Field Effect Modulation of Electrocatalytic Hydrogen Evolution at Back-Gated Two-Dimensional MoS₂ Electrodes

Electrocatalytic activity for hydrogen evolution at monolayer MoS₂ electrodes can be enhanced by the application of an electric field normal to the electrode plane. The electric field is produced by a gate electrode lying underneath the MoS₂ and separated from it by a dielectric. Application of a voltage to the back-side gate electrode while sweeping the MoS₂ electrochemical potential in a conventional manner in 0.5 M H₂SO₄ results in up to a 140 mV reduction in overpotential for hydrogen evolution at current densities of 50 mA/cm². Tafel analysis indicates that the exchange current density is correspondingly improved by a factor of four to 0.1 mA/cm² as gate voltage is increased. Density functional theory calculations support a mechanism in which the higher hydrogen evolution activity is caused by gate-induced increase in the electronic charge on Mo metal centers adjacent to the S vacancies (the active sites), leading to enhanced Mo-H bond strengths. Overall, our findings indicate that the back-gated working electrode architecture is a convenient and versatile platform for investigating the connection between tunable electronic charge at active sites and overpotential for electrocatalytic processes on ultrathin electrode materials.