Catalytic investigation of ceria-zirconia solid solutions for solar hydrogen production

This study addresses the solar thermochemical production of hydrogen from water-splitting cycles using ceria-zirconia solid solutions prepared via soft chemistry methods. The effect of zirconium doping on the catalytic activity of ceria for hydrogen production was studied using thermogravimetric analysis. The influence of the zirconium content between 10% and 50% on the redox properties of the Ce_1−δZr_δO₂ material was investigated. The higher the amount of zirconium, the higher the reduction yields. The reduction yield at 1400 °C in inert atmosphere was 9% for 10% Zr, 16% for 25% Zr, and 28% for 50% Zr. However, increasing the Zr content did not automatically lead to the highest amount of hydrogen produced during cycling. Indeed, the powder with 25% Zr produced 334 and 298 μmol H₂/g at 1050 °C during the first and the second cycle, respectively. In contrast, the powder with 50% Zr yielded 468 and 266 μmol H₂/g during the two successive cycles. Moderate Zr contents thus favored H₂ production during repeated cycles without any significant reactivity losses. A kinetic study of the reduction and the hydrolysis steps was proposed. The activation energies for the thermal reduction and the hydrolysis of Ce_0.75Zr_0.25O₂ were 221 kJ/mol and 51 kJ/mol, respectively. Finally, the use of a template molecule during synthesis was considered, which improved the reduction yield markedly (up to 52%) but strong sintering phenomena limited the hydrogen production and the material cyclability.     

A new class of bubble-free water electrolyzer that is intrinsically highly efficient

A new class of ‘bubble-free’ alkaline electrolyzer with electrodes comprising of PTFE-based Gortex gas diffusion layers coated with catalysts, is described (PTFE = poly(tetrafluoroethylene)). At ≥80 °C (E^o_cell 1.18 V), the electrolyzers displayed the lowest cell onset potentials (≥1.28 V) yet reported, indicating that they exhibit the highest-known intrinsic efficiency when the influence of impedance is stripped out. The overpotentials at each electrode, particularly the oxygen-generating anode, were significantly diminished by the presence of the porous Gortex substrate, which exhibited a powerful ‘gas-philic’ capillary action (6.3 bar capillary pressure). The bubble-free process arose from preferential coalescence of newly-formed gases on the PTFE surfaces, where the capillary action of the Gortex continuously extracted them before they could nucleate bubbles. In so doing, observable bubble formation was avoided, along with the energy penalties associated with the formation and release of gas bubbles.