The HER involves two main steps: the Volmer reaction and either the Heyrovsky or Tafel reactions. In the Volmer step, a proton (H+) from the aqueous solution adsorbs onto the catalyst surface and gains an electron to form an adsorbed hydrogen atom (H*). The subsequent steps can either be:
Heyrovsky Reaction: H* combines with another proton and electron to form H2. Tafel Reaction: Two H* atoms combine to form H2.
