Oxygen evolution activity of nickel-based phosphates and effects of their electronic orbitals†

Abstract

Metal phosphate-type compounds have been utilized in diverse applications, and their distinctive chemical properties have recently opened avenues for their use as catalysts. Metal phosphates have previously demonstrated significant electrocatalytic activity for the anodic oxygen evolution reaction (OER) in electrochemical water splitting. However, the critical factors influencing OER electrocatalysis on Ni-based phosphates have been insufficiently explored. We herein demonstrate nickel (Ni)-based phosphates—monoclinic Ni₃(PO₄)₂, monoclinic Ni₂P₂O₇, and monoclinic Ni₂P₄O₁₂—as exemplary materials exhibiting outstanding OER activity in alkaline media. These Ni-based phosphates exhibit superior OER overpotentials compared to conventional Ni-based oxides (NiO) and phosphides (Ni₂P). Additionally, their OER-specific activity surpasses that of the rare metal-based benchmark, IrO₂, and previously reported state-of-the-art crystalline electrocatalysts comprising nonprecious metals. Long-term durability tests show that Ni₃(PO₄)₂ maintains its OER activity even after 1000 repeated potential cycles while retaining its elemental composition and Raman spectrum. To understand the excellent OER activities of Ni-based phosphates, the atomic configurations within their crystals are examined. Remarkably, a clear correlation between Ni–O bond length and OER overpotentials is observed in both Ni-based phosphates and NiO, i.e., shorter Ni–O bond lengths are highly beneficial for the OER. Density functional theory (DFT) calculations revealed that the outstanding OER activities of Ni-based phosphates are facilitated by their favorable electronic orbitals, which strengthen the Ni–O bond and improve the adsorption of OER intermediates on Ni sites. This mechanism is substantiated by DFT calculations employing surface slab models, where the adsorption of OER intermediates on the surface of Ni-based phosphates is more energetically favorable than on the surface of NiO. Hence, Ni-based phosphates are promising OER electrocatalysts, and this study provides important guidelines to further improve Ni-based electrocatalysts.