Solar hydrogen generation from ambient humidity using functionalized porous photoanodes.

Abstract

Solar hydrogen is a promising sustainable energy vector and steady progress has been made in the development of photoelectrochemical cells. Most research in this field has focused on using acidic or alkaline liquid electrolytes for ionic transfer. However, the performance is limited by (i) scattering of light and blocking of catalytic sites by gas bubbles and (ii) mass transport limitations. An attractive alternative to a liquid water feedstock is to use the water vapor present as humidity in ambient air, which has been demonstrated to mitigate the above problems and can expand the geographical range where these devices can be utilized. We show here how the functionalization of porous TiO2 and WO3 photoanodes with solid electrolytes - proton conducting Aquivion® and Nafion® ionomers, enables the capture of water from ambient air and allows subsequent photoelectrochemical hydrogen production. The optimization strategy of the photoanode functionalization was examined through testing the effect of ionomer loading and the ionomer composition. Optimized functionalized photoanodes operating at 60% relative humidity (RH) and Tcell=30-70°C were able to recover up 90% of the performance obtained at 1.23 V vs RHE when water is introduced in liquid phase (i.e. conventional PEC operation). Full performance recovery is achieved at higher applied potential. In addition, long term experiments have shown remarkable stability at 60% RH for 64 h of cycling (8 h continuous illumination - 8 h dark) demonstrating that the concept can be applicable outdoors.