Synthesis and characterization of transition metal fluorides for electrochemical energy conversion devices

• Transition metal (Co, Cu, Fe, Ni, Zn) fluorides effectively synthesised by co-precipitation. • The same synthesis method resulted in different fluoride morphologies depending on TM. • The most efficient catalysis of oxygen electrode reactions achieved with CoF 2. • Electron-withdrawing from TM affects/optimises the intermediates adsorption during ORR. • Higher metal oxidation state and dynamic surface reconstruction lead to efficient OER. The increasing demand for efficient and sustainable energy technologies has intensified interest in advanced materials for electrochemical energy conversion and storage. Among these, metal fluorides constitute a unique class of compounds that offer high ionic potential, chemical stability, and favorable redox properties. In this study, five transition metal fluorides (CoF 2 , NiF 2 , ZnF 2 , CuF 2 , and FeF 3 ) were synthesized by the coprecipitation method. X-ray diffraction analysis and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX) revealed the presence of oxygenated species alongside the targeted fluoride phases and distinct morphologies. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performances of the fluorides were analyzed, with CoF 2 showing the most promising performance. CoF 2 exhibited the highest electric double-layer capacitance and electrochemically active surface area. ORR study revealed a four-electron reduction mechanism and the lowest Tafel slope (112 mV dec −1 ) for CoF 2 . CoF 2 further exhibited the lowest OER Tafel slope (137 mV dec −1 ) and charge-transfer resistance, resulting in high current densities during OER. The stability of transition-metal fluorides during both reactions was investigated using chronoamperometry, with post-mortem SEM/EDX analysis witnessing surface reconstruction.