Dual Alkaline Ion Route to Chemical De-insertion in Oxygen Evolution Olivine Electrocatalysts

The oxygen evolution reaction (OER) is an essential reaction for the sustainable synthesis of fuels from renewable feedstocks. In recent years, cathode materials for Li-ion batteries have been successfully utilized as OER electrocatalysts, and among them are the lithiated transition-metal phosphates possessing an olivine crystallographic phase. The LiMPO₄ (M = Ni, Fe, Co) olivine materials display a potentially suitable overpotential for OER, which can be further enhanced by delithiation. Herein, we report the synthesis of olivine phases with both Li and Na as alkaline ions. The combination of alkaline ions stabilizes the olivine structure with low alkaline ion content, which is instrumental to reach low OER overpotential. The dual alkaline ion approach has been demonstrated for a single transition metal (Co) as well as different combinations of Co–Fe–Ni to form olivine solid solutions of the chemical formula Li­(Na)­Co­(Fe,Ni)­PO₄. This screening of eight olivine materials revealed a trend in OER activity, and the Li_0.6Na_0.2Ni_0.7Fe_0.1PO₄ solid solution displays an overpotential as low as 0.28 V versus RHE at 10 mA cm^–2, with a low Tafel slope of 30 mV dec^–1 and a stability tested over 20 h.