Electrochemical CO2 reduction toward multicarbon alcohols - The microscopic world of catalysts & process conditions

Composition effects of electrodeposited Cu-Ag nanostructured electrocatalysts for CO2 reduction

The electrochemical carbon dioxide reduction (CO2RR) on Cu-based catalysts is a promising strategy to store renewable electricity and produce valuable C2+ chemicals. We investigate the CO2RR on Cu-Ag nanostructures that have been electrodeposited in a green choline chloride and urea deep eutectic solvent (DES). We determine the electrochemically active surface area (ECSA) using lead underpotential deposition (UPD) to investigate the CO2RR intrinsic activity and selectivity. We show that the addition of Ag on electrodeposited Cu primarily suppresses the production of hydrogen and methane. While the production of carbon monoxide slightly increases, the partial current of the total C2+ products does not considerably increase. Despite that the production rate of C2+ is similar on Cu and Cu-Ag, the addition of Ag enhances the formation of alcohols and oxygenates over ethylene. We highlight the potential of metal electrodeposition from DES as a sustainable strategy to develop bimetallic Cu-based nanocatalysts for CO2RR.

Accelerating the Reaction Kinetics of CO2 Reduction to Multi-Carbon Products by Synergistic Effect between Cation and Aprotic Solvent on Copper Electrodes

Improving the selectivity of electrochemical CO2 reduction to multi-carbon products (C2+ ) is an important and highly challenging topic. In this work, we propose and validate an effective strategy to improve C2+ selectivity on Cu electrodes, by introducing a synergistic effect between cation (Na+ ) and aprotic solvent (DMSO) to the electrolyte. Based on constant potential ab initio molecular dynamics simulations, we first revealed that Na+ facilitates C-C coupling while inhibits CH3 OH/CH4 products via reducing the water network connectivity near the electrode. Furthermore, the water network connectivity was further decreased by introducing an aprotic solvent DMSO, leading to suppression of both C1 production and hydrogen evolution reaction with minimal effect on *OCCO* hydrogenation. The synergistic effect enhancing C2 selectivity was also experimentally verified through electrochemical measurements. The results showed that the Faradaic efficiency of C2 increases from 9.3 % to 57 % at 50 mA/cm2 under a mixed electrolyte of NaHCO3 and DMSO compared to a pure NaHCO3 , which can significantly enhance the selectivity of the C2 product. Therefore, our discovery provides an effective electrolyte-based strategy for tuning CO2 RR selectivity through modulating the microenvironment at the electrode-electrolyte interface.

Acidic CO2 electroreduction for high CO2 utilization: catalysts, electrodes, and electrolyzers

The electrochemical carbon dioxide (CO2) reduction reaction (CO2RR) is considered a promising technology for converting atmospheric CO2 into value-added compounds by utilizing renewable energy. The CO2RR has developed in various ways over the past few decades, including product selectivity, current density, and catalytic stability. However, its commercialization is still unsuitable in terms of economic feasibility. One of the major challenges in its commercialization is the low single-pass conversion efficiency (SPCE) of CO2, which is primarily caused by the formation of carbonate (CO32-) in neutral and alkaline electrolytes. Notably, the majority of CO2RRs take place in such media, necessitating significant energy input for CO2 regeneration. Therefore, performing the CO2RR under conditions that minimize CO32- formation to suppress reactant and electrolyte ion loss is regarded an optimal strategy for practical applications. Here, we introduce the recent progress and perspectives in the electrochemical CO2RR in acidic electrolytes, which receives great attention because of the inhibition of CO32- formation. This includes the categories of nanoscale catalytic design, microscale microenvironmental effects, and bulk scale applications in electrolyzers for zero carbon loss reactions. Additionally, we offer insights into the issue of limited catalytic durability, a notable drawback under acidic conditions and propose guidelines for further development of the acidic CO2RR.

Mechanistic Insights into the Formation of Hydroxyacetone, Acetone, and 1,2-Propanediol from Electrochemical CO2 Reduction on Copper

Studies focused on the mechanism of CO₂ electroreduction (CO₂RR) aim to open up opportunities to optimize reaction parameters toward selective synthesis of desired products. However, the reaction pathways for C₃ compound syntheses, especially for minor compounds, remain incompletely understood. In this study, we investigated the formation pathway for hydroxyacetone, acetone, and 1,2-propanediol through CO₍₂₎RR, which are minor products that required long electrolysis times to be detected. Our proposed reaction mechanism is based on a systematic investigation of the reduction of several functional groups on a Cu electrode, including aldehydes, ketones, ketonealdehydes, hydroxyls, hydroxycarbonyls, and hydroxydicarbonyls, as well as the coupling between CO and C₂-dicarbonyl (glyoxal) or C₂-hydroxycarbonyl (glycolaldehyde). This study allowed us to derive the fundamental principles of the reduction of functional groups on Cu electrodes. Our findings suggest that the formation of ethanol does not follow the glyoxal pathway, as previously suggested but instead likely occurs via the coupling of CH₃* and CO. For the C₃ compounds, our results suggest that 1,2-propanediol and acetone follow the hydroxyacetone pathway during CO₂RR. Hydroxyacetone is likely formed through the coupling of CO and a C₂-hydroxycarbonyl intermediate, such as a glycolaldehyde-like compound, as confirmed by adding glycolaldehyde to the CO₍₂₎-saturated solution. This finding is consistent with CO₂RR product distribution, as glycolaldehyde formation during CO₂RR is limited, which, in turn, limits hydroxyacetone production. Our study contributes to a better understanding of the reaction mechanism for hydroxyacetone, acetone, and 1,2-propanediol synthesis from CO₂RR and gives insights into these interesting compounds that may be formed electrochemically.

Nanoreactor Engineering Can Unlock New Possibilities for CO2 Tandem Catalytic Conversion to C-C Coupled Products

Climate change is becoming increasingly more pronounced every day while the amount of greenhouse gases in the atmosphere continues to rise. CO₂ reduction to valuable chemicals is an approach that has gathered substantial attention as a means to recycle these gases. Herein, some of the tandem catalysis approaches that can be used to achieve the transformation of CO₂ to C-C coupled products are explored, focusing especially on tandem catalytic schemes where there is a big opportunity to improve performance by designing effective catalytic nanoreactors. Recent reviews have highlighted the technical challenges and opportunities for advancing tandem catalysis, especially highlighting the need for elucidating structure-activity relationships and mechanisms of reaction through theoretical and in situ/operando characterization techniques. In this review, the focus is on nanoreactor synthesis strategies as a critical research direction, and discusses these in the context of two main tandem pathways (CO-mediated pathway and Methanol-mediated pathway) to C-C coupled products.

Electroreduction of CO2 in a Non-aqueous Electrolyte-The Generic Role of Acetonitrile

Transition metal carbides, especially Mo2C, are praised to be efficient electrocatalysts to reduce CO2 to valuable hydrocarbons. However, on Mo2C in an aqueous electrolyte, exclusively the competing hydrogen evolution reaction takes place, and this discrepancy to theory was traced back to the formation of a thin oxide layer at the electrode surface. Here, we study the CO2 reduction activity at Mo2C in a non-aqueous electrolyte to avoid such passivation and to determine products and the CO2 reduction reaction pathway. We find a tendency of CO2 to reduce to carbon monoxide. This process is inevitably coupled with the decomposition of acetonitrile to a 3-aminocrotonitrile anion. Furthermore, a unique behavior of the non-aqueous acetonitrile electrolyte is found, where the electrolyte, instead of the electrocatalyst, governs the catalytic selectivity of the CO2 reduction. This is evidenced by in situ electrochemical infrared spectroscopy on different electrocatalysts as well as by density functional theory calculations.

Surface hydroxide promotes CO2 electrolysis to ethylene in acidic conditions

Performing CO₂ reduction in acidic conditions enables high single-pass CO₂ conversion efficiency. However, a faster kinetics of the hydrogen evolution reaction compared to CO₂ reduction limits the selectivity toward multicarbon products. Prior studies have shown that adsorbed hydroxide on the Cu surface promotes CO₂ reduction in neutral and alkaline conditions. We posited that limited adsorbed hydroxide species in acidic CO₂ reduction could contribute to a low selectivity to multicarbon products. Here we report an electrodeposited Cu catalyst that suppresses hydrogen formation and promotes selective CO₂ reduction in acidic conditions. Using in situ time-resolved Raman spectroscopy, we show that a high concentration of CO and OH on the catalyst surface promotes C-C coupling, a finding that we correlate with evidence of increased CO residence time. The optimized electrodeposited Cu catalyst achieves a 60% faradaic efficiency for ethylene and 90% for multicarbon products. When deployed in a slim flow cell, the catalyst attains a 20% energy efficiency to ethylene, and 30% to multicarbon products.

A crystal growth kinetics guided Cu aerogel for highly efficient CO2 electrolysis to C2+ alcohols

To realize commercial CO₂ electrochemical reduction to C₂₊ alcohols, the selectivity and production rate should be further improved. Establishing controllable surface sites with a favorable local environment is an interesting route to guide the C₂₊ pathway. Herein, we report a room-temperature one-step synthetic strategy to fabricate a highly stable Cu aerogel as an efficient CO₂ reduction electrocatalyst. Controlling crystal growth kinetics using different reductants is an efficient strategy to modulate the nucleation and growth rate of Cu aerogels, enabling the creation of efficient surface sites for the C₂₊ pathway. Over the Cu aerogel obtained by reducing Cu²⁺ using a weak reductant (NH₃·BH₃), the faradaic efficiency of C₂₊ products could reach 85.8% with the current density of 800 mA cm^-2 at the potential of -0.91 V vs. reversible hydrogen electrode, and the C₂₊ alcohol selectivity was 49.7% with a partial current density of 397.6 mA cm^-2, while the Cu aerogel prepared using a strong reductant (NaBH₄) was favorable to generating CO. Experimental and theoretical studies showed that the selectivity of the reaction depended strongly on the desorption and dimerization of *CO intermediates on the catalysts. The strong reductant induced a defective Cu surface that could facilitate the desorption of the *CO intermediate, subsequently producing CO, whereas the low defect Cu produced using a weak reductant could significantly enhance the selectivity for the C₂₊ product by improving *CO adsorption and the C-C coupling on the catalyst. This work opens a new way for constructing efficient electrocatalysts for CO₂ electroreduction to C₂₊ alcohols.

Electro-Synthesis of Organic Compounds with Heterogeneous Catalysis

Electro-organic synthesis has attracted a lot of attention in pharmaceutical science, medicinal chemistry, and future industrial applications in energy storage and conversion. To date, there has not been a detailed review on electro-organic synthesis with the strategy of heterogeneous catalysis. In this review, the most recent advances in synthesizing value-added chemicals by heterogeneous catalysis are summarized. An overview of electrocatalytic oxidation and reduction processes as well as paired electrocatalysis is provided, and the anodic oxidation of alcohols (monohydric and polyhydric), aldehydes, and amines are discussed. This review also provides in-depth insight into the cathodic reduction of carboxylates, carbon dioxide, CC, C≡C, and reductive coupling reactions. Moreover, the electrocatalytic paired electro-synthesis methods, including parallel paired, sequential divergent paired, and convergent paired electrolysis, are summarized. Additionally, the strategies developed to achieve high electrosynthesis efficiency and the associated challenges are also addressed. It is believed that electro-organic synthesis is a promising direction of organic electrochemistry, offering numerous opportunities to develop new organic reaction methods.

Design strategy of a Cu-based catalyst for optimizing the performance in the electrochemical CO2 reduction reaction to multicarbon alcohols

The electrochemical CO₂ reduction reaction (ECRR) is a promising method to reduce excessive CO₂ emissions and achieve a sustainable carbon cycle. Due to the high reaction kinetics and efficiency, copper-based catalysts have shown great application potential for preparing multicarbon (C₂₊) products. C₂₊ alcohols have high economic value and use-value, playing an essential role in modern industry. Therefore, we summarize the latest research progress of the ECRR to synthesize C₂₊ alcohols on Cu-based catalysts and discuss the state-of-the-art catalyst design strategies to improve CO₂ reduction performance. Moreover, we analyzed in detail the specific reaction pathways for the conversion of CO₂ to C₂₊ alcohols based on DFT calculations. Finally, we propose the problems and possible solutions for synthesizing C₂₊ alcohols with copper-based catalysts. We hope that this review can provide ideas for devising ECRR catalysts for C₂₊ alcohols.

Tuning the Electronic Properties of Homoleptic Silver(I) bis-BIAN Complexes towards Efficient Electrocatalytic CO2 Reduction

We report herein the preparation and characterization of six readily assembled bis-coordinated homoleptic silver(I) N,N′-bis(arylimino)acenaphthene (BIAN) complexes of general structure [Ag(I)(BIAN)2]BF4 and the influence of the electronic properties of the ligand substitution pattern on their performance in electrochemical CO2 reduction (CO2R). All the explored catalysts displayed substantial current enhancements in carbon-dioxide-saturated solvents dependent on the ligated BIAN and no significant concurrent H2 evolution when utilizing 2% H2O as a proton source. Additionally, preliminary studies, employing a drop-casted ink of 0.4 mg cm−2 [Ag(I)(4-OMe-BIAN)2]BF4 (Ag4) immobilized onto carbon paper gas diffusion electrodes in a flow cell with 1M KHCO3 aqueous electrolyte, resulted in a propitious Faradaic efficiency of 51% for CO at a current density of 50 mA cm−2.