H2 binding energy may be an auxiliary criterion of H adsorption free energy to estimate hydrogen evolution catalyst

Abstract

The H adsorption free energy (ΔG_H) is commonly used as a descriptor to estimate the activity of hydrogen evolution catalyst and a ΔG_H close to zero is desired for the hydrogen evolution catalyst. The whole hydrogen evolution reaction (HER) process includes the generation *H and *H₂ and *H₂ desorption. However, the value of ΔG_H only decides the generation rate of adsorbed H (*H) and H₂ (*H₂) and is not related with the rate of *H₂ desorption. The H₂ binding energy ($∆E_{H_2}$) may influence the step of *H₂ desorption. Therefore, in this paper, H₂ binding energy ($∆E_{H_2}$) and ΔG_H on the commonly used HER catalysts were calculated using the density functional theory (DFT) method. The results show that ΔG_H is irrelevant to $∆E_{H_2}$and the excellent HER catalysts such as Pt and [Fe-Fe] hydrogenase have a desired ΔG_H and low $∆E_{H_2}$. The value of ΔG_H decides the generation rate of adsorbed H (*H) and H₂ (*H₂) and the low $∆E_{H_2}$can promote *H₂ desorption from the surface of catalyst. The activity of HER catalyst is codetermined by ΔG_H and $∆E_{H_2}$. Based on the calculations, we firstly propose that $∆E_{H_2}$ can act as an auxiliary criterion of ΔG_H to estimate the activity of HER catalyst.