Electrochemical evolution of oxygen on the material of the Hf2B2Ir5 electrode

Water electrolysis is an electrochemical way of producing hydrogen, which is regarded as one of the future energy-carrying molecules. Therefore, looking at the numerous advantages of proton exchange membrane electrolysis over the classic alkaline variant, its efficiency and large-scale applicability is of enormous importance nowadays. However, the slow kinetics of the anodic oxygen evolution reaction (OER) limits the overall electrolysis process and requires an active and stable electrocatalyst.

This need inspired scientists from the departments of Metal Chemistry and Physics of Related Matter of MPI CPfS together with the Fritz-Haber-Institut in Berlin to employ their long-standing experience in the chemistry of intermetallic compounds, in the electronic characteristics of solid matter. and in electrocatalysis to take a step forward. forward in this challenging direction. As a result of fruitful teamwork, the concept of cooperative phases with different stability under OER conditions has been successfully demonstrated with the intermetallic compound Hf₂B.₂Ir₅ as a self-optimizing electrocatalyst for OER.

Based on the chemical bond analysis, the intermetallic compound Hf₂B.₂Ir₅ has a type of crystal structure cage: the two-dimensional layers of B.₂Ir₈ the units are interconnected by two- and three-center Ir-Ir interactions with the polyanionic structure and the hafnium atoms are hosts in such anionic cages. The characteristics of atomic interactions are reflected in the electronic structure of Hf₂B.₂Ir₅ and its chemical behavior under OER conditions.

The initial electrochemical OER activity of Hf₂B.₂Ir₅ sustains during continuous operation at processed current densities of 100 mA cm^-2 for at least 240 h and places this material among the state-of-the-art Ir-based electrocatalysts. The harsh oxidative conditions of OER activate the limited changes to the surface of the uncontaminated material and consequently the electrochemical performance is related to the cooperative work of the Ir-terminated surface of the ternary compound itself and the agglomerates of IrO_X(OH)_Y(SO₄) z particles.

The latter are formed mainly due to oxidation of the secondary phase HfB4Ir3 and oxidation near the surface of the studied compound. The presence of at least two OER-active states of Ir, originating from Hf₂B.₂Ir₅ under OER conditions, it was confirmed by XPS analysis. The experimental data (electrochemical results, characterization of the material by mass and surface sensitive methods, elemental analysis of the electrolyte used) are consistent with the chemical bond analysis. The illustrated concept of cooperative phases with different chemical stability under OER conditions can be explored with other systems and offers a prospective knowledge-based way for the discovery of novel effective OER electrocatalysts.