Evaluating electrocatalytic activity of metal-substituted hafnium diboride (Hf1-xTMxB2; TM = Ni and Co) toward water splitting

Abstract

Developing non-precious, competent, and environmentally benign electrocatalysts to efficiently drive water electrolysis is critical but at the same time demanding. Layered metal diborides (MDbs) holding attractive features mainly due to the presence of borophene subunits in its crystal structure have lately sparked a great deal of attention for electrocatalytic applications. Here, we highlight the synthesis and electrocatalytic properties of HfB2-based electrocatalysts as Hf1-xTMxB2 (TM = Ni and Co; x = 0.1, 0.2, and 0.3) for both hydrogen and oxygen evolution reactions (HER/OER) under 1 M KOH. Amidst as-prepared samples, Hf0.8Ni0.2B2 and Hf0.8Co0.2B2 appeared to be the best OER and HER catalysts, generating the current density of 10 mA cm−2 at overpotentials of 320 and 430 mV, individually. Concerning the OER, Hf0.8Ni0.2B2 performed an oxygen-evolving process similar to benchmarking RuO2 (an overpotential of 290 mV at 10 mA cm−2) and even more facile kinetics compared to RuO2 with Tafel slopes of 39.5 mV dec−1 vs. 66.2 mV dec−1. In addition, regarding HER, Hf0.8Co0.2B2 displayed an enhanced performance relative to pristine HfB2 (an overpotential of 620 mV at 10 mA cm−2), still inferior to noble metal 10% Pt/C (an overpotential of 198 mV at 10 mA cm−2). Furthermore, the interpretation of Tafel slopes unveils that all samples follow the Volmer path as HER mechanism. Finally, both best-performing electrocatalysts tolerated the current density of 10 mA cm−2 for 20 h, manifesting stable bifunctional catalysts.