Liquid-Solid Boundaries Dominate Activity of CO₂Reduction on Gas-Diffusion Electrodes

Electrochemical CO2 electrolysis to produce hydrocarbon fuels or material feedstocks offers a renewable alternative to fossilized carbon sources. Gas-diffusion electrodes (GDEs), composed of solid electrocatalysts on porous supports positioned near the interface of a conducting electrolyte and CO2 gas, have been able to demonstrate the substantial current densities needed for future commercialization. These higher reaction rates have often been ascribed to the presence of a three-phase interface, where solid, liquid, and gas provide electrons, water, and CO2, respectively. Conversely, mechanistic work on electrochemical reactions implicates a fully two-phase reaction interface, where gas molecules reach the electrocatalyst's surface by dissolution and diffusion through the electrolyte. Because the discrepancy between an atomistic three-phase versus two-phase reaction has substantial implications for the design of catalysts, gas-diffusion layers, and cell architectures, the nuances of nomenclatures and governing phenomena surrounding the three-phase-region require clarification. Here we outline the macro, micro, and atomistic phenomena occurring within a gas-diffusion electrode to provide a focused discussion on the architecture of the often-discussed three-phase region for CO2 electrolysis. From this information, we comment on the outlook for the broader CO2 electroreduction GDE cell architecture. </p

Electrochemical CO2 reduction in membrane-electrode assemblies

Electrochemical conversion of gaseous CO2 to value-added products and fuels is a promising approach to achieve net-zero CO2 emission energy systems. Significant efforts have been achieved in the design and synthesis of highly active and selective electrocatalysts for this reaction and their reaction mechanism. To perform an efficient conversion and desired product selectivity in practical applications, we need an active, cost-effective, stable, and scalable electrolyzer design. Membrane-electrode assemblies (MEAs) can be an efficient solution to address the key challenges in the aqueous gas diffusion electrodes (GDE), e.g., ohmic resistances and complex reactor design. This review presents a critical overview of recent advances in experimental design and simulation of MEAs for CO2 reduction reaction, including the shortcomings and remedial strategies. In the last section, the remaining challenges and future research opportunities are suggested to support the advancement of CO2 electrochemical technologies.Lei Ge, Hesamoddin Rabiee, Mengran Li, Siddhartha Subramanian, Yao Zheng, Joong Hee Lee, Thomas Burdyny, Hao Wan

Liquid-Solid Boundaries Dominate Activity of CO₂Reduction on Gas-Diffusion Electrodes

Electrochemical CO2 electrolysis to produce hydrocarbon fuels or material feedstocks offers a renewable alternative to fossilized carbon sources. Gas-diffusion electrodes (GDEs), composed of solid electrocatalysts on porous supports positioned near the interface of a conducting electrolyte and CO2 gas, have been able to demonstrate the substantial current densities needed for future commercialization. These higher reaction rates have often been ascribed to the presence of a three-phase interface, where solid, liquid, and gas provide electrons, water, and CO2, respectively. Conversely, mechanistic work on electrochemical reactions implicates a fully two-phase reaction interface, where gas molecules reach the electrocatalyst's surface by dissolution and diffusion through the electrolyte. Because the discrepancy between an atomistic three-phase versus two-phase reaction has substantial implications for the design of catalysts, gas-diffusion layers, and cell architectures, the nuances of nomenclatures and governing phenomena surrounding the three-phase-region require clarification. Here we outline the macro, micro, and atomistic phenomena occurring within a gas-diffusion electrode to provide a focused discussion on the architecture of the often-discussed three-phase region for CO2 electrolysis. From this information, we comment on the outlook for the broader CO2 electroreduction GDE cell architecture. Accepted Author ManuscriptChemE/Materials for Energy Conversion & Storag

Electroreduction of Carbon Dioxide to Acetate using Heterogenized Hydrophilic Manganese Porphyrins

The electrochemical reduction of carbon dioxide (CO2) to value-added chemicals is a promising strategy to mitigate climate change. Metalloporphyrins have been used as a promising class of stable and tunable catalysts for the electrochemical reduction reaction of CO2 (CO2RR) but have been primarily restricted to single-carbon reduction products. Here, we utilize functionalized earth-abundant manganese tetraphenylporphyrin-based (Mn-TPP) molecular electrocatalysts that have been immobilized via electrografting onto a glassy carbon electrode (GCE) to convert CO2 with overall 94 % Faradaic efficiencies, with 62 % being converted to acetate. Tuning of Mn-TPP with electron-withdrawing sulfonate groups (Mn-TPPS) introduced mechanistic changes arising from the electrostatic interaction between the sulfonate groups and water molecules, resulting in better surface coverage, which facilitated higher conversion rates than the non-functionalized Mn-TPP. For Mn-TPP only carbon monoxide and formate were detected as CO2 reduction products. Density-functional theory (DFT) calculations confirm that the additional sulfonate groups could alter the C−C coupling pathway from *CO→*COH→*COH-CO to *CO→*CO-CO→*COH-CO, reducing the free energy barrier of C−C coupling in the case of Mn-TPPS. This opens a new approach to designing metalloporphyrin catalysts for two carbon products in CO2RR.</p

Elucidating the effects of solvent-ionomer interactions on copper catalyst layers for CO2 electrolysis to multicarbon products

We report the influence of ionomer and catalyst dispersion solvent interaction on the structure and ionomer film wettability in copper (Cu) catalyst layers (CLs) in a gas diffusion electrode (GDE). Our results show that acetone and methanol dispersion solvents interact differently with the perfluorinated sulfonic acid (PFSA) ionomer Aquivion, which is composed of hydrophobic backbones and hydrophilic ionic heads. Acetone solvates more with the hydrophobic backbones in the PFSA compared to methanol. Consequently, the ionomer film fabricated from casting Aquivion and acetone mixture on a flat surface is more continuous and hydrophobic than its methanol counterpart. Such ionomer-solvent interaction also leads to a more uniform and flooding-tolerant GDE when producing the copper catalyst layer with acetone (acetone-CL) compared to methanol (methanol-CL). As a result, acetone-CL yields higher selectivity to C2+ products at high current density, up to 29 % greater than methanol-CL at 500 mA cm-2. Ethylene is the primary product for both CLs, reaching 47.5 4.0 % and 43.9 5.5 % at 300 mA cm-2 for acetone-CL and methanol-CL, respectively. The improvement in C2+ product selectivity for the acetone-CL is attributed to the CLs high resistance against flooding at current densities above 300 mA cm-2. Our findings offer a new strategy to advance CO2 electrolysis by manipulating solvent-ionomer interactions