Oxygen Reduction Catalytic Activity of Hollandite-Type Manganese Oxides

Abstract

Hollandite-type Mn oxides were synthesized by a co-precipitation (CP) and a hydrothermal (HT) method as the cathode catalysts for anion-exchange membrane fuel cells (AEMFCs), and their oxygen reduction reaction (ORR) activities and AEMFC single cell performances were evaluated. In this study, we prepared two kinds of hollandite oxides K0.14MnO2·0.12H2O(KMO-CP) and K0.12MnO2·0.06H2O(KMO-HT) and their partially Co-substituted ones K0.11(Mn0.88 Co0.12)O2・0.16H2O(KMC0.12-CP) and K0.11(Mn0.88Co0.12)O2· 0.08H2O(KMC0.12-HT), and examined the ORR activities with a rotating ring-disk electrode (RRDE) in 0.1 M KOH at 50°C. All the samples showed high onset potentials of ORR, ca. 0.9 V vs. reversible hydrogen electrode (RHE), and relatively high ORR currents at 0.75 V and high efficiencies for 4-electron reduction (Eff4) of around 90% were obtained for the HT samples owing to higher crystallinity and higher specific surface area than those of the CP ones. The AEMFC single cells prepared with KMO-HT and KMC0.12-HT cathode catalysts were operated stably and exhibited the maximum power densities of ca. 49 and 42 mW cm-2, respectively, which were comparable to that of the single cell using a conventional 50 wt% Ag/C catalyst.