Trends Sci. 2026; 23(3): 12134

Introduction

The rising global requirement for advanced energy storage systems has become a major catalyst for research, focusing on the creation of devices that mitigate the performance trade-off between high-power supercapacitors and high-energy batteries [1]. Consequently, the escalating need for efficient clean energy storage in industrial applications has placed supercapacitors and batteries at the center of significant research and attention over the past decade [2]. The battery possesses a high energy density (~8 - 600 Wh/kg), but it is limited by poor cycling stability and low power density [3]. Meanwhile, a supercapacitor has high power density (10 kW/kg), long and stable cycle life, but poor energy density [4]. Hybrid asymmetric supercapacitors, or “supercapatteries,” are an emerging class of hybrid devices that synergistically combine the distinct advantages of 2 electrode types: The high energy capacity of a battery and the high power delivery (rapid kinetics) of a capacitor [5]. The supercapacitor’s EDLC electrode, which stores charge via ion adsorption at the electrolyte interface, acts as the power source. Conversely, the battery-type electrode, leveraging Faradaic redox characteristics, functions as the energy source [6]. The success of these devices hinges on the development of advanced electrode materials capable of sustaining both high power and high energy densities. In this context, 2-dimensional (2D) transition metal carbides and nitrides, known as MXenes, and specifically Ti₃C₂T_x, have garnered immense attention as a revolutionary material for electrochemical energy storage [7].

MXene, a 2-dimensional transition metal carbide, nitride, and carbonitride, is emerging as a promising electrode material for supercapacitors. Owing to its unique combination of metallic conductivity, a hydrophilic surface, and a layered structure analogous to graphene, Ti₃C₂T_x MXene offers an excellent platform for rapid ion transport and charge storage [8]. MXene was first discovered by Gogotsi at Drexel University in 2011 [9]. MXene (M_n+1X_nT_x) can be obtained through selective etching, which removes the A layer from the MAX (M_n+1AX_n) phase. M refers to an early transition metal (Ti, Zr, Mo, etc.), A represents an element group of IIIA or IVA (Si, Ga, Al, etc), X is a carbon (C), and nitrogen (N), and T_x is surface terminal groups (-F, -O, or -OH) [8,10]. Among the discovered 30 MXene phases, Ti₃C₂T_x has an advantage in ultra-high conductivity (~15.000 S/cm) [8]. Therefore, MXene can be a potential intrinsic electro-active material 2D material for high-performance supercapacitor electrodes [11].

First, the strong van der Waals forces between individual MXene nanosheets lead to inevitable restacking during electrode processing, which severely diminishes the electrochemically active surface area and obstructs electrolyte ion diffusion pathways. This restacking limits the redox reaction, thus leading to poor supercapacitor performance [12]. In addition, MXene has an intrinsic drawback, namely, poor oxidative stability, which leads to easy oxidation in ambient atmospheres [13]. Second, the charge storage mechanism in pristine MXene is predominantly based on electric double-layer capacitance (EDLC) and limited surface pseudocapacitance from its native functional groups (e.g., -OH, -O, -F). This capacitive behavior, while enabling high power, inherently limits the material’s specific capacity and overall energy density, falling short of battery-level performance. This drawback was exploited to form a hybrid with TiO₂ using annealing treatment, acting as a spacer to enhance ion transportation and stabilize the MXene structure. The interaction between the Ti element of MXene (Ti₃C₂T_x ) with the presence of oxygen in air leads to TiO₂ formation [14]. Because of its low cost, non-toxicity, chemical stability, and easy availability, TiO₂ has drawn interest in supercapacitor research [15]. However, the functional group of -F contribution to the redox storing mechanism is still unknown, and hardly provides a capacitive contribution [9,16]. Meanwhile, the oxygen and titanium vacancies will improve the storage capability, thus enhancing the performance of redox material [17]. Therefore, during the annealing treatment, exposed Ti due to the vacancy of -F functional group will be oxidized and replaced by –OH or =O [18].

TiO₂ can demonstrate enhanced redox activity and charge transport properties for a supercapacitor [17]. Kumar et al. [19] demonstrate that a supercapacitor-based MXene-TiO₂ has a higher capacity, up to 25%, compared to a pure MXene layer. MXene TiO₂ can be addressed using several methods, such as wet chemical (hydrothermal), flashlight, and annealing treatment in the air. Compared to other methods, annealing treatment is a facile approach that does not require additional chemical processes, making it more environmentally friendly. It also offers a short and simple synthesis process and supports scalable production with consistent results. However, MXene TiO₂ by annealing treatment still exhibits poorer electrochemical performance compared to the wet chemical methods. Xie et al. [15] synthesized a fluorine-free MXene/TiO₂ composite by the hydrothermal method, resulting in a high specific capacitance (321 F/g) with excellent stability (81.7% after 10.000 cycles) [15]. Hong et al (2024) fabricate a porous MXene-TiO₂ anode with the help of a flashlight as a lithium-ion battery anode exhibiting improved Li⁺ storage capacity (148 mAh/g) than pristine MXene with cycle stability up to 1,500 cycles [20]. Meanwhile, Sahu et al. [17] synthesized MXene-TiO₂ by annealing treatment for 1h at a temperature of 400 °C in the air, achieving a high specific capacitance of 186 F/g and excellent stability, showing 94% capacitance retention after 5,000 cycles. Furthermore, the modification of MXene TiO₂ by the annealing treatment in the air requires further investigation at various temperatures to determine the optimal conditions. When MXene is heated at low temperatures (150 - 350 °C), the intercalated H₂O and the other physically adsorbed molecules, such as -OH, -F, and -Cl, are removed, while the MXene structure remains intact [21]. At the higher temperatures, the defunctionalization and surface oxidation occur, leading to the formation of transition metal oxide, CO, and water terminations [22].

The previous studies have demonstrated that incorporating TiO₂ into MXene effectively enhances redox activity, hence improving the overall electrochemical energy storage performance. However, the most reported synthesis method relies on complex, multi-step wet-chemical methods (e.g., hydrothermal), which can be difficult to scale and often result in non-uniform material deposition and poor interfacial contact. Although one-step annealing treatment has been explored as a simpler alternative, it still faces challenges in preserving the MXene interlayer structure and stability caused by restacking and excessive oxidation. To address these limitations, there remains a critical need for a facile, scalable, and in situ strategy that can simultaneously engineer the MXene TiO₂ surface chemistry and morphology to unlock its full faradaic potential. This study presents a novel and highly scalable approach to overcome these challenges through a controlled thermal oxidation process. This research proposes a simple, one-step annealing treatment in an ambient air atmosphere as an elegant method to engineer the electrochemical properties of Ti₃C₂T_x MXene. The novelty of this strategy lies in its dual functionality: (1) it facilitates the in-situ formation of anatase TiO₂ nanocrystals directly on the MXene surface, which serve as both redox-active sites and robust pillars to prevent nanosheet restacking; and (2) it simultaneously modifies the surface chemistry by removing inert functional groups (e.g., -F) and introducing more electrochemically active Ti-O bonds. Hypothesize that by precisely tuning the annealing temperature, we can achieve an optimal balance between creating these beneficial features and preserving the conductive Ti₃C₂ core, thereby fundamentally shifting the charge storage mechanism from capacitive to diffusion-controlled, battery-type behavior. To validate this approach, we systematically investigate the effect of annealing temperature (350, 450, 550, and 650 °C) on the structural, chemical, and electrochemical properties of Ti₃C₂T_x. We demonstrate that the optimized material, annealed at 350 °C, delivers a remarkable specific capacitance and exhibits clear battery-type characteristics. Furthermore, we validate its practical utility by fabricating a high-performance asymmetric supercapattery device, pairing the modified MXene with an activated carbon electrode. This work establishes controlled thermal oxidation as a powerful and commercially viable route for engineering next-generation MXene-based electrodes for advanced energy storage systems.

Materials and methods

Materials

The materials used in this study include MAX phase Ti₃AlC₂ (Luoyang Tongrun Info Technology CO., LTD., specific 98 wt% pure, 200 mesh), hydrofluoric acid (HF) (Sigma-Aldrich, 48% purity), DI water (OneMed, 99%), alcohol (OneMed, 96%), aquadest (Brataco), dimethylacetamide (DMAC, Merck, Darmstadt, Germany), ET₄NBF₄ (Gelon, Shandong, China), acetonitrile (Merck, Darmstadt, Germany), Cu foil, polyvinylidene fluoride (PVDF) (Sigma-Aldrich, Burlington, MA, USA), carbon black (CB) (Imerys, La Hulpe, Belgium), cellulose separator, commercial activated carbon (AC) (CGC, Bangkok, Thailand), and coin cell sets (TOB Machine, Fujian, China).

Synthesis and modification of MXene

The Al layer from Ti₃AlC₂ was removed using a selective etching process using an HF solution. The 1 g MAX powder was completely dissolved in 20 mL of HF and then mixed for 24 h. The sediments were washed with DI water until the supernatant pH was higher than 6. The sediment was dried using a hot plate at 100 °C for 6 h, resulting in MXene powders. The MXene TiO₂ was achieved by annealing the MXene powder at various temperatures (350, 450, 550, and 650 °C) using a furnace under air conditions for 1 h. MXene synthesis and modification illustrations are shown in Figure 1(a).

Fabrication coin cells

The MXene slurry was mixed in a DMAC solution in an 8:1:1 ratio of MXene as the active material, PVDF as the binder, and CB as the conductive agent, respectively. First, PVDF was dissolved in DMAC at 80 °C until it was completely homogenized. After that, CB and MXene are slowly mixed into the slurry, then stirred for 6 h. The slurry was stirred for 6 h at room temperature until the solution was completely homogeneous. Next, the slurry was deposited using a doctor blade on Cu foil with a thickness of 20 µm. The deposited samples were then dried in an oven at 100 °C for 24 h. The electrodes were manufactured into coin cells with 1 M ET₄NBF₄ as the electrolyte and cellulose as a separator, as shown in Figure 1(b).

Characterization

The morphological characteristics and composition of the samples were characterized using Scanning Electron Microscopy (SEM-EDX, SEM-EDX FEI Type INSPECT-S50). The crystal phase of the samples was characterized using X-Ray Diffraction (XRD, PAN Analytical type EXPERT-3). The chemical bonds formed were determined using Fourier Transform Infrared (FTIR, Shimadzu type IR Prestige 21). The sample’s surface area and porosity were characterized using Brunauer-Emmett-Teller (BET SA, Bellsorp mini X). Then, the electrochemical measurements of coin cells were carried out using Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), and Galvanostatic Charge Discharge (GCD) by Cortest and Neware 3000. The MXene electrodes’ performance was characterized with CV using a 3-electrode system, including the MXene electrode as a working electrode, Ag/AgCl as a reference, and Pt as a counter electrode. Meanwhile, the supercapattery device was characterized using a 2-electrode system in the form of a coin cell device, characterized with CV, EIS, and GCD.

Figure 1 Schematic illustration of the (a) MXene synthesis by HF etching and subsequent annealing in air at varying temperatures (350 - 650 °C), (b) Fabrication process of the coin cell supercapattery, including slurry preparation, electrode deposition, and cell assembly with activated carbon as the cathode and MXene or MXene-TiO₂ composites as the anode, (c) the temperature-dependent optimization of MXene, and (d) Final appearance of sample powder.

Electrochemical measurements

The performance of the supercapattery was analyzed after calculating several parameters, including specific capacitance, energy density, and power density. Capacitance is the ability of a material to hold an electrical charge [23]. Meanwhile, specific capacitance can be defined as capacitance related to a specific entity, such as area, volume, or mass [24]. In this research, gravimetric capacitance has been calculated based on the mass of the electrode’s active material, expressed in Farads per gram (F/g) [25]. CV test was carried out in a 3-electrode system for a single electrode (C_g)and a 2-electrode system for a coin cell device (C_cell). CV curves can be used to calculate the specific capacitance of a single electrode (C_g^CV) and the 2-electrode cell (C_cell^CV) using Eq. (1) [26].

where is the CV curves area (W), v is the scan rate, and m is the mass of the electrode active material, and ΔV is the potential window.

The specific capacitance, energy density, and power density of the supercapattery device were obtained through GCD using Eqs. (2) - (4), respectively [27].

where Δt are discharging time (s), I is current, ED represents the energy density (Wh/kg), and PD is power density (W/kg).

To confirm the storage mechanism of materials, the relationship between peak current and scan rates according to Power’s law is used, shown in Eq. (5), where i, a, and v^b represent the peak current, scan rate, and constants, respectively. The b value can be determined from the slope of log (v) vs log (i) plot from Eqs. (6) - (7). According to the Randles-Sevcik equation, b = 0.5 indicates that the electrochemical storage mechanism is dominated by diffusion control for battery-like materials. Meanwhile, the value b = 1 indicates the dominance of the capacitive-controlled for EDLC and pseudocapacitive materials [6,28]. For further analysis of the capacitive and diffusive contributions, the power law equation can be modified into Eq. (8). If the relationship between i and v is linear (i ∝ v¹), the charge storage reaction is limited by the capacitive control. While if the relationship between i and v is nonlinear (i ∝ v^1/2), the charge storage reaction is limited by diffusion [28]. The constants k₁ and k₂ represent the contribution of capacitive-controlled and diffusive-controlled, and can be evaluated by the slope and the intercept of v ½ vs i (V) /v ½ [29].

Results and discussion

The key findings of this research are the successful utilization of controlled thermal oxidation as a scalable strategy to transform MXene (Ti₃C₂T_x) into an MXene-TiO₂ composite. The annealing temperature serves as a key parameter controlling the material’s final properties, as demonstrated by various characterization techniques. By visual observation, the appearance of the thermally treated MXene powder serves as preliminary evidence of TiO₂ production, shown in Figure 1(d). As the temperature rises, MXene gradually loses color from its original black into a greyish tone, and finally to opaque (MXene TiO₂ 650 °C). This steadily fading demonstrated the rising formation of TiO₂, which normally has an opaque white color [30].

Controlled thermal oxidation: Driving phase transformation and crystallinity in MXene TiO₂

To clarify the transformation due to thermal oxidation, X-Ray Diffraction (XRD) analysis was used to observe the phase evolution and crystallinity from MAX phase to MXene and subsequently to the MXene TiO₂. The XRD spectra in Figure 2(a) chart the structural evolution from the MAX phase to MXene and finally to MXene-TiO₂ composites. The complete disappearance of Al peaks confirms the successful etching of the MAX phase (Ti₃AlC₂) to produce MXene (Ti₃C₂T_x), characterized by its distinct (003), (004), (011), (013), (008), and (111) peaks [20]. After annealing, a significant structural transformation occurs. The original MXene peak decreases, leaving peaks at orientations such as (008), (013), and (111), indicating disruption of the layered nanosheet structure of MXene due to oxidation [31]. The d-spacing of MXene before and after annealing was 1.90 and 1.89 Å, respectively, indicating that the annealing process has a negligible effect on the interlayer spacing. Crucially, new peaks appear corresponding to the anatase phase of TiO₂ - specifically (101), (103), (004), (105), (211), (116), (220), (301), and (224) [19]. The dominance of the anatase phase, known to form under rapid oxidation conditions, confirms that our annealing strategy successfully induces a fast conversion of the MXene surface to TiO₂ [32]. This transformation is a direct result of the controlled thermal oxidation, where the growth of TiO₂ peaks occurs at the expense of the original MXene signatures. This signifies a clean phase evolution where the original MXene structure is consumed to form TiO₂ crystals.

Figure 2 The histograms of (a) X-ray Diffraction (XRD), (b) Crystallinity as a function of annealing temperature.

Figure 2(b) shown derived from the XRD data, quantitatively shows the MXene lattice’s crystallinity as a function of annealing temperature. Crucially, increasing the annealing temperature resulted in a direct, positive correlation with the material’s overall crystallinity (from 44.69% at 350 °C to 48.30% at 650 °C), substantially higher than that of pure MXene (25.23%). This observation confirms that despite the inherent structural weakening caused by rapid oxidation, the highly crystalline anatase TiO₂ phase successfully dominates the composite structure, dictating the enhancement in crystallinity. Theoretically, the strong oxidation rate weakens the crystal structure of the material, thus also reducing the crystallinity [15]. This result confirms that the controlled thermal treatment promotes the formation of a highly crystalline anatase TiO₂ phase, which acts as the dominant crystalline structure, thus enhancing the overall crystallinity of the composite.

Chemical and elemental evolution: Confirming the oxidation mechanism

The profound phase and structural changes confirmed by XRD and crystallinity data are further quantified and validated by Energy-Dispersive X-ray Spectroscopy (EDX) and Fourier Transform Infrared (FTIR) analysis.

Figure 3 (a) Elemental composition by Energy-Dispersive X-ray Spectroscopy (EDX), and (b) functional bonds spectra by Fourier-Transform Infrared Spectroscopy (FTIR) of the samples

Figure 3(a) shows the EDX data provide elemental evidence of the oxidation process: as the annealing temperature increased, the atomic ratio shown in Table 1. Carbon (C) decreased gradually (from 21.41% to 6.10%), while the ratio of Oxygen (O) increased substantially (from 48.95% to 72.01%). This inverse elemental relationship directly confirms the chemical mechanism where exposed MXene interacts with ambient oxygen. C atoms are consumed by forming CO_x​ gases, leading to the observed loss of C content, thus generating pore formation. While Ti atoms readily bond with the increasing amount of O, resulting in TiO₂​. The schematic process can be illustrated as in Figure 1(c). TiO₂ formation was definitively detected in the EDX data beginning at 350 °C, consistent with the initial appearance of anatase peaks in the XRD pattern at this same temperature.

Figure 3(b) shows the FTIR spectra, revealing the evolution of chemical functional groups across the samples. FTIR analysis provides further evidence of the chemical changes induced by HF etching and thermal oxidation. First, the FTIR region corresponding to the fingerprint vibration of the material in the range of 400 - 1,400 cm⁻¹ [33]. Materials fingerprint vibration, including Ti-O stretching [34], Ti-C stretching, Ti-F bending [33], Ti-O-Ti [35], and C-F stretching [33], are detected at wavenumbers of 400 - 430, 631, 756, 923 and 1,300 - 1,500 cm⁻¹, respectively. Second, the FTIR region corresponding to confined water and carbon bond vibrations, in the range of 1,400 - 4,000 cm⁻¹. Confined water vibrations, including O-H stretching [36], were detected at wavenumbers of 2,900 - 3,500 cm⁻¹. The carbon bond vibration, including C-O stretching [37], and O=C=O bonds [38] were detected at wavenumbers of 1,557 and 2,300 - 2,400 cm⁻¹. The spectrum of MXene, compared to the MAX phase, shows the appearance of new vibrations - including C-F stretching, Ti-F bending, Ti-O stretching, and Ti-O-Ti-confirming that HF etching successfully replaced Al atoms with oxygen- and fluorine-containing functional groups (-O, -OH, -F) [7]. The FTIR spectra of the annealed samples reveal the progression of oxidation. The Ti-F bond disappears entirely after heat treatment, and the C-F bond weakens significantly in MXene TiO₂ 350°C and vanishes at higher temperatures. Concurrently, the intensities of the Ti-O and Ti-O-Ti peaks deepen with increasing temperature, confirming the increasing formation of more TiO₂ [20]. The appearance and intensification of C-O stretching and O=C=O bonds indicate carbon oxidation and CO₂ adsorption on the material surface during annealing [34,39]. The persistence of the Ti-C bond across all samples except MXene TiO₂ 650 °C suggests the core MXene structure remains largely intact up to 550 °C, with severe oxidation occurring only at the highest temperature.

The chemical and morphological evolution resulting from this controlled oxidation process profoundly influences the material’s suitability for electrochemical applications. FTIR spectroscopy provided molecular evidence of the oxidation, showing the complete removal of unstable surface fluorides (Ti-F and C-F), concurrent with the deepening of Ti-O and Ti-O-Ti bonds, confirming the increased formation and structural integration of TiO₂. The FTIR spectroscopy provided a molecular link to this elemental shift: the progressive deepening of the Ti-O and Ti-O-Ti bond peaks confirmed the structural establishment of TiO₂, while the appearance of C-O and O=C=O bonds corroborated the EDX finding that C oxidation occurred during heat treatment. The loss of C atoms and the simultaneous generation of CO_x​ gas are also the fundamental drivers for pore formation observed in the SEM and N₂​ adsorption analysis, reinforcing the interconnected nature of the controlled thermal process.

Table 1 The atomic composition (%) of the samples by EDX.

Textural Properties and the Oxidation Trade-Off (Porosity and SSA)

Figures 4(a) and 4(b) Adsorption-Desorption Isotherms and Pore Size Distribution. The textural properties, which are critical for ion transport in supercapatteries, reveal a crucial trade-off driven by the oxidation level. N₂ adsorption-desorption isotherms are shown in the graph from Figure 4(a). All samples demonstrate a type IV isotherm and H3 hysteresis loop. The isotherm IV type is related to mesoporous materials (2 - 50 nm in diameter) [40]. The H3 loop type suggests the presence of plate-like particles forming non-rigid aggregates, typical of the MXene-derived structure. The loss of C atoms and the generation of CO_x​ gas during oxidation directly led to the formation of these pores; consequently, pore size generally increased with annealing temperature due to the accelerating oxidation rate (as confirmed by EDX. The pore diameter and specific surface area of the samples are shown in Table 2. All the materials have diameters in the range of 4 - 49 nm, indicating mesoporous materials, as confirmed by the Barret-Joyner-Halenda (BJH) pore size distributions in Figure 4(b). From these results, it can be confirmed that the higher the annealing temperature, the higher the oxidation level, and thus the pore size formed is larger, except for MXene at 650°C. However, despite the larger pore size, the Specific Surface Area (SSA) exhibited a drastic decrease as the temperature rose (from 35.6 m²g⁻¹ for pure MXene down to a minimum of 10.3 m²g⁻¹ at 550 °C). This reduction is the direct consequence of the high TiO₂ formation (validated by XRD and high O content in EDX) at elevated temperatures. Specifically, the large amount of TiO₂ acts detrimentally by covering the exposed MXene pores and filling the critical interlayer spacing. This TiO₂ blockage prevents the surface area from increasing and, critically, inhibits ion transportation rather than acting as a beneficial spacer to prevent self-restacking. MXene 650 °C has the highest number of TiO₂ confirmed by EDX and XRD. The specific surface area of pure MXene, MXene 350 °C, MXene 450 °C, MXene 550 °C, and MXene 650 °C are 35.6, 25.6, 14.5, 10.3, and 15.7 m²g⁻¹, respectively. The surface area value decreases with increasing annealing temperature, despite the pore size increasing. This is because the surface area is also affected by other factors, such as the number and size of MXene pores, the value of MXene interlayer spacing, and the amount and size of TiO₂ [19,41]. Based on the XRD data, the annealing treatment (from 350 to 650 °C) has a negligible effect on the interlayer spacing; therefore, the decrease in surface area is mainly attributed to the formation of TiO₂. The surface area will not increase when TiO₂ covers the pores and the MXene interlayer spacing, despite the increase in the number of pores. Additionally, a TiO₂ size that is too large will occupy the interlayer space, preventing it from acting as a spacer and inhibiting ion transportation.

Figure 4 (a) N₂ adsorption-desorption isotherms of the samples, and (f) BJH pore size distribution of MXene samples.

Table 2 The pore diameter of pure MXene and modified MXene samples.

Morphological evolution of MXene (Ti₃​C₂​T_x​) during controlled thermal

The precursor MAX phase (Ti₃​AlC₂​) initially exhibits its characteristic bulk structure (Figure 5(a)). Following the Hydrofluoric acid (HF) etching process, the selective removal of the Al layer - a reaction that produces easily dissolved AlF₃​ and H₂​ gas - leads to the formation of the MXene Ti₃C₂T_x structure [42,43]. This is distinctly observed as an accordion-like structure composed of multilayered nanosheets (Figure 5(b)). The subsequent controlled thermal oxidation induces significant morphological changes, primarily manifested as surface defects and pore formation, which directly affect ion transport kinetics.

Figure 5 SEM-EDX images of (a) MAX, (b) pure MXene, (c) MXene TiO₂ 350 °C, (d) MXene TiO₂ 450 °C, (e) MXene TiO₂ 550 °C and (f) MXene TiO₂ 650 °C.

The heat treatment process induces morphological defects and pore formation on the surface of MXene. Clear evidence of pore formation is observed on MXene TiO₂ 350 °C and MXene TiO₂ 450 °C, with sizes of 0.8 µm (Figure 5(c)), 2.4 and 3.0 µm (Figure 5(d)). However, pore formation is not obvious in MXene TiO₂ 550 °C and MXene TiO₂ 650 °C, as a significant amount of TiO₂ completely covers the MXene flakes, as seen in Figures 5(e) - 5(f). When the temperature increases to 550 and 650 °C, too many defects will form. Hence, oxidation will happen faster in the big defect than in the smaller defect, which can lead to the rapid formation of TiO₂ particles [32]. TiO₂ Overgrowth and Surface Blocking: However, at higher annealing temperatures (550 and 650 °C), the morphology shifts dramatically. The rapid increase in temperature creates numerous defects, which accelerate the oxidation rate. This leads to the rapid formation and overgrowth of TiO₂ particles at these defect sites. As seen in Figures 5(e) - 5(f), a significant amount of TiO₂ now completely covers the MXene flakes. The oxidation process, which occurs at increasing annealing temperatures, plays a crucial role in forming pores and TiO₂ particles. This phenomenon is related to MXene’s intrinsic property, which causes it to oxidize and degrade easily in ambient atmospheric conditions [13]. In the oxidation process, oxygen reacts and separates the carbon atoms from MXene Ti₃C₂T_x. CO_x gases will be created so that pores are successfully formed [44]. Moreover, oxygen will also easily bond with the Ti element from Ti₃C₂T_x to create TiO₂ during the heat treatment [45].

Electrochemical Performance of MXene-TiO₂ anodes and asymmetric supercapatteries

This discussion analyzes the electrochemical performance of thermally oxidized MXene-TiO₂ electrodes, linking the findings directly to the research purpose of developing a scalable annealing strategy for high-performance asymmetric supercapatteries.

Three-electrode electrochemical analysis of MXene-TiO₂ Anodes

To discover the sample’s performance and storage mechanism, all samples were tested in a 3-electrode system with Ag/AgCl reference at the various scan rates of 20, 30, 40, 50, 60, 80, and 100 mV/s.

Figure 6 (a) CV curves of all samples at the scan rate of 20 mV/s, (b) CV curves of MXene-Cu foil at the various scan rates, (c) CV curves of MXene TiO₂ 350 °C-Cu foil at the various scan rates, (d) Plot of the square root of scan rate vs the peak current of MXene-Cu foil, (e) Plot of the square root of scan rate vs the peak current of MXene TiO₂ 350 °C-Cu foil, (f) Plot of log(v) vs log (I).

All sample’ CV profiles show the non-rectangular and faradaic shape as a feature of the battery-like material [46], shown in Figures 6(a). The occurrence of the faradaic redox reaction is indicated by the appearance of the redox peaks, including anodic (pa) and cathodic peaks (pc). MXene TiO₂ 350 °C electrode demonstrated the highest specific capacitance (289.58 F/g at 20 mV/s), surpassing pristine MXene (228.03 F/g) and significantly outperforming materials annealed at higher temperatures, such as MXene TiO₂ 650 °C (104.71 F/g). The complete capacitance values at 20 mV/s follow the sequence: MXene TiO₂ 350 °C (289.58 F/g) > MXene TiO₂ 450 °C (254.98 F/g) > pristine MXene (228.03 F/g) > MXene TiO₂​ 550 °C (147.32 F/g) > MXene TiO₂ 650 °C (104.71 F/g). The specific capacitance at the other scan rates also demonstrates that MXene TiO₂ 350 °C has the highest performance, shown in Table 3. This optimal result at MXene TiO₂ 350 °C is a direct consequence of balancing TiO₂ anatase formation (confirmed by XRD and EDX) to introduce redox activity, with the preservation of sufficient specific surface area (SSA). Higher temperatures cause excessive TiO₂ growth, leading to particle blocking and catastrophic SSA reduction, thus hindering performance. The overall enhanced performance is attributed to the increase in redox reaction due to the diffusion of ions into the newly formed TiO₂ crystalline anatase phase, improved hydrophilicity, and the creation of electrochemically active defect sites on the MXene TiO₂ 350 °C surface [17].

The mechanism that occurs inside the MXene-Cu foil and the MXene TiO₂ 350 °C-Cu foil can be analyzed using CV curves at scan rates of 20, 30, 40, 50, and 60 mV/s (Figures 6(b) and 6(c)). As the scan rates applied increase, the anodic and cathodic peaks are shifted into the higher and lower potential values, respectively, with the most obvious shift occurring in the MXene sample. This phenomena indicate the incomplete intercalation and deintercalation process of the electrolyte ion into the electrode material [29]. The fundamental charge storage mechanism was investigated by analyzing the kinetics of the electrochemical reaction. A linear relationship between the peak current and the square root of the scan rate (Figures 6(d) and 6(e)) for both anodic and cathodic peaks, evidenced by R² values approaching 0.99, signifies an electrochemical process predominantly governed by semi-infinite diffusion. This behavior is characteristic of battery-type materials, where Faradaic charge transfer is limited by the solid-state diffusion of ions within the bulk electrode [47]. Further quantitative analysis of the charge storage kinetics was performed using the power-law relationship (i = av^b). The calculated b-values for MXene and MXene TiO₂ 350 were 0.58 and 0.54, respectively (Figure 6(f)). These values, being closer to the theoretical limit of 0.5 for a diffusion-controlled process than to 1.0 for a surface-controlled capacitive process, confirm that the storage mechanism is primarily battery-type. Generally, from the previous studies, TiO₂ is suitable as a battery anode due to its redox kinetics [28]. The marginally higher b-value for pristine MXene implies a more non-negligible capacitive contribution, attributable to its larger specific surface area and wider interlayer spacing that facilitate electric double-layer formation and surface-mediated ion storage. The current response was deconvoluted into capacitive (k₁) and diffusion-controlled (k₂) contributions to provide a definitive quantification. The k₂ values (~7.19×10⁻³ for MXene and ~8.03×10⁻³ for MXene TiO₂ 350 °C) were found to be an order of magnitude greater than the corresponding k₁ values (~10⁻⁴), unambiguously verifying the dominance of the diffusion-controlled pathway.

Table 3 Specific capacitance of all samples deposited on Cu foil substrate at the various scan rates from the CV curves.

Figures 7 (a) Capacitive and diffusive contribution, and (b-c) CV curve with capacitive and diffusive distribution at the scan rate of 20 mV/s of MXene-Cu foil and MXene TiO₂ 350 °C Cu foil.

Table 4 Electrode-based MXene performance comparison with existing research.

The percentage of capacitive and diffusive contributions as a function of the scan rates can be shown in Figure 7(a). Both samples show that the diffusion-controlled process was more dominant than the capacitive-controlled process. At the highest scan rate of 60 mV/s, MXene and MXene TiO₂ 350 °C show that 85% and 89% of their capacitance was driven by the diffusion-controlled process. As the scan rates increase, the diffusion-controlled process contribution also increases. At the slow scan rate of 20 mV/s, MXene and MXene TiO₂ 350 °C show that 91% and 93% of their capacitance was driven by the diffusion-controlled process, shown in Figures 7(b) and 7(c). The increase of diffusion-controlled contribution is caused by the rise of redox reaction due to TiO₂ addition in MXene TiO₂ 350 °C [17]. This enhancement is directly correlated with the material’s evolution; the controlled thermal oxidation at 350 °C generates electroactive anatase TiO₂ phases, which provide a high density of accessible sites for reversible redox reactions, thereby amplifying the diffusion-limited, Faradaic charge storage component. The optimized MXene TiO₂ 350 °C sample demonstrates competitive electrochemical performance when benchmarked against composites synthesized via alternative methods, as cataloged in Table 4. This comparative analysis substantiates that controlled annealing at 350 °C constitutes a scalable and efficacious strategy for fabricating high-performance MXene-derived TiO₂ composites, positioning it as a viable and superior synthesis route for advanced energy storage materials. However, the electrode performances tend to deteriorate at the higher annealing temperatures (450, 550 and 650 °C). This is because the electrode-electrolyte contact area is reduced by too large a TiO₂ formation [8]. The excessive TiO₂ production will hinder the continuous electron transfer in the electrode, thus increasing the electrode resistance and lowering the electrochemical performance [20].

Performance of asymmetric supercapattery devices

The electrochemical performance of a supercapattery device based on MXene and MXene TiO₂ 350 °C as an anode paired with an AC commercial cathode_, was investigated using Cyclic Voltammetry (CV), Galvanostatic Charge Discharge (GCD), and Electrochemical Impedance Spectroscopy (EIS) in a 2-electrodel system configuration.

Figure 8 (a) CV curves of supercapattery based AC//MXene and AC//MXene TiO₂ 350 °C at the scan rate of 5 mV/s, (b) CV curves of AC//MXene at the various scan rates, (c) Plot of the square root of scan rate vs the peak current of AC//MXene, (d) CV curves of AC//MXene TiO₂ 350 °C at the various scan rates, (e) Plot of the square root of scan rate vs the peak current of AC//MXene TiO₂ 350 °C, (f) Plot of log(v) vs log (I).

The CV curves were established at the scan rates of 5, 10, 15, 20, 25, 30, 40, 60, 80, and 100 mV/s. The CV curves show energy storage mechanism from the combination of an EDLC by AC material and a battery-type by MXene material (Figure 8(a)). The specific capacitance from the supercapattery device can be calculated using Eq. (1), shown in Table 5. Compared to AC//MXene, AC//MXene TiO₂ 350 °C represents the highest specific capacitance in all scan rates. The highest capacitance was achieved by AC//MXene TiO₂ 350 °C at a scan rate of 5 mV/s (50.50 F/g), surpassing that of AC//MXene (49.02 F/g). This result is compatible with the result of the CV 3-electrode system as described before. To confirm the mechanism inside the supercapattery devices, the CV curves at the scan rates of 5, 10, 15, 20, 25, and 30 mV/s can be used (Figures 8(b) and 8(c)). The alignment of redox peaks serves as an indicator of the reversibility mechanism. The semi-reversible mechanism has the rapid peaks, and the irreversible mechanism shows the divergent peaks [46].

Table 5 Specific capacitance of AC//MXene and AC//MXene TiO₂ 350 °C at the various scan rates from CV curves.

The R² values close to 1 indicate the linear relationship, thus showing the good reversibility and rapid interfacial reaction [50]. The square root of scan rate vs the peak current from AC//MXene (Figures 8(d)) shows R² values for Ipa and Ipc are 0.31 and 0.99, respectively. Therefore, the CV of AC//MXene exhibits an irreversible reaction as evidenced by broad divergence and significantly shifted peaks observed at various scan rates. Meanwhile, the square root of scan rate vs the peak current from AC//MXene TiO₂ 350 °C (Figure 8(e)) shows R² values for Ipa and Ipc are 0.94 and 0.99. This indicates that the redox reaction that occurs inside the supercapattery is reversible. The CV curves at the various scan rates also show the retained shape with the increment of the scan rates, thus indicating the good reversibility and capability. The oxidation peak is primarily caused by the redox reaction of MXene functional groups (-OH, -F, -O), while the reduction peak is related to the redox activity of TiO₂ [51]. This statement is linear to the appearance of the reduction peak at AC//MXene TiO₂ 350 °C at the various scan rates.

Figure 9 (a) Capacitive and diffusive contribution, and (b-c) CV curve with capacitive and diffusive distribution at the scan rate of 5 mV/s of AC//MXene and AC//MXene TiO₂ 350 °C.

Figure 8(f) shows the constant b to obtain the dominant storage reaction mechanism of supercapattery. AC//MXene and AC//MXene TiO₂ 350 °C exhibit b 0.78 and 0.73, respectively. The b values of supercapattery are higher than the b values of electrodes estimated before by the CV 3-electrode system. The detailed contribution of the diffusion-controlled process and the capacitive-controlled process can be evaluated by obtaining the k₁ and k₂ values from Eq. (8). AC//MXene has k₁ and k₂ values of 6.71×10⁻⁵ and 2.76×10⁻⁴, respectively, while AC//MXene TiO₂ 350 °C has k₁ and k₂ values of 6.58×10⁻⁵ and 3.26×10⁻⁴, respectively. Furthermore, Figure 9(a) shows the percentage of capacitive and diffusive contributions as a function of the scan rates. At the highest scan rate of 30 mV/s, MXene and MXene TiO₂ 350 °C show that 43% and 48% of their capacitance was driven by the diffusion-controlled process. As the scan rates increase, the diffusion-controlled process contribution also increases. At the lowest scan rate of 5 mV/s, MXene and MXene TiO₂ 350 °C show that 65% and 69% of their capacitance was driven by the diffusion-controlled process, shown in Figures 9(b) and 9(c). Thus, increasing the capacitive-controlled contribution of supercapattery was due to the EDLC storage mechanism performed by AC.

Figure 10 (a) GCD curves at a current density of 0.16 A/g, (b) Capacity retention at a current density of 1.6 A/g, and (c) Nyquist plot of supercapattery based AC//MXene and AC//MXene TiO₂ 350 °C.

The GCD test was performed on a supercapattery coin cell device at the potential window of 0 - 2.5 V, same as the CV test. At the current density of 0.16 A/g (Figure 10(a)), both AC//MXene and AC//MXene TiO₂ 350 °C performed a non-symmetric curve as a signature of the faradaic contribution on the energy storage mechanism [51]. MXene//AC exhibits specific capacitance of 38.51 F/g, energy density of 24.58 Wh/kg, and power density of 686.02 W/kg. After the annealing treatment at a temperature of 350 °C, AC//MXene TiO₂ 350 °C shows higher electrochemical performance, including specific capacitance of 49.42 F/g, energy density of 32.88 Wh/kg, and power density of 700.38 W/kg. GCD curves of AC//MXene TiO₂ 350 also perform a smaller IR drop (0.12 V) compared to AC//MXene (0.16 V). Furthermore, to analyze the cyclic stability, the supercapattery device was tested at a current density of 1.6 A/g, shown in Figure 10(b). After 5,000 cycles, the AC//MXene TiO₂ 350 °C supercapattery retains 96% of the initial capacity, higher than the AC//MXene supercapattery (59%). This result shows that TiO₂ addition can improve the MXene stability [15]. Both supercapattery devices show the gradual increment up to 170% at the 18^th cycle for the AC//MXene supercapattery and 159% at the 862^nd cycle for the AC//MXene TiO₂ 350 °C supercapattery. This is due to the electrode material activation caused by the diffusion of electrolytes into the electrode materials [51]. The surface functional groups have been modified during the cycling process, which facilitates the increasing capacity. Some research shows that the ion intercalation, modification, and reaction among the functional groups can increase the electronic conductivity and charge transport efficiency, leading to an increase in specific capacitance during the cycling process [52]. Supercapattery-based AC//MXene TiO₂ 350 °C from this investigation can be compared with the previous research, which is listed in Table 6. These results demonstrate that AC//MXene TiO₂ 350 °C supercapattery exhibits a practical approach to improving the energy density without sacrificing power density or capacity retention of the device.

Table 6 Supercapattery performance-based AC and MXene comparison to the previous research.

The impedance analysis was carried out from the Nyquist plot (Figure 10(c)) with the possible circuit model shown in Figure 10(d). This model shares similar characteristics with those found in battery components. Equivalent series resistance (ESR) was obtained from the intercept of the Z’-axis, which relates to the resistance of the current collector/electrode interface, electrolyte ions, and electrode materials [51]. The ESR value of AC//MXene TiO₂ 350 °C is 6.13 Ω, lower than that of AC//MXene (8.36 Ω). The first semicircle at the high frequency appeared due to the ion diffusion through the SEI layer during the formation and decomposition of the electrolyte. This region, represented by R and CPE elements, represents the resistance and capacitance of SEI [54]. The first semicircle of AC//MXene TiO₂ 350 °C is much lower R_sei = 22.62 Ω than AC//MXene R_sei = 31.67 Ω. The second semicircle at the middle frequency represents the charge transfer during the electrode reaction, as indicated by the R and CPE elements. The ions adsorb at the surface of the electrode to form a double layer on the interface, and the energy storage process occurs [54]. The second semicircle of AC//MXene TiO₂ 350 is also lower, R_ct = 87.05 Ω, than AC//MXene R_ct = 339.29 Ω. At high frequency, a straight line at an angle close to 45° related to the Warburg impedance represents ion diffusion or transport in the electrolyte and the electrode surface [55]. The slope of AC//MXene TiO₂ 350 °C is much higher than that of AC//MXene, indicating the low ion diffusion resistance from the electrolyte to the electrodes [53].

Conclusions

This study successfully demonstrated a controlled thermal oxidation strategy of MXene TiO₂ as a scalable and effective route for fabricating superior electrochemical kinetics and stability of asymmetric supercapattery based on TiO₂ MXene. Crucially, the research identified a key thermal threshold: excessive oxidation at high temperatures led to massive TiO₂ overgrowth, which​ decreased the SSA by blocking the MXene pores and interlayer spacing (e.g., SSA decreased to 10.3 m²g⁻¹), even though TiO₂ formation is beneficial. The MXene TiO₂ 350 °C sample emerged as the optimal material, striking a precise balance between sufficient TiO₂ growth, successful surface functional group modification by replacing unstable −F groups with oxide groups (Ti-O and Ti-O-Ti bonds) for redox activity, and adequate SSA preservation (25.6 m²g⁻¹), while maintaining its interlayer spacing (1.89 Å). These characteristics leading kinetic optimization confirmed by CV analysis, which revealed a battery-type behavior dominated by the diffusion-controlled charge storage (up to 93% contribution) rather than surface-limited capacitance. This electrode exhibited a superior specific capacitance of 289.58 F/g at 20 mV/s. Subsequently, the AC//MXene TiO₂ 350 °C asymmetric supercapattery device revealed outstanding performance, providing excellent stability (∼92% capacity retention after 5,000 cycles) and high metrics (Cs​ = 49.42 F/g, ED​ = 32.88 Wh/kg, PD​ = 700.38 W/kg). Compared to AC//MXene, AC//MXene TiO₂ 350 °C showed enhanced kinetics, significantly reduced resistance parameters (ESR, Rsei​, Rct​), and lower ion diffusion resistance. These findings emphasize the significance of precise oxidation control in tailoring MXene TiO₂ 350 °C composites and their surface chemistry, leading to enhanced faradaic kinetics and overall device performance. In summary, this study offers a novel, scalable, and single-step strategy for creating MXene composites for next-generation energy storage applications.

Acknowledgements

This research was funded by the internal competitive Matching Fund grant from the PNBP UM 2024 under contract No. 4.4.624/UN32.14.1/LT/2024.

Declaration of generative AI in scientific writing

For the preparation of this manuscript, especially for the language editing and grammar correction, the authors acknowledge the use of generative AI tools (e.g., QuillBot and ChatGPT by OpenAI). These tools were not used for content generation or data interpretation. The authors take full responsibility for the content and conclusions presented in this work.

CRediT author statement

Nida Usholihah: Conceptualization, Methodology, Writing - Original draft preparation; Ishmah Luthfiyah: Data curation, Visualization, Investigation, Writing - Reviewing; Worawat Meevasana: Supervision, Investigation, Herlin Pujiarti: Supervision, Writing - Reviewing; Aripriharta: Software, Validation; Malik Maaza: Conceptualization, Methodology; Muhammad Subhan: Methodology; Markus Diantoro: Supervisio, Writing - Reviewing and Editing.

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