Insights into the Carbon Balance for CO2 Electroreduction on Cu using Gas Diffusion Electrode Reactor Designs

Abstract

In this work, the carbon balance during high-rate CO2 reduction in flow electrolyzers is rigorously analyzed. The CO2 consumption at gas-diffusion electrodes due to electrochemical reduction and reaction with OHat the electrode-electrolyte interface leads to a substantial reduction in the volumetric flowrate of gas flow out of the electrolyzer, especially when highly alkaline electrolytes and elevated current densities are utilized, mainly owing to elevated pH at cathode/electrolyte interface. Without considering the CO2 consumption, the Faradaic efficiencies for major gas products could be significantly overestimated during high current density CO2 reduction conditions, particularly in the case of high pH electrolyte. In addition, a detailed carbon balance path is elucidated via a two-step procedure of CO2 reaction with OHat cathode/electrolyte interface and subsequent CO2 generation at anode/electrolyte interface caused by a relatively low pH in the vicinity of the anode. Based on the proposed two-step carbon balance path, a systemic exploration of gases released in anolyte reveals the transformation of a HCO3-or OHcatholyte to a CO3 2catholyte, which was further confirmed by pH measurement.

The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem

“…1c ). In order to protonate CO 3 2– , the anode pH equilibrates to near-neutral (pH ~ 8) 2 , which increases the cell voltage compared to high pH because the anode thermodynamic potential moves in the positive direction (Fig. 1a ).…”

“…Leveraging insights from fuel cells and membrane water electrolyzers, researchers have developed gas diffusion electrode (GDE) cells demonstrating synthetically relevant CO 2 electrolysis current densities (>100 mA cm −2 ) and promising stability. Despite these advances, the energy efficiency (power-to-product) and carbon efficiency (CO 2 -to-product) of low-temperature CO 2 electrolysis remain too low to support large-scale applications 1 , 2 . While much current research is focused on CO 2 reduction catalyst design, the biggest obstacle to improving performance is an often overlooked basic chemistry problem: the rapid and thermodynamically favorable reaction of CO 2 with hydroxide (OH – ) to form carbonate (CO 3 2– ) imposes steady state electrolysis conditions that result in large voltage and CO 2 losses.…”

“…While much current research is focused on CO 2 reduction catalyst design, the biggest obstacle to improving performance is an often overlooked basic chemistry problem: the rapid and thermodynamically favorable reaction of CO 2 with hydroxide (OH – ) to form carbonate (CO 3 2– ) imposes steady state electrolysis conditions that result in large voltage and CO 2 losses. Although recent work has brought attention to some aspects of the CO 3 2– problem 2 – 4 , it is far more pernicious than what is widely appreciated. Here we explain how CO 3 2– formation compromises efficiency to highlight the need for new research directions that address this problem.…”

Molecular electrocatalysts transform CO into C2+ products effectively in a flow cell

“…1-2), which also inducing solid salt formation that compromises flow cell operation. 6,7 Bicarbonate formation:…”

Electrocatalysts derived from copper complexes transform CO into C2+ products effectively in a flow cell

“…1-2). 6,7 This formation of (bi)carbonates not only impacts the energy efficiency of the reactor, it also induces solid salt formation that compromises flow cell operation. 8,9 Bicarbonate formation: CO 2 + OH -⇌ HCO 3 -Eq.…”