Coverage-driven selectivity switch from ethylene to acetate in high-rate CO2/CO electrolysis

Switching CO-to-Acetate Electroreduction on Cu Atomic Ensembles

The electrocatalytic reaction pathway is highly dependent on the intrinsic structure of the catalyst. CO2/CO electroreduction has recently emerged as a potential approach for obtaining C2+ products, but it is challenging to achieve high selectivity for a single C2+ product. Herein, we develop a Cu atomic ensemble that satisfies the appropriate site distance and coordination environment required for electrocatalytic CO-to-acetate conversion, which shows outstanding overall performance with an acetate Faradaic efficiency of 70.2% with a partial current density of 225 mA cm-2 and a formation rate of 2.1 mmol h-1 cm-2. Moreover, a single-pass CO conversion rate of 91% and remarkable stability can be also obtained. Detailed experimental and theoretical investigations confirm the significant advantages of the Cu atomic ensembles in optimizing C-C coupling, stabilizing key ketene intermediate (*CCO), and inhibiting the *HOCCOH intermediate, which can switch the CO reduction pathway from the ethanol/ethylene on the conventional metallic Cu site to the acetate on the Cu atomic ensembles.

Recent advances in microenvironment regulation for electrocatalysis

High-efficiency electrocatalysis could serve as the bridge that connects renewable energy technologies, hydrogen economy and carbon capture/utilization, promising a sustainable future for humankind. It is therefore of paramount significance to explore feasible strategies to modulate the relevant electrocatalytic reactions and optimize device performances so as to promote their large-scale practical applications. Microenvironment regulation at the catalytic interface has been demonstrated to be capable of effectively enhancing the reaction rates and improving the selectivities for specific products. In this review we summarize the latest advances in microenvironment regulation in typical electrocatalytic processes (including water electrolysis, hydrogen-oxygen fuel cells, and carbon dioxide reduction) and the related in situ/operando characterization techniques and theoretical simulation methods. At the end of this article, we present an outlook on development trends and possible future directions.

Enhancing CO2 Electroreduction Precision to Ethylene and Ethanol: The Role of Additional Boron Catalytic Sites in Cu-Based Tandem Catalysts

The electrocatalytic conversion of carbon dioxide (CO2) into valuable multicarbon (C2+) compounds offers a promising approach to mitigate CO2 emissions and harness renewable energy. However, achieving precise selectivity for specific C2+ products, such as ethylene and ethanol, remains a formidable challenge. This study shows that incorporating elemental boron (B) into copper (Cu) catalysts provides additional adsorption sites for *CO intermediates, enhancing the selectivity of desirable C2+ products. Additionally, using a nickel single-atom catalyst (Ni-SAC) as a *CO source increases local *CO concentration and reduces the hydrogen evolution reaction. In situ experiments and density functional theory (DFT) calculations reveal that surface-bound boron units adsorb and convert *CO more efficiently, promoting ethylene production, while boron within the bulk phase of copper influences charge transfer, facilitating ethanol generation. In a neutral electrolyte, the bias current density for ethylene production using the B-O-Cu2@Ni-SAC0.05 hybrid catalyst exceeded 300 mA cm-2, and that for ethanol production with B-O-Cu5@Ni-SAC0.2 surpassed 250 mA cm-2. This study underscores that elemental doping in Cu-based catalysts not only alters charge and crystalline phase arrangements at Cu sites but also provides additional reduction sites for coupling reactions, enabling the efficient synthesis of distinct C2+ products.

Direct low concentration CO2 electroreduction to multicarbon products via rate-determining step tuning

Direct converting low concentration CO2 in industrial exhaust gases to high-value multi-carbon products via renewable-energy-powered electrochemical catalysis provides a sustainable strategy for CO2 utilization with minimized CO2 separation and purification capital and energy cost. Nonetheless, the electrocatalytic conversion of dilute CO2 into value-added chemicals (C2+ products, e.g., ethylene) is frequently impeded by low CO2 conversion rate and weak carbon intermediates' surface adsorption strength. Here, we fabricate a range of Cu catalysts comprising fine-tuned Cu(111)/Cu2O(111) interface boundary density crystal structures aimed at optimizing rate-determining step and decreasing the thermodynamic barriers of intermediates' adsorption. Utilizing interface boundary engineering, we attain a Faradaic efficiency of (51.9 ± 2.8) % and a partial current density of (34.5 ± 6.4) mA·cm-2 for C2+ products at a dilute CO2 feed condition (5% CO2 v/v), comparing to the state-of-art low concentration CO2 electrolysis. In contrast to the prevailing belief that the CO2 activation step (C O 2 + e - + * → C O 2 - * ) governs the reaction rate, we discover that, under dilute CO2 feed conditions, the rate-determining step shifts to the generation of *COOH (C O 2 - * + H 2 O → C * O O H + O H - ( a q ) ) at the Cu0/Cu1+ interface boundary, resulting in a better C2+ production performance.

Bridging activity gaps between batch and flow reactor configurations in the electroreduction of carbon dioxide

To date, the understanding of various modes of CO2 mass transport remains incomplete, impeding the transfer of catalysts identified in the more accessible electrochemical batch cells to high-performance flow cells. In this work, we demonstrate that the meniscus region formed between the electrode and the convex liquid level due to the electrowetting of the catalyst plays a vital role in the CO2RR in batch cells. CO2RR in the meniscus region in batch cells exhibits similar performance with that in flow cells, and the performance disparity between these two configurations largely disappears when conducting CO2RR primarily in the meniscus region. An assembled double-sided gas diffusion electrode with a gas channel is developed to maximize the meniscus-like region, achieving a CO2RR partial current density of 640 mA/cm2geo on commercial Cu in the KHCO3 electrolyte. This performance represents the highest CO2RR activity in neutral buffered media.

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.

Sulfite-Assisted Acetate Conversion from CO Electroreduction

The efficient acetate conversion from CO electroreduction is challenging due to the poor selectivity at high reaction rate, which requires the competition with H2 and other C2+ (i. e., ethylene, ethanol, n-propanol) reduction products. Electrolyte engineering is one of the efficient strategies to regulate the reaction microenvironment. In this work, the adding of sulfite (SO3 2-) with high nucleophilicity in KOH electrolytes was demonstrated to enable improving the CO-to-acetate conversion via generating a S-O chemical bond between SO3 2- and oxygenated *C2 intermediates (i. e., *CO-CO, *CO-COH) compared with that in pure KOH system on both synthesized Cu(200)- and normal commercial Cu(111)-facets-exposed metallic Cu catalysts. As a result, the prepared Cu(200)-facets-exposed metallic Cu catalyst with surface ions modification showed an superior Faradaic efficiency of 63.6 % at -0.6 A ⋅ cm-2, and extraordinary absolute value of peak partial current density as high as 1.52 A ⋅ cm-2 with adding SO3 2- in KOH electrolytes, compared to the best reported values in both CO and CO2 electroreduction. Our work suggests an attractive strategy to introduce the oxyanion with high nucleophilicity in electrolytes to regulate the microenvironment for industrial-current-density electrosynthesis of acetate from CO electroreduction.

Electrochemical Formation of C2+ Products Steered by Bridge-Bonded *CO Confined by *OH Domains

During the electrochemical CO2 reduction reaction (eCO2RR) on copper catalysts, linear-bonded CO (*COL) is commonly regarded as the key intermediate for the CO-CO coupling step, which leads to the formation of multicarbon products. In this work, we unveil the significant role of bridge-bonded *CO (*COB) as an active species. By combining in situ Raman spectroscopy, gas and liquid chromatography, and density functional theory (DFT) simulations, we show that adsorbed *OH domains displace *COL to *COB. The electroreduction of a 12CO+13CO2 cofeed demonstrates that *COB distinctly favors the production of acetate and 1-propanol, while *COL favors ethylene and ethanol formation. This work enhances our understanding of the mechanistic intricacies of eCO(2)RR and suggests new directions for designing operational conditions by modifying the competitive adsorption of surface species, thereby steering the reaction toward specific multicarbon products.

A Click Chemistry Strategy Toward Spin-Polarized Transition-Metal Single Site Catalysts for Dynamic Probing of Sulfur Redox Electrocatalysis

Catalytic conversion of lithium polysulfides (LiPSs) is a crucial approach to enhance the redox kinetics and suppress the shuttle effect in lithium-sulfur (Li-S) batteries. However, the roles of a typical heterogenous catalyst cannot be easily identified due to its structural complexity. Compared with the distinct sites of single atom catalysts (SACs), each active site of single site catalysts (SSCs) is identical and uniform in their spatial energy, binding mode, and coordination sphere, etc. Benefiting from the well-defined structure, iron phthalocyanine (FePc) is covalently clicked onto CuO nanosheet to prepare low spin-state Fe SSCs as the model catalyst for Li-S electrochemistry. The periodic polarizability evolution of Fe-N bonding is probed during sulfur redox reaction by in situ Raman spectra. Theoretical analysis shows the decreased d-band center gap of Fe (Δd) and delocalization of dxz/dyz after the axial click confinement. Consequently, Li-S batteries with Fe SSCs exhibit a capacity decay rate of 0.029% per cycle at 2 C. The universality of this methodological approach is demonstrated by a series of M SSCs (M = Mn, Co, and Ni) with similar variation of electronic configuration. This work provides guidance for the design of efficient electrocatalysis in Li-S batteries.

Highly Tensile Strained Cu(100) Surfaces by Epitaxial Grown Hexagonal Boron Nitride for CO2 Electroreduction to C2+ Products

Copper (Cu) has been considered as the most promising catalyst for the electrochemical conversion of CO2 to multicarbon (C2+) products. However, insufficient coverage of the *CO intermediate on the C2+ formation Cu(100) facet largely hinders the C-C coupling process and thus the C2+ conversion efficiency. Herein, we developed an epitaxial growth strategy to generate highly tensile-strained Cu(100) facets via the epitaxial growth of hexagonal boron nitride (hBN) on Cu(100) facets to promote *CO coverage for efficient CO2 to C2+ conversion. The highest ∼6% tensile strain on the Cu(100) facets was obtained by lattice mismatch between the Cu(100) and hBN(002) facets. Theory calculations indicated that tensile-strained Cu(100) facets deliver a notable d-band center upshift to enhance *CO adsorption. As a result, the obtained highly tensile-strained Cu(100) facets enabled an 8-fold improvement of *CO coverage and thus a 83.4% C2+ Faradaic efficiency at 1.2 A cm-2 in strongly acidic electrolyte (pH = 1).

Flow-electrode capacitive separation of organic acid products and recovery of alkali cations after acidic CO2 electrolysis

Acidic CO2 electrolysis, enhanced by the introduction of alkali cations, presents a strategic approach for improving carbon efficiency compared to processes conducted in neutral and alkaline environments. However, a significant challenge arises from the dissolution of both organic acids and alkali cations in a strongly acidic feed stream, resulting in a considerable energy penalty for downstream separation. In this study, we investigate the feasibility of using flow-electrode capacitive deionization (FCDI) technology to separate organic acids and recover alkali cations from a strongly acidic feed stream (pH ~ 1). We show that organic acids, such as formic acid and acetic acid, are retained in molecular form in the separation chamber, achieving a rejection rate of over 90% under all conditions. Alkali cations, such as K+ and Cs+, migrate to the cathode chamber in ionic form, with their removal and recovery significantly influenced by their concentration and the pH of the feed stream, but responding differently to the types and concentrations of organic acids. The energy consumption for the removal and recovery of K+ is 4 to 8 times higher than for Cs+, and the charge efficiency is significantly influenced by the types of organic acid products and alkali cations. We conduct a series of electrochemical measurements and analyze the impedance spectroscopy, identifying that hindered mass transfer governed the electrode process. Our findings underscore the potential of FCDI as an advanced downstream separation technology for acidic electrocatalysis processes.

Tensile-Strained Cu Penetration Electrode Boosts Asymmetric C-C Coupling for Ampere-Level CO2-to-C2+ Reduction in Acid

The synthesis of multicarbon (C2+) products remains a substantial challenge in sustainable CO2 electroreduction owing to the need for sufficient current density and faradaic efficiency alongside carbon efficiency. Herein, we demonstrate ampere-level high-efficiency CO2 electroreduction to C2+ products in both neutral and strongly acidic (pH=1) electrolytes using a hierarchical Cu hollow-fiber penetration electrode (HPE). High concentration of K+ could concurrently suppress hydrogen evolution reaction and facilitate C-C coupling, thereby promoting C2+ production in strong acid. By optimizing the K+ and H+ concentration and CO2 flow rate, a faradaic efficiency of 84.5 % and a partial current density as high as 3.1 A cm-2 for C2+ products, alongside a single-pass carbon efficiency of 81.5 % and stable electrolysis for 240 h were demonstrated in a strong acidic solution of H2SO4 and KCl (pH=1). Experimental measurements and density functional theory simulations suggested that tensile-strained Cu HPE enhances the asymmetric C-C coupling to steer the selectivity and activity of C2+ products.

Cu-In Dual Sites with Sulfur Defects toward Superior Ethanol Electrosynthesis from CO2 Electrolysis

The electrosynthesis of multi-carbon chemicals from excess carbon dioxide (CO2) is an area of great interest for research and commercial applications. However, improving both the yield of CO2-to-ethanol conversion and the stability of the catalyst at the same time is proving to be a challenging issue. Here it is proposed to stabilize active Cu(I) and In dual sites with sulfur defects through an electro-driven intercalation strategy, which leads to the delocalization of electron density that enhances orbital hybridizations between the Cu-C and In-H bonds. Hence, the energy barrier for the rate-limiting *CHO formation step is reduced toward the key *OCHCHO* formation during ethanol production, which is also facilitated by the combined Cu site enabling C-C coupling and In site with a higher oxygen affinity based on both thermodynamic and kinetic calculations. Accordingly, such dual-site catalyst achieves a high partial current density toward ethanol of 409 ± 15 mA cm⁻2 for over 120 h. Furthermore, a scaled-up flow cell is assembled with an industrial-relevant current of 5.7 A for over 36 h, in which the carbon loss is less than 2.5% and single-pass carbon efficiency is ≈19%.

Switching Product Selectivity in CO2 Electroreduction via Cu-S Bond Length Variation

Regulating competitive reaction pathways to direct the selectivity of electrochemical CO2 reduction reaction toward a desired product is crucial but remains challenging. Herein, switching product from HCOOH to CO is achieved by incorporating Sb element into the CuS, in which the Cu-S ionic bond is coupled with S-Sb covalent bond through bridging S atoms that elongates the Cu-S bond from 2.24 Å to 2.30 Å. Consequently, CuS with a shorter Cu-S bond exhibited a high selectivity for producing HCOOH, with a maximum Faradaic efficiency (FE) of 72 %. Conversely, Cu3SbS4 characterized by an elongated Cu-S bond exhibited the most pronounced production of CO with a maximum FE of 60 %. In situ spectroscopy combined with density functional theory calculations revealed that the altered Cu-S bond length and local coordination environment make the *HCOO binding energy weaker on Cu3SbS4 compared to that on CuS. Notably, a volcano-shaped correlation between the Cu-S bond length and adsorption strength of *COOH indicates that Cu-S in Cu3SbS4 as double-active sites facilitates the adsorption of *COOH, and thus results in the high selectivity of Cu3SbS4 toward CO.

Mesostructure-Specific Configuration of *CO Adsorption for Selective CO2 Electroreduction to C2+ Products

The multi-carbon (C2+) alcohols produced by electrochemical CO2 reduction, such as ethanol and n-propanol, are considered as indispensable liquid energy carriers. In most C-C coupling cases, however, the concomitant gaseous C2H4 product results in the low selectivity of C2+ alcohols. Here, we report rational construction of mesostructured CuO electrocatalysts, specifically mesoporous CuO (m-CuO) and cylindrical CuO (c-CuO), enables selective distribution of C2+ products. The m-CuO and c-CuO show similar selectivity towards total C2+ products (≥76 %), but the corresponding predominant products are C2+ alcohols (55 %) and C2H4 (52 %), respectively. The ordered mesostructure not only induces the surface hydrophobicity, but selectively tailors the adsorption configuration of *CO intermediate: m-CuO prefers bridged adsorption, whereas c-CuO favors top adsorption as revealed by in situ spectroscopies. Computational calculations unravel that bridged *CO adsorbate is prone to deep protonation into *OCH3 intermediate, thus accelerating the coupling of *CO and *OCH3 intermediates to generate C2+ alcohols; by contrast, top *CO adsorbate is apt to undergo conventional C-C coupling process to produce C2H4. This work illustrates selective C2+ products distribution via mesostructure manipulation, and paves a new path into the design of efficient electrocatalysts with tunable adsorption configuration of key intermediates for targeted products.

Oriented Synthesis of Glycine from CO2, N2, and H2O via a Cascade Process

Air contains carbon, hydrogen, oxygen, and nitrogen elements that are essential for the constitution of amino acids. Converting the air into amino acids, powered with renewable electricity, provides a green and sustainable alternative to petrochemical-based methods that produce waste and pollution. Here, taking glycine as an example, we demonstrated the complete production chain for electrorefining amino acids directly from CO2, N2, and H2O. Such a prospective Scheme was composed of three modules, linked by a spontaneous C-N bond formation process. The high-purity bridging intermediates, separated from the stepwise synthesis, boosted both the carbon selectivity from CO2 to glycine of 91.7 % and nitrogen selectivity from N2 to glycine of 98.7 %. Under the optimum condition, we obtained glycine with a partial current density of 160.8 mA cm-2. The high-purity solid glycine product was acquired with a separation efficiency of 98.4 %. This work unveils a green and sustainable method for the abiotic creation of amino acids from the air components.

High-efficiency C3 electrosynthesis on a lattice-strain-stabilized nitrogen-doped Cu surface

The synthesis of multi-carbon (C2+) fuels via electrocatalytic reduction of CO, H2O using renewable electricity, represents a significant stride in sustainable energy storage and carbon recycling. The foremost challenge in this field is the production of extended-chain carbon compounds (Cn, n ≥ 3), wherein elevated *CO coverage (θco) and its subsequent multiple-step coupling are both critical. Notwithstanding, there exists a "seesaw" dynamic between intensifying *CO adsorption to augment θco and surmounting the C-C coupling barrier, which have not been simultaneously realized within a singular catalyst yet. Here, we introduce a facilely synthesized lattice-strain-stabilized nitrogen-doped Cu (LSN-Cu) with abundant defect sites and robust nitrogen integration. The low-coordination sites enhance θco and concurrently, the compressive strain substantially fortifies nitrogen dopants on the catalyst surface, promoting C-C coupling activity. The n-propanol formation on the LSN-Cu electrode exhibits a 54% faradaic efficiency and a 29% half-cell energy efficiency. Moreover, within a membrane electrode assembly setup, a stable n-propanol electrosynthesis over 180 h at a total current density of 300 mA cm-2 is obtained.

Sustainable Gas Storage: CO2 Activation of Edge-Functionalized Graphitic Nanoplatelets

Graphitic nanoplatelets (GnPs), edge-selectively carboxylated graphitic nanoplatelets (ECGnPs), are functionalized with a carboxylic acid at the edge increasing their surface area, and are highly dispersible in various solvents. However, there is a limit in that the basal plane remains intact because it is functionalized only in the part where the radical is generated at the edge. Here, we activate ECGnPs to have porous structures by flowing CO2 at 900 °C. Etching of the ECGnPs structure was performed through the Boudouard reaction, and the surface area increased from 579 m2 g-1 to a maximum of 2462 m2 g-1. In addition, the pore structure was investigated with various adsorption gases (CH4, Ar, CO2, H2, and N2) according to the reaction time. This study provides the overall green chemistry in that it utilizes CO2 from manufacturing to activation compared to the process of activating with conventional chemical treatment.

Sulfur-Bridged Asymmetric CuNi Bimetallic Atom Sites for CO2 Reduction with High Efficiency

Double-atom catalysts (DACs) with asymmetric coordination are crucial for enhancing the benefits of electrochemical carbon dioxide reduction and advancing sustainable development, however, the rational design of DACs is still challenging. Herein, this work synthesizes atomically dispersed catalysts with novel sulfur-bridged Cu-S-Ni sites (named Cu-S-Ni/SNC), utilizing biomass wool keratin as precursor. The plentiful disulfide bonds in wool keratin overcome the limitations of traditional gas-phase S ligand etching process and enable the one-step formation of S-bridged sites. X-ray absorption spectroscopy (XAS) confirms the existence of bimetallic sites with N2Cu-S-NiN2 moiety. In H-cell, Cu-S-Ni/SNC shows high CO Faraday efficiency of 98.1% at -0.65 V versus RHE. Benefiting from the charge tuning effect between the metal site and bridged sulfur atoms, a large current density of 550 mA cm-2 can be achieved at -1.00 V in flow cell. Additionally, in situ XAS, attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), and density functional theory (DFT) calculations show Cu as the main adsorption site is dual-regulated by Ni and S atoms, which enhances CO2 activation and accelerates the formation of *COOH intermediates. This kind of asymmetric bimetallic atom catalysts may open new pathways for precision preparation and performance regulation of atomic materials toward energy applications.

Site-Selective Growth of fcc-2H-fcc Copper on Unconventional Phase Metal Nanomaterials for Highly Efficient Tandem CO2 Electroreduction

Copper (Cu) nanomaterials are a unique kind of electrocatalysts for high-value multi-carbon production in carbon dioxide reduction reaction (CO2RR), which holds enormous potential in attaining carbon neutrality. However, phase engineering of Cu nanomaterials remains challenging, especially for the construction of unconventional phase Cu-based asymmetric heteronanostructures. Here the site-selective growth of Cu on unusual phase gold (Au) nanorods, obtaining three kinds of heterophase fcc-2H-fcc Au-Cu heteronanostructures is reported. Significantly, the resultant fcc-2H-fcc Au-Cu Janus nanostructures (JNSs) break the symmetric growth mode of Cu on Au. In electrocatalytic CO2RR, the fcc-2H-fcc Au-Cu JNSs exhibit excellent performance in both H-type and flow cells, with Faradaic efficiencies of 55.5% and 84.3% for ethylene and multi-carbon products, respectively. In situ characterizations and theoretical calculations reveal the co-exposure of 2H-Au and 2H-Cu domains in Au-Cu JNSs diversifies the CO* adsorption configurations and promotes the CO* spillover and subsequent C-C coupling toward ethylene generation with reduced energy barriers.

Electrocatalytic Reduction of Carbon Dioxide in Acidic Electrolyte with Superior Performance of a Metal-Covalent Organic Framework over Metal-Organic Framework

CO2 electroreduction (CO2RR) to generate valuable chemicals in acidic electrolytes can improve the carbon utilization rate in comparison to that under alkaline conditions. However, the thermodynamically more favorable hydrogen evolution reaction under an acidic electrolyte makes the CO2RR a big challenge. Herein, robust metal phthalocyanine(Pc)-based (M = Ni, Co) conductive metal-covalent organic frameworks (MCOFs) connected by strong metal tetraaza[14]annulene (TAA) linkage, named NiPc-NiTAA and NiPc-CoTAA, are designed and synthesized to apply in the CO2RR in acidic electrolytes for the first time. The optimal NiPc-NiTAA exhibited an excellent Faradaic efficiency (FECO) of 95.1% and a CO partial current density of 143.0 mA cm-2 at -1.5 V versus the reversible hydrogen electrode in an acidic electrolyte, which is 3.1 times that of the corresponding metal-organic framework NiPc-NiN4. The comparison tests and theoretical calculations reveal that in-plane full π-d conjugation MCOF with a good conductivity of 3.01 × 10-4 S m-1 accelerates migration of the electrons. The NiTAA linkage can tune the electron distribution in the d orbit of metal centers, making the d-band center close to the Fermi level and then activating CO2. Thus, the active sites of NiPc and NiTAA collaborate to reduce the *COOH formation energy barrier, favoring CO production in an acid electrolyte. It is a helpful route for designing outstanding conductive MCOF materials to enhance CO2 electrocatalysis under an acidic electrolyte.

Selective and stable CO2 electroreduction at high rates via control of local H2O/CO2 ratio

Controlling the concentrations of H2O and CO2 at the reaction interface is crucial for achieving efficient electrochemical CO2 reduction. However, precise control of these variables during catalysis remains challenging, and the underlying mechanisms are not fully understood. Herein, guided by a multi-physics model, we demonstrate that tuning the local H2O/CO2 concentrations is achievable by thin polymer coatings on the catalyst surface. Beyond the often-explored hydrophobicity, polymer properties of gas permeability and water-uptake ability are even more critical for this purpose. With these insights, we achieve CO2 reduction on copper with Faradaic efficiency exceeding 87% towards multi-carbon products at a high current density of -2 A cm-2. Encouraging cathodic energy efficiency (>50%) is also observed at this high current density due to the substantially reduced cathodic potential. Additionally, we demonstrate stable CO2 reduction for over 150 h at practically relevant current densities owning to the robust reaction interface. Moreover, this strategy has been extended to membrane electrode assemblies and other catalysts for CO2 reduction. Our findings underscore the significance of fine-tuning the local H2O/CO2 balance for future CO2 reduction applications.

Bioinspired molecule-functionalized Cu with high CO adsorption for efficient CO electroreduction to acetate

Electrochemical reduction of carbon dioxide (CO2) or carbon monoxide (CO) to valuable multi-carbon (C2+) products like acetate is a promising approach for a sustainable energy economy. However, it is still challenging to achieve high activity and selectivity for acetate production, especially in neutral electrolytes. Herein, a bioinspired hemin/Cu hybrid catalyst was developed to enhance the surface *CO coverage for highly efficient electroreduction of CO to acetate fuels. The hemin/Cu electrocatalyst exhibits a remarkable faradaic efficiency of 45.2% for CO-to-acetate electroreduction and a high acetate partial current density of 152.3 mA cm-2. Furthermore, the developed hybrid catalyst can operate stably at 200 mA cm-2 for 14.6 hours, producing concentrated acetate aqueous solutions (0.235 M, 2.1 wt%). The results of in situ Raman spectroscopy and theoretical calculations proved that the Fe-N4 structure of hemin could enhance the CO adsorption and enrich the local concentration of CO, thereby improving C-C coupling for acetate production. In addition, compared to the unmodified Cu catalysts, the Cu catalysts functionalized with cobalt phthalocyanine with a Co-N4 structure also exhibit improved acetate performance, proving the universality of this bioinspired molecule-enhanced strategy. This work paves a new way to designing bioinspired electrolysis systems for producing specific C2+ products from CO2 or CO electroreduction.

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.

Recent advances in dynamic reconstruction of electrocatalysts for carbon dioxide reduction

Electrocatalysts undergo structural evolution under operating electrochemical CO2 reduction reaction (CO2RR) conditions. This dynamic reconstruction correlates with variations in CO2RR activity, selectivity, and stability, posing challenges in catalyst design for electrochemical CO2RR. Despite increased research on the reconstruction behavior of CO2RR electrocatalysts, a comprehensive understanding of their dynamic structural evolution under reaction conditions is lacking. This review summarizes recent developments in the dynamic reconstruction of catalysts during the CO2RR process, covering fundamental principles, modulation strategies, and in situ/operando characterizations. It aims to enhance understanding of electrocatalyst dynamic reconstruction, offering guidelines for the rational design of CO2RR electrocatalysts.

Facet-switching of rate-determining step on copper in CO2-to-ethylene electroreduction

Reduction of carbon dioxide (CO2) by renewable electricity to produce multicarbon chemicals, such as ethylene (C2H4), continues to be a challenge because of insufficient Faradaic efficiency, low production rates, and complex mechanistic pathways. Here, we report that the rate-determining steps (RDS) on common copper (Cu) surfaces diverge in CO2 electroreduction, leading to distinct catalytic performances. Through a combination of experimental and computational studies, we reveal that C─C bond-making is the RDS on Cu(100), whereas the protonation of *CO with adsorbed water becomes rate-limiting on Cu(111) with a higher energy barrier. On an oxide-derived Cu(100)-dominant Cu catalyst, we reach a high C2H4 Faradaic efficiency of 72%, partial current density of 359 mA cm-2, and long-term stability exceeding 100 h at 500 mA cm-2, greatly outperforming its Cu(111)-rich counterpart. We further demonstrate constant C2H4 selectivity of >60% over 70 h in a membrane electrode assembly electrolyzer with a full-cell energy efficiency of 23.4%.

Revealing the structural evolution of CuAg composites during electrochemical carbon monoxide reduction

Comprehending the catalyst structural evolution during the electrocatalytic process is crucial for establishing robust structure/performance correlations for future catalysts design. Herein, we interrogate the structural evolution of a promising Cu-Ag oxide catalyst precursor during electrochemical carbon monoxide reduction. By using extensive in situ and ex situ characterization techniques, we reveal that the homogenous oxide precursors undergo a transformation to a bimetallic composite consisting of small Ag nanoparticles enveloped by thin layers of amorphous Cu. We believe that the amorphous Cu layer with undercoordinated nature is responsible for the enhanced catalytic performance of the current catalyst composite. By tuning the Cu/Ag ratio in the oxide precursor, we find that increasing the Ag concentration greatly promotes liquid products formation while suppressing the byproduct hydrogen. CO2/CO co-feeding electrolysis and isotopic labelling experiments suggest that high CO concentrations in the feed favor the formation of multi-carbon products. Overall, we anticipate the insights obtained for Cu-Ag bimetallic systems for CO electroreduction in this study may guide future catalyst design with improved performance.

CO electrolysis to multicarbon products over grain boundary-rich Cu nanoparticles in membrane electrode assembly electrolyzers

Producing valuable chemicals like ethylene via catalytic carbon monoxide conversion is an important nonpetroleum route. Here we demonstrate an electrochemical route for highly efficient synthesis of multicarbon (C2+) chemicals from CO. We achieve a C2+ partial current density as high as 4.35 ± 0.07 A cm-2 at a low cell voltage of 2.78 ± 0.01 V over a grain boundary-rich Cu nanoparticle catalyst in an alkaline membrane electrode assembly (MEA) electrolyzer, with a C2+ Faradaic efficiency of 87 ± 1% and a CO conversion of 85 ± 3%. Operando Raman spectroscopy and density functional theory calculations reveal that the grain boundaries of Cu nanoparticles facilitate CO adsorption and C - C coupling, thus rationalizing a qualitative trend between C2+ production and grain boundary density. A scale-up demonstration using an electrolyzer stack with five 100 cm2 MEAs achieves high C2+ and ethylene formation rates of 118.9 mmol min-1 and 1.2 L min-1, respectively, at a total current of 400 A (4 A cm-2) with a C2+ Faradaic efficiency of 64%.

Boosting Electrocatalytic Ethylene Epoxidation by Single Atom Modulation

The electrochemical synthesis of ethylene oxide (EO) using ethylene and water under ambient conditions presents a low-carbon alternative to existing industrial production process. Yet, the electrocatalytic ethylene epoxidation route is currently hindered by largely insufficient activity, EO selectivity, and long-term stability. Here we report a single atom Ru-doped hollandite structure KIr4O8 (KIrRuO) nanowire catalyst for efficient EO production via a chloride-mediated ethylene epoxidation process. The KIrRuO catalyst exhibits an EO partial current density up to 0.7 A cm-2 and an EO yield as high as 92.0 %. The impressive electrocatalytic performance towards ethylene epoxidation is ascribed to the modulation of electronic structures of adjacent Ir sites by single Ru atoms, which stabilizes the *CH2CH2OH intermediate and facilitates the formation of active Cl2 species during the generation of 2-chloroethanol, the precursor of EO. This work provides a single atom modulation strategy for improving the reactivity of adjacent metal sites in heterogeneous electrocatalysts.

Addressing the Carbonate Issue: Electrocatalysts for Acidic CO2 Reduction Reaction

Electrochemical CO2 reduction reaction (CO2RR) powered by renewable energy provides a promising route to CO2 conversion and utilization. However, the widely used neutral/alkaline electrolyte consumes a large amount of CO2 to produce (bi)carbonate byproducts, leading to significant challenges at the device level, thereby impeding the further deployment of this reaction. Conducting CO2RR in acidic electrolytes offers a promising solution to address the "carbonate issue"; however, it presents inherent difficulties due to the competitive hydrogen evolution reaction, necessitating concerted efforts toward advanced catalyst and electrode designs to achieve high selectivity and activity. This review encompasses recent developments of acidic CO2RR, from mechanism elucidation to catalyst design and device engineering. This review begins by discussing the mechanistic understanding of the reaction pathway, laying the foundation for catalyst design in acidic CO2RR. Subsequently, an in-depth analysis of recent advancements in acidic CO2RR catalysts is provided, highlighting heterogeneous catalysts, surface immobilized molecular catalysts, and catalyst surface enhancement. Furthermore, the progress made in device-level applications is summarized, aiming to develop high-performance acidic CO2RR systems. Finally, the existing challenges and future directions in the design of acidic CO2RR catalysts are outlined, emphasizing the need for improved selectivity, activity, stability, and scalability.

High yield electrosynthesis of oxygenates from CO using a relay Cu-Ag co-catalyst system

As a sustainable alternative to fossil fuel-based manufacture of bulk oxygenates, electrochemical synthesis using CO and H2O as raw materials at ambient conditions offers immense appeal. However, the upscaling of the electrosynthesis of oxygenates encounters kinetic bottlenecks arising from the competing hydrogen evolution reaction with the selective production of ethylene. Herein, a catalytic relay system that can perform in tandem CO capture, activation, intermediate transfer and enrichment on a Cu-Ag composite catalyst is used for attaining high yield CO-to-oxygenates electrosynthesis at high current densities. The composite catalyst Cu/30Ag (molar ratio of Cu to Ag is 7:3) enables high efficiency CO-to-oxygenates conversion, attaining a maximum partial current density for oxygenates of 800 mA cm-2 at an applied current density of 1200 mA cm-2, and with 67 % selectivity. The ability to finely control the production of ethylene and oxygenates highlights the principle of efficient catalyst design based on the relay mechanism.

Revolutionizing CO2 Electrolysis: Fluent Gas Transportation within Hydrophobic Porous Cu2O

The success of electrochemical CO2 reduction at high current densities hinges on precise interfacial transportation and the local concentration of gaseous CO2. However, the creation of efficient CO2 transportation channels remains an unexplored frontier. In this study, we design and synthesize hydrophobic porous Cu2O spheres with varying pore sizes to unveil the nanoporous channel's impact on gas transfer and triple-phase interfaces. The hydrophobic channels not only facilitate rapid CO2 transportation but also trap compressed CO2 bubbles to form abundant and stable triple-phase interfaces, which are crucial for high-current-density electrocatalysis. In CO2 electrolysis, in situ spectroscopy and density functional theory results reveal that atomic edges of concave surfaces promote C-C coupling via an energetically favorable OC-COH pathway, leading to overwhelming CO2-to-C2+ conversion. Leveraging optimal gas transportation and active site exposure, the hydrophobic porous Cu2O with a 240 nm pore size (P-Cu2O-240) stands out among all the samples and exhibits the best CO2-to-C2+ productivity with remarkable Faradaic efficiency and formation rate up to 75.3 ± 3.1% and 2518.2 ± 8.1 μmol h-1 cm-2, respectively. This study introduces a novel paradigm for efficient electrocatalysts that concurrently addresses active site design and gas-transfer challenges.

Multiscale effects in tandem CO2 electrolysis to C2+ products

CO2 electrolysis is a sustainable technology capable of accelerating global decarbonisation through the production of high-value alternatives to fossil-derived products. CO2 conversion can generate critical multicarbon (C2+) products such as drop-in chemicals ethylene and ethanol, however achieving high selectivity from single-component catalysts is often limited by the competitive formation of C1 products. Tandem catalysis can overcome C2+ selectivity limitations through the incorporation of a component that generates a high concentration of CO, the primary reactant involved in the C-C coupling step to form C2+ products. A wide range of approaches to promote tandem CO2 electrolysis have been presented in recent literature that span atomic-scale manipulation to device-scale engineering. Therefore, an understanding of multiscale effects that contribute to selectivity alterations are required to develop effective tandem systems. In this review, we use relevant examples to highlight the complex and interlinked contributions to selectivity and provide an outlook for future development of tandem CO2 electrolysis systems.

A surface strategy boosting the ethylene selectivity for CO2 reduction and in situ mechanistic insights

Electrochemical reduction of carbon dioxide into ethylene, as opposed to traditional industrial methods, represents a more environmentally friendly and promising technical approach. However, achieving high activity of ethylene remains a huge challenge due to the numerous possible reaction pathways. Here, we construct a hierarchical nanoelectrode composed of CuO treated with dodecanethiol to achieve elevated ethylene activity with a Faradaic efficiency reaching 79.5%. Through on in situ investigations, it is observed that dodecanethiol modification not only facilitates CO2 transfer and enhances *CO coverage on the catalyst surfaces, but also stabilizes Cu(100) facet. Density functional theory calculations of activation energy barriers of the asymmetrical C-C coupling between *CO and *CHO further support that the greatly increased selectivity of ethylene is attributed to the thiol-stabilized Cu(100). Our findings not only provide an effective strategy to design and construct Cu-based catalysts for highly selective CO2 to ethylene, but also offer deep insights into the mechanism of CO2 to ethylene.

Acidic media enables oxygen-tolerant electrosynthesis of multicarbon products from simulated flue gas

Renewable electricity powered electrochemical CO2 reduction (CO2R) offers a valuable method to close the carbon cycle and reduce our overreliance on fossil fuels. However, high purity CO2 is usually required as feedstock, which potentially decreases the feasibility and economic viability of the process. Direct conversion of flue gas is an attractive option but is challenging due to the low CO2 concentration and the presence of O2 impurities. As a result, up to 99% of the applied current can be lost towards the undesired oxygen reduction reaction (ORR). Here, we show that acidic electrolyte can significantly suppress ORR on Cu, enabling generation of multicarbon products from simulated flue gas. Using a composite Cu and carbon supported single-atom Ni tandem electrocatalyst, we achieved a multicarbon Faradaic efficiency of 46.5% at 200 mA cm-2, which is ~20 times higher than bare Cu under alkaline conditions. We also demonstrate stable performance for 24 h with a multicarbon product full-cell energy efficiency of 14.6%. Strikingly, this result is comparable to previously reported acidic CO2R systems using pure CO2. Our findings demonstrate a potential pathway towards designing efficient electrolyzers for direct conversion of flue gas to value-added chemicals and fuels.

Elucidation of Critical Catalyst Layer Phenomena toward High Production Rates for the Electrochemical Conversion of CO to Ethylene

This work utilizes EIS to elucidate the impact of catalyst-ionomer interactions and cathode hydroxide ion transport resistance (RCL,OH-) on cell voltage and product selectivity for the electrochemical conversion of CO to ethylene. When using the same Cu catalyst and a Nafion ionomer, varying ink dispersion and electrode deposition methods results in a change of 2 orders of magnitude for RCL,OH- and ca. a 25% change in electrode porosity. Decreasing RCL,OH- results in improved ethylene Faradaic efficiency (FE), up to ∼57%, decrease in hydrogen FE, by ∼36%, and reduction in cell voltage by up to 1 V at 700 mA/cm2. Through the optimization of electrode fabrication conditions, we achieve a maximum of 48% ethylene with >90% FE for non-hydrogen products in a 25 cm2 membrane electrode assembly at 700 mA/cm2 and <3 V. Additionally, the implications of optimizing RCL,OH- is translated to other material requirements, such as anode porosity. We find that the best performing electrodes use ink dispersion and deposition techniques that project well into roll-to-roll processes, demonstrating the scalability of the optimized process.

Site-selective protonation enables efficient carbon monoxide electroreduction to acetate

Electrosynthesis of acetate from CO offers the prospect of a low-carbon-intensity route to this valuable chemical--but only once sufficient selectivity, reaction rate and stability are realized. It is a high priority to achieve the protonation of the relevant intermediates in a controlled fashion, and to achieve this while suppressing the competing hydrogen evolution reaction (HER) and while steering multicarbon (C2+) products to a single valuable product--an example of which is acetate. Here we report interface engineering to achieve solid/liquid/gas triple-phase interface regulation, and we find that it leads to site-selective protonation of intermediates and the preferential stabilization of the ketene intermediates: this, we find, leads to improved selectivity and energy efficiency toward acetate. Once we further tune the catalyst composition and also optimize for interfacial water management, we achieve a cadmium-copper catalyst that shows an acetate Faradaic efficiency (FE) of 75% with ultralow HER (<0.2% H2 FE) at 150 mA cm-2. We develop a high-pressure membrane electrode assembly system to increase CO coverage by controlling gas reactant distribution and achieve 86% acetate FE simultaneous with an acetate full-cell energy efficiency (EE) of 32%, the highest energy efficiency reported in direct acetate electrosynthesis.

Continuously Producing Highly Concentrated and Pure Acetic Acid Aqueous Solution via Direct Electroreduction of CO2

It is crucial to achieve continuous production of highly concentrated and pure C2 chemicals through the electrochemical CO2 reduction reaction (eCO2RR) for artificial carbon cycling, yet it has remained unattainable until now. Despite one-pot tandem catalysis (dividing the eCO2RR to C2 into two catalytical reactions of CO2 to CO and CO to C2) offering the potential for significantly enhancing reaction efficiency, its mechanism remains unclear and its performance is unsatisfactory. Herein, we selected different CO2-to-CO catalysts and CO-to-acetate catalysts to construct several tandem catalytic systems for the eCO2RR to acetic acid. Among them, a tandem catalytic system comprising a covalent organic framework (PcNi-DMTP) and a metal-organic framework (MAF-2) as CO2-to-CO and CO-to-acetate catalysts, respectively, exhibited a faradaic efficiency of 51.2% with a current density of 410 mA cm-2 and an ultrahigh acetate yield rate of 2.72 mmol m-2 s-1 under neutral conditions. After electrolysis for 200 h, 1 cm-2 working electrode can continuously produce 20 mM acetic acid aqueous solution with a relative purity of 95+%. Comprehensive studies revealed that the performance of tandem catalysts is influenced not only by the CO supply-demand relationship and electron competition between the two catalytic processes in the one-pot tandem system but also by the performance of the CO-to-C2 catalyst under diluted CO conditions.

Evoking C2+ production from electrochemical CO2 reduction by the steric confinement effect of ordered porous Cu2O

Selective conversion of carbon dioxide (CO2) to multi-carbon products (CO2-to-C2+) at high current densities is in essential demand for the practical application of the resultant valuable products, yet it remains challenging to conduct due to the lack of efficient electrocatalysts. Herein, three-dimensional ordered porous cuprous oxide cuboctahedra (3DOP Cu2O-CO) were designed and synthesized by a molecular fence-assisted hard templating approach. Capitalizing on the merits of interconnected and uniformly distributed pore channels, 3DOP Cu2O-CO exhibited outstanding electrochemical CO2-to-C2+ conversion, achieving faradaic efficiency and partial current density for C2+ products of up to 81.7% and -0.89 A cm-2, respectively, with an optimal formation rate of 2.92 mmol h-1 cm-2 under an applied current density of -1.2 A cm-2. In situ spectroscopy and simulation results demonstrated that the ordered pores of 3DOP Cu2O-CO can effectively confine and accumulate sufficient *CO adsorption during electrochemical CO2 reduction, which facilitates efficient dimerization for the formation of C2+ products. Furthermore, the 3DOP structure induces a higher local pH value, which not only enhances the C-C coupling reaction, but also suppresses competing H2 evolution.

High-Power CO2-to-C2 Electroreduction on Ga-Spaced, Square-like Cu Sites

The electrochemical conversion of CO2 into multicarbon (C2) products on Cu-based catalysts is strongly affected by the surface coverage of adsorbed CO (*CO) intermediates and the subsequent C-C coupling. However, the increased *CO coverage inevitably leads to strong *CO repulsion and a reduced C-C coupling efficiency, thus resulting in suboptimal CO2-to-C2 activity and selectivity, especially at ampere-level electrolysis current densities. Herein, we developed an atomically ordered Cu9Ga4 intermetallic compound consisting of Cu square-like binding sites interspaced by catalytically inert Ga atoms. Compared to Cu(100) previously known with a high C2 selectivity, the Ga-spaced, square-like Cu sites presented an elongated Cu-Cu distance that allowed to reduce *CO repulsion and increased *CO coverage simultaneously, thus endowing more efficient C-C coupling to C2 products than Cu(100) and Cu(111). The Cu9Ga4 catalyst exhibited an outstanding CO2-to-C2 electroreduction, with a peak C2 partial current density of 1207 mA cm-2 and a corresponding Faradaic efficiency of 71%. Moreover, the Cu9Ga4 catalyst demonstrated a high-power (∼200 W) electrolysis capability with excellent electrochemical stability.

Mechanistic Implications of Low CO Coverage on Cu in the Electrochemical CO and CO2 Reduction Reactions

Electrochemical CO or CO2 reduction reactions (CO(2)RR), powered by renewable energy, represent one of the promising strategies for upgrading CO2 to valuable products. To design efficient and selective catalysts for the CO(2)RR, a comprehensive mechanistic understanding is necessary, including a comprehensive understanding of the reaction network and the identity of kinetically relevant steps. Surface-adsorbed CO (COad) is the most commonly reported reaction intermediate in the CO(2)RR, and its surface coverage (θCO) and binding energy are proposed to be key to the catalytic performance. Recent experimental evidence sugguests that θCO on Cu electrode at electrochemical conditions is quite low (∼0.05 monolayer), while relatively high θCO is often assumed in literature mechanistic discussion. This Perspective briefly summarizes existing efforts in determining θCO on Cu surfaces, analyzes mechanistic impacts of low θCO on the reaction pathway and catalytic performance, and discusses potential fruitful future directions in advancing our understanding of the Cu-catalyzed CO(2)RR.

In Situ Studies of Copper-Based CO2 Reduction Electrocatalysts by Scanning Transmission Soft X-ray Microscopy

A microfluidic-enabled electrochemical device has been developed to investigate electrochemically active nanomaterials under reaction conditions using in situ scanning transmission soft X-ray microscopy (STXM). In situ STXM measurements were conducted on electrodeposited Cu catalysts under electrochemical CO2 reduction (CO2R) conditions. The study provides detailed, quantitative results about the changes in the morphology and chemical structure of the catalytic nanoparticles as a function of applied potentials. The deposited Cu nanoparticles initially contain both Cu(0) and Cu(I). As an increasingly cathodic potential is applied, the Cu(I) species gradually convert to Cu(0) over the potential range of +0.4 to 0 V versus the reversible hydrogen electrode (VRHE). During this process, Cu(I) particles of various sizes are converted to metallic Cu at different reaction rates and at slightly different potentials, indicating a degree of heterogeneity in the electrochemical response of discrete particles. At CO2R relevant potentials, only metallic Cu is observed, and the morphology of the particles is fairly stable within the spatial resolution limits of STXM (∼40 nm). We also report in situ STXM studies of a working electrode with relatively thick Cu-based electrodeposits. The spatially resolved chemical analysis identifies that Cu-oxide species can persist under CO2R conditions, but only when the catalytic nanoparticles are electronically isolated from the working electrode and therefore are catalytically irrelevant. In summary, in situ STXM is presented as a technique to gain advanced morphological and spatially resolved chemical structure insights into electrochemically active nanomaterials, which was used to provide improved understanding regarding Cu nanomaterial catalysts under CO2 reduction conditions.

Gerhardtite as a Precursor to an Efficient CO-to-Acetate Electroreduction Catalyst

Carbon-carbon coupling electrochemistry on a conventional copper (Cu) catalyst still undergoes low selectivity among many different multicarbon (C2+) chemicals, posing a grand challenge to achieve a single C2+ product. Here, we demonstrate a laser irradiation synthesis of a gerhardtite mineral, Cu2(OH)3NO3, as a catalyst precursor to make a Cu catalyst with abundant stacking faults under reducing conditions. Such structural perturbation modulates electronic microenvironments of Cu, leading to improved d-electron back-donation to the antibonding orbital of *CO intermediates and thus strengthening *CO adsorption. With increased *CO coverage on the defect-rich Cu, we report an acetate selectivity of 56 ± 2% (compared to 31 ± 1% for conventional Cu) and a partial current density of 222 ± 7 mA per square centimeter in CO electroreduction. When run at 400 mA per square centimeter for 40 h in a flow reactor, this catalyst produces 68.3 mmol of acetate throughout. This work highlights the value of a Cu-containing mineral phase in accessing suitable structures for improved selectivity to a single desired C2+ product.

Directing the Selectivity of CO Electrolysis to Acetate by Constructing Metal-Organic Interfaces

Electrochemically converting CO2 to valuable chemicals holds great promise for closing the anthropogenic carbon cycle. Owing to complex reaction pathways and shared rate-determining steps, directing the selectivity of CO2 /CO electrolysis to a specific multicarbon product is very challenging. We report here a strategy for highly selective production of acetate from CO electrolysis by constructing metal-organic interfaces. We demonstrate that the Cu-organic interfaces constructed by in situ reconstruction of Cu complexes show very impressive acetate selectivity, with a high Faradaic efficiency of 84.2 % and a carbon selectivity of 92.1 % for acetate production, in an alkaline membrane electrode assembly electrolyzer. The maximum acetate partial current density and acetate yield reach as high as 605 mA cm-2 and 63.4 %, respectively. Thorough structural characterizations, control experiments, operando Raman spectroscopy measurements, and density functional theory calculation results indicate that the Cu-organic interface creates a favorable reaction microenvironment that enhances *CO adsorption, lowers the energy barrier for C-C coupling, and facilitates the formation of CH3 COOH over other multicarbon products, thus rationalizing the selective acetate production.

Promoting CO2 Electroreduction to Acetate by an Amine-Terminal, Dendrimer-Functionalized Cu Catalyst

Acetate derived from electrocatalytic CO2 reduction represents a potential low-carbon synthesis approach. However, the CO2-to-acetate activity and selectivity are largely inhibited by the low surface coverage of in situ generated *CO, as well as the inefficient ethenone intermediate formation due to the side reaction between CO2 and alkaline electrolytes. Tuning catalyst microenvironments by chemical modification of the catalyst surface is a potential strategy to enhance CO2 capture and increase local *CO concentrations, while it also increases the selectivity of side reduction products, such as methane or ethylene. To solve this challenge, herein, we developed a hydrophilic amine-tailed, dendrimer network with enhanced *CO intermediate coverage on Cu catalytic sites while at the same time retaining the in situ generated OH- as a high local pH environment that favors the ethenone intermediate toward acetate. The optimized amine-network coordinated Cu catalyst (G3-NH2/Cu) exhibits one of the highest CO2-to-acetate Faradaic efficiencies of 47.0% with a partial current density of 202 mA cm-2 at -0.97 V versus the reversible hydrogen electrode.

Insights into the mechanism in electrochemical CO2 reduction over single-atom copper alloy catalysts: A DFT study

Copper single-atom alloy catalysts (M@Cu SAAs) have shown great promise for electrochemical CO2 reduction reaction (CO2RR). However, a clear understanding of the CO2RR process on M@Cu SAAs is still lacking. This study uses density functional theoretical (DFT) calculations to obtain a comprehensive mechanism and the origin of activity of M@Cu SAAs. The importance of the adsorption mode of M@Cu is revealed: key intermediates either adsorbed in the adjacent hollow site around Cu atoms (AD mode) or adsorbed directly on the top site of M (SE mode). AD mode generally exhibits finely tuned binding strengths of key intermediates, which significantly enhances the activity of the catalysts. Increasing the coverage of ∗CO on the M@Cu with SE mode leads to relocation of the active site, resulting in improved activity of C2 products. The insights gained in this work have significant implications for rational design strategy toward efficient CO2RR electrocatalysts.

Tuning N coordination environment in AgNC single atom catalysts for efficient electrochemical CO2-to-Ethanol conversion

The rational modulation of the coordination environment of the active center of single-atom catalysts (SACs) is a potential strategy to improve the activity and selectivity of electrocatalytic CO2 reduction. We developed a series of single-atom Ag-anchored on N-doped carbon catalysts with different N-coordination environments by regulating the carbon source (glucose/graphene oxide, GO). Results demonstrated that the CO2-to-CO/C2H5OH pathway can be converted through reasonable regulation of the content of pyrrolic N and pyridine N. At a highly dominant pyrrolic N content of 40.0 %, the electrocatalyst of Ag-N-rGO0.025 M facilitates the formation of CO with a Faradaic efficiency (FE) of 69.7 %. By contrast, when the pyridine N content is 70.5 %, the best Ag-N-C0.05 M catalyst in the H-cell shows an FE of 42 % for C2H5OH at -0.97 V vs. RHE; moreover, the FE is stable at 40 % within 31 h. Density functional theory calculations further confirm that the pyrrolic N site is conducive to the conversion of CO2 to intermediate *CO, whereas pyridine N exhibits the highest binding energy (ΔG = -4.73 eV) to hold *CO intermediates for dimerization, consequently steering the pathway to the formation of C2H5OH. The results of this study provide novel insights into the rational design of carbon-based SACs based on efficient electrochemical CO2 reduction reaction and the regulation of CO2 selectivity and activity for C2H5OH production.

Oxophilicity-Controlled CO2 Electroreduction to C2+ Alcohols over Lewis Acid Metal-Doped Cuδ+ Catalysts

Cu-based electrocatalysts have great potential for facilitating CO2 reduction to produce energy-intensive fuels and chemicals. However, it remains challenging to obtain high product selectivity due to the inevitable strong competition among various pathways. Here, we propose a strategy to regulate the adsorption of oxygen-associated active species on Cu by introducing an oxophilic metal, which can effectively improve the selectivity of C2+ alcohols. Theoretical calculations manifested that doping of Lewis acid metal Al into Cu can affect the C-O bond and Cu-C bond breaking toward the selectively determining intermediate (shared by ethanol and ethylene), thus prioritizing the ethanol pathway. Experimentally, the Al-doped Cu catalyst exhibited an outstanding C2+ Faradaic efficiency (FE) of 84.5% with remarkable stability. In particular, the C2+ alcohol FE could reach 55.2% with a partial current density of 354.2 mA cm-2 and a formation rate of 1066.8 μmol cm-2 h-1. A detailed experimental study revealed that Al doping improved the adsorption strength of active oxygen species on the Cu surface and stabilized the key intermediate *OC2H5, leading to high selectivity toward ethanol. Further investigation showed that this strategy could also be extended to other Lewis acid metals.

Phase Engineering and Synchrotron-Based Study on Two-Dimensional Energy Nanomaterials

In recent years, there has been significant interest in the development of two-dimensional (2D) nanomaterials with unique physicochemical properties for various energy applications. These properties are often derived from the phase structures established through a range of physical and chemical design strategies. A concrete analysis of the phase structures and real reaction mechanisms of 2D energy nanomaterials requires advanced characterization methods that offer valuable information as much as possible. Here, we present a comprehensive review on the phase engineering of typical 2D nanomaterials with the focus of synchrotron radiation characterizations. In particular, the intrinsic defects, atomic doping, intercalation, and heterogeneous interfaces on 2D nanomaterials are introduced, together with their applications in energy-related fields. Among them, synchrotron-based multiple spectroscopic techniques are emphasized to reveal their intrinsic phases and structures. More importantly, various in situ methods are employed to provide deep insights into their structural evolutions under working conditions or reaction processes of 2D energy nanomaterials. Finally, conclusions and research perspectives on the future outlook for the further development of 2D energy nanomaterials and synchrotron radiation light sources and integrated techniques are discussed.

Multiscale CO2 Electrocatalysis to C2+ Products: Reaction Mechanisms, Catalyst Design, and Device Fabrication

Electrosynthesis of value-added chemicals, directly from CO2, could foster achievement of carbon neutral through an alternative electrical approach to the energy-intensive thermochemical industry for carbon utilization. Progress in this area, based on electrogeneration of multicarbon products through CO2 electroreduction, however, lags far behind that for C1 products. Reaction routes are complicated and kinetics are slow with scale up to the high levels required for commercialization, posing significant problems. In this review, we identify and summarize state-of-art progress in multicarbon synthesis with a multiscale perspective and discuss current hurdles to be resolved for multicarbon generation from CO2 reduction including atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes, and macroscale electrolyzers with guidelines for future research. The review ends with a cross-scale perspective that links discrepancies between different approaches with extensions to performance and stability issues that arise from extensions to an industrial environment.