Critical influence of phase transition on the hydrogen evolution reaction activity of Heusler alloys

Unraveling structure-property relationships is of critical importance for designing superior catalysts. Heusler alloys are a family of materials whose atomic packing structure may change with a change in the external field, for example, temperature, magnetic field, and/or stress, and are ideal models for studying the structure-property relationship. In this study, a series of Ni–Mn–Sn–Co Heusler alloys with controlled phases and compositions, including the tetragonal martensite phase (Ni46Mn36Sn11Co7, labeled Co7), cubic austenite phase (Ni42Mn43Sn10Co5, labeled Co5), and tetragonal/cubic mixing phase (Ni46Mn38Sn12Co4, labeled Co4) were investigated. The alloys with the martensitic phase exhibited superior catalytic activity toward hydrogen evolution reaction (HER) than those with the austenitic phase. At an overpotential of 350 mV, the HER current density of the martensite phase of the Co7 sample was almost 2 times higher than that of the austenite phase of the Co5 sample. In-situ temperature-dependent catalytic measurements for the Co4 alloy suggest that the catalytic activity decreases significantly when phase transformation from martensite to austenite occurs. The efficient electron transfer kinetics and low electron work function are responsible for the high activity of the martensite phase, according to Tafel and work function analyses. These results provide a promising strategy for the development of high-performance Heusler alloys by controlling their phase structures and phase transitions.