Divergent Paths, Same Goal: A Pair-Electrosynthesis Tactic for Cost-Efficient and Exclusive Formate Production by Metal-Organic-Framework-Derived 2D Electrocatalysts

Self-supported bimetallic array superstructures for high-performance coupling electrosynthesis of formate and adipate

The coupling electrosynthesis involving CO2 upgrade conversion is of great significance for the sustainable development of the environment and energy but is challenging. Herein, we exquisitely constructed the self-supported bimetallic array superstructures from the Cu(OH)2 array architecture precursor, which can enable high-performance coupling electrosynthesis of formate and adipate at the anode and the cathode, respectively. Concretely, the faradaic efficiencies (FEs) of CO2-to-formate and cyclohexanone-to-adipate conversion simultaneously exceed 90% at both electrodes with excellent stabilities. Such high-performance coupling electrosynthesis is highly correlated with the porous nanosheet array superstructure of CuBi alloy as the cathode and the nanosheet-on-nanowire array superstructure of CuNi hydroxide as the anode. Moreover, compared to the conventional electrolysis process, the cell voltage is substantially reduced while maintaining the electrocatalytic performance for coupling electrosynthesis in the two-electrode electrolyzer with the maximal FEformate and FEadipate up to 94.2% and 93.1%, respectively. The experimental results further demonstrate that the bimetal composition modulates the local electronic structures, promoting the reactions toward the target products. Prospectively, our work proposes an instructive strategy for constructing adaptive self-supported superstructures to achieve efficient coupling electrosynthesis.

Synergistic effects of CuS/TiO2 heterointerfaces: Enhanced cathodic CO2 reduction and anodic CH3OH oxidation for paired electrosynthesis of formate

The electrochemical reduction of carbon dioxide into energy-carrying compounds or value-added chemicals is of great significance for diminishing the greenhouse effect. However, it is still imperative to replace the less-value anodic oxygen evolution reaction (OER) to improve the technical economy. Herein, we firstly reported a bifunctional CuS/TiO2 catalyst for both anodic methanol oxidation reaction (MOR) and cathodic carbon dioxide reduction (CO2R). The in-built abundant CuS/TiO2 heterointerfaces are found to boost the CO2R and MOR to produce formate. Based on the unique bifunctionality of CuS/TiO2, a paired electrosynthesis of formate was performed with a total Faradaic efficiency (FE) of about 170 %, in which the cathodic CO2R achieved a formate FE of about 70 %, and the anodic MOR exhibited an almost 100 % formate FE.

Trivalent Nickel-Catalyzing Electroconversion of Alcohols to Carboxylic Acids

The comprehension of activity and selectivity origins of the electrooxidation of organics is a crucial knot for the development of a highly efficient energy conversion system that can produce value-added chemicals on both the anode and cathode. Here, we find that the potential-retaining trivalent nickel in NiOOH (Fermi level, -7.4 eV) is capable of selectively oxidizing various primary alcohols to carboxylic acids through a nucleophilic attack and nonredox electron transfer process. This nonredox trivalent nickel is highly efficient in oxidizing primary alcohols (methanol, ethanol, propanol, butanol, and benzyl alcohol) that are equipped with the appropriate highest occupied molecular orbital (HOMO) levels (-7.1 to -6.5 eV vs vacuum level) and the negative dual local softness values (Δsk, -0.50 to -0.19) of nucleophilic atoms in nucleophilic hydroxyl functional groups. However, the carboxylic acid products exhibit a deeper HOMO level (<-7.4 eV) or a positive Δsk, suggesting that they are highly stable and weakly nucleophilic on NiOOH. The combination (HOMO, Δsk) is useful in explaining the activity and selectivity origins of electrochemically oxidizing alcohols to carboxylic acid. Our findings are valuable in creating efficient energy conversions to generate value-added chemicals on dual electrodes.

Supramolecular assemblies of Cu(II) with a tetraphenylethene-imidazole ligand for tuning photocatalytic CO2 reduction

Cu(II) supramolecular assemblies [Cu2(tipe)2(H2O)2](NO3)4·2.5H2O (CuN4) and [Cu2Cl4(tipe)(CH3CN)]·H2O (CuN2Cl2) (tipe = 1,1,2,2-tetrakis(4-(imidazole-1-yl)phenyl)ethene) were synthesized and utilized for photocatalytic CO2 reduction. CuN4 exhibits CO production of up to 891 μmol gcat-1 with a selectivity of 79.9%, while CuN2Cl2 gives low CO production of 206 μmol gcat-1 but with a high selectivity of >99.9% in 5 h. The experimental and DFT calculation results indicate that the coordination environment and non-covalent interactions within the assemblies have a great impact on the photocatalytic CO2 reduction behavior. This work provides useful insights on Cu(II) assembly catalyzed CO2 photoreduction.

Electrocatalytic CO2 Reduction to C2+ Products in Flow Cells

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

Catalysts and electrolyzers for the electrochemical CO2 reduction reaction: from laboratory to industrial applications

To cope with the urgent environmental pressure and tight energy demand, using electrocatalytic methods to drive the reduction of carbon dioxide molecules and produce a variety of fuels and chemicals, is one of the effective pathways to achieve carbon neutrality. In recent years, many significant advances in the study of the electrochemical carbon dioxide reduction reaction (CO2RR) have been made, but most of the works exhibit low current density, small electrode area and poor long-term stability, which are not suitable for large-scale industrial applications. Herein, combining the research achievements obtained in laboratories and the practical demand of industrial production, we summarize recent frontier progress in the field of the electrochemical CO2RR, including the fundamentals of catalytic reactions, catalyst design and preparation, and the construction of electrolyzers. In addition, we discuss the bottleneck problem of industrial CO2 electrolysis, and further present the prospect of the essential issues to be solved by the available technology for industrial electrolysis. This review can provide some basic understanding and knowledge accumulation for the development and practical application of electrochemical CO2RR technology.

Enhanced CO2 Electroreduction Selectivity toward Ethylene on Pyrazolate-Stabilized Asymmetric Ni-Cu Hybrid Sites

Metal-organic frameworks (MOFs) possess well-defined, designable structures, holding great potential in enhancing product selectivity for electrochemical CO2 reduction (CO2R) through active site engineering. Here, we report a novel MOF catalyst featuring pyrazolate-stabilized asymmetric Ni/Cu sites, which not only maintains structural stability under harsh electrochemical conditions but also exhibits extraordinarily high ethylene (C2H4) selectivity during CO2R. At a cathode potential of -1.3 V versus RHE, our MOF catalyst, denoted as Cu1Ni-BDP, manifests a C2H4 Faradaic efficiency (FE) of 52.7% with an overall current density of 0.53 A cm-2 in 1.0 M KOH electrolyte, surpassing that on prevailing Cu-based catalysts. More remarkably, the Cu1Ni-BDP MOF exhibits a stable performance with only 4.5% reduction in C2H4 FE during 25 h of CO2 electrolysis. A suite of characterization tools─such as high-resolution transmission electron microscopy, X-ray absorption spectroscopy, operando X-ray diffraction, and infrared spectroscopy─and density functional theory calculations collectively reveal that the cubic pyrazolate-metal coordination structure and the asymmetric Ni-Cu sites in the MOF catalyst synergistically facilitate the stable formation of C2H4 from CO2.

Ni-Carbon Microtube/Polytetrafluoroethylene as Flexible Electrothermal Microwave Absorbers

Flexible microwave absorbers with Joule heating performance are urgently desired to meet the demands of extreme service environments. Herein, a type of flexible composite film is constructed by homogeneously dispersing a hierarchical Ni-carbon microtube (Ni/CMT) into a processable polytetrafluoroethylene (PTFE) matrix. The Ni/CMT are interconnected into a 3D conductive network, in which the huge interior cavity of the carbon microtube (CMT) improves impedance matching and provides additional hyper channels for electromagnetic (EM) waves dissipation, and the hierarchical magnetic Ni nanoparticles enhance the synergistic interactions between confined heterogeneous interfaces. Such an ingenious structure endows the composites with excellent electrothermal performance and improves their serviceability for application under extreme environments. Moreover, under a low fill loading of 3 wt.%, the Ni/CMT/PTFE (NCP) can achieve excellent low-frequency microwave absorption (MA) property with a minimum reflection loss of -59.12 dB at 5.92 GHz, which covers almost the entire C-band. Relying on their brilliant MA property, an EM sensor is designed and achieved by the resonance coupling of the patterned NCP. This work opens up a new way for the design of next-generation microwave absorbers that meet the requirements of EM packaging, proofing water and removing ice, fire safety, and health monitoring.

Designs of Tandem Catalysts and Cascade Catalytic Systems for CO2 Upgrading

Upgrading CO2 into multi-carbon (C2+) compounds through the CO2 reduction reaction (CO2 RR) offers a practical approach to mitigate atmospheric CO2 while simultaneously producing high value chemicals. The reaction pathways for C2+ production involve multi-step proton-coupled electron transfer (PCET) and C-C coupling processes. By increasing the surface coverage of adsorbed protons (*Had ) and *CO intermediates, the reaction kinetics of PCET and C-C coupling can be accelerated, thereby promoting C2+ production. However, *Had and *CO are competitively adsorbed intermediates on monocomponent catalysts, making it difficult to break the linear scaling relationship between the adsorption energies of the *Had /*CO intermediate. Recently, tandem catalysts consisting of multicomponents have been developed to improve the surface coverage of *Had or *CO by enhancing water dissociation or CO2 -to-CO production on auxiliary sites. In this context, we provide a comprehensive overview of the design principles of tandem catalysts based on reaction pathways for C2+ products. Moreover, the development of cascade CO2 RR catalytic systems that integrate CO2 RR with downstream catalysis has expanded the range of potential CO2 upgrading products. Therefore, we also discuss recent advancements in cascade CO2 RR catalytic systems, highlighting the challenges and perspectives in these systems.

Boosting nucleophilic attack to realize high current density biomass valorization on a tunable Prussian blue analogue

Electrochemical biomass valorization provides a promising approach to generating value-added chemicals. Herein, we have creatively utilized a Prussian blue analogue as a structure template of the anodic catalyst and improved its catalyst capacity by adjusting its electronic structure. The nickel-based Prussian blue analogue/Ni foam (NiFe-PBA/NF) exhibits excellent performance for methanol (MeOH) oxidation and achieves almost 94.1% FE of formic acid at a high current density of 500 mA cm-2. Apart from formic acid, NiFe-PBA/NF also has good catalytic ability for ethanol, glycerol, glucose, and 5-hydroxymethylfurfural (HMF). In short, this work has developed a promising class of catalysts for biomass valorization.

Modulating the Density of Catalytic Sites in Multiple-Component Covalent Organic Frameworks for Electrocatalytic Carbon Dioxide Reduction

It is generally assumed that the more metal atoms in covalent organic frameworks (COFs) contribute to higher activity toward electrocatalytic carbon dioxide reduction (CO2RR) and hindered us in exploring the correlation between the density of catalytic sites and catalytic performances. Herein, we have constructed quantitative density of catalytic sites in multiple COFs for CO2RR, in which the contents of phthalocyanine (H2Pc) and nickel phthalocyanine (NiPc) units were preciously controlled. With a molar ratio of 1/1 for the H2Pc and NiPc units in COFs, the catalyst achieved the highest selectivity with a carbon monoxide Faradaic efficiency (FECO) of 95.37% and activity with a turnover frequency (TOF) of 4713.53 h-1. In the multiple H2Pc/NiPc-COFs, the electron-donating features of the H2Pc units provide electron transport to the NiPc centers and thus improved the binding ability of CO2 and intermediates on the NiPc units. The theoretical calculation further confirmed that the H2Pc units donated their electrons to the NiPc units in the frameworks, enhanced the electron density of the Ni sites, and improved the binding ability with Lewis acidic CO2 molecules, thereby boosting the CO2RR performance. This study provides us with new insight into the design of highly active catalysts in electrocatalytic systems.

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.

Bi-MOF-Derived Carbon Wrapped Bi Nanoparticles Assembly on Flexible Graphene Paper Electrode for Electrochemical Sensing of Multiple Heavy Metal Ions

The development of nanohybrid with high electrocatalytic activity is of great significance for electrochemical sensing applications. In this work, we develop a novel and facile method to prepare a high-performance flexible nanohybrid paper electrode, based on nitrogen-doped carbon (NC) wrapped Bi nanoparticles (Bi-NPs) assembly derived from Bi-MOF, which are decorated on a flexible and freestanding graphene paper (GP) electrode. The as-obtained Bi-NPs encapsulated by an NC layer are uniform, and the active sites are increased by introducing a nitrogen source while preparing Bi-MOF. Owing to the synergistic effect between the high conductivity of GP electrode and the highly efficient electrocatalytic activity of Bi-NPs, the NC wrapped Bi-NPs (Bi-NPs@NC) modified GP (Bi-NPs@NC/GP) electrode possesses high electrochemically active area, rapid electron-transfer capability, and good electrochemical stability. To demonstrate its outstanding functionality, the Bi-NPs@NC/GP electrode has been integrated into a handheld electrochemical sensor for detecting heavy metal ions. The result shows that Zn²⁺, Cd²⁺, and Pb²⁺ can be detected with extremely low detection limits, wide linear range, high sensitivity, as well as good selectivity. Furthermore, it demonstrates outstanding electrochemical sensing performance in the simultaneous detection of Zn²⁺, Cd²⁺, and Pb²⁺. Finally, the proposed electrochemical sensor has achieved excellent repeatability, reproducibility, stability, and reliability in measuring real water samples, which will have great potential in advanced applications in environmental systems.

Simultaneous Anodic and Cathodic Formate Production in a Paired Electrolyzer by CO2 Reduction and Glycerol Oxidation

Electrochemical CO₂ conversion is a key technology to promote the production of carbon-containing molecules, alongside reducing CO₂ emissions leading to a closed carbon cycle economy. Over the past decade, the interest to develop selective and active electrochemical devices for electrochemical CO₂ reduction emerged. However, most reports employ oxygen evolution reaction as an anodic half-cell reaction causing the system to suffer from sluggish kinetics with no production of value-added chemicals. Therefore, this study reports a conceptualized paired electrolyzer for simultaneous anodic and cathodic formate production at high currents. To achieve this, CO₂ reduction was coupled with glycerol oxidation: a BiOBr-modified gas-diffusion cathode and a Ni_x B on Ni foam anode keep their selectivity for formate in the paired electrolyzer compared to the half-cell measurements. The paired reactor here reaches a combined Faradaic efficiency for formate of 141 % (45 % anode and 96 % cathode) at a current density of 200 mA cm^-2 .

Atomic bismuth induced ensemble sites with indium towards highly efficient and stable electrocatalytic reduction of carbon dioxide

Structural reconstruction is commonly observed during electrocatalytic CO₂ reduction (CO₂RR) process. However, the proper modulation of interface and defect sites remains challenging with the mechanism understanding to realize the favorable electrocatalysis. Herein, the atomic bridging of bismuth with indium atoms is elaborately designed for improving electrocatalysis of CO₂RR via electrochemical reduction and in situ anchoring strategy. As revealed by in situ structure analysis and theoretical studies, the ensemble sites supported on carbon matrix enable the charge density gradient to significantly promote the adsorption of *OCHO intermediate by the regulation of σ bonding and π* back-donation. Consequently, such unique electrocatalyst achieves the high formate faradaic efficiency of 95.1% over the entire potential range tested and the long-lived stability for 9 d. With coupling of CO₂RR, the solar-driven full cell demonstrates the spontaneous production of formate and 2,5-furandicarboxylic acid via the efficient oxidation of 5-hydroxymethylfurfural with an outstanding yield of 88.2%, highlighting the impressive solar-to-fuel conversion selectivity. Monitoring and understanding the intrinsic active sites of biatomic bridge are crucial to elucidate the synergic electrocatalysis for rationally designing high-performance electrocatalysts.

Selective CO2 Electroreduction to Formate on Polypyrrole-Modified Oxygen Vacancy-Rich Bi2 O3 Nanosheet Precatalysts by Local Microenvironment Modulation

Challenges remain in the development of highly efficient catalysts for selective electrochemical transformation of carbon dioxide (CO₂ ) to high-valued hydrocarbons. In this study, oxygen vacancy-rich Bi₂ O₃ nanosheets coated with polypyrrole (Bi₂ O₃ @PPy NSs) are designed and synthesized, as precatalysts for selective electrocatalytic CO₂ reduction to formate. Systematic material characterization demonstrated that Bi₂ O₃ @PPy precatalyst can evolve intoBi₂ O₂ CO₃ @PPy nanosheets with rich oxygen vacancies (Bi₂ O₂ CO₃ @PPy NSs) via electrolyte-mediated conversion and function as the real active catalyst for CO₂ reduction reaction electrocatalysis. Coating catalyst with a PPy shell can modulate the interfacial microenvironment of active sites, which work in coordination with rich oxygen vacancies in Bi₂ O₂ CO₃ and efficiently mediate directional selective CO₂ reduction toward formate formation. With the fine-tuning of interfacial microenvironment, the optimized Bi₂ O₃ @PPy-2 NSs derived Bi₂ O₂ CO₃ @PPy-2 NSs exhibit a maximum Faradaic efficiency of 95.8% at -0.8 V (versus. reversible hydrogen electrode) for formate production. This work might shed some light on designing advanced catalysts toward selective electrocatalytic CO₂ reduction through local microenvironment engineering.

Inhibitor and Activator: Dual Role of Subsurface Sulfide Enables Selective and Efficient Electro-Oxidation of Methanol to Formate on CuS@CuO Core-Shell Nanosheet Arrays

Selective electro-oxidation of aliphatic alcohols into value-added carboxylates at lower potentials than that of the oxygen evolution reaction (OER) is an environmentally and economically desirable anode reaction for clean energy storage and conversion technologies. However, it is challenging to achieve both high selectivity and high activity of the catalysts for the electro-oxidation of alcohols, such as the methanol oxidation reaction (MOR). Herein, a monolithic CuS@CuO/copper-foam electrode for the MOR with superior catalytic activity and almost 100% selectivity for formate is reported. In the core-shell CuS@CuO nanosheet arrays, the surface CuO directly catalyzes MOR, while the subsurface sulfide not only serves as an inhibitor to attenuate the oxidative power of the surface CuO to achieve selective oxidation of methanol to formate and prevent over-oxidation of formate to CO₂ but also serves as an activator to form more surface O defects as active sites and enhances the methanol adsorption and charge transfer to achieve superior catalytic activity. CuS@CuO/copper-foam electrodes can be prepared on a large scale by electro-oxidation of copper-foam at ambient conditions and can be readily utilized in clean energy technologies.

Engineering Under-Coordinated Active Sites with Tailored Chemical Microenvironments over Mosaic Bismuth Nanosheets for Selective CO2 Electroreduction to Formate

Selective electrochemical reduction of CO₂ into fuels or chemical feedstocks is a promising avenue to achieve carbon-neutral goal, but its development is severely limited by the lack of highly efficient electrocatalysts. Herein, cation-exchange strategy is combined with electrochemical self-reconstruction strategy to successfully develop diethylenetriamine-functionalized mosaic Bi nanosheets (mBi-DETA NSs) for selective electrocatalytic CO₂ reduction to formate, delivering a superior formate Faradaic efficiency of 96.87% at a low potential of -0.8 V_RHE . Mosaic nanosheet morphology of Bi can sufficiently expose the under-coordinated Bi active sites and promote the activation of CO₂ molecules to form the OCHO^- * intermediate. Moreover, in situ attenuated total reflectance infrared spectra further corroborate that surface chemical microenvironment modulation of mosaic Bi nanosheets via DETA functionalization can improve CO₂ adsorption on the catalyst surface and stabilize the key intermediate (OCHO^- *) due to the presence of amine groups, thus facilitate the CO₂ -to-HCOO^- reaction kinetics and promote formate formation.

Metal-Organic Frameworks for Greenhouse Gas Applications

Using petrol to supply energy for a car or burning coal to heat a building generates plenty of greenhouse gas (GHG) emissions, including carbon dioxide (CO₂ ), water vapor (H₂ O), methane (CH₄ ), nitrous oxide (N₂ O), ozone (O₃ ), fluorinated gases. These up-and-coming metal-organic frameworks (MOFs) are structurally endowed with rigid inorganic nodes and versatile organic linkers, which have been extensively used in the GHG-related applications to improve the lives and protect the environment. Porous MOF materials and their derivatives have been demonstrated to be competitive and promising candidates for GHG separation, storage and conversions as they shows facile preparation, large porosity, adjustable nanostructure, abundant topology, and tunable physicochemical property. Enormous progress has been made in GHG storage and separation intrinsically stemmed from the different interaction between guest molecule and host framework from MOF itself in the recent five years. Meanwhile, the use of porous MOF materials to transform GHG and the influence of external conditions on the adsorption performance of MOFs for GHG are also enclosed. In this review, it is also highlighted that the existing challenges and future directions are discussed and envisioned in the rational design, facile synthesis and comprehensive utilization of MOFs and their derivatives for practical applications.

Coupling Value-Added Anodic Reactions with Electrocatalytic CO2 Reduction

Electrocatalytic CO₂ reduction features a promising approach to realize carbon neutrality. However, its competitiveness is limited by the sluggish oxygen evolution reaction (OER) at anode, which consumes a large portion of energy. Coupling value-added anodic reactions with CO₂ electroreduction has been emerging as a promising strategy in recent years to enhance the full-cell energy efficiency and produce valuable chemicals at both cathode and anode of the electrolyzer. This review briefly summarizes recent progresses on the electrocatalytic CO₂ reduction, and the economic feasibility of different CO₂ electrolysis systems is discussed. Then a comprehensive summary of recent advances in the coupled electrolysis of CO₂ and potential value-added anodic reactions is provided, with special focus on the specific cell designs. Finally, current challenges and future opportunities for the coupled electrolysis systems are proposed, which are targeted to facilitate progress in this field and push the CO₂ electrolyzers to a more practical level.

Iridium-incorporated Co3O4 with lattice expansion for energy-efficient green hydrogen production coupled with glycerol valorization

The iridium-incorporated Co₃O₄ (Ir-Co₃O₄) catalyst is obtained from binary CoIr-based metal-organic framework precursors via controlled calcination treatment. Structural characterization reveals that in situ incorporation of Ir cations can cause lattice expansion of Co₃O₄ and regulate its electronic structure, thus in turn favoring electrocatalytic performance improvement. With a lattice expansion-induced strain effect, the Ir-Co₃O₄ catalyst shows superior performance for both electrocatalytic glycerol-to-formate conversion and the hydrogen evolution reaction.

Self-Supporting Metal-Organic Framework-Based Nanoarrays for Electrocatalysis

The replacement of powdery catalysts with self-supporting alternatives for catalyzing various electrochemical reactions is extremely important for the large-scale commercial application of renewable energy storage and conversion technologies. Metal-organic framework (MOF)-based nanoarrays possess tunable compositions, well-defined structure, abundant active sites, effective mass and electron transport, etc., which enable them to exhibit superior electrocatalytic performance in multiple electrochemical reactions. This review presents the latest research progress in developing MOF-based nanoarrays for electrocatalysis. We first highlight the structural features and electrocatalytic advantages of MOF-based nanoarrays, followed by a detailed summary of the design and synthesis strategies of MOF-based nanoarrays, and then describe the recent progress of their application in various electrocatalytic reactions. Finally, the challenges and perspectives are discussed, where further exploration into MOF-based nanoarrays will facilitate the development of electrochemical energy conversion technologies.

Electrocatalytic Reduction of CO2 Coupled with Organic Conversion to Selectively Synthesize High-Value Chemicals

The electrochemical process of coupling electrocatalytic CO₂ reduction and organic conversion reaction can effectively reduce the reaction overpotential and obtain value-added chemicals. Moreover, because of the diversity of substrates and the designability of coupling forms, more and more attention has been paid to this field. This review systematically summarizes the research progress of coupling electrolysis in recent years, (1) co-electrolysis of CO₂ and organics at the cathode to obtain specific products with high selectivity, (2) replacing traditional anodic oxygen evolution reaction (OER) with other valuable oxidation reactions to improve energy utilization efficiency and economic benefits of CO₂ conversion, (3) in an electrolytic cell without membrane, the cathode and anode jointly transform CO₂ and organics to redox products. We hope that the examples and insights on coupling electrolysis introduced in this review can inspire researchers to further explore and innovate in this direction.

A Pair-Electrosynthesis for Formate at Ultra-Low Voltage Via Coupling of CO2 Reduction and Formaldehyde Oxidation

Formate can be synthesized electrochemically by CO₂ reduction reaction (CO₂RR) or formaldehyde oxidation reaction (FOR). The CO₂RR approach suffers from kinetic-sluggish oxygen evolution reaction at the anode. To this end, an electrochemical system combining cathodic CO₂RR with anodic FOR was developed, which enables the formate electrosynthesis at ultra-low voltage. Cathodic CO₂RR employing the BiOCl electrode in H-cell exhibited formate Faradaic efficiency (FE) higher than 90% within a wide potential range from - 0.48 to - 1.32 V_RHE. In flow cell, the current density of 100 mA cm^-2 was achieved at - 0.67 V_RHE. The anodic FOR using the Cu₂O electrode displayed a low onset potential of - 0.13 V_RHE and nearly 100% formate and H₂ selectivity from 0.05 to 0.35 V_RHE. The CO₂RR and FOR were constructed in a flow cell through membrane electrode assembly for the electrosynthesis of formate, where the CO₂RR//FOR delivered an enhanced current density of 100 mA cm^-2 at 0.86 V. This work provides a promising pair-electrosynthesis of value-added chemicals with high FE and low energy consumption.

The Critical Role of Initial/Operando Oxygen Loading in General Bismuth-Based Catalysts for Electroreduction of Carbon Dioxide

Operando reconstruction of solid catalyst into a distinct active state frequently occurs during electrocatalytic processes. The correlation between initial and operando states, if ever existing, is critical for the understanding and precise design of a catalytic system. Inspired by recently established intermediate metallic state of Bi-based catalysts during electrocatalytic carbon dioxide reduction (CO₂RR), here we investigate a series of Bi oxide catalysts (Bi, Bi₂O₃, BiO₂) and demonstrate that the operando surface/subsurface oxygen loading, positively correlated to the initial oxygen content, plays a critical role in determining Bi-based CO₂RR performance. Higher initial oxygen loading indicates a better electrocatalytic efficiency. Further analysis shows that this conclusion generally applies to all Bi-based electrocatalysts reported up to date. Following this principle, cost-effective BiO₂ nanocrystals demonstrated the highest formate Faradaic efficiency (FE) and current density compared to Bi/Bi₂O₃, further allowing a pair-electrolysis system with 800 mA/cm² current density and an overall 175% FE for formate production.

In situ grown oxygen-vacancy-rich copper oxide nanosheets on a copper foam electrode afford the selective oxidation of alcohols to value-added chemicals

Selective oxidation of low-molecular-weight aliphatic alcohols like methanol and ethanol into carboxylates in acid/base hybrid electrolytic cells offers reduced process operating costs for the generation of fuels and value-added chemicals, which is environmentally and economically more desirable than their full oxidation to CO₂. Herein, we report the in-situ fabrication of oxygen-vacancies-rich CuO nanosheets on a copper foam (CF) via a simple ultrasonication-assisted acid-etching method. The CuO/CF monolith electrode enables efficient and selective electrooxidation of ethanol and methanol into value-added acetate and formate with ~100% selectivity. First principles calculations reveal that oxygen vacancies in CuO nanosheets efficiently regulate the surface chemistry and electronic structure, provide abundant active sites, and enhance charge transfer that facilitates the adsorption of reactant molecules on the catalyst surface. The as-prepared CuO/CF monolith electrode shows excellent stability for alcohol oxidation at current densities >200 mA·cm² for 24 h. Moreover, the abundant oxygen vacancies significantly enhance the intrinsic indicators of the catalyst in terms of specific activity and outstanding turnover frequencies of 5.8k s^-1 and 6k s^-1 for acetate and formate normalized by their respective faradaic efficiencies at an applied potential of 1.82 V vs. RHE.

Ligand-Assisted Controllable Growth of Self-Supporting Ultrathin Two-Dimensional Metal-Organic Framework Nanosheet Electrodes for an Efficient Oxygen Evolution Reaction

Rational design of metal-organic frameworks (MOFs) into ultrathin two-dimensional (2D) nanosheets with controllable thickness is significantly attractive but is also a significant challenge. Herein, the authors report, for the first time, the synthesis of ultrathin 2D nickel-based MOF nanosheets with a thickness of only about 2 nm via a ligand-assisted controllable growth strategy, which cannot be acquired from the exfoliation of their bulky counterparts or the conventional hydrothermal method. The correlation between 2D nanosheets and crystal growth preference was demonstrated through a judicious choice of a specific [Ni(BIP)(p-BDC)(H₂O)₂]_n framework (BIP = (3,5-bis(1-imidazoly)pyridine), p-H₂BDC = terephthalic acid) to underlie the geometry of the resultant morphology. Under the modulation by the dosage of terephthalic acid through a corrosion-dissolution-coordination process, the nanosheets of Ni-MOFs with a controllable thickness can be tuned to 50 and 100 nm. Ultrathin 2D Ni-MOF nanosheet-derived N-doped Ni@carbon exhibits a satisfactory electrocatalytic performance with a small overpotential of 170 mV to achieve a current density of 10 mA cm^-2, much outperforming the bulk Ni-MOF and the most reported non-noble-metal oxygen evolution reaction electrocatalysts to date. It is believed that this ligand-assisted controllable growth strategy represents a novel and simple path to prepare high-performance MOF-based electrocatalysts for wide applications.

Enzyme-Inspired Microenvironment Engineering of a Single-Molecular Heterojunction for Promoting Concerted Electrochemical CO2 Reduction

Challenges remain in the development of novel multifunctional electrocatalysts and their industrial operation on low-electricity pair-electrocatalysis platforms for the carbon cycle. Herein, an enzyme-inspired single-molecular heterojunction electrocatalyst ((NH_x )₁₆ -NiPc/CNTs) with specific atomic nickel centers and amino-rich local microenvironments for industrial-level electrochemical CO₂ reduction reaction (eCO₂ RR) and further energy-saving integrated CO₂ electrolysis is designed and developed. (NH_x )₁₆ -NiPc/CNTs exhibit unprecedented catalytic performance with industry-compatible current densities, ≈100% Faradaic efficiency and remarkable stability for CO₂ -to-CO conversion, outperforming most reported catalysts. In addition to the enhanced CO₂ capture by chemisorption, the sturdy deuterium kinetic isotope effect and proton inventory studies sufficiently reveal that such distinctive local microenvironments provide an effective proton ferry effect for improving local alkalinity and proton transfer and creating local interactions to stabilize the intermediate, ultimately enabling the high-efficiency operation of eCO₂ RR. Further, by using (NH_x )₁₆ -NiPc/CNTs as a bifunctional electrocatalyst in a flow cell, a low-electricity overall CO₂ electrolysis system coupling cathodic eCO₂ RR with anodic oxidation reaction is developed to achieve concurrent feed gas production and sulfur recovery, simultaneously decreasing the energy input. This work paves the new way in exploring molecular electrocatalysts and electrolysis systems with techno-economic feasibility.

Metal-Organic Framework-Derived BiIn Bimetallic Oxide Nanoparticles Embedded in Carbon Networks for Efficient Electrochemical Reduction of CO2 to Formate

Bismuth-based catalysts exhibit excellent activity and selectivity for the electroreduction of carbon dioxide (CO₂). However, single-component bismuth-based catalysts are not satisfactory for the electrochemical reduction of CO₂ to formic acid, mainly due to their high hydrogen production, low electrical conductivity, and small catalytic current density. Herein, we used a coordination strategy to recombine Bi and In at the molecular level to form Bi/In bimetallic metal-organic frameworks (MOFs), which were then calcined to obtain MOF-derived Bi/In bimetallic oxide nanoparticles embedded in carbon networks. Thanks to the synergistic effect of bimetallic components, high specific surface area, suitable pore size distribution, and high electrical conductivity of the carbon network, the material exhibits excellent activity and selectivity for electroreduction of CO₂ to formate. In H-type electrolyzers, the formate Faradaic efficiency reaches 91% at -0.9 V (vs RHE) and does not decrease significantly within 48 h. In situ Fourier transform infrared spectroscopy confirms the reaction intermediates and reveals that CO₂ electroreduction is dominant by the *OCHO pathway.

Eliminating the need for anodic gas separation in CO2 electroreduction systems via liquid-to-liquid anodic upgrading

Electrochemical reduction of CO₂ to multi-carbon products (C₂₊), when powered using renewable electricity, offers a route to valuable chemicals and fuels. In conventional neutral-media CO₂-to-C₂₊ devices, as much as 70% of input CO₂ crosses the cell and mixes with oxygen produced at the anode. Recovering CO₂ from this stream adds a significant energy penalty. Here we demonstrate that using a liquid-to-liquid anodic process enables the recovery of crossed-over CO₂ via facile gas-liquid separation without additional energy input: the anode tail gas is directly fed into the cathodic input, along with fresh CO₂ feedstock. We report a system exhibiting a low full-cell voltage of 1.9 V and total carbon efficiency of 48%, enabling 262 GJ/ton ethylene, a 46% reduction in energy intensity compared to state-of-art single-stage CO₂-to-C₂₊ devices. The strategy is compatible with today’s highest-efficiency electrolyzers and CO₂ catalysts that function optimally in neutral and alkaline electrolytes.

In the electrified conversion of CO2 to multicarbon products, CO2 crossover to the O2-rich anodic stream adds a further, energy-intensive, chemical separation step. Here, the authors demonstrate a strategy that eliminates the separation requirement.

Efficient Electrooxidation of 5-Hydroxymethylfurfural Using Co-Doped Ni3 S2 Catalyst: Promising for H2 Production under Industrial-Level Current Density

Replacing oxygen evolution reaction (OER) by electrooxidations of organic compounds has been considered as a promising approach to enhance the energy conversion efficiency of the electrolytic water splitting proces. Developing efficient electrocatalysts with low potentials and high current densities is crucial for the large-scale productions of H₂ and other value-added chemicals. Herein, non-noble metal electrocatalysts Co-doped Ni₃ S₂ self-supported on a Ni foam (NF) substrate are prepared and used as catalysts for 5-hydroxymethylfurfural (HMF) oxidation reaction (HMFOR) under alkaline aqueous conditions. For HMFOR, the Co_0.4 NiS@NF electode achieves an extremely low onset potential of 0.9 V versus reversible hydrogen electrode (RHE) and records a large current density of 497 mA cm^-2 at 1.45 V versus RHE for HMFOR. During the HMFOR-assisted H₂ production, the yield rates of 2,5-furandicarboxylic acid (FDCA) and H₂ in a 10 mL electrolyte containing 10 × 10^-3 M HMF are 330.4 µmol cm^-2 h^-1 and 1000 µmol cm^-2 h^-1 , respectively. The Co_0.4 NiS@NF electrocatalyst displays a good cycling durability toward HMFOR and can be used for the electrooxidation of other biomass-derived chemicals. The findings present a facile route based on heteroatom doping to fabricate high-performance catalyses that can facilitate the industrial-level H₂ production by coupling the conventional HER cathodic processes with HMFOR.

Toward High-Performance CO2 -to-C2 Electroreduction via Linker Tuning on MOF-Derived Catalysts

Copper (Cu)-based metal-organic frameworks (MOFs) and MOF-derived catalysts are well studied for electroreduction of carbon dioxide (CO₂ ); however, the effects of organic linkers for the selectivity of CO₂ reduction are still unrevealed. Here, a series of Cu-based MOF-derived catalysts is investigated with different organic linkers appended, named X-Cu-BDC (BDC = 1,4-benzenedicarboxylic acid, X = NH₂ , OH, H, F, and 2F). It is found that the linkers affect the faradaic efficiency (FE) for C₂ products with an order of NH₂  < OH < bare Cu-BDC < F < 2F, thus tuning the FE_C2 :FE_C1 ratios from 0.6 to 3.8. As a result, the highest C₂ FE of ≈63% at a current density of 150 mA cm^-2 on 2F-Cu-BDC derived catalyst is achieved. Using operando Raman measurements, it is revealed that the MOF derives to Cu₂ O during eCO₂ RR but organic linkers are stable. The fluorine group in organic linker can promote the H₂ O dissociation to *H species, further facilitating the hydrogenation of *CO to *CHO that helps CC coupling.

Integrated 3D Open Network of Interconnected Bismuthene Arrays for Energy-Efficient and Electrosynthesis-Assisted Electrocatalytic CO2 Reduction

Electrocatalytic CO₂ reduction reaction (CO₂ RR) toward formate production can be operated under mild conditions with high energy conversion efficiency while migrating the greenhouse effect. Herein, an integrated 3D open network of interconnected bismuthene arrays (3D Bi-ene-A/CM) is fabricated via in situ electrochemically topotactic transformation from BiOCOOH nanosheet arrays supported on the copper mesh. The resulted 3D Bi-ene-A/CM consists of 2D atomically thin metallic bismuthene (Bi-ene) in the form of an integrated array superstructure with a 3D interconnected and open network, which harvests the multiple structural advantages of both metallenes and self-supported electrodes for electrocatalysis. Such distinctive superstructure affords the maximized quantity and availability of the active sites with high intrinsic activity and superior charge and mass transfer capability, endowing the catalyst with good CO₂ RR performance for stable formate production with high Faradaic efficiency (≈90%) and current density (>300 mA cm^-2 ). Theoretical calculation verifies the superior intermediate stabilization of the dominant Bi plane during CO₂ RR. Moreover, by further coupling anodic methanol oxidation reaction, an exotic electrolytic system enables highly energy-efficient and value-added pair-electrosynthesis for concurrent formate production at both electrodes, achieving substantially improved electrochemical and economic efficiency and revealing the feasibility for practical implementation.

Two-dimensional materials for electrochemical CO2 reduction: materials, in situ/operando characterizations, and perspective

Electrochemical CO₂ reduction (CO₂ ECR) is an efficient approach to achieving eco-friendly energy generation and environmental sustainability. This approach is capable of lowering the CO₂ greenhouse gas concentration in the atmosphere while producing various valuable fuels and products. For catalytic CO₂ ECR, two-dimensional (2D) materials stand as promising catalyst candidates due to their superior electrical conductivity, abundant dangling bonds, and tremendous amounts of surface active sites. On the other hand, the investigations on fundamental reaction mechanisms in CO₂ ECR are highly demanded but usually require advanced in situ and operando multimodal characterizations. This review summarizes recent advances in the development, engineering, and structure-activity relationships of 2D materials for CO₂ ECR. Furthermore, we overview state-of-the-art in situ and operando characterization techniques, which are used to investigate the catalytic reaction mechanisms with the spatial resolution from the micron-scale to the atomic scale, and with the temporal resolution from femtoseconds to seconds. Finally, we conclude this review by outlining challenges and opportunities for future development in this field.