Abstract: A light-driven electrolytic cell that uses water vapor as the feedstock and that has no wires or connections whatsoever to an external electrical power source of any kind. In one embodiment, the electrolytic cell uses a proton exchange membrane (PEM) with an IrRuOx water oxidation catalyst and a Pt black water reduction catalyst to consume water vapor and generate molecular oxygen and a chemical fuel, molecular hydrogen. The operation of the electrolytic cell using water vapor supplied by a humidified carrier gas has been demonstrated under varying conditions of the gas flow rate, the relative humidity, and the presence or absence of oxygen. The performance of the system with water vapor was also compared to the performance when the device was immersed in liquid water.