Rechargeable Photoactive Zn-Air Batteries Using NiCo2S4 as an Efficient Bifunctional Photocatalyst towards OER/ORR at the Cathode

Using free solar energy in oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysts to enhance the efficiency of zinc-air batteries (ZABs) has not yet been widely investigated. Herein, we report a photoactive bifunctional catalyst of spinel-type NiCo2S4 (NCS) urchin-like structure with rich mesopores and direct band gap energies of ca. 1.4 and 2.4 eV. The NCS catalyst exhibits high catalytic activities for both OER (eta = 338 mV at 10 mA cm(-2)) and ORR (eta = 475 mV at E-1/2), comparable to that of the state-of-the-art counterparts (e. g., Pt/C for ORR, RuO2 for OER). Under light illumination, the p-type photoactive NCS catalyst can absorb visible light, generating photogenerated holes and photoelectrons via the photoelectric effect for direct conversion of photoenergy into electric energy with increasing kinetics charge transfer process and provides ca. 10 and 18.5 % lower OER and ORR overpotentials, respectively than those under the dark condition. In addition, the as-fabricated zinc-air battery with the photoactive NCS as the cathode exhibits decrease in voltage gap from 0.82 to 0.60 V with an increasing round-trip efficiency from 59.2 % to 68.8 % after exposed to visible light. The zinc-air battery with a reversible redox reaction for the simultaneous conversion of chemical and photoenergy into electric energy could open a new pathway for the utilization of a single energy conversion and storage device.