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Comprehensive Summary: Cu‐catalyzed electrochemical CO2 reduction reaction (CO2RR) and CO reduction reaction (CORR) are of great interest due to their potential to produce carbon‐neutral and value‐added multicarbon (C2+) chemicals. In practice, CO2RR and CORR are typically operated at industrially relevant current densities, making the process exothermal. Although the increased operation temperature is known to affect the performance of CO2RR and CORR, the relationship between temperatures and kinetic parameters was not clearly elaborated, particularly in zero‐gap reactors. In this study, we detail the effect of the temperature on Cu‐catalyzed CO2RR and CORR. Our electrochemical and operando spectroscopic studies show that high temperatures increase the activity of CO2RR to CO and CORR to C2H4 by enhancing the mass transfer of CO2 and CO. As the rates of these two processes are highly influenced by reactant diffusion, elevating the operating temperature results in high local CO2 and CO availability to accelerate product formation. Consequently, the *CO coverage in both cases increases at higher temperatures. However, under CO2RR conditions, *CO desorption is more favorable than carbon‐carbon (C—C) coupling thermodynamically at high temperatures, causing the reduction in the Faradaic efficiency (FE) of C2H4. In CORR, the high‐temperature‐augmented CO diffusion overcomes the unfavorable adsorption thermodynamics, increasing the probability of C—C coupling. [ABSTRACT FROM AUTHOR]

CO2 electroreduction is a potential pathway to achieve net-zero emissions in the chemical industry. Yet, CO2 loss, resulting from (bi)carbonate formation, renders the process energy-intensive. Acidic environments can address the issue but at the expense of compromised product Faradaic efficiencies (FEs), particularly for multi-carbon (C2+) products, as rapid diffusion and migration of protons (H+) favors competing H2 and CO production. Here, we present a strategy of tuning the 2-position substituent length on benzimidazole (BIM)-based copper (Cu) coordination polymer (CuCP) precatalyst – to enhance CO2 reduction to C2+ products in acidic environments. Lengthening the substituent from H to nonyl enhances H+ diffusion retardation and decreases Cu-Cu coordination numbers (CNs), favoring further reduction of CO. This leads to a nearly 24× enhancement of selectivity towards CO hydrogenation and C-C coupling at 60 mA cm−2. We report the highest C2+ product FE of more than 70% at 260 mA cm−2 on pentyl-CuCP and demonstrate a CO2-to-C2+ single-pass conversion (SPC) of ~54% at 180 mA cm−2 using pentyl-CuCP in zero-gap electrolyzers. Acidic CO2 electroreduction suffers poor selectivity for multi-carbon products. Here, the authors report that lengthening the 2-position substituents of benzimidazole-based copper coordination polymer precatalysts retards H+ diffusion, reduces Cu-Cu coordination numbers, and promotes C-C coupling. [ABSTRACT FROM AUTHOR]

The electrochemical CO2 reduction reaction (CO2RR) is an emerging carbon utilisation technology. The use of ionomers in fabricating catalytic electrodes was found to exert substantial effects on activity and product selectivity, particularly for Cu-catalyzed multicarbon (C2+) product formation. Here, we study the CO2RR performance of Cu catalysts coated with 8 different commercial ionomers to understand how ionomers impact C2+ product formation in zero-gap membrane electrode assembly (MEA) reactors. We find that the ionomer hydrophobicity plays the most decisive role in determining the faradaic efficiency (FE) and partial current density of C2+ products. Cu coated with the most hydrophobic ionomer converts CO2 into C2+ products at an FE of about 75% and 180 mA cm−2. This is due to the hydrophobicity-induced *CO adsorption stabilisation. The ionised side chains of ionomers may also impact the selectivity and activity of C2+ products by altering the electric double layer (EDL) structures. Ionised side chains with bulkier molecular structures and smaller hydration numbers presumably result in less compact EDLs, pertinent to enhanced C–C coupling activity. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: Cu‐catalyzed electrochemical CO2 reduction reaction (CO2RR) and CO reduction reaction (CORR) are of great interest due to their potential to produce carbon‐neutral and value‐added multicarbon (C2+) chemicals. In practice, CO2RR and CORR are typically operated at industrially relevant current densities, making the process exothermal. Although the increased operation temperature is known to affect the performance of CO2RR and CORR, the relationship between temperatures and kinetic parameters was not clearly elaborated, particularly in zero‐gap reactors. In this study, we detail the effect of the temperature on Cu‐catalyzed CO2RR and CORR. Our electrochemical and operando spectroscopic studies show that high temperatures increase the activity of CO2RR to CO and CORR to C2H4 by enhancing the mass transfer of CO2 and CO. As the rates of these two processes are highly influenced by reactant diffusion, elevating the operating temperature results in high local CO2 and CO availability to accelerate product formation. Consequently, the *CO coverage in both cases increases at higher temperatures. However, under CO2RR conditions, *CO desorption is more favorable than carbon‐carbon (C—C) coupling thermodynamically at high temperatures, causing the reduction in the Faradaic efficiency (FE) of C2H4. In CORR, the high‐temperature‐augmented CO diffusion overcomes the unfavorable adsorption thermodynamics, increasing the probability of C—C coupling. [ABSTRACT FROM AUTHOR]