Abstract: Using phenolic resin-based nanoporous glassy carbon (NPGC) as the electrode, the relationship between electrolyte ions and the double-layer capacitance of the porous carbon electrode was studied by measuring differential capacitance voltammetry curves. The results showed that in dilute solutions, the differential capacitance curve of the porous carbon electrode exhibited a concave point at the point of zero charge (PZC), indicating a decrease in capacitance. The double-layer capacitance was significantly affected by the diffusion layer. Smaller pore sizes resulted in greater intra-ionic diffusion resistance, leading to a more rapid decrease in capacitance and a greater influence of the diffusion layer on the double-layer capacitance. Increasing the pore size of the carbon material or the electrolyte concentration could significantly weaken or even eliminate the influence of the diffusion layer on the capacitance. A high differential capacitance per unit area of ​​the carbon electrode only indicates high pore surface utilization; a larger specific surface area is required to obtain higher capacitance. Ion hydration adversely affects the capacitance of the carbon electrode. Using large ions and increasing the pore size of the carbon material can effectively reduce the impact of ion hydration on the capacitance performance of the carbon electrode. Keywords: Electrolyte, Ion, Carbon electrode, Double-layer capacitance. Double-layer capacitors achieve energy storage by forming a double layer at the solid electrode/solution interface. Therefore, the properties of solid electrodes and solution ions, as well as the interfacial charge and its distribution, have a significant impact on the electric double layer. While the structure and properties of solid carbon electrode materials have been extensively studied, the influence of electrolyte ions, which are closely related to interfacial charge and distribution, on the electric double layer capacitance is less well investigated. Acidic or alkaline solutions are commonly used as electrolytes in aqueous double-layer capacitors. However, due to irreversible side reactions and the potential formation of pseudocapacitors in acidic and alkaline solutions, some researchers have explored the double-layer capacitance characteristics of carbon electrodes using neutral solutions. Salitra et al. studied the relationship between ions and pores in dilute alkali metal chloride solutions using microporous activated carbon cloth as an example. They concluded that the smallest hydrated ion, K, is more conducive to increasing the capacitance of the carbon electrode; and that the smallest pore size that can effectively form the electric double layer is 0.36 nm. Electrolyte ions are best matched with micropores of similar size, resulting in the highest capacitance per unit area for small-pore carbon electrodes. However, since electrolytes have a certain viscosity, especially high-concentration electrolytes, the pore size of carbon electrodes should generally not be less than 2 to 4 times the diameter of the ions in order to reduce the resistance to ion diffusion and migration. Therefore, mesoporous carbon aerogels have been extensively studied due to their excellent capacitance performance. We used our self-developed nanoporous glassy carbon (NPGC) I... as electrodes to study the double-layer capacitance characteristics of NPGC series samples with microporous and mesoporous features in dilute and concentrated KCI solutions. Then, using Li, Na, and K from the same group as cation probes in aqueous solutions, we further explored the factors affecting the capacitance performance of carbon electrodes by hydrated ions, aiming to provide some theoretical guidance for the application research of inorganic electrolytes in double-layer capacitors. 1 Experimental Section 1.1 Sample Preparation The carbon electrode was a self-developed nanoporous glassy carbon material. Briefly, the following steps were taken: Phenolic resin after thermosetting was crushed and pressed into sheets, placed in a tube furnace, and heated to 600℃ under nitrogen protection for carbonization, followed by the introduction of CO. The samples were further heated to 900℃ and activated for 5–35 minutes to obtain carbon electrode samples with different pore structure characteristics (at0–at35, where the numbers represent activation time). 1.2 Test Methods The N2 adsorption-desorption isotherms of the samples were determined using an Autosorb 1 MP gas adsorption instrument from Quantachrome, USA. The specific surface area and mesopore volume were calculated using the multi-point BET method and the BJH method, respectively. Differential specific capacitance (cv) was tested in a three-electrode system using a Solartron 1280Z electrochemical workstation from the UK. The working temperature was 25℃, and the scan rate was 1 mV·s⁻¹. Both the counter electrode and the study electrode were NPGC electrodes, with a saturated calomel electrode (SCE) as the reference electrode. 2 Results and Discussion 2.1 Pore Structure Characteristics Figure 1 shows the N2 adsorption-desorption isotherms of NPGC samples with different activation times. It can be seen that the unactivated sample at0 and samples at5 and at15 with activation times shorter than 25 min exhibit the characteristics of microporous type I in the N adsorption-desorption isotherm. The N adsorption of at0 is close to saturation at lower pressures, indicating that its micropore size is the smallest. With prolonged activation time, not only does the adsorption amount increase, but there is also significant adsorption at higher pressures, indicating an increase in specific surface area and pore volume, and a slight increase in micropore size. For activation times ≥25 min, the N adsorption-desorption isotherms of samples at25 and at35 show hysteresis loops, exhibiting the characteristics of mesoporous type IV. However, at25 has a low adsorption amount, small specific surface area, and small pore volume; at35 has a high adsorption amount, large specific surface area, and large pore volume. To better reflect the comparison of the strength of pore-forming and pore-expanding effects with prolonged activation time, this characteristic can be examined using the ratio r of the total pore volume to the specific surface area of ​​the sample. Table 1 shows the changes in pore structure parameters and r values ​​for each sample with activation time. It can be seen that for NPGC samples activated for less than 25 min, extending the activation time results in a slow increase in mesopore volume with little change in r value, indicating that activation is mainly pore-forming, and the increase in micropores leads to an increase in the specific surface area and pore volume of the sample. When the activation time is extended to 25 min, the r value and mesopore volume of at25 increase rapidly, while the specific surface area decreases accordingly, indicating that activation has shifted to pore expansion. When the activation time is further extended to 35 min, the specific surface area and pore volume of at35 are significantly improved, while the r value is the same as that of sample at25, indicating that the pore-forming effect is further enhanced due to the easier diffusion of the activator gas, resulting in an increase in micropores. The N2 adsorption-desorption isotherm not only exhibits the characteristics of mesopore type IV, but also shows an increase in adsorption capacity, pore volume, and specific surface area. 2.2 Relationship between double-layer capacitance characteristics and pore structure Figure 2(a) and Figure 2(b) show the area and mass differential specific capacitance (cV) curves of NPGC electrodes with different activation times in 0.2 mol·L⁻¹ KCl solution, respectively. It can be seen that the CV curves in the positive/negative polarization regions are symmetrical, indicating that no Faraday reaction occurred within the measured potential range, which is characteristic of double-layer capacitance. However, near -0.2 V (VS SCE), each CV curve shows a concave point similar to that of the planar electrode to varying degrees; this concave point is the zero-charge point (Pzc). In dilute solutions, the double layer has the most diffuse structure, and the capacitance shows the lowest value. The micropore size of sample at0 is the smallest, and the resistance to ion diffusion into the pore is large, which further aggravates the diffusion of the double layer at the PZC point, resulting in the lowest concave point of its CV curve, causing the rectangular CV curve to bend and deform. At both ends of the PZC point, the differential capacitance per unit area of ​​sample at0 increases rapidly, and the pore surface shows a high utilization rate. With the extension of activation time, the pore size of the carbon electrode gradually increases, the resistance to ion diffusion into the pore decreases rapidly, the charge density on the electrode surface increases, the diffusion of the double layer at the PZC point weakens, the capacitance increases, and the concave point of the CV curve gradually becomes blurred. When the activation time is less than 25 min, the specific surface area of ​​the microporous carbon increases with the extension of the activation time, but the capacitance does not increase with the activation time. This leads to a decrease in the utilization rate of the differential capacitance per unit area, i.e., the pore surface area, indicating that more small micropores that cannot effectively form an electric double layer are formed during the activation process. When the activation time is extended to 25 min, sample at25 exhibits a mesoporous structure, the differential capacitance per unit area reaches its maximum, and the utilization rate of the pore surface increases rapidly. When the activation time is further extended to 35 min, due to the influence of the increased micropores, the differential capacitance per unit area decreases compared to sample at25, and the utilization rate of the pore surface decreases. As can be seen from the mass differential capacitance curve in Figure 2(b), although the carbonized sample at0 has a high pore surface utilization rate, it has the lowest specific surface area and the lowest mass specific capacitance. After activation with CO₂, the specific capacitance of sample at5 increased rapidly with increasing specific surface area. With prolonged activation time, the specific capacitance of microporous carbon at15 only increased slightly due to its low pore surface utilization. While mesoporous carbons at25 and at35 had lower specific surface areas than at15, their high pore surface utilization resulted in a continued increase in specific capacitance with prolonged activation time. However, Salitra et al., studying the relationship between pore size and ions in microporous activated carbon, concluded that the sample with the shortest activation time had pore sizes similar to ion sizes, resulting in strong interactions and the highest pore surface specific capacitance, thus exhibiting optimal performance. However, the above studies indicate that the differential capacitance of porous carbon electrodes only reflects the pore surface utilization; the specific capacitance also depends on the size of its specific surface area. In summary, in dilute solutions, a decrease in the pore size of porous carbon electrodes increases the resistance to ion diffusion into the pores, exacerbating diffusion in the double layer at PZC points, leading to a rapid decrease in capacitance and severe distortion of the differential capacitance curve. Furthermore, the differential capacitance (pore surface utilization) of the porous carbon electrode does not completely determine its capacitance. The changes in the differential capacitance curves of each NPGC electrode when the KCl electrolyte concentration was increased to 3 mol·L are shown in Figure 3. It can be seen that the CV curves of the activated samples tend to be horizontal, indicating that the charge density on the electrode surface increases rapidly with the increase of electrolyte concentration, the diffusion of the double layer at PZC points is significantly weakened, and the double layer capacitance is basically unaffected. However, the CV curve of the carbonized sample at0 still shows a concave point at PZC points. This is because its pore size is too small, and the diffusion resistance of ions within the pores is too great. Even in concentrated solutions, the double layer at PZC still exhibits obvious diffusion characteristics. The differential capacitance of each sample is in the same order as that measured in 0.2 mol·L KCl solution. 2.3 Effect of Cation Hydration on Double Layer Capacitance Electrolyte ions with smaller radii migrate more easily into the pores, the structural characteristics of the diffused double layer are not obvious, and the double layer capacitance is improved. However, in aqueous systems, ions mostly exist in hydrated form. Therefore, the size of the hydrated ions determines the effective radius of the ions forming the electric double layer. Ion hydration mainly depends on the size of the ions and the strength of the intermolecular forces with H₂O molecules. The smaller the ion, the stronger the intermolecular forces with H₂O molecules, and the more severe the ion hydration. Figure 4 shows the CV curves of samples at0 and mesoporous carbon at25 in 0.2 mol·L LiCl, NaCl, and KCl electrolytes. It can be seen that the pore size of sample at0 is close to the ion size, making it more sensitive to ion size, and the "molecular sieve" effect is obvious. In the negative polarization region where cations undergo adsorption and desorption, the CV curves show a large difference. Since the size order of hydrated ions is Li > Na > K, the capacitance of the negative polarization region is the highest measured in KCl solution, and is close to the capacitance of the positive polarization region. The CV curve is the most symmetrical, indicating that Cl⁻ and H₂O have the same hydration mechanism and degree of hydration due to their similar ion size and intermolecular forces with H₂O molecules. The CV curves of the positive polarization region exhibiting Cl- adsorption and desorption are basically identical in NaCl and KCl solutions. In LiCl solution, the charge and discharge capacities are reduced, possibly due to the small ionic radius of Li-, resulting in very strong hydration. The adsorption of hydrated Li- ions on the carbon electrode surface by hydrated anions Cl- also has a certain impact, reducing the charge and discharge capacity of the positive polarization region. The large pore size of the mesoporous carbon at25 greatly reduces the resistance to the entry and exit of hydrated ions, narrowing the difference in charge and discharge capacities in the negative polarization region. The influence of ion hydration on the double-layer capacitance is significantly weakened, but the capacitance measured in KCl solution is still the highest. Compared with sample atO, the diffusion of the double layer at the PZC point is weakened in dilute solution, and the concave point of the differential capacitance curve is no longer obvious. Figure 3 shows the differential capacitance curves per unit area (a) and mass (b) of NPGC electrodes at different activation times. Figure 4 shows the differential capacitance curves of samples at0 and at25 at O. CV curves in LiCl, NaCl, and KCl solutions at 2 tool·L 3 Conclusions (1) Microporous carbon electrodes with small pore sizes have high resistance to ion diffusion, which further aggravates diffusion in the double layer at the zero charge point (Pzc), resulting in the lowest capacitance value. Increasing the pore size of the carbon electrode or the electrolyte concentration can significantly reduce the diffusion in the double layer. (2) A high differential specific capacitance per unit area of ​​the carbon electrode only indicates high pore surface utilization. A porous carbon electrode with both high pore surface utilization and large specific surface area can obtain a high capacitance. (3) Ion hydration has an adverse effect on the double layer capacitance of the carbon electrode. Larger ionic radii and lower charge density result in less hydration. In addition, larger pore sizes reduce the resistance to the entry and exit of hydrated ions, thus reducing the effect of ion hydration on capacitance.