Hausmannite-Carbon Nanofiber Composite Electrocatalyst for High Areal-Discharge Energy Rechargeable Zinc-Air Battery

Rechargeable zinc-air batteries (RZABs) have been described as one of the most viable next-generation battery technologies, especially due to their low cost, high capacity, and being environmental-friendly. In this work, hausmannite Mn₃O₄ nanoparticles, obtained from low-cost commercial electrolytic manganese dioxide, were dispersed on conductive multiwalled carbon nanotubes (CNTs) and carbon nanofibers (CNFs) and investigated for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in an alkaline medium and then applied in RZAB cell. The high performance of the CNFs (in terms of electron transfer kinetics) over the CNTs has been associated with its inherent defects and nitrogen content. Density functional theory (DFT) calculations predict that CNF give higher partial density of states (PDOS, i.e., 67 eV vs 51 eV for CNT) and can allow for a more favorable distribution of the d-electrons of the Mn and enhanced synergistic effect with Mn₃O₄ for weaker adsorption energies and p-band centers of the oxygen intermediates (O*, OH*, and OOH*). In a proof-of-concept, Mn₃O₄ + CNF was investigated as the air cathode for RZAB in a micro-3D-printed cell configuration. The RZAB showed good performance in terms of open circuit voltage (OCV = 1.77 V), areal-discharge energy (≥40 mW h/cm² _geometric) and cycling stability (∼25 cycles at 8 h per cycle for 140 h at 10 mA cm^-2; and ∼17 cycles at 16 h per cycle for 270 h at 5 mA cm^-2) better than 100 catalysts used in RZAB cells in recent articles including the state-of-the-art Pt/C-IrO₂ catalysts. The findings here provide fresh physicochemical perspectives on the future design and utility of CNFs for developing Mn-based RZABs that meet or even outperform the new literature-recommended benchmark areal-discharge energy density of 35 mW h/cm² _geometric at 10 mA cm^-2 current loading for any possible application in real devices.