Title : Intermetallic Hf2B2Ir5 as OER electrocatalyst
The transition from fossil fuels to renewable energy sources requires multiple solutions, among which water electrolysis will play an important role. Considering proton-exchange-membrane (PEM) electrolysis with its advantages as a route for hydrogen production, the kinetics of both electrode reactions should be sufficient for large-scale application. However, at the current stage, the slow kinetics of anodic oxygen evolution reaction (OER) limits the overall efficiency of the PEM electrolyzer and requires an active and stable OER electrocatalyst. Additionally, the harsh oxidative conditions of OER restrict the catalyst choice to noble metals and their oxides. The implementation of intermetallic compounds (IMCs) as electrocatalyst materials introduces the possibility to modify the electronic state of the active centers (influence on OER activity) and realize the combined scenario of the chemical bonding through complex atomic interactions in IMC (contribution to electrode stability).
Based on the chemical bonding analysis, the orthorhombic structure of ternary compound Hf2B2Ir5 has a cage-like character . The structural units B2Ir8 form the anionic covalently bonded framework and hafnium cations occupy corresponding 14-vertices B-Ir cages. The hafnium atoms are positively charged (+1.83), whereas B and Ir possess negative charges (B: -0.19, Ir: -0.66). These features of chemical bonding are reflected in the chemical behaviour of Hf2B2Ir5 under OER conditions and have an influence on the OER activity of this material. The prominent initial OER activity of Hf2B2Ir5 is accompanied with the suppressed dissolution of Ir (compared with Ir anode), revealing the enhancement of the material stability in the case of IMC. The long-term operation at current densities of 100 mA cm-2 for 246 h reveals the continuous improvement of OER activity with time, which can be explained by the “team” OER activity of Ir-terminated surface of Hf2B2Ir5 and in situ-formed IrOx particles as a catalyst of 2nd generation. The covalent bonding in the Ir-B framework hinders the massive Ir dissolution and leads to the long-term bulk stability of Hf2B2Ir5 material. The electrochemical data are supported by comprehensive bulk- and surface material characterization of the electrode after electrochemical experiments. The assessment of this material at different scales and different states highlights that Hf2B2Ir5 is a self-optimizing stable OER electrocatalyst.