A band-aid-based MoSe₂/Nb₂C wearable supercapacitor for integrated ammonium-ion energy storage and real-time pressure monitoring

Abstract

The market for wearable electronic devices is rapidly growing, with increasing potential for future development. Researchers around the globe are striving to enhance these devices, focusing on achieving a balance between functionality and wearability to drive commercialization. Supercapacitors are regarded as one of the most promising energy storage technologies, bridging the gap between conventional batteries and dielectric capacitors to support high-power applications. This study presents the hybrid supercapacitor application of molybdenum sulfide, molybdenum selenide, and niobium carbide on band-aid-based wearable electrodes tested in three different electrolytes (sodium sulfate, ammonium sulfate, and zinc sulfate). Among the three electrolytes, the ammonium sulfate electrolyte exhibited exceptional electrochemical performance, including high specific capacitance, excellent power density, and remarkable cycling stability. Band-aid-based wearable ammonium ion hybrid supercapacitors were designed to explore their real-time applicability, utilizing activated carbon as the negative electrode and molybdenum selenide/niobium carbide as the positive electrode. The fabricated ammonium ion hybrid supercapacitor offers a maximum specific capacitance of 120 F g⁻¹ with 92% capacitance retention after 20 000 cycles. Also, it exhibits an outstanding energy density and power density. This suggested multipurpose integrated system opens new possibilities for creating flexible and adaptive wearable electronics. A glucose sensor is shown to be powered by the fabricated ammonium ion hybrid supercapacitor. The molybdenum selenide/niobium carbide-coated band-aid is also sandwiched between tiny layers of copper foil to create a pressure sensor. It is powered by an ammonium ion hybrid supercapacitor, which enables precise and steady real-time monitoring of the radial pulse pressure on the wrist of a person. This work creates new opportunities to explore the potential of wearable technology and nanomaterials to develop self-sufficient, cost-effective healthcare systems for monitoring health parameters in real-time.

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1 Introduction

The severe environmental degradation brought on by greenhouse gas emissions from burning fossil fuels necessitates the advancement of energy storage and conversion technologies. Because of their portability and great energy density, batteries—particularly lithium-ion batteries—can meet the demands of the modern world.^1,2 However, the current battery technologies' limited power density restricts their wider range of applications. In contrast, supercapacitors are a possible substitute for traditional batteries since they provide a far higher power density. Electric double-layer capacitors (EDLCs) and pseudocapacitors are two varieties of supercapacitors that differ in how they store charge. EDLCs are often based on carbonaceous substances that store charge at the interface between the electrode and electrolyte through electrostatic.³ Meanwhile, conducting polymers and transition metal oxides are examples of pseudocapacitive electrode materials.⁴ The reversible faradaic redox mechanisms that underlie pseudocapacitors generally offer higher specific capacitance and energy density than EDLCs.^5,6 However, it is also important to mention that batteries currently offer a higher energy density than supercapacitors. Therefore, creating new electrode materials for supercapacitors is essential to boosting their energy density.

Active materials in wearable technology are crucial for achieving sensing, along with energy storage devices. These devices should continue to perform as intended even when subjected to various human motions. However, energy storage devices and sensors operate in entirely different ways, so it is typically necessary to use separate device architectures made of other materials using various fabrication techniques.⁷ This makes it challenging, time-consuming, and limits manufacturing scalability because sensors and energy storage devices are frequently made separately before being assembled on the same substrate. Thus, it is crucial and urgently necessary to develop a straightforward and scalable process for producing active materials that may be utilized for energy storage and sensing in wearable electronics.^8,9 Paper,¹⁰ textiles,¹¹ band-aids,¹² and other flexible materials have emerged as the most viable recyclable substrates for energy applications in recent years. For instance, supercapacitors,¹³ solar cells,¹⁴ and micro-supercapacitors¹⁵ have all made extensive use of certain band-aid-based components. Because they are reasonably priced, highly breathable, and easy to wear, band-aids made of textile fabric and a cotton pad can be used as a substrate for wearable technology.¹² Meanwhile, scientists have created several extremely effective materials for energy storage devices. Layered transition metal dichalcogenides are currently receiving much attention in energy-related applications because of their low cost, strong catalytic activity, and great chemical stability.¹⁶ Because two-dimensional (2D) materials have large surface areas and sheet-like morphology that enhance the exposure of active regions, they are being thoroughly studied for use in supercapacitor electrodes.¹⁷ For high-performance supercapacitor electrodes, the 2D transition metal carbides and nitrides (M_n+1X_nT_x), also known as MXenes, have been extensively studied because of their remarkable electrical conductivity, abundance of active sites, and fascinating lamellar structures.¹⁸ 2D Nb₂C MXenes display the best performance and very good cycle performance. Unfortunately, their relatively low specific capacitance still limits the actual applications of MXene-based electrode materials. To overcome the drawbacks of Nb₂C MXenes, compositing the material with pseudocapacitive materials is a crucial tactic frequently employed to increase the capacitance of Nb₂C MXene-based supercapacitors.¹⁹ In this study, multilayer Nb₂C MXenes were employed without further delamination into single/few-layer nanosheets.²⁰

Numerous studies have been conducted on pseudocapacitive materials, such as transition metal selenides, oxides, and sulfides, owing to their remarkable electrochemical performance and theoretical capacitance. An MX₂ structure is produced in 2D transition metal dichalcogenides (TMDs) when a metal layer (M = Mo) is sandwiched between two chalcogen layers (X = S and Se). Because of its layered structure, weak van der Waals interactions between the layers, and several characteristics, molybdenum disulfide (MoS₂) and molybdenum diselenide (MoSe₂) have dragged the interest of researchers.²¹ The hexagonal structures of the layered TMDs MoS₂ and MoSe₂ are composed of molybdenum atoms stacked among two chalcogen layers (S and Se) by weak van der Waals interactions.²² We created a layered structure by fusing Nb₂C with MoS₂ and MoSe₂ nanosheets to improve capacitance performance. To ensure more active sites for redox reactions, the two distinct (Nb₂C/MoS₂ and Nb₂C/MoSe₂) nanocomposites are assumed to prevent MXenes or MoS₂ and MoSe₂ nanosheets from aggregating. Additionally, this structure guarantees robust interaction between the constituents, which is advantageous for preserving excellent electrical and mechanical contact. Motivated by these goals, it is highly likely that the hybrids of Nb₂C-MXenes (a high conductivity material) and MoS₂ and MoSe₂ (a pseudocapacitive material) can combine their benefits to improve electrochemical performance. In recent years, MXene/TMD heterostructures have attracted increasing attention because they combine the high electrical conductivity of MXenes with the layered structure and pseudocapacitance of TMDs. Such hybrids can suppress MXene restacking, provide additional redox-active sites, and facilitate rapid ion transport. Importantly, previous reports have demonstrated that the capacitance of MXene heterostructure electrodes can exceed that of the individual components, confirming the presence of strong synergistic effects.^23,24 This motivates the design of Nb₂C/MoS₂ and Nb₂C/MoSe₂ composites in our work, where Nb₂C provides high conductivity and structural stability, while MoS₂ and MoSe₂ contribute interlayer ion storage and pseudocapacitive charge transfer.

These days, metal ions are used as charge carriers in the majority of electrochemical energy storage devices, including Li-ion batteries, Zn-ion batteries, and Na-ion batteries.²⁵ The lack of innovative nonmetal-ion electrochemical capacitors and batteries is due to the contempt for nonmetal cation charge carriers. Various characteristics set NH₄⁺ apart from metal ions such as Na⁺, Mg²⁺, and Zn²⁺. Because NH₄⁺ has the lightest molar mass of 18 g mol⁻¹ and the lowest hydrated ionic size among aqueous electrolytes, it may diffuse quickly. Interest in the unique properties of NH₄⁺ has increased ever since the growth of the “shaking chair” ammonium-ion battery (AIB).²⁶ Most research on NH₄⁺ charge carriers has been restricted to batteries and has focused mostly on creating appropriate host materials for NH₄⁺ storage and investigating the related chemistry. Other NH₄⁺ charge carrier-based rechargeable energy storage technologies have not been developed, particularly for hybrid supercapacitors.^27,28 Compared to commonly used cations such as Na⁺, K⁺, or H⁺ in acidic electrolytes, NH₄⁺ offers unique advantages for flexible supercapacitors. Its small hydrated radius allows faster ion diffusion and higher ionic conductivity, contributing to enhanced power density. In addition, NH₄⁺ can form hydrogen-bonding interactions with the electrode surface and exhibits proton-like pseudocapacitance, thereby improving charge storage and energy density. These reversible interactions also contribute to superior cycling stability, as evidenced by 92% capacitance retention after 20 000 cycles in our device. Importantly, (NH₄)₂SO₄ is a neutral, biocompatible electrolyte, which makes it safe for skin-contact applications and highly suitable for integration into flexible, wearable band-aid platforms.

As illustrated in Scheme 1, using an ammonium ion hybrid supercapacitor, we created a multipurpose smart band-aid with applications for health monitoring and sensing. The band-aid's woven fabric is coated with carbon ink, and then the two active materials (Nb₂C/MoSe₂ and Nb₂C/MoS₂) are produced and applied. The doctor blade method is adopted to create flexible electrodes. MoS₂ and MoSe₂ are layered transition metal dichalcogenides (TMDs) with the layered structure that allows for ion intercalation between layers, making them promising candidates for energy storage. MoSe₂ has a larger interlayer spacing and higher electrical conductivity compared to MoS₂, which can facilitate faster ion and electron transport. Conductive carbon is added to the electrode composite to improve overall electrical conductivity and support efficient charge transfer during electrochemical cycling. Smart wearable band-aid opens up a new path for the growth of multipurpose integrated band-aid-based wearable electronics, which have great promise for applications including health monitoring. More significantly, the prototype supercapacitor demonstrated its immense potential for wearable technology by powering a flexible, self-powered integrated pressure sensor that could be directly attached to the human body to detect motions.

Scheme 1 Real-time bio monitoring applications of an integrated wearable band-aid based Nb₂C/MoSe₂‖AC ammonium ion hybrid supercapacitor flexible device: (a) schematic illustration of the Cu/Nb₂C/MoSe₂/Cu flexible sensor and Nb₂C/MoSe₂‖AC ammonium ion hybrid supercapacitor flexible device fabrication. (b) Fabricated ammonium ion hybrid supercapacitor powering a glucose sensor and (c) fabricated pressure sensor powered by an ammonium ion hybrid supercapacitor for real-time radial pulse monitoring.

Herein, we present a band-aid-based wearable ammonium-ion hybrid supercapacitor (AIHSC) that combines innovations at both the material and device levels. At the materials level, the Nb₂C/MoSe₂ composite electrode leverages the high conductivity of MXenes and the redox activity with enlarged interlayer spacing of MoSe₂, thereby providing enhanced ion transport, high specific capacitance, and excellent long-term cycling stability compared to conventional Nb₂C/MoS₂ and pristine MXene electrodes. At the device level, this work demonstrates for the first time a flexible, band-aid-integrated AIHSC employing NH₄⁺ ions as charge carriers. The small hydrated radius and proton-like pseudocapacitance of NH₄⁺ contribute to superior energy and power densities, together with outstanding cycling stability. Importantly, the integration of the AIHSC with glucose and pressure sensors highlights its practical potential as a multifunctional, wearable health-monitoring platform. Thus, the core novelty of this study lies in the synergistic combination of material design and device architecture to realize high-performance and multifunctional energy storage in real-world wearable electronics.

2 Results and discussion

Wearable health monitoring sensors require highly efficient and flexible energy storage devices. To meet this demand, we have developed a band-aid-based flexible energy storage device that seamlessly integrates with wearable sensors while ensuring electrochemical stability through a Nb₂C/MoSe₂ AIHSC framework. AIHSCs were fabricated using a band-aid coated with Nb₂C/MoSe₂ as the cathode, activated carbon as the anode, and a PVA/(NH₄)₂SO₄ gel electrolyte. From Fig. 1A Nb₂C exhibits an accordion-like layered structure, which is typical of multilayer Nb₂C. Fig. S1 shows the SEM images of pristine MoS₂ (a) and MoSe₂ (b). MoS₂ shows stacked, platelet-like nanosheets with an agglomerated morphology, while MoSe₂ exhibits larger, smoother, and more loosely packed lamellae with visible interflake porosity. Fig. 1B and C display an interfacial integration and partial accommodation of MoS₂ and MoSe₂ petal-like morphologies in the multilayered structure of Nb₂C. This result confirms that the existence of MoS₂ and MoSe₂ could effectively inhibit the stacking of the MXene sheets and enhance the charge transport, thus improving the corresponding electrochemical performance. The EDX spectrum for the Nb₂C/MoSe₂ sample in Fig. 1D confirms the presence of Nb, Se, C, O, and Mo. The discovered elements of these materials demonstrate a good synthesis and a homogeneous mixing of MoSe₂ nanoplatelets and Nb₂C nanoparticles, which is consistent with the SEM results.

Fig. 1 Structural and morphological characterization: (A) SEM images of Nb₂C MXenes; (B) Nb₂C/MoS₂; (C) Nb₂C/MoSe₂; (D) elemental mapping from the SEM image of the Nb₂C/MoSe₂ nanocomposite. (E) XRD spectra of (a) Nb₂C MXenes; (b) Nb₂C/MoSe₂; (c) Nb₂C/MoS₂. (F) Surface area analysis of the Nb₂C/MoSe₂ nanocomposite. (G) Zeta potential measurement of Nb₂C/MoSe₂ and Nb₂C/MoS₂ nanocomposites.

Fig. 1E shows the X-ray diffraction patterns of Nb₂C, Nb₂C/MoSe₂, and Nb₂C/MoS₂. Due to the Nb–Al bond breaking, which widened the interlayer gap, a broader characteristic peak, represented by the Nb₂C MXenes' (002) plane, appeared at 9.04°. The XRD pattern of Nb₂C shows a relatively low-intensity (002) reflection, which is characteristic of multilayer MXenes and indicates that complete delamination did not occur. This observation is consistent with prior reports.²⁹ In our study, no additional intercalation or exfoliation steps were applied; therefore, the Nb₂C used is confirmed to be multilayer MXenes, exhibiting the stacked, accordion-like morphology seen in SEM (Fig. 1A). Additionally, three diffraction peaks were observed at 60°, 39.1°, and 33.8°. These align with the crystallographic planes (100), (101), and (110) of hexagonal Nb₂C MXenes (JCPDS no. 00-015-0127).² After MoSe₂ is added to the MXene surface, the (002) peak position moves to the right side, indicating that the order that operates between the Nb₂C layers is disrupted. The diffraction peaks were found at angles of 2θ = 12.7°, 31.9°, 38.2°, and 56.4°, which correspond to the planes (002), (100), (103), and (110), respectively, based on the XRD pattern. The hexagonal structure of MoSe₂ is linked to the observed peaks, and this structure matches the conventional powder diffraction pattern of MoSe₂ from JCPDS card no. 87-2419.27.³⁰ According to ICCD 03-065-0160, MoS₂ displays a broad peak (2θ = 14.08°) connected to the (002) plane in the case of Nb₂C/MoS₂. Four other diffraction peaks were also seen at 2θ = 33°, 39°, 58°, and 62°, which were attributed to the (100), (103), (110) and (201) planes, respectively.^31,32 The confinement of MoS₂ between the MXene layers was confirmed by low-intensity peaks at 2θ angles of 33°, 39°, and 57°.³³ Nevertheless, because of the nanostructured MoS₂ creation linked to Nb₂C, MoS₂ peaks such as (002) at 14.08° and (100) at 33° were overlapped by the Nb₂C/MoS₂ pattern. When the XRD patterns of Nb₂C and Nb₂C/MoS₂ were compared, the (002) peak was discovered to be displaced to the right, indicating that MXenes had a greater interplanar spacing because MoS₂ was restricted between their layers. The XRD patterns show that the full width at half maximum (FWHM) of the Nb₂C/MoSe₂ and Nb₂C/MoS₂ lattices has increased with the incorporation of MoSe₂ and MoS₂. The Nb₂C/MoSe₂ sample has an average crystallite size of 39 nm, while the Nb₂C/MoS₂ sample has a crystallite size of 47 nm. The normal crystallite size for both the MoSe₂ and MoS₂ phases has shrunk with the addition of Nb₂C, which contributes to good electrochemical performance. Although the Nb₂C/MoSe₂ sample has a smaller crystallite size (39 nm) compared to Nb₂C/MoS₂ (47 nm), as indicated by its broader FWHM, this suggests more disorder and exfoliation in the MoSe₂ sample. This finding is another confirmation that MoSe₂ more effectively disrupts the interlayer order, suggesting better prevention of restacking.

For Nb₂C/MoSe₂, N₂ adsorption–desorption experiments are carried out, resulting in BET surface areas of 127 m² g⁻¹, respectively (Fig. 1F). The pristine Nb₂C and Nb₂C/MoS₂ nanocomposite are shown in Fig. S2. The Nb₂C/MoSe₂ composite isotherms suggest the existence of a micro mesoporous structure. Together with the interlayer aggregation of MoSe₂, the layers of Nb₂C MXenes offer a greater volume and a more accessible specific surface area. These elements help to lower the initial resistance and increase the specific capacitance. The overall electrochemical behavior is further enhanced by the presence of effective ion-transfer channels, which are indicated by the combination of micro and mesopores. As a result of stacking, Nb₂C/MoSe₂ possessed a comparatively large surface area compared to Nb₂C/MoS₂. Furthermore, Table S1 (SI) provides the pore size and the pore volume distribution. Zeta potential analysis was used to ascertain charge at the surface, as illustrated in Fig. 1G. Nanosuspension stability is represented by a greater negative or positive zeta potential, which results from electrostatic repulsive forces between nanoparticles.³⁴ The input of repulsive forces, primarily van der Waals forces, causes nanoparticles to aggregate at the lowest zeta potential value. The zeta potential value of −9.3 mV for the Nb₂C/MoSe₂ nanocomposite indicates low stability, while the zeta potential value of −23 mV for the Nb₂C/MoS₂ nanocomposite suggests higher stability. This indicates that the Nb₂C/MoSe₂ is significantly more agglomerated, suggesting that van der Waals forces caused the intercalation of MoSe₂ nanoplatelets and Nb₂C nanosheets.

The electrochemical performance of pristine Nb₂C, MoSe₂ and MoS₂ electrodes was tested in a half-cell assembly, which consisted of a working electrode, reference electrode, and counter electrode using three different electrolytes (Fig. S3). Fig. S3A shows the cyclic voltammogram (CV) curves of the Nb₂C electrode at a scan rate of 30 mV s⁻¹ across the potential window for the three different electrolytes (1 M (NH₄)₂SO₄, 1 M ZnSO₄, and 1 M Na₂SO₄). From the CV curves, symmetric charge–discharge curves are observed, which prove the good reversibility and capacitive behavior of the electrode. Fig. S3B shows the CV curves of the MoSe₂ electrode at a scan rate of 30 mV s⁻¹, and the curves exhibit a pair of prominent redox peaks. This results in the battery-type behaviour of the MoSe₂ electrode. In the case of the MoS₂ electrode, the CV curves in all the electrolytes are symmetric, which suggests that the electrode has excellent reversibility (Fig. S3C). Fig. S3D–F shows the galvanostatic charge–discharge curves of pristine Nb₂C, MoSe₂ and MoS₂ electrodes at all three electrolytes. The specific capacitance of each electrode is shown in Table S2.

The electrochemical behavior of the Nb₂C/MoSe₂ and Nb₂C/MoS₂ electrodes was also investigated using a half cell consisting of a working electrode (Nb₂C/MoSe₂ and Nb₂C/MoS₂), reference electrode (Ag/AgCl), and counter electrode (platinum wire) in three different electrolytes (1 M (NH₄)₂SO₄, 1 M ZnSO₄, and 1 M Na₂SO₄). Fig. 2A depicts the CV curves recorded at a 30 mV s⁻¹ scan rate for the Nb₂C/MoSe₂ electrode with the potential window recorded from −0.4 to 0.4 V, 0.0 to 1.0 V, and 0.0 to 1.0 V for 1 M Na₂SO₄, 1 M (NH₄)₂SO₄ and 1 M ZnSO₄, respectively. All CV curves show quasi-rectangular shapes. CV curve (b) shows the highest current response, indicating it has the highest capacitance among the three. Curve (a) has the lowest current response, suggesting the lowest capacitance. The curves are not perfectly rectangular, indicating the presence of both electric double-layer capacitance (EDLC) and some faradaic (pseudocapacitive) contributions. Fig. 2B presents the GCD data of Nb₂C/MoSe₂ at three different electrolytes, and the obtained potential window well matched with the potential obtained from CV analysis. All curves exhibit a nearly symmetric triangular shape, which is typical for capacitive behavior (ideal supercapacitor). This symmetry indicates good reversibility and low internal resistance. In the case of GCD, curve (b) has the longest discharge time, which means it stored and delivered the most charge and eventually attained the highest specific capacitance. Curve (c) has a moderate discharge time, and curve (a) has the shortest discharge time and has the lowest capacitance. The symmetrical charge–discharge curves suggest good capacitive behavior, and possibly a mix of EDLC and pseudocapacitance. The specific capacitance contribution of each different electrolyte is shown in Fig. 2C. A greater specific capacitance of 252 F g⁻¹ at 1 A g⁻¹ was found for Nb₂C/MoSe₂ electrode in 1 M (NH₄)₂SO₄ than in 1 M ZnSO₄ (82 F g⁻¹) and 1 M Na₂SO₄ (38 F g⁻¹). In the Nb₂C/MoSe₂ system, compared to the storage properties of various metal ions (Na⁺ and Zn²⁺), the specific capacitance of nonmetal NH₄⁺ is noticeably higher. NH₄⁺ ions have a smaller hydrated radius than Na⁺ and Zn²⁺, which allows easier and faster ion diffusion into the electrode's pores. Also, NH₄⁺ ions can behave similarly to protons (H⁺) and engage in pseudo-capacitive redox reactions, contributing additional faradaic charge storage.

Fig. 2 Electrochemical performances of the Nb₂C/MoSe₂ flexible electrode: (A) comparison of cyclic voltammograms (30 mV s⁻¹) of (a) 1 M Na₂SO₄; (b) 1 M (NH₄)₂SO₄; (c) 1 M ZnSO₄. (B) Galvanostatic charge–discharge curves of (a) 1 M Na₂SO₄; (b) 1 M (NH₄)₂SO₄; (c) 1 M ZnSO₄. (C) Comparison bar graph of electrolytes versus specific capacitance. (D) Cyclic voltammograms of Nb₂C/MoSe₂ at various scan rates. (E) Galvanostatic charge–discharge curves of Nb₂C/MoSe₂ at different current densities: (a) 1 A g⁻¹; (b) 1.5 A g⁻¹; (c) 2 A g⁻¹; (d) 2.5 A g⁻¹. (F) Graph of specific capacitance versus current density. (G) CV curve of total capacitive and diffusion-controlled contribution at 50 mV s⁻¹. (H) Capacitive and diffusion-controlled contribution calculated at different scan rates of the Nb₂C/MoSe₂ electrode. (I) Stability performance of Nb₂C/MoSe₂ electrodes for 10 000 cycles at 5 A g⁻¹.

This hybrid mechanism (EDLC + pseudocapacitance) enhances total capacitance. This improves charge storage efficiency, particularly in porous materials. NH₄⁺ < Na⁺ < Zn²⁺ is the general trend of hydrated ionic radii in aqueous electrolytes, which correlates with the observed capacitance: smaller hydrated ions diffuse more easily into electrode pores, leading to faster ion transport and higher rate capability. This is because a smaller charge carrier typically results in faster ion diffusion and, thus, better rate capability.³⁵ The superior performance of NH₄⁺ over Na⁺ and Zn²⁺ arises from its smaller hydrated ionic radius, which facilitates rapid diffusion and efficient pore utilization. Moreover, NH₄⁺ ions engage in hydrogen-bonding interactions with the Nb₂C/MoSe₂ electrode, stabilizing the electrode/electrolyte interface and enhancing long-term cycling stability. In contrast, Na⁺ and Zn²⁺ ions exhibit slower diffusion and weaker interfacial interactions, resulting in lower specific capacitance and poorer rate capability. Fig. 2D displays the scan speeds of the CV curves for Nb₂C/MoSe₂ electrode in 1 M (NH₄)₂SO₄ within a potential range of 0.0 to 1.0 V. The CV curves suggest that the electrode has excellent reversibility. CV curves retain shape but get distorted at higher scan rates, which is typical due to increased resistance and limited ion diffusion. The increasing current response with scan rate suggests good rate capability. Some deviation from the ideal rectangular shape indicates a combination of capacitive and faradaic behavior. The wearable Nb₂C/MoSe₂ performance in 1 M (NH₄)₂SO₄ electrolytes at different current densities was assessed by analysing GCD data (Fig. 2E). The triangular shape of the GCD curve exhibits good reversibility, which was well agreed with the performance observed from CV curves and a quick ion transport mechanism. As the current density increases from (a) to (d), the discharge time decreases, leading to lower specific capacitance at higher currents. Curve (a) has the longest discharge and highest capacitance at the lowest current density. The curves are still fairly linear and symmetric, indicating stable charge/discharge performance. The specific capacitance (C_sp) of Nb₂C/MoSe₂ at 1 M (NH₄)₂SO₄ is gradually decreased at varying current densities (Fig. 2F). Additionally, it is observed that the discharge time decreases as the current density increases. Rapid ion transport at the electrode–electrolyte interface and inadequate use of the effective surface area are the causes of this phenomenon. Fig. 2G compares the total capacitive and diffusion control of the Nb₂C/MoSe₂ electrode at a scan rate of 50 mV s⁻¹. Using the power law (SI), diffusion-controlled or surface/capacitive-controlled charge storage is predicted.³⁶ The contribution ratio of the response mechanism at various scan rates was computed using Dunn's method (Fig. 2H).^37,38

The sandwiched Nb₂C/MoSe₂ electrode's diffusion-controlled and capacitive contributions are displayed in the bar diagram at various scan speeds (2, 5, 10, 30, 50, 70, and 90 mV s⁻¹). These correspond to 31%, 46%, 53%, 65%, 77%, 83%, and 91% capacitive contributions. The cycling stability of the Nb₂C/MoSe₂ hybrid flexible electrode is demonstrated by the GCD test in Fig. 2I for 10 000 charge and discharge cycles at a current density of 5 A g⁻¹. The capacitance of the Nb₂C/MoSe₂ electrode decreases by 8% after 10 000 cycles. The electrode exhibits exceptional electrochemical robustness by maintaining a 92% capacitance retention rate. Coulombic efficiency stays near 97%, showing high reversibility and minimal degradation over time.

Similarly, Fig. 3A and B depict the CV curves recorded at a 30 mV s⁻¹ scan rate and GCD data of the Nb₂C/MoS₂ electrode within an operating potential range observed from 0.0 to 1.0 V, 0.0 to 1.0 V, and 0.0 to 0.8 V for 1 M Na₂SO₄, 1 M (NH₄)₂SO₄ and 1 M ZnSO₄, respectively. The non-ideal CV shape suggests a combination of electric double-layer capacitance and pseudocapacitance. The symmetric charge–discharge profiles imply good reversibility and low internal resistance. Fig. 3C displays the specific capacitance contribution of the electrolytes. Among the three electrolytes, Nb₂C/MoS₂ in 1 M (NH₄)₂SO₄ exhibits very high specific capacitance. Electrolytes containing NH₄⁺ have a specific capacitance of 110 F g⁻¹, higher than that of Zn²⁺ (50 F g⁻¹) and Na⁺ (28 F g⁻¹). NH₄⁺ can engage in pseudocapacitive redox reactions due to proton-like behavior. Higher ionic conductivity and efficient interaction with the electrode surface facilitate both EDLC and faradaic contributions, maximizing charge storage. The CV scan rates for the Nb₂C/MoS₂ electrode in 1 M (NH₄)₂SO₄ with a potential range of 0.0 to 1.0 V are shown in Fig. 3D. The as-fabricated Nb₂C/MoS₂ electrode displays electric double-layer performance. Increased current with scan rate confirms good rate capability. Slight distortion of curves at higher scan rates is typical due to ion diffusion limits but still maintains a stable shape, indicating fast charge propagation and stable electrochemical behavior. The wearable Nb₂C/MoS₂ electrochemical performance in 1 M (NH₄)₂SO₄ electrolytes at various current densities was analyzed (Fig. 3E). The linear charge–discharge curve exhibits exceptional EDLC behavior and reversibility, which was well agreed with the performance observed from CV curves, indicating good electrochemical performance and a quick ion transport mechanism. Specific capacitance drops at higher currents due to incomplete ion diffusion into porous structures, leaving less time for redox reactions to contribute at high current densities. The specific capacitance (C_sp) of Nb₂C/MoS₂ in 1 M (NH₄)₂SO₄ is gradually decreased at varying current densities (Fig. 3F). Fig. 3G compares the diffusion-controlled and total capacitance of the Nb₂C/MoS₂ electrode at a scan rate of 50 mV s⁻¹. The faradaic mechanism is observed to dominate the electrode's capacitance at low scan rates. The contribution of intercalation/deintercalation decreases as the scan rate increases, while the non-diffusion controlled capacitive contribution begins to increase.

Fig. 3 Electrochemical performances of the Nb₂C/MoS₂ flexible electrode: (A) comparison of cyclic voltammograms (30 mV s⁻¹) of (a) 1 M Na₂SO₄; (b) 1 M (NH₄)₂SO₄; (c) 1 M ZnSO₄. (B) Galvanostatic charge–discharge curves of (a) 1 M Na₂SO₄; (b) 1 M (NH₄)₂SO₄; (c) 1 M ZnSO₄. (C) Comparison bar graph of electrolytes versus specific capacitance. (D) Cyclic voltammograms of Nb₂C/MoS₂ at various scan rates. (E) Galvanostatic charge–discharge curves of Nb₂C/MoS₂ at different current densities: (a) 1 A g⁻¹; (b) 1.5 A g⁻¹; (c) 2 A g⁻¹; (d) 2.5 A g⁻¹. (F) Graph of specific capacitance versus current density. (G) CV curve of total capacitive and diffusion-controlled contribution at 50 mV s⁻¹. (H) Capacitive and diffusion-controlled contribution calculated at different scan rates of the Nb₂C/MoS₂ electrode. (I) Stability performance of Nb₂C/MoS₂ electrodes for 10 000 cycles at 5 A g⁻¹.

In Fig. 3H, the bar diagram shows the diffusion-controlled and capacitive contributions of the fabricated Nb₂C/MoS₂ electrode at different scan rates (2, 5, 10, 30, 50, 70, and, 90 mV s⁻¹). The corresponding capacitive contributions are 31%, 39%, 52%, 63%, 72%, 84%, and 91%, respectively. Fig. 3I shows the cycling stability of Nb₂C/MoS₂ in 1 M (NH₄)₂SO₄ over 10 000 cycles at a current density of 5 A g⁻¹. The electrode shows a 15% decline in specific capacitance after 10 000 cycles, demonstrating high reversibility and minimal structural degradation over extended operation. Therefore, the Nb₂C/MoSe₂ electrode's comparatively intact sheet structure in the NH₄⁺ electrolyte suggests that NH₄⁺ and the electrode form a hydrogen bond, which improves NH₄⁺ adsorption onto the electrode material and contributes to the exceptional cycling stability.³⁹ Additionally, NH₄⁺ exhibits superior electrochemical stability, ion migration rate, and hydrogen bonding interactions with the Nb₂C/MoSe₂ electrode compared to Nb₂C/MoS₂, giving supercapacitors a very high specific capacitance and noticeable cycling stability.

Moreover, the different electrochemical behavior of Nb₂C/MoSe₂ and Nb₂C/MoS₂ is due to MoSe₂ being more redox-active, having lower electronegativity of Se, better electronic conductivity, and different redox potentials and ion intercalation behavior. Therefore, MoSe₂ may reach its redox limits or decompose earlier/later, requiring a different optimized potential window to accurately capture its behavior in each electrolyte. In contrast, MoS₂ may operate stably within a single common window across all three electrolytes, especially if it exhibits more capacitive (surface-controlled) rather than battery-type (redox) behavior. This comparison aims to investigate how replacing sulfur (MoS₂) with selenium (MoSe₂) affects the electrochemical performance of the Nb₂C layered structure, due to differences in conductivity, interlayer spacing, and redox behavior. The Nb₂C/MoSe₂ composite demonstrated superior electrochemical performance compared to Nb₂C/MoS₂. This improvement is attributed to the higher electrical conductivity and larger interlayer spacing of MoSe₂, which facilitate faster ion diffusion and charge transfer. Moreover, MoSe₂ forms more favorable interfaces with Nb₂C, enhancing electron mobility at each layered junction. These advantages are reflected in the CV and GCD profiles, which show higher current response, improved capacitance, and lower internal resistance for MoSe₂-based composites. To further validate the Nb₂C/MoSe₂ electrode performance, electrochemical impedance analysis of all three electrolytes was carried out, as shown in Fig. S4. The 1 M Na₂SO₄ electrolyte exhibits maximum resistance behaviour, indicated by the largest semicircle (highest R_ct = 20 Ω). The less steep line in the low-frequency region exhibits poor capacitive behavior because of the presence of Na⁺, which has a relatively large hydrated radius, causing sluggish ion transport and poor wettability or weaker interaction with the electrode. In the case of 1 M ZnSO₄, it exhibits low R_ct = 9 Ω, but is limited by diffusion kinetics even though the Zn²⁺ has a high charge density (strong attraction to the electrode). However, it has a larger hydrated radius and slower mobility than NH₄⁺. The lowest total resistance (lowest real Z′ value) is observed for 1 M (NH₄)₂SO₄, which exhibits a very small semicircle (very low R_ct = 4 Ω), and the steep linear region exhibits the excellent capacitive behavior (closer to vertical). Low internal resistance facilitates fast charge transfer and efficient ion transport. The EIS analysis confirms that (NH₄)₂SO₄ offers the lowest impedance, highest ion mobility, and fastest charge transfer, making it the most efficient electrolyte among the three electrolytes.

A wearable ammonium ion supercapacitor (AIHSC) device was constructed with PVA/(NH₄)₂SO₄ gel electrolyte. The flexible Nb₂C/MoSe₂‖AC scheme is shown in Fig. 4A. Fig. 4B and C illustrate CV curves over various scan rates and the GCD with various current densities for the fabricated band-aid AIHSC. The CV curves exhibit clear symmetric charging and discharging curves at all scan rates and deliver the ideal capacitive behavior of Nb₂C/MoSe₂‖AC. The maximum specific capacitance of 120 F g⁻¹ at a current density of 1 A g⁻¹, which is two times lower than the value observed in the 1 M (NH₄)₂SO₄ aqueous electrolyte because of the PVA/(NH₄)₂SO₄ electrolyte's lower ionic coefficient and the inadequate transport of ions toward the interface between the electrolyte and the active material. Minimal voltage drop and linear slopes indicate low internal resistance and good rate performance. The specific capacitance of the fabricated device at different current densities is provided in Fig. 4D, where it decreases linearly. At higher current densities, there is less time for ions to penetrate the deeper pores, reducing capacitance due to the limited usage of the active material. The higher current leads to a significant internal resistance drop (V = IR), which reduces the effective discharge voltage window. Capacitance inherently decreases at higher current densities, which is consistent with kinetic limitations. This behavior is commonly observed in hybrid supercapacitors and does not necessarily reflect poor performance but rather is a limitation of ion transport at high rates. Fig. 4E shows the contribution at different scan rates for Nb₂C/MoSe₂‖AC, and the capacitive contribution gradually increases from 44%, 49%, 67%, 74%, 84%, and 89% at scan rates of 5, 10, 30, 50, 70, and 90 mV s⁻¹. Slow migration of NH₄⁺ ions at low scan rates results in a longer residence period on the electrode surface. CV analysis of the diffusion-controlled and total capacitance of the Nb₂C/MoSe₂‖AC electrode at a scan rate of 50 mV s⁻¹ is shown in Fig. 4F, which facilitates the adsorption of NH₄⁺ ions on the Nb₂C/MoSe₂ electrode surface. Conversely, when the rapid migration of NH₄⁺ ions at a high scan rate disrupts the charging and discharging processes of ions on the surface of the Nb₂C/MoSe₂ electrode, it affects the charge transfer between NH₄⁺ and Nb₂C/MoSe₂. This results in a reaction mechanism where the Nb₂C/MoSe₂ electrode is capacitive at high scan rates and pseudocapacitive at low scan rates. A constant current density of 2 A g⁻¹ was used to evaluate the cycling stability of the band-aid-based AIHSC across 20 000 cycles (Fig. 4G). After 20 000 cycles, the AIHSC device provides a coulomb efficiency of 96.6% and capacitance retention of 92% respectively. This retains excellent structural stability and low degradation over cycles. After 20 000 charge–discharge cycles, the SEM image and XRD pattern show (Fig. S5) no significant structural or phase changes. This confirms excellent long-term structural stability and durability of the electrode material.

Fig. 4 The electrochemical performance of band-aid-based Nb₂C/MoSe₂‖AC in PVA/(NH₄)₂SO₄ nanoparticle gel electrolyte. (A) Schematic illustration of the cell assembly. (B) Cyclic voltammograms at various scan rates. (C) Galvanostatic charge–discharge curves at different current densities: (a) 1 A g⁻¹; (b) 1.5 A g⁻¹; (c) 2.0 A g⁻¹; (d) 2.5 A g⁻¹. (D) Specific capacitance versus different current densities. (E and F) Capacitive and diffusion-controlled contributions calculated at various scan rates. (G) Cycling stability and coulombic efficiency (%) (first 10 cycles). (H) Nyquist plot. (I) Ragone plot. (J and K) Mechanical and flexibility study of Nb₂C/MoSe₂‖AC AIHSCs at various bending angles.

To investigate the charge transfer and electrolyte diffusion at the electrode/electrolyte interface, electrochemical impedance spectroscopy (EIS) was performed on the AIHSC, as shown in Fig. 4H. The Nyquist plot displays a small semicircle in the high-frequency region, attributed to the charge-transfer resistance (R_ct), and an inclined line in the low-frequency region, associated with Warburg impedance. The Nb₂C/MoSe₂ electrode exhibits an R_ct of approximately 2.6 Ω, reflecting low internal resistance and excellent conductivity. The steep slope in the low-frequency region indicates efficient ion diffusion and nearly ideal capacitive behavior. To further analyze these features, the spectra were fitted using an equivalent circuit composed of series resistance (R_s), charge-transfer resistance (R_ct), a constant phase element (CPE), and a Warburg element (W) (inset of Fig. 4H).^40,41 Compared with Nb₂C/MoS₂, the Nb₂C/MoSe₂ electrode shows smaller R_s and R_ct values, confirming its lower intrinsic resistance, faster charge-transfer kinetics, and superior ion accessibility. Together, these characteristics demonstrate that Nb₂C/MoSe₂ enables improved charge transport and diffusion pathways, which underpin its enhanced electrochemical performance. The fundamental metrics used to evaluate the performance of supercapacitors are their energy and power densities. The supercapacitor's power density is the ratio of the power it can absorb or disperse to its mass. This indicator assesses a supercapacitor's capacity to quickly release or absorb energy, enabling quick charging, discharging, and a longer lifespan. The AIHSC has a noteworthy energy density of 60 Wh kg⁻¹ and a power density of 1236 W kg⁻¹, which is higher than the values found in other materials, as shown in Fig. 4I. However, wearable technology requires a flexible and sturdy device. To evaluate mechanical robustness under different bending angles, the AIHSC retained its capacitive profile at 0°, 45°, 90°, 135°, and 180° (Fig. 4J), indicating minimal distortion of charge-storage kinetics under deformation. Capacity retention after 5000 bending cycles at these angles was 95%, 94%, 90%, 85%, and 82%, respectively (Fig. 4K), evidencing durable electrochemical performance during repeated flexure. Together with the 92% capacitance retention after 20 000 charge–discharge cycles, these results confirm that the electrode/gel–electrolyte interfaces and conductive pathways remain intact during prolonged mechanical actuation.

Two devices coupled in series and parallel at 50 mV s⁻¹ have their assembled AIHSC cyclic voltammetry curves shown in Fig. 5A. Interestingly, compared to a single device, the current density of two parallel devices is almost double. In contrast, the potential is increased to 0.0–3.0 V by two serial devices. The charge–discharge period of two devices in parallel is double that of a single device, and the operating potential of two devices is 3.0 V (Fig. 5B). These findings highlight the Nb₂C/MoSe₂‖AC AIHSC's exceptional integrability and consistency. Fig. 5C displays the self-discharge curve of the AIHSC band-aid with devices linked in series. After charging to ∼3 V (series of 2 devices), the device shows minimal voltage drop over 2000 s, which indicates good electrolyte stability, low leakage current, and a strong electrode–electrolyte interface. This is important for long-term energy retention in wearable/bioelectronic devices. Fig. 5D displays the serially connected AIHSC powering and testing a glucose meter as proof of concept.

Fig. 5 Wearable band-aid-based Nb₂C/MoSe₂‖AC AIHSC for powering biomonitoring systems: (A) and (B) Cyclic voltammograms and galvanostatic charge–discharge curves of Nb₂C/MoSe₂‖AC in both (a, c) series and (b, d) parallel configurations. (C) Self-discharge curve of two devices connected in series. (D) Schematic illustration and photographs of the powering glucose sensing system with the Nb₂C/MoSe₂‖AC band-aid based AIHSC. (E) Schematic diagram of a self-powered pressure sensor integrated device. (F) Self-powered pressure sensor exhibiting the measurement of radial artery pressure. (G) Real-time monitoring of finger touching.

The fabricated ultrasensitive pressure sensor was coupled with the manufactured Nb₂C/MoSe₂‖AC AIHSC to measure real-time arterial pulse pressure. As seen in Fig. 5E, we created a closed circuit by connecting the flexible pressure sensor and supercapacitor vertically to accomplish device integration. To use a self-powered pressure sensor (Cu‖Nb₂C/MoSe₂‖Cu) integrated device for monitoring the human body's physiological signals, a PI film was used to secure the flexible pressure sensor and supercapacitor to the skin. The Nb₂C/MoSe₂‖AC device was charged at the electrochemical workstation, and the charged supercapacitor powered the pressure sensor. Additionally, a multimeter was used to measure the change in output current for the applied pressure. As shown in Fig. 5F, the fabricated supercapacitor and pressure sensor were attached to a human wrist for on-body testing. The integrated AIHSC successfully powered the Cu‖Nb₂C/MoSe₂‖Cu pressure sensor, producing stable radial-pulse recordings with a heart rate of ≈70 bpm. The current traces exhibited periodic, high-contrast peaks with a steady baseline, confirming stable signal output during wrist movement. In addition, deliberate finger-tapping stimuli generated sharp, reproducible current responses, with variations in peak magnitude and sharpness corresponding to the applied pressure and tapping rate (Fig. 5G). To further evaluate mechanical robustness, the device was mounted on an index finger and tested under fixed-angle and angle-varying bending. As shown in Fig. S6, the pressure sensor delivered consistent current signals in both cases, indicating low hysteresis, good repeatability, and reliable performance under dynamic deformation. The performance of the assembled MoSe₂-based supercapacitor for real-time powering of flexible electronic devices with comparable reports is compared in Tables S3 and S4. However, in this work, the full unit, which included a pressure sensor and a supercapacitor, was utilized to measure the arterial pulse pressure of humans in real time. Furthermore, the supercapacitor of the Nb₂C/MoSe₂‖AC AIHSC showed a respectable discharge voltage of 3 V and a charging time of 60 s. Finally, it demonstrates that the fabricated sensor has the advantage of being modular (series/parallel configuration works efficiently), stable (excellent self-discharge performance), practical (capable of powering real-world medical devices like glucose meters), wearable-compatible (supports flexible, on-body biosensing), repeatable and responsive (shows reliable charge/discharge and sensing cycles).

3 Conclusions

In summary, we report a smart wearable band-aid integrating an ammonium-ion hybrid supercapacitor (AIHSC) and pressure sensing, enabled by a Nb₂C/MoSe₂ nanocomposite electrode. The Nb₂C/MoSe₂ flexible electrode delivers a high specific capacitance of 252 F g⁻¹ in 1 M (NH₄)₂SO₄ with excellent cycling stability, while the assembled Nb₂C/MoSe₂‖AC AIHSC achieves 120 F g⁻¹, an energy density of 60 Wh kg⁻¹, and a power density of 1236 W kg⁻¹, retaining 92% capacitance after 20 000 cycles. Beyond electrochemical performance, the device exhibits remarkable flexibility and practical applicability by powering glucose and pressure sensors for real-time biosensing. The innovation of this work lies in the synergistic integration of material and device advances: (i) the Nb₂C/MoSe₂ nanocomposite electrode that maximizes conductivity, redox activity, and ion transport, and (ii) the first realization of a band-aid-based ammonium-ion supercapacitor as a multifunctional wearable platform. Together, these advances enable high-performance, flexible, and practical energy storage for next-generation health monitoring and sensing applications. Furthermore, the use of a neutral, biocompatible (NH₄)₂SO₄ electrolyte ensures both high electrochemical performance and safety advantages over acidic electrolytes, making this system uniquely suited for skin-contact wearable devices.

4 Experimental section

4.1. Materials and chemicals

Ammonium persulfate (99.5%), ammonium sulfate (99.5%), zinc sulfate (99.5%), sodium sulfate (99%), hydrochloric acid, polyvinyl alcohol (PVA, M_w; 8000), lithium fluoride (99.9%), molybdenum sulfide powder (99.9%), and molybdenum selenide powder (99.9%) were purchased from Sigma Aldrich. Niobium aluminium carbide powders were acquired from Hong Kong's Laizhou Kai Ceramic Materials Co. Ltd. Sheets of polyimide were purchased from Fiedler Scientific Instruments in the Czech Republic. A stretchable band-aid was purchased from a local shop in Brno, Czech Republic. All of the compounds were used as analytical reagents without any further purification.

4.2. Synthesis of niobium carbide (Nb₂C)

A 100 ml reaction kettle was first filled with 10 ml of deionized water, and then 30 ml of 12 M HCl was gradually added while being vigorously agitated for 10 minutes. Then, in an ice water bath, 3 g of LiF was gradually added and agitated for 30 minutes to completely react. The reaction kettle was then gradually filled with 2 g of Nb₂AlC powder, which was agitated for 30 minutes. The reactor was then placed in an oven set to 180 °C for twenty-four hours. The MXene solution was washed multiple times by centrifugation in distilled water at 3500 rpm until the pH reached 7. Ultimately, the Nb₂C suspension was freeze-dried to produce MXene powder.⁴²

4.3. Fabrication of band-aid-based Nb₂C/MoSe₂ electrodes

0.25 g of conductive carbon was initially added into 25 ml of deionized water and stirred for 3 h at 60 °C. Then, the slurry was smoothly coated onto the surface of the band-aid and dried for 8 h at 70 °C. Then, the as-synthesized Nb₂C/MoSe₂ and Nb₂C/MoS₂ were coated on the wearable band-aid substrate. The cotton pad in the band-aid was dipped with the prepared PVA/(NH₄)₂SO₄ and sandwiched between the activated carbon fabric electrode and the Nb₂C/MoSe₂ band-aid electrode. The PI tape was used to seal the device. The device was connected to Cu foil with silver paste for further measurement.

4.4. Integrated glucose monitoring system

The glucose sensor and the Nb₂C/MoSe₂ AIHSC band-aid were coupled in series. After dropping the aqueous glucose solution onto the chip, a 5 s waiting period was required. Then an accurate glucose concentration measurement range was displayed on the sensor display.

4.5. Fabrication of the pressure sensor

Nb₂C/MoSe₂ was used to create a highly lightweight, flexible, piezoelectric sensor. The Nb₂C/MoSe₂ band-aid is positioned between two sheets of copper foil, and copper wires were soldered to the electrodes. To increase its longevity, PI tape was used to enclose the manufactured sensor, which was set up as a Cu‖Nb₂C/MoSe₂‖Cu electrode.

4.6. Materials characterization

Powder X-ray diffraction (XRD) was used to characterize the produced materials using a Rigaku Smart Lab 3kW equipped with an accelerator detector (Cu-Kα radiation, λ = 1.5418 Å). MIRA with an EDX detector (MIRA3 XMU (MIRA-STAN)) was used to record the surface morphologies and associated elemental mapping. Brunauer–Emmett–Teller (BET) analysis was performed using a Quantachrome instrument. Zeta potential analysis was carried out using a Malvern Zetasizer Nano ZS instrument.

Author contributions

Both M. P. and K. M. conceptualized and designed the idea. K. M. prepared the materials, executed the experiments, and characterized the material. M. P. supervised the work and reviewed the manuscript. The manuscript was written through the contributions of the authors. Both authors have approved the final version of the manuscript.

Conflicts of interest

There is no conflict of interest.

Data availability

Data for this article are available at ZENODO at https://zenodo.org.

Supplementary information: (1) Experimental section, figures and tables. (2) Application of the biomonitoring sensor: fabricated AIHSC to power a glucose sensor (Video S1). See DOI: https://doi.org/10.1039/d5ta06175j.

Acknowledgements

The work was supported by the ERDF/ESF project TECHSCALE (No. CZ.02.01.01/00/22_008/0004587). This research was co-funded by the European Union under the REFRESH – Research Excellence For Region Sustainability and High-tech Industries project (No. CZ.10.03.01/00/22_003/0000048) via the Environment Programme Just Transition. K. M. would like to acknowledge Dr Senthil Nathan for his help in zeta potential measurements and Dr Bindu Kalleshappa for help with sensor experiments. M. P. acknowledges project ANGSTROM for funding. Project ANGSTROM was selected in the Joint Transnational Call 2023 of M-ERA.NET 3, which is an EU-funded network of about 49 funding organisations (Horizon 2020 grant agreement no. 958174). This project “Advancing Supercapacitors with Plasma-designed Multifunctional Hybrid Materials” (no. TQ05000001) is co-financed from the state budget by the Technology Agency of the Czech Republic under the SIGMA Programme within the M-ERA-NET 3 Call 2023. This project/result was funded under the National Recovery Plan from the European Recovery and Resilience Facility.

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