Fig. 1. (a) The time-dependent SEM images of Cu-CO2 and Cu-HER during electrodepositions. (b) The ratio of Cu(100) to Cu(111) facets, as measured by OH- electroadsorption; The error bars represent the standard deviation of three independent measurements. (a,b) Reproduced with permission from Ref. [34]. Copyright 2019, Springer Nature. TEM images of ED-Cu catalysts (c) and GB-Cu catalysts (d). Lines of orange dots indicate different orientations of the lattice. The XRD patterns (e) and XPS (f) of ED-Cu and GB-Cu catalysts as prepared. (g) FEs of both C2+ products for ED-Cu and GB-Cu. (c-g) Reproduced with permission from Ref. [33]. Copyright 2020, American Chemical Society.

Fig. 2. Fitting results of the operando EXAFS spectra to R space (a) and K space (b). The working electrode was biased at a few different potentials between the open circuit voltage (OCV) and 0.78 V vs. RHE during the measurements. (a,b) Reproduced with permission from Ref. [54]. Copyright 2019, Springer Nature. (c) CV scan (at 1 mV/s) of Cu foil in the prepared electrolyte (KOH solution, with 2.3 mol/L lactate ions, pH 13.7). (d) During SW treatment, the copper foil was subjected to alternate potentials of -0.45 and -0.75 V vs. SCE for 30 minutes. (e) Oxidation and reduction current densities during the SW treatment of Cu foil in the prepared electrolyte (3.2 mol/L KOH solution, with 2.3 mol/L lactate ions, 40 °C). (f) XRD pattern of SW-Cu2O/Cu, with a strong signal from Cu2O (111)-facets. (g) The aberration-corrected scanning transmission electron microscopy (STEM) image of a single nanoparticle on the SW-Cu2O/Cu surface. (h) The enlarged image of the region marked in (g). The corresponding fast Fourier transform (FFT) pattern shows the lattice spacing (0.248 nm) relevant to the orientation of Cu2O (111). (c-h) Reproduced with permission from Ref. [55]. Copyright 2020, American Chemical Society.

Fig. 3. (a) Based on the diameter of Ag nanocrystals, the percentage of different active sites. Reproduced with permission from Ref. [63]. Copyright 2020, American Chemical Society. (b) Atomic force microscopy (AFM) images of CuZn NPs with different compositions. From left to right are Cu100, Cu50Zn50, and Zn100 (For the symbol CuxZn100-x, X/100-X means the ratio of Cu to Zn. Scale bar: 200 nm). (c) Geometric current density and (d) products distribution (0.1 mol/L KHCO3, -1.35 V vs. RHE, 1 h electrolysis) as a function of the NP composition. (e) A time-dependent stability test of Cu50Zn50 NPs for the main products of CO2RR at -1.35 V vs. RHE. (c-e) Reproduced with permission from Ref. [67]. Copyright 2019, American Chemical Society.

Fig. 4. (a) TEM image of triangular Cu nanosheets. (b) AFM image of a single Cu nanosheet. (c) HRTEM image showing a Cu nanosheet in its basal plane. (d) The (111) peak is more prominent in the XRD pattern of Cu nanosheets deposited on a Si wafer; Inset: the selected area electron diffraction (SAED) pattern of copper nanosheets. (a-d) Reproduced with permission from Ref. [103]. Copyright 2019, Springer Nature. Typical (e) TEM image and (f) AFM image of Bismuthene. (g) The height profiles of three Bismuthene nanosheets shown in (f). (h) Typical lateral high angle annular dark-field scanning TEM (HAADF-STEM) image of a Bismuthene nanosheet, a zig-zag pattern directly displaying the single atom thickness. (e-g) Reproduced with permission from Ref. [115]. Copyright 2020, Springer Nature.

Fig. 5. Typical SEM image (a) and HRTEM image (b) of the F-Cu catalysts. (c) XPS of the F-Cu catalysts (Fluorine 1s). (d) The CO2RR energy diagram (C2H4 products) on Cu(111) and F-Cu(111), direct *CO dimerization or *CO hydrogenation (formation of *CHO) followed by dimerization. (e) A proposed CO2RR mechanism (C2H4 products); Purple, blue, red, grey, and white colours represent K, F, O, C, and H, respectively. (a-e) Reproduced with permission from Ref. [121]. Copyright 2020, Springer Nature. SEM image (f) and the corresponding HAADF-STEM image (g) of N-C/Cu. (h) Scheme of N-C/Cu/PTFE nanofibre; Green, yellow, and white colours represent N-C, copper, and PTFE, respectively. (i) The density of electrons for N-C/Cu with two adsorbed *COs and a charged water layer. Charge accumulations are indicated by yellow contours, and depressions by blue contours. (j) On Cu, C/Cu, and N-C/Cu, the CO dimerization energy profiles are shown for the initial states (IS), transition states (TS), and final states (FS). (f-j) Reproduced with permission from Ref. [122]. Copyright 2020, Springer Nature.

Fig. 6. (a) Illustration of the heterogenization of molecules on the Cu surface; Due to a local reaction environment with sufficient CO, the ethanol pathway on the Cu surface is preferred over the ethylene pathway. (b) Structure of iron porphyrin metal complexes. (a,b). Reproduced with permission from Ref. [129]. Copyright 2020, Springer Nature. (c) Investigated iron porphyrins. Reproduced with permission from Ref. [139]. Copyright 2012, The American Association for the Advancement of Science. (d) Results of the modifications of secondary ligands. (e) Synthesized Bipyridine-Mn Complexes with different ligand structures (number 1-8). (f) Products distribution of Bipyridine-Mn complexes (number 1-8) after 1 h CO2RR (at -2.17 V vs. Fc+/Fc in CO2-saturated 0.2 mol/L Bu4NBF4/MeCN). (g) Based on IR analysis, a catalytic cycle was proposed (CO2 to HCOOH); The formation of manganese-hydride intermediate (Mn-H) is a key step. (d-f) Reproduced with permission from Ref. [140]. Copyright 2020, American Chemical Society.

Fig. 7. (a) Ni-N4-TPP and Ni-N3O-TPP synthesized by modified Lindsey's methods. (b) 3D structure determined by single-crystal X-ray diffraction of Ni-N3O-TPP. (c) Ni K-edge XANES spectra; Red, blue, and dark lines represent Ni-N4-TPP, Ni-N3O-TPP, and Ni foil, respectively. (d) Polarization curves of Ni-N4-TPP and Ni-N3O-TPP (CO2 saturated 0.5 mol/L KHCO3 electrolyte, scan rate 50 mV s-1); Red and blue represent Ni-N4-TPP and Ni-N3O-TPP, respectively. (e) FECO and FETotal acquired at different potentials (in a two-compartment H-cell for 1 h). CV results of (f) Ni-N3O-TPP and (g) Ni-N4-TPP with different lower potential limit; Voltammograms were obtained by varying the lower potential limit from -0.5 to -0.8 VRHE (CO2 saturated 0.5 mol/L KHCO3 electrolyte, scan rate 50 mV s-1). Reproduced with permission from Ref. [170]. Copyright 2021, American Chemical Society.

Fig. 8. Reaction pathways for CO2RR. C, H, and O elements are represented by the black, grey, and red circles, respectively. The blue layer represent the catalytic surface.

Fig. 9. Various copper coverages on an Ag-Cu model catalyst. SEM images of Ag-Cu surfaces at Cu2+ concentrations of 0.5 ppm (a), 1.5 ppm (b), and 2.5 ppm (c). (d) Comparison of the FEs of Ag-Cu surfaces with different Cu coverages at -1.10 V (vs. RHE). At least three independent measurements are used to calculate the error bars. Operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) studies of CO surface-adsorption on Cu, bare Ag, and Ag-Cu films. (e) Atop-bonded CO and (f) bridge-bonded CO on different surfaces under -0.4?V (vs. RHE). Reproduced with permission from Ref. [196]. Copyright 2019, Springer Nature.

Fig. 10. (a) Activation energies (at U = -0.9 V) of CO2 on MM (metallic matrix, blue), FOM (fully oxidized matrix, red), and MEOM (Cu metal embedded in the oxidized matrix, green) models, as well as the resulting chemisorbed CO2 structures. (b) The free energy profiles for CO dimerization (pH = 7) in MM, FOM, and MEOM models, and for CO hydrogenation to form surface *CHO species in MEOM (grey-green) models; The right image shows the surface OCCO structures, while the left image shows the initial structure from the MEOM simulation. Reproduced with permission from Ref. [199] Copyright 2017, American National Academy of Sciences. (c) A catalytic cycle with TPY-MOL-CoPP and BTB-MOL-CoPP for CO2RR; BTB-MOL and TPY-MOL were MOLs with triangular benzenetribenzoate (BTB) and 4′-(4-benzoate)-(2,2′,2″-terpyridine)-5,5″-dicarboxylate (TPY) sites, respectively. The DFT calculations also provide the pKa values and redox potentials. (d) The calculated HER and CO2RR energy profiles of TPY-MOL-CoPP and BTB-MOL-CoPP. Reproduced with permission from Ref. [130]. Copyright 2019, American Chemical Society.

Fig. 11. Plastron effects: trapping a layer of gas between the solution-solid interfaces with a hydrophobic surface; An illustration of this is shown on a diving bell spider (a) and a hydrophobic dendritic Cu surface (b) for subaquatic breathing and aqueous CO2 reduction; The photo of the diving bell spider is adapted from Seymour and Hetz [218] with permission from the company of biologists. The contact angle measurements of the (c) wettable and (d) hydrophobic dendrite. (e) HRTEM image of 1-octadecanethiol-treated Cu dendrite demonstrate the alkanethiol layer attached to the Cu surface. (a-e) Reproduced with permission from Ref. [216] Copyright 2019, Springer Nature. (f) Raman spectra of the multi-hollow Cu2O catalyst when 60 mA cm-2 of current is applied; At 2053 cm-1, a peak of *CO vibrations can be seen. (g) Cu+/Cu0 ratio calculated from Cu K-edge XAS at -0.61 V (vs. RHE). (f,g) Reproduced with permission from Ref. [217]. Copyright 2020, American Chemical Society.