Hydrolysis rate of submicron Zn particles for solar H2 synthesis

The hydrolysis rate of Zn particles by up to 50 mol% water vapor in Ar gas was measured by thermogravimetric analysis at atmospheric pressure and 330–360 °C and quantified by a core-shell model. An initial ZnO layer led to an initially linear conversion profile attributed to a fast surface reaction (half-order with respect to water vapor mole fraction, y) followed by a parabolic conversion profile independent of y but dependent on Zn ion diffusion through a ZnO layer. The latter is most important for solar H2 formation by the Zn/ZnO water-splitting cycle as it determines the required process residence time for Zn hydrolysis. A ready-to-use equation for calculation of ZnO and H2 formation during Zn hydrolysis is proposed and compared to literature data revealing enhanced hydrolysis rates for submicron Zn particles.