Hf<sub>2</sub>B<sub>2</sub>Ir<sub>5</sub>: A Self-Optimizing Catalyst for the Oxygen Evolution Reaction
Ana María Barrios Jiménez, Ulrich Burkhardt, Raúl Cardoso‐Gil, Katharina Höfer, S. G. Altendorf, Robert Schlögl, Yuri Grin, Iryna Antonyshyn
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.