One-step synthesis of bifunctional iron-doped manganese oxide nanorods for rechargeable zinc-air batteries

Rechargeable zinc-air batteries for portable electrical devices have attracted considerable attention due to their high energy density combined with light weight, environmentally benign nature and moderate cost. However, one of the major operational concern for the successful commercialization of Zn-air batteries lies in the cost of the electrodes required for oxygen catalytic activities. In this study, we report a series of inexpensive first row transition metal oxide-based electrocatalysts that show excellent bifunctionality towards both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). A bifunctional catalyst having efficiency both for ORR and OER would be highly desirable as it can not only reduce cost but also suppress undesired side reactions. Here, Fe-doped manganese oxide (MnO2) nanorods were synthesized by a one-step, low-cost, facile hydrothermal method. Different ratios of Fe-doping were invorporated in the MnO2 nanorods and their bifunctional ORR and OER activities were compared with the undoped MnO2, commercially available ORR catalysts (Pt/C) and OER catalysts (RuO2), respectively. The most efficient Fe-doped MnO2 showed an onset potential of 0.89 V (vs. RHE), which is very close to commercial Pt/C (onset potential = 1.01 V) for ORR. The optimal Fe-doped MnO2 material exhibited an overpotential of 660 mV for OER at a current density of 10 mA cm(-2) and a Tafel slope of 58 mV per decade. The overall oxygen electroactivity Delta E = E-j(OER)=10 - E-1/2(ORR) = 1.17 V for the best Fe-doped MnO2 was lower than that of the various state-of-the-art bifunctional electrocatalysts. Finally, a solid-state zinc-air battery was made for powering a light-emitting diode. This work provides an easy strategy to develop low-cost doped oxide nanostructures with multiple functionality for energy conversion and storage devices.