Water and Solute Activities Regulate CO2 Reduction in Gas-Diffusion Electrodes

Nathan Nesbitt, Wilson Smith

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15 Scopus Citations


Electrolysis of CO2at gas-diffusion electrodes (GDEs) has typically been limited by the supply of gas to the electrocatalyst, overshadowing the importance of the supply of water. However, at high current densities that approach 1 A cm-2, where the electrolyte becomes highly concentrated in the catalyst layer of a GDE, the activity of water and solutes deviate from their bulk dilute solution values, potentially slowing reaction rates and changing reaction equilibrium potentials. In addition, as flow plates for the gas stream are introduced to enable larger electrodes and high single pass conversion of CO2to product, variations in the gas composition will become important. By drawing upon literature for the oxygen reduction reaction (ORR), here we explain how to account for these effects in future modeling and experimental work, with particular attention to accurate use of the Nernst equation for electrode potentials and the Arrhenius equation for reaction rates. Specifically, using measurements of KOH solvent and solute activity reported in literature, and assuming the second protonation of CO2by water as the rate-determining step, we show the Nernst equation dilute-solution approximation of the CO2to CO equilibrium potential to be accurate below 5 M KOH, but it has a 74 mV error when increasing the concentration up to 10 M KOH. Finally, a simple one-dimensional model of a serpentine flow-field on a GDE demonstrated that a reactor with constant pressure of 1 bar and 1 A cm-2at the inlet had only ∼0.3 A cm-2at the outlet for a conversion in CO2partial pressure from 0.90 to 0.48 bar, showing the significant practical implications of this work.

Original languageAmerican English
Pages (from-to)13085-13095
Number of pages11
JournalJournal of Physical Chemistry C
Issue number24
StatePublished - 24 Jun 2021

Bibliographical note

Publisher Copyright:
© 2021 American Chemical Society

NREL Publication Number

  • NREL/JA-5900-78623


  • CO2 electrolysis
  • CO2 reduction
  • concentrated electrolytes
  • gas-diffusion electrodes
  • high current density
  • KOH
  • NaOH
  • solute activity
  • water activity


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