▎ 摘　　要

Molecular hydrogen is widely considered as a potential carbon-free and renewable energy carrier for a sustainable energy strategy. The electrocatalytic hydrogen evolution reaction is a major pathway to obtain ultra-pure hydrogen, and it vastly relies on the development of inexpensive electrocatalysts. In recent years, graphene has been proposed as a "metal-free" catalyst for the hydrogen evolution reaction. Here, we showed that metallic impurities are introduced during the synthesis process and these impurities act as the catalyst for the hydrogen evolution reaction.