Solar charging of a Zn-air battery

Solar-charging of Zn-air batteries has been studied by employing a photoelectrocatalytic or a photovoltaic system. Discharging of a Zn-air battery corresponds to oxidation of Zn and formation of ZnO. Charging is the reverse action, i.e., the reduction of ZnO and deposition of metallic Zn on the Zn electrode. In a typical Zn-air battery, charging also corresponds to water oxidation on the counter electrode and oxygen evolution. Because of the high overpotential for the oxygen evolution reaction, charging of the battery necessitates very high voltages, much higher than the voltage produced by the battery. This problem can be overcome by substituting oxygen evolution reaction by photocatalytic oxidation of an organic substance, which may be a biomass waste. This photoelectrocatalytic charging was presently realized with 100% Faradaic efficiency. More than 2 V of power gain was achieved through photoelectrocatalytic charging, i.e., a small input voltage was then necessary in order to charge the battery. Battery charging was also possible by using a photovoltaic cell as long as the cell provides enough open circuit voltage to overcome battery potential, overpotentials and losses. In the present case charging was possible with a commercial photovoltaic cell generating 2.25 V at an incident light intensity of 100 mW cm−2.