Electrospun zein-based conductive composites engineered with versatile nanomaterials for high-performance supercapacitors

Abstract

Corn farm profitability has declined because of mounting problems in the agricultural sector caused by shifting consumer preferences and environmental changes. Therefore, scientists have investigated high-value uses for agricultural waste, and zein, a corn-derived protein, is a viable option for long-term energy storage. Although zein has benefits, including mechanical durability and biodegradability, its low electrical conductivity necessitates modification for use in supercapacitor (SC) electrodes. To improve electrochemical performance, this study describes the fabrication of zein-based composite electrodes incorporating metal oxides (MOs), graphene oxide (GO), polypyrrole (PPy), and indium phosphide quantum dots (InP QDs). To increase conductivity, pyrrole was polymerized in the vapour phase after electrospinning zein into a fibrous structure as a PPy-based nanoporous matrix. Moreover, nanomaterials such as MOs, GO, and QDs were added to the matrix to improve its ion transport and charge storage capabilities. The successful integration of essential components was validated via material characterisation using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. A densely packed fibrous structure promoting effective ion diffusion was observed using scanning electron microscopy. Meanwhile, the electrochemical performance of the zein-PPy-QD-GO-MO-composite-based SC demonstrated a low charge transfer resistance (R_ct) and a fairly good specific capacitance (135 mF cm⁻²) among the electrodes of the bio-resource-derived composite. The device exhibited nearly 100% coulombic efficiency and excellent capacitance retention, retaining >70% of its initial capacitance after 6000 cycles. Thus, zein-based bio-composites are sustainable substitutes for the traditional SC matrix materials. By integrating conductive nanostructures and biodegradable polymers, this study promotes high-performance and ecologically acceptable energy storage solutions made from agricultural waste.

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1. Introduction

The agricultural industry has undergone significant transformations in recent decades due to evolving dietary trends and environmental concerns. While maize remains a key raw material across various industries, unpredictable climate conditions and price volatility have posed financial difficulties for farmers.^1,2 To overcome these challenges, corn producers seek to diversify into high-value businesses. Zein, a protein obtained from corn biomass,³ has garnered considerable attention due to its exceptional characteristics, including nontoxicity, biodegradability, and biocompatibility. Zein nanofibers have found applications in encapsulation of essential oils, controlled drug delivery with enhanced cell adhesion and proliferation,⁴ edible antifungal coatings,^5–7 and wound-dressing mats combined with Ag nanoparticles. In addition, zein-based polymers have been explored for wastewater treatment⁸ and photocatalytic dye removal.^9,10 Electrospun zein can also be functionalized for drug delivery¹¹ and antibacterial wound dressings.¹² Unlike biomaterials such as chitosan, collagen, and glycerin—which are often animal-derived or sourced from limited feedstocks—zein is a plant-based prolamin protein extracted as a byproduct of maize milling: through dry milling, yields of approximately 7–12 wt% are obtained from dried distillers' grains (DDG), dried distillers' grains with solubles (DDGS), and wet distillers' grains (WDG), while wet milling of corn gluten meal (CGM) affords 20–40 wt%, thereby enabling sustainable, large-scale production and ensuring resource abundance and cost-effectiveness (Fig. 1).

Fig. 1 A schematic overview of zein generation and its utilization in advanced products and technologies.

Moreover, despite these advantages, exploration of zein in electrochemical energy storage remains relatively limited, presenting substantial opportunities for novel electrode architectures and performance improvements. Based on these features, zein exhibits excellent chemical modifiability, high surface-area potential via electrospinning, and suitability for wearable devices.

Supercapacitors (SCs), often referred to as electrochemical capacitors, have garnered significant interest from a variety of industries, including wearable electronics, mobile phones, and automobiles, because of their long cycle stability and high power delivery capabilities.¹³ Electric double-layer capacitance (EDLC), which involves electrostatic charge accumulation at the electrode/electrolyte interface, and pseudocapacitance, arising from surface or near-surface redox processes, are the two general categories into which the charge storage mechanisms of SCs can be divided. Conventional EDLC-type SCs mostly use carbon-based materials,^14,16–18 such as carbide-derived carbons, graphene, activated carbon, and carbon onions.¹⁹ Although these materials usually have high specific surface areas and good conductivity, their pseudocapacitive contributions are limited because they frequently lack the functional groups required for redox processes.

Biomass-derived carbonaceous materials have attracted more attention recently as sustainable, economical, and environmentally beneficial substitutes for SC electrode production.¹⁵ In addition to providing hierarchical porous architectures, these materials support the objectives of sustainable production. Precursors for SC electrode materials have been studied from a variety of biomass sources, neem leaves,²⁰ coconut shells,²¹ banana peels,²² egg whites,²³ silk cocoons,²⁴ human hair,²⁵ maize cobs,²⁶ corn grains,²⁷ and rice husks²⁸ Among these, zein protein from corn is especially appealing because it is simple to prepare using electrospinning, which may produce nanostructured fibrous networks with a high specific surface area (SSA), which is essential for optimizing the charge storage between electrodes and electrolytes.^29,30 Zein is a viable platform for the development of functional electrodes because of its chemical stability and functional modifiability, despite the fact that it is insoluble in water and contains fatty acids that make it unfit for direct eating.

To the best of our knowledge, previous studies have not thoroughly explored the viability of zein as an electrode material in electrochemical energy storage applications. Therefore, in this study, we fabricated sustainable and eco-friendly zein-based supercapacitors (SCs) using electrospinning and incorporated various nanomaterials—namely, polypyrrole (PPy), graphene oxide (GO), metal oxides (MOs) such as copper oxide (CuO) and manganese dioxide (MnO₂), and indium phosphide quantum dots (InP QDs)—to enhance their performance.

Every component was chosen for the composite according to its complementary functional contribution. Through the use of redox active nitrogen sites, PPy, a commonly used conducting polymer, was added to improve electrical conductivity and provide pseudocapacitive behavior.³¹ GO has strong mechanical strength and a large number of oxygen-containing surface groups (such as –OH and –COOH), which enhance electrolyte accessibility and promote electron transport. In order to increase pseudocapacitance and further improve the charge storage capacity through reversible faradaic reactions,³² MOs, such as CuO and MnO₂, were added. Because of their size-dependent quantum confinement effects and superior surface reactivity,^33,34 InP QDs were used to improve charge separation and lower recombination losses during electrochemical cycling.³⁵ In order to create a hierarchical hybrid electrode system with enhanced charge storage capacity, conductivity, and stability, these substances were supposed to work in concert with zein.

Zein is the perfect matrix for integrating QDs, GO, and MOs because of its intrinsic chemical composition, which comprises a high density of amide (–CONH₂) and hydroxyl (–OH) functional groups. In order to provide uniform dispersion and strong interfacial contact within the fibrous scaffold, these groups allow for strong hydrogen bonding interactions with the surface functional groups of the inserted nanomaterials. By preserving a large surface area and avoiding nanoparticle aggregation, this promotes both structural integrity and electrochemical efficiency.^36,37

The amide groups in zein create hydrogen bonds and electrostatic attractions with the surface ligands of InP QDs. This helps zein spread evenly across the network and speeds up the transfer of charge.³⁸ GO also has a number of functional groups that contain oxygen (–OH, –COOH). These groups join with zein to form hydrogen bonding networks that make charge transport channels that are linked.³⁹ This synergy considerably increases the composite's electronic conductivity, which is necessary for improving the charge storage efficiency in SCs.

Moreover, CuO and MnO₂ nanoparticles are stabilized inside the zein matrix by forming hydrogen bonds with its functional groups. By participating in reversible faradaic redox processes, these MOs not only play a crucial role in structural reinforcement but also greatly enhance pseudocapacitance.^40,41 To evaluate their suitability for energy storage devices, we carried out extensive structural and electrochemical investigations based on the structural framework depicted in Fig. 2(b). These included assessments of electrochemical performance (areal capacitance, retention, and stability), surface morphology, and chemical structure. The resultant zein-based SCs exhibit excellent biocompatibility and powerful charge storage capacity, which may pave the way for the creation of wearable, environmentally friendly energy storage devices.

Fig. 2 (a) Chemical structure of zein and interactions of InP QDs, GO and MOs, and (b) schematic representation of the fabrication process of the electrospun zein-based composites.

2. Experimental

2.1. Chemicals

Zein powder (Z3625), citric acid anhydrous (C2404), oxidant iron(III) p-toluenesulfonate (FTS), and monomer pyrrole (Py, 98%) were purchased from Sigma-Aldrich. Indium phosphide quantum dots (InP QDs, model: InP/ZnSeS QDs, average particle diameter: ∼9 nm) were obtained from Uniam Korea. GO was supplied by Grapheneall (Lot No. 20200205). Ethylene glycol, ethanol, and citric acid were purchased from Samchun Pure Chemical (Korea) and used without further purification.

2.2. Preparation of CuO and MnO₂ nanoparticles

The CuO/MnO₂ nanoparticles were successfully synthesized using a coprecipitation approach, with the parameters and synthesis process previously documented in the literature.⁴²

2.3. Preparation of zein-PPy and zein-PPy-QD-GO-MO composites

To guarantee total homogeneity, zein powder (10 g) was dissolved in a mixture solvent (25 mL) of 70% ethanol and 30% citric acid and agitated at 1000 rpm for 24 hours. Porous zein nanofibers were created by electrospinning the resultant solution at 25 kV with a spinneret-to-collector distance of 10 cm and a solution feed rate of 0.35 mL h⁻¹ through a 23G stainless steel needle. The fibers were moved to a N₂-purged Vapour Phase Polymerization (VPP) chamber (1 L min⁻¹) where pyrrole was polymerized for 6 hours at 25 °C to create the zein-PPy composite. To create the final zein-PPy-QD-GO-MO material, zein-PPy was surface-coated first with 25 weight percent InP QDs, then with GO and metal oxides (CuO or MnO₂) via solution casting. Fig. 2(b) provides a schematic representation of the complete fabrication process. Fig. 2(a) provides a brief illustration of the chemical structure of zein and its interfacial interactions with PPy, QDs, GO, and MOs, which are driven by hydrogen bonding between the amide/hydroxyl groups of zein and the functional groups of each nanomaterial.

2.4. Characterization and electrochemical analysis of zein-based composites

The surface structure and morphological characteristics of the zein-based composites were investigated using field-emission scanning electron microscopy (FE-SEM, MIRA LMH, TESCAN). The distribution and content of the elements were examined using energy-dispersive X-ray spectroscopy (EDS, Bruker AXS X Flash detector 5010), which provided information on the uniformity of material deposition and the dispersion of functional components. The chemical composition and bonding states within the composite were examined using X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific), which was calibrated to the C 1s hydrocarbon peak at a binding energy of 284.8 eV to enable accurate spectral interpretation. Fourier-transform infrared spectroscopy (FT-IR, PerkinElmer) was used to determine the surface chemistry and functional groups of the materials.

The electrochemical performances of the fabricated SC electrodes were then systematically evaluated using a Biologic SP-150 workstation. Electrochemical impedance spectroscopy (EIS) was used to analyse charge transfer resistance, ionic conductivity, and electrode/electrolyte interface behavior. In addition, cyclic voltammetry (CV) was used to assess the redox activity and capacitive behavior at different scan rates, while galvanostatic charge–discharge (GCD) measurements were performed to determine the specific capacitance, charge storage efficiency, and rate capability. Long-term cycling stability experiments were also conducted to evaluate the durability and retention of the electrochemical performance over many charge–discharge cycles.

3. Results and discussion

3.1. Chemical properties of zein-based composites

To evaluate the functional group changes and interactions within the matrix, the FT-IR spectra of the zein, zein-PPy, zein-PPy-QD, and zein-PPy-MO-GO-QD composite samples were examined (Fig. 3). The secondary structure of the protein is responsible for the signature amide-I (1600–1700 cm⁻¹) and amide-II (1480–1570 cm⁻¹) bands, which are seen as characteristic absorption peaks at 1650 cm⁻¹ and 1550 cm⁻¹, respectively, in the spectra of the zein-only sample. These peaks correlated with N–H bending vibrations and C=O stretching vibrations, respectively.⁴³ Following polymerization with Py, the broad absorption peak of the N–H stretching vibration at 3319 cm⁻¹ (zein alone) was moved to 3407 cm⁻¹, signifying the successful incorporation of PPy.⁴⁴ Furthermore, the presence of polymerization is confirmed by the identification of distinctive PPy peaks at 1031 cm⁻¹ and 737 cm⁻¹, which correspond to C–H in-plane and out-of-plane bending vibrations, respectively.⁴⁵

Fig. 3 (a) FT-IR spectra of zein, zein-PPy, zein-PPy-QD and zein-PPy-MO-GO-QD; (b) high-frequency regions highlighting the presence of metal oxides.

The FT-IR spectra showed increased functionalization with GO and MO. The peak at around 3400 cm⁻¹ due to O–H stretching vibration became broader, showing that hydroxyl (O–H) groups from GO were present. The two peaks at 480 cm⁻¹ and 529 cm⁻¹ were attributed to Mn–O and Cu–O stretching vibrations, respectively (see Fig. 3(b)), which showed that MnO₂ and CuO had been successfully combined.^46,47 These spectral data show that all of the functional components of the zein-based composite were successfully incorporated, indicating their presence and interactions with each other. This thorough investigation also shows that each modification step has been successfully integrated, making sure that the structure and chemical properties needed for better electrochemical performance are in place.

Fig. 4 shows that XPS analysis revealed the elemental composition and bonding characteristics of the zein-PPy-QD composite in great detail. The survey spectrum (Fig. 4(a)) showed that the most prominent peaks corresponded to carbon (C 1s), oxygen (O 1s), nitrogen (N 1s), and indium (In 3d). This means that the appropriate components were added throughout the manufacturing process. This broad range of elemental detection shows how well the synthetic process was able to include these elements into the composite structure. We broke down the high-resolution C 1s spectra (Fig. 4(b)) even further to find clear peaks that correspond to C–H and C–N bonds at 284.6 eV and 285.8 eV, respectively. These peaks are characteristic of aromatic/aliphatic hydrocarbons and nitrogen-containing groups. The peaks show how the matrix and the conductive polymer work together, which is due to both the PPy layer and the zein protein scaffold. This finding is in line with what other research has shown about how proteins and polymers link in composites.^48,49

Fig. 4 (a) Full XPS spectrum of zein-PPy-QD; (b) high-resolution XPS spectra of C 1s (c) O 1s (d) N 1s and (e) In 3d.

The O 1s spectra also included peaks at 530.3, 531.3, and 533.0 eV. These peaks are attributed to In–O bonds from InP QDs, carbonyl groups (C=O), and C–O functionalities, respectively (Fig. 4(c)). The C=O groups mostly come from zein and PPy,⁵⁰ while the O–H signals might indicate that the surface has been modified or that there is still moisture left over from the synthesis of the product. All of these oxygen-containing species work together to improve the composite's electrochemical performance and structural stability by making it easier for electrons and ions to move around and strengthening the bonds across the electrode network.

The high-resolution N 1s spectra showed three separate peaks at 399.0, 399.6, and 401.5 eV. These peaks were attributed to =N-(pyrrolic nitrogen), –NH– (amine/amide nitrogen), and N⁺–H (protonated nitrogen), respectively (Fig. 4(d)). The 399.0 eV pyrrolic signal shows that the pyrrole was successfully oxidatively polymerized into PPy on the zein nanofiber scaffolds. The 399.6 eV –NH– peak comes from the zein protein's native amino groups and shows that the PPy chains are covalently bonded to the zein protein. This suggests that these functional groups were kept to serve as active sites for further surface modification or pseudocapacitive charge storage. Finally, the 401.5 eV peak of protonated nitrogen species shows that PPy has been effectively doped, which is important for improving the electrical conductivity of the composite. These nitrogen environments show that zein and PPy interact strongly at their interfaces, which helps keep the structure stable and makes it easier for charges to move through the composite.^51,52

We deconvoluted the high-resolution In 3d spectrum (Fig. 4(e)) into four separate parts that matched the In–P and In–O bonding states. The peaks at 444.9 eV (In 3d₅/₂) and 452.0 eV (In 3d₃/₂) were attributed to In–P bonds, which showed that the InP QDs had been successfully added to the composite. Also, the peaks at 445.1 eV (In 3d₅/₂) and 453.6 eV (In 3d₃/₂) were caused by In–O species, which means that the surfaces of InP QDs were partially oxidized. People often see this kind of surface oxidation in colloidal InP systems. It is known to make the interface more compatible and speed up the transfer of charge between the QDs and the surrounding matrix.^53–55

3.2. Morphologies of zein-based composites

The morphological development of electrospun zein-based composites, as shown in Fig. 5(a–h), showed the increasing structural improvements made possible by the successive addition of PPy, InP QDs, GO, and MO nanoparticles. The virgin zein nanofibers showed a homogeneous fibrous network and a smooth surface (Fig. 5(a and e)). The fibers' rougher surface after the VPP of Py (Fig. 5(b and f)) demonstrated that the PPy deposition process using FTS-mediated oxidative polymerization was successful. Due to interactions between zein's amino groups and the π-electrons in PPy, this conductive layer stuck firmly to the fibers, increasing electrical conductivity and offering more surface area for charge storage.^50,56 By integrating InP QDs, a denser and more interconnected network was able to partially cover the interfiber gaps (Fig. 5(c and g)).

Fig. 5 SEM images of (a and e) electrospun zein, (b and f) zein-PPy, (c and g) zein-PPy-QD and (d and h) zein-PPy-QD-GO-MO.

The interaction of InP QDs with PPy and zein made the composite more stable. Their quantum confinement effect accelerated charge transfer and made electrons move more freely. These changes to the structure may have increased the active surface area and the number of redox-active sites, which improved the capacitance and rate performance.^57,58 When GO and MO were eventually added, the composite became more denser, the surface got rougher, and many pseudocapacitive active sites were added (Fig. 5(d and h)). GO and PPy interacted via π–π interactions to make the conductive network better. CuO and MnO₂ nanoparticles filled in any gaps that were still there, which might have resulted in increased redox activity. These factors work together to make the structure stronger, the chemicals more stable, and the electrochemical performance better, which makes the composite an excellent candidate for high-performance SCs.^59,60

The surface area and pore structure were systematically probed at each functionalization stage via N₂ adsorption–desorption measurements (Fig. S1 and Table S1†). All samples showed the typical adsorption–desorption curves and hysteresis loops of a material with medium-sized pores, confirming that the zein fiber's porous network stayed intact throughout each modification. After VPP, the Bruauer–Emmett–Teller (BET) surface area (S_BET) went down from 2.83 m² g⁻¹ for pure zein to 0.31 m² g⁻¹ for zein-PPy. This is in line with what was seen in SEM, which showed that the material was becoming denser. When InP QDs were added, the average 5 nm pores of the zein fiber were blocked by around 9 nm QDs, which made the surface area drop even further to 0.004 m² g⁻¹. After adding GO and MO nanoparticles, the surface area went back to 0.63 m² g⁻¹ and even became bigger since GO has a naturally large surface area. In addition, the average pore diameter stayed between ∼5 and 6 nm in all samples, which shows that mesoporosity was still present even after further functionalization.

To further correlate these surface and network changes with electrode morphology, cross-sectional SEM images (Fig. S2†) reveal that the overall electrode thickness increases progressively from ∼29 μm for pristine zein to ∼37 μm for zein-PPy, ∼62 μm for zein-PPy-QD, and ∼140 μm for zein-PPy-QD-GO-MO, confirming that each functionalization step adds material and potentially enhances charge storage per unit area.

EDS mapping was utilized to make sure that the main components of the zein-PPy-QD-GO-MO composites were spread out evenly (Fig. S3†). The zein matrix was able to hold onto the PPy in such a way that it formed a continuous conductive network. This was supported even further by using maps of carbon (C) and nitrogen (N). The oxygen (O) signal highlighted the mixture of GO and MO with a lot of oxygen, which made the surface more wettable and increased redox activity. Also, manganese (Mn) and copper (Cu) mappings were utilized to confirm that MnO₂ and CuO nanoparticles are important pseudocapacitive contributors. Indium and phosphorus mappings were also employed to show that InP QDs were evenly embedded, which made the material more stable overall and improved its capacity to transmit charge. These results illustrate how successfully the composites worked together to make a zein SC electrode.

3.3. Electrochemical performances of zein-based composites

Electrospun zein-based fibrous sheets were applied to the nickel foam surface as symmetric SC electrodes. A 6 M KOH electrolyte was used to assess the electrochemical performance.

The EIS properties of the fabricated zein and zein-based composite SC were studied. The charge transport and ion diffusion properties of each composite electrode were determined via EIS analysis of the electrospun zein-based SCs (Fig. 6(a and b)). The corresponding sheet resistance was obtained for all the zein-based electrodes; the initial resistance decreased dramatically for the zein-PPy-QD-GO/MO composite electrode, possibly due to the incorporation of the conductive materials GO, QD, and MO on the surface of the SC electrode. Intrinsic resistance of the electrolyte and electrode material, which correlated with the intercept at the real axis in the high-frequency domain, was present in all samples, according to the Nyquist plots.⁶¹

Fig. 6 (a) Nyquist plots for electrospun zein-based material composites and (b) in the high frequency regions.

The electrospun zein samples generally showed a noticeable semicircle in the high-frequency range (see Fig. 6(b)), suggesting significant charge transfer resistance (R_ct), which resulted from inadequate interfacial charge transport characteristics and restricted electrical conductivity.⁶² Prior to electrochemical characterization, the sheet resistance of each composite was measured via a four-point probe, and corresponding bulk conductivities were calculated (Table S2†). The conductivities σ were calculated according to eqn S1.† Upon PPy deposition, the zein-PPy electrode exhibited a sheet resistance of 1.5 × 10⁷ Ω/□ (σ = 1.80 × 10⁻⁵ S cm⁻¹), and the high-frequency semicircle markedly compressed, confirming enhanced electron transport at the electrode/electrolyte interface.⁶³ A more compressed semicircle was produced by further incorporating InP QDs into the zein-PPy-QD composite, reducing the sheet resistance to 2.9 × 10⁶ Ω/□ (σ = 5.56 × 10⁻⁵ S cm⁻¹) and indicating the QDs' quantum-confinement-facilitated charge hopping.^57,58 Owing to the synergistic effect of GO and the transition MO supplying additional conductive pathways and electroactive sites, the final composite, zein-PPy-QD-GO-MO, achieved the lowest sheet resistance of 4.9 × 10⁵ Ω/□ (σ = 1.44 × 10⁻⁴ S cm⁻¹) and the smallest high-frequency semicircle, confirming the combined benefits of π–π stacking with GO and faradaic contributions from CuO/MnO₂.^64,65

The dominance of ion-diffusion-controlled processes, which greatly affect SC performance, was linked to the Warburg impedance: a sloped line at intermediate frequencies became more pronounced in the zein-PPy and zein-PPy-QD samples, indicating improved ion diffusion. In contrast, the interconnected GO/MO network in zein-PPy-QD-GO-MO minimized the Warburg feature, signifying enhanced ionic conductivity and reduced diffusion resistance. By improving both electronic and ionic transport, this hierarchical multifunctionalization reduces polarization effects and amplifies charge-storage capacity. Zein-PPy-QD-GO-MO is thus the most attractive option for high-performance SCs, owing to its substantially higher energy storage performance, which results from increased electroactive surface area and elevated electronic and ionic conductivities.

The impacts of PPy, GO, QD, and MO inclusion were investigated using CV and GCD measurements of the electrospun zein-based composite SCs. Fig. 7(a–d) displays the CV curves obtained at scan rates between 25 and 250 mV s⁻¹. A steady shift toward a more distorted conventional rectangular form was evident in the obtained CV curves, suggesting that pseudocapacitive behavior is influenced by an increasing CV area. As the scanning speed increased, the closed portions of the CV curves grew as a result of the rising current levels. The quick reaction kinetics seen at high scan speeds were the cause of this.⁶⁶ In contrast to those of pure zein-PPy and zein-PPy-QD, the zein-PPy-QD-GO-MO SC's larger surface area suggested that the PPy, QD, GO, and MO interactions had better energy storage capacities and a greater specific capacitance than the other SCs. The CV curves of the pure zein and modified zein composite SC electrodes differed, indicating that GO and MO also encouraged faster ionic and electrical transmission inside the PPy polymer network. Superior capacitance characteristics were indicated by the electrode's wider curve area. These findings demonstrate that the addition of additives in the study promote the development of electron transfer pathways within the active material and between the active material and the current collector, all of which exhibit the largest region bounded by the CV curve. Accordingly, these characteristics may enhance the functionalization of zein-based composites' specific capacitance, indicating improved charge storage via both electrical double-layer and faradaic processes. This enhancement was caused by the faradaic redox changes associated with the integrated components. Specifically, the PPy and MO redox processes enabled pseudocapacitive charge storage.⁶⁷

Fig. 7 (a–d) CV and (e–h) GCD of electrospun zein-based material composites.

The GCD profiles acquired at currents ranging from 0.25 mA to 2.0 are shown in Fig. 7(e–h). The longer charge–discharge endurance of the zein-PPy-QD-GO-MO composite confirmed its improved energy storage performance compared to that of the other composites. This outcome is consistent with the CV data and confirms the higher energy-storage capabilities of the zein-PPy-QD-GO-MO composite. The addition of MO increased the pseudocapacitance through surface redox processes.⁶⁸ These elements worked together to maximise the electrochemical activity of the material by promoting effective charge transport paths and increasing the total charge storage capacity. Specifically, compared to the other composite materials, the zein-PPy-QD-GO-MO composite had a notably higher storage capacity (135 mF cm⁻²). This enhancement indicates a synergistic effect of the combined changes. The increased number of redox-active sites and improved electron transfer kinetics were likely the results of the addition of InP QDs. In addition, GO enhanced the electrical conductivity of the composite matrix and offered a versatile framework for charge storage. As shown in Fig. S4,† both CV and GCD data at 250 mV s⁻¹ and 0.25 mA cm⁻² are combined. In the CV curves (Fig. S4(a)†), zein-PPy and zein-PPy-QD show larger enclosed areas and higher currents than pristine zein, with zein-PPy-QD-GO-MO reaching the highest current density. Likewise, the GCD profiles (Fig. S4(b)†) display progressively longer discharge times and smaller IR drops in the same order, confirming the best electrochemical performance among all the samples.

A number of analytical parameters were used to fully assess the improved SC performances of the zein-based composites that were made. Fig. 8(a) shows the particular areal capacitance of each sample, which makes it evident how the composition changes have led to better performance over time. The original zein film had a low capacitance of 11.5 mF cm⁻². When it was coated with PPy (zein-PPy), the capacitance increased to 61.5 mF cm⁻². Adding InP QDs (zein-PPy-QD) later gave 91 mF cm⁻², and the fully functionalized zein-PPy-QD-GO-MO composite achieved 135 mF cm⁻², which showed that the hybrid structure had a synergistic impact.

Fig. 8 Electrochemical performance of zein-based composite supercapacitors: (a) arial capacitance, (b) Ragone plot of energy vs. power density, (c) coulombic efficiency over 6000 cycles and (d) capacitance retention over 6000 cycles.

The trends in energy and power density (Fig. 8(b)), coulombic efficiency (Fig. 8(c)), and capacitance retention (Fig. 8(d)) as functions of the cycle number were also examined. With an energy density of 1.6 × 10⁻⁵ Wh cm⁻² and a competitive power density of approximately 20 W cm⁻², the zein-PPy-QD-GO-MO composite exhibited the highest energy density among the prepared composite electrodes. In stark contrast, the pristine zein showed much lower values (a power density of approximately 10 W cm⁻² and an energy density of less than 0.4 × 10⁻⁵ Wh cm⁻²). This significant performance difference demonstrates how PPy, QD, GO, and MO integration work together. The improved charge transport channels and larger surface areas were responsible for the higher electrochemical activity. In particular, MO maintained the energy density by enabling pseudocapacitance, whereas GO, with a strong electrical conductivity, further increased the power density. Fig. 8(c) shows the coulombic efficiency of each composite as a function of the cycle number. All composites exhibited excellent reversibility, retaining a coulombic efficiency of approximately 100%. Notably, the zein-PPy-QD-GO-MO composite demonstrated outstanding cycling stability, retaining an efficiency above 99% even after 6000 cycles. Coulombic efficiency, calculated based on the charge ratio during the GCD process, is a key indicator of SC performance,⁶⁹ and the capacitance retention over the cycle life of each composite is shown in Fig. 8(d). As shown in the figure, pure zein did not exhibit electrical conductivity; therefore, the capacitance retention of the electrolyte dominated. Further modification with QD, GO, and MO after PPy vapour-phase polymerization effectively alleviated the capacitance decay, and the cycling stability gradually improved. The addition of QD improved charge transport, while GO and MO contributed to the structural stability and increased the active surface area. As a result, after 6000 cycles, the zein-PPy-QD-GO-MO composite retained more than 70% of its initial capacitance, showing the highest capacitance retention among all the samples. Thus, the elevated coulombic efficiency and remarkable cycle stability indicated the superior properties and uniformity of the zein-PPy-QD-GO-MO electrode within this system. This work shows how PPy, GO, QD, and MO affect the characteristics of the zein-based composite zein-PPy-QD-GO-MO. It also shows how vital and unique these materials are for improving the electrochemical performance of SC applications.^70–72 We looked at the main impacts in a systematic way utilizing methodologies including EIS, CV, and GCD, as well as cycling stability. The zein-only sample had a significant drop in capacitance because it had lower electrical conductivity and poor electrochemical activity. Adding PPy, QD, GO, and MO helped to enhance the capacitance retention and prevented this drop from happening. Fig. 8(b–d) shows that the zein-PPy-QD-GO-MO composite might be a high-performance SC electrode material because it has a high energy density, power density, coulombic efficiency, and capacitance retention.

We compared the performance of a previously unexplored electrospun zein scaffold electrode with that of established biomass-derived nanofiber systems, as summarized in Table S3.†^73–83Gelatin-based mats functionalized with PPy⁷³ deliver modest capacitance in organic electrolytes but exhibit limited high-rate and alkaline stability. Cellulose-derived scaffolds,^74–78 incorporating PPy, CNTs, GO and/or MnO₂, achieve areal capacitances of 600–8000 mF cm⁻² in acidic or neutral media with > 80% retention, yet rarely report power densities above ∼10 W cm⁻² or operation in strong alkali. Lignin-derived fibers and hydrogels^79–81 similarly offer F g⁻¹ level pseudocapacitance and moderate cycle life, while chitosan-based electrodes^82,83 show mixed performance under acid/neutral conditions. In contrast, our zein-PPy-QD-GO-MO composite achieves 135 mF cm⁻² at 0.25 mA cm⁻² in 6 M KOH, retains ∼70% of its initial capacitance after 6000 cycles, and delivers an energy density of 16 μWh cm⁻² alongside a power density of 20 W cm⁻². This combination of moderate areal capacitance with exceptional high-rate capability and alkaline resilience comes from the hierarchical zein scaffold's high surface area and mechanical strength, as well as the synergistic layering of PPy, InP QDs, GO, and metal oxides, which provide fast electron/ion pathways and plenty of redox sites. These findings show that clean, non-pyrolyzed zein is a great base for next-generation sustainable supercapacitor electrodes since it is eco-friendly and can be processed in large amounts.

4. Conclusion

The zein-PPy-QD-GO-MO composite was used to make a high-performance SC electrode. After making the porous nanofiber zein scaffolds by electrospinning, VPP was utilized to coat them with conductive PPy. Adding InP QDs, GO, and MO after that made the material's electrochemical characteristics much better. The optimized zein-PPy-QD-GO-MO composite has a specific capacitance of 135 mF cm⁻², in addition to its competitive power density (20 W cm⁻²), high energy density (1.6 × 10⁻⁵ Wh cm⁻²), almost 100% coulombic efficiency, and excellent capacitance retention (over 70% after 6000 cycles). These great performance indicators show how the components of the composite work together, making it a good choice for uses that include both bio-waste and sophisticated energy storage.

Data availability

All authors confirm that the data supporting the investigation of this study are available within the article and its ESI.† Raw data files supporting this study are available from the corresponding authors upon reasonable request.

Author contributions

Young-Hwi Jo: writing original draft, validation, methodology, investigation, data curation, conceptualization. Jonghyun Choi: writing review & editing, resources. Yen Bach Truong: resources. Jin-Heong Yim: writing original draft, validation, supervision, resources, project administration, investigation, conceptualization.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Basic Science Research Program through a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. RS-2023-00221237). This work was also supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. RS-2024-00335799).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta02307f

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