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

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