Microwave-triggered low temperature thermal reduction of Zr-modified high entropy oxides with extraordinary thermochemical H2 production performance

Solar-driven two-step thermochemical H2O splitting has emerged as a promising strategy for hydrogen production, but the conversion efficiency of solar-to-fuel has to be improved to make it more economically viable. Here, we present insights on microwave-triggered low temperature (600 degrees C) oxygen exchange for water splitting based on using Zr-modified high entropy oxides. The as-synthesized nanoparticles were characterized by XRD, FTIR, SEM, TEM, BET and EIS. The characterization results showed that the introduction of Zr4+ enlarged the Metal-Oxygen bond length of the spinel phase of FeMgCoNiOx and produced more oxygen vacancies with the increase of Zr4+ concentration, which greatly improved the thermochemical performance of FeMgCoNiOx. Water was splitted via reaction with FeMgCoNiOx/Zr-y that was previously reduced by microwave irradiation. By tuning the level of Zr4+ content from 0.0 to 1.0, FeMgCoNiOx/Zr-0.6 was found to show the best trade-off, giving rise to a H-2 yield of 4.84 mmol/g that was two times higher than that of FeMgCoNiOx (2.35 mmol/g) and outstanding thermodynamic energy efficiency (50 %). This work demonstrates that designing materials with abundant oxygen vacancies is a very efficient strategy to improve their thermochemical H-2 evolution activity. Moreover, the energy efficiency will be further improved through tighter control of microwave reduction energetics.