Mn₃O₄ Supported on Glassy Carbon:\nAn Active Non-Precious Metal Catalyst for the Oxygen Reduction Reaction

Abstract

In this work, we explore the interplay between manganese\noxide\n(MnO_<i>x</i>) nanomaterials and a glassy carbon\n(GC) support in catalyzing the oxygen reduction reaction (ORR) in\nan alkaline environment. Initially, we characterize the ORR activity\nof bare GC electrodes as a function of heat treatments in air, and\nfind that ORR activity increases with increasing temperature up to\n500 °C. Modification of GC with size-selected 1 nm MnO_<i>x</i> nanoparticles prior to the 500 °C heat treatment\nyields a highly porous GC (pGC) structure, devoid of MnO_<i>x</i>. This pGC sample exhibits the highest ORR performance\nof the bare carbon electrodes reaching an onset potential of 0.75\nV vs the reversible hydrogen electrode (RHE) and a complete 2-electron\nreduction of oxygen to peroxide. Having established ORR activity of\nbare GC electrodes, we deposit size-selected 14 nm MnO nanoparticles\nonto the GC and pGC electrodes and then incite phase changes in MnO\nthrough heat treatments in air. Electrochemical characterization of\nthe resulting electrodes reveals that MnO nanoparticles offer no improvement\nin the ORR onset potential over bare GC or pGC and only slightly increase\nthe number of electrons transferred. By contrast, thermal oxidation\nof MnO nanoparticles to Mn₃O₄ at 500 °C,\nconfirmed by Mn L-edge X-ray absorption spectroscopy, results in an\nimproved ORR onset potential of 0.80 V and a 4-electron reduction\nof oxygen. Thus at low overpotentials, where GC and pGC were inactive\nfor the ORR, MnO_<i>x</i> sites must contribute\nto all steps of the reaction. The catalyst’s estimated specific\nactivity of 3700 μA·cm^–2_cat at 0.75 V compares favorably with specific activities of Pt/C as\nwell as the best nonprecious metal catalysts. This establishes Mn₃O₄ as another MnO_<i>x</i> phase\nwith high activity for the ORR.