XAFS-CT. The difference between the electrochemical measurement and XAFS measurement results is possibly, as mentioned before for the results shown in Fig. 3, because the area close to the PEFC anode gas inlet underwent greater AGEX degradation than the entire MEA. In addition, in XAFS-CT, for the XANES spectrum measured at each detection pixel of the 2D X-ray image detector, the Pt valence change is calculated from the height of the white line at the Pt L_III absorption edge. However, with the detector pixels that measure the XANES of the area with a low Pt amount, the amount of change in the white line, and thus, the analysis accuracy decreases as AGEX degradation progresses.

Ex situ same-view nano-XAFS/STEM-EDS maps

Fig. S6 (ESI†) shows the site and size of an MEA section cut out for the same-view nano-XAFS/STEM-EDS mapping from the MEA of 680 μm × 680 μm size imaged by 3D XAFS-CT after 300 AGEX cycles (Fig. S6, ESI† is a cross-sectional Pt amount map). Fig. 9(a–c) show the STEM image of the MEA section near the cathode catalyst layer, and the Pt amount map and Pt valence map obtained by 2D nano-XAFS imaging at a nanometer scale of 167 nm × 156 nm. In addition, an enlarged view of the electrolyte membrane-side boundary area (E), the center of the cathode (M), and the GDL-side boundary area (G) are shown. Fig. 10 shows the size and number distribution of the voids in each of the E, M, and G areas obtained from the STEM image, and Table S1 (ESI†) shows the number, average size, and fraction of the voids. The STEM image (Fig. 9(a)) shows the presence of many voids of several hundred nanometers throughout the cathode catalyst layer. As shown in Table S1 (ESI†), the number, average size, and fraction of the voids in the boundary area (E) on the electrolyte membrane side are largest among the three areas (E, M, and G), and the averaged sizes and fractions of the voids in the center part of the catalyst layer (M) and the GDL-side boundary area (G) are similar. As shown in Fig. 9(a-E), in the boundary area of the electrolyte membrane side, in addition to many large voids, marked corrosion degradation occurred in the boundary layer, and the thickness of the cathode catalyst layer decreased by 3% as compared with that before AGEX degradation (∼400 nm). The Pt valence map obtained by the nano-XAFS analysis (Fig. 9(c-E)) shows that Pt existed as a metal and/or ions on the carbon support discretely present in the boundary layer. In this experiment, the corrosion and voids of the carbon support in the boundary area on the electrolyte membrane side are larger than those observed in our previous study.⁵⁷ This also corresponds to the large ratio of the inactive Pt catalyst obtained from the change in the white line peak intensity of QXAFS shown in Fig. 3. The difference in the experimental results is that it is difficult to perform ADT experiments that repeat the electrochemical reaction many times with high reproducibility in addition to the existence of individual differences between the MEA samples. This shows that the multi-analysis of the same observation area of the same sample with simultaneous electrochemical measurements is important for high-precision spatiotemporal analysis.

On the other hand, in the GDL boundary area (Fig. 9(a-G)) and in the center of the catalyst layer (Fig. 9(a-M)), small and medium-sized voids are formed, and many Pt nanoparticles are present in the voids (Fig. 9(a-V)). The Pt valence map obtained by nano-XAFS (Fig. 9(c-G)) shows that some Pt species inside the void exist as Pt ions in addition to Pt metal particles. The morphological change in the MEA Pt/C cathode catalyst layer and the change in the distribution of the amount and chemical state of Pt occurred at a scale of 1 μm or less. This was inadequate to capture changes with a spatial resolution of 2 μm of in situ 3D XAFS-CT. In this study, we were able, for the first time, to observe the nanometer-scale changes with the same MEA (an MEA section) as that in Fig. 4–6 by 2D resolution at a nanometer scale of 167 nm × 156 nm by the same-view nano-XAFS/STEM-EDS.

Few Pt aggregates are present among the distributed Pt particles in the catalyst layer after 300 AGEX cycles, as observed by nano-XAFS imaging (Fig. 9(b)); however, no notable heterogeneity was seen, and similar to the results of XAFS-CT imaging, the movement range of Pt elution, detachment and aggregation is small. On the other hand, a low-density, wide Pt band was formed in the electrolyte membrane (Fig. 9(a-B)), and the Pt amount in the Pt band was 5% of the Pt amount in the catalyst layer. The Pt in the corrosion-degraded portion (Fig. 9(a-E)) of the cathode catalyst layer near the boundary with the electrolyte membrane was considered to have moved.

Fig. S7 and Table S2 (ESI†) show the particle sizes of the Pt nanoparticles and carbon support in the cathode catalyst layer, as evaluated by TEM. The diameter of the Pt nanoparticles increased slightly from 2.9 ± 0.6 nm before AGEX degradation to 3.0 ± 0.3 nm after 300 AGEX cycles; 3.5 nm in the electrolyte boundary area, 3.0 nm in the center of the cathode layer, 2.9 nm in the GDL boundary area, and 3.3 nm in the spaces inside and around the voids. This showed that the elution and aggregation of Pt nanoparticles occurred locally in these areas. In addition, the particle size of the carbon carrier was 40–45 nm before AGEX degradation; however, after 300 AGEX cycles, it reduced to 32 nm only in the area around the voids. This showed that the carbon support underwent corrosion, which resulted in the formation of voids. The particle size of the carbon support changed insignificantly to 40–41 nm in the other areas.

Mechanism of AGEX degradation

In the case of a degraded MEA Pt/C sample obtained by square wave (0.6–1.0 V_RHE) ADT treatment, Pt²⁺ elution occurred over the entire cathode catalyst layer, which then moved to the electrolyte membrane and formed a Pt band. Moreover, Pt aggregation occurred in the cathode catalyst layer.^54,56,57 These issues occurred exceeding the 2 μm spatial scale and decreased the Pt catalyst performance.⁵⁴ On the other hand, as seen by in situ XAFS-CT imaging (Fig. 5 and Fig. S4, ESI†) and nano-XAFS/STEM-EDS imaging (Fig. 9), the AGEX treatment led to a sharp voltage change (voltage spikes) of approximately 1.4–1.6 V_RHE when the gas was switched.⁶³ Pt elution and dropout from the carbon support occurred due to carbon degradation; however, the resulting change in the 3D Pt distribution in the MEA is in the 1 μm scale or less.

According to our previous study using near-ambient pressure (NAP)-HAXPES,^60,62 the MEA has a large potential difference at the boundary between the cathode catalyst layer and the electrolyte membrane. At this cathode interface, the change in potential (spike) caused by AGEX is particularly large compared with that in the other sites, and significant corrosive degradation of the carbon support is suggested to occur (Fig. 9E). On the other hand, the carbon erosion (carbon gasification) effect by Pt nanoparticles that is triggered by the change in potential due to AGEX is considered to occur locally at the interface between the materials with different electrical conductivity (for example, the contact surface between the carbon support and the ionomer). The vicinity of the GDL boundary, where the O₂ concentration is high, was more susceptible to carbon erosion than the center of the cathode layer, which tended to have more voids (Fig. 9M and G). As a result, as seen from the 3D Pt oxidation degree distribution of the MEA Pt/C cathode by in situ XAFS-CT imaging after 100 AGEX cycles, the degradation of catalytic activity was likely to occur near both the boundary layers (Fig. 7). With increasing AGEX cycles, small and medium-sized voids gradually formed in the center part of the catalyst layer. Above 200 AGEX cycles, voids formed in the entire cathode catalyst layer, which caused degradation to progress.

Carbon gasification reactions at the MEA Pt/C cathode occurred to form CO and CO₂. Under the AGEX conditions, humidified air always flows at the cathode, where CO oxidation to CO₂ is promoted, resulting in a low or nonexistent level of CO concentration.^64,65 Thus, inhibition of Pt catalysis by CO (transformation of active Pt to inactive Pt by CO poisoning) can be regarded to be insignificant under the AGEX degradation conditions.

Atomically-resolved HAADF-STEM images revealed almost no significant change in the structure and exposed planes in Pt nanoparticles after 300 AGEX cycles as observed in our recent report.⁷⁰ However, the catalytic properties of Pt nanoparticles in the MEA Pt/C cathode electrocatalyst after AGEX cycles have been classified according to the rate constants (k₁ and k₂) and the amounts (A₁, A₂ and A₃) in Table 1 for initial active Pt nanoparticles, less active Pt nanoparticles (5.7% of the initial activity), and inactive Pt nanoparticles (no contribution to ORR) by means of the transient-response QXAFS (Fig. 2 and 3). The exact physical and chemical states of the less active Pt nanoparticles are not definitely characterized in the molecular level at the moment, but the TEM measurement results (Fig. 9 and Table S2, ESI†) revealed that Pt nanoparticles were present on the eroded carbon support in the area around the voids due to carbon corrosion by AGEX cycles. This type of Pt nanoparticles appears to have a weak interaction with the carbon support and is regarded as a precursor state of Pt detachment, which corresponds to the less active Pt species. The comprehensive data by the combination technique of the time-resolved QXAFS/3D XAFS-CT/2D nano-XAFS/STEM-EDS along with the electrochemical data suggest that inactive Pt nanoparticles are regarded as Pt species detached from the carbon support. The inactive Pt species may involve minor Pt²⁺ ions in large voids after 300 AGEX cycles. In Fig. 3, the ratio of the less-active Pt species hardly changed due to AGEX degradation, while the ratio of inactive Pt species increased. This is due to the relative rate constants of the transitions from active Pt species to less active Pt species, less active Pt species to inactive Pt species, and active Pt species directly to inactive Pt species, respectively, as explained before.⁴⁸ In addition, as the size of the voids increased with the progress of AGEX degradation, the ratio of the void area to the void peripheral area increased with an increase in the size of the void, resulting in an increase in the inactive Pt species in voids, which promoted the degradation of the MEA Pt/C cathode.

Conclusions

We developed an in situ time-resolved QXAFS/in situ 3D XAFS-CT same-view multi-measurement system for practical PEFCs. In addition, we developed a combination multi-measurement method combining the ex situ 2D nano-XAFS/STEM-EDS same-view measurement method for MEA slices, which were cut out from predetermined areas from the measurement area of the MEA sample after 3D XAFS-CT imaging. In this study, we applied the combination multi-measurement method to an MEA Pt/C cathode electrocatalyst in PEFC for the first time and achieved visualized evaluation for elucidating the regulation factor and mechanism of the anode-gas exchange (AGEX) degradation process. Among the various degradation processes, severe MEA degradation due to the AGEX cycles (start-up/shut-down operation) of PEFCs is one of the major issues that must be solved.

The same visual field measurement of the AGEX degradation process (simulation process of start-up/shut-down of FC vehicles) of the Pt/C cathode in the MEA in the time axis and space axis clarified the changes in the ratios of three types of Pt species in the Pt/C cathode catalyst nanoparticles at each stage of degradation, the distribution of 3D active Pt species, and the 3D morphology of the Pt/C cathode layer. Furthermore, the same-view measurement by 2D nano-XAFS/STEM-EDS was performed at a predetermined location of the 3D-imaged MEA under a water vapor-saturated N₂ atmosphere. Thus, the Pt amount distribution, Pt oxidation state distribution, Pt particle size distribution, and carbon morphology could be measured, and results complementary to in situ XAFS-CT imaging were obtained. The obtained data showed that the AGEX degradation phenomenon occurred by the mechanism described below. The change in the electrode potential to a spike-like high potential during AGEX generates nano-cracks and voids in the cathode catalyst layer. Then, the oxidation elution and dropping of Pt from the carbon into these voids occur, resulting in the formation of inactive Pt species. However, the contact of the ionomer or carbon with the Pt species on the corroded carbon around the voids becomes insufficient, which leads to its transformation to less active Pt species. In the initial stage of degradation, nano-cracks and voids are formed in the carbon support at the boundary area between the cathode catalyst layer and the electrolyte membrane, which is susceptible to voltage change, and they are slightly formed near the boundary area with the GDL that has a high O₂ concentration. With the progression of AGEX degradation, the degradation proceeds to the center of the cathode catalyst layer. Most of the generated voids are less than 1 μm, and the above-mentioned degradation Pt species do not move exceeding the 1 μm region. This was inadequate to capture the changes with a spatial resolution of 2 μm of in situ 3D XAFS-CT. We were able to observe the nanometer-scale changes with the same MEA (an MEA section) as that for the 3D XAFS-CT by 2D resolution at a nanometer scale of 167 nm × 156 nm by the same-view nano-XAFS/STEM-EDS.

This multi-measurement method is recognized as a powerful tool for elucidating the mechanism of reaction and degradation of various FC and EV batteries in various types of vehicles and for evaluating measurements through visualization during operations of many kinds of catalysts and functional materials.

Author contributions

The manuscript was written through contributions of all the authors. All authors have given approval to the final version of the manuscript.

Funding sources

Funding is received from the New Energy and Industrial Technology Development Organization (NEDO) of the Ministry of Economy, Trade, and Industry (METI), Japan.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

The XAFS measurements were performed with the approval of SPring-8 subject number 2017A7807, 2017A7840, 2017B7801, 2017B7802, 2017B7840, 2018A7801, 2018A7804, 2018A7840, 2018B7804, 2018B7840, 2019A7804, 2019A7840, 2019B7804, and 2019B7840. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of the Ministry of Economy, Trade, and Industry (METI), Japan.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S7 and Tables S1–S2 for in situ XAFS cell, AGEX setup, electrochemical data, transient response curves, cross-sectional XAFS-CT images of Pt amount and valence, a sliced MEA sample, histograms of Pt particle sizes, and statistics of carbon sizes in each area of MEA. See DOI: 10.1039/d0cp01356k

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