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 <i>U</i><sub>kin</sub> is not biased by
product H<sub>2</sub>
and O<sub>2</sub>
concentrations, and it represents a reliable intrinsic reference point for electrolysis kinetics. At standard temperature <i>T</i> = 25<sup>◦</sup>C, its value is <i>U</i><sub>kin</sub> = 1.441 V, which is in remarkable
agreement with commonly observed onset voltages for macroscopic electrolysis rates. We define
the reactive excess overvoltage <i>η</i><sub>rxs</sub> = <i>U</i><sub>kin</sub> − <i>U</i><sub>rev</sub> 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 <i>η</i><sub>rxs</sub> among HER and OER. Increasing the electrolysis gas pressure
results in a reduction of <i>η</i><sub>rxs</sub> 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.<br>
Kinetic Reference Potential, pH-Effect, and Energy Recovery in Electrolysis of Water
