Carbonation in Low-Temperature CO₂ Electrolyzers: Causes, Consequences, and Solutions

Abstract

Electrochemical reduction of carbon dioxide (CO2) to useful products is an emerging power-to-X concept, which aims to produce chemicals and fuels with renewable electricity instead of fossil fuels. Depending on the catalyst, a range of chemicals can be produced from CO2 electrolysis at industrial-scale current densities, high Faraday efficiencies, and relatively low cell voltages. One of the main challenges for up-scaling the process is related to (bi)­carbonate formation (carbonation), which is a consequence of performing the reaction in alkaline media to suppress the competing hydrogen evolution reaction. The parasitic reactions of CO2 with the alkaline electrolytes result in (bi)­carbonate precipitation and flooding in gas diffusion electrodes, CO2 crossover to the anode, low carbon utilization efficiencies, electrolyte carbonation, pH-drift in time, and additional cost for CO2 and electrolyte recycling. We present a critical review of the causes, consequences, and possible solutions for the carbonation effect in CO2 electrolyzers. The mechanism of (bi)­carbonate crossover in different cell configurations, its effect on the overall process design, and the economics of CO2 and electrolyte recovery are presented. The aim is to provide a better understanding of the (bi)­carbonate problem and guide research directions to overcome the challenges related to low-temperature CO2 electrolysis in alkaline media.

Advances and challenges in membrane electrode assembly electrolyzers for CO2 reduction

“…However, the detailed mechanism behind this phenomenon is still under question, as a recent report also suggests that cracks in the microporous layer can cause flooding. 73 In general, the factors influencing the hydrophobicity of the GDE can be categorized as electrowetting effects, 66,67 hydrophilic carbonate salt precipitation, 74,75 pressure fluctuations, 73 and defects or cracks in the GDE, 76 as summarized by Ramdin et al 77…”

“…This principle also applies to alkaline anolytes, although no H + ions are formed, and HCO 3 − or CO 3 2− may undergo direct electro–oxidation reactions, as shown in Table 3. Ramdin et al 77 summarized recent studies on AEM-based CO 2 RR and proposed that bicarbonate was the main charge carrier at low current density ( e.g. , for CO 2 to CO, j < 100 mA cm −2 ), while carbonate dominated at higher current density.…”

Bipolar Membrane with Porous Anion Exchange Layer for Efficient and Long‐Term Stable Electrochemical Reduction of CO2 to CO

“…When (bi)carbonate ions from the cathode side arrive at the anode, the pH of the electrolyte is neutralized over time. [ 13,15 ] To maintain the high pH needed for the employment of non‐noble metal catalysts on the anode side (e.g., Ni or Co oxides) the hydroxide ions have to be recovered in an additional process, which increases the overall energy demand. [ 16 ] While the pH decrease can be mitigated by using neutral electrolytes from the beginning, the formation of the (bi)carbonate ions and in consequence the CO 2 crossover is inherent to AEM‐based setups.…”

Direct Membrane Deposition for CO2 Electrolysis

“…− and CO 3 2− anion migration to the anode, where these anions oxidize and form CO 2 . 17 The result is high CO 2 crossover (up to 70%), 7 evidenced by the low percentage of CO 2 detected at the cathode outlet (∼6% CO 2 ) despite high HER (i.e., low CO 2 conversion is expected with 61% H 2 FE at 300 mA cm −2 ; Figure S2). When a standalone commercial CEM is employed as the sole membrane, high H 2 evolution is obtained (Figure S2), a result attributed to the high concentration of protons at the cathode surface.…”

“…We started by employing a commercial standalone AEM in the electrolyzer. The AEM facilitated HCO 3 – and CO 3 2– anion migration to the anode, where these anions oxidize and form CO 2 . The result is high CO 2 crossover (up to 70%), evidenced by the low percentage of CO 2 detected at the cathode outlet (∼6% CO 2 ) despite high HER (i.e., low CO 2 conversion is expected with 61% H 2 FE at 300 mA cm –2 ; Figure S2).…”