Developing nonprecious hydrogen evolution electrocatalysts that can work well at large current densities (e.g., at 1000 mA/cm(2): a value that is relevant for practical, large-scale applications) is of great importance for realizing a viable water-splitting technology. Herein we present a combined theoretical and experimental study that leads to the identification of alpha-phase molybdenum diboride (alpha-MoB2) comprising borophene subunits as a noble metal-free, superefficient electrocatalyst for the hydrogen evolution reaction (HER). Our theoretical finding indicates, unlike the surfaces of Pt- and MoS2-based catalysts, those of alpha-MoB2 can maintain high catalytic activity for HER even at very high hydrogen coverage and attain a high density of efficient catalytic active sites. Experiments confirm alpha-MoB2 can deliver large current densities in the order of 1000 mA/cm(2), and also has excellent catalytic stability during HER. The theoretical and experimental results show alpha-MoB2's catalytic activity, especially at large current densities, is due to its high conductivity, large density of efficient catalytic active sites and good mass transport property.