Bifunctional oxygen electrocatalysts based on non-critical raw materials: Carbon nanostructures and iron-doped manganese oxide nanowires

Abstract

Alkaline metal-air batteries are unique systems for energy storage. These devices require a bifunctional catalyst in the positive electrode that must perform both the oxygen evolution and reduction reactions (OER and ORR, respectively). Generally, cobalt-based oxides are employed as air electrodes; however, cobalt is a critical raw material. Future battery devices will mandatorily need non-critical raw materials based on highly abundant metals. Here we investigate the feasibility of iron-doped manganese oxide in the form of nanowires (Fe-MONW) combined with carbon nanofibers. MnO2 is known for being active for the ORR, however its activity towards the OER is not yet fully understood. Carbon nanofibers (CNF) on the other hand, provide the necessary electrical conductivity to the catalytic system. Simple methods and economic materials are employed to synthesize the Fe-MONW/CNF composites. Our results show that there is a synergistic effect between CNF and MONW, especially for the ORR, which manifests in an increase in the number of exchanged electrons– from 2.9 to 3.5 – and a shift in the onset potential of 70 mV. Doping MONW with iron further enhances the catalytic activity, for both the ORR and OER. Fe ions generate defects in the manganese oxide structure, favoring the adsorption of oxygen and eventually enhancing the catalytic activity. Fe-doped-MONW shows onset potentials for OER comparable to the benchmark catalyst, IrO2. The improvement on the catalytic activity is particularly evident in terms of the reversibility gap, ΔE. ΔE is the difference between the potential when the current density is 10 mA cm−2 in OER and the half-wave potential for the ORR, being a fundamental parameter to assess the performance of metal-air batteries. The reversibility gap for the best catalyst, 5Fe-MONW/CNF, is ΔE = 922 mV (140 mV lower than non-doped MONW/CNF and between 160 and 320 mV lower than the individual components, MONW and CNF). Endurance tests show remarkable stability of the iron-doped MONW/CNF, with a stable potential and an even lower ΔE of 800 mV for ca. 20 h of operation (charge-discharge cycles at ± 10 mA cm−2).