Mild chemical-activated hydrothermal porous carbon derived from durian peel biomass for an electrochemical supercapacitor

Abstract

Because waste biomass is the ideal precursor for the preparation of porous carbon, the reuse of waste biomass resources has become a current research hotspot. However, because of the complexity of waste biomass and its microstructure, the quality reproduction of discarded biomass is poor. Therefore, it is of great significance to develop a reliable method for the preparation of porous carbon. In this paper, a hydrothermal carbonization treatment could complete the sphere/nanosheet morphology structure adjustment and KHCO₃ could activate the hydrothermal porous carbon while maintaining the spherical morphology. The activated hydrothermal porous carbon with a carbon sphere/nanosheet structure facilitated ion/electrolyte diffusion and increased accessibility between the surface area and electrolyte ions. The durian peel-derived activated hydrothermal porous carbon had a high specific surface area (2100.5 m² g⁻¹), good specific capacitance (267 F g⁻¹ at 1 A g⁻¹) and good cycling stability, with a capacitance loss of only 6.7% after 10 000 charge–discharge cycles. A Na₂SO₄-based cell achieved a maximum energy density of 14.45 W h kg⁻¹ at 225 W kg⁻¹; even at a higher power density of 4500 W kg⁻¹, the specific energy remained at 10.75 W h kg⁻¹.

---

1. Introduction

The increasing fossil fuel demand and consumption in the growing global economy have diminished our ecology and already attracted a lot of concern. To meet the growing demand for energy storage and conversion in modern society, environmentally friendly, safe, and high-energy-density devices are urgently required for the energy storage market.^1–4 Hence, designing and exploring a sustainable energy storage system are needed to ensure continuity, such as Li-ion batteries,⁵ ammonium-ion batteries,^6–9 fuel cells,¹⁰ lithium–sulfur batteries,¹¹ zinc-ion batteries,¹² supercapacitors (SC)¹³etc.

Among the various types of energy storage devices, SC have been developed as a result of the superior durableness, instantaneous recharging capability and high power density.^14–19 Since the energy storage of SC depends on electrostatic charge accumulation on the electrode surface, the properties of the electrode material play the most crucial part in the SC performance of devices. It is highly desirable that the electrode material exhibit a high specific surface area (SSA) for high specific capacitance.^20–23 It is still an important challenge to prepare a carbon material with a suitable pore size distribution and a large SSA in a simple and environmentally friendly manner. Biomass material is one of the most economical and environmentally friendly materials. It not only shows better electrochemical performance but also is relatively less costly, and thus it could solve all the above problems.^24–26 However, biomass has inherent defects: insolubility, complex composition and different pore structure of different biomass, which make it difficult to control the morphology and pore structure of biomass char. Therefore, in order to make biomass porous carbon (PC) material with better electrochemical properties in SC, it is important to develop suitable biomass carbon materials and related activation synthesis methods.^27–30 However, the synthesis process of these porous materials always employs chemical reagents (e.g., potassium hydroxide, zinc chloride and potassium bicarbonate) as activation agents. Summarily, high-performance PC electrode materials could be prepared using these methods but generally exhibit low carbon yields, and the heavy use of traditional corrosive activating agents would place a great burden on the environment.³¹

Pyrolysis and hydrothermal carbonization (HTC) are two common methods to carbonize biomass. Pyrolysis involves an inert or limited-oxygen atmosphere at elevated temperatures while the HTC refers to a thermo-chemical process to convert biomass to carbonaceous material.³² As a thermo-chemical conversion technique, HTC can be influenced by several parameters, such as temperature, residence time, precursor concentration and catalyst.³³ It uses subcritical water for the conversion of a biomass to carbonaceous products, resulting in efficient hydrolysis and dehydration of precursors and bestowing the hydrochar with a high and tunable content of oxygen-containing functional groups (OFG).^34,35 Hao et al. synthesized PC from ginkgo leaves by “HTC + KOH activation” with 364 F g⁻¹ at 0.5 A g⁻¹ being attained as a result of the exceptional textural properties of the porous structures.³⁶ Chen et al. performed HTC integrated with KOH post-activation to transform elm flower into porous activated hydrochar, the as-formed activated hydrochar material in a three-electrode system of SC presenting a high specific capacitance of 275 F g⁻¹ at 1.0 A g⁻¹ using 6.0 M KOH solution.³⁷ On the basis of the “HTC + activation” strategy, heteroatom-doped PC was further synthesized, which could achieve better performance.³⁸ Zhang et al. reported that N-doping in hydrothermal lignin improved conductivity and surface wettability with electrolyte ions, whilst the hierarchical bowl-like pore structure and large SSA (2218 m² g⁻¹) resulted in a high specific capacitance of 312 F g⁻¹ at 1 A g⁻¹ and excellent cycling stability (98% after 20 000 cycles at 10 A g⁻¹).³⁹ Heteroatoms can not only improve the specific capacitance of a material through the Faraday reaction, but also improve the conductivity and wettability of material, which could significantly enhance the energy storage performance of SC.⁴⁰ Indeed, conventional KOH activation with high-temperature activation has some disadvantages such as lower yields, pore shrinking effects and loss of surface heteroatoms that would reduce the active surface area of the electrode materials.⁴¹ To solve these problems, we explored a new strategy introducing heteroatoms through the HTC process and using green weakly basic potassium bicarbonate (KHCO₃) as activator to maintain the spherical morphology of hydrothermal carbon.^42,43

Durian peels are an agricultural waste in Asian countries. The global durian production in 2023 was about 3.4 million tons, which could be used as a precursor for the production of activated carbon. The main components of durian peels are cellulose and lignin, the structure of which is easy to convert into small molecules during the HTC process. In this study, HTC combined with ferric sulfate (Fe₂(SO₄)₃xH₂O) was used to regulate the sphere/nanosheet structure for durian peel-based hydrothermal carbon.^44–46 Subsequent mild KHCO₃ activation could maintain the mixed morphology of sphere/nanosheet structure and accelerate ion or electrolyte transport. Because morphology control is an important way to improve the electrochemical performance of PC, compared with KOH activation, retaining the precursor morphology is one of the significant advantages of the KHCO₃ activation method. This provides a new way for converting biomass into high-performance electrode material.

2. Experimental

2.1 Materials

Ferric sulfate (Fe₂(SO₄)₃xH₂O, AR. 99.5%) and potassium bicarbonate (KHCO₃, AR. 99.5%) were purchased from Shanghai Macklin Biochemical Technology Co. Ltd. Hydrochloric acid (HCl, AR. 37%) was purchased from Sinopharm Chemical Reagent Co, Ltd. The waste durian peels were obtained from a fruit market. Polytetrafluoroethylene (PTFE; condensed liquid binder for Li-ion battery, 46%) and foamed nickel (99.9%) were purchased from Shenzhen Kejing Star Technology Company. Conductive carbon black (lithium-ion battery electrode material) was purchased from Jiuding Chemical (Shanghai) Technology Co. Ltd.

2.2 Preparation of hydrothermal porous carbon

Initially, the waste durian peels underwent a series of processing steps including washing, drying, cutting into small pieces, crushing into powder of 30 mesh (300–500 μm) using a ball mill, and subsequent drying at 100 °C for a duration of 24 h. Subsequently, three portions of durian peel powder weighing 6 g each, along with three portions of Fe₂(SO₄)₃·xH₂O weighing 0.3 g, 0.6 g, and 1.2 g respectively and 60 ml of deionized water were meticulously measured. The resulting mixture was then placed in a Teflon liner (100 mL), subjected to sonication at room temperature for 15 min, and subsequently sealed in a stainless-steel autoclave. The autoclave containing the mixture was then placed in an oven set at 180 °C for a duration of 24 h. Following the completion of the reaction, the hydrothermal product underwent vacuum filtration and subsequent washing with deionized water and absolute ethanol multiple times until the filtrate achieved a colorless appearance. Subsequently, the product was dried at 100 °C for a duration of 24 h, resulting in the formation of hydrothermal porous carbon (HPC) denoted as HPC-Fe-X (X = 5, 10, and 20), with X representing the mass ratio of Fe₂(SO₄)₃·xH₂O to durian peel powder. In contrast, hydrothermal porous carbon labeled as HPC was prepared using pure water without the addition of Fe₂(SO₄)₃·xH₂O during the HTC process under identical conditions.

2.3 Preparation of activated hydrothermal porous carbon

The experimental procedure for the activation of durian peel-based HPC involved the mixing of 2 g of HPC-Fe-X with 12 g of KHCO₃ in an agate mortar to ensure an even distribution. The resulting mixture was then transferred to a corundum boat and subjected to heat treatment in a tube furnace under an Ar atmosphere. The temperature of the furnace was gradually increased to 850 °C at a rate of 5 °C min⁻¹, held at this temperature for 2 h, and allowed to cool naturally to ambient temperature. The activated porous carbon (APC) was neutralized using a specific quantity of dilute HCl solution and then rinsed multiple times with deionized water. Subsequently, the product was obtained following a drying process at 100 °C for 24 h and identified as APC-Fe-X. For comparative purposes, HPC underwent activation using the identical procedure as for APC-Fe-X and was denoted as APC. To investigate the impact of the KHCO₃ activator, HPC-Fe-10% was subjected to direct heat treatment in a tube furnace without the KHCO₃ activator, resulting in a product referred to as HPC-Fe-10%.

2.4 Characterization methods

X-ray diffraction (XRD) analysis was conducted utilizing a Bruker D8 diffractometer equipped with Cu Ka radiation (λ = 1.5418 Å, 40 kV, 40 mA). Field emission scanning electron microscopy (FE-SEM) was carried out using an FEI Quanta 250 FEG. Raman spectroscopy was performed with a Thermo Fisher DXR2xi LabRAM HR Evolution instrument (λ = 532 nm). Nitrogen physisorption–desorption measurements were conducted at 77 K following out-gassing at 200 °C for a minimum of 4 h. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo Fisher Escalab Xi⁺ instrument.

2.5 Electrochemical measurements

A three-electrode system was used to investigate the electrochemical performance of the samples. Specifically, a mixture of PTFE (10 wt%), conductive carbon black (10 wt%), and APC (80 wt%) was blended in an agate mortar with ethanol, followed by vigorous grinding to create a slurry. The slurry was then rolled into thin sheets using a glass rod, cut into 1 × 1 cm² sheets with a blade, and pressed onto foamed nickel measuring 1 × 2 cm² using a powder tablet machine at a pressure of 10 MPa. Subsequently, the electrodes were dried at 80 °C for 12 h. The mass loading of active material in each working electrode was approximately 2–3 mg cm⁻². In a standard three-electrode configuration, five types of APC were utilized as the working electrode, an Hg/HgO electrode as the reference electrode, a Pt foil as the counter electrode, and a 6 M KOH aqueous solution as the electrolyte. In a symmetric system, two identical electrodes were placed on a coin cell (CR2032) and divided by a hydrophilic PVDF membrane filter. All electrochemical properties were assessed through cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements conducted using an electrochemical workstation (IVIUM Ivium Stat.h, Netherlands) at ambient temperature. Additionally, electrochemical impedance spectroscopy (EIS) was performed at 0 V within the frequency range of 10⁻² to 10⁵ Hz.

The capacitance of a single electrode was determined by

The capacitance of a symmetric capacitor was estimated by

The energy density was calculated by

The power density was estimated by

where ΔV, Δt, m, and I are the discharge voltage without IR drop (V), the discharge time (s), the mass of APC on a single electrode (g), and the discharge current (A), respectively.

3. Results and discussion

The SEM images of the durian peel-based APC and durian peel 30-mesh raw material are presented in Fig. 1. Among the five samples examined, those shown in Fig. 1(a)–(d) were activated by KHCO₃, whereas that in Fig. 1(e) underwent only hydrothermal treatment without activation by KHCO₃. Compared to the SEM image of durian peel 30-mesh raw material, the SEM images of all five samples showed that the original structure of durian peels was destroyed. The primary chemical constituents of durian peels consist of cellulose and lignin, which have the potential to undergo partial conversion into small organic compounds following a hydrolysis reaction. Subsequently, these compounds could be transformed into spherical carbon particles through condensation polymerization. As depicted in Fig. 1, the sizes of the carbon spheres varied among the latter four samples; however, carbon spheres were not found in the SEM image of sample APC. This disparity could be attributed to the introduction of Fe₂(SO₄)₃ in the hydrothermal process of the latter four samples, which led to an increased production of small organic molecules, further facilitating the formation of spherical carbon particles. Moreover, while both sample 3 and sample 5 contained the same quantity of Fe³⁺ during the HTC process, sample 3 underwent activation with KHCO₃. The microscopic morphology of the two samples depicted in Fig. 1 appeared nearly identical, suggesting that the spherical structure was effectively maintained following mild KHCO₃ activation. Additionally, alongside the spherical carbon materials, the SEM images of the latter four samples revealed a significant presence of carbon nanosheet structures. The elemental mapping of APC-Fe-10% is shown in Fig. S1 (ESI†), which suggested the uniform distribution of all elements. Prior research had demonstrated that porous spherical carbon materials exhibited improved packing properties and reduced ion diffusion distances.⁴⁷ The spherical structure enhanced dispersion and fluidity of APC materials, thereby mitigating the viscosity effect.⁴⁸ Additionally, the combination of microspheres and nanosheets effectively hindered the stacking of nanosheets, creating additional space for electrolyte storage and diffusion. This structural design promoted optimal contact between electrolyte and APC, resulting in a high SSA of APC being fully utilized, thus improving the performance of the SC.⁴⁶

Fig. 1 SEM images of (a) APC, (b) APC-Fe-5%, (c) APC-Fe-10%, (d) APC-Fe-20%, (e) HPC-Fe-10% and (f) durian peel 30-mesh raw material.

Fig. 2 illustrates the results of N₂ adsorption–desorption tests conducted on five samples. Fig. 2(a) displays adsorption and desorption curves for five samples, with the HPC-Fe-10% sample exhibiting a type I isotherm, while the remaining four samples displayed combined isotherms of type I and type IV with hysteresis loops. Notably, when the relative pressure P/P₀ was below 0.1, all samples demonstrated a significant increase in nitrogen adsorption capacity, indicating the presence of abundant micropores in APC materials.⁴⁹ As the relative pressure P/P₀ increased to 0.1 and 0.4, the nitrogen adsorption capacity of APC, APC-Fe-5%, APC-Fe-10% and APC-Fe-20% exhibited a gradual increase, suggesting the presence of mesopores in the respective carbon materials. At a relative pressure of 0.4 and 0.95, the observation of a hysteresis loop indicated the existence of both mesopores and macropores in the carbon materials. Analysis of pore size distributions shown in Fig. 2(b) revealed a higher concentration of micropores in the APC, APC-Fe-5%, APC-Fe-10% and APC-Fe-20% samples compared to the HPC-Fe-10% sample, with a significantly larger micropore area in the former four samples. The micropore area of the four APC-based samples under consideration was found to be significantly greater than that of HPC-Fe-10%. The SSA and pore structure parameters of the five samples are detailed in Table S1 of the (ESI†).⁵⁰ Specifically, the SSA values for HPC-Fe-10%, APC, APC-Fe-5%, APC-Fe-10% and APC-Fe-20% were determined to be 370.07 m² g⁻¹, 1995.3 m² g⁻¹, 1883.6 m² g⁻¹, 2100.5 m² g⁻¹ and 1251 m² g⁻¹, respectively. Notably, the SSA and pore volume of APC-Fe-10% were found to be the largest among the five samples. These findings suggested that the hydrothermal carbon activated by KHCO₃ exhibits superior SSA and pore volume characteristics. The findings suggested that the porous carbon material exhibited a tri-modal pore structure, comprising micropores, mesopores, and macropores. Micropores are known to play a crucial role in providing a high SSA for carbon materials, facilitating charge storage, and enhancing specific capacitance. Mesopores and macropores, on the other hand, serve as storage sites and buffer zones for charged ions, thereby promoting the diffusion rate of electrolyte ions within electrodes and enhancing rate performance under varying current densities. In summary, the APC materials demonstrated exceptional electrochemical properties due to their large SSA and abundant pore structure.^46,50

Fig. 2 (a) N₂ adsorption–desorption isotherms and (b) pore size distributions.

Fig. 3(a) and (b) display XRD patterns and Raman spectra of five samples, respectively. XRD and Raman analyses were utilized to assess the crystallinity and graphitization of the samples. The results revealed the presence of two prominent graphite characteristic peaks at approximately 2θ = 24° and 43° in the XRD patterns of the five samples, corresponding to the (002) and (100) crystal planes of graphite, respectively. The low intensity of the (002) and (100) peaks suggested that the carbon materials in the five samples consisted primarily of an amorphous structure with some graphite structure present.^51,52 All samples exhibited higher diffraction peaks in the low-angle region, suggesting the presence of numerous micropores in the samples, a finding consistent with the results of SSA analysis. A comparison between the HPC-Fe-10% sample and the other four samples revealed a significantly higher diffraction peak intensity for the (002) and (100) crystal planes in the former, attributed to the disruption of the APC graphite structure and the introduction of additional defects resulting from the use of activators. The disruption of the crystal structure of carbon materials resulted in a decrease in diffraction peak intensity of crystal planes. The Raman spectra in Fig. 3(b) provide additional evidence of the level of graphitization in the five samples. The absorption peaks observed at 1350 cm⁻¹ and 1590 cm⁻¹ correspond to the defects and disorder (D) band and graphitic (G) band of carbon materials, respectively.^53,54 The D band signifies the sp³ vibration of defective graphite, while the G band signifies the sp² vibration of graphitic carbon. Hence, the I_D/I_G ratio, which represents the absorption peak intensity ratio of the D band to the G band, is commonly utilized in characterizing the defect or graphitization levels of carbon materials. The findings indicated that the I_D/I_G ratios of APC, APC-Fe-5%, APC-Fe-10%, APC-Fe-20% and HPC-Fe-5% were 1.00, 1.11, 1.04, 1.00 and 1.04 respectively. A higher I_D/I_G value suggests more defects and a lower degree of graphitization in the carbon materials. This observation aligns with the results obtained from XRD analysis.

Fig. 3 (a) XRD patterns and (b) Raman spectra of the five samples.

Fig. 4 presents XPS measurement results of APC-Fe-10%. Fig. 4(a) illustrates the full XPS spectrum of APC-Fe-10%. Fig. 4(b) illustrates the division of the C 1s spectrum into four distinct carbon binding states at 288.6 eV (C=O), 286.7 eV (C–O), 284.8 eV (C–C), and 283.7 eV (C=C), indicating the presence of various oxygen functional groups in the carbon material.⁴⁶ Additionally, the high-resolution XPS spectrum of O 1s in Fig. 4(c) clearly shows three fitted peaks, which could be attributed to lattice oxygen (O_L, 530.1 eV), C=O (531.1 eV), and C–OH/C–O–C (532.5 eV).⁵¹ Additionally, Fig. 4(d) demonstrates the presence of Fe element in the carbon material, with characteristic peaks at 724.9 eV (Fe 2p_1/2) and 710.2 eV (Fe 2p_3/2).⁵⁵ Generally, these results revealed that heteroatoms are essential to improve the wettability of carbon materials.

Fig. 4 XPS spectra of APC-Fe-10%: (a) full XPS spectrum, (b) high-resolution C 1s spectrum, (c) high-resolution O 1s spectrum and (d) high-resolution Fe 2p spectrum.

Fig. 5(a) and (b) display the results of electrochemical performance tests conducted with the five samples in a three-electrode system with 6 M KOH as the electrolyte. Specifically, Fig. 5(a) depicts the CV curves of the five samples, all of which exhibited quasi-rectangular shapes when scanned at a rate of 100 mV s⁻¹. Fig. S2 (ESI†) shows the CV curves of five samples at 5–100 mV s⁻¹. The CV curve analysis indicated that the electric capacity of the five samples predominantly originated from the double electric layer capacitance. A comparison of the CV curves of the five samples revealed that the HPC-Fe-10% sample exhibited the smallest cross-sectional area, while the APC-Fe-10% sample displayed the largest cross-sectional area. The APC-Fe-5% sample demonstrated the second largest cross-sectional area, with the cross-sectional areas of the APC sample and APC-Fe-20% sample following in decreasing order. The cross-sectional area of the CV curve serves as a reflection of the energy storage performance of carbon materials. The variation in cross-sectional areas observed in this case can be primarily attributed to the discrepancies in SSA among the various samples. Consequently, the SSA of carbon materials emerged as the principal determinant influencing the specific capacitance of APC materials, aligning with prior literature findings.⁴⁶ A comparison between HPC-Fe-10% and APC specimens revealed that the cross-sectional area of the CV curve for the APC sample surpassed that for the HPC-Fe-10% sample, suggesting that the activation of durian peels by KHCO₃ effectively facilitated the formation of pores with a greater SSA, thus enabling the sample to have better charge storage capacity. The APC-Fe-10% sample exhibited the highest specific capacitance among all samples, suggesting the formation of a composite structure consisting of spherical carbon material and carbon nanosheets following hydrothermal treatment of durian peels with Fe₂(SO₄)₃. This composite structure facilitated the creation of additional contact interfaces between APC material and electrolyte, leading to enhanced diffusion rates of electrolyte ions within the electrode and ultimately improving the electrochemical properties of the material.⁵⁰

Fig. 5 Electrochemical behaviors of the five samples. (a) CV curves of five samples at 100 mV s⁻¹, (b) GCD curves of five samples at 1 A g⁻¹, (c) C_g of five samples estimated at 1–0 A g⁻¹ and (d) the cycling stability of APC-Fe-10% sample at 10 A g⁻¹.

Fig. 5(b) illustrates the GCD curves of five samples under a current density of 1 A g⁻¹. The curves exhibited linear symmetrical triangles with no discernible voltage drop, indicating favorable capacitive properties and electrochemical reversibility. Fig. S3 (ESI†) shows the GCD curves of five samples at 1–10 A g⁻¹. Notably, the APC-Fe-10% sample demonstrated the longest charge–discharge time, while the HPC-Fe-10% sample exhibited the shortest duration, the charge–discharge times of APC-Fe-5%, APC, and APC-Fe-20% samples following in decreasing order. Based on the capacitance calculation formula (1), the capacitance values of APC, APC-Fe-5%, APC-Fe-10%, APC-Fe-20%, and HPC-Fe-10% at a current density of 1 A g⁻¹ were determined to be 220, 237.6, 267, 189, and 121.8 F g⁻¹, respectively, in accordance with the order of CV curve area. Compared with specific capacitance in the literature, it was found that the specific capacitance of activated hydrothermal carbon prepared in this paper was larger (Table S3, ESI†).

Subsequently, the specific capacitance of these samples within the current density range of 1–10 A g⁻¹ was calculated using the capacitance calculation formula (1), and the results are presented in Fig. 5(c). The comparison revealed that the APC-Fe-10% sample exhibited the highest specific capacitance across all current density ranges, with the HPC-Fe-10% sample exhibiting the lowest specific capacitance across all current density ranges, in line with the findings of previous SSA analyses. This enhanced specific capacitance could be attributed to the substantial SSA and abundant pore structure composition of the APC materials, which facilitated superior electrochemical properties. Specifically, the specific capacitance of the APC-Fe-10% sample was measured as 267 F g⁻¹ at 1 A g⁻¹ and as 228 F g⁻¹ at 10 A g⁻¹, with a capacitance retention rate of 85.4%, indicating exceptional performance. In Fig. 5(c), the specific capacitance values of the five samples exhibited a gradual decline as current density increased. This phenomenon could be attributed to the shortened charge and discharge times associated with higher current densities, which hindered the efficient entry of charged ions into the pores. As current density increased, the transfer resistance of electrons also increased, impeding the transfer of electrolyte ions and leading to suboptimal utilization of the SSA, ultimately resulting in a decrease in capacitance.⁵⁶

Fig. 5(d) demonstrates that the APC-Fe-10% sample exhibited a capacitance retention rate of 93.3% after undergoing 10 000 consecutive cycles of charging and discharging at a current density of 10 A g⁻¹. Additionally, the GCD curves of the initial and final 10 cycles displayed in Fig. 5(d) both exhibit consistent isosceles triangles, providing further evidence of the excellent cyclic stability of the APC-Fe-10% sample.

Fig. 6 illustrates the EIS diagram of the five samples. It is evident from the diagram that all samples exhibited a semicircular arc in the high-frequency region and a straight line in the low-frequency region. The angle formed between the straight line in the low-frequency region and the Z′ axis signifies the electrolyte resistance to the internal charge of the electrode material. Additionally, the intersection of the semicircle in the high-frequency region on the real axis indicates the equivalent series resistance (R_s) of the sample, while the diameter of the high-frequency region in the EIS diagram corresponds to the charge transfer resistance (R_ct) at the electrode–electrolyte interface.^50,57 A comparison of the EIS diagrams of the five samples, namely APC, APC-Fe-5%, APC-Fe-10%, APC-Fe-20%, and HPC-Fe-10%, reveals that the values of R_s for these samples are 0.011 Ω, 0.056 Ω, 0.076 Ω, 0.10 Ω and 0.164 Ω respectively. It is observed that the R_s values for the five samples are comparable, with slight variations possibly attributed to variances in the inherent resistance of the carbon materials. The R_ct values for five samples APC, APC-Fe-5%, APC-Fe-10%, APC-Fe-20%, and HPC-Fe-10% were determined to be 0.44 Ω, 1.67 Ω, 2.27 Ω, 1.98 Ω, and 2.02 Ω, respectively.

Fig. 6 EIS diagram of the five samples in 6 M KOH.

Analysis of the R_ct values revealed that the sequence of resistance values for the samples was as follows: APC < APC-Fe-5% < APC-Fe-20% < HPC-Fe-10% < APC-Fe-10%. Notably, the APC sample exhibited the lowest R_ct value, while the APC-Fe-10% sample displayed the highest. APC-Fe-10% exhibited superior electrochemical properties attributed to its significantly larger SSA and unique carbon sphere/nanosheet structure. In comparison to APC and APC-Fe-20%, APC-Fe-5% demonstrated the second-best electrochemical performance despite its larger SSA and larger R_ct. APC displayed a smaller R_ct and larger SSA than APC-Fe-20%, resulting in better electrochemical performance than APC-Fe-20%. Conversely, HPC-Fe-10% showcased larger R_ct but significantly smallest SSA compared to the other samples, leading to the least favorable electrochemical performance. These findings suggested that the electrochemical properties of APC materials were influenced by SSA and R_ct. The nearly vertical line observed in the low-frequency range was indicative of diffusive resistance of the electrolyte within the electrode and ion diffusion into the electrode, suggesting capacitive behavior in all five samples.

The potential practical application of the APC-Fe-10% sample was assessed through the use of a symmetric SC, constructed by placing a filter paper between two identical APC-Fe-10% electrodes within a CR2032 cell. The assembled device underwent initial testing in 6 M KOH, with voltage cycling between 0 and 1 V. Analysis of the CV curves, as depicted in Fig. 7(a), revealed quasi-rectangular shapes indicative of ideal EDLC performance, with minimal distortion observed across scanning rates ranging from 5 to 100 mV s⁻¹. In a similar vein, the GCD curves displayed linearity and symmetrical triangular shapes at 0.5–10 A g⁻¹, indicating the superior coulombic efficiency and electrochemical reversibility of the system. The EIS diagram of the assembled cell exhibited a small semicircle and a vertical line, confirming the rapid charge transfer rate and exceptional capacitive performance, respectively. Additionally, the device's R_s and R_ct values were measured as 0.39 and 0.76 Ω, respectively. The rapid charge transfer and ion diffusion into the surface of the electrode were demonstrated through simulation using the equivalent circuit model (Fig. 7(c) inset). The Ragone plot for the symmetric SC is depicted in Fig. 7(d), showing a maximum specific energy of 4.67 W h kg⁻¹ at 125 W kg⁻¹. Even at a higher power density of 2500 W kg⁻¹, the specific energy remained at 4.17 W h kg⁻¹, the result showing that the assembled device had good stability.

Fig. 7 Electrochemical behaviors of the APC-Fe-10%-based symmetric device in 6 M KOH. (a) CV curves at 5–100 mV s⁻¹, (b) GCD curves at 0.5–10 A g⁻¹, (c) EIS diagram and (d) Ragone plot of APC-Fe-10%.

The KOH electrolyte-based cell exhibited a narrow voltage window. In order to increase the voltage window, the assembly and utilization were realized of a Na₂SO₄ electrolyte-based symmetric SC in various potential windows ranging from 0–1.0 to 0–2.0 V at a scan rate of 100 mV s⁻¹. Analysis of Fig. 8(a) revealed the stability of the device at 1.8 V, with slight deformation observed at a higher potential of 2.0 V, indicating a reversible electrochemical cycle at 0–1.8 V. The neutral electrolyte demonstrated the ability to achieve a wider voltage window compared to the alkaline electrolyte. The observed phenomena can be attributed to (1) elevated overpotentials of oxygen/hydrogen evolution reactions (OER/HER) resulting from the equilibrium of OH⁻ and H⁺ ions, and (2) enhanced stability at high potentials due to the strong solvation of Na⁺ and SO₄²⁻ ions.^58,59 Additionally, the deformation observed during passage of anodic current in the range of 1.8–2.0 V may be linked to the hydrolysis reaction of electrolytes. Furthermore, CV curves at various scanning rates (Fig. 8(b)) and GCD curves at different current densities (Fig. 8(c)) were recorded within the voltage window of 0–1.8 V. The device demonstrated the ability to maintain quasi-rectangular and symmetrical triangular shapes as scanning rates and current densities increase, indicating excellent electrochemical reversibility and rate capability. Fig. S4 (ESI†) shows the Nyquist plot of the APC-Fe-10%-based symmetric device in 1 M Na₂SO₄. The device's R_s and R_ct values were measured as 3.24 and 5.22 Ω, respectively. The Na₂SO₄-based cell, with its large working voltage and specific capacitance, achieved a maximum energy density of 14.45 W h kg⁻¹ at 225 W kg⁻¹ (Fig. 8(d)); even at a higher power density of 4500 W kg⁻¹, the specific energy remained at 10.75 W h kg⁻¹. The combination of fast charging and high energy density highlighted the significant practical application potential of the APC-Fe-10% electrode.

Fig. 8 Electrochemical behaviors of the APC-Fe-10%-based symmetric device in 1 M Na₂SO₄. (a) CV curves at 100 mV s⁻¹ with various potential windows, (b) CV curves at 5–100 mV s⁻¹, (c) GCD curves at 0.5–10 A g⁻¹ and (d) Ragone plot of APC-Fe-10%.

4. Conclusion

In summary, we used a synthetic strategy of hydrothermal treatment and mild activation to prepare Fe-modified APC with a high SSA (2100.5 m² g⁻¹) and carbon sphere/nanosheet structure by utilizing durian peels as a carbon precursor. Using this method, carbon form adjustment could be achieved by adding Fe₂(SO₄)₃ to the hydrothermal treatment, and the mild activation of KHCO₃ could activate the porous carbon while maintaining the spherical morphology. The carbon sphere/nanosheet structure could not only prevent the stacking of carbon nanosheets, but also provide sufficient space for rapid ion/electrolyte diffusion to effectively utilize the SSA. When the obtained product was applied as an electrode material, a high capacity of 267 F g⁻¹ can be achieved at 1 A g⁻¹ and good cycling stability (6.7% capacitance loss after 10,000 cycles). The Na₂SO₄-based cell, with its large working voltage and specific capacitance, achieved a maximum energy density of 14.45 W h kg⁻¹ at 225 W kg⁻¹. The combination of hydrothermal treatment and mild activation provided an effective method for the conversion of waste biomass into high-performance electrode materials.

Author contributions

Liujie Wang: methodology, investigation, validation, conceptualization, data curation, writing – original draft preparation. Xueji Ma: methodology, writing – reviewing and editing draft preparation, supervision. Zhihua Ma: methodology, investigation, validation, data curation, writing – original draft preparation, visualization. Pengfa Li: conceptualization, methodology, writing – reviewing and editing draft preparation. Wenbo Li: methodology, conceptualization, writing – reviewing, editing draft preparation, supervision.

Data availability

Data are available on request from the authors. The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by Henan Province Key R&D and Promotion Special Project (Science and Technology Research) (232102310375), the Key Research Programs of Higher Education Institutions in Henan Province (22A480007) and Xinxiang City Major Science and Technology Projects (ZD2020008).

References

1. H. Chen, Y. Zheng, J. Li, L. Li and X. Wang, AI for Nanomaterials Development in Clean Energy and Carbon Capture, Utilization and Storage (CCUS), ACS Nano, 2023, 17(11), 9763–9792.
2. J. D. Hunt, J. Jakub, Z. Behnam, T. Wenxuan, N. Andreas, G. Fei and Đ. Bojan, Electric truck gravity energy storage: An alternative to seasonal energy storage, Energy Storage, 2024, 6(1), e575.
3. J. Baigorri, F. Zaversky and D. Astrain, Massive grid-scale energy storage for next-generation concentrated solar power: A review of the potential emerging concepts, Renewable Sustainable Energy Rev., 2023, 185, 113633.
4. J. Chang and Y. Yang., Recent advances in zinc–air batteries: self-standing inorganic nanoporous metal films as air cathodes, Chem. Commun., 2023, 59, 5823–5838.
5. Y. Wang, Y. Weisen, Z. Chunmei, H. Xiaoshuai, H. Jingquan, H. Shuijian, H. Jiapeng and J. Shaohua, Effect of precursor selection on the structure and Li-storage properties of wood-based hard carbon thick electrodes, Ind. Crops Prod., 2023, 198, 116664.
6. T. Guo, K. Xiang, X. Wen, W. Zhou and H. Chen., Facile construction on flower-like CuS microspheres and their applications for the high-performance aqueous ammonium-ion batteries, Mater. Res. Bull., 2024, 170, 112595.
7. Y. Huang, L. Xing, S. Pei, W. Zhou, Y. Hu, W. Deng, L. Chen, H. Zhu and H. Chen., Co₉S₈/CNTs microspheres as superior-performance cathodes in aqueous ammonium-ion batteries, Trans. Nonferrous Met. Soc. China, 2023, 33(11), 3452–3464.
8. X. Wen, L. Jinhua, X. Kaixiong, Z. Wei, Z. Changfan and C. Han, High-performance monoclinic WO₃ nanospheres with the novel NH⁴⁺ diffusion behaviors for aqueous ammonium-ion batteries, Chem. Eng. J., 2023, 458, 141381.
9. Y. Liu, K. Xiang, W. Zhou, W. Deng, H. Zhu and H. Chen., Investigations on tunnel-structure MnO₂ for utilization as a high-voltage and long-life cathode material in aqueous ammonium-ion and hybrid-ion batteries, Small, 2024, 20(20), 2308741.
10. S. Jiang, H. Liu, W. Zhang and Y. Lu, Bioanode boosts efficacy of chlorobenzenes-powered microbial fuel cell: Performance, kinetics, and mechanism, Bioresour. Technol., 2024, 405, 130936.
11. L. Li, J. S. Nam, M. S. Kim, Y. Wang, S. Jiang, H. Hou and I.-D. Kim, Sulfur-Carbon Electrode with PEO-LiFSI-PVDF Composite Coating for High-Rate and Long-Life Lithium-Sulfur Batteries, Adv. Energy Mater., 2023, 13(36), 2302139.
12. F. Xiankai, K. Xiang, W. Zhou, W. Deng, H. Zhu, L. Chen and H. Chen, A novel improvement strategy and a comprehensive mechanism insight for α-MnO₂ energy storage in rechargeable aqueous zinc-ion batteries, Carbon Energy, 2024, e536.
13. Z. Tian, C. Yang, C. Zhang, X. Han, J. Han, K. Liu, S. He, G. Duan, S. Jian, J. Hu, W. Yang and S. Jiang, In situ activation of resorcinol-furfural resin derived hierarchical porous carbon for supercapacitors and zinc-ion hybrid capacitors, J. Energy Storage, 2024, 85, 111130.
14. C. Song, H. Lin, T. Wang, B. Li, R. Lu and S. Zhang, Targeted Synthesis of “Urechis Unicinctus”-Like Nitrogen-Doped Porous Carbon Nanorods for Supercapacitors, Energy Fuels, 2023, 37(10), 7511–7521.
15. Y. Sun, D. Xu, Z. He, Z. Zhang, L. Fan and S. Wang, Green fabrication of pore-modulated carbon aerogels using a biological template for high-energy density supercapacitors, J. Mater. Chem. A, 2023, 11(37), 20011–20020.
16. Z. Xu, Z. Sun, J. Shan, S. Jin, J. Cui, Z. Deng, M. H. Seo and X. Wang, O, N-Codoped, Self-Activated, Holey Carbon Sheets for Low-Cost And High-Loading Zinc-Ion Supercapacitors, Adv. Funct. Mater., 2024, 34(14), 2302818.
17. A. Zhang, Q. Zhang, H. Fu, H. Zong and H. Guo, Metal–Organic Frameworks and Their Derivatives-Based Nanostructure with Different Dimensionalities for Supercapacitors, Small, 2023, 19(48), 2303911.
18. J. Xiao, H. Li, H. Zhang, S. He, Q. Zhang, K. Liu, S. Jiang, G. Duan and K. Zhang, Nanocellulose and its derived composite electrodes toward supercapacitors: Fabrication, properties, and challenges, J. Bioresour. Bioprod., 2022, 7(4), 245–269.
19. Z. Shang, X. An, S. Nie, N. Li, H. Cao, Z. Cheng, H. Liu, Y. Ni and L. Liu., Design of B/N Co-doped micro/meso porous carbon electrodes from CNF/BNNS/ZIF-8 nanocomposites for advanced supercapacitors, J. Bioresour. Bioprod., 2023, 8(3), 292–305.
20. Y. Zhu, Z. Wang, X. Zhu, Z. Feng, C. Tang, Q. Wang, Y. Yang, L. Wang, L. Fan and J. Hou, Optimizing Performance in Supercapacitors through Surface Decoration of Bismuth Nanosheets, ACS Appl. Mater. Interfaces, 2024, 16(13), 16927–16935.
21. X. Li, J. Ren, D. Sridhar, B. B. Xu, H. Algadi, Z. M. El-Bahy, Y. Ma, T. Li and Z. Guo., Progress of layered double hydroxide-based materials for supercapacitors, Mater. Chem. Front., 2023, 7, 1520–1561.
22. E. Pameté, L. Köps, F. Alexander Kreth, S. Pohlmann, A. Varzi, T. Brousse, A. Balducci and V. Presser, The Many Deaths of Supercapacitors: Degradation, Aging, and Performance Fading, Adv. Energy Mater., 2023, 13(29), 2301008.
23. K. Gong, H. Lee, Y. Choi, G. Jung, K. Keum, J. W. Kim and J. S. Ha, A flexible supercapacitor with high energy density and wide range of temperature tolerance using a high-concentration aqueous gel electrolyte, Electrochim. Acta, 2024, 475, 143585.
24. Y. Zhang, C.-G. Zhou, X.-H. Yan, Y. Cao, H.-L. Gao, H.-W. Luo, K.-Z. Gao, S.-C. Xue and X. Jing, Recent advances and perspectives on graphene-based gels for superior flexible all-solid-state supercapacitors, J. Power Sources, 2023, 565, 232916.
25. K. Paritosh and A. Bose, Application of biogenic carbon in renewable energy vectors and devices: A step forward to decarbonization, Renewable Sustainable Energy Rev., 2024, 197, 114399.
26. Z. Guo, H. Xiaoshuai, Z. Chunmei, H. Shuijian, L. Kunming, H. Jiapeng, Y. Weisen, J. Shaoju, J. Shaohua and D. Gaigai, Activation of biomass-derived porous carbon for supercapacitors: A review, Chin. Chem. Lett., 2021, 35(7), 109007.
27. Y.-L. Bai, C.-C. Zhang, F. Rong, Z.-X. Guo and K.-X. Wang., Biomass-Derived Carbon Materials for Electrochemical Energy Storage, Chem. – Eur. J., 2024, 30(23), e202304157.
28. S. Wei, W. Caichao and W. Yiqiang, Recent advances in wood-based electrode materials for supercapacitors, Green Chem., 2023, 25(9), 3322–3353.
29. C. Xiong, C. Zheng, X. Jiang, X. Xiao, H. Wei, Q. Zhou and Y. Ni., Recent progress of green biomass based composite materials applied in supercapacitors, sensors, and electrocatalysis, J. Energy Storage, 2023, 72, 108633.
30. J. Tamuly, D. Bhattacharjya and B. K. Saikia., Graphene/Graphene Derivatives from Coal, Biomass, and Wastes: Synthesis, Energy Applications, and Perspectives, Energy Fuels, 2022, 36(21), 12847–12874.
31. Y. Zhang, H. Pan, Q. Zhou, K. Liu, W. Ma and S. Fan., Biomass-derived carbon for supercapacitors electrodes-A review of recent advances, Inorg. Chem. Commun., 2023, 153, 110768.
32. R. O. Hasan, B. Ercan, A. N. Acikkapi, S. Ucar and S. Karagöz., Effects of Metal Chlorides on the Hydrothermal Carbonization of Grape Seeds, Energy Fuels, 2021, 35(10), 8834–8843.
33. K. Liang, Y. Chen, D. Wang, W. Wang, S. Jia, N. Mitsuzakic and Z. Chen, Post-modified biomass derived carbon materials for energy storage supercapacitors: a review, Sustainable Energy Fuels, 2023, 7(15), 3541–3559.
34. S. Fakudze and J. Chen., A critical review on co-hydrothermal carbonization of biomass and fossil-based feedstocks for cleaner solid fuel production: Synergistic effects and environmental benefits, Chem. Eng. J., 2023, 457, 141004.
35. Y. Yin, Q. Liu, Y. Zhao, T. Chen, J. Wang, L. Gui and C. Lu., Recent Progress and Future Directions of Biomass-Derived Hierarchical Porous Carbon: Designing, Preparation, and Supercapacitor Applications, Energy Fuels, 2023, 37(5), 3523–3554.
36. E. Hao, W. Liu, S. Liu, Y. Zhang, H. Wang, S. Chen, F. Cheng, S. Zhao and H. Yang, Rich sulfur doped porous carbon materials derived from ginkgo leaves for multiple electrochemical energy storage devices, J. Mater. Chem. A, 2017, 5(5), 2204–2214.
37. H. Chen, F. Yu, G. Wang, L. Chen, B. Dai and S. Peng, Nitrogen and Sulfur Self-Doped Activated Carbon Directly Derived from Elm Flower for High-Performance Supercapacitors, ACS Omega, 2018, 3(4), 4724–4732.
38. S. A. Nicolae, H. Au, P. Modugno, H. Luo, A. E. Szego, M. Qiao, L. Li, W. Yin, H. J. Heeres, N. Berge and M.-M. Titirici, Recent advances in hydrothermal carbonisation: from tailored carbon materials and biochemicals to applications and bioenergy, Green Chem., 2020, 22(15), 4747–4800.
39. T. You L. Zhang, X. Zhou T. Zhou and F. Xu, Interconnected Hierarchical Porous Carbon from Lignin-Derived Byproducts of Bioethanol Production for Ultra-High Performance Supercapacitors, ACS Appl. Mater. Interfaces, 2016, 8(22), 13918–13925.
40. X. Zhang, T. Cao, G. Zhang, Q. Liu, G. Kong, K. Wang, Y. Jiang, X. Zhang and L. Han., Sustainable hydrothermal carbon for advanced electrochemical energy storage, J. Mater. Chem. A, 2024, 12(9), 4996–5039.
41. J. Fan, L. Duan, X. Zhang, Z. Li, P. Qiu, Y. He and P. Shang, Selective Adsorption and Recovery of Silver from Acidic Solution Using Biomass-Derived Sulfur-Doped Porous Carbon, ACS Appl. Mater. Interfaces, 2023, 15(33), 40088–40099.
42. M. Cavali, N. Libardi Junior, J. D. de Sena, A. L. Woiciechowski, C. R. Soccol, P. Belli Filho, R. Bayard, H. Benbelkacem and A. B. de Castilhos Junior, A review on hydrothermal carbonization of potential biomass wastes, characterization and environmental applications of hydrochar, and biorefinery perspectives of the process, Sci. Total Environ., 2023, 857(3), 159627.
43. J. Yang, Z. Zhang, J. Wang, X. Zhao, Y. Zhao, J. Qian and T. Wang, Pyrolysis and hydrothermal carbonization of biowaste: A comparative review on the conversion pathways and potential applications of char product, Sustainable Chem. Pharm., 2023, 33, 101106.
44. S. Liu, X. Hu, J. Ma, M. Li, H. Lin and S. Han, N/P Codoped Carbon Materials with an Ultrahigh Specific Surface Area and Hierarchical Porous Structure Derived from Durian Peel for High-Performance Supercapacitors, Energy Fuels, 2020, 34(11), 14948–14957.
45. Z. Xu, X. Zhang, X. Yang, Y. Yu, H. Lin and K. Sheng, Synthesis of Fe/N Co-doped Porous Carbon Spheres Derived from Corncob for Supercapacitors with High Performances, Energy Fuels, 2021, 35(17), 14157–14168.
46. K. MacDermid-Watts, E. Adewakun, O. Norouzi, T. Deb Abhi, R. Pradhan and A. Dutta., Effects of FeCl₃ Catalytic Hydrothermal Carbonization on Chemical Activation of Corn Wet Distillers’ Fiber, ACS Omega, 2021, 6(23), 14875–14886.
47. M. Sevilla and A. B. Fuertes., A Green Approach to High-Performance Supercapacitor Electrodes: The Chemical Activation of Hydrochar with Potassium Bicarbonate, ChemSusChem, 2016, 9(14), 1880–1888.
48. M. Xu, Q. Yu, Z. Liu, J. Lv, S. Lian, B. Hu, L. Mai and L. Zhou, Tailoring porous carbon spheres for supercapacitors, Nanoscale, 2018, 10(46), 21604–21616.
49. M. Vijayakumar, A. Bharathi Sankar, D. Sri Rohita, T. Narasinga Rao and M. Karthik., Conversion of Biomass Waste into High Performance Supercapacitor Electrodes for Real-Time Supercapacitor Applications, ACS Sustainable Chem. Eng., 2019, 7(20), 17175–17185.
50. Y. Wu, J.-P. Cao, X.-Y. Zhao, Z.-Q. Hao, Q.-Q. Zhuang, J.-S. Zhu, X.-Y. Wang and X.-Y. Wei., Preparation of porous carbons by hydrothermal carbonization and KOH activation of lignite and their performance for electric double layer capacitor, Electrochim. Acta, 2017, 252, 397–407.
51. L. Hu, J. Hou, Y. Ma, H. Li and T. Zhai, Multi-heteroatom self-doped porous carbon derived from swim bladders for large capacitance supercapacitors, J. Mater. Chem. A, 2016, 4(39), 15006–15014.
52. M. Karnan, A. G. Karthick Raj, K. Subramani, S. Santhoshkumara and M. Sathish, The fascinating supercapacitive performance of activated carbon electrodes with enhanced energy density in multifarious electrolytes, Sustainable Energy Fuels, 2020, 4(6), 3029–3041.
53. N. Yadav, Ritu, Promila and S. A. Hashmi., Hierarchical porous carbon derived from eucalyptus-bark as a sustainable electrode for high-performance solid-state supercapacitors, Sustainable Energy Fuels, 2020, 4(4), 1730–1746.
54. J. Gao, D. Fan and X. Liu, Simultaneous gas expansion and nitrogen doping strategy to prepare licorice root residues-derived nitrogen doped porous carbon for supercapacitors, New J. Chem., 2021, 45(34), 15469–15474.
55. C. Guan, W. Zhao, Y. Hu, Q. Ke, X. Li, H. Zhang and J. Wang, High-Performance Flexible Solid-State Ni/Fe Battery Consisting of Metal Oxides Coated Carbon Cloth/Carbon Nanofiber Electrodes, Adv. Energy Mater., 2016, 6(20), 1601034.
56. B. Duan, X. Gao, X. Yao, Y. Fang, L. Huang, J. Zhou and L. Zhang, Unique elastic N-doped carbon nanofibrous microspheres with hierarchical porosity derived from renewable chitin for high rate supercapacitors, Nano Energy, 2016, 27, 482–491.
57. D. Li, Y. Sun, S. Chen, J. Yao, Y. Zhang, Y. Xia and D. Yang., Highly Porous FeS/Carbon Fibers Derived from Fe-Carrageenan Biomass: High-capacity and Durable Anodes for Sodium-Ion Batteries, ACS Appl. Mater. Interfaces, 2018, 10(20), 17175–17182.
58. Y. Zhang, S. Yang, S. Wang, X. Liu and L. Li., Microwave/freeze casting assisted fabrication of carbon frameworks derived from embedded upholder in tremella for superior performance supercapacitors, Energy Storage Mater., 2019, 18, 447–455.
59. Y. Zhao, P. Chen, S. Tao, X. Zu, S. Li and L. Qiao, Nitrogen/oxygen co-doped carbon nanofoam derived from bamboo fungi for high-performance supercapacitors, J. Power Sources, 2020, 479, 228835.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj03624g

---