Direct Chemical Synthesis of Lithium Sub-Stochiometric Olivine Li_0.7Co_0.75Fe_0.25PO₄ Coated with Reduced Graphene Oxide as Oxygen Evolution Reaction Electrocatalyst

The oxygen evolution reaction (OER) is of fundamental importance for the development of renewable energies. Research on earth-abundant, efficient, and stable OER electrocatalysts is a key factor for the technological improvement of water-splitting systems. In this article, we demonstrate OER performance of olivine-type lithium earth-abundant transition-metal phosphates coated with reduced graphene oxide (rGO). We report on LiCoPO₄/rGO and the solid solutions Li_1-xCo_yFe_1-yPO₄/rGO synthesized by a nonaqueous sol-gel microwave reaction. Interestingly, such conditions allow for the direct synthesis of substochiometric Li_0.7Co_0.75Fe_0.25PO₄/rGO compound with improved OER activity. The materials crystallize in the olivine orthorhombic phase with nanoplatelet-like shape with nanometer-size thickness and micrometer-size width. The substochiometric phase Li_0.7Co_0.75Fe_0.25PO₄/rGO exhibits the best OER activity in alkaline medium, generating a current density of 5 mA cm^-2 at an overpotential of 0.39 V and a Tafel slope of 44.5 mV dec^-1 in 6 M KOH with a low 51 μg cm^-2 loading, outperforming previously reported results in terms of mass effectivity and Tafel slope.