Sisyphus effects in hydrogen electrochemistry on metal silicides enabled by silicene subunit edge.

Nonmetal elements strictly govern the electrochemical performance of molybdenum compounds. Yet, the exact role played by nonmetals during electrocatalysis remains largely obscure. With intermetallic MoSi2 comprising silicene subunits, we present an unprecedented hydrogen evolution reaction (HER) behavior in aqueous alkaline solution. Under continuous operation, the HER activity of MoSi2 shows a more than one order of magnitude improvement in current density from 1.1 to 21.5 mA cm-2 at 0.4 V overpotential. Meanwhile, this activation behavior is highly reversible, such that voltage withdrawal leads to catalyst inactivation but another operation causes reactivation. Thus, the system shows dynamics strikingly analogous to the legendary Sisyphus' labor, which drops and recovers in a stepwise manner repeatedly, but never succeeds in reaching the top of the mountain. Isomorphic WSi2 behaves almost the same as MoSi2, whereas other metal silicides with silicyne subunits, including CrSi2 and TaSi2, do not exhibit any anomalous behavior. A thin amorphous shell of MoSi2 is observed after reaction, within which the Si remains partially oxidized while the oxidation state of Mo is basically unchanged. First-principles calculations further reveal that the adsorption of hydroxide ions on silicene subunit edges and the subsequent Si vacancy formation in MoSi2 jointly lead to the anomalous HER kinetics of the adjacent Mo active centers. This work demonstrates that the role of nonmetal varies dramatically with the electronic and crystallographic structures of silicides and that silicene structural subunit may serve as a promoter for boosting HER in alkaline media.