Electrolysis of CO2 at 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 CO2 to 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 CO2 by water as the rate-determining step, we show the Nernst equation dilute-solution approximation of the CO2 to 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–2 at the inlet had only ~0.3 A cm–2 at the outlet for a conversion in CO2 partial pressure from 0.90 to 0.48 bar, showing the significant practical implications of this work.
Nesbitt, Nathan T. and Smith, Wilson A.. "Water and Solute Activities Regulate CO<sub>2</sub> Reduction in Gas-Diffusion Electrodes." Journal of Physical Chemistry. C, vol. 125, no. 24, May. 2021. https://doi.org/10.1021/acs.jpcc.1c01923
Nesbitt, Nathan T., & Smith, Wilson A. (2021). Water and Solute Activities Regulate CO<sub>2</sub> Reduction in Gas-Diffusion Electrodes. Journal of Physical Chemistry. C, 125(24). https://doi.org/10.1021/acs.jpcc.1c01923
Nesbitt, Nathan T., and Smith, Wilson A., "Water and Solute Activities Regulate CO<sub>2</sub> Reduction in Gas-Diffusion Electrodes," Journal of Physical Chemistry. C 125, no. 24 (2021), https://doi.org/10.1021/acs.jpcc.1c01923
@article{osti_1787245,
author = {Nesbitt, Nathan T. and Smith, Wilson A.},
title = {Water and Solute Activities Regulate CO<sub>2</sub> Reduction in Gas-Diffusion Electrodes},
annote = {Electrolysis of CO2 at 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 CO2 to 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 CO2 by water as the rate-determining step, we show the Nernst equation dilute-solution approximation of the CO2 to 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–2 at the inlet had only ~0.3 A cm–2 at the outlet for a conversion in CO2 partial pressure from 0.90 to 0.48 bar, showing the significant practical implications of this work.},
doi = {10.1021/acs.jpcc.1c01923},
url = {https://www.osti.gov/biblio/1787245},
journal = {Journal of Physical Chemistry. C},
issn = {ISSN 1932-7447},
number = {24},
volume = {125},
place = {United States},
publisher = {American Chemical Society},
year = {2021},
month = {05}}