Electrode Materials Engineering in Electrocatalytic CO2 Reduction: Energy Input and Conversion Efficiency

Electrocatalytic CO₂ reduction (ECR) is a promising technology to simultaneously alleviate CO₂-caused climate hazards and ever-increasing energy demands, as it can utilize CO₂ in the atmosphere to provide the required feedstocks for industrial production and daily life. In recent years, substantial progress in ECR systems has been achieved by the exploitation of various novel electrode materials. The anodic materials and cathodic catalysts that have, respectively, led to high-efficiency energy input and effective heterogenous catalytic conversion in ECR systems are comprehensively reviewed. Based on the differences in the nature of energy sources and the role of materials used at the anode, the fundamentals of ECR systems, including photo-anode-assisted ECR systems and bio-anode-assisted ECR systems, are explained in detail. Additionally, the cathodic reaction mechanisms and pathways of ECR are described along with a discussion of different design strategies for cathode catalysts to enhance conversion efficiency and selectivity. The emerging challenges and some perspective on both anode materials and cathodic catalysts are also outlined for better development of ECR systems.     

Isolation of Highly Reactive Cobalt Phthalocyanine via Electrochemical Activation for Enhanced CO2 Reduction Reaction

Electrochemical CO₂-to-CO conversion offers an attractive and efficient route to recycle CO₂ greenhouse gas. Molecular catalysts, like CoPc, are proved to be possible replacement for precious metal-based catalysts. These molecules, a combination of metal center and organic ligand molecule, may evolve into single atom structure for enhanced performance; besides, the manipulation of molecules’ behavior also plays an important role in mechanism research. Here, in this work, the structure evolution of CoPc molecules is investigated via electrochemical-induced activation process. After numbers of cyclic voltammetry scanning, CoPc molecular crystals become cracked and crumbled, meanwhile the released CoPc molecules migrate to the conductive substrate. Atomic-scale HAADF-STEM proves the migration of CoPc molecules, which is the main reason for the enhancement in CO₂-to-CO performance. The as-activated CoPc exhibits a maximum FE_CO of 99% in an H-type cell and affords a long-term durability at 100 mA cm⁻² for 29.3 h in a membrane electrode assembly reactor. Density-functional theory (DFT) calculation also demonstrates a favorable CO₂ activation energy with such an activated CoPc structure. This work provides a different perspective for understanding molecular catalysts as well as a reliable and universal method for practical utilization.     

Sn-Ag Synergistic Effect Enhances High-Rate Electrocatalytic CO2-to-Formate Conversion on Porous Poly(Ionic Liquid) Support

The electrocatalytic transformation of carbon dioxide (CO₂) to formate is a promising route for highly efficient conversion and utilization of CO₂ gas, due to the low production cost and the ease of storage of formate. In this work, porous poly(ionic liquid) (PPIL)-based tin-silver (Sn-Ag) bimetallic hybrids (PPIL^m-Sn_xAg_10-x) are prepared for high-performance formate electrolytic generation. Under optimal conditions, an excellent formate Faradaic efficiency of 95.5% with a high partial current density of 214.9 mA cm⁻² is obtained at −1.03 V (vs reversible hydrogen electrode). Meanwhile, the high selectivity of formate (>≈83%) is maintained in a wide potential range (>630 mV). Mechanistic studies demonstrate that the presence of Ag-species is vital for the formation, maintenance, and high dispersion of tetravalent Sn(IV)-species, which accounts for the active sites for CO₂-to-formate conversion. Further, the introduction of Ag-species significantly enhances the activity by increasing the electron density near the Fermi energy level.     

Highly Active,Durable Ultrathin MoTe2 Layers for the Electroreduction of CO2 to CH4

The electroreduction of CO₂ to CH₄ is a highly desirable, challenging research topic. In this study, an electrocatalytic system comprising ultrathin MoTe₂ layers and an ionic liquid electrolyte for the reduction of CO₂ to methane is reported, efficiently affording methane with a faradaic efficiency of 83 ± 3% (similar to the best Cu‐based catalysts reported thus far) and a durable activity of greater than 45 h at a relatively high current density of 25.6 mA cm^?2 (?1.0 V_RHE). The results obtained can facilitate research on the design of other transition‐metal dichalcogenide electrocatalysts for the reduction of CO₂ to valuable fuels.     

Harnessing the Hybridization of a Metal-Organic Framework and Superbase-Derived Ionic Liquid for High-Performance Direct Air Capture of CO2

Direct air capture (DAC) of CO₂ has emerged as the most promising “negative carbon emission” technologies. Despite being state-of-the-art, sorbents deploying alkali hydroxides/amine solutions or amine-modified materials still suffer from unsolved high energy consumption and stability issues. In this work, composite sorbents are crafted by hybridizing a robust metal-organic framework (Ni-MOF) with superbase-derived ionic liquid (SIL), possessing well maintained crystallinity and chemical structures. The low-pressure (0.4 mbar) volumetric CO₂ capture assessment and a fixed-bed breakthrough examination with 400 ppm CO₂ gas flow reveal high-performance DAC of CO₂ (CO₂ uptake capacity of up to 0.58 mmol g⁻¹ at 298 K) and exceptional cycling stability. Operando spectroscopy analysis reveals the rapid (400 ppm) CO₂ capture kinetics and energy-efficient/fast CO₂ releasing behaviors. The theoretical calculation and small-angle X-ray scattering demonstrate that the confinement effect of the MOF cavity enhances the interaction strength of reactive sites in SIL with CO₂, indicating great efficacy of the hybridization. The achievements in this study showcase the exceptional capabilities of SIL-derived sorbents in carbon capture from ambient air in terms of rapid carbon capture kinetics, facile CO₂ releasing, and good cycling performance.     

Evolution of Electrocatalytic CO2 Reduction Activity Induced by Charge Segregation in Atomically Precise AuAg Nanoclusters Based on Icosahedral M13 Unit 3D Assembly

The precise self-assembly of building blocks at atomic level provides the opportunity to achieve clusters with advanced catalytic properties. However, most of the current self-assembled materials are fabricated by 1/2D assembly of blocks. High dimensional (that is, 3D) assembly is widely believed to improve the performance of cluster. Herein, the effect of 3D assembly on the activity for electrocatalytic CO₂ reduction reaction (CO₂RR) is investigated by using a range of clusters (Au₈Ag₅₅, Au₈Ag₅₇, Au₁₂Ag₆₀) based on 3D assembly of M₁₃ unit as models. Although three clusters have almost the same sizes and geometric structures, Au₈Ag₅₅ exhibits the best CO₂RR performance due to the strong CO₂ adsorption capacity and effective inhibition of H₂ evolution competition reaction. The deep insight into the superior activity of Au₈Ag₅₅ is the unique electronic structure attributed to the charge segregation. This study not only demonstrates that the assembly mode greatly affects the catalytic activity, but also offers an idea for rational designing and precisely constructing catalysts with controllable activities.     

Identifying the Origin of Ti3+ Activity toward Enhanced Electrocatalytic N2 Reduction over TiO2 Nanoparticles Modulated by Mixed-Valent Copper

The ambient electrocatalytic N₂ reduction reaction (NRR) enabled by TiO₂ has attracted extensive recent attention. Previous studies suggest the formation of Ti³⁺ in TiO₂ can significantly improve the NRR activity, but it still remains unclear what kinds of Ti³⁺ are effective. Herein, it is demonstrated that mixed-valent Cu acts as an effective dopant to modulate the oxygen vacancy (V_O) concentration and Ti³⁺ formation, which markedly improves the electrocatalytic NRR performance. In 0.5 m LiClO₄, this electrocatalyst attains a high Faradic efficiency of 21.99% and a large NH₃ yield of 21.31 µg h⁻¹ mg_cat.⁻¹ at –0.55 V vs reversible hydrogen electrode, which even surpasses most reported Ti-based NRR electrocatalysts. Using density function theory calculations, it is evidenced that mixed-valent Cu ions modulate the TiO₂ (101) surface with multiple oxygen vacancies, which is beneficial for generating different Ti³⁺ 3d¹ defect states localized below the Fermi energy. N₂ activation and adsorption are effectively strengthened when Ti³⁺ 3d¹ defect states present the splitting of e_g and t_2g orbitals, which can be modulated by its coordination structure. The synergistic roles of the three ion pairs formed by the V_O defect, including Cu¹⁺–Ti⁴⁺, Ti³⁺–Ti⁴⁺ and Ti³⁺–Ti³⁺, are together responsible for the enhanced NRR performance.     

Metal–CO2 Batteries on the Road: CO2 from Contamination Gas to Energy Source

Rechargeable nonaqueous metal–air batteries attract much attention for their high theoretical energy density, especially in the last decade. However, most reported metal–air batteries are actually operated in a pure O₂ atmosphere, while CO₂ and moisture in ambient air can significantly impact the electrochemical performance of metal–O₂ batteries. In the study of CO₂ contamination on metal–O₂ batteries, it has been gradually found that CO₂ can be utilized as the reactant gas alone; namely, metal–CO₂ batteries can work. On the other hand, investigations on CO₂ fixation are in focus due to the potential threat of CO₂ on global climate change, especially for its steadily increasing concentration in the atmosphere. The exploitation of CO₂ in energy storage systems represents an alternative approach towards clean recycling and utilization of CO₂. Here, the aim is to provide a timely summary of recent achievements in metal–CO₂ batteries, and inspire new ideas for new energy storage systems. Moreover, critical issues associated with reaction mechanisms and potential directions for future studies are discussed.     

锡量子点制备及其电催化还原二氧化碳产甲酸性能

锡基材料在自然界含量丰富、价格低廉, 在电催化还原CO₂制液体燃料反应中具有巨大潜力。但是较低的产物选择性和较差的稳定性限制了其应用。本工作制备的锡量子点电催化剂(Sn-QDs), 具有高效、高稳定性和高选择性的电催化还原CO₂产HCOOH活性。Sn-QDs的平均颗粒尺寸仅为2~3 nm, 结晶性良好。小的颗粒尺寸增大了电化学活性面积(ECSA), Sn-QDs的ECSA约为锡颗粒的4.4倍。ECSA增大以及CO₂还原反应动力学加速, 促进了CO₂电化学转化。在-1.0 V (vs RHE)下, Sn-QDs/CN催化剂的HCOOH法拉第效率(FE_HCOOH)达到95%, 并且在宽约0.5 V的电势范围内能够保持在83%以上。此外, Sn-QDs/CN可以在24 h内保持良好的电化学稳定性。     

Interface-Induced Electrocatalytic Enhancement of CO2-to-Formate Conversion on Heterostructured Bismuth-Based Catalysts

Electrochemical CO₂ reduction reaction (CO₂RR) is a promising approach to convert CO₂ to carbon-neutral fuels using external electric powers. Here, the Bi₂S₃-Bi₂O₃ nanosheets possessing substantial interface being exposed between the connection of Bi₂S₃ and Bi₂O₃ are prepared and subsequently demonstrate to improve CO₂RR performance. The electrocatalyst shows formate Faradaic efficiency (FE) of over 90% in a wide potential window. A high partial current density of about 200 mA cm^?2 at ?1.1 V and an ultralow onset potential with formate FE of 90% are achieved in a flow cell. The excellent electrocatalytic activity is attributed to the fast-interfacial charge transfer induced by the electronic interaction at the interface, the increased number of active sites, and the improved CO₂ adsorption ability. These collectively contribute to the faster reaction kinetics and improved selectivity and consequently, guarantee the superb CO₂RR performance. This study provides an appealing strategy for the rational design of electrocatalysts to enhance catalytic performance by improving the charge transfer ability through constructing a functional heterostructure, which enables interface engineering toward more efficient CO₂RR.     

Engineering Single Cu Sites into Covalent Organic Framework for Selective Photocatalytic CO2 Reduction

Photocatalytic CO₂ conversion into value-added chemicals is a promising route but remains challenging due to poor product selectivity. Covalent organic frameworks (COFs) as an emerging class of porous materials are considered as promising candidates for photocatalysis. Incorporating metallic sites into COF is a successful strategy to realize high photocatalytic activities. Herein, 2,2′-bipyridine-based COF bearing non-noble single Cu sites is fabricated by chelating coordination of dipyridyl units for photocatalytic CO₂ reduction. The coordinated single Cu sites not only significantly enhance light harvesting and accelerate electron–hole separation but also provide adsorption and activation sites for CO₂ molecules. As a proof of concept, the Cu-Bpy-COF as a representative catalyst exhibits superior photocatalytic activity for reducing CO₂ to CO and CH₄ without photosensitizer, and impressively, the product selectivity of CO and CH₄ can be readily modulated only by changing reaction media. Experimental and theoretical results reveal the crucial role of single Cu sites in promoting photoinduced charge separation and solvent effect in regulating product selectivity, which provides an important sight onto the design of COF photocatalysts for selective CO₂ photoreduction.     

Regulation of Electrocatalytic Behavior by Axial Oxygen Enhances the Catalytic Activity of CoN4 Sites for CO2 Reduction

Recent studies have found that the existence of oxygen around the active sites may be essential for efficient electrochemical CO₂-to-CO conversion. Hence, this work proposes the modulation of oxygen coordination and investigates the as-induced catalytic behavior in CO₂RR. It designs and synthesizes conjugated phthalocyanine frameworks catalysts (CPF-Co) with abundant CoN₄ centers as an active source, and subsequently modifies the electronic structure of CPF-Co by introducing graphene oxide (GO) with oxygen-rich functional groups. A systematic study reveals that the axial coordination between oxygen and the catalytic sites could form an optimized O-CoN₄ structure to break the electron distribution symmetry of Co, thus reducing the energy barrier to the activation of CO₂ to COOH*. Meanwhile, by adjusting the content of oxygen, the proper supports can also facilitate the charge transfer efficiency between the matrix layer and the catalytic sites. The optimized CPF-Co@LGO exhibits a high TOF value (2.81 s⁻¹), CO selectivity (97.6%) as well as stability (24 h) at 21 mA cm⁻² current density. This work reveals the modulation of oxygen during CO₂RR and provides a novel strategy for the design of efficient electrocatalysts, which may inspire new exploration and principles for CO₂RR.     

Regulated Surface Electronic States of CuNi Nanoparticles through Metal-Support Interaction for Enhanced Electrocatalytic CO2 Reduction to Ethanol

Developing stable catalysts with higher selectivity and activity within a wide potential range is critical for efficiently converting CO₂ to ethanol. Here, the carbon-encapsulated CuNi nanoparticles anchored on nitrogen-doped nanoporous graphene (CuNi@C/N-npG) composite are designedly prepared and display the excellent CO₂ reduction performance with the higher ethanol Faradaic effiency (FE_ethanol ≥ 60%) in a wide potential window (600 mV). The optimal cathodic energy efficiency (47.6%), Faradaic efficiency (84%), and selectivity (96.6%) are also obtained at −0.78 V versus reversible hydrogen electrode (RHE). Combining with the density functional theory (DFT) calculations, it is demonstrated that the stronger metal-support interaction (Ni-N-C) can regulate the surface electronic structure effectively, boosting the electron transfer and stabilizing the active sites (Cu⁰-Cu^δ+) on the surface of CuNi@C/N-npG, finally realizing the controllable transition of reaction intermediates. This work may guide the designs of electrocatalysts with highly catalytic performance for CO₂ reduction to C₂₊ products.     

Highly Enhancing CO2 Photoreduction by Metallization of an Imidazole-linked Robust Covalent Organic Framework

Converting CO₂ into value-added chemicals to solve the issues caused by carbon emission is promising but challenging. Herein, by embedding metal ions (Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) into an imidazole-linked robust photosensitive covalent organic framework (PyPor-COF), effective photocatalysts for CO₂ conversion are rationally designed and constructed. Characterizations display that all of the metallized PyPor-COFs (M-PyPor-COFs) display remarkably high enhancement in their photochemical properties. Photocatalysis reactions reveal that the Co-metallized PyPor-COF (Co-PyPor-COF) achieves a CO production rate as high as up to 9645 µmol g⁻¹ h⁻¹ with a selectivity of 96.7% under light irradiation, which is more than 45 times higher than that of the metal-free PyPor-COF, while Ni-metallized PyPor-COF (Ni-PyPor-COF) can further tandem catalyze the generated CO to CH₄ with a production rate of 463.2 µmol g⁻¹ h⁻¹. Experimental analyses and theory calculations reveal that their remarkable performance enhancement on CO₂ photoreduction should be attributed to the incorporated metal sites in the COF skeleton, which promotes the adsorption and activation of CO₂ and the desorption of generated CO and even reduces the reaction energy barrier for the formation of different intermediates. This work demonstrates that by metallizing photoactive COFs, effective photocatalysts for CO₂ conversion can be achieved.     

Electrocatalytic Reduction of CO2 to Ethylene by Molecular Cu‐Complex Immobilized on Graphitized Mesoporous Carbon

The electrochemical reduction of carbon dioxide (CO₂) to hydrocarbons is a challenging task because of the issues in controlling the efficiency and selectivity of the products. Among the various transition metals, copper has attracted attention as it yields more reduced and C2 products even while using mononuclear copper center as catalysts. In addition, it is found that reversible formation of copper nanoparticle acts as the real catalytically active site for the conversion of CO₂ to reduced products. Here, it is demonstrated that the dinuclear molecular copper complex immobilized over graphitized mesoporous carbon can act as catalysts for the conversion of CO₂ to hydrocarbons (methane and ethylene) up to 60%. Interestingly, high selectivity toward C2 product (40% faradaic efficiency) is achieved by a molecular complex based hybrid material from CO₂ in 0.1 m KCl. In addition, the role of local pH, porous structure, and carbon support in limiting the mass transport to achieve the highly reduced products is demonstrated. Although the spectroscopic analysis of the catalysts exhibits molecular nature of the complex after 2 h bulk electrolysis, morphological study reveals that the newly generated copper cluster is the real active site during the catalytic reactions.     

DBD转化CO2的研究现状及进展

为了更高效地进行CO₂的转化利用,许多学者开展了多种CO₂转化方法的研究。介电阻挡放电（dielectric barrier discharge,DBD）能在低温常压的环境下还原CO₂,是CO₂转化利用的重要方法之一。为了设计出更好的DBD反应器、更佳的反应条件和催化剂,总结了近年来DBD转化CO₂的相关研究。通过分析不同研究中过程参数、稀释性气体和催化剂等因素对CO₂转化量和能量效率的影响,发现催化剂能大幅度提高DBD转化CO₂的反应效率,并介绍了DBD中一些常用的催化剂。最后简要分析了此类研究的可能发展趋势。     

Multilayer-Cavity Tandem Catalyst for Profiling Sequentially Coupling of Intermediate CO in Electrocatalytic Reduction Reaction of CO2 to Multi-Carbon Products

Electrochemical CO₂ reduction reaction (CO₂RR) is an effective approach to address CO₂ emission, promote recycling, and synthesize high-value multi-carbon (C₂₊) chemicals for storing renewable electricity in the long-term. The construction of multilayer-bound nanoreactors to achieve management of intermediate CO is a promising strategy for tandem to C₂₊ products. In this study, a series of Ag@Cu₂O nanoreactors consisting of an Ag-yolk and a multilayer confined Cu shell is designed to profile electrocatalytic CO₂RR reactions. The optimized Ag@Cu₂O-2 nanoreactor exhibits a 74% Faradaic efficiency during the C₂₊ pathway and remains stable for over 10 h at a bias current density of 100 mA cm⁻². Using the finite element method, this model determines that the certain volume of cavity in the Ag@Cu₂O nanoreactors facilitates on-site CO retention and that multilayers of Cu species favor CO capture. Density functional theory calculations illustrate that the biased generation of ethanol products may arise from the (100)/(111) interface of the Cu layer. In-depth explorations in multilayer-bound nanoreactors provide structural and interfacial guidance for sequential coupling of CO₂RR intermediates for efficient C₂₊ generation.     

Solvent‐Free Self‐Assembly to the Synthesis of Nitrogen‐Doped Ordered Mesoporous Polymers for Highly Selective Capture and Conversion of CO2

A solvent‐free induced self‐assembly technology for the synthesis of nitrogen‐doped ordered mesoporous polymers (N‐OMPs) is developed, which is realized by mixing polymer precursors with block copolymer templates, curing at 140–180 °C, and calcination to remove the templates. This synthetic strategy represents a significant advancement in the preparation of functional porous polymers through a fast and scalable yet environmentally friendly route, since no solvents or catalysts are used. The synthesized N‐OMPs and their derived catalysts are found to exhibit competitive CO₂ capacities (0.67–0.91 mmol g^?1 at 25 °C and 0.15 bar), extraordinary CO₂/N₂ selectivities (98–205 at 25 °C), and excellent activities for catalyzing conversion of CO₂ into cyclic carbonate (conversion >95% at 100 °C and 1.2 MPa for 1.5 h). The solvent‐free technology developed in this work can also be extended to the synthesis of N‐OMP/SiO₂ nanocomposites, mesoporous SiO₂, crystalline mesoporous TiO₂, and TiPO, demonstrating its wide applicability in porous material synthesis.