Core–shell metal sulfide composite materials and PEDOT:PSS decorated with MnO₂ on nickel foam for high-performance super-capacitor applications

Dinah Punnoose^a and Hee-Je Kim*^b
aFremont, California 94536, USA
^bSchool of Electrical Engineering, Pusan National University, Gumjeong-Ku, Jangjeong-Dong, Busan 46241, South Korea. E-mail: heejekim734@gmail.com; Tel: +82 10-2295-0613

First published on 20th August 2025

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1. Introduction

Supercapacitors (SCs) and energy storage devices have attracted growing interest over the past few years for their high power density, long cycling ability, reliability and fast charge–discharge rates compared to conventional rechargeable batteries.^1,2 Hence, they are nowadays widely used in applications such as memory backup systems, industrial power management and hybrid electric vehicles.^3–6 However, a major problem that hinders many practical applications of supercapacitors is their low energy density, which is a challenging issue in finding good electrode materials with high specific capacitance, good rate capability, low cost and natural abundance.⁷ According to the charge–discharge mechanism and the materials used, supercapacitors are classified as electric double-layer capacitors and pseudocapacitors. (1) In electric double-layer capacitors (EDLCs), charge is stored electrostatically using charge accumulation at the electrode/electrolyte interface; they employ carbon-active materials (carbon spheres, activated carbon, carbon nanotubes, reduced graphene, and so on^8,9) with high specific area. (2) Pseudocapacitors rely on fast and reversible redox reactions for charge storage (between electrolyte ions and electroactive materials) and possess higher capacitance and superior energy density compared to EDLCs.¹⁰ Pseudocapacitor electrode materials are mainly conducting polymers and transition metal oxides/hydroxides, considering their high specific capacitance. Transition metal oxides, besides conducting polymers, have attracted considerable interest as active electrode materials for supercapacitors. As a layered transition metal dichalcogenide (TMD) with a two-dimensional structure akin to graphene's, molybdenum disulfide (MoS₂) features a unique architecture that enhances surface area and facilitates ion transport.^11,12

Among the transition metal oxides reported to date, zinc oxide (ZnO) is considered one of the promising candidates that can be used for supercapacitor applications as a result of its good electrochemical activity, low cost and environmental friendliness.^11,12 ZnO and its hybrid structures can be exploited in a variety of applications such as solar cells,^13,14 transparent conducting oxides,^15,16 gas sensors,^17,18 and photodetectors.¹⁹ Although ZnO is an encouraging material, regrettably, its applicability is hindered by its low rate capability, poor electrical conductivity and cyclic stability in its pure form.^20–22 One way to address these concerns is by forming 1D nanocomposites with other materials. Semiconducting 1D ZnO nanostructures usually have a high aspect ratio, high surface area, and high crystalline quality, which provide a direct conduction path for electrons.^23,24 A synergetic enhancement in the properties arises in the composite, which helps to improve the performance of the material.²⁵

The benefits of both the capacitances of the metal oxides and the double layer capacitance improve the capacitance and cyclic stability.^26,27 Additionally, the core–shell structure not only guarantees a stable structure and favorable kinetics but also provides a high surface area that increases the amount of active sites on the surface of materials, ensuring that the device demonstrates satisfactory electrochemical performance.²⁸ Comparable to metal oxides, metal sulfides have been established as capable anode materials for SCs. Recently, numerous metal sulfides such as NiS, MnS, CuS, CoS₂, ZnS, and MoS₂ have been investigated as the electrode material for SCs.^29–34 Metal oxide and metal sulfide-based electrode materials are more suited for SC applications owing to their high specific capacitance, enhanced redox reversibility, and high electrical conductivity.^35–38 The complex hierarchical core–shell nanostructures are considered a promising approach that can improve the supercapacitor performances.³⁹

To enhance the electrochemical capacitance of supercapacitors, various types of composite nanomaterials have been used as active electrode materials, such as metal oxides, metal sulfides, and conducting polymers. Among the metal sulfides, NiS and CuS have been extensively studied due to their high theoretical specific capacitances and electrical conductivity.⁴⁰ The electronegativity of sulfur is low when compared to oxygen, which makes it easy for electron transport, and the replacement of oxygen with sulfur provides flexibility in the fabrication of nanomaterials.⁴¹ Copper sulfide (CuS) is one of the most attractive multifunctional energized nanomaterials with many different morphological phases. Chalcocite (Cu₂S), villamaninite (CuS₂), djurleite (Cu1.95S), anilite (Cu1.75S), and covellite (CuS) have been studied extensively for electronic devices, solar cells, gas sensors, and photo-catalysis due to their ease of electric conduction, optoelectronic properties, and excellent chemical and thermal stability.^42–44 Nickel sulfide (NiS) has also been widely examined and has boundless applications in supercapacitors, lithium ion batteries, dye-sensitized solar cells, and ceramic tougheners, etc. NiS is a high-performance supercapacitor electrode, which has excellent electrochemical performance due to its unique structure, high surface area, large in-plane conductivity, and enhanced structural stability. It has provoked significant attention due to its advantage of high electronic conduction, ease of fabrication, low cost, and low toxicity.^45–47 Although NiS has a larger capacitance, a drawback of this material is its short life cycle when compared with CuS.⁴⁸ CuS is widely used in photo-catalysts, solar cells, lithium-ion batteries, and so on.^49–51 This is because CuS shows a metal-like electronic conductivity (103 S cm⁻¹), a high theoretical capacity (561 mAh g⁻¹), complex structures, and valence states.^52–54 Recently, preparations of nanostructures of CuS composites have been demonstrated to show improved energy storage properties. MnO₂ is generally considered one of the most encouraging candidates due to its high theoretical specific capacitance, low cost, environmental friendliness, and abundance in nature.^55,56 However, the poor electrical conductivity of MnO₂ results in a low gravimetric capacitance, thus hindering its wide application in energy storage systems.^57,58 MnO₂ nanoflower intercalation on Ti₃C₂T_x MXenes with expanded interlayer spacing for flexible asymmetric supercapacitors improved capacitance retention and ion diffusion, surpassing the performance of each material.⁵⁹ Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) was discovered in the 1980 s, and it has since been used in many industries because of its optical properties, high conductivity, ease of synthesis, and stability in high humidity and high temperature.^60,61 Moreover, PEDOT:PSS is considered a promising material for SC electrodes because of its excellent chemical and electrochemical stability and the ability to be dispersed in various solvents. An effective strategy is to combine MnO₂ with intrinsic conductive polymers, e.g., polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PEDOT),⁶² poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS),^63,64 and polyaniline (PANI). PEDOT:PSS effectively strengthens the electrochemical properties. The outstanding electrochemical performances of the hierarchical films are very difficult to achieve from a single common material. These approaches are slowly becoming more important for producing effective supercapacitors. A myriad of transition oxide/sulfide materials are fabricated on nickel foam (NF) due to its high porosity and highly accessible open structure, which guarantees large amounts of active sites for redox reaction and ion diffusion. NF acts as a template for the growth of materials due to its high conductivity and porous structure.

In an attempt to increase the performance of SCs, this article discusses the overall electrochemical behavior of metal sulfides. We have proposed a supercapacitor with a novel architecture and synthesized a hybrid electrode material with enhanced capacitive performance. This new hybrid electrode material is based on the effects of vertically aligned ZnO NR arrays as nano-structured collectors for support in a core–shell nanostructure of the PEDOT:PSS conductive polymer and MnO₂, both being pseudo-capacitive materials. ZnO structures have a high aspect ratio, high surface area, and very high crystalline quality, which provides a direct conduction path for electrons.^23,24 Herein, for the first time, hierarchically assembled ZnO nanorods (NRs) with catalytically active metal sulfide (NiS, CuS) materials were fabricated to effectively improve the electron transfer to the NR current collector, facilitating the growth of the polymer (PEDOT:PSS) shell on them. Moreover, we tried to compare the NiS and CuS monocrystalline layer effects on the hybrid electrode. The effect of the metal sulfide plays a major role in enhancing the capacitance of the fabricated electrode. The ZnO NR@NiS/CuS@PEDOT:PSS nanostructures were also decorated with MnO₂. The techniques employed for the synthesis of this hybrid electrode material have the advantages of low growth temperature, low cost, and are less time-consuming compared to those based on hard templates. Moreover, the hierarchical structure with high specific area improves the density of active sites and enhances the density of active sites, electron transfer, and ion diffusion rate, respectively.⁶⁵ Jorge Rodríguez-Moreno et al. reported a hybrid nanostructured electrode on ITO/glass substrates; we have explained the effect of metal sulfides and the effect of each layer on the fabrication of hybrid nanostructured electrodes on NF. The obtained interesting electrode structure plays a vital role in providing more active sites for electrochemical reactions, short ion and electron diffusion pathways, and facilitating ion transport.

2. Materials and methods

2.1. Materials

Zn(CH₃COOH)₂·2H₂O, Zn (NO₃)₂·6H₂O, HMT, C₆H₁₂N₄, NiCl₂·6H₂O, thiourea, 3-MPA, ethylene glycol, copper chloride dehydrate, and sulfur were purchased from Aldrich and used without further purification.

2.2. Preparation of Nickel foam substrate

Before the deposition of ZnO@NiS/CuS@PEDOT:PSS@MnO₂, the nickel foam (approximately 2 cm × 1.5 cm) was cleaned carefully with a concentrated HCl solution (37 wt%) by sonication for 10 min to remove the surface oxide layer. The foam was then washed sequentially with acetone, ethanol, and deionized water (DI) for 10 min each to ensure that the surface of the nickel foam was well cleaned. After drying with a hair dryer, the well-cleaned nickel foam was transferred to a flask.

2.3. Synthesis of ZnO nanorods

ZnO NRs were prepared by a two-step facile chemical bath method. The first step was to coat a ZnO seed layer on the NF. The ZnO seed layer was prepared using zinc acetate dihydrate, 10 mM [Zn (CH₃COOH)₂·2H₂O], as a precursor, dissolved in ethanol, and stirred for 30 min. After the growth of the seed layer, the samples were annealed at 65 °C for 5 h in ambient air.

The second step was to grow ZnO NRs on the seeded NF using a chemical bath deposition method. The seeded substrate was suspended upside down in an aqueous solution containing 0.015 M zinc nitrate hexahydrate (Zn (NO₃)₂·6H₂O) and hexamethylenetetramine (HMT) (C₆H₁₂N₄) at 95 °C for 15 h to grow ZNO NRs. The samples were then rinsed with DI water and annealed at 65 °C for 5 h.

2.4. Synthesis of NiS and CuS nanoparticles

2.5. Fabrication of electrodes

NF with applied ZnO NRs is further used here. CuS or NiS powder and a small amount of ethanol were added to produce a homogeneous paste. This paste was then pressed onto a ZnO-coated NF to produce ZnO@NiS/ZnO@CuS electrodes, which were vacuum dried for 4 h at 55 °C. For the industrial use of high-performance PEDOT:PSS as an efficient and conductive platform matrix on ZnO@NiS/CuS nanoparticles, the prepared electrodes were dip-coated twice with PEDOT:PSS for 10 min each. The polymer-coated electrodes (ZnO@NiS/CuS@PEDOT:PSS) were dried in a vacuum for 12 h at 60 °C. To coat the ZnO NR@NiS/CuS@PEDOT with MnO₂ nanoparticles, these samples (ZnO@NiS/CuS@PEDOT:PSS@MnO₂) were drop-cast using 10 mM KMnO₄ aqueous solution. The step-by-step coating procedure for the resultant electrodes is shown in Fig. 1.

Substrates were densely coated with ZnO, ZnO@CuS, ZnO@ NiS, ZnO@NiS @PEDOT:PSS, ZnO@CuS@PEDOT:PSS, ZnO@NiS @PEDOT:PSS@ MnO₂, and ZnO@CuS@PEDOT:PSS@MnO₂ and kept nearly opaque, and named Z, ZC, ZCP, ZCPM, ZN, ZNP, ZNPM, respectively.

2.6. Electrochemical measurements

Electrochemical measurements were performed using a CHI 660C electrochemical workstation (CH Instruments, Inc., China) interfaced to a computer system with electrochemical software. A 7-mg amount of the as-prepared electrodes was incorporated into nickel foam (1.0 cm × 1.0 cm). We used a three-electrode glass cell setup in the experiment, consisting of a working electrode, platinum wire as the counter electrode, and saturated Ag/AgCl as the reference electrode at 25 °C in 2 M KOH. The electrodes were soaked in a 2 M KOH solution and degassed in a vacuum for 5 h before the electrochemical test. All the electrodes were tested in 2 M KOH aqueous solution with a potential range between 1 to −1 V (vs. Ag/AgCl). The galvanostatic charge/discharge curves were obtained at current densities of 0.71, 1.42, 2.14, 4.28, 5.71, 7.14, 8.57, 10.00, 11.42, 12.85, and 14.28 A g⁻¹. The energy and power density of a single electrode from charge–discharge were calculated using eqn (1) and (2). The specific capacitance of the electrode was calculated using eqn (3) (galvanostatic charge/discharge curves):

(a) Energy and power density of a single electrode

From charge–discharge

	(1)

	(2)

(b) Capacitance of device:

From charge–discharge,

	(3)

Where C = specific capacitance (F g⁻¹), m = mass of the single electrode (g), ΔV = voltage window during the discharge process (V), i = discharge current, Δt = discharge time difference, E = energy density (Wh g⁻¹ or Wh cm⁻² or Wh cm⁻³), P = power density (W g⁻¹ or W cm⁻² or W cm⁻³) I_d = constant current used for charging and discharging process.

2.7. Characterization

The microstructure of the synthesized samples was measured by Schottky emission scanning electron microscope (SEM; SU-70, Hitachi) at the Busan KBSI. The elemental compositions of the electrodes were investigated using a field emission scanning electron microscope (FE-SEM, S-2400, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDX) operated at 15 kV. The crystalline structure and morphology of the prepared electrodes were investigated by high-resolution transmission electron microscopy (HRTEM; JEM 2011, JEOL). High-resolution X-ray diffraction (XRD) analysis was performed on a D8 ADVANCE with a DAVINCI (Bruker AXS) diffractometer using Cu Kα radiation and operated at 40 kV and 40 mA. X-ray photoelectron spectroscopy was also conducted (XPS, PHI 5000C ESCA System). Prior to the measurement, the samples were dehydrated at 90 °C for 5 h. Transmission electron microscopy (TEM) images were obtained (Jeol Corporation, JEM 2011, and 200 kV) at the Busan KBSI center and operated at 200 kV, using a CCD camera, 4k × 4k.

3. Results and discussion

The large contact area between the current collector (ZnO nanorods) and active material (NiS/CuS) can significantly shorten the electron transportation path length. Next, the decoration of the active nanomaterial (MnO₂) on ZnO (current collectors) can lead to an escalation in the number of electrochemically active sites for the redox reaction that enables rapid diffusion of the cations during the charge–discharge process. Densely arranged ZnO NRs@NiS/CuS @PEDOT:PSS on the NF substrate offers more electrochemically active sites for nucleation and growth of MnO₂. To conclude, the existence of the NiS/CuS@PEDOT:PSS shell layer can provide more active charge transport routes along each single ZnO NR. The fabrication procedure is graphically represented in Fig. 2.

3.1. Surface Morphology

SEM images of electrodes Z, ZN, ZNP, ZNPM and their corresponding high magnification images are shown in Fig. 3. The extensive vertical growth of bare ZnO NRs (Fig. 3(a)) was realized, and was uniformly distributed throughout the substrate. The morphology of the well-aligned ZnO NRs (Z) and the resultant cone-like morphology are shown in Fig. 3(a1), and were densely formed on the substrates. Moreover, at higher magnifications, the hexagonal end surface of the NRs can be seen, and their length and diameter are approximately 1–3 μm.

On the other hand, SEM reveals that the surface of the NRs becomes rough after NiS (ZN) deposition. The rough texture of the nanoscale NiS coatings on these NRs is visible in Fig. 3(b and b1). NiS is well crystallized with uniform nanospheres, and is around 700 to 800 nm long. NF is inactive in KOH electrolyte and has a three-dimensional structure, which contributes to a high specific surface area and escalates the adhesion of NiS film. The strong adhesion plays a pivotal role in thin film deposition, which affects the capacitance. Good adhesion of the material depends on the good wettability of the bonded surfaces by the liquid adhesives.⁶⁶ When PEDOT:PSS was deposited, the structure was degraded slightly and was employed to investigate the changes in the microstructures of the hybrid films. The SEM images of ZNP and ZNPM film are revealed in Fig. 3(c and d), respectively. Fig. 3(c) displays a compact and rough morphology for the PEDOT:PSS film on ZN that depicts a wrinkled surface morphology as well as a typical porous network structure (Fig. 3(c1)). Fig. 3(d) reveals the across the ZCPM film, which shows that the material spreads out onto the NF substrate SEM images of Z, ZC, ZCP, and ZCPM and their corresponding high magnification images are shown in Fig. 4(a, b, c, d and a1, b1, c1, d1). For the deposition of CuS on the Z (ZC), the CuS nanoparticles are uniformly decorated on the surface of the Z. It was found that the deposition of CuS (Fig. 4(b)) did not destroy the morphology of the ZC. The uniform distribution of CuS nanoparticles arose from the growth mechanism, and CuS aggregates are formed on the ZnO NRs, which increases the number of reactive sites for the reaction.⁶⁵ Consequently, the void spaces between the NRs were filled with metal sulfides that formed a network wrapping the ZnO NRs. This results in improved electronic conductivity, yielding more defects and an enhancement of the charge transport process compared to the bare ZnO material. CuS layer is deposited onto the ZnO NRs not only to enhance charge-transport ability along each single Z but also to provide heterogeneous nucleation sites for the growth of the PEDOT:PSS layer.⁶⁷ It was demonstrated that this CuS layer is composed of nano-crystalline aggregates. The observed SEM images also suggested that during the CuS coating on networks of ZnO NRs and NF, the particle sizes were tremendously reduced to hold the high surface area of NF. It is well recognized that the nanoparticles on porous structures can enable the transport of the electrolyte and ensure efficient contact between the active material and the electrolyte for electrodes.⁶⁸

When PEDOT:PSS (Fig. 4(c)) conductive polymer shell layer is grown on the ZC using dip coating onto the surface of the ZC, it ultimately forms a core–shell structure. To grow the MnO₂ nanoparticles, through a redox exchange process, decoration of the ZCP nanostructures by MnO₂ nanoparticles was achieved. The addition of PEDOT:PSS facilitated the cohesiveness between MnO₂, and both PEDOT:PSS and MnO₂ combined well with each other. Fig. 4(d) shows a SEM image of a single ZCPM nanostructure, which suggests that a PEDOT layer could be successfully deposited. Furthermore, over the smooth PEDOT layer, the presence of the MnO₂ NPs can be confirmed by the roughness of the ZCPM nanostructure. Nearly all the hybrid core–shell NRs are highly accessible to electrolytes for electrochemical energy storage due to the presence of convenient diffusion channels. EDX analysis was used to examine the elemental compositions of Z, ZC, ZCP, ZCPM, ZN, ZNP, ZNPM, and the results are shown in Fig. S1 and S2 in the SI.

The TEM image (Fig. 5(a)) identifies that the ZnO NRs with an average diameter of approximately 1 to 3 μm. The distribution of ZnO NRs is shown in the magnified image (Fig. 5(a1)). When NiS is coated on ZnO, the TEM image of the prepared ZnO NRs appears to be smooth on the surface. The outer layers revealed that every Z was formed with a solid core and a shell. Fig. 5(c) is a representative TEM image of the obtained ZNP. The strong contrast in the nanoparticles from ZN to ZNP, where NRs are converted to distributed particles. Fig. 5(d) also shows that these products are shorter compared to ZnO NRs, indicating that they were broken into smaller sections during the chemical conversion process.

The intrinsic morphologies of Z, ZC, ZCP, and ZCPM were further investigated by TEM. The TEM images (Fig. 6(b)) of CuS on the ZnO NRs (ZC) nanostructure indicate that the core shell was formed. The average particle size of ZC is found to be ∼1 to 1.5 μm from the TEM images. When PEDOT:PSS is coated, there are no interlayer gaps between nanoparticles, indicating that the CuS NPs were strongly attached to ZnO NRs. The synergetic interaction between them may help the enhancement of the catalytic activity and long cycle life for supercapacitor and photocatalytic applications. In contrast, TEM images of ZCP (Fig. 6(c)) showed large, agglomerated particles, which are consistent with FE-SEM analysis. For comparison, the intrinsic morphology of ZCPM was evaluated by using TEM (Fig. 6(d)), which further confirms the fabrication of the ZCPM. These results reveal that a uniform-sized CuS NP with a hierarchical core–shell structure, which may boost electron transfer rates and increase the cycle life of the materials in supercapacitor and photo-catalytic applications.

The chemical composition and surface electronic state of the electrode with certain regularity in distribution is one of the most significant characteristics that guarantee greater electrochemical properties. X-ray photoelectron spectroscopy was used to determine all the elements valence states and their specific ratios in the compound (Fig. 7). The strong resolution Zn 2p spectrum is presented in (Fig. 7(a)), of which two strong peaks at 1022.02 and 1045.05 eV can be clearly seen, corresponding to the binding energy of Zn 2p_3/2 and Zn 2p_1/2, respectively, indicating the presence of Zn²⁺ in the ZnO wurtzite structure.⁶⁹ It is observed that there is an energy separation of 23 eV between the Zn 2p_3/2 and Zn 2p_1/2 peaks, which is in agreement with an earlier report on ZnO.⁶⁹

From the O 1s spectrum (Fig. 7(b)), it can be seen that the spectrum can be fitted to two Gaussian peaks, namely, O_latt (530.51eV) and O_ads (531.77eV). O_latt is defined as oxygen ions in the crystal lattice, while O_ads is the absorbed oxygen ions in the oxygen-deficient regions. These functional groups are beneficial to capacitive performance because they can contribute to additional pseudo-capacitance and improve the wettability between the electrode and electrolyte.⁷⁰ The survey spectrum of ZnO is given in Fig. 7(c), ZnO peaks, which confirms the presence of Zn species in the electrode along with O 1s and Cls peaks.

XPS measurements were carried out for ZN, and the results are shown in Fig. 8(a). The survey spectrum of ZN shows S 2p, C 1s, O 1s, Ni 2p3, Zn 2p3, and N 1s at 162.89, 284.6, 531.14, 855.5, and 1072.12, respectively. The O 1s, C 1s, and N 1s peaks were attributed to the exposure of the fabricated electrode to air. The binding energy of NiS at 853.2 and 870.5 eV corresponds to the presence of Ni 2p3, and for S 2p spectrum of NiS (Fig. S3(a) of SI), the binging energy at 161.4 eV suggests that the S element exist as S²⁻, and the peaks of S2p_1/2 at 162.5 eV(Fig. S3(b) of SI) can be owed to sulfur ion in low coordination at NiS surface.⁷¹ Meanwhile, the peaks at 164.2 eV and 165.3 eV correspond to the sulfur atom of PEDOT, and the higher binding energy peak at 168.5 eV corresponds to the sulfur atom present in PSS (Fig. 8(b and e)).^72,73 The survey spectrum of electrode ZNPM is shown in Fig. 8(c) where the binding energy peaks at 642.48 eV for Mn 2p3 is shown along with Ni 2p3 (855.16) and S 2p (167.3). The survey spectrum is almost similar to ZN and ZNP, where the addition of Mn 2p3 can be seen. The high-resolution XPS scan of Mn 2p has two peaks at 642.48 and 653.32 eV, which were assigned to Mn 2p_3/2 and Mn 2p_1/2, respectively, and were separated by 10.84 eV (Fig. S3(d) of SI).

The XPS results show that the ZC electrode consists of Cu and S elements with ZnO (Fig. 8(d)). The two peaks of Cu 2p are obtained at binding energies in the range of 932 to 952 eV, relative to the 2p_1/2 and 2p_3/2 regions,⁷⁴ which are shown in Fig. S3(c) of the SI. Sulfur peaks were also obtained at 162 eV (Fig. S3(d) of SI). The absence of a sulfonate peak suggests an increase in stability when using CuS and a lower reactive rate with atmospheric oxygen.⁷⁵ These elements with corresponding binding energies are consistent with previously stated CuS,⁷⁶ though the S 2p_3/2 and S 2p_1/2 binding energies.

The survey spectrum of electrode ZNPM & ZCPM during the electrode preparation with respect to MnO₂ is given in the SI (Fig. S4). CuS crystal that designates the strong bonding between CuS and ZnO. This is beneficial to improve the charge transfer rate and also increase the cycle life of the electrode materials in supercapacitors and photocatalytic applications. These results indicate that the CuS NPs are well sum up by the ZnO/CuS/PEDOT:PSS and are also successfully fabricated by the current coating techniques. The presence of mammoth numbers of surface functional groups is responsible for preventing agglomeration of CuS in the ZnO/CuS and ZnO/CuS/PEDOT:PSS/MnO₂. The XRD analysis of ZCPM is given in the SI (Fig. S3).

3.2. Electrochemical studies

The functional integrity of as-prepared hierarchical supercapacitor electrodes was investigated by characterizing electrochemical studies. The obtained Z, ZN, ZNP, ZNPM, ZC, ZCP, and ZCPM on nickel foam were evaluated as electrodes for supercapacitors. Electrochemical measurements were carried out in a three-electrode electrochemical cell with 2 M KOH aqueous solution used as the electrolyte.

Fig. 9(a–h) shows the CV curves of the electrodes recorded at various scan rates in the potential range of −0.1 to 0.7 at different scan rates for ZN, ZNP, ZNPM, ZC, ZCP, and ZCPM electrodes. A pair of redox peaks can be observed in all the CV curves that indicate the electrochemical performance of electrodes resulting from pseudo-capacitive behaviour. Since the process is faradaic, the CV curve is different from the usual curve of a double-layer supercapacitor, which shows normal rectangular peak shapes.⁶⁴ With increasing scan rates, the positions of cathodic peak current increase to a more cathodic direction, which could be attributed to the high conductivity of the electrode. The capacitance increases with a decrease in the scanning rate and it is evident that this is associated with the adsorption and desorption of ions at available sites. When moving on to higher scan rates, the ions do not get enough time to get into the available sites. At lower scan rates, the ions diffuse and are adsorbed at available sites with an increase in the specific capacity of the material.⁶⁴

The CV of electrode Z (Fig. 9(a)) consists of a pair of redox peaks, which are observed in the CV curve caused by the redox reactions of ZnO. This redox process is mainly governed by the intercalation and de-intercalation of K⁺ from the electrolyte into ZnO.

ZnO + K+ + e− ↔ ZnOK

When NiS/CuS (ZN/ZC) (Fig. 9(b) and (e)) is coated on ZnO (Z), there is no doubt that such a structure will have a high specific area and can improve the adsorption efficiency of ions, resulting in the strong capacitance of the material. The ZC electrode showed a larger CV area, which suggests a higher capacitance and the high performance of the ZC electrode was attributed to the material with a high specific surface area and high porosity, which improves the transport of ions to the active sites of the electrodes which is shown in Fig. 9(e). When PEDOT:PSS is added to ZN and ZC the there is an enhanced specific current for the composite film, which is due to its good synergetic effect between them. This enlarges the contact area between the active materials and electrolytes. ZCP electrode shows higher current when compared with ZNP electrode because of its fast electron transfer kinetics of ions on the resultant electrode surface and hence offers more active sites for the nucleation and growth of MnO₂ (Fig. 9(c) and (f)).

MnO₂ is coated on ZNP (ZNPM) (Fig. 9(d)) and ZCP (ZCPM) (Fig. 9(g)) the polymer-supported NR structures enable fast access of ions to the surface of MnO₂. The reaction between KMnO4 and PEDOT:PSS is shown in Fig. 10. The capacitance of manganese oxides comes mainly from pseudo-capacitance ascribed to reversible redox reactions in conjunction with the intercalation/adsorption of protons and/or cations. ZCPM shows the highest current in comparison with that of Z, ZC, ZN, ZNP, ZCP, and ZNPM, suggesting that the ZCPM electrode exhibits much better pseudo-capacitive performance and electrochemical reversibility. Fig. 9(h) shows the current densities of ZNPM and ZCPM at different scan rates. The ZCPM shows enhanced conductivity, increased areal capacitance, and hence higher current values when compared to ZNPM. ZCPM has better conductivity than ZNPM because of CuS than ZNPM. As a result, the composite electrode architecture has a synergistic effect between PEDOT:PSS and ZnO@CuS, the high electronic conductivity of CuS, and shortens the ion diffusion pathway. The CV curves of the ZCPM composite illustrate that the electrode material has good reversibility. The detailed oxidation and reduction current values of each sample is given in the SI (Tables S1 and S2).

Galvanostatic charge–discharge (GCD) is the most accurate technique for capacitance measurements, hence, GCD measurements were conducted for samples Z, ZN, ZNP, ZNPM, ZC, ZCP, and ZCPM (Fig. 11(a–h)) within the potential window of −0.1–0.7 V at different current densities of 0.71, 1.42, 2.14, 4.28, 5.71, 7.14, 8.57, 10.00, 11.42, 12.85, and 14.28 A g⁻¹. A small internal resistance drop (IR drop) was observed, which is caused by equivalent series resistance, which includes electrode resistance, electrolyte resistance, and contact resistance between the electrode and the electrolyte. Discharge profile of the supercapacitor electrodes was found to depend on the applied current, and similar curve shapes have been obtained for different current densities.⁷⁷ The ZCPM electrode exhibited the longest discharge duration compared to the Z, ZN, ZNP, ZNPM, ZC, and ZCP, reflecting the substantially superior performance. These results show that the specific surface area and mesoporous structures with a uniform morphology are crucial factors for obtaining high supercapacitor performance. The detailed specific capacitance values of each electrode are given in the SI (Tables S3 and S4).

Moreover, the specific capacitance of ZCPM was calculated to be 2554.61, 2143.61, 1704.76, 1466.34, 1259.16, 1095.92, 954.28, 840.4, 736.8, 650.91, and 568.14 F g⁻¹ at current densities of 0.71, 1.42, 2.14, 4.28, 5.71, 7.14, 8.57, 10.00, 11.42, 12.85, and 14.28 A g⁻¹, respectively. As the current density increased from 0.71 to 14.28 A g⁻¹, the capacitance of the ZCPM electrode decreased from 722.41 to 470.25 F g⁻¹, which is a relatively high capacitance retention (i.e., 65.09%). The capacitive performance of the ZCPM min electrode was superior to that of the other electrodes due to the higher diffusion and migration of electrolytic ions, as well as to the use of the inner active surface area for charge storage at high current densities.⁷⁸

The electrochemical performance of the nanostructured optimized ZNPM and ZCPM electrodes was examined by standard cyclic voltammetry (CV), the galvanostatic charge–discharge technique, and electrochemical impedance spectroscopy (EIS). Fig. 12(a) shows CV curves of the ZCPM and ZNPM electrodes at a scan rate of 5 mV s⁻¹ in a 3 M KOH electrolyte solution. The CV curve shape of the core–shell nanocomposite electrodes decorated with MnO₂ (ZNPM and ZCPM) is different from the electric double-layer capacitance, showing that the capacitance was due mainly to the pseudocapacitive capacitance. Moreover, from the CV curve shapes, the integrated area of the ZCPM electrode was higher than that of the ZNPM electrode materials, showing that the ZCPM electrode has the largest specific capacitance. The optimized ZNPM and ZCPM electrode GCD curves are shown in Fig. 12(b). The lowest capacitance of 2072.52 F g⁻¹ @ 0.71 A g⁻¹ is observed for ZNPM, and the highest capacitance of 2554.61 F g⁻¹ @ 0.71 A g⁻¹ is observed for the composite sample ZCPM. The excellent electrochemical performances of the core–shell structures (ZCPM) can be attributed to their unique hierarchical structure, which can provide more active sites for electrochemical reactions. Comparison of specific capacitance values of different composite electrode materials with previously reported nanostructures using a standard three-electrode cell is given in the SI (Table S5).

To discuss the transport characteristics of the charge carriers in sample of ZCPM and ZNPM electrodes, the electrochemical impedance spectroscopy (EIS) was also performed at open circuit potential over the frequency range 100 kHz to 0.1 Hz. Using the EIS spectra, in Fig. 12(c), the Nyquist plots show a semicircle at high frequency regions and an inclined line in low frequency regions. At higher frequencies, the point that intersects with the real axis exhibits an internal resistance (R_S) that includes the intrinsic resistance of the electrode active material, bulk resistance of electrolyte, and contact resistance between active material and current collector interface. Besides, the diameter of the semicircle corresponds to the interfacial charge transfer resistance (R_ct). At the lower frequency, the linear line is related to the Warburg impedance or diffusion resistance, and hence, the higher value indicates lower impedance.⁷⁹ It can be seen that the slope of the straight line for the ZCPM nanocomposite electrode is larger than that of the ZNPM electrode. That is to say, the ZCPM nanocomposites have better ion diffusion in the electrode, which is seen in Fig. 12(c). At high frequencies, the point intersecting with the real axis of the ZCPM nanocomposite electrode is smaller than that of the ZNPM electrode which means that the conduction of the ZCPM nanocomposites was improved by compositing the active material with good electrical conductivity. The analysis for all the remaining electrodes (ZN, ZNP, ZNPM, ZC, ZCP and ZCPM) are provided in the SI (Fig. S6). The better ion diffusion can be ascribed to the morphology of the ZCPM nanocomposites possessed core–shell structure ZCP as well as larger specific surface area, which are beneficial to the ion diffusion, and the specific capacitance was improved.⁸⁰

The stability of the electrode materials is one of the most important requirements for supercapacitor applications. The cycling stability test of the ZCPM and ZNPM electrodes were evaluated at a constant charge–discharge current density of 50 mA cm⁻² for 3000 cycles, as shown in Fig. 12(d). The ZNPM electrode for the 1^st cycle showed a specific capacitance of 559.85 F g⁻¹, which decreased slowly to 453.58 F g⁻¹ after 3000 cycles, showing 19% loss. On the other hand, the ZCPM electrode, initially for first 500 cycles the capacitance increased from 1095.92 F g⁻¹ to 1205.52 F g⁻¹. After 500 cycles the capacitance reduced and finally it exhibited only 2% loss of a specific capacitance after more than 3000 cycles, highlighting the good long-term stability of the ZCPM electrode.

Fig. 13 shows the comparison of scanning electron microscopy images before and after cyclic testing of ZCPM: (a), (a1) are before the cycle; (b), (b1) are after the cycle. Nearly all the hybrid core–shell NRs are highly accessible to electrolytes for electrochemical energy storage due to the presence of convenient diffusion channels. Comparing the surface morphology of the aggregates before and after the cycle reveals they retain their rough and slotted texture, though their porosity has increased. Increased porosity offers several benefits for electrode materials: it reduces charge transfer resistance, enabling faster electron transfer, and minimizes volume changes during charge–discharge cycles. These improvements positively impact the electrode's electrochemical performance and stability.^81,82

Fig. 14 shows the Comparison of transmission electron microscopy images before and after cyclic testing of ZCPM: (a), (a1) are before the cycle; (b), (b1) are after the cycle. These results demonstrate that the hierarchical core–shell ZnO@CuS@PEDOT:PSS@MnO₂ structure remains uniform and intact even after cycling. This preserved architecture, featuring uniform-sized CuS nanoparticles, is crucial for enhancing electron transfer rates and extending the cycle life of the material in both supercapacitor and photocatalytic applications. The entire multi-layered structure maintains its cohesion and mechanical stability, ensuring that the ion and electron transport pathways through all layers remain intact. Furthermore, the integrity of the PEDOT:PSS conductive binder is preserved, and the accessibility of active sites within MnO₂ is maintained.

The power density and energy density are two essential parameters to examine the performance of supercapacitors. Fig. 15 shows the Ragone plots of ZNPM and ZCPM. The ZCPM exhibited a high energy density (169.9 Wh Kg⁻¹) and power density (278.57 W kg⁻¹), which were much higher than those of ZNPM (31.03 Wh Kg⁻¹ and 107.14 W kg⁻¹) at a current density of 5 mA cm⁻².

To summarize, a scalable strategy and an approach have been developed to fabricate transparent hierarchical ZCPM hybrid nanoarchitectures with an enhanced electrochemical performance for supercapacitors. The enhanced capacitive behavior is attributed to the unique hierarchical core–shell hybrid nanorod array configuration and the synergistic effects of ZnO NR current collectors and the combined pseudo-capacitive contributions from the PEDOT@NPs MnO₂ shell layer. The metal sulfide nanostructures would provide more active sites to catalyze the reduction of electrolyte and connect with the vertically aligned ZnO NR network to ensure conductivity, accelerate the electrolyte transport, and promote fast charge transport as well as increase the diffusion velocity of the redox couple.

4. Conclusion

We have successfully demonstrated a strategy for enhancing the capacitance of supercapacitor electrodes. We report a facile and cost-effective hydrothermal, chemical bath deposition method, along with dip coating, to synthesize ZnO@NiS/CuS@PEDOT:PSS@MnO₂ (ZCPM) hetero-structures. The resultant electrode (ZCPM) exhibits advantages such as a higher deployment rate of the electrode in KOH solution, easy penetration of OH⁻¹ ions into the inner region of the electrode, and an enhanced faradaic reaction of the electrode surface area with electrolyte compared to other electrodes (Z, ZN, ZNP, ZNPM, ZC, and ZCP). The ZCPM electrode shows a high capacitance of 2554.61 F g⁻¹ at a current density of 0.71 A g⁻¹ and good long-term cycling stability of 98%, which is superior to that of the ZNPM electrode. The outstanding performances profit from its unique porous structure constructed by interlaced nano particles. Our future work will be focused on further improving the cycling stability of the remaining electrodes with different metal sulfides. The overall improved electrochemical performance of the ZCPM electrode makes it promising for potential applications in high-performance supercapacitors.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

Data for this article have been included as part of the main manuscript and its SI. The document includes results from several analyses: EDX analysis, XRD analysis, XPS analysis, EIS analysis and comparison table of specific capacitance. See DOI: https://doi.org/10.1039/d5nj01656h

Acknowledgements

This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korea Ministry of Education (2016R1D1A1B02009234) and the Ministry of Trade, Industry, and Energy (Grant no. 0002310). This work was supported by the National Research Foundation of Korea (NRF) (Grant no. RS-2022-00166999).

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