Transition Metal-Based High Entropy Alloy Microfiber Electrodes with Improved Corrosion Behavior and Hydrogen Activity

The hydrogen evolution reactions using advanced alloy systems have received growing interest in recent years owing to the poor kinetics, i.e., sluggish water dissociation step, of the conventionally used materials in alkaline electrolytes. The main difference between acidic and alkaline media is the low number of protons in an alkaline environment. Hence, a water-dissociation step is expected to detach the bonded hydrides and form H₂ molecules. However, the slow kinetics challenge the further advancement of anionic exchange membrane water electrolyzers.<br/>So far, studies of various high and medium entropy alloys, HEA and MEA, respectively, have been performed in aqueous environments to understand the electrocatalytic and electrocorrosion behavior along with diffusion and electron transfer kinetics. Due to the differences in electronic structures and valence states, transition group elements display remarkable differences in terms of hydrogen evolution reaction (HER). For example, group IVB (Ti, Zr, Hf) and VB (V, Nb, Ta) elements have strong M–H bond strengths and are excellent hydride formers. Besides, the large difference in elemental atomic radii (cf. 208 pm and 171 pm for Hf and V, respectively) is expected to induce an enhanced electrocatalytic behavior in analogy to the large lattice distortions upon hydrogen absorption during gas-solid reactions of a TiVZrNbHf HEA alloy. Moreover, for bio-implant applications, similar multicomponent alloy systems have been recently tested in different buffer solutions and saline environments.<br/>This study focuses on the (bio)corrosion kinetics and electrocatalytic activity of newly developed TiZrNb and TiZrNbVTa high entropy alloys. Some of the alloys exhibit ultra-high corrosion resistance in alkaline environment, adverting their use for battery/fuel cell components. Formation of several nanometer thick passive oxide layers confirmed by scanning transmission electron microscopy accounts for improved corrosion resistance which increases with increasing TiO<i>_x </i>content. The Cathodic Tafel slope of 67 mV dec–1 and a large transfer coefficient of 0.82 obtained for Ti₂₀Zr₂₀Nb₂₀V₂₀Ta₂₀ suggest its usefulness for hydrogen electrocatalysis. High amounts of hydrogen storage, e.g., 1.7 wt% in Ti₂₅Zr₂₅Nb₁₅V₁₅Ta₂₀, were confirmed by gas-solid reactions. This HEA also has high corrosion resistance in acidic and saline environments ideal for coatings and surgical tools/implants.<br/>These results suggest that composition and structure optimized HEAs can be viable candidates for advanced hydrogen systems and/or as electrocatalytically active materials.