Volmer-Tafel mechanism: Platinum, as a well-known catalyst for HER, is thought to follow the this process , which requires the combination of two MHads species. The observed bond length of the hydrogen molecule is just 0.74, 3 but the distance between two neighbouring atoms in pure platinum (cubic, space group: Fm-3m) is 2.774. This demonstrates that the distance between nearby platinum atoms is 3.74 times the length of the hydrogen bond, and that extra energy is required for atomic hydrogen transfer across a long distance. In the Volmer-Heyrovsky mechanism, the MHads is formed during the first Volmer stage, there is no obvious and persuasive procedure to explain adsorbing another H atom from water to form a H2 molecule.