Effect of Cu doping on morphology and properties of calcium ferrite and its application as oxygen carrier in chemical looping hydrogen production

Abstract

Chemical looping hydrogen production with inherent CO₂ capture has been widely recognized as a clean and efficient approach to high-purity hydrogen production because of the ultra-pure H₂ product without any purification facilities. It is crucial to develop oxygen carriers with better performance to improve fuel conversion and hydrogen production efficiency. The potential of Ca₂Fe₂O₅ in steam converting has been confirmed. However, its lower oxygen transfer capacity, that is, it tends to produce lower fuel conversion in fuel reactors, which will limit its practical application. Here, we report the development of Cu-doped Ca₂Fe₂O₅-based oxygen carriers using sol-gel technology. The effects of B-site substitution of Cu element on the morphological properties and redox properties of Ca₂Fe_2-xCu_xO₅ (x = 0, 0.1, 0.25, 0.5, 1) oxygen carriers were evaluated based on experiments and density functional theory calculations. The results show that Cu doping not only elevated the surface oxygen content and enhanced the oxygen activity of the oxygen carriers, but also increased the Fe³⁺ at the B-site, thus enhanced their binding ability with oxygen molecules. Vacancy formation was a rate-determining step in the chemical looping hydrogen production (CLH), and Cu-doped Ca₂Fe₂O₅ reduced the energy of oxygen vacancy formation. In the CLH process, the doping of Cu significantly improved the hydrogen productivity and fuel conversion rate. The fuel conversion rate was positively correlated with the doping amount of Cu. When x = 1, Ca₂Fe_2-xCu_xO₅ had the maximum fuel conversion rate, and its average conversion was 54.2% more than that of undoped Ca₂Fe₂O₅. Ca₂Fe_2-xCu_xO₅ with x = 0.25 was the most suitable for CLH with the highest H₂ yield, which was 20.3% more than that of Ca₂Fe₂O₅. Moreover, its properties remained stable over multiple redox cycles with high activity and stability for CO-CLH.