How to Select Each Component of CO2 Electrolyzers

Abstract

Electrochemical CO₂ electrolyzers are increasingly recognized for their potential to convert CO₂ into valuable chemical feedstocks, addressing critical environmental and economic challenges. Traditionally, the catalytic properties of the cathode, where CO₂RR directly occurs, have been the main focus of research due to their control over product selectivity. More recently, however, membrane-based electrolyzers-commonly used in fuel cells and water electrolyzers-have shown substantial potential for commercial CO₂ reduction, offering improved scalability and efficiency. Nevertheless, the complex components in membrane-based electrolyzers require precise optimization, as each unit directly impacts system performance and product selectivity. In this review, the structures and components of membrane-based CO₂ electrolyzers are systematically examined, including the electrolyzer design, flow channels, membranes, electrolytes, CO₂ supply units, and electrodes. Recent innovations in the optimization of these components are highlighted to provide insights into advancing CO₂RR technology toward commercially feasible applications. This approach can assist considerably in improving the CO₂RR electrolyzer performance, thereby helping predict optimal pathways for commercial realization and guide future development.

Keywords: CO2 electrolyzer; CO2 reduction reaction; flow channel; membrane; porous transfer electrode.

FIGURE 1

Schematic illustration of CO₂RR electrolyzers. (A) H‐type cell, (B) flow‐type electrolyzer, and (C) MEA‐type electrolyzer.

FIGURE 2

(A) Dioxide materials commercial electrolyzer: In the case of the above figure, customized parts are used for in‐situ experiments. Reproduced with permission [21]. Copyright 2023, American Chemical Society. (B) Electrolyzer with all parts customized.

FIGURE 3

(A) Three basic configurations of the gas flow channel. Reproduced with permission [46]. Copyright 2023, American Chemical Society. (B) CFD simulation results of the studied central flow channel. (C) Schematic illustration of CO₂RR flow‐type electrolyzers. Reproduced with permission [50]. Copyright 2023, Royal Society of Chemistry.

FIGURE 4

Ion transport mechanisms, polymer structures, and interfaces of bipolar membranes. (A) Schematic diagram of the ion transport mechanism in the membrane [60]. (B) Polymer structure of Sustainion [60]. (C) Quaternary ammonium poly(N‐methyl‐piperidine‐co‐p‐terphenyl) (QAPPT). Reproduced with permission [71]. Copyright 2023, American Chemical Society. (D) Perfluorinated sulfonic acid (PFSA) [60]. (E) Schematic diagrams of different interfaces of the bipolar membrane.^[81] Copyright 2021, Elsevier.

FIGURE 5

Research about different electrolytes in CO₂RR. (A) pH value of catholyte and anolyte to reaction time on both CEM and AEM, diagrams about CO₂RR in aqueous KHCO₃. Reproduced with permission.^[85] Copyright 2024, American Chemical Society. (B) Schematic diagram showing cation effects with alkali metal electrolyte.^[91] Copyright 2022, Elsevier. (C) FE values of the gaseous products generated during the CO₂RR with different organic cations and anions.^[96] (D) Schematic diagram showing replacement of metal cations with bulk organic cations and their weakly coordinated local environment. Reproduced with permission.^[97] Copyright 2023, American Chemical Society.

FIGURE 6

(A) Schematic of an electrochemical gaseous CO₂RR PTE cathode and its fundamental components. Reproduced with permission [12]. Copyright 2024, American Chemical Society. (B) Schematic of CO₂ conversion in the cathode compartment of a liquid‐fed bicarbonate electrolyzer. Reproduced with permission [124]. Copyright 2023, American Chemical Society. (C) Faradaic efficiencies of the major products and the applied potential at various CO₂ feed flow rates at different current densities. Reproduced with permission [113]. Copyright 2020, Elsevier. (D) Illustration and FE of the three gas conditions: Ar (0% CO₂), dry CO₂, and humidified CO₂, explored in the gas chamber of the hybrid reactor. Reproduced with permission [114]. Copyright 2022, American Chemical Society. (E) FEs toward formate and CO₂RR partial current densities on Au, Ag, and Sn catalysts under different pressures at −1.1 V versus RHE. Reproduced with permission [128]. Copyright 2022, Springer Nature. (F) Illustration of the FTDT cell for pumping CO₂‐saturated catholyte throughout a porous electrode with CO₂ exsolution from the dynamic equilibria E1 and E2. Reproduced with permission [129]. Copyright 2022, Springer Nature. CEM, cation exchange membrane; GDL, gas diffusion layer.

FIGURE 7

(A) Structure of different commercially available carbon gas diffusion layers observed through X‐ray computed tomography. (B) Comparing commercially available carbon PTL‐based electrodes in the CO₂ reduction reaction at MEA with Ag catalyst. Reproduced with permission [144]. Copyright 2023, Springer Nature. (C) SEM image of Cu nanoparticles sputtered on the PTFE PTL. Reproduced with permission [196]. Copyright 2021, Springer Nature. (D) Sketches of a traditional ePTFE/Cu (top) and ePTFE/NICC/ Cu electrode (bottom). Current collection lines are depicted in blue. (E) Observed selectivity toward ethylene for the three designs at a constant potential of ≈–0.55 V versus RHE. Reproduced with permission [35]. Copyright 2023, Springer Nature.

FIGURE 8

(A–C) Cross‐sectional SEM images and their energy‐dispersive X‐ray spectroscopy (EDS) elemental mappings of Cu for catalyst layers prepared by dropcasting (A), hand‐painting (B), and airbrushing (C). (D) FEs of the major products and the applied potential at different current densities for various ink‐based catalyst deposition methods. Reproduced with permission [113]. Copyright 2020, Elsevier. (E) C₂H₄ FEs in the current density range of 200 to 300 mA cm⁻², showing the increased C₂H₄ selectivity of the abrupt reaction interface samples (10 and 25 nm) compared with that of thicker samples (1000 nm and 1000 mg) that allow for a more distributed reaction. Reproduced with permission [154]. Copyright 2020, American Association for the Advancement of Science. (F) SEM image of hierarchical Cu dendrites. Reproduced with permission [215]. Copyright 2021, American Chemical Society. (G) The time‐dependent morphological change of Cu‐CO₂ (upper arrows) and Cu‐HER (lower arrows) during electrodepositions at 400 mA cm⁻² in 1 M KOH containing the copper precursor. Reproduced with permission [216]. Copyright 2019, Springer Nature.