# From local wood charcoal to high-performance supercapacitors: The role of pore accessibility

Pauline Blyweert, Jimena Castro-Gutiérrez, Solène Gentil, Vanessa Fierro, Alain Celzard

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Pauline Blyweert, Jimena Castro-Gutiérrez, Solène Gentil, Vanessa Fierro, Alain Celzard. From local wood charcoal to high-performance supercapacitors: The role of pore accessibility. Renewable Energy, In press, 258, pp.124992. 10.1016/j.renene.2025.124992. hal-05414713

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# From local wood charcoal to high-performance supercapacitors: The role of pore accessibility

Pauline Blyweert<sup>a</sup>, Jimena Castro-Gutiérrez<sup>a</sup>, Solène Gentil<sup>b</sup>, Vanessa Fierro<sup>a</sup>, Alain Celzard<sup>a,c,*</sup>

## ARTICLE INFO

### Keywords:

Electrochemistry

Supercapacitors

Activated carbon

Wood charcoal

## ABSTRACT

Locally produced wood charcoal was explored as a sustainable and competitive precursor for activated carbon (ACs) applied to advanced energy storage applications. Three different activation strategies were applied: steam activation, KOH activation after physical mixing (*KOH-p*), and KOH activation after impregnation (*KOH-imp*). All materials demonstrated good electrochemical performance at low charging rates, achieving specific cell capacitance above 40 F g⁻¹ at 0.5 A g⁻¹ in acidic aqueous electrolyte with the best-performing material demonstrating a remarkable cell capacitance value of 64 F g⁻¹ at 0.5 A g⁻¹ (*i.e.*, a specific electrode capacitance of 257 F g⁻¹) while retaining 85 % and 73 % of its performance at 5 and 40 A g⁻¹, respectively. Moreover, despite comparable BET areas (~1500 m² g⁻¹) and microporous volumes (~0.5 cm³ g⁻¹), ACs produced by *KOH-p* exhibited restricted micropore accessibility, leading to lower performance at high charging rates. In contrast, ACs with increased pore volume and wider pore size from the *AC-KOH-p* series retained 73 % of their capacitance, while the *KOH-imp* series stabilized at 55 %. These findings highlight the critical role of hierarchical porosity and effective micropore accessibility in enhancing charge transport and overall electrochemical performance, reinforcing the value of local biomass sources in the development of sustainable energy storage materials.

## 1. Introduction

With the gradual phasing out of combustion engines and the rapid expansion of telecommunications, the demand for electrochemical energy storage devices is rising exponentially. These devices collect excess energy during production and release it as needed. While batteries provide medium- and short-term storage solutions for remote distribution, electric vehicles, small-scale generation and off-grid systems, supercapacitors (SCs) are high-power devices that rapidly store and deliver energy [1]. SCs are particularly useful for applications requiring bursts of electricity and long-term stability, such as electrical grids, power tools, and renewable energy systems [2].

Over the years, various materials, including conducting polymers, metal oxides [3], and carbon-based materials [4], have been explored to enhance supercapacitor efficiency. Among these, activated carbon (AC) is the most extensively studied [5]. Currently, commercial supercapacitors primarily use ACs derived from agricultural sources such as coconut shells [6] or from synthetic [7] and mineral resources [8]. While

effective and economical, these sources often rely on non-local and environmentally unsustainable supply chains, particularly in Europe.

Many studies have explored the potential of various biomasses [9] such as lignin [10], agricultural products [11] and by-products, including fibrous matrices [12], molasses [13], leaves [14] or flowers [15], fruits [16], fruit stones [17] and seeds [18], husks [19], hard [20] and soft [21] wood, bamboo [22], wood bark [23] and hulls [24] for producing highly porous ACs. Developing innovative materials from carbonized, industrially produced, and sustainably sourced precursors such as vegetable coal, could create new market opportunities and help decarbonize the supercapacitor industry.

It is generally reported in the literature that, for supercapacitor applications in aqueous electrolytes, key properties of porous carbon materials include a high surface area obtained through activation, [25], and a controlled amount and nature of functional groups that enhance capacitance through reversible redox reactions [26]. Proper pore size design and analysis are thus crucial to establishing a strong correlation between electrode material texture and electrolyte ion transport. Among

<footnote><sup>a</sup> Université de Lorraine, CNRS, IJL, F-88000, Epinal, France</footnote>

<footnote><sup>b</sup> Groupe Bordet, Froidvent, F-21290, Leuglay, France</footnote>

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

<footnote>* Corresponding author. Université de Lorraine, CNRS, IJL, F-88000, Epinal, France.</footnote>

<footnote>E-mail address: alain.celzard@univ-lorraine.fr (A. Celzard).</footnote>
activation methods, KOH activation is widely used to produce micro-
porous ACs with high surface areas. Extensive research has focused on
optimizing AC synthesis and textural properties by varying precursors,
activation temperatures [27], activating agents (CO₂ for physical acti-
vation [23] and potassium salt for chemical activation [28]),
precursor-to-activator ratios [27] and activation sequence (i.e., one- or
two-step activation) [29].

This study aimed to prepare ACs with diverse textural and chemical
properties using readily available wood charcoal as a precursor. The
goal was to investigate how different activation routes influence the
physicochemical properties of ACs, and the electrochemical perfor-
mance of derived supercapacitors. The ACs were obtained from
hardwood-derived charcoal via physical (steam) and chemical (KOH in
powder or aqueous solution) activation methods. Their electrochemical
performance was assessed over a broad range of conditions using
potentiostatic, galvanostatic and electrochemical impedance spectros-
copy (EIS) techniques in an acidic aqueous electrolyte. The results were
then correlated with physicochemical properties, including specific
surface area, pore size and chemical composition, which are key pa-
rameters influencing the performance of supercapacitors. This study
thus highlights the role of finely tuned pore structure and surface
functionality beyond surface area alone, offering new perspectives for
the design of high-performance carbon design.

## 2. Experimental

### 2.1. Materials

Wood charcoal, produced by Groupe Bordet (Leuglay, France), was
used as the precursor for AC electrode fabrication for SCs. This charcoal,
with a fixed carbon content exceeding 80 %, is derived from the slow
pyrolysis of hardwood sawmill by-products sourced within a 150 km
radius of the plant.

The benchmark ACs known as YP-50F and YP-80F, currently used in
commercial supercapacitors, were purchased from Kuraray (Japan).
KOH pellets, 1M H₂SO₄, HCl (37 %), polytetrafluoroethylene (PTFE, 60
wt% suspension in water) and carbon black were purchased from Sigma
Aldrich and used as received without further purification.

### 2.2. Activation

For this study, charcoal activation was performed *via* three routes:

(i) Physical activation under a simultaneous flow of nitrogen (300 mL min⁻¹) and steam at 900 °C (5 °C·min⁻¹ heating rate) for 120 min. Steam was generated in situ by continuous evaporation of deionized water introduced into the furnace at a rate of 6 mL min⁻¹. The resulting material was ground using an automatic mortar grinder (RM 200, Retsch).

(ii) Chemical activation after impregnation with a 6M KOH aqueous solution, followed by drying at 70 °C under vacuum until a slurry was obtained.

(iii) Chemical activation after physical mixing with KOH powder.

For routes (ii) and (iii), the charcoal was first ground ( < 30 µm) with an automatic mortar grinder, mixed with KOH at various KOH-to-carbon weigh ratios, and treated at 800 °C (3 °C·min⁻¹ heating rate, under 150 mL min⁻¹ of N₂) for 1h. The resulting materials were washed with 1M HCl and deionized water until neutral pH was obtained, then dried at 105 °C.

In the following, ACs are labeled as AC-x-z, where “x” represents the
activation method (i.e., S, *KOH-imp* and *KOH-p* for routes (i), (ii) and
(iii), respectively) and “z” denotes the KOH-to-carbon weight ratio
(ranging from 1 to 6) for chemically activated samples.

### 2.3. Morphological and textural characterization

Scanning Electron Microscopy (SEM) observations were carried out using a Zeiss GeminiSEM 500 microscope at a magnification of 1000×, using secondary electrons accelerated under 5 kV.

The bulk density of the ACs was determined according to the ASTM D2854 standard method. Briefly, a precision glass cylinder of known volume (e.g., a graduated measuring cylinder) was filled with the granular material, ensuring minimal compaction or settling, as prescribed by the method. The filled cylinder was then accurately weighed. The apparent density (g·cm⁻³) was calculated by dividing the mass of the carbon material by the internal volume of the cylinder.

The porous structures of all ACs were characterized using N₂ adsorption-desorption isotherms (-196 °C) carried out with an ASAP 2020 (Micromeritics) automatic adsorption device. Samples were degassed at 110 °C, under secondary vacuum for 24h, before analysis. The Brunauer-Emmett-Teller (BET) method was applied to determine the BET area ($A_{BET}$, m²·g⁻¹) [30]. Pore size distribution (PSD), surface area ($S_{NLDFT}$, m²·g⁻¹), total pore volume ($V_t$, cm³·g⁻¹), ultramicropore volume ($V_{uµ}$, cm³·g⁻¹, pore size w < 0.7 nm), supermicropore volume ($V_{sµ}$, cm³·g⁻¹, 0.7 < w < 2 nm), mesopore volume ($V_{meso}$ = $V_t$-$V_{sµ}$-$V_{uµ}$, cm³·g⁻¹, 2 < w < 50 nm), and associated surface areas, were calculated using the 2D nonlocal density functional theory (2D-NLDFT) for heterogeneous surface *via* SAIEUS software (Micromeritics) applied to N₂ adsorption isotherms [31].

### 2.4. Chemical characterization

Elemental analysis (EA) was performed using an Elementar Vario EL Cube analyzer, for which around 2 mg of samples were burned; the gases produced were then separated using a chromatographic column to determine carbon, hydrogen, nitrogen, sulphur and oxygen contents (wt.%).

The ash content of the samples was determined using a gravimetric technique similar to the ASTM E1755-01 standard test method for ash in biomass. Approximately 1 g of sample, previously dried at 105 °C for at least 12 h, was placed in a porcelain crucible (preheated at 600 °C in air and stored in a desiccator), and then heated in open air in a muffle furnace up to 600 °C at a heating rate of 3 °C min⁻¹. The temperature was maintained at 600 °C for 4 h, after which the crucible was allowed to cool naturally to 100 °C. The ash content was calculated as the mass fraction of the residue remaining at the end of the process relative to the initial dry mass of the sample.

Surface chemistry was studied through potentiometric titration, according to the following protocol. A 0.1 g sample of AC was dispersed in 50 mL of 0.01M NaNO₃ solution, acidified with 1 mL of 0.1M HCl, and stirred overnight for equilibration. The dispersion was then titrated with 0.1M NaOH under N₂ saturation using an automatic titrator (905 Titrando, Metrohm) controlled by tiamo® software (v2.2). The resulting proton-binding isotherm was analyzed using SAIEUS® software to determine the density of functional groups as a function of pKa [32].

### 2.5. Electrochemical characterization

A paste was prepared by mixing the AC under study with carbon black and PTFE in an 85:5:10 dry weight ratio. Then, self-standing disc electrodes (carbon load ~10 mg cm⁻²) were cut from the paste and impregnated with 1M H₂SO₄. Electrochemical measurements were carried out using a VMP3 (BioLogic) electrochemical workstation in a two-electrode set-up with an accuracy of 0.1 % of the full-scale range. Two-electrode configurations were used to provide a realistic assessment of electrode performance, as recommended by Stoller and Ruoff [33].

Two electrodes of similar mass and thickness (5 mm diameter, 220–330 µm thickness), separated by a porous quartz fiber mat (Tissu-quartz, Pallflex®), were placed between two gold current collectors.

The resistive behavior of the different materials was assessed by
electrochemical impedance spectroscopy (EIS) at open-circuit voltage, using a 10 mV sinusoidal signal over a frequency ranging from 10 mHz to 100 kHz. Cyclic voltammetry (CV) was carried out within a 1V potential window. The specific cell capacitance ($C_{cell}$, F·g⁻¹) was calculated using Equation (1) [1]:

$$C_{cell} = \frac{\int I dV}{s \Delta V m} \qquad (1)$$

where $I$ (A) is the measured current, $s$ (V·s⁻¹) in the scan rate, $\Delta V$ (V) is the potential window, and $m$ (g) is the mass of active material in both electrodes.

Galvanostatic charge-discharge (GCD) tests were conducted over a 1V range with current ranging from 0.2 to 40 A g⁻¹. The change of $C_{cell}$ with the applied current was calculated using Equation (2) [1]:

$$C_{cell} = \frac{I}{4 \left( \frac{dV}{dt} \right) m_e} \qquad (2)$$

where $I$ (A·g⁻¹) is the applied current density (based on the mass of one electrode), $dV/dt$ is the slope of the discharge curve after removing the $iR$ drop, determined from the upper half of the curve, considering that supercapacitors are usually operated in the range of $V_{max}$ to $1/2V_{max}$ [1, 33], and $m_e$ is the mass of the active material in one electrode.

To enable the comparison between the results from different studies, the specific electrode capacitance, $C_e$ (F·g⁻¹) will be considered as the reference metric. Considering that the cell normalization factor is twice that of the electrode [1]:

$$C_e = 4 \cdot C_{cell} \qquad (3)$$

The specific energy $E$ (Wh·kg⁻¹) and specific power $P$ (W·kg⁻¹) were then calculated between the two operating potentials $V_1 = V_{max} - iR$ and $V_2 = 1/2 V_{max}$, according to Equations (4) and (5), respectively [1]:

$$E = \frac{1}{2} C_{cell} (V_1^2 - V_2^2) \qquad (4)$$

$$P = \frac{E}{\Delta t_{12}} \qquad (5)$$

where $\Delta t_{12}$ (s) is the discharge time between these two operating potentials.

Cycling tests were performed at a current density of 10 A g⁻¹ for 10,000 cycles using a Biologic VMP3 electrochemical workstation in a 1V potential window.

# 3. Results and discussion

## 3.1. Morphological and textural properties

SEM observations of samples AC-S, AC-KOH-p-3 and AC-KOH-imp-4, representative of the different activation series (Fig. 1), reveal the presence of irregularly shaped, slightly elongated particles with a rough

texture in all samples, which is characteristic of wood-derived carbon materials. Although the cell wall structures are not clearly resolved, the presence of some macropores and heterogeneous surface features is likely to facilitate ion diffusion, which is advantageous for super-capacitor applications.

Surface area and pore volume are key parameters in AC materials used for energy storage. Depending on the activation method, distinct textural properties emerge. For instance, the N₂ adsorption-desorption isotherms of charcoal and KOH-p-1 AC (Fig. 2a) are classified as type Ia isotherms, characteristic of microporous materials with narrow micropores. In contrast, KOH-p-2, KOH-p-3 and carbon activated from the impregnation method shows a type Ib isotherm (Fig. 2b), indicating the presence of both larger micropores and narrow mesopores [30]. In all cases, increasing gas uptake at low relative pressures (P/P° < 0.2) confirms the development of microporosity as the KOH/C ratio (z ratio) increases, which is also evident in the pore size distribution (PSD, Fig. 2c and d). Notably, KOH activation by physical mixing primarily enhances ultramicroporosity, while the impregnation route broadens microporosity.

Meanwhile, the steam-AC exhibits a combination of Type I and Type II isotherms, characteristic of micro-mesoporous materials. These materials display significant adsorption at relative pressures below 0.1 and a discernible Type H4 hysteresis loop at P/P° > 0.4. The PSD reveals a broad peak centered at ~3 nm, indicating the formation of narrow mesopores (Fig. 2c).

The textural properties of the various materials, including BET surface area, total pore volume, and pore volumes for the different ranges of pores, are reported in Table S1. BET areas range from 1325 to 2726 m² g⁻¹, significantly exceeding that of the pristine charcoal (340 m² g⁻¹) and increase with the z ratio for both chemical activation routes (Fig. 2e). The differences in activation efficiency and burn-off values (see again Table S1) between AC-KOH-p and AC-KOH-imp can be attributed to the high water affinity of charcoal, which enhances the penetration of KOH solution into the internal porosity of the carbon, facilitating greater contact between the activation agent and the carbon [23]. Studies on coal activation suggest that this close interaction [34], combined with a long contact time, intensifies the reaction within the carbon matrix [35].

As a result, excessive pore enlargement may occur, leading to the formation of macropores and a reduction in BET area. Consequently, physical mixing (p route) appears to be the most efficient activation method, as it results in lower burn-off and higher surface area at the same z ratio (see again Table S1).

Despite differences in activation efficiency, the ultramicroporous ($V_{uµ}$) and supermicroporous ($V_{sµ}$) volumes of materials from both p- and imp-routes follow the same trend relative to surface area (Fig. 2f). Specifically, while $V_{sµ}$ increases linearly with $A_{BET}$, from 0.08 to 0.87 cm³ g⁻¹, $V_{uµ}$ increases from 0.04 cm³ g⁻¹ to a maximum of 0.22 cm³ g⁻¹ at ~1450 m² g⁻¹ but then decreases to 0.10 cm³ g⁻¹ as the BET area increases further. The mesopore volume ($V_{meso}$, see again Fig. 2f) also increases with surface area, but more significantly for steam- and

<figure><img src="image_3.png" /><figcaption>Fig. 1. SEM images of AC-S, AC-KOH-p-3 and AC-KOH-imp-4 samples.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Textural properties of the carbon materials: (a–b) Nitrogen adsorption-desorption isotherms; (c–d) PSDs for pristine charcoal, steam-AC, and the series of KOH-ACs from physical mixing (KOH-p-z (a,c) and impregnation (KOH-imp-z (b, d)); (e) BET area ($A_{BET}$, $m^2·g^{-1}$) as a function of the KOH/carbon weight ratio (z); (f) pore volumes as a function of $A_{BET}$ for ultramicropores ($V_{μμ}$, $cm^3·g^{-1}$), supermicropores ($V_{μμ}$, $cm^3·g^{-1}$) and mesopores ($V_{meso}$, $cm^3·g^{-1}$).</figcaption></figure>
impregnation-ACs (from 0 to 0.53 cm³ g⁻¹) than for those obtained by physical mixing (from 0 to 0.11 cm³ g⁻¹). These trends are consistent with previous studies on steam and KOH activation at temperatures exceeding 700 °C [23].

## 3.2. Chemical properties

The ash content of the different activated carbons is shown in Table S1. The pristine charcoal exhibited a relatively low ash content for a wood-derived material, with a value of 1.4 %. Steam activation, which does not require a washing step, resulted in a slightly higher ash content (2 %) compared to KOH-activated carbons (<0.1 % for both AC-KOH-p and AC-KOH-imp series), for which ash removal takes place during the preparation process. The resulting high purity of the carbons is beneficial for supercapacitor applications, as residual inorganic content can adversely affect conductivity and capacitance.

The elemental composition (C, H, N, S and O) of wood charcoal and its derived AC is detailed in Table S2. Compared to pristine charcoal, which already has a notably high carbon content for this type of material (89.1 wt%), both physical and chemical activation result in a general increase in carbon content (up to 95.9 wt%) and a simultaneous decrease in hydrogen and oxygen concentrations (from 8.7 to 3.4 wt% for oxygen, see Fig. 3a). It is worth noting that the oxygen content tends to increase with increasing burn-off (see again Fig. 3a). In the meantime, the nitrogen content remains within the same range (0.2–0.3 wt%), with

a slight increase observed in the impregnation route at low KOH/C ratios (i.e., 1 and 2, where 0.8 wt% nitrogen is detected). A high oxygen content in activated carbon is known to improve the wettability of the material [36], facilitating ion access from the electrolyte and the formation of the electrochemical double layer. This can in turn improve capacitance by promoting faradaic contributions, particularly in the presence of hydroxyl and quinone groups in acidic electrolytes [37].

Fig. 3b, c and d present the results of potentiometric titration for charcoal and its steam-activated and KOH-activated derivatives. The pKa distributions indicate a predominance of strongly basic species such as lactol- or hydroxyl-moieties (see Fig. 3b and c) with pKa values above 8. Activation significantly alters the surface chemistry by increasing acidity, as evidenced by the emergence of functional groups with pKa values between 3 and 8 (primarily carboxylic acid moieties) [38]. Interestingly, despite the reduction in total oxygen content, steam activation decreases the functional group density (from 1.14 to 0.84 mmol g⁻¹, see Fig. 3d), whereas chemical activation significantly increases it (2.82 and 6.75 mmol g⁻¹ for AC-KOH-p-3 and AC-KOH-imp-4, respectively). Thus, with higher densities of hydroxyl and quinone functional groups, systems based on chemically activated carbons (namely AC-KOH-p and AC-KOH-imp) should exhibit slightly higher capacitance values than those based on materials activated with steam.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** (a) Van Krevelen plot, indicating the elemental composition of the different activated carbons and O/C ratio as a function of burn-off (inset); (b) density of functional groups as a function of pKa for charcoal and its derived ACs; (c) surface groups on carbon (in color, the groups dosed by potentiometric titration); (d) potentiometric titration measurements: pKₐ of the main functional groups, and in brackets, their corresponding quantities (in mmol·g⁻¹). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
### 3.3. Electrochemical performance

#### 3.3.1. Cyclic voltammetry

Fig. 4a and b shows the cyclic voltammograms (CVs) recorded at 5 mV s⁻¹ in 1M H₂SO₄ for ACs from the three activation routes. The near-rectangular shape of the CV curves indicates supercapacitor behavior. However, samples produced via impregnation activation exhibit slight humps, suggesting the occurrence of Faradaic reactions at low scan rates. Notably, despite a lower overall oxygen content, these redox humps become more pronounced at higher KOH/C ratios (z = 4 and 6, see gain Fig. 4b), consistent with an increase in electrochemically active basic sites and higher nitrogen content [37].

Due to their high surface area and pore volumes, the different samples achieved high specific cell capacitance (Ccell, A·g⁻¹) at a low scan rate of 5 mV s⁻¹, ranging between 36 F g⁻¹ for AC-S and 61 F g⁻¹ for AC-KOH-p-3, see Fig. 4c and d. As the scan rate increases, Ccell decreases significantly across all samples, with capacitance retention ranging from 12% for AC-KOH-p-1 to 52% for AC-KOH-p-3 at 1000 mV s⁻¹, assuming that 100% is the value obtained at 5 mV s⁻¹. Although the AC-KOH-imp series displayed capacitance values comparable to or exceeding those of commercial references YP-50F and YP-80F at low scan rates (<100 mV s⁻¹), its capacitance retention upon increasing scan rate was significantly lower. Specifically, YP-50F and YP-80F maintained 54% and 58% of their initial capacitance, respectively, under high-rate conditions. In contrast, within the AC-KOH-p series, only the AC-KOH-p-3 sample exhibited a capacitance retention that matched the performance of the commercial benchmarks, indicating improved rate capability associated with optimized textural properties.

#### 3.3.2. Galvanostatic charge-discharge

Galvanostatic charge-discharge (GCD) tests were performed using a

specific applied current (I) ranging from 0.5 to 40 A g⁻¹. Fig. 5a presents representative GCD curves for three samples with comparable BET areas (~1400 m² g⁻¹), illustrating the impact of activation routes. Results for all ACs are provided in Fig. S1a and b. An in-depth analysis of the GCD curves at 0.5 A g⁻¹ reveals Coulombic efficiency (CE = tdischarge/tcharge) > 1 for AC-S and AC-KOH-imp-1 (Fig. S2), whereas CE < 1 for AC-KOH-p-1. A CE > 1 suggests electrolyte decomposition, which is favored at low charging rates due to the presence of numerous oxygenated functions. For I > 1 A g⁻¹, the three samples exhibit CE ≤ 1, and their GCD curves take on a triangular shape, confirming their supercapacitor (SC) behavior (see again Fig. S2).

The specific capacitance values derived from discharge curves and plotted as a function of I (Fig. 5b) emphasize the role of pore volume and hierarchical porosity in electrochemical performance. Indeed, at low currents, AC-KOH-p-1, which has the highest Vµ among the three samples, outperforms both AC-S and AC-KOH-imp-1. However, as the current increases (for I > 4 A g⁻¹), its performance declines sharply, retaining only 13% capacitance at 40 A g⁻¹, whereas AC-S and AC-KOH-imp-1 retain 52 and 59%, respectively. This drastic drop suggests that the electrode surface is not easily accessible to electrolyte ions, likely due to limited pore hierarchy, which restricts the formation of the electric double layer at high current densities.

As porosity and pore width increase, capacitance retention improves. The AC-KOH-p series retains up to 73% capacitance at 40 A g⁻¹, while the KOH-imp series stabilizes around 55% (Fig. S1b,e and Fig. S3). While the commercial activated carbon references exhibited higher capacitance retention values (68% for YP-50F and 80% for YP-80F), the best-performing sample from this study, AC-KOH-p-3, which has the highest Vµ among all tested materials, consistently outperformed the commercial benchmarks (Fig. 6a).

These findings highlight the importance of high pore volumes and

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Electrochemical performance of wood charcoal-derived ACs: (a,b) CV curves at 5 mV s⁻¹ and (c,d) specific cell capacitance (Ccell, F·g⁻¹) as a function of scan rate for steam (S) and physical mixing (p-z) activation routes (a,c) and impregnation route (imp-z) (b,d).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Electrochemical performance of steam-activated and KOH-ACs prepared by impregnation and physical mixing routes, AC-S, AC-KOH-imp-1 and KOH-p-1, respectively: (a) charge-discharge curves at 1 and 2 A g⁻¹; (b) specific cell capacitance (Ccell) as a function of applied current (I), with capacitance retention at 40 A g⁻¹; (c) comparison of gravimetric and volumetric capacitance at 0.5 A g⁻¹; (d) Ragone-like plot. Electrochemical impedance spectroscopy: (e) Nyquist and (f) Bode plots.</figcaption></figure>

hierarchical porosity in facilitating ion diffusion to the active surface. In addition, capacitance retention, C<sub>ret</sub> (%), obtained from GCD tests, is consistently higher than that from CV measurements. This indicates that the different ACs have, as expected, less resistance to electron mobility than to ion mobility, which are the respective charging mechanisms of GCD and CV tests [39].

An often-overlooked aspect in supercapacitor research is volumetric capacitance. While electrochemical performance is traditionally evaluated based on gravimetric capacitance (Equations (2) and (3)), the

thickness and density of the electrodes are known to affect the overall device performance significantly. Consequently, for comprehensive optimization, it is crucial to consider electrochemical properties in terms of volume, along with factors that influence them.

At a current of 0.5 A g⁻¹, the gravimetric capacitance of electrodes derived from AC-KOH-p-1 is higher than that of AC-S and AC-KOH-imp-1 (Fig. 5b and c). However, at the same current density, the volumetric capacitance of AC-KOH-imp-1 (20 F cm⁻³) significantly surpasses that of both AC-S (14 F cm⁻³) and AC-KOH-p-1 (16 F cm⁻³). This demonstrates
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Electrochemical performance of the best material compared to commercial benchmarks: (a) specific cell capacitance ($C_{cell}$, F·g⁻¹) as a function of applied current (I), and (b) Ragone-like plot.</figcaption></figure>

that considering volumetric capacitance can change the ranking of materials in terms of electrochemical performance. Similar trends are observed within the AC-KOH-imp and AC-KOH-p series (see Fig. S4), which can be partially attributed to differences in electrode density. Although the bulk densities of AC-S, AC-KOH-p-1, and AC-KOH-imp-1 are identical at 0.15 g cm⁻³ (see Table S1), the density of the electrodes made from AC-KOH-imp-1 is approximately 0.50 g cm⁻³, compared to 0.45 g cm⁻³ for AC-S and 0.37 g cm⁻³ for AC-KOH-p-1 (Table S3).

These results highlight the critical influence of electrode preparation methods and the textural properties of ACs on the volumetric performance of supercapacitors. Given an identical formulation and shaping process, intraparticle porosity, represented by electrode density and associated with the particle size of the AC, plays a decisive role in supercapacitor performance [40]. Therefore, optimizing electrochemical performance requires particular attention to these parameters [41].

Fig. 5d and Fig. S1c, f present Ragone-like plots, showing the relationship between specific energy (E) and specific power (P) for supercapacitor cells, derived from GCD experiments. At the lowest specific applied current, 0.5 A g⁻¹, the three ACs (AC-S, AC-KOH-imp-1 and AC-KOH-p-1) exhibit a maximum storage capacity of ~6 Wh·kg⁻¹ at 150 W kg⁻¹. However, the best-performing materials at low current (i.e. samples with a BET area >1500 m² g⁻¹ and $V_T$ > 0.85 cm³ g⁻¹) achieve a maximum storage capacity of 8 Wh·kg⁻¹ at the same power. These E and P values align with previous reports on bio-based carbon EDLCs derived from biochar [42], flowers [43], agricultural products such as corncob [11] and sugarcane [44], as well as peanut [45] and walnut husks [46], wood bark [23] and wood residues [20], tested in similar conditions in H₂SO₄ (see Table S4). They remain higher than those obtained with benchmarked commercial reference, i.e. YP-50F and YP-80F, displaying values of ~5 Wh·kg⁻¹ at 150 W kg⁻¹ (Fig. 6b).

### 3.3.3. Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) tests reveal key differences in resistive behavior. From the Nyquist plot (Fig. 5e), the equivalent distributed resistance (EDR) is one order of magnitude higher for AC-KOH-p-1 (~1.5 Ω cm²) than for AC-S and AC-KOH-imp-1 (~0.15 Ω cm²). The most important contribution to the EDR of AC-KOH-p-1 (and AC-KOH-p-z series, see Fig. S5a) arises from limited ion diffusion [47]. The mass transfer of redox species to the electrode, represented by the so-called Warburg impedance, appears as a long diagonal line at 45° in the Nyquist plot and as a nearly constant phase region in the Bode plot (Fig. 5f and Fig. S5b and i). In contrast, the presence of larger pores in AC-S and AC-KOH-imp-1 (and KOH-imp series, Fig. S5b, ii) facilitates ion

transport, reducing diffusion limitations as shown by a shorter diagonal line in the Nyquist plot and the absence of visible constant phase in the Bode plot. Thus, despite the high surface area and significant microporous volume, the slow diffusion of ions within the materials tends to reduce charge accumulation at the electrode-electrolyte interface, leading to lower performance. This observation is consistent with the conclusions drawn from the GCD curves.

### 3.3.4. Cycling stability

In practical applications, the cycling stability of electrode materials is a key parameter that directly influences device lifetime. Therefore, cycling tests were conducted on samples AC-S, AC-KOH-imp-4 and AC-KOH-p-3, which exhibited the highest volumetric cell capacitance among the three activation routes, to evaluate their long-term electrochemical stability (see Fig. S6).

All samples showed excellent stability under continuous cycling at 10 A g⁻¹, with gravimetric cell capacitance retention ranging from 95 % for AC-KOH-p-3 to 97 % for AC-S and AC-KOH-imp-4 after 10,000 cycles. Overall, the performance of the activated materials can be attributed to key differences in their physicochemical properties. While it may be tempting to associate superior electrochemical performance solely with a high BET area, pore volume and hierarchical porosity are also crucial factors. These properties indeed influence ion diffusion within the material and the ability to form an electric double layer at higher currents.

## 3.4. Comparison with the literature

Due to the wide variety of carbon precursors, synthesis methods, electrode formulations, carbon loading and electrochemical setups (including differences in cell type, electrolyte composition, concentration and potential window), direct comparisons can prove difficult and should be approached with caution.

For example, thin electrodes with low carbon loading experience minimal mass transfer limitations, which can enhance their performance but may lead to an overestimation of capacitance and rate capability [23]. Similarly, when comparing capacitance retention reported in the literature, it is important to consider that the reference maximum capacitance value may be obtained at charging rates ranging from 0.05 to 1 A g⁻¹. To ensure a fair comparison, the performance of the materials developed in this study was thus evaluated under conditions similar to those used for ACs derived from biomass. These include biochar [42], flowers [43], agricultural products like corncob [11], sugarcane bagasse [44] and bio-waste such as peanut [45] and walnut husks [46], wood bark [23] and wood residues [20], as well as two state-of-the-art
commercial references, YP-50F and YP-80F, from Kuraray (Table S4).

Fig. 7a shows the specific electrode capacitance (calculated using Equation (3), where applicable) and capacitance retention at 5 A g⁻¹ as a function of A<sub>BET</sub>. Given the variability in reported results and the factors discussed above, it is important to acknowledge that establishing a direct correlation between BET surface area and capacitance can be oversimplified. This is particularly true for microporous materials, where BET measurements often overestimate the surface area accessible to electrolyte ions. Therefore, pore size distribution and total pore volume should also be considered, as they significantly influence energy storage performance. Nevertheless, BET area remains a commonly used metric for comparing results across different studies.

In general, physical activation of char derived from agricultural biomass and wood tends to produce materials with smaller surface areas and lower capacitances, whereas chemical activation produces larger surface areas, leading to enhanced capacitances. Notably, even the materials in this study with the least developed surface area have capacitances comparable to those of commercial references tested under the same conditions. In addition, as shown in Fig. 7b, regardless of the activation method, ACs produced in this work exhibited capacitance retention values above 65 % at 5 A g⁻¹. These values are comparable to, or in some cases exceed, those of ACs derived from wood and agricultural biomass, including those developed in laboratory research as well as commercial references. These findings highlight the potential of these materials, produced from local resources, to rival commercially available ACs for electrochemical energy storage applications in an aqueous electrolyte. While direct extrapolation of performance between aqueous and organic electrolytes is not straightforward for microporous materials, these remarkable results nevertheless suggest strong application potential in organic electrolytes and pave the way for further in-depth studies.

## 4. Conclusion

This study highlights the critical influence of carbon textural

properties, beyond surface area alone, on the electrochemical performance of supercapacitor electrodes. Using industrially produced wood charcoal from a short supply chain as a precursor, three activation methods: steam activation, KOH physical mixing and KOH impregnation, were explored. A direct comparison of materials with similar surface areas and pore volumes revealed that steam-activated and KOH-impregnated carbons facilitated faster ion diffusion and exhibited superior capacitance compared to KOH-physically mixed ACs. Enhanced activation, coupled with increased pore width, alleviated ion diffusion limitations in highly microporous materials produced via the KOH physical mixing route, ultimately leading to improved performance retention. Beyond surface area, pore volume and hierarchical porosity thus emerged as critical factors influencing electrochemical energy storage performance.

By comparing them with commercial ACs and those from the literature, our results confirm the strong performance potential of wood-derived carbon achievable in aqueous electrolytes, while also clearly identifying the most effective activation route. The best chemically activated samples achieved specific cell capacitance of about 65 F g⁻¹ (i. e., a specific electrode capacitance, C<sub>e</sub>, of 257 F g⁻¹), clearly out-performing commercial carbons such as YP-50F and YP-80F, which remained below 40 F g⁻¹ (C<sub>e</sub> of 156 F g⁻¹) under the same testing conditions. More importantly, these findings underscore the potential of locally sourced French charcoal as a sustainable and high-performance alternative for electrochemical energy storage applications, positioning it as a competitive option compared to conventional materials. Future work will aim to explore scale-up opportunities based on these findings.

### CRediT authorship contribution statement

**Pauline Blyweert:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation. **Jimena Castro-Gutiérrez:** Writing – review & editing, Methodology, Formal analysis, Data curation. **Solène Gentil:** Resources, Funding acquisition. **Vanessa**

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Electrochemical performance of ACs from this study compared to literature data obtained from biochar [42], flowers [43], agricultural products such as corncob [11] and sugarcane [44], as well as peanut [45] and walnut husks [46], wood bark [23] and wood residues [20], tested in similar conditions in H₂SO₄: (a) Specific electrode capacitance (C<sub>e</sub>) and (b) capacitance retention (C<sub>ret</sub>) at 5 A g⁻¹ as a function of BET area (A<sub>BET</sub>). C<sub>ret</sub> was calculated by considering the capacitance at 0.5 A g⁻¹ as the maximum (i.e., C<sub>ret</sub> = 100 %).</figcaption></figure>
**Fierro:** Writing – review & editing, Resources, Funding acquisition.
**Alain Celzard:** Writing – review & editing, Resources, Project administration, Funding acquisition.

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.

Acknowledgements

The authors would like to thank Pauline Haraux for her help in producing SEM images. This research was funded by the French National Research Agency (ANR) through the "CarBioLab" project (ANR-21-LCV3-0001), as part of the LabCom "Joint Laboratories" program, 2021 edition.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2025.124992.

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