Oxygen Reduction Electrode Properties of Manganese Oxide Nanosheet-Based Materials

Two types of oxide nanosheet-based materials, H 3 O + -form regularly stacked manganese oxide nanosheets (H 3 O + -RG(Mn)) and H 3 O + -form randomly restacked manganese oxide nanosheets (H 3 O + -RE(Mn)) were synthesized by soft chemical methods, and their oxygen reduction reaction (ORR) activities were examined by cyclic voltammetry (CV) and semi-steady-state voltammetry (SSV) with a rotating ring-disc electrode at 70 °C in 0.1 M KOH. Both samples showed high onset potentials ( E on ) of the ORR current and high efficiencies ( Eff 4 ) of the 4-electron reduction of oxygen, and E on and Eff 4 values were improved by electrochemical oxidation up to 1.2 V (vs. reversible hydrogen electrode) in the CV measurement prior to the SSV measurement. As a result, the nanosheet-based samples exhibited higher ORR activities than the starting materials, K + -form layered manganese oxide K 0.5 MnO 2 (K + -RG(Mn)) and Mn 2 O 3 , and a well-known ORR catalyst, MnO 2 . The H 3 O + -RE(Mn) sample electrochemically oxidized up to 1.2 V showed the highest ORR activity, E on = 0.97 V and Eff 4 = 99%, which were comparable to those of a conventional 20 mass% Pt/C catalyst. The comparison of their ORR activities, BET surface areas and X-ray photoelectron spectra suggests that the enhancement of the ORR activity is attributed to an increase in the numbers of the ORR active sites and a large amount of H 2 O in the interlayers and on the surface of the nanosheets because of rapid of H 2 O-supply enough for ORR in alkaline solution.