# Probing the role of isoelectric point in charge storage mechanisms of functionalized carbon by electrochemical quartz crystal microbalance (EQCM)

Sreya Babu Nambiar, Barbara Daffos, Julie Segalini, Patrice Simon, Pierre-Louis Taberna

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Sreya Babu Nambiar, Barbara Daffos, Julie Segalini, Patrice Simon, Pierre-Louis Taberna. Probing the role of isoelectric point in charge storage mechanisms of functionalized carbon by electrochemical quartz crystal microbalance (EQCM). Electrochemistry Communications, 2025, 180, pp.108054. 10.1016/j.elecom.2025.108054. hal-05377295

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# Probing the role of isoelectric point in charge storage mechanisms of functionalized carbon by electrochemical quartz crystal microbalance (EQCM)

Sreya Babu Nambiar<sup>a,b</sup>, Barbara Daffos<sup>a,b</sup>, Julie Segalini<sup>a,b</sup>, Patrice Simon<sup>a,b,*</sup>, Pierre-Louis Taberna<sup>a,b,*</sup>

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

### Keywords:

Isoelectric point

EQCM (electrochemical quartz crystal microbalance)

Charge storage

Oxygen-containing functional groups

Carbon

Aqueous electrolytes

## ABSTRACT

In this work, we aim to explore the synergetic influence of the functionalization of the carbon electrode and that of the electrolyte pH on the charge storage mechanism by utilizing the technique of EQCM (Electrochemical Quartz Crystal Microbalance). A comparative analysis has been carried out on YP50 and o-YP50 (carbon with oxygen functionalities), which revealed crucial differences in the evolution of ionic contribution to charge storage under varying pH. A progression from anionic to cationic mechanism was observed with an increase in pH, depending on the isoelectric point of the carbon under study. A combination of TPD-MS (Temperature Programmed Desorption Mass Spectrometry) and EQCM was used to probe pH-dependent surface transformations occurring on the functionalized and non-functionalized carbon electrode during electrochemical cycling. The electrolyte pH governs the protonation state of the surface functional groups, which influences the charge on the electrode and hence the mechanism of charge storage.

## 1. Introduction

Charge storage mechanism in electrochemical double-layer capacitors is achieved by electrostatic physical separation of charges at the electrode-electrolyte interface, resulting in the formation of an electrical double layer [1,2]. Over time, several models have been proposed to provide a better understanding of the double layer. The Helmholtz model (1879) proposes the balance of electrode surface charges by a redistribution of ions in solution, forming two layers of opposite charges separated by some distance at the molecular level. Gouy-Chapman model, on the other hand, takes into consideration the thermal distribution of ions, suggesting the involvement of a diffuse layer. Stern model (1924) attempts to develop a combination of the previous models to arrive at a double layer consisting of a compact and diffuse layer [3,4]. The Grahame model (1947) incorporates the concept of specific ion adsorption in which the inner Helmholtz plane consists of specifically adsorbed ions that are partially or completely de-solvated, enabling them to approach the electrode more closely. The interactions in this case are not just limited to electrostatic forces but can also involve

chemical interactions with the electrode surface, like in the case of functionalized carbon [5]. Surface functionalization, doping of hetero atoms (N, O, S) [6], chemical or electrochemical treatment to increase surface area, modification of structural design (mesoporous carbon spheres, carbon nanofiber arrays) [7] are some of the popular strategies for the enhancement of energy density of carbon-based supercapacitors.

The introduction of Oxygen-containing Functional Groups (OFGs) such as carboxyl (-COOH), hydroxyl (-OH), and carbonyl (-C=O) imparts additional capacitance to the electric-double-layer capacitance through reversible redox reactions and also enhances the wettability of the carbon surface [8–10]. The presence of these functionalities on the carbon surface can give rise to a positive or negative charge depending on the nature of these functionalities and the pH of the aqueous medium [11]. Isoelectric point (IEP) is the pH value of an electrolyte when the net surface charge of the sample is zero under given conditions of temperature and pressure (also referred to as pH<sub>PZC</sub> in some articles) [12]. The evolution of ionic contribution to charge storage is heavily dependent on the electrolyte pH and hence could be affected by a shift in the isoelectric point of the carbon [13]. The isoelectric point is expected

<footnote><sup>a</sup> Université de Toulouse, CIRIMAT UMR CNRS 5085, 118 Route de Narbonne, 31062 Toulouse, France</footnote>

<footnote><sup>b</sup> Réseau sur le Stockage Electrochimique de l'Energie (RS2E), FR CNRS 3459, France</footnote>

<footnote>* Corresponding authors at: Université de Toulouse, CIRIMAT UMR CNRS 5085, 118 Route de Narbonne, 31062 Toulouse, France.</footnote>

<footnote>E-mail addresses: sreya.babu-nambiar@utoulouse.fr (S.B. Nambiar), patrice.simon@utoulouse.fr (P. Simon), pierre-louis.taberna@utoulouse.fr (P.-L. Taberna).</footnote>
to play a critical role in determining the charge storage properties of carbon materials, particularly in electrochemical systems. The IEP is dictated by the nature and abundance of surface functional groups, as these groups directly influence how the material interacts with its surroundings under different pH conditions [14]. For activated carbon like YP50, it was observed that the capacitance decreases with an increase in pH [15]. Earlier studies [16] have attributed this dependence on proton concentration to redox reactions involving the surface functionalities, ruling out the possibility of electrolyte ion adsorption. Though these groups have been well explored, relatively less attention has been paid to understanding the underlying charge storage mechanism. [17]

To gain a deeper understanding of the impact of these groups on the performance of supercapacitors, it is necessary to use characterization techniques that can detect electrochemical and mass changes at the electrode-electrolyte interface, in real-time. EQCM (Electrochemical Quartz Crystal Microbalance) is one such technique with nanogram sensitivity that enables us to monitor ion-adsorption and desorption under varying potentials [18,19]. When applied to functionalized carbon electrodes, EQCM provides critical insights into the nature of ion-surface interactions.

Previous studies from our group [20,21] have demonstrated the relevance of EQCM in elucidating ion dynamics and charge storage mechanisms in porous carbon electrodes. Herein, a comparative analysis of the oxidized and non-oxidized carbon is provided with emphasis on the electrolyte pH. This study, to the best of our knowledge, is the first attempt to decipher the charge storage mechanism of carbon with OFGs in aqueous electrolytes at different pH by using EQCM. Moreover, we highlight the utility of EQCM frequency responses to give insight into the isoelectric pH (IEP), enabling us to predict the mechanism of charge storage at a given electrolyte pH relative to the isoelectric point of the material under study. This study not only provides us with an understanding of the charge storage mechanism but also lays the foundation for future modifications in carbon materials and optimizing the electrolyte pH to improve supercapacitor performance.

## 2. Experimental

### 2.1. Synthesis of o-YP50

YP50 purchased from Kurray was oxidized by following the protocol mentioned by Li et al. [22] Briefly, 1 g of carbon was added to 20 mL of 0.7 mol/L ammonium persulfate solution in 1 mol/L sulfuric acid and stirred for 3 h at 60 °C. The oxidized carbon samples, post-filtering and washing, were dried at 80 °C for 24 h.

### 2.2. Material characterization

#### 2.2.1. Gas sorption measurements and pore structure analysis

The powders were characterized by gas sorption using Argon gas with an ASAP 2020 Micrometrics equipment. Before measurement, the samples were degassed at 150 °C for 10 h under vacuum to eliminate traces of water and other impurities that could obstruct access to the pores. The Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface area (SSA). Pore size distribution was determined from the adsorption branch of the isotherms using the 2D non-local density functional theory (2D-NLDFT) model for heterogeneous carbon surfaces, implemented via the SAIEUS software (Micromeritics).

#### 2.2.2. Scanning Electron Microscopy (SEM)

The surface morphology of YP50 and oxidized YP50 (o-YP50) coated quartz crystals was examined using Scanning Electron Microscopy (SEM). The measurements were performed using a MEB TESCAN VEGA 3 system. Before imaging, the carbon powders were coated onto Au/Ti-coated quartz crystals and allowed to dry.

#### 2.2.3. Wettability assessment by contact angle measurements

The wettability of the carbon samples (YP50 and o-YP50) was evaluated using static contact angle measurements. A KRÜSS contact angle goniometer was employed to deposit a 2 µL droplet of deionized water on the surface of the carbon films coated on Au/Ti-coated quartz crystals to assess the hydrophilicity of the coatings.

### 2.3. Temperature Programmed Desorption Mass Spectrometry (TPD-MS)

TPD-MS analysis was performed to identify the nature of surface functional groups through the detection of evolved gases during thermal decomposition. The measurements were conducted using a Netzsch STA 449 Jupiter thermal analyzer coupled with a QMS 403 Aeolos quadrupole mass spectrometer. The carbon samples were heated to a temperature of 900 °C at a constant heating rate of 5 °C/min under a continuous flow of Argon. The desorbed gases, including H₂O, CO, and CO₂, were monitored as a function of temperature to infer the types of oxygen-containing surface functionalities present on the samples.

### 2.4. Electrochemical measurements using Swagelok-type cell

Cyclic voltammetry (CV) experiments were conducted using free-standing electrodes composed of YP50 and oxidized YP50 (o-YP50), prepared with a carbon to PTFE binder ratio of 8:2. Electrochemical testing was performed in a three-electrode Swagelok-type cell configuration, employing a Hg/Hg₂SO₄ reference electrode. The electrodes were tested in aqueous electrolytes: 0.5 M H₂SO₄ (pH ≈ 1) and 0.5 M K₂SO₄ (pH ≈ 7.6). The K₂SO₄ solution was adjusted to pH values of 3.0 and 9.4 by the controlled addition of H₂SO₄ and KOH to study the effect of pH on electrochemical behavior.

### 2.5. EQCM (Electrochemical Quartz Crystal Microbalance)

The pristine YP50 and the oxidized YP50 (o-YP50) were used to prepare a slurry in NMP (N-Methyl-2-pyrrolidone) with a carbon to PVDF (polyvinylidene fluoride) binder ratio of 8:2. The slurry was then drop-coated on Au-coated quartz crystals (1.28 cm²) with an oscillating frequency of 5 MHz. The coated quartz crystals (with a controlled loading between 40 and 60 µg/cm²) were then dried at 85 °C for 2–3 h. The coated quartz crystal was used as the working electrode in a 3-electrode cell with a Pt counter electrode and Hg/HgSO₄ reference electrode.

Potassium sulfate purchased from Alfa Aesar was used to prepare a 0.5 M K₂SO₄ electrolyte, which was then adjusted to different pH values with the addition of appropriate amounts of H₂SO₄ and KOH solutions.

## 3. Results and discussion

Pore size distribution (PSD) analysis reveals an average pore diameter of about 1.2 nm for both carbons YP50 and oxidized YP50, termed as o-YP50 (Fig. S1a). However, the total pore volume of o-YP50 was comparatively less than that of YP50, as shown in Fig. S1b. According to those results, the oxidation step may have led to the partial oxidation of the outer carbon grains' surface, which in turn makes a part of the pore volume not accessible without a significant change in the pore size distribution. This suggests that the OFGs might obstruct access to the porous network, particularly since the smallest pores are reached through larger ones. This effect on the amount of pores but not on pore size distribution is in alignment with the results reported by Calvo et al. [23]. The SEM images of YP50 and o-YP50 (Fig. S2) samples coated on the Au/Ti quartz did not reveal any significant morphological differences. Interestingly, the average contact angle for YP50 was found to be around 76°, and that of o-YP50, 32°. (Fig. S3). The lower contact angle is consistent with a higher hydrophilicity, which is expected after carbon oxidation [24,25]. This is primarily due to the introduction of hydrophilic groups on the carbon surface that may favor electrolyte access to
the carbon's porosity.

Fig. 1 presents the TPD-MS analysis of the samples. To begin with, the oxidized carbon display a higher percentage of weight loss than the pristine carbon (Fig. 1a and b), which aligns with the presence of oxygenated functional groups introduced during the oxidation process. Additionally, the decomposition peaks observed at various temperatures suggest the presence of different types of functional groups on the carbon surface.

The pristine YP50 contains fewer functional groups (indicated by the absence of sharp peaks), most of them being thermally stable lactones that decompose at higher temperatures [26,27] along with a minimal quantity of carboxylic acids (150–350 °C range) as seen from CO₂ evolution in Fig. 1c. Higher relative intensity of CO and H₂O signals at lower temperatures is an indication of the presence of physisorbed carbon monoxide (also we cannot rule out N₂ with m/z = 28) and water [28].

On the other hand, after oxidation, the carbon predominantly contains carboxylic acid functional groups as sharp peaks of CO₂ in the temperature range of 150–350 °C show up. It is worth mentioning that two neighboring carboxylic acid groups can undergo a dehydration reaction, releasing H₂O in this temperature range (possibly phenolic groups with acidic behavior [29]), and the simultaneous desorption of CO could be from the decomposition of the anhydride that was formed after dehydration [30,31]. The broad peak of CO₂ with less intensity in the high-temperature region mainly corresponds to thermally stable groups like cyclic esters, anhydrides, and lactones, which are formed after the dehydration condensation of carboxylic acid groups at lower temperatures (Fig. 1d). This could also imply the presence of multiple OFGs introduced in minimal quantities with overlapping peaks of evolution. The treatment of the carbon in a harsh oxidative environment with sulfate radicals could have resulted in the removal or transformation of stable groups into species like carboxylic acids, as evidenced by the shift in CO₂ evolution to lower temperatures in TPD-MS analysis for o-YP50 [32].

According to TPD-MS analysis, oxidized carbon samples exhibit distinct gas evolution peaks, indicating a higher content and variety of

oxygenated functional groups compared to pristine carbon. Notably, sharp CO₂ signals between 150 and 350 °C suggest the presence and decomposition of carboxylic acid groups, while broader high-temperature peaks correspond to more thermally stable species like lactones or quinones. It is in line with what was reported in previous works [33,34].

Since the carbon surface oxidation led to the presence of acidic groups, it is expected that the zeta potential will be different from that of pristine YP50F, and will depend as well on the pH of the electrolyte the carbon is in contact. A modification of the zeta potential should affect the electrochemical double layer and, in turn, the ionic fluxes at the interface under polarization, such as we previously reported with carbide-derived carbons used as model materials [35]. As a first step, cyclic voltammetry measurements were carried out using free-standing electrodes of YP50 and o-YP50 in different electrolytes having different pH.

In Fig. 2a presented are the cyclic voltammograms obtained for the YP50 electrode in electrolytes at different pH values. As observed, except for the solution at pH = 1, a nearly ideal rectangular-shaped cyclic voltammogram, characteristic of electric double-layer behavior commonly observed in carbon-based supercapacitors, is obtained. However, at a low pH of 1 (in H₂SO₄), a deviation from this rectangular shape is observed as indicated by the presence of bumps in the cyclic voltammogram, in agreement with previous results, which have demonstrated that redox phenomena are more favored when the electrode surface is positively charged, which is the case in acidic electrolytes [23]. The calculated specific capacitance in this case was around 67 F/g, whereas for the other three values of pH, it was around 50 F/g. A similar trend of higher capacitance for YP50 in acidic aqueous LiTFSI electrolyte (pH = 0.5) was attributed to an increase in in-pore ion concentrations from the NMR experiments carried out by Lyu et al. [15]

Regarding the electrochemical behavior of the oxidized carbon samples, despite a lower total porous volume, they exhibit higher current densities across all electrolyte pH values compared to the pristine carbon. At pH = 1, both pristine and oxidized carbons display a similar

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 1. TG curves of a) YP50 and b) o-YP50 as a function of temperature. Ionic current variation (for m/z = 18(H₂O), 28(CO), 44(CO₂)) as a function of temperature for c) YP50 and d) o-YP50.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 2. Swagelok cells with YP50 (Fig. 2a) and o-YP50 (Fig. 2b) free-standing electrodes in 0.5 M H₂SO₄ and 0.5 M K₂SO₄ (pH ~ 3, 7.6, 9.4).</figcaption></figure>

electrochemical signature; however, at other pH levels, the oxidized samples show distinctly different curve shapes and a marked increase in current in the cathodic region (Fig. 2b). The calculated capacitance value for o-YP50 was around 115 F/g at pH = 1, which decreased to 55 F/g at pH = 9.4. When comparing the performance with pristine activated carbon, oxidized samples showed an increase in capacitance by about 73% at acidic pH, which decreases to around 13% at a pH of 9.4. This difference in behavior could be attributed to the electrochemical activity of oxygen-containing functional groups, as proposed by Andreas et al. [16], and/or to variations in surface charge with electrolyte pH. The effect may be more pronounced in oxidized carbons due to their higher concentration of oxygenated groups.

To gain insight into the charge storage mechanism, EQCM measurements were conducted. Fig. 3 presents cyclic voltammograms along with the corresponding frequency shifts for both carbon samples at different pH values. It is important to note that a negative or positive slope in the frequency-potential relationship indicates a net mass uptake negatively (anions) or positively (cations) charged counter-ions, respectively, during the charging process. According to the average

slope variation, the EQCM responses of both YP50 and o-YP50 indicate a transition from anion-based mechanism at low pH – weight increase during anodic scan - to a process where cations play an increased contribution with the increase in pH (Fig. 3). However, this progression can be of varying degrees depending on the isoelectric point (IEP) of the material under study. Indeed, the IEP shifts to a lower pH value with the increase in oxygen-containing acidic surface functional groups [36], as expected to occur when moving from YP50 to o-YP50. This has also been shown from TPD-MS data and contact angle measurement that the surface exhibits more oxygen-based groups, leading to a more hydrophilic surface. A detailed diagram with pH ~ 9.4 is provided in Fig. S4.

According to the results presented in Fig. 3, the IEP of both carbons can be assessed: for o-YP50, it is expected to be at a pH value of 3, while it should be >7 for YP50. This assumption is based on the frequency responses observed in Fig. 3. The presence of both anionic and cationic contributions to charge storage suggests a net zero charge on the electrode surface at that pH, thus indicating the proximity of the electrolyte pH to the isoelectric point of the carbon. Nevertheless, it was not possible to confirm via a zeta measurement on the suspension, as the

<figure><img src="image_8.png" /><figcaption>b) o-YP50 pH~1</figcaption></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>pH ~ 3</figcaption></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 3. CV along with frequency responses of EQCM cells assembled with YP50 (Fig. 3a) and o-YP50 (Fig. 3b) in 0.5 M H₂SO₄(pH ~ 1) and 0.5 M K₂SO₄ (pH ~ 3, 7.6).</figcaption></figure>
particle size was not stable for a proper measurement to be performed.

To go further in the analysis, Fig. 4 presents the electrode mass variation translated from frequency variation using the Sauerbrey equation, which was made possible as the motional resistance variations were ensured to be negligible. (Fig. S5). The mass variations are plotted vs. charge variation of the electrode -the zero was set at the minimum of mass for comparison.

In Fig. 4a, which presents the mass variation of pristine YP50 versus charge, the positive slopes observed for positive charge are consistent with an overall anion uptake as the charge-compensating species. Since the slope values are close to those expected for sulfate ions, this suggests that sulfate dominates the charge compensation mechanism for positive polarization across all pH values. Interestingly, in our highly acidic electrolyte (pH = 1), a deviation in slope from the theoretical line at high positive charge could be due to a possible deformation of the carbon at higher charge densities, leading to enhanced adsorption as reported by S.Sigalov et al. [37] The isoelectric pH of YP50 was experimentally determined to be around 9 (Fig. S6 a). This suggests that the carbon surface has a basic IEP and would have almost no net charge as the pH approaches this value, and hence, we observe the cationic contribution in addition to the anion at higher pH values. The slope observed in this case is less than the expected slope for potassium but is closer to the theoretical slope for $H_3O^+$. But it could be predicted that at more basic values of pH, potassium ions could be involved in charge compensation rather than $H_3O^+$ due to their reduced concentration in alkaline media. Note that the highly basic pH values could not be studied by EQCM due to the instability of the Au/Ti quartz while cycling in alkaline electrolytes.

Analyzing the trend with slopes observed in the case of YP50 carbon in acidic pH were the difference between theoretical and experimental slopes for anions is minimized with the increase in acidity. It can be predicted that the charge storage would be dominated by cations at highly alkaline pH, as is observed from the progressive transition from positive to negative slopes with the increase in pH. The TPDMS data confirms the presence of lactone groups on the carbon surface, though in small quantities. At very basic pH values, these groups could undergo base-catalyzed hydrolysis, leading to ring-opening and formation of carboxylate anions [38]. This negatively charged surface could exhibit a cation-based mechanism in an extremely alkaline medium.

o-YP50, on the other hand, shows significant cationic contribution at pH 7.6 and 9.4 as indicated by the negative slopes in the region of negative charge in Fig. 4b when compared to YP50. The slope in this region seems to approach the theoretical slope for potassium ions with an increase in pH, indicating that potassium ions predominantly govern

the charge compensation at basic pH values. This uptake of potassium ions with an increase in pH could be because the carboxylic acid groups introduced on the carbon surface are present in their deprotonated form at these pH values, and hence impart a negative charge to the surface. This degree of deprotonation and the resulting negative charge decrease with the decrease in pH, eventually reaching the IEP, where both cationic and anionic contributions could be observed. This is seen close to the pH value of 3 in Fig. 4b, which is close to the experimentally determined isoelectric point of the carbon with OFGs (Fig. S6 b). At pH values of 1 and 3, the charge compensation mechanism seems to be dominated by $H_3O^+$ due to their higher concentration in acidic values of pH. The observed slope in this case and at pH 1 is less than the theoretical slope value due to perm-selective failure of the carbon electrode, resulting in an ion-mixing region consisting of cationic and anionic fluxes directed oppositely, which decreases the overall measured slope due to the adsorbed ions.

Compared to the traditional m-Q plots, plotting the instantaneous mass change rate (dm/dt) versus the potential (E) can give more insights into overlapping processes such as simultaneous adsorption and desorption. A positive dm/dt value reflects net ion uptake, while a negative dm/dt indicates ion release from the electrode surface. [39,40]

Both YP50 (Fig. 5a) and o-YP50(Fig. 5d) in 0.5 M $H_2SO_4$ show sulphate ion adsorption in the positive scan and its desorption during the negative scan as indicated by the positive and negative mass change rates. A similar anion adsorption-desorption mechanism could be observed in YP50 at pH = 3 (Fig. 5b). pH = 7.6 is less than the IEP of YP50 (Fig. 5c), and hence we should expect an almost positively charged surface. Note that the electrode surface would be less positively charged than the pH values of 3 and 1 as we are approaching the basic IEP of YP50. Hence, we begin to observe cationic contribution to charge storage, which is illustrated by the appearance of a crossover point at this pH. On the contrary, the pH = 7.6 > IEP of o-YP50 (Fig. 5f), which means a negatively charged electrode surface in this medium. As mentioned before in the case of YP50, the negative charge in this case would be less than in highly alkaline electrolytes, and hence there could be a minimal contribution of anion in addition to cation. The butterfly-shaped plot with a cross-over point at about -0.3 V vs. $Hg/Hg_2SO_4$ in Fig. 5e indicates the proximity of the electrolyte pH to the isoelectric point of carbon. Both anion and cation uptake and desorption from the electrode surface could be observed, hinting at a net zero surface charge.

The crossover point observed in Figs. 5b,5c, 5e, and 5f may correspond to the potential of zero charge (PZC) when crossing the IEP, which shifts toward more anodic values as the pH of the electrolyte increases. The absence of this crossover point at highly acidic pH suggests a highly

<figure><img src="image_14.png" /><figcaption>**Fig. 4.** Mass change (Δm) versus charge (ΔQ) plots from EQCM measurements on YP50(Fig. 4a) and o-YP50(Fig. 4b) at four different pH values. The dashed lines represent the theoretical mass response for each of the ions predicted by Faraday's law.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 5. dm/dt versus E plots at three different values of electrolyte pH (1,3, and 7.6) of YP50(5a,5b,5c) and o-YP50(5d,5e,5f).</figcaption></figure>

cathodic PZC at this pH, which could be observed on extrapolation of the curves to more negative values of potential. This could be justified by the fact that this pH value being less than the IEP of both the carbon materials, they are bound to have a positively charged surface and hence a negative PZC. Hence, with the instantaneous rate of change of mass, we were able to get more insights into the charge storage mechanism, which was not possible with the cumulative mass-charge plots.

## 4. Conclusion

In this study, we tried to understand the influence of oxygen-containing functional groups on the charge storage behavior of carbon-based supercapacitors through a combination of Temperature-Programmed Desorption Mass Spectrometry (TPD-MS) and Electrochemical Quartz Crystal Microbalance (EQCM) analysis. The ionic current evolution of H₂O, CO, and CO₂ enabled the identification and differentiation of surface functionalities. The electrochemical performance in aqueous electrolytes of varying pH revealed a strong dependence of capacitance on both surface chemistry and proton availability. Moreover, the reduction in the IEP of carbon after the oxidation treatment led to significant differences in its underlying charge storage mechanism. EQCM analysis provided deeper insights about the type of ion dominating the charge storage at a given electrolyte pH, which was further elucidated with the understanding of the behavior of the surface functional groups. These findings demonstrate the critical role of surface chemistry and electrolyte conditions in tuning the electrochemical behavior of carbon electrodes, offering valuable insights for the design of high-performance supercapacitor materials.

### CRediT authorship contribution statement

**Sreya Babu Nambiar:** Writing – original draft, Investigation, Formal analysis, Data curation.
**Barbara Daffos:** Validation, Formal analysis, Data curation.
**Julie Segalini:** Validation, Formal analysis.
**Patrice Simon:** Writing – review & editing, Validation, Supervision, Formal analysis.
**Pierre-Louis Taberna:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Formal analysis.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Sreya Babu Nambiar reports financial support was provided by Inter-university Center of Materials Research and Engineering. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgement

The authors gratefully acknowledge the financial support received from ANR-22-CE05-0011.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.elecom.2025.108054.

### Data availability

Data will be made available on request.

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