Simultaneous Operando Time-Resolved XAFS–XRD Measurements of a Pt/C Cathode Catalyst in Polymer Electrolyte Fuel Cell under Transient Potential Operations

We have succeeded in simultaneous <i>operando</i> time-resolved quick X-ray absorption fine structure (QXAFS)–X-ray diffraction (XRD) measurements at each acquisition time of 20 ms for a Pt/C cathode catalyst in a polymer electrolyte fuel cell (PEFC), while measuring the current/charge of the PEFC during the transient voltage cyclic processes (0.4 V_RHE → 1.4 V_RHE → 0.4 V_RHE) under H₂(anode)–N₂(cathode). The rate constants for Pt–O bond formation/dissociation, Pt charging/discharging, Pt–Pt bond dissociation/reformation, and decrease/increase of Pt metallic-phase core size under the transient potential operations were determined by the combined time-resolved QXAFS–XRD technique. The present study provides a new insight into the transient-response reaction mechanism and structural transformation in the Pt surface layer and bulk, which are relevant to the origin of PEFC activity and durability as key issues for the development of next-generation PEFCs

The Relationship between the Active Pt Fraction in a PEFC Pt/C Catalyst and the ECSA and Mass Activity during Start-Up/Shut-Down Degradation by in Situ Time-Resolved XAFS Technique

Transient-response kinetics of the transformations (six elementary steps) of the Pt valence, coordination number of Pt–Pt bonds, and coordination number of Pt–O bonds of a Pt/C cathode catalyst in a polymer electrolyte fuel cell (PEFC) under cyclic voltage operations (0.4 → 1.4 → 0.4 <i>V</i>_RHE) during anode–gas exchange (AGEX) treatments (start-up/shut-down) has been studied by in situ time-resolved quick X-ray absorption fine structure (QXAFS, 100 ms/spectrum). The transient-response analysis identified the existence and fractions of three different kinds (active, less active, and inactive) of Pt nanoparticles in the Pt/C cathode. The active Pt nanoparticles degraded to less active and inactive Pt nanoparticles by the AGEX cycles. The degradation probability and mechanism were clarified by the transient-response kinetics. The electrochemical surface area (ECSA) and mass activity (MA) of the Pt/C cathode catalyst also decreased with increasing AGEX cycles. It was found that the change in the sum of the fractions of the active and less active Pt nanoparticles correlates with the change in the ECSA and MA during the AGEX treatments. The in situ time-resolved QXAFS analysis provides direct information on the dynamic behavior of the Pt/C catalyst relevant to the electrochemical performance and property under the operando conditions for thorough understanding of the degradation process toward PEFC improvement

Disappearance of the Superionic Phase Transition in Sub‑5 nm Silver Iodide Nanoparticles

Bulk silver iodide (AgI) is known to show a phase transition from the poorly conducting β/γ-phases into the superionic conducting α-phase at 147 °C. Its transition temperature decreases with decreasing the size of AgI, and the α-phase exists stably at 37 °C in AgI nanoparticles with a diameter of 6.3 nm. In this Letter, we investigated the atomic configuration, the phase transition behavior, and the ionic conductivity of AgI nanoparticles with a diameter of 3.0 nm. The combination of pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) modeling based on high-energy X-ray diffraction (XRD) revealed for the first time that they formed the β/γ-phases with atomic disorder. The results of extended X-ray absorption fine structure (EXAFS) analysis, differential scanning calorimetry (DSC), and AC impedance spectroscopy demonstrated that they did not exhibit the superionic phase transition and their ionic conductivity was lower than that of crystalline AgI. The disappearance of the superionic phase transition and low ionic conductivity in the very small AgI nanoparticles originates from their small size and disordered structure

Rate Enhancements in Structural Transformations of Pt–Co and Pt–Ni Bimetallic Cathode Catalysts in Polymer Electrolyte Fuel Cells Studied by in Situ Time-Resolved X‑ray Absorption Fine Structure

In situ time-resolved X-ray absorption fine structure spectra of Pt/C, Pt₃Co/C, and Pt₃Ni/C cathode electrocatalysts in membrane electrode assemblies (catalyst loading: 0.5 mg_metal cm^–2) were successfully measured every 100 ms for a voltage cycling process between 0.4 and 1.0 V. Systematic analysis of in situ time-resolved X-ray absorption near-edge structure and extended X-ray absorption fine structure spectra in the molecular scale revealed the structural kinetics of the Pt and Pt₃M (M = Co, Ni) bimetallic cathode catalysts under polymer electrolyte fuel cell operating conditions, and the rate constants of Pt charging, Pt–O bond formation/breaking, and Pt–Pt bond breaking/re-formation relevant to the fuel cell performances were successfully determined. The addition of the 3d transition metals to Pt reduced the Pt oxidation state and significantly enhanced the reaction rates of Pt discharging, Pt–O bond breaking, and Pt–Pt bond re-forming in the reductive process from 1.0 to 0.4 V

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

The degradation of Pt electrocatalysts in membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells under working conditions is a serious problem for their practical use. Here we report the kinetics and mechanism of redox reactions at the surfaces of Pt/C and Pt₃Co/C cathode electrocatalysts during catalyst degradation processes by an accelerated durability test (ADT) studied by operando time-resolved X-ray absorption fine structure (XAFS) spectroscopy. Systematic analysis of a series of Pt L_III-edge time-resolved XAFS spectra measured every 100 ms at different degradation stages revealed changes in the kinetics of Pt redox reactions on Pt/C and Pt₃Co/C cathode electrocatalysts. In the case of Pt/C, as the number of ADT cycles increased, structural changes for Pt redox reactions (charging, surface, and subsurface oxidation) became less sensitive because of the agglomeration of catalyst particles. It was found that their rate constants were almost constant independent of the agglomeration of the Pt electrocatalyst. On the other hand, in the case of Pt₃Co/C, the rate constants of the redox reactions of the cathode electrocatalyst gradually reduced as the number of ADT cycles increased. The differences in the kinetics for the redox processes would be differences in the degradation mechanism of these cathode electrocatalysts

Simultaneous Improvements in Performance and Durability of an Octahedral PtNi_<i>x</i>/C Electrocatalyst for Next-Generation Fuel Cells by Continuous, Compressive, and Concave Pt Skin Layers

Simultaneous improvements in oxygen reduction reaction (ORR) activity and long-term durability of Pt-based cathode catalysts are indispensable for the development of next-generation polymer electrolyte fuel cells but are still a major dilemma. We present a robust octahedral core–shell PtNi_<i>x</i>/C electrocatalyst with high ORR performance (mass activity and surface specific activity 6.8–16.9 and 20.3–24.0 times larger than those of Pt/C, respectively) and durability (negligible loss after 10000 accelerated durability test (ADT) cycles). The key factors of the robust octahedral nanostructure (core–shell Pt₇₃Ni₂₇/C) responsible for the remarkable activity and durability were found to be three continuous Pt skin layers with 2.0–3.6% compressive strain, concave facet arrangements (concave defects and high coordination), a symmetric Pt/Ni distribution, and a Pt₆₇Ni₃₃ intermetallic core, as found by STEM-EDS, in situ XAFS, XPS, etc. The robust core–shell Pt₇₃Ni₂₇/C was produced by the partial release of the stress, Pt/Ni rearrangement, and dimension reduction of an as-synthesized octahedral Pt₅₀Ni₅₀/C with 3.6–6.7% compressive Pt skin layers by Ni leaching during the activation process. The present results on the tailored synthesis of the PtNi_<i>x</i> structure and composition and the better control of the robust catalytic architecture renew the current knowledge and viewpoint for instability of octahedral PtNi_<i>x</i>/C samples to provide a new insight into the development of next-generation PEFC cathode catalysts

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

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

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

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