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

• Published in: Energy conversion and management 2022-01, Vol.252, p.115125, Article 115125
• Main Authors: Gao, Yibo, Zhang, Miaomaio, Mao, Yanpeng, Cao, Han, Zhang, Shujuan, Wang, Wenlong, Sun, Chenggong, Song, Zhanlong, Sun, Jing, Zhao, Xiqiang
• Format: Article
• Language: English
• Subjects: Energy conversion efficiency; Energy efficiency; Entropy; High entropy oxide; Hydrogen; Hydrogen evolution; Hydrogen production; Irradiation; Low temperature; Microwave; Nanoparticles; Oxides; Oxygen; Oxygen exchange; Thermal reduction; Thermochemical water splitting; Vacancies; Water splitting; Zirconium oxides
• ISSN: 0196-8904; 1879-2227
• DOI: 10.1016/j.enconman.2021.115125

•Zr-modified high entropy FeMgCoNiOx oxides was firstly used for solar thermochemical hydrogen production.•An operating strategy of microwave-driven reduction combined with conventional heating oxidization was proposed.•An exceptionally high H2 yield was obtained by high entropy FeMgCoNiOx/Zr0.6 oxide during water splitting cycle.•Microwave-triggered low temperature thermal reduction improved the thermodynamic energy efficiency (50 %). 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 °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/Zry that was previously reduced by microwave irradiation. By tuning the level of Zr4+ content from 0.0 to 1.0, FeMgCoNiOx/Zr0.6 was found to show the best trade-off, giving rise to a H2 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 H2 evolution activity. Moreover, the energy efficiency will be further improved through tighter control of microwave reduction energetics.