Constructing vacancy-rich metal phosphates by the spatial effect of ionic oligomers for enhanced OER activity

Abstract

The Oxygen Evolution Reaction (OER) is a critical half-reaction in green energy devices and has become a bottleneck in improving energy conversion efficiency. Introducing vacancies in electrocatalysts is recognized as a promising method to improve their OER reactivity. However, the production of catalysts through the ion-by-ion nucleation pathway limits the generation of vacancies due to the closely-packed ionic structure. Here, we developed nickel–cobalt phosphate (NiCoPi) oligomers with an average size of 1–2 nm, which were used as building blocks for the NiCoPi catalyst. The oligomer-by-oligomer crosslinking process has a spatial effect, which could introduce more sub-nano-sized gaps between oligomer blocks. These gaps acted as the vacancies in the solid phase of NiCoPi, where the exposed oxygen largely improved the OER activity of NiCoPi (with a low overpotential of 254 mV at 10 mA cm⁻¹ and a small Tafel slope of 45.75 mV dec⁻¹) through lattice oxygen-mediated (LOM) reaction pathways, and this reactivity was superior to those of all the reported NiCoPi catalysts. The oligomer building blocks were extendable to more bimetallic transition metal phosphates (e.g. Ni–Fe phosphate, Co–Fe phosphate, etc.) for vacancy manufacturing in solid catalysts, and their performances exceeded those of all the analogous phosphate catalysts. This work developed an alternative way to build high performance electrocatalysts by rationally introducing vacancies. Moreover, it presented the important role of ionic oligomers in the regulation of solid structures at sub-nano-sizes.