Kinetic Reference Potential, pH-Effect, and Energy Recovery in Electrolysis of Water

The electrolysis of water will likely become of superior importance for a sustainable energy economy. However, the electrocatalysis of electrochemical water splitting is complicated and the origin of significant energy losses. Among the heavily discussed open questions in this field at present is the origin of experimentally observed differences between electrolysis kinetics in acidic vs. alkaline electrolyte, and the effect of high-pressure operation on electrolyser performance. Our thermodynamic analysis reveals answers and fundamental connections between these questions by the definition of balanced reactive conditions and the kinetic reference voltage of the electrolysis reaction. Unlike the reversible cell voltage, the kinetic reference voltage Ukin is not biased by product H2 and O2 concentrations, and it represents a reliable intrinsic reference point for electrolysis kinetics. At standard temperature T = 25◦C, its value is Ukin = 1.441 V, which is in remarkable agreement with commonly observed onset voltages for macroscopic electrolysis rates. We define the reactive excess overvoltage ηrxs = Ukin − Urev as the difference between the kinetic reference voltage and the reversible cell voltage. Comparing the hydrogen evolution (HER) and oxygen evolution (OER) half-cell reactions in acidic vs. alkaline electrolyte, we find an asymmetric and pH-dependent distribution of ηrxs among HER and OER. Increasing the electrolysis gas pressure results in a reduction of ηrxs due to an increased free energy content of the evolved gases. Our analysis provides a new perspective on activation losses in water electrolysis, on pH-effects in hydrogen and oxygen evolution electrocatalysis, and on high-pressure electrolysis as a means for energy recovery.