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

07 August 2020, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

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 = 25C, 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 = UkinUrev 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.

Keywords

Electrolyser
overvoltage
Oxygen Evolution Reaction
Hydrogen Evolution Reaction
pH effect
Thermodynamic analyses

Supplementary materials

Title
Description
Actions
Title
electrolyser efficiency kinetic potential SI
Description
Actions

Comments

Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.