Review Fundamentals and Challenges of Electrochemical CO2 Reduction Using Two-Dimensional Materials Zhenyu Sun,1,* Tao Ma,1 Hengcong Tao,1 Qun Fan,1 and Buxing Han2,* Electrochemical CO2 reduction (ECR) to value-added fuels and chemicals provides a ‘‘clean’’ and efficient way to mitigate energy shortages and to lower the global carbon footprint. The unique structures of two-dimensional (2D) nanosheets and their tunable electronic properties make these nanostructured materials intriguing in catalysis. Various 2D nanosheets are showing promise for CO2 reduction, depending on the preferred reaction product (HCOOH, CO, CH4, CH3OH, or CH3COOH). In this review, we focus on recent progress that has been achieved in using these 2D materials for ECR. We highlight procedures available for tuning catalytic activities of 2D materials and describe the fundamentals and future challenges of CO2 catalysis by 2D nanosheets. INTRODUCTION Energy consumption continues to grow rapidly and is expected to reach a level approximately two times greater than that of current consumption by 2050.1 Fossil fuels will likely continue to be a major source of energy for the forseeable future. It has been predicted that in the coming few decades (2010–2060), about 500 gigatons of CO2 will be generated from combustion of fossil fuels.2 Excessive anthropomorphic CO2 emission has caused problems associated with resources, environment, and climate, known as the ‘‘greenhouse effect.’’ To alleviate these adverse effects, converting CO2 into fuels or commodity chemicals ushers in hope for an energy transition from today’s ‘‘fossil fuel economy’’ to a sustainable ‘‘CO2 economy.’’ CO2 can be converted through diverse routes, including biochemical,3 electrochemical,4 photochemical,5 radiochemical,6 and thermochemical reactions.7,8 Among these approaches, direct electroreduction of CO2 into hydrocarbons, oxygenates, or CO is attractive because of (1) environmental compatibility coupling with carbon-free renewable energy resources (solar, tidal, and wind), (2) operating under ambient temperature and pressure, (3) control of reactions by adjusting external parameters such as electrolytes and applied voltages, and (4) engineering and economic feasibility. Nonetheless, the linear molecule CO2 is stable and chemically inert with a low electron affinity and a large energy gap (13.7 eV) between its lowest unoccupied molecular orbital and highest occupied molecular orbital. CO2 transformation is dominated by nucleophilic attacks at the carbon, which is an uphill process requiring a substantial input of energy (750 kJ mol1 required for dissociation of the C=O bond). Many studies of electrochemical CO2 reduction (ECR) have used water as electrolyte, but CO2 dissolves poorly in water at acidic or neutral pH values. Homogeneous9 and heterogeneous catalysts10 have been applied to accelerate this kinetically slow reduction reaction. Although high selectivity is achieved in homogeneous catalysis, most homogeneous systems suffer from drawbacks, such as high cost, toxicity, poor stability, and complex post-separation, limiting practical 560 Chem 3, 560–587, October 12, 2017 ª 2017 Elsevier Inc. The Bigger Picture Transforming CO2 into fuels and chemicals by using electrocatalysis is a promising strategy for providing a long-term solution to mitigating global warming and energy supply problems. Two-dimensional (2D) nanosheets featuring abundant surface atoms of low coordination and large specific surface area appear to be beneficial for fast interfacial charge transfer and facile electrochemical catalysis. The properties of nanosheet can be readily tuned by variations in their thickness, by modification with heteroatoms, or by external stimuli such as electric field, strain, and illumination, providing new routes for engineering 2D materials for CO2 electrocatalysis. We describe the latest advances in electrocatalytic CO2 reduction using 2D materials and their hybrids. Relationships between structure and properties in these emerging 2D electrocatalysts are discussed. A foundational summary for electrocatalytic CO2 conversion and possible reaction pathways are highlighted. application, but which are less likely to occur in heterogeneous catalysis. Therefore, major efforts have been made to develop heterogeneous electrocatalysts including (bi-)metals, metal oxides and sulfides, carbon-based materials, and organic frameworks.10 Despite advances achieved in the electrocatalytic reduction of CO2, this field still faces challenges of (1) large overpotential (or low energetic efficiency), (2) slow electron transfer kinetics resulting in low exchange current densities, (3) unsatisfactory selectivity, implying costly separation steps, and (4) deactivation of electrodes in less than 100 hr, restricting practical use and technological commercialization. Most CO2 electrocatalysts reported so far operate below 20 mA cm2, which is, however, far less than commercial electrolyzers typically operating with over 70% efficiency at current densities above 200 mA cm2.11 In addition, the scarcity and high cost of noble metal catalysts (Pt, Au, and Pd) are obstacles to their large-scale applications. From these scenarios, heightened research efforts have focused on the design and synthesis of novel, cost-effective, and robust electrocatalysts that can reduce CO2 at high rates with a minimum amount of overpotential. Research on two-dimensional (2D) nanosheets has burgeoned in the past decade after the isolation of graphene. A large range of 2D compounds, including group IV and V elements, metals, chalcogenides, oxides, halides, nitrides, carbides, hydroxides, hydrides, phosphates, phosphonates, and covalent organic frameworks, have been reported. The family of 2D materials display insulating, semiconducting (with direct and indirect band gaps ranging from UV to infrared throughout the visible region), and metallic behavior. Of particular interest is that enhanced and novel electronic and optical properties emerge when layered materials are thinned to a single layer or to few-layer flakes (referred to as ‘‘nanosheets’’) as a result of quantum confinement. Semiconducting 2D materials exhibit strong enhancement of Coulomb interactions among charge carriers and defects, leading to longer lived excitons and trions than those in bulk.12 Another important feature is the specific surface area created with reduced dimensionality. In addition, the physicochemical properties of 2D materials can be effectively tuned in diverse ways, such as doping, heterostructuring, chemical functionalization, and alloying.13 Layered materials with interlayer cohesive energies of 200 meV or less per atom can be exfoliated down to nanosheets via top-down methods (e.g., mechanical or chemical exfoliation to overcome interlayer van der Waals forces).14 Singleand few-layer nanosheets can also be grown through bottom-up approaches (e.g., chemical vapor deposition, wet-chemical synthesis). These strategies, in combination with characterization using spectroscopic techniques and theoretical calculations, open new opportunities to engineer nanosheets for catalysis and energy applications. Catalysis with 2D nanosheets has sparked increasing interest because such sheets have unique structural and electronic properties, albeit this area is still in its infancy.15 Ultrathin nanosheets with open double-sided surfaces possess abundant exposed surface atoms that can easily escape from the respective lattice to form vacancy-type defects. Such vacancy defects, together with the structural disorder that usually appears in nanosheets, can reduce the coordination number of the surface atoms, leading to dangling bonds and enhancing catalytic performance. An increase in low-coordinated surface sites promotes chemisorption of reactants. Control of vacancy defects allows one to modify the electronic structure and thereby tailor corresponding catalytic activities. Equally importantly, atomic sites at the edges of nanosheets with low coordination can afford interesting catalytic properties. The 1State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China 2Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid and Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China *Correspondence: sunzy@mail.buct.edu.cn (Z.S.), hanbx@iccas.ac.cn (B.H.) https://doi.org/10.1016/j.chempr.2017.09.009 Chem 3, 560–587, October 12, 2017 561 well-defined structure of 2D nanosheets also provides an applicable platform for investigation of catalytic mechanisms at the atomic level. 2D nanosheets with a substantial number of exposed active sites appear to be beneficial for fast interfacial charge transfer and facile electrochemical catalysis. Very promising results have already been realized in developing nanosheets for clean energy conversion (involving water, hydrogen, and oxygen) competitive with noble metal electrocatalysts.16 The properties of nanosheets can be readily tuned by varying their thickness, and by external stimuli such as electric field, strain, illumination, providing new routes to engineer 2D materials for CO2 (photo)electrocatalysis. Nanosheets can be chemically doped with heteroatoms or surface decorated with catalytic nanoparticles or heterostructures to direct particularly desired CO2 chemistries, and such surface modifications can be used to design multiple mechanistic catalytic steps. Further, the limiting specific surface area imparted by nanosheets offers a promise of limiting catalytic activity. Recently, a diversity of 2D materials ranging from metals, metal oxides to chalcogenides, and even metal-free catalysts have been demonstrated to have great potential for electrocatalytic CO2 reduction.17–22 In the following sections, we review recent advances in electrocatalytic CO2 reduction using 2D materials. Relationships between structure and properties in these emerging 2D electrocatalysts are discussed. A foundational summary for electrocatalytic CO2 conversion and possible reaction pathways are highlighted. An outlook of future prospects for conversion of CO2 into fuels by nanosheet electrocatalysis is described. A PERSPECTIVE OF ELECTROCHEMICAL CO2 REDUCTION Heterogeneous electrochemical reduction of CO2 occurs at electrode-electrolyte interfaces. Three steps are mainly considered when modeling these heterogeneous catalytic processes: (1) Chemical adsorption of CO2 on the surface of a catalyst (cathode). (2) Electron transfer and/or proton migration to break C–O bonds and/or form C–H bonds. (3) Rearrangement of product species followed by desorption from electrode surface and diffusion into electrolyte. Aqueous electrocatalytic reduction of CO2 is particularly challenging because of occurrences of competitive proton reduction, leading to poor selectivity for carbonaceous products. Another issue is the low solubility of CO2 in water (0.034 M), adversely affecting such diffusion-controlled reactions. There are several approaches that can be used to enhance CO2 dissolution, involving (1) the use of nonaqueous solutions; (2) low-temperature working conditions; (3) increasing CO2 partial pressure. The CO2 pressure also influences the thermodynamics of the ECR. Some metal catalysts that are inactive at atmospheric pressure, however, can reduce CO2 to CO, formate, or hydrocarbons at elevated pressures. Alternatively, the rate at which CO2 flows into the catholyte below 50 mL CO2 min1 affects catalytic activity. Higher CO2 flow rates yield larger reaction rates, faradic efficiencies, and product selectivity.23 Parameters that have been used as a gauge for evaluating the performance of a catalyst include the following: (1) onset potential or overpotential (h, i.e., the difference between the thermodynamic and actual electrode reduction voltages); (2) current density (current divided by the geometric surface area of working electrode); (3) 562 Chem 3, 560–587, October 12, 2017 faradic efficiency (EFaradic = anF=Q, where a is the number of electrons transferred, n is the number of moles for a given product, F is Faraday’s constant [96,485 C mol1], and Q is all the charge passed throughout the electrolysis process); (4) energetic efficiency (Eenergetic = ðE 0 =E 0 + hÞ3EFaradic , where E0 is standard potential); (5) turnover frequency (TOF), a measure of per-site activity of catalysts, such as TOF (s1) for CO formation, i0 ðA cm2 Þ 3 EFaradic ðCOÞ ; ½active site densityðsites cm2 Þ 3 ½1:602 3 1019 C=e 3 2e =CO2 and (6) Tafel slope. A Tafel plot relates to overpotential verses logarithm of the partial current density of a specific product. The Tafel slope derived is an indicator of reaction pathways and the rate-determining step (RDS). A slope of 118 mV dec1 implies that the RDS for CO2 reduction is the initial step of the CO2,– generation, whereas the RDS is the chemical step following the fast one-electron pre-equilibrium when the Tafel slope is 59 mV dec1.24 A good catalyst should minimize the activation barrier for CO2 reduction in relation to proton reduction, driving CO2 reduction selectively at low overpotential (i.e., high energy efficiency) with satisfactory reaction rates (i.e., high turnover number). The total cell voltage required for ECR includes potentials for both anodic and cathodic processes (Ecell = Eanode Ecathode). Real-world ECR and oxygen evolution reaction (OER) catalysts require overpotentials of several hundred millivolts to attain satisfactory reaction rates. As a consequence, cell voltages usually exceed the reaction formal potentials. Note that the anode catalyst plays an important role in CO2 conversion because it consumes almost half of the electrical input. Therefore, improving the anode efficiency and lowering OER overpotentials can reduce the overall energy requirements for CO2 reduction, making this carbon mitigation strategy more practical. Thermodynamics and Kinetics of CO2 Reduction Direct reduction of CO2 involves one-electron transfer to form CO2 anion radical (CO2,–). However, this step is highly unfavorable, having a negative formal redox potential 1.97 V (versus standard hydrogen electrode [SHE]) in an aprotic solvent (N,N0 -dimethylformamide [DMF]) and 1.90 V in water (pH 7). Catalytic strategies have thus been developed to bypass the formation of CO2,– through proton-assisted multiple-electron transfer to reduce CO2 at lower energetic costs. Depending on the number of electrons and protons transferred, CO2 can be reduced to 16 different products, including CO, oxalic acid (H2C2O4) (C2O42, basic medium), formic acid (HCOOH) (HCOO, basic medium), formaldehyde (HCHO), C, methanol (CH3OH), methane (CH4), ethylene (C2H4), ethanol (C2H5OH), ethane (C2H6), and n-propanol (C3H7OH) (Equations 1–20; CO2 reduction potentials versus SHE at pH 7). The competing proton reduction feeding hydrogen evolution is a two-electron process (Equation 21).4,10,25 CO2 + 2H + + 2e /HCOOH E 0 redox = 0:610 V CO2 + 2H2 O + 2e /HCOOH + OH E 0 redox = 1:491 V CO2 + 2H + + 2e /CO + H2 O E 0 redox = 0:530 V CO2 + 2H2 O + 2e /CO + 2OH E 0 redox = 1:347 V 2CO2 + 2H + + 2e /H2 C2 O4 E 0 redox = 0:913 V E 0 redox = 1:003 V 2CO2 + 2e /C2 O2 4 + CO2 + 4H + 4e /HCHO + H2 O E 0 redox = 0:480 V CO2 + 3H2 O + 4e /HCHO + 4OH E 0 redox = 1:311 V (Equation (Equation (Equation (Equation (Equation 1) 2) 3) 4) 5) (Equation 6) (Equation 7) (Equation 8) Chem 3, 560–587, October 12, 2017 563 CO2 + 4H + + 4e /C + 2H2 O E 0 redox = 0:200 V CO2 + 2H2 O + 4e /C + 4OH E 0 redox = 1:040 V CO2 + 6H + + 6e /CH3 OH + H2 O E 0 redox = 0:380 V CO2 + 5H2 O + 6e /CH3 OH + 6OH E 0 redox = 1:225 V CO2 + 8H + + 8e /CH4 + 2H2 O E 0 redox = 0:240 V CO2 + 6H2 O + 8e /CH4 + 8OH E 0 redox = 1:072 V 2CO2 + 12H + + 12e /C2 H4 + 4H2 O E 0 redox = 0:349 V 2CO2 + 8H2 O + 12e /C2 H4 + 12OH E 0 redox = 1:177 V 2CO2 + 12H + + 12e /C2 H5 OH + 3H2 O E 0 redox = 0:329 V 2CO2 + 9H2 O + 12e /C2 H5 OH + 12OH E 0 redox = 1:157 V 2CO2 + 14H + + 14e /C2 H6 + 4H2 O E 0 redox = 0:270 V 3CO2 + 18H + + 18e /C3 H7 OH + H2 O E 0 redox = 0:310 V 2H + + 2e /H2 E 0 redox = 0:42 V (Equation 9) (Equation 10) (Equation 11) (Equation 12) (Equation 13) (Equation 14) (Equation 15) (Equation 16) (Equation 17) (Equation 18) (Equation 19) (Equation 20) (Equation 21) The effects of conditions such as the types of electrocatalysts (composition, size, shape, oxidation state, and crystallographic structure), electrolytes (cation, anion, concentration, and pH), temperature, pressure, and applied potential can be superimposed onto the reaction scheme. A reduction reaction with a more positive E0 is thermodynamically more favorable according to the relationship DG = nFE0, where n is the number of electrons transferred during the redox reaction and F is the Faraday constant. On the basis of E0, the reductions of CO2 toward hydrocarbon or alcohol products should be thermodynamically more favorable than CO, HCOOH, HCHO, and H2 production. However, that is actually not the case because in addition to a thermodynamic barrier, CO2 reduction also has a kinetic dependence on the concentration of available protons in solution. This implies that a preferred catalyst likely entails catalytic sites transferring electrons that are close to sites providing protons. The hydrogenation of adsorbed C1 intermediates is kinetically easier than the formation of C–C bonds, limiting the rate and selectivity of C2 and higher hydrocarbon production. The maximum faradic efficiency for C2H4, the simplest C2 product, was reported to be 60%,26 whereas that of C3 product (C3H7OH) is no more than 30%.27,28 In the multi-electron transfer reduction of CO2, the adsorption energies of reaction intermediates seem to follow linear scaling relationships, thus limiting catalytic efficiency.29,30 In order to improve catalytic activity, breaking such linear scaling relations is required, which can be realized by: (1) reducing coordination numbers, (2) doping with p-block elements, (3) introducing oxophilic sites, and (4) coating the catalyst surface with active ligands.30 Electrolytes In CO2 electroreduction, electrolytes provide a medium to transfer coupled electrons and protons (e/H+). The type and concentration of electrolytes affect catalyst activity and selectivity. It should also be noted that trace impurities (metal and organic) from electrolytes can cause the deactivation of catalytic sites and affect catalyst performance. Aqueous Solutions Most studies in electrochemical CO2 reduction have focused on weakly acidic or alkaline CO2-saturated aqueous electrolytes containing inorganic salts with HCO3, SO42, or Cl anions and alkali metal cations (e.g., Na+ and K+). Cationic species alter the outer Helmholtz plane potential and can also influence the hydrogen coverage on the electrode by delivering water molecules from their 564 Chem 3, 560–587, October 12, 2017 solvation shell to the electrode. Meanwhile, cations can affect relative concentrations of charged species (such as anion radical intermediates) close to the electrode, thus affecting product selectivity and current density.31 The rate of CO2 electroreduction to CH3OH was found to increase with the surface charge of electrolyte cations in the following order: Na+ < Mg2+ < Ca2+ < Ba2+ < Al3+ < Zr4+.32 Cation size was shown to influence product selectivity. Large cations favor the formation of HCOOH on a Hg electrode, C2H4 on a Cu electrode,33 and CO on a Ag electrode.31 Large cations were also demonstrated to suppress H2 evolution.31,34 H2 evolution prevailed over CO2 reduction in Li+ electrolytes, whereas CO2 reduction was favorable in Na+, K+, and Cs+ solutions. Anionic species (i.e., Cl, ClO4, SO42, HCO3, and H2PO4) with different buffer capacities affect the local pH at the electrode and hence the nature and selectivity of products formed.35 A high local pH inhibits H2 evolution because of low proton concentration. C2H4 and alcohols are favored in KCl, K2SO4, KClO4, and dilute KHCO3 solutions under a non-equilibrium local high pH, whereas CH4 is preferentially generated in KH2PO4 and concentrated KHCO3 solutions.35 The CH4/C2H4 product ratio increases with increasing HCO3 concentration. The effects of halogens on ECR were also studied with a Cu mesh electrode in aqueous electrolytes of 3 M KCl, KBr, and KI.36 It was found that the bond between adsorbed halides (e.g., Br, Cl, or I) and Cu facilitated electron transfer from these anions to the vacant orbital of CO2 and promoted CO2 reduction. A stronger halide adsorption to the electrode made CO2 more intensely restrained, thus enhancing reduction current. In addition, specifically adsorbed halides inhibited proton adsorption and induced a higher hydrogen overvoltage. A recent study from Strasser and colleagues revealed that different halogen anions in electrolytes resulted in distinct product distributions for ECR on a Cu electrode.37 Addition of both Cl and Br increased CO selectivity. On the contrary, the presence of I decreased the selectivity toward CO, whereas CH4 formation was six times higher than with halide-free electrolytes. Organic Electrolytes Nonaqueous electrolytes were applied for ECR to increase CO2 solubility and also to suppress hydrogen evolution, thereby improving faradic efficiency. The solubility of CO2 in DMSO and acetonitrile (AN) is approximately four times that in water, whereas its solubility in CH3OH and propylene carbonate is about five and eight times greater, respectively. DMF is a good solvent for CO2, with 20 times larger solubility than in water at ambient conditions. Nonaqueous solvents (DMF, DMSO, and AN) were demonstrated to facilitate CO2,– dimerization with adsorbed CO2 molecules to produce C2O42 on Sn, In, Pb, and Hg electrodes,38 whereas the solvent of CH3OH increases CH4 selectivity on a Cu electrode.39 Manipulating the amount of water in an organic electrolyte to tune proton availability can effectively control faradic efficiency and selectivity for CO2 reduction. Ionic Liquids The use of ionic liquids (ILs), especially those featuring imidazolium cations, is beneficial to: (1) lowering CO2 reduction overpotential, most likely by complexation to reduce the energy of the CO2,– intermediate; (2) inhibiting the hydrogen evolution reaction (HER); and (3) increasing the selectivity for CO formation.40,41 The ILs that are commonly used in ECR include 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6), 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM-TFO), Chem 3, 560–587, October 12, 2017 565 1-butyl-3-methylimidazolium triflate (BMIM-OTF), and 1-ethyl-3-methyl-imidazolium triflate (EMIM-OTF), among others.42 BMIM-BF4 was shown to exhibit the highest current density with large faradic efficiency for CH4 formation on an N-doped graphene-like electrode among five different ILs, BMIM-BF4, BMIM-PF6, BMIMTFO, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-TF2N), and 1-butyl-3-methylimidazolium dicyanamide (BMIM-DCA).43 Tuning the cation structure of imidazolium-BF4 ILs showed that the C4 and C5 protons on the imidazolium ring are vital for efficient CO2 reduction to yield CO on a Ag electrode.44 In order to overcome the relatively high cost and viscosity of ILs, they are usually dissolved in organic solvents or water for use as electrolytes. The concentrations of ILs reported in the literature vary from as low as millimolar to neat ILs. Finding an optimum ratio of ILs remains an important unanswered question. The ratio between an IL and H2O affects the pH and viscosity of the electrolyte. Using an electrolyte comprising 75 mol % water and 25 mol % EMIM-BF4 resulted in an enhanced CO2 reduction current density that was approximately five times higher than that of pure EMIM-BF4.45 The decrease in pH caused by the formation of hydroxyl ions (such as [BF3OH], [BF2(OH)2], or [BF(OH)3]) from EMIMBF4 hydrolysis led to a greater proton availability, thus enhancing the rate of CO2 reduction. Likewise, [Bmim]BF4–H2O binary electrolytes were observed to afford both large current density and high CH4 selectivity when the water content in the mixture was less than 5 wt %.43 High faradic efficiencies for CO formation were reported for electrolytes containing R20 mol % EMIM-TFO at potentials more negative than 1.46 V versus Ag/ AgCl over a Ag electrode. The CO faradic efficiency reached 95.6% G 6.8% in a 50 mol % IL electrolyte.46 Despite IL-based electrolytes providing benefits of high CO2 solubility and low overpotentials, their high cost and poor stability in the presence of H2O need to be addressed for practical CO2 electrolyzers. Solid-Oxide Electrolytes Molten carbonate or solid-oxide electrolytes are used in electrolyzers operating at high temperatures (>673 K) for the co-electrolysis of H2O and CO2. Such electrolyte is typically zirconia stabilized by yttrium oxide.47 A BaCeO0.5Zr0.3Y0.16Zn0.04O3d electrolyte also showed utility in converting CO2 into CO and CH4 in the presence of H2 and H2O.48 Electrochemical Cells Electrolyzer design has a profound effect on mass transport. There is no standard experimental setup or methodology for ECR. A variety of electrolyzers and flow cells have been developed. Two representative lab-scale prototypes are presented below. H-Type Cells A typical and commonly used lab-scale electrochemical cell (H-type) in ECR encompasses two compartments and three electrodes (i.e., working electrode, reference electrode, and counter electrode).49 The cathodic and anodic chambers are separated by a proton-conducting (e.g., Nafion 117) or an anion-exchange polymer membrane. This setup allows ionic conductivity, preventing the transport of cathodic products to the anode where they can be oxidized while keeping the electrodes in close proximity. The cathode compartment can be directly connected to a mass spectrometer or gas chromatography spectrometer for product analysis. Continuous-Flow Cells Microfluidic flow cells have been recently used in ECR. This type of setup comprises two electrodes separated by a flowing liquid electrolyte50 and can test both cathode 566 Chem 3, 560–587, October 12, 2017 and anode performance by using an external reference electrode. Gas diffusion electrodes (GDEs) are often used for better control of the three-phase boundary where reactions occur. This continuous-flow reactor presents advantages for online product detection. Other versions of flow reactors include filter-press and parallelplate type electrochemical cells.51 A scale-up ‘‘trickle bed’’ cell with a two-phase flow of catholyte was designed by Li and co-workers, which can achieve a current efficiency up to 91% of the efficiency obtained in a lab prototype.52 Possible Reaction Pathways Efforts have been devoted to exploring the reaction mechanism of CO2 activation and reduction both theoretically and experimentally.25,53 However, most mechanisms are hypothesized on the basis of the results of either the Tafel slope or density functional theory (DFT) calculations. Further experimental work combining DFT calculations and in situ spectroscopy is needed to fully elucidate mechanistic pathways. Formation of Formate and Formic Acid HCOO or HCOOH evolves possibly via an intermediate that binds to a transitionmetal electrode either through one oxygen atom (monodentate) or two oxygen atoms (bidentate) (Figure 1A and top route in Figure 1B).25 This intermediate can form through reaction with *H via CO2 insertion into the metal–hydrogen bond or through direct protonation with H+ from solution. An alternative pathway is presumed to proceed via a CO2,– radical that reacts with a neighboring water or proton to generate HCOO or HCOOH on p-block metals, such as In, Sn, Pb, and Hg (middle route in Figure 1B).25,54 In addition, the presence of HCO3 ions was found to enhance HCOO production.54 Bocarsly and co-workers recently proposed that the formation of a surface-bound tin carbonate is a key chemical step involved in the reduction of CO2 to HCOO on Sn electrodes (Figure 1C).55 Specifically, a two-electron reduction of the electrode from a native SnO2 to a SnII oxyhydroxide, which is the catalytic resting state, occurs before the reduction of CO2. This species then reacts with CO2 to form a surface-bound carbonate. The carbonate undergoes transfers of two electrons and one proton to form HCOO—, which desorbs to return the surface to the SnII oxyhydroxide. Formation of Carbon Monoxide The electrochemical reduction of CO2 to CO is a simple two-electron transfer process (Figure 2). By transferring a concerted proton-electron (H+/e) from solution to adsorbed species, a CO2 molecule is reduced to a carboxyl intermediate *COOH. A second H+/e can subsequently attack the oxygen atom (OH) in the *COOH to form H2O (l) and CO, which then desorbs from the electrode.53 The conversion of COOH* to *CO was found to take place readily. But the initial step of CO2 conversion to *COOH is inhibited by weak COOH binding. Likewise, the last step of CO desorption from the surface is hindered by strong CO binding. These two steps were calculated to be rate limiting.56 Another possible route to generate *COOH is through a decoupled electron and proton transfer, involving the formation of a CO2,2– radical that is adsorbed at electrode surfaces (bottom route in Figure 2). The two pathways were suggested to show different pH dependences. Increases in pH or pressure likely promote the yield of *COOH over *CO.57 Metals such as Au, Ag, Zn, and Pd bind *COOH tightly enough for further reduction to produce a *CO intermediate. The *CO is weakly bound to the electrode surface, and CO desorbs from the electrode as a major product. Chem 3, 560–587, October 12, 2017 567 Figure 1. Possible Reaction Pathways for the Electrocatalytic Reduction of CO2 to HCOO or HCOOH (A) Monodentate or bidentate intermediate route. 25 (B) CO 2 ,– radical intermediate route. 25,54 (C) Surface-bound carbonate intermediate route. 55 Formation of Formaldehyde, Methanol, and Methane (C1 Products) *CO is likely a common intermediate for the production of HCHO, CH3OH, and CH4.25 DFT calculations of ECR on Cu (211) surfaces by Peterson et al. showed that the thermodynamically most favorable pathway involves the initial formation of *CO and subsequent hydrogenation to *HCO, *H2CO (desorb as HCHO), and *H3CO (methoxy) (Figure 3A).58 CH3OH can be formed by *H3CO reduction. The methoxy intermediate can be alternatively reduced to CH4 and *O. The *O is finally reduced to H2O. However, C–H bond formation coupled with C–O dissociation to yield CH4 was calculated to be kinetically prohibitive with a barrier of 1.21 eV, much higher than the barrier of 0.15 eV for CH3OH production.53 Another possible reaction path, as proposed by Nie et al.,53 is through a *COH intermediate, which is reduced to an adsorbed C (Figure 3B). Such graphitic carbon species has been observed by in situ X-ray photoelectron spectroscopy and Auger electron spectroscopy during ECR on Cu electrodes.59 The surface carbon can be further reduced to *CH, *CH2, *CH3, and finally to CH4. The 568 Chem 3, 560–587, October 12, 2017 Figure 2. Possible Reaction Pathways for the Electrocatalytic Reduction of CO2 to CO See Kortlever et al. 25 oxophilicity of the surface, as determined by the binding energy of Oads, could play an important role in the reaction for determining selectivity between CH4 and CH3OH. Only CH3OH was detected on the poorly oxophilic Au, whereas for the strongly oxophilic Fe, only CH4 was identified.60 From this scenario, engineering a surface by modifying the binding energy for Cads and Oads to favor or disfavor C–O bond breakage allows one to improve selectivity toward one product. Formation of Acetaldehyde, Ethylene, and Ethanol (C2 Products) C2H4, acetaldehyde (CH3CHO), and C2H5OH have been produced at less negative potentials than CH4, especially at high (alkaline) pH on Cu-based catalysts.26,61 The formation of these C2 products is a complex process with many possible pathways involving both electrochemical and chemical steps, which is still the focus of debate. In a ‘‘carbene’’ mechanism, C2H4 is formed via either coupling of two *CH2 species53 or CO insertion in a Fischer-Tropsch-like step (Figure 4A), which has also been suggested to be the pathway for the formation of C2H5OH.62 An alternative pathway involves *CO dimerization with an electron transfer to form a *C2O2 key intermediate, which is subsequently protonated to *CO–COH (Figure 4B).25,63 The intermediate of ethenol can be further reduced to C2H4, or CH3CHO and then C2H5OH, with C2H4 being the kinetically favored product on Cu surfaces.64 Formation of Acetic Acid and Acetate Another C2 product, CH3COOH or CH3COO, has been reported from ECR on Cubased electrodes65 and also on an N-doped nanodiamond/Si rod array electrode.66 The formation of acetic acid was attributed to a nucleophilic attack from adsorbed CO2,– on the reduced –CH3 species (Figure 5).65,67 A faradic efficiency of 91.2%– 91.8% was achieved for CH3COO formation at 0.8 to 1.0 V (versus RHE) on N-doped nanodiamond.66 Electrokinetic data in combination with in situ infrared spectroscopy suggested the coupling of two adsorbed CO2,– to generate OOC– COO, which is possibly an important intermediate to form acetate. Formation of n-Propanol (C3 Product) Electrochemical reduction of CO2 to yield long-chain products, such as C3H7OH, remains a challenge. To date, only graphene/ZnO/Cu2O hybrid surfaces are known to Chem 3, 560–587, October 12, 2017 569 Figure 3. Possible Reaction Paths for Electrocatalytic CO2 Reduction to Produce HCHO, CH3OH, and CH4 on Cu Electrodes (A) A thermodynamic analysis. 58 (B) A combined thermodynamic and kinetic analysis.53 catalyze this process with maximum faradic efficiency of about 30%.28 It was hypothesized that an adsorbed C2 intermediate could undergo intermolecular C–C coupling with an adjacent C1 intermediate, followed by proton/electron transfers to form propionaldehyde (CH3CH2CHO) (Figure 6).62 CH3CH2CHO is further reduced to C3H7OH. 2D NANOSHEET CATALYSIS OF CO2 ELECTROREDUCTION Searching for catalysts with low cost but high activity and selectivity remains a challenge to promote the kinetically sluggish CO2 reduction process for large-scale production. Recently, 2D nanosheets featuring a large fraction of low-coordinated surface atoms have gained much attention for CO2 reduction (Table 1). 570 Chem 3, 560–587, October 12, 2017 Figure 4. Possible Reaction Paths for Electrocatalytic CO2 Reduction to Produce C2H4, CH3CHO, and C2H5OH (A) Coupling of two *CH 2 species or CO insertion in a Fischer-Tropsch-like step. 62 (B) *CO dimerization. 63 Formic Acid and Formate Selectivity Metal Nanosheets Xie and colleagues recently prepared freestanding 4-atom-thick Co sheets with and without surface Co oxide by using a ligand-confined growth strategy (Figure 7).18 The authors showed an onset potential (Eonset) of 0.68 V (versus saturated calomel Chem 3, 560–587, October 12, 2017 571 Figure 5. Possible Reaction Paths for Electrocatalytic CO2 Reduction to Produce CH3COO or CH3COOH See Genovese et al. 65 electrode [SCE]) for formate production in a CO2-saturated 0.1 M Na2SO4 solution on partially oxidized Co nanosheets (z0.84 nm), with an overpotential of only 0.07 V. A high current density (j) of 10.59 mA cm2 and a formate faradic efficiency (Efaradic) of 90.1% were obtained at a low overpotential (0.24 V) over 40 hr, competitive with a high-cost Pd-based catalyst with a j of 3.92 mA cm2 and Efaradic of 88% at an overpotential of 0.15 V in 0.5 M aqueous NaHCO3.82 Compared with bulk Co, these atomically thin layer catalysts achieved a 260-fold increase in catalytic activity . This enhancement could result from a large number of active sites provided by increased electrochemical surface area and the presence of a surface Co oxide with higher intrinsic activity. The Tafel slope of these partially oxidized Co layers was estimated to be about 59 mV dec1, suggesting a fast pre-equilibrium involving 1e transfer to form CO2,– and subsequent slower chemical reactions as the RDS. It was thus inferred that Co atoms confined in atomically thin layers accelerated CO2 activation by stabilizing the CO2,– intermediate more effectively than the bulk counterpart. An even cheaper Bi nanosheet catalyst was demonstrated to catalyze electrochemical CO2 reduction to HCOO in 0.5 M KHCO3 solution with a much larger faradic efficiency (92%) at 1.5 V (versus SCE) than that of commercial Bi powder (55%).19 Such Bi nanosheets also exhibited 100 mV lower onset potential (1.3 V versus SCE) but about five times larger current density (3.7 mA cm2 at 1.8 V versus SCE) than commercial Bi. Ultrathin metal layers can be highly active for CO2 reduction but can also be prone to oxidation, causing a rapid decay in cyclability. To solve this issue, confining metal sheets in graphene seems to be a good choice (Figure 8). Indeed, metallic Sn quantum sheets, confined in few-layer graphene, retained stability in air up to 570 C, whereas Sn nanoparticles (15 nm in diameter) without the protection of a graphene interlayer started to be oxidized at 200 C.68 Such Sn quantum sheets displayed a more positive onset potential (0.85 V versus SCE), larger current density (21.1 mA cm2 at 1.8 V versus SCE), and better formate faradic efficiency (89%) than 15 nm 572 Chem 3, 560–587, October 12, 2017 Figure 6. Possible Reaction Paths for Electrocatalytic CO2 Reduction to Produce C3H7OH See Hori et al. 62 Sn nanoparticles physically mixed with graphene (j 10.6 mA cm2; Efaradic 61.4%), 15 nm Sn nanoparticles (j 8.4 mA cm2; Efaradic 59.3%), and bulk Sn (j 1.6 mA cm2; Efaradic 44.5%). This enhancement could be due to a combination of high electrochemical surface area favoring CO2 adsorption, rapid rate-limiting electron transfer from CO2 to CO2,– benefiting from conductive graphene, and low Sn-Sn coordination numbers enabling stabilization of CO2,–. Metal-Oxide Nanosheets Enhanced electrocatalytic activity is expected upon reduction in thickness as a result of corresponding increases in active sites and electrical conductivity. This has been confirmed by a recent work in which 1.72-nm-thick Co3O4 layers exhibited a CO2 reduction current density of 0.68 mA cm2 at 0.88 V (versus SCE), 1.5 and 20 times higher than those of 3.51-nm-thick Co3O4 layers and bulk Co3O4, respectively.69 DFT calculations, along with CO2 adsorption isotherms and X-ray absorption fine structure spectroscopy, demonstrated that oxygen (II) vacancies facilitated CO2 adsorption and also HCOO– desorption. Such oxygen (II) vacancies played a role in lowering the rate-limiting activation barrier via stabilizing the HCOO–* intermediate, as reflected by the decrease of onset potential from 0.81 to 0.78 V (versus SCE) and Tafel slope from 48 to 37 mV dec1.70 Co3O4 nanosheets (z0.84 nm) rich in oxygen (II) vacancies exhibited a current density of 2.7 mA cm2 with 85% formate selectivity in 40 hr, showing promise in CO2 reduction catalysis. Mesoporous SnO2 nanosheets were also reported as an efficient, selective, and durable electrocatalyst for CO2 reduction.22 A high partial current density for HCOO (45 mA cm2) at a moderate overpotential (0.88 V) with large faradic efficiency (87% G 2%) was achieved (Figure 9), outperforming many GDEs in aqueous media. This superior performance was correlated to the porous hierarchical structure, which provided a large surface area and facilitated charge and mass transfer. Heteroatom-Doped Graphene and Graphene-Based Hybrids Despite that pristine graphene has low activity requiring high CO2 activation free energy (over 1.3 eV),83 doping graphene with heteroatoms (such as B, N, etc.) can reduce the CO2 adsorption barrier markedly to catalyze the electrochemical reduction of CO2 to C1 fuels. Boron-doped graphene has been used as a metal-free catalyst for ECR in 0.1 M KHCO3 aqueous solution.71 HCOO with a faradic efficiency of 66% was attained at 1.4 V (versus SCE). DFT calculations suggested that the presence of boron introduces asymmetric charge and spin density distribution throughout the ground state geometry, resulting in a high spin density. Those B and C atoms with positive spin densities were speculated to be catalytically active sites, favoring the chemisorption of protonated CO2 (COOH*). An even higher Chem 3, 560–587, October 12, 2017 573 Table 1. Summary of 2D-Based Materials Reported for Electrocatalytic CO2 Reduction Catalysts Electrolytes Ja (mA$cm2) Onset Potential (V) or hb (V) Main Products (FE)c Stability Reference Partially oxidized Co 4-atomic-layers 0.1 M Na2SO4 10.59 mA cm2 @ 0.85 V (versus SCE) h: 0.24 V @ 10.59 mA cm2 HCOO: 90.1% @ 0.85 V (versus SCE) 40 hr Gao et al.18 BiOCl-derived Binanosheets 0.5 M KHCO3 3.7 mA cm2 @ 1.8 V (versus SCE) onset potential: 1.3 V (versus SCE) HCOO: 92% @ 1.5 V (versus SCE) 7 hr Zhang et al.19 Sn sheets confined in graphene 0.1 M NaHCO3 21.1 mA cm2 @-1.8 V (versus SCE) onset potential: 0.85 V (versus SCE) HCOO: 89% @ 1.8 V (versus SCE) 50 hr Lei et al.68 Ultrathin Co3O4 nanosheets 0.1 M KHCO3 0.68 mA cm2 @ 0.88 V (versus SCE) onset potential: 0.82 V (versus SCE) HCOO: 64.3% @ 0.88 V (versus SCE) 20 hr Gao et al.69 Oxygen-deficient Co3O4 atomic layers 0.1 M KHCO3 2.7 mA cm2 @ 0.87 V (versus SCE) onset potential: 0.78 V (versus SCE) HCOO: 87.6% @ 0.87 V (versus SCE) 40 hr Gao et al.70 Mesoporous SnO2 nanosheets 0.5 M NaHCO3 50 mA cm2 @ 1.6 V (versus Ag/AgCl) h: 0.88 V @ 45 mA cm2 HCOO: 89% @ 1.6 V (versus Ag/AgCl) 24 hr Li et al.22 B-doped graphene 0.1 M KHCO3 2.0 mA cm2 @ 1.4 V (versus SCE) onset potential: 1.1 V (versus SCE) HCOO: 66% @ 1.4 V (versus SCE) 4 hr Sreekanth et al.71 N-doped graphene 0.5 M KHCO3 7.5 mA cm2 @ 0.84 V (versus RHE) onset potential: 0.3 V (versus RHE) HCOO: 73% @ 0.84 V (versus RHE) 12 hr Wang et al.20 SnS2/rGO nanosheets 0.5 M NaHCO3 13.7 mA cm2 @ 0.75 V (versus RHE) h: 0.68 V @ 13.9 mA cm2 HCOO: 84.5% @ 0.76 V (versus RHE) 14 hr Li et al.72 Ag nanosheets 0.5 M NaHCO3 10 mA cm2 @ 0.8 V (versus RHE) h: 0.29 V @ 5 mA cm2 CO: 95% @ 0.7 V (versus RHE) NA Lee et al.73 WSe2 nanoflakes 50 vol % EMIMBF4 aqueous solution 330 mA cm2 @ 0.764 V (versus RHE) h: 54 mV @ 18.95 mA/cm2 CO: 90% @ 0.764 V (versus RHE) 27 hr Asadi et al.74 MoSeS alloy monolayers 4 mol % EMIMBF4 aqueous solution 43 mA cm2 @ 1.15 V (versus RHE) onset potential: 0.55 V (versus RHE) CO: 45.2% @ 1.15 V (versus RHE) 10 hr Xu et al.75 Nb-doped vertically aligned MoS2 50 vol % EMIMBF4 aqueous solution 237 mA cm2 @ 0.8 V (versus RHE) onset potential: 0.142 V (versus RHE) CO: 82% @ 0.8 V (versus RHE) NA Abbasi et al.76 N-doped graphene foam 0.1 M KHCO3 1.8 mA cm2 @ 0.58 V (versus RHE) onset potential: 0.3 V (versus RHE) CO: 85% @ 0.58 V (versus RHE) 5 hr Wu et al.77 Ni-N modified graphene 0.1 M KHCO3 0.2 mA cm2 @ 0.65 V (versus RHE) h: 0.54 V @ 0.2 mA cm2 CO: > 90% @ 0.7 0.9 V (versus RHE) 5 hr Su et al.78 Re-SURMOF thin films 0.1 M tetrabutylammonium hydroxide in acetonitrile with 5 vol % trifluoroethanol 2.5 mA cm2 @ 1.6 V (versus normal hydrogen electrode, NHE) onset potential: 1.3 V (versus NHE) CO: 93% @ 1.6 V (versus NHE) NA Ye et al.79 g-C3N4/MWCNTs 0.1 M KHCO3 0.55 mA cm2 (JCO) @ 0.75 V (versus RHE) h: 0.64 V @ 0.92 mA cm2 CO: 60% @ 0.75 V (versus RHE) 50 hr Lu et al.80 Mo-Bi bimetallic chalcogenide nanosheets [BMIM]BF4/MeCN 12.1 mA cm2 @ 0.7 V (versus SHE) h: 0.16 V @ 1.0 mA cm2 CH3OH: 71.2% @ 0.7 V (versus SHE) 5 hr Sun et al.81 N-doped graphenelike materials [BMIM]BF4 with 3 wt % H2O 3.26 mA cm2 @ 1.4 V (versus SHE) h: 1.16 V @ 3.26 mA cm2 CH4: 93.5% @ 1.4 V (versus SHE) 5 hr Sun et al.43 J refers to geometric current density or partial current density with specific subscript. h refers to overpotential to generate a specific product. c FE refers to maximum faradic efficiency of the main products. a b HCOO selectivity (73%) was reported on N-doped graphene at 0.84 V (versus RHE) for the electroreduction of CO2 in 0.5 M aqueous KHCO3.20 N doping was supposed to modify the electronic properties of graphene, lowering the free energy barrier of*COOH formation, which is the potential limiting step, enhancing *COOH adsorption but weakening CO or HCOOH adsorption. However, 574 Chem 3, 560–587, October 12, 2017 Figure 7. Synthetic Scheme and Characterizations of Co Four-Atom-Thick Layers with and without Surface Oxide (A) Schematic formation processes of partially oxidized and pure-Co with four atomic layers. (B) Lateral HAADF-STEM image. Scale bar, 1 nm. (C) Corresponding intensity profile along the marked rectangle in (B) shows the four-atom thickness of the layer. (D and E) Corresponding crystal structures. Reprinted with permission from Gao et al.18 Copyright 2016 Macmillan Publishers Limited. the active sites and mechanism for selectivity remain to be explored. DFT calculations performed by Liu et al. showed that pyrrolic N performs the best for the ECR to yield HCOOH as a result of its higher *COOH adsorption energy (3.24 eV) and lower overpotential (0.24 V) than those of graphitic and pyridinic N configurations in graphene.84 In contrast, Chai and Guo proposed that graphitic N-doped edges have a low CO2 activation barrier (0.58 eV) and are likely the most active sites for CO2 reduction relative to pyridinic and pyridinium N forms in graphene-based materials.83 The authors found that the overpotential for a given product can be tuned by a curvature effect. Recent work showed that tri-pyridinic defects in graphene have suitable thermodynamic energies to undergo 2H+/2e reductions for efficient CO2 conversion.85 2D SnS2 nanosheets supported on reduced graphene oxide were reported to be an active catalyst for electrocatalytic reduction of CO2 into HCOO in aqueous bicarbonate media.72 Such 2D/2D hybrids provided an overpotential of 0.23 V for HCOO formation and a HCOO faradic efficiency of 84.5% at an overpotential of 0.68 V, comparable with those of most state-of-the-art Sn or N-doped carbon-based catalysts. Carbon Monoxide Selectivity Metal Nanosheets Au, Ag, and Zn all bind CO weakly and exhibit relatively high CO2 reduction efficiencies to CO, with edge-rich Au nanowires being the most active (CO Efaradic, 94% at an overpotential of 0.23 V) in aqueous solutions.4 CO2 can be selectively Chem 3, 560–587, October 12, 2017 575 Figure 8. Schematic Illustration Depicts the Several Advantages of Ultrathin Metal Layers Confined in Graphene for CO2 Electroreduction into Hydrocarbon Fuels Reprinted with permission from Lei et al. 68 Copyright 2016 Macmillan Publishers Limited. converted to CO (Efaradic R 95%) on both Ag and Bi cathodes at an overpotential below 0.2 V in IL solutions.40 Pd nanoparticles (NPs) of 3.7 nm also provide a CO faradic efficiency of 91.2% at 0.89 V (versus RHE) in 0.1 M aqueous KHCO3.86 There are few research works about metal nanosheets used for selective CO2 electroreduction to CO. Recently, 50-nm-thick interconnected Ag nanosheets were prepared by an electrochemical oxidation-reduction approach.73 Such nanosheets provided a CO faradic efficiency of 95% at an overpotential of 0.29 V and a current density 37 times larger than that obtained for polycrystalline Ag at 0.6 V (versus RHE), which are among the best performances for aqueous CO2 reduction to CO. A faradic efficiency of 93% at 1.6 V (versus SCE) was achieved for CO on Zn nanoplates 1 mm long and 40 nm thick decorated with NPs (30–50 nm) in aqueous NaCl, whereas the CO faradic efficiency is only 18% on bulk Zn in NaHCO3 solution.87 Transition-Metal Dichalcogenide Nanosheets 2D nanoflakes (NFs) of transition-metal dichalcogenides (TMDs) are a new class of catalysts that display remarkable CO2 reduction performances for CO generation in ILs (such as 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM-BF4]).17,74 Vertically aligned WSe2, MoSe2, WS2, and MoS2 NFs all showed very high current densities above 130 mA cm2 (up to 330 mA cm2 for WSe2 nanosheets) with 90% CO selectivity (Figure 10),17,74,75 in contrast to 3.3 mA cm2 with 0% CO selectivity for bulk Ag, 10 mA cm2 with 65% CO selectivity for 40 nm Ag NPs. WSe2 NFs gave the best performance and exhibited a current density of 18.95 mA cm2, CO faradic efficiency of 24%, and CO TOF of 0.28 s1 at an overpotential of 54 mV. The current density decreased in the order of WSe2 > MoSe2 > WS2 > MoS2, which might correlate with their respective electron transfer properties. WSe2 has the lowest work function and thus performs the best for CO2 activation in these TMDs. It was further inferred that the metal edges of these TMDs had strong binding of COOH* and CO* intermediates during the catalytic reaction to maintain a high turnover rate. These TMD nanosheet/IL systems exhibit a synergy between metallic edge CO binding and bulk IL CO2 solubility, thus providing promising TMD-based electrocatalysts for CO production. Increasing the number of exposed edges by doping with metal atoms to modify the binding energies of intermediates can effectively enhance the CO2 conversion efficiency of TMDs.29 In this regard, 5% niobium (Nb)-doped MoS2 nanosheets were demonstrated to exhibit an order of magnitude higher CO formation TOF than 576 Chem 3, 560–587, October 12, 2017 Figure 9. Electroreduction of CO2 over Mesoporous SnO2 Nanosheets on a Carbon-Cloth Electrode in CO2-Saturated 0.5 M NaHCO3 Solution (A) Current density across the entire potential range. (B) Corresponding faradic efficiency for HCOO , CO, and H 2 . Error bars represent standard error. Reprinted with permission from Li et al.22 Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. pristine MoS2 in ILs at an overpotential range of 50–150 mV.76 This catalyst yielded a very low onset overpotential of 31 mV for CO2 reduction. Heteroatom-Doped Graphene, Metal-Organic Framework Thin Films, and 2DNanosheet-Based Hybrids N-doped graphene (NG) foam has been used as a catalyst for aqueous CO2 reduction.77 Such metal-free catalyst exhibited an overpotential as low as 0.19 V for CO formation, a CO faradic efficiency of 85% at an overpotential of 0.47 V, and stability over 5 hr, comparable with polycrystalline Ag and Au. The maximum faradic efficiency for CO increased with the pyridinic N content, and the corresponding potential shifted anodically, which was, however, not observed for pyrrolic N and graphitic N. DFT calculations based on a computational hydrogen electrode model revealed that pristine graphene has the highest free energy barrier for the first step of COOH* adsorption (Figure 11). Whereas the free energy barrier for COOH* adsorption decreased substantially upon introducing N defects because the dangling N bonds formed bind the COOH* intermediate much more strongly. Triple-pyridinic N lowered the barrier of COOH* adsorption the most followed by single pyridinic and graphitic N. COOH* adsorption at a pyrrolic N-defect site is exergonic. However, an energy barrier of 0.6 eV is required to desorb chemisorbed CO from pyrrolic N. Pyridinic N was suggested to be likely the most active site for CO2 reduction to CO. The introduction of transition-metal atoms coordinated with N into the sp2 networks of graphene can create a synergy and further enhance CO2 reduction catalysis. For example, Ni-N modified graphene (Ni-N-Gr) efficiently catalyzed the electrochemical reduction of CO2 to CO with a CO faradic efficiency exceeding 90% at 0.7 to – 0.9 V (versus RHE).78 The TOFs for CO formation on per electroactive Ni atoms of Ni-N-Gr reached 2,700 and 4,600 hr1 at 0.7 and 0.8 V (versus RHE) in neutral solutions, respectively, comparable with those of the best Co-porphyrin-based covalent organic framework catalysts for CO2 reduction.88 On the basis of DFT and microkinetics modeling, an adjacent graphene single-vacancy-supported Cu dimer was demonstrated to be catalytically active for CO2 electroreduction to CO, with a reaction rate and CO partial current density comparable Chem 3, 560–587, October 12, 2017 577 Figure 10. DFT Analysis (A) Calculated free-energy diagrams for ECR to CO on Ag (111), Ag 55 NPs, MoS 2 , WS 2 , MoSe 2 , and WSe 2 NFs at 0 V (versus RHE). (B) Calculated partial density of states of the d band (spin-up) of the surface Ag atom of Ag 55 . (C) Calculated partial density of states of the surface bare metal edge atom (W) of the WSe 2 NFs. Reprinted with permission from Asadi et al.74 Copyright 2016 American Association for the Advancement of Science. with or even higher than those of Au electrodes.89 The selectivity for CO formation was attributed to the fact that the positively charged metal sites and the electronic interaction between Cu and graphene weakened CO adsorption. Thin films of metal-organic frameworks (MOFs), such as Co-porphyrin MOF,90 have been demonstrated as catalysts for electroreduction of CO2 to CO with CO selectivity in excess of 76% in aqueous electrolytes. In particular, highly oriented MOF thin films grafted with ReL(CO)3Cl (L = 2,20 -bipyridine-5,50-dicarboxylic acid) (Figure 12), grown on a fluorine-doped tin oxide (FTO) electrode displayed a faradic efficiency of up to 93% G 5% toward CO production at 1.6 V (versus NHE), exceeding those reported for most electrocatalytically active MOF thin films.79 However, the stability of such Re-based MOF catalysts needs to be further improved for future applications. g-C3N4, an important metal-free 2D semiconductor material, has been widely used in photocatalysis. Recently, Amal and co-workers first reported the potential of utilizing g-C3N4/multi-walled carbon nanotube (MWNT) composites as a stable and selective electrocatalyst for CO2 reduction in 0.1 M KHCO3. The as-prepared composites exhibited a CO faradic efficiency of 60%, and no decay in catalytic activity was observable after 50 hr. The covalent C–N bonds formed between g-C3N4 and MWNTs were proposed as active sites for ECR. In addition, high specific surface area and improved material conductivity of the composite led to enhancement in CO2 reduction.80 Methanol Selectivity Metal oxides, such as Cu2O91 and RuO2,92 have demonstrated direct ECR to CH3OH. However, using 2D nanosheets for ECR selectively to CH3OH has been rarely reported. Han and colleagues81 showed that Mo-Bi bimetallic chalcogenide (BMC) nanosheets were able to reduce CO2 to CH3OH with a faradic efficiency of 71.2% and a current density of 12.1 mA cm2, which outperform earlier reported catalysts. In contrast, CH3OH was not generated when bulk MoS2/carbon paper (CP) or Bi2S3/CP electrode was used. These Mo-Bi BMC nanosheets performed better than 578 Chem 3, 560–587, October 12, 2017 Figure 11. DFT Modeling of Electrocatalysis of CO2 on NG (A) Free energy diagram of electrochemical reduction of CO 2 to CO on NG. (B) Schematic of N configuration and CO 2 reduction pathway. Reprinted with permission from Wu et al.77 Copyright 2016 American Chemical Society. both Mo-Ag and Mo-Cu BMC nanosheets (Figures 13A–13C). This catalytic process relies on an ionic liquid acetonitrile solution, wherein the IL, 1-butyl-3-methylimidazolium tetrafluoroborate, provides increasing faradic efficiency as the concentration of IL is decreased from 1 M to 0.2 M and a maximum current density at about 0.5 M (Figure 13D). This selectivity was attributed to a synergistic effect between Mo and Bi, wherein the IL coordinated with CO2 for reduction to radical anion, which was transformed by a second one-electron process to adsorbed CO. This nanosheet result surpasses about 40% faradic efficiencies afforded by Ru/Cu (0.5 M aqueous NaHCO3) and Cu (LiCl in ethanol/water), and 55% faradic efficiency from Mo (0.2 M aqueous Na2SO4). Incorporation of Ni-Cu dopant pairs into adjacent single vacancies in graphene (NiCu@SV) was calculated to lower the free energy barrier of the CO / CHO step from 0.99 eV on Cu (111) to 0.71 eV on Ni-Cu@SV, leading to an 0.3–0.4 V reduction in overpotential.89 The O adsorption energy on the Ni-Cu dopant is also lower than that of the Ni2 and Cu2 dopant pairs, making CH3OH production more thermodynamically favorable than CH4 production, which promotes CH3OH selectivity. More recently, Mn2 dimers supported on 2D expanded phthalocyanine nanosheets were predicted to be the best catalyst for ECR to CH3OH among single Mn atom, or Fe, Co, Ni, Cu dimers.93 The two Mn atoms in the dimer contribute to the bonding between COOH* adsorbate and catalyst. The bridge adsorption of Mn-C-O-Mn enhanced the metal-to-adsorbate p-back bonding, resulting in an easy transition between C-end adsorbate and O-end adsorbate with lowered energy barrier in CH3OH desorption. Methane Selectivity Nitrogen-doped graphene-like materials (NG) coated on CP electrodes were shown to catalyze the electroreduction of CO2 to CH4 in an IL, 1-butyl-3-methylimidazolium tetrafluoroborate.43 The faradic efficiency of CH4 formation increased significantly with increasing N content, approaching 94% at 4.8 atom % of N. It was proposed that pyridinic N and pyridonic or pyrrolic N species promoted CO2 adsorption; the ionic liquid helped drive the transformation of adsorbed CO2 molecules to CO2, radical anions. Meanwhile, a strong interaction between COads and the electrode inhibited release of CO from the electrode, which is favorable for its further hydrogenation to form CH4. This process was found to retain selectivity when 3% of Chem 3, 560–587, October 12, 2017 579 Figure 12. Schematic of the Fabrication Process of Re-surface-Grafted MOF on Functionalized FTO Substrate in a Layer-by-Layer Fashion (A) Zn acetate. (B) Re-linker. (C) Idealized structure of Re-surface-grafted MOF on FTO. Reprinted with permission from Ye et al. 79 Copyright 2016 Royal Society of Chemistry. water was added to the IL while improving the current density (from 1.4 to 3.3 mA cm2). The current densities obtained with this NG system were about 6-fold higher than that of copper foil used under similar conditions. Nitrogen doping is critical, as only CO and H2 are generated in its absence. 2D IrTe2 (T), RhTe2 (T), PFeLi, and TiS2 (T) were calculated to have theoretical reduc tion potentials (E ECR) of 0.58 V, 0.61 V, 0.65 V, 0.69 V (versus RHE), respectively. They all demonstrated lower overpotentials for CO2 conversion into CO than the best metal catalyst Au (calculated E ECR = 0.71 V versus RHE), whereas 2D LiFeAs and ScS2 (T) were calculated to have strong CO affinity but low E ECR of 0.55 and 0.62 V (versus RHE) for CH4 formation, respectively, with lower overpo tential over 0.35 V than with Cu (calculated E ECR = 0.97 V versus RHE).94 Mn-Cu dimers supported on graphene with adjacent single vacancies were demonstrated to have strong *OH or *COOH adsorption, favoring further reduction of CO. In addition, such a catalyst can reduce the free energy barrier of the CO / CHO step to facilitate CH4 generation.93 Despite such promising theoretical predictions, future work needs to synthesize these 2D materials and further investigate their catalytic properties for ECR. Acetic Acid Selectivity It is difficult to produce C2 and C2+ chemicals in CO2 electrochemical reduction because of the higher energy required and slower kinetics for the formation of C–C bonds than for the formation of C–H and H–H bonds. Generation of C2 products has been rarely reported with 2D materials. Sun and co-workers demonstrated ECR to CH3COOH over N-based Cu(I)/C-doped BN sheets.67 A high CH3COOH faradic efficiency was obtained up to 80.3%, surpassing previously reported values. Such an outstanding performance was attributed to a synergistic effect of C-doped BN (BNC), the Cu metal center, the N-based ligand (N,N,N0 ,N0 -tetra(2-pyridyl)-2,6-pyridinediamine), and the electrolyte ([Emim]BF4/LiI/H2O solution). Specifically, BNC facilitated adsorption conversion of CO2 to CO2,. The Cu complex helped the formation and protonation of *CO, *CHO, and *CH3O, and the C–C coupling in the presence of LiI. 580 Chem 3, 560–587, October 12, 2017 Figure 13. Current Density and Product Faradic Efficiency at Different Applied Potentials in CO2Saturated 0.5 M [BMIM]BF4 MeCN Solution (A) Mo-Bi BMC/CP electrode. (B) Mo-Ag BMC/CP electrode. (C) Mo-Cu BMC/CP electrode. Curve (a): current density; curves (b)–(e): faradic efficiency of (b) CH3 OH, (c) CH 4 , (d) CO, and (e) H2 . (D) CH3 OH faradic efficiency and current density at 0.7 V (versus SHE) as a function of [BMIM]BF 4 molar concentration in MeCN over Mo-Bi BMC/CP electrode. Reprinted with permission from Sun et al. 81 Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Note that only Cu27, NG quantum dots95 and graphene/ZnO/Cu2O hybrid28 have been found to convert CO2 into C2+ products, the yields of which are however % 30% in both cases. 2D catalysts and strategies to afford C2+ products with higher efficiency need to be explored. STRATEGIES FOR IMPROVING CO2 ELECTROCATALYTIC ACTIVITY OF 2D NANOSHEETS Strategies that can be used to improve nanosheet electrocatalytic activity should take into account the following: (1) enhancing current density at low overpotential toward a single energy-rich product especially toward C2–C4 products, (2) slowing down the competitor reactions such as HER, and (3) improving long-term stability. This section provides basic information and understanding of some possible effective strategies. Surface Engineering Surface Modification Surfaces of 2D nanosheets with a high level of exposed low-coordination atoms are easily modified with metallic and non-metallic species. Alloying of different metals can optimize the binding energies of reaction intermediates such as *CO, and might also be good for breaking scaling relations between similarly adsorbed intermediates to reduce reaction overpotential.96 For instance, *CO tends to bind to metals exclusively via the C atom, but embedding an oxophilic atom into the surface can selectively stabilize *COOH and *CHO over *CO. Alternatively, doping with Chem 3, 560–587, October 12, 2017 581 adatoms (Co, Mn, Bi, As, Li, H, C, S, N, B, P, O, F, and I) can substantially alter the electronic and geometric properties of nanosheets. Such doping can tune the binding strengths of different intermediates and reaction energetics, thereby modulating the activity and selectivity of CO2 conversion. Recently, we found that doping of Pd with Te enables highly selective electrocatalytic reduction of aqueous CO2 to CO. DFT calculations show that Te atoms preferentially bind at terrace sites of Pd, thereby suppressing hydrogen evolution, whereas CO2 adsorption and activation on high index sites of Pd give rise to CO (unpublished data). Chemical doping of 2D materials with controllable configuration and doping level remains a challenge. Surface functional groups (such as amines) play a role in reaction activity and selectivity. For example, amine functionalization of a catalyst can concentrate CO2 on the electrode surface from the bulk solution, increasing its effective local concentration. In addition, amine can stabilize CO2,– via a H-bond interaction, thus lowering the onset potential for reducing CO2 to CO2,– by creating a stabilizing environment. Likewise, amine can help the bonding of *CO, favoring subsequent reduction of CO to methanol and C–C coupling for C2 formation.67 Pure graphene-like carbon materials exhibit small catalytic activities for CO2 activation, whereas doping with heteroatoms, such as B, N, and/or Ni, can drastically lower CO2 adsorption barriers to facilitate electrocatalytic CO2 reduction. A remarkable enhancement in carrier concentration can be achieved by quaternary N doping up to 2.6 3 1013 cm2 (four times higher than that for pristine graphene) even at a very small doping level of 0.6 atom %.97 We note that single- and co-doped graphene materials with other p-block and d-block elements in addition to B, N, and Ni, remain to be explored for ECR. Surface-Structure Tuning Structural tuning of 2D nanosheets is realized mainly by modification of four important aspects: (1) surface defects, (2) surface porosity, (3) exposed crystal facets, and (4) surface phase. Surface tuning by an oxygen vacancy can increase the adsorption of CO2 on metal oxides or hydroxides. CO2 molecules are prone to adsorb at oxygen vacancies with one oxygen atom of CO2 situated by bridging oxygen vacancy defects, thus decreasing the energy barrier for CO2 activation.98 Likewise, chalcogen vacancies on chalcogenide nanosheets can act as adsorption sites for CO2, facilitating CO2 reduction selectively to CO.17 The surface vacancy density can be adjusted via high-temperature reduction or by doping with a lower-valent foreign element. Creating hierarchical micro-, meso-, and macroporous structure on 2D nanosheets offers a way to enhance adsorption and capture of CO2 and to improve CO2 conversion efficiency. Macropores (>50 nm) facilitate facile transfer and diffusion of reactants and products over electrodes, whereas mesopores (2–50 nm) and micropores (<2 nm) provide a large number of surface active sites with high dispersion. In addition, tuning the diameters of pores or channels according to the molecular sizes of reactants or products can tailor selectivity correspondingly. Crystal facets play a significant role in determining reaction activity and selectivity. Different surface facets exhibit different Lewis acidity and polarizing power, thus influencing CO2 adsorption and activation. Facets with high adsorption energies and low activation barriers are preferred for electrocatalytic reactions. 582 Chem 3, 560–587, October 12, 2017 The catalytic activity and selectivity of TMDs and many metals are sensitive to surface composition and order. The octahedral (1T) phase of TMD possesses inherently low charge-transfer resistance. Active-site density increases as a result of strained lattice distortion in comparison with the trigonal prismatic (2H) phase. Controlling adsorbed ions near surface vacancies permits tuning of the electronic properties and activity of catalysts. In particular, introducing the partially oxidized surface of metal electrocatalysts, typically Sn, Bi, and Co, affords improved activity over that of pure metals.18 Construction of Nanosheet Composites Creation of nanosheet composites can enhance charge transport, imparting electronic and physical coupling effects to facilitate CO2 conversion.80 Important parameters for an interface between components include interfacial compositions and facets, areas, interfacial defects, and electronic coupling. Interfacial charge transfer can be accelerated by addition of conductive sheets such as graphene.72,89 When CO2 adsorption energy can be also increased, a synergy is created, promoting CO2 reduction. SUMMARY AND OUTLOOK Fossil fuels are likely to continue to be a major source of energy for the next few decades. Alleviating the effects caused by waste CO2 emission remains a critical issue to modern society. Electrocatalytic reduction of CO2 offers an intriguing way for CO2 mitigation, by which CO2 as a feedstock can be converted to fuels or value-added chemicals. Notable results have been achieved in developing 2D materials with electrocatalytic performances superior to their bulk counterparts. Partially oxidized 4atom-thick Co layers have provided large selectivity for HCOO, competitive with the use of Pd nanoparticles supported on carbon (Pd/C). N doping of graphene has been shown to offer high promise for metal-free catalysis of the ECR to produce CO and HCOO in aqueous electrolytes, and CH4 in ILs, but the impact of quaternary basal-plane nitrogen versus pyridinic nitrogen on edges has not yet been settled and requires further analytical work. Interconnected Ag nanosheets have yielded one of the best performances for aqueous CO2 reduction to CO so far. 2D TMDs have been found to drastically promote CO2 reduction for CO generation in ILs, and the selenides appear to be the most effective. Mo-Bi BMC nanosheets are capable of catalyzing CO2 reduction to CH3OH, outperforming earlier reported catalysts. For real-world CO2 electrolysis application, further improvement in electrocatalytic activity is required, which should fulfill the following demand: (1) small overpotential and high energy efficiency, (2) satisfactory selectivity and faradic efficiency, and (3) good electrocatalytic stability (>100 hr). To promote CO2 adsorption and activation, some useful strategies can be adopted by surface modification, surface-structure tuning (surface defect, porosity, crystallographic orientation, composition, strain, and curvature), and formation of multiphase hybrid nanostructures. Scaling relations that exist between adsorption energies of different reaction intermediates need to be circumvented, which can be realized possibly by selectively stabilizing the intermediates through some external stimuli. Other technical aspects, including acidic or alkaline working conditions, operating parameters (such as pressure, electrolyte concentration, etc.), and electrolytic cells, must be optimized. For the sake of desirable overall cell efficiency, high-performance electrocatalysts to promote the anode OER are required as well. Large-scale production of 2D nanosheets is the key to their practical use in CO2 catalysis. Chemical vapor deposition and liquid-phase exfoliation methods hold Chem 3, 560–587, October 12, 2017 583 promise for the preparation of layered materials. But scalable routes to prepare atomically thin non-lamellar nanosheets, especially those being intrinsically catalytic active, remain to be developed. The electronic properties of particular 2D materials can be tuned by varying dimensionality (thickness and lateral dimension) to improve efficiency of electron transfer and of CO2 reduction. Control in the number of layers, flake dimensions, and defect levels, in addition to yield, throughput, and cost are thus needed. Equally importantly, developing in situ (spectroscopic) characterization techniques will help develop more precise theoretical models to provide deeper insights into catalytic reaction pathways involving scaling relations among reactive intermediates and structure-property relationships peculiar to these materials. The combination of theory and experiment will further aid catalyst design to increase the number of active sites and intrinsic activity of each active site as well as development of novel robust 2D catalysts for this clean energy reaction. Exploration of low-cost, non-noble metal 2D nanosheets that can convert CO2 to target products at sufficiently high reaction rate and efficiency continues to be a focus of research. Porous carbon nanosheets modified with p-block and d-block elements, or with NG quantum dots, 2D mixed-metal (phosphorus) chalcogenides, appear to be promising candidates. Design of heterostructures comprising different 2D materials (van der Waals heterostructures and lateral heterostructures) and/or nanoparticles enables great tuning over the structural and electronic properties of such crystals and hybrids, creating enormous diversity in properties and functionality. CO2 catalytic conversion making use of strong electron interactions, confined space, and creation of sandwich structures offers much interest. Initial positive results have been obtained from selectively producing CO and HCOO by using 2D materials. Whereas the established Fischer-Tropsch process is capable of taking generated CO and H2 to liquids, these kinds of two- to four-electron steps can be followed by the addition of extra catalytic phases through doping or hybrid design for sequential reaction steps that lead to C2 and even higher-order hydrocarbons and oxygenates. Research in this direction is an exciting arena. AUTHOR CONTRIBUTIONS Z.S. proposed the topic of the review and wrote the manuscript. T.M., H.T., and Q.F. searched the literature and edited figures and the table. B.H. revised the manuscript. ACKNOWLEDGMENTS This work was supported by the State Key Laboratory of Organic-Inorganic Composites (oic-201503005), Fundamental Research Funds for the Central Universities (buctrc201525), Beijing National Laboratory for Molecular Sciences (BNLMS20160133), Key Laboratory of Photochemical Conversion and Optoelectronic Materials (Technical Institute of Physics and Chemistry, Chinese Academy of Sciences [CAS]), and Key Laboratory of Materials for High-Power Laser (Shanghai Institute of Optics and Fine Mechanics, CAS). REFERENCES AND NOTES 1. McCollum, D., Bauer, N., Calvin, K., Kitous, A., and Riahi, K. (2014). Fossil resource and energy security dynamics in conventional and carbonconstrained worlds. Climatic Change 123, 413–426. 2. Davis, S.J., Caldeira, K., and Matthews, H.D. (2010). Future CO2 emissions and climate change from existing energy infrastructure. Science 329, 1330–1333. 584 Chem 3, 560–587, October 12, 2017 3. 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