The hydrogen overpotential—hydrogen adsorption energy relationship. A new approach to the problem

Highlights: • At the barrierless discharge activation and reorganization energies are independent. • Real equilibrium barrierless activation energy equals to hydrogen adsorption energy. • Kinetic data on the barrierless discharge allows estimating the reorganization energy. • Reorganization energies for mercury, silver, and gold electrodes are different. • The differences of overpotentials and of hydrogen adsorption energies are not equal. - Abstract: The hydroxonium barrierless discharge can proceed at electrodes having positive hydrogen adsorption free energy. A characteristic feature of this process is that its activation energy equals to the free energy of the discharge elementary act; hence, it is independent of the reorganization energy. This allows obtaining the reorganization energy and the adsorption free energy in a self-consistent way from the experimental data. For the existence of the barrierless discharge the reorganization energy should be less than the adsorption free energy. The data on mercury electrode give the corresponding values of 0.95 eV and 1.18 eV. These data conform to the requirement stated above. On silver and gold electrodes the adsorption free energies determined from the data in the barrierless region are 0.65 eV and 0.47 eV. It means that the reorganization energy for these electrodes is lower than for mercury. Hence, it was for the first time shown on the basis of experimental data that the hydroxonium discharge reorganization energies for different electrodes are different. The traditional opinion that the difference of overpotentials equals to the difference of the hydrogen adsorption energies implies the constancy of reorganization energy. Therefore, this approach reveals to be incorrect.