Proton exchange membrane electrolysis sustained by water vapor

Abstract

The current–voltage characteristics of a proton exchange membrane (PEM) electrolyzer constructed with an IrRuO_x water oxidation catalyst and a Pt black water reduction catalyst, under operation with water vapor from a humidified carrier gas, have been investigated as a function of the gas flow rate, the relative humidity, and the presence of oxygen. The performance of the system with water vapor was also compared to the performance when the device was immersed in liquid water. With a humidified Ar(g) input stream at 20 °C, an electrolysis current density of 10 mA cm⁻² was sustained at an applied voltage of ∼1.6 V, with a current density of 20 mA cm⁻² observed at ∼1.7 V. In the system evaluated, at current densities >40 mA cm⁻² the electrolysis of water vapor was limited by the mass flux of water to the PEM. At <40 mA cm⁻², the electrolysis of water vapor supported a given current density at a lower applied bias than did the electrolysis of liquid water. The relative humidity of the input carrier gas strongly affected the current–voltage behavior, with lower electrolysis current density attributed to dehydration of the PEM at reduced humidity values. The results provide a proof-of-concept that, with sufficiently active catalysts, an efficient solar photoelectrolyzer could be operated only with water vapor as the feedstock, even at the low operating temperatures that may result in the absence of active heating. This approach therefore offers a route to avoid the light attenuation and mass transport limitations that are associated with bubble formation in these systems.