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.

The structural kinetics of surface events on a Pt/C cathode catalyst in a membrane electrode assembly (MEA) with a practical catalyst loading (0.5 mgPt cm(-2)) for a polymer electrolyte fuel cell were investigated by in situ time-resolved X-ray absorption fine structure analysis (XAFS; time resolution: 100 ms) for the first time. The rate constants of structural changes in the Pt/C cathode catalyst in the MEA during voltage cycling were successfully estimated. For voltage-cycling processes, all reactions (electrochemical reactions and structural changes in the Pt catalyst) in the MEA were found to be much faster than those in an MEA with a thick cathode catalyst layer, but the in situ time-resolved XAFS analysis revealed that significant time lags similarly existed between the electrochemical reactions and the structural changes in the Pt cathode catalyst. The time-resolved XAFS also revealed differences in the structural kinetics of the Pt/C cathode catalyst for the voltage-cycling processes under N2 and air flows at the cathode.

There is limited information on the mechanism for platinum oxidation and dissolution in Pt/C cathode catalyst layers of polymer electrolyte fuel cells (PEFCs) under the operating conditions though these issues should be uncovered for the development of next-generation PEFCs. Pt species in Pt/C cathode catalyst layers are mapped by a XAFS (X-ray absorption fine structure) method and by a quick-XAFS(QXAFS) method. Information on the site-preferential oxidation and leaching of Pt cathode nanoparticles around the cathode boundary and the micro-crack in degraded PEFCs is provided, which is relevant to the origin and mechanism of PEFC degradation.

Three types of bimetallic Pt–Pd nanoparticles with different core–shell structures besides Pt and Pd nanoparticles were synthesized by coreduction and sequential reduction methods in ethylene glycol. The synthesized nanoparticles were supported on carbon to prepare five different electrocatalysts Pt/C, Pd/C, PdPt alloy/C, Pd­(core)–Pt­(shell)/C, and Pt­(core)–Pd­(shell)/C for oxygen reduction reaction (ORR) in fuel cells. The nanoparticles and supported catalysts were characterized by means of transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), extended X-ray absorption fine structure (EXAFS), and cyclic voltammetry (CV). It was proposed by these characterizations that the PdPt alloy/C, Pd­(core)–Pt­(shell)/C, and Pt­(core)–Pd­(shell)/C catalysts constituted Pd4Pt1(core)–Pt­(two-layers shell), Pd (core)–Pd2Pt1(three-layers)–Pt­(three-layers shell), and Pt­(core)–Pt2Pd1(two-layers)–Pd (microcrystal shell), respectively. The Pt surface-enriched catalysts were more stable than the Pd surface-enriched catalysts in long-term CV scanning in acid electrolyte. The Pt/C, PdPt alloy/C, and Pd­(core)–Pt­(shell)/C catalysts with Pt-enriched surfaces showed much higher ORR specific activity than the Pd/C and Pt­(core)–Pd­(shell)/C catalysts with Pd-enriched surfaces. The Pt surface-enriched bimetal catalysts with core–shell structures showed the higher Pt-based mass activity than the Pt monometal catalyst. The PdPt catalysts with Pd/Pt = 2 and 4 in an atomic ratio were also prepared by the coreduction method. The Pt-enriched surfaces formed also with these samples, but the ORR specific activity and (Pd + Pt)-based mass activity decreased with increasing Pd/Pt ratios (1, 2, and 4). The present study provided core–shell catalysts with better ORR activity, which may be useful for understanding key issues to develop next-generation fuel-cell cathode catalysts.

The misfit compound [CoO2][Ca2CoO3−δ]0.62 is well-known for its good potentialities in the field of thermoelectric oxides combining good electronic transport, high Seebeck coefficient, and low thermal conductivity. Its 2D-crystal structure can be regarded as a natural intergrowth between electronic-conducting Co3+/Co4+ hexagonal layers and oxygen deficient Co2+/Co3+ rock-salt layers with low thermal conductivity. Their lacunar character suggests a possible anionic conductivity. We took advantage of this model for application as a SOFC cathode material. Additional advantages appear from the good chemical and mechanical adaptability (TEC = 9−10 × 10−6 °C−1) with intermediate temperature electrolyte, namely, CGO. The manufactured symmetrical cells show a good electrode/electrolyte adherence, stable after long-time experiments. Our promising preliminary electrochemical tests show a rather low electrode overpotential (4Ω·cm2) for ∼40 μm thick layers with a rather dense microstucture. The porosity and electric performances are improved in the composite with 30 wt % CGO (∼1 Ω·cm2). In general, from polarization experiments versus temperature and oxygen pressure, we found two distinct processes, frequency-separated, that is, HF, charge transfer at the TPB with intrinsic O2− diffusion; LF, gas transfer/oxygen dissociation. This latter is largely fastened in the CGO/Ca3Co4O9−δ, reminiscent of the existing but limiting ionic mobility in the single phase of the title compound.