Hf₂B₂Ir₅: A Self-Optimizing Catalyst for the Oxygen Evolution Reaction

Abstract

The ternary compound Hf<sub>2</sub>B<sub>2</sub>Ir<sub>5</sub> was assessed as an electrocatalyst for the oxygen evolution reaction (OER) in 0.1 M H<sub>2</sub>SO<sub>4</sub> . The oxidative environment restructures the studied material in the near-surface region, creating cavities in which agglomerates of IrO<sub>x</sub>(OH)<sub>y</sub>(SO<sub>4</sub>)<sub>z</sub> particles are incorporated. These in situ generated particles result from the oxidation of secondary phases in the matrix as well as from self-controlled near-surface oxidation of the ternary compound itself. The oxidation is controlled by the structural and chemical bonding features of Hf<sub>2</sub>B<sub>2</sub>Ir<sub>5</sub>. The cage-like motif, exhibiting mostly ionic interactions between positively charged Hf atoms and a covalently bonded Ir–B network, selectively controls the extent and kinetics of the transformation process induced during the operation of the electrocatalyst. The resulting self-optimized composite material, formed by a Hf<sub>2</sub>B<sub>2</sub>Ir<sub>5</sub> matrix surrounding IrO<sub>x</sub>(OH)<sub>y</sub>(SO<sub>4</sub>)<sub>z</sub> particles, was used in the OER over 240 h at 100 mA cm<sup>-2</sup> current density. The chemical changes, as well as the OER performance, were studied via a combination of bulk- and surface-sensitive experimental techniques as well as by employing a quantum-chemical bonding analysis.