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# Activated Carbon from Birch Wood as an Electrode Material for Aluminum Batteries and Supercapacitors

Menestreau Paul,[a, d] Alejandro Grimm,[a] Glaydson Simões Dos Reis,[a, b] Gopinathan Manavalan,[a, c] Sruthy E S,[e] Mikael Thyrel,[a] and Shaikshavali Petnikota*[a]

Due to its sustainable approach, biomass is the subject of much research focused on the synthesis of multifunctional materials including electrodes for batteries and supercapacitors. In this work, sawdust from the processing of birch logs was used to produce a highly porous carbon material (CBW) that is employed for the construction of electrodes for aluminum batteries (ABs) and supercapacitors (SCs). A multitude of characterizations indicated that CBW is built in with highly disordered amorphous carbons and an extremely high specific surface area of $3029 \text{ m}^2\text{g}^{-1}$ which is predominant with micro-porous features. The chemical analysis of CBW indicated the

presence of a significant amount of oxygen functionalities. As a cathode of AB, CBW achieved discharge capacities 115, 74, 54, 50, 47, 43, and $29 \text{ mAh g}^{-1}$ at current rates 0.1, 1.0, 2.0, 3.0, 4.0, 5.0, and $10.0 \text{ Ag}^{-1}$, respectively. Similarly, SC with CBW symmetric electrodes exhibited capacitances 143, 94, 87, 79, 74, 69, 65, and $51 \text{ Fg}^{-1}$ at current rates 0.1, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and $10.0 \text{ Ag}^{-1}$, respectively. The electrochemical characterization revealed that CBW is promising for ABs and SCs, and controlling the porosity type could further enhance the performance.

## 1. Introduction

The growing demand for efficient energy storage solutions linked to the proliferation of renewable energy sources and electric vehicles is driving a growing need for durable, high-performance electrode materials that can be produced on a large scale from abundant, renewable resources like activated carbon derived from biomass residues.[1] Lithium-ion batteries (LIBs) are the most widely used today, offering a technical solution to today's energy storage needs, but they present

problems such as a limited lifespan, safety and environmental concerns. The main problems with LIBs are their cost and resource scarcity, while biomass is still an under-exploited resource that offers a wide range of materials, high availability, and great potential for cathode application in batteries, especially aluminum batteries (ABs). On the other hand, supercapacitors (SCs) are used worldwide with electrode materials that are attracting interest, such as graphene, molybdenum trioxide, molybdenum disulfide, black phosphorus, manganese dioxide, etc. but these materials encounter several difficulties in SC applications.[2] These include graphene's lack of a band gap, which limits its operation as a semiconductor material, the limited mobility of molybdenum disulfide carriers, and the complex, multi-step, and time-consuming exfoliation procedures required for black phosphorus, which often result in structural defects,[3] manganese dioxide performance in SCs is far from satisfactory and an in-depth understanding of the underlying storage mechanisms alongside with cycling stability improvements are highly required.[2a] Whereas SCs require electrode materials with high specific surface area (SSA), surface chemistry, and electrical conductivity, which are the characteristics of biomass-derived activated carbon (AC).[4]

ABs and SCs represent two promising categories of energy storage devices which, combined with biomass-derived ACs like carbon from birch wood (CBW), can offer a unique combination of high capacity, durability, and environmental safety. They exhibit distinct characteristics and are therefore suited for different applications. Batteries are characterized by high energy densities but relatively low power densities while SCs deliver high power densities and show rapid charge/discharge cycles with high capacitance though they have lower energy densities.[4,5] The formation of an electric double layer of charge (EDLC) allows SCs to attain better energy storage (millions of times higher) than in capacitors, but of course still lower than in

[a] M. Paul, A. Grimm, G. Simões Dos Reis, G. Manavalan, M. Thyrel, S. Petnikota
Biomass Technology Centre, Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences, Umeå SE-90183, Sweden
E-mail: shaikshavali.petnikota@slu.se
psvali85@gmail.com

[b] G. Simões Dos Reis
Faculty of Natural Sciences and Technology, Industrial Chemistry and Reaction Engineering, Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Åbo-Turku 20500, Finland

[c] G. Manavalan
Technical Chemistry, Department of Chemistry, Chemical-Biological Centre, Umeå University, Umeå SE-90187, Sweden

[d] M. Paul
Public Research University Engineering School, IMT Mines Albi, Albi 81000, France

[e] S. E S
School of Electronic Systems and Automation, Digital University Kerala, Thiruvananthapuram, India

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Supporting information for this article is available on the WWW under
https://doi.org/10.1002/celc.202400549

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© 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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batteries.[6] AB is an atypical type of battery composed of an aluminum metal anode, a carbonaceous cathode, and an electrolyte composed of a mixture of AlCl₃ and 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl).[6b,7] The reaction between these two components creates complex species, AlCl₄⁻ and Al₂Cl₇⁻ which together serve as charge carriers as described in equations 1–3.[6b,7]

$$ \text{Anode : Al} + 7 \text{AlCl}_4^{-} \rightleftharpoons 4 \text{Al}_2\text{Cl}_7^{-} + 3 \text{e}^{-} \quad (1) $$

$$ \text{Cathode : } C_n[\text{AlCl}_4] + \text{e}^- \rightleftharpoons C_n + \text{AlCl}_4^{-} \quad (2) $$

$$ \text{Net reaction : } \begin{aligned} & \text{Al} + 4 \text{AlCl}_4^{-} + 3 C_n[\text{AlCl}_4] \\ & \rightleftharpoons 4 \text{Al}_2\text{Cl}_7^{-} + 3 C_n \end{aligned} \quad (3) $$

Graphite and graphene are most widely studied as cathode materials in ABs associated with low experimental capacities, high volume expansion, and high manufacturing costs.[7b,c,8] Typically, ABs operate with a graphite cathode, but they encounter several problems in the process of intercalating AlCl₄⁻ into the graphite cathode. Indeed, the low capacity of ABs is mainly due to the larger anionic size of the AlCl₄⁻ charge carrier, which is twice the d-spacing of graphite, leading to structural damage of the cathode.[7b,8–9] In contrast, ACs derived from biomass are a potential solution due to their well-developed porosity, non-toxicity, and high SSA, with several functional groups and numerous micro- and mesopores which could favor the ion intercalation and improve overall performance.[6b,10] The unique properties of CBW, such as its tunable porosity, non-toxicity, eco-friendly nature, and abundance of surface functional groups, make it an ideal candidate for addressing the challenges faced by both ABs and SCs, particularly in terms of improving capacity and cycle stability. CBW is best suited as an alternative to graphite and graphene owing to its aforesaid unique characteristics. To meet these challenges, this study aims to develop and optimize CBW as a dual-purpose electrode material for ABs and SCs, addressing the key challenges of energy storage efficiency, durability, and material abundance but also overcoming the capacity and stability limitations of the graphitic electrode materials in ABs and SCs. The CBW is reproduced using birch wood (BW) sawdust obtained from a sawmill, and it was activated using a 2 step procedure according to previous work and references therein.[6a]

In this work, a highly porous carbon material with efficient dual application on ABs and SCs was prepared using a sustainable precursor from wastes generated from the harvesting of birch trees (Betula spp.), which includes dozens of tree species densely distributed in North America, Asia and Europe, including northern Finland and Sweden.[11] Birch tree when cut down and processed at sawmill and pulp industries generate wastes which is considered as a low-value side-product, which could successfully employed to produce advanced materials for energy storage application including batteries and supercapacitors. Birch waste is mainly composed of cellulose, hemicellulose, and lignin, which makes it suitable to be employed to produce porous carbon with low level of impurities that is desired for AB

and SC applications. Of late, our group demonstrated BW-AC as a promising electrode material for SCs with 1.0 M TEA-BF₄ organic electrolyte. Similarly, it is intriguing to use CBW as an electrode for ABs as well as for SCs with high voltage electrolytes such as 1.0 M [BMIM]BF₄ organic electrolyte which is the aim of the present study.

# Materials and Methods

## Materials

The raw material used for the preparation of activated biochar was birch (Betula spp.) sawdust obtained from a sawmill located in the province of Västerbotten, Sweden. Potassium hydroxide (KOH) pellets (Supelco) were used as an activation agent. Super P Carbon and sodium carboxymethyl cellulose (NaCMC) from Alfa Aesar, AlCl₃ (99.99%), [EMIM]Cl (98%), molybdenum foil (thickness 0.1 mm, ≥99.9%) from Sigma Aldrich, aluminum foil (thickness 0.25 mm, ≥99.999%) from Advent Research Materials Ltd, U.K., aluminum laminated polymer film battery cases from PI-KEM, U.K., 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF₄ ≥98%), acetonitrile (AN, 99.999%) from Sigma Aldrich, AvCarbon from Fuelcell store, Whatman glass microfiber filters, and stainless steel 304 CR2032 coin cell cases were used for the production of ABs and SCs.

## Biochar and CBW Preparation

The activated biochar was prepared using the chemical activation procedure in two steps using KOH as the activation agent to promote the enhancement of the specific surface area as widely reported in the literature.[12] First, the precursor was carbonized at 500°C in a stainless steel reactor heated externally by a muffle furnace. Nitrogen gas (99%, AGA) was used to keep an inert atmosphere during the process. The sample temperature was increased from room temperature to the final treatment temperature at 10°C/min rate and kept at this temperature for 1 h. After this, the furnace was switched off and allowed to cool naturally to room temperature. The carbon obtained was activated using an impregnation ratio of 1:8 wt.% of the biochar and KOH, respectively. The KOH and biochar were mixed and water was added under constant stirring until all KOH was dissolved. This mixture was then stirred for 7 hours and then dried in an oven at 90°C overnight. The impregnated carbon was pyrolyzed at 900°C using the reactor described previously, under a nitrogen atmosphere. The temperature of the sample was increased from room temperature to the final treatment temperature at a rate of 10°C/min and kept for 1 h. The obtained carbon was washed with water several times until the pH of the wash water became neutral. Afterward, the carbon was dried in an oven at 105°C overnight and stored in glass bottles until further use.

## Fabrication of CBW Electrodes

The CBW electrodes have been prepared according to these steps: First, CBW powder, NaCMC binder, and Super P Carbon in 80:10:10 wt.%, respectively, mixed in 1:1 (v/v) solvent of deionized water and ethanol for preparing the electrode slurry. Then the slurry was bath-sonicated for 30 minutes before coating. A 50 µL of CBW slurry was coated onto Mo (1.5×1.4 cm²) and AvCarbon discs (Ø~16 mm) substrates by drop casting. Then the electrodes were dried in the oven at 105°C overnight. The active mass loading of the electrodes was in the range of 4–8 mg. The aluminum anodes
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(1.5×1.4 cm²) were prepared by welding Al to Mo current collectors. Then all the electrodes, separators, and battery cases were dried under vacuum at 80°C for 12 hours using Glass Oven B-585 from Buchi before entering the glovebox.

## Electrolyte Preparation

The electrolyte preparation was done in an Argon atmosphere inside a glovebox (LABstar workstation, MBraun, Germany). Electrolyte for ABs was prepared as detailed in our recent publication.[7a,b,9] In brief, at first, the residual water content in [EMIM]Cl was removed by vacuum heating it at 140 °C for 48 hours using the Glass Oven B-585. Then it was transferred to the glove box and mixed with AlCl₃ powder in a 1:1.3 mole ratio, respectably. The reaction between AlCl₃ and [EMIM]Cl is highly exothermic and both solids become liquids instantaneously but the liquid stirred vigorously overnight. On the other hand, electrolytes for SCs prepared simply by mixing [BMIM]BF₄ and AN in a 1:1 weight ratio.[13] The mixture was stirred overnight before using to fabricate coin cell SCs.

## ABs and SCs Assembly

The assembly of both ABs and SCs was carried out inside the Ar-filled glovebox. For the ABs, the CBW cathode on Mo substrate and aluminum anode were put together separated by a glass microfiber filter, and placed into a pouch that sealed three sides in an open atmosphere. Then the pouches were vacuum dried at 80 °C overnight inside the Glass Oven B-585 before entering into the glovebox. Then the pouches were filled with 1.3 M AlCl₃: [EMIM]Cl electrolyte about 400 µL inside the glovebox and the fourth side was sealed. On the other hand, two CBW electrodes on AvCarbon substrates with identical mass were used to assemble CR2032 coin cell SCs. The two electrodes were separated with glass microfiber filters and filled with the AN:[BMIM]BF₄ electrolyte about 200 µL before sealing the coins.

## Characterization

The activated carbon was imaged using a Carl Zeiss Merlin Field Emission Scanning Electron Microscope (SEM) at an accelerating voltage of 5 kV and a current of 120 pA, equipped with a Gemini column. EDS Elemental mapping and analysis were performed using this SEM. The surface area of CBW powder was determined by measuring liquid nitrogen physisorption using Micromeritics TriStar 3000 porosimeter, with values extracted using the BET equilibrium model. Adsorption-desorption isotherms of CBW powder were recorded at -196 °C after it was degassed at 110 °C for 2 hours. Raman spectra were acquired using a Raman spectroscope (Renishaw) equipped with a 514 nm laser. X-ray diffraction (XRD) pattern of CBW was collected using a PANalytical Empyrean X-ray diffractometer equipped with a Cu Kα X-ray tube in the 2θ range of 10°-58°. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra spectrometer. Transmission electron microscope (TEM) imaging was done using JEOL JEM-2200FS (JEOL Ltd., Japan) with electron beam of 200 kV. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker IFS 66 v/S FT-IR spectrometer equipped with a vacuum bench, diffuse reflectance carouse!, and a DTGS detector. Dry solid samples were diluted with KBr powder before measurement.

## Electrochemical Characterization

Both ABs and SCs were tested after relaxing for 8 hours at least and all the testing was done at room temperature. The ABs were tested

in a voltage window of 0.01–2.2 V whereas SCs tested in 0.0–2.5 V. The cyclic voltammetry (CV) measurements were carried out with multiple scan rates of 0.1–500 mV s⁻¹ using BioLogic SP-150e potentiostat. The galvanostatic charge-discharge (GCD) measurements were done with current densities ranging from 0.1–10 A g⁻¹ using a BioLogic 800 series battery cycler. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 200000–0.1 Hz with an AC signal of 10 mV amplitude using BioLogic SP-150e. The data obtained was plotted as Nyquist plots and fitted into equivalent circuits (ECs) using ZView2 software.

# 2. Results and Discussion

## 2.1. Characterization of CBW Materials

### 2.1.1. Physicochemical Characterization of the CBW Electrode Material

The morphology of the CBW material was examined using SEM (Figure 1a–b), which shows small and elongated particles, which could be beneficial for improving the particle dispersion and coating of the electrode substrate, thus facilitating uniform ion transport in the electrode material. Figure 1c further evaluated the morphology of the materials as well as the element composition through EDS. Big cavities are observed which provide channels for electrolyte penetration and ion transport, which are essential for the efficient operation of AC as an electrode material in ABs and SCs. Figure 1c also exhibits the mapping of the elements and a high concentration of carbon was observed (90.41 wt.%) and 7.63 wt.% of oxygen. The large presence of oxygen indicates the presence of residual functional groups that may contribute to pseudo-capacitance, enhancing capacity through faradaic reactions in SCs. Even for ABs, the oxygen functionalities won't pose any threat as the operating voltage cut-off, 2.2 V was far below to trigger the side reactions with the electrolyte ( > 2.35 V).[7c,13]

The microstructure of the CBW was examined by high-resolution TEM images (Figure 2), which confirms the highly porous characteristics of the CBW material. The individual particles are found in random shapes and sizes with semi-transparent and transparent to electron beam at the center and at the edges, respectively, as seen Figure 2a, b, d and e. The images were taken from different particles and different magnifications to understand and summarize the structure of CBW powder. The edges and surfaces of the particles have smooth finishing in line with SEM surface features. The atomic projections seen in Figure 2b and d lacking a distinct order, they appear randomly forming wave/curve patterns. These random atomic projections in both images suggest the dominant presence of disordered/amorphous structures and rich in defects. Defects in carbon anode materials could have positive electrochemical effect since they can act as active sites and enhance the interaction with the electrolyte and improve the intercalation process. The corresponding selected area electron diffraction (SAED) patterns (Figure 2c and f) confirm that the CBW carbon structure was completely amorphous. The
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<figure><img src="image_7.png" /><figcaption>Figure 1. a) and b) SEM images of CBW material, and c) EDS image and mapping of C and O elements.</figcaption></figure>

diffused ring patterns seen in Figure 2c and f are characteristics of amorphous carbon and clarify that CBW was not graphitic carbonaceous material such as graphite, graphene, few-layered graphene (FLG) etc.

The specific surface area and size of the pores of the carbon used in electrodes for ABs and SCs are important parameters to obtain devices of good quality. The nitrogen adsorption/desorption curve was exhibited in Figure 3a. The curve shows features of type-I isotherms, meaning that CBW was dominated by microporous structures.[14] The SSA calculated according to the BET principle, was 3029 m²g⁻¹, which was extremely higher than the majority of the carbon reported in the literature (see Table S1 of Supporting Information (SI)). Figure 3b shows the pore size distribution, whereas a large portion of micropores at the range of less than 2 nm and small mesopores at 2.5 nm can be seen. The high existence of micropores and small mesopores contributes to the extremely high surface area, which can be beneficial for electrochemistry applications since the combined micro and mesopores feature alongside an elevated SSA interconnected with macropores offer suitable properties for

effectively improving the mass transfer pathway for ion diffusion into the porous structure of the electrode.

The carbon network configuration has great implications for its electrochemical performance, and Raman analysis was employed to analyze the degree of order/disorder and graphitization of the CBW.[14,15] Figure 3c shows that CBW exhibits a D band (around 1340 cm⁻¹) and an evident sp² G band (around 1585 cm⁻¹) corresponding to the disordered and imperfect structures of carbon materials and the vibrations of carbon atoms with an sp² electronic configuration in graphite structures, respectively. A ratio for the $I_D/I_G$ was obtained from these peaks ($I_D/I_G$=1.4), suggesting the presence of more structural defects in the carbon lattice than the formation of graphitic structure. The complete absence of a 2D band (around 2710 cm⁻¹) seen in Figure S1a that was acquired for a different region also confirms CBW does not possess any graphitic basal planes those stalked along the c-direction. The same was referenced from the XRD pattern of CBW shown in Figure S1b in which neither crystalline nor characteristic (002) graphitic peaks appeared. Therefore, both Raman and XRD analyses confirm the complete amorphous nature of CBW. It was
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<figure><img src="image_9.png" /><figcaption>**Figure 2.** High-resolution TEM image of CBW at different magnifications (a, b, d, e) and their corresponding SAED patterns (c, f).</figcaption></figure>

reported that defect-rich carbon lattice can improve the electronic properties of the carbon electrode by enhancing the exposed active sites and accelerating the redox reaction, which boosts the electrochemical performance of supercapacitors and batteries.[16]

In carbon-based materials, the surface elemental chemical composition plays a critical role in its electrochemical performance due to the redox activities. The surface chemical composition of the CBW was evaluated by XPS to better assess its elemental composition (Figure 3d–f and Table 1). By the survey spectra (Figure 3d), two main peaks can be observed near 284.0 and 532.0 eV, corresponding to the peaks of C1s and O1s, respectively, which are coherent with the results regarding

the other carbons prepared with KOH.[6a] To further evaluate the nature state of the C and O elements, these peaks were deconvoluted. The C1s spectrum (Figure 3e) indicates that the CBW sample built in with non-graphitic sp³ carbons C–C (~284.3 eV), phenolic hydroxyl C–OH (~285.6 eV), ether groups C–O–C (~286.8 eV), carboxylic groups O=C–OH (~288.6 eV), and carbonate –CO₃ (~290.5 eV) bonds.[6a,7b,c,17] The O1s spectrum (Figure 3f) was deconvoluted to three oxygen states at 530.9 eV, 532.7 eV, and 535.4 eV, which can be assigned to oxygen bonded to carbon C–O (531.1 eV) and C=O bonds (532.7 eV), and the peak at around 535.4 eV can be related to the oxygen singly bounded to carbon in phenols and ethers functional groups.[6a,7b,c,17] The high presence of oxygen groups

<table><caption>**Table 1.** XPS-based elemental analysis of CBW.</caption><thead><tr><th>Element</th><th>Bonding</th><th>Position (eV)</th><th>Atomic%</th></tr></thead><tbody><tr><td rowspan="6">C</td><td>C–C</td><td>284.3</td><td>50.55</td></tr><tr><td>C–OH</td><td>285.6</td><td>10.46</td></tr><tr><td>C–O–C</td><td>286.8</td><td>9.45</td></tr><tr><td>O=C–OH</td><td>288.6</td><td>5.48</td></tr><tr><td>CO<sub>3</sub></td><td>290.5</td><td>10.56</td></tr><tr><td>π–π*</td><td>293.3</td><td>0.71</td></tr><tr><td rowspan="3">O</td><td>C–O</td><td>530.9</td><td>1.52</td></tr><tr><td>C=O</td><td>532.7</td><td>9.15</td></tr><tr><td>C–OH</td><td>535.4</td><td>2.12</td></tr></tbody></table>
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<figure><img src="image_11.png" /><figcaption>**Figure 3.** a) N₂ adsorption-desorption curves, b) pore size distribution, c) Raman spectrum, d) XPS survey spectra, e) deconvoluted C 1s peaks, and f) deconvoluted O 1s peaks of CBW powder sample.</figcaption></figure>

on the CBW surface suggests a high presence of functional
groups that can boost its electrochemical performances
through redox reactions.[18] Table 1 shows the atomic percen-
tages of carbon and oxygen. The carbon and oxygen atomic
percentages of the CBW are 87.21% and 12.79%, respectively.

The high oxygen content in the CBW carbon could introduce a
redox pseudocapacitance effect through oxygen-containing
functional groups, which helps to improve the specific
capacitance of the electrode. FTIR response of CBW (Figure S2)
was pursued to further understand its chemical nature by
<figure><img src="image_12.png" /></figure>

visualizing type of residual functional groups attached to it. As seen in Figure S2, CBW mainly contain absorption bands corresponding to O–H (3450 cm⁻¹), asymmetric C–H (2920 cm⁻¹), symmetric C–H (2852 cm⁻¹), C=C (1628 cm⁻¹) and C–O (1055 cm⁻¹) vibration modes.[17a,19] The strong O–H band suggest major share of oxygen was attached to CBW through C–OH and O=C–OH functional groups as seen in XPS peaks. The weak C–O band indicate that minor share of oxygen was from either carbonates and/or directly attached to carbons of CBW through carboxylic groups as seen in its C 1s spectrum.[17a,19] Therefore, the high extent of elemental oxygen found in EDS plausibly stem out mainly from various hydroxyl functional groups as perceived by both XPS and FTIR techniques.

## 2.2. Electrochemical Assay

### 2.2.1. ABs

The AB performance of the CBW was assessed through GCD, rate capability tests, and the type of redox reactions involved are evaluated with the CV test. This analysis aims to understand electrochemical behavior, charge storage mechanism, and

Coulombic efficiency (CE) over several hundred cycles at different current densities. The CV in Figure 4a was analyzed to understand the charge storage mechanism. It shows no diffusion or intercalation peaks, indicating pseudocapacitive behavior with cathodic and anodic currents forming a quasi-rectangular shape. Unlike graphite/multi-layered graphene, CBW has no ordered structure and cannot exhibit intercalation of charge carriers, such as $AlCl_4^-$ anions.[7b,c,20] CV profiles of CBW have overlapping curves with constant currents, which was promising for stable performance, it's comparable to other biomass-derived ACs mentioned in Table 2. In Figure 4b of 0.1 $Ag^{-1}$ current rate, the GCD profiles show characteristic charge-discharge curves without distinct voltage plateaus, indicating that the charge storage mechanism was in majority capacitive rather than intercalation-based. As cycling progresses, the curves tend to overlap, suggesting good cyclic stability of the AC, correlated with the overlap of the CV curves. This was superior to materials such as the carbonized pomegranate peel, P-10 as mentioned in Table 2, which shows significantly lower capacity retention (90 $mAh g^{-1}$ after 200 cycles) compared to the stable performance of CBW. The first CV and GCD charge curves are different due to initial electrolyte percolation and electrode formation effects. The discharge capacity for 0.1 $Ag^{-1}$ starts at around 128 $mAh g^{-1}$ stabilized

<table><caption>**Table 2.** Comparative electrochemical performance of biobased carbon materials as AB cathodes.</caption><thead><tr><th>Source</th><th>Synthesis Method</th><th>SSA (m<sup>2</sup>g<sup>-1</sup>)</th><th>Voltage window (V)</th><th>Current Rate (mA g<sup>-1</sup>)</th><th>Initial DC</th><th>CR/ (Cycle)</th></tr></thead><tbody><tr><td>CBW (This work)</td><td>BW was carbonized at 500 °C under N<sub>2</sub> flow. Then it was activated with KOH (1:8). And pyrolyzed at 900 °C under N<sub>2</sub> atmosphere with 10 °C/min temperature rate.</td><td>3029</td><td>0.01–2.2</td><td>100</td><td>120</td><td>115 (300)</td></tr><tr><td>LS–C-1:2<sup>[20]</sup></td><td>Synthesized via a one-step pyrolysis activation process. With a 1:2 ratio (LS: KOH) and pyrolyzed with a heating rate of 10 °C/min for 1 hour and under an inert atmosphere.</td><td>1708</td><td>0.01–2.2</td><td>100</td><td>60</td><td>70 (2500)</td></tr><tr><td>Sucrose<sup>[22]</sup></td><td>Sucrose was carbonized over customized silica template particles and washed with NaOH etching. The carbonization was carried out at 900 °C under H<sub>2</sub>/Ar atmosphere.</td><td>1185</td><td>0.01–2.25</td><td>500</td><td>64</td><td>74 (6700)</td></tr><tr><td>Tar pitch<sup>[22]</sup></td><td>The reaction mixture of tar pitch and KOH (1:4 wt.%) was heated at 850 °C to produce activated carbon (AC). The porosity of AC was further modified by heating at 750 °C under N<sub>2</sub> flow.</td><td>1980</td><td>0.01–2.25</td><td>500</td><td>51</td><td>95 (1000)</td></tr><tr><td>CMK-3<sup>[23]</sup></td><td>–</td><td>1000–2000</td><td>0.5–2.3</td><td>980</td><td>~27</td><td>33 (36,000)</td></tr><tr><td>Birch sawdust<sup>[24]</sup></td><td>Carbonized birch sawdust (400 °C) activated with 4 wt.% of NaOH under Ar at 600–850 °C.</td><td>3300</td><td>0.0–2.5</td><td>283</td><td>82</td><td>69 (40)</td></tr><tr><td>Coconut shell<sup>[21]</sup></td><td>coconut shell chars were activated with 5 wt.% of KOH at 850 °C under an argon atmosphere.</td><td>2686</td><td>0.01–2.2</td><td>1000</td><td>90</td><td>81 (1500)</td></tr><tr><td>Human Hair<sup>[25]</sup></td><td>Hair samples brunt at 300 °C were activated with 2 wt.% of NaOH at 750 °C in Ar flow.</td><td>–</td><td>0.2–2.45</td><td>50</td><td>103</td><td>100 (50)</td></tr><tr><td>P-0<sup>[26]</sup></td><td>Pomegranate peel carbonized at 900 °C under Ar atmosphere. 0 min of ball milling.</td><td>4.24</td><td>0.01–2.4</td><td>200</td><td>100</td><td>80 (200)</td></tr><tr><td>P-5<sup>[26]</sup></td><td>Pomegranate peel carbonized at 900 °C under Ar atmosphere. 5 min of ball milling.</td><td>81.38</td><td>0.01–2.4</td><td>200</td><td>150</td><td>125 (200)</td></tr><tr><td>P-10<sup>[26]</sup></td><td>Pomegranate peel carbonized at 900 °C under Ar atmosphere. 10 min of ball milling.</td><td>53.80</td><td>0.01–2.4</td><td>200</td><td>169</td><td>90 (200)</td></tr><tr><td colspan="7">DC-discharge capacity and CR-capacity retention (mAh g<sup>-1</sup>)</td></tr></tbody></table>
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 4.** (a) CV profiles at 0.1 mV/s, (b) GCD profiles and corresponding (c) cycling behavior at 0.1 Ag⁻¹, (d) rate capability, (e) GCD profiles and corresponding (f) cycling behavior at 1.0 Ag⁻¹ of CBW as a cathode of AB.</figcaption></figure>

after the first few cycles and gradually decreases to around 115 mAh g⁻¹ after 300 cycles as shown in Figure 4c. The slight decrease in capacity can be attributed to structural adjustments of the carbon matrix and possible electrolyte decomposition.[7b,c,20] Despite this, CE remains close to 100% after initial few cycles onwards, demonstrating highly reversible charge-discharge processes and minimal side reactions. The starting CE values exceeded 100% during initial few cycles due to several factors including electrode formation, ide reactions of the impurities to decompose electrolyte, structural changes etc. However, despite these discrepancies, the overall CE stabilizes

after the first few cycles, maintaining high reversibility as evidenced by the near 100% efficiency seen in the later cycles. There are no distinct voltage plateaus found at 1.0 Ag⁻¹ current rate cycling too, as seen in Figure 4e, the curves take time to overlap, but finally do so between the 1000th and 2500th cycles, and capacity continues to increase throughout the cycling. All this was a sign of the good cyclic stability of the activated carbon. The initial discharge capacity was lower than in the 0.1 Ag⁻¹ condition, starting at around 60 mAh g⁻¹ and increasing significantly to around 70 mAh g⁻¹ over the cycles (Figure 4e, f). This increase in capacity at higher cycles could be due
to the progressive activation of deeper pores in the carbon structure, a behavior also observed in other biomass-based carbon materials such as LS-C-1:2,[20] which showed an increasing trend in capacity to 64 mAh g⁻¹ after 6700 cycles (Table 3), reinforcing the idea that the porous structure significantly influences electrochemical performance. The structural readjustments such as expansion could also be a plausible reason behind the increased capacity.[7c,20] The CE value remains consistently high even after 2500 cycles, close to 100%, reaffirming the stability and efficiency of CBW at higher rates. This capacity could further increase to say, 75 mAh g⁻¹ if cycling continues to say, 5000 cycles as seen in the rate pursued after 300 cycles at 0.1 Ag⁻¹ (Figure 4d). The charge and discharge profiles of CBW exhibit continuous slopes in both cases in line with the CV curves. The consistent shape of these curves throughout the low and high current density cycling indicates that the charge storage mechanism remains stable and unchanged and also confirms that the charge storage mechanism was not based on intercalation but rather on surface processes irrespective of the current rate.[7c,20] We can see the same behavior with AC from coconut shells,[21] which retain a capacity of 81 mAh g⁻¹ after 1500 cycles, as indicated in Table 2, although CBW offers a slightly higher retention at lower current rates. For the rate capability, the results in Figure 4f show a progressive decrease in specific capacity as the current density increases from 1–10 Ag⁻¹, which was typical of less efficient ionic diffusion at higher current densities. However, the battery retains significant capacity even at 10 Ag⁻¹, indicating good performance compared to other similar biochars mentioned in Table 2 under these conditions. The recovery of initial capacity after high current density cycles as seen in Figure 4f underlines the superior stability of the electrode matrix that can be explained by the highly disordered amorphous structure of CBW which promotes rapid reversible adsorption of AlCl₄⁻ anions without the need for intercalation as seen in the case of coconut shell derived AC (Table 2).[21] In addition, CE remained close to 100% throughout the multiple current rate testing, demonstrating the high reversibility of charge-discharge proce-

esses. Overall, CBW achieved discharge capacities of 115, 74, 54, 50, 47, 43, and 29 mAh g⁻¹ at current rates of 0.1, 1.0, 2.0, 3.0, 4.0, 5.0, and 10 Ag⁻¹, respectively as seen in Figure 4d and corresponding GCD profiles shown in Figure S3. There was an increase in the capacity noticed when current density decreased from 10.0 Ag⁻¹ to 1.0 Ag⁻¹ that was due to C-rate effect as well as electrode matrix formatting.[7c]

The electrochemical performances of CBW cathode are compared to many reported works of literature (see Table 2). It was emphasized that CBW carbon exhibited a competitive performance compared to the cathodes reported in the literature, delivering high initial discharge of 120 mAh g⁻¹, and 110 mAh g⁻¹ at the end of 300 cycles at 100 mAh g⁻¹ that further slightly increased to 115 mAh g⁻¹ after the rate capability test (Figure 4d). The low current density performance was superior to the similar category material such as LS-C-1:2, human hair, CMK-3, P-0 etc., as mentioned in Table 2. The performance of the cathode at the end of 2500 cycles was 70 mAh g⁻¹, showing to be a yardstick against the other state-of-the-art- carbon cathodes as detailed in Table 2. It was possible to boost the performance further by engineering the porosity of CBW to contain mesopores dominant. In general, carbon materials with mesopores are favorable for AB application.[6b] Further, using equations 4–7,[7b] we obtain an energy density of 126 Wh kg⁻¹ and a power density of 109 W kg⁻¹ at 0.1 Ag⁻¹, and an energy density of 82 Wh kg⁻¹ and a power density of 1218 W kg⁻¹ at 1.0 Ag⁻¹. So energy density therefore tends to decrease with the current rate, but power density increases, which follows the usual behavior of an electrical storage device on a Ragone plot (Figure 6a). However, the measured energy densities are on par with commercial materials such as graphene but power densities are found far better than the graphene.[7b]

$$ \text{Specific capacity, } C = \frac{l \times \Delta t}{m \times 3600} \qquad (4) $$

$$ \text{Energy density, } Ed = \Delta V \times C \qquad (5) $$

<table><caption>**Table 3.** Comparative electrochemical performance of biobased carbon material electrodes for non-aqueous and aqueous SCs.</caption><thead><tr><th>Material</th><th>BET surface area [m²/g]</th><th>Electrolyte</th><th>Specific capacitance [F g⁻¹]</th><th>Energy density [Wh kg⁻¹]</th><th>Power density [W kg⁻¹]</th></tr></thead><tbody><tr><td>CBW (This work)</td><td>3029</td><td>[BMIM]BF₄</td><td>80 (2 Ag⁻¹)</td><td>74</td><td>2379</td></tr><tr><td>Soybean oil<sup>[32]</sup></td><td>685</td><td>0.5 M NaPF₆</td><td>71 (~4 Ag⁻¹)</td><td>2.46</td><td>158</td></tr><tr><td>SMS<sup>[6a]</sup></td><td>2913</td><td>TEA-BF₄</td><td>87 (1 Ag⁻¹)</td><td>2.04</td><td>6932</td></tr><tr><td>BW-AC<sup>[6a]</sup></td><td>3160</td><td>TEA-BF₄</td><td>105 (1 Ag⁻¹)</td><td>12.7</td><td>9164</td></tr><tr><td>Nanocellulose<sup>[33]</sup></td><td>1510</td><td>BMPYTFSI</td><td>65.2 (1 Ag⁻¹)</td><td>21</td><td>0</td></tr><tr><td>CL-700<sup>[34]</sup></td><td>1500</td><td>[EMIM]BF₄</td><td>92 (0.05 Ag⁻¹)</td><td>29</td><td>771</td></tr><tr><td>CC-AA-600<sup>[35]</sup></td><td>800</td><td>EMIM borate</td><td>80 (0.1 Ag⁻¹)</td><td>25</td><td>–</td></tr><tr><td>CC-AA-600<sup>[35]</sup></td><td>800</td><td>BMIM phosphate</td><td>45 (0.1 Ag⁻¹)</td><td>25</td><td>–</td></tr><tr><td>CC-AA-600<sup>[35]</sup></td><td>800</td><td>EMIM-TFSA</td><td>35 (0.1 Ag⁻¹)</td><td>25</td><td>–</td></tr><tr><td>AHPCS<sup>[36]</sup></td><td>1650,7</td><td>[EMIM]BF₄</td><td>64.4 (0.5 Ag⁻¹)</td><td>80.5</td><td>750</td></tr><tr><td>ANT–K–ACT<sup>[37]</sup></td><td>2934,60</td><td>[EMIM]BF₄</td><td>42 (1 Ag⁻¹)</td><td>72.51</td><td>1312</td></tr><tr><td>ANT–K–ACT<sup>[37]</sup></td><td>2934,60</td><td>1 M Et₄NBF₄/ACN</td><td>38 (1 Ag⁻¹)</td><td>33.32</td><td>1250</td></tr><tr><td>AC<sup>[12b]</sup></td><td>1505</td><td>6 M KOH</td><td>263 (0.5 Ag⁻¹)</td><td>37.5</td><td>989</td></tr></tbody></table>
<figure><img src="image_15.png" /></figure>

$$ \text{Voltage, } \Delta V = \frac{V_{max} + V_{min}}{2} \qquad (6) $$

$$ \text{Power density, } P_d = \frac{Ed \times 3600}{\Delta t} \qquad (7) $$

Where m was active mass in g, Δt was step time in s, ΔV was the voltage window in V and specific capacity C in Ah g⁻¹ which gives energy density E_d and power density P_d in Wh kg⁻¹ and W kg⁻¹, respectively.

### 2.2.2. SCs

The performance of the CBW electrodes in symmetric SCs was evaluated by CV at multiple scan rates, GCD, and rate capability tests. This analysis focuses on understanding electrochemical behavior, capacitance retention, and CE over several thousand cycles at different current densities. The CV curves (Figure 5a) have a quasi-rectangular shape, typical of EDLC. This indicates that the energy storage mechanism takes place mainly by electrostatic adsorption/desorption of ions on the material

surface, rather than by faradaic reactions. The absence of pronounced redox peaks confirms that the charge storage process was characterized by non-faradic mechanisms. The anodic and cathodic curves are relatively symmetrical concerning the potential axis, which indicates good reversibility of the charge/discharge process. This was comparable to the behavior observed for other similar classes of reported materials as mentioned in Table 3. The observed reversibility was confirmed by the GCD charge-discharge profiles (Figure 5b), which show an almost symmetrical triangular shape concerning the time axis, indicating high electrochemical storage stability and efficiency. The compliance between the CV and GCD curves shows that the charge storage mechanism remains constant over several thousand cycles, which was essential for the cyclic stability and durability of the SC. Due to limited ion diffusion at high scan speeds,[17c] the curves become less rectangular yet possess a high extent of symmetry between the cathodic and anodic scans, a promising sign of good reversibility. However, the material exhibits good rectangular shapes at lower scan rates (10, 50, and 100 mV s⁻¹) as seen in Figure 5a, indicating that the material has good access to ions over a wide range of potentials, like CL-700, which shows a comparable capacitance at low current densities (Table 3). The triangular shape charge-

<figure><img src="image_16.png" /><figcaption>Figure 5. (a) CV profiles at multiple scan rates, (b) GCD profiles and corresponding (c) long-term cycling at 2 Ag⁻¹, and (d) rate capability of CBW electrodes in symmetric SCs.</figcaption></figure>
<figure><img src="image_17.png" /></figure>

discharge profiles recorded with 2 Ag⁻¹ current density (Figure 5b), further indicate good capacitive behavior with relatively symmetrical charging and discharging processes in good agreement with the CV profiles (Figure 5a). This symmetry suggests efficient electrochemical reversibility and the stability of CBW electrodes. The first three cycles (Figure 5b) show a steep slope, implying higher resistance to EDLC and charge transfer, perhaps due to initial electrode conditioning.[27] As the cycling progresses (up to 500 cycles), the slope of the discharge curve decreases slightly, which indicates an improvement in conductivity and a reduction in the resistance,[27] similar to the performance improvements seen in materials like spent mushroom substrate (SMS) (Table 3). Finally, between 2000 and 5000 cycles, the GCD curves shift slightly towards higher times, showing minor changes in the potential profile. This indicates good cycle stability with minimal changes in capacitance over extended cycles, demonstrating the durability of the electrode material. The capacitance change between 500–1500 cycles as seen in Figure 5c was plausibly due to impurities such as residual ash contents, or can be correlated with the oxygen functional groups of CBW.[28] From the XPS and FTIR inferences, it was clear that CBW contains a large number of oxygen-based functional groups, such as –OH, –COOH, –CO₃ etc. on its surface. These groups can cause side reactions or redox reactions during cycling, potentially creating covalent by-products that are electrochemically inactive and end up blocking the active sites of the material. The anionic part of the electrolyte i.e. BF₄⁻ similar to AlCl₄⁻ anions plausibly undergo side reactions with the impurities as demonstrated in the recent reports.[7c,29] Over time, this would explain why discharge capacity starts to drop around the thousandth cycle, even if charge capacity remains more or less stable. In addition, the capacitance values for the charging and discharging processes in Figure 5c, remain relatively stable at 80 Fg⁻¹ from start to finish, with charging capacitance slightly higher than discharging capacitance, which compares favorably with other materials like AHPCS, which shows a slightly lower capacitance of 64.4 Fg⁻¹ at 0.5 Ag⁻¹ (Table 3). This slight difference can be attributed to the change in the internal resistance.[27] The CE remains consistently high (above 99% except for 500–1500 cycles) throughout the cycling, indicating minimal charge loss and excellent reversibility of charge-discharge processes. This suggests that these electrodes are well suited to long-term applications in SCs. The electrode shows good capacitance even at high current rates as seen in Figure 5d, but particularly maximum at 0.5 Ag⁻¹ and 0.1 Ag⁻¹. This indicates efficient charge storage and distribution, even under high-power conditions. Specific capacitance decreases at higher current densities, with values falling from 142 Fg⁻¹ at 0.1 Ag⁻¹ to 50 Fg⁻¹ at 10 Ag⁻¹, we can see the same behavior in Figure S4 in which step time Δt decreases with increasing the current density. Overall, CBW exhibited capacitances of 143, 94, 87, 79, 74, 69, 65, and 51 Fg⁻¹ at current rates of 0.1, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 10.0 Ag⁻¹, respectively. There are some excellent performances at low currents where the electrode can easily store the charge carrier, but they tend to decrease with increasing current density, which was typical for supercapacitor

materials, where high current densities reduce the time available for ion transport in the material pores, thus decreasing charge storage efficiency.

Table 3 shows the performances of several biomass-derived doped carbons, including the CBW sample. The table shows the highly competitive performance of our CBW SCs compared to the others as furnished in Table 3, showing higher than many devices reported worldwide. For instance, our SCs exhibited a very high energy density of 74.7 Whkg⁻¹ at a power density of 2379 Whkg⁻¹, and a specific capacitance of 80 Fg⁻¹ at 2.0 Ag⁻¹. The SC rate capability values of CBW outperform some of the other similar category materials, such as Nanocellulose, CC-AA-600, AHPCS, and ANT–K-ACT, among others, which show lower capacitance at similar current densities as mentioned in Table 3. However, the CBW performs slightly less compared to activated carbon derived from birch wood (BW-AC), which achieves a higher specific capacitance of 105 Fg⁻¹ at 1 Ag⁻¹ but the corresponding energy density was very low compared to CBW. CBW still demonstrates competitive performance in terms of energy and power densities, especially when compared to other materials listed in Table 3, such as CC-AA-600 and Nanocellulose. Thus, we can confidently state that our sustainable CBW electrode and its respective supercapacitors’ performance, make it a promising candidate not only for AB cathode but also an excellent electrode for efficient SCs. The SC performance of CBW further improved by containing its porosity within the micropores range only.[6b] Incorporating transition metals or their oxides and secondary source of carbon into CBW could also improve the performance further.[30] Energy density and power density for CBW SC are calculated with equations 8–11,[12b,30a,b] respectively and results are plotted in Figure 6.[31]

$$ \text{Specific Capacitance, } C_s = \frac{l \times \Delta t}{m \times \Delta V} \qquad (8) $$

$$ \text{Voltage, } \Delta V = V_{max} - V_{min} \qquad (9) $$

$$ \text{Energy density, } E_d = \frac{C_s \times \Delta V^2}{2 \times 3600} \qquad (10) $$

$$ \text{Power density, } P_d = \frac{E_d \times 3600}{\Delta t} \qquad (11) $$

Where m was active mass in g, Δt was step time in s, ΔV was the voltage window (2.5 V) and C_s was the specific capacitance in Fg⁻¹ which gives an energy density E_d in Whkg⁻¹ and power density P_d in Wkg⁻¹.

### 2.2.3. Ragone Plots

The Ragone plot of the supercapacitor shows a typical relationship where the energy density progressively decreases as the power density increases. Initially, at relatively low power densities, the supercapacitor reaches a maximum energy density of around 125 Whkg⁻¹. However, as the power density increases, the energy density decreases, reaching more modest
<figure><img src="image_18.png" /></figure>

values. This behavior was characteristic of SCs, which are capable of delivering high power but with lower energy density than conventional batteries. CBW bagged energy densities 125, 82, 76, 69, 65, 61, 57, 45 Wh kg⁻¹ correspond to power densities of 124, 622, 1249, 2516, 3800, 4947, 6428 and 14813 W kg⁻¹ at current densities 0.1, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 and 10.0 Ag⁻¹ respectively, as seen in Figure 6a. On the other hand, the Ragone plot for the AB also shows a decrease in energy density with increasing power density. However, the initial energy density was slightly lower, but the AB can maintain a higher energy density at higher power densities. The maximum energy density observed was around 100 Wh kg⁻¹, while power density reaches values above 10,000 W kg⁻¹. This behavior reflects the ability of ABs to deliver competitive energy density while maintaining acceptable performance at higher power densities, owing to fast redox reactions involving AlCl₄⁻ complexes. Overall, CBW as a cathode in ABs delivered energy densities 127, 82, 56, 53, 51, 46, and 31 Wh kg⁻¹ those corresponding to power densities of 109, 1218, 2125, 3277, 4337, 5264 and

10080 W kg⁻¹ at current densities 0.1, 1.0, 2.0, 3.0, 4.0, 5.0 and 10.0 Ag⁻¹ respectively, as seen in Figure 6b.

### 2.2.4. EIS Results

The EIS results of CBW for AB and SC interpreted as Nyquist diagrams and corresponding equivalent circuits (ECs) are presented in Figure 7. For AB, the Nyquist diagrams at various charge and discharge conditions (Figure 7a, b) are best fitted to EC shown in the inset of Figure 7b. The values of EC components shown in Table 4 reveal that resistance to charge transfer (R<sub>ct</sub>) continuously decreased with the cycling. The constant phase element (CPE<sub>dl</sub>) values indicate minor changes with an increasing trend as cycling progresses. The increasing CPE<sub>dl</sub> which was equivalent to EDLC, and decreasing R<sub>ct</sub> together reflect the increase in the capacity seen in GCD profiles and rate capability test. Here in the AB case, it was worth noticing that bulk resistance (R<sub>b</sub>) was several hundred

<figure><img src="image_19.png" /><figcaption>Figure 6. Ragone plots of CBW in (a) SC and (b) AB.</figcaption></figure>

<table><caption>Table 4. Pre and post-cycling EIS data of CBW electrodes of AB and SC in various charge-discharged conditions.</caption><thead><tr><th></th><th>Re (Ω)</th><th>Rct (Ω)</th><th>Rb (Ω)</th><th>CPEdl (F)</th></tr></thead><tbody><tr><td><strong>AB</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>OCV</td><td>1.67</td><td>6.76</td><td>905.3</td><td>0.42</td></tr><tr><td>1C@2.20 V</td><td>2.67</td><td>9.75</td><td>241.1</td><td>0.26</td></tr><tr><td>1D@0.01 V</td><td>1.72</td><td>3.78</td><td>131.6</td><td>0.34</td></tr><tr><td>2501C@2.20 V</td><td>6.56</td><td>3.17</td><td>188.5</td><td>0.46</td></tr><tr><td>2501D@0.01 V</td><td>7.19</td><td>2.03</td><td>111.7</td><td>0.47</td></tr><tr><td><strong>SC</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>OCV</td><td>1.12</td><td>3.14</td><td>-</td><td>0.69</td></tr><tr><td>1C@2.5 V</td><td>2.39</td><td>2.42</td><td>-</td><td>0.43</td></tr><tr><td>1D@0.0 V</td><td>1.80</td><td>2.84</td><td>-</td><td>0.42</td></tr><tr><td>5001C@2.5 V</td><td>3.93</td><td>2.70</td><td>-</td><td>0.41</td></tr><tr><td>5001D@0.0 V</td><td>4.01</td><td>2.33</td><td>-</td><td>0.40</td></tr></tbody></table>
<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 7.** Nyquist diagrams and ECs of CBW in (a–b) AB, and (c–d) SC at different charge-discharged conditions before and after long-term cycling (hollow symbols correspond to fitted data).</figcaption></figure>

times the R<sub>ct</sub> that classifies CBW's behavior closest toward an ideal conductor.<sup>[7c,38]</sup> Further, R<sub>ct</sub> ≪ R<sub>b</sub> suggests the redox reactions occurring at very fast rates.<sup>[7c,38]</sup> At the end of the cycling R<sub>ct</sub> was found feeble compared to R<sub>b</sub> which was a condition of negligible resistance to charge transfer by CBW, and the charge transfer whatever resistance experienced was mainly from electronic conductivity of the substrate and ionic conductivity of the electrolyte present in the electrode matrix.<sup>[7c,38]</sup> Because of these improvements in charge transfer kinetics and fast faradaic redox reactions, CBW as a cathode of AB delivered very high energy and power densities seen in Figure 6b. The values of R<sub>b</sub>, decreased continuously with the cycling prolonged indicates structural optimization of the electrode material over time which was also a compound reason behind the capacity increase noticed.

On the other hand, for the SC, Nyquist plots of the first cycle fitted to simple EC shown in Figure 7c whereas they fitted to complex EC shown in Figure 7d at the end of the long-term cycling. In this case, also, CBW shows a reduction in R<sub>ct</sub> from 3.14 Ω at open circuit voltage (OCV) to 2.42 Ω at first charge (1 C) to 2.5 V, with a slight increase after discharging (1D) to

0.0 V. Interestingly, after 5000 cycles, R<sub>ct</sub> decreased slightly to 2.33 Ω, underlining the excellent stability and durability of the supercapacitor electrodes under prolonged cycling. The slight variations in CPE<sub>dl</sub> values reflect the stable capacitance of the supercapacitor's double layer, indicating that the electrode's surface properties remain largely unchanged after prolonged cycling. The evolution of resistance due to surface film formation, R<sub>sf</sub> (~0.5–1.0 Ω) after the long-term cycling indicates that the electrode matrix has undergone permanent structural and chemical changes. However, the similar initial and final capacitance values in conjunction with similar and stable CPE<sub>dl</sub> and R<sub>ct</sub> values suggest that the effect of the R<sub>sf</sub> associated changes seems intermediatory, say related to 500–1500 cycles only in which the capacitance dramatically changed. Overall, the EIS results suggest that CBW as an electrode of ABs and SCs exhibit excellent long-term stability and efficient charge transfer kinetics, with minimal degradation of their electrochemical properties after thousands of charge-discharge cycles.
<figure><img src="image_22.png" /></figure>

## 3. Conclusions

In conclusion, this study demonstrates the potential of activated carbon derived from birch wood as an effective electrode material for aluminum batteries and supercapacitors. Results indicate that CBW, with its high specific surface area (3029 m²g⁻¹) and essentially microporous structure, offers several advantages over conventional carbonaceous materials, including improved capacity retention, high Coulombic efficiency, and excellent cycling stability. Electrochemical characterization shows that CBW offers competitive performance in both ABs and SCs. Specifically, as a cathode material for ABs, CBW achieved discharge capacities of 110 mAhg⁻¹ at 0.1 Ag⁻¹, retaining over 70 mAhg⁻¹ after 2500 cycles at 1.0 Ag⁻¹, which was superior to other its class of biomass-derived carbons and comparable to the latest commercial materials. Overall, this work highlights the potential of CBW as a sustainable, high-performance material for use in advanced energy storage devices. The results suggest that biomass-derived carbons such as CBW could be used as a viable alternative to the more expensive and less durable materials currently used in industry. Future research could focus on optimizing the physicochemical properties of CBW to maximize its performance in different energy storage configurations, thus contributing to the development of cost-effective, environmentally friendly, and highly efficient energy storage solutions.

## Acknowledgements

This research was funded by Bio4Energy - a Strategic Research Environment appointed by the Swedish government and the Swedish University of Agricultural Sciences. The authors are thankful to EU/Interreg Aurora (Project GreenBattery, grant No. 20357574) and KEMPE Foundations. Dr. Grimm acknowledges financial support from Interreg Aurora (Project Nature Refines, grant No. 20361711) and the Swedish Research Council Formas (grant No. 2021-00877). Dr. dos Reis gratefully acknowledges financial support from the Research Council of Finland (Academy Research Fellows 2024, Project: Bio-Adsorb&Energy, grant no. 361583.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

Research data are not shared.

**Keywords:** Birch wood · Activated carbon · Biomass carbon electrodes · Aluminum battery · Supercapacitor

[1] a) L. Zhou, H. Cao, S. Zhu, L. Hou, C. Yuan, *Green Chem.* **2015**, *17*, 2373-2382; b) J. Zhang, D. Wu, Q. Zhang, A. Zhang, J. Sun, L. Hou, C. Yuan, *J. Mater. Chem. A* **2022**, *10*, 2932-2944; c) L. Hou, Z. Chen, Z. Zhao, X. Sun, J. Zhang, C. Yuan, *ACS Appl. Energy Mater.* **2019**, *2*, 548-557; d) C. Hernández-Rentero, V. Marangon, M. Olivares-Marín, V. Gómez-Serrano, Á. Caballero, J. Morales, J. Hassoun, *J. Colloid Interface Sci.* **2020**, *573*, 396-408.

[2] a) D. Henghan, Z. Ruicong, Z. Zhao, Z. Jinyuan, S. Gengzhi, *Energy Mater.* **2022**, *2*, 200040; b) H. Liu, X. Liu, S. Wang, H.-K. Liu, L. Li, *Energy Storage Mater.* **2020**, *28*, 122-145.

[3] A. Rabiei Baboukani, I. Khakpour, V. Drozd, A. Allagui, C. Wang, *J. Mater. Chem. A* **2019**, *7*, 25548-25556.

[4] R. M. Lima, G. S. dos Reis, U. Lassi, E. C. Lima, G. L. Dotto, H. P. de Oliveira, *C. Nanosyst.* **2023**, *9*, 59.

[5] a) H. Liu, X. Cai, X. Zhi, S. Di, B. Zhai, H. Li, S. Wang, L. Li, *Nano-Micro Lett.* **2022**, *15*, 24; b) Y. Wang, H. Li, B. Zhai, X. Li, P. Niu, J. Odent, S. Wang, L. Li, *ACS Nano* **2024**, *18*, 3456-3467.

[6] a) N. Boulanger, A. V. Talyzin, S. Xiong, M. Hultberg, A. Grimm, *Colloid Surface A: Physicochem. Eng. Asp.* **2024**, *680*, 132684; b) G. S. Reis, S. Petnikota, C. M. Subramaniyam, H. P. de Oliveira, S. Larsson, M. Thyrel, U. Lassi, F. García Alvarado, *Nanomaterials* **2023**, *13*, 765.

[7] a) S. Petnikota, R. Chua, K. M. Boopathi, R. Satish, F. Bonaccorso, V. Pellegrini, M. Srinivasan, *J. Electrochem. Soc.* **2020**, *167*, 100514; b) S. Petnikota, D. Koch, M. Imran, J. Buha, J. K. Panda, M. Akbari Garakani, L. Marasco, A. Gamucci, F. Bonaccorso, V. Pellegrini, *Sustain. Energy Fuels* **2022**, *6*, 4311-4321; c) S. Petnikota, G. Simões Dos Reis, F. A. Kayakool, V. S. S. S. Vadali, J. Välikangas, U. Lassi, M. Thyrel, *ACS Appl. Energy Mater.* **2024**, *7*, 6862-6872.

[8] a) Y. Uemura, C.-Y. Chen, Y. Hashimoto, T. Tsuda, H. Matsumoto, S. Kuwabata, *ACS Appl. Energy Mater.* **2018**, *1*, 2269-2274; b) K. Zhang, K. O. Kirlikovali, J. M. Suh, J.-W. Choi, H. W. Jang, R. S. Varma, O. K. Farha, M. Shokouhimehr, *ACS Appl. Energy Mater.* **2020**, *3*, 6019-6035.

[9] S. Petnikota, G. Dos Reis, F. A. L. I. Kayakool, V. V. S. S. Srikanth, J. Välikangas, U. Lassi, M. Thyrel, *ACS Appl. Energy Mater.* **2024**, *7*(16), 6862-6872.

[10] S. Saini, P. Chand, A. Joshi, *J. Energy Storage* **2021**, *39*, 102646.

[11] J. Hynynen, P. Niemistö, A. Viherä-Aarnio, A. Brunner, S. Hein, P. Velling, *For.: An Int. J. For. Res.* **2010**, *83*, 103-119.

[12] a) Z. Wang, S. Yun, X. Wang, C. Wang, Y. Si, Y. Zhang, H. Xu, *Ceram. Int.* **2019**, *45*, 4208-4218; b) X. Wang, S. Yun, W. Fang, C. Zhang, X. Liang, Z. Lei, Z. Liu, *ACS Sustain. Chem. Eng.* **2018**, *6*, 11397-11407.

[13] S. Petnikota, S. V. S. S. Vadali, J. J. Toh, M. Srinivasan, C. B. V. R. S. Adams, M. V. V. Reddy, *New J. Chem.* **2019**, *43*, 9793-9801.

[14] G. Simões dos Reis, C. Mayandi Subramaniyam, A. D. Cárdenas, S. H. Larsson, M. Thyrel, U. Lassi, F. García-Alvarado, *ACS Omega* **2022**, *7*, 42570-42581.

[15] E. Laisné, J. Thivet, G. Manavalan, S. Petnikota, J.-P. Mikkola, M. Thyrel, T. Hu, E. C. Lima, M. Naushad, U. Lassi, G. S. dos Reis, *Colloids Surf. A* **2024**, *692*, 133899.

[16] Z. Zhao, H. Chen, W. Zhang, S. Yi, H. Chen, Z. Su, B. Niu, Y. Zhang, D. Long, *Mater. Adv.* **2023**, *4*, 835-867.

[17] a) S. Petnikota, H. Maseed, V. V. S. S. Srikanth, M. V. Reddy, S. Adams, M. Srinivasan, B. V. R. Chowdari, *J. Phys. Chem. C* **2017**, *121*, 3778-3789; b) S. Petnikota, J. J. Toh, J. Y. Li, R. Chua, M. Srinivasan, *ChemElectroChem* **2019**, *6*, 493-503; c) S. Petnikota, V. V. S. S. Srikanth, J. J. Toh, M. Srinivasan, C. V. R. Bobba, S. Adams, M. V. Reddy, *New J. Chem.* **2019**, *43*, 9793-9801.

[18] H. Cao, X. Peng, M. Zhao, P. Liu, B. Xu, J. Guo, *RSC Adv.* **2018**, *8*, 2858-2865.

[19] B. N. Kuznetsov, N. V. Chesnokov, S. I. Tsyganova, N. M. Mikova, I. P. Ivanov, N. M. Ivanchenko, *Solid Fuel Chem.* **2016**, *50*, 23-30.

[20] F. A. Kayakool, H. Pant, P. Menestreau, G. Dos Reis, G. Manavalan, V. V. S. S. Srikanth, M. Thyrel, S. Petnikota, *Carbon Resour. Convers.* **2025**, *8*, 100301.

[21] P. Thanwisai, N. Chaiyapo, P. Phuenhinlad, Y. Kanaphan, J. Nash, C. Chotsuwan, A. Klamchuen, Y. Wang, T. Nann, N. Meethong, *Carbon* **2022**, *191*, 195-204.

[22] C. Li, P. C. Rath, S.-X. Lu, J. Patra, C.-Y. Su, D. Bresser, S. Passerini, J.-K. Chang, *Chem. Eng. J.* **2021**, *417*, 129131.

[23] Z. A. Zafar, S. Imtiaz, R. Li, J. Zhang, R. Razaq, Y. Xin, Q. Li, Z. Zhang, Y. Huang, *Solid State Ion.* **2018**, *320*, 70-75.

[24] E. I. Shkolnikov, E. M. Novaev, S. A. Kochanova, A. S. Ilyukhin, D. E. Vervikishko, T. F. Yudina, I. V. Bratkov, L. M. Glukhov, *Mater. Today: Proc.* **2018**, *5*, 26073-26077.

[25] S. Divya, T. Nann, *ChemElectroChem* **2021**, *8*, 492-499.
<figure><img src="image_23.png" /></figure>

[26] B. Wang, Z. Zhang, F. Yuan, D. Zhang, H. Sun, H. Wang, J. Wang, Q. Wang, Z. Li, *J. Phys. Chem. Solids* **2023**, *179*, 111394.

[27] L. E. Helseth, *J. Energy Storage* **2022**, *55*, 105440.

[28] W. Yang, Y. Li, Y. Feng, *Materials* **2018**, *11*, 2455.

[29] T. Romann, O. Oll, P. Pikma, H. Tamme, E. Lust, *Electrochim. Acta* **2014**, *125*, 183–190.

[30] a) Z. Gao, S. Yun, C. Yang, Y. Zhang, J. Dang, G. Yang, T. Yang, D. Qiao, K. Wang, *J. Colloid Interface Sci.* **2023**, *639*, 33–48; b) C. Yang, S. Yun, J. Shi, M. Sun, N. Zafar, A. Arshad, Y. Zhang, L. Zhang, *Chem. Eng. J.* **2021**, *419*, 129636; c) Y. Abbas, S. Yun, M. S. Javed, J. Chen, M. F. Tahir, Z. Wang, C. Yang, A. Arshad, S. Hussain, *Ceram. Int.* **2020**, *46*, 4470–4476.

[31] M. Yaseen, M. A. Khattak, M. Humayun, M. Usman, S. S. Shah, S. Bibi, B. S. Hasnain, S. M. Ahmad, A. Khan, N. Shah, A. A. Tahir, H. Ullah, *Energies* **2021**, *14*, 7779.

[32] R. Kumari, V. Singh, C. Ravi Kant, *Mater. Chem. Phys.* **2023**, *305*, 127882.

[33] Z. Li, J. Liu, K. Jiang, T. Thundat, *Nano Energy* **2016**, *25*, 161–169.

[34] M. Karnan, A. G. K. Raj, K. Subramani, S. Santhoshkumar, M. Sathish, *Sustain. Energy Fuels* **2020**, *4*, 3029–3041.

[35] M. Karnan, K. Subramani, P. K. Srividhya, M. Sathish, *Electrochim. Acta* **2017**, *228*, 586–596.

[36] J. Yang, J. Liu, Y. Chen, B. Chang, X. Zhang, X. Wang, *ACS Appl. Energy Mater.* **2021**, *4*, 13766–13775.

[37] O. Boujibar, F. Ghamouss, A. Ghosh, O. Achak, T. Chafik, *J. Power Sources* **2019**, *436*, 226882.

[38] S. Petnikota, S. K. Marka, A. Banerjee, M. V. Reddy, V. V. S. S. Srikanth, B. V. R. Chowdari, *J. Power Sources* **2015**, *293*, 253–263.

Manuscript received: September 24, 2024

Revised manuscript received: November 22, 2024

Version of record online: ■■, ■■
RESEARCH ARTICLE

<figure><img src="image_24.png" /></figure>

This work investigates activated carbon derived from birch wood as a sustainable material for electrodes in aluminum batteries (ABs) and supercapacitors (SCs). With a specific surface area of 3029 m²g⁻¹ and a microporous structure, the material

shows excellent electrochemical performance, delivering competitive capacities and stability, positioning biomass-derived carbon as a promising solution for future energy storage technologies.

M. Paul, A. Grimm, G. Simões Dos Reis, G. Manavalan, S. E S, M. Thyrel, S. Petnikota*

1 – 16

**Activated Carbon from Birch Wood as an Electrode Material for Aluminum Batteries and Supercapacitors**

<figure><img src="image_25.png" /></figure>