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. 2026 Sep 28. Online ahead of print. doi: 10.1039/d6ra08646b

Engineering multi-source biomass carbon hybrid integrated with MWCNTs and TiO2 for high-performance supercapacitors and oxygen evolution catalysis

Nasima Arshad a,✉, Muhammad Shahroz a, Maryam Rizwan a, Muhammad A U R Qureshi a
PMCID: PMC13618356  PMID: 42807971

Abstract

This project was designed to investigate the electrode performance of a sustainable biomass-derived carbon hybrid P1 (AC) sourced from mango leaves, moringa leaves, and spent tea waste, alongside its binary P2, P3 (AC@MWCNTs, AC@TiO2) and ternary P4 (AC-MWCNTs@TiO2) composites for supercapacitors and oxygen evolution reaction (OER) water splitting applications. X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), and Fourier-transform infrared spectroscopy (FTIR) were used to confirm the successful synthesis of the as-prepared materials via structural, compositional, and morphological attributes. Electrochemical characterization using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) chronopotentiometry revealed significantly enhanced supercapacitive performance for the composites compared to the pristine biomass carbon hybrid. Among all tested materials, the P4 ternary composite emerged as the most promising supercapacitor electrode, delivering a superior specific capacitance (Cs) of 651 F g−1 at 2 mV s−1 (via CV) and 312.5 F g−1 at 0.8 A g−1 (via GCD), a longer discharge time (dt) of 168 s, a high energy density (Ed) of 12.13 Wh kg−1, and an excellent capacitance retention of 96%. Furthermore, OER electrocatalytic evaluations conducted via CV and linear sweep voltammetry (LSV) demonstrated that the same P4 composite exhibited a low onset potential of 1.50 V, a minimized overpotential (η) of 198 mV at 10 mA cm−2, and a low Tafel slope of 64.28 mV dec−1. Long-term electrocatalytic stability was successfully authenticated via controlled potential electrolysis (CPE) chronoamperometry, showing steady performance at a current density of 98 mA cm−2. Electrochemical impedance spectroscopy (EIS) corroborated the rapid charge kinetics of P4, which yielded the lowest solution resistance (R1 = 1.20 ohm) and charge transfer resistance (R2 = 1.70 ohm). Therefore, driven by these benchmark performances, the P4 ternary composite stands out as a viable, dual-functional electrode candidate for next-generation supercapacitors and OER applications.

Biomass-derived carbon hybrid sourced from mango leaves, moringa leaves, and spent tea waste, and its binary and ternary composites with MWCNTs and TiO2 as electrode candidates for supercapacitors and OER catalysis applications.graphic file with name d6ra08646b-ga.webp

Introduction

Today's world is facing extreme energy crises; hence, it is needed to search for eco-friendly, abundant, non-toxic, and renewable sources for energy conversion and storage. The world is moving towards sustainable energy solutions to ensure carbon neutrality, and this trend has resulted in a greater demand for storage technologies that are efficient, economical and green. Consequently, significant research interest has focused on developing advanced materials and systems as viable pathways toward renewable energy storage and conversion.1 Hence, advanced storage devices must be capable of responding rapidly, withstanding numerous charge–discharge cycles, and ensuring overall operational safety. In this scenario, supercapacitors emerge as highly promising candidates that can easily be integrated into hybrid energy systems, while electrochemically driven water splitting provides an efficient and sustainable route for renewable energy generation and storage.2,3

Supercapacitors have been increasingly recognized as a key technology for next-generation energy storage applications, with performance characteristics that overcome most shortcomings of conventional batteries and capacitors.4 A long cycle life, high power density, rapid charge–discharge rates, and operational reliability make them ideal for applications requiring instantaneous power delivery or capture.2,5–9 These exceptional properties stem largely from the high surface area of the constituent electrode materials and the inherent chemical and thermal stability of the systems. Furthermore, these devices are relatively cost-effective, eco-friendly, and require low maintenance, which further enhances their suitability for contemporary energy infrastructures.10 Similarly, exploration of new, cost-effective, and efficient electrode materials for water-splitting catalysis is one of the most crucial demands today, where reducing energy barriers is directly related to the cathodic and anodic catalytic activities of materials driving the hydrogen evolution (HER) and oxygen evolution (OER) reactions.3,11,12

Metal oxides and carbon-based materials are considered the most generic and reliable sources for electrode preparation to be used in energy conversion and storage.13 Among metal oxides, titanium dioxide (TiO2) remains a prior choice in energy applications due its photochemical stability, abundance, low cost, non-toxicity, and simplicity of processing.7 Despite TiO2 exhibit nanostructured morphology, that increases its usability through high surface to volume ratio and active site exposure, it has some intrinsic disadvantages like poor electrical conductivity and poor long-term mechanical stability that limit its application as an independent component in high-performance systems.12 Similarly multi-walled carbon nanotubes (MWCNTs) have substantially drawn attention in energy applications.14 MWCNTs exhibit high mechanical strength, electrical conductivity, structural stability, and ion-transport pathways, and have interconnecting conduits that assist in the quicker and efficient transfer of charges.15,16 However, agglomeration in MWCNTs reduces the surface area and create hurdles in electrolyte induction into the carbon matrix, that definitely effect on its electrochemical performance.15 Consequently, there is a need to develop new electrode materials to cope with aforesaid problems in a more efficient manner.

Owing to high porosity, large specific surface areas, presence of heteroatoms, tunable microstructures, natural occurrence, and renewability, biomass precursors have been the promising choice and turned out to be an appealing starting materials in sustainable energy development.17,18 A biomass is considered a carbon-neutral resource when harvested and converted in a responsible manner.19 The use of a biomass to generate a functional carbon material is very much in line with the global movement to lessen reliance on non-renewable resources as well as to decrease the environmental impact of energy technologies.20 With the continued growth of green technologies – especially biofuels, bioproducts, and other energy systems that are based on the circular economy – biomass is becoming a major focus in mitigating greenhouse gas emissions and ensuring long-term energy security.21 The distinguished properties of biomass carbon materials render them very applicable in electrochemical processes, such as the electrocatalysis and energy-storage.22 Recent research studies showed amazing results of nano-structured carbon synthesized from biomass materials, which despite their inexpensiveness, exhibited enhanced electrochemical performances.23–25

Biomass derived carbon materials are being investigated at length as independent, as synergistically derived biomass hybrid carbon, and as integrated electrode materials for energy storage devices.26 Despite tremendous advantages, low energy density and capacitance fading are the two major concerns that limit biomass-activated carbon when used alone.27 However, biomass synergies and their composite formation presents a promising avenue and help to improve the electrode performance in diverse applications, such as supercapacitors and electrocatalytic hydrogen and oxygen evolution reactions (HER/OER).24,27–32 Such modifications provide tremendous potential for the development of electrode materials that align with the global trend of using resource-efficient and sustainable materials, minimizing environmental impact, and offering state-of-the-art electrochemical performance.

In this work, three different biomass sources (mango leaves, Moringa oleifera leaves, and spent tea waste) were used to synthesize a synergistic activated carbon (AC) hybrid. Based on the respective advantages and limitations of biomass carbon, MWCNTs and TiO2, binary and ternary composites were prepared as AC@MWCNTs, AC@TiO2, and (AC-MWCNTs@TiO2. Moreover, systematic assessments of the as-prepared materials were carried out to evaluate their structural, morphological, and electrochemical properties and to explore their potential as electrode materials. The primary rationale behind this study is to engineer multi-component architectures that can serve as a sustainable, eco-friendly, and cost-effective dual-functional electrode for advanced energy applications. Our core scientific hypothesis is that integrating high-conductivity 1D networks (MWCNTs) and redox-active metal oxides (TiO2) with a porous 3D carbon matrix provides powerful, interconnected network architecture for the enhancement of electrochemical performance.

Experimental

Materials and methods

All the necessary chemicals and reagents were purchased from Merck and Sigma Scientific, while Mango leaves, Moringa leaves, and spent tea waste were collected from local sources. Activated carbon was synthesized from the biomass materials using an integrated approach of hydrothermal carbonization followed by chemical activation. Ethanol, KOH, and HCl were used for the chemical activation of the biomass materials, with purities of 99.9%, 99.99%, and 37%, respectively. Prior to conducting electrochemical measurements, a three-electrode cell was assembled with a working electrode, Platinum (Pt) wire counter electrode, and a silver/silver chloride (Ag/AgCl) reference electrodes immersed in 1.0 M KOH electrolyte. Working electrode surface was prepared by cutting a sheet of nickel-foam into 1 × 1 cm chunks. A slurry of each synthesized material was prepared, separately, by adding 0.25 mg of it into 1 mL of ethanol. Subsequently, the mixture was sonicated for 1 hour, followed by the addition of 10 µL of a 5 wt% Nafion binder solution. The resulting paste was coated onto a nickel foam substrate, dried at 80 °C for 15 hours, and stored at room temperature for 24 hours. The fabricated carbon paste-modified nickel foam surface was then utilized as the working electrode.

Synthesis

Activated carbon from biomasses

Activated carbon was synthesized, separately, from three distinct biomass precursors-mango leaves, moringa leaves, and spent tea waste-using a series of well-controlled steps. Each biomass was first washed well with tap water, distilled water, sun dried for 02 weeks, and then oven dried at 80 °C. After drying, each biomass was ground into powder. The obtained powder was soaked into ethanol for few hours followed by stirring to remove organic impurities. The mixture was then filtered, washed with distilled water, and allowed to dry completely in air. The powder sample was then subjected to pre-carbonization at 350 °C for 02 hours in a furnace under an inert atmosphere, with the ramping rate set at 2 °C per minute. The pre-carbonized sample thus obtained was impregnated with KOH in a 1 : 3 weight ratio using 10 mL of distilled water and allowed to soak for 14 hours. The mixture was dried in an oven at 80 °C for 08 hours and crushed once again after cooling to room temperature. It was then carbonized at 600 °C for 01 hour under nitrogen flow, using a heating rate of 5 °C per minute. After carbonization, the product was washed with 1.0 M HCl, followed by repeated rinsing with deionized water until a neutral pH was reached. Finally, the material was dried at 80 °C for 08 hours to obtain the activated carbon from each biomass.

Biomass carbon hybrid (AC)

A hybrid activated carbon was prepared by blending equal weights (100 mg each) of the activated carbon derived from mango leaves, moringa leaves, and spent tea waste. The three carbon powders were dispersed uniformly in absolute ethanol under continuous stirring for 03 hours. The mixture was then sonicated for 01 hour and subsequently filtered to retrieve the biomass carbon hybrid. The collected sample was dried at 80 °C for 08 hours and further heat-treated at 200 °C for 01 hour to yield the finalized activated carbon hybrid, denoted as AC.

Composite formation

Initially, multi-walled carbon nanotubes (MWCNTs) were functionalized by adding them to a mixture of sulfuric acid and nitric acid in 3 : 1 ratio. The mixture was ultrasonicated for 04 hours, washed with distilled water to achieve the neutral pH, and finally dried in an oven at 60 °C for 12 hours.

For composite formation, 100 mg of functionalized MWCNTs was dispersed in 50 mL of ethanol. Then, 100 mg of AC was added to the dispersion, followed by ultrasonication for 45 minutes and subsequent magnetic stirring for 03 hours to ensure uniform distribution. The resulting mixture was filtered, dried and ground to fine powder to obtain the binary AC@MWCNTs composite. For the integration of the binary AC@TiO2 composite, a uniform slurry was prepared by dispersing 100 mg of AC in 50 mL of ethanol. Subsequently, 60 mg of TiO2 nanoparticles were incorporated into the mixture, which was then subjected to ultrasonication and stirring for two hours. The solvent was then evaporated in an oven at 80 °C for 08 hours. The resulting dried AC@TiO2 was further annealed at 200 °C for 01 hour under inert atmosphere at the heating rate of 2 °C per minute.

AC-MWCNTs@TiO2 ternary composite was prepared by dispersing AC@MWCNTs in 50 mL of ethanol; following this, 60 mg of TiO2 nanoparticles was introduced into the suspension. The mixture was ultrasonicated 02 hours, followed by a continuous stirring for 6 hours to achieve a uniform distribution of TiO2 on the AC-MWCNTs framework. The resulting suspension was dried at 80 °C, gently ground, and further annealed at 200 °C for 02 hours to enhance interfacial adhesion and structural stability, yielding the AC-MWCNTs@TiO2 ternary composite. The specific weight compositions for AC, and its binary (AC@MWCNTs, AC@TiO2) and ternary (AC-MWCNTs@TiO2) composites are summarized in Table 1, and the samples were designated as P1, P2, P3, and P4, respectively.

Table 1. Sample designations, chemical compositions, and constituent weight ratios of the prepared activated carbon hybrid and its respective compositesa.
Samples and codes Weight wt. ratio
CMMS CMWCNTs CTiO2
P1 (AC) 100 mg 1 : 1 : 1
P2 (AC@MWCNTs) 100 mg 100 mg 1 : 1
P3 (AC@TiO2) 100 mg 60 mg 1 : 0.6
P4 (AC-MWCNTs@TiO2) 100 mg 100 mg 60 mg 1 : 1 : 0.6
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a

CMMS = 100 mg each of activated carbon of CMango leaves, CMoringa leaves, and CSpent tea waste in 1 : 1 : 1 ratio.

Instrumentation for characterization

PW-3040/60 Xpert-Pro diffractometer (Panalytical, Netherlands) having Cu Kα radiation source (λ = 1.544 Å, V = 45 kV, I = 30 mA, step size = 0.033°, and 2θ = 20° − 80°) was used in X-ray diffraction (XRD) analysis of the synthesized materials. A high-resolution scanning electron microscope (SEM, JSM-6490A, JEOL, Japan) equipped with a tungsten filament and an Everhart–Thornley (E.T.) detector was used for the surface morphological analysis of the synthesized samples. The microscope was operated with a resolution of 3 nm over a magnification range of 5×–300 000×. For structural elucidation, the Fourier transform infrared (FTIR) spectrum of each sample was recorded at a resolution of 2 cm−1 using a Nicolet iS10 FTIR spectrometer (Thermo Fisher Scientific, USA). The FTIR spectra were acquired over a scanning range of 4000–400 cm−1. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronopotentiometry (galvanostatic charge discharge; GCD), electrochemical impedance spectroscopy (EIS), and chronoamperometry (controlled potential analysis; CPE) were employed to investigate the electrochemical performance of the fabricated electrodes using electrochemical workstation (GAMRY-3000, Potentiostat/Galvanostat, USA).

Electrochemical analysis

The electrochemical activities of fabricated electrode materials were monitored in a 1.0 M KOH aqueous electrolyte. To evaluate the suitability of the prepared materials as supercapacitor electrodes, CV of each fabricated electrode was performed at selected potential scanning range of 0–0.7 V at scan rates from 2 to 100 mV s−1. Capacitance measurements were done by using current data at selected scan rates. Further, for capacitance retention, repeated cycling at low scan rate (5 mV s−1) was carried out till 1000 cycles. Additionally, GCD profiles were recorded at various current densities within a potential window of 0.65 V. Optimal performance was achieved at low current densities of 0.8, 1.2, and 2.0 A g−1. The OER catalytic activity of the fabricated anode candidates was evaluated using CV and LSV. These experiments were conducted over a potential range of 1.0 to 1.8 V vs. the reversible hydrogen electrode (RHE) at a scan rate of 2 mV s−1. From the LSV curves, the onset potential, overpotential (η), and Tafel slope (b) were determined by using equations; [(η = ERHE − 1.23 V), (η = a + b log j)], where ERHE represents the potential vs. the reversible hydrogen electrode, a and b are Tafel constant and Tafel slope, respectively, and j is the current density. Potential conversion to the reversible hydrogen electrode (RHE) scale was performed using the Nernst eq. Inline graphic, where Inline graphic is 0.1976 V (vs. SHE at 25 °C) for the saturated Ag/AgCl reference electrode. CPE was conducted at a constant potential of 1.65 V vs. RHE for 15 hours. The resulting current density versus time profile was recorded to evaluate the long-term stability of the fabricated anode during OER activity. EIS measurements were performed at the open-circuit potential (OCP) to record the Nyquist plots. The frequency range was set from 100 kHz to 10 mHz with a sinusoidal perturbation amplitude of 5 mV.

Results and discussion

Synthesis scheme

The overall synthesis pathway of the materials is outlined in Scheme 1, while the detailed procedures are provided in Experimental Section 2.1. Initially, pre-carbonization, chemical activation, and final carbonization of each biomass were carried out using controlled thermal methods. The resulting carbon of each biomass were then blended in equal weight ratios and subjected to further heat treatment to yield the hybrid activated carbon matrix (AC). Subsequent blending and annealing processes served as the primary steps for fabricating the AC-based binary and ternary composites with MWCNTs and TiO2.

Scheme 1. Schematic illustration of the systematic synthesis protocol for individual biomass-derived activated carbon from mango leaves, moringa leaves, and spent tea waste, their carbon hybrid (AC), and corresponding binary (AC@MWCNTs, AC@TiO2) and ternary (AC-MWCNTs@TiO2) composites.

Scheme 1

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Material characterizations

XRD

The XRD pattern of the P1 sample retains characteristic peaks of an amorphous carbon system (JCPDS file, 41-1487). The peak at about 22–25° (2θ), representing the (002) plane, signifies random interlayer carbon arrangements. This broad peak signifies that the nature of the prepared activated carbon is predominantly amorphous with minimal graphitic structure. Additionally, the reflection at approximately 43° (2θ) corresponds to the (100) plane, indicating the in-plane structural arrangement of sp2-hybridized carbon atoms. The broad, low-intensity peak observed at 62°–65° (2θ) is assigned to the (110) crystallographic plane, further confirming the presence of localized graphitic domains within the amorphous framework. Overall, the XRD pattern confirmed that the synthesized activated carbon is predominantly amorphous, with a minor crystalline fraction indicating partial graphitization. Since P2 composite exhibits the combined characteristics of both AC and MWCNTs, the appearance of distinct diffraction peaks indicates a heightened level of crystallinity, which is primarily imparted by MWCNTs. Specifically, the sharp reflections corresponding to the (002) and (004) crystalline planes confirm the successful integration of MWCNTs within the composite matrix.33

The XRD patterns of P3 and P4 confirms the successful incorporation of TiO2 in both binary AC@TiO2 and ternary AC-MWCNTs@TiO2 composites. A well-defined set of diffraction peaks observed at 2θ values of approximately 25.3°, 37.8°, and 54.0° can be indexed to the (101), (004), and (105) crystallographic planes, respectively. These reflections are in accordance with JCPDS no. 21-1272, confirming the presence of the crystalline anatase phase of TiO2 in both composite architectures.31,34 The diffractogram of P4 reveals a combination of both carbon-related broad reflections and sharp TiO2 peaks, demonstrating that the ternary hybrid composite consists of coexisting amorphous and crystalline phases. A prominent diffraction feature consisting of a sharp peak superimposed on a wide hump within the 2θ range of 24°–26° further confirmed the formation of the composite. In this region, the sharp peak corresponds to the (002) graphitic reflection of the MWCNTs, while the broad hump represents the diffuse scattering background characteristic of the amorphous AC phase, illustrating the successful co-existence of distinct structural phases. The general shape of this peak indicates the presence of roughly stacked carbon layers in little graphitic regions, hence dominating the amorphous nature in the structure over graphitic nature, which is the characteristic of activated carbon obtained from biowaste.35 The addition of MWCNTs also overlaps in this area, which further strengthens the carbonaceous structure. In general, the broadness of peaks in XRD patterns indicated the dominance of amorphous nature in AC and its composites. The XRD patterns, with appended hkl values, of P1 and P2 are provided in Fig. S1 (in SI file), while these patterns for P3 and P4 are provided in Fig. 1.

Fig. 1. X-ray diffraction (XRD) patterns of the P3 (AC@TiO2) and P4 (AC-MWCNTs@TiO2) composites appended with respective hkl values.

Fig. 1

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Furthermore, the average crystallite sizes of P3 and P4 composites were calculated from their respective X-ray diffraction (XRD) patterns using Scherer's equation (D = Kλ/β cos θ), where D, K, λ, β, and θ, respectively, represent crystallite size, Scherrer shape factor (normally 0.9), X-ray wavelength (1.5406 Å for Cu-Kα radiation), FWHM (full width at half maximum of the diffraction peak in radians), and Bragg angle. The evaluated values are provided in Table 2. The XRD spectra of both samples revealed prominent diffraction peaks for (101), (004), and (105) planes, confirming that the anatase form of TiO2 is present in both nanocomposites. The average crystallite size was reduced from 16.1 nm in sample P3 (AC@TiO2) to 9.1 nm in sample P4 (AC-MWCNTs@TiO2). In addition, the FWHM showed broadening in all peaks, suggesting that the TiO2 crystallite size is more refined in P4 than in P3. The reason behind this refinement in P4 is due to the network of MWCNTs as structural templates/scaffolds which inhibit the TiO2 grain growth and hence favour the formation of smaller and more uniformly dispersed TiO2 grains.

Table 2. Average crystallite sizes and related XRD peak characteristics for P3 and P4 composites.
Peak (hkl) 2θ (°) P3 P4
FWHM 2θ (°)/β (rad) D (nm) FWHM 2θ (°)/β (rad) D (nm)
(101) 25.3 0.50/0.00855 16.6 0.89/0.01501 9.2
(004) 37.8 0.45/0.00925 15.9 0.91/0.01641 9.2
(105) 54.0 0.65/0.00995 15.8 0.99/0.01763 9.0
Average — — 16.1 — 9.1
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SEM-EDX

SEM analysis reveals a distinct morphological transition across the series of prepared materials. The SEM image of P1 exhibits a highly disordered, stratified, and flake-like arrangement of sheets. This structure features an abundance of interconnected micro- and mesopores, confirming successful biomass carbonization. This well-developed porosity offers a high specific surface area that is critical for enhancing ion transport kinetics and maximizing charge storage capacity.36 The SEM image of the P2 composite demonstrates a highly entangled, tubular morphology rather than spherical particles, which is a characteristic feature of MWCNTs. These carbon nanotubes form elongated, interconnected fibers that intertwine and overlap across the carbon matrix, creating a robust web-like network. During synthesis, the biomass-derived activated carbon hybrid (AC) can deposit on or bridge the nanotubes, which reinforces the fibrous framework instead of producing discrete spherical particles. Upon the incorporation of MWCNTs (P2), morphology is transformed into a highly interconnected and integrated conductive network. In this structural arrangement, the MWCNTs effectively bridge the individual activated carbon particles, thereby inhibiting particle agglomeration, forming open ion-diffusion channels, and significantly improving overall electrical connectivity. Overall, the observed morphology aligns precisely with the expected structure of P2 composite. The highly intertwined fibrous network confirms the presence of MWCNTs, while the rough surface texture indicates the successful integration of activated carbon within and along the nanotube framework.37

In contrast, the P3 composite, which incorporates TiO2 without MWCNTs, maintains a porous carbon framework. This structure exhibits minor surface irregularities due to the decoration of TiO2 clusters on the activated carbon particles, which preserves the porous cavities and enhances structural uniformity across the composite.31 Due to the absence of MWCNTs, however, the resulting conductive network is less continuous. The concurrent incorporation of TiO2 and MWCNTs in the ternary composite P4 leads to the decoration of the activated carbon matrix with disordered TiO2 particles, establishing well-defined heterointerfaces. Although partial TiO2 agglomeration occurs, the intimate contact maintained among the TiO2, MWCNTs, and activated carbon components ensures effective charge transport pathways and enhances synergistic catalytic performance. Ultimately, the systematic and progressive transformation of morphology from P1 to P4—driven by the sequential integration of AC, MWCNTs, and TiO2—demonstrates that this hierarchically porous, electrically integrated framework is highly optimized to promote electrode performance in both supercapacitor and OER applications.

EDX data revealed a high carbon percentage in P1 and its corresponding composites (P2–P4), whereas non-carbon elements were present only in trace or fractional amounts. The oxygen content observed across all samples was primarily derived from the inherent oxygen groups of the three biomass precursors used to synthesize the initial activated carbon hybrid (AC). This elemental profile can be attributed to the complete or partial elimination of volatile matter and non-carbonaceous constituents during the high-temperature chemical activation and annealing processes.38 The presence of TiO2 in the P3 and P4 composites was further confirmed by their respective EDX spectra. The SEM micrographs and corresponding EDX spectra of P3 and P4 are presented in Fig. 2, whereas those for P1 and P2, along with the detailed elemental compositions, are provided in Fig. S2 and Table S1 (in SI file).

Fig. 2. SEM micrographs and corresponding EDX spectra of the P3 (AC@TiO2) and P4 (AC-MWCNTs@TiO2) composites. The quantitative elemental compositions from EDAX are provided in Table S1 (in SI file).

Fig. 2

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Moreover, the size distribution analysis was done using ImageJ software on SEM images which showed an enormous decrease in particle diameter from 340.5 nm in P3 (AC@TiO2) to 71.8 nm in P4 (AC-MWCNTs@TiO2), which is in line with the crystallite size distribution pattern obtained through XRD analysis. Histogram analysis of the P3 sample showed a broad distribution pattern, which is skewed to the right, with a long tail ranging from 1200 to 1400 nm, indicating high particle agglomeration, while the histogram pattern of P4 sample showed a narrow distribution pattern below 700 nm, Fig. 3. The fine and uniform particle distribution in P4 supports the nucleation effect provided by MWCNTs that hinder TiO2 particle growth and agglomeration.

Fig. 3. Particle size histogram profiles of P3 (AC@TiO2) and P4 (AC-MWCNTs@TiO2) nanocomposites.

Fig. 3

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FTIR

The FTIR spectra of all prepared samples were recorded, with those of the P3 and P4 composites presented in Fig. 4, while the corresponding spectra for P1 and P2 are provided in Fig. S3 (in SI file). Because all samples contain the AC matrix, the FTIR spectra of P2–P4 composites are directly comparable with the spectrum of P1 regarding baseline biomass functionalities. Following the thermal carbonization and activation processes, the FTIR spectrum of P1 displays several prominent absorption peaks corresponding to the presence of functional surface groups. Particularly a broad peak at ∼3226 cm−1, a strong reflection at ∼1578 cm−1, and an absorption band at ∼1048 cm−1 correspond to O–H, C Created by potrace 1.16, written by Peter Selinger 2001-2019 C, and C–O stretching vibrations, respectively. Meanwhile, a lower-intensity peak at ∼2346 cm−1 and a sharp band around ∼750 cm−1 correspond to the adsorption of atmospheric CO2 and C–H out-of-plane bending modes in aromatic rings, respectively.39–41 In the FTIR spectrum of the P2 composite, a weak band at ∼3029 cm−1 corresponds to the C–H stretching vibrations of aromatic or unsaturated carbon structures, confirming the presence of localized graphitic domains. Furthermore, the peak observed at ∼1588 cm−1 is assigned to the aromatic C Created by potrace 1.16, written by Peter Selinger 2001-2019 C stretching vibrations of sp2-hybridized carbon frameworks contributed by both AC and MWCNTs.42

Fig. 4. FTIR spectra of the P3 (AC@TiO2) and P4 (AC-MWCNTs@TiO2) composites.

Fig. 4

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From the FTIR spectra of the samples P3 (AC@TiO2) and P4 (AC-MWCNTs@TiO2), the presence of TiO2 can be confirmed based on the appearance of characteristic bands at values below 600 cm−1. The peaks of Ti–O and Ti–O–Ti in the carbon/TiO2 are found at 473 and 623 cm−1, respectively.41 Also, TiO2 tends to exhibit transmittance peaks at 500–1000 cm−1 due to Ti–O and Ti–O–Ti bond vibrations.41 Indeed, P3 is seen to have a cluster at 564.7/511.4/455.6 cm−1, while P4 is seen to have a sharper doublet at 468.2/460.9 cm−1. An absorption band at ∼1070 cm−1 in the FTIR spectrum of P3 is assigned to Ti–O–C stretching vibrations, confirming the successful integration of TiO2 into the carbon matrix.31 The FTIR spectrum of P4 confirms the effective concurrent integration of AC, MWCNTs, and TiO2 within the ternary composite framework. A broad, shallow absorption band spanning 3200–3600 cm−1 in the spectra of both the P3 and P4 composites is assigned to surface –OH stretching vibrations. These groups originate from adsorbed moisture and surface hydroxyl moieties on the TiO2 nanoparticles, as well as the oxygenated functional groups of the activated carbon matrix, thereby enhancing the surface hydrophilicity and electrochemical wettability of the electrodes. The band at ∼1555 cm−1 is assigned to C Created by potrace 1.16, written by Peter Selinger 2001-2019 C stretching vibrations of the sp2-hybridized carbon, that is, graphitic domains in AC and MWCNTs, which show that the conductive carbon frameworks are maintained. Importantly, a distinct, sharp absorption peak appeared at 1068 cm−1 in P4, suggests a stronger carbon–TiO2 interaction in the ternary composite. The establishment of this localized AC/Ti–O bridging provides clear evidence of a strong chemical interfacial interaction between TiO2 and the hybrid carbon framework.31 Notably, the low-wavenumber extra peaks, attributed to Ti–O–Ti (500–800 cm−1) and Ti–O–C (1210 cm−1) vibrations, are seen in the CNTs-based TiO2 system due to the vibration caused by oxygen-containing groups on the surface of nanotubes serving as anchor sites for TiO2 nanoparticles and C–O–Ti linkage formation. Therefore, the presence of MWCNTs in the AC/TiO2 system results in enhanced TiO2 lattice. Overall, the FTIR findings confirm the successful coexistence of conductive carbon species and crystalline TiO2. The abundance of surface functional groups across the samples facilitates rapid charge-transfer kinetics, rendering these hybrid materials highly, favorable for electrode fabrication of these materials in supercapacitor and OER catalysis.

Electrochemical studies

Supercapacitor performance

The capacitive behavior and specific capacitance (Cs) of each fabricated electrode were evaluated in a 1.0 M KOH electrolyte using CV scanning at various scan rates and GCD cycling at three optimized current densities. The Cs values from both CV and GCD were determined using equations; (Cs = ∫Idv/2mνΔV) and (Cs = Idt/mΔV), respectively.24,31 Where ∫Idv, I, dt, m, ν, and ΔV represent the area under CV curve, charge/discharge current (A), discharge time (s), mass of active material (g), scan rate (V s−1), and potential window (V), respectively.

The CV curves of the fabricated electrodes are presented in Fig. 5 (for P3 and P4) and Fig. S4 (for P1 and P2) of the SI file, while their evaluated Cs values are summarized in Table 3. CV responses indicated pseudocapacitive behavior for all fabricated electrodes, as scanning at various rates revealed well-defined redox peaks in the resulting voltammograms. The broad and drawn-out nature of the redox peaks indicates the quasi-reversible character of the electrochemical process. This redox behaviour is maintained across all scan rates, confirming a sustained pseudocapacitive charge-storage mechanism.24,31,43,44 Particularly, the continuous increase in peak current with accelerating scan rates indicates that the redox activity at each fabricated electrode is governed by mixed kinetics, involving both surface-controlled pseudocapacitive and diffusion-controlled processes.45 This kinetic profile reveals that the observed performance originates from a rapid surface faradaic reactions and fast electrolyte ion diffusion through the hybrid framework.45,46 As shown by the calculated Cs values, electrode performance decreases at elevated scan rates. This behaviour stems from restricted ion transport, which limits electrochemical reactions to the outer electrode surface during rapid scanning. At lower scan rates, the extended response time allows electrolyte ions to fully penetrate the inner active pores, promoting extensive faradaic reactions and achieving superior capacitive storage.24,31,45,47

Fig. 5. (Top) Overlay of CV curves for the P3 (AC@TiO2) and P4 (AC-MWCNTs@TiO2) composite electrodes in a 1.0 M KOH electrolyte at scan rates of 2, 5, 10, 20, 30, 50, and 100 mV s−1 (in-set: specific capacitance as a function of scan rate). (Bottom) long-term electrochemical stability profiles for all fabricated electrodes showing capacitance retention versus the number of cycles over 1000 continuous runs.

Fig. 5

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Table 3. Specific capacitance (Cs) values of the biomass-derived carbon hybrid and its respective composites, evaluated from CV data at scan rates ranging from 2 to 100 mV s−1.
Scan rate (mV s−1) C s (F g−1)
Fabricated electrode material
P1 (AC) P2 (AC@MWCNTs) P3 (AC@TiO2) P4 (AC-MWCNTs@TiO2)
2 162 265 541 651
5 137 185 419 566
10 128 150 397 431
20 112 117 351 370
30 98 101 323 340
50 90 86 273 298
100 87 66 190 261
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The decrease in Cs values with increasing scan rates is consistent across all fabricated electrodes, as shown in the in-sets of Fig. 5 and S4 (in SI file). Compared to P1 and P2, the composites P3 and P4 exhibited greater Cs values at the lowest scan rate of 2 mV s−1. Notably, the capacitance reached 651 F g−1 for P4 which was almost 498, 386, and 110 times higher than those evaluated for P1, P2, and P3, respectively. The distinct charge-storage behavior is directly linked to unique compositional variations of each fabricated electrode material. Because P1 is an activated carbon hybrid (AC) derived from three biomass sources (mango leaves, moringa leaves, and spent tea waste), its formulation altered the overall porosity.24,48,49 The reason for low Cs of P1 could be attributed to the structural degradation via pore collapse, which induced a localized graphitization effect, as confirmed by XRD analysis.48,49 Conversely, P2 combines the properties of both AC and MWCNTs; its enhanced Cs is allied with an extensive conductive network that significantly improves the overall electrical conductivity.38,50

The CV profile of composite P3 exhibited a marked improvement in pseudocapacitive performance, yielding an evaluated Cs value of 541 F g−1 at a low scan rate of 2 mV s−1. This enhancement in electrical conductivity and pseudo capacitance can be ascribed to its multilayered architecture. More precisely, the coating of the synergistic triple-biomass carbon hybrid onto the TiO2 nanoparticles facilitates ion penetration by providing more active redox sites for faradaic reactions.31,51 Furthermore, the integration of AC-MWCNTs with TiO2 in the P4 composite electrode successfully optimizes the electrocatalytic kinetics, leading to the highest observed pseudocapacitive activity.

Compared to P3, the incorporation of MWCNTs into the P4 matrix fosters the development of a highly interconnected conductive network. This network accelerates electron transfer and maximizes active material utilization, culminating in a notable enhancement in electrochemical performance. This is quantitatively confirmed by the superior specific capacitance of P4 (651 F g−1) relative to P3 at a scan rate of 2 mV s−1. Remarkably, even at an elevated scan rate of 100 mV s−1, P4 retains a high capacitance of 261 F g−1, whereas P3 drops to 190 F g−1. Overall, owing to the synergistic interactions among the porous biomass-derived carbon hybrid, redox-active anatase TiO2, and highly conductive MWCNTs, the P4 composite stands out as the most effective electrode candidate for advanced supercapacitor applications. These collective interactions significantly enhance ion diffusion, charge-transfer kinetics, and capacitance retention.

To evaluate long-term electrochemical stability, CV cycling was performed for up to 1000 cycles for each fabricated electrode at a low scan rate, as graphically illustrated in the bottom panel of Fig. 5. All fabricated electrodes successfully retained over 80% of their initial capacitance after 1000 cycles. Among the tested candidates, P4 demonstrated the maximum capacitance retention of 94%, whereas P1, P2, and P3 exhibited retentions of 81%, 85%, and 89%, respectively. These retention values are highly competitive with existing benchmark electrode stabilities.52 Moreover, owing to superior Cs and enhanced stability, P4 stands out as the most promising electrode material.

The super capacitive performance of each fabricated electrode was further evaluated via chronopotentiometry, with GCD measurements carried out at optimized and constant current densities of 0.8, 1.2, and 2.0 A g−1. The potential-time profiles (GCD curves) are presented in Fig. 6 (for P3 and P4) and Fig. S5 (for P1 and P2) in the SI file, while the corresponding electrochemical data are summarized in Table 4. In-sets in Fig. 6 provide a magnified view of the initial discharge regions.

Fig. 6. (Top) Overlay of GCD curves for the P3 (AC@TiO2) and P4 (AC-MWCNTs@TiO2) composite electrodes in a 1.0 M KOH electrolyte at current densities of 0.8, 1.2, and 2.0 A g−1 (in-sets: magnified view of the initial discharge regions). (Bottom) Ragone plots illustrating energy density as a function of power density for fabricated electrodes.

Fig. 6

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Table 4. Performance parameters of the fabricated electrodes evaluated from GCD measurements at the lowest optimized current density (0.8 A g−1).
Fabricated electrode material Current density (A g−1) Charge time (s) Discharge time dt (s) C s (F g−1) E d (Wh kg−1)/Pd (W kg−1)
P1 (AC) 0.8 48 36 52.30 2.6/260
P2 (AC@MWCNTs) 67 57 91.20 4.12/260
P3 (AC@TiO2) 91 61 157.4 4.41/260
P4 (AC-MWCNTs@TiO2) 306 168 312.5 12.13/260
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All the fabricated electrodes exhibited a prolonged discharge time (dt) at the lowest current density of 0.8 A g−1, a trend that aligns consistently with the calculated specific capacitance (Cs) values. This behavior is primarily linked to the porous architecture and abundant surface functional groups of the biomass-derived activated carbon, which enhance electrolyte wettability and ion adsorption. Because rapid charge–discharge processes hinder ion transport into deeper pores, the drop in Cs at elevated current densities arises from restricted pore accessibility. Consequently, shorter discharge time at higher current densities indicate internal resistance, limited ion diffusion, and reduced active site utilization.53 This inverse relation reflected superior charge storage at slower rate, confirming the classic capacitive nature of the material.53 Furthermore, the presence of a distinct potential plateau in the GCD curves signifies interfacial faradaic activity, consistent with the redox peaks observed in the cyclic voltammograms, thereby authenticating the pseudocapacitive nature of the fabricated materials.24,31

The discharge times of the composite electrodes (P2–P4) were consistently longer than that of the pristine carbon hybrid (P1) across all current densities. Specifically, at a low current density of 0.8 A g−1, the discharge durations for P2, P3, and P4 were extended by 21, 25, and 132 seconds, respectively, compared to P1. The extended discharge time and resulting higher Cs of P2 are ascribed to the conductive pathways introduced by the MWCNTs, which minimize the internal voltage drop and enhance charge mobility. For P3, the integrated TiO2 nanoparticles augment the pseudocapacitive performance through surface redox reactions, though its inherent semi-conductive nature limits rapid electron transport. Nevertheless, P3 delivers stable and reproducible GCD profiles with a longer discharge time compared to P1. Crucially, P4 exhibits the longest discharge duration and superior Cs, demonstrating a powerful collective effect among the AC, MWCNTs, and TiO2 constituents. Within this structure, the MWCNTs network provides continuous electron-transport channels between the AC and TiO2 particles. This mitigation of internal resistance enables the simultaneous, efficient utilization of both the electric double-layer capacitance (EDLC) from the AC matrix and the faradaic pseudo capacitance from the TiO2 active sites.

The energy storage performance of each fabricated electrode was further evaluated by calculating the energy density and power density by using equations; (Ed (Wh kg−1) = Cs × ΔV2/2 × 3.6) and (Pd W kg−1 = Ed × 3600/dt), respectively. The resulting values across all optimized current densities are summarized in Table 4 and S2 (in SI file). The correlation between Ed and Pd is graphically illustrated via Ragone plots in the bottom panel of Fig. 6 and S5 (in SI file). Notably, the P4 composite delivered the highest energy density of 12.13 Wh kg−1 at a power density of 260 W kg−1. These findings reinforce the superior super capacitive activity of P4 under a low current density of 0.8 A g−1.24,31,54

The electrochemical findings obtained from the CV and GCD analyses were mutually complementary. Overall, the cooperative synergy among the AC, MWCNTs, and TiO2 nanoparticles in the P4 composite successfully enhanced the redox activity, cycling stability, discharge capacity, specific capacitance, and energy density. Consequently, this composite stands out as the most promising electrode candidate among all tested materials for high-performance supercapacitor applications.

OER performance

The electrocatalytic OER activity of each fabricated electrode was further investigated using iR-corrected CV and LSV measurements. While CV was employed to assess the electrochemical behaviour and stability of the electrode surface, LSV provided quantitative insights into the OER kinetics through the determination of onset potentials and overpotentials.55 A high overpotential acts as a major kinetic barrier for the inherently sluggish oxygen evolution reaction; conversely, a minimized overpotential signifies enhanced charge-transfer kinetics and superior electrocatalytic performance. The CV and LSV overlapping curves, as shown in Fig. 7 for P3 and P4 and in Fig. S6 for P1 and P2 (in SI file), demonstrate highly consistent electrochemical behavior across the samples. The onset potentials were evaluated against the reversible hydrogen electrode (RHE), and the overpotentials were determined at a standard current density of 10 mA cm−2. As summarized in Table 5, the onset potentials and overpotentials ranged between 1.50–1.53 V and 198–318 mV, respectively.

Fig. 7. (Top) Overlay representations of CV and LSV profiles of the P3 (AC@TiO2) and P4 (AC-MWCNTs@TiO2) composite electrodes in 1.0 M KOH electrolyte for determining onset potential and the overpotential at 10 mA cm−2; the in-sets show the respective Tafel plots. (Bottom) CPE curves depicting current density versus time for electrodes P1 through P4.

Fig. 7

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Table 5. Performance parameters evaluated for the electrocatalytic OER activity of the fabricated electrodes.
Fabricated electrode material Onset potential (V) Over potential (mV) at 10 mA cm−2 Tafel slope (mV dec−1)
P1 (AC) 1.51 318 99.46
P2 (AC@MWCNTs) 1.52 298 81.47
P3 (AC@TiO2) 1.53 238 74.86
P4 (AC-MWCNTs@TiO2) 1.50 198 64.28
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Among all tested samples, the P4 composite exhibited the lowest onset potential and corresponding overpotential at a current density of 10 mA cm−2. These values are highly comparable to state-of-the-art advanced materials reported for high-performance OER applications, which typically feature onset potentials and overpotentials in the ranges of 1.45–1.60 V and 170–400 mV, respectively.56–58 Particularly, the remarkably low overpotential of P4 (<200 mV) highlights its outstanding OER catalytic activity; making it an excellent candidate for advanced electrocatalysis. Furthermore, the efficient OER performance of the remaining fabricated electrodes (P1–P3) is well-demonstrated, as their respective onset and overpotential values fall well within these benchmark ranges.56

To further evaluate the OER reaction kinetics, Tafel slope analysis was conducted by using the LSV data, with the resulting plots presented as in-sets in Fig. 7 and S6 (in SI file). The Tafel slope is a critical kinetic parameter derived from the relationship between the applied overpotential and the logarithmic current density, serving as a vital criterion for assessing inherent electrocatalytic activity. Because a lower Tafel slope corresponds to faster charge-transfer kinetics, it serves as a key indicator of enhanced catalytic activity. For reference, efficient OER electrocatalysts in alkaline media typically exhibit Tafel slopes within the 60–120 mV dec−1 range.56 Tafel slopes for P3 and P4 were found to be comparatively lower than those for P1 and P2, implying significantly fast reaction kinetics and enhanced catalytic activity. Notably, the Tafel slope of the P4 composite is particularly remarkable, as it falls below the values of commercially available noble-metal benchmarks such as RuO2 (65 mV dec−1) and IrO2 (82 mV dec−1). This rapid reaction kinetics underscores the outstanding electrocatalytic activity of the P4 matrix toward the OER.24

The CPE measurements were conducted over 15 hours at an applied potential of 1.65 V versus the reversible hydrogen electrode (RHE), as shown in the bottom panel of Fig. 7. The CPE profiles demonstrate the robust electrocatalytic durability and operational stability of the fabricated electrodes under long-term conditions.24,31 Among the tested electrodes, the binary P2 electrode (composed of AC and MWCNTs) exhibited the highest initial current density, which decreases gradually from 104 to 90 mA cm−2, suggesting minor catalyst degradation or active-site blocking during prolonged operations. In contrast, the ternary P4 electrode (consisting of AC, MWCNTs, and TiO2) maintained a steady current density of 98 mA cm−2 throughout the 15 hours electrolysis process. This behavior demonstrates a superior degree of long-term stability, which is directly attributed to the symbiotic interactions between the highly conductive nanotubes and the structurally resilient TiO2 framework. The pristine activated carbon monolith (P1) exhibited a gradual decline in current density from 87 to 76 mA cm−2, indicating progressive electrochemical degradation coupled with its moderate electrical conductivity. In contrast, the P3 composite displayed a highly stable current signal within the range of 50–57 mA cm−2, signifying excellent structural integrity despite its lower overall catalytic performance. Collectively, these findings demonstrate that the ternary P4 composite possesses the most optimized balance between a high operational current density and long-term durability.

EIS analysis

Interfacial characteristics regarding charge transfer and ion transport were investigated using EIS. Fig. 8 and S7 (in SI file) display the Nyquist plots (ZREALvs. ZIMAG) for electrodes P3–P4 and P1–P2, respectively. The corresponding electrochemical parameters are listed in Table 6. In the high-frequency region of the Nyquist plot, a small semicircle diameter indicates low charge-transfer resistance (R2). Meanwhile, the slope greater than 45° in the low-frequency region signifies reduced ion diffusion resistance, highlighting the efficient charge-storage capability of the electrode materials (Fig. 8, in-sets).31 To verify the internal electrochemical processes, an equivalent circuit model was fitted to the experimental data (Fig. 8). The circuit configuration consists of ohmic resistance (R1), a constant phase element (CPE1), bulk capacitance (C1), charge-transfer resistance (R2), and a Warburg element (W1) representing low-frequency diffusion impedance. To account for the depressed semicircle arcs of the fabricated electrodes, the ideal double-layer (Cdl) capacitance was replaced with a CPE to optimize the fit.31,59,60 The parallel arrangement of R2 and CPE1 indicates a transition from ideal capacitive behavior to a pseudocapacitive nature.61

Fig. 8. (Top) Nyquist plots showing the EIS analysis of the P3 (AC@TiO2) and P4 (AC-MWCNTs@TiO2) composite electrodes in 1.0 M KOH electrolyte, in-sets presenting a magnified view of the high-frequency region. (Bottom) equivalent circuit model fitted to the experimental data.

Fig. 8

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Table 6. Electrochemical impedance parameters evaluated from Nyquist plots for the biomass-derived carbon hybrid and its composite electrodes in 1.0 M KOH electrolyte.
Fabricated electrode material Solution resistance R1 (ohm) Charge transfer resistance R2 (ohm)
P1 (AC) 1.82 3.00
P2 (AC@MWCNTs) 1.51 2.02
P3 (AC@TiO2) 1.30 2.00
P4 (AC-MWCNTs@TiO2) 1.20 1.70
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All fabricated electrode materials exhibited low solution resistance (R1), indicating excellent electrolyte conductivity and intimate electrode–electrolyte contact. Notably, R1 steadily decreased from sample P1 (1.82 ohm) to P4 (1.20 ohm), reflecting the progressive expansion of conductive pathways facilitated by optimized composite architecture. The P3 electrode (AC/TiO2 blend) demonstrated a low charge-transfer resistance (2.0 ohm), highlighting the efficient faradaic activity of the anatase TiO2 phase. Upon introducing MWCNTs to form the P4 electrode, a robust, highly conductive network was established to enable rapid electron transport. Consequently, P4 exhibited the lowest charge-transfer resistance (1.7 ohm) among all samples, demonstrating significantly accelerated interfacial charge-transfer kinetics. Furthermore, in the low-frequency region, the capacitive vertical line of P4 was steeper than that of P3, confirming its enhanced capacitive behaviour and reduced ion diffusion resistance.

Overall, the fitted impedance parameters R1 and R2 were in the range of 1.20–1.82 (ohm), and 1.7–3.0 (ohm), respectively, confirming the pseudocapacitive nature of all fabricated electrode materials. Typically, minimized R1 and R2 values support a high electrochemical potential. These EIS insights closely align with our other electrochemical characterizations, validating the suitability of the P4 electrode as a promising candidate for next-generation supercapacitors and OER electrocatalysis.

Performance comparison with recent benchmarks

To contextualize the performance of the P4 (AC-MWCNTs@TiO2) ternary composite, its key electrochemical metrics for supercapacitor and OER activities were benchmarked against recently reported ternary electrode materials (with and without biomass components) as summarized in Table 7. The P4 composite demonstrated outstanding competitive advantages over these literature benchmarks. Given its strategic balance of low production cost, environmental sustainability, and superior electrochemical properties, the engineered ternary composite P4 represents a highly promising electrode candidate for integrated next-generation supercapacitor energy storage and OER-driven water-splitting applications.

Performance comparison of the optimized P4 electrode with recently reported ternary electrode benchmarks for supercapacitor and OER applicationsa.
Supercapacitor
Electrode material C s CV/GCD (F g−1) E d/Pd (Wh kg−1)/(W kg−1) Retention (%)/cycles R 1 (ohm) R 2 (ohm) Ref.
P4 (AC-MWCNTs@TiO2) 651@2 mV s−1/312.5@0.8 A g−1 12.13/260@0.8 A g−1 94/1000 1.20 1.70 Current work
Ternary titanium dioxide/polypyrrole/phosphorene 502.6@10 mV s−1/481.3@1 A g−1 153.92@1 A g−1/— 53/1500 7.0 — 62
graphene/TiO2/polypyrrole 431.2@10 mV s−1/14.25@3.03 A g−1 8.94/3030@3.03 A g−1 ∼100/1000 0.762 4585 63
PPy-PT/TiO2 —/271.8@1 A g−1 — ∼100/1000 — — 64
Co-doped NiFe2O4/rGO/MWCNT —/295.8@1 A g−1 0.495/3447@5 A g−1 85.5/5000 — — 65
Ternary MnO2/Fe3O4@ AC 87.6@10 mV s−1/322.5@1 A g−1 18.3/33.4@1 A g−1 81.83/700 — — 66
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a

— = Value not reported.

OER
Electrode material Onset-potential (V) Overpotential (mV) in 1 M KOH Tafel slope (mV dec−1) CPE-current density retention (mA cm−2 h−1) Ref.
P4 (AC-MWCNTs@TiO2) 1.50 198@10 mA cm−2 64.28 98/16 Current work
CuS/Co3O4@g-C3N4 — 191@10 mA cm−2 213 —/60 67
Ternary Cu–Co–Ni — 411@50 mA cm−2 84 —/30 68
Ni80Fe8P12/Ni75Fe12P12 — 412/432@10 mA cm−2 96/91 79/10 69
NiCoFe/NF 1.45 220@10 mA cm−2 50 100/10 70
Heteroatom doped NiFe-LDH using biomass carbon source (NiFe@NCS-600) — 318.6@10 mA cm−2 128.6 — 71
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Conclusions

This study demonstrates the successful synthesis of a sustainable biomass-derived activated carbon hybrid P1 (AC) from mango leaves, moringa leaves, and spent tea waste via hydrothermal route and formulated into its binary and ternary composites with MWCNTs and TiO2, namely P2 (AC@MWCNTs), P3 (AC@TiO2), and P4 (AC-MWCNTs@TiO2). Integrating these natural and synthetic components aimed to engineer cost-effective, non-toxic, eco-friendly, and renewable electrode materials for energy conversion and storage. Physical characterization via XRD, SEM, and FTIR confirmed the successful synthesis and structural integrity of the materials. Detailed electrochemical evaluations using CV, LSV, GCD, and EIS validated their significant performance in both supercapacitors and OER water-splitting catalysis. Among all fabricated samples, the P4 ternary composite exhibited the most outstanding electrochemical performance, demonstrating superior specific capacitance, prolonged discharge time, enhanced energy density, excellent capacitance retention, and robust long-term stability. This remarkable performance is supported by its minimized overpotential, Tafel slope, solution resistance, and charge-transfer resistance. The overall electrochemical performance enhancement followed the trend of P4 > P3 > P2 > P1. Consequently, these findings validate the engineered P4 ternary composite as a promising, high-performance, and sustainable dual-functional electrode candidate for future energy storage and electrocatalytic applications.

Author contributions

Nasima Arshad: conceptualization, project administration, supervision, methodology, formal analysis, visualization, validation, resources, writing – original draft, writing – review & editing; Muhammad Shahroz: investigation, software, writing – original draft, writing – review & editing; Maryam Rizwan: data curation, investigation, formal analysis, software, writing – original draft, writing – review & editing; Muhammad A. U. R. Qureshi: software, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

RA-OLF-D6RA08646B-s001

Acknowledgments

All possible research facilities of Chemistry department, Allama Iqbal Open University (AIOU), Islamabad Pakistan are highly acknowledged. We extend our gratitude to Office of Research Innovation & Commercialization (ORIC), Allama Iqbal Open University (AIOU), Islamabad, Pakistan for research funding that helped to cover the research expenses of current project.

Data availability

Data will be made available on request.

Supplementary information (SI): X-ray diffraction (XRD) patterns of the P1 (AC) and its P2 AC@MWCNTs composites appended with respective hkl values (Fig. S1); SEM micrographs and corresponding EDX spectra of the P1 (AC) and its P2 AC@MWCNTs composite (Fig. S2); FTIR spectrum of P1 (AC) and its P2 AC@MWCNTs composite (Fig. S3); overlay of CV curves for the P1 (AC) and P2 AC@MWCNTs electrode materials in 1.0 M KOH electrolyte at scan rates of 2, 5, 10, 20, 30, 50, and 100 mV s−1, (in-set: specific capacitance as a function of scan rate) (Fig. S4); (Top) overlay of GCD curves for the P1 (AC) and P2 (AC@MWCNTs) electrode materials in 1.0 M KOH electrolyte at current densities of 2 A g−1, 1.2 A g−1, and 0.8 A g−1 (Bottom) Ragone plots illustrating energy density as a function of power density for fabricated electrodes (Fig. S5); overlay representations of CV and LSV profiles of the P1 (AC) and P2 AC@MWCNTs electrode materials in 1.0 M KOH electrolyte for determining onset potential (V) vs. RHE and the overpotential at 10 mA cm−2; the in-sets show the respective Tafel plots (Fig. S6); Nyquist plots showing the EIS analysis of the P1 (AC) and P2 AC@MWCNTs electrode materials in 1.0 M KOH electrolyte. (Fig. S7); elemental composition of the biomass carbon hybrid and its corresponding MWCNTs and TiO2 based composites determined by EDX analysis (Table S1); performance parameters of the fabricated electrodes evaluated from GCD measurements at the optimized current densities (1.2 and 2 A g−1) (Table S2). See DOI: https://doi.org/10.1039/d6ra08646b.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-OLF-D6RA08646B-s001
RA-OLF-D6RA08646B-s001.pdf (1.2MB, pdf)

Data Availability Statement

Data will be made available on request.

Supplementary information (SI): X-ray diffraction (XRD) patterns of the P1 (AC) and its P2 AC@MWCNTs composites appended with respective hkl values (Fig. S1); SEM micrographs and corresponding EDX spectra of the P1 (AC) and its P2 AC@MWCNTs composite (Fig. S2); FTIR spectrum of P1 (AC) and its P2 AC@MWCNTs composite (Fig. S3); overlay of CV curves for the P1 (AC) and P2 AC@MWCNTs electrode materials in 1.0 M KOH electrolyte at scan rates of 2, 5, 10, 20, 30, 50, and 100 mV s−1, (in-set: specific capacitance as a function of scan rate) (Fig. S4); (Top) overlay of GCD curves for the P1 (AC) and P2 (AC@MWCNTs) electrode materials in 1.0 M KOH electrolyte at current densities of 2 A g−1, 1.2 A g−1, and 0.8 A g−1 (Bottom) Ragone plots illustrating energy density as a function of power density for fabricated electrodes (Fig. S5); overlay representations of CV and LSV profiles of the P1 (AC) and P2 AC@MWCNTs electrode materials in 1.0 M KOH electrolyte for determining onset potential (V) vs. RHE and the overpotential at 10 mA cm−2; the in-sets show the respective Tafel plots (Fig. S6); Nyquist plots showing the EIS analysis of the P1 (AC) and P2 AC@MWCNTs electrode materials in 1.0 M KOH electrolyte. (Fig. S7); elemental composition of the biomass carbon hybrid and its corresponding MWCNTs and TiO2 based composites determined by EDX analysis (Table S1); performance parameters of the fabricated electrodes evaluated from GCD measurements at the optimized current densities (1.2 and 2 A g−1) (Table S2). See DOI: https://doi.org/10.1039/d6ra08646b.

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