Effect of HClO4 Concentration on Oxygen Reduction Reaction Activity at Pt and Pt-Co Alloy Single Crystal Electrodes

Abstract

Pt-based alloys such as Pt-Fe, Pt-Co and Pt-Ni, have attracted much attention as cathode catalysts with enhanced activities for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs).1 Recently, we have found by use of an electrochemical quartz crystal microbalance (EQCM) and a rotating disk electrode (RDE) that the specific adsorption of perchlorate anion on a Pt-skin/Pt-Co alloy polycrystalline electrode was stronger than that on bulk-Pt, and that the kinetically-controlled current densities jk for the ORR at the Pt-skin/Pt-Co decreased with increasing HClO4 concentration more steeply than that in the case of bulk-Pt, due to the blocking of the active sites by the specifically adsorbed perchlorate anion. 2 In practical PEFCs, Pt-based nanoparticles dispersed on carbon supports are employed in the catalyst layer, and the nanocatalyst surfaces usually consist of low-index crystal facets such as (111) and (100). Recently, we have established a facile method for preparation of Pt-Co alloy single crystals with desired Co contents,3 and have revealed the strong dependence of the ORR activity jk on the Co content at Pt-skin/Pt100-xCox(111) electrodes prepared by annealing under pure H2 atmosphere.4 In the present research, we have evaluated jk at well-defined Pt-skin/Pt-Co alloy single crystal electrodes as a function of HClO4 concertation [HClO4] by using the rotating disk electrode (RDE) method in order to clarify the effect of specific adsorption of perchlorate on the ORR activity. The preparation of Pt-Co alloy single crystal electrodes has been described elsewhere.3 Prior to the electrochemical measurements, well-defined (1 × 1) surfaces of polished crystal electrodes were freshly prepared by heating to 1170 K in H2, and subsequent cooling in H2 for the (111) and (100) planes or 1% CO/He streams for the (110) plane.3 This heat treatment resulted in the formation of a Pt-skin layer on the Pt-Co alloy single crystal surface with the same atomic arrangement as the underlying crystal, confirmed by low energy ion scattering (LEIS) and low energy electron diffraction (LEED).4 Figure 1 shows cyclic voltammograms (CVs) at a Pt-skin/Pt81Co19(111) electrode acquired in various concentrations of HClO4 purged with N2. The ohmic loss was corrected based on impedance measurements. The CVs exhibited characteristic, highly reversible features, i.e., the hydrogen underpotential deposition (HUPD) wave and the so-called butterfly wave, with a broad peak around 0.85 V, in the surface-oxidation region. At 0.05 M ≤ [HClO4] ≤ 0.2 M, no significant changes were observed in the HUPD region. In contrast, with increasing [HClO4], it was found that the double layer currents increased and the surface oxidation wave shifted to higher potentials due to specific adsorption of perchlorate anion, similar to the polycrystalline Pt-Co alloy.2 Figure 2 shows jk values of Pt73Co27(111)-RDEs at 0.9 V vs. RHE in air-saturated HClO4 solutions as a function of [HClO4]. As [HClO4] increased from 0.05 to 0.2 M, the jk values decreased, similar to the case of the polycrystalline Pt-Co alloy.2 The decrease in jk can be ascribed to the blocking of the ORR active sites on the Pt-skin layer by perchlorate anions. However, the jk value in the 0.01 M HClO4 solution was significantly lower than those for other concentrations. The lower jk value in 0.01 M HClO4was presumably due to the low activity of protons, which are consumed in the ORR on the surface. This work was supported by the funds for the “SPer-FC Project’’ of NEDO (Japan) and a Grant-in-Aid No. 25410007 for Scientific Research MEXT of Japan.