Water‐In‐Salt Electrolyte Induced Fast Ion Diffusion and Moisture Driven Charging of CNT Inks Based Sustainable Flexible Microsupercapacitor

ABSTRACT

In today's era, the rise of flexible, biodegradable, and micro‐energy storage devices represents the next frontier of sustainable electronics. Despite promising conductivity, carbon nanotube (CNT)‐based supercapacitors display modest capacitance and compromised rate performance due to densely packed layers or diffusion paths of ions. To make a promising and sustainable device, we synthesized CNT‐inks with enhanced and improved intertubular spacing. Herein, we report improved ion diffusion paths and enhanced interwall spacing by insertion of ions. For the scalable production of microsupercapacitors (MSCs), the Li‐ion modified CNT‐ink was further processed via screen printing. Coupled with the engineered electrode architecture and wide electrochemical window of bis(trifluoromethane)sulfonamide lithium (LiTFSI) water‐in‐salt (WIS) electrolyte, the device with limited interdigitated‐electrode (IDE) fingers achieved a high areal capacitance (42 mF/cm²) and excellent cyclic durability (∼95% after 20,000  cycles). Additionally, Kelvin‐probe force microscopy (KPFM) revealed dynamic surface potential modulation, directly evidencing Fermi level tuning through Li⁺ ion adsorption. Notably, the MSC also performed with commendable electrochemical behavior even at ultra‐low temperatures down to −60°C and demonstrated effective power generation at high humidity. Also, we tested device reproducibility with remarkable performance and excellent flexibility. These outstanding performances underscore their strong potential for next‐generation wearable and environmentally adaptive electronics.

A flexible paper‐based Li⁺‐engineered CNT microsupercapacitor is developed for simultaneous energy storage and humidity‐driven self‐charging. The device exhibits enhanced electrochemical performance, excellent structural stability, sustainable design, and reliable operation over an ultrawide temperature range (−60°C to 80°C), making it highly promising for next‐generation self‐powered wearable and portable electronics.

1. Introduction

Rapid expansion in the Internet of Things (IoT), wearable electronics, and distributed sensor systems has led to a growing need for small, flexible, and high‐performance on‐chip energy storage components [1, 2, 3, 4]. Among the various devices, microsupercapacitors (MSCs) are considered to be excellent power sources because they have a high‐power density, can be charged and discharged very quickly, are highly stable, and are highly compatible with miniaturized electronic architectures [5, 6, 7]. Still, large‐scale production of MSCs capable of high energy density and effective ion transport simultaneously is a real challenge, especially for devices that are made up of printable electrode arrangements [8].

Currently, spraying, plasma cutting, laser engraving, and other patterning methods are the most common processing techniques to fabricate supercapacitors. Despite their effectiveness, such methods generally require complex multistep processing, advanced equipment, and have limited scalability, all of which can lead to higher manufacturing costs and longer processing times [9, 10]. In comparison, direct ink‐based printing technologies present a simpler and more versatile solution by permitting the direct laying down of the active materials on different substrates [11]. Extrusion printing, inkjet printing, laser direct writing, and screen printing are some of the methods that not only help in achieving electrode designs with precision but also substantially lower the complexity of fabrication [12, 13]. Among them, screen printing is particularly appealing due to its low cost, high throughput, and suitability for large‐area production. It allows the rapid fabrication of supercapacitors with customizable shapes and dimensions using functional inks [14, 15, 16]. However, most conventional printable inks require binders or rheology modifiers and additional post‐printing treatments, such as thermal annealing, to achieve adequate electrical performance [17]. Therefore, developing additive‐free ink systems that enable room‐temperature printing is highly important for advancing scalable screen‐printed supercapacitor technologies.

Among various electrode materials explored for MSCs, carbon nanotubes (CNTs), with their one‐dimensional tubular morphology, continuum of conductive pathways, and mechanical flexibility, represent a class of carbon nanomaterials that can significantly enhance MSC performance. The high electron mobility, structural robustness, and large theoretical surface area of CNT networks facilitate efficient charge transport and electrolyte ion diffusion, both crucial for achieving superior electrochemical kinetics and high‐power densities [18]. Numerous studies have shown that CNT‐based electrodes can successfully enhance the performance of micro‐supercapacitors by increasing the accessible surface area through structural engineering or hybridization with pseudocapacitive materials. In contrast to other cutting‐edge hybrid nanostructures, pristine CNTs may have a lower capacitance due to their comparatively small number of electrochemically active sites [19, 20].

Although the enlargement of ion diffusion channels markedly improves the capacitance, the resultant energy density of the fabricated device remains limited. This limitation can mainly be explained by the fact that the electrochemical stability window (ESW) of the aqueous gel electrolyte is not wide enough, thus limiting the operating voltage that can be attained [21, 22]. A highly concentrated “water‐in‐salt” (WIS) electrolyte system, in which the amount of salt is greater than that of the solvent in both mass and volume, has lately been introduced as a very good option for next‐generation electrochemical energy storage devices [23]. For instance, a 21 mol/kg lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte has attracted a lot of attention due to its wide ESW and non‐flammability features [24, 25]. At such high salt concentration, most of the water molecules are bound strongly to lithium ions, thus, stable solvation structures are formed. Hence, the activity of free water is lowered to a great extent, resulting in the suppression of chemical reactions. The hydrogen evolution reaction (HER), in particular, is almost eliminated, which leads to a significant broadening of the ESW [26, 27]. While LiTFSI‐based WIS electrolytes are naturally hygroscopic, getting moisture from the air does not always affect the ESW of the WIS. In concentrated WIS systems, the strong ion‐dipole interaction between the Li⁺ ions and water molecules forms a tightly‐held solvation shell around the Li⁺ ions that remains even after moderate amounts of moisture have been taken into the WIS [23, 28]. Owing to the highly concentrated nature of the WIS electrolyte and the relatively small amount of moisture adsorbed from the surrounding environment, the bulk electrolyte composition remains largely unchanged. Consequently, the wide ESW of the WIS electrolyte is effectively preserved, while the interfacial adsorption of water provides an additional pathway for humidity‐driven charge generation and transport. The humidity‐driven self‐charging behavior originates from the adsorption of a limited amount of atmospheric water at the electrode/electrolyte interface and within the porous CNT network, rather than from extensive dilution of the bulk electrolyte [29]. When moisture is taken from the environment, excess moisture will coordinate into the current Li⁺ solvation structures instead of being free water, and therefore will not compromise the suppression of the hydrogen evolution reaction and the maintenance of a wide ESW [29].

Besides traditional externally charged systems, there has also been work on self, powered MSCs that collect their energy from the environment, especially the moisture in the air, thus operating independently. Energy production with moisture is based on the contact of water molecules with ion‐conductive materials. A multi‐step physicochemical process can be used to illustrate how the humidity‐driven, self‐charging mechanism functions in CNT‐WIS composites. When the ambient environment has moisture, the water molecules will be attracted to the CNT network's surface and will adsorb to the surfaces, the adsorbed water molecules facilitate dissociation and ion transport, thereby enhancing ionic conductivity and promoting electrostatic charge accumulation during humidity exposure and form a directional ionic current via a Grotthuss hopping mechanism [28, 30]. The hygroscopic ionic liquid LiTFSI in the CNT‐WIS composite will also absorb moisture from the ambient environment, allowing the dissociation of additional Li⁺ and TFSI⁻ ions and further establishing a potential difference between the electrodes [31, 32]. This interaction results in the movement of ions and the formation of a voltage difference between the electrodes [33]. Carbon nanomaterials, such as CNTs, based networks still represent very attractive materials in this track due to their superior intrinsic electrical conductivity, especially CNT films, which have a large specific surface area and strong adsorption affinity for water molecules. Also, CNT‐based architectures, upon the addition of hygroscopic salts or working polymers, can convert changes in humidity into measurable electrical signals through ion diffusion and electrochemical potential differences [31, 32].

Herein, we report the development of screen‐printable CNT‐WIS composite inks targeted for the fabrication of advanced CNT‐based quasi‐solid‐state MSCs. The ink composition was deliberately set to introduce Li⁺ ions in the CNT matrix, which will then serve as nanoscale separators in the conductive network. This modification of the CNT structure engenders more accessible pathways for the ions throughout the electrode, diminishes bundling of nanotubes, and favors infiltration of electrolyte. By raising the level at which the electroactive surfaces get exposed and facilitating a faster ionic mobility in the WIS electrolyte environment confines, the tailored setting ultimately results in higher charge storage ability and faster redox reactions.

2. Experimental Section

2.1. Insertion of Li⁺ Ions into CNTs

High‐purity CNTs were procured from Sigma‐Aldrich (purity > 95%) with a specific surface area of >1075 m²/g and used as received without any further purification. Initially, 0.21 g of CNTs was added to 30 cm³ of deionized water and sonicated at 240 W for 2 h, followed by centrifugation for 1 h at 2500 rpm. The supernatant was collected and further centrifuged at 10000 rpm for 1 h, and then an aqueous lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solution (21 mol/kg) was added and ultrasonicated for 4 h to form a homogeneous Li‐modified CNT ink (Li–CNT ink). Subsequently, various LiTFSI concentrations (2–10 wt%, relative to the mass of CNT)in the Li–CNT ink as a WIS gel electrolyte were systematically investigated, where Li‐CNT_1 (2 wt%), Li‐CNT_2 (5 wt%), Li‐CNT_3 (10 wt%) refer to the samples with increasing Li⁺‐ion concentration. Finally, the obtained Li–CNT ink was stored at a low temperature to maintain the viscosity and ensure long‐term dispersion stability.

2.2. MSC Preparation via Screen Printing

A (32 × 32 cm²) screen plate was initially fixed onto the printing table, and Whatman filter paper (Advantec JIS P 3801 5C) was utilized as a substrate for printing. The synthesized Li‐CNT ink was brushed onto the mesh and uniformly coated with a flat scraper at an angle of nearly 45° with the screen plate. Upon coating, the devices were vacuum‐dried at 60°C for 24 h, and the electrode parts were coated with a WIS gel electrolyte containing LiTFSI. The WIS gel electrolyte was prepared by first dissolving LiTFSI in deionized water to obtain a 21 m LiTFSI electrolyte. Subsequently, poly(vinyl alcohol) (PVA) was added to the as‐prepared LiTFSI electrolyte at a mass ratio of 10:1 (LiTFSI solution:PVA). Ethylene glycol (EG) was then added in small amount to the mixture to facilitate the complete dissolution of PVA and to impart antifreezing properties to the gel electrolyte. The amount of EG was adjusted to 30 wt% of the solvent composition. The resulting mixture was heated at 80°C under continuous stirring at 600 rpm for 4 h until a homogeneous transparent gel was obtained. Due to the excellent conductivity of CNTs, they can be used as an electrode directly on a substrate without a conductive layer as a current collector. To ensure good electrical contact, electrochemical measurements were made with gold paste and gold wires sealed.

2.3. Electrochemical Characterization

Electrochemical characterization was carried out using BioLogic SP‐150. The device behavior was examined using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). All measurements were conducted in a two‐electrode system, where the micro‐supercapacitor operated with a LiTFSI (WIS) electrolyte. The devices were examined in a potential window of 1.5 V at various CV scan rates (5 to 100 mV/s) and GCD current densities (0.01 to 0.15 mV/cm²). The specific areal capacitance (mF/cm²) based on the CV curve was determined using the formula

$$C_s=\frac{\int I\left(V\right)dV}{2Av\mathrm{\Delta }V}$$ (1)

where, $\int I(V)dV$ represents the area under the CV loop, A is the area of the electrode, ν is the scan rate, and ΔV illustrates the applied potential window.

The specific areal capacitance obtained from the GCD curves (Cs [mF/cm²]) were determined using

$$C_{\mathrm{s}}=\frac{I\times \mathrm{\Delta }t}{\mathrm{\Delta }V\times A}$$ (2)

The corresponding areal energy density (E [µWh/cm²]) and power density (P [µW/cm²]) were obtained from,

$$E=\frac{C_s{\left(\mathrm{\Delta }V\right)}^2}{2\times 3.6}$$ (3)

$$P=\frac{E\times 3600}{\mathrm{\Delta }t}$$ (4)

3. Results and Discussion

3.1. Design and Fabrication of Li–CNT Based MSC

Figure 1 depicts the conceptual framework used in the fabrication of LiTFSI‐functionalized CNT ink‐based MSCs. The devices were developed by screen‐printing the prepared ink onto a Whatman filter paper substrate (see Figure 1a,b). The schematic illustrates the introduction of LiTFSI into the CNT network. Nanotube bundles had small radii, allowing the distribution of lithium ions (Li⁺) along and between the bundles (Figure 1c). The presence of Li⁺ in the CNT framework improves separation within adjacent nanotubes, minimizing agglomeration and improving surface accessibility. The interaction between Li⁺ ions and the CNT framework enhances ionic compatibility in the ink system. Li⁺ species form interconnected hydration structures with surrounding water molecules, resulting in three‐dimensional ion transport pathways. These hydrated ionic networks allow for rapid ion migration throughout the electrode matrix, which contributes to an increased charge storage capacity. Moreover, the confined water molecules within the CNT network demonstrated long‐term cycling durability.

FIGURE 1.

(a) Schematic illustration of Li–CNT‐based MSC fabrication, (b) fabricated electrodes over paper substrate, (c) schematic of intertubular separation by Li⁺‐ion insertion, (d) gel electrolyte coating over electrode, and (e) scalable fabrication using Li‐CNT ink.

The screen‐printing process was used to fabricate interdigitated electrodes, followed by coating of gel over the electrodes (Figure 1d). Furthermore, the magnified image of the printed region clearly distinguishes between the electrode layer and the paper substrate.

Following ion incorporation, the Li⁺‐modified CNT dispersion was further processed into printable conductive inks. This approach enables scalable fabrication of patterned interdigital electrodes (see Figure 1e).

3.2. Structural and Morphological Characteristics

The optical microscopic image, as shown in Figure 2a, reveals the surface characteristics of the CNT ink deposited by screen printing onto the paper substrate. The image depicts a continuous, interlinked network of nanotubes with distinct surface roughness. The enlarged inset clearly distinguishes the printed electrode from the paper substrate region, verifying consistent coating of the CNT layer. The 3D topographical map shows how the height changes over the printed film. The colour scale represents thickness variation, which can be seen in Figure 2b. The surface has some uneven features instead of being a smooth morphology. Such topographical changes help electrolyte infiltration and increase the electrode, electrolyte contact area, thus resulting in better electrochemical activity. Figure 2c displays a screen‐printed interdigitated micro‐supercapacitor fabricated on a Whatman filter paper substrate.

FIGURE 2.

(a) Height profiles with (b) contour plot Li–CNT ink‐coated substrates, (c) image of the fabricated MSC, (d) XRD plots of bare and coated substrates, (e) synthesized Li–CNT ink, (f) viscosity plot as a function of shear rate, and (g) zeta potentials of CNT and Li‐CNT_2 inks.

The structural properties of the Whatman filter paper substrate, CNT powder, printed CNT film, and Li‐modified (2%, 5%, and 10%) CNT films were investigated using X‐ray diffraction, as shown in Figure 2d. The XRD pattern of untreated Whatman filter paper displays an obvious peak around 2θ ≈ 22°, which is assigned to the (200) plane of crystalline cellulose. Besides, there are faint reflections within the 15–17° range, which are considered the (110) planes. These features indicate that the paper substrate has a semi‐crystalline structure [34]. The CNT powder has a significantly broad peak at 2θ ≈ 26°, which is mainly attributed to the (002) plane of graphitic carbon. This peak is due to the graphene layers being stacked between the multiwalled CNT structures [35, 36]. After embedding the CNT ink into the paper substrate, the characteristic (002) plane peak was still visible, thus confirming the formation of a conductive CNT layer on the filter paper. A detailed look at the zoomed, in (002) region shows systematic changes after Li modification. Compared to pristine CNT, the Li–CNT samples show a slight shift of the (002) plane peak toward lower diffraction angles. This shift signifies a larger space between layers, which can be explained by lithium ions interacting with the CNT network and thus expanding the graphitic stacking domains. With increasing the Li content from Li‐CNT_1 (2% Li) to Li‐CNT_3 (10% Li), a broadening of the peak is observed, suggesting increased structural disorder or induced microstrain within the modified CNT framework.

Figure 2e illustrates the process of preparing the CNT‐based ink. The dispersion appeared dark and uniform. This homogeneous appearance indicates effective dispersion of nanotubes within the solvent mixture, which is essential for consistent printing performance.

The rheological properties of 5% Li, modified (Li‐CNT_2) and unmodified CNT inks were first compared. As we can see from the relative viscosity change over the shear rate graph in Figure 2f, both components show very strong shear‐thinning behavior. The viscosity continued to decrease as the shear rate increased, which is due to the slow orientation and rearrangement of the CNT network [37].

This response confirms that both inks behaved as non‐Newtonian fluids, a desirable feature for screen‐printing applications because it enables smooth flow under applied force while maintaining structural integrity after deposition [38]. Notably, the Li‐CNT_2 ink displayed a significantly higher maximum viscosity (10.7 Pa·s) than the pristine CNT ink (8.4 Pa·s), indicating stronger internal interactions and a more developed network structure within the dispersion. To create stable and uniform CNT ink dispersions, it is very important to have a very high negative zeta potential. The zeta potentials of CNT and Li–CNT determined (see Figure 2g) were −37.6 and −36.8 mV, respectively, which means there is strong electrostatic stabilization in both cases. The negatively charged surface of the CNT network provides a large number of active sites for the interaction between the surface and the positively charged hydrated Li⁺ ions. Through electrostatic attraction, these Li⁺ ions will be adsorbed on the CNT surface, causing a potential difference in that area. Such interaction results in the formation of an electric double layer (EDL) at the interface that not only helps the dispersion remain stable but also changes the ions' movement within the conductive network [9].

FESEM was used to examine the structural and morphological properties of the Li–CNT‐deposited electrodes. Figure 3a shows the FESEM image of a printed Li–CNT electrode on a paper substrate. The uniform dark region represents the Li–CNT coating, whereas the nearby lighter area reflects the base paper substrate. Figure 3b demonstrates the higher magnified image of the electrode and substrate, which reveals a clear interface. The red boundary denotes the electrode‐substrate junction, demonstrating good adhesion and intimate contact of the Li–CNT layer with the fibrous paper matrix. A porous paper substrate was chosen for its interlinked fibrous network and pores, as shown in Figure 3c. The active material is incorporated into the pores and facilitates efficient electrolyte transport. Figure 3d demonstrates the thickness of interdigitated electrodes. Higher resolution micrograph shown in the inset of Figure S1 reveals a rough and textured surface morphology following lithium incorporation, which indicates an increase in defect density and irregularity on the surface. Additionally, the HRTEM images reveal that the interlayer spacing increases from approximately 0.342 nm for the pristine CNTs to 0.356 nm after Li treatment, indicating a slight expansion of the graphitic layers as demonstrated in Figures 3e and S2. Furthermore, the (002) diffraction peak in the XRD pattern exhibits a small shift toward a lower diffraction angle, which is consistent with an increase in the average interlayer spacing according to Bragg's law. These observations suggest that Li⁺ ions are incorporated within the graphitic domains and defect‐rich regions of the CNT network, leading to a slight expansion of the graphitic layers. The successful incorporation of Li⁺ can be attributed to its small ionic radius, which enables the ions to penetrate the defect sites and interstitial graphitic regions of the CNT network with only a minor expansion of the (002) lattice spacing and the inter‐bundle spacing. As per the observation, it is stated that the insertion of lithium ions will possibly be held by interstitial channeling within the CNTs in the tubes [39]. The process of ion insertion is expected to modify the spacing of the van der Waals toward expanded CNT bundles [40]. The observed peak shift in XRD may be obtained from the possible electrostatic interaction between Li ions and the delocalized π electrons along the axis of CNT, which produce the axial and radial strains of C‐C bonds. The nanoscale variations in the structure were further confirmed by the height profile image, as shown in Figure 3g, which promotes the improved charge storage behavior. As illustrated in Figure 3f, the hydrophilicity of CNT increases following pre‐insertion with the WIS electrolyte. This improvement is reflected by a reduction in the contact angle from 48° for pure CNT to 39° for Li‐modified CNT. As a result of its stronger affinity for water, Li–CNT readily forms stable aqueous inks without requiring additional binders, making it suitable for subsequent screen printing.

FIGURE 3.

FESEM images of (a) fabricated MSC, (b) electrode–substrate interface, (c) bare substrate, (d) magnified IDE pattern (inset: electrode thickness), (e) HRTEM image of Li‐CNT with d‐spacing, (f) contact angle measurements of pure CNT and Li–CNT, (g) sensor height profile using AFM, and XPS spectra of (h) C 1s and (i) Li 1s with peak deconvolution.

X‐ray photoelectron spectroscopy (XPS) analysis was carried out to understand the chemical states of the synthesized materials. The high‐resolution spectra of C 1s and Li 1s is shown in Figure 3h,i, confirming the successful formation of Li‐modified CNT. The C 1s spectrum was deconvoluted into six prominent peaks at 281.6, 282.4, 238.2, 284.1, 285.0, and 289.3 eV, which correspond to C = C, C‐C, C‐OH, C‐O‐C, C = O groups, and π‐π^* transition, respectively [41]. Furthermore, the peak at 53.7 eV in the Li 1s spectrum reveals the successful insertion of Li⁺ ions into the CNT matrix.

3.3. Electrochemical Performance of Li‐CNT Inks

Flexible coplanar quasi‐solid‐state MSCs were fabricated by screen printing of Li–CNT‐based inks on a paper substrate, followed by coating with a WIS electrolyte. The ion transport kinetics of pristine CNT and Li⁺‐ion‐modulated CNT electrodes were comprehensively evaluated using different techniques like CV, GCD, and EIS. CV measurements of the fabricated electrodes were conducted over a wide range of scan rates, and all the obtained results revealed that Li–CNT electrodes exhibited well‐defined, symmetric, and quasi‐rectangular voltammograms across an extended potential range of −0.5 to 1.0 V. As observed in Figure 4a,b, the pristine CNT electrode showed lower current densities and more CV curve distortion at elevated scan rates, reflecting the less or limited access to the active sites and lower rate capability. After the incorporation of Li⁺, an enhanced current density was observed (see Figure 4c–f), which is attributed to the introduction of an additional charge‐storage pathway created by Li⁺, by making a significant gap within the two layers of CNT [42]. Among all the fabricated electrodes using various concentrations of Li⁺ ions, Li‐CNT_2 exhibited the largest enclosed CV area at all scan rates (see Figure 4c). At a lower concentration of Li⁺ added into the CNT network (Li‐CNT_1), a moderate enhancement in the areal capacitance with increased discharging time (see Figure 4d) suggests that the number of Li⁺‐induced active sites is insufficient to fully exploit the electrochemically available surface area. Parallelly, excessive Li⁺ incorporation (Li‐CNT_3) likely resulted in partial pore blockage and an increase in the charge‐transfer resistance [43, 44], which, in combination, hinders electrolyte penetration and the process of charge‐transfer. Out of all the Li⁺ concentrations, Li‐CNT_2 showed an enhanced capacitive response contributed to the synergistic combination of surface‐controlled electric double‐layer capacitance and pseudocapacitive contributions. The observed results define the critical role of optimizing the Li⁺⁻ion concentration in CNT electrodes.

FIGURE 4.

(a) CV and (b) GCD curves of pristine CNT, comparison analysis of (c) CV and (d) GCD curves with various Li⁺ concentrations, (e) CV and (f) GCD curves, and areal capacitances with (g) current density and (h) scan rate of Li‐CNT_2, and (i) comparison of areal capacitance of pristine CNT and Li‐CNT_2.

As shown in Figure 4b, GCD curves of the pristine CNT electrode were measured at current densities ranging of 10–150 µA/cm². It shows comparatively lower discharging time and a noticeable IR drop at the onset of discharging confirms it higher internal resistance and lower efficiency of charge storage. The enhanced polarization with increasing current densities reflects the limitation of kinetics with transport of ion and accumulation of charge at interface of electrode and electrolyte. The electrode with the optimum concentration of Li⁺ ions, that is, the Li‐CNT_2 electrode, exhibits markedly improved capacitive performance, as observed from its CV and GCD curves (see Figure 4e,f). The recorded CV curves at identical scan rates (Figure 4e) displays a tremendous increase in the current density and also improved rectangularity compared to pristine CNT. The characteristics indicated a substantial improvement upon Li⁺ modification. Additionally, the modification of Li⁺ ions improved interfacial wettability and stabilized charge storage at defect‐rich carbon surfaces, resulting in a combined Electric Double Layer Capacitor (EDLC)‐pseudocapacitive behavior [45, 46]. A similar trend of improved performance was observed in case of GCD curves (see Figure 4f) with nearly symmetric and linear curves of charge‐discharge. The minimal IR drop at the beginning of the discharging indicated reduced equivalent series resistance. Notably, even at the highest current density, Li‐CNT_2 preserved stable voltage profiles, underscoring its excellent reversibility and structural stability under fast charge‐discharge operation. The current‐ density dependence of the areal capacitance of Li‐CNT_2 is shown in Figure 4g. As the current density increased from 0.01 to 0.15 mA/cm², the areal capacitance gradually decreased from 40.8 to 14.2 mF/cm². A similar trend was observed in the scan‐rate dependent capacitance analysis (Figure 4h), where the areal capacitance of Li‐CNT_2 decreased with increasing scan rate from 5 to 100 mV/s. The radar plot demonstrated in Figure 4i offers a comprehensive comparison of pristine CNT and Li‐CNT_2 electrodes by simultaneously comparing areal capacitance and current density. The larger enclosed area of Li‐CNT_2 than that of pristine CNT undoubtedly confirms its superior multidimensional electrochemical performance. A table of comparison shown in table S1 also confirms its superior performance as compared to other MSCs not only in terms of capacitance but also in the following aspects: (i) the introduction of a simple and effective Li⁺ insertion strategy; (ii) the realization of a humidity‐driven self‐charging mechanism; (iii) the electrochemical performance at extreme temperatures; and (iv) the excellent flexibility and long‐term cycling stability of the fabricated microsupercapacitor.

As shown in the Nyquist plot (Figure S3), Li CNT_2 showed a lower charge transfer resistance, indicating better electronic conductivity and quicker ion transport, both of which enhance its overall electrochemical performance. The long‐term cycling durability of electrode materials is a key factor in improving the efficiency and reliability of energy storage systems, especially for supercapacitor applications. In this context, Figure 5a represents the long‐term cycling performance of the Li‐CNT_2 electrode over approximately nine days, extending upto 20,000 successive charge‐discharge cycles at a current density of 0.01 mA/cm². According to the results, the electrode capacity remained constant over 20,000 cycles, with no significant loss of capacitance and a retention of 95% over that period, demonstrating excellent long‐term stability.

FIGURE 5.

(a) Cyclic stability of Li‐CNT_2 MSC up to 20,000 cycles, (b) XRD pattern of the MSC after multiple cycles, (c,d) SEM image of Li‐CNT_2 before and after retention, (e) Ragone plot of fabricated electrode with other works, and (f) comparison of capacitive and diffusive contributions.

Furthermore, the GCD curves before and after 20,000 repetitive cycles are demonstrated in Figures S4 and S5. This performance can be attributed to the porous structure of Li‐modified CNT, which provides a microporous network, an improved electrical pathway, and an increased active surface. These characteristics enhance overall electrochemical behavior by facilitating effective electron mobility and quick ion diffusion. Figure 5b displays the XRD patterns recorded after different cycle numbers, ranging from 10 to 20,000 cycles. The diffraction peaks remained unchanged with increasing cyclability and the absence of noticeable peak shifts, suggesting structural durability during charge–discharge processes. Furthermore, Figure 5c,d demonstrate the surface morphologies of the electrodes before and after the cycling tests. Prior to cycling, the electrode exhibited a uniform material distribution and a well‐defined porous structure. After prolonged cycling, the morphology remained largely intact. The clear distinction between the substrate and electrode regions after cycling indicates strong adhesion and mechanical integrity. Also, the cyclic measurement was also performed to determine the stability of the device after 40 days, as demonstrated in Figure S6.

Moreover, the areal energy density (E) and power density (P) were determined from the GCD measurements and are illustrated via a Ragone plot, as shown in Figure 5e. The resulting plot highlights the device's ability to deliver both high energy and power density. The electrochemical characteristics significantly surpassed those of previously reported electrodes, as shown in Figure 5e [47, 48, 49, 50, 51, 52, 53]. Furthermore, Dunn's method was applied to investigate the charge storage behavior of the Li‐CNT electrode through detailed analysis of its CV profiles. In accordance with the power law,

$$I=a{V}^b$$ (5)

where ‘a’ and ‘b’ represent the tunable parameters. The value of ‘b’ was determined from the slope of the log(I) vs. log(scan rate) plot, as shown in Figure S7. The value of ‘b’ provides insight into the dominant charge storage mechanism of the electrode material. A value approaching 0.5 suggests that the storage mechanism is mainly governed by ion diffusion, while a value near 1 indicates capacitive‐dominated behavior, generally for EDLC or pseudocapacitive storage. In the present instance, as shown in Figure S7, the calculated slope value of 1.0624 (nearly 1) indicates that surface‐controlled charge storage predominates, while diffusion‐controlled processes make up a small contribution.

The respective contributions from surface‐controlled and diffusive‐governed capacitance were determined by applying the Trasatti approach

$$I\left(V\right)=k_{1}V+K_2{V}^{1/2}$$ (6)

The terms k ₁ V and k ₂ V ^1/2 correspond to the capacitive and diffusive contributions, respectively, where k ₁ and k ₂ are determined from the slope and intercept of the plot between I/k ₂ V ^1/2 and V ^1/2 (see Figure S8). Figure 5f illustrates the combined effects of both capacitive and diffusive contributions for the Li–CNT electrode at a scan rate of 5 mV/s. It can be seen from Figure S9, as the scan rate increases from 5 to 100 mV/s, the capacitive contribution increases from 93.5% to 98.4%.

3.4. Device Integration and Environmental Sustainability of MSCs

The most critical parameters for commercialization of the device and practical viability of MSCs in real‐world applications are its scalability, device‐to‐device statistical analysis, and environmental robustness, as shown in Figure 6a–h). The charge storage behavior of a combination of microsupercapacitor was evaluated by CV and GCD curves, as shown in Figure 6a,b. The CV curves of multiple electrode combinations (1 MSC, 2 MSCs, 3 MSCs) exhibit nearly identical shapes, with progressively increasing current densities as the number of devices connected increases. Similarly, the GCD characteristics further corroborated these findings (Figure 6b). The curve of GCD exhibited symmetric and linear profiles for the combination of multiple devices with increasing discharge time systematically from single to multiple MSCs. The absence of IR drops or curve distortion evidenced robust interfacial integrity, enabling reliable integration of Li‐CNT_2‐based MSCs to tailor output performance for microelectronic and wearable applications. The obtained results confirmed that the fabricated Li‐CNT_2 device could be reliably combined to tailor the output performance without any change in the electrochemical integrity, which is essential for powering microelectronics. To determine the MSC scalability and fabrication reproducibility, CV and GCD measurements were conducted for device‐to‐device consistency (see Figure 6c). For a similar set of devices, Figure 6d shows their GCD profiles. The symmetric nature and nearly overlapping behavior of both CV and GCD curves indicated excellent reproducibility and high uniformity in electrode fabrication and device assembly processes. A quantitative areal capacitance comparison derived from the GCD curves is shown in Figure 6e.

FIGURE 6.

(a) CV and (b) GCD curves of multiple combined MSCs, (c) CV and (d) GCD curves of multiple fabricated Li‐CNT_2 MSCs, with their (e) areal capacitance, (f) CV curves under different bending conditions, (g) experimental setup for temperature dependent electrochemical property and (h) CV curves at different operating temperatures.

The areal capacitance values for different devices clustered within a narrow range, with only minor statistical deviations. In addition, the error bars reflected minimal performance dispersion, coming to the conclusion of Li‐CNT_2‐based MSCs are highly reproducible, a prerequisite for large scale manufacturing and integration of the device. Finally, to determine the mechanical robustness of the biodegradable Li‐CNT_2 device, multiple bending tests were performed with 0°, 45°, and 90° bending, and after repeated bending of 1000 times, as shown in Figure 6f. Even after the serve mechanical stress, the CV curve shape retained and exhibited minimal variation in current density. This shows that with such deformation in the shape of the MSC, no significant change was observed in area enclosed and in ionic conduction. Almost every supercapacitor performance failed at elevated temperatures, and negligible reports are there on sub‐zero temperature, due to the loss of ionic conducting medium [54]. Figure 6g shows the experimental setup used for the temperature dependent electrochemical measurement. So, to determine the thermal stability of the device, CV measurements of Li‐CNT_2‐based MSCs were evaluated at varied temperatures as shown in Figure 6h, spanning ultralow conditions (−60°C) to elevated temperatures (80°C). With an increase in temperature, the CV curves exhibited increased current density and slight expansion in the enclosed area, which was caused by the electrolyte's improved ionic conductivity and strong electrode–electrolyte behavior [55, 56].

Conversely, at lower temperatures (20°C to −60°C), a gradual reduction in current density was observed, reflecting a slow decrease in ionic mobility. Despite the reduction in current density, the CV curves retained their characteristic shape even at −60°C, demonstrating high thermal stability under such extreme temperature conditions. Also, the areal capacitance has been demonstrated with varied temperature at a scan rate of 100 mV/s (see Figure S10). To determine the thermal stability of the electrode interface and the rheological property of the electrolyte, SEM, DSC (Differential Scanning Calorimetry) and rheology measurements were performed. Specifically, cross‐sectional SEM images (Figure S11) of the microsupercapacitor were obtained after electrode treatment at −60°C, room temperature, and 80°C. The SEM observations reveal that the electrode/substrate architecture remains well preserved over the entire temperature range, with no discernible delamination, cracking, or structural degradation at the electrode‐substrate interface, demonstrating excellent mechanical and interfacial stability. Furthermore, DSC measurement was carried out to investigate the thermal behavior of the gel electrolyte (Figure S12). The DSC thermogram shows no significant crystallization or melting transitions between −60°C and 80°C, suggesting that the electrolyte remains predominantly amorphous throughout the investigated temperature range. A very weak thermal feature is observed near 20°C, however, its intensity is extremely small and does not correspond to a major phase transition. Instead, it is likely associated with a minor polymer‐chain relaxation or subtle structural rearrangement within the gel matrix. These results support the conclusion that electrolyte freezing is effectively suppressed under the experimental conditions. In addition, temperature‐dependent viscosity measurements of the gel electrolyte were performed to evaluate its rheological behavior (Figure S13). The viscosity results demonstrate that the electrolyte maintains suitable flow characteristics over a broad temperature range with no significant change in the viscosity, providing favorable conditions for ion transport and supporting stable electrochemical operation from −60°C to 80°C.

The fabricated Li‐CNT_2 device not only works as a normal MSC but also as a moisture‐assisted energy harvester, where the environmental humidity condition actively contributes to the energy generation and influences the charge–discharge dynamics (see Figure 7a–i). As demonstrated in Figure 7a, the charging curves slope rises more steeply with increasing humidity. In the present system, adsorption of moisture simultaneously generates an intrinsic electrochemical potential that adds to the externally applied charging bias. In other words, the voltage induced by the moisture effectively reduces the burden on the external power source. The same phenomenon was observed during the discharging behavior, as shown in Figure 7b, where discharging time increases significantly with increased humidity, the continuous adsorption and redistribution of water persist, effectively slowing the rate of decay. Due to the concurrent energy harvesting, the device does not simply behave as passive capacitor instead it acts as a hybrid system, reflecting the superposition of energy storage and hydrovoltaic energy generator. To determine the voltage generation with increasing humidity, the voltage was measured as a function of time while varying the relative humidity (see Figure 7c,d). Without giving external bias, an open circuit voltage was measured with a stepwise increase in relative humidity. The measurable amount of voltage contributes proportionally to the level of moisture concentration. This integration of moisture enabled energy generation with microsupercapacitor behavior using a single architecture, demonstrating a significant advancement towards self‐powered systems.

FIGURE 7.

Galvanostatic (a) charge and (b) discharge curves of Li‐CNT_2 at different humidity levels, (c,d) voltage generation profiles under stepwise humidity exposure, (e) moisture‐enabling response at 90%RH, (f) KPFM measurement in presence of varied relative humidity, (g) ESI spectra at different humidity, (h) schematic illustration of moisture‐enabled ion diffusion mechanism, and (i) wearable device integration with powering LED.

Figure 7e shows the voltage sustainability under a constant humidity of 90%RH. The sharp rise and subsequent saturation plateau reflect the fast adsorption kinetics of H₂O molecules onto the active surface of the Li–CNT network. The presence of oxygen‐containing groups on CNTs, together with intrinsic nanopores and voids between CNT networks, encourages the creation of charged nanochannels when water molecules are present (see Figure 7h). As water moves through these channels via capillary action and transpiration, protons are guided along the negatively charged CNT surfaces, establishing an ion concentration gradient and generating a streaming potential between the wet and dry ends of the device. Specifically, once water is introduced into one end of the CNT‐coated substrate, water permeation within the CNT/filter paper nanochannels occurs, which then reacts with the channel surfaces, producing H⁺ ions. Lithium ions, due to their strong hygroscopic nature, readily form hydrated ions with water molecules, enhancing the ionic conductivity within the ion‐conducting material. The Li ions rapidly absorb moisture and partially dissociate into mobile ions, establishing both a water content gradient and an ion concentration gradient within the structure. Consequently, an EDL forms when the positively charged counterions align with the negatively charged oxygen‐containing functional groups on the nanochannel walls of the Li–CNT/filter paper composite. Along with the water, the majority of cations can flow through the channels and pores; however, the Debye screening effect and the formation of an electrical double layer oppose the anions. Once a balance between adsorbed water and exhaled vapor is achieved, a persistent water‐flowing cycle without a saturation adsorption state is created because the transported water is expelled in the evaporative layer. The directional movement of these positive ions along the water pathway, facilitated by capillary action, produces an ion concentration gradient, which consequently creates a potential difference between the dry and wet areas, enabling the generation of electrical output [57, 58, 59]. The proposed mechanism is supported by experimental characterizations. Specifically, the KPFM measurements demonstrate a progressive increase in surface potential with increasing humidity, indicating humidity‐induced charge accumulation. KPFM measurement (Figure 7f) was performed to directly investigate the evolution of the local surface potential under humid conditions. KPFM provides spatially resolved information on the contact potential difference (CPD), which is highly sensitive to the local charge distribution and surface electronic states. As shown in Figure 7f, the surface potential increases markedly from 65.4 to 293.9 mV under high relative humidity, indicating a significant increase in the electrostatic potential across the Li‐treated CNT network. Also, the electrochemical impedance spectroscopy (EIS) results reveal a decrease in impedance under higher relative humidity (see Figure 7g), confirming enhanced ion transport and reduced charge‐transfer resistance resulting from the adsorption of water molecules. Furthermore, the carrier concentration measurements show a significant increase with increasing humidity, providing direct evidence that humidity promotes the generation and transport of charge carriers within the Li‐treated CNT network (see Figure S14). As shown in the Figure S15, the diffraction patterns remain essentially unchanged throughout the humidity exposure period, with no noticeable peak shift or the appearance of additional phases. These results confirm that the crystal structure of the active material remains stable even after prolonged exposure to a humid environment, demonstrating excellent structural robustness against moisture.

Further, the fabricated devices were integrated with the flexible band to make it into a wearable format (see Figure 7i). The image confirms the mechanical adaptability and flexibility. The inset image shows the glow of the LED powered by the integrated device. In addition, Figure 7i provides the power generation capability by showing the device connected to an external load. By connecting a combination of MSCs, it can be manufactured on a broad scale while maintaining high‐precision printing.

3.5. Analysis Under Actual Operation of MSCs

To determine the structural behavior of the electrode after charging and discharging, XPS analysis of Li‐CNT_2 was performed before and after charging, as well as after discharging, as shown in Figure 8a–c. At the initial stage, the dominant component at 281.4 eV corresponds to graphitic carbon (C = C), which is characteristic of a well‐conjugated π‐electron system. Upon electrochemically charging, a significant C 1s peak shift was observed towards a lower binding energy. This blue shift signifies a reduction in the core‐level binding energy of carbon atoms, which is explained by the impact of a higher local charge density within the π‐conjugated framework. As in the charged state, electrochemical charging induces the accumulation of charge in the Li–CNT framework. This injection of charge elevates the Fermi level towards high energies. This electronic redistribution confirms the active participation of the carbon lattice in charge compensation [60, 61]. In addition, after discharging, the C 1s spectrum returns to its pristine state. This confirms the multiple‐time reusability of the electrodes while maintaining retention. Furthermore, Figure 8b shows the Li 1s peak under three different states. In the uncharged state, very weak and broad peak of the Li signal is observed, indicating the surface‐associated and loosely bound Li species. After charging, a significant peak enhancement is observed with a slight shift and sharper peak, which confirms the electrostatic interaction between Li⁺ ions and the CNT framework. The observed peak sharpening indicated the preferential confinement of Li⁺ within energetically favorable defect sites. After discharging, the Li peak intensity reduces to its initial state. This reversible modulation of Li chemical state supports a surface‐controlled adsorption–desorption mechanism. Figure 8d–h demonstrate the surface morphology and surface potential while charging and discharging. This film's height profile, as displayed in Figure 8d, confirms the continuous and uniform electrode formation. At the initial state, a very low potential was observed, indicating a uniform charge distribution prior to significant electrochemical polarization (see Figure 8e). In contrast, the surface potential magnitude rises to ∼0.85 V after electrochemical charging. The broadened distribution of potential and elevated amplitude reflects strong surface polarization induced by lithium adsorption under external bias (see Figure 8f). In addition, the work function decreases significantly (see Figure S16) as calculated using [62, 63]:

$$CPD=\frac{\mathrm{\Delta }\mathrm{\Phi }}{e}=\frac{{\mathrm{\Phi }}_{\mathrm{tip}}-{\mathrm{\Phi }}_{\mathrm{m}}}{e}$$ (7)

FIGURE 8.

XPS profiles of (a–c) C 1s and Li 1s before and after charging and discharging, (d) AFM image of CPD site, and KPFM 3D spectra of CPD at (e) initial state, (f) charged state, (g) first discharging state, and (h) second discharging state.

Where, Φ_tip is the work function of the tip, Φ_m is the material work function and e defines the elementary charge. This can be attributed to the upward Fermi level shift by charge accumulation and Li⁺‐stabilized interfacial dipole formation at defect‐rich sites. Corresponding to the discharging state, the surface potential returns to its pristine distribution (see Figure 8g), as does its work function. Importantly, the surface potential mapping demonstrates polarity inversion and a significant reduction in the accumulated positive charge, promoting the re‐injection of electrons into the framework and Li⁺ ion redistribution, as demonstrated in Figure 8h. This operando study shows that the Li‐CNT_2 electrode operates through a reversible adsorption‐driven electrostatic mechanism, enabling efficient charge storage behavior and cyclic behavior without structural degradation.

4. Conclusions

In this paper, we demonstrate a novel approach for direct screen printing of MSCs using lithium‐ion pre‐insertion CNT aqueous ink. The fabricated MSCs demonstrated impressive areal capacitance of 42 mF/cm² with retention of ∼95% even after 20,000 cycles. Capacitive performance at different environmental conditions was also tested, even under ultra‐low temperature (−60°C). Notably, at high humidity MSC device demonstrated efficient power output for self‐powered operation. This research innovates Li‐modified CNT‐based MSCs from three key perspectives: (1) Firstly, we introduce a novel strategy of inter‐tubular space expansion by insertion of hydrated lithium ions into electrode‐electrolyte composite ink. (2) Secondly, a highly concentrated LiTFSI WIS electrolyte with superior wettability was employed to widen the window of electrochemical stability (ESW), improving the device performance with enhanced energy density. (3) Finally, the additive‐free formulation of Li–CNT ink for scalable manufacturing and high‐precision preparation of MSCs via screen printing. We believe the formulation strategy provides a key pathway to sustainable and scalable energy devices for the future.

Supporting information

Supporting File: smll74674‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.