Synthesis and characterisation of Cu1-xNi0.5xZn0.5xFe2O4 nanoferrites for hydrogen production from the hydrogen carrier NaBH4 via methanolysis

Abstract

The transition to sustainable hydrogen energy requires efficient and cost-effective catalysts to replace noble metals in hydrogen release reactions. Addressing this challenge, this study designs a novel series of multi-cationic spinel nanoferrites with a balanced ternary cation substitution, Cu1-xNi0.5xZn0.5xFe2O4 (x = 0.2, 0.6, 0.8, 1), synthesised via a sol-gel self-combustion method. These materials were investigated as catalysts for H2 production from the chemical hydrogen carrier, sodium borohydride (NaBH4) via methanolysis. Fundamental characterisations (XRD, SEM and FTIR) were applied to the entire Cu1-xNi0.5xZn0.5xFe2O4 system and on the reference materials (NiFe2O4 and ZnFe2O4) to verify the formation of the spinel phase and morphological homogeneity. Advanced characterisation, including Raman spectroscopy performed on both the optimal catalyst and the references (NiFe2O4 and ZnFe2O4), supplemented by TPR and TPD analyses specifically on the best-performing Ni0.5Zn0.5Fe2O4 sample, provided a better understanding of the structure-activity relationships. This revealed the synergistic effects between Ni and Zn cations, driving the catalytic performance. This catalyst demonstrated superior activity in methanolysis, achieving a hydrogen generation rate of 29661 mL.min-1.gcat-1 with a low activation energy of 25.66 kJ.mol-1, outperforming its performance in both hydrolysis (10,096 mL. min-1.gcat-1) and ethanolysis (16,587mL.min-1.gcat-1). Although recyclability tests showed a gradual activity decrease over five cycles with the hydrogen evolution rate declining from 29,661.6 to 18,010.3 mL.min-1.gcat-1 due to partial active site deactivation, this work fundamentally underscores the critical role of tailored cation synergy in spinel nanoferrites, providing valuable insights for designing sustainable, cost-effective heterogeneous catalysts for clean energy applications.