B-Cu-Zn Gas Diffusion Electrodes for CO2 Electroreduction to C2+ Products at High Current Densities

Construction synergetic adsorption and activation surface via confined Cu/Cu2O and Ag nanoparticles on TiO2 for effective conversion of CO2 to CH4

High-performance photocatalysts for catalytic reduction of CO2 are largely impeded by inefficient charge separation and surface activity. Reasonable design and efficient collaboration of multiple active sites are important for attaining high reactivity and product selectivity. Herein, Cu-Cu2O and Ag nanoparticles are confined as dual sites for assisting CO2 photoreduction to CH4 on TiO2. The introduction of Cu-Cu2O leads to an all-solid-state Z-scheme heterostructure on the TiO2 surface, which achieves efficient electron transfer to Cu2O and adsorption and activation of CO2. The confined nanometallic Ag further enhances the carrier's separation efficiency, promoting the conversion of activated CO2 molecules to •COOH and further conversion to CH4. Particularly, this strategy is highlighted on the TiO2 system for a photocatalytic reduction reaction of CO2 and H2O with a CH4 generation rate of 62.5 μmol∙g-1∙h-1 and an impressive selectivity of 97.49 %. This work provides new insights into developing robust catalysts through the artful design of synergistic catalytic sites for efficient photocatalytic CO2 conversion.

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.

Anionic Coordination Control in Building Cu-Based Electrocatalytic Materials for CO2 Reduction Reaction

Renewable energy-driven conversion of CO2 to value-added fuels and chemicals via electrochemical CO2 reduction reaction (CO2RR) technology is regarded as a promising strategy with substantial environmental and economic benefits to achieve carbon neutrality. Because of its sluggish kinetics and complex reaction paths, developing robust catalytic materials with exceptional selectivity to the targeted products is one of the core issues, especially for extensively concerned Cu-based materials. Manipulating Cu species by anionic coordination is identified as an effective way to improve electrocatalytic performance, in terms of modulating active sites and regulating structural reconstruction. This review elaborates on recent discoveries and progress of Cu-based CO2RR catalytic materials enhanced by anionic coordination control, regarding reaction paths, functional mechanisms, and roles of different non-metallic anions in catalysis. Finally, the review concludes with some personal insights and provides challenges and perspectives on the utilization of this strategy to build desirable electrocatalysts.

Chlorine-Coordinated Unsaturated Ni-N2 Sites for Efficient Electrochemical Carbon Dioxide Reduction

Heteroatom-doping is an effective method for modifying the geometric symmetry of metal-nitrogen-carbon (M-N-C) single-atom catalysts and thereby tuning the electronic structure. Up to now, most of the current reports have concentrated on introducing heteroatoms into the highly symmetrical M-N4 structure. The coordination-unsaturated M-N2 structure is more sterically favorable for the insertion of alien atoms to optimize the electronic structure. Herein, a Ni-N2 catalyst with out-of-plane coordinated chlorine (Cl) atoms (Ni-N2 Cl/C) is successfully constructed on chlorine-functionalized carbon supports (C-Cl) for an efficient carbon dioxide reduction reaction (CO2 RR). Density functional theory calculations demonstrate that the prepared Ni-N2 Cl/C catalyst exhibits a higher capability in balancing COOH* formation and CO* desorption. In addition, in situ Raman spectra confirm that the lower CO binding energy on the Ni-N2 Cl/C facilitates CO escape, leading to excellent CO2 RR performance. A high CO Faradaic efficiency (FECO ) of more than 80% is achieved from -0.6 to -1.2 V versus reversible hydrogen electrode on the Ni-N2 Cl/C and it exhibits negligible FECO and current declination over a 40-h stability test. Furthermore, a high turnover frequency (TOF) value of 15 808 h-1 is obtained, which is more than ten times that of Ni-N2 /C (1476 h-1 ) without coordinated Cl atoms.

Boron-Doped Nickel-Nitrogen-Carbon Single-Atom Catalyst for Boosting Electrochemical CO2 Reduction

Tuning the coordination environment of the metal center in metal-nitrogen-carbon (M-N-C) single-atom catalysts via heteroatom-doping (oxygen, phosphorus, sulfur, etc.) is effective for promoting electrocatalytic CO2 reduction reaction (CO2 RR). However, few studies are investigated establishing efficient CO2 reduction by introducing boron (B) atoms to regulate the M-N-C structure. Herein, a B-C3 N4 self-sacrifice strategy is developed to synthesize B, N co-coordinated Ni single atom catalyst (Ni-BNC). X-ray absorption spectroscopy and high-angle annular dark field scanning transmission electron microscopy confirm the structure (Ni-N3 B/C). The Ni-BNC catalyst presents a maximum CO Faradaic efficiency (FECO ) of 98.8% and a large CO current density (jCO ) of -62.9 mA cm-2 at -0.75 and -1.05 V versus reversible hydrogen electrode, respectively. Furthermore, FECO could be maintained above 95% in a wide range of potential windows from -0.65 to -1.05 V. In situ experiments and density functional theory calculations demonstrate the Ni-BNC catalyst with B atoms coordinated to the central Ni atoms could significantly reduce the energy barrier for the conversion of *CO2 to *COOH, leading to excellent CO2 RR performance.

Direct OC-CHO coupling towards highly C2+ products selective electroreduction over stable Cu0/Cu2+ interface

Electroreduction of CO2 to valuable multicarbon (C2+) products is a highly attractive way to utilize and divert emitted CO2. However, a major fraction of C2+ selectivity is confined to less than 90% by the difficulty of coupling C-C bonds efficiently. Herein, we identify the stable Cu0/Cu2+ interfaces derived from copper phosphate-based (CuPO) electrocatalysts, which can facilitate C2+ production with a low-energy pathway of OC-CHO coupling verified by in situ spectra studies and theoretical calculations. The CuPO precatalyst shows a high Faradaic efficiency (FE) of 69.7% towards C2H4 in an H-cell, and exhibits a significant FEC2+ of 90.9% under industrially relevant current density (j = -350 mA cm-2) in a flow cell configuration. The stable Cu0/Cu2+ interface breaks new ground for the structural design of electrocatalysts and the construction of synergistic active sites to improve the activity and selectivity of valuable C2+ products.

Less-Coordinated Atomic Copper-Dimer Boosted Carbon-Carbon Coupling During Electrochemical CO2 Reduction

This work reports a metal-organic framework (MOF) with less-coordinated copper dimers, which displays excellent electrochemical CO2 reduction (eCO2 RR) performance with an advantageous current density of 0.9 A cm-2 and a high Faradaic efficiency of 71% to C2 products. In comparison with MOF with Cu monomers that are present as Cu1 O4 with a coordination number of 3.8 ± 0.2, Cu dimers exist as O3 Cu1 ···Cu2 O2 with a coordination number of 2.8 ± 0.1. In situ characterizations together with theoretical calculations reveal that two *CO intermediates are stably adsorbed on each site of less-coordinated Cu dimers, which favors later dimerization via a key intermediate of *CH2 CHO. The highly unsaturated dual-atomic Cu provides large-quantity and high-quality actives sites for carbon-carbon coupling, achieving the optimal trade-off between activity and selectivity of eCO2 RR to C2 products.

Bridging Trans-Scale Electrode Engineering for Mass CO2 Electrolysis

Electrochemical CO2 upgrade offers an artificial route for carbon recycling and neutralization, while its widespread implementation relies heavily on the simultaneous enhancement of mass transfer and reaction kinetics to achieve industrial conversion rates. Nevertheless, such a multiscale challenge calls for trans-scale electrode engineering. Herein, three scales are highlighted to disclose the key factors of CO2 electrolysis, including triple-phase boundaries, reaction microenvironment, and catalytic surface coordination. Furthermore, the advanced types of electrolyzers with various electrode design strategies are surveyed and compared to guide the system architectures for continuous conversion. We further offer an outlook on challenges and opportunities for the grand-scale application of CO2 electrolysis. Hence, this comprehensive Perspective bridges the gaps between electrode research and CO2 electrolysis practices. It contributes to facilitating the mixed reaction and mass transfer process, ultimately enabling the on-site recycling of CO2 emissions from industrial plants and achieving net negative emissions.

From Traditional to New Benchmark Catalysts for CO2 Electroreduction

In the last century, conventional strategies pursued to reduce or convert CO₂ have shown limitations and, consequently, have been pushing the development of innovative routes. Among them, great efforts have been made in the field of heterogeneous electrochemical CO₂ conversion, which boasts the use of mild operative conditions, compatibility with renewable energy sources, and high versatility from an industrial point of view. Indeed, since the pioneering studies of Hori and co-workers, a wide range of electrocatalysts have been designed. Starting from the performances achieved using traditional bulk metal electrodes, advanced nanostructured and multi-phase materials are currently being studied with the main goal of overcoming the high overpotentials usually required for the obtainment of reduction products in substantial amounts. This review reports the most relevant examples of metal-based, nanostructured electrocatalysts proposed in the literature during the last 40 years. Moreover, the benchmark materials are identified and the most promising strategies towards the selective conversion to high-added-value chemicals with superior productivities are highlighted.

CO2 electrolysis towards large scale operation: rational catalyst and electrolyte design for efficient flow-cell

The electrochemical CO₂ reduction reaction (CO₂RR) to renewable fuels/chemicals is a potential approach towards addressing the carbon neutral economy. To date, a comprehensive analysis of key performance indicators, such as an intrinsic property of catalyst, reaction environment and technological advancement in the flow cell, is limited. In this study, we discuss how the design of catalyst material, electrolyte and engineering gas diffusion electrode (GDE) could affect the CO₂RR in a gas-fed flow cell. Significant emphasis is given to scale-up requirements, such as promising catalysts with a partial current density of ≥100 mA cm^-2 and high faradaic efficiency. Additional experimental hurdles and their potential solutions, as well as the best available protocols for data acquisition for catalyst activity evaluation, are listed. We believe this manuscript provides some insights into the making of catalysts and electrolytes in a rational manner along with the engineering of GDEs towards CO₂RR.

Competitive Trapping of Single Atoms onto a Metal Carbide Surface

Controlling atomic adjustment of single-atom catalysts (SACs) can directly change its local configuration, regulate the energy barrier of intermediates, and further optimize reaction pathways. Herein, we report an atom manipulating process to synthesize Ni atoms stabilized on vanadium carbide (Ni_SA-VC) through a nanofiber-medium thermodynamically driven atomic migration strategy. Experimental and theoretical results systematically reveal the tunable migration pathway of Ni atom from Ni nanoparticles to neighboring N-doped carbon (NC) and finally to metal carbide that was obtained by regulating the competitive adsorption energies between VC and NC for capturing Ni atoms. For CO₂-to-CO electroreduction, Ni_SA-VC exhibits an industrial current density of -180 mA cm^-2 at -1.0 V vs reversible hydrogen electrode and the highest Faradaic efficiency for CO production (FE_CO) of 96.8% at -0.4 V vs RHE in a flow cell. Significant electron transfers occurring in Ni_SA-VC structures contribute to the activation of CO₂, facilitate the reaction free energy, regulate *CO desorption as the rate-determining step, and promote the activity and selectivity. This study provides an understanding on how to design powerful SACs for electrocatalysis.

Ni Nanoclusters Anchored on Ni-N-C Sites for CO2 Electroreduction at High Current Densities

Transition metal catalyst-based electrocatalytic CO₂ reduction is a highly attractive approach to fulfill the renewable energy storage and a negative carbon cycle. However, it remains a great challenge for the earth-abundant VIII transition metal catalysts to achieve highly selective, active, and stable CO₂ electroreduction. Herein, bamboo-like carbon nanotubes that anchor both Ni nanoclusters and atomically dispersed Ni-N-C sites (NiNCNT) are developed for exclusive CO₂ conversion to CO at stable industry-relevant current densities. Through optimization of gas-liquid-catalyst interphases via hydrophobic modulation, NiNCNT exhibits as high as Faradaic efficiency (FE) of 99.3% for CO formation at a current density of -300 mA·cm^-2 (-0.35 V vs reversible hydrogen electrode (RHE)), and even an extremely high CO partial current density (j_CO) of -457 mA·cm^-2 corresponding to a CO FE of 91.4% at -0.48 V vs RHE. Such superior CO₂ electroreduction performance is ascribed to the enhanced electron transfer and local electron density of Ni 3d orbitals upon incorporation of Ni nanoclusters, which facilitates the formation of the COOH* intermediate.

Back-illuminated photoelectrochemical flow cell for efficient CO2 reduction

Photoelectrochemical CO2 reduction reaction flow cells are promising devices to meet the requirements to produce solar fuels at the industrial scale. Photoelectrodes with wide bandgaps do not allow for efficient CO2 reduction at high current densities, while the integration of opaque photoelectrodes with narrow bandgaps in flow cell configurations still remains a challenge. This paper describes the design and fabrication of a back-illuminated Si photoanode promoted PEC flow cell for CO2 reduction reaction. The illumination area and catalytic sites of the Si photoelectrode are decoupled, owing to the effective passivation of defect states that allows for the long minority carrier diffusion length, that surpasses the thickness of the Si substrate. Hence, a solar-to-fuel conversion efficiency of CO of 2.42% and a Faradaic efficiency of 90% using Ag catalysts are achieved. For CO2 to C2+ products, the Faradaic efficiency of 53% and solar-to-fuel of 0.29% are achieved using Cu catalyst in flow cell.

Tuning d-Band Structure of CuII in Coordinated Polymer via d-π Conjugation for Improving CO2 Electroreduction Selectivity toward C2 Products

Copper-coordinated catalysts are reported to be effective for electrocatalytic CO₂ reduction reaction (CO₂ RR) to C₂ products but suffer from low selectivity. Herein a strategy was developed to tune the d-band structure of Cu^II via coordinating with aromatic ligands to form Cu-based conjugated polymers for CO₂ RR to C₂ chemicals. The catalysts derived from copper chloride coordinating with tetraminobenzoquinone (TABQ) and with 1,2,4,5-benzenetetramine possessed high-density and compact Cu single-atom sites and displayed high activity for CO₂ RR to C₂ products. Especially, Cu-TABQ exhibited a maximum C₂ faradaic efficiency of 63.2 % with a current density of 423 mA cm^-2 at -1.17 V (vs. reversible hydrogen electrode). Density functional theory calculations indicated that the TABQ linker possessing C=O groups significantly widened the d-band of coordinated Cu^II , which facilitated binding of *CO intermediate on the catalyst and thus enhanced C-C coupling. This work provides mechanistic insight into the Cu^II -coordinated polymers for CO₂ RR with high selectivity toward C₂ products.

Preparation of an electrochemical immunosensor based on a Cu/Cu2O-rGO@Au signal synergistic amplification strategy and efficient and sensitive detection of alpha-fetoprotein

The effective amplification of the signal is the prerequisite for the ultrasensitive detection of electrochemical immunosensors. To quantitatively detect alpha-fetoprotein (AFP), we prepared a sandwich-type electrochemical immunosensor. Using a gold nanoparticles (Au NPs) modified glassy carbon electrode (GCE) as the sensing platform and Cu/Cu₂O-rGO@Au as the signal label, differential pulse voltammetry (DPV) was used to achieve sensitive detection of AFP. We found that the nanomaterials can undergo electro-oxidation and electro-reduction reactions between Cu(I) and Cu(II) in a buffer solution of pH = 6.0. It is worth mentioning that the incorporation of metals into metal oxide substrates is a new strategy to combine the catalytic activity of metal oxides with the electrical conductivity of metals. Reduced graphene oxide (rGO), which is rich in oxygen-containing groups, can load more Cu/Cu₂O and Au NPs and increase the conductivity. The modification of Au NPs makes them have better biocompatibility and conductivity. Under the best detection conditions, the prepared immunosensor realizes the specific and ultrasensitive detection of AFP. The detection range is 0.01-50 ng mL^-1 and the limit of detection (LOD) was as low as 0.589 pg mL^-1 (S/N = 3); meanwhile it also has good practical application ability. Therefore, this immunosensor provides an important means for the early screening and detection of AFP.

N-Graphene Sheet Stacks/Cu Electrocatalyst for CO2 Reduction to Ethylene

Renewable energy resources (wind, solar) are unpredictable, so it is wise to store the electricity they generate in an energy carrier X. Various PtX (power to useful energy-intensive raw material such as hydrogen, synthetic natural gas, fuel) applications have been proposed. At the heart of our work is widely used idea to convert residual CO2 from biogas plant into higher hydrocarbons using electricity from renewables (e.g., sun, wind, hydro). The specific goal is to produce ethylene-highly demanded hydrocarbon in plastics industry. The process itself is realised on electrocatalytic carbon/copper cathode which must be selective to reaction: 2CO2 + 12e− + 12H+→C2H4 + 4H2O. We propose a bottom-up approach to build catalyst from the smallest particles-graphene sheet stacks (GSS) coated with metallic copper nanocrystals. Composite GSS-Cu structure functions as a CO2 and proton absorber, facilitating hydrogenation and carbon–carbon coupling reactions on Cu-nanocluster/GSS for the formation of C2H4. In our design electrocatalytic electrode is made from nitrogen-doped graphene sheet stacks coated with copper nanostructures. The N-GSSitself can be drop-casted or electrophoretically incorporated onto the carbon paper and gas diffusion electrode. Electrochemical deposition method was recognized as successful and most promising to grow Cu nanocrystals on N-GSS incorporated in conducting carbon substrate. Gaseous products from CO2 electro-catalytic reformation on the cathode were investigated by mass-spectrometer but the electrode surface was analysed by SEM/EDS and XRD methods.

Nanoporous Intermetallic SnTe Enables Efficient Electrochemical CO2 Reduction into Formate via Promoting the Fracture of Metal-Oxygen Bonding

Electrochemical reduction of CO₂ into formate product is considered the most practical significance link in the carbon cycle. Developing cheap and efficient electrocatalysts with high selectivity for formate on a wide operated potential window is desirable yet challenging. Herein, nanoporous ordered intermetallic tin-tellurium (SnTe) is synthesized with a greater reduction performance for electrochemical CO₂ to formate reduction compared to bare Sn. This nanoporous SnTe achieves 93% Faradaic efficiency for formate production and maintains over 90% Faradaic efficiency at a wide voltage range from -1.0 to -1.3 V versus reversible hydrogen electrode (RHE), together with 60 h stability. Combining operando Raman spectroscopy studies with density functional theory calculations reveals that strong orbital interaction between Sn and neighboring tellurium (Te) in the intermetallic SnTe can lower the barriers of the oxygen cutoff hydrogenation and desorption steps by promoting the fracture of bond between metal and oxygen, leading to the significant enhancement of formate production.

Redox Replacement of Silver on MOF-Derived Cu/C Nanoparticles on Gas Diffusion Electrodes for Electrocatalytic CO2 Reduction

Bimetallic tandem catalysts have emerged as a promising strategy to locally increase the CO flux during electrochemical CO₂ reduction, so as to maximize the rate of conversion to C-C-coupled products. Considering this, a novel Cu/C-Ag nanostructured catalyst has been prepared by a redox replacement process, in which the ratio of the two metals can be tuned by the replacement time. An optimum Cu/Ag composition with similarly sized particles showed the highest CO₂ conversion to C₂₊ products compared to non-Ag-modified gas-diffusion electrodes. Gas chromatography and in-situ Raman measurements in a CO₂ gas diffusion cell suggest the formation of top-bound linear adsorbed *CO followed by consumption of CO in the successive cascade steps, as evidenced by the increasingνC-H bands. These findings suggest that two mechanisms operate simultaneously towards the production of HCO₂ H and C-C-coupled products on the Cu/Ag bimetallic surface.

Strain Relaxation in Metal Alloy Catalysts Steers the Product Selectivity of Electrocatalytic CO2 Reduction

Strain engineering in bimetallic alloy structures is of great interest in electrochemical CO₂ reduction reactions (CO₂RR), in which it simultaneously improves electrocatalytic activity and product selectivity by optimizing the binding properties of intermediates. However, a reliable synthetic strategy and systematic understanding of the strain effects in the CO₂RR are still lacking. Herein, we report a strain relaxation strategy used to determine lattice strains in bimetal MNi alloys (M = Pd, Ag, and Au) and realize an outstanding CO₂-to-CO Faradaic efficiency of 96.6% and show the outstanding activity and durability toward a Zn-CO₂ battery. Molecular dynamics (MD) simulations predict that the relaxation of strained PdNi alloys (s-PdNi) is correlated with increases in synthesis temperature, and the high temperature activation energy drives complete atomic mixing of multiple metal atoms to allow for regulation of lattice strains. Density functional theory (DFT) calculations reveal that strain relaxation effectively improves CO₂RR activity and selectivity by optimizing the formation energies of *COOH and *CO intermediates on s-PdNi alloy surfaces, as also verified by in situ spectroscopic investigations. This approach provides a promising approach for catalyst design, enabling independent optimization of formation energies of reaction intermediates to improve catalytic activity and selectivity simultaneously.

Electronic structure modulation of bismuth catalysts induced by sulfur and oxygen co-doping for promoting CO2 electroreduction

Carbon dioxide electroreduction into green fuels and value-added chemicals is an attractive method for the utilization of renewable energy to mitigate global warming. High-efficiency catalysts are necessary for mild and...

Highly Ethylene-Selective Electrocatalytic CO2 Reduction Enabled by Isolated Cu-S Motifs in Metal-Organic Framework Based Precatalysts

Copper-based materials are efficient electrocatalysts for the conversion of CO₂ to C₂₊ products, and most these materials are reconstructed in situ to regenerate active species. It is a challenge to precisely design precatalysts to obtain active sites for the CO₂ reduction reaction (CO₂ RR). Herein, we develop a strategy based on local sulfur doping of a Cu-based metal-organic framework precatalyst, in which the stable Cu-S motif is dispersed in the framework of HKUST-1 (S-HKUST-1). The precatalyst exhibits a high ethylene selectivity in an H-type cell with a maximum faradaic efficiency (FE) of 60.0 %, and delivers a current density of 400 mA cm^-2 with an ethylene FE up to 57.2 % in a flow cell. Operando X-ray absorption results demonstrate that Cu^δ+ species stabilized by the Cu-S motif exist in S-HKUST-1 during CO₂ RR. Density functional theory calculations indicate the partially oxidized Cu^δ+ at the Cu/Cu_x S_y interface is favorable for coupling of the *CO intermediate due to the modest distance between coupling sites and optimized adsorption energy.

B-Cu-Zn Gas Diffusion Electrodes for CO2 Electroreduction to C2+ Products at High Current Densities

Electroreduction of CO2 to multi-carbon products has attracted considerable attention as it provides an avenue to high-density renewable energy storage. However, the selectivity and stability under high current densities are rarely reported. Herein, B-doped Cu (B-Cu) and B-Cu-Zn gas diffusion electrodes (GDE) were developed for highly selective and stable CO2 conversion to C2+  products at industrially relevant current densities. The B-Cu GDE exhibited a high Faradaic efficiency of 79 % for C2+  products formation at a current density of -200 mA cm-2 and a potential of -0.45 V vs. RHE. The long-term stability for C2+ formation was substantially improved by incorporating an optimal amount of Zn. Operando Raman spectra confirm the retained Cu+ species under CO2 reduction conditions and the lower overpotential for *OCO formation upon incorporation of Zn, which lead to the excellent conversion of CO2 to C2+ products on B-Cu-Zn GDEs.