Potential-Dependent Restructuring and Hysteresis in the Structural and Electronic Transformations of Pt/C, Au(Core)-Pt(Shell)/C, and Pd(Core)-Pt(Shell)/C Cathode Catalysts in Polymer Electrolyte Fuel Cells Characterized by in Situ X-ray Absorption Fine Structure

Abstract

Potential-dependent transformations of surface structures, Pt oxidation states, and Pt−O bondings in Pt/C, Au(core)-Pt(shell)/C (denoted as Au@Pt/C), and Pd(core)-Pt(shell)/C (denoted as Pd@Pt/C) cathode catalysts in polymer electrolyte fuel cells (PEFCs) during the voltagestepping processes were characterized by in situ (operando) Xray absorption fine structure (XAFS). The active surface phase of the Au@Pt/C for oxygen reduction reaction (ORR) was suggested to be the Pt 3 Au alloy layer on Au core nanoparticles, while that of the Pd@Pt/C was the Pt atomic layer on Pd core nanoparticles. The surfaces of the Pt, Au@Pt and Pd@Pt nanoparticles were restructured and disordered at high potentials, which were induced by strong Pt−O bonds, resulting in hysteresis in the structural and electronic transformations in increasing and decreasing voltage operations. The potential-dependent restructuring, disordering, and hysteresis may be relevant to hindered Pt performance, Pt dissolution to the electrolyte, and degradation of the ORR activity.

<i>In</i>-<i>Situ</i> XAFS Characterization of PtPd Nanoparticles Synthesized by Galvanic Replacement

“…The results of the curve fitting (Table 5) For the PtPd nanoparticles after 0.6 h and 2 h, the double peak shows a slight asymmetry with a higher peak height at 2.2 Å, which occurs for small potentials and disappears progressively with increasing potential. This was also observed by other researchers working on PtPd nanoparticles [25]. For the PtPd nanoparticles at 8 h, no significant changes are present and the spectra can be reproduced with only one component.…”

“…Like that, the evolution of the relative concentrations ( Figure 6) was monitored: When the potential is decreasing, the concentration of metal is increasing, while, in contrast, the concentration of the oxide is increasing with increasing potential, which was already observed in former studies on Pt and PtPd nanoparticles [25]. Obviously, the PtPd NP after 8 hours, whose spectra are based only on one component, cannot be analyzed using this model.…”

K-Edge X-ray Absorption Fine Structure Analysis of Pt/Au Core–Shell Electrocatalyst: Evidence for Short Pt–Pt Distance

“…Hence, reliable curve-fitting analysis of the Pt and Au L III edges becomes difficult. 30,35,36 If Pt and Au K-edge XAFS analyses can be performed, we can obtain well-separated Pt and Au spectra (Pt and Au K-edges are located at 78.4 and 80.7 keV, respectively), although this had been considered to be impossible due to machine limitations and finite lifetime broadening. After Ce K-edge XAFS was shown to be possible, 37 the extension of K-edge XAFS measurements to higher energies has progressed.…”

Key Factors Affecting the Performance and Durability of Cathode Electrocatalysts in Polymer Electrolyte Fuel Cells Characterized by In Situ Real Time and Spatially Resolved XAFS Techniques

“…It is to be noted that the Pt valence in most parts of the micro-crack region was calculated as 2? by the linear relationship between the normalized WLPA and Pt valence [17,76,77], whereas Pt nanoparticles in the other cathode areas were metallic as shown in Fig. 7C.…”

“…have extensively been studied [3][4][5][6][7][8][9][10][11][12][13], but they are difficult to observe the dynamic and spatial behavior and transformation of Pt nanoparticles under PEFC operating conditions. In situ time-resolved X-ray absorption fine structure (XAFS) techniques are very powerful for in situ/ operando investigation of the local coordination structures and oxidation states of supported nanoparticle catalysts under working conditions particularly in such complex systems as PEFCs, involving Pt valence change and Pt-O/ Pt-Pt bonds transformation [14][15][16][17][18][19][20]. These key reaction processes, which regulate the durability of PEFCs as well as the ORR activity, may occur heterogeneously in the space of the cathode layer due to the spatially heterogeneous property and distribution of Pt nanoparticles and carbon supports and the microscopically non-uniform potentials loaded in the cathode layer under PEFC potential operations [21].…”