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

Abstract

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.

Is Hydroxide Just Hydroxide? Unidentical CO2 Hydration Conditions during Hydrogen Evolution and Carbon Dioxide Reduction in Zero-Gap Gas Diffusion Electrode Reactors

“…This CO 2 -rich film in turn affects the reaction kinetics of chemical reactions at the gas−liquid interface. However, during high current electrolysis, the local hydroxide concentration increases (>7 M KOH) in this shallow electrolyte layer along with the substantial decrease in water activity, 54 that is, the ion hydration ability of water molecules. 53 A further factor to be considered is the near-electrode concentration gradient formed during electrolysis.…”

2022 roadmap on low temperature electrochemical CO2 reduction

“…Optimization of such metrics is associated with fluid management in the device, a device-scale modeling challenge. Improvement of this device CO 2 conversion rate decreases the CO 2 partial pressure along the path and all related parameters (CO 2 solubility, bicarbonate equilibrium, equilibrium potential, etc) drift away from the 1 atm pressure assumption [72]. At such high CDs, other typical assumptions may not be valid anymore.…”

“…At such high CDs, other typical assumptions may not be valid anymore. This is the case for the acid-base reaction that might drift from equilibrium or solvent activity that could decrease below unity [71,72].…”

Deposition of Bismuth Nanoplatelets onto Graphene Foam for Electrocatalytic CO2 Reduction

“…2,3 Formic acid is a high volumetric capacity hydrogen storage media (53.4 g/L at standard temperature and pressure) and could be a possible long-term energy storage solution to the seasonal intermittency of renewable energy sources through the storage of energy in chemical bonds. 4−6 Yet, to facilitate implementation of electrocatalytic CO 2 rr, further improvement in electrocatalyst efficiency (activity and selectivity), 7 overcoming mass transport limitations, 8 and novel process design are needed. 9 In the past two decades, there has been an intensive scientific and engineering research effort to realize a carbonneutral energy cycle through identifying new electrocatalysts, 10 catalysts, 11,12 and electrolyte engineering, 13−15 development of new electrolyzer designs, 16 optimization of process conditions, 17 and varying electrode structure and preparation techniques that improve the overall process efficiency.…”

“…Valorizing carbon dioxide to fuels and valuable chemical compounds is a prominent technological and scientific challenge . A recent techno-economic analysis of the viability of the electrocatalytic CO 2 reduction reaction (CO 2 rr) to various C1 and C2 products by utilizing electricity from a renewable energy source identified CO and HCOOH as favorable products. , Formic acid is a high volumetric capacity hydrogen storage media (53.4 g/L at standard temperature and pressure) and could be a possible long-term energy storage solution to the seasonal intermittency of renewable energy sources through the storage of energy in chemical bonds. − Yet, to facilitate implementation of electrocatalytic CO 2 rr, further improvement in electrocatalyst efficiency (activity and selectivity), overcoming mass transport limitations, and novel process design are needed …”