Compositional Engineering of NiSe₂ Precatalysts with IrO_x for Controlled Reconstruction toward Improved Alkaline OER

The development of efficient and durable oxygen evolution reaction (OER) electrocatalysts remains a challenge in alkaline water electrolysis. Nickel-based materials are well explored as OER precatalysts due to their ability to undergo structural reconstruction into an active nickel oxyhydroxide phase (NiOOH) under anodic conditions. Interestingly, nickel-based chalcogenides, such as nickel selenides, also undergo reconstruction but form a more active surface compared to conventional nickel oxide catalysts, making them appealing for water electrolysis application in alkaline media. Here, we explore a strategy to modulate the chemical composition of nickel selenide by incorporating small amounts of iridium oxide (IrO_x) nanoparticles. A series of catalysts with varying Ni:Ir ratio proportions were synthesized, and we demonstrate that IrO_x incorporation leads to a reduction in particle size, an increase in surface area, and a modification of surface composition, resulting in a lower OER overpotential. Online electrochemical dissolution measurements reveal that selenium acts as a sacrificial species during the structural reconstruction to form the NiOOH phase, while IrO_x suppresses excessive Se leaching. This behavior is rationalized using hard-soft acid-base theory, where the replacement of soft-base Se₂^2- by hard-base oxygen species is driven by stronger interactions with Ni³⁺, a hard acid. The optimal composition (IrO_x-NiSe₂ with Ir atomic percentage of 2.4%) achieves a balance between accessible active sites and favorable intermediate adsorption. In contrast, higher IrO_x content led to performance decline due to site blocking, as predicted by Sabatier's principle. This work highlights how compositional control and surface reconstruction can guide the design of OER precatalysts, while also deepening the understanding of oxidative transformation mechanisms in nickel chalcogenides, particularly selenides.