Bond Exchange Mechanism: Unveiling the Volmer–Tafel Pathway and an Electronic Descriptor for Predicting Hydrogen Evolution Reaction Activity of Borophene

Hydrogen as a fuel is a promising alternative to harmful fossil fuels, thereby its production by electrocatalytic hydrogen evolution reaction (HER) is an important research problem. Using density functional theory, H 2 evolution through the Volmer–Tafel (V–T) pathway is studied on borophene sheets. The results reveal that the sheets exhibit high activity, comparable to platinum. A better theoretical understanding of the mechanism that explains the experimental results is an existing issue. To address this issue, we have proposed that H 2 evolves through a bond-exchange mechanism in the Tafel step. The activation energy through this mechanism is drastically reduced, compared to the existing model of direct H 2 evolution and matches well with the experimental results in terms of activation energy for HER. It is also tested and observed on other HER catalysts, such as nitrogen-doped graphene, 2H-MoS 2, and Pt(111), which proves the proposed mechanism is correct for HER. An electronic property-based descriptor is proposed that shows a good correlation with HER activity on the borophene sheets. This work sheds insights into theoretical modeling of HER and can further lead to better methods toward the understanding of the reaction. Also the electronic descriptor can be tested on similar boron allotropes and reactions.