The continuously increasing CO2 released from human activities poses a great threat to human survival by fluctuating global climate and disturbing carbon balance among the four reservoirs of the biosphere, earth, air, and water. Converting CO2 to value-added feedstocks via electrocatalysis of the CO2 reduction reaction (CO2RR) has been regarded as one of the most attractive routes to re-balance the carbon cycle, thanks to its multiple advantages of mild operating conditions, easy handling, tunable products and the potential of synergy with the rapidly increasing renewable energy (i.e., solar, wind). Instead of focusing on a special topic of electrocatalysts for the CO2RR that have been extensively reviewed elsewhere, we herein present a rather comprehensive review of the recent research progress, in the view of associated value-added products upon selective electrocatalytic CO2 conversion. We initially provide an overview of the history and the fundamental science regarding the electrocatalytic CO2RR, with a special introduction to the design, preparation, and performance evaluation of electrocatalysts, the factors influencing the CO2RR, and the associated theoretical calculations. Emphasis will then be given to the emerging trends of selective electrocatalytic conversion of CO2 into a variety of value-added products. The structure-performance relationship and mechanism will also be discussed and investigated. The outlooks for CO2 electrocatalysis, including the challenges and opportunities in the development of new electrocatalysts, electrolyzers, the recently rising operando fundamental studies, and the feasibility of industrial applications are finally summarized.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

Managing the gas-liquid interface within gas-diffusion electrodes (GDEs) is key to maintaining high product selectivities in carbon dioxide electroreduction. By screening silver-catalyzed GDEs over a range of applied current densities, an inverse correlation was observed between carbon monoxide selectivity and the electrochemical double-layer capacitance, a proxy for wetted electrode area. Plotting current-dependent performance as a function of cumulative charge led to data collapse onto a single sigmoidal curve indicating that the passage of faradaic current accelerates flooding. It was hypothesized that high cathode alkalinity, driven by both initial electrolyte conditions and cathode half-reactions, promotes carbonate formation and precipitation which, in turn, facilitates electrolyte permeation. This mechanism was reinforced by the observations that post-test GDEs retain less hydrophobicity than pristine materials and that water-rinsing and drying electrodes temporarily recovers peak selectivity. This knowledge offers an opportunity to design electrodes with greater carbonation tolerance to improve device longevity.

Investigating electrode flooding in a flowing electrolyte, gas‐fed carbon dioxide electrolyzer

Electrochemical reduction of gaseous feeds such as CO2, CO, and N2 holds promise for sustainable energy and chemical production. Practical application of this technology is impeded by slow mass transport of the sparingly soluble gases to conventional planar electrodes. Gas diffusion electrodes (GDEs) maintain a high gas concentration in the vicinity of the catalyst and improve mass transport, thereby resulting in current densities higher by orders of magnitude. However, gaseous feeds cause changes to the GDE environment, and specific features are required to efficiently tune the product selectivity and improve reaction stability. Herein, with a comprehensive review of the challenges and advances in GDE development for various electrocatalytic reactions, we intend to complement the body of material-focused reviews. This review outlines GDE fundamentals and highlights key advantages of GDE over conventional electrodes. Through critical discussion about steps in GDE fabrication, and specific shortcomings and remedial strategies for various electrochemical applications, this review discusses connections, unique design criteria, and potential opportunities for gas-fed reactions and desired products. Finally, priorities for future studies are suggested, to support the advancement and scale-up of GDE-based electrochemical technologies.

Gas diffusion electrodes (GDEs) for electrochemical reduction of carbon dioxide, carbon monoxide, and dinitrogen to value-added products: a review

The deployment of gas diffusion electrodes (GDEs) for the electrochemical CO2 reduction reaction (CO2RR) has enabled current densities an order of magnitude greater than those of aqueous H cells. The gains in production, however, have come with stability challenges due to rapid flooding of GDEs, which frustrate both laboratory experiments and scale-up prospects. Here, we investigate the role of carbon gas diffusion layers (GDLs) in the advent of flooding during CO2RR, finding that applied potential plays a central role in the observed instabilities. Electrochemical characterization of carbon GDLs with and without catalysts suggests that the high overpotential required during electrochemical CO2RR initiates hydrogen evolution on the carbon GDL support. These potentials impact the wetting characteristics of the hydrophobic GDL, resulting in flooding that is independent of CO2RR. Findings from this work can be extended to any electrochemical reduction reaction using carbon-based GDEs (CORR or N2RR) with cathodic overpotentials of less than -0.65 V versus a reversible hydrogen electrode.

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Role of the Carbon-Based Gas Diffusion Layer on Flooding in a Gas Diffusion Electrode Cell for Electrochemical CO 2 Reduction

Electrochemical CO2 capture technologies are gaining attention due to their flexibility, their ability to address decentralized emissions (e.g., ocean and atmosphere) and their fit in an electrified industry. In the present work, recent progress made in electrochemical CO2 capture is reviewed. The majority of these methods rely on the concept of “pH-swing” and the effect it has on the CO2 hydration/dehydration equilibrium. Through a pH-swing, CO2 can be captured and recovered by shifting the pH of a working fluid between acidic and basic pH. Such swing can be applied electrochemically through electrolysis, bipolar membrane electrodialysis, reversible redox reactions and capacitive deionization. In this review, we summarize main parameters governing these electrochemical pH-swing processes and put the concept in the framework of available worldwide capture technologies. We analyse the energy efficiency and consumption of such systems, and provide recommendations for further improvements. Although electrochemical CO2 capture technologies are rather costly compared to the amine based capture, they can be particularly interesting if more affordable renewable electricity and materials (e.g., electrode and membranes) become widely available. Furthermore, electrochemical methods have the ability to (directly) convert the captured CO2 to value added chemicals and fuels, and hence prepare for a fully electrified circular carbon economy.

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Electrochemical carbon dioxide capture to close the carbon cycle

Influence of flow and pressure distribution inside a gas diffusion electrode on the performance of a flow-by CO2 electrolyzer

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Electrochemical Reduction of CO2: Effect of Convective CO2 Supply in Gas Diffusion Electrodes

A zero-gap flow electrolyzer with a tin-coated gas diffusion electrode as the cathode was used to convert humidified gaseous CO2 to formate. The influence of humidification, flow pattern and the type of membrane on the faradaic efficiency (FE), product concentration, and salt precipitation were investigated. We demonstrated that water management in the gas diffusion electrode was crucial to avoid flooding and (bi)carbonate precipitation, to uphold a high FE and formate concentration. Direct water injection was validated as a novel approach for water management. At 100 mA/cm2, direct water injection in combination with an interdigitated flow channel resulted in a FE of 80 % and a formate concentration of 65.4+/−0.3 g/l without salt precipitation for a prolonged CO2 electrolysis of 1 h. The use of bipolar membranes in the zero-gap configuration mainly produced hydrogen. These results are important for the design of commercial scale CO2 electrolyzers.

Direct Water Injection in Catholyte-Free Zero-Gap Carbon Dioxide Electrolyzers

Sn-Based Electrocatalyst Stability: A Crucial Piece to the Puzzle for the Electrochemical CO2 Reduction toward Formic Acid

Direct electrochemical reduction of CO2 to C2 products such as ethylene is more efficient in alkaline media, but it suffers from parasitic loss of reactants due to (bi)carbonate formation. A two-st...

https://pubs.acs.org/doi/pdf/10.1021/acs.iecr.1c03592

Bert De Mot

Papers

Influence of flow and pressure distribution inside a gas diffusion electrode on the performance of a flow-by CO2 electrolyzer

Electrochemical Reduction of CO2: Effect of Convective CO2 Supply in Gas Diffusion Electrodes

Direct Water Injection in Catholyte-Free Zero-Gap Carbon Dioxide Electrolyzers

Sn-Based Electrocatalyst Stability: A Crucial Piece to the Puzzle for the Electrochemical CO2 Reduction toward Formic Acid

Electroreduction of CO2/CO to C2 Products: Process Modeling, Downstream Separation, System Integration, and Economic Analysis