Operando Time-Resolved X-ray Absorption Fine Structure Study for Surface Events on a Pt₃Co/C Cathode Catalyst in a Polymer Electrolyte Fuel Cell during Voltage-Operating Processes

The structural kinetics of surface events on a Pt₃Co/C cathode catalyst in a polymer electrolyte fuel cell (PEFC) was investigated by operando time-resolved X-ray absorption fine structure (XAFS) with a time resolution of 500 ms. The rate constants of electrochemical reactions, the changes in charge density on Pt, and the changes in the local coordination structures of the Pt₃Co alloy catalyst in the PEFC were successfully evaluated during fuel-cell voltage-operating processes. Significant time lags were observed between the electrochemical reactions and the structural changes in the Pt₃Co alloy catalyst. The rate constants of all the surface events on the Pt₃Co/C catalyst were significantly higher than those on the Pt/C catalyst, suggesting the advantageous behaviors (cell performance and catalyst durability) on the Pt₃Co alloy cathode catalyst

Same-View Nano-XAFS/STEM-EDS Imagings of Pt Chemical Species in Pt/C Cathode Catalyst Layers of a Polymer Electrolyte Fuel Cell

We have made the first success in the same-view imagings of 2D nano-XAFS and TEM/STEM-EDS under a humid N₂ atmosphere for Pt/C cathode catalyst layers in membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells (PEFCs) with Nafion membrane to examine the degradation of Pt/C cathodes by anode gas exchange cycles (start-up/shut-down simulations of PEFC vehicles). The same-view imaging under the humid N₂ atmosphere provided unprecedented spatial information on the distribution of Pt nanoparticles and oxidation states in the Pt/C cathode catalyst layer as well as Nafion ionomer-filled nanoholes of carbon support in the wet MEA, which evidence the origin of the formation of Pt oxidation species and isolated Pt nanoparticles in the nanohole areas of the cathode layer with different Pt/ionomer ratios, relevant to the degradation of PEFC catalysts

Surface-Regulated Nano-SnO₂/Pt₃Co/C Cathode Catalysts for Polymer Electrolyte Fuel Cells Fabricated by a Selective Electrochemical Sn Deposition Method

We have achieved significant improvements for the oxygen reduction reaction activity and durability with new SnO₂-nanoislands/Pt₃Co/C catalysts in 0.1 M HClO₄, which were regulated by a strategic fabrication using a new selective electrochemical Sn deposition method. The nano-SnO₂/Pt₃Co/C catalysts with Pt/Sn = 4/1, 9/1, 11/1, and 15/1 were characterized by STEM-EDS, XRD, XRF, XPS, in situ XAFS, and electrochemical measurements to have a Pt₃Co core/Pt skeleton-skin structure decorated with SnO₂ nanoislands at the compressive Pt surface with the defects and dislocations. The high performances of nano-SnO₂/Pt₃Co/C originate from efficient electronic modification of the Pt skin surface (site 1) by both the Co of the Pt₃Co core and surface nano-SnO₂ and more from the unique property of the periphery sites of the SnO₂ nanoislands at the compressive Pt skeleton-skin surface (more active site 2), which were much more active than expected from the d-band center values. The white line peak intensity of the nano-SnO₂/Pt₃Co/C revealed no hysteresis in the potential up–down operations between 0.4 and 1.0 V versus RHE, unlike the cases of Pt/C and Pt₃Co/C, resulting in the high ORR performance. Here we report development of a new class of cathode catalysts with two different active sites for next-generation polymer electrolyte fuel cells

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

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 voltage-stepping processes were characterized by in situ (operando) X-ray absorption fine structure (XAFS). The active surface phase of the Au@Pt/C for oxygen reduction reaction (ORR) was suggested to be the Pt₃Au 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

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

The structural kinetics (rate constants for electronic and structural transformations) of the Pt charging/discharging, Pt–Pt bond dissociation/re-formation, and Pt–O bond formation/dissociation of Pt/Ketjenblack, Pt/acetylene black, and Pt/multiwalled carbon nanotube cathode catalysts in polymer electrolyte fuel cell (PEFC) membrane electrode assemblies (MEAs) under transient potential operations (0.4 V_RHE → 1.4 V_RHE → 0.4 V_RHE) has been studied by in situ/operando time-resolved quick X-ray absorption fine structure (QXAFS; 100 ms/spectrum), while measuring electrochemical currents/charges in the MEAs under the potential operations. From the systematic QXAFS analysis for potential-dependent surface structures and rate constants (<i>k</i> and <i>k</i>′) for the transformations of Pt nanoparticles under the operations (0.4 V_RHE → 1.4 V_RHE and 1.4 V_RHE → 0.4 V_RHE), respectively, we have found the structural kinetics (<i>k</i>′_Pt–O and <i>k</i>′_valence) controlling the oxygen reduction reaction (ORR) activity and also the structural kinetics (<i>k</i>′_Pt–Pt/<i>k</i>_Pt–Pt) reflecting the durability of the cathode catalysts. The relaxation time of the Pt–Pt bond re-formation and Pt–O bond dissociation processes in the activated MEAs was also suggested to predict the relative durability of similar kinds of cathode catalysts. The in situ time-resolved XAFS analysis provided direct information on the key structural kinetics of the Pt/C catalysts themselves for thorough understanding of the cathode catalysis toward PEFC improvement