Oxygen Reduction Catalytic Activity of Hollandite-type Manganese Oxides

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 (ORB.) activities and AEMFC single cell performances were evaluated. In this study, we prepared two kinds of hollandite oxides K0.14MnO2 center dot 0.12H(2)O(KMO-CP) and K0.12MnO2 center dot 0.06H(2)O(KMO-HT) and their partially Co-substituted ones K-0.11(Mn-0.88 Co-0.12)O-2 center dot 0.16H(2)O(KMC0.12-CP) and K-0.11(Mn0.88Co0.12)O-2 center dot 0.08H(2)O(KMC0.12-HT), and examined the ORR activities with a rotating ring-disk electrode (RRDE) in 0.1 M KOH at 50 degrees C. All the samples showed high onset potentials of ORB., 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 (Eff(4)) 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.