# Ion docking drives HER carbon-based electrocatalyst performance

Javier Quílez-Bermejo, Romain Dupuis, Rafael Canevesi, Katerina Ioannidou, Alain Celzard, Roland Pellenq, Vanessa Fierro

► To cite this version:

Javier Quílez-Bermejo, Romain Dupuis, Rafael Canevesi, Katerina Ioannidou, Alain Celzard, et al. Ion docking drives HER carbon-based electrocatalyst performance. Carbon, 2025, 241, pp.120383. 10.1016/j.carbon.2025.120383. hal-05220983

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Submitted on 24 Aug 2025

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# Ion docking drives HER carbon-based electrocatalyst performance

Javier Quílez-Bermejo<sup>a, * ID</sup>, Romain Dupuis<sup>b ID</sup>, Rafael Canevesi<sup>a ID</sup>, Katerina Ioannidou<sup>b ID</sup>, Alain Celzard<sup>a, c ID</sup>, Roland Pellenq<sup>b, d ID</sup>, Vanessa Fierro<sup>a, ** ID</sup>

## ARTICLE INFO

### Keywords:

Electrocatalysis

Hydrogen evolution reaction

carbon electrodes

Electric double layer

ion docking

## ABSTRACT

Despite tremendous progress in understanding electrocatalysis in carbon electrodes, existing approaches overlook the key role of the electric double layer, which is essential to fully understand the complex nature of electrochemical reactions. The remaining challenge is to develop an electrocatalytic model that fully encompasses all phenomena, in order to capture a more accurate representation of the intricate processes taking place. Here we present a comprehensive experimental approach using carbon electrocatalysts, together with advanced reactive atomistic simulations, to explore the fundamental aspects of the hydrogen evolution reaction. Contrary to the state of knowledge, our findings reveal essential ion effects that govern electrocatalysis. We uncovered dynamic mechanisms where cations dock in the microporosity of carbon electrodes under the effect of an applied negative potential, resulting in a strong polarization of the electrode. This leads to the localization of charge density on the outermost surface of the carbon, resulting in boosted electrocatalytic efficiency. This study establishes fundamental dynamic mechanisms regarding crucial catalytic impact of ion docking in carbon-based electrocatalysis.

## 1. Introduction

Understanding the mechanistic phenomena underlying electrochemical water splitting is crucial to providing a solid foundation for electrochemical energy conversion systems like electrolyzers [1,2]. In particular, a comprehensive understanding of the hydrogen evolution reaction (HER) within nanoporous carbon electrodes is essential for designing highly efficient electrocatalysts [3,4]. All electrochemical reactions, including the HER, depend on two factors: the structure of the electrocatalyst, including the active sites, and the conditions of the reaction environment [5-7]. However, the vast majority of the existing literature is limited to simple electrode-reagent models, often neglecting the key role of the electrolyte and, in particular, the electrochemical double layer (EDL). In fact, the EDL is only considered when metal-electrolyte interfaces are involved [8,9], and limited attention is paid to its relevance when using carbon-based systems. Furthermore, the influence of the electrochemical environment plays a substantial role in the HER, since the source of protons is the aqueous electrolyte, and local

pH, counter ions and solubility effects strongly influence the HER performance of carbon electrodes [10,11].

EDL results from the variation of molecular assemblies at the electrode/electrolyte interface in response to the application of an external electric potential [9,12]. Even in the absence of potential, an excess of charge carriers with opposite sign could also be formed at the electrode/electrolyte interface, where charges accumulate. In the case of HER with a negatively applied potential, the local environment is strongly influenced by electrostatic interactions between electrolyte ions and the negatively charged electrode surface [13]. Additionally, a negative potential can induce the formation of an extended EDL that can span over several nanometers within the carbon electrode. Understanding the EDL has emerged as a key element in elucidating the physical basis behind various innovative electrochemical systems in the advancement of energy storage devices [14-16]. Ion docking in micropores and charge localization on non-sp² defective carbon sites play crucial roles in charge storage in supercapacitors [16]. However, the EDL effect in electrochemical reactions is more complex, requiring consideration not only of

<footnote><sup>a</sup> Université de Lorraine, Centre National de la Recherche Scientifique (CNRS), Institut Jean Lamour (IJL), F-88000, Épinal, France</footnote>

<footnote><sup>b</sup> LMGC, Université de Montpellier, CNRS, F-34090, Montpellier, France</footnote>

<footnote><sup>c</sup> Institut Universitaire de France (IUF), F-75231, Paris, France</footnote>

<footnote><sup>d</sup> IEM, Université de Montpellier, CNRS, F-34090, Montpellier, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: javier.quilez-bermejo@univ-lorraine.fr (J. Quílez-Bermejo), vanessa.fierro@univ-lorraine.fr (V. Fierro).</footnote>
the solution phase, but also of the presence of a reagent and subsequent reaction mechanisms. This complexity has led to a limited understanding of the mechanisms involved in electrochemical reactions, with the EDL often overlooked by theorists and experimentalists studying carbon electrodes [17,18].

Inspired by this research gap, we conducted electrocatalytic HER measurements on meticulously characterized carbon materials possessing well-developed microporosity and distinct textural properties. Simultaneously, leveraging recent advancements in simulations, we employed a reactive molecular dynamics (MD) simulation under polarization in a microporous carbon model. Through our extensive research, we have uncovered sub-nanometric mechanisms underlying the fundamental physics of HER in carbon electrodes. Our findings highlight the key involvement of ion docking in micropores, leading to a redistribution of electronic charges in the carbon electrode. Contrary to common belief about HER, we observed that the majority of H₂O splitting events occur predominantly outside the nano-porosity. This observation highlights the critical role of charge dynamics in HER facilitation, particularly in the regions surrounding micropores. By combining experimental and MD research, we unveil the hidden factors that govern electrochemical reactions in carbon electrodes.

## 2. Experimental

### 2.1. Chemicals and reagents

The activated carbon (AC) materials of this work were obtained commercially: MSC30 (Kansai Coke and Chemicals Co., Ltd. Japan), MSC20 (Kansai Coke and Chemicals Co., Ltd. Japan), CW30 (Silcarbon & Aktivkohle), TH90I (Silcarbon & Aktivkohle - Germany), and SA20 (Ingevity - USA). Ultrapure water (18.2 MΩ cm, Milli-Q, Merk) was used for the electrochemical experiments. KOH, NaOH and CsOH were purchased from Sigma-Aldrich.

### 2.2. Physicochemical characterization

Textural properties of carbon materials were obtained by two-dimensional density functional theory (2D-NLDFT-HS) using nitrogen and hydrogen as gas probes. N₂ and H₂ adsorption isotherms were performed at -196 °C, using a fully automated 3Flex manometric adsorption unit (Micromeritics, Atlanta, GA). All adsorption experiments were carried out after outgassing under high vacuum (1.33 × 10⁻⁵ mbar and 110 °C for 12h). 2D-NLDFT-HS calculations were performed using SAIEUS software (Micromeritics, Atlanta, GA). Total pore volume and pore size distribution (PSD) were calculated using the 2D-NLDFT-HS model. By means of mathematical integration of volumes of pore smaller than 20 Å (micropore), 10 Å and 5 Å were calculated. More details are given elsewhere [19].

The bulk chemistry of the materials was studied using a Vario EL cube elemental analyzer (Elementar). Carbon, hydrogen and nitrogen contents were determined by burning 10 mg of the sample at 1700 °C in a mixed atmosphere of helium and oxygen in excess. The gases formed were separated and monitored by chromatographic columns, and then quantified using a thermal conductivity detector. The oxygen content was calculated by difference.

The surface chemistry related to oxygen functionalities was studied using thermogravimetric (TG) equipment coupled to a mass spectrometer, and using temperature-programmed desorption (TPD) measurements [20–22]. The m/z lines of 18 (H₂O), 28 (CO) and 44 (CO₂) were followed to characterize the surface oxygen groups. To this end, the carbon materials were heated in a thermobalance at 900 °C with a heating rate of 10 °C·min⁻¹. Prior to measurements, the thermobalance was purged by three vacuum – argon flow cycles to ensure an inert atmosphere during the experiment. The argon flow rate was adjusted to 50 mL min⁻¹ during the experiments.

Raman spectra were collected with an XploRa Raman spectrometer

(Horiba Scientific), using a 50× long-range objective for all carbon materials. A holographic grating of 1200 lines per mm was used with a circularly polarized laser of wavelength 532 nm, filtered at 10% of its nominal power to avoid heating the samples. Each sample was subjected to 3–5 spectrum acquisitions lasting 120–180 s each, and averaged.

### 2.3. Electrochemical experiments

The HER catalytic activity for all samples was studied using a rotating ring-disk electrode (RRDE, Metrohm), equipped with a carbon disk electrode and a platinum ring. The experiments were performed in a three-electrode electrochemical cell using an Autolab PGSTAT302 potentiostat. A reversible hydrogen electrode (RHE) in a capillary housing was used as reference electrode, while a graphite rod was used as counter electrode (CE).

Prior to the experiments, the working electrode was prepared by drop-casting the materials onto the carbon disk electrode. To this end, 1 mg of the samples was dispersed in 0.25 mL of an aqueous solution of 0.2 wt% Nafion and 20 wt% isopropanol. 4 drops of 8.40 µL were deposited on the carbon disk electrode to achieve an electrode loading of 0.68 mg cm⁻². Then, the material was immersed in the working electrolyte under vacuum to ensure adequate contact with the solution [23]. Linear sweep voltammetry (LSV) was carried out between 0.2 and -1.0 V vs RHE in nitrogen-saturated 0.1 and 1.0 M KOH solutions at a scan rate of 5 mV s⁻¹ and a rotation speed of 1600 rpm. 0.1 M NaOH and CsOH solutions were also used for electrocatalytic measurements. The current density (mA·cm⁻²) recorded at -1 V vs. RHE (E<sub>V</sub>) was used as an indicator of HER activity.

During HER experiments, the platinum ring of the RRDE was fixed at a constant potential of 0.8 V vs RHE, a value at which the platinum ring can oxidize the generated hydrogen molecules to H⁺. The current density recorded at the platinum ring was used to determine whether the current density of the carbon disk was related to the HER or to other electrochemical redox processes. It is worth mentioning that this is a qualitative rather than quantitative analysis, since H₂ bubbles at negative potentials cause significant distortion of the platinum ring's current density.

### 2.4. Molecular dynamics

To simulate adsorption processes, we have produced a symmetrical system consisting of two microporous carbon electrodes separated by a 3 nm gap in an electrolyte solution. The model electrolyte consists of an aqueous electrolyte with the incorporation of 60 cations and 60 anions into a 6000 water molecules system in a simulation cell of 52 × 52 × 100 Å³, which corresponds to a concentration of 0.7 M. In this work, only the cathode is considered in the discussion. Notably, the pore size distribution (PSD) of the model, as depicted in Fig. 2D, exhibits a well-developed microporosity that aligns well with the narrowest micropores of carbon materials. This enables a meaningful comparison between theoretical predictions and experimental data.

To capture the effect of texture and chemistry on the excess charge distribution due to polarization, we used the Echemid algorithm [24] to apply the voltage to the electrodes. To simulate water splitting, we employed the reactive force field known as ReaxFF [25]. More specifically, the Na/K/Cs/O/H force field has recently been parameterized to improve the accuracy of simulation of ionic alkaline solutions, which corresponds to the solution of interest in our study [26]. The carbon structure parameters were obtained from a training set derived from the original paper by A. Van Duin et al. [25], which includes a comprehensive dataset. This dataset incorporates quantum calculations, including reaction paths for H₂ formation and dissociation, and was developed in Rahaman et al. [27].

The voltage applied in molecular dynamic simulations (MD voltage) was adjusted to meet the experimental threshold using the following equation:
$$Experimental threshold = E_{OCV} - \frac{MD voltage}{2}$$

where EOCV is the open-circuit potential and was experimentally determined to be around 0.8 V vs. RHE for all ACs.

# 3. Results

## 3.1. Physicochemical characterization of carbon electrodes

One of the most important aspects of carbon materials in electrocatalysis is the large variety of textures and surface chemistry that can be found in carbon electrodes. We thus selected five commercial ACs (MSC20, MSC30, TH90I, CW30, and SA20) to evaluate their HER electrocatalytic performance. As observed from the N₂ and H₂ adsorption isotherms shown in Fig. 1A and Fig. S1, respectively, the set of materials is composed of MSC20 and TH90I, which are dominated by micropores (pore size <20 Å), SA20 and CW30, mainly by mesopores (20–500 Å), and MSC30 with a combination of micropores and mesopores. Pore size distributions (Fig. S2) and cumulated pore volumes (Fig. 1B) were calculated from the aforementioned isotherms using the 2D-NLDFT-HS model proposed by Jagiello et al. [28,29]. As demonstrated by an excellent linear trend (Fig. 1C), the surface area of all materials comes mainly from the microporosity, i.e., pores with a diameter of less than 20 Å. Moreover, Table S1 presents a comprehensive list of all textural parameters, and Supplementary Note 1 explains the textural properties of all ACs in detail.

The chemical composition of the ACs was also studied by elemental analysis and the results are presented in Fig. 1D and Table S2, indicating the mass contents of C, H, and O. While MSC20 and MSC30 have a high carbon content (above 90 wt%), CW30 and TH90I have C contents

slightly below 80 wt%. These findings show that carbon electrodes differ considerably in terms of textural properties and chemical composition. To investigate further the chemical composition of the materials, temperature-programmed desorption (TPD) analyses coupled with mass spectrometry were carried out, especially to gain insight into the surface oxygen groups. TPD is a well-established technique used for the quantitative assessment of oxygen functionalities in carbon-based materials [20,30]. Fig. 1E illustrates the CO desorption profile and Fig. S3 shows the evolution of oxygen groups from all carbon electrodes. The desorption profiles for H₂O and CO₂ are shown in Fig. S4 and Fig. S5, respectively. Although the surface O content determined by TPD differs slightly from the bulk content, the trend agrees with elemental analysis, with MSC20 and MSC30 exhibiting the fewest O functional groups. Most O groups desorb in the form of CO gas at 700–900 °C, indicating a predominance of phenol- and quinone-type groups in the surface chemistry of all carbon electrodes [30]. A minor contribution of CO₂ and H₂O desorption was detected by TPD for these ACs. Further details on TPD desorption groups can be found in Supplementary Note 2.

The surface chemistry of all AC materials was studied by XPS. Table S3 shows similar C and O atomic contents, with carbon content ranging from 92.3 at.% in CW30 to 97.2 at.% in MSC30, which is consistent with TPD results. Additionally, Fig. S6 shows similar C 1s spectra, with a dominant contribution from the C=C bond at 286.4 eV, along with smaller peaks related to C–O species. Raman spectroscopy was performed to gain insights into the structural order of the ACs. As observed in Fig. S7, all ACs display the typical features of highly disordered porous carbon materials, with the contribution of the D band at 1350 cm⁻¹, which is associated with defects in the carbon structure, and the contribution of the G band at 1590 cm⁻¹, which is associated with the vibration mode of the graphitic layers [31–34]. Their intensity ratios (I<sub>D</sub>/I<sub>G</sub>) determined were 1.01, 1.00, 0.87, 0.77 and 0.78 for

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Physicochemical characterization of carbon-based electrocatalysts. (A) N₂ adsorption (thick lines) – desorption (thin lines) isotherms obtained at –196 °C. (B) Cumulative pore volumes obtained from N₂ and H₂ adsorption isotherms. (C) Linear fit of micropore volume with NLDFT surface area for all carbon materials. (D) Chemical composition of commercial carbon materials, where gray, white and red colors represent C, H and O weight contents, respectively. (E) CO-TPD profiles of commercial carbon materials. (F) I<sub>D</sub>/I<sub>G</sub> indicator obtained from Raman spectra at 532 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Electrocatalytic measurements of carbon materials and MD results. (A) LSV curves of all materials in N₂-saturated 0.1 M KOH solution at a scan rate of 5 mV s⁻¹ and a rotation speed of 1600 rpm. (B) Tafel plots of all materials obtained from LSV curves. (C) Linear fit of E_v in N₂-saturated 0.1 and 1.0 M KOH solutions with the volume of pores smaller than 5 Å. (D) Pore size distribution of the simulated carbon model. (E) Charge distribution projected on the axis perpendicular to the surface of the electrodes, averaged over all C atoms of the cathode at 0 V (black), 4 V (orange, before K⁺ docking) and 4 V (blue, after K⁺ docking). (F) Evolution of the number of H₂ molecules as a function of time at 0, 2 and 4 V. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

MSC20, MSC30, TH90I, CW30 and SA20, respectively. The results show that MSC20 and MSC30 have a higher degree of defects, while a greater structural order is detected for the CW30 and SA20 sample.

## 3.2. Microporous-dependent HER activity on carbon electrodes

Focusing on electrocatalytic performance, we measured the electrocatalytic activity of all ACs for HER in an N₂-saturated 0.1 M KOH solution. Linear sweep voltammetry (LSV) was performed from 0.2 V to -1.0 V vs RHE at a scan rate of 5 mV s⁻¹ and 1600 rpm. Before recording the LSV curves, we also assessed the current densities and capacitance values of the ACs by cyclic voltammetry (Fig. S8). The values obtained for these materials appear to be consistent with the surface area reported previously. Fig. 2A shows LSV curves obtained for all carbon electrodes, and Table S4 summarizes the most relevant electrochemical results. Notably, TH90I exhibits the highest HER catalytic activity among all ACs, followed closely by MSC20. TH90I is the AC having the lowest total pore volume and the highest volume of micropores with diameters below 5 Å. Conversely, MSC30, which is the activated carbon with the most developed textural characteristics, shows significantly lower catalytic activity, while CW30 and SA20 show negligible HER, barely noticeable at the most negative potentials. We can conclude from this that electrochemical performance is not governed by ion diffusion, but rather by ion docking, as we will confirm in the next section. However, it is worth mentioning that the catalytic performance of these ACs is significantly lower than that of state-of-the-art platinum-based electrocatalysts (Fig. S9). In the HER experiments, the platinum ring potential

of the RRDE was kept constant at 0.8 V vs RHE. At this potential, the Pt ring can oxidize the hydrogen molecules formed in the disk electrode, allowing the recorded current to serve as a qualitative indicator of HER activity [35]. Fig. S10 shows the current density of the platinum ring, confirming that the observed current is associated with electrochemical H₂ production, as evidenced by similar but inverse profiles. Tafel slopes were calculated from LSV curves to get further insights into HER mechanisms. As observed in Fig. 2B, all ACs exhibit Tafel slopes higher than 160 mV·dec⁻¹, which indicates that the HER rate-limiting step is the Volmer step (H₂O + e⁻ → Hads + OH⁻) [36,37]. Additionally, Tafel slopes exhibit a linear regression up to potentials close to -1 V vs. RHE for almost all the materials studied, indicating that HER kinetics remain under a reaction-limited control at these conditions.

To deepen the understanding of HER mechanisms in AC-based electrodes, we have assigned the current density (mA·cm⁻²) recorded at -1 V vs. RHE (hereafter referred to as E_v) as an indicator of HER activity. All other electrochemical parameters relevant to HER are also summarized in Table S4. As shown in Fig. 2C, the HER performance of ACs is strongly related to their textural properties. In particular, a noteworthy linear relationship (R² = 0.90) is observed between E_v and the volume associated with pores with diameters less than 5 Å. We also examined possible links with other textural parameters, structural order or chemical composition, but no correlation was found (Fig. S11). It is important to highlight that the different surface chemistry does not appear to play a key role in the HER activity. Further details are given in the Supplementary Note 3. To corroborate this linear trend between E_v and micropore volume, additional catalytic experiments were
performed in an N₂-saturated 1.0 M KOH solution (Fig. 2C), which confirm the close relationship between these parameters in HER ($R^2$ = 0.83). LSV curves and Tafel slopes are shown in Fig. S12 and electro-catalytic parameters are gathered in Table S5. Interestingly, the linear dependence is not perfect and shows some deviations, particularly in the case of TH90I. Despite having a pore volume associated with diameters below 5 Å, similar to other materials, TH90I presents higher current density values. The higher $E_V$ in the 1.0 M KOH solution might be attributed not only to increased ionic conductivity but also to enhanced docking of K⁺ ions into the porosity, as will be demonstrated in the following section.

### 3.3. Ion docking mechanism for HER activity

To establish the fundamental physical basis of the phenomena underlying these observations, we used a model symmetrical system based on a microporous carbon model and we performed MD simulations involving chemical reactivity on flexible carbon electrodes. The PSD assumed in the simulated carbon model electrodes (Fig. 2D) represents accurately the narrowest micropores of carbon materials, with a pore range of 5–10 Å, enabling a meaningful comparison between theoretical predictions and experimental data. The model electrolyte consisted of an aqueous electrolyte with the incorporation of 60 cations and 60 anions into a 6000 water molecules system in a simulation cell of 52 × 52 × 100 Å³, which corresponds to a concentration of 0.7 M. Two identical microporous carbon electrodes were placed 30 Å apart in the electrolyte solution. However, for clarity in the discussion of the HER, only the cathode is considered and discussed. The phenomena occurring at the anode are not addressed in this manuscript. The voltage was adjusted to simulate potentials similar to those of the cathode in experimental analyses. It is worth noting that when the carbon electrode is negatively polarized at 4 V (i.e., -1.2 V vs RHE in the experimental setup), the density of water inside the pores increases slightly (+3 %). However, the carbon electrode, which was initially empty of K⁺ ions, docked 20 K⁺ (33 %) in the microporosity and the remaining 40 ions (67 %) were adsorbed on the outer surface. The ion docking process is also accompanied by changes in the solvation shell of K⁺.

To understand the key role of K⁺ docking in the HER mechanisms, we have plotted the charge distribution perpendicular to the surface of the carbon-based cathode (Fig. 2E), with zero being the midpoint of the electrode. The charge distribution appears mostly uniform in a non-polarized carbon system (blue line), with minor fluctuations related to electrode texture or defects. However, when a polarization of 4 V is applied to a K⁺-free aqueous electrolyte (orange line), the charge becomes heterogeneously distributed over the electrode, with a strong negative charge on the outer surface (indicated by the dashed line). Interestingly, when K⁺ ions are introduced into the electrolyte and dock in the micropores (black line), charge localization on the outer surface increases substantially, while the inner charge remains unchanged (Fig. 2E). As a result of this stronger charge localization, the majority of HER events occur on the outer surface of the carbon electrode, with a smaller proportion of water splitting occurring within the microporosity (Fig. S13). Contrary to the traditional understanding of HER, we have discovered a more subtle mechanism that involves the EDL, in which cations enter the micropores after the application of an external voltage and generate a localized electric field that strongly polarizes the outer carbon surface, leading to the majority of HER events. It is worth reiterating that the high surface polarization is due to the amplification of the applied external polarization through the ion docking process. This enhanced polarization is consistent with the trend deviation in the TH90I sample. Despite a slightly higher volume associated with pore diameters of less than 5 Å compared with other materials, TH90I shows very high activity. Materials with a smaller outer surface area, such as TH90I, may achieve higher HER performance because the charge localization induced by the ion docking effect is more concentrated per unit outer surface area, leading to a stronger local electric field.

In contrast, materials with a larger outer surface area, such as MSC30, experience dispersion of the polarization induced by ion docking, which reduces its efficiency.

We have also studied the effect of the strength of the external electric potential. The effect of applied voltage was simulated by applying 0, 2 and 4 V. These values correspond to the experimental thresholds, which are around 0.8, -0.2 and -1.2 V vs. RHE, respectively. As shown in Fig. 2F, we observe the slow formation of H₂ molecules at 2 V, which increases when 4 V is applied, in agreement with experiments.

### 3.4. Modulation of EDL through cation replacement

To further confirm the EDL impact behind the HER mechanisms, electrochemical measurements and MD models were performed by replacing K⁺ with Na⁺ or Cs⁺ in the electrolyte. LSV curves and Tafel plots in N₂-saturated 0.1 M NaOH and CsOH solutions are displayed in Fig. 3. Fig. 3A shows that the highest catalytic activity is achieved with the sample MSC20 ($E_V$ = 24.1 mA cm⁻²) in the 0.1 M CsOH solution, closely followed by MSC30 ($E_V$ = 21.5 mA cm⁻²). Then, MSC20, CW30, TH90I and SA20 show an $E_V$ of -12.4, -12.4 and -7.9 mA cm⁻², respectively. In general, the LSV curves of ACs show different catalytic activities when they are compared to the results obtained in the KOH solution. Tafel slopes (Fig. 3B) were calculated from LSV curves, showing values between 200 and 400 mV·dec⁻¹, which also indicates lower kinetics and different HER mechanisms in CsOH solutions. At the same time, HER measurements were also performed in a N₂-saturated 0.1 M NaOH solution. Fig. 3C and D display the LSV curves and Tafel plots of all carbon materials. MSC20 shows the best catalytic performance with an $E_V$ of 15 mA cm⁻², closely followed by MSC30 and TH90I, both with an $E_V$ of 11.5 mA cm⁻². Nevertheless, it is worth mentioning that the HER results are far from those obtained in 0.1 M KOH or 0.1 M CsOH solutions, demonstrating significant differences in the effect of ions depending on the working solution. Tafel plots also demonstrate poor kinetics in HER of all materials, showing Tafel slopes in excess of 200 mV·dec⁻¹.

Table S6 and S7 summarize the most important electrocatalytic parameters in N₂-saturated 0.1 M NaOH and CsOH solutions, respectively. As observed in Fig. 4A–C, the electrochemical linear correlation of the KOH solution is different when using NaOH and CsOH electrolytes. The upward trend between $E_V$ and the volume of pores smaller than 5 Å is lost when using a 0.1 M NaOH solution (Fig. 4A). Electrocatalytic performance was found to be independent of carbon electrode microporosity, with similar $E_V$ values in the range of 10–15 mA cm⁻² (Fig. 4B). In the case of Cs⁺, catalytic activity also appears to be strongly related to micropores, but in this case to larger micropores than in the case of K⁺. As observed in Fig. 4C, this is supported by an excellent correlation ($R^2$ = 0.93) between $E_V$ and the volume of pores smaller than 10 Å (instead of 5 Å as in the case of K⁺).

We carried out MD simulations to understand the dynamics of Na⁺, K⁺ and Cs⁺ in the carbon system, considering the size and strength of their hydration shell. As depicted in Fig. 4D, Na⁺ ions tend to remain solvated and close to the outer electrode surface, without docking themselves significantly in the micropores. We calculated the average hydration shell of docked ions alongside the corresponding average pore size of these ions (Fig. 4G). Even after 2 ns, around half of the Na⁺ ions remain solvated in the bulk solution, indicating that their strong hydration shell restricts insertion into the micropores. This limitation of Na⁺ docking by the hydration shell implies that the ion docking effect of the EDL disappears in Na⁺-based electrolytes. As Na⁺ is not docked in the smallest micropores, the redistribution of charges does not reach the outer surface, as confirmed in Fig. S14, resulting in poorer H₂ production. Indeed, H₂ molecules were not produced after the same external electric voltage was applied in the case of Na⁺ (Fig. S15), which corroborates the essential role of ion docking in HER electrocatalysis.

Cs⁺, with a less tightly bound hydration shell (Fig. 4I), adsorbs easily into microporosity (Fig. 4F). This is corroborated in simulations of Cs⁺,
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (A) LSV curves for all ACs in N₂-saturated 0.1 M NaOH solution at 5 mV s⁻¹ and 1600 rpm. (B) Tafel plot obtained from the LSV curves under the same conditions. (C) LSV curves for all carbon materials in N₂-saturated 0.1 M CsOH solution at 5 mV s⁻¹ and 1600 rpm. (D) Tafel plot obtained from the LSV curves under the same conditions. In the Tafel plots, black line represents MSC20, violet MSC30, orange TH90I, blue CW30, and red SA20. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

in which Cs⁺ ions are found in the micropores of the carbon system. As with K⁺ (Fig. 4E), Cs⁺ docking contributes to greater charge localization (Fig. S14) and improved HER catalytic performance at the outermost surface (Fig. S15), which explains our experimental linear correlations. We also plotted the EDL calculated based on the surface ion density (Fig. S16), and Debye lengths for Na⁺ (1.53 Å), K⁺ (2.17 Å) and Cs⁺ ions (4.54 Å) were determined. As observed, Na⁺ and K⁺ shows an EDL that extends around 4 and 6 Å, respectively, while Cs⁺ notably influences the solution up to 10 Å. It is worth noting here that the influence of surface chemistry on the EDL is discarded in this study when comparing AC materials, as the C 1s spectra reveal similar oxidation states for all carbons.

This work demonstrates that the EDL is extremely important in HER, and we corroborate the fact that the EDL in carbon materials must be understood in a much more complex way than what we find in metal-electrolyte interfaces, as the ion docking effect in nanopores generates a greater charge localization on the surface, making the HER much more intense. Not only do these findings confirm our system works within the framework of EDL theories but they also unveil a previously unknown and significant mechanism in the electrocatalysis of carbon electrodes.

## 4. Conclusion

Our results thus introduce a groundbreaking perspective into the understanding of electrocatalysis on carbon-based electrodes. Contrary to common belief, the combination of experimental observations and

theoretical analysis shows the high relevance of the EDL in the HER mechanism, in which cations present in solution play a key role by docking in microporosity under polarization conditions. By docking in the smallest pores, these ions induce a redistribution of the electronic charge on the outermost surface, leading to the production of H₂. While most water splitting events occur at the surface, it is essential to emphasize that ultramicropores are crucial in electrocatalysis for ion docking and charge distribution on the outermost surface. Furthermore, this study reveals that not all cations induce the same electrocatalytic effect, and the reasons behind this phenomenon are related to their hydration shell. Ions with strong hydration shell, such as Na⁺, maintain their associated water molecules, thereby inhibiting their penetration into micropores and, consequently, hindering the ion docking effect. In contrast, larger ions such as K⁺ and Cs⁺ readily lose their water molecules, resulting in better ion docking and, consequently, improved electrocatalytic performance.

### CRediT authorship contribution statement

**Javier Quílez-Bermejo:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization. **Romain Dupuis:** Writing – review & editing, Investigation, Formal analysis, Data curation. **Rafael Canevesi:** Formal analysis, Data curation. **Katerina Ioannidou:** Investigation, Formal analysis, Data curation. **Alain Celzard:** Writing – review & editing, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Evaluation of ion docking effect as a function of cation (Na⁺, K⁺ and Cs⁺). (A) E~V~ in N~2~-saturated 0.1 M NaOH solution for all carbon electrodes. (B) Linear fit of the E~V~ in N~2~-saturated 0.1 M KOH solution with volume of pores smaller than 5 Å. (C) Linear fit of the E~V~ in N~2~-saturated 0.1 M CsOH solution with volume of pores smaller than 10 Å. Snapshots of ion position at the cathode electrode after 2 ns of simulations at 4 V: (D,G) Na⁺ ions in blue, (E,H) K⁺ in yellow, and (F,I) Cs⁺ in green. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

**Roland Pellenq:** Writing – review & editing, Investigation, Funding acquisition, Conceptualization. **Vanessa Fierro:** Writing – review & editing, Visualization, Supervision, Investigation, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

We thank the French PIA project "Lorraine Université d'Excellence", reference ANR-15-IDEX-04-LUE, and the TALiSMAN and TALiSMAN2 projects funded by ERDF.

**Appendix A. Supplementary data**

Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2025.120383.

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