Two-dimensional hydrogen evolution reaction (HER) electrocatalysts are outstanding alternatives to high-cost Pt due to their good material properties. However, the few existing two-dimensional HER electrocatalysts have shortcomings that restrict their performance. Here, we report a first-principles study of single-layer A₂BS₄ (A = Ag, Cu; B= Mo, W) as HER electrocatalysts and identify single-layer Cu₂WS₄ as a promising candidate. Single-layer A₂BS₄ is found to be chemically, dynamically, and thermally stable. They require only a small energetic cost to be created from their layered bulks, suggesting the possibility of their exfoliation in experiments. Most importantly, without significant density of vacancies and in the absence of large applied strain, the basal plane of single-layer Cu₂WS₄ shows excellent electrocatalytic activity toward HER. Such an activity is attributed to the introduced in-gap states and d band center shifting upon adsorbing hydrogen. These characteristics suggest that single-layer Cu₂WS₄ is an extraordinary two-dimensional HER electrocatalyst.

Keywords: d band center; electrocatalyst; first-principles; hydrogen evolution reaction; single-layer.