Steering the Site Distance of Atomic Cu-Cu Pairs by First-Shell Halogen Coordination Boosts CO2-to-C2 Selectivity.

Electrocatalytic reduction of CO2 into C2 products of high economic value provides a promising strategy to realize resourceful CO2 utilization. Rational design and construct dual sites to realize the CO protonation and C-C coupling to unravel their structure-performance correlation is of great significance in catalysing electrochemical CO2 reduction reactions. Herein, Cu-Cu dual sites with different site distance coordinated by halogen at the first-shell are constructed and shows a higher intramolecular electron redispersion and coordination symmetry configurations. The long-range Cu-Cu (Cu-I-Cu) dual sites show an enhanced Faraday efficiency of C2 products, up to 74.1%, and excellent stability. In addition, the linear relationships that the long-range Cu-Cu dual site is accelerated to C2H4 generation and short-range Cu-Cu (Cu-Cl-Cu) dual site is beneficial for C2H5OH formation are disclosed. In situ electrochemical attenuated total reflection surface enhanced infrared absorption spectroscopy, in situ Raman and theoretical calculations manifest that long-range Cu-Cu dual sites can weaken reaction energy barriers of CO hydrogenation and C-C coupling, as well as accelerating deoxygenation of *CH2CHO. This study uncovers the exploitation of site-distance-dependent electrochemical property to steer the CO2 reduction pathway, as well as a potential generic tactic to target C2 synthesis by constructing the desired Cu-Cu dual sites.

Dual-metal sites drive tandem electrocatalytic CO2 to C2+ products.

The electrochemical conversion of CO2 into valuable chemicals is a promising route for renowable energy storage and the mitigation of greenhouse gas emission, and production of multicarbon (C2+) products is highly desired. Here, we report a 1.4%Pd-Cu@CuPz2 comprising of dispersive CuOx and PdO dual nanoclusters embedded in the MOF CuPz2 (Pz = Pyrazole), which achieves a high C2+ Faradaic efficiency (FEC2+) of 81.9% and C2+ alcohol FE of 47.5% with remarkable stability when using 0.1 M KCl aqueous solution as electrolyte in a typical H-cell. Particularly, the FE of alcohol is obviously improved on 1.4%Pd-Cu@CuPz2 compared to Cu@CuPz2. Theoretical calculations have revealed that revealed that the enhanced interfacial electron transfer facilitates the adsorption of *CO intermediate and *CO-*CO dimerization on the Cu-Pd dual sites bridged by Cu nodes of CuPz2. Additionally, the oxophilicity of Pd can stabilize the key intermediate *CH2CHO and promote subsequent proton-coupled electron transfer more efficiently, confirming that the formation pathway is skew towards *C2H5OH. Consequently, the Cu-Pd dual sites play a synergistic tandem role in cooperatively improving the selectivity of alcohol and accelerating reductive conversion of CO2 to C2+.

Electrochemical CO2 Reduction to Multicarbon Products on Non-Copper Based Catalysts.

Electrochemical CO2 reduction reaction (eCO2RR) to value-added multicarbon (C2+) products offers a promising approach for achieving carbon neutrality and storing intermittent renewable energy. Copper (Cu)-based electrocatalysts generally play the predominant role in this process. Yet recently, more and more non-Cu materials have demonstrated the capability to convert CO2 into C2+, which provides impressive production efficiency even exceeding those on Cu, and a wider variety of C2+ compounds not achievable with Cu counterparts. This motivates us to organize the present review to make a timely and tutorial summary of recent progresses on developing non-Cu based catalysts for CO2-to-C2+. We begin by elucidating the reaction pathways for C2+ formation, with an emphasis on the unique C-C coupling mechanisms in non-Cu electrocatalysts. Subsequently, we summarize the typical C2+-involved non-Cu catalysts, including ds-, d- and p-block metals, as well as metal-free materials, presenting the state-of-the-art design strategies to enhance C2+ efficiency. The system upgrading to promote C2+ productivity on non-Cu electrodes covering microbial electrosynthesis, electrolyte engineering, regulation of operational conditions, and synergistic co-electrolysis, is highlighted as well. Our review concludes with an exploration of the challenges and future opportunities in this rapidly evolving field.

Unconventional Electrocatalytic CO Conversion to C2 Products on Single-Atomic Pd-Agn Sites.

The electrochemical reduction of CO or CO2 into C2+ products has mostly been focused on Cu-based catalysts. Although Ag has also been predicted as a possible catalyst for the CO-to-C2+ conversion from the thermodynamic point of view, however, due to its weak CO binding strength, CO rapidly desorbs from the Ag surface rather than participates in deep reduction. In this work, we demonstrate that single-atomic Pd sites doped in Ag lattice can tune the CO adsorption behavior and promote the deep reduction of CO toward C2 products. The monodispersed Pd-Agn sites enable the CO adsorption with both Pd-atop (PdL) and Pd-Ag bridge (PdAgB) configurations, which can increase the CO coverage and reduce the C-C coupling energy barrier. Under room temperature and ambient pressure, the Pd1Ag10 alloy catalyst exhibited a total CO-to-C2 Faradaic efficiency of ~37% at ‒0.83 V, with appreciable current densities and electrochemical stability, thus featuring unconventional non-Cu electrocatalytic CO-to-C2 conversion capability.

Dual-Anion-Stabilized Cuδ+ Sites in Cu2(OH)2CO3 for High C2+ Selectivity in the CO2 Electroreduction Reaction.

The excessive emission of CO2 has aroused increasingly serious environmental problems. Electrochemical CO2 reduction reaction (CO2RR) is an effective way to reduce CO2 concentration and simultaneously produce highly valued chemicals and fuels. Cuδ+ species are regarded as promising active sites to obtain multi-carbon compounds in CO2RR, however, they are easily reduced to Cu0 during the reaction and fail to retain the satisfying selectivity for C2+ products. Herein, via a one-step method, we synthesize Cu2(OH)2CO3 microspheres composed of nanosheets, which has achieved a superior Faraday efficiency for C2+ products as high as 76.29 % at -1.55 V vs. RHE in an H cell and 78.07 % at -100 mA cm-2 in a flow cell. Electrochemical measurements, in situ Raman spectra and attenuated total reflectance infrared spectra (ATR-IR) as well as the theoretic calculation unveil that, compared with Cu(OH)2 and CuO, the dual O-containing anionic groups (OH- and CO3 2-) in Cu2(OH)2CO3 can effectively stabilize the Cuδ+ species, promote the adsorption and activation of CO2, boost the coverage of *CO and the coupling of *CO-*COH, thus sustain the flourishment of C2+ products.

Polyacrylate modified Cu electrode for selective electrochemical CO2 reduction towards multicarbon products.

Highly selective production of value-added multicarbon (C2+) products via electrochemical CO2 reduction reaction (eCO2RR) on polycrystalline copper (Cu) remains challenging. Herein, the facile surface modification using poly (α-ethyl cyanoacrylate) (PECA) is presented to greatly enhance the C2+ selectivity for eCO2RR over polycrystalline Cu, with Faradaic efficiency (FE) towards C2+ products increased from 30.1% for the Cu electrode to 72.6% for the obtained Cu-PECA electrode at -1.1 V vs. reversible hydrogen electrode (RHE). Given the well-determined FEs towards C2+ products, the partial current densities for C2+ production could be estimated to be -145.4 mA cm-2 for the Cu-PECA electrode at -0.9 V vs. RHE in a homemade flow cell. In-situ spectral characterizations and theoretical calculations reveal that PECA featured with electron-accepting -C≡N and -COOR groups decorated onto the Cu electrode could inhibit the adsorption of *H intermediates and stabilize the *CO intermediates, given the redistributed interfacial electron density and the raised energy level of d-band center (Ed) of Cu active sites, thus facilitating the C-C coupling and then the C2+ selective production. This study is believed to be guidable to the modification of electrocatalysts and electrodes with polymers to steer the surface adsorption behaviors of reaction intermediates to realize practical eCO2RR towards value-added C2+ products with high activity and selectivity.

Metal-Organic Framework Supported Low-Nuclearity Cluster Catalysts for Highly Selective Carbon Dioxide Electroreduction to Ethanol.

It is still a great challenge to achieve high selectivity of ethanol in CO2 electroreduction reactions (CO2RR) because of the similar reduction potentials and lower energy barrier of possible other C2+ products. Here, we report a MOF-based supported low-nuclearity cluster catalysts (LNCCs), synthesized by electrochemical reduction of three-dimensional (3D) microporous Cu-based MOF, that achieves a single-product Faradaic efficiency (FE) of 82.5 % at -1.0 V (versus the reversible hydrogen electrode) corresponding to the effective current density is 8.66 mA cm-2. By investigating the relationship between the species of reduction products and the types of catalytic sites, it is confirmed that the multi-site synergism of Cu LNCCs can increase the C-C coupling effect, and thus achieve high FE of CO2-to-ethanol. In addition, density functional theory (DFT) calculation and operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy further confirmed the reaction path and mechanism of CO2-to-EtOH.

Critical Role of Cu Nanoparticle-Loaded Cu(100) Surface Structures on Structured Copper-Based Catalysts in Boosting Ethanol Generation in CO2 Electroreduction.

Presently, realizing high ethanol selectivity in CO2 electroreduction remains challenging due to difficult C-C coupling and fierce product competition. In this work, we report an innovative approach for improving the efficiency of Cu-based electrocatalysts in ethanol generation from electrocatalytic CO2 reduction using a crystal plane modification strategy. These novel Cu-based electrocatalysts were fabricated by electrochemically activating three-dimensional (3D) flower-like CuO micro/nanostructures grown in situ on copper foils and modifying with surfactants. It was demonstrated that the fabricated Cu-based electrocatalyst featured a predominantly exposed Cu(100) surface loaded with high-density Cu nanoparticles (NPs). The optimal Cu-based electrocatalyst displayed considerably improved CO2 electroreduction performance, with a Faraday efficiency of 37.9% for ethanol and a maximum Faraday efficiency of 68.0% for C2+ products at -1.4 V vs RHE in an H-cell, accompanied by a high current density of 69.9 mA·cm-2, much better than the particulate Cu-based electrocatalyst. It was unveiled that the Cu(100)-rich surface of nanoscale petals with abundant under-coordinated copper atoms from CuNPs was conducive to the formation and stabilization of key *CH3CHO and *OC2H5 intermediates, thereby promoting ethanol generation. This study highlighted the critical role of CuNP-loaded Cu(100) surface structures on structured Cu-based electrocatalysts in enhancing ethanol production for the CO2 electroreduction process.

State of Play of Critical Mineral-Based Catalysts for Electrochemical E-Refinery to Synthetic Fuels.

The pursuit of decarbonization involves leveraging waste CO2 for the production of valuable fuels and chemicals (e.g., ethanol, ethylene, and urea) through the electrochemical CO2 reduction reactions (CO2RR). The efficacy of this process heavily depends on electrocatalyst performance, which is generally reliant on high loading of critical minerals. However, the supply of these minerals is susceptible to shortage and disruption, prompting concerns regarding their usage, particularly in electrocatalysis, requiring swift innovations to mitigate the supply risks. The reliance on critical minerals in catalyst fabrication can be reduced by implementing design strategies that improve the available active sites, thereby increasing the mass activity. This review seeks to discuss and analyze potential strategies, challenges, and opportunities for improving catalyst activity in CO2RR with a special attention to addressing the risks associated with critical mineral scarcity. By shedding light onto these aspects of critical mineral-based catalyst systems, this review aims to inspire the development of high-performance catalysts and facilitates the practical application of CO2RR technology, whilst mitigating adverse economic, environmental, and community impacts.

Enhancing CO2 Electroreduction to C2 Products on Metal-Nitrogen Sites by Regulating H2O Dissociation.

Water dissociation remarkably affects the CO2 reduction to CO and HCOOH, but whether it is effective for two-carbon product formation on M-Nx-containing catalysts is still ambiguous. Herein, by using a fluorinated metal phthalocyanine (MPc-F) as the M-N4-based model electrocatalyst, experimental and theoretical results reveal that the H2O-dissociation-induced active H species decrease the overpotential of the *CO hydrogenation to *CHO and facilitate the C-C coupling between *CHO and neighboring CO. Such an effect is strengthened by an increase in the *CO binding strength on the metal center. By introducing CuPc as the H2O dissociation catalyst into MPc-F (MPc-F/CuPc) to accurately regulate the H2O dissociation, the faradic efficiency of C2 products on FePc-F/CuPc and MnPc-F/CuPc increases from 0% (FePc-F and MnPc-F) to 26 and 36%, respectively. This work develops a novel strategy for enhancing the selectivity of M-Nx-containing catalysts to C2 products and reveals the correlation between H2O dissociation and C2 product formation.

Breakthrough in CO2 Electroreduction to Multi-Carbon Products at Ampere-Level Enabled by Active Sites Engineering.

Efficient production of value-added chemicals with high selectivity from CO2 electroreduction at industrial-level current density is highly demanded, yet remains a big challenge. In a recent issue of Angewandte Chemie, Han and colleagues have elegantly increased the Faradaic efficiency (FE) of multi-carbon (C2+) products to over 70 % at amperes level (1.4 A cm-2) by engineering the active sites for the key reactions involved in the CO2 electroreduction. In this study, the highly dispersed Pd atoms have two unique functions: active sites for water dissociation and to induce the electron rearrangement of the surrounding Cu atoms to form new active sites for CO conversion, while the Cu far from Pd are the active sites for efficient CO2 conversion to CO, the synergistic functions of these three active sites result in high FE and yields of C2+ products at industrial-level current density. This research is a remarkable step forward in the methodology for developing efficient and durable catalysts for CO2 electroreduction and beyond.

Bottom-up Growth of Convex Sphere with Adjustable Cu(0)/Cu(I) Interfaces for Effective C2 Production from CO2 Electroreduction.

One challenge confronting the Cu2O catalysts in the electrocatalysis of carbon dioxide reduction reaction (CO2RR) is the reduction of active Cu(I) species, resulting in low selectivity and quick deactivation. In this study, we for the first time introduce a bottom-up growth of convex sphere with adjustable Cu(0)/Cu(I) interfaces (Cux@Cu2O convex spheres). Interestingly, the interfaces are dynamically modulated by varying hydrothermal time, thus regulating the conversion of C1 and C2 products. In particular, the 4 h hydrothermal treatment applied to Cu0.25@Cu2O convex sphere with the favorable Cu(0)/Cu(I) interface results in the highest selectivity for C2 products (90.5 %). In situ Fourier-transform infrared spectroscopy measurements and density functional theory calculations reveal that the Cu(0)/Cu(I) interface lowers the energy barrier for the production of ethylene and ethanol while increasing the coverage of localized *CO adsorbate for increased dimerization. This work establishes a novel approach for transforming the state of valence-sensitive electrocatalysts into high-value energy-related engineering products.

Enhancing electrochemical carbon dioxide reduction efficiency through heat-induced metamorphosis of copper nanowires into copper oxide/copper nanotubes with tunable surface.

Electrochemical CO2 reduction reaction (CO2RR) presents a unique opportunity to convert carbon dioxide (CO2) to value-added products while simultaneously storing renewable energy in the form of chemical energy. However, particle applications of this technology are limited due to the poor efficiency and product selectivity of the existing catalyst. In this study, we demonstrate a facile method for the heat-induced transformation of copper nanowires into CuOx/Cu nanotubes with defect-enriched surfaces. During this transformation, the outward migration of copper results in the formation of tubular structures encased within nanosized oxide grains. Notably, the hydrogen faradaic efficiency (FE) decreases with extended heat treatment, while carbon monoxide (CO) FE increases. As compared to Cu NWs, Cu NTs exhibit lower selectivity towards H2 and single-carbon (C1) products and favor the formation of multi-carbon (C2+) products. Consequently, a 2-fold increase in the single pass CO2 conversion (SPCC) and C2+ half-cell energy efficiency (EEhalf cell) was noted after heat treatment. The Cu NT-4 variant, synthesized under optimized conditions, exhibits the highest FE of 72.1 % for C2+ products at an operating current density (ID) of 500 mA cm-2.

Highly dispersed Cu-Cu2O-CeOx interfaces on reduced graphene oxide for CO2 electroreduction to C2+ products.

The Cu0-Cu+ interfaces play a key role in the electrochemical CO2 reduction reaction (CO2RR) to produce multi-carbon products (C2+), however, it is difficult for Cu+ to exist stably under reducing conditions. Herein, we construct highly dispersed and stable Cu-Cu2O-CeOx interface on reduced graphene oxide (rGO) for CO2 electroreduction to C2+ products. During the synthesis process, utilizing strong electrostatic interactions, the complex ions of Cu2+ and Ce3+ are uniformly adsorbed on the surface of graphene oxide. Then, under the solvothermal reaction of ethylene glycol and thiourea, the two metal complex ions are converted into highly dispersed and ultrafine Cu2S-CeOx nanocomposites on rGO. Interestingly, CeOx and thiourea synergistically regulate the generation of only Cu+. Under the CO2RR process, the reconstruction of Cu2S promotes the formation of Cu0 and Cu2O species. CeOx stabilizes partial Cu+ species and promotes the formation of Cu-Cu2O-CeOx composite interface. With the help of synergistic effect of Cu0, Cu+ and CeOx, the optimized reaction interface achieves the Faradaic efficiency (FE) of 74.5 % for C2+ products with the current density of 230 mA cm-2 at -0.9 V versus the reversible hydrogen electrode. In situ attenuate total reflectance-infrared absorption spectroscopy (ATR-IRAS) spectra show that the composite interfaces promote the adsorption and activation of H2O and CO2, improve the surface coverage of CO intermediates (*CO), and thus accelerate the CC coupling process.

Cooperation of Different Active Sites to Promote CO2 Electroreduction to Multi-carbon Products at Ampere-Level.

Electroreduction of CO2 to C2+ products provides a promising strategy for reaching the goal of carbon neutrality. However, achieving high selectivity of C2+ products at high current density remains a challenge. In this work, we designed and prepared a multi-sites catalyst, in which Pd was atomically dispersed in Cu (Pd-Cu). It was found that the Pd-Cu catalyst had excellent performance for producing C2+ products from CO2 electroreduction. The Faradaic efficiency (FE) of C2+ products could be maintained at approximately 80.8 %, even at a high current density of 0.8 A cm-2 for at least 20 hours. In addition, the FE of C2+ products was above 70 % at 1.4 A cm-2. Experiments and density functional theory (DFT) calculations revealed that the catalyst had three distinct catalytic sites. These three active sites allowed for efficient conversion of CO2, water dissociation, and CO conversion, ultimately leading to high yields of C2+ products.

Nanoreactor Confined and Enriched Intermediates for Electroreduction of CO2 to C2+ Products.

In the process of electroreduction of carbon dioxide (eCO2RR) to multi-carbon (C2+) products, it is imperative to enhance the concentration of key intermediate species on the catalyst surface. The utilization of micro-nano reactors to achieve confinement effects has been widely observed in various catalytic reactions, yet it has seldom been employed in eCO2RR. Here, we present a novel nanoreactor composed of stacked CuS nanosheets for eCO2RR to C2+ products. In comparison to catalyst comprising of nanosheet with open space, the C-C coupling within this confined nanospace is significantly enhanced, resulting in the increase of Faraday efficiency (FE) of C2+ products to 53 %. In situ infrared (IR) spectroscopy reveals the confinement and enrichment of key intermediate by the nanoreactor. Our research findings demonstrate that a meticulously designed nanoreactor can elevate the selectivity of C2+ products, thereby aiding in the design of eCO2RR catalysts.

Electrocatalytic CO2 Reduction to C2+ Products in Flow Cells.

Electrocatalytic CO2 reduction into value-added fuels and chemicals by renewable electric energy is one of the important strategies to address global energy shortage and carbon emission. Though the classical H-type electrolytic cell can quickly screen high-efficiency catalysts, the low current density and limited CO2 mass transfer process essentially impede its industrial applications. The electrolytic cells based on electrolyte flow system (flow cells) have shown great potential for industrial devices, due to higher current density, improved local CO2 concentration, and better mass transfer efficiency. The design and optimization of flow cells are of great significance to further accelerate the industrialization of electrocatalytic CO2 reduction reaction (CO2 RR). In this review, the progress of flow cells for CO2 RR to C2+ products is concerned. Firstly, the main events in the development of the flow cells for CO2 RR are outlined. Second, the main design principles of CO2 RR to C2+ products, the architectures, and types of flow cells are summarized. Third, the main strategies for optimizing flow cells to generate C2+ products are reviewed in detail, including cathode, anode, ion exchange membrane, and electrolyte. Finally, the preliminary attempts, challenges, and the research prospects of flow cells for industrial CO2 RR toward C2+ products are discussed.

Recent Progress on Perovskite-Based Electrocatalysts for Efficient CO2 Reduction.

An efficient carbon dioxide reduction reaction (CO2RR), which reduces CO2 to low-carbon fuels and high-value chemicals, is a promising approach for realizing the goal of carbon neutrality, for which effective but low-cost catalysts are critically important. Recently, many inorganic perovskite-based materials with tunable chemical compositions have been applied in the electrochemical CO2RR, which exhibited advanced catalytic performance. Therefore, a timely review of this progress, which has not been reported to date, is imperative. Herein, the physicochemical characteristics, fabrication methods and applications of inorganic perovskites and their derivatives in electrochemical CO2RR are systematically reviewed, with emphasis on the structural evolution and product selectivity of these electrocatalysts. What is more, the current challenges and future directions of perovskite-based materials regarding efficient CO2RR are proposed, to shed light on the further development of this prospective research area.

Insight on Atomically Dispersed Cu Catalysts for Electrochemical CO2 Reduction.

Electrochemical CO2 reduction (ECO2R) with renewable electricity is an advanced carbon conversion technology. At present, copper is the only metal to selectively convert CO2 into multicarbon (C2+) products. Among them, atomically dispersed (AD) Cu catalysts have received great attention due to the relatively single chemical environment, which are able to minimize the negative impact of morphology, valence state, and crystallographic properties, etc. on product selectivity. Furthermore, the completely exposed atomic Cu sites not only provide space and bonding electrons for the adsorption of reactants in favor of better catalytic activity but also provide an ideal platform for studying its reaction mechanism. This review summarizes the recent progress of AD Cu catalysts as a chemically tunable platform for ECO2R, including the atomic Cu sites dynamic evolution, the catalytic performance, and mechanism. Furthermore, the prospects and challenges of AD Cu catalysts for ECO2R are carefully discussed. We sincerely hope that this review can contribute to the rational design of AD Cu catalysts with enhanced performance for ECO2R.

Cu2O Nanoparticles Wrapped by N-Doped Carbon Nanotubes for Efficient Electroreduction of CO2 to C2 Products.

Electroreduction of carbon dioxide (CO2) to C2 products (ethylene and ethanol) using efficient catalysts is a feasible approach to alleviate the climate crisis. Cuprous oxide nanoparticles (Cu2O NPs) are a promising catalyst for C2 production but suffer from inherent selectivity and durability. To address this challenge, a Cu2O NPs-nitrogen-doped carbon nanotube (Cu2O NPs-NCNT) composite was prepared with carbon nanotubes (CNTs), Cu2O NPs, and phthalocyanine (Pc). The results indicate that Cu2O NPs-NCNT has excellent Faradic efficiency of C2 products (77.61%) at -1.1 V vs RHE, which is 103.43% higher than that of Cu2O NPs. In the potentiostatic electrolysis combined with Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements, Cu2O NPs-NCNT exhibited structural and catalytic current stability over 10 h. Finally, density functional theory calculations combined with XPS demonstrated that the NCNT in Cu2O NPs-NCNT can selectively absorb CO2 through specific N-CO2 interactions. Our work provides a unique strategy to promote the selectivity of Cu2O NPs for C2 production by introducing N-doped linear carbon materials to fabricate composite.