Progress and Perspective of Electrocatalytic CO2 Reduction for Renewable Carbonaceous Fuels and Chemicals

Abstract

The worldwide unrestrained emission of carbon dioxide (CO₂) has caused serious environmental pollution and climate change issues. For the sustainable development of human civilization, it is very desirable to convert CO₂ to renewable fuels through clean and economical chemical processes. Recently, electrocatalytic CO₂ conversion is regarded as a prospective pathway for the recycling of carbon resource and the generation of sustainable fuels. In this review, recent research advances in electrocatalytic CO₂ reduction are summarized from both experimental and theoretical aspects. The referred electrocatalysts are divided into different classes, including metal-organic complexes, metals, metal alloys, inorganic metal compounds and carbon-based metal-free nanomaterials. Moreover, the selective formation processes of different reductive products, such as formic acid/formate (HCOOH/HCOO^-), monoxide carbon (CO), formaldehyde (HCHO), methane (CH₄), ethylene (C₂H₄), methanol (CH₃OH), ethanol (CH₃CH₂OH), etc. are introduced in detail, respectively. Owing to the limited energy efficiency, unmanageable selectivity, low stability, and indeterminate mechanisms of electrocatalytic CO₂ reduction, there are still many tough challenges need to be addressed. In view of this, the current research trends to overcome these obstacles in CO₂ electroreduction field are summarized. We expect that this review will provide new insights into the further technique development and practical applications of CO₂ electroreduction.

Keywords: carbon cycle; catalytic mechanisms; electrocatalysts; electrochemical CO2 reduction; renewable fuels.

Scheme 1

Illustration of the electrochemical CO₂ reduction process and the possible products generated in an electrochemical reaction cell.

Figure 1

Metal–macrocyclic complexes as electrocatalysts for CO₂ reduction. a) Investigated iron porphyrins. Reproduced with permission.39 Copyright 2016, American Association for the Advancement of Science. b) Schematic mechanism of the electrochemical CO₂ reduction using Co protoporphyrin. Reproduced with permission.40 Copyright 2015, Macmillan Publishers Limited.

Figure 2

a) Redox mechanism of [Mn(mesbpy)(CO)₃]⁻ and Mg²⁺ at −1.5 V versus Fc^+/0 for electroreduction of CO₂ to CO. Reproduced with permission.52 Copyright 2016, American Chemical Society. b) Proposed mechanism for electroreduction of CO₂ to HCOO⁻ using iridium pincer dihydride electrocatalyst. Reproduced with permission.54

Figure 3

Schematic mechanism of different metal electrocatalysts for CO₂ reduction reaction in aqueous solution.

Figure 4

a) Potential‐dependent Faradaic efficiencies of different Au NPs (4, 6, 8, 10 nm) during electrocatalytic reduction of CO₂ to CO. b) Current densities (mass activity) for electrocatalytic reduction of CO₂ to CO on the Au NPs with different sizes at various applied potentials. Free energy diagrams for electrochemical reduction of c) CO₂ to CO and d) protons to hydrogen on Au (111), Au (211), and a 13‐atom Au cluster at −0.11 V (vs RHE), respectively. Reproduced with permission.79 Copyright 2013, American Chemical Society. Free energy diagrams for electrochemical reduction of e) CO₂ to CO and f) H⁺ to H₂ on Au(111), Au(211), Au₅₅ NPs, and Au₃₈ NPs at 0 V versus RHE. Reproduced with permission.80 Copyright 2014, American Chemical Society. g) Morphological model of concave rhombic dodecahedron Au NPs with different exposed facets. h) Faradaic efficiencies of different Au NPs and Au film for CO production at applied potential (vs RHE). Reproduced with permission.81 Copyright 2015, American Chemical Society.

Figure 5

a) DFT calculation results on the binding energies of *COOH intermediates as a function of the size of Ag NPs. Reproduced with permission.86 Copyright 2015, American Chemical Society. b) Schematic diagram of nanoporous Ag (scale bar, 500 nm). c) The partial current density of CO production under different overpotentials on polycrystalline silver and nanoporous Ag, respectively. Reproduced with permission.87 Copyright 2014, Macmillan Publishers Limited. d) Free energy diagrams for the electroreduction of CO₂ to CO on flat (Ag(100) and Ag(111)) and edge (Ag(221) and Ag(110)) sites. Reproduced with permission.88 Copyright 2015, American Chemical Society.

Figure 6

a) Applied potential dependence of Faradaic efficiencies for CO production over Pd NPs with different sizes. b) Adsorption of *COOH (top) and DFT results on the free energy for CO₂ reduction to CO (bottom) on Pd(111), Pd(211), Pd55, and Pd38. Reproduced with permission.94 Copyright 2015, American Chemical Society. c) SEM image of hierarchical hexagonal Zn. d) Free‐energy diagrams of CO₂ reduction (left) and HER (right) on Zn (002) and Zn (101). Reproduced with permission.99 (e) Faradaic efficiencies of CO production under different applied potentials on 36 nm freshly reduced Bi/C. f) Faradaic efficiencies and mass activities of CO production on electrodeposited Bi films (Bi‐ED), 36 or 7 nm freshly reduced Bi/C by hydrazine (36 nm Bi/C or 7 nm Bi/C). Reproduced with permission.100 Copyright 2016, American Chemical Society.

Figure 7

Comparison of current densities and Faradaic efficiencies of n‐Cu/C and copper foil. a) Total current density of n‐Cu/C and copper foil. b) Faradaic efficiencies for CH₄ generation. c) Methanation current densities. d) Faradaic efficiencies for H₂ generation, showing suppressed H₂ evolution on n‐Cu/C catalyst. e) Proposed mechanism for the electrochemical reduction of CO₂ to CH₄, including the rate‐limiting step (RLS), consistent with the electrochemical data and known intermediates identified in the literature. Reproduced with permission.110 Copyright 2014, American Chemical Society.

Figure 8

a) Hydrocarbon selectivity of plasma‐treated Cu foils. Reproduced with permission.114 Copyright 2016, the Author, published under CC‐BY 4.0 license. b) The DFT calculated free energy change of CO₂ and CO protonation without glycine (blue lines) and with glycine (red lines). Reproduced with permission.116 Copyright 2016, The Royal Society of Chemistry.

Figure 9

a) Schematic illustration of the species involved in the reaction pathways to generate C₂H₄ (blue) and C₂H₅OH (green). Reproduced with permission.118 b) Bar graph reporting the Faradaic efficiencies for each product produced by Cu foil and Cu nanocubes with different sizes at −1.1 V versus RHE. The glassy carbon signal has been subtracted. Reproduced with permission.119

Figure 10

a) Relative turnover rates (TORs) for CO generation and (b–d) proposed mechanism for CO₂ reduction on the Au–Cu bimetallic NPs. Reproduced with permission.121 Copyright 2014, Macmillan Publishers Limited. Free energy diagrams for e) H₂ evolution and f) CO₂ electroreduction to CH₄ or CH₃OH on W/Au and Cu electrodes. Reproduced with permission.125 Copyright 2014, American Chemical Society.

Figure 11

a) Lateral high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) image of partially oxidized Co 4‐atom‐thick layers and (b) the corresponding intensity profile along the pink rectangle in (a). c,d) Corresponding crystal structures. e) Electrochemical active surface area (ECSA) corrected Tafel plots for HCOO⁻ production. f) CO₂ adsorption isotherms of partially oxidized Co 4‐atom‐thick layers (red), Co 4‐atom‐thick layers (blue), partially oxidized bulk Co (violet) and bulk Co (black). Reproduced with permission.131 Copyright 2016, Macmillan Publishers Limited.

Figure 12

a) Binding configurations of *COOH, *CO, and *CHO on the Mo edge of MoS₂. *COOH and *CHO preferably bind to the bridging S atoms, while *CO binds to the Mo atoms. Reproduced with permission.135 b) Cyclic voltammograms (CVs) of rGO–PEI–MoS_x modified glassy carbon electrode in N₂‐saturated and CO₂‐saturated 0.5 m aqueous NaHCO₃ solution, respectively. Inset: Structure of PEI. c) Faradaic efficiency for CO (red bars) and H₂ (blue bars) production at different applied potentials. Reproduced with permission.136 Copyright 2016, The Royal Society of Chemistry. Free energy diagrams of CO₂ conversion to CH₄ over d) Cu (211) and e) Mo₂C (100) surfaces at 0 V (vs RHE), respectively. The most endergonic step in the overall process is designated with an arrow. Reproduced with permission.141 Copyright 2016, American Chemical Society.

Figure 13

a) TEM image of bamboo‐shaped NCNTs. b) Schematic of CO formation on NCNTs and free‐energy diagram at equilibrium potential for CO₂ reduction on pyridinic‐N, pyrrolic‐N, and graphitic‐N defects compared to original CNTs. Reproduced with permission.144 c) The corresponding N functionality content and d) Faradaic efficiency of CO production versus applied potential on N‐doped graphene with different doping temperatures (700–1000 °C). e) Free energy diagrams of electrocatalytic CO₂ conversion on N‐doped graphene and f) schematic of nitrogen defects and CO₂ reduction mechanism. Reproduced with permission.150 Copyright 2015, American Chemical Society.