High-Aspect-Ratio Ag Nanowire Mat Electrodes for Electrochemical CO Production from CO2

Trilayer Polymer Electrolytes Enable Carbon-Efficient CO2 to Multicarbon Product Conversion in Alkaline Electrolyzers

The electrochemical CO2 reduction reaction (CO2RR) is an appealing method for carbon utilization. Alkaline CO2 electrolyzers exhibit high CO2RR activity, low full-cell voltages, and cost-effectiveness. However, the issue of CO2 loss caused by (bi)carbonate formation leads to excessive energy consumption, rendering the process economically impractical. In this study, we propose a trilayer polymer electrolyte (TPE) comprising a perforated anion exchange membrane (PAEM) and a bipolar membrane (BPM) to facilitate alkaline CO2RR. This TPE enables the coexistence of high alkalinity near the catalyst surface and the H+ flux at the interface between the PAEM and the cation exchange layer (CEL) of the BPM, conditions favoring both CO2 reduction to multicarbon products and (bi)carbonate removal in KOH-fed membrane electrode assembly (MEA) reactors. As a result, we achieve a Faradaic efficiency (FE) of approximately 46 % for C2H4, corresponding to a C2+ FE of 64 % at 260 mA cm-2, with a CO2-to-C2H4 single-pass conversion (SPC) of approximately 32 % at 140 mA cm-2-nearly 1.3 times the limiting SPC in conventional AEM-MEA electrolyzers. Furthermore, coupling CO2 reduction with formaldehyde oxidation reaction (FOR) in the TPE-MEA electrolyzer reduces the full-cell voltage to 2.3 V at 100 mA cm-2 without compromising the C2H4 FE.

Fluorinated polymer zwitterions on gold nanoparticles: patterned catalyst surfaces guide interfacial transport and electrochemical CO2 reduction

We report the use of fluorinated polymer zwitterions to build hybrid systems for efficient CO2 electroreduction. The unique combination of hydrophilic phosphorylcholine and hydrophobic fluorinated moieties in these polymers creates a fractal structure with mixed branched cylinders on the surface of gold nanoparticles (AuNPs). In the presence of these polymers, the CO faradaic efficiency improves by 50-80% in the range of -0.7 V to -0.9 V. The fractal structures have a domain size of ∼3 nm, showing enhanced mass transfer kinetics of CO2 approaching the catalyst surfaces without limiting ion diffusion. The phase-separated hydrophilic and hydrophobic domains offer separated channeling to water and CO2, as confirmed by attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and molecule dynamic (MD) simulations. H2O molecules permeate extensively into the polymer layer that adsorbs on zwitterions, forming continuous chains, while CO2 molecules strongly associate with the fluorinated tails of fluorinated polyzwitterions, with oxygen facing the positively charged amine groups. Overall, this coupling of zwitterion and fluorocarbon in a polymer material creates new opportunities for defining microenvironments of metallic nanocatalysts in hybrid structures.

Introduction of a Conductive Layer into Flood-Resistant Gas Diffusion Electrodes with Polymer Substrate for an Efficient Electrochemical CO2 Reduction with Copper Oxide

Conversion of atmospheric carbon dioxide (CO2) into valuable feedstocks is a crucial technology, and electrochemical reduction of CO2 is a promising approach that can provide a useful source of ethylene (C2H4). Gas diffusion electrodes (GDEs) placed at the interface of the CO2 gas and electrolyte can achieve high current density through a sufficient supply of dissolved CO2 to the reaction site, making them indispensable in industrial applications. However, conventional GDEs with carbon substrate have suffered from electrolyte flooding and consequent loss of efficiency, posing an obstacle for practical application. While flood-resistant GDEs with hydrophobic polymer substrate have been proposed recently, only conductive materials can be employed as electrocatalysts because of their insulative properties, despite the high activities of oxide materials such as copper oxide. Here, we introduce an aluminum conductive layer in GDE with polymer substrate to enable the use of electrically resistive catalysts. Cuprous oxide (Cu2O) with silver particles was tested as a model material and has shown prolonged stability (>17 h) with high C2H4 Faraday efficiency (>50%) while suppressing flooding. A thorough characterization revealed that the conductive layer makes Cu2O an efficient electrocatalyst, even on the polymer substrate, by providing sufficient electrons through its conduction path. This research significantly expands the scope of electrode design by enabling the incorporation of a wide range of nonelectrically conductive materials on GDEs with polymer substrate.

Copper and silver nanowires for CO2 electroreduction

Copper and silver nanowires have been extensively investigated as the next generation of transparent conductive electrodes (TCEs) due to their ability to form percolating networks. Recently, they have been exploited as electrocatalysts for CO2 reduction. In this review, we present the most recent advances in this field summarizing different strategies used for the synthesis and functionalization/activation of copper and silver nanowires, as well as, the state of the art of their electrochemical performance with particular emphasis on the effect of the nanowire morphology. Novel perspectives for the development of highly efficient, selective, and stable electrocatalysts for CO2 reduction arise from the translation of NW-based TCEs in this challenging field.

Polymer-Regulated Electrochemical Reduction of CO2 on Ag

The influence of polymer overlayers on the catalytic activity of Ag for electrochemical CO2 reduction to CO is explored. Polystyrene and poly(4-vinylpyridine) films of varying thicknesses are applied as catalysis-directing overlayers atop Ag electrodes. For polystyrene, substantial suppression of CO2 reduction activity is observed while the hydrogen evolution reaction (HER) increases. The addition of a nitrogen heteroatom into the phenyl groups of polystyrene (e.g., a pyridine ring) results in an increase in the conversion of CO2 to CO and suppression of HER. Block copolymer variants containing both phenyl and pyridyl functionalities exhibit similar activity for CO evolution but appear to suppress HER further than the polymer layer containing only pyridine groups. The size of the blocks for the copolymer influences the catalytic output of the Ag electrode, suggesting that the hierarchical structure that forms in the block copolymer layer plays a role in catalytic activity at the electrode surface. Analysis of the polymer overlayers suggests that polystyrene significantly inhibits all ion transport to the metal electrode, while poly(4-vinylpyridine) enables CO2 transport while modifying the electronics of the Ag active site. Therefore, the engineered application of polymer overlayers, especially those containing heteroatoms, enables new avenues of electrochemical CO2 reduction to be explored.

Functionalized Ag with Thiol Ligand to Promote Effective CO2 Electroreduction

It is challenging while critical to develop efficient catalysts that can achieve both high current density and high energy efficiency for electrocatalytic CO₂ reduction (CO₂R). Herein, we report a strategy of tailoring the surface electronic structure of an Ag catalyst via thiol ligand modification to improve its intrinsic activity, selectivity, and further energy efficiency toward CO₂R. Specifically, interconnected Ag nanoparticles with residual thiol ligands on the surface were prepared through electrochemical activation of a thiol-ligand-based Ag complex. When it was used as a catalyst for CO₂R, the thiol-ligand modified Ag exhibited high CO selectivity (>90%) throughout a wide electrode-potential range; furthermore, high cathodic energy efficiencies of >90% and >70% were obtained for CO formation at high current densities of 150 and 750 mA cm^-2, respectively, outperforming the state-of-the-art Ag-based electrocatalysts for CO₂ to CO conversion. The first-principle calculations on the reaction energetics suggest that the binding energies of the key intermediate -*COOH on Ag are optimized by the adsorbed thiol ligand, thus favoring CO formation while suppressing the competing H₂ evolution. Our findings provide a rational design strategy for CO₂ reduction electrocatalyst by electronic modulation through surface-adsorbed ligands.

Cracks as Efficient Tools to Mitigate Flooding in Gas Diffusion Electrodes Used for the Electrochemical Reduction of Carbon Dioxide

The advantage of employing gas diffusion electrodes (GDEs) in carbon dioxide reduction electrolyzers is that they allow CO₂ to reach the catalyst in gaseous state, enabling current densities that are orders of magnitude larger than what is achievable in standard H-type cells. The gain in the reaction rate comes, however, at the cost of stability issues related to flooding that occurs when excess electrolyte permeates the micropores of the GDE, effectively blocking the access of CO₂ to the catalyst. For electrolyzers operated with alkaline electrolytes, flooding leaves clear traces within the GDE in the form of precipitated potassium (hydrogen)carbonates. By analyzing the amount and distribution of precipitates, and by quantifying potassium salts transported through the GDE during operation (electrolyte perspiration), important information can be gained with regard to the extent and means of flooding. In this work, a novel combination of energy dispersive X-ray and inductively coupled plasma mass spectrometry based methods is employed to study flooding-related phenomena in GDEs differing in the abundance of cracks in the microporous layer. It is concluded that cracks play an important role in the electrolyte management of CO₂ electrolyzers, and that electrolyte perspiration through cracks is paramount in avoiding flooding-related performance drops.

The formation mechanism of fibrous metal oxalate prepared by ammonia coordination method

Anisotropic microstructures normally lead to different or better optical, electrical and magnetic properties for functional materials. We recently found some metal oxalates (nickel, cobalt, iron, silver, etc.) prepared in aqueous solutions will form fibrous morphology in the presence of ammonia (ammonia coordination method). Metals or metal oxides obtained by heating the fibrous metal oxalates in inert or oxidizing atmospheres can also inherit the high-aspect-ratio morphology. Ammonia coordination method is a simple and economical way to synthesize fibrous metal oxalates, metals, and metal oxides. In this work, fibrous nickel oxalate with a formula of Ni(NH₃)_1.7C₂O₄·2.2H₂O and its corresponding single crystal were synthesized to investigate the morphology transitions, structure transitions, and formation mechanism of fibrous particles. Ammonia molecules gradually coordinating with nickel atoms, which caused the increase of surface energy and atomic stacking rate of (020) crystal plane, was the fundamental reason for the oriented growth of nickel oxalate. Our results demonstrate a feasible method to synthesize high-aspect-ratio metallic materials and show the important influences of coordination ligand ammonia on the crystal growth stage of metallic materials which may provide references for synthesizing metallic materials with extraordinary microstructures and better properties by simple ammonia coordination method.

Ammonia coordination method was proposed to synthesize fibrous metal oxalates and its mechanism was studied.

CuZnAl-Oxide Nanopyramidal Mesoporous Materials for the Electrocatalytic CO2 Reduction to Syngas: Tuning of H2/CO Ratio

Inspired by the knowledge of the thermocatalytic CO₂ reduction process, novel nanocrystalline CuZnAl-oxide based catalysts with pyramidal mesoporous structures are here proposed for the CO₂ electrochemical reduction under ambient conditions. The XPS analyses revealed that the co-presence of ZnO and Al₂O₃ into the Cu-based catalyst stabilize the CuO crystalline structure and introduce basic sites on the ternary as-synthesized catalyst. In contrast, the as-prepared CuZn- and Cu-based materials contain a higher amount of superficial Cu⁰ and Cu¹⁺ species. The CuZnAl-catalyst exhibited enhanced catalytic performance for the CO and H₂ production, reaching a Faradaic efficiency (FE) towards syngas of almost 95% at −0.89 V vs. RHE and a remarkable current density of up to 90 mA cm⁻² for the CO₂ reduction at −2.4 V vs. RHE. The physico-chemical characterizations confirmed that the pyramidal mesoporous structure of this material, which is constituted by a high pore volume and small CuO crystals, plays a fundamental role in its low diffusional mass-transfer resistance. The CO-productivity on the CuZnAl-catalyst increased at more negative applied potentials, leading to the production of syngas with a tunable H₂/CO ratio (from 2 to 7), depending on the applied potential. These results pave the way to substitute state-of-the-art noble metals (e.g., Ag, Au) with this abundant and cost-effective catalyst to produce syngas. Moreover, the post-reaction analyses demonstrated the stabilization of Cu₂O species, avoiding its complete reduction to Cu⁰ under the CO₂ electroreduction conditions.