Electrochemically Activated Tetra-Metallic Phosphate (Mn–Fe–Co–Ni) Electrocatalyst for the Oxygen Evolution Reaction (OER) in Alkaline Media

Abstract The development of highly active and stable oxygen evolution reaction (OER) catalysts based on earth-abundant and low-cost metals remains a major challenge in the field of electrochemical water splitting. Previous studies have shown that metallic and bimetallic phosphates exhibit promising catalytic activity but often suffer from limited long-term stability. In this work, tetrametallic phosphate CoNiFeMn(PO4) microparticles were synthesized using a simple and cost-effective precipitation method with thermal annealing. These microflakes exhibit an average length and width of 4.14 and 1.21 μm, respectively. An electrochemical activation of the catalyst was done by cyclic voltammetry between 1.05 and 1.50 V vs RHE at a scan rate of 10 mV/s in 0.1 M KOH. X-ray photoelectron spectroscopy (XPS) analysis after activation revealed a decrease in the atomic percentage of P from 7.3% to 2.7% and a concomitant increase in O from 74.8% to 78.0%, while the formation of cobalt species with coexisting Co2+ and Co3+ oxidation states, consistent with Co2O3, was observed at a lower scan rate (10 mV s–1). The activated CoNiFeMn(PO4) microflakes demonstrated enhanced OER catalytic activity compared to the RuO2 reference catalyst, achieving an overpotential of 429 mV at 10 mA cm–2 in 0.1 M KOH and 360 mV at 10 mA cm–2 in 1.0 M KOH, with a Tafel slope of 40 mV dec–1, corresponding to the lowest overpotential among the investigated catalysts and 17 mV lower than that of RuO2. In addition, the long-term stability, evaluated by chronopotentiometry on a catalyst-modified GC-RDE at 1600 rpm and at constant current density of 10 mA cm–2 in 0.1 M KOH, showed that RuO2 exhibited a continuous increase in overpotential, exceeding that of CoNiFeMn(PO4) by 235 mV at 9 h, reflecting poor stability due to catalyst dissolution and structural degradation. CoNiFeMn(PO4) maintained a stable overpotential of ∼400 mV over 24 h with only a 7.4 mV variation, confirming its superior durability. The synergistic effect among the four transition metals plays a key role in achieving enhanced OER performance relative to single-metal phosphate catalysts.