Atomic-Scale insights into oxygen evolution reaction on Pristine and reconstructed IrO2(110) surfaces

Electrocatalytic water splitting in proton‑exchange‑membrane electrolysers is limited at the anode by the sluggish kinetics of the oxygen evolution reaction (OER) — 2H2O → O2(g) + 4H+ + 4e-. IrO2(110) is the benchmark OER catalyst in acidic media, yet the reaction mechanism that governs its high activity remains unsettled, with contradictory reports in the literature regarding the dominant pathway and rate‑determining step. This work combines density functional theory computations with thermodynamic and kinetic analysis to construct a complete mechanistic picture of OER on both pristine and reconstructed IrO2(110) surfaces. Several thermodynamically accessible surface configurations of the pristine IrO2(110) are first identified with the help of Pourbaix diagram and their OER mechanisms evaluated using the free‑energy span descriptor Gmax(U). Conventional pathways display comparable Gmax(U) values, which obscures the assignment of a unique dominant mechanism. Introducing Walden‑type pathways, in which O2 formation occurs concertedly with water adsorption, resolves this ambiguity: across all investigated surface terminations, these pathways lower Gmax(U) by ~0.3 – 0.6 eV relative to the conventional mechanisms. Grand‑canonical calculations confirm that these trends from canonical calculations are reliable. Bader charge analysis reveals markedly smaller fluctuations in the oxidation state of the active iridium centers along the Walden-type pathway, and barrier calculations for the chemical Walden step show that the corresponding transition state are kinetically accessible under OER conditions. Investigation is then turned to the bridge sites of IrO2(110), which are commonly regarded as spectators. Under acidic OER potentials reveals that these sites can trigger a reconstruction into single‑atom‑like motifs. On this reconstructed site, mononuclear and mononuclear‑Walden mechanisms are analyzed using the same computational framework. The Walden-type pathway again emerges as the most favorable mechanistic description, with Gmax(U) values that are lower than for pristine IrO2(110) surfaces and a chemical Walden step whose activation barrier is likewise reduced relative to the pristine surface. A Born–Haber cycle indicates that dissolution of the reconstructed iridium SAC-like site into Ir3+ or Ir4+ becomes thermodynamically favorable only at potentials well above typical operating OER conditions. Taken together, these results show that Walden‑type concerted steps provide a unifying mechanistic principle for OER on IrO2(110), simultaneously improving activity, reducing electronic reorganization, and remaining compatible with long‑term stability. They further demonstrate that bridge sites, once reconstruction is taken into account, can host the most active centers on the surface, overturning their traditional classification as mere spectators. The mechanistic and stability insights obtained here establish an insightful picture of IrO2(110) under OER conditions and identify Walden‑type pathways on reconstructed single‑atom‑like motifs as a promising design motif for future iridium‑based OER catalysts.