Efficient generation of hydrogen by two-step thermochemical cycles: Successive thermal reduction and water splitting reactions using equal-power microwave irradiation and a high entropy material

Hydrogen production through two-step thermochemical water splitting cycle based on metal oxide has emerged as a promising strategy to store dilute and intermittent solar energy. However, a typical reaction time of the two-step thermochemical water splitting cycle is lengthy, with at least 0.5 h for thermal reduction step and 1 h for water splitting step, and the energy required in the thermal reduction process for hydrogen regeneration is higher than the generated hydrogen energy. In this work, to overcome the problem of an energy efficiency imbalance, we investigated the possibility of the rapid and successive reactions of thermal reduction and water splitting, using short-term irradiation from a low-energy microwave. To this end, a high entropy material, as a poly-metal oxide used to generate hydrogen, was produced by simultaneously introducing four cations onto a SiC foam - (FeMgCoNi)O-1.2@SiC. It was found that the oxygen vacancy of the (FeMgCoNi)O-1.2@SiC could be significantly increased by short-term microwave irradiation, and hence the thermal reduction process took only 4 min, which is much less than normal. High H-2 generation rates were achieved by re-oxidation of the Fe (II) to Fe (III) of the (FeMgCoNi)O-1.2@SiC. In addition, the microwave plasma generated by microwave irradiation induced (FeMgCoNi)O-1.2@SiC discharge could enhance the water splitting process. The maximum hydrogen yield was 122 mL/g at 700 W, due to the coupling effect of the thermochemical cycle and microwave plasma. In this way, the power consumption of microwave process is only 3% of that of conventional high-temperature heat treatment during thermal reduction process.