Fig 1

Schematic diagram of four classic double layer models, wherein the orange area is the electrode side with positive surface charges, and the blue area is the electrolyte side. (a) The Helmholtz-Perrin model, where the electrode with positive surface charges and the ions with negative charges on the solution side (green solid circles) form an electrical double layer, and the potential φ decreases linearly with the distance x from the surface; (b) the Gouy-Chapman model, where the negatively charged ions on the solution side form a diffusion layer, and the potential φ decreases exponentially with the distance x from the surface; (c) the GCS model modified by Stern, the ions on the solution side have a certain distance d (d ≠ 0) from the electrode, that is, the ions can only reach the (outer) Helmholtz layer at the nearest point, and the potential φ first decreases linearly with the increase of the distance x from the surface, and then decreases exponentially; (d) the GCSG model, modified by Grahame, incorporates specific adsorption of positive and negative ions and organic molecules on the electrode surface (represented by dark gray solid ellipse, green solid ellipse and yellow solid ellipse respectively), namely IHP. The Helmholtz layer in (c) is OHP in (d), and the potential φ varies with the distance x from the surface in a relatively complex manner. The curves presented here are intended to be schematic only and should not be taken as an accurate representation of the actual data."

Table 1

List of Symbols."

Fig 2

Theoretical model of electrical double layer, modified on the solution side and electrode side. Adapted based on the literature 31."

Table 2

Summary of different electrocatalytic EDL characterization techniques and their technical properties."

Fig 3

(a) Schematic illustration of the principle of the external reflection mode for in situ infrared spectroscopy, where CE is the counter electrode, WE is the working electrode and RE is the reference electrode (the same notation applies to subsequent figures) [75]. (b) Schematic illustration of the principle of internal reflection mode [75]. (c) Schematic representation of the apparatus configuration for in situ infrared spectroscopy [80]. (d) In situ IRRAS spectrum of ethanol electrooxidation catalyzed by Pt/TaC under alkaline conditions [76]. (e) In situ ATR-SEIRAS spectrum of the CO2 reduction reaction process using CO2-saturated KHCO3 as the electrolyte and a Cu thin film as the working electrode, with reference to the spectrum at 0.3 V vs. RHE [77]. (f) Evolution of current density and integrated intensities of characteristic infrared peaks as a function of electrode potential during in situ testing [77]. (g) In situ IRRAS spectra of the alcohol oxidation reaction in the 0.5 mol·L−1 NaOH + 0.5 mol·L−1 ethylene glycol system, with Pd CNCs as the catalyst for panel A and Bi-Pd CNCs for panel B, referenced to the spectrum at 0.15 V vs. RHE in the same system [81]. (h) In situ ATR-SEIRAS spectra of the alcohol oxidation reaction in the 0.5 mol·L−1 NaOH + 0.5 mol·L−1 ethylene glycol system, referenced to the spectrum at 1.02 V vs. RHE in the 0.5 mol·L−1 NaOH system [81]. The evolution of the current density and integrated intensity of the CO peak with electrode potential during the test is also shown, with panels A–C using Pd CNCs as the catalyst and panels D–F using Bi-Pd CNCs."

Fig 4

(a) Schematic diagram of the in situ shell-isolated nanoparticle-enhanced Raman spectroscopy setup; (b) Numerical simulation results of the electromagnetic enhancement field in Au@SiO2 core-shell structures for in situ surface-enhanced Raman spectroscopy; (c) In situ surface-enhanced Raman spectroscopy results of Pd(111)|0.1 mol·L−1 NaClO4 interface, all adapted from reference [88]; (d)–(g) In situ Raman spectra of probe molecules on cyanobenzoic acid (10 mmol·L−1) in four different electrolytes [82]."

Fig 5

(a) Schematic diagram illustrating the fundamental principle of SFG-VS, redrawn based on reference [91]; (b) Schematic diagram depicting the basic principles of SHG; (c) Schematic diagram representing the device for SFG-VS [93]; (d) Schematic diagram showcasing the setup for SHG installation [95]; (e) Experimental and theoretical calculations of IR spectra (A and D) of PDI-W(CO)5 in solution, along with perpendicular SFG-VS spectra of PDI-W(CO)5 relative to the Au electrode surface at −0.4 V vs. Ag/Ag+ reference electrode (B and C denote experimental values, while E and F represent theoretical simulated values) [94]; (f) In situ SFG spectrum at the interface of Au|PDI-W(CO)5 | electrolyte denoted by A, and the relationship between the peak frequency ω(φ) and electrode potential φ, where B represents Au―C≡N, C corresponds to W―C≡W, and D signifies W―CO [94]. (g) For Au|PDI-W(CO)5 | electrolyte interface, (A) the relationship between interface electric field and electrode potential, (B) schematic diagram of the structure, and (C) electric field intensity at different positions in the double layer under varying electrode potentials [94]. (i) The CV curves and SHG measurements at the Pt | aqueous solution interface under different pH conditions, using electrolytes composed of specific ratios of HClO4 and NaOH [95]."

Fig 6

Schematic diagram of a typical experimental apparatus for electrochemical scanning tunneling microscopy, adapted from reference [101]."

Fig 7

(a) Schematic representation of a conventional experimental setup for scanning electrochemical probe microscopy, adapted from reference 104; (b) Schematic representation of the substrate generation-tip collection mode in scanning electrochemical microscopy."

Fig 8

(a) ORR current-potential curves and current-pKa relationships (corresponding to C and D) after 7 ionic liquids with different pKa were modified on the Au/C (A) and Pt/C (B) working electrode surfaces, respectively [110]; (b) In situ ATR-SEIRAS using [MTBD][NTf2] modified Au/C as the working electrode, where the electrolyte is 0.1 mol·L−1 HClO4 saturated with oxygen: C=N stretching region (A), O-O-H bending vibration region (B) and X-H (X = N, O) stretching vibration region, all with open-circuit potential spectra as reference [110]."

Fig 9

(a) Double layer structure under the hydrogen evolution potential on the surface of Pt(111) electrode in acid system (A) and alkaline system (B) simulated by AIMD, where the gray, red, white, green, purple and yellow spheres represent Pt, O, H, H3O+, Na+ and Had respectively, while the dashed blue lines represent hydrogen bonds. And the relationship between the concentration of O atom in water and the distance from the metal surface (C) and the number of hydrogen bonds perpendicular to the surface and the distance from the metal surface (D) [24]; (b) The ATR-SEIRAS spectra of alkaline system (A) and acidic system (B) and their respective O-H characteristic peaks (E, F) under 0.20 – −0.05 V vs. RHE test conditions were compared with the simulation results (C, D, G, H) [24]; (c) ATR-SEIRAS spectra without (A) and with (B) 1,2-dimethylimidazole added to the electrolyte [78]."

Fig 10

(a) Schematic diagram illustrating the conventional cationic effect and our newly discovered cationic effect [79]; (b) Experimental setup depicting the capture of Li+ and K+ by crown ether (A), changes in Faradaic efficiency of products before and after crown ether capture (B), in situ IRRAS spectrum recorded during electrolysis of 2 mol·L−1 LiOH + 0.1 mol·L−1 glycerol at 1.50 V vs. RHE for 30 minutes (C), In situ IRRAS spectra obtained during a 30-minute electrolysis of 2 mol·L−1 KOH + 0.1 mol·L−1 glycerol at 1.50 V vs. RHE (D). Changes in adsorption free energy (E) and adsorption structures (F) of glycerol, glyceraldehyde, and glycolaldehyde on NiOOH materials under different ion conditions [79]."

Fig 11

(a) Schematic diagram of cationic control interface electric field [118]; (b) The adsorption free energy change of Co2RR-related intermediates on the Cu(111) plane - electric field intensity curve [118]; (c) The energy-electrode potential relationship required for the hydrated cation in the bulk phase to enter the OHP of Cu(111) [118]."

Fig 12

Five future directions for electrocatalytic reactions regulated by electrode side surface modification, adapted from the literature[120]. (a) Regulation of electrocatalytic reactions by altering interactions within the second coordination layer, such as hydrogen bonding and Lewis acid-base equivalence; (b) Control of reaction selectivity by surface modification to prevent the formation of by-products; (c) Modification of the surface hydrophilicity and pH to affect the electrocatalytic reaction; (d) Modification of different additives on the catalyst surface to promote a single reaction process, and coupling can achieve a series reaction; (e) Stabilization of the catalyst surface, such as maintaining a specific crystal surface with high catalytic activity."