Fig. 1. (a) Volcano plot for the HER on metal electrodes in acidic media^[20]. Reprinted with permission, Copyright 2010, American Chemical Society. (b) Activity trends for OER as a function of for rutile and anatase oxides. The activity is expressed by the value of overpotential to achieve a certain value of current density^[17]. Reprinted with permission, Copyright 2011, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 2. (Color online) (a) Microstructure and interfaces of the molybdenum sulfide@NPG. (b) HER polarization curves of NPG, MoS₂@GCE, and MoS₂@NPG^[50]. Reprinted with permission, Copyright 2014, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Cross-sectional SEM image of the NiCeO _x–Au film. (d) Activity difference between NiCeO _x supported on either Au-coated or bare GC disk, as well as OER performance of NiCeO_x–Au compared to those benchmarking catalysts reported. (e) Representation of the theoretical overpotential as a function of the difference in O^* and HO^* adsorption Gibbs energies. (f) DFT+U calculations illustrates the support effects on modifying performance of NiCeO_x–Au^[52]. Reprinted with permission, Copyright 2016, Springer Nature. (g) Comparison of activity and (h) impedance spectra for Ni-based electrodes with various mass loading and composition. (i) In situ XAS of NiOOH and NiCeOOH on GC and Au substrates^[53]. Reprinted with permission, Copyright 2017, American Chemical Society. (j) Adsorption sites and adsorption energies of OH on FeNi LDH and FeNi LDH with hydroxide interfacial layer, respectively. (k) Mass activity and (l) stability test of the FeNi LDH on foils at 1.5 V vs RHE for 10 h^[49]. Reprinted with permission, Copyright 2018, American Chemical Society.

Fig. 3. (Color online) (a) SEM image of an α-Ni(OH)₂ hollow sphere and a digital photo of the nanosheet-assembled α-Ni(OH)₂ hollow spheres deposited on glassy carbon. (b) Comparison of CVs recorded at 100th cycle for bare GC electrode and modified GC electrodes comprising the α- and β-Ni(OH)₂ nanocrystals, RuO₂, and 20 wt% Pt/C and the corresponding Tafel slope^[74]. Reprinted with permission, Copyright 2014, American Chemical Society. (c) SEM and TEM images for Cu@NiFe LDH for morphology characterization^[60]. Reprinted with permission, Copyright 2017, Royal Society of Chemistry. (d) Polarization curves of CoS₂ film and CoS₂ nanowire/microwire array towards the morphology-dependent enhancement of both performance and stability. Bottom sketch describe the effect of morphology changing of CoS₂ towards the bubble evolution^[68]. Reprinted with permission, Copyright 2014, American Chemical Society. (e) Illustration of 3D-nanomesh nickel electrode and the long-term stability tests of Ni foam, the 3D-nanomesh Ni electrode and the 3D-nanomesh NiFe electrode^[75]. Reprinted with permission, Copyright 2018, Elsevier.

Fig. 4. (Color online) (a) SEM image of the large area SP superlattices and inserted photograph of the SP superlattices with dimensions 0.7 × 0.7 cm². (b) High-magnification SEM image of the long-rang-ordered Pt nanocrystals and TEM image of the nanocrystal superlattice. (c) Polarization curves of Pt SP₅, Pt SP₁₀, Pt SP₂₀, Pt SP₅₀, Pt NP film, Pt/C film and Pt foil electrocatalysts, the current density was normalized by geometry area. (d) Stability testing on Pt SP₅ for 11 h. (e) Stability testing on Pt NP films. (f) Schematic illustration of the growth of gas bubbles on a flat film electrode. (g) Schematic illustration of the stability difference between flat film and SP electrode. (h–j) Snapshots of digital videos taken during electrolysis at 10 mA/cm², magnified observations and schematic illustration of single bubble behavior on (h) Pt SP₅, (i) Pt SP₂₀ and (j) Pt NP film. (k) i–t curves with a stable and straight curve in the case of Pt SP₅ and serrated curves on the Pt SP₂₀ and Pt NP films^[37]. Reprinted with permission, Copyright 2019, American Chemical Society.

Fig. 5. (Color online) Possible mechanistic pathways of CO₂ reduction to C₁ and C₂ products^[12]. Reprinted with permission, Copyright 2019, American Chemical Society.

Fig. 6. (Color online) (a) Microstructural analysis and bulk compositions of the catalysts. (b) Calculated reaction energy profiles for CO₂ electroreduction to form CO (top) and HCOOH (bottom) on the PdSnO₂ surface^[113]. Reprinted with permission, Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Surface valence band photoemission spectra of Au–Cu bimetallic nanoparticles. (d) Schematic showing the proposed mechanism for CO ₂ reduction on the catalyst surface of Au–Cu bimetallic nanoparticles^[120]. Reprinted with permission, Copyright 2014, Springer Nature. (e) The sketch of atomic ordering transformation of AuCu nanoparticles and the corresponding structural investigation. (f) Computational results of CO₂ reduction on AuCu surfaces. (g) The illustration of CuPd nanoalloys with different structures and the corresponding comparison of FE^[125]. Reprinted with permission, Copyright 2017, American Chemical Society. (h) The SEM image of Ni_xGa_y alloying and the terminal products^[95]. Reprinted with permission, Copyright 2016, American Chemical Society.

Fig. 7. (Color online) (a) A schematic illustration of the Ag₃Sn core-shell structure. (b) CO₂ reduction Faradaic efficiencies of AgSn/SnO_x. (c) The most favorable free energy pathways of CO and HCOOH formation on SnO with Ov and –(O)H*, respectively^[112]. Reprinted with permission, Copyright 2017, American Chemical Society. (d) Schematic and TEM characterization of the three types of Ag/Cu nanocrystals. (e) The schematic illustration and activity comparison of Ag NPs, Cu NPs, and AgCu alloy with various composition ratio^[137]. Reprinted with permission, Copyright 2019, American Chemical Society. (f) Interaction between CeO_x/Au(111) and CO₂ (g) DFT calculations of CO₂RR at 0 V vs RHE on Au(111) and Ce₃O₇H₇/Au(111) surfaces^[134]. Reprinted with permission, Copyright 2017, American Chemical Society. (h) Schematic illustration of the synthesis process and the corresponding morphology and composition characterization of the Ag@Al-PMOF. (i) FEs and total current densities for Ag NCs and Ag@Al-PMOF hybrids with different MOF thicknesses^[138]. Reprinted with permission, Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 8. (Color online) (a) Schematic illustration of the structure of Cu nanofoams and the terminal products. (b) Product distribution as a function of applied potential during the electrochemical reduction of CO₂^[145]. Reprinted with permission, Copyright 2014, American Chemical Society. (c) SEM images of the Cu cube surface and cyclic voltammograms of Cu cube and polycrystalline cupper surface towards formation of H₂, methane and ethylene^[146]. Reprinted with permission, Copyright 2015, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Schematic showing the Cu overlayer structure-dependency over the formation of hydrocarbons. (e) Ratios of RFSs for CH ₄/C₂H₄ on different thicknesses of the Cu layer deposited on a pure Pt substrate obtained from the CO₂ reduction experiments^[147]. Reprinted with permission, Copyright 2013, American Chemical Society.

Fig. 9. (Color online) (a) This is illustrated on a diving bell spider for subaquatic breathing. (b) A hydrophobic dendritic Cu surface for aqueous CO₂ reduction. (c) The contact angle measurements of the wettable and hydrophobic dendrite. (d) SEM image of the hydrophobic dendrite. (e) Illustration of the hydrophobic dendrite gaining a solid–liquid interface on the application of a negative potential. (f) CPE product FEs from the wettable and hydrophobic dendrite at various potentials. (g) The proposed role of hydrophobicity in promoting CO₂ reduction over proton reduction^[140]. Reprinted with permission, Copyright 2019, Springer Nature. (h) SEM images of EDTA modified porous hollow cupper sphere. (i) FE of all the reduction products for H-Cu MPs at various potentials. (j) Free energy profiles for the CO dimerization reaction on surfaces with or without EDTA modification^[149]. Reprinted with permission, Copyright 2020, American Chemical Society.