High-rate and selective conversion of CO2 from aqueous solutions to hydrocarbons

A tutorial on the modeling of the heterogenous captured CO2 electroreduction reaction and first principles electrochemical modeling

As the energy demands of the world continue to grow, the electroreduction of captured CO2 (c-CO2RR) is an appealing alternative to the traditional CO2 reduction reaction (CO2RR) as it does not include the energetically unfavorable release of CO2 from the capture agent. In this tutorial we cover the motivation behind the c-CO2RR and CO2RR, their respective mechanisms, and computational tools that have been used to model these reactions and to compare their reactivities. Emphasis is given to methods that have already been used to model the c-CO2RR but a comparison to the methods used to explore the more understood CO2RR is covered as well.

Simultaneous High Current Density and Selective Electrocatalytic CO2-to-CH4 through Intermediate Balancing

The electrochemical reduction of CO2 to CH4 is promising for carbon neutrality and renewable energy storage but confronts low CH4 selectivity, especially at high current densities. The key challenge lies in promoting *CO intermediate and *H coupling while minimizing side reactions including C-C coupling and H-H coupling, which is particularly difficult at high current density due to abundant intermediates. Here we report a cooperative strategy to address this challenge using Cu-based catalysts comprising Cu-N coordination polymer and CuO component that can simultaneously manage the key intermediates *CO and *H. A fast CO2-to-CH4 conversion rate of 3.14 mmol cm-2 h-1 is achieved at 1,300 mA cm-2 with a Faradaic efficiency of 51.7 %. In situ spectroscopy and theoretical calculations show that the increased Cu-Cu distance in the Cu-N coordination polymer component favors multistep *CO hydrogenation over the dimerization, and the CuO component ensures an adequate supply of *H, together contributing to the selective CO2-to-CH4 conversion at high current densities. This work develops a cooperative strategy for the electrosynthesis of CH4 with simultaneous high current density and high selectivity by rational catalyst design, paving the way for its applications.

Solvent Mediated Interfacial Microenvironment Design for High-Performance Electrochemical CO2 Reduction to C2+ Products

Electrochemical CO2 reduction (CO2RR) in membrane electrode assembly (MEA) represents a viable strategy for converting CO2 into value-added multi-carbon (C2+) compounds. Therefore, the microstructure of the catalyst layer (CL) affects local gas transport, charge conduction, and proton supply at three-phase interfaces, which is significantly determined by the solvent environment. However, the microenvironment of the CLs and the mechanism of the solvent effect on C2+ selectivity remains elusive. Herein, a tailored interfacial structure is designed by introducing a solvent-mediated catalyst-ionomer-solvent microenvironment. The acetone surface promotion strategy is beneficial for the unfolded ionomers to uniformly coat the catalysts, which contributes to enhancing interfacial hydrophobicity and inhibiting hydrogen evolution. Furthermore, molecular dynamics (MD) simulation and in situ ATR-SEIRAS are employed to elucidate the appropriate interfacial network with a balanced distribution of CO2 and H2O. The uniform and continuous network in acetone is advantageous for CO2-to-C2+. The optimized structure favors the production of C2+ products in Cu-based MEA, exhibiting a high C2+ faradaic efficiency (FE) of 80.27% at 400 mA cm-2.

Copper-Catalysed Electrochemical CO2 Methanation via the Alloying of Single Cobalt Atoms

The electrochemical reduction of carbon dioxide (CO2) to methane (CH4) presents a promising solution for mitigating CO2 emissions while producing valuable chemical feedstocks. Although single-atom catalysts have shown potential in selectively converting CO2 to CH4, their limited active sites often hinder the realization of high current densities, posing a selectivity-activity dilemma. In this study, we developed a single-atom cobalt (Co) doped copper catalyst (Co1Cu) that achieved a CH4 Faradaic efficiency exceeding 60 % with a partial current density of -482.7 mA cm-2. Mechanistic investigations revealed that the incorporation of single Co atoms enhances the activation and dissociation of H2O molecules, thereby lowering the energy barrier for the hydrogenation of *CO intermediates. In situ spectroscopic experiments and density functional theory simulations further demonstrated that the modulation of the *CO adsorption configuration, with stronger bridge-binding, favours deep reduction to CH4 over the C-C coupling or CO desorption pathways. Our findings underscore the potential of Co1Cu catalysts in overcoming the selectivity-activity trade-off, paving the way for efficient and scalable CO2-to-CH4 conversion technologies.

Electrochemical valorization of captured CO2: recent advances and future perspectives

The excessive emission of CO2 has led to severe climate change, prompting global concern. Capturing CO2 and converting it through electrochemistry into value-added products represent promising approaches to mitigating CO2 emissions and closing the carbon cycle. Traditionally, these two processes have been performed independently, involving multiple steps, high energy consumption, and low efficiency. Recently, the electrochemical conversion of captured CO2, which integrates the capture and conversion processes (also referred to as electrochemically reactive CO2 capture), has garnered increasing attention. This integrated approach bypasses the energy-intensive steps involved in the traditional independent process, including CO2 release, purification, compression, transportation, and storage. In this review, we discuss recent advances in the electrochemical conversion of captured CO2, focusing on four key aspects. First, we introduce various capture media, emphasizing the thermodynamic aspects of carbon capture and their implications for integration with electrochemical conversion. Second, we discuss product control mediated by the selection of different catalysts, highlighting the connections between the conversion of captured CO2 and gas-fed CO2. Third, we examine the effect of reactor systems and operational conditions on the electrochemical conversion of captured CO2, shedding light on performance optimization. Finally, we explore real integration systems for CO2 capture and electrochemical conversion, revealing the potential of this new technology for practical applications. Overall, we provide insights into the existing challenges, potential solutions, and thoughts on opportunities and future directions in the emerging field of electrochemical conversion of captured CO2.

Macro- and Nano-Porous Ag Electrodes Enable Selective and Stable Aqueous CO2 Reduction

Electrochemical carbon dioxide (CO2) reduction from aqueous solutions offers a promising strategy to overcome flooding and salt precipitation in gas diffusion electrodes used in gas-phase CO2 electrolysis. However, liquid-phase CO2 electrolysis often exhibits low CO2 reduction rates because of limited CO2 availability. Here, a macroporous Ag mesh is employed and activated to achieve selective CO2 conversion to CO with high rates from an aqueous bicarbonate solution. It is found that activation of Ag surface using oxidation/reduction cycles produces nanoporous surfaces that favor CO2-to-CO conversion. Notably, it is found that a combination of dissolved CO2 in bicarbonate solution with CO2 generated in situ from bicarbonate ions enables increased CO2 availability and a CO2-to-CO conversion rate over 100 mA cm-2. By optimizing the oxidation/reduction cycles to fine-tune the structure of Ag surface, CO2-to-CO conversion is reported from a bicarbonate solution with CO Faradaic efficiency of over 85% at current density of 100 mA cm-2, high concentration of 24.7% at outlet gas stream and stability of over 100 h with maintaining CO FE over 85% during whole reaction time.

Large Dipole Moment Enhanced CO2 Adsorption on Copper Surface: Achieving 68.9% Catalytic Ethylene Faradaic Efficiency at 1.0 A cm-2

The electrochemical conversion of carbon dioxide (CO2) into hydrocarbon products emerges as a pivotal sustainable strategy for carbon utilization. Cu-based catalysts are currently prioritized as the most effective means for this process, yet it remains a long-term goal to achieve high product selectivity at elevated current densities. This study delved into exploring the influence of a topological poly(2-aminoazulene) with a substantial dipole moment on modulating the Cu surface dipole field to augment the catalytic activity involved in CO2 reduction. The resulting Cu/poly(2-aminoazulene) heterojunction showcases a remarkable ethylene Faradaic efficiency of 68.9% even at a substantial current density of 1 A cm-2. Through in situ Raman and in situ Fourier-transform infrared spectroscopy, poly(2-aminoazulene)-modified Cu electrode exhibits a heightened concentration of intermediates as compared to the bare Cu, proving advantageous for C-C dimerization. Theoretical calculations demonstrate the reduced energy barrier for C-C dimerization, and meanwhile impeding hydrogen evolution reaction on Cu/poly(2-aminoazulene) heterojunction, which are beneficial to CO2 reduction. The catalyst design in this study, incorporating dipole moment considerations, not only investigates the influence of dipole moment on electrochemical carbon dioxide reduction but also pioneers an innovative strategy to augment catalytic activity by elevating the micro-concentration of reactants on catalyst surfaces.

Synthesis of ethane from CO2 by a methyl transferase-inspired molecular catalyst

Molecular catalysts with a single metal center are reported to reduce CO2 to a wide range of valuable single-carbon products like CO, HCOOH, CH3OH, etc. However, these catalysts cannot reduce CO2 to two carbon products like ethane or ethylene and the ability to form C-C from CO2 remains mostly limited to heterogeneous material-based catalysts. We report a set of simple iron porphyrins with pendant thiol group can catalyze the reduction of CO2 to ethane (C2H6) with H2O as the proton source with a Faradaic yield >40% the rest being CO. The mechanism involves a CO2-derived methyl group transfer to the pendant thiol akin to the proposal forwarded for methyl transferases and a follow-up C-C bond formation of the thioether thus formed and a Fe(II)-CH3 species generated by the reduction of a second molecule of CO2. The availability of a "parking space" in the molecular framework for the first reduced C1 product from CO2 reduction allows C-C bond formation resulting in a unique case where a component of natural gas can be generated from direct electrochemical reduction of CO2.

Self-Assembled Controllable Cu-Based Perovskite/Calcium Oxide Hybrids with Strong Interfacial Interactions for Enhanced CH4 Electrosynthesis

Cu-based perovskite oxide catalysts show promise for CO2 electromethanation, but suffer from unsatisfactory CH4 selectivity and poor stability. Here, we report self-assembled, controllable Cu-based perovskite/calcium oxide hybrids with strongly interacting interfaces for high-performance CH4 electrosynthesis. As proof-of-concept catalysts, the La2CuO4/(CaO)x (x from 0.2 to 1.2) series has tunable CaO phase concentrations and thus controllable interface sizes. The La2CuO4 and CaO components are intimately connected at the interface, leading to strong interfacial interactions mainly manifested by marked electron transfer from Ca2+ to Cu2+. In CH4 electrosynthesis, their activity and selectivity show a volcano-type dependence on the CaO phase concentrations and are positively correlated with the interface sizes. Among them, the La2CuO4/(CaO)0.8 delivers the optimal activity and selectivity for CH4, together with good stability, much better than those of a physical-mixture counterpart and most reported Cu-based perovskite oxides. Moreover, La2CuO4/(CaO)0.8 stands out as one of the most effective Cu-based catalysts for CH4 electrosynthesis, achieving a high CH4 selectivity of 77.6% at 300 mA cm-2. Our experiments and theoretical calculations highlight the significant role of self-assembly-induced strong interfacial interactions in promoting *CO adsorption/hydrogenation, intensifying resistance to structural degradation, and consequently underpinning the achievement of such optimized performance.

Electrochemical CO2 Conversion Commercialization Pathways: A Concise Review on Experimental Frontiers and Technoeconomic Analysis

Technoeconomic analysis (TEA) studies are vital for formulating guidelines that drive the commercialization of electrochemical CO2 reduction (eCO2R) technologies. In this review, we first discuss the progress in the field of eCO2R processes by providing current state-of-the-art metrices (e.g., faradic efficiency, current density) based on the recent heterogeneous catalysts' discovery, electrolytes, electrolyzers configuration, and electrolysis process designs. Next, we assessed the TEA studies for a wide range of eCO2R final products, different modes of eCO2R systems/processes, and discussed their relative competitiveness with relevant commercial products. Finally, we discuss challenges and future directions essential for eCO2R commercialization by linking suggestions from TEA studies. We believe that this review will catalyze innovation in formulating advanced eCO2R strategies to meet the TEA benchmarks for the conversion of CO2 into valuable chemicals at the industrial scale.

A hybrid electro-thermochemical device for methane production from the air

Coupling direct air capture (DAC) with methane (CH4) production is a potential strategy for fuel production from the air. Here, we report a hybrid electro-thermochemical device for direct CH4 production from air. The proposed device features the cogeneration of carbon dioxide (CO2) and hydrogen (H2) in a single compartment via a bipolar membrane electrodialysis module, avoiding a separate water electrolyzer, followed by a thermochemical methanation reaction to produce CH4. H2-induced disturbances lead to efficient CO2 extraction without pumping requirement. The energy consumption and techno-economic analysis predict an energy reduction of 37.8% for DAC and a cost reduction of 36.6% compared with the decoupled route, respectively. Accordingly, CH4 cost is reduced by 12.6%. Our proof-of-concept experiments show that the energy consumption for CO2 release and H2 production is 704.0 kJ mol-1 and 967.4 kJ mol-1, respectively with subsequent methanation achieving a 97.3% conversion of CO2 and a CH4 production energy of 5206.4 kJ mol-1 showing a promising pathway for fuel processing from the air.

Time-Resolved Infrared Spectroscopic Evidence for Interfacial pH-Dependent Kinetics of Formate Evolution on Cu Electrodes

By deployment of rapid-scan (second time scale) electrochemical FT-IR reflection-absorption spectroscopy, we studied the reduction of CO2 in 0.1 M Na2SO4 in deuterated water at a pD of 3.7. We report on the impact of dynamic changes in the bicarbonate equilibrium concentration in the vicinity of a polycrystalline Cu electrode, induced by step changes in applied electrode potential. We correlate these changes in interfacial composition and concentrations of dissolved species to the formation rate of formate, and provide evidence for the following conclusions: (i) the kinetics for the conversion of dissolved CO2 to formate (formic acid) are fast, (ii) bicarbonate is also converted to formate, but with less favorable kinetics, and (iii) carbonate does not yield any formate. These results reveal that formate formation requires (mildly) acidic conditions at the interface for CO2 to undergo a proton-coupled conversion step, and we postulate that bicarbonate reduction to formate is driven by catalytic hydrogenation via in situ formed H2. Interestingly CO was not observed, suggesting that the kinetics of the CO2 to CO reaction are significantly less favorable than formate formation under the experimental conditions (pH and applied potential). We also analyzed the feasibility of pulsed electrolysis to enhance the (average) rate of formation of formate. While a short positive potential pulse enhances the CO2 concentration, this also leads to the formation of basic copper carbonates, resulting in electrode deactivation. These observations demonstrate the potential of rapid-scan EC-IRRAS to elucidate the mechanisms and kinetics of electrochemical reactions, offering valuable insights for optimizing catalyst and electrolyte performance and advancing CO2 reduction technologies.

Pressure-Dependent CO2 Electroreduction to Methane over Asymmetric Cu-N2 Single-Atom Sites

Single-atom catalysts (SACs) with unitary active sites hold great promise for realizing high selectivity toward a single product in the CO2 electroreduction reaction (CO2RR). However, achieving high Faradaic efficiency (FE) of multielectron products like methane on SACs is still challenging. Herein, we report a pressure-regulating strategy that achieves 83.5 ± 4% FE for the CO2-to-CH4 conversion on the asymmetric Cu-N2 sites, representing one of the best CO2-to-CH4 performances. Elevated CO2 pressure was demonstrated as an efficient way to inhibit the hydrogen evolution reaction via promoting the competing adsorption of reactant CO2, regardless of the nature of the active sites. Meanwhile, the asymmetric Cu-N2 structure could endow the Cu sites with stronger electronic coupling with *CO, thus suppressing the desorption of *CO and facilitating the following hydrogenation of *CO to *CHO. This work provides a synergetic strategy of the pressure-induced reaction environment regulating and the electronic structure modulating for selective CO2RR toward targeted products.

Solid Electrolytes for Low-Temperature Carbon Dioxide Valorization: A Review

One of the most promising approaches to address the global challenge of climate change is electrochemical carbon capture and utilization. Solid electrolytes can play a crucial role in establishing a chemical-free pathway for the electrochemical capture of CO2. Furthermore, they can be applied in electrocatalytic CO2 reduction reactions (CO2RR) to increase carbon utilization, produce high-purity liquid chemicals, and advance hybrid electro-biosystems. This review article begins by covering the fundamentals and processes of electrochemical CO2 capture, emphasizing the advantages of utilizing solid electrolytes. Additionally, it highlights recent advancements in the use of the solid polymer electrolyte or solid electrolyte layer for the CO2RR with multiple functions. The review also explores avenues for future research to fully harness the potential of solid electrolytes, including the integration of CO2 capture and the CO2RR and performance assessment under realistic conditions. Finally, this review discusses future opportunities and challenges, aiming to contribute to the establishment of a green and sustainable society through electrochemical CO2 valorization.

Stabilizing the oxidation state of catalysts for effective electrochemical carbon dioxide conversion

In the electrocatalytic CO2 reduction reaction (CO2RR), metal catalysts with an oxidation state generally demonstrate more favorable catalytic activity and selectivity than their corresponding metallic counterparts. However, the persistence of oxidative metal sites under reductive potentials is challenging since the transition to metallic states inevitably leads to catalytic degradation. Herein, a thorough review of research on oxidation-state stabilization in the CO2RR is presented, starting from fundamental concepts and highlighting the importance of oxidation state stabilization while revealing the relevance of dynamic oxidation states in product distribution. Subsequently, the functional mechanisms of various oxidation-state protection strategies are explained in detail, and in situ detection techniques are discussed. Finally, the prevailing and prospective challenges associated with oxidation-state protection research are discussed, identifying innovative opportunities for mechanistic insights, technology upgrades, and industrial platforms to enable the commercialization of the CO2RR.

Ni-Based Catalysts for CO2 Methanation: Exploring the Support Role in Structure-Activity Relationships

The catalytic hydrogenation of CO2 to methane is one of the highly researched areas for the production of chemical fuels. The activity of catalyst is largely affected by support type and metal-support interaction deriving from the special method during catalyst preparation. Hence, we employed a simple solvothermal technique to synthesize Ni-based catalysts with different supports and studied the support role (CeO2, Al2O3, ZrO2, and La2O3) on structure-activity relationships in CO2 methanation. It is found that catalyst morphology can be altered by only changing the support precursors during synthesis, and therefore their catalytic behaviours were significantly affected. The Ni/Al2O3 with a core-shell morphology prepared herein exhibited a higher activity than the catalyst prepared with a common wet impregnation method. To have a comprehensive understanding for structure-activity relationships, advanced characterization (e. g., synchrotron radiation-based XAS and photoionization mass spectrometry) and in-situ diffuse reflectance infrared Fourier transform spectroscopy experiments were conducted. This research opens an avenue to further delve into the role of support on morphologies that can greatly enhance catalytic activity during CO2 methanation.

Dynamics of bulk and surface oxide evolution in copper foams for electrochemical CO2 reduction

Oxide-derived copper (OD-Cu) materials exhibit extraordinary catalytic activities in the electrochemical carbon dioxide reduction reaction (CO2RR), which likely relates to non-metallic material constituents formed in transitions between the oxidized and the reduced material. In time-resolved operando experiment, we track the structural dynamics of copper oxide reduction and its re-formation separately in the bulk of the catalyst material and at its surface using X-ray absorption spectroscopy and surface-enhanced Raman spectroscopy. Surface-species transformations progress within seconds whereas the subsurface (bulk) processes unfold within minutes. Evidence is presented that electroreduction of OD-Cu foams results in kinetic trapping of subsurface (bulk) oxide species, especially for cycling between strongly oxidizing and reducing potentials. Specific reduction-oxidation protocols may optimize formation of bulk-oxide species and thereby catalytic properties. Together with the Raman-detected surface-adsorbed *OH and C-containing species, the oxide species could collectively facilitate *CO adsorption, resulting an enhanced selectivity towards valuable C2+ products during CO2RR.

Surface coating combined with in situ cyclic voltammetry to enhance the stability of gas diffusion electrodes for electrochemical CO2 reduction

Electrochemical CO2 reduction (CO2RR), fueled by clean and renewable energy, presents a promising method for utilizing CO2 effectively. The electrocatalytic reduction of CO2 to CO using a gas diffusion electrode (GDE) has shown great potential for industrial applications due to its high reaction rate and selectivity. However, guaranteeing its long-term stability still poses a significant challenge. In this study, we conducted a comprehensive investigation into various strategies to enhance the stability of the GDE. These strategies involved modifying the structure of the substrate, such as the gas diffusion layer (GDL) and the back side of the GDL (macroporous layer side). Additionally, we explored modifications to the catalyst layer (CL) and the front of the CL. To address these stability concerns, we proposed a practical approach that involved surface coating using carbon black in combination with in situ cyclic voltammetry (CV) cycles on Ag/Ag300/polytetrafluoroethylene (PTFE). The partial Faradaic efficiency exceeded 80 % within a span of 70 h. Electron microscopy and electrochemical characterization revealed that the implementation of in situ CV led to a reduction in catalyst particle size and the formation of a porous surface structure. By enhancing the stability of the GDE, this research opens up possibilities for the advancement of hybrid systems that focus on the production and utilization of syngas.

Developing Catalysts Integrated in Gas-Diffusion Electrodes for CO2 Electrolyzers

ConspectusAs the demand for a carbon-neutral society grows rapidly, research on CO2 electrolysis has become very active. Many catalysts are reported for converting CO2 into value-added products by electrochemical reactions, which have to perform at high Faradaic and energy efficiency to become commercially viable. Various types of CO2 electrolyzers have been used in this effort, such as the H-cell, flow cell, and zero-gap membrane-electrode assembly (MEA) cell. H-cell studies are conducted with electrodes immersed in CO2-saturated electrolyte and have been used to elucidate reaction pathways and kinetic parameters of electrochemical CO2 reduction on many types of catalytic surfaces. From a transport phenomenological perspective, the low solubility and diffusion of CO2 to the electrode surface severely limit the maximum attainable current density, and this metric has been shown to have significant influence on the product spectrum. Flow and MEA cells provide a solution in the form of gas-diffusion electrodes (GDEs) that enable gaseous CO2 to closely reach the catalyst layer and yield record-high current densities. Because GDEs involve a complicated interface consisting of the catalyst surface, gaseous CO2, polymer overlayers, and liquid electrolyte, catalysts with high intrinsic activity might not show high performance in these GDE-based electrolyzers. Catalysts showing low overpotentials at low current densities may suffer from poor electron conductivity and mass transfer limitations at high current densities. Furthermore, the stability of the GDE-based catalysts is often compromised as CO2 electrolysis is pursued with high activity, most notoriously by electrolyte flooding.In this Account, we introduce recent examples where the electrocatalysts were integrated in GDEs, achieving high production rates. The performance of such systems is contingent on both GDE and cell design, and various parameters that affect the cell performance are discussed. Gaseous products, such as carbon monoxide, methane, and ethylene, and liquid products, such as formate and ethanol, have been mainly reported with high partial current density using the flow or MEA cells. Different strategies to this end are described, such as controlling microenvironments by the use of polymers mixed within the catalyst layer or the functionalization of catalyst surfaces with ligands to increase local concentrations of intermediates. Unique CO2 electrolyzer designs are also treated, including the incorporation of light-responsive plasmonic catalysts in the GDE, and combining the electrolyzer with a fermenter utilizing a microbial biocatalyst to synthesize complex multicarbon products. Basic conditions which the catalyst should satisfy to be adapted in the GDEs are listed, and our perspective is provided.