Combining strategies of structural design and oxidation states control of electrocatalysts for cost-effective polymer electrolyte membrane water electrolysis

Abstract

For the efficient utilization of intermittent electricity produced by renewable energies, the proton exchange membrane water electrolyzer (PEMWE) has been attracting significant attention by converting surplus electric energy into a form of transportable hydrogen energy. However, the high cost of noble metal (Ir and Ru) based catalysts for oxygen evolution reaction (OER) hinders the wide application of PEMWE. It is a challenging issue to improve OER catalytic activity while decreasing the amount of the noble metal to the level of 0.01 g kW-1 in the membrane-electrode-assembly (MEA) for wide application of PEMWE in near future. Three major strategies are introduced in this dissertation to improve OER performance while reducing Ir usage to develop cost-effective PEMWE. By combining those strategies, the key factors for the OER performances and each role of them were investigated. The first one is developing macropores in mesoporous structures using silica or polymer-based nanospheres while maintaining a high specific surface area to increase both OER active sites and mass transport. Each role of meso- and macropores were revealed and the enhancement of OER activity was observed in both RuO2 and IrO2. And then, Ir oxidation states were controlled using glycine as an additive while obtaining ultrahigh specific surface area. The ratio of active Ir(III) was controlled by annealing temperature and the structure of IrOx@IrO2 was demonstrated by X-ray-based spectroscopies. Owing to the stable Ir(IV) species in the catalysts, balanced OER activity and stability were obtained. Finally, combining the other two strategies, mesoporous Ta2O5 was developed as an OER support to reduce Ir usage. In addition, the introduction of macropores in mesoporous Ta2O5 demonstrated their role of them to reduce mass transport resistance even in metal oxide support. The uniform distribution of Ir nanostructure on the porous Ta2O5 increased ECSA and strong interaction between Ir and Ta2O5, electron transfer from Ta to Ir, maintained Ir species at a relatively low oxidation state of the Ir (III). A significant reduction in Ir usage was demonstrated by comparing specific power density with OER catalysts developed in this dissertation and recently reported literature, resulting in a decrement in the PEMWE cost. To prove the enhancement of catalytic performance in a practical device, all the catalysts were analyzed at the single-cell level and demonstrated the improvement of catalytic activity and stability in PEMWE.