McPherson, H., Hodges, T., Chuma, M.H., Sherwin, C., Podbevšek, U., Rigg, K., Celorrio, V., Russell, A., Corbos, E.C., Cathodes for Electrochemical Carbon Dioxide Reduction to Multi-Carbon Products: Part I and Part II. Johnson Matthey Technol. Rev., 2023, 67, (1), 97–109.
Part I: https://doi.org/10.1595/205651323X16672291226135 - Part II: https://doi.org/10.1595/205651323X16703459968311 - OPEN ACCESS
Abstract : This is a focused review of recent highlights in the literature in cathode development for low temperature electrochemical carbon dioxide and carbon monoxide reduction to multi-carbon (C2+) products. The major goals for the field are to increase Faradaic efficiency (FE) for specific C2+ products, lower cell voltage for industrially relevant current densities and increase cell lifetime. A key to achieving these goals is the rational design of cathodes through increased understanding of structure-selectivity and structure-activity relationships for catalysts and the influence of catalyst binders and gas diffusion layers (GDLs) on the catalyst microenvironment and subsequent performance.
Ju, W., Bagger, A., Saharie, N.R. et al. Electrochemical carbonyl reduction on single-site M–N–C catalysts. Commun Chem 6, 212 (2023). https://doi.org/10.1038/s42004-023-01008-y - OPEN ACCESS
Abstract : Electrochemical conversion of organic compounds holds promise for advancing sustainable synthesis and catalysis. This study explored electrochemical carbonyl hydrogenation on single-site M–N–C (Metal Nitrogen-doped Carbon) catalysts using formaldehyde, acetaldehyde, and acetone as model reactants. We strive to correlate and understand the selectivity dependence on the nature of the metal centers. Density Functional Theory calculations revealed similar binding energetics for carbonyl groups through oxygen-down or carbon-down adsorption due to oxygen and carbon scaling. Fe–N–C exhibited specific oxyphilicity and could selectively reduce aldehydes to hydrocarbons. By contrast, the carbophilic Co–N–C selectively converted acetaldehyde and acetone to ethanol and 2-propanol, respectively. We claim that the oxyphilicity of the active sites and consequent adsorption geometry (oxygen-down vs. carbon-down) are crucial in controlling product selectivity. These findings offer mechanistic insights into electrochemical carbonyl hydrogenation and can guide the development of efficient and sustainable electrocatalytic valorization of biomass-derived compounds.
Möller, T., Filippi, M., Brückner, S. et al. A CO2 electrolyzer tandem cell system for CO2-CO co-feed valorization in a Ni-N-C/Cu-catalyzed reaction cascade. Nat Commun 14, 5680 (2023). https://doi.org/10.1038/s41467-023-41278-7 - OPEN ACCESS
Abstract : Coupled tandem electrolyzer concepts have been predicted to offer kinetic benefits to sluggish catalytic reactions thanks to their flexibility of reaction environments in each cell. Here we design, assemble, test, and analyze the first complete low-temperature, neutral-pH, cathode precious metal-free tandem CO2 electrolyzer cell chain. The tandem system couples an Ag-free CO2-to-CO2/CO electrolyzer (cell-1) to a CO2/CO-to-C2+ product electrolyzer (cell-2). Cell-1 and cell-2 incorporate selective Ni-N-C-based and Cu-based Gas Diffusion Cathodes, respectively, and operate at sustainable neutral pH conditions. Using our tandem cell system, we report strongly enhanced rates for the production of ethylene (by 50%) and alcohols (by 100%) and a sharply increased C2+ energy efficiency (by 100%) at current densities of up to 700 mA cm−2 compared to the single CO2-to-C2+ electrolyzer cell system approach. This study demonstrates that coupled tandem electrolyzer cell systems can offer kinetic and practical energetic benefits over single-cell designs for the production of value-added C2+ chemicals and fuels directly from CO2 feeds without intermediate separation or purification.
Sahin B, Raymond S.K., Ntourmas F. , et al., Accumulation of Liquid Byproducts in an Electrolyte as a Critical Factor That Compromises Long-Term Functionality of CO2-to-C2H4 Electrolysis ACS Applied Materials & Interfaces 15 (39), 45844-45854 ( 2023) https://doi.org/10.1021/acsami.3c08454
Abstract : Electrochemical conversion of CO2 using Cu-based gas diffusion electrodes opens the way to green chemical production as an alternative to thermocatalytic processes and a storage solution for intermittent renewable electricity. However, diverse challenges, including short lifetimes, currently inhibit their industrial usage. Among well-studied determinants such as catalyst characteristics and electrode architecture, possible effects of byproduct accumulation in the electrolyte as an operational factor have not been elucidated. This work quantifies the influence of ethanol, n-propanol, and formate accumulation on selectivity, stability, and cell potential in a CO2-to-C2H4 electrolyzer. Alcohols accelerated flooding by degrading the hydrophobic electrode characteristics, undermining selective and stable ethylene formation. Furthermore, high alcohol concentrations triggered the catalyst layer’s abrasion and structural disfigurements in the Nafion 117 membrane, leading to high cell potentials. Therefore, continuous removal of alcohols from the electrolyte medium or substantial modifications in the cell components must be considered to ensure long-term performing CO2-to-C2H4 electrolyzers.
Filippi, M., Möller, T., Liang, L., and Strasser, P., Understanding the impact of catholyte flow compartment design on the efficiency of CO2 electrolyzers, Energy Environ. Sci., 2023, 16, 5265-5273, https://doi.org/10.1039/D3EE02243A - OPEN ACCESS
Abstract : This work explores and provides new understanding how catholyte flow compartment design and catholyte bubble flow characteristics of a gas diffusion electrode inside a CO2 flow cell electrolyzer affect its electrocatalytic reactivity and product selectivity. Focusing on Cu-based GDEs for CO2 electroreduction to hydrocarbons at high current densities (50–700 mA cm−2), four basic compartment designs were selected, 3D printed and investigated. Experiments were coupled to computational fluid dynamics simulation of catholyte flow and bubble dynamics. The findings from this work suggest a homogenous fluid velocity distribution combined with fluid velocity in the range between 0.1–0.01 m s−1 to be optimal for high yields in C2+ products at high current densities. Special focus was placed on the role and relation between gas bubble dynamics and local pH, both strongly affected by the design architecture. From our experimental observations and simulations, we propose a hydrodynamic “volcano” model addressing the competition between bubble release rate and local pH, both controlled by catholyte flow velocity. The balance between fast bubble release and high enough local pH across the electrode surface puts the electrolyzer operation at the top of the performance volcano.