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

The ternary compound Hf₂B₂Ir₅ was assessed as an electrocatalyst for the oxygen evolution reaction (OER) in 0.1 M H₂SO₄ . The oxidative environment restructures the studied material in the near-surface region, creating cavities in which agglomerates of IrO_x(OH)_y(SO₄)_z 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 Hf₂B₂Ir₅. 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 Hf₂B₂Ir₅ matrix surrounding IrO_x(OH)_y(SO₄)_z particles, was used in the OER over 240 h at 100 mA cm^-2 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.