Intermetallic Compounds Mo2 TMB2 (TM: Fe, Co, Ni) for Hydrogen Evolution Reaction

Abstract

Due to rising energy demand, renewable energy sources integrated processes have gained great interest in the recent years. Water electrolysis powered with renewable energy have potential to contribute to the development of sustainable hydrogen economy enormously. Despite the fact that “green” hydrogen is produced in the water electrolyzers with eco-friendly product O2, the excess energy input still hinders this technology to replace the fossil fuel-based technologies. Even with tremendous attempts and considerable contribution to the electrocatalyst development/optimization/improvement over last decades, there is a continued massive endeavor on the way of noble-metal based catalyst replacement for oxygen evolution reaction (OER) as well as for hydrogen evolution reaction (HER). From this point of view, transition metal-containing intermetallic compounds with well-defined electronic and crystal structures, are considered as promising electrocatalysts or precursors for them [1, 3]. Up to now, Mo-Ni and Mo-B systems have been explored for the hydrogen evolution reaction, and noticeable HER activities have been demonstrated for the binary compounds in these systems [4-6].The chemical behavior of ternary borides Mo2FeB2, Mo2CoB2 and Mo2NiB2 under hydrogen evolution (HER) reactions is investigated. Material synthesis and electrode manufacturing include the arc melting of elements in 2:1:2 atomic ratio, homogenization annealing at 1300-1400 °C and densification of grinded powder into cylindrically shaped electrodes via spark plasma sintering (SPS). The electrochemical measurements are carried out in 1M KOH and ambient conditions. Electrocatalytic activity and stability are monitored with cyclic voltammetry (CV) and 2-hour chronopotentiometry (CP) measurements performed at reducing current density of 10 mA cm-2, which are considered as conditions of standard benchmarking HER experiment. Furthermore, long-term stability of HER activity at elevated reducing current densities such as 50, 100, 200 mA cm-2 are essential to prove the preserved catalytic performance under harsh reaction conditions. For the comprehensive pre- and post-characterization of the electrode materials, bulk and surface-sensitive techniques are utilized.The pristine Mo2FeB2, Mo2CoB2 and Mo2NiB2 in HER region outperform their constituent TM components and indeed, their HER activity is close to that of the state-of-art Pt catalyst (Figure). During short-term CP experiment, Mo2NiB2 and Mo2FeB2 show continuing activation during the measurement, whereas Mo2CoB2 maintained its stability. Energy dispersive X-ray analysis (EDXS) of electrochemically treated areas indicate an enrichment in Fe, Co and Ni due to the noticeable leaching of Mo into electrolyte. Also, chemical analysis via ICP-OES reveals only Mo among all three constituent elements in the exploited electrolyte solution.As a conclusion, Mo2 TMB2 intermetallic compounds show outstanding HER activity and its stability over 2h of CP with respect to corresponding reference materials. Long-term chronopotentiometry in alkaline media were carried out and will be extensively presented. Figure. HER activity of Mo2 TMB2 (TM: Fe, Co, Ni) with respect to elemental references Fe, Co, Ni and state-of-art Pt. Inlets: Backscattered secondary electron images after 2h CP, including energy dispersive X-ray (EDXS) analysis results.