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. 2026 Jul 7;16(36):38455–38467. doi: 10.1039/d6ra04144b

Waste leather-derived pumice-like N, S-codoped porous carbon for high-performance supercapacitors

Mohy Menul Islam a,b, Ahammad Musa a,c, Md Mominul Islam a,✉
PMCID: PMC13339986  PMID: 42416730

Abstract

Activated carbon (AC) is a viable material showing extensive potential in several economic sectors including energy applications. We report a facile method for converting waste leather into pumice-like hierarchically porous AC for supercapacitor applications. This study focuses on a novel synthetic strategy for preparing N, S-codoped hierarchical porous AC (A-NSC) with pumice-like morphology from waste leather (WL), which inherently contains, in addition to C, H, and O, elemental N and S and ca. 35% trapped water stabilizing the folded structure of collagen. A two-step carbonization strategy was followed: (i) char was prepared from WL at low pyrolysis temperature, and (ii) it was then pyrolyzed at different elevated temperatures by mixing with excess KOH in an N2 environment. The hydrophilic surface of N, S-codoped carbon with a high specific surface area (2215 m2 g−1) developed at 800 °C exhibits exceptional electrical charge storage of 1158 F g−1, along with an energy density of 40 W h kg−1 in aqueous KOH solution. The developed A-NSCs offered electric double layer-type storage with a negligible decay in consecutive charging–discharging tested for 10 000 cycles. This study opens a new insight into the formation of highly robust, valuable carbon from WL.

Activated carbon (AC) is a viable material showing extensive potential in several economic sectors including energy applications.graphic file with name d6ra04144b-ga.jpg

1. Introduction

The increasing energy demand and wider energy consumption spectrum have compelled researchers worldwide to devote tremendous efforts to search for alternative energy sources and storage protocols such as fuel cells, lithium-ion batteries, conventional capacitors, and supercapacitors.1–3 Supercapacitors, also known as electrochemical capacitors, have outperformed other energy storage devices due to their superior electrochemical characteristics, high specific power, rapid charging/discharging rate, and excellent long-term cycling stability. Supercapacitors truly fill the gap between batteries and conventional capacitors. These facts make them important devices for wider practical application in hybrid vehicles, portable electronic devices, and large-scale energy production.4,5

The two most important concerns of supercapacitors are to be highlighted, such as energy density and stability against repeated charging–discharging cycles, i.e., the retention of initial capacitance for a long-term application.6 The energy density limitation of electrochemical double-layer capacitors (EDLCs), which store electrical charge through non-faradaic processes at the electrolyte/electrode interface, can be addressed by increasing the specific surface area (SSA) of electrode materials.3,7 On the other hand, a large population of functional groups capable of undergoing faradaic reactions is required for pseudocapacitors, which store charge through reversible redox reactions.8 Other technological concerns include the materials being cost-effective, non-toxic, scalable, environmentally-friendly, and recyclable. In general, inorganic, organic, oxides, complexes, polymers, structured compounds, composites, etc., varieties of materials have been attempted in designing high-performance supercapacitors.9,10 Carbonaceous materials, such as activated carbon (AC),11,12 graphene,5,13 carbon nanotubes (CNTs),14 and carbon aerogel15 electrode materials, are of prime choice in devising EDLCs because of their affordability, eco-friendliness, adjustable pore structures, excellent conductivity, and strong chemical stability, among other attributes.4,5,16

In recent times, about 80% of supercapacitor devices are assembled with AC as the electrode material, because it can be developed with an adjustable porous structure, chemical stability, and excellent electrical conductivity.9,17 CNTs and graphene demonstrate comparable or even superior electrochemical performance compared to AC. However, their broader application is limited by the complex preparation process, resulting in high costs and constraints on mass production. In addition, CNTs and graphene nanosheets tend to restack because of weak van der Waals forces and π–π interactions between them, which reduces the accessibility of active sites and consequently decreases their stability in charge-storage applications.12,14 Hu et al. designed highly stable 3D carbon superstructures to prevent restacking, in which aromatic π–π stacking interactions and intermolecular hydrogen bonding cause the organic motifs to assemble into spherical nanoflower-like superstructures. These properly developed carbon superstructures can provide a robust framework that effectively prevents structural collapse and stacking. Consequently, the zinc-ion hybrid capacitors based on carbon superstructures offer an ultra-long life of 30 000 cycles at 20 A g−1, a high energy density of 152 Wh kg−1, and a specific capacitance (Csp) of 245 mA h g−1 at 0.5 A g−1.4,18

In this context, it is crucial to develop affordable, robust carbon materials derived from biowaste, which are plentiful, sustainable, environmentally beneficial, and an excellent source of carbon. Some selective instances utilizing biomass to prepare AC for supercapacitor devices are typically highlighted in Table 1. Biomass precursors reported in the literature include tea waste,19 corncobs,20 rice husk,21 sugarcane bagasse,22 onion husk,23 olive pits,24 banana peel,25 pinecones,26 watermelon rind,27 peanut shell,28 walnut shell,29 almond shell,30 coconut shell,31 neem and Ashoka leaves,32 among others. Peng et al. synthesized AC from tea leaves using KOH activation. The thus-derived AC is well-performed as a supercapacitor material with a Csp of 330 F g−1 in a 2.0 M KOH solution with a capacitance retention of 92% studied for 2000 cycles.33 Wu et al. observed a cycle stability of 97% after 5000 cycles, with a Csp of 406 F g−1 at 0.5 A g−1 in 6.0 M KOH.34

Table 1. Charge storage performances of carbons derived from various biomasses.

Materials Operation conditions C sp (F g−1) Cycle stability@ current density Ref.
Precursor Activating agents Preparation method SSA (m2 g−1) Electrolyte Potential window (V)
Tea leaves KOH Carbonization and activation 2841 2.0 M KOH −0.1 to −1.1 330 98% @ 1.0 A g−1 after 2000 cycles 33
Albizia procera leaves NaHCO3 Pyrolysis and activation 910 1.0 M H2SO4 0 to 1.0 231 97.3% @ 1.0 A g−1 after 1000 cycles 50
Jujube fruit NaOH Pre-carbonization and activation 1135 6.0 M KOH −0.4 to 1.4 460 92.2% @ 1.0 A g−1 after 130 000 cycles 51
Rice husk NaOH Pre-carbonization and activation 2376 0.5M K2SO4 −1.1 to 0 198.4 99% @ 1.0 A g−1 after 1000 cycles 52
Sugarcane bagasse KOH Pyrolysis and activation 1939 1.0 M H2SO4 −0.4 to 0.6 298 94.5% @ 1.0 A g−1 after 50 000 cycles 53
Onion husk K2CO3 Carbonization and activation 2571 188 92.5% @ 1.0 A g−1 after 2000 cycles 54
Albizia flowers KOH Pyrolysis and activation 2758 6.0 M KOH −1.0 to 0 406 98% @ 0.5 A g−1 after 5000 cycles 34
Mangosteen peels NaOH Carbonization and activation 2623 6.0 M KOH 0 to 1.4 357 94.5% @ 1.0 A g−1 after 130 000 cycles 55
Rice straw KHCO3 Hydrothermal and activation 2188 6.0 M KOH −1.0 to 0 317 93.2% @ 1.0 A g−1 after 5000 cycles 7
Corncob KOH Carbonization and activation 800 1.0 M H2SO4 −0.4 to 0.8 390 94% @ 0.5 A g−1 after 5000 cycles 56
Soybean pods NaOH Carbonization and activation 2612 1.0 M Na2SO4 0 to 1.7 352.6 94.2% @ 0.5 A g−1 after 50 000 cycles 57
Tea waste buds KOH Pre-carbonization and activation 1610 6.0 M KOH −0.95 to 0.05 332 97.8% @ 1.0 A g−1 after 100 000 cycles 58
Watermelon rind KOH Hydrothermal and activation 2277 6.0 M KOH −1.0 to 1.0 333.4 96.8% @ 1.0 A g−1 after 10 000 cycles 59
Orange peel KOH Carbonization and activation 2160 1.0 M H2SO4 −0.7 to 0.3 460 98% @ 1.0 A g−1 after 10 000 cycles 60
Leather waste H4P2O7 Pyrolysis and activation 381 6.0 M KOH 0 to 0.55 550 82% @ 1.0 A g−1 after 2000 cycles 38
Waste leather KOH Pre-carbonization and activation 2215 1.0 M KOH 0 to 0.5 1158 99% @ 1.0 A g−1 after 10 000 cycles This work
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Various strategies, including structure optimization, heteroatom doping, and compositing have been widely followed to improve the performance of the energy storage materials. It has been found that a well-developed porous structure including micro-, meso-, or macro-pore interconnected with each other can expose enough electrode/electrolyte interfaces and shorten the path of ion diffusion for the electrochemical process, and thus enhance the energy storage performance.35 Currently, researchers focus on designing carbon material with tunable porous nanostructure for supercapacitor electrodes. On the other hand, doping heteroatoms such as N, B, O, S, and F into the carbon skeleton has been proven to be an effective strategy for enhancing supercapacitor performance by altering the surface physicochemical properties, wettability, and conductivity, as well as generating more active sites while retaining the intrinsic properties of the carbon material.36 Wu et al. studied N, S-self-doped carbon obtained from Chinese rice wine lees, revealing a Csp of 433.5 F g−1 at 1.0 A g−1 in a 6 M KOH electrolyte, along with a cycle stability of 98.9% after 60 000 cycles.37 Recently, Hout et al. reported AC derived from leather waste through H4P2O7 activation and found a Csp of 550 F g−1 along with 82% capacitance retention studied for 2000 cycles in a 6.0 M KOH electrolyte.38

Regardless of other biomass sources, the use of leather as a carbon source has several advantages. Leather mainly consists of collagen fiber, a natural source of C, N, O, and S elements in its protein moiety made of different amino acids. These elements reside in the chemical skeleton, and hence during the pyrolysis, N, S, and O would be doped into the carbon structure, providing an uneven surface charge distribution and creating defects on the electrode surface.15,39 Thus, this route of heteroatoms-doped carbon development does not require additional chemicals like urea, melamine, etc., generally used as doping agents. Another important feature helping to get the special morphology of carbon is its trapped water stabilizing the folded structure of collagen. The collagen structure contains large amounts of free and bound water (ca. 35%) to the protein backbones stabilizing the fibrous structure, a hydrogel.40,41 The slow release of this trapped water at ca. 60–200 °C leads to the denaturation (i.e., cooking) of collagenous proteins before carbonization. In addition to doping, the controlled carbonization of leather leads to pore formation with a larger SSA and the introduction of surface functional groups. Such doping with heteroatoms of carbon not only improves charge mobility and wettability but also acts as an active site to enhance the charge storage capacity of carbon.15,41,42 However, a proper attempt to exploit these advantages of leather in the preparation of carbon materials has not yet been made, although limited research has been devoted to producing carbon from leather for different applications.

This study focuses on a novel synthetic strategy for preparing heteroatom-doped porous AC with pumice morphology from waste leather (WL) used to fabricate highly robust supercapacitor electrodes with extraordinary energy storage performances. Looseness, a quality problem, has been causing around 5–10% of finished bovine leather produced currently as WL or lower-priced materials.41 In addition, converting WL biowaste, an environmental burden, to produce highly stable carbon materials used in technological applications is important from both environmental and economic points of view. A two-step carbonization strategy of WL was followed to prepare highly robust AC, wherein char was prepared at low pyrolysis temperature first, and then the obtained mass was pyrolyzed further at elevated temperatures in excess KOH. The charge storage performance of the prepared materials, characterized by the state-of-the-art techniques, is optimized and compared with those reported. This study opens a new insight into the formation of highly efficient charge storage carbon from WL.

2. Experimental

2.1. Materials

The leather sample, specifically pickle pelt leather that is basically loose leather left as waste, utilized for preparing precursor material, was obtained from Anjuman Tannery, Hemayetpur, Dhaka, Bangladesh. Other chemicals such as absolute ethanol, CH3–CH2–OH (Sigma, Germany), potassium hydroxide, KOH (Sigma, Germany), hydrochloric acid, HCl (Sigma, Germany), poly(vinylidene fluoride), PVDF (Sigma, Germany), N,N-dimethylformamide, DMF (Alfa Aesar, England), and N-methyl-2-pyrrrolidone, NMP (SRL, India) were used without further purification. De-ionized (DI) water (conductivity: 0.055 µS cm−1 at 25.0 °C) obtained from HPLC grade water purification systems (BOECO, BOE 8082060, Germany) was used throughout this study.

2.2. Preparation of activated carbon

A two-stage process produced highly porous carbon utilizing WLs as the precursor material, depicted in Scheme 1. WL was cleaned, dried, and grounded into tiny bits during pre-carbonization. Then, these small segments were dried at 100 °C in an electric oven, and these bits of WL were pyrolyzed at 400 °C under an N2 atmosphere in a tube furnace for 1 h. This carbon was designated as heteroatom-doped carbon (NSC). For activation, the prepared NSC was mixed with KOH at a ratio of 1 : 3 (NSC : KOH), and pyrolyzed at 400 °C, with a heating rate of 10 °C min−1, and kept for 1 h. Similarly, this mixture was allowed to be pyrolyzed at temperatures of 600, 700, and 800 °C for 1 h in the N2 atmosphere. These carbons produced were labeled as A-NSCx, where the letter x denotes the specific temperature at which the activation process occurred, and the letter A stands for activation.

Scheme 1. Schematic representation for the preparation of A-NSC from WL, chemical structures of typical amino acids present in leather, and electrode fabrication.

Scheme 1

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2.3. Characterizations

The characterizations of the samples involved the analysis of the morphology, structure, and chemical stability through field emission scanning electron microscopy (FESEM) (JSM-7600F, JEOL, USA) and Fourier transform infrared (FTIR) (Frontier FT-NIR/MIR model from PerkinElmer, USA). The samples underwent phase analysis and chemical characterization using X-ray diffraction (XRD) (Philips PW 1724). The XRD investigation was conducted utilizing an XDC-700 Guinier–Hagg focusing camera and employing strictly monochromatic CuKα radiation (λ = 1.540598). Furthermore, thermogravimetric analysis (TGA) (Hitachi TG/DTA 7200) was performed to investigate the thermal stability of the material. The chemical composition of the prepared AC materials has been examined using X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, Czech Republic). The analysis of the surface area, pore volume, and pore diameter of the as-prepared samples was conducted using N2 gas adsorption–desorption through the BET method, employing a BET surface analyzer (Belsorpmini-II, BEL, Japan). Ossila contact angle goniometer (L2004A1) was used to analyze the wettability by contact angle measurement.

2.4. Electrode fabrication and electrochemical measurements

Electrochemical studies were carried out with a CHI 660E potentiostat using a three-electrode setup consisting of a spiral platinum wire counter electrode, a graphite rod working electrode (diameter 6.15 mm, Alfa Aesar, USA), and a standard reference electrode, Ag/AgCl/KCl (saturated). The graphite electrode was modified using a solution-casting method. A slurry consisting of 95 wt% carbon material and 5 wt% PVDF in 1000 µL of NMP solvent was prepared by ultrasonication. Subsequently, a total of 25–30 µL of the dispersed slurry was drop-cast onto the polished, clean graphite electrode surface and dried in open air. The casting process was repeated 2–3 times to obtain a homogeneous coating, followed by final drying overnight in an oven at 80 °C. The active mass loading was determined and used to calculate the Csp (in F g−1) as described below. 1.0 M KOH was employed as an aqueous electrolyte to evaluate the electrochemical performance of the electrodes. Cyclic voltammetric measurements were carried out at different potential scan rates. Galvanostatic charging–discharging (GCD) tests were conducted on these materials at different current densities to evaluate their capacity and stability across numerous cycles. In addition, electrochemical impedance spectroscopic measurements were carried out in the frequency range of 0.1 Hz to 1.0 MHz with a potential amplitude of 5 mV to reveal various important parameters including electrode resistance, pore capacitance, charge transfer resistance, etc.

The values of Csp were calculated from the GCD curves using the equation:

2.4. 1

where Csp (F g−1) is the specific capacitance, i (A) is the current, Δt (s) is the discharging time, ΔV (v) is the potential difference, and m (g) is the mass of electroactive material on the electrode.5,10 The energy density (E, W h kg−1) and power density (P, W kg−1), two important parameters of supercapacitors have been calculated using the following equations:

2.4. 2
2.4. 3

Cycle stability was evaluated within the same potential window as the CV of the prepared composites, using Csp for 10 000 cycles at elevated current densities of 10.0 A g−1 in a 1.0 M KOH solution. The assessment of Coulombic efficiency plays a significant role in determining the cycle stability of electrode materials. The Coulombic efficiency of a supercapacitor is defined as the ratio of discharging time to charging time as follows:

2.4. 4

where η is the Coulombic efficiency, td is the discharging time (s), and tc is the charging time (s).43,45

3. Results and discussion

The prepared highly porous heteroatom-doped AC underwent several characterizations, including elemental, crystallographic, morphological, and surface wettability investigations. The charge storage characteristics of these materials deposited on graphite electrode substrates were thoroughly investigated.

3.1. Carbonization of WL

Controlling the temperature of carbonization, which must be carried out in the absence of O2, is one of the vital factors in producing carbonaceous materials with the desired morphology and functionality. Biomass may contain metallic components, and the knowledge of their presence in the final product is technically important. TGA was employed to trace the mass retention throughout the carbonization process of WL, as depicted in Fig. 1A. This study guides to know the mechanism of thermal degradation of WL, and hence assists in selecting the carbonization temperature in producing the desired material. However, the carbonization of WL occurs in the N2 atmosphere via three distinct stages. Initially, at the range of 30–100 °C, dehydration occurred due to the elimination of water. Then, a drastic weight loss occurred within the temperature range of 100 to 400 °C due to removing polysaccharide and collagen-bound water, the evaporation of volatile chemicals, and the carbonization of WL. Finally, a stable carbon structure was obtained, and the weight was reduced slowly at 400 to 800 °C.15,44 During the whole carbonization, the WL experienced acute weight loss, and the yield of the carbon materials was only 20 to 30%, as summarized in Table 2. The TGA of WL was also studied in the O2 atmosphere, and no residue was retained at nearly 700 °C (Fig. S1). This observation suggests that WL contains only carbon materials, and no metallic residue is present. This is rational since WL is technically known as wet blue leather that does not pass the impregnation stage for adding or filling different metallic or non-metallic additives in the tanning process.

Fig. 1. (A) TGA curve of WL measured in N2 atmosphere, (B) FT-IR spectra of NSC, A-NSC6, A-NSC7, and A-NSC8; XRD patterns of (C) NSC and (D) A-NSC8.

Fig. 1

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Table 2. Characteristics of the prepared carbons and their charge storage performances.

Materials Yield (%) aElement (%) SSA (m2 g−1) Total pore volume, V (cm3 g−1) Average pore diameter, D (nm) Contact angle (°) Charge storage performance @ 5.0 A g−1
C N S O C sp (F g−1) η (%)
NSC 32.3 57.32 31.35 0.37 10.95 2 0.005 3.07 43 20 80
A-NSC6 29.4 68.17 24.72 0.65 6.47 532 0.170 1.31 38 130 71
A-NSC7 27.6 60.57 27.23 0.39 11.81 36 241 73
A-NSC8 22.2 59.35 (59.03) 28.59 (31.12) 0.94 (0.52) 11.12 (8.91) 2215 0.810 1.47 28 380 78
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a

Outside and inside parenthesis values were determined by EDS and XPS, respectively.

3.2. Surface chemical composition and functional group analysis

The characterization of surface functional groups was performed by FTIR spectroscopy. At a glance, the FTIR spectra showed that the positions of the peaks are the same for char and AC (Fig. 1B). A broad band centered at ca. 3440 cm−1 is attributed to O–H or N–H stretching or overlapping both. The 2920 and 1622 cm−1 bands represent C–H and either C Created by potrace 1.16, written by Peter Selinger 2001-2019 O or C Created by potrace 1.16, written by Peter Selinger 2001-2019 N stretching, respectively. Additionally, the most significant and characteristic bands were found at 2371 cm−1 for C Created by potrace 1.16, written by Peter Selinger 2001-2019 N and 1080 cm−1 for S Created by potrace 1.16, written by Peter Selinger 2001-2019 O stretching.15,45 These observations suggest that the materials prepared from WL contain various functional groups, and the resulting carbon materials became self-doped with nitrogen and sulfur originating from the amino acids present in the collagen fibers of WL.

Further confirmation of the presence of different elements in the materials can be ensured by the results of the EDS and XPS analyzer. Table 2 represents the summary of the elemental compositions of the materials. These results support the FTIR spectroscopic results indicating the presence of N, S, and O, along with C, in the AC prepared at different carbonization conditions.

The EDS spectrum (Fig. 3E, F, and S2) and the survey-level XPS scan (Fig. 2A), both verify the elements found in the prepared materials. Fig. 3E shows the %composition of N, O, and S in NSC with 31.35, 10.95, and 0.37% composition, respectively. The %composition is more or less consistent in A-NSCs samples. For instance, the compositions of elemental N, O, and S in ANSC8 were 28.59%, 11.12%, and 0.94%, respectively. Only minor changes in elemental composition were observed for A-NSC6 and A-NSC7, as shown in Fig. 3F and S2. However, the elemental composition and the chemical state of heteroatoms (N, S, and O) present were carried out by XPS measurement, as typically illustrated in Fig. 2 for A-NSC8.

Fig. 3. FESEM images of (A) NSC, (B) A-NSC6, (C) A-NSC7, and (D) A-NSC8. EDS spectra of (E) NSC and (F) A-NSC8.

Fig. 3

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Fig. 2. XPS measurement of A-NSC8. (A) Survey spectrum, and high-resolution spectra of (B) C 1s, (C) N 1s, (D) S 2p, and (E) O 1s.

Fig. 2

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The survey spectrum shown in Fig. 2A displays characteristic signals corresponding to C 1s (285 eV), N 1s (401 eV), O 1s (531 eV), and S 2p (166 eV), suggesting the successful incorporation of N, S, and O into the carbon materials. The high-resolution spectrum of C 1s, as illustrated in Fig. 2B, can be deconvoluted into four distinct peaks. The peaks observed at 282.4, 283.2, 284.1, 284.2, and 286.5 eV correspond to the configurations of C–C, C Created by potrace 1.16, written by Peter Selinger 2001-2019 C, C–N, C–S, and C–O, respectively. The high-resolution N 1s spectrum reveals three distinct peaks at 398.9, 400.1, and 401.5 eV, corresponding to pyridine-N, pyrrole-N, and graphite-N/quaternary-N, respectively (Fig. 2C). On the other hand, in the S 2p spectrum (Fig. 2D), the peaks at 163.1 and 166.6 eV are associated with the configuration of C–S and C Created by potrace 1.16, written by Peter Selinger 2001-2019 S. The other peak indicates the presence of oxidized sulfur groups (C–SOx). Additionally, the O 1s spectrum indicates the existence of C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (quinone, approximately 529.6 eV) and C–O (phenol and/or ether, around 531.1 eV), respectively. In our previous study, it was shown that the pyridine-N and pyrrole-N increase the wettability of carbon, as further clarified later. It is well established that oxygen groups may also contribute to pseudocapacitance. However, the elemental compositions determined using EDS and XPS are supportive of each other and suggest that the developed A-NSC contains expected elemental N and S.

3.3. Microstate analysis

XRD analysis was applied to identify the phase formation and investigate the structural properties of the prepared carbon. XRD patterns of NSC and A-NSC samples at different temperatures are shown in Fig. 1C, D and S3. Two characteristic peaks at the 2θ angle of ca. 25° and 45° were observed, indexed with (002) and (100) planes of the graphitic-like carbon with the amorphous character. Furthermore, the absence of diffraction peaks associated with impurities indicates a high purity level in the A-NSC8 samples.7,30 However, the origin of the peaks at 2θ angles of 31°, 56°, and 66° in NSC is not well understood. These peaks may indicate a semi-crystalline nature of the material.

3.4. Morphological analysis

The heteroatom-doped carbon prepared with or without an activating agent was studied via FESEM to identify the surface morphology and microstructure of the prepared carbon. As shown in Fig. S4, leather is primarily composed of natural collagen fibers that are arranged in a 3D structure, resulting in the formation of intermolecular bonds that create a tightly-knit material. In addition, the highly fragmented morphology, comprising sheared collagen fiber bundles and irregular, dense microflakes, reduces the bulk matrix to discrete frameworks, enhancing the material for subsequent carbonization, in which the volatilization of organic phases creates internal porosity without collapsing the stable fibrillar framework. During the pre-carbonization process at 400 °C, the WL converted into fibrous NSC, as shown in Fig. 3A and S4. An insufficient pre-carbonization temperature prevented the formation of well-defined porosity and the removal of decomposable components. An increase in activation temperature and the use of an activating agent have been observed to enhance the porosity and surface roughness of A-NSC.1,4,15,30,33 The pumice-like structure was observed in samples activated at 600, 700, and 800 °C, as depicted in Fig. 3 and S4. Furthermore, A-NSC8 possesses a hierarchical porous structure as can clearly be seen in Fig. 3D.

3.5. Porosity and surface wettability

The porosity characteristics of the synthesized material were further investigated by N2 adsorption/desorption measurements. According to the IUPAC categorization, all samples exhibit type I isotherms (Fig. 4A). The substantial increase in adsorption at extremely low pressures (P/P0 < 0.1) demonstrates the prevalence of micropores. The incremental rise in absorption volume at P/P0 ∼0.4 indicates the presence of larger pores, as supported by the BJH pore size distribution curve (Fig. 4B) and structural parameters (Table 2).1,4,18 The SSA of NSC, A-NSC6, and A-NSC8 were determined to be 2, 531.71, and 2214.50 m2 g−1 with the pore volumes of 0.005, 0.17, and 0.81 cm3 g−1, respectively. These results demonstrate that temperature and activating agents are key factors influencing SSA and pore volume.10,15,46 The A-NSC8 demonstrates a commendable porous architecture characterized by numerous micropores, a moderate quantity of mesopores, and macropores. It is noted that this hierarchical porous structure would be beneficial for charge storage in EDLCs, as macropores and mesopores facilitate ion transport and diffusion, while micropores increase the charge storage capacity.

Fig. 4. (A) N2 adsorption/desorption isotherms and (B) corresponding pore size distribution of (a) NSC, (b) A-NSC6, and (c) A-NSC8. (C) Water contact angle measurements of (a) NSC, (b) A-NSC6, (c) A-NSC7, and (d) A-NSC8.

Fig. 4

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It is widely recognized that surface wettability is a crucial factor for enhancing electrode performance. The wettability of the NSC and A-NSC was examined employing the contact angle test, and the corresponding results are presented in Fig. 4C. The contact angle measurements for NSC, A-NSC6, A-NSC7, and A-NSC8 were 43°, 38°, 36°, and 28°, respectively. These values suggest that the wettability increases in the following order: NSC < A-NSC6 < A-NSC7 < A-NSC8. It is important to point out that high temperatures increase the feasibility of co-doping of N and S, which might result in an uneven distribution of surface charges and the creation of active sites with polar properties on the surface. In addition, the distinctive hierarchical porous configuration of A-NSC8 in pumice-like morphology can also augment wettability through capillary action. Increased wettability can improve the attraction of materials to water, leading to faster adsorption of ions at the electrode/electrolyte interface and greater degrees of EDLC.

3.6. Electrochemical charge storage performances

3.6.1. Charge storage behavior and energy density

The cyclic voltammograms (CVs) of all prepared materials recorded at the same potential scan rate (ν) are displayed in Fig. 5A. By monitoring the CVs over the potential range of −0.4 to 1.2 V (Fig. S5), the optimal potential window was determined, and no Faradaic reactions were observed between 0.0 and 0.5 V. It should be noted that hydrogen evolution at negative potentials and oxygen evolution at positive potentials are known to occur outside this potential range. The CVs of NSC and A-NSCs recorded at a ν of 0.05 V s−1 are also shown in Fig. 5A. All of the samples exhibit the quasi-rectangular form of CV, indicating the expected behavior of an EDLC.4,20,43 The areas of these CVs increased as the activation temperature of A-NSC was increased. However, A-NSC8 exhibits the greatest rectangular area, which corresponds to superior structural properties as clarified later.

Fig. 5. (A) CVs and (B) GCD curves of (a) NSC, (b) A-NSC6, (c) A-NSC7, and (d) A-NSC8 at the scan rate of 0.05 V s−1 and the current densities of 5.0 A g−1, respectively; (C) the Csp value of the studied materials at the current densities of 5.0 A g−1; (D) the Csp of A-NSC8 at the different current densities; (E) Nyquist plots of (a) NSC, (b) A-NSC6, (c) A-NSC7, and (d) A-NSC8; (F) cycling performance of A-NSC8 at the current density of 10 A g−1.

Fig. 5

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To thoroughly assess the capacitive characteristics of the produced ANSCs, GCD measurements were performed at a current density of 5.0 A g−1 (Fig. 5B). As expected from CV results, all the profiles exhibit characteristic symmetrical triangles and linear patterns without notable IR drops. At a glance, A-NSC8 has the longest duration for discharging, a signature of the highest Csp. However, the Csp values were determined from the GCD response using eqn (1). A-NSC8 had a maximum Csp of 380 F g−1 determined from the GCD response measured by applying a higher current density of 5.0 A g−1. The effects of applying current density on the values of Csp are exhibited in Fig. 5C. A non-linear decay of Csp against the applied current density is noticed. However, the lower the current density, the higher the Csp is.6,45 The maximum value of Csp obtained is 1158 F g−1 at the current density of 1.0 A g−1 for A-NSC8. Irrespective of the applied current density, the other A-NSCs prepared did not exhibit such an extraordinary charge storage performance. It is also worth noting that the pumice-like AC derived from WL outperformed the supercapacitive performance of those derived from similar biomasses, as can be seen in Table 1.

3.6.2. Cycle stability of charging–discharging

The charging–discharging cycle stability of A-NSC8 was assessed by subjecting the cells to rigorous charge–discharge conditions within a voltage range of 0–0.5 V at a current density of 10 A g−1 (Fig. 5F). The supercapacitor revealed consistent cycling performance for up to 10 000 cycles. This demonstrates exceptional stability with a high-energy capacitor that aligns with the KOH electrolyte findings,34,55,59 occasionally exceeding the cycling stability reported in earlier studies involving biomass-derived AC, as illustrated in Table 1. The impressive capacitance retention of A-NSC8 indicates its remarkable stability, which is attributed to its well-ordered hierarchical porous structure in pumice-like AC with a large SSA.

3.6.3. Type of charge storage and electrode kinetics

The charge storage pathway of A-NSC8 can be examined by following the methodology provided by Trasatti et al.43,45,47 This method quantifies the impact of non-surface-controlled diffusion compared to the surface-controlled capacitive mechanism on the overall charge stored (Csp)total in a material. According to Trasatti et al., the charge stored (i.e., Csp) at the infiniteν is called the outer charge (Csp)outer. It may be estimated by finding the intercept of the plot of Cspvs. ν−0.5 using eqn (5).

Csp = (Csp)outer + Kv−0.5 5

where the capacity, Csp (F g−1), is determined by analyzing the CV curves at different ν (V s−1), and K represents a constant. The value of Csp (F g−1) is obtained using eqn (6).

3.6.3. 6
3.6.3. 7

By employing a sufficiently low ν, one can determine the overall charge stored (i.e., (Csp)total) from the plot of 1/Cspvs. ν0.5 (eqn (7)). The A-NSC8 has a (Csp)total of 1076 F g−1 and (Csp)outer of 36 F g−1 (in Fig. 6B and C), wherein (Csp)total is essentially comparable to the Csp derived from the GCD response of A-NSC8 (Fig. 5D).38,45,48 However, this analysis was carried out for all ν studied and the obtained results are compared in Fig. 6D. It indicates that the diffusion-controlled process predominantly (maximum of 97%) contributes to the overall charge. The linear relationship of the i vs. ν0.5 plots (Fig. 6E) derived from the experimental data further supports this observation, i.e., the diffusion of ions is the main factor influencing the charge storage response.38,49

Fig. 6. (A) CVs of A-NSC8 at the scan rate of (a) 0.01, (b) 0.02, (c) 0.03, (d) 0.04, and (e) 0.05 V s−1. (B) Plot 1/Cspversus ν0.5 and (C) plot Cspversus ν−0.5 for calculating the total charge and outer charge of A-NSC8, respectively. (D) Comparative study of the surface contribution of A-NSC8 at different potential scan rates, (E) plot of current vs. square root of potential scan rate, and (F) Ragone plot of this work.

Fig. 6

Open in a new tab

3.6.4. Coulombic efficiency

Coulombic efficiency (η) serves as an indicator of the cycle stability of an energy storage device. A high η typically suggests an extended cycle life. The efficiency of synthesized materials was determined from GCD curves measured at a current density of 5.0 A g−1 by following eqn (4). The values of η were determined to be 80, 71, 73, and 78% for NSC, A-NSC6, A-NSC7, and A-NSC8, respectively, i.e., A-NSC8 has superior η. These results reflect that these materials are highly efficient not only for storing charges but also for minimizing the electric current loss during the charging process.

3.6.5. Energy–power relationship

The Ragone plot helps to evaluate the practical applicability of an energy-storage device. It compares energy density (charge storage per unit weight of active material) and power density (a measure of the rate at which the stored electrical energy is released). In this study, the energy and power densities were determined using eqn (2) and (3), respectively, and the results are illustrated in Fig. 6F. Benefiting from the higher Csp and low IR drop, A-NSC8 reaches a maximum energy density of 40 W h kg−1 at a power density of 250 W kg−1. This value is comparable to those of other carbon-based electrodes reported previously.53–58 In this study, a three-electrode system was employed. Although a two-electrode system is generally preferred for evaluating the energy–power relationship and practical applicability of electrode materials, the A-NSC8 electrode remains a promising candidate for advanced supercapacitor applications.

3.7. EIS measurements and the mechanism of charge storage

All materials including A-NSC8 studied exhibit EDLC-type charge storage with excellent cycle stability. EIS is a powerful tool that helps to reveal deeply the underlying factors associated with different modes of charge storage by a material. Generally, the typical Nyquist plots consist of a straight line in the low-frequency region and a semicircular segment in the high-frequency region. The diameter of the semicircle indicates the charge transfer resistance (Rct), determined by the electrical conductivity and porous structural properties. The solution resistance (Rs), which encompasses the resistance of the electrolyte, internal electrode, and contact, is determined by the intercept on the X-axis.5–9,55 In this study, the underlying functions of each micro-domain of the materials providing extraordinary performance in storing charge were revealed with EIS measurements, as illustrated in Fig. 5E and Table S1. The Rct of these materials is quite negligible. Diminishment of Rct in the composites indicates quicker charge transfer between the electrode/electrolyte interfaces promoting improved capacitance. Hence, better storage of charges is possible to explain the cause of superior supercapacitive performances of the materials.7,33,59

The A-NSC8 electrode has the lowest Rs resistance (6.73 Ω), suggesting that a greater carbonization temperature leads to enhanced graphitization, reduced resistance, and improved conductivity. Furthermore, the A-NSC8 delivered smaller Warburg resistance (ZW) values exhibiting steeper, vertical, and diagonal lines close to the imaginary axis. This depicts better diffusion of ions into the deep region of the material. Better ionic transport and deep penetration are possible when the material itself is porous and has a defective, cracked structure.34,58 To summarize, the A-NSC8 electrode has a low level of internal resistance and exhibits good conductivity characteristics. This is because A-NSC8 has a greater number of mesopores and micropores, which leads to the development of a denser pore network. As a result, this improved pore shape allows for the movement of salt ions, decreasing resistance. Consequently, the Cp and Cdl of A-NSC8 also exhibit an increase. Conversely, the NSC has fewer pore structures and is mainly made up of micropores, which hinders the movement of ions and results in higher resistance. The FESEM data displayed above are consistent with the EIS results, specifically indicating a larger value of Cp. Therefore, the A-NSC8 electrode demonstrates more advantageous channels for transporting salt ions, rendering it a more appropriate selection for a supercapacitor.

The exceptional Csp of the A-NSC8 electrode can be attributed to its highly porous architecture and enhanced hydrophilicity due to heteroatom doping, facilitating ion adsorption and fast ion transfer, and hence maximizing the charge-storage capacity. The GCD profile of A-NSC8 exhibits a minor departure from the ideal symmetrical triangle, possibly attributed to the Faradaic reactions induced by doped heteroatoms such as nitrogen, oxygen, and sulfur. Nevertheless, when the current density rises, the charge–discharge curve becomes nearly linear, suggesting the presence of unstable pseudocapacitance caused by the heteroatoms.7,57 This is because, at lower current densities, the active material on the electrode surface has ample time to establish a double layer with solvated ions from the electrolyte during the charging–discharging process. The charging and discharging process is prolonged when the creation of the double-layer is inflexible due to the insertion or release of solvated ions from the electrolyte under optimal conditions, specifically when there is a greater surface area with porosity.36,37,57 The electrochemical results indicate that the synthesized A-NSC8 is an effective electrode material for high-performance supercapacitors (Table 1). The present strategy of heteroatom-doping of carbon (Table 2) tends to a more hydrophilic surface that lowers functional sites like oxygenated groups and facilitates ion adsorption required for EDL-type charge storage.

4. Conclusions

A distinctive, highly porous heteroatom-doped carbon with excess SSA can be synthesized by a simple two-step approach from waste leather with an expected yield of 22–32%. The microstates and SSA of A-NSC can be varied by changing the pyrolysis temperatures. A-NSC8 prepared at 800 °C from the mixture of solid KOH and char that initially developed at low-temperature carbonization at 400 °C in an N2 environment exhibited a pumice-like morphology. The collagen structure of leather, containing N and S, and the trapped water (ca. 35%) within the protein moiety, which stabilizes the fibrous structure, aids the formation of N, S-codoped carbon with a pumice-like morphology. The slow release of this trapped water occurring in the temperature range of 60–200 °C leads to the denaturation (i.e., cooking) of collagenous proteins before carbonization, helping to get a unique morphology of carbon. The activation and the increase of the pyrolysis temperature were found to decrease the %yield of A-NSC. The doping of elemental N was identified as pyridinic-N, pyrrolic-N, and graphitic-N/quaternary-N. The configuration of C–S and C Created by potrace 1.16, written by Peter Selinger 2001-2019 S was identified for S-doping along with a few amounts of C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (quinone) and C–O (phenol and/or ether)-type functional groups. Both SSA and surface hydrophilicity of the A-NSC can be enhanced by using a solid KOH activating agent and elevating pyrolysis temperature. The obtained carbon surface with pumice-like morphology was studied to be suitable for EDLC-type supercapacitive charge storage in aqueous KOH solutions. A-NSC8 exhibited a specific capacitance of 1158 F g−1 at a current density of 1.0 A g−1, with exceptional cycle stability, sustaining over 10 000 consecutive charge/discharge cycles. Such materials with high EDLC-type capacitance would be useful for designing capacitive deionization electrodes,9 and studies in this direction are currently in progress. Thus, this study introduces novel possibilities for the development of carbon materials for advanced applications due to the environmentally friendly processing, cost-effectiveness of the precursor, and straightforward activation procedure. The proposed strategy also provides new opportunities for producing heteroatom-doped activated carbon for advanced energy-storage applications.

Author contributions

Mohy Menul Islam: methodology, investigation, visualization formal analysis, writing – original draft; Ahammad Musa: investigation, validation, review & Editing; Md. Mominul Islam: conceptualization, resources, writing – review and editing, funding acquisition, visualization & supervision.

Conflicts of 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.

Supplementary Material

RA-016-D6RA04144B-s001
RA-016-D6RA04144B-s001.pdf (1,001.9KB, pdf)

Acknowledgments

The financial support of the University Grants Commission (UGC), Bangladesh provided through a research project is gratefully acknowledged. The instrumental support for advanced characterization of the materials taken from IGCRT, Bangladesh Council of Scientific and Industrial Research, Dhaka 1205, and the Centre for Advanced Research in Sciences (CARS), University of Dhaka is also acknowledged.

Data availability

The data supporting the findings of this study are available within the article and its supplementary information (SI). Additional datasets generated during the current study, including raw electrochemical measurements and characterization data, are available from the corresponding author upon reasonable request. Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra04144b.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-016-D6RA04144B-s001
RA-016-D6RA04144B-s001.pdf (1,001.9KB, pdf)

Data Availability Statement

The data supporting the findings of this study are available within the article and its supplementary information (SI). Additional datasets generated during the current study, including raw electrochemical measurements and characterization data, are available from the corresponding author upon reasonable request. Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra04144b.

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