Revisiting the origin of electrochemical activity in the topological semimetal PtGa.

Topological materials have been reported as active and selective electrochemical catalysts for a variety of small-molecule energy-conversion reactions. The exceptional activity and selectivity of these materials have been attributed to their topologically non-trivial surface states, which support chiral spin currents and boast high carrier mobilities. For the topological semimetal PtGa, we find that these states are not robust under practical electrochemical conditions. During electrochemical operation, Ga rapidly corrodes from the surface, yielding a nanoporous, Pt-rich layer. Inductively coupled plasma mass spectrometry and scanning transmission electron microscopy imaging independently confirm Ga corrosion. Hydrogen evolution reaction activity measurements demonstrate that the electrocatalytic performance of PtGa is correlated with the number of Pt active sites, and first-principle calculations further show that introducing Ga vacancies into the PtGa crystal structure disrupts the topologically non-trivial surface states. These observations suggest that previous reports detailing the high electrochemical activity of PtGa might be better explained by an enrichment in Pt active sites following corrosion, rather than by a genuine increase in the intrinsic reaction rate mediated by PtGa's topological surface states. These results urge caution in attributing enhanced catalytic activity to topological surface states without verifying surface composition and electronic structure. They also highlight the distinct meaning of robustness in physics and chemistry.