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Article

Supercapacitors with Composite Fibrous Electrodes

by
Victoria P. T. Cosmas
1,
Ioanna Savva
2,
Maria Karouzou
2,
Vasileios Drakonakis
2,
Mark A. Baker
1 and
Constantina Lekakou
1,*
1
School of Engineering, University of Surrey, Guildford GU2 7XH, UK
2
Advanced Materials Design & Manufacturing (AMDM) Ltd., Lefkosia 1027, Cyprus
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2026, 10(2), 105; https://doi.org/10.3390/jcs10020105
Submission received: 29 December 2025 / Revised: 29 January 2026 / Accepted: 14 February 2026 / Published: 17 February 2026
(This article belongs to the Special Issue Feature Papers in Journal of Composites Science in 2025)
Browse Figures
Versions Notes

Abstract

We present an investigation to develop innovative composite fibrous electrodes optimized for a supercapacitor with a “green” low-cost aqueous electrolyte, superconcentrated potassium formate, which raises the maximum energy storage device voltage beyond the water electrolysis limit. Three types of electrospun nanofiber mats are investigated for optimum pseudocapacitance with this electrolyte: polyaniline (PANI)/polyacrylonitrile (PAN) fibers, without or with 1 wt% or 10 wt% graphene nanoplatelets (GNP). These nanofiber mats are considered as standalone electrodes or in bilayer formations with a phenolic-derived activated carbon fabric. Supercapacitor cells with these electrodes are tested electrochemically via electrical impedance spectroscopy, cyclic voltammetry and galvanostatic charge–discharge at different current densities. The supercapacitor with hybrid electrode bilayers of activated carbon fabric and electrospun fiber mat consisting of PANI:PAN at 50:50 w/w with 10 wt% GNP exhibited the best performance with an energy and a power density of 39 Wh/kg and 6057 W/kg of electrodes, respectively.

1. Introduction

Supercapacitors store charge in the form of ions, so they are able to charge and discharge rapidly without the delay of slow electrochemical reactions as in batteries. On the other hand, the energy density of supercapacitors is much lower than that of batteries [1,2,3,4,5,6,7,8,9]. The most commercialized supercapacitors are those with organic electrolytes, such as 1.5 M TEABF4 in acetonitrile or propylene carbonate (PC), and many electrode materials have been designed and optimized for such electrolytes [10,11,12,13]. Activated carbon (AC) is a popular material for high specific surface area electrodes and originates from different precursors, including lignocellulosic biomass waste [14,15,16,17] such as coconut shells [18,19,20,21], peat [22,23], and synthetic polymers such as phenolics [24,25,26,27]. A common feature of the pore size distribution (PSD) of the AC materials is a pore size peak at 0.60–0.65 nm, which is close to the minimum size of the desolvated TEA+ ion of organic electrolytes [22,28]. Given the maximum electrode capacitance is observed in electrode pores of similar size as the desolvated electrolyte ion [29], this confirms the trend of the AC electrode materials to be optimized for the common organic electrolytes.
As organic electrolyte solvents are flammable, especially highly volatile, low-viscosity acetonitrile that is associated with high-performance supercapacitors [7,30], alternative electrolytes are sought for supercapacitors. Ionic liquids (ILs) are characterized by high viscosity, which necessitates higher operating temperatures for supercapacitors with IL electrolytes [13,31]. Hence, intensive research worldwide has been directed towards green aqueous electrolytes, with the focus of raising the operating voltage above 1.2 V, where water electrolysis takes place [32]. Great progress has been made based on the “water-in-salt” (WIS) principle, according to which, in superconcentrated aqueous solutions, water molecules are trapped between salt species via electrostatic or other secondary forces, which delay water electrolysis [33]. Examples of superconcentrated aqueous solutions from the literature include the following: electrochemical double layer capacitors (EDLCs) with 66.7 wt% sucrose aqueous electrolyte which reached a maximum voltage of 3.3 V but had a high resistance [34], and EDLCs with aqueous electrolytes 2 M NaNO3 or 17 M NaClO4 which reached 1.5 V and 2.3 V, respectively [34,35,36]; however, these salts are strong oxidizing agents and pose explosion hazards in certain circumstances. EDLCs with aqueous electrolytes 18.6 m CH3COONH4 and 26.1 m CH3COOK reached a maximum voltage of 1.5 V [34,37,38]. EDLCs with 35 m and 40.1 m HCOOK aq. reached a maximum voltage of 1.7 and 2.3 V, respectively [39,40,41]. Hence, the low cost, non-toxic superconcentrated aqueous electrolyte 40.1 m HCOOK has been selected for this study.
Molecular modeling yielded a minimum size of the desolvated ions of 0.266 nm and 0.366 nm for K+ and HCOO−, respectively [41], and a minimum size of the solvated ions of 0.65 nm and 1.31 nm for K+·6H2O and HCOO−·6H2O, respectively [41]. The pore size distribution (PSD) of phenolic-derived, activated carbon fabrics (ACFs) may contain a peak matching the size of desolvated ions and also appropriate peaks for the motion of the solvated ions of the HCOOK aqueous electrolyte [28]. Other materials such as graphene display a wide PSD with pore size peaks down to 0.3 nm and large specific surface area, inviting the trapping of K+ ions in the ultramicropores and carbon interlayers [42,43,44,45]. Supercapacitor electrodes may be in the form of a coating consisting mainly of graphene or a coating with added graphene nanoplatelets (GNPs) at low to medium concentrations. Hence, graphene may be used in the role of a highly conductive material of large specific surface area and a wide PSD, supporting both rapid ion transport through macro- and mesopores, and ion trapping in ultramicropores.
Nanofibrous electrodes have also been considered for supercapacitors, given their increased surface area. Such a type of electrodes includes single and multiwall carbon nanotubes (SWCNT and MWCNT), which also contribute to the enormous increase in power density to 990 kW/kg [46]. Combined with carbon fabric electrodes, they are employed to create structural supercapacitors [47]. Other studies have employed electrospun MWCNT–polymer fibrous interlayers in composite laminates to increase their penetration energy under impact, interlaminar shear strength and fatigue properties [48,49]. On the other hand, electrospun polymer separator membranes have been reported, sandwiched between graphene electrodes to create an EDLC with a 6 M KOH aqueous electrolyte, which yielded an electrode specific capacitance of 150 F/g [50]. Adding graphene in a polymer solution for electrospinning leads to changes in the electrospun fiber diameter depending on the GNP concentration, increase in the nucleation and degree of crystallization in the polymer matrix, and increase in the tensile modulus to a maximum value at an optimum GNP concentration [51,52,53,54,55].
Pseudocapacitance is another means of increasing the energy density of supercapacitors [56]. Two main mechanisms have been identified in pseudocapacitive electrodes: surface redox and intercalation with partial redox [56]. A large surface area favors the surface redox type of pseudocapacitance, which means that the pseudocapacitive material or groups need be distributed across a large surface area without blocking any micro- and ultramicropores of this 3D area, so the electrode also benefits from double layer capacitance. Short intercalation paths favor the intercalation with partial redox type of pseudocapacitance, which is translated to nanometer-thin electrode films or nanofiber electrodes.
Pseudocapacitive materials include transition metal oxides and sulfides, which have the disadvantage of inferior electronic conductivity compared to carbon electrodes [57,58]. Two-dimensional transition metal carbides in the format of MXene films in acid electrolyte offer both good pseudocapacitance and ionic conductivity but are limited to the maximum voltage of 1.2 V of aqueous electrolytes [59,60,61,62,63]. Graphene oxide functionalized with pseudocapacitive groups of metal oxides and carbides has also been explored for supercapacitor electrodes [64,65,66]. Conductive polymers offer the possibility of pseudocapacitance in suitable electrolytes while they can be easily processed to form films, fibers, or nanofibers [67,68,69,70,71,72]. Such polymers include PEDOT:PSS and polyaniline (PANI), which have exhibited pseudocapacitance [73] in acid aqueous electrolytes [74,75] or in Li-ion electrolytes [76,77]. Hybrid pseudocapacitors, such as PEDOT:PSS and graphene [78] or MXene and PEDOT:PSS [79], have also been explored.
The development of our innovative supercapacitors combines distinct features contributing to high energy density while maintaining good power density. This study centers on the enhancement and optimization of supercapacitor electrodes in combination with the “green” aqueous superconcentrated electrolyte 40 m HCOOK. Taking into account the good performance of the phenolic-derived ACF electrodes with this electrolyte [27], their enhancement is sought via PANI-based pseudocapacitance to be investigated for the first time in this study in combination with the HCOOK aqueous electrolyte. Given that a thin nanolayer is typically required to enable intercalation with partial redox type of pseudocapacitance [56,73,74,75,76,77] and a thin PANI or other polymer coating has been found to block the micropores of the ACF electrodes, electrospun PANI-containing nanofiber mats will be developed and evaluated in this study. In this case, the increased surface area of nanofibrous electrodes and the added graphene nanoplatelets are expected to expand the surface area of the active electrode material. The GNP additive aims to also increase the electronic conductivity of electrodes. Consequently, novel hybrid bilayer fibrous electrodes of ACF and electrospun composite nanofiber mats will be explored in aqueous supercapacitor devices.

2. Materials and Methods

The symmetric supercapacitors of this study consisted of the following components. The electrolyte was a superconcentrated aqueous solution, 40.1 m HCOOK aq. of a viscosity of 15 mPas [41,80] and an ionic conductivity of 52 mS/cm [40,41] at 20 °C. This electrolyte was demonstrated in a previous study of our group to have the potential of reaching a maximum cell voltage of 2.3 V [41] on the basis of the WIS principle. The separator was NKK TF4060 cellulose paper (NKK-Nippon Kodoshi Corp., Japan) of density 0.40 g/cm3 and thickness 60 μm, which is compatible with aqueous electrolytes due to its good wetting properties with water [81]. Current collectors were Toyal-Carbo® aluminum foil (Toyal Toyo Aluminium K.K., Japan), used to lower the contact resistance between the current collector and the ACF electrodes [82]. Furthermore, the carbon coating protects the aluminum substrate against corrosion in the presence of aqueous electrolytes [83]. Different types of fibrous electrodes were tested. Phenolic-derived activated carbon fabric (ACF) Kynol ACC-507-15 (Kynol Europe Gmbh, Germany) of a nominal thickness of 0.5 mm, a measured average areal density of 11.75 mg/cm2, and a BET specific surface area of 1461 m2/g. Three types of pseudocapacitor-type, electrospun fiber mat electrodes were investigated alone or in bilayer forms with the ACF, with the pseudocapacitor electrospun fiber layer facing the separator. The pseudocapacitive polymer polyaniline (PANI) was combined with a polyacrylonitrile (PAN) matrix to enable electrospinning. The role of PAN has been to reinforce the fibers to avoid fiber break up during electrospinning. The fiber mats were electrospun by AMDM using needless electrospinning (Figure 1) and were as follows: an electrospun porous PANI/PAN 50:50 w/w membrane, electrospun from a feed of 9 wt% PANI/PAN solution in DMF (dimethylformamide); an electrospun porous PANI/PAN 20:80 w/w membrane with 1 wt% GNP (graphene nanoplatelets), electrospun from a feed of 9 wt% PANI/PAN/GNP in DMF; an electrospun porous PANI/PAN 50:50 w/w membrane with 10 wt% GNP, electrospun from a feed of 9 wt% PANI/PAN/GNP in DMF. The GNP material was purchased from Nanografi Nanotechnology (Ankara, Turkey), product no. NG01GNP0109, and used as received. According to the manufacturer’s specifications, the GNPs have a purity of 99.9%, an average thickness of approximately 3 nm, a lateral diameter of ~1.5 μm, and a specific surface area of about 800 m2 g−1.
Supercapacitor cells were fabricated, each consisting of two outer discs of 1.9 cm diameter of current collector foil, each in inner contact with its corresponding electrode (single layer or bilayer) disc of 1.9 cm diameter, sandwiching a porous separator disc of 2.5 cm diameter. Each inner cell component, i.e., both electrode and separator, were well wetted with electrolyte 40.1 m HCOOK aq. The cells were subjected to a series of electrochemical tests, including electrical impedance spectroscopy (EIS) scanning through the frequency range from 1 MHz to 10 mHz, cyclic voltammetry at 5 mV/s, and galvanostatic charge–discharge (GCD) at different current densities. The electrode specific capacitance, Celec,spec, was determined from the CV or GCD data according to the following equation:
C e l e c , s p e c = 4 I m e l e c s d V / d t
where I is the current, dV/dt is the voltage (V) change rate with time t (constant voltage rate of 5 mV/s in CV) and melecs is the total mass of the positive and negative electrodes. Ragone plots were constructed from the discharge data of GCD tests at different current densities. At each current density, the power was determined as the maximum discharge power and the energy was determined from the integral under the voltage–time discharge curve multiplied by the constant current. Power density and energy density values were then calculated by dividing the power and energy values by the total mass of the positive and negative electrodes, melecs.
The electrodes were characterized by scanning electron microscopy (SEM) using an FEG SEM JEOL-7100 F instrument (JEOL Ltd., Tokyo, Japan).

3. Results

3.1. Material Characterization

Figure 2 presents the SEM micrographs of the electrode materials investigated in this study. The ACF Kynol ACC-507-15 is a plain-woven fabric (Figure 2a) with an average fiber diameter of 10 μm [84], macropores as shown in Figure 2b, and a PSD with peaks at 0.37 nm, 0.58 nm, 0.64 nm, and 1.31 nm [28]. The electrospun fiber mats have nanofibers with an average diameter of 100 nm (PANI/PAN 50:50 w/w), 300 nm (PANI/PAN 20:80 w/w with 1 wt% GNP), and 70 nm (PANI/PAN 50:50 w/w with 10 wt% GNP) as shown in Figure 2c, Figure 2d, and Figure 2e, respectively. Reducing the PANI concentration to 20 wt% seems to bring an increase in the fiber diameter, whereas increasing the GNP concentration to 10 wt% appears to decrease the fiber diameter. The latter may be attributed to the fact that more polymer volume is occupied to wrap the GNPs, resulting in thinner fibers for the 10 wt% GNP concentration. Spindle and bead formation is evident in the electrospun fibers in Figure 2c–e, which might be beneficial for the electrode capacitance as it increases the electrode surface area.

3.2. Electrochemical Testing Results

Figure 3 presents the Nyquist plots from the EIS test data for the as-fabricated cells for all the symmetric supercapacitor types investigated in this study, with an electrode area of 2.89 cm2 for each cell. It is clear from Figure 3(a1,a2) that the cells with the electrospun electrodes have negligible equivalent in-series resistance (ESR) of 157, 105 and 116 milli-ohm, respectively, but also very low capacitance. A comparison of the symmetric cells with hybrid electrodes against the cell with ACF electrodes in Figure 3b shows that the addition of the electrospun layers brings a small increase in the contact resistance for PANI/PAN without and with 1 wt% GNPs, which may be attributed to the increased number of interfaces. However, the contact resistance is eliminated in the hybrid ACF-PANI/PAN/10 wt% GNP electrode cells, which is attributed to the conductivity of graphene nanoplatelets and their role in creating conductive bridges across the interfaces. The capacitance seems also higher for this cell.
Figure 4 presents CV plots in terms of electrode specific capacitance, Celec,spec, as a function of cell potential, for a voltage scan from 0 to 2.3 V forward and reverse, at a rate of 5 mV/s. Symmetric cells with different types of electrodes were tested. It seems that for all cells with electrospun electrodes, either alone or in bilayer with the ACF, a Faradaic process leading to rapid voltage rise occurs after 1.5 to 1.8 V, so the cell cannot cycle to a maximum voltage of 2.3 V unlike the ACF-only-based cell [41]. Figure 4a–c present the CV plots of symmetric cells with electrospun electrodes of PANI/PAN without or with GNPs: a peak appears around 1.8 V during charge that partially reverses around 0.5 V in discharge. The peak becomes more prominent when GNPs are present. The ACF electrode also presents a small reversible peak at 1.5 V in charge and 1.1 V in discharge that has been attributed to K+ intercalation in ultramicropores [41]. The cells with the hybrid bilayer electrodes exhibit a small peak at 1.35 to 1.5 V in charge and 1.25 to 1.17 V in discharge, respectively.
Figure 5 presents the results of the GCD tests for symmetric cells with the different types of electrodes evaluated in this study. The cells with electrospun electrodes PANI/PAN without and with 1 wt% GNP in Figure 5a and Figure 5b, respectively, exhibit a very low capacitance, compared with the cell with electrospun electrodes PANI/PAN with 10 wt% GNP in Figure 5c that shows significant capacitance in discharge, an electrode specific capacitance Celec,spec = 347 F/g. In fact, taking into account the low mass of the electrodes, this cell demonstrates higher specific capacitance than the cell with ACF electrodes in Figure 5d (see Figure 6a). As also seen in Figure 4a–c, pseudocapacitance is the dominant capacitance mechanism in the cells with the electrospun PANI/PAN electrodes without or with GNPs, with the disadvantage of low energy efficiency (ratio of the areas under the GCD curve in discharge versus charge in Figure 5) due to the high overpotential between K+ ion intercalation and deintercalation in the anode during charge and discharge, respectively. Furthermore, K+ ion intercalation and deintercalation seems to be slow, encountered only at low areal current densities less than or equal to 1 mA/cm2.
In contrast, the symmetric cells with ACF electrodes or with hybrid bilayer ACF–electrospun PANI/PAN electrodes without or with 1 or 10 wt% GNP in Figure 5d, Figure 5e, Figure 5f, and Figure 5g, respectively, offer consistently high capacitance and good energy efficiency, and operate at current densities of much wider range (Figures S1–S3). Figure 5h presents the Ragone plots of the ACF-based [41] and hybrid electrode-based cells from their galvanostatic discharge data at different current densities. The cell with the highest energy and power density is that with bilayer electrodes ACF–electrospun PANI/PAN/10 wt% GNP, which reaches 39 Wh/kg and 6057 W/kg. Table 1 summarizes the major metrics of all cells with the different types of fibrous electrodes assessed in this study.
Figure 6a presents the electrode specific capacitance as a function of current density, as determined from the discharge phase of the GCD tests for supercapacitor cells based on ACF or hybrid bilayer electrodes. It can be seen that capacitance peaks appear at 2 and 5 mA/cm2 for electrodes ACF-PANI/PAN and ACF-PANI/PAN/1 wt% GNP, respectively, although the energy efficiency is relatively lower at these current densities, as seen in Figures S1 and S2. The symmetric supercapacitor with electrodes ACF-PANI/PAN/10 wt% GNP shows an electrode specific capacitance peak at 10 mA/cm2 (Figure 6a), but at that current density the supercapacitor with electrodes ACF-PANI/PAN/1 wt% GNP exhibits even higher specific capacitance. Increasing the current density, Figure 6a depicts that the symmetric supercapacitor with electrodes ACF-PANI/PAN/10 wt% GNP exhibits consistently superior specific capacitance than the supercapacitors with all the other types of electrodes, for current densities equal to or greater than 20 mA/cm2. The best supercapacitor with electrodes ACF-PANI/PAN/10 wt% GNP, according to the Ragone plot (Figure 5h), was cycled at 20 mA/cm2. The results in Figure 6b demonstrate that after an initial 10% decrease in the specific capacitance in the first few cycles, the capacitance remains fairly constant up to 2000 cycles tested in this study.

4. Discussion

Following the protonation of polyaniline from emeraldine base to emeraldine salt, in the presence of a protonic acid [85,86,87,88], the process of K+ ion intercalation in the presence of HCOOK aq. is proposed in Figure 7 to explain the pseudocapacitance of the electrospun PANI-based fiber mats in this study. Starting with the surface redox type of pseudocapacitance for the electrospun PANI/PAN 50:50 w/w electrode (Figure 4a), distinct CV peaks are evident for the electrospun electrodes of PANI/PAN 20:80 w/w with 1 wt% GNP and PANI/PAN 50:50 w/w with 10 wt% GNP (Figure 4b,c), indicating intercalation with the partial redox type of pseudocapacitance [56]. It seems that the dispersion of GNPs in the PANI/PAN matrix creates paths that enable the intercalation of K+ ions through and along the GNP-PANI interfaces. The specific electrode capacitance of 347 F/g of PANI/PAN/10 wt% GNP (Figure 5c) is higher than that of the other pseudocapacitive electrodes in this study, and also of PANI electrodes in strong acid electrolytes in other studies in the literature which reached 270 F/g in 1 M H2SO4 and 302 F/g in 1 M HCl [89]. Following the GCD curves in Figure 5a–c, the 10 wt% GNP appears to create the maximum interfacial area for intercalation and deintercalation of K+ ions in charge and discharge, respectively, yielding the highest specific capacitance of the electrospun fiber mats in discharge in GCDs (Figure 5c) and, hence, the highest energy density of the corresponding supercapacitor cells of all three types of cells with electrospun fiber mat electrodes only, as displayed in Table 1.
Cyclic voltammetry of PANI electrodes in the presence of acid or Li-ion electrolytes in other studies [89,90,91] have revealed fully reversible CV peaks as the cell is cycled from charge to discharge. In contrast, partial reversibility is observed for the peak of the charge curves in the CVs of Figure 4b,c in this study as well as in the study by Vujković et al. [92], associated with the intercalation and deintercalation of the cations of electrolytes HCOOK and Al(NO3)3, respectively, where the ion diffusion coefficient seems to be lower during deintercalation compared to that during intercalation.
Employing the Rendles–Shevchik equation [93], according to which the ion diffusivity is proportional to the square of the CV peak current driving the intercalation or deintercalation process, the ion diffusivity values in the first intercalation are in the ratio of 1:4:5.4 for the electrodes PANI/PAN 50:50 w/w (Figure 4a), PANI/PAN 20:80 w/w with 1 wt% GNP (Figure 4b), and PANI/PAN 50:50 w/w with 10 wt% GNP (Figure 4c), respectively. Furthermore, the ion diffusion coefficient through the pseudocapacitive layer during deintercalation is 44, 11.1, and 12.3% of that during the intercalation process shown in Figure 4a, Figure 4b, and Figure 4c, respectively. Hence, it seems that the addition of GNPs creates interfacial paths between the GNPs and PANI that facilitate the diffusion of K+ ions during their intercalation, raising their diffusivity. However, it is possible that the intercalated K+ ions block the diffusion path and, hence, decrease the ion diffusivity during deintercalation.
The hybrid cells with bilayer ACF–electrospun electrodes exhibit improved metrics, as shown in Table 1, benefitting from both the PANI pseudocapacitance and the double layer capacitance of the electrodes in relation with the aqueous HCOOK electrolyte. Figure 8 displays the PSD of the phenolic-derived, activated carbon fabric (ACF) employed in this study, which exhibits PSD peaks at 0.37 nm, 0.58 nm, 0.64 nm, and 1.31 nm [28]. It also depicts the relation of different pore sizes along the PSD with the ion sizes of 0.27 and 0.37 nm for the desolvated K+ and HCOO−, respectively, and ion sizes of 0.65 and 1.31 nm for the solvated K+·6H2O and HCOO−·6H2O, respectively [41]. As shown in Figure 8, the ACF anode and cathode electrodes offer about 30% of pore surface area matching the desolvated ion size of K+ and HCOO− ions, respectively, which corresponds to the highest double layer capacitance [28,29]. Furthermore, the ACF electrodes also contain larger micropores of suitable size to accommodate the solvated ions K+·6H2O and HCOO−·6H2O [41] and enable their transport to smaller pores promoting high double layer capacitance. On the other hand, the electrospun PANI/PAN/10 wt% GNP electrodes offer the highest surface area of all electrospun fibrous mats, due to their thin nanofibers, the thinnest from all electrospun fiber mats as depicted in Figure 2c–e.
Furthermore, the 10 wt% GNP enables a high degree of intercalation type of pseudocapacitance in the PANI matrix of the nanofibers, contributing to the cell with the bilayer electrodes. Most importantly, 10 wt% GNP improves the electrode conductivity, which also contributes to the improvement of the specific capacitance at medium and high rates. As demonstrated in Figure 6a, the hybrid bilayer electrode of ACF–electrospun PANI/PAN/10 wt% GNP achieves the highest specific capacitance at current densities of 20 mA/cm2 or greater, in comparison with all the other electrodes shown in Figure 6a. Consequently, the supercapacitor with ACF–electrospun PANI/PAN/10 wt% GNP electrodes reaches the highest energy and power density of 39 Wh/kg and 6057 W/kg of electrodes, as presented in Table 1. This is a 13% and 260% increase in the energy and power density of the ACF-only-based supercapacitor, resulting from the incorporation of the electrospun nanofiber layers.
It is important that such high performance has been achieved using a low-cost, green aqueous electrolyte, the superconcentrated 40 m HCOOK aqueous solution. The best aqueous supercapacitor with hybrid double layer pseudocapacitive fibrous electrodes of this study has more than doubled the energy density of other aqueous supercapacitors in the literature either with an HCOOK electrolyte [39,40] or other aqueous electrolytes [94,95]. Our best aqueous supercapacitor has even just surpassed the energy density of the best EDLCs with organic electrolyte 1.5 M TEABF4 in acetonitrile of 38 Wh/kg and 6 kW/kg of ACF electrodes [30,96]. Given the high volatility, flammability, and toxicity of the acetonitrile in the organic electrolyte, the aqueous supercapacitor with the hybrid fibrous electrodes of this study offers great benefits in terms of both safety and performance.

5. Conclusions

This investigation led to an optimized novel supercapacitor with an aqueous, non-toxic, low-cost electrolyte 40 m HCOOK, and bilayer electrodes of a phenolic-derived activated carbon fabric and electrospun composite nanofiber mat consisting of PANI:PAN at 50:50 w/w and 10 wt% GNP. This supercapacitor just exceeded the performance of the best EDLCs with organic electrolyte, reaching an energy and power density of 39 Wh/kg and 6057 W/kg of electrodes, respectively. The enhanced performance of this innovative supercapacitor was achieved via a combination of double layer capacitance of the ACF and nanofiber electrodes, also containing conductivity enhancing GNPs, ion intercalation in the ACF micropores matching the size of the desolvated electrolyte ions [41], and intercalation with partial redox-type pseudocapacitance of K+ ions in the PANI domains, where the 10 wt% of dispersed GNPs in the nanofibers is thought to facilitate the ion intercalation through the increased PANI-GNP interface. The addition of 10 wt% GNP in the electrospun nanofiber layer of the hybrid electrodes raised the specific capacitance, energy density, and power density of the innovative supercapacitor, maximizing its performance for current densities equal to or greater than 20 mA/cm2.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/jcs10020105/s1: Figure S1. Results of the GCD tests of symmetric cell with bilayer electrodes ACF-PANI/PAN 50:50 w/w. Figure S2. Results of the GCD tests of symmetric cell with bilayer electrodes ACF-PANI/PAN 20:80 w/w with 1 wt% GNP. Figure S3. Results of the GCD tests of symmetric cell with bilayer electrodes ACF-PANI/PAN 50:50 w/w with 10 wt% GNP.

Author Contributions

Conceptualization, V.P.T.C., I.S., V.D. and C.L.; methodology, V.P.T.C., I.S., M.K., V.D., M.A.B. and C.L.; formal analysis, V.P.T.C., I.S. and C.L.; investigation, V.P.T.C., I.S., M.K. and C.L.; resources, V.D., M.A.B. and C.L.; data curation, V.P.T.C., I.S., V.D. and C.L.; writing—original draft preparation, V.P.T.C.; writing—review and editing, V.P.T.C., I.S., M.K., V.D., M.A.B. and C.L.; visualization, V.P.T.C., I.S. and M.K.; supervision, V.D., M.A.B. and C.L.; project administration, V.D. and C.L.; funding acquisition, V.D. and C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partially funded by the FOUNDATION OF RESEARCH & INNOVATION (RIF), Cyprus, grant number CODEVELOP-REPowerEU/1223/0124.

Data Availability Statement

All data are supplied in the manuscript of this paper and the Supplementary Information (SI) File.

Conflicts of Interest

Authors IS, MK and VD were employed by Advanced Materials Design & Manufacturing (AMDM) Ltd. However, their involvement in this work was part of a research grant funded by the FOUNDATION OF RESEARCH & INNOVATION (RIF), Cyprus, grant number CODEVELOP- REPowerEU /1223/0124; it was not part of their commercial activities. The remaining authors (VPTC, MAB and CL) declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ACActivated carbon
ACFActivated carbon fabric
CVCyclic voltammetry
EDLCElectrochemical double layer capacitor
EISElectrical impedance spectroscopy
ESREquivalent in-series resistance
GCDGalvanostatic charge–discharge
GNPGraphene nanoplatelets
PANPolyacrylonitrile
PANIPolyaniline
PCPropylene carbonate
PSDPore size distribution
SEMScanning electron microscopy
TEABF4Tetraethylammonium tetrafluoroborate
WISWater-in-salt

References

  1. Gopi, C.V.V.M.; Alzahmi, S.; Narayanaswamy, V.; Vinodh, R.; Issa, B.; Obaidat, I.M. Supercapacitors: A promising solution for sustainable energy storage and diverse applications. J. Energy Storage 2025, 114, 115729. [Google Scholar] [CrossRef]
  2. Lakshmi, K.C.S.; Vedhanarayanan, B. High-performance supercapacitors: A comprehensive review on paradigm shift of conventional energy storage devices. Batteries 2023, 9, 202. [Google Scholar] [CrossRef]
  3. Baboo, J.P.; Jakubczyk, E.; Yatoo, M.A.; Phillips, M.; Grabe, S.; Dent, M.; Hinder, S.J.; Watts, J.F.; Lekakou, C. Investigating battery-supercapacitor material hybrid configurations in energy storage device cycling at 0.1 to 10C rate. J. Power Sources 2023, 561, 232762. [Google Scholar] [CrossRef]
  4. Huang, S.; Zhu, X.; Sarkar, S.; Zhao, Y. Challenges and opportunities for supercapacitors. APL Mater. 2019, 7, 100901. [Google Scholar] [CrossRef]
  5. Liu, W.; Placke, T.; Chau, K.T. Overview of batteries and battery management for electric vehicles. Energy Rep. 2022, 8, 4058–4084. [Google Scholar] [CrossRef]
  6. Baboo, J.P.; Yatoo, M.A.; Dent, M.; Najafabadi, E.H.; Lekakou, C.; Slade, R.; Hinder, S.J.; Watts, J.F. Exploring different binders for a LiFePO4 battery, battery testing, modelling and simulations. Energies 2022, 15, 2332. [Google Scholar] [CrossRef]
  7. Lei, C.; Fields, R.; Wilson, P.; Lekakou, C.; Amini, N.; Tennison, S.; Perry, J.; Gosso, M.; Martorana, B. Development and evaluation of a composite supercapacitor-based 12 V transient start–stop power system for vehicles: Modelling, design and fabrication scaling up. Proc. Inst. Mech. Eng. Part A J. Power Energy 2021, 235, 914–927. [Google Scholar] [CrossRef]
  8. Xu, X.; Han, X.; Lu, L.; Wang, F.; Yang, M.; Liu, X.; Wu, Y.; Tang, S.; Hou, Y.; Hou, J.; et al. Challenges and opportunities toward long-life lithium-ion batteries. J. Power Sources 2024, 603, 234445. [Google Scholar] [CrossRef]
  9. Gopi, C.V.V.M.; Ramesh, R. Review of battery-supercapacitor hybrid energy storage systems for electric vehicles. Results Eng. 2024, 24, 103598. [Google Scholar] [CrossRef]
  10. Pal, B.; Yang, S.; Ramesh, S.; Thangadurai, V.; Jose, R. Electrolyte selection for supercapacitive devices: A critical review. Nanoscale Adv. 2019, 1, 3807–3835. [Google Scholar] [CrossRef]
  11. Lu, J.; Zhang, J.; Wang, X.; Zhang, J.; Tian, Z.; Zhu, E.; Yang, L.; Guan, X.; Ren, H.; Wu, J.; et al. A review of advanced electrolytes for supercapacitors. J. Energy Storage 2024, 103, 114338. [Google Scholar] [CrossRef]
  12. Acharjee, A.; Saha, B. Organic electrolytes in electrochemical supercapacitors: Applications and developments. J. Mol. Liq. 2024, 400, 124487. [Google Scholar] [CrossRef]
  13. Saha, M.; Kumar, A.; Kanaoujiya, R.; Behera, K.; Trivedi, S. A Comprehensive review of novel emerging electrolytes for supercapacitors: Aqueous and organic electrolytes versus ionic liquid-based electrolytes. Energy Fuels 2024, 38, 8528–8552. [Google Scholar] [CrossRef]
  14. Lu, W.; Si, Y.; Zhao, C.; Chen, T.; Li, C.; Zhang, C.; Wang, K. Biomass-derived carbon applications in the field of supercapacitors: Progress and prospects. Chem. Eng. J. 2024, 495, 153311. [Google Scholar] [CrossRef]
  15. Yuan, C.; Xu, H.; El-khodary, S.A.; Ni, G.; Esakkimuthu, S.; Zhong, S.; Wang, S. Recent advances and challenges in biomass-derived carbon materials for supercapacitors: A review. Fuel 2024, 362, 130795. [Google Scholar] [CrossRef]
  16. Rashidi, N.A.; Chai, Y.H.; Ismail, I.S.; Othman, M.F.H.; Yusup, S. Biomass as activated carbon precursor and potential in supercapacitor applications. Biomass Conv. Bioref. 2025, 15, 29597–29611. [Google Scholar] [CrossRef]
  17. Chen, Z.; Weng, X.; Zeng, Q.; Xie, Y.; Liu, Y.; Lv, Y.; Liu, M. Regulation of nitrogen species on biomass-based porous activated carbon surfaces by varying nitrogen sources for high-performance supercapacitors. J. Power Sources 2025, 636, 236387. [Google Scholar] [CrossRef]
  18. Neme, I.; Gonfa, G.; Masi, C. Activated carbon from biomass precursors using phosphoric acid: A review. Heliyon 2022, 8, e11940. [Google Scholar] [CrossRef]
  19. Hassan, M.F.; Sabri, M.A.; Fazal, H.; Hafeez, A.; Shezad, N.; Hussain, M. Recent trends in activated carbon fibers production from various precursors and applications—A comparative review. J. Anal. Appl. Pyrolysis 2020, 145, J104715. [Google Scholar] [CrossRef]
  20. Ioannidou, O.; Zabaniotou, A. Agricultural residues as precursors for activated carbon production—A review. Renew. Sustain. Energy Rev. 2007, 11, 1966–2005. [Google Scholar] [CrossRef]
  21. González-García, P. Activated carbon from lignocellulosics precursors: A review of the synthesis methods, characterization techniques and applications. Renew. Sustain. Energy Rev. 2018, 82, 1393–1414. [Google Scholar] [CrossRef]
  22. Markoulidis, F.; Bates, J.; Lekakou, C.; Slade, R.; Laudone, G.M. Supercapacitors with lithium-ion electrolyte: An experimental study and design of the activated carbon electrodes via modelling and simulations. Carbon 2020, 164, 422–434. [Google Scholar] [CrossRef]
  23. Paalo, M.; Adamson, A.; Härmas, M.; Lust, E.; Janes, A. Peat derived carbon–perfect fit as an electrode material for high power supercapacitors and high capacity sodium-ion batteries. ECS Meet. Abstr. 2024, MA2024-02, 1515. [Google Scholar] [CrossRef]
  24. Lei, C.; Amini, N.; Markoulidis, F.; Wilson, P.; Tennison, S.; Lekakou, C. Activated carbon from phenolic resin with controlled mesoporosity for an electric double-layer capacitor (EDLC). J. Mater. Chem. A 2013, 1, 6037–6604. [Google Scholar] [CrossRef]
  25. Quílez-Bermejo, J.; Prez-Rodríguez, S.; Celzard, A.; Fierro, V. Progress in the use of biosourced phenolic molecules for electrode manufacturing. Front. Mater. 2022, 9, 810575. [Google Scholar] [CrossRef]
  26. Yu, Z.-L.; Gao, Y.-C.; Qin, B.; Ma, Z.-Y.; Yu, S.-H. Revitalizing traditional phenolic resin toward a versatile platform for advanced materials. Acc. Mater. Res. 2024, 5, 146–159. [Google Scholar] [CrossRef]
  27. Kolmakova, A.A.; Ivanov, V.K.; Baranchikov, A.E.; Malkova, A.N.; Lermontov, S.A.; Filippova, A.D.; Ivanova, O.S.; Kolmakov, A.G.; Kaplan, M.A. Main service properties of carbon aerogels based on resorcinol–formaldehyde. Russ. Metall. 2025, 2025, 804–811. [Google Scholar] [CrossRef]
  28. Bates, J.; Markoulidis, F.; Lekakou, C.; Laudone, G.M. Design of porous carbons for supercapacitor applications for different organic solvent-electrolytes. C 2021, 7, 15. [Google Scholar] [CrossRef]
  29. Huang, J.; Sumpter, B.G.; Meunier, V. Theoretical model for nanoporous carbon supercapacitors. Angew. Chem. Int. Ed. 2008, 47, 520–524. [Google Scholar] [CrossRef]
  30. Fields, R.; Lei, C.; Markoulidis, F.; Lekakou, C. The composite supercapacitor. Energy Technol. 2016, 4, 517–525. [Google Scholar] [CrossRef]
  31. Yu, L.; Chen, G.Z. Ionic Liquid-based electrolytes for supercapacitor and supercapattery. Front. Chem. 2019, 7, 00272. [Google Scholar] [CrossRef]
  32. Li, J.; Ma, Y.; Mu, X.; Wang, X.; Li, Y.; Ma, H.; Guo, Z. Recent advances and perspectives on coupled water electrolysis for energy-saving hydrogen production. Adv. Sci. 2025, 12, 2411964. [Google Scholar] [CrossRef] [PubMed]
  33. McEldrew, M.; Goodwin, Z.A.H.; Kornyshev, A.A.; Bazant, M.Z. Theory of the double layer in water-in-salt electrolytes. J. Phys. Chem. Lett. 2018, 9, 5840–5846. [Google Scholar] [CrossRef]
  34. Bi, H.; Wang, X.; Liu, H.; He, Y.; Wang, W.; Deng, W.; Ma, X.; Wang, Y.; Rao, W.; Chai, Y.; et al. A universal approach to aqueous energy storage via ultralow-cost electrolyte with super-concentrated sugar as hydrogen-bond-regulated solute. Adv. Mater. 2020, 32, 2000074. [Google Scholar] [CrossRef] [PubMed]
  35. Bu, X.; Su, L.; Dou, Q.; Lei, S.; Yan, X. A low-cost “water-in-salt” electrolyte for a 2.3 V high-rate carbon-based supercapacitor. J. Mater. Chem. A 2019, 7, 7541. [Google Scholar] [CrossRef]
  36. Thareja, S.; Kumar, A. “Water-in-salt” electrolyte-based high-voltage (2.7 V) sustainable symmetric supercapacitor with superb electrochemical performance—An analysis of the role of electrolytic ions in extending the cell voltage. ACS Sustain. Chem. Eng. 2021, 9, 2338–2347. [Google Scholar] [CrossRef]
  37. Lukatskaya, M.R.; Feldblyum, J.I.; Mackanic, D.G.; Lissel, F.; Michels, D.L.; Cui, Y.; Bao, Z. Concentrated mixed cation acetate “water-in-salt” solutions as green and low cost high voltage electrolytes for aqueous batteries. Energy Environ. Sci. 2018, 11, 2876–2883. [Google Scholar] [CrossRef]
  38. El Halimi, M.S.; Poli, F.; Mancusoa, N.; Olivieri, A.; Mattioli, E.J.; Calvaresi, M.; Chafikc, T.; Zanelli, A.; Soavi, F. Circumneutral concentrated ammonium acetate solution as water-in-salt electrolyte. Electrochim. Acta 2021, 389, 138653. [Google Scholar] [CrossRef]
  39. Liu, S.; Klukas, R.; Porada, T.; Furda, K.; Fernandez, A.M.; Balducci, A. Potassium formate-based electrolytes for high performance aqueous electrochemical capacitors. J. Power Sources 2022, 541, 231657. [Google Scholar] [CrossRef]
  40. Liu, T.; Tang, L.; Luo, H.; Cheng, S.; Liu, M. A promising water-in-salt electrolyte for aqueous based electrochemical energy storage cells with a wide potential window: Highly concentrated HCOOK. Chem. Commun. 2019, 55, 12817. [Google Scholar] [CrossRef]
  41. Ayere, O.; Cosmas, V.P.T.; Hinder, S.J.; Baker, M.A.; Lekakou, C. An investigation into green, high voltage, aqueous supercapacitors. In Ionics; Springer: Berlin/Heidelberg, Germany, 2026; Available online: https://link.springer.com/article/10.1007/s11581-025-06931-9 (accessed on 22 December 2025).
  42. Zhu, Y.; Murali, S.; Stoller, M.D.; Ganesh, K.J.; Cai, W.; Ferreira, P.J.; Pirkle, A.; Wallace, R.M.; Cychosz, K.A.; Thommes, M.; et al. Carbon-based supercapacitors produced by activation of graphene. Science 2011, 332, 1537–1541. [Google Scholar] [CrossRef] [PubMed]
  43. Baboo, J.P.; Babar, S.; Kale, D.; Lekakou, C.; Laudone, G.M. Designing a graphene coating-based supercapacitor with lithium ion electrolyte: An experimental and computational study via multiscale modeling. Nanomaterials 2021, 11, 2899. [Google Scholar] [CrossRef] [PubMed]
  44. Ran, J.; Liu, Y.; Feng, H.; Shi, H.; Ma, Q. A review on graphene-based electrode materials for supercapacitor. J. Ind. Eng. Chem. 2024, 137, 106–121. [Google Scholar] [CrossRef]
  45. Wang, L.; Yao, H.; Yan, F.C.; Cheng, H.; Li, Y.; Jiang, L.; Qu, L. Spatial-interleaving graphene supercapacitor with high area energy density and mechanical flexibility. ACS Nano 2022, 16, 12813–12821. [Google Scholar] [CrossRef]
  46. Izadi-Najafabadi, A.; Yamada, T.; Futaba, D.N.; Yudasaka, M.; Takagi, H.; Hatori, H.; Iijima, S.; Hata, K. High-power supercapacitor electrodes from single-walled carbon nanohorn/nanotube composite. ACS Nano 2011, 5, 811–819. [Google Scholar] [CrossRef]
  47. Senokos, E.; Anthony, D.B.; Rubio, N.; Ribadeneyra, M.C.; Greenhalgh, E.S.; Shaffer, M.S.P. Robust single-walled carbon nanotube-infiltrated carbon fiber electrodes for structural supercapacitors: From reductive dissolution to high performance devices. Adv. Funct. Mater. 2023, 33, 2212697. [Google Scholar] [CrossRef]
  48. Sadykov, D.; Nouicer, L.; Lekakou, C. Hybrid woven glass fibre fabric-multi-walled carbon nanotube-epoxy composites under low rate impact. J. Comp. Sci. 2017, 1, 10. [Google Scholar] [CrossRef]
  49. Wable, V.; Biswas, P.K.; Moheimani, R.; Aliahmad, N.; Omole, P.; Siegel, A.P.; Agarwal, M.; Dalir, H. Engineering the electrospinning of MWCNTs/epoxy nanofiber scaffolds to enhance physical and mechanical properties of CFRPs. Comp. Sci. Technol. 2021, 213, 108941. [Google Scholar] [CrossRef]
  50. Luengchavanon, M.; Chuenchom, L.; Limbut, W.; Kantakapun, K.; Choowang, R.; Putson, C.; Chowdhury, S. Effect of applied graphene nanosheets (electrode) from the electrolysis process and nanofiber-based separator (electrolyte) from an electrospinning technique for high-performance supercapacitors. ACS Omega 2025, 10, 31348–31358. [Google Scholar] [CrossRef]
  51. Selatile, K.; Ray, S.S.; Ojijo, V.; Sadiku, R.E. Morphological, thermal, and mechanical properties of electrospun recycled poly(ethylene terephthalate)/graphene oxide composite nanofiber membranes. ACS Omega 2021, 6, 21005–21015. [Google Scholar] [CrossRef]
  52. Das, S.; Wajid, A.S.; Bhattacharia, S.K.; Wilting, M.D.; Rivero, I.V.; Green, M.J. Electrospinning of polymer nanofibers loaded with noncovalently functionalized graphene. J. Appl. Polym. Sci. 2012, 128, 4040–4046. [Google Scholar] [CrossRef]
  53. Maccaferri, E.; Mazzocchetti, L.; Benelli, T.; Ortolani, J.; Brugo, T.M.; Zucchelli, A.; Giorgini, L. Is graphene always effective in reinforcing composites? the case of highly graphene-modified thermoplastic nanofibers and their unfortunate application in CFRP laminates. Polymers 2022, 14, 5565. [Google Scholar] [CrossRef] [PubMed]
  54. Shakil, U.A.; Hájek, P. Electrospun nanofibres for load-bearing composite materials—A review of techniques, mechanical properties and durability. Polym. Compos. 2025. [Google Scholar] [CrossRef]
  55. Kim, H.C.; Panicker, P.S.; Kim, D.; Adil, S.; Kim, J. High-strength cellulose nanofiber/graphene oxide hybrid filament made by continuous processing and its humidity monitoring. Sci. Rep. 2021, 11, 13611. [Google Scholar] [CrossRef]
  56. Gogotsi, Y.; Penner, R.M. Energy storage in nanomaterials − capacitive, pseudocapacitive, or battery-like. ACS Nano 2018, 12, 2081–2083. [Google Scholar] [CrossRef]
  57. He, X.; Chen, Q.; Mao, X.; Liu, W.; Zhou, Y.; Yang, W.; Yanga, Y.; Xu, J. Pseudocapacitance electrode and asymmetric supercapacitor based on biomass juglone/activated carbon composites. RSC Adv. 2019, 9, 30809–30814. [Google Scholar] [CrossRef]
  58. Wei, Y.-M.; Dasha Kumar, K.D.; Zhang, L.; Li, J.-F. Pseudocapacitive materials for energy storage: Properties, mechanisms, and applications in supercapacitors and batteries. Front. Chem. 2025, 13, 1636683. [Google Scholar] [CrossRef]
  59. Lukatskaya, M.R.; Kota, S.; Lin, Z.; Zhao, M.-Q.; Shpigel, N.; Levi, M.D.; Halim, J.; Taberna, P.-L.; Barsoum, M.W.; Simon, P.; et al. Ultra-high-rate pseudocapacitive energy storage in two-dimensional transition metal carbides. Nat. Energy 2017, 2, 17105. [Google Scholar] [CrossRef]
  60. Panchu, S.J.; Raju, K.; Swart, H.C. Emerging two–dimensional intercalation pseudocapacitive electrodes for supercapacitors. ChemElectroChem 2024, 11, e202300810. [Google Scholar] [CrossRef]
  61. Li, J.; Wang, H.; Xiao, X. Intercalation in two-dimensional transition metal carbides and nitrides (MXenes) toward electrochemical capacitor and beyond. Energy Environ. Mater. 2020, 3, 306–322. [Google Scholar] [CrossRef]
  62. Gyu Jung, M.; Gund, G.S.; Gogotsi, Y.; Seok Park, H. Electrochemical Activation of 2D MXene-based hybrid for high volumetric mg-ion storage capacitance. Batter. Supercaps 2020, 3, 354–360. [Google Scholar] [CrossRef]
  63. Nakhanivej, P.; Dou, Q.; Xiong, P.; Seok Park, H. High-energy- and high-power-oriented applications of supercapacitors. Acc. Mater. Res. 2021, 2, 86–96. [Google Scholar] [CrossRef]
  64. Askari, M.B.; Salarizadeh, P. State-of-the-art review on reduced graphene oxide for supercapacitor electrode applications. Results Eng. 2025, 28, 107429. [Google Scholar] [CrossRef]
  65. Mousavi, S.M.; Hashemi, S.A.; Kalashgrani, M.Y.; Gholami, A.; Binazadeh, M.; Chiang, W.-H.; Rahman, M.M. Recent advances in energy storage with graphene oxide for supercapacitor technology. Sustain. Energy Fuels 2023, 7, 5176–5197. [Google Scholar] [CrossRef]
  66. Kumar, R.; Sahoo, S.; Joanni, E.; Singh, R.K. A review on the current research on microwave processing techniques applied to graphene-based supercapacitor electrodes: An emerging approach beyond conventional heating. J. Energy Chem. 2022, 74, 252–282. [Google Scholar] [CrossRef]
  67. Dallaev, R. Conductive polymer thin films for energy storage and conversion: Supercapacitors, batteries, and solar cells. Polymers 2025, 17, 2346. [Google Scholar] [CrossRef]
  68. Bryan, A.M.; Santino, L.M.; Lu, Y.; Acharya, S.; D’Arcy, J.M. Conducting polymers for pseudocapacitive energy storage. Chem. Mater. 2016, 28, 5989–5998. [Google Scholar] [CrossRef]
  69. Liu, J.; Tang, D.; Hou, W.; Ding, D.; Yao, S.; Liu, Y.; Chen, Y.; Chi, W.; Zhang, Z.; Ouyang, M.; et al. Conductive polymer electrode materials with excellent mechanical and electrochemical properties for flexible supercapacitor. J. Energy Storage 2023, 74, 109329. [Google Scholar] [CrossRef]
  70. Bhattacharya, S.; Roy, I.; Tice, A.; Chapman, C.; Udangawa, R.; Chakrapani, V.; Plawsky, J.L.; Linhardt, R.J. High-conductivity and high-capacitance electrospun fibers for supercapacitor applications. ACS Appl. Mater. Interfaces 2020, 12, 19369–19376. [Google Scholar] [CrossRef]
  71. Serrano-Garcia, W.; Bonadies, I.; Thomas, S.W.; Guarino, V. New insights to design electrospun fibers with tunable electrical conductive–semiconductive properties. Sensors 2023, 23, 1606. [Google Scholar] [CrossRef]
  72. Prasannakumar, A.T.; Mohan, R.R.; Rohith, R.; Manju, V.; Varma, S.J. Progress in conducting polymer-based electrospun fibers for supercapacitor applications: A review. ChemistrySelect 2023, 8, e202203564. [Google Scholar] [CrossRef]
  73. Gascón, N.C.; Terryn, H.; Hubin, A. The role of PEDOT:PSS in (super)capacitors: A review. Next Nanotechnol. 2023, 2, 100015. [Google Scholar] [CrossRef]
  74. Kim, K.; Park, J.; Lee, J.; Suh, S.; Kim, W. Ultrafast PEDOT:PSS/H2SO4 electrical double layer capacitors: Comparison with polyaniline pseudocapacitors. ChemSusChem 2023, 16, e202202057. [Google Scholar] [CrossRef] [PubMed]
  75. Li, L.; Ai, Z.; Wu, J.; Lin, Z.; Huang, M.; Gao, Y.; Bai, H. A robust polyaniline hydrogel electrode enables superior rate capability at ultrahigh mass loadings. Nat. Comm. 2024, 15, 6591. [Google Scholar] [CrossRef] [PubMed]
  76. Markoulidis, F.; Dawe, A.; Lekakou, C. Electrochemical double-layer capacitors with lithium-ion electrolyte and electrode coatings with PEDOT: PSS binder. J. Appl. Electrochem. 2021, 51, 373–385. [Google Scholar] [CrossRef]
  77. Luo, Y.; Guo, R.; Li, T.; Li, F.; Liu, Z.; Zheng, M.; Wang, B.; Yang, Z.; Luo, H.; Wan, Y. Application of polyaniline for li-ion batteries, lithium–sulfur batteries, and supercapacitors. ChemSusChem 2019, 12, 1591–1611. [Google Scholar] [CrossRef]
  78. Jimoh, M.F.; Carson, G.S.; Anderson, M.B.; El-Kady, M.F.; Kaner, R.B. Direct fabrication of 3D electrodes based on graphene and conducting polymers for supercapacitor applications. Adv. Funct. Mater. 2025, 35, 2405569. [Google Scholar] [CrossRef]
  79. Gund, G.S.; Park, J.H.; Harpalsinh, R.; Kota, M.; Shin, J.H.; Kim, T.I.; Gogotsi, Y.; Park, H.S. MXene/Polymer hybrid materials for flexible AC-filtering electrochemical capacitors. Joule 2019, 3, 164–176. [Google Scholar] [CrossRef]
  80. Longo, G.A.; Gasparella, A. Experimental measurement of thermophysical properties of H2O/KCOOH (potassium formate) desiccant. Int. J. Refrig. 2016, 62, 106–113. [Google Scholar] [CrossRef]
  81. Etale, A.; Onyianta, A.J.; Turner, S.R.; Eichhorn, S.J. Cellulose: A review of water interactions, applications in composites, and water treatment. Chem. Rev. 2023, 123, 2016–2048. [Google Scholar] [CrossRef]
  82. Lei, C.; Markoulidis, F.; Lekakou, C.; Ashitaka, Z. Reduction of porous carbon/Al contact resistance for an electric double-layer capacitor (EDLC). Electrochim. Acta 2013, 92, 183–187. [Google Scholar] [CrossRef]
  83. Chomkhuntod, P.; Sawangphruk, M. First prototype 18650 aqueous supercapacitors using graphite coated Al foil as low cost and anti-corrosion current collector. ECS Meet. Abstr. 2021, MA2021-01, 491. [Google Scholar] [CrossRef]
  84. Grabe, S.; Baboo, J.P.; Tennison, S.; Zhang, T.; Lekakou, C.; Andritsos, E.I.; Cai, Q.; Downes, S.; Hinder, S.; Watts, J.F. Sulfur infiltration and allotrope formation in porous cathode hosts for lithium-sulfur batteries. AIChE J. 2022, 68, e17638. [Google Scholar] [CrossRef]
  85. Majeed, A.H.; Mohammed, L.A.; Hammoodi, O.G.; Sehgal, S.; Alheety, M.A.; Saxena, K.K.; Dadoosh, S.A.; Mohammed, I.K.; Jasim, M.M.; Salmaan, N.U. A review on polyaniline: Synthesis, properties, nanocomposites, and electrochemical applications. Int. J. Polym. Sci. 2022, 2022, 9047554. [Google Scholar] [CrossRef]
  86. Scotto, J.; Marmisollé, W.A.; Posadas, D. About the capacitive currents in conducting polymers: The case of polyaniline. J. Solid State Electrochem. 2019, 23, 1947–1965. [Google Scholar] [CrossRef]
  87. Llorens, E.; Armelin, E.; Del Mar Pérez-Madrigal, M.; Del Valle, L.J.; Alemán, C.; Puiggalí, J. Nanomembranes and nanofibers from biodegradable conducting polymers. Polymers 2013, 5, 1115–1157. [Google Scholar] [CrossRef]
  88. Tritscher, F.; Mularczyk, A.; Forner-Cuenca, A.; Hacker, V.; Bodner, M. Surfactant doped polyaniline coatings for functionalized gas diffusion layers in low temperature fuel cells. Mater. Adv. 2023, 4, 2573–2585. [Google Scholar] [CrossRef]
  89. Zhu, H.; Peng, S.; Jiang, W. Electrochemical properties of PANI as single electrode of electrochemical capacitors in acid electrolytes. Sci. World J. 2013, 2013, 940153. [Google Scholar] [CrossRef]
  90. Han, J.-J.; Zhang, N.; Liu, D.-L.; Ma, H.; Han, T.; Sun, D.-D. Cyclic voltammetry for the determination of the selectivity of PANI-HClO4 sensor to different acids. Ionics 2020, 26, 1029–1038. [Google Scholar] [CrossRef]
  91. Gharahcheshmeh, M.H.; Chowdhury, K. Fabrication methods, pseudocapacitance characteristics, and integration of conjugated conducting polymers in electrochemical energy storage devices. Energy Adv. 2024, 3, 2668–2703. [Google Scholar] [CrossRef]
  92. Vujković, M.J.; Etinski, M.; Vasić, B.; Kuzmanović, B.; Bajuk-Bogdanović, D.; Dominko, R.; Mentus, S. Polyaniline as a charge storage material in an aqueous aluminum-based electrolyte: Can aluminum ions play the role of protons? J. Power Sources 2021, 482, 228937. [Google Scholar] [CrossRef]
  93. Galaguz, V.A.; Potapenko, O.V.; Panov, E.V. Diffusion characteristics of the LiFePO4/C composite in an electrolyte based on LiBOB. RREC 2015, 6, 49–57. Available online: https://www.tsijournals.com/abstract/diffusion-characteristics-of-the-lifepo4c-composite-in-an-electrolyte-based-on-libob-3920.html#:~:text=The%20behavior%20of%20a%20composite,Awards%20Nomination%2020+%20Million%20Readerbase (accessed on 22 December 2025).
  94. Bo, Z.; Zhou, M.; Deng, Y.; Yan, J.; Cen, K.; Zhou, Y.; Yang, H. Ultralow-freezing-point (−70 °C), organic solvent-free aqueous electrolyte with superior ionic conductivity and thermodynamics stability. J. Energy Storage 2024, 93, 112379. [Google Scholar] [CrossRef]
  95. Yang, Y.; Qi, Y.; Wang, Z.; Pan, Q.; Zhang, C.; Yan, J.; Cen, K.; Bo, Z.; Ostrikov, K. Sodium nitrate/formamide deep eutectic solvent as flame-retardant and anticorrosive electrolyte enabling 2.6 V safe supercapacitors with long cyclic stability. Energy Environ. Mater. 2024, 7, e12641. [Google Scholar] [CrossRef]
  96. Markoulidis, F.; Lei, C.; Lekakou, C. Investigations of activated carbon fabric-based supercapacitors with different interlayers via experiments and modelling of electrochemical processes of different timescales. Electrochim. Acta 2017, 249, 122–134. [Google Scholar] [CrossRef]
Jcs 10 00105 g001
Figure 1. Needless electrospinning, with an applied voltage of −20 to 50 kV (depending on the electrospun material) between the spinning electrode in the feed solution bath (at the bottom) and the collector electrode just over the rolling substrate (at the top) separated by a distance of 210 to 240 mm (depending on the electrospun material).
Figure 1. Needless electrospinning, with an applied voltage of −20 to 50 kV (depending on the electrospun material) between the spinning electrode in the feed solution bath (at the bottom) and the collector electrode just over the rolling substrate (at the top) separated by a distance of 210 to 240 mm (depending on the electrospun material).
Jcs 10 00105 g001
Jcs 10 00105 g002
Figure 2. SEM micrographs of the electrodes investigated and assessed in this study: (a,b) ACF Kynol ACC-507-15; (c) electrospun PANI/PAN 50:50 w/w, scale: 1 μm; (d) electrospun PANI/PAN 20:80 w/w with 1 wt% GNP, scale: 10 μm; (e) electrospun PANI/PAN 50:50 w/w with 10 wt% GNP, scale: 10 μm.
Figure 2. SEM micrographs of the electrodes investigated and assessed in this study: (a,b) ACF Kynol ACC-507-15; (c) electrospun PANI/PAN 50:50 w/w, scale: 1 μm; (d) electrospun PANI/PAN 20:80 w/w with 1 wt% GNP, scale: 10 μm; (e) electrospun PANI/PAN 50:50 w/w with 10 wt% GNP, scale: 10 μm.
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Figure 3. Nyquist plots from the EIS test data for as-fabricated symmetric cells, with different types of electrodes: (a1,a2) Electrospun fiber mat electrodes: (a1) full scale; (a2) scale magnified around the origin; (b) bilayer electrodes of ACF–electrospun fiber mat, compared against the ACF-only electrode-based cell [41].
Figure 3. Nyquist plots from the EIS test data for as-fabricated symmetric cells, with different types of electrodes: (a1,a2) Electrospun fiber mat electrodes: (a1) full scale; (a2) scale magnified around the origin; (b) bilayer electrodes of ACF–electrospun fiber mat, compared against the ACF-only electrode-based cell [41].
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Figure 4. CV plots of the electrode specific capacitance results from CV tests at 5 mV/s of the symmetric cells with different types of electrodes. (a–c) Electrospun fiber mat electrodes; (d–f) Bilayer electrodes of ACF–electrospun fiber mat (red lines), also with the CV of a symmetric cell with ACF electrodes (black line [41]). Yellow circular regions indicate pseudocapacitance effects. Red elliptical regions indicate possible onset of electrolysis.
Figure 4. CV plots of the electrode specific capacitance results from CV tests at 5 mV/s of the symmetric cells with different types of electrodes. (a–c) Electrospun fiber mat electrodes; (d–f) Bilayer electrodes of ACF–electrospun fiber mat (red lines), also with the CV of a symmetric cell with ACF electrodes (black line [41]). Yellow circular regions indicate pseudocapacitance effects. Red elliptical regions indicate possible onset of electrolysis.
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Figure 5. Results of the GCD tests of symmetric cells with different types of electrodes. (a–g) Results of 3 consecutive GCD cycles at the stated current density: color from darker-to-lighter indicates increasing cycle number; (a) Electrodes PANI/PAN 50:50 w/w; (b) Electrodes PANI/PAN 20:80 w/w with 1 wt% GNP; (c) Electrodes PANI/PAN 50:50 w/w with 10 wt% GNP; (d) Electrodes ACF; (e) Electrodes bilayer ACF- PANI/PAN 50:50 w/w; (f) Electrodes bilayer ACF- PANI/PAN 20:80 w/w with 1 wt% GNP; (g) Electrodes bilayer ACF- PANI/PAN 50:50 w/w with 10 wt% GNP; (h) Ragone plots for the different cells (d–g) from their galvanostatic discharge data at different current densities; energy and power density values are given with respect to the total mass of electrodes (positive and negative).
Figure 5. Results of the GCD tests of symmetric cells with different types of electrodes. (a–g) Results of 3 consecutive GCD cycles at the stated current density: color from darker-to-lighter indicates increasing cycle number; (a) Electrodes PANI/PAN 50:50 w/w; (b) Electrodes PANI/PAN 20:80 w/w with 1 wt% GNP; (c) Electrodes PANI/PAN 50:50 w/w with 10 wt% GNP; (d) Electrodes ACF; (e) Electrodes bilayer ACF- PANI/PAN 50:50 w/w; (f) Electrodes bilayer ACF- PANI/PAN 20:80 w/w with 1 wt% GNP; (g) Electrodes bilayer ACF- PANI/PAN 50:50 w/w with 10 wt% GNP; (h) Ragone plots for the different cells (d–g) from their galvanostatic discharge data at different current densities; energy and power density values are given with respect to the total mass of electrodes (positive and negative).
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Figure 6. Summary results from GCD tests: (a) Average electrode specific capacitance versus current density from the discharge phase of GCD tests at different current densities for symmetric supercapacitors with ACF or hybrid bilayer fibrous electrodes; (b) Cyclability data from the discharge phase of cyclic GCD tests at 20 mA/cm2 for symmetric supercapacitor with electrodes ACF-PANI/PAN/10 wt% GNP.
Figure 6. Summary results from GCD tests: (a) Average electrode specific capacitance versus current density from the discharge phase of GCD tests at different current densities for symmetric supercapacitors with ACF or hybrid bilayer fibrous electrodes; (b) Cyclability data from the discharge phase of cyclic GCD tests at 20 mA/cm2 for symmetric supercapacitor with electrodes ACF-PANI/PAN/10 wt% GNP.
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Figure 7. Schematic of the proposed pseudocapacitance mechanism in polyaniline (PANI) in the presence of HCOOK aq. solution.
Figure 7. Schematic of the proposed pseudocapacitance mechanism in polyaniline (PANI) in the presence of HCOOK aq. solution.
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Figure 8. PSD of the ACF electrodes of this study (Kynol 507-15) with the concept of pores of different size corresponding to the PSD, also indicating their relation with respect to the size of desolvated and solvated ions in the anode and cathode for the aqueous electrolyte HCOOK: in anode: K+ of 0.27 nm and hydrated K+ of 0.65 nm; in cathode: HCOO− of 0.37 nm and hydrated HCOO− of 1.31 nm.
Figure 8. PSD of the ACF electrodes of this study (Kynol 507-15) with the concept of pores of different size corresponding to the PSD, also indicating their relation with respect to the size of desolvated and solvated ions in the anode and cathode for the aqueous electrolyte HCOOK: in anode: K+ of 0.27 nm and hydrated K+ of 0.65 nm; in cathode: HCOO− of 0.37 nm and hydrated HCOO− of 1.31 nm.
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Table 1. Metrics of all types of symmetric cells from the test data of this study, including maximum energy density (Emax) and maximum power density (Pmax) from the Ragone plots.
Table 1. Metrics of all types of symmetric cells from the test data of this study, including maximum energy density (Emax) and maximum power density (Pmax) from the Ragone plots.
ElectrodeAreal Density
from EIS, Post-CV (ohm cm2)		Celec,spec (F/g)
from CV Discharge		Emax (Wh/kg)
from GCD Discharge		Pmax (W/kg)
from GCD Discharge
ACF [41]6.925534.61690
PANI/PAN0.4114.40.41671
PANI/PAN/1% GNP0.2935.31.191540
PANI/PAN/10% GNP0.327.511.5475
ACF-PANI/PAN8.327038.31611
ACF-PANI/PAN/1% GNP5.128037.32304
ACF-PANI/PAN/10% GNP1.027139.06057
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MDPI and ACS Style

Cosmas, V.P.T.; Savva, I.; Karouzou, M.; Drakonakis, V.; Baker, M.A.; Lekakou, C. Supercapacitors with Composite Fibrous Electrodes. J. Compos. Sci. 2026, 10, 105. https://doi.org/10.3390/jcs10020105

AMA Style

Cosmas VPT, Savva I, Karouzou M, Drakonakis V, Baker MA, Lekakou C. Supercapacitors with Composite Fibrous Electrodes. Journal of Composites Science. 2026; 10(2):105. https://doi.org/10.3390/jcs10020105

Chicago/Turabian Style

Cosmas, Victoria P. T., Ioanna Savva, Maria Karouzou, Vasileios Drakonakis, Mark A. Baker, and Constantina Lekakou. 2026. "Supercapacitors with Composite Fibrous Electrodes" Journal of Composites Science 10, no. 2: 105. https://doi.org/10.3390/jcs10020105

APA Style

Cosmas, V. P. T., Savva, I., Karouzou, M., Drakonakis, V., Baker, M. A., & Lekakou, C. (2026). Supercapacitors with Composite Fibrous Electrodes. Journal of Composites Science, 10(2), 105. https://doi.org/10.3390/jcs10020105

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