Free energy curves for the Volmer reaction obtained from molecular dynamics simulation based on quantum chemistry

Hydrogen desorption from Au(111) at a constant electrode charge has been studied by a DFT-based tight binding theory (DFTB). Free energy curves have been obtained from molecular dynamics with umbrella sampling. The activation energy increases linearly with the applied negative charge and decreases with temperature, so that the Butler-Volmer law does not hold in its strict form. The activation energy is determined by the reorganization of the accepting water molecule and its surroundings, which becomes more facile with temperature. A separation of the free energy curves into a constant chemical part and an electrostatic part, which varies with the charge, does not hold. Because of its speed DFTB is very suitable for the investigation of electrochemical reactions, but is presently hampered by a lack of sets of interaction parameters.