The process of the catalytic HER is often regarded as the Volmer-Tafel and the Volmer-Heyrovsky mechanism. Both mechanisms involve electrochemical hydrogen adsorption, hydrogen atoms were adsorbed on active atoms of the catalyst to form adsorbed H -Volmer reaction (Equation ), then followed by the combination of two adsorbed hydrogen atoms to produce H2 molecule -Tafel reaction (Equation ) or the reaction between one adsorbed hydrogen and a solvated proton from the water layer to form a H2 molecule -Heyrovsky reaction (Equation ). 𝐻 + + 𝑀 + 𝑒 -↔ 𝑀𝐻 𝑎𝑑𝑠 Volmer (1)