Oxygen vacancy modulation for enhanced hydrogen production via chemical looping water-gas shift

Abstract

Chemical looping water gas shift (CL-WGS)is prospective to generate high-purity hydrogen with integrated CO₂ capture. However, this technology is impeded by the lack of active oxygen carriers at mid-temperatures. Here, we synthesized several Ni-doped CoFe₂O_4-δ to modulate oxygen vacancies and investigate its effect on promoting hydrogen production reaction via chemical looping water gas shift at 650 °C. The findings delineate that doping Ni considerably lowers the energy barriers associated with the oxygen vacancies formation, thereby augmenting their concentration. The underlying mechanism elucidates that within the CL-WGS process, the transfer of lattice oxygen acts as the rate-limiting step. Ni_xCo_1-xFe₂O₄ lowers the formation energy of oxygen vacancies and facilitates the bulk lattice oxygen diffusion through the bulk. Hence, Ni_0.5Co_0.5Fe₂O₄ demonstrates the most reduction depth and reversibility via redox reactions, resulting in an elevated hydrogen yield (∼15.5 mmol g⁻¹) at 650 °C, which surpasses the yield from undoped CoFe₂O₄ by 1.4 times. This performance remains consistently high with only a minimal decline over 100 cycles. The findings introduce a promising approach to promote the reactivity of oxygen carriers, particularly for mid-temperature applications.