Oxygen Reduction Electrode Properties of Manganese Oxide Nanosheet-Based Materials

Two types of oxide nanosheet-based materials, H₃O⁺-form regularly stacked manganese oxide nanosheets (H₃O⁺-RG(Mn)) and H₃O⁺-form randomly restacked manganese oxide nanosheets (H₃O⁺-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_0.5MnO₂ (K⁺-RG(Mn)) and Mn₂O₃, and a well-known ORR catalyst, MnO₂. The H₃O⁺-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 H₂O in the interlayers and on the surface of the nanosheets because of rapid of H₂O-supply enough for ORR in alkaline solution.