Various states of water species in an anion exchange membrane characterized by Raman spectroscopy under controlled temperature and humidity.

Anion exchange membrane fuel cells (AEMFCs) hold the key to future mass commercialisation of fuel cell technology, even though currently, AEMFCs perform less optimally than proton exchange membrane fuel cells (PEMFCs). Unlike PEMFCs, AEMFCs have demonstrated the capability to operate independently of Pt group metal-based catalysts. Water characterization inside the membrane is one factor that significantly influences the performance of AEMFCs. In this paper, different water species inside an anion exchange membrane (AEM), QPAF-4, developed at the University of Yamanashi, were studied for the first time using micro-Raman spectroscopy. Spectra of pure water, alkaline solutions, and calculations based on density functional theory were used to identify the water species in the AEM. The OH stretching band was deconvoluted into nine unique Gaussian bands. All the hydrogen-bonded OH species increased steadily with increasing humidity, while the CH and non-H-bonded OH remained relatively constant. These results confirm the viability of micro-Raman spectroscopy in studying the various water-related species in AEMs. The availability of this technique is an essential prerequisite in improving the ionic conductivity and effectively solving the persisting durability challenge facing AEMFCs, thus hastening the possibility of mass commercialisation of fuel cells.

Effects of Sulfate on the Oxygen Reduction Reaction Activity on Stabilized Pt Skin/PtCo Alloy Catalysts from 30 to 80 °C.

The effects of the concentration of H2SO4 ([H2SO4]), which is the major decomposition product of polymer electrolyte membranes during the operation of fuel cells, on the performance of stabilized Pt skin/PtCo alloy nanocatalysts supported on high-surface-area carbon (PtxAL-PtCo/C) were investigated. Kinetically controlled activities for the oxygen reduction reaction (ORR) and the H2O2 yields ( P(H2O2)) on the PtxAL-PtCo/C were examined based on hydrodynamic voltammograms in O2-saturated 0.1 M HClO4 + X M H2SO4 ( X = 0 to 5 × 10-2) by use of the channel flow double electrode method at temperatures between 30 and 80 °C. At X ≤ 10-6 (1 µM) and all temperatures examined, the apparent ORR rate constants kapp@0.85 V (per unit electrochemically active surface area) on PtxAL-PtCo/C at 0.85 V vs the reversible hydrogen electrode (RHE) were nearly identical with those in sulfate-free 0.1 M HClO4 and were at least twice as high as those on a commercial Pt/C catalyst (c-Pt/C). The values of kapp@0.85 V on both PtxAL-PtCo/C and c-Pt/C decreased linearly with log[H2SO4] in the concentration range 10-6 < X ≤ 5 × 10-2. The detrimental effect by H2SO4 was less pronounced on PtxAL-PtCo/C than on c-Pt/C at high temperatures; the kapp@0.85 V value at X = 5 × 10-2 on the former at 80 °C was maintained as high as 87%, whereas that of the latter was 66% (34% loss). The values of peroxide production percentage P(H2O2) on PtxAL-PtCo/C at 80 °C were nearly constant (ca. 0.22% at 0.76 V vs RHE) up to X = 5 × 10-2. These superior characteristics are ascribed to weakened adsorption of sulfate on the Pt skin surface, supported by DFT calculations, which provides the great advantage of robustness in the presence of impurities, maintaining active sites for the ORR during the PEFC operation.

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.

NiFe Alloy Integrated with Amorphous/Crystalline NiFe Oxide as an Electrocatalyst for Alkaline Hydrogen and Oxygen Evolution Reactions.

The rational design of efficient and low-cost electrocatalysts based on earth-abundant materials is imperative for large-scale production of hydrogen by water electrolysis. Here we present a strategy to prepare highly active catalyst materials through modifying the crystallinity of the surface/interface of strongly coupled transition metal-metal oxides. We have thermally activated the catalysts to construct amorphous/crystalline Ni-Fe oxide interfaced with a conductive Ni-Fe alloy and systematically investigated their electrocatalytic performance toward the hydrogen evolution and oxygen evolution reactions (HER and OER) in alkaline solution. It was found that the Ni-Fe/oxide material with a crystalline surface oxide phase showed remarkably superior HER activity in comparison with its amorphous or poorly crystalline counterpart. In contrast, interestingly, the amorphous/poorly crystalline oxide significantly facilitated the OER activity in comparison with the more crystalline counterpart. On one hand, the higher HER activity can be ascribed to a favorable platform for water dissociation and H-H bond formation, enabled by the unique crystalline metal/oxide structure. On the other hand, the enhanced OER catalysis on the amorphous Ni-Fe oxide surfaces can be attributed to the facile activation to form the active oxyhydroxides under OER conditions. Both are explained based on density functional theory calculations. These results thus shed light onto the role of crystallinity in the HER and OER catalysis on heterostructured Ni-Fe/oxide catalysts and provide guidance for the design of new catalysts for efficient water electrolysis.

Electronic States and Transport Phenomena of Pt Nanoparticle Catalysts Supported on Nb-Doped SnO2 for Polymer Electrolyte Fuel Cells.

Semiconducting oxide nanoparticles are strongly influenced by surface-adsorbed molecules and tend to generate an insulating depletion layer. The interface between a noble metal and a semiconducting oxide constructs a Schottky barrier, interrupting the electron transport. In the case of a Pt catalyst supported on the semiconducting oxide Nb-doped SnO2 with a fused-aggregate network structure (Pt/Nb-SnO2) for polymer electrolyte fuel cells, the electronic conductivity increased abruptly with increasing Pt loading, going from 10-4 to 10-2 S cm-1. The Pt X-ray photoemission spectroscopy (XPS) spectra at low Pt loading amount exhibited higher binding energy than that of pristine Pt metal. The peak shift for the Pt XPS spectra was larger than that of the Pt hard X-ray photoemission spectroscopy (HAXPES) spectra. For all of the spectra, the peaks approached the binding energy of pristine Pt metal with increasing Pt loading. The Sn XPS spectral peak proved the existence of Sn metal with increasing Pt loading, and the peak intensity was larger than that for HAXPES. These spectroscopic results, together with the scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDX) spectra, proved that a PtSn alloy was deposited at the interface between Pt and Nb-SnO2 as a result of the sintering procedure under dilute hydrogen atmosphere. Both Nb spectra indicated that the oxidation state of Nb was +5 and thus that the Nb cation acts as an n-type dopant of SnO2. We conclude that the PtSn alloy at the interface between Pt and Nb-SnO2 relieved the effect of the Schottky barrier, enhanced the carrier donation from Pt to Nb-SnO2, and improved the electronic transport phenomena of Pt/Nb-SnO2.

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.