Quantitative Understanding of the Sluggish Kinetics of Hydrogen Reactions in Alkaline Media Based on a Microscopic Hamiltonian Model for the Volmer Step

• Published in: Journal of physical chemistry. C Vol. 123; no. 28; pp. 17325 - 17334
• Main Authors: Huang, Jun, Li, Peng, Chen, Shengli
• Format: Journal Article
• Language: English
• Published: American Chemical Society 18.07.2019
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.9b03639

The sluggish kinetics of hydrogen evolution/oxidation reactions in alkaline media remains a technical barrier for alkaline membrane fuel cells and a scientific puzzle under heated discussion in fundamental electrocatalysis. Much attention has been centered around thermodynamic origins, whereas microscopic kinetics is less understood and a quantitative account of key factors is yet missing. To fill in this gap, a microscopic Hamiltonian model is developed for the alkaline Volmer step, an elementary step of hydrogen reactions, encompassing electronic interactions, bond breaking, solvent reorganization, and double-layer electrostatic effects. The model gives out a simple yet informative analytical formula for the activation barrier of the alkaline Volmer step, quantifying the contributions of various factors; roughly speaking, one quarter of the H–OH bond energy enters into the activation energy. This model elucidates that the larger activation energy seen at a more charged interface is not because it is more difficult to reorganize the solvents but rather because it consumes more work in bringing OH– to the double layer, namely, a larger work term. Previous strategies used to boost the activity of hydrogen reactions in alkaline media are rationalized in a coherent framework.