Electrochemical Transformation of CO 2 to Value-Added Chemicals and Fuels

Open AccessCCS ChemistryMINI REVIEW3 Oct 2022Electrochemical Transformation of CO2 to Value-Added Chemicals and Fuels Shunhan Jia, Xiaodong Ma, Xiaofu Sun and Buxing Han Shunhan Jia Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Xiaodong Ma Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Xiaofu Sun *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author and Buxing Han *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049 Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202094 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail CO2 is the main greenhouse gas and a renewable carbon resource. Electrochemical transformation of CO2 (CO2ET) to value-added chemicals and fuels is one of the promising routes to reduce CO2 emission and contributes to sustainability and carbon neutrality. In this review, we discuss recent developments on apparatuses used in CO2ET, electrocatalytic reactions of CO2 with water, organics, nitrogen, and nitrogen-containing compounds to synthesize chemicals and fuels by the construction of different chemical bonds (e.g., C–H, C–C, C–O, and C–N), and related reaction mechanisms. Also, an outlook was considered to highlight the opportunities and challenges in CO2ET. Download figure Download PowerPoint Introduction The skyrocketed atmospheric CO2 level resulting from the overuse of fossil fuels is breaking the carbon cycle and degrading our living environment, which compels us to adjust and optimize the current energy supply structure.1–3 At the same time, CO2 also demonstrates potential as a low-cost, abundant, toxic-free, and renewable C1 feedstock.2–4 Hence, the transformation of CO2 to value-added chemicals is significant for reducing the utilization of fossil resources and building a carbon-neutral society. Numerous routes have been developed for CO2 transformation, including thermochemical, electrochemical, photochemical, biochemical approaches, and some coupled strategies.2,3,5,6 Among these strategies, CO2 electrochemical transformation (CO2ET), with many advantages such as eco-friendly reaction conditions, clean power sources, tunable reaction pathways, and ideal technology compatibility (Scheme 1), is one of the most promising candidates for sustainable chemical production and renewable energy storage, achievable by multiple chemical bond formations (e.g., C–H, C–C, C–O, and C–N bonds). CO2 electrochemical reduction (CO2ER) is an important route to realizing the reaction between CO2 and water, in which C–H and C–C bonds could be established to form hydrocarbons, acids, and alcohols.7 On the other hand, electrochemical organic transformation of CO2 (CO2EOT) with different substrates is also emerging. An example is the CO2-derived intermediates coupled with substrates through C–C bonds in electro-carboxylation.8 C–O and C–N bond formations are key steps in CO2 cycloaddition with alcohols, epoxy compounds, and organic ammonia.9–13 The establishment of C–N bonds is also the basis for the production of urea and organic amines using CO2 as a feedstock.14–23 However, the linear CO2 molecule is chemically inert and hard to activate, making CO2ET a challenging subject. Researchers have conducted numerous experiments to achieve efficient CO2ET. Our group has also designed a series of electrocatalysts and green-solvent-based electrolytes to achieve enhanced performance for CO2ET and gained mechanic understanding by employing in situ technology.24–32 Aiming at high-rate CO2ET, gas diffusion electrodes (GDEs), and flow cells have been extensively investigated.33–35 Additionally, many high-quality reviews have been published for CO2ET, including electrocatalysts, electrolytes, and electrolyzers.7,8,33,34,36–42 Nevertheless, a timely and comprehensive review of CO2ET covering both CO2ER and CO2EOT is lacking at present. Herein, we have reviewed recent advances in CO2ET via chemical bond formation (e.g., C–H, C–C, C–O, and C–N). Finally, some perspectives are provided towards building the next-generation CO2ET system. Besides, we hope that the insights obtained from the current review would inspire researchers to focus on inert chemical bond activation and transformation (e.g., biomass upgrading and plastic waste degradation) through electrochemical strategies, which could be utilized in some chemical reactions not feasible to be conducted by other methods. Scheme 1 | Schematic illustration of CO2ET through various chemical bond formations toward artificial carbon cycle. Download figure Download PowerPoint Basic Issues of CO2ET Building blocks of CO2ET system A CO2ET system is integrated by electrodes, electrolytes, and electrolyzers. Metal-based materials, metal complexes, and carbon materials are widely utilized electrocatalysts in CO2ET due to their low-cost, high conductivity, and tunable properties.25,42–46 The electrodes for CO2ET are usually fabricated with powder catalysts, polymeric binders, and current collectors. However, the binders usually result in pore blocks of the catalysts, which hinders the high electrochemical performance of powder-based electrodes due to the loss of mass transfer channels. Fortunately, the emerging self-assembly and electrochemical deposition technology provide freestanding and binder-free strategies for electrode preparation with enhanced catalytic activity, stability, and lower cost.26,41 Considerable efforts have been made to design aqueous, organic, and ionic liquid (IL)-based electrolytes. Aqueous electrolytes have advantages such as low cost and wild availability. Nonetheless, the poor CO2 solubility limits the mass-transportation of CO2ET, and the narrow potential window also hinders the performance improvement of the aqueous CO2ET severely.38 Despite the higher CO2 solubility and broadened potential window in organic electrolytes providing possibilities for enhancing catalytic performance, potential toxicity, electrochemically stability, and safety hazards are inevitable shortcomings of organic solvents that are supposed to be assessed critically.39,47 ILs, an emerging family of green solvents with tailorable chemical properties, are attractive for applications such as gas absorption, material preparation, and sustainable catalysis.2,48,49 Their inherently molecular structure renders them features as low-melting salts (Figures 1a and 1b), high gas solubility, high ionic conductivity, and wide potential window, making them promising for CO2ET.7,37 In addition, the activation of CO2 molecule by ILs and suppression of competing hydrogen evolution reaction (HER) in IL-based electrolytes during CO2ET have been discussed recently.50 Figure 1 | Structural formulas of (a) cations and (b) anions of some typical IL-based electrolytes. Reprinted with permission from ref 7. Copyright 2021 Oxford University Press. Schematics of (c) H-cell, (d) microfluidic cell, and GDE-based MEA cells with (e) liquid and (f) gas-phase CO2 supply. Reprinted with permission from ref 33. Copyright 2020 Elsevier. Download figure Download PowerPoint According to techno-economic analysis, a high current (>200 mA cm−2) is crucial for implementing the CO2ET system.51 Aiming at highly efficient CO2ET, and intensive research has been focused on the design of electrolyzers.35 Typically, in the cathode chamber of the H-cell (Figure 1c), the working electrode and reference electrode are designed, while in the anode chamber, the counter electrode is located. The separator serves to control the mass transportation between two chambers. Particularly, ion conductivity is enabled, while the reaction product diffusion and re-oxidation are prevented.33 In microfluidic reactors (MFRs) (Figure 1d), a thin spacer is fixed between the anode and cathode. CO2 gas supplied by the cathodic GDE and flowing stream of electrolytes in MFR overcome the restriction of CO2 solubility and product transport, but it may lead to re-oxidation of the product. To solve the drawbacks of H-cell and MFR, three separate chambers for CO2 gas, catholyte, and anolyte are designed in membrane electrode assembly (MEA) electrolyzers (Figures 1e and 1f). Enhanced energy efficiency and stability can be achieved in MEA electrolyzers due to the contact between the GDE and the member.34 Overall, highly efficient CO2ET is realized in the coordination of electrodes, electrolytes, and electrolyzers described above. Reaction pathways of CO2ET Typically, CO2ER and CO2EOT are discussed separately, but their reaction pathways have some similarities. In this review, we discuss CO2ET pathways combining the two types of reactions. CO2ET is a multi-step process enabled by a proton-coupled electron transformation, in which 2, 4, 6, 8, 12, or more electrons are transferred to CO2 molecules, and involves the following steps (Figure 2): The CO2 molecule is chemically adsorbed to the electrode, which is then transformed into *COOH and *CO intermediates. *CO is usually a central intermediate in CO2ET and serves as a bridge between CO2 and various products. For CO2ET producing C1 chemicals such as carbon monoxide (CO), formic acid/formate (COOH/COO−), methanol (MeOH), and methane (CH4), the products would desorb from the electrolyte-electrode interface after rearrangements of configuration and C–H bond formation. *CO intermediates can also form C–C bonds, producing C2 and C2+ chemicals such as ethanol (EtOH), ethene (C2H4), and n-propanol (C3H8OH). Figure 2 | The typical reaction pathways of CO2ET via C–H, C–C, C–O, and C–N bond formation. Download figure Download PowerPoint Besides direct desorption and *CO–*CO dimerization, coupling with other activated intermediates from substrates is also a possible pathway of *CO through CO2EOT, which widens the classes of products of CO2ET significantly. For example, MeOH can be activated and form *OMe intermediate. The *OMe intermediate can, in turn, be coupled with *CO to form C–O bond to produce carbonate.52 Tuning the molecular structures of alcohol substrates leads to other linear and cycle carbonate products via C–O bond formation. However, in some typical carboxylation reactions, only organic substrates were reported to be activated, while CO2 molecules could be coupled with organic intermediates directly without activation.53–56 In addition, since the development of nitrogen reduction reaction (NRR), co-reduction of CO2 and N2 has been demonstrated.57 In such a reaction, a *CO couples with a “lying” *N=N* intermediate to produce a urea precursor *NCON* on the electrode via the formation of C–N bonds.19,20 Moreover, secondary electroreduction of intermediates after coupling is a common phenomenon in CO2EOT pathways involving C–N bond formation. For example, in the co-reduction of CO2 and nitrate (NO3−) to produce methylamine (MeNH2), it is proved that a secondary electroreduction serves to break the N–O bond and form the amino in the products.14 Additionally, in the electrosynthesis of dimethyl ammonia, a C–O bond is broken and converted into the methyl in the products.27 Due to the technical compatibility of CO2ET, many coupled strategies were integrated into traditional CO2ET pathways to enhance the electrochemical performance, mechanistic understanding, energy efficiency, and economic benefits of CO2ET. For instance, direct electroreduction of CO2 in capture media such as (bi)carbonate and amine solution is able to save energy and cost for CO2ET system.58 Pulsed CO2ET was demonstrated as a simple but useful strategy to improve the CO2ET selectivity and stability.59,60 The diverse CO2ET pathways with different theoretical onset potentials provide the basis for designing net reactions, which can save energy consumption and improve economic viability.61 To date, CO2ET coupled with organic electro-transformation, ammonia production, and oxygen evolution have been reported.22,62–64 Physical fields such as a magnetic field were reported as a promoter for CO2ET due to the enhanced mass transfer in electrolyte and tuned spin state in electrocatalysts.65–67 Moreover, coupling CO2ET with product utilization units such as chemical vapor deposition (CVD), electrooxidation reaction, and bioreactor were reported recently. Researchers could first transform CO2 to platform molecules (e.g., CO and C2H4) and then convert these generated products to high-value chemicals (e.g., propylene oxide and glucose) and functional materials (e.g., graphene).6,68–70 On-chip electrocatalytic microdevices for CO2ET were utilized to uncover the intermediate surface on electrocatalysts, which could hardly be seen in typical electrochemical cells.71,72 Electroreduction of CO2 to C1 Chemicals Electrosynthesis of CO and COOH/COO− Owing to the considerable economic and technological feasibility, the selective electrochemical CO2-to-CO transformation is of great importance in CO2ET.73 Many electrocatalysts have been developed to approach highly efficient CO2-to-CO transformation, such as carbon-based catalysts, single-atom catalysts (SACs), and molecular catalysts.25,28,74–78 Our group has reported the facile preparation of N,P-co-doped carbon aerogels (NPCA).25 The incorporation of N and P enhanced the catalytic performance significantly. NPCA could reduce CO2 to CO with >99% Faradaic efficiency (FE) at a high current density (CD) of −143.6 mA cm−2. SACs are substantially different from traditional bulk catalysts due to their maximal atomic utilization.79 Both main group and transition metal active sites were reported for CO2-to-CO.28,40,80,81 For instance, Sun and co-workers synthesized Sb single-atom sites on porous N-doped carbon, achieving the turnover frequency (TOF) of 16,500 h−1 for CO electrosynthesis.80 Liu et al. introduced s-block Mg sites for CO2ET to CO recently.82 Our lab reported that atomic In catalysts anchored on N-doped carbon (InA/NC) could reduce CO2 to CO, while COO−/COOH is the main product over In nanoparticle (NP) catalysts (Figure 3a).28 Notably, the TOF of InA/NC is up to ∼40,000 h−1, which is very high compared with other CO2-to-CO electrocatalysts (Figure 3b). Wang and co-workers fabricated MEA reactors based on Ni SACs, yielding CO at a CD of >100 mA cm−2 and selectivity of ∼100%.50 Also, molecular catalysts can be utilized in CO2ER.40 Cao and co-workers reported that the enhanced conductivity and in-plan π-d conjugation make 2D metal–organic frameworks (MOFs) exhibit better selectivity and CD.75 Graphdiyne/graphene (GDY/G) heterostructure was demonstrated as the scaffold of cobalt phthalocyanine (CoPc).77 CoPc supported on GDY/G can provide better performance (e.g., CD, FE, and TOF) than CoPc/GDY and CoPc/G. Figure 3 | Recent advances in electrocatalyst design for CO2ET. (a) TEM image and (b) TOFMS compared with other CO2-to-CO catalysts of InA/NC. Reprinted with permission from ref 28. Copyright 2020 American Chemical Society. (c) Schematic diagram for the electrosynthesis of MOF catalysis. Reprinted with permission from ref 44. Copyright 2020 American Chemical Society. (d) Optical image of CuSAs/THCF catalysis. Reprinted with permission from ref 45. Copyright 2019 American Chemical Society. (e) MeOH FE over atomic Sn catalysis in IL-based system. Reprinted with permission from ref 24. Copyright 2019 John Wiley and Sons. (f) Comparison of different Cu-based catalysts. Reprinted with permission from ref 29. Copyright 2019 Springer Nature. (g) Structure and (h) CO2ER performance of copolymer modified Cu electrode. Reprinted with permission from ref 83. Copyright 2021 American Chemical Society. TEM, transmission electron microscopy; TOFMS, time-of-flight mass spectrometry; MOF, metal–organic framework; IL, ionic liquid. Download figure Download PowerPoint COOH/COO− is widely used as chemical feedstocks in industries and is regarded as a future alternative energy carrier.84 High-performance CO2ET electrocatalysts based on MOFs have been wildly reported for their inherent single-dispersion of metal sites and high capacity and selectivity of CO2 absorption.26,43,44,85,86 Kang et al. proposed the electro-synthesized MFM-300(In) based on an In foil (MFM-300(In)-e/In) as an electrode for CO2-to-COOH transformation (Figure 3c).44 Compared with MFM-300(In) from thermo-synthesis, based on carbon paper, MFM-300(In)-e/In electrode exhibits higher CD and FE of 99.1%. Kang and co-workers also reported an IL-templated electro-synthesis of MOF [Cu2(L)] on a copper electrode.43 The electrode leads to 90.5% FE in the generation of COOH at a CD of 65.8 mA cm−2. According to density functional theory (DFT) calculation, defect Cu2(L) from electrosynthesis showed the most facile pathway, resulting in the selective production of COOH. Zhu et al. reported the quick and in situ synthesis of 3D hierarchical Cu dendrite with preferentially exposed active sites for CO2-to-COO− transformation.26 Qiao and co-workers studied the intricate reconstruction of Bi-MOF mediated by electrolyte and potential with ex situ electron microscopy during CO2-to-COO− transformation recently, noting that the reconstruction of pre-catalysts is critical in designing highly efficient CO2ET catalyst.87 In the above-mentioned CO2ET systems, CO2-to-COO− transformations were carried out in solutions, producing a mixture with the dissolved COOH/ COO−, in which extra separation was needed. However, the Wang group recently constructed an MEA cell based on solid electrolytes and realized the stable and continuous production of 0.1 mol L−1 COOH.88 Electrosynthesis of MeOH and CH4 MeOH is a renewable fuel and a platform molecule for the chemical industry.89 Wang and co-workers showed that carbon nanotube immobilized CoPc (CoPc/CNT) could perform the CO2-to-MeOH transformation at high FE (>40%) and CD (>10 mA cm−2) through a *CO intermediated domino process, but its catalytic performance would decay with the reduction of Pc.90 This phenomenon can be suppressed by adding electro-donation amino substitutes to the Pc ring and forming CoPc-NH2. Utilizing CoPc-NH2/CNT as a catalyst, the transformation of CO2 to MeOH can achieve high efficiency, selectivity, and stability. In addition, He et al. reported the synthesis of atomic Cu catalysts on carbon fiber (CuSAs/TCNFs), which is inherent self-standing and can be used as the cathode for CO2-to-MeOH transformation directly (Figure 3d).45 Reduced embedment of atomic Cu in CuSAs/TCNFs with through-hole structure increases the atomic sites and catalytic performance. Notably, CuSAs/TCNFs produce nearly pure MeOH with FE of 44% and CD of 93 mA cm−2 in liquid production since the by-products are mainly in the gas phase (H2 and CO). Our group has prepared atomically dispersed Sn catalysts on defective CuO for CO2 electroreduction to MeOH.24 The FE of MeOH can reach 88.6% with a CD of 67.0 mA cm−2 (Figure 3e). The catalyst is beneficial for CO2 activation by decreasing the energy barrier of *COOH dissociation to form *CO. Then the *CO is bound to the Cu species for further reduction, leading to high selectivity toward MeOH. Cu-based materials are among the efficient electrocatalysts for CO2-to-CH4 transformation.91 Hydrogenation of *CO intermediate on the surface of Cu catalysts is the key step for CH4 production, while the *CO dimerization and HER are the main factors limiting the selectivity of CH4 production. The surface structure of Cu catalysts and CO2 concentration is crucial for the selectivity of CO2-to-CH4 transformation. For example, Peng and co-workers synthesized [email protected]2O with a yolk-shell structure as a nanoreactor for CH4 production.92 The *CO coverage and *H adsorption at the oxide derived (OD) Cu surface can be tuned by the Cu2O envelope size. As a result, CH4 is obtained in the flow cell at an FE of 74% and a partial CD of 178 mA cm−2. Sargent and co-workers reported the control of *CO coverage via CO2 concentration in the gas stream.93 The reduced CO2 concentration leads to CH4 production with FE of 48% and energy efficiency of 20% over GDE. Guided by theoretical calculations, they also introduced Au into Cu catalysts (Au–Cu) to reduce the *CO coverage and *H adsorption.94 This strategy optimized the ratio of CH4 and hydrogen from ∼1.5 (Cu) to ∼2.7 (3% Au–Cu) and achieved the CO2-to-CH4 transformation at the CD of 100 mA cm−2 and the FE of 56%. Recently, Qiao and colleagues utilized CeO2 to stabilize the Cu2+ active species in the Cu–Ce–Ox solid solution, reaching highly efficient CH4 production with an FE of 67.8%.95 CO2ET via C–C Bonds Formation Electroreduction of CO2 to C2 and C2+ chemicals Production of C2 and C2+ chemicals such as EtOH, C2H4, ethane, and C3H8OH is a vital way to improve the economic efficiency of CO2 transformation.73 Cu-based catalysts are widely studied because Cu is the most efficient for coupling of C1 intermediates found to date. Our group discovered that the N-doped graphene quantum dots and OD Cu composites (NGQ/Cu-nr) exhibit high performance for CO2 transformation to EtOH and C3H8OH.30 NGQ/Cu-nr can provide dual sites for the stabilization of *CH2CHO intermediate. As a result, NGQ/Cu-nr displays better performance in producing C2 and C2+ chemicals at a commercial CD of 282.1 mA cm−1 and FE of 52.4% (Figure 3f). Notably, Zhuang and co-workers reported that a thick polyaniline (PANI) modifier on the Cu surface exhibits distinctive FE and stability for producing C2+ products due to the modified interaction of the Cu/PANI interface.96 Recently, Grubbs et al. reported that Cu electrode coated with tricomponent copolymer could yield C2+ selectivity of 77% (Figures 3g and 3h).83 Bao and co-workers mixed Cu NPs and CuI powders physically to obtain a Cu-CuI composite.97 The catalyst had many Cu/Cu+ sites that enhanced the electrocatalytic performance. Research performed using flow cells with Cu-CuI catalyst achieved C2+ partial CD of 591 mA cm−2 at −1 V versus reversible hydrogen electrode (RHE). Iodide-based Cu catalysts were also reported by Qiao et al. for efficient CO2-to-ethane transformation. In situ studies revealed the pathway of CO2-to-ethane on iodide-derived copper (ID Cu), showing the importance of stabilization of ethoxy intermediate for ethane production.98 In addition, Gao et al. reported bioinspired hierarchical and inherent hydrophobic Cu catalysts in GDE to enhance the C2+ production in flow cells.99 Hydrophobic Cu could capture more CO2 molecules to Cu surface and provide a more robust gas–liquid–solid triple-phase interface for catalytic CO2ET with C2+ CD up to 255 mA cm−2 and the FE of 64%. Direct production of C2 and C2+ chemicals from CO2 is widely studied but suffers from many side reactions and poor selectivity. CO electrochemical reduction (COER) provides an avenue as a two-step pathway for CO2ET following CO2-to-CO technology.100 Many Cu-based catalysts are also used to produce C2+ chemicals from CO. Kanan et al. compared the COER performance of traditional Cu NPs and OD Cu. It was found that the catalysts prepared via the electrochemical method can reduce CO to C2+ chemicals at a FE of 57%, while the traditional Cu NPs mainly produce H2.101 Jiao and co-workers fabricated a high-performance flow cell for COER with a well-defined liquid-gas interface, reaching CO electroreduction at CD up to 1 A cm−2 with enhanced C2+ selectivity (FE of 91%).102 Further, co-reduction of CO2 and CO mixture on Cu catalysts has been studied by Strasser et al.103 The authors also examined the mechanism via isotope labeling experiments. Electrochemical carboxylation reaction Organic halides pollute the environment due to their high toxicity and are difficult to degrade by conventional techniques.104 Electrosynthesis of value-added carboxylic acid through electrochemical carboxylation and C–C bond formation is a promising strategy. Typical CO2ET, including electrochemical carboxylation mechanisms, are presented in Figure 4, which will be discussed further. Recently, nanostructured heterogeneous catalysis provides opportunities for highly efficient electrochemical carboxylation. Lu and co-workers synthesized Ag NPs as the electrocatalysts for electrochemical carboxylation of 2-phenyl bromide, yielding 98% of 2-phenyl propionic acid.106 In addition, Ag NPs displayed excellent stability and reusability and can be reused >10 times. Rajagopal et al. coated Ag, Cu, and Ni on carbon electrodes and utilized the AgCuNi alloy with different compositions as electrocatalysts for electrochemical carboxylation of benzyl bromide with CO2 in IL electrolytes.107 Ag46Cu40Ni14 displays the highest peak CD and lowest peak potential and exhibits better catalytic CO2ET performance than other trimetallic alloys with different compositions. He et al. prepared a [email protected]@CC composite through an electrospinning strategy as a binder-free electrode for electrochemical carboxylation.108 2-Phenyl propionic acid with a 99% yield can be obtained (Figure 5a-I). Moreover, [email protected]@CC is bifunctional and can also perform CO2-to- COO− transformation with a FE of 91%. Atobe and co-workers developed a CO2ET system with MFR for high-rate electrochemical carboxylation of benzyl halides and CO2 at a yield of 95% (Figure 4a).105 This MFR system could control unstable carboxylate ions without sacrificial anode utilization such as Mg. Notably, sensitive, toxic, and harmful chemicals can be avoided in this reaction, and this non-metal-ion-containing MFR system is promising in electrosynthesis. The possibility of asymmetric electrochemical carboxylation of halides was reported by Lu and co-workers.111 Chiral [CoI(salen)]− was used to active 1-phenylethyl chloride and produce chiral 2-phenylpropionic acid in 37% yield, with an ee value of 83%. However, most of the reported electrochemical asymmetric carboxylation reactions are based on homogeneous catalysts, while the heterogeneous catalytic systems based on asymmetric nanoscale catalysts still need to be further explored.112 Figure 4 | Typical reaction mechanism of CO2ET with organic substrates. (a) Reaction mechanism of electrochemical carboxylation of halides in the flow cell. Reprinted with permission from ref 105. Copyright 2015 Royal Society of Chemistry. Possible reaction pathways of electrosynthesis of (b) ethylamine and (c) dimethylamine. Reprinted with permission from refs 15 and 27, respectively. Copyright 2015 Elsevier and Copyright 2017 Royal Society of Chemistry. Download figure Download PowerPoint Further, CO2 can be transformed with ketones, α,β-unsaturated ketones, and imines via electrochemical carboxylation (Figure 5a-II).53 However, unlike other CO2ET reactions, CO2 is usually not electrochemically activated when reacted with ketones. Typically, CO2 could be coupled directly with organic intermediates to form target products. Lu et al. examined the asymmetric electrochemical carboxylation of 4-methylpropiophenone with alkaloid-based catalysts and produced α-ethyl-α-hydroxy-4-methylbenzeneacetic acid with optical activity.54 Zhang and co-workers demonstrated a one-pot electrochemical synthesis of β-carboxyl ketones with CO2 and α,β-unsaturated ketones as feedstocks.55 Si nanowires (NWs) were applied in this