Influence of OxideReservoirs on the Performance ofDirect Air Electrolysis Using NaClO₄ as a DeliquescentNeutral Electrolyte Salt

Water electrolysis using solar power is a key technology for producing green hydrogen. However, in many areas with high densities of solar radiation and stable weather conditions, for example, arid regions, it is difficult to even access freshwater for daily life. On the other hand, even in the arid regions, the atmosphere contains a certain amount of water vapor. In this study, we investigated water electrolysis using water vapor from the air, specifically direct air electrolysis (DAE) using NaClO₄ as a deliquescent neutral electrolyte salt and SiO₂, Al₂O₃, MFI-type and LTA-type zeolites, and TiO₂ as reservoirs. After the DAE modules remained in the air at ∼83% relative humidity (RH), the water vapor was captured by NaClO₄ loaded onto all reservoirs, forming an electrolyte solution. The amount of water captured from the humid air increased with an increasing amount of NaClO₄. Water electrolysis began after the stay in the humid air for 5–12 h in all cases. After water absorption for 20 h under 83% RH, the current densities of DAE with Al₂O₃, MFI-type zeolite, and TiO₂ as reservoirs were comparable to that measured in a NaClO₄ aqueous solution. Water captured in the TiO₂ reservoir was efficiently electrolyzed even when less than 40 vol % of the reservoir was filled with the electrolyte solution.