Akihiro Iiyama

Theoretical Investigation of the H2O2-Induced Degradation Mechanism of Hydrated Nafion Membrane via Ether-Linkage Dissociation

A H2O2-induced degradation mechanism is presented for the hydrated Nafion membrane proceeding through the dissociation of the ether linkages of the side chains. Although the durability of proton-exchange membrane fuel cells clearly depends on the degradation rate of the membrane, typically Nafion, the degradation mechanism still has not been resolved. It has often been assumed that the principal mode of degradation involves OH• radicals; in contrast, we show here that a H2O2-induced degradation mechanism is more likely. On the basis of state-of-the-art theoretical calculations and detailed comparison with experimental results, we present such a mechanism for the hydrated Nafion membrane, proceeding through the dissociation of the ether linkage of the side chains, with a relatively low activation energy. In this mechanism, (H2O)λHO3S–CF2–CF2–O–O–H (λ is the hydration number) is obtained as a key degradation fragment. Possible subsequent decomposition-reaction mechanisms are also elucidated for this fragment. The calculated vibrational spectra for the intermediates and products proposed in these mechanisms were found to be consistent with the experimental IR spectra. Further consideration of this H2O2-mediated degradation mechanism could greatly facilitate the search for ways to combat membrane degradation.

Evaluation of Cell Performance and Durability for Cathode Catalysts (Platinum Supported on Carbon Blacks or Conducting Ceramic Nanoparticles) During Simulated Fuel Cell Vehicle Operation: Start-Up/Shutdown Cycles and Load Cycles

We summarized investigations on the evaluation of cell performance and durability for cathode catalysts on two types of supports, carbon blacks (CBs) and conducting ceramic nanoparticles, during simulated fuel cell vehicle (FCV) operation, including start-up/shutdown (SU/SD) cycles and load cycles. In cathode catalyst layers (CLs) using Pt supported on CBs (Pt/CBs), the effects of graphitized CB (GCB) and Pt nanoparticle size, as well as its dispersion state on the GCB, were investigated on both the performance and durability. The negative effects of the interim cyclic voltammetric measurements on the Pt/CB catalyst degradation during SU/SD cycling evaluation, which led to an overestimation of the degradation process, were also suggested. We found that catalyst degradation occurred not only in the outlet region but also in the inlet region during the gas-exchange SU. Degradation of CBs during a hydrogen passivation SU/SD process was found to decrease but still to occur, due to local cells arising from nonuniform distributions of ionomer and Pt particles. The effects of load cycle conditions, which involved open circuit and load holding times, and variations of current density, and humidity, on the durability of the cathode were also investigated. The buildup of Pt oxides at higher potentials during open circuit and re-reduction at lower potentials during high current density operation led to accelerated degradation; these conditions have relevance to ordinary operation with drastic load changes. For the intrinsic improvement of SU/SD durability, we synthesized conducting ceramic nanoparticles. The durability of the cathode CLs, using Pt supported on conducting ceramic nanoparticles with a fused-aggregate network structure, was superior to that of Pt/GCB. We also proposed that the cathode CL degradation can be mitigated by the use of ceramic nanoparticles in the anode because of the significant reduction of the reverse current due to the high resistivity in the air, termed the “atmospheric resistive switching mechanism” (ARSM).

Analysis of the Surface Oxidation Process on Pt Nanoparticles on a Glassy Carbon Electrode by Angle-Resolved, Grazing-Incidence X-ray Photoelectron Spectroscopy

We have analyzed the surface oxidation process of Pt nanoparticles that were uniformly dispersed on a glassy carbon electrode (Pt/GC), which was adopted as a model of a practical Pt/C catalyst for fuel cells, in N2-purged 0.1 M HF solution by using angle-resolved, grazing-incidence X-ray photoelectron spectroscopy combined with an electrochemical cell (EC-ARGIXPS). Positive shifts in the binding energies of Pt 4f spectra were clearly observed for the surface oxidation of Pt nanoparticles at potentials E > 0.7 V vs RHE, followed by a bulk oxidation of Pt to form Pt(II) at E > 1.1 V. Three types of oxygen species (H2Oad, OHad, and Oad) were identified in the O 1s spectra. It was found for the first time that the surface oxidation process of the Pt/GC electrode at E < ca. 0.8 V (OHad formation) is similar to that of a Pt(111) single-crystal electrode, whereas that in the high potential region (Oad formation) resembles that of a Pt(110) surface or polycrystalline Pt film.

Weakened CO adsorption and enhanced structural integrity of a stabilized Pt skin/PtCo hydrogen oxidation catalyst analysed by in situ X-ray absorption spectroscopy

In situ X-ray absorption spectroscopy has afforded detailed structural and electronic characterization of a newly developed stabilized Pt-skin/PtCo alloy nanoparticle catalyst for CO-tolerant H2 oxidation. The X-ray absorption near-edge structure (XANES) results show the significant effects of H and CO adsorption on white lines for pure Pt nanoparticles, which are correlated with the depletion of the density of states (DOS) just below the Fermi level (FL) calculated with density-functional theory (DFT). In contrast, for the Pt-skin/PtCo surface, there is little effect on the XANES for either H or CO adsorption. The lack of effect is consistent with the lack of variation of the DOS near the FL. The extended X-ray absorption fine structure (EXAFS) results for pure Pt nanoparticles have exhibited significant changes caused by H and CO adsorption. Such changes are suggested to be associated with the selected surface expansion of Pt atoms, as revealed from DFT-calculated radial distribution functions (RDFs). In contrast, for the Pt-skin/PtCo surface, there were negligible changes in the EXAFS, either with H or CO adsorption, which is consistent with the lack of changes in the DFT-calculated RDF. Notably, the rigidity of the Pt skin and the lower degree of overlap between Pt and CO orbitals, due to the narrowing of the d-band, are thought to be responsible for the weak adsorption.