Self-supporting ternary metal–organic framework-based electrocatalysts for efficient oxygen evolution: Roles of inorganic anions

Self-supporting metal–organic frameworks (MOFs) have attracted considerable attention as promising catalysts for water electrolysis. The role of anions in the precursor solution in modulating their growth and performance has been increasingly recognised. This study reports the systematic investigation of the impact of anion species and their concentrations in precursors on the oxygen evolution reaction (OER) performance of self-supported MOFs. X-ray diffraction and field emission scanning electron microscopy results revealed significant differences in their crystallinity, crystal structure and morphology. Furthermore, inductively coupled plasma mass spectrometry, ion chromatography and nitrogen adsorption–desorption results indicated that the precursor containing chloride ion (Cl⁻) formed MOFs with the highest metal content and largest specific surface area. When acetate ions (CH₃COO⁻) were used exclusively, the resulting MOFs exhibited a moderate metal content, with the largest pore volume and average pore size. Moreover, the presence of nitrate ion (NO₃⁻) resulted in MOFs with the smallest specific surface area. Electrochemical analysis results indicated that the OER activity of the catalysts decreased with increasing Cl⁻ and NO₃⁻ concentrations, while CH₃COO⁻ was the most favourable anion for enhancing MOF catalytic performance. The self-supported ternary catalyst FCN-MOF-AA, synthesised via the solvothermal method using CH₃COO⁻ precursors, exhibited superior OER performance with low overpotentials of 191 and 242 mV at 10 and 100 mA cm⁻², respectively, a Tafel slope of 22.79 mV dec⁻¹ and stable operation for 500 h at 15 mA cm⁻². This study highlights the critical role of precursor anions in modulating the catalytic properties of MOFs.