Fabrication of Biomimetic Ultralight Carbon Aerogel for High-Performance Flexible Supercapacitors with Enhanced Voltage Windows and Energy Density

Fabrication of Biomimetic Ultralight Carbon Aerogel for High-Performance Flexible Supercapacitors with Enhanced Voltage Windows and Energy Density | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 13 April 2025 V1 Latest version Share on Fabrication of Biomimetic Ultralight Carbon Aerogel for High-Performance Flexible Supercapacitors with Enhanced Voltage Windows and Energy Density Authors : Ao Song , Yunchao Li 0000-0002-6470-9639 [email protected] , Jie Wu , Dingkun Yuan , Maosheng Liu , and Guangxue Zhang Authors Info & Affiliations https://doi.org/10.22541/au.174452411.16415110/v1 168 views 118 downloads Contents Abstract 1. Introduction 2. Experiment 3. Results and discussion 4. Conclusions ToC figure Supplementary Material References Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract jabbrv-ltwa-all.ldf jabbrv-ltwa-en.ldf This study presents a high-performance ultralight carbon aerogel (denoted as PFSCA) synthesized via hydrothermal self-assembly, solvent exchange, and carbonization. Compared to passion fruit shell-derived carbon (PFSC) produced by direct pyrolysis, PFSCA exhibits a significantly lower packing density (0.056 g cm⁻³ vs. 0.210 g cm⁻³ for PFSC). Despite its reduced specific surface area (SSA: 455 m² g⁻¹ vs. 1485 m² g⁻¹ for PFSC), PFSCA achieves a comparable specific capacitance (167 F g⁻¹ vs. 170 F g⁻¹ for PFSC) and superior electrochemical performance. These enhancements are attributed to PFSCA’s unique structural and chemical features: (1) helical hollow tubes and surface nanospheres that shorten ion diffusion pathways, (2) a high degree of graphitization with enlarged d₀₀₂ interlayer spacing (0.38 nm vs. 0.35 nm for PFSC), and (3) surface-functional C=O and -COOH groups that improve hydrophilicity and charge transfer. A flexible symmetric supercapacitor (FASSC) fabricated with PFSCA electrodes operates at a 2.0 V voltage window, delivering an energy density of 30.6 Wh kg⁻¹ at 2000 W kg⁻¹. The FASSC retains 100% Coulombic efficiency over 6,000 cycles and demonstrates exceptional mechanical flexibility under bending angles of 0–180°. This work provides a cost-effective and scalable strategy for carbon aerogel synthesis, highlighting their potential for advanced flexible energy storage applications. Article category: (Research Article) Subcategory: (Supercapacitor) Abstract: This study presents a high-performance ultralight carbon aerogel (denoted as PFSCA) synthesized via hydrothermal self-assembly, solvent exchange, and carbonization. Compared to passion fruit shell-derived carbon (PFSC) produced by direct pyrolysis, PFSCA exhibits a significantly lower packing density (0.056 g cm⁻³ vs. 0.210 g cm⁻³ for PFSC). Despite its reduced specific surface area (SSA: 455 m² g⁻¹ vs. 1485 m² g⁻¹ for PFSC), PFSCA achieves a comparable specific capacitance (167 F g⁻¹ vs. 170 F g⁻¹ for PFSC) and superior electrochemical performance. These enhancements are attributed to PFSCA’s unique structural and chemical features: (1) helical hollow tubes and surface nanospheres that shorten ion diffusion pathways, (2) a high degree of graphitization with enlarged d ₀₀₂ interlayer spacing (0.38 nm vs. 0.35 nm for PFSC), and (3) surface-functional C=O and -COOH groups that improve hydrophilicity and charge transfer. A flexible symmetric supercapacitor (FASSC) fabricated with PFSCA electrodes operates at a 2.0 V voltage window, delivering an energy density of 30.6 Wh kg⁻¹ at 2000 W kg⁻¹. The FASSC retains 100% Coulombic efficiency over 6,000 cycles and demonstrates exceptional mechanical flexibility under bending angles of 0–180°. This work provides a cost-effective and scalable strategy for carbon aerogel synthesis, highlighting their potential for advanced flexible energy storage applications. Keywords: Biomass waste; Ultralight carbon aerogel; Hydrothermal self-assembly; Flexible supercapacitor 1. Introduction In recent years, flexible wearable electronic devices (FWED) have developed rapidly. Further development of flexible portable energy storage technologies is critical. [1, 2] Flexible supercapacitors offer long service life, high power density, low production and maintenance costs, and diverse applications in portable electronics. [3, 4] The electrode, as the core component of a flexible supercapacitor, has a significant influence on the overall property of the device. [5] Carbon materials (CMs) are widely used for flexible supercapacitor electrodes because of their abundant availability, low cost, high chemical stability, and flexibility after design. [6-8] Currently, low-dimensional CMs such as graphene sheets as well as carbon nanotubes are mainly used in flexible supercapacitors. However, these materials are often associated with complex manufacturing processes and high costs. The development of novel carbon-based electrodes that are environmentally friendly, cost-effective, and offer high energy density and flexibility remains a key research area. [9] Carbon aerogels are widely used in flexible supercapacitor electrodes owing to their light weight, low density, high porosity and good mechanical flexibility. The three-dimensional porous structure of carbon aerogels will enhance ion/electron transfer and improve electrical conductivity. [10] Various precursors, such as nanocellulose and chitosan, are used to produce carbon aerogels. [11, 12] For instance, Liu et al. [13] synthesized a composite of cellulose nanofibers and graphene oxide-carbon aerogels by freeze-drying at -56°C. The resulting flexible supercapacitor demonstrates excellent flexibility with a capacity retention rate of 82% after 5000 cycles. Gao et al. [14] prepared a zinc-chitosan-carbon aerogel composite by freeze-drying at -60°C. The specific energy density of the resulting symmetrical supercapacitor was 0.78 μWh cm - ² and it retained 99% of its capacity after 10,000 cycles. While carbon aerogels have good flexibility and electrochemical performance, their production typically requires cryogenic freeze-drying technology (< -50°C). Furthermore, most precursors used are expensive, making the production process costly, complex and unsuitable for continuous manufacturing [15, 16] . Biomass-derived carbon materials offer a cost-effective alternative for carbon aerogel production. Biomasses such as grapefruit, watermelon, and durian etc. with high moisture content and spongy structures are ideal precursors for carbon aerogels. [17] Researchers have successfully used biomass-derived carbon aerogels for applications in pollutant adsorption, [18] reaction catalysis, [19] and electromagnetic wave absorption. [20] However, few studies have been focused on the use of biomass-derived carbon aerogels in flexible electrodes. In this study, an ultralight carbon aerogel (PFSCA) derived from passion fruit shell were synthesized through hydrothermal self-assembly, solvent exchange, and carbonization. For comparison, passion fruit shell-derived carbon (PFSC) was prepared via direct pyrolysis under identical carbonization conditions. PFSCA was employed as an electrode material in flexible supercapacitors. Despite PFSCA’s lower specific surface area (SSA: 455 m² g⁻¹ vs. 1485 m² g⁻¹ for PFSC), it delivered a comparable specific capacity (167 F g⁻¹ vs. 170 F g⁻¹ for PFSC) and superior electrochemical performance. This can be attributed to: PFSCA features helical hollow tubes and surface nanospheres dominated by mesopores, which optimize ion transport pathways and reduce diffusion resistance. The high degree of graphitization and enlarged d 002 interlayer spacing in PFSCA improve electrical conductivity, facilitating charge transfer. C=O and -COOH functional groups on PFSCA’s surface enhance hydrophilicity, promoting electrolyte interaction and compensating for its lower SSA. A flexible symmetrical supercapacitor (FASSC) constructed with PFSCA electrodes achieved a 2 V operating voltage in a PVA/KOH gel electrolyte, delivering an energy density of 30.6 Wh kg⁻¹ at 2000 W kg⁻¹. Notably, the FASSC retains 100% coulombic efficiency over 6,000 cycles and maintained stable electrochemical properties under repeated bending at various angles, demonstrating robust mechanical flexibility. 2. Experiment 2.1. Materials Passion fruits are purchased in Beiliu City, Yulin, Guangxi. The CO 2 gas used is a highly pure gas with a purity of 99.99%. Poly (vinyl alcohol) (PVA) was purchased from the Macklin Biochemical Technology Co. LTD. All chemical reagents used are analytically pure. 2.2. PFSCA and PFSC preparation and testing Fig. 1 (a) illustrates the synthesis process of carbon aerogel from passion fruit shell (PFS). The PFS fragments were first washed with deionized water (DW) to eliminate surface impurities and cut into 2×2 cm pieces. These pieces underwent hydrothermal treatment at 180°C for 10 hours in a stainless steel autoclave. Given that cellulose and hemicellulose constitute over 70% of PFS composition, these polysaccharides were hydrolyzed into glucose and fructose during the hydrothermal process. These monosaccharides subsequently self-assembled into cross-linked hydrogel structures via hydrogen bonding ( Fig. 1 (b)). [21, 22] The resulting hydrogel was frozen at −25°C for 12 hours and then subjected to solvent exchange in ethanol under ultrasonication (30-minute intervals, repeated three times). The gel was dried at 30°C for 10 hours to produce passion fruit shell aerogel (PFSA). The PFSA was carbonized in a tube furnace under a CO₂ atmosphere by heating to 700°C at a rate of 10°C min⁻¹, followed by a 30-minute hold at the target temperature to yield carbonized material. This material was further purified through sequential washing with 1 M HCl, anhydrous ethanol, and DW, yielding the final passion fruit shell-derived carbon aerogel (PFSCA). For comparison, raw PFS was directly pyrolyzed at 700°C under identical CO₂ conditions to produce passion fruit shell-derived carbon (PFSC). Both PFSCA and PFSC were processed into electrode materials using methods detailed in our prior work. [23] The electrochemical properties of the samples were tested using a three-electrode system (CHI660E, Chenhua, China). Details of the material characterization and test methods can be found in the supporting information. 2.3. FASSC preparation and testing FASSC was prepared using a PVA-KOH gel (PKG) electrolyte. Briefly, adding about 3 g of PVA into 30 ml of DW. The mixture is stirred at 90 °C until clear. Then slowly drop 10 mL of 6M KOH into the aqueous PVA solution and stir for 30 minutes. The mixture is cooled to indoor temperature to obtain a PKG electrolyte. Prepare two electrode materials with nearly the same weight as the working electrode. Using nickel foam as the collector liquid, a symmetrical carbon electrode was immersed in a PKG electrolyte. After drying in the oven, FASSC was obtained. The specific capacitance ( C s ), power density ( P d ) as well as energy density ( E d ) of the FASSC sample are calculated as follows. The meaning of the characters can be found in the literature [24, 25] ：\(C_{s}=\frac{2I\mathrm{\Delta}t}{m\mathrm{\Delta}V}\) (1)\(P_{d}=\frac{E}{\mathrm{\Delta}t}\times 3600\) (2)\(E_{d}=\frac{C_{S}\times{\mathrm{\Delta}V}^{2}}{2\times 3.6}\) (3) 3. Results and discussion 3.1. Physical and chemical properties SEM was employed to characterize the micromorphology of the samples ( Fig. 2 ). The raw PFS sample exhibits hollow, threaded tubular structures with diameters averaging 10.9 µm ( Fig. 2 (a-c)). In contrast, the PFSCA sample retains the intrinsic bio-derived architecture of PFS but with reduced hollow tube diameters ( 2–5 μm , Fig. 2 (d)) and the wrinkle structure on the outer surface of the pipeline collapsed ( Fig. 2 (e)), likely due to pore contraction during pyrolysis. Additionally, PFSCA displays a porous surface network composed of carbon nanospheres ( ~100 nm diameter , Fig. 2 ( f )). This hierarchical structure—helical hollow tubes and surface nanospheres—is anticipated to shorten ion/electron diffusion pathways within electrolytes. By comparison, the PFSC sample shows partially collapsed hollow tubes with smooth inner walls ( Fig. 2 (g-h)). Its surface features abundant open pores ( <500 nm diameter , Fig. 2 (i)), a morphology that may reduce internal series resistance. Fig. 3 (a) and 3 (b) display the adsorption isotherms as well as pore size distribution (PSD) curves for the PFSCA and PFSC samples, respectively. Both materials exhibit a type I isotherm curve and a type H4 hysteresis loop, suggesting the presence of both microporous and mesoporous structures. [26, 27] The PFSCA sample mainly has a pore-size around 3.7 nm, while the PFSC sample has a pore-size around 1.7 nm. Detailed pore structure parameters can be seen in Table S1 . The PFSCA sample has a specific surface area (SSA) of 455 m² g⁻¹, with a pore volume (PV) of 0.24 cm³ g⁻¹ and a micropore coverage of 63%. In contrast, the PFSC sample exhibits significantly higher SSA (1485 m² g⁻¹), with a PV of 0.85 cm³ g⁻¹, and micropore fraction (80%), all exceeding PFSCA values. This divergence arises from structural differences: PFSCA retains hollow tubular architectures during gelation, favoring mesopore formation, while direct pyrolysis of PFSC generates abundant micropores and open pores via volatile release. The mesoporous dominance in PFSCA likely enhances ion diffusion kinetics, improving rate performance at high current densities. [28, 29] The crystal structure of the PFSCA sample was compared with that of the PFSC sample ( Fig. 3 (c)). Both materials exhibit broad peaks around 43.4° and 24°, which match with the (100) and (002) crystal faces of the graphitic carbon structure, respectively. [30] The results indicate that both passion fruit shell-derived carbon samples exhibit a typical amorphous carbon structure. Further analysis reveals that the (002) diffraction peaks of the PFSCA and PFSC samples are located at 23.1° and 25.3°, respectively. The (002) peak of the PFSCA sample is shifted to a smaller angle, indicating a larger d 002 distance. [31, 32] Bragg’s law was used to calculate the d 002 spacing (\(d_{002}=\frac{\lambda}{2\sin\theta_{002}}\), (Cu-Kα, λ=1.540 nm)). The d 002 distance of the PFSCA sample was 0.38 nm, larger than the d 002 distance of the PFSC sample (0.35 nm). The larger layer spacing in the PFSCA sample will facilitate ion diffusion. Besides, the intensity of the crystal plane diffraction peak for the PFSCA sample is larger than that for the PFSC sample, implying that the PFSCA sample has a more graphitic carbon structure. The Raman spectrum of the sample is displayed in Fig. 3 (d). The characteristic peaks for PFSCA and PFSC samples appear at 1590 cm⁻¹ and 1350 cm⁻¹, corresponding to the G band and D band, respectively. The G band corresponds to the graphitized carbon structure, while the D band represents the disordered carbon structure of the material. [33, 34] The ratio of the peak intensities (I D /I G ) provides insight into the degree of graphitization and defects in the material. The results show the I D /I G values for the PFSCA and PFSC are 1.10 and 1.18, respectively. This suggests that the PFSCA sample has a higher degree of graphitization, while the PFSC sample contains more defects. This finding is consistent with the XRD results. The higher degree of graphitization and the larger d 002 spacing in the PFSCA sample will contribute to their better electrical conductivity. [35] In contrast, the higher number of defects in the PFSC sample may enhance their specific capacity. Fig. 4 (a) presents the overall XPS spectra of the PFSCA and PFSC samples. The spectra reveal that the main elements in both samples are carbon (C), oxygen (O), and nitrogen (N), with corresponding peaks at 284.8 eV, 532.8 eV, and 400.5 eV, respectively. Table S2 provides the relative atomic composition of each element. The results show that carbon is the predominant element in both samples, with atomic percentages of 92.8% for PFSCA and 87.8% for PFSC. The oxygen and nitrogen contents on the surface of the PFSCA sample is lower than that of the PFSC sample. This reduction may because of the hydrothermal treatment and solvent replacement, which probably led to the degradation of nitrogen- and oxygen- functional groups of the sample. [36] The fine XPS spectra of the C1s, N1s, and O1s orbitals were further analyzed. The C1s spectra of the PFSCA and PFSC can be deconvoluted into four peaks located at 284.8, 286.2, 288.2, and 291.2 eV ( Fig. S1 ). These peaks match to the C=C, C-O/C-N, C=O as well as π -π* bonds, respectively. [37, 38] The N1s spectrum can be divided into three main peaks: pyridine N (N-6) at 398.38 eV, pyrrolidine N (N-5) at 400.38 eV, as well as nitric oxide (N-X) at 402.78 eV ( Fig. 4 (b, c)). The relative content of these three nitrogen species is displayed in Fig. 4 (d). There are significant differences in nitrogen species between the PFSCA and PFSC samples. The PFSCA sample contains mainly N-5 and have a higher proportion of N-X. In contrast, the PFSC sample contains more N-5 and N-6 species. It is generally believed that N-5 and N-6 species provide additional redox active sites that increase the pseudocapacitance of the material. [39, 40] The presence of N-X may enhance the electric conductivity. Based on these observations, it can be inferred that the PFSCA sample will exhibit better conductivity, while the PFSC sample may offer a higher capacitance. The O1s spectrum of the PFSCA sample can be deconvoluted into four peaks located at 531.18, 532.68, 534.18, and 537.38 eV ( Fig. 4 (e)), which match with the C=O, C-O-H, -COOH, and oxygen adsorbed from water or carbon dioxide, respectively. [41, 42] In contrast, the O1s spectrum of the PFSC sample mainly consists of the C-O-H peak ( Fig. 4 (f)). This shows that the process of hydrothermal treatment and solvent exchange may lead to the partial evolution of C-O-H functional groups into C=O and -COOH groups in samples. The presence of C=O and -COOH groups will increase the surface hydrophilicity of the carbon material, which facilitates electrolyte penetration and ion transport. [43] 3.2. Electrochemical properties Fig. 5 (a) displays the CV (cyclic voltammetry) curves of the samples in a scan rate (SR) of 5 mV s⁻¹. Both CV curves of PFSCA and PFSC exhibit a regular, quasi-rectangular shape, suggesting that the samples exhibit good double layer capacitance behavior. Area enclosed by the CV curve is proportional to the electrode capacity. The ranges of the CV curves for PFSCA and PFSC samples are similar, suggesting that the samples may have comparable specific capacities. As the SR further raised, the CV curve of the PFSCA electrode remained largely unchanged ( Fig. 5 (b)), while the curve for the PFSC electrode gradually deviated from the rectangular shape ( Fig. S2 (a)). This indicates that the PFSCA electrode has better reversibility. Notably, additional redox peaks appear in the PFSC electrode between -0.2 and -0.4 V, suggesting the PFSC electrode exhibits a pseudocapacitive effect. This behavior can be due to the higher nitrogen and oxygen content in the PFSC sample, which promotes electrochemical reactions [44] : \begin{equation} -COOH\leftrightarrow H^{+}+-COO+e^{-}\nonumber \\ \end{equation}\begin{equation} C-OH\leftrightarrow H^{+}+C=O+e^{-}\nonumber \\ \end{equation}\begin{equation} {2H}^{+}+2e^{-}+-C=NH\leftrightarrow-CH-NH_{2}\nonumber \\ \end{equation}\begin{equation} {2H}^{+}+2e^{-}+-C-NHOH\leftrightarrow-C-NH_{2}+H_{2}O\nonumber \\ \end{equation} Fig. 5 (c) displays the GCD (galvanostatic charge-discharge) curves of the PFSCA and PFSC samples. Both curves are almost symmetrical triangles, with no obvious voltage drop. This indicates that both electrode samples exhibit excellent coulombic efficiency and low internal resistance. [45] The capacitances of the PFSCA and PFSC are 167 and 170 F g⁻¹, respectively, at the current density (CD) of 1 A g⁻¹. The specific capacitance of the PFSC sample was slightly higher than that of the PFSCA sample, consistent with previous prediction. Fig. 5 (d) and Fig. S2 (b) show the GCD curves of the PFSCA and PFSC electrode sample at different current densities (CDs) (1–10 A g⁻¹). As the CD increases, the curves remain symmetrical with minimal deformation. This suggests that both materials maintain stable even at higher CDs. Further analysis of the rate capability is displayed in Fig. 5 (e). The capacitance of the PFSCA electrode at 1, 2, 5 and 10 A g⁻¹ is 167, 145, 130 and 116 F g⁻¹, respectively. For the PFSC electrode, the capacities at these densities are 170, 153, 130 and 95 F g⁻¹. It is obvious that the rate performance of PFSCA samples is significantly better than that of PFSC samples, which is consistent with the predicted results of pore characterization. The impedance properties of the PFSCA and PFSC electrode samples were evaluated using electrochemical impedance spectroscopy (EIS). The Nyquist diagram after fitting the EIS spectrum to an equivalent circuit is shown in Fig. 5 (f). The diagram shows two different areas: the high frequency area, which appears as a semicircle, and the low frequency area, which is almost vertical. In the high frequency range, the semicircles for both the PFSCA and PFSC samples show a small intersection with the Z’ axis. This suggests that the internal series resistance (R s ) between the electrolyte and electrode is very low for both samples. The R s value for the PFSCA sample was 1.1 Ω, slightly larger than that of the PFSC sample (0.81 Ω). This difference can be due to the larger SSA of the PFSC sample, which allows better electrolyte penetration. Meanwhile, this result is consistent with that predicted by morphological analysis. The diameter of the semicircle match with the R ct (charge transfer resistance) of the material. The values of R ct for the PFSCA and PFSC samples were 1.3 Ω and 12 Ω, respectively. The much lower R ct value of the PFSCA sample indicates better electrical conductivity. This is due to the higher degree of graphitization and the lower content of oxygen functional groups in the PFSCA sample, both of which improve charge transfer. These results are consistent with the results of Raman spectroscopy as well as XPS characterization of the PFSCA sample. Fig. 1 (c) further highlights the lightweight nature of the PFSCA sample. When placed on delicate petals, PFSCA caused no observable deformation ( Fig. 1 (c)), confirming its exceptionally low density. Packing density measurements further revealed a PFSCA density of 0.056 g cm⁻³ , nearly fourfold lower than that of PFSC ( 0.210 g cm⁻³ ). This disparity arises from solvent displacement during PFSCA synthesis, where ethanol replaces water in the gel matrix. Ethanol’s low capillary tension minimizes structural shrinkage caused by hydrogen bonding and capillary forces during drying, [46] facilitating the formation of a highly porous aerogel. Subsequent pyrolysis preserves this three-dimensional architecture ( Fig. 1 (c)), yielding an ultralight carbon aerogel. 3.3. Electrochemical kinetic analysis To further assess the energy storage mechanism of the PFSCA and PFSC, we calculated the kinetic parameters of the electrode. Electrochemical processes are typically divided into two categories: internal diffusion control and external surface capacitance control, as displayed in Fig. 6 (a). First, the b value of the carbon sample was determined using the equation \(i=av^{b}\), where the symbols are defined in the literature. [47, 48] The fitting results were illustrated in Fig. 6 (b). The b value of the PFSCA sample was 0.93, which is close to 1. This suggests the charge storage process is mainly determined by the surface capacitance. In contrast, the b value of the PFSC sample is 0.83, indicating the charge storage process is influenced by both diffusion and capacitance behavior. This observation is consistent with the cyclic voltammetry curve results. Furthermore, Dunn’s formula is used to quantitatively evaluate the contributions of capacitance and diffusion at different CDs. The formula is given by:\(i=k_{2}v^{1/2}+k_{1}v\) (7) Where \(k_{2}v^{1/2}\) and \(k_{1}v\) represent the diffusion control contribution and the capacity contribution of the sample, respectively. [49] Fig. 6 (c-d) show the contributions of diffusion and capacitance to energy storage of the PFSCA and PFSC samples at different scan rates (SRs) of 5–100 mV s -1 . At the SR of 5 mV s -1 , the PFSCA sample exhibits a high capacitance contribution of 75%, suggesting that the charge storage process is primarily determined by the surface capacitance. In contrast, the PFSC sample shows a larger contribution of diffusion control (52%) at low SRs. As the SR increases, the capacitance contribution gradually becomes dominant in both samples. These trends are consistent with the b -value results. At a high SR of 100 mV s -1 , the capacitive contribution for the PFSCA and PFSC samples was 92% and 78%, respectively (see Fig. 6 (e) and 6 (f)). The high capacitive contribution of the PFSCA sample reflects its fast charge storage kinetics, which favors the reversible adsorption and release of electrolyte ions. Compared to the PFSC sample, the PFSCA sample has superior energy storage performance, especially double-layer storage capability. In summary, although PFSCA exhibits a lower SSA (455 m² g⁻¹) compared to PFSC (1485 m² g⁻¹), PFSCA demonstrates comparable specific capacity (167 F g⁻¹ vs. 170 F g⁻¹ for PFSC) alongside superior electrochemical performance. The reasons are as follows: 1) Hierarchical structure and pore optimization : The reduced SSA of PFSCA does not exclusively govern its capacity. PFSCA’s unique hierarchical architecture—comprising helical hollow-tube and surface-nanosphere structures—shortens ion/electron diffusion pathways within the electrolyte. This structure further mitigates damage during electrochemical cycling and resists volume expansion-induced degradation. [50, 51] Additionally, the predominance of mesopores in PFSCA enhances ion diffusion kinetics, leading to improved rate performance at high current densities. 2) Enhanced surface hydrophilicity : The presence of C=O and -COOH functional groups enhances the carbon material’s surface hydrophilicity, promoting electrolyte penetration and accelerating ion transport. This mechanism effectively counterbalances the limitations posed by its lower SSA. 3) Improved electrical conductivity : PFSCA exhibits a higher degree of graphitization and larger d 002 interlayer spacing compared to PFSC, both of which enhance electrical conductivity. These experimental findings align with characterization data, providing robust validation for the observed performance. 3.4. Performance of FASSC To further evaluate the energy storage and flexibility of PFSCA, we assembled a flexible asymmetric supercapacitor (FASSC) using a PKG electrolyte. The device structure is displayed in Fig. 7 (a). Fig. 7 (b) displays the CV curves of the FASSC at various SRs of 5–100 mV s -1 . The CV curves maintain a nearly rectangular shape even at high SRs, indicating excellent reversibility, coulomb efficiency and rate capability. Fig. 7 (c) shows the GCD curves of FASSC at different CDs (1–10 A g⁻¹). The curves are symmetrical triangles with no significant voltage drop or polarization plateau, and the device maintains a 2V high voltage window. The capacitances calculated from the GCD curves are 55, 44, 34, and 25 F g⁻¹ at CDs of 1, 2, 5, and 10 A g⁻¹, respectively (seen in Fig. 7 (d)). After the device was assembled, the capacitance of the PFSCA electrode decreased. This reduction is attributed to the relatively low electrochemical activity of the PKG electrolyte, which hinders the rapid diffusion of ions and reduces the overall capacity. The impedance characteristics of the FASSC were further explored and the corresponding EIS (electrochemical impedance spectroscopy) curve is displayed in Fig. 7 (e). The R s and R ct values of the FASSC calculated from the high frequency range are 1.1 and 1.2 Ω, respectively. These impedance values are similar to those of the three-electrode system and indicate good conductivity of the PFSCA electrode under the PKG electrolyte. At the low frequency range, the linear slope of the FASSC is obviously smaller than that of the PFSCA electrode, suggesting a reduced ion diffusion rate after the FASSC arrangement. Finally, Fig. 7 (f) illustrates the cycle performance of the FASSC. After 6000 cycles, the device retained 81.4% of its original capacity and demonstrated a coulombic efficiency of 100% at 1 A g⁻¹. These results demonstrate the superb cyclic stability and reversibility of the FASSC device. The mechanical flexibility of the FASSC device is further evaluated. The specific capacitances at bending angles of 0°, 60°, 90°, and 180° are 55, 57, 53, and 55 F g⁻¹, respectively (shown in Fig. 8 (a)). The capacity remains almost constant across various bending angles. At a SR of 50 mV s -1 , the CV curves of the FASSC have little fluctuation at different bending angles ( Fig. 8 (b)), proving the excellent mechanical flexibility of the devices. The relationship between P d and E d for the FASSC is shown in Fig. 8 (c). With a high P d of 2000 W kg -1 , the specific E d reaches 30.6 Wh kg -1 . When three FASSC devices are connected in series, a 20-second charge is enough to power a calculator and an LED (video available) ( Fig. 8 (d)). Compared to previous studies, the FASSC device in this work has a higher voltage window, larger P d , and larger E d than other carbon-based flexible supercapacitors ( Fig. 8 (e)). [44, 52-55] These results highlight the significant potential of passion fruit shell-derived carbon aerogel for use in flexible energy storage devices with high-voltage windows as well as energy densities. 4. Conclusions This study successfully synthesized an ultralight biomass-derived carbon aerogel (PFSCA) through hydrothermal self-assembly, solvent exchange, and carbonization using passion fruit shells as a precursor. PFSCA exhibited a remarkably low packing density (0.056 g cm⁻³ vs. 0.210 g cm⁻³ for carbon obtained by direct pyrolysis, PFSC) and achieved a specific capacitance of 167 F g⁻¹ at 1 A g⁻¹, comparable to PFSC (170 F g⁻¹), despite its lower specific surface area (455 m² g⁻¹ vs. 1485 m² g⁻¹). The superior electrochemical performance of PFSCA stems from three key characteristics: (1) a hierarchical structure of helical hollow tubes and surface nanospheres that optimize ion diffusion, (2) enhanced graphitization with enlarged d ₀₀₂ interlayer spacing (0.38 nm vs. 0.35 nm for PFSC), and (3) surface-functional C=O and -COOH groups that improve hydrophilicity and charge transfer kinetics. Kinetic analysis revealed that 92% of PFSCA’s capacitance originates from capacitive processes. A flexible symmetric supercapacitor (FASSC) fabricated with PFSCA electrodes achieved a 2.0 V operating voltage in a PKG electrolyte, delivering an energy density of 30.6 Wh kg⁻¹ at 2000 W kg⁻¹. The FASSC retained 100% coulombic efficiency over 6,000 cycles and demonstrated exceptional mechanical stability under bending angles of 0–180°. This work underscores the potential of biomass-derived carbon aerogels as scalable, cost-effective materials for advanced flexible energy storage systems. 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. Data availability Data will be made available on request. Acknowledgment The authors acknowledge the financial support from the National Natural Science Foundation of China (52376216). ToC figure Supplementary Material File (figure.docx) Download 4.09 MB References 1. Article category 1 Subcategory 1 Abstract 1 1. Introduction 1 2. Experiment 3 2.1. Materials 3 2.2. PFSCA and PFSC preparation and testing 4 2.3. FASSC preparation and testing 4 3. Results and discussion 5 3.1. Physical and chemical properties 5 3.1.1. Morphology analysis 5 3.1.2. Pore characteristics 5 3.1.3. Crystal structure analysis 6 3.1.4. Surface element composition 7 3.2. Electrochemical properties 8 3.3. Electrochemical kinetic analysis 10 3.4. Performance of FASSC 12 4. Conclusions 13 References 14 ToC figure 19 [1] Y. 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Keywords biomass waste flexible supercapacitor hydrothermal self-assembly ultralight carbon aerogel Authors Affiliations Ao Song China Jiliang University View all articles by this author Yunchao Li 0000-0002-6470-9639 [email protected] China Jiliang University View all articles by this author Jie Wu China Jiliang University View all articles by this author Dingkun Yuan China Jiliang University View all articles by this author Maosheng Liu China Jiliang University View all articles by this author Guangxue Zhang China Jiliang University View all articles by this author Metrics & Citations Metrics Article Usage 168 views 118 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Ao Song, Yunchao Li, Jie Wu, et al. Fabrication of Biomimetic Ultralight Carbon Aerogel for High-Performance Flexible Supercapacitors with Enhanced Voltage Windows and Energy Density. Authorea . 13 April 2025. 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