MnO–nitrogen doped graphene as a durable non-precious hybrid catalyst for the oxygen reduction reaction in anion exchange membrane fuel cells

Abstract

A promising non-precious metal catalyst containing manganese oxide and N-graphene is synthesized and is recognised as an efficient and durable electrocatalyst for the oxygen reduction reaction (ORR) in an alkaline medium. Hydrothermally synthesized Mn₃O₄ is incorporated into graphene mixed with melamine as a nitrogen source and annealed at a temperature between 700 °C and 1000 °C. The phase formation, purity and morphological behaviours of pre-formed Mn₃O₄ and MnO after annealing are systematically studied by X-ray diffraction (XRD), high resolution scanning electron microscopy (HR-SEM) and transmission electron microscopy (TEM). The MnO and N-doped graphene (MnO/NG) catalyst enhances ORR activity compared to other combinations of hybrid catalysts, such as Mn₃O₄/G and MnO/G, as observed from the linear sweep voltammograms. It is found that MnO/NG annealed at 900 °C demonstrates excellent ORR catalytic activity in O₂ saturated 0.1 M aqueous KOH electrolyte with a dominant 4e⁻ transfer process, as confirmed by both RDE and RRDE measurements. The stability and durability of this optimized catalyst is ascertained by potential cycling between −0.8 and 0.2 V vs. Ag/AgCl up to 10 000 potential cycles; superior durability was found in comparison to the state-of-art Pt/C catalyst. This MnO/NG-900 hybrid catalyst is used as a cathode catalyst, and we fabricate a membrane electrode assembly (MEA) to validate the catalyst in an anion exchange membrane fuel cell (AEMFC). A peak power density of 13 mW cm⁻² at 30 °C under ambient pressure is realized; thus, our catalyst appears to be promising as an alternative non-precious metal catalyst for AEMFCs.