Graphene-based materials for electrochemical CO2 reduction

Journal of CO₂ Utilization 30 (2019) 168–182

Contents lists available at ScienceDirect

Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou

Graphene-based materials for electrochemical CO2 reduction Tao Ma

a,1

, Qun Fan

a,1

a

a

, Xin Li , Jieshan Qiu , Tianbin Wu

b,⁎

, Zhenyu Sun

a,c,⁎

T

a

State Key Laboratory of Organic-Inorganic Composites, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid and Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China c Beijing Key Laboratory of Energy Environmental Catalysis, Beijing University of Chemical Technology, Beijing 100029, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 reduction Electrocatalysis Graphene Doping Composite Single atom catalyst

Electrocatalytic CO2 reduction (ECR) using renewable electricity provides an alternative strategy for alleviating energy shortage and global warming issues. To facilitate this kinetically sluggish process, the design of highly selective, energy-efficient, and cost-effective electrocatalysts is key. Graphene-based materials have features of relatively low cost, excellent electrical conductivity, tunability in structure and surface chemistry, and renewability, rendering them competitive for CO2 electroreduction. In particular, by doping with heteroatoms, it’s possible to create unique active sites on graphene for CO2 adsorption and activation. Besides, integration of graphene with other materials enables creation of a synergistic effect, thereby boosting CO2 conversion. This review focuses on recent advances of graphene-based catalysts in ECR. The relationship between structure and property with regard to CO2 electroreduction is highlighted. Leading electrocatalysts are discussed and compared with some metal benchmark materials to provide an evolutionary perspective of performance progress. Development opportunities and challenges in the field are also summarized.

1. Introduction

kinetically more facile hydrogen evolution reaction (HER) usually occurs at a similar thermodynamic potential to that of ECR in aqueous solutions. The adsorbed *H intermediate is more stable than adsorbed CO or *COOH intermediates [11]. The competition between ECR and the parasitic HER reduces selectivity toward any carbonaceous product. Since pioneering works in the 1980s and 1990s investigated different bulk metal electrodes for the ECR [12,13], tremendous advances have been made in the exploitation of novel nanostructured catalysts [14]. Nevertheless, the industrial realization of ECR is still hampered by its high overpotential (about 1.0 V), poor durability (less than 100 operating hours), low current density (below 100 mA cm−2), and insufficient selectivity (leading to increase in separation cost) or faradaic efficiency (also referred to as current efficiency which is the fraction of electric charge that is used for the formation of a specific product) at practical reaction rates. In this context, the development of efficient, robust, and selective electrocatalysts especially toward a single multicarbon product remains a key challenge. Efforts are also required on engineering of CO2 reduction with a suitable anode half oxidation reaction. Both homogenous and heterogeneous catalysts have been used for CO2 electroreduction. Homogeneous catalytic systems (enzymes and molecular catalysts) bear advantages of efficiency and product

The progress of human society and development of industrial economy cause increasing upstream energy consumption, over twice the current global consumption by the year 2050 [1,2]. Fossil fuels will likely continue to be the major source of energy in the next few decades. Their utilization and accompanying excessive CO2 emissions have led to severe problems associated with resources, environment, and climate (referred to as “global warming”) [3,4]. To circumvent these dire consequences and decrease the atmospheric CO2 concentration, direct electrochemical CO2 reduction (ECR) to fuels and commodity chemicals ((oxygenated) hydrocarbons) appears to be a promising early-stage technology [5–7]. Electricity from renewable sources (solar, tidal, wind, and geothermal power) with a foreseen cheap price provides a power to drive reaction of waste CO2 with water, enabling an energy transition from “fossil fuel economy” to a sustainable “CO2 economy” [8–10]. The use of CO2 reduction products (such as methanol and formic acid) may offer an alternative to H2 in fuel cell technology owing to easier fuel storage, transport, and handling. Whereas CO2 conversion is an uphill energy process, requiring a high activation barrier (large overpotentials) because the linear molecule is highly stable (ΔGf ° = −394.4 kJ mol−1) and chemically inert. Meanwhile, the ⁎

Corresponding authors. E-mail addresses: [email protected] (T. Wu), [email protected] (Z. Sun). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jcou.2019.02.001 Received 29 May 2018; Received in revised form 30 January 2019; Accepted 1 February 2019 2212-9820/ © 2019 Published by Elsevier Ltd.

Journal of CO₂ Utilization 30 (2019) 168–182

T. Ma, et al.

for direct ECR, but this process requires a very negative equilibrium potential (−1.97 V vs. SHE in N, N-dimethylformamide (DMF); −1.90 V vs. SHE in neutral aqueous solution). An alternative and more favorable route to reduce CO2 is through a proton-coupled multiple electron transfer to obviate the formation of CO2%−. The simultaneous transfer of electrons and protons in pairs facilitates CO2 reduction at lower energy penalty with a thermodynamic potential in the range −0.2 to −0.6 V (vs. SHE) [56,57].

specificity via precise control over individual active centers [15]. While they suffer from drawbacks of high cost, low stability, and complicated post-separation, limiting scalable and practical applications. An emerging strategy to solve these issues is immobilization on conductive supports such as graphene [16]. A variety of heterogeneous electrocatalysts including transition (bi-)metals (having vacant orbits and active d electrons) (e.g., Pt, Rh, Ni, Pd, Ag, Au, and Cu) [17–20], p-block (bi-)metals (e.g., Pb, Sn, In, Bi) [21], oxides (e.g., RuO2 [22], IrO2 [23], TiO2 [24], SnO2 [25]), chalcogenides [26–28], carbides [29–31], carbon-based materials [32–34], and metal organic frameworks [35] have been extensively explored. Of these, carbon-based materials are showing much promise in ECR [10,36,37]. They provide distinctive benefits, such as abundance, low cost, high conductivity, good structural stability, environmental friendliness, and renewability. Recently, graphene has emerged as an attractive carbon allotrope owing to its unique structural features and many outstanding properties. An appealing advantage is its large specific surface area (theoretically about 2630 m2 g−1), affording a potentially high density of surface active sites. It also possesses remarkable electrical conductivity and high mechanical strength and stability, which are beneficial when used as either catalysts or catalyst supports. Equally importantly is that the surface of graphene can be modified using metallic and nonmetallic species. This offers degrees of freedom to tune the corresponding catalytic activities [38]. Pristine graphene and graphene oxide (GO) both are inactive toward the ECR because the neutral carbon atoms have negligible ability to activate CO2. While doping with heteroatoms such as boron, nitrogen, and phosphorus (including heteroatom single-, dual-, and multiple-doping) can dramatically alter the electronic structure of graphene, which substantially enhance its activities in ECR. Alternatively, construction of graphene-based heterostructures can enhance charge transport and create a synergy, promoting CO2 reduction [25,39–45]. Recent and excellent perspectives on carbon materials for ECR [36,37,46–48] and other electrocatalysis [49–54] are available. While the use of graphene in ECR has been only slightly described, mechanistic discussions on the relationships between structures and properties are rare. Here graphene refers to two-dimensional allotrope of carbon. Other carbon structures such as carbon nanotubes, diamond, and amorphous/porous carbons are not included except the case of formation of hybrids with graphene. In this review, we examine how graphene has been applied to the ECR. We first provide an overview on the fundamentals of CO2 reduction, reiterating important parameters in the ECR. Various graphene-based electrocatalysts (as shown in Table 1) are then presented with emphasis of functionalization of graphene for ECR. These reported graphene-based materials are compared with benchmark metal catalysts in terms of performance. Strategies for the optimization of ECR performance is highlighted. An outlook towards future opportunities in the conversion of CO2 into fuels by electrocatalysis with graphene is provided.

CO 2 (g) + 2H+ + 2e− → HCOOH (1) E• redox = −0.250 V

(1)

CO 2 (g) + 2H+ + 2e− → CO (g) + H2 O (1) E• redox = −0.117 V

(2)

2H+

2CO2 (g) +

+

2e−

→ H2 C2 O4 (1) E• redox = −0.500 V

(3)

CO 2 (g) + 4H+ + 4e− → HCHO (1) + H2 O (1) E• redox = −0.067 V (4)

CO 2 (g) +

6H+

+

6e−

→ CH3 OH (1) + H2 O (1) E• redox = 0.033 V

CO 2 (g) + 8H+ + 8e− → CH 4 (g) + 2H2 O (1) E• redox = 0.173 V

(5) (6)

2CO 2 (g) + 12H+ + 12e− → C2 H 4 (g) + 4H2 O (1) E• redox = 0.064 V (7)

2CO 2 (g) + 12H+ + 12e− → C2 H5 OH (1) + 3H2 O (1) E• redox = 0.084 V (8)

2CO 2 (g) +

14H+

3CO 2 (g) +

12H+

3CO 2 (g) +

18H+

+

14e−

+

12e−

+

18e−

→ C2 H6 (g) + 4H2 O (1) E• redox = 0.143 V (9)

→ C3 H 4 O2 (1) + 4H2 O (1) E• redox = 0.020 V (10)

→ C3 H7 OH (1) + 5H2 O (1) E• redox = 0.103 V (11)

4CO 2 (g) + 14H+ + 14e− → C4 H 4 O3 (1) + 5H2 O (1) E• redox = 0.010 V (12)

2H+

+

2e−

→ H2 (g) E• redox = 0.000 V

CO 2 (g) + e− → CO2• − E• redox = −1.900 V

(13) (14)

In a CO2 electrolyzer, an anodic half oxidation reaction (such as water oxidation, i.e. the oxygen evolution reaction, OER, 2H2O → O2 + 4H+ + 4e−) is associated with the cathodic ECR. The overall cell voltage required for ECR includes potentials for the paired reactions (Ecell = Eanode − Ecathode). To gain satisfactory reaction rates at low overpotentials, highly active and stable electrocatalysts for both cathode and anode processes, in addition to electrodes and electrolytes that have good conductivity to permit sufficient mass transport (diffusion overpotential) and charge transport (ohmic overpotential), are demanded [10]. For more favorable thermodynamics and production of higher-value chemicals than O2 from OER, researchers attempt to develop a useful oxidative half-reaction allowing for paired electrosynthesis such as chloralkali electrolysis. The ECR is diffusion controlled and hence the CO2 bulk and electrode-electrolyte interface concentrations substantially influence the process. But CO2 has low solubility in water, which is only about 30 mM at 1 atm and 20 °C. To enhance CO2 dissolution and accelerate reduction kinetics, three ways can be adopted: i) employing low operating temperature to reduce the Henry constant; ii) using a non-aqueous solvent; iii) increasing CO2 partial pressure both in liquid and in gas phases. Solvated CO2 is in equilibrium with carbonic acid, bicarbonate, and carbonate, whereby bicarbonate and carbonate are more difficult to be reduced than CO2 under electrolysis conditions. Ionic gradients in the boundary layer adjacent to the electrode result in diffusion losses. The thickness of the diffusion layer correlates with the degree of turbulence. A flow cell with a gas diffusion electrode can avoid CO2

2. CO2 electrolysis 2.1. Fundamentals CO2 reduction can proceed through different reactions that involve transfer of 2–18 electrons. Depending on the number of electrons and protons transferred, CO2 can be converted to HCOOH (HCOO−, alkaline medium), H2C2O4 (carboxylates, alkaline environment), CO, CH2O, CH3OH, CH4, C2H4, C2H5OH, C2H4O2 (CH3COO−, alkaline environment), C2H6, C3H4O2, C3H7OH, and C4H4O3, respectively [Eqs. (1)–(12), all reduction potentials are given relative to standard hydrogen electrode (SHE) at pH 0, 1.0 atm, and 25 °C in aqueous solution] [55]. The competing HER is a two-electron reduction process [Eq. (13)]. Reduction of CO2 often requires nucleophilic attacks at the C atom to bend the O]C]O bond. A single-electron transfer to generate a radical anion (CO2%−) [Eq. (14)] has been considered as the first step 169

170

−2

∼1.4 mA cm @ −1.4 V (vs. SCE) 7.5 mA cm−2 @ −0.84 V (vs. RHE) 22.1 mA cm−2 @ −1.8 V (vs. SCE) 10.5 mA cm−2 @ −0.958 V (vs. SCE) ∼8 mA cm−2 @ −1.06 V (vs. RHE) 5 mA cm−2 @ −1.6 V (vs. Ag/AgCl) ∼1.8 mA cm−2 @ −0.58 V (vs. RHE) 2.7 mA cm−2 @ −2.2 V (vs. Ag/Ag+) ∼2.5 mA cm−2 @ −0.6 V (vs. RHE) 30 mA cm−2 @ −0.88 V (vs. RHE) 22 mA cm−2 @ −0.7 V (vs. RHE) 11 mA cm−2 @ −0.64 V (vs. RHE) ∼0.2 mA cm−2 @ −0.65 V (vs. RHE) ∼0.3 mA cm−2 (JCO) @ −1.0 V (vs. Ag/AgCl) ∼5 mA cm−2 @ −0.47 V (vs. RHE) ∼3 mA cm−2 @ −0.6 V (vs. RHE) 50.95 mA cm−2 @ −1.50 V (vs. RHE) ∼70 μA cm−2 @ −0.759 V (vs. RHE) 4.4 mA cm−2 @ −1.0 V (vs. RHE) 3.26 mA cm−2 @ −1.4 V (vs. SHE) N/A 90.3 mA cm−2 @ −0.75 V (vs. RHE) 19.0 mA mg−1 @ −1.2 V (vs. NHE) 0.745 mA cm−2 @ −0.4 V (vs. RHE) ∼1.7 mA cm−2 @ −1.2 V (vs. RHE) 12.2 mA cm−2 @ −1.7 V (vs. Ag/AgCl) ∼1.43 mA cm−2 @ −0.75 V (vs. RHE) 0.31 mA cm−2 @ −0.5 V (vs. RHE) 8 mA cm−2 @ −1.8 V (vs. Ag/AgCl)

Ja

J refers to geometric current density or partial current density with specific subscript.

0.1 M KHCO3 0.5 M KHCO3 0.1 M NaHCO3 0.1 M KHCO3 0.5 M NaHCO3 0.5 M KHCO3 0.1 M KHCO3 90 wt% [Bmim]BF4/ MeCN 0.1 M KHCO3 0.5 M KHCO3 0.5 M KHCO3 0.5 M KHCO3 0.1 M KHCO3 0.1 M KHCO3 0.5 M KHCO3 0.1 M NaHCO3 0.5 M NaHCO3 0.1 M KHCO3 0.1 M KHCO3 [Bmim]BF4 with 3 wt% H2O 0.1 M KNO3 1 M KOH 0.5 M KHCO3 0.1 M KHCO3 0.1 M KHCO3 0.5 M NaHCO3 0.2 M KI 0.1 M KHCO3 0.5 M NaHCO3

B-doped graphene (G) N-doped G (NG) Sn sheets confined in G Co3O4/G Sb nanosheets/G Pd-In/3D-rGO NG foam GO/MWCNT Fe/NG Ni2+@NG A-Ni-NG Ni/NG Ni-N-modified G Cu NPs/rGO Au NPs/GNR Cu/rGO Au/rGO Ag2S/N-S-doped rGO PdTe/FLG NG like materials Pt/NG NGQDs Cu NPs/NG NG Cu NPs/NG Cu2O/G Cu/TiO2/NG Ag/NG/carbon foam Cu2O/ZnO/G

a

Electrolyte

Catalyst

Table 1 Summary of graphene-based catalysts reported for ECR.

HCOOH: 66% @ −1.4 V (vs. SCE) HCOOH: 73% @ −0.84 V (vs. RHE) HCOOH: 89% @ −1.8 V (vs. SCE) HCOOH: 83% @ −0.95 V (vs. SCE) HCOOH: 84% @ −1.06 V (vs. RHE) HCOOH: 85.3% @ −1.6 V (vs. Ag/AgCl) CO: 85% @ −0.58 V (vs. RHE) CO: 85% @ −2.2 V (vs. Ag/Ag+) CO: ∼80% @ −0.6 V (vs. RHE) CO: 92% @ −0.68 V (vs. RHE) CO: 97% @ −0.61 V (vs. RHE) CO: 95% @ −0.49 V (vs. RHE) CO: > 90% @ −0.7∼ −0.9 V (vs. RHE) CO: ∼50% @ −1.0 V (vs. Ag/AgCl) CO: 92% @ −0.66 V (vs. RHE) CO: 21.7% @ −0.4 V (vs. RHE) CO: N/A CO: 87.4% @ −0.76 V (vs. RHE) CO: ∼90% @ −0.8 V (vs. RHE) CH4: 93.5% @ −1.4 V (vs. SHE) CH3OH: 41% @ −0.3 V (vs. RHE) C2H4: 31% @ −0.75 V (vs. RHE) C2H4: 19% @ −0.9 V (vs. NHE) C2H5OH: 36.4% @ −0.4 V (vs. RHE) C2H5OH: 63% @ −1.2 V (vs. RHE) C2H5OH: 9.93% @ −0.9 V (vs. Ag/AgCl) C2H5OH: 43.6% @ −0.75 V (vs. RHE) C2H5OH: 85.2% @ −0.7 V (vs. RHE) PrOH: 30% @ −0.9 V (vs. Ag/AgCl)

Main product (Maximum FE)

Onset potential: −1.1 V (vs. SCE) Onset potential: −0.3 V (vs. RHE) Onset potential: −0.85 V (vs. SCE) Onset potential: −0.82 V (vs. SCE) η: 0.97 V N/A Onset potential: −0.3 V (vs. RHE) N/A N/A Onset potential: −0.34 V (vs. RHE) Onset potential: −0.3 V (vs. RHE) Onset potential: −0.31 V (vs. RHE) η: 0.54 V @ 0.2 mA cm−2 Onset potential: −0.9 V (vs. Ag/AgCl) Onset potential: −0.14 V (vs. RHE) N/A Onset potential: −0.24 V (vs. RHE) Onset potential: −0.34 V (vs. RHE) η: 0.69 V η: 1.16 V @ 3.26 mA cm−2 N/A Onset potential: −0.26 V (vs. RHE) Onset potential (CO2 reduction): −0.7 V (vs. NHE) N/A N/A N/A Onset potential: −0.1 V (vs. RHE) Onset potential (CO2 reduction): −0.38 V (vs. RHE) N/A

Onset potential or reaction overpotential (η)

4h 12 h 50 h 8h 12 h 24 h 5h 5h 10 h 20 h 100 h 20 h 5h 4000 s 24 h 15 h 1h 40 h 5h 5h N/A N/A 12 h N/A 6h 20 min 20 h 10 h 20 min

Stability

[104] [110] [129] [130] [39] [131] [108] [109] [134] [123] [122] [120] [91] [135] [142] [143] [136] [44] [42] [137] [139] [140] [40] [141] [127] [144] [43] [41] [45]

Reference

T. Ma, et al.

Journal of CO₂ Utilization 30 (2019) 168–182

Journal of CO₂ Utilization 30 (2019) 168–182

T. Ma, et al.

2.2.1. *CO2%− radical anion (* denotes surface adsorbed species) Despite extensive experimental and theoretical studies of heterogeneous ECR [60,61], the reaction mechanisms remain elusive. On metals (Au, Cu, and Ag) with medium to high overpotentials for HER, electron transfer to generate a carboxylate CO2%− was proposed by Hori as the initial step in ECR [62], which is supported by reaction kinetics analysis [63,64]. This step requires noticeable energy input to rearrange the linear molecule into a bent radical anion. Note that *CO2%− has extremely short lifetime in either aqueous or nonaqueous solutions. Effort has been made to explore microscopic and spectroscopic methods, such as scanning electrochemical microscopy [61], Raman [60,65] and Fourier transform infrared (FTIR) [66] spectroscopy for detection of the common intermediate on metal electrode surface. Nevertheless, it is still not fully clear what the structure of *CO2%− and how it controls the reaction pathway. As shown in Fig. 3a, strongly adsorbed η1(C)-CO2%− (C-down) or η2(C,O)-CO2%− (C,O-down) was speculated to transform to CO after subject to protonation to carboxyl *COOH followed by reductive dissociation. While η2(O,O)-CO2%− (O,Odown) or weakly adsorbed η1(C)−CO2%− (C-down) may be hydrogenated by either a direct Tafel-like reaction with surface hydride *H or by proton-coupled electron transfer (PCET) to produce formate. Recently, a carboxylate intermediate with the η2(C,O)-CO2%− structure on Cu was deduced by combining operando surface-enhanced Raman scattering and density functional theory (DFT) calculations [60]. In the proposed model (Fig. 3b), η2(C,O)-CO2%− is generated at potentials anodic of the electrocatalytic current. Cathodic polarization gradually elongates the CeO bonds and decreases the OeCeO angle, coupling with weakening of the CueC bond and stabilization of the CueO bond. Further, η2(C,O)-CO2%− reacts with *H, leading to formate. At circumneutral and basic pH, η2(C,O)-CO2%− directly dissociates to form CO as observed in gas phase.

depletion and induce turbulent flow at the catalytic surface, benefiting CO2 electrolysis. It also enables a short CO2 diffusion distance to the catalyst surface (minimizing concentration gradients), allowing for reaction before it is converted to bicarbonate. Ohmic overpential occurs through the Joule effect. Such charge limitation can be reduced by salient choice of electrolyte, membrane, and geometry [21]. For parameters that may be useful in mechanism analysis and benchmarking of a catalyst, readers are referred to earlier reviews [10,58,59]. We would like to stress that to obtain a meaningful comparison between different catalysts, a combined figures of merit is required. In some cases, a single metric may be inadequate to describe exact catalytic activity. For instance, faradaic efficiency (FE) is commonly used as a gauge for selectivity. However, an increase in FE to a product may be not necessarily coupled with an increase in production rate. The latter is proportional to the product partial current density. As such, only comparing FE can give incomplete picture of catalyst performance. Equally importantly, measured rates for CO2 reduction should be normalized based on the number of available active sites. The electrochemical surface area (ECSA) for normalization is preferred instead of geometric surface area to reflect the intrinsic performance of a catalyst. To attain an efficient CO2 electrolyzer, we need to maximize the overall product generation rate per unit of energy input, the partial current density of the desirable product at the highest energy efficiency as well as the energy efficiency at the largest partial current density. The maximum FE is plotted against corresponding overpotential for about 260 reported ECR catalysts, as shown in Fig. 1. It reveals that a large number of reported systems selectively produce CO and HCOO− (FE ≈ 90%) with considerably lower overpotentials for CO. Products of CH4, CH3OH, C2H4O2, C2H4, and C2H6 have been obtained with modest FEs in the range 30–80%. CH3OH displays an apparently lower overpotential than the others, while CH4 and C2H4 typically demand overpotentials of ∼1 V with scattered efficiency. C2H5OH and C3H6OH have been only produced with relatively lower FEs (generally < 40%). Both require higher overpotentials than CH3OH.

2.2.2. Formyloxyl *OCHO *OCHO has been postulated as an intermediate for HCOOH. There are three possible paths for *OCHO. Nørskov and co-workers proposed that *OCHO originated through PCET (CO2 + e– + H+ → *OCHO) [67]. While Goddard et al. [68] suggested the hydride transfer mechanism for *OCHO (CO2 + *H → *OCHO). *OCHO then coupled e– to yield stabilized HCOO–. Besides these two DFT models, an alternative is the direct two-electron reduction of adsorbed bicarbonate HCO3–, as observed on Sn electrode using in situ attenuated total reflectance infrared spectroscopy (ATR-IR) [69].

2.2. Possible key reaction intermediates during ECR Controlling the binding affinities of key intermediates is critical to achieve high activity and selectivity in the ECR. As such, possible reaction intermediates during ECR, as illustrated in Fig. 2 are discussed in the following.

2.2.3. Carboxyl *COOH *COOH can convert to CO via concerted proton and electron transfer (*COOH + e– + H+ → CO + H2O) [70]. *COOH may be generated either by protonation of *CO2%− [62] or through PCET of aqueous CO2 [71]. It was calculated that the pathway for the formation of *COOH on nitrogen doped graphene is more favorable than that for *OCHO [72]. 2.2.4. *CO and *HCO *CO, resulting from PCET of *COOH, is the common species for C1 (CO, HCHO, CH4, and CH3OH) and a range of C2+ products (containing two or more carbon atoms such as C2H4, C2H6, C2H5OH, C2H4O, C2H4O2, C2H6O2, and C3H7OH) [59]. For Au, Ag, Zn, Pd, and Ga metals [42,73,74], *CO binds weakly and easily desorbs from the metal electrode to form CO. DFT calculations on Cu(211) showed that *CO may be subsequently hydrogenated to *HCO, *H2CO (desorb as HCHO), and *H3CO (methoxy) [70]. CH3OH can be generated by *H3CO reduction. Dimerization of *CO to form *C2O2− [55] or *HCO [75] is likely a crucial and rate-limiting step for production of C2+ compounds. Fig. 1. Activity map for the ECR toward the widely reported products. The maximum FE is plotted against corresponding overpotential for approximately 260 reported catalysts as shown in Table S1 in Supporting Information. The background color intensity correlates with the density of points, indicating characteristic values of FE − overpotentials for different products.

2.2.5. *CH2 *CH2 can be produced by protonation and deoxygenation of *CO [76]. In a “carbene” mechanism, C2H4 is formed through either coupling of two *CH2 species or CO insertion in a Fischer-Tropsch-like step 171

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Fig. 2. Proposed reaction pathways with key intermediates for different CO2 electroreduction products.

then reduced to *CH2]CHO which appears to be the common intermediate for C2H4 [82], C2H6 [83], and C2H5OH [84]. 2.3. Effect of metallic impurities on electrocatalytic activity 2.3.1. The role of metal impurities in graphene-based electrocatalysts for CO2 reduction Graphene and heteroatom-doped graphene are always referred to as “metal-free” electrocatalysts. However, this needs to be cautious since metal residues may be present even after repeated acid washing when using metal precursor during synthesis [85,86]. For instance, graphene prepared by the (modified) Hummers’ method may contain manganese resulting from potassium permanganate [87]. The methods such as XPS (detection of only < 5 nm surface composition; detection limit > 0.1 atomic percent), EDX, and XRD that are usually employed are not sufficiently sensitive to rule out the presence of trace metal impurities (< 0.1 atomic percent) in graphene-based catalysts. Metallic impurities can profoundly influence the electronic structure of graphene even when present at only trace levels, and in some cases serve as dominant active sites [87–90]. As an example, impurities of Ni, Fe, Mn, and Cu were found to significantly contribute to the electrochemical activity of GO for ECR [88]. In particular, a small amount of Cu species (119 ppm/ W) in GO could facilitate a high level of CH4 generation (54.67 mmol g−1 s−1). Furthermore, metals may coordinate with heteroatoms (such as N) or defects in modified graphene (or GO), dramatically enhancing ECR performance [38,91]. From these scenarios, the concentration of residual metals must be determined to avoid misinterpretation. To this end, inductively coupled plasma mass spectrometry (ICP-MS) as well as ion scattering spectroscopy (ISS) likely provide more accurate qualitative and even quantitative information about the metal contents in those “so-called” metal-free catalysts.

Fig. 3. (a) Possible first intermediates of ECR in aqueous electrolytes. (b) Proposed model of CO2 activation on a Cu electrode in aqueous electrolyte. Atom colors: red, oxygen; gray, carbon. Adapted from Ref. [60] with permission from the National Academy of Sciences of the United States of America.

[77], which has also been postulated to be the pathway for the formation of C2H4O and C2H5OH [78]. Whereas C2H4O is rarely obtained as the final product because it can be further converted to C2H5OH via rapid reduction or through base-catalyzed Cannizzaro-type disproportionation [79,80]. *CH2 can be also obtained from subsequent reductions of *HCO, *C, and *CH. Further reduction of *CH2 gives rise to *CH3 and finally to CH4 [81].

2.3.2. The influence of metallic impurities from other sources on CO2 reduction Apart from the impact of possible impurities arising from catalysts, trace amounts of metallic impurities in electrolytes originating from solvents and electrolyte salts, and metal cations evolved from anode electrocatalysts (such as Pt) which may reach the cathode, can cause

2.2.6. *COCO Major pathways for C2+ products are usually associated with *CO dimerization to form a *C2O2 intermediate. It can be hydrated to *COCOH via the hydride transfer (Eley-Rideal) mechanism. *COCOH is 172

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affording enhanced activities in ECR. Additionally, doping with some heteroatoms may improve surface wettability of pristine graphene [103], which facilitates transportation of hydrated CO2 to active sites. Relative to carbon atom (2s22p2), boron atom (2s22p1) has one fewer valence electron, thus inducing charge polarization and p-type doping in the graphene matrix. In B-doped graphene (B-G), boron mainly exists in the forms of in-plane doping BC3 and out of plane doping B4C, boronic BC2O, and borinic BCO2. B-G has been shown to possess a high spin density. Boron doping induces charge polarization in the carbon framework, which stabilizes the negatively polarized O atoms of CO2 [36]. Hence the chemisorption of CO2 onto the carbon surface is enhanced. The positively charged B and C atoms were speculated to be catalytically active sites for CO2 reduction, which facilitated chemisorption *COOH to generate formate [104]. In contrast to B doping, N doping induces polarization in the carbon network due to the higher electronegativity of N (3.04) than C. Four major bonding configurations have been reported in N-doped graphene (N-G): basal plane quaternary N (central graphitic), edge pyrrolic N, pyridinic N, and nitrilic N [105]. Graphitic N results in an n-type material while pyrrolic-, pyridinic- and nitrilic N atoms leads to a p-type material. N dopant (such as pyridinic N site) has been demonstrated to modify the electronic structure of the surrounding carbon atoms and induce a local density of states of ∼200 − 400 meV below the Fermi level [106], which may provide CO2 adsorption and reaction sites. DFT calculations showed that N doping results in decreased energy barrier to *COOH intermediate, weakened adsorption energy of *CO (ΔECO), and strengthened adsorption energy of *COOH (ΔECOOH). In addition, pyridinic N with a lone pair of electrons can create Lewis base sites for CO2 adsorption [107]. As a consequence, electroreduction of CO2 to CO [108,109] or formate (formic acid) [110] was improved. The decisive active sites in N-G for CO2 reduction are still not clearly. Pyridinic N (triple-and single-pyridinic N [108]) and a C atom next to pyridinic N [111] were proposed to be the most active [36]. It was revealed that the pyridinic N at the edge sites of graphene quantum dots could induce CeC bond formation more likely than the basal plane, leading to enhancement in the yield of C2 and C3 products [112]. Alternatively, pyrrolic N was inferred to prompt ECR to HCOOH with the lowest overpotential of 0.24 V [113]. The graphitic N at the edge was also shown as the most active site in planar graphene, with CO2 activation barrier of 0.58 eV [72]. Likewise, the efficiency in promoting CO2 conversion to CO was stated to follow the trend graphitic N > pyridinic N > pyrrolic N [114]. These discrepancies in the role of N dopant for ECR may be attributed to the variation of content of each species and difficulty in the formation of a single configuration. The electronegativities of sulfur (2.58) and phosphorous (2.19) are closer to that of C (2.55) and lower than N (3.04). So P and S atoms have a smaller effect on the distribution of charge density (negligible polarization for S doping). Both P and S doping tend to occur at edges of graphene because of the larger atomic radii of P (1.10 Å) and S (1.04 Å) than that of C (0.77 Å). The interaction between the 3p orbitals of P and the 2p orbitals of C can cause sp3 hybridization of carbon atoms, forming a tetrahedral-like distorted structure with three proximal carbon atoms. The mismatch of the outermost orbitals of the S and C atoms enables a non-uniform distribution of spin density, imparting graphene with catalytic activity. S-doping results in two major forms: C–S–C (thiophene) and C–SOx–C (x = 2, 3 or 4). S doping was claimed to result in lower ECR activity than N doping. This was assumed to be due to the less positively charged carbon atoms adjacent to S atoms [111]. In contrast to pure pyridinic N atoms that require a higher Gibbs free energy (1.34 eV) to overcome the barrier of *COOH adsorption, a lower Gibbs free energy barrier of 1.01 eV for *COOH occurs at pyridinic N neighboring carbon-bonded S atoms [32]. F- or Cl-doping leads to p-type material with sp3 bonding of carbon atoms. Whereas Br and I dopants interact with graphene by physisorption or formation of charge-transfer complexes without perturbing the sp2 carbon network. Fluorine doping was calculated to activate

CO2 reduction electrocatalysts to lose their activities due to metal ion deposition [92,93]. In these cases, the loss of activity can be solved by 1) periodic oxidative pulsing of the electrode [94], 2) long-term preelectrolysis using a sacrificial electrode to scavenge trace metal ion impurities [88,95], and 3) complexing trace metal ion impurities with a chelating agent [90], and 4) using an inert counter such as graphite rod [96]. 2.4. Confirmation of product origin Despite a growing number of papers on ECR, only a few of them confirmed the source of CO2 reduction products. When performing ECR, traces of organic species in the electrocatalyst originating from its synthesis (e.g., solvents, precursor molecules) or contaminants from gas cylinders (99.999% pure CO2 is preferred to minimize contamination) may contribute to product formation. This necessitates especially for chemically modified carbon electrodes to identify the predominant origin of the CO2 reduction product. It is useful to conduct a blank test in an Ar or He gas flow. Product formation detected in this case would suggest analytical contamination. We would like to point out that the use of N2 is not suggested in such experiment taking into account that N2 reduction may occur under operating conditions. Meanwhile, one needs to carry out a control experiment with CO2 feed gas but no potential applied. If products are detectable, this unequivocally indicates the existence of contaminants. Isotopic labeling using 13CO2 molecules and/or H13CO3− ions in combination with gas chromatography/mass spectrometry (GC–MS) and nuclear magnetic resonance (NMR) provides an effective tool to examine the source of CO2 reduction products [97–99]. Electron paramagnetic resonance (EPR), surface-sensitive infrared spectroscopy and other analytical techniques can offer supplementary mechanistic insight. The exclusive formation of 13C products indicates the actual reduction of the labeled component. When the CO2 feed gas is labeled, if 12C species are produced, carbon residues in the electrocatalyst and/ or bicarbonate ions in the electrolyte are reacting sources. Conducting electrolysis using labeled bicarbonate in Ar can further be used to identify the most likely product origin. To ascertain whether the bicarbonate or the aqueous CO2 contribute to the ECR, the two carbon sources can be selectively labeled. Analyses of the 13C/12C product ratio within the nonequilibrium time frame and under equilibrium conditions can disclose possible reacting species. To ascertain the exact reacting species (the CO2 or the HCO3−), a series of control experiments can be operated at different pH values where only one species dominates (in the form of CO2 (aq) at pH below 5; in the form of bicarbonate at pH between 7 and 9). Note that the origin of CO2 reduction product remains a debate. It may be related with electrocatalyst type and electrolyte applied. CO was observed to stem from the aqueous CO2 supplied through the fast equilibrium with the bicarbonate anions (not from the dissolved CO2 in the bulk solution) on Au [100], Cu [101], and N-doped porous carbon [99] electrodes. 3. CO2 electroreduction using graphene-based materials 3.1. Modification of graphene to enhance CO2 reduction Surfaces of graphene (or GO) featuring a high level of exposed atoms can be modified using metallic and nonmetallic species. 3.1.1. Nonmetallic doping Pristine graphene cannot effectively catalyze the ECR owing to negligible ability to adsorb and activate the CO2 molecule as well as the very high free energy barrier for the elementary step of *COOH formation. Nevertheless, doping with single or multiple heteroatoms (pblock elements such as B, N, P, S, F, Cl, Br, and I, as shown in Fig. 4) [102] can greatly modify the structure and properties (electronic properties, spin densities, edge strain, hydrophilicity) of graphene, 173

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Fig. 4. Illustration of doped graphene with different heteroatoms (B, N, S, P, F, Cl, Br, and I).

3.1.3. Construction of graphene composites Graphene featuring large accessible surface areas and excellent conductivity is typically used as a support for active phases (nanoparticles (NPs) or nanosheets). Such combination can enhance charge transport and result in electronic and structural coupling effects [126] to accelerate CO2 conversion. A synergy can be created at the interface, which increases CO2 adsorption, intermediate binding and stability, benefiting ECR. It has been shown that immobilization of Cu [127] or Ag [41] on the surface of N − doped graphene could transform stabilized *CO (by N − doped carbon) into *OC-COH, promoting ethanol or n−propanol generation [41,127]. Pyridinic − N was reported to serve as a CO2 and proton absorber, facilitating hydrogenation and CeC coupling reactions on Cu to form ethylene [40]. On the basis of largescale screening DFT and microkinetics modeling, Cu2, CuMn, and CuNi dimers supported on graphene with adjacent vacancies were predicted to be potential electrocatalysts for CO, CH4, and CH3OH production, respectively [128].

neighbor carbon atoms by creating asymmetrical charge distributions and causing larger polarizations. Meanwhile, asymmetrical spin distributions were induced, which benefited COOH* interacting with activated carbon [33]. These carbon atoms with high positive charge and asymmetrical spin densities exhibited reduced energy barrier for COOH* formation, and were speculated to be active sites for ECR.

3.1.2. Transition metal doping Doping of graphene with metallic species (e.g., Ni, Fe, Co, Zn, Mn, Ru, Rh, Ir, Os, Ag, Cu, and Pt) can significantly enhance CO2 electrocatalysis with higher efficiency than the aforementioned heteroatom substitution. On the basis of calculated energy changes, Co-, Rh-, and Irdoped porphyrin-like graphene structures were stated to be potential electrocatalysts for hydrocarbon production [115]. But the undesirable HER was also favored in these systems, causing low ECR efficiency. Following this work, Mn, Fe, and Ni dopants were found to effectively reduce CO2 to CO at low overpotentials [116–118]. Of these dopants, Ni atoms embedded in graphene [119] and N-substituted graphene [91,120–123] exhibit superior ECR activities. Compared with metallic Ni, the Ni single sites possess different electronic structures that facilitate CO2-to-CO conversion and substantially inhibit the competitive HER. In many examples, Ni, mainly in the form of either Ni+ [122] or Ni2+ [123], was supposed to be coordinated by four N atoms (Ni-N4). The geometry around Ni(I) in N-doped graphene was observed to be highly distorted possibly due to a non-centrosymmetric ligand strength, which helps promote better adsorption of reactants and intermediates on the catalyst surface. The CO FE and partial current density followed the order of Ni-N4 > Fe-N4 > Co-N4 [123]. Mixed Ni–C and Ni–N coordination structures such as Ni @ single vacancies (Ni-C3), Ni @ double vacancies (Ni-C4), Ni-N @ single vacancies (Ni-N-C2), and Ni-N @ double vacancies (Ni-N-C3) were also suggested for those Ni SACs dispersed on N-doped graphene [120]. By a combination of operando hard X-ray absorption fine structure (XAFS) spectroscopy, ambient pressure soft X-ray photoelectron spectroscopy (XPS) and DFT simulations, nitrogen-coordinated Fe(II) single atoms at the interface between Fe-OOH and N-doped C were presumed to facilitate CeC coupling to selectively form acetic acid [124]. The N dopants in carbon network were expected to not only coordinate CO2-related species but also stabilize Fe(II) moieties to avoid their further reduction, thereby suppressing hydrogen evolution. By computational screening, single Os [125], Ru [125], and Ag [38] atoms on graphene were identified to be active for CH4 formation, while isolated Pt [125] and Cu [38] atoms tended to promote CH3OH formation. We would like to point out that the catalytic properties of emerging single metal atoms can be tuned by a ligand–field effect, which differs depending on the identity of the metal centers and their interaction with the coordination configuration.

3.2. Reaction products 3.2.1. Formic acid or formate Both boron-doped graphene [104] and nitrogen-doped graphene have been reported as effective metal-free catalysts for ECR to produce formate. The maximum FE toward HCOO− for B-G was about 66% at −1.4 V (vs. SCE). A slightly higher HCOO− selectivity for N-G was achieved to be 73% at −0.84 V (vs. RHE) [110]. A variety of graphene supported metal or metal oxide (Sn [129], Sb [39], Co3O4 [130], and PdIn [131]) catalysts have been shown to be active for formate formation via ECR with better performance than nonmetallic doped graphene catalysts. The maximum formate FE was higher than 80% in all cases. Notably, Sn quantum sheets confined in graphene were synthesized through a spatially confined reduction strategy (Fig. 5a) [129]. Such ultrathin metal layers displayed a current density of 21.1 mA cm−2 and a formate FE of 89% at −1.8 V (vs. SCE) (Fig. 5b) as well as good long-term stability (Fig. 5c), comparable to carbon supported Pd NPs [132]. The Sn-Sn coordination numbers were reduced in this hybrid, which can efficiently stabilize CO2%− with a lower potential (0.13 V) for hydroxyl ion adsorption compared with bulk tin. A similar formate FE of 88.5% at –1.06 V (vs. RHE) was attained for Sb nanosheet–graphene composites that were prepared by using an electrochemical exfoliation method [39]. Co3O4 NPs supported on N-G were able to produce 3.14 mmol of formate in 8 h at −0.95 V (vs. SCE) with a FE of 83% [130]. On the one hand, the N-G support is beneficial to maximizing exposed cobalt sites and minimizing NP aggregation. On the other hand, it can help the transformation toward reduced CoO (the true active phase under electrocatalytic conditions). A unique structure comprising bimetallic Pd-In NPs supported on three-dimensional (3D) reduced graphene 174

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Fig. 5. (a) Scheme illustration for the formation of Sn quantum sheets confined in graphene. (b) FEs for formate at each applied potentials for 4 h. (c) Chronoamperometry results at −1.8 V for the Sn quantum sheets confined in graphene, 15 nm Sn NPs mixed with graphene, 15 nm Sn NPs and bulk Sn. The error bars in (b) represent the standard deviations of five independent measurements of the same sample. Adapted from Ref. [129] with permission from Springer Nature.

of 87.4% at −0.76 V (vs. RHE) [44]. However, this composite catalyst has drawback of low current density, limiting practical applications in ECR.

oxide (rGO) was shown to deliver a formate FE of 85.3% at –1.6 V (vs. Ag/AgCl) [131]. 3.2.2. Carbon monoxide and syngas A 3D N-G foam [108] was recently demonstrated as a metal-free electrocatalyst for aqueous CO2 reduction to CO with a smaller onset overpotential (about 0.19 V) and overpotential (0.47 V) to yield a maximum CO FE (∼85%), and better stability (≥5 h) than polycrystalline Au and Ag [133]. Interestingly, a composite consisting of multi-walled carbon nanotubes (MWCNTs) and GO [109] showed a synergetic effect in ECR and exhibited higher efficiency for CO formation than each single counterpart in an ionic liquid (IL)/organic solvent ([Bmim]BF4/MeCN) electrolyte. A CO FE of 85% was obtained with a CO partial current density of 2.3 mA·cm−2 at −2.2 V (vs. Ag/Ag+). Metal-doped graphene performed excellently in ECR to produce CO [91,119,120,122,123,134]. Fe atomically dispersed on N-G in the form of Fe–N4 can catalyze ECR to CO with a FE of 80% [134]. The Fe–N4 structure and nitrogen species worked collaboratively and enhanced CO2 adsorption and activation by lowering *COOH formation barrier and facilitating *CO desorption. Single Ni sites [91,119,120,122,123] performed better than Fe atoms for CO2 conversion to CO. A CO FE of 97% as well as a specific current of 350 A per gram of catalyst was accomplished at a mild overpotential of 0.61 V by using atomically dispersed Ni on nitrogenated graphene (A-Ni-NSG) (Fig. 6a) [122]. The calculated turnover frequency (TOF) reached 14,800 h−1, one order of magnitude larger than those of the best cobalt protoporphyrin and noble metal catalysts in aqueous solution (Fig. 6b). Further, the catalyst preserved 98% of its initial activity after 100 h of continuous electrolysis at a CO formation current density of 22 mA cm−2 (Fig. 6c). Graphene supported metals such as Cu [135], Au [136], and PdTe [42], and metal sulfide of Ag2S [44] were also used for ECR. A Cu NPs/ rGO composite electrocatalyst enabled generation of syngas with various ratios of H2 to CO from 1.16:1 to 6.38:1 by tuning applied potentials [135]. Both Au [136] and PdTe [42] NPs dispersed on graphene have outstanding catalytic activities for ECR with CO FE values exceeding 90%. Ag2S NPs anchored on N, S co-doped reduced graphene oxide were also reported for stable and efficient ECR, affording a CO FE

3.2.3. Methane Graphene-based materials have been rarely explored for ECR selectively to CH4. Only N-doped graphene-like materials (NGMs) was demonstrated to reduce CO2 to yield CH4 in an ionic liquid (1-butyl-3methylimidazolium tetrafluoroborate) organic solvent electrolyte [137]. This process was found to maintain selectivity when adding a small amount of water (3%) to the ionic liquid (Fig. 7a), while the current density increased from 1.4 to 3.3 mA cm−2 (Fig. 7b). The current densities obtained with this NGMs electrode were approximately six-fold higher than that of Cu foil used under similar conditions. Pyridinic N and pyridinic/pyrrolic N species were speculated as active sites to promote CO2 adsorption. The CH4 FE could be significantly improved from 20.8% to 93.5% by increasing the active N (pyridinic or pyridinic or pyrrolic N) content from 1.8% to about 4.8%. The ionic liquid favored the transformation of adsorbed CO2 molecules to CO2%− [138]. Meanwhile, the strong interaction between *CO and the electrode inhibited the escape of CO from the electrode, which is favorable for its further hydrogenation to form CH4. A mechanism proposed for this eight-electron reduction is illustrated in Fig. 7c. 3.2.4. Methanol Pt-modified N-G provided a methanol FE of 41% at −0.30 V (vs. Ag/AgCl) [139]. It was inferred that pyridine in N-G at low pH accepted a proton to form pyridinium ion (−PyH+), whereby hydrogen atoms were transformed to the surface of Pt. The hydrogen atoms were susceptible to electrophilic attack by CO2, forming *COOH. The intermediate interacted with PyH+, leading to the formation of PyH+⋯COOH which was further reduced to methanol. 3.2.5. C2+ products (ethylene, ethanol, and n-propanol) It is difficult to produce C2+ chemicals from ECR due to the high energy required and sluggish kinetics for C-C coupling. N-doped graphene quantum dots (NGQDs) (Fig. 8a) showed such possibility for 175

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Fig. 6. (a) Aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of A-Ni-NG, scale bar: 5 nm. (b) TOF of A-Ni-NSG compared with those of other state-of-the-art CO2 to CO reduction catalysts, highlighted in the shaded region. (c) Current–time response of A-Ni-NSG on carbon fibre paper for CO2 reduction at an overpotential of 0.61 V. All measurements were conducted under the same conditions: 0.5 M KHCO3 (pH = 7.3), 1 atm CO2, room temperature. Adapted from Ref. [122] with permission from Springer Nature.

C3 products on NGQDs than on undoped graphene quantum dots (GQDs) (Fig. 8c) and N-doped reduced GO (NRGO) (Fig. 8d). As a catalyst support, graphene can promote electron transport to the key potential limiting step species of *CHO, and also help prevent sintering of metal NPs possibly arising from the strong interplay between metal and the surface of graphene. A synergistic effect was observed on Cu NPs (7 nm) assembled on a pyridinic-N rich graphene (pNG) [40]. The p-NG itself only reduced CO2 to formate. While in the composite, the pyridinic-N served as a CO2 and proton absorber,

metal-free CO2 catalysis so far [140]. NGQDs rich in defects and N species at edges exhibited a C2H4 partial current density of over 40 mA/ cm2 at potentials more positive than −0.8 V (vs. RHE), with C2 product FE reaching about 31%, comparable to Cu nanocube catalyst but with significantly lower overpotential (Fig. 8b). Multi-carbon oxygenate liquid fuels were also obtained with a FE of 26% at −0.78 V (vs. RHE). The major component of C2H5OH accounted for a FE of 16%. Pyridinic N at the edge sites of NGQDs were claimed to induce CeC bond formation more likely than the basal plane, therefore favoring more C2 and

Fig. 7. (a) CH4 FEs and (b) partial current densities as a function of water contents at various applied potentials. (c) Nitrogen-doped graphenic catalysis mechanism of CO2 (1) reduction to methane (8). Adapted from Ref. [137] with permission froVm The Royal Society of Chemistry. 176

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Fig. 8. (a) HRTEM image of NGQDs. Scale bar, 2 nm. Inset shows a single NGQD containing zigzag edges as circled. The yellow line outlines the zigzag edge. Scale bar in inset, 1 nm. (b) FEs of CO, CH4, C2H4, HCOO−, C2H5OH, CH3COO−, and C3H7OH at various applied cathodic potentials for NGQDs. (c) FEs of CO2 reduction products for pristine GQDs. (d) Selectivity for NRGOs. Adapted from Ref. [140] with permission from Springer Nature.

4.1. Synergistic structural engineering

facilitating hydrogenation and C-C coupling reactions on Cu for the formation of C2H4. At p-NG/Cu mass ratio of 1:1, the C2H4 FE and hydrocarbon selectivity reached 19% and 79%, respectively at −0.9 V (vs. RHE) with a mass activity of 2.9 A/gCu. Pyridoxine modified GO sheets were demonstrated to produce ethanol with a FE of 36.4% at −0.4 V (vs. RHE) [141]. CeC bonding was speculated to be associated with the presence of pyridinic N species and pyridine derivative structure. Higher selectivity of ethanol was attained by using N-doped graphene supported Ag [41] and Cu/TiO2 [43] heterostructures. The former even afforded a surprisingly high ethanol FE of 85.2% at −0.7 V (vs. RHE). Pyridinic N on graphene was supposed to strengthen the binding energy to adsorb CO* which was gradually converted to OC–COH on Ag surfaces, leading to the formation of ethanol. Graphene supported mixed metal oxides of ZnO/Cu2O were recently shown to catalyze ECR to n-propanol [45]. At a potential of −0.9 V (vs. Ag/AgCl.), the propanol FE approached 30% by using the catalyst with a ZnO/Cu2O weight ratio of 2:1.

Construction of a 3D hierarchical graphene structure with well-defined porosity and cascade active sites likely promote ECR kinetics and break the scaling relation limitation. A 3D geometry can accelerate facile diffusion of species and shorten ion and electron migration distances. Control of distinct active centers in close vicinity in a 3D porous space can bind different intermediates, beneficial to facilitating C-C coupling. Among others, local pH and CO2 concentration may be modulated in a hierarchical 3D structure, making it possible to tune reaction activity and selectivity.

4.2. Multiple doping Tailoring the surface structure of graphene by doping appears to benefit ECR performance. Single doping (as listed in 3.1.1 and 3.1.2.) has been commonly used. While double doping and multiple doping are rarely explored in CO2 electroreduction. Doping graphene with two elements, one with higher and the other with lower electronegativity than that of C, can create a unique electronic structure with a synergistic coupling effect between heteroatoms [102]. Two concerted effects may be expected by multiple doping: 1) jointly alter the polarization and spin density of the carbon atom to reduce Gibbs free energy for key intermediates (such as the potential-limiting *COOH species [32]); 2) introduce distinct active sites for different intermediates to enable tandem reactions for C2+ generation. Note that the role of different doping species and configurations in ECR are not fully understood. The balance of doping components to maximum ECR efficiency demands further investigation.

4. Strategies for enhancing the ECR activity of graphene-based materials Schemes that are useful for improving graphene-based ECR need to consider the following: 1) increasing reduction current density at low overpotential selectively toward a single product especially C2+ compounds, (2) blocking competing reactions such as HER, and (3) enhancing stability and durability. To this end, we provide some possible effective strategies in this section.

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should be also considered to reduce errors. Moreover, future new calculation methods need to critically take into account the electrodeelectrolyte interface. To deepen understanding of CO2 reduction intermediates, real catalytic active sites and their structure variation during reaction as well as mechanistic pathways, in-situ (or ex-situ) and in operando techniques are necessary. Increasing effort has been made on exploring the ECR by using spectroscopic methods, such as in situ/operando Raman spectroscopy, attenuated total reflectance (ATR)-(surface-enhanced) infrared (IR) spectroscopy [100,159], hard X-ray absorption fine structure (XAFS) spectroscopy [122,124], and ex-situ XPS analysis. Ultrafast electrochemical ATR-IR spectroscopy and electrochemical tip-enhanced Raman spectroscopy are two recently developed techniques that may provide solid information on surface adsorbates and active species [160,161]. Scanning electrochemical microscopy with a tip generation/ substrate collection mode has been also used to detect CO2%− in the ECR [61]. Albeit recent advances in this regard, the development of more powerful analytical tools for detection of extremely unstable and short-lived intermediates and study of reaction mechanism deserves much more endeavors in the future. These techniques, combined with accurate theoretical modeling, may allow one to gain insight into reaction pathways, which in turn aids new (efficient and selective) catalyst design and optimize catalysts’ properties for this clean energy reaction.

4.3. Surface modification Modification of graphene surfaces with organic species such as amino acids [145], salen ligand (H2LNO2) [146], pyridine derivative [147], and N−arylpyridinium [148] provides a strategy to tune CO2 electrocatalysis. Their roles in affecting ECR involve: 1) regulation of proton adsorption and electron/proton transfer to boost reactions of species generated in CO2 reduction; 2) stabilization of intermediates (such as *CHO) and their proper binding (according to Sabatier’s principle); 3) formation of a radical carbamate by reacting with CO2 to lower reduction overpotential. In particular, surface modification can be linked with decoration of metal phases (such as Cu and its alloys). This permits creation of synergistic/cooperative effects to boost CO2 reduction towards multicarbon products [149]. Equally importantly, the versatile surface chemistry of graphene offers advantages in immobilization of homogeneous molecules such as metal phthalocyanines [150,151] and porphyrin complexes [152,153]. Such combination allows inhibition of aggregation or dimerization of molecular catalyst, exposure of catalytic sites, efficient electron shuttling, and stabilization of the catalyst, hence enhancing catalytic performance. 4.4. Optimization of electrolyte and electrolyzer The type, composition, and concentration of electrolytes profoundly affect CO2 electrolysis [59]. Alkaline electrolytes (such as KOH) are favorable for CO2 reduction by suppressing undesirable HER and reducing activation energy barriers of both CO2 reduction and CO-CO coupling [97]. Another promising electrolyte type in ECR is ionic liquid (IL). The promotion of CO2 reduction using IL electrolytes over different electrodes has been reported [138,154,155]. The ion-pairing between IL cation and CO2%− radical anion to form a complex was supposed to decrease the activation overpotential [138]. The current density achieved in ILs was reported as being 10 times higher than the current density obtained in aqueous electrolytes [154]. Of interest is that a majority of ILs have a dramatically larger CO2 solubility (up to 100 times) than water, favoring a high concentration of CO2 at the electrode-electrolyte interface. Further, the presence of adsorbed ILs at the interface can effectively impede hydrogen evolution. Consequently, diluted aqueous ILs with ion-pairing and HER-inhibition properties may have a potential for CO2 reduction in industrial applications [156]. However, the challenge is that ILs may be unstable under high current density conditions required for commercial systems. To overcome mass transport limitation associated with the low solubility of CO2 in aqueous electrolytes, a flow-cell electrolyzer with gas diffusion electrode (GDE) cathode can be engineered where gaseous CO2 is fed to the electrode-electrolyte interface to form a triple-phase boundary [157,158]. This allows one to convert CO2 at high reaction rates. For stable cell operation and commercialization of CO2 electrolyzer, flooding and salt accumulation should be alleviated by interface engineering. Meanwhile, retaining a robust electrode-electrolyte interface where gaseous CO2/products can readily migrate in/out of the GDE without deteriorating ionic and electrical conductivity at high reaction rates is essentially required.

5. Summary and outlook CO2 electroreduction will likely continue to be the subject of extensive research driven by the need to mitigate environment problems and energy shortages. Graphene-based materials provide both technological and new fundamental insight into the ECR. Doping of graphene with nitrogen enables selective CO2 reduction to form formate in aqueous electrolytes and CH4 in ionic liquid solvent mixtures (at a high FE of 93.5% at −1.4 V vs. SHE). Construction of three-dimensional foam of N-doped graphene can convert CO2 selectively to CO. Unprecedented catalytic performance for CO formation was achieved by creation of single Ni sites on N-doped graphene (yielding a specific current of 350 A per gcatalyst, 97% CO FE, and turnover frequency of 14,800 h−1 at an overpotential of 0.61 V). Modification of N-doped graphene surface with Pt promotes CO2 reduction to produce CH3OH with reasonable FE (41% at −0.30 V vs. RHE). Reduction in two dimensional structure to zero-dimensional N-doped graphene quantum dots allows one to catalyze CO2 reduction to C2H4 with activity comparable to Cu nanocube catalyst. Production of C2H5OH with very high FE (85.2% at −0.7 V vs. RHE) has been attained by anchoring Ag nanoparticles on N-doped graphene. Furthermore, C3 product such as n-propanol with the highest FE (30% at −0.9 V vs. Ag/AgCl) to date has been obtained by using graphene supported ZnO/Cu2O composites. Heteroatom doped graphene shows promise in CO2 electrocatalysis. However, the number and nature of catalytic active sites are still unclear. Both dopant atoms and their neighboring carbon atoms (with high charge and spin densities) have been shown to facilitate the ECR (by serving as preferable adsorption sites and/or active centers). The role of different heteroatom species (such as graphitic N, pyridinic N, pyrrolic N) also remains controversial. Further, topological defects and edge defects (zigzag and armchair edges) may redistribute graphene’s local electron density and increase affinity to reaction intermediates. Their effects on the ECR deserve future research attention. In this context, the controllable synthesis of heteroatom-doped graphene materials with precise tunability in structure (dimensions, defects, and porosity), composition, functionality, and doping content is critically important. It is especially challenging to manipulate doping with an exclusively specific configuration. Also, for practical application, a scalable, facile, and low cost preparation process is required. Residual metals, even at trace amounts undetectable by XPS and EDX, significantly affect the ECR. Therefore, we recommend that

4.5. Development of precise DFT models, in-situ characterization techniques, and their combination DFT provides a useful tool for simulation on CO2 electrocatalysis systems [70]. Commonly used models include computational hydrogen electrode (CHE) model (under constant-charge approximation), implicit solvation model (under constant chemical potential condition), and some recent methodologies including a combination of DFT, large-scale Monte-Carlo and first-principles-based neural network atomic potentials, and machine learning. However, employed calculations require further empirical correction on energetics to obtain matching thermodynamics results with experiments. Correction of long-range interaction 178

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catalyst activity be evaluated under conditions with definitive proof of the absence of metal impurities in graphene-based catalysts. Although recent efforts have made very good progress for C1 production via ECR, efficient generation of C2+ compounds still remains as a major challenge. Astute chemical formulation and design of electrodeelectrolyte interface are expected to trigger further advances in improving multicarbon product selectivity at high current density. Graphene offers a highly tunable platform for realization of multifunctional interfaces that are capable of both coordinating CO2 reduction processes and optimizing bonding chemical environments for critical reduction intermediates. Surface modification chemistries of graphene by combining metal-ligand cooperativity appear a promising direction. In parallel to the development of new catalysts, the pursuit of mechanical understanding will likely continue to be the focus of future research. Aside from electroanalytical methods, in-situ/operando spectroscopy and microscopy techniques provide further potential for deep studies in the field of ECR. Coupling cathodic CO2 reduction with useful anodic processes to generate valuable products may attract increasing attention.

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Conflict of interest

[20]

The authors declare no conflict of interest. [21]

Acknowledgments [22]

This work was supported by the State Key Laboratory of OrganicInorganic Composites (No. oic-201503005); Fundamental Research Funds for the Central Universities (No. buctrc201525); Beijing Municipal Natural Science Foundation (No. 2192039); Beijing National Laboratory for Molecular Sciences (BNLMS20160133); State Key Laboratory of Separation Membranes and Membrane Processes (Tianjin Polytechnic University), No. M2-201704.

[23] [24]

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Appendix A. Supplementary data

[26]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2019.02.001. [27]

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