Hausmannite–Carbon\nNanofiber Composite Electrocatalyst\nfor High Areal-Discharge Energy Rechargeable Zinc–Air Battery

Rechargeable zinc–air batteries (RZABs) have been\ndescribed\nas one of the most viable next-generation battery technologies, especially\ndue to their low cost, high capacity, and being environmental-friendly.\nIn this work, hausmannite Mn<sub>3</sub>O<sub>4</sub> nanoparticles,\nobtained from low-cost commercial electrolytic manganese dioxide,\nwere dispersed on conductive multiwalled carbon nanotubes (CNTs) and\ncarbon nanofibers (CNFs) and investigated for oxygen reduction reaction\n(ORR) and oxygen evolution reaction (OER) in an alkaline medium and\nthen applied in RZAB cell. The high performance of the CNFs (in terms\nof electron transfer kinetics) over the CNTs has been associated with\nits inherent defects and nitrogen content. Density functional theory\n(DFT) calculations predict that CNF give higher partial density of\nstates (PDOS, i.e., 67 eV vs 51 eV for CNT) and can allow for a more\nfavorable distribution of the d-electrons of the Mn and enhanced synergistic\neffect with Mn<sub>3</sub>O<sub>4</sub> for weaker adsorption energies\nand p-band centers of the oxygen intermediates (O*, OH*, and OOH*).\nIn a proof-of-concept, Mn<sub>3</sub>O<sub>4</sub> + CNF was investigated\nas the air cathode for RZAB in a micro-3D-printed cell configuration.\nThe RZAB showed good performance in terms of open circuit voltage\n(OCV = 1.77 V), areal-discharge energy (≥40 mW h/cm<sup>2</sup><sub>geometric</sub>) and cycling stability (∼25 cycles at\n8 h per cycle for 140 h at 10 mA cm<sup>–2;</sup> and ∼17\ncycles at 16 h per cycle for 270 h at 5 mA cm<sup>–2</sup>)\nbetter than 100 catalysts used in RZAB cells in recent articles including\nthe state-of-the-art Pt/C–IrO<sub>2</sub> catalysts. The findings\nhere provide fresh physicochemical perspectives on the future design\nand utility of CNFs for developing Mn-based RZABs that meet or even\noutperform the new literature-recommended benchmark areal-discharge\nenergy density of 35 mW h/cm<sup>2</sup><sub>geometric</sub> at 10\nmA cm<sup>–2</sup> current loading for any possible application\nin real devices.