An anion-sorted Li-ion electrolyte and flexible MnVO@SWCNT hybrid electrode for an efficient supercapacitor system

Abstract

Portable and wearable electronics create a wide application opportunity for flexible energy storage materials. This study focuses on electrode and electrolyte modification to fabricate a highly flexible energy storage device with exceptional performance characteristics. Herein, a binary transition metal oxide (BTMO), leveraging the combined advantages of manganese (Mn) and vanadium (V) with their diverse oxidation states, was selected as the electrode material. To construct a flexible and efficient hybrid electrode (MnVO@SWCNT), the BTMO was integrated in the mesh of highly conductive and ductile single-walled carbon nanotubes (SWCNT). Consequently, the impact of electrolyte anions, such as OH⁻, SO₄²⁻, Cl⁻, and NO₃⁻, on the electrochemical efficiency of a MnVO@SWCNT electrode was analyzed in detail to boost the supercapacitor performance. The selection of anion species strongly influenced the fundamental capacitance due to the dissimilarities in ionic mobility and size of anions. As a result, the hybrid electrode operating in an aqueous electrolyte containing Li⁺ and OH⁻ ions exhibited superior capacitive performance and attained a maximum areal capacitance of 1886 mF cm⁻². To demonstrate the high application potential of such a material, a free-standing binder-free hybrid electrode devoid of an extra counter electrode was used to synthesize a flexible planar supercapacitor, MVO//OH@SWCNT (FpSC), which offered 718 mF cm⁻² capacitance and 105.79 µWh cm⁻² energy density.

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1. Introduction

The electronics industry is gradually shifting to sustainable power sources, creating a need for high-performance, lightweight, and eco-friendly energy storage devices like supercapacitors and batteries. These devices can power various electronics, from flexible portable devices to electric vehicles. However, their development poses technological challenges, such as the design of electrodes and electrolytes,^1–5 which should exhibit high and ideally time-invariant electrochemical properties. As an electrode material, transition metal oxides, such as vanadium, nickel, molybdenum, cobalt, and ruthenium, have garnered significant attention as active materials for electrochemical energy storage devices due to their high theoretical specific capacitance. Despite their outstanding promise, the experimental capacitive values of these systems still require optimization, which can be improved by adding dual distinct metals in the oxide matrix and developing binary transition metal oxides (BTMO). The BTMOs are unique in this respect due to the coexistence of multiple cations of distinct metals within a single crystal structure, which can enhance the capacitive performance and widen the potential window.^6,7 BTMOs provide an appreciable number of electrons, which can participate in electrochemical processes and have high porosity,⁷ leading to better electrical properties compared to metal oxides composed of a single element. For example, spinel oxides like NiCo₂O₄ have two to three orders of magnitude higher conductivity than NiO and Co₃O₄.^6,8 These advances of BTMO prove to be beneficial for energy storage and offer pseudo-capacitive mechanisms to store charge.^9,10 At the same time, single-walled carbon nanotubes (SWCNTs) are flexible and conductive materials, which are employed for energy storage by electric double-layer charge (EDLC) storage.^7,11 Hence, they can be used as a framework to complement the BTMO material, providing the possibility of preparing free-standing sheet electrodes without any binders, which is crucial for making supercapacitor electrodes. Besides that, low-density SWCNTs can reduce the weight of the device by acting as a current collector and also providing high conductivity to the oxide electrode material for rapid ion and electron transportation.^12,13 This final electrode can utilize the intrinsic charge storage mechanism of BTMO as well as SWCNT, as a hybrid combination of pseudo-capacitive and EDLC mechanisms.¹⁴ Recent studies on hybrid architectures, including the SWCNT/ZnO nanocomposite, CoMn–N-doped CNT, and In₂O₃-SWCNT composites, have shown significantly improved electrochemical performance compared to pristine pseudocapacitive or EDLC materials.^15–17 Such a hybrid electrode, with the right electrolyte, has the potential to offer superior capacity for energy storage.^14,18 As per the existing research on electrolytes, an aqueous solution can be a perfect choice for developing energy storage devices, which provides the possibility of establishing high ionic conductivity and high concentrations of ions.^19,20 In particular, aqueous electrolytes formed by simple salt solutions (LiA, NaA, and KA where A = Cl⁻, NO₃⁻, OH⁻, and SO₄²⁻) reduce the need for extensive purification or handling under a controlled atmosphere, eliminating the requirement for a dry room or glovebox, which ultimately simplifies the fabrication process. Furthermore, using water-based electrolytes provides cost and safety benefits as an advantage over energy storage technologies that use non-aqueous electrolytes.²¹ After carefully considering all the limitations in current state-of-the-art supercapacitors, this research focuses on the development of stable, porous, and highly conductive hybrid sheet electrodes exploiting Mn and V-based BTMO integrated with the SWCNT framework (MnVO@SWCNT). MnVO combines the advantages of Mn and V oxides in a single particle. As a single-phase compound, Mn and V are uniformly distributed in the lattice, enabling synergistic redox activity. When integrated with SWCNTs, MnVO particles are evenly dispersed, ensuring consistent electrochemical behavior, unlike the non-uniform distribution of MnO₂ or V₂O₅ particles. The uneven distribution of metal oxide particles in the SWCNT matrix can lead to phase separation and uneven redox activity, reducing performance due to poor contact and mismatched kinetics.^22–25Fig. 1 shows the schematic of the facile process used to fabricate the MnVO@SWCNT hybrid electrode with a top-view SEM micrograph, revealing an interconnected network of SWCNTs with homogeneously distributed metal particles. Li-ions were selected as the electrolyte cation because of their diffusion enhancement mechanism, while the anion was chosen after detailed experimental investigations.^3,26 A thorough study was conducted on anion-sorted Li⁺-based aqueous electrolytes to optimize the performance of the fabricated electrode material and the flexible supercapacitor device. Considering the recorded performance, the chosen electrode material and electrolyte hold promise as a viable option for next-generation energy storage devices.

Fig. 1 Synthesis of the BTMO material and free-standing MnVO@SWCNT hybrid electrodes.

2. Experimental section

2.1 BTMO powder preparation

The BTMO material was prepared using a simple method involving dry grinding and a calcination process. First, a mixture of MnO₂ and V₂O₅ in a 2 : 1 ratio was prepared. These powders were procured from Sigma-Aldrich. After 2 hours of dry grinding, the resulting yellow powder was transferred to a furnace and calcined at 650 °C in an argon (Ar) gas atmosphere for 5 hours. A black powder was obtained and removed from the furnace after the calcination process. This black powder was then subjected to another 2-hour dry grinding process and transferred to the furnace to remove humidity in an argon gas atmosphere at 300 °C for 3 hours. After these procedures, a dark brown powder of BTMO (Mn₂V₂O₇) was obtained.

2.2 MnVO@SWCNT sheet electrode fabrication

To prepare a hybrid electrode, 75 mg of MnVO and 75 mg of SWCNTs were added to a beaker. Then, 15 cm³ of a mixture of solvents containing 2-propanol and toluene in equal parts was introduced and stirred overnight. Subsequently, an ultrasonic homogenizer (Hielscher UP200St) was used to homogenize the mixture with concurrent magnetic stirring to improve the SWCNT dispersion homogeneity. The mixture was processed twice for 20 minutes (with a delay of 5 minutes in between) at 100% amplitude with 30 W power while keeping it in an ice bath to prevent heating and the resulting potential degradation of the material. After homogenization, the produced black paste was transferred to a 3D-printed PP Mold (9 cm × 9 cm × 0.6 cm) containing a nomex substrate. The paste was spread in the mold uniformly and then left overnight under a fume hood to allow the solvent to evaporate. The as-obtained free-standing sheet was peeled off the Nomex substrate, transferred to a furnace, and heated for 2 hours in an Ar atmosphere at 300 °C to remove traces of solvents and water vapor present in the environment, which could have deposited on the surface of the material. As a result, a mechanically stable and flexible 9 cm × 9 cm sheet with a thickness of 0.048 mm, suitable for electrochemical applications, was produced. Fig. 1 shows the schematic of the process used to fabricate the MnVO@SWCNT hybrid electrode with a top-view SEM micrograph. It revealed an interconnected network of SWCNTs with homogeneously distributed metal particles.

2.3 Flexible supercapacitor device fabrication

To utilize this research work in practical application, a flexible planar symmetric supercapacitor, MVO//OH@SWCNT (FpSC), was assembled using an MnVO@SWCNT hybrid electrode as a cathode and an anode, separated by a GF/C glass microfiber soaked in anion-optimized 1 M LiOH aqueous electrolyte. The LiOH-soaked microfiber sheet was sandwiched between the hybrid electrodes to construct a 1 × 1 cm² FpSC device with a thickness of around ∼0.098 mm. Further, this device was pressed between external uniform forces to reduce any gap or gas bubbles between the electrode and electrolyte. This step creates better contact between the electrode surface and the aqueous electrolyte, ensuring easy movement of ions in the device from one electrode to another through the electrolyte medium.

3. Characterization methods

The electrode surface underwent crystallographic structure analysis by X-ray diffraction (XRD, Rigaku Smart Lab XRD). X-ray photoelectron spectroscopy (XPS) measurements were performed using PHI Versa Probe III electron spectrometers in an ultrahigh vacuum with a base pressure of ∼2 × 10⁻⁷ Pa. High-resolution measurements were obtained at 25 eV, while survey scans were carried out at a constant energy of 200 eV. The multipack software was used to examine the composition of the synthesized MnVO and MnVO@SWCNT. The specific surface area and pore characteristics were determined by N₂ adsorption–desorption isotherms using a Quantachrome Brunauer–Emmett–Teller (BET) analyzer. Samples were degassed at 200 °C under vacuum for 6 h before analysis. Raman spectra were acquired using a Renishaw inVia Raman microscope (Leica) equipped with a 532 nm excitation laser. Additionally, the surface morphologies and topographies of the prepared samples were examined by field emission scanning electron microscopy (FE-SEM) with a Carl Zeiss ultra plus system and a Scanning-Transmission Electron Microscope, S/TEM 80–300 kV (FEI), equipped with an Energy Dispersive X-ray (EDX) detector. An electrochemical investigation of the electrodes in various electrolytes with fabricated planar symmetric devices was conducted using an Autolab PGSTAT302N potentiostat/galvanostat electrochemical workstation.

4. Results and discussion

4.1 X-ray analysis

XPS was used in this study to get a deeper understanding of the chemical compositions of both BTMO (MnVO) and MnVO@SWCNT. Fig. 2(a) displays the XPS survey spectra of MnVO and MnVO@SWCNT, wherein the C 1s, Mn 2p, O 1s, V 3p, V 2p, OKLL, and VLMM features were observed. The C 1s peak intensity was notably higher for MnVO@SWCNT than MnVO, attributed to the presence of SWCNTs in the composite. SWCNTs, known for their larger surface area and efficient electron emission, result in a higher detection rate of carbon atoms in the XPS analysis compared to bare MnVO powder survey spectra. Fig. 2(b) shows the deconvoluted V 2p and O 1s spectra. The V 2p spectrum includes peaks at 516.31, 517.77, and 518.23 eV corresponding to V 2p_3/2, and a peak at 523.23 eV along with peaks at 525.43 and 527.35 eV associated with V 2p_1/2. These peaks indicate the presence of vanadium in the +3, +4, and +5 oxidation states. Multiple oxidation states (+3, +4, +5) found in XPS data highlight that vanadium (V) can undergo various redox processes. These multiple oxidations are promising for reaching better capacitive performance of the electrode. In the O 1s spectra, peaks at approximately 530.6 and 532.5 eV are attributed to metal–oxygen (V/Mn–O) and O–H bonds, respectively, which confirms the formation of metal oxide, along with the presence of some atmospheric humidity.^27,28

Fig. 2 X-ray study of the electrode material: (a) XPS survey spectra of electrodes and (b–d) deconvolution of elements associated with MnVO@SWCNT.

The Mn2p spectrum, shown in Fig. 2(c), also displays mixed valence states. Peaks at 641.0 eV and 653.0 eV correspond to the Mn 2p_3/2 and Mn 2p_1/2 of Mn²⁺, while peaks at 642.1 eV and 653.6 eV correspond to the Mn 2p_3/2 and Mn 2p_1/2 of Mn³⁺. Another pair of peaks at 643.6 eV and 654.6 eV is associated with Mn⁴⁺. These peaks confirm the presence of Mn²⁺, Mn³⁺, and Mn⁴⁺. Additionally, the presence of a 2⁺ satellite peak indicates multiplet splitting caused by the spin–orbit coupling of the electrons at the Mn 2p core level, which can provide further insights into the electronic structure of the manganese atoms in the material. The mixed valence states of manganese (Mn²⁺, Mn³⁺, Mn⁴⁺) again suggest the material's capability to engage in multiple redox reactions, enhancing its pseudocapacitive properties, which would manifest as broad redox peaks in CV due to the overlapping contributions from the different manganese oxidation states.^29,30Fig. 2(d) presents deconvoluted spectra of C 1s, highlighting the carbon bonding and metal oxide interactions. The calibration of the C 1s peak for the reference SWCNT was set at 284.6 eV relative to the sp² carbon phase. The deconvolution of the C 1s peak for these samples was challenging due to the presence of the p–p* shake-up feature. The deconvoluted C 1s peaks were observed at 284.5, 285.9, 286.6, 288.5, and 291.4 eV, corresponding to C=C, C–C/C–H, C–O/C–O–C, C=O, and shake-up satellite (π → π*) process, respectively. The C=C and C–C/C–H indicate the presence of sp² and sp³ carbon atoms, respectively. The peaks attributed to C–O/C–O–C and C=O suggest the existence of covalently bonded oxygen atoms with carbon, which may come from the used isopropanol or a combination of metal oxide with SWCNTs.^31,32 The XPS analysis confirms the successful fabrication of MnVO and MnVO@SWCNT, with all relevant peaks present in the material. Further, XRD spectra of the hybrid electrode with MnVO materials exhibit relevant diffractogram peaks associated with the monoclinic crystal system of Mn₂V₂O_7, with XRD peaks representing SWCNT presence, as shown in the inset of Fig. S1(a).

4.2 Surface analysis

Fig. 3(a and b) displays the surface morphology of the fabricated sheet electrode, including a close-up view of the MnVO nanoparticles. The MnVO powder grew as cube-like nanoparticles composed of combined oxide elements of Mn and V, as revealed in the surface micrograph of the powder in the inset of Fig. 3(a). The as-prepared sheet of MnVO@SWCNT exhibits well-connected tubes with attached nanoparticles of MnVO at the wall of the tube or tightly squeezed nanoparticles between SWCNTs to form a hybrid electrode. A clear and interconnected nanostructure exhibiting a certain degree of porosity is advantageous in electrochemical applications because it supports effective ion diffusion and promotes efficient charge transfer during electrochemical reactions. Also, this self and tightly interlinked structure provides a free-standing electrode without a current collector or binder. In addition, EDS analysis was conducted to get a brief understanding of the elements present in the sheet and their distribution with interlinked SWCNT.

Fig. 3 Surface morphology of MnVO@SWCNT: (a) SEM top view image (with a SEM micrograph of MnVO powder in the inset), (b) surface micrograph at high magnification, and (c and d) EDS analysis with elemental spectra at different spots and magnifications, (e and f) color mapping overlay micrographs.

Fig. 3(c and d) presents an EDS analysis, where spectra were obtained from localized points (spots 1 and 2) at different resolutions, detailing the elemental composition of the MnVO@SWCNT hybrid electrode material. These spectra indicate the presence of Mn, V, O, and C elements, which are in accordance with the above-discussed XPS findings. The low oxygen concentration detected in EDS can be impacted by the inherent limitations of the EDS method to quantify light elements.^33–35 The EDS spectra display MnVO elements even for a selected tiny spot on the wall of the SWCNT for analysis (spot 2), in Fig. 3(d), revealing the presence of MnVO throughout the SWCNT mesh. The presence of Mn and V suggests their involvement in the electrochemical redox processes during charge and discharge, while O and C contribute to the material's structural integrity and conductivity. Color mapping overlay displayed both with and without carbon elements. Carbon is predominantly concentrated (derived from SWCNTs) and covers all colors representing various elements (illustrated in Fig. 3(e)), so an overlay without carbon is also present in Fig. 3(f). This validation confirms the uniform distribution of elements on the material surface, ensuring consistent electrochemical performance throughout the electrode surface. In addition, the structure of the MnVO powder was analyzed using TEM, as depicted in Fig. S2(a and d). The TEM analysis was consistent with the XRD and SEM results and confirmed the growth of the monoclinic system MnVO with a cube-like nanostructure.

4.3 BET and Raman study of the electrodes

The specific surface area and pore volume characteristics of the synthesized SWCNT and MnVO@SWCNT sheet electrodes were analyzed using Brunauer–Emmett–Teller (BET) surface area measurements. Fig. 4(a and b) depicts the N₂ adsorption–desorption isotherms of the SWCNT and MnVO@SWCNT samples, respectively, both exhibiting type IV isotherms with mesoporous features. However, MnVO@SWCNT reveals a noticeably higher nitrogen uptake at high relative pressure (P/P₀ ≈ 1.0), indicating a significantly larger total pore volume and a hierarchical meso/macroporous structure, which can be attributed to the integrated particle of MnVO in the SWCNT mesh. SWCNT, with a BET surface area of 131 m² g⁻¹ and an average pore diameter of 3.41 nm, has narrower mesopores that limit electrolyte penetration and restrict charge storage primarily to EDLC, which is also reflected in the CV curve shapes. In contrast, MnVO@SWCNT shows a slightly higher surface area (138.9 m² g⁻¹) and a similar pore diameter (3.06 nm), but with much greater pore accessibility. This improves LiOH infiltration and accelerates ion transport. However, the key enhancement in the electrochemical performance of the hybrid electrode will be due to the presence of multivalent Mn²⁺/Mn³⁺/Mn⁴⁺ and V³⁺/V⁴⁺/V⁵⁺ species. MnVO@SWCNT thus develops numerous surface redox centers that enable rapid pseudocapacitive reactions, working together with the conductive SWCNT network to achieve extremely high capacitance.^36–40

Fig. 4 Nitrogen adsorption–desorption isotherms of (a) SWCNT and (b) MnVO@SWCNT, with the corresponding pore volume distributions shown in the inset. Raman spectra of (c) SWCNT and (d) MnVO@SWCNT.

Raman spectra of the pristine SWCNT and MnVO@SWCNT sheet electrodes are shown in Fig. 4(c and d), displaying the carbon bands along with new features from the oxide. In both spectra, the SWCNT D-band (∼1350 cm⁻¹) and G-band (∼1590 cm⁻¹), as well as a G′ overtone (∼2670 cm⁻¹) are present. In the SWCNT sample, the D-band is very weak (I_D/I_G ≈ 0.012), while in MnVO@SWCNT, the I_D/I_G ratio increases (∼0.036), indicating a small amount of defect sites created by MnVO decoration.^41,42 Furthermore, in the Raman spectra of MnVO@SWCNT (Fig. 4(d)), the strong band at ∼868 cm⁻¹ corresponds to O–Mn–O–V–O stretching vibrations in manganese vanadate oxide, and broad modes around 430–470 cm⁻¹ confirm the presence of vanadate vibrations. These oxide-related bands and the lack of significant shifts in the SWCNT G-band confirm the successful integration of BTMO in the mesh of SWCNTs.^40,43

4.4 Electrode ion kinematics in various electrolytes

As a hybrid electrode possessing both EDLC and pseudo-capacitance charge–discharge nature, it was important to select an appropriate electrolyte capable of minimizing the internal electrical resistance to support both behaviors to achieve a high-power performing supercapacitor. Here, a systematic study was done to adjust the electrolyte characteristics by choosing the optimal type of anion. To understand the effect of anions on the electrochemical properties of hybrid electrodes, a series of electrochemical studies was performed as depicted in Fig. 5. The cyclic voltammetry (CV) experiments were performed in various electrolytes having a range of anions (Cl⁻, NO₃⁻, OH⁻, and SO₄²⁻) and Li⁺ as the counter-ion in all the cases considered, as shown in Fig. 5(a). The hybrid electrode displayed superior capacitive performance with LiOH compared to other electrolytes, taking into account the wide integrated area of the CV curve for LiOH. The electrodes worked at different voltage window ranges depending on the anion-selected electrolyte. In the case of LiOH, the electrode exhibited a 0.8 V working voltage. These voltage windows of the electrode with electrolytes were optimized after considering and balancing the current values of the cathode and anode (I_a and I_c) with the same magnitude. A CV study in LiOH of a bare SWCNT sheet was also done to see the influence of BTMO particles in hybrid electrodes, as presented in Fig. S3(a and b), which revealed that including BTMO particles in SWCNT increased the capacitive performance by five times. The higher capacitance can be attributed to multiple oxidation states and increased active sites provided by BTMO particles for chemical reactions to store charge.⁴⁴ Further, a CV kinematic study of bare SWCNT with a hybrid electrode was performed using eqn S(1–4) and presented in Fig. S4(a and b). CV kinematics revealed that hybrid electrodes had good capacitive stability and stored maximum charge by surface phenomena, possibly because of the high surface area and pseudo-capacitive behavior acquired from inserted BTMO particles. Fig. S5(a–d) shows galvanostatic charging–discharging (GCD) curves at various current densities with anion-sorted electrolytes. The capacitance values as a function of current density are plotted and shown in Fig. 5(b), calculated from the GCD curves using eqn (S5). The hybrid electrode in LiOH electrolyte offered a high areal capacitance of about 1886 mF cm⁻² at the current density of 1 mF cm⁻², which was 5–6 times higher compared to electrolytes with Cl⁻, NO₃⁻, and SO₄²⁻ anions. To better understand the anion effect, ion transport properties and kinematics were evaluated using impedance spectroscopy (EIS). The Nyquist plots of the electrode with anion-selected electrodes are shown in Fig. 5(c), with data fitted to an electronic circuit model. All the Nyquist plots had a linear pattern developing in the lower-frequency region, which confirmed that the hybrid electrode exhibited ideal capacitive behavior for all the Li-based electrolytes. Further, the effect of anions on the ion intercalation of the Li⁺ cation was observed by the chemical diffusion coefficient (D_A+) and Warburg impedance coefficient (σ_w), calculated from the frequency response data and using eqn (1)–(3) (with other constants), as shown in Fig. 5(d).^45,46

Z′ = R_s + R_ct + σ_Wω^−1/2 (1)

Eqn (1) was plotted as a straight line in the Z′ versus ω^−1/2 graph, and the slope of the fitted straight line gave the value of σ_w. The calculated values of σ_w of Li⁺ for the different anions Cl⁻, NO₃⁻, OH⁻, and SO₄²⁻ were found to be 2.35, 1.59, 0.90, and 1.34, by fitting a straight line. Further, D_A+ was obtained using the values of σ_w in eqn (2) and (3). The D_A+ of Li⁺ exhibited a higher value with OH⁻ anions (β* 1.21 cm² s⁻¹) than Cl⁻ (β* 0.179 cm² s⁻¹), NO₃⁻ (β* 0.393 cm² s⁻¹), and SO₄²⁻ (β* 0.556 10⁻⁵ cm² s⁻¹). The higher value of diffusion-coefficient for the OH⁻ anion revealed that the MnVO@SWCNT hybrid electrode with the Li⁺ cation and OH⁻¹ anion had lower ion diffusion resistance and shorter diffusion distance compared to Cl⁻, NO₃⁻, and SO₄²⁻ anions. The ion interaction with the surface of the electrode became more prominent as the ion diffusion coefficient increased, implying higher capacitance. The estimated parameters from the impedance study are depicted in Fig. 5(e). The hybrid electrode showed the lowest resistance value and the highest chemical diffusion coefficient for OH⁻ anions compared to electrolytes with Cl⁻, NO₃⁻, and SO₄²⁻ Anions. This suggests efficient charge transport and electrochemical kinetics of the hybrid electrode in a 1 M LiOH environment, as indicated by the shifted intersection at real impedance in the Nyquist plot. The better electrochemical performance of the MnVO@SWCNT electrode, confirmed by CV and impedance analysis (LiOH > LiNO₃ > LiCl > Li₂SO₄), can be attributed to the differences in the ion diffusion coefficients and anion ionic size/mobility. The ion hydrated diameters (ionic conductivity) of Cl⁻, NO₃⁻, OH⁻, and SO₄²⁻ ions are 3.32 Å (76.31 S cm² mol⁻¹), 3.16 Å (72 S cm² mol⁻¹), 3.00 Å (198 S cm² mol⁻¹), and 3.79 Å (79.8 S cm² mol⁻¹), respectively.^47,48 The small ionic size prompted the fast intercalation/deintercalation reactions and high ionic conductivity led to rapid transportation of ions, which resulted in better capacitive performance for the LiOH electrolyte.

Fig. 5 Electrochemical study of the prepared MnVO@SWCNT hybrid electrode in Li-based electrolyte and elucidation of the effect of anions, (a) CV curves at a scan rate of 100 mV s⁻¹, (b) calculated areal capacitance values by using GCD curves, (c) Nyquist plot, (d) Warburg coefficient and diffusion coefficient calculation, and (e) calculated parameters from EIS.

Further, the hybrid electrode subjected to electrochemical tests was analyzed by XPS to elucidate the effect of the reaction occurring at the electrode surface with an anion-selected electrolyte. The deconvoluted high-resolution spectra of the elements are depicted in Fig. 6(a–c). The spectra of V and O, in Fig. 6(a), exhibited a visible variation confirming the major effect of anion changes. These changes may be linked to the expansion/compression of planes during the charge/discharge process, due to the intercalation/deintercalation of ions into/from the V-oxide lattice.⁴⁹ In Fig. 6(b), the binding energy difference between Mn 2p_3/2 and Mn 2p_1/2 slightly decreased to around 11.11 eV after electrochemical testing in LiOH (11.74 eV, 11.5 eV, and 11.78 eV for Li₂SO₄, LiNO₃, and LiCl, respectively) compared to the pristine sample (ΔBE = 12.17 eV). This is likely due to chemical reactions during charging and discharging. Compared to other anions, OH⁻ exhibits the largest reduction in ΔBE, indicating higher electrochemical activity, which correlates with its superior performance in the LiOH electrolyte.^15,50 Concomitantly, the spectra of C, as depicted in Fig. 6(c), had almost identical shapes, indicating no effect of the anion. This behavior can be attributed to the porous architecture of the MnVO-based electrode, which accommodates volume changes during ion intercalation/deintercalation. This helps to mitigate stress and preserve the integrity of the monoclinic crystal structure, reducing the risk of distortion or degradation over cycling.⁴⁹

Fig. 6 XPS peak deconvolution of post-electrochemical tested electrode elements, in 1 M aqueous solution of LiCl, LiNO₃, Li₂SO₄, and LiOH electrolyte. Deconvoluted spectra and deconvoluted peak area ratios (a and d) of vanadium and oxygen, (b and e) manganese, and (c and f) carbon.

Further, SEM surface micrographs of the electrochemically treated electrodes still displayed a spaghetti-type structure, as shown in Fig. S6(a–d). The structural integrity of the SWCNT confirmed that the SWCNT did not exhibit any discernible chemical changes during the process⁷ since the spectra of carbon presented in Fig. 6(c) appeared analogous. The deconvoluted peak area ratios of the elements subjected to the electrochemical study of the anions with elements of the pristine electrode are depicted in Fig. 6(d–f). The vanadium peak ratio reduced considerably for OH⁻ compared to other anion-based electrolytes, as analyzed in Fig. 6(d). This indicated that the split doublet state of vanadium took the lead in element conversion during charging–discharging or redox reactions with the anion. The M–O peak had a maximum area ratio for OH⁻ (67.7%). At the same time, pristine (37.63%), Cl⁻ (28.64%), NO₃⁻ (38.23%), and SO₄²⁻ (36.92%) had lower peak ratios, indicating the excessive presence of OH⁻ anions in the electrolyte, with fast kinematics in OH based solvent (H₂O) rapidly reacting and producing more conversion element from the metal of M–O. Possible electrochemical reactions may occur during charging–discharging and could lead to different products and outcomes, which is also consistent with the previous explanation of the electrochemical process for hybrid electrodes.^49,51

Mn₂V₂O₇ + 10OH⁻ ↔ 2VO₄ + 2MnO₂ + 5H₂O + 10e⁻ (4)

MnO₂ + Li⁺ + e⁻ ↔ MnOOLi (5)

In Fig. 6(e), the deconvoluted peak area ratios associated with the Mn element are consistent for every case. This can be attributed to the stability of MnO_x during the charging–discharging reaction, leading to the production of MnO_x as the elemental conversion product of electrochemical reactions. Carbon peaks come from the SWCNT, taking part in the charging–discharging by forming EDLC layers and providing easy facilitation of electrons and electrolyte ions. As shown in Fig. 6(f), there is an increase in the deconvoluted peak area of C–C/C–H after the electrochemical process, which can be attributed to the presence of hydroxide in an aqueous or humid environment. The OH⁻ hydration sphere had a narrower radius compared to the Cl⁻, NO₃⁻, and SO₄²⁻ hydration spheres, increasing the number of ion accessing pores and the formation of a lower electric double layer, evidenced by the deconvoluted spectra of vanadium. Also, interactions with OH⁻ ions can lead to the formation of conductive surface layers (for example, Mn–OH or V–OH). These layers improve surface conductivity, decrease overall resistance, and enhance charge transfer. Additionally, relative to other anions, OH⁻ ions exhibited better conductivity and ionic mobility, which also helped in improving electrochemical reactions to get better capacitive performance.⁵² Considering appropriate porosity of the flexible MnVO@SWCNT hybrid electrode (Fig. 4(b)) and suitable mechanical properties making the material flexible (vide infra), its combination with the anion-sorted Li-ion electrolyte can be regarded as synergistic.

4.5 Capacitive performance of the fabricated FpSC

Further, to utilize the findings from the above analysis and combine the hybrid electrode with the optimized electrolyte, a flexible supercapacitor MVO//OH@SWCNT-FpSC was fabricated for the practical application, as shown in Fig. 7(a). The fabricated FpSC worked with a voltage of 1.0 V and had an ideal bending performance confirmed by CV curves shown in Fig. 7(b). The CV curves obtained in the flat and bent states on a circular surface were symmetrical in shape, indicating that the fabricated device could be a good option for energy storage in flexible electronics. CV curves taken after unbending the device exhibit a similar shape to the flat device, confirming the good flexible stability of the device. Fig. 7(c) depicts GCD curves at various current densities. These curves had symmetric triangular shapes, confirming that the device offered good rate capability. The areal capacitance (C_a) and coulombic efficiency (η%) of the device were calculated from the GCD curves after considering IR drop and presented in Fig. 7(d). The device attained a high areal capacitance of about 718 mF cm⁻² and around 90% coulombic efficiency at 1 mA cm⁻² current density.

Fig. 7 (a) Graphical illustration of the constructed MVO//OH@SWCNT FpSC device. (b) CV curves for flexibility, (c) GCD curves at various current densities, (d) capacitive performance calculated by GCD, (e) cycling stability for 5000 GCD cycles in the bent state, (f) Nyquist plot with the Ragone plot in the inset, and 3 cell assembly. (g) CV at 100 mV s⁻¹ and (h) GCD at 5 mA cm⁻².

Regarding stability, this device was tested for several GCD cycles at a current density of 15 mA cm⁻², as depicted in Fig. 7(e), and the device retained 74.8% capacitance even after 5000 cycles, indicating good stability and a long lifespan. The iR drop in Fig. 7(f) of the GCD curves may be due to the high current density of 15 mA cm⁻², where the slow irreversible faradic reaction could not match the fast charge–discharge process.⁵³Fig. 7(g) shows the ion kinematics of the device in the flat and bent states with an electrochemically tested device. The Nyquist plot of the device in all cases tended to a straight line in low-frequency regions, confirming the device's good capacitive behavior. The electrochemical series resistance, obtained by fitting data with an electronic circuit and intersection on the real axis, had very low resistance values, confirming that this device with LiOH electrolyte transports ions rapidly and offers a good capacitive response. The device's energy and power density were also calculated using eqn (S6) and (S7), which are present in the inset of Fig. 7(h), as the Ragone plot. This device offered a good energy density of ∼105.79 µWh cm⁻² with optimum power density (∼509 µW cm⁻²) at 1 mA cm⁻² current density. Further, a three-device assembly was also fabricated to achieve a wide voltage window, as shown in Fig. 7(g). The assembly of the device enabled a wide 3 V operating voltage, confirmed by CV and GCD (Fig. 7(h)). The device displayed higher electrochemical stability and performance compared to previous reports. For comparison, the capacitive parameters of several recent works are tabulated in Table 1. The assembly of the three devices and a single device with outstanding performance is a promising potential choice for high-voltage and flexible electronics. The improved performance can be credited to the exceptional electrical conductivity of SWCNT and the increased surface area with various oxidation states of MnVO in the hybrid electrode. Additionally, the chosen electrolyte with highly mobile anions (OH⁻) substantially enhanced electron transport efficiency and improved the electrochemical rate capability.

Table 1 Comparison of previously reported supercapacitor performance with this work^a

No.	Fabricated device	Max. attained areal capacitance (mF cm^−2)	Energy density (µWh cm^−2)	Power density (µW cm^−2)	Cyclability (number of cycles)	Reference
a CF-Carbon Fiber, ASC-asymmetric supercapacitor, PMMPA/PMPA-polymerized micropillar array, CNT-carbon nanotube, LDH-Layered Double Hydroxide, PEDOT-conductive poly 3,4-ethylenedioxythiophene, GF-Graphene Flakes, CC-Carbon Cloth.
1	Ni wire/Co3O4@MnO2//CF/graphene ASC	13.9	4.34	750	82% (1000)	54
2	RuO2//Ti3C2Tx	60	45	6000	86% (2000)	55
3	PMMPA//PMPA	66.9	18.22	420	88.3% (10 000)	56
4	VN//Na-MnOx	109.5	87.62	12 110	—	57
5	MnO2/CNT/nylon fiber	40.9	2.6	66.5	—	58
6	Ti3C2//Co–Al-LDH	28.5	8.84	230	92% (1000)	59
7	MnO2/V2O5/PEDOT@GF	89.3	24	350	82% (2500)	60
8	α-MnO2//γ-MnO2@CNT	—	93.8	193	98% (1000)	61
9	Ag @Ni–Co LDHs/CC//ASC	230	78.33	785	80.47% (2000)	62
10	ZnCo2O4–Ni foams/solid electrolyte/ZnCo2O4–Ni	94	—	—	98% (1000)	63
11	Graphene/amorphous carbon/Mn3O4 ASC	285.5	120	1800	88% (10 000)	64
12	Lignin-based carbon fibre/PANI	34.3	3.051	44.18	—	65
13	Carbon fibre multilayer@MnO2	218	18	391	—	66
14	rGO CNT-c film supercapacitor	233	10.4	1113	90% (200)	67
15	MVO//OH@SWCNT (FpSC)	718 (149.6 F cm^− ^3 )	105.79	525	74.6% (5000)	This work

---

5. Conclusion

In conclusion, this study demonstrates that a high surface area is not the sole crucial aspect for achieving high capacitance values. Instead, specific capacitance depends on a range of factors, including redox stability, conductivity, connected framework, and the selection of electrolytes, particularly the conductive salt of the anion and cation. These factors significantly influence the power performance of any energy storage system. To address these factors, this research used a free-standing, binder-free, and cost-effective hybrid electrode (MnVO@SWCNT) fabricated by a facile process. To achieve high-power performance, an electrolyte with an optimized combination of the cation (Li⁺) and anion (OH⁻) was selected by a thorough study for the hybrid electrode. The hybrid electrode with LiOH electrolyte delivers a high area capacitance of about 1886 mF cm⁻². Further, a flexible supercapacitor, MVO//OH@SWCNT (FpSC), was constructed with the hybrid electrode and anion-sorted electrolyte. The FpSC offers good bending performance and works on 1 V with a maximum attained areal capacitance of about 718 mF cm⁻². This device showed good stability, retained 74.8% capacitance even after 5000 GCD cycles, and displayed an optimum areal energy density of 105.79 µWh cm⁻² with a power density of 811 µWh cm⁻².

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available within the article and its supplementary information (SI). Supplementary information: analysis and equations used to calculate the capacitive parameters of hybrid electrodes and MVO//OH@SWCNT FpSC, along with supporting electrochemical and physical analysis. See DOI: https://doi.org/10.1039/d5ta06247k.

Acknowledgements

The authors would like to thank the Polish National Science Center (UMO-2020/39/D/ST5/00285) and National Agency for Academic Exchange (BPI/PST/2021/1/00039) for funding the research.

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