Room-temperature thermochemical water splitting: efficient mechanocatalytic hydrogen production

Abstract

Considering climate change and the environmental pollution crisis, CO₂-emission-free H₂ production based on green, low-energy processes is critically important. In this article, we report the accidental discovery and subsequent investigation of room-temperature thermochemical water splitting in a planetary ball mill. H₂ yields of ∼20–1600% were obtained by milling six metals (Al, Zn, Fe, Ti, Mn, and Sn) and their oxides in water at 30–38 °C. Remarkably, when Ti was milled, H₂ was generated continuously until the water was consumed. Experimental and theoretical investigations—based on redox potentials, Gibbs energies, collision energies, and local temperatures and pressures—revealed that the milling medium acts as a mechano-cocatalyst, regenerating the catalyst (Ti and/or titanium oxides). Thus, thermochemical water splitting, via a cyclic mechanocatalytic TiO₂/Ti–water reaction, was responsible for the continuous H₂ production. The high H₂-production rate was attributed to reactions occurring in supercritical water at impact sites between balls, where the instantaneous local pressure and temperature were 11 GPa and 1300 °C, respectively. The production of high-purity hydrogen (>99%) using seawater feedstocks in a compact (∼50 cm), low-power (∼0.26 kW) reactor indicates that on-site, on-demand H₂ production is achievable.