Nitrogen Plasma-Assisted Surface Engineering on Multilayer Ti₃C₂T_x Electrodes for Enhanced Interfacial Charge Dynamics and Energy Storage in Ascorbic Acid Electrolyte

Abstract

The intrinsic limitations of Ti₃C₂T_x electrodes, specifically low interfacial charge-transfer efficiency and structural degradation in strongly acidic environments, hinder their performance in high-rate aqueous supercapacitors. Herein, we report a synergistic strategy combining nitrogen plasma surface engineering with a redox-active ascorbic acid electrolyte to optimize the electrode/electrolyte interfacial kinetics. By systematic investigation, the Ti₃C₂T_x supercapacitor obtained by a 10-min plasma duration (N10P-AA) achieved the optimal balance between activating surface sites and preserving the conductive Ti–C framework integrity. The ascorbic acid electrolyte broadened the potential window to approximately 0.7 V, and N10P-AA exhibited the lowest charge-transfer impedance and superior rate capability, retaining a relatively high Coulombic efficiency (>72%) even at a high scan rate of 10,000 mV·s⁻¹. The EIS results and kinetics analysis (b values) confirmed that the moderate plasma activation effectively promoted more surface-dominated charge storage kinetics and mitigated diffusion limitation, consistent with reduced charge-transfer resistance and a smaller Warburg slope. The XPS results revealed that the 10-min treatment suppressed detrimental oxidation during cyclings and facilitated the formation of electrochemically favorable hydroxylated surface functional groups. This work demonstrates a feasible surface electrolyte co-engineering strategy for modulating the interfacial behavior of MXene, which is of great significance for future high-efficiency aqueous electrochemical energy storage and potential biosensing applications.

1. Introduction

MXene represents a novel and rapidly growing family of two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides [1,2], generally expressed by the formula M_n+1X_nT_x, which typically originate from etching processes such as acidic or molten salt treatment [2]. Since the initial synthesis of layered Ti₃C₂T_x in 2011 through the selective etching of the A-layer element in MAX phases [1], MXenes have become a research hotspot. The layered structures and the presence of surface terminal groups (T_x, such as –O, –OH, –F, etc.) derived from etching endow these materials with a unique combination of metallic-level conductivity (10⁴ S·cm⁻¹–10⁵ S·cm⁻¹), excellent mechanical flexibility, and intrinsic hydrophilicity [3]. Compared to other 2D materials such as highly conductive but surface-inert graphene, or semiconducting transition metal dichalcogenides (TMDs) with low inherent conductivity, MXenes offer the distinct advantage of combining metallic conductivity with tunable surface chemistry [4], making them promising candidates for diverse applications. For example, MXenes have demonstrated high specific capacitance and rapid ion transport in supercapacitors and Li/Na ion batteries [5], served as efficient supports for electrocatalytic hydrogen evolution, oxygen reduction, and CO₂ reduction [6], and acted as effective adsorbents for the removal of heavy metal ions and organic pollutants from water [7]. Furthermore, their exceptional electromagnetic shielding ability and mechanical flexibility render MXenes particularly attractive for applications in wearable electronics, sensors, and electromagnetic interference (EMI) shielding materials [8].

Among MXenes, Ti₃C₂T_x is the most extensively studied material, particularly for high-power energy storage systems such as supercapacitors. Its structure consists of highly conductive Ti–C layers and hydrophilic surfaces [2,3], enabling rapid electron transport and efficient ion diffusion, which leads to a high specific capacitance. The charge storage mechanism of Ti₃C₂T_x often involves intercalation pseudocapacitance, which allows reversible intercalation of various cations (e.g., Li⁺, Na⁺) accompanied by charge-transfer reactions, significantly enhancing energy storage density [9] compared to conventional electric double-layer capacitors (EDLCs). Aqueous electrolytes (e.g., H₂SO₄, KOH, Na₂SO₄) are widely used due to their high ionic conductivity (>0.1 S·cm⁻¹) [10] and power output. In an acidic H₂SO₄ electrolyte, the small, highly mobile hydrated protons reversibly intercalate into the Ti₃C₂T_x interlayers [11], enabling high volumetric capacitance and excellent rate performance. For instance, Lukatskaya et al. reported a volumetric capacitance of 900 F·cm⁻³ for Ti₃C₂T_x with a high retention rate in 1 M H₂SO₄ [12]. However, acidic environments can lead to surface corrosion and degrade structural integrity after prolonged cycling [13]. More importantly, despite the promising theoretical prospects, maximizing the performance of MXene, especially at high-rate operation, remains fundamentally constrained by interfacial charge-transfer inefficiencies where proton and ion transport become kinetically limited.

While non-aqueous electrolytes offer a broader electrochemical stability window, which is critical for enhancing energy density [12], they often come with trade-offs. Organic electrolytes are limited by their lower ionic conductivity and larger solvent molecules, leading to sluggish intercalation kinetics [14]. Ionic liquids, despite offering outstanding chemical and thermal stability, typically exhibit high viscosity and low ion mobility [15]. To overcome these limitations and enhance energy density without sacrificing power, integrating redox-active electrolytes has become a key strategy. Ascorbic acid (AA), a water-soluble and biocompatible organic molecule, represents a highly promising candidate. It exhibits unique reversible redox activity via its enediol structure, undergoing reversible oxidation to dehydroascorbic acid (DHA) [16]. Its low oxidation potential makes AA an ideal probe molecule for investigating charge-transfer behavior, conductivity, and catalytic activity on the surface of electrode materials [17]. In electrochemical energy storage, AA can act as a functional electrolyte or additive, enhancing pseudocapacitive contribution [18] and increasing the energy density of the storage device. However, the inherently larger molecular size of AA compared to simple protons, along with its susceptibility to surface-induced side reactions, frequently leads to diffusion bottlenecks and unstable reaction pathways when interfacing with MXene surfaces.

Surface modification is thus a critical strategy for precisely tuning the interfacial properties and electronic structure of MXenes [2,3]. Notably, when a molecular redox electrolyte such as AA is employed, the electrolyte is no longer a passive medium; instead, the electrode surface must be engineered to match the interfacial charge-transfer and mass-transport requirements of the redox-active molecules. In this context, nitrogen (N₂) plasma surface engineering has emerged as a powerful and low-damage route to reconstruct the outermost atomic layers. This treatment utilizes high-energy nitrogen species to interact with the MXene surface, effectively introducing the desired nitrogen-doped atoms or nitrogen-containing functional groups (e.g., –NH₂, O–Ti–N, Ti–O–N) while simultaneously removing detrimental fluorine-containing (–F) groups, thereby enhancing surface polarity and electronic structure [19]. This chemical modification approach promotes interfacial affinity with electrolyte ions and enhances pseudocapacitive contributions. For instance, Zhang et al. reported that nitrogen-doped Ti₃C₂T_x exhibited a high specific capacitance of up to 415 F·g⁻¹ in 1 M H₂SO₄ and maintained over 90% capacity after 18,000 cycles [20]. However, the key hypothesis of the present study is that, under AA electrolyte conditions, plasma duration functions as a kinetic dial that regulates the MXene–AA interfacial charge-transfer pathway and mitigates diffusion bottlenecks/side reactions that are more pronounced for molecular redox species than for simple ions. Accordingly, an optimized plasma duration is expected to actively modulate the complex MXene-AA interfacial environment, thereby accelerating charge transport and enhancing surface-controlled charge storage, which is necessary to suppress the side reactions and diffusion constraints typical in molecularly active electrolytes.

Based on the above background and hypothesis, this study aims to systematically explore the effects of N₂ plasma treatment duration on the electrochemical performance of multilayer Ti₃C₂T_x electrodes and supercapacitors. The modified Ti₃C₂T_x films were evaluated in two different aqueous systems: a 3 M H₂SO₄ solution (as a reference benchmark), and a 500 μM AA solution (as the target redox-active electrolyte). Through a complementary approach utilizing electrochemical analysis, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and kinetics assessment (b value analysis), along with X-ray photoelectron spectroscopy (XPS) for surface composition confirmation, this work establishes a mechanistically explainable strategy. The resulting insights clarify why moderate plasma activation can simultaneously enhance interfacial kinetics in AA while preserving the conductive Ti–C framework, thereby informing the design of high-rate supercapacitors and electrolyte-sensitive electrochemical systems.

2. Experimental

2.1. Preparation of Electrodes and Supercapacitors

Electrodes were fabricated by mixing polytetrafluoroethylene (PTFE, 60 wt% dispersion) with activated carbon powder (Kurary YP-50F) in 95 wt% ethanol to form a mixture, which was stirred until a viscous carbon paste was obtained. The resulting paste was rolled into a uniform thin film and dried overnight at 80 °C to obtain a stable activated carbon film, serving as the counter electrode. Two electrolytes were employed: 3 M sulfuric acid (H₂SO₄) solution and 500 μM AA in 10× PBS. The pH of the 500 μM AA in 10× PBS was verified to be approximately 7 at room temperature using the universal pH paper (pH 0–14). The reference electrode depended on the electrolyte: a saturated calomel electrode (SCE, Hg₂Cl₂/KCl, CHI150) for the AA solution, and a mercury/mercurous sulfate electrode (Hg/Hg₂SO₄, CHI151) for the 3 M H₂SO₄ solution. The working electrode was prepared by attaching a multilayer Ti₃C₂T_x film (Jilin 11 Technology Co., Ltd., Changchun, China) to a glassy carbon electrode (GCE, CHI104) with an inner diameter of 3 mm. The mass loading of the Ti₃C₂T_x film was 0.38 mg per 3-mm-diameter disc (average of n = 10), corresponding to ~5.38 mg·cm⁻² based on the geometric area. Celgard 2400 served as the separator, permitting ion transport while preventing direct contact between the two electrodes and thus avoiding internal short circuits. Plasma treatment was performed using a Harrick plasma cleaner (PDC-32G) operated at the medium power setting (10.5 W). Pure N₂ was used as the process gas, and the chamber pressure was maintained at 590 mTorr. Samples were placed at the center of the tubular chamber on the standard sample stage with the same orientation for all runs.

2.2. Electrochemical Analysis and Materials Characterization

Electrochemical measurements were performed in a three-electrode configuration using a potentiostat/galvanostat (CHI6273C, CH Instruments, Austin, TX, USA). CV was recorded at scan rates from 2 mV·s⁻¹ to 10,000 mV·s⁻¹. EIS was conducted at various open-circuit potentials with a 5 mV sinusoidal perturbation over a frequency range of 0.1 Hz to 100 kHz, to evaluate the impedance at the electrode/electrolyte interface, charge-transfer kinetics, and ion diffusion characteristics. Surface chemical compositions of Ti₃C₂T_x electrodes were analyzed by XPS (PHI 5000 VersaProbe, ULVAC-PHI, Kanagawa, Japan). High-resolution elemental energy spectra including C 1s, F 1s, O 1s, and Ti 2p were acquired, with binding energies calibrated to the C–C/C–H peak of adventitious carbon at 284.8 eV.

3. Results and Discussion

The electrochemical behavior of multilayer Ti₃C₂T_x electrodes with and without nitrogen plasma modification was compared and evaluated by CV in 3 M H₂SO₄ and 500 μM AA electrolytes at different scan rates. As shown in Figure 1a, the unmodified Ti₃C₂T_x supercapacitor in 3 M H₂SO₄ exhibits an extremely narrow stable potential window of approximately 0.14 V (−0.30 V to −0.16 V), which is likely restricted by hydrogen evolution and/or oxidation reactions. In contrast, owing to its neutral to weakly acidic nature and less stringent redox potential constraints, the AA electrolyte significantly extends the stable potential window to approximately 0.7 V. Although AA is redox-active, under our conditions (500 µM AA in 10× PBS on a multilayer Ti₃C₂T_x film), its faradaic contribution may not appear as distinct peak pairs. Instead, it can contribute as a broadened, pseudocapacitive-like current superimposed on electric double-layer capacitance (EDLC), leading to near rectangular CVs despite the enlarged current response and widened potential window relative to H₂SO₄. Although H₂SO₄ provides high ionic conductivity, the excessive proton concentration can accelerate oxidation or hydrogen evolution at the Ti₃C₂T_x surface, particularly when the applied voltage is close to the decomposition potential of water. Figure 1a,b show that at the scan rates of 2 mV·s⁻¹ and 10 mV·s⁻¹, the CV curves of various Ti₃C₂T_x supercapacitors treated under different plasma conditions all resemble blunt-rounded rectangles, indicating that charge storage mechanism is primarily EDLC accompanied by some surface pseudocapacitance contributions. At these low scan rates, both the device without plasma treatment (noP-AA) and the device treated with nitrogen plasma for 10 min (N10P-AA) display superior performance; even the unmodified electrode maintains sufficient ion/electron exchange, resulting in a CV area comparable to that of N10P-AA. With increasing scan rate, the CV curves gradually evolve from rectangular to spindle-like shapes, accompanied by larger current responses and more pronounced potential hysteresis, indicating that charge-transfer kinetics and ion diffusion become rate-limiting. As shown in Figure 1e,f, at the high scan rates of 500 mV·s⁻¹ and 1000 mV·s⁻¹, the current difference between devices caused by different plasma treatment conditions becomes more pronounced. N10P-AA consistently retains the largest CV area and a faster ion response, demonstrating superior rate capability. In contrast, N15P-AA exhibits a performance decline under certain conditions, which can be attributed to over-etching that induces structural damage and conductivity loss. Meanwhile, noP-H₂SO₄ exhibits the smallest CV curve area with a different shape, reflecting the poorest rate performance.

Nitrogen plasma treatment introduces surface functionalities such as pyridinic N, pyrrolic N, and graphitic N, while simultaneously removing surface contaminants and altering oxide states. The modification increases the density of active sites, enhances pseudocapacitance contributions, and improves wettability and conductivity, thereby facilitating charge transport. The optimized nitrogen plasma duration of 10 min strikes a balance between the number of active sites and the integrity of the conductive network, enabling higher current output and a wide potential window even at high scan rates. Additionally, Figure 1d–f reveal that electrolyte composition significantly influences electrochemical behavior. The devices operated in the 500 μM AA solution display markedly larger currents than those in H₂SO₄, presumably attributed to the participation of AA molecules in reversible Faradaic reactions and the enhanced surface pseudocapacitance. However, the chemical activity of AA may also trigger irreversible side reactions, potentially compromising long-term stability and necessitating further testing and verification. Based on the results in Figure 1, it has been confirmed that nitrogen plasma modification is beneficial for improving the potential window and rate capability of Ti₃C₂T_x electrodes and supercapacitors, with N10P identified as the optimal plasma treatment condition. Notably, the performance enhancement does not scale linearly with plasma duration. Prolonged treatment can inhibit performance improvement, suggesting the need to optimize treatment parameters for optimal results in practical applications. An appropriate nitrogen plasma treatment balances introducing active sites and retaining conductivity, thus enabling higher energy storage performance over a wider potential window and at a high scan rate.

As displayed in Figure 2a, all Ti₃C₂T_x supercapacitors exhibit nearly 100% coulombic efficiency at low scan rates (2 mV·s⁻¹ to 10 mV·s⁻¹). This indicates a high charge utilization efficiency, as ions have sufficient time to react completely at the electrode surfaces under slow charge–discharge conditions. However, as the scan rate increases to 10,000 mV·s⁻¹, coulombic efficiency drops significantly. This reduction is most pronounced for the untreated device using the 3 M H₂SO₄ electrolyte (noP), where the efficiency sharply declines below 40%. In contrast, the nitrogen plasma-treated supercapacitors using the AA electrolyte (N5P, N10P, and N15P) maintain relatively high coulombic efficiencies (>72%) at high scan rates, demonstrating that nitrogen plasma treatment effectively improves the reversibility of the Ti₃C₂T_x electrodes during rapid charge–discharge cycles. Figure 2b presents the rate capability for all Ti₃C₂T_x supercapacitors. The nitrogen plasma-treated devices consistently outperform untreated ones, particularly in the medium to high scan rate range (100 mV·s⁻¹ to 1000 mV·s⁻¹), indicating their better electrochemical kinetics. The phenomenon can be attributed to introducing nitrogen-containing functional groups and surface defects during nitrogen plasma treatment, which enhances interfacial polarization and electronic conductivity, thereby reducing the ion transport impedance under high-rate conditions. Additionally, the rate capability curves of N10P-AA and N15P-AA are flatter than that of N5P-AA, suggesting that extending the nitrogen plasma treatment duration can further optimize surface chemical properties and pore structures, contributing to high-rate capability. Figure 2c shows the corresponding specific capacitance results supporting the above observations. At low scan rates, the noP-H₂SO₄ supercapacitor exhibits the highest initial specific capacitance (approximately 120 F·g⁻¹), but it decays most rapidly with increasing scan rate. This indicates that while an acidic environment enhances the energy storage at low rates, it limits the cycling stability at high rates. Conversely, while N10P-AA and N15P-AA exhibit slightly lower specific capacitances at lower rates (approximately 100 F·g⁻¹), their capacitance retention at high rates is better, implying the effectiveness of nitrogen plasma treatment in enhancing both long-term cycling tolerance and high-rate charge and discharge adaptability. The results in Figure 2 confirm that nitrogen plasma modification can improve the electrochemical stability of Ti₃C₂T_x under high rates and the overall performance across different scan rates.

EIS results further elucidate the influences of bias potential and plasma treatment. As seen in Figure 3a,b, the slope of the low-frequency tail in the Nyquist plot gradually flattens and shifts toward 45° as the applied bias becomes more negative. This reflects that the real impedance (Z′) and imaginary impedance (Z″) become more equivalent, indicating that the system is gradually controlled by ion diffusion limitation. At more negative voltages (−0.8 to −1.2 V), a reduction reaction occurs on the Ti₃C₂T_x surface, rapidly oxidizing AA to DHA, resulting in a rapid depletion of AA and a steep concentration gradient between the electrode surface and the electrolyte. The system enters a diffusion-controlled regime since the diffusion rate cannot keep up. On the contrary, when a more positive voltage is applied (close to 0 V), the low-frequency tail of the Nyquist plot goes more vertical, signifying a more capacitive behavior with more efficient mass transport to the electrode surface. At this time, the reactants can more effectively diffuse and replenish the electrode surface, preventing the formation of a concentration gradient and thus avoiding a diffusion-limited regime. Thanks to the high conductivity and large specific surface area of Ti₃C₂T_x, the electrode exhibits supercapacitor-like behavior, and the interfacial electrochemical reaction is primarily surface-controlled. Figure 3c shows that for the untreated Ti₃C₂T_x supercapacitor noP-AA, the semicircle in the high-frequency region gradually increases as the bias sweeps from 0 V to −1.2 V, indicating a rise in charge-transfer impedance (R_CT). Figure 3d demonstrates that N10P-AA obtained by 10-min nitrogen plasma modification exhibits a similar trend. However, its overall R_CT is lower than that in noP-AA, indicating that nitrogen plasma treatment can reduce R_CT and enhance electron transfer ability. This finding aligns with the aforementioned CV results, where N10P-AA caused a larger current and lower polarization potential, revealing its higher conductivity and more effective interfacial reaction kinetics compared to noP-AA. These also demonstrate that the appropriate nitrogen plasma modification promotes electrode surface activation and accelerates AA redox kinetics. The Nyquist plots in Figure 3 obtained from EIS analysis have clearly shown that nitrogen plasma treatment effectively reduces R_CT across the entire bias voltage range and mitigates the impact of diffusion limitations at high bias voltages. These findings underscore the potential of nitrogen plasma-modified MXenes for future electrochemical energy storage and biosensing applications.

Figure 4 presents the magnified high-frequency regions of the Nyquist plots for Ti₃C₂T_x supercapacitors under different plasma treatment conditions at the applied bias voltages of −0.3 V and −0.7 V. The intercept on the real axis corresponds to the series resistance (R_s), while the semicircle diameter reflects the magnitude of R_CT. As shown in Figure 4a, under −0.3 V, the untreated device in H₂SO₄ (noP-H₂SO₄) exhibits the smallest R_s (approximately 1.8 Ω), primarily due to the high ionic conductivity of the protonic acid electrolyte. In comparison, the noP-AA device displays a larger R_s and a larger semicircle, indicating an increased R_CT. Figure 4a,b show that with an increasing nitrogen plasma treatment duration, the Nyquist plot gradually shifts towards a higher real impedance, corresponding to an increase in R_s. As shown in Figure 4b, under −0.7 V, the plots shift further to the right relative to those at −0.3 V in Figure 4a, reflecting a more substantial polarization effect at a more negative potential, leading to increases in both R_s and v. N10P-AA maintains a relatively minor R_CT, highlighting its better interfacial charge-transfer capability across different operating bias voltages. By contrast, N15P-AA exhibits the largest R_s and R_CT, again confirming that excessive nitrogen plasma treatment can deteriorate charge transport. Collectively, N10P-AA achieves an optimal balance of Rs and a consistently low R_CT in the AA electrolyte across different operating bias voltages, thus delivering the most favorable energy-storage characteristics.

The magnitude of the bias voltage also pronouncedly influences diffusion behavior. As the applied bias shifts from negative to less negative (−1.0 V to −0.3 V), the Warburg slope consistently decreases for all Ti₃C₂T_x supercapacitors. This trend can be attributed to the physicochemical changes at the electrode/electrolyte interface. The variation in bias voltage alters the local electric field intensity and the structure of the electrical double layer near the electrode surface. Under a more negative potential (e.g., −1.0 V), more negative charges are on the electrode surface, exerting a stronger electrostatic interaction on the electrolyte ions and thereby increasing the diffusion resistance. As the potential becomes less negative, the electric field on the electrode surface weakens, reducing electrostatic hindrance, facilitating ion diffusion, and improving transport efficiency. During this process, the electrochemical behavior in the system gradually transits from being diffusion-limited towards being more surface-controlled, exhibiting a more capacitance-dominated behavior. Furthermore, the change in bias potential not only affects the diffusion process but also the activation energy of the electrochemical reactions. At more favorable potentials (e.g., −0.3 V), faster charge-transfer kinetics can more effectively sustain the concentration gradients at the interface and accelerate diffusion, reducing Warburg impedance.

The findings in Figure 5 highly align with the preceding CV and EIS results. The relatively minor Warburg slope observed for N10P-AA indicates that ions can be rapidly replenished at the reaction interface during fast surface reactions, effectively avoiding diffusion bottlenecks. This explains its ability to maintain a larger CV curve area and high current density at high scan rates. In contrast, N15P-AA exhibits larger Warburg slopes across all potentials, in agreement with its larger R_CT observed in its Nyquist plots. These suggest that both electron transport and mass transfer are limited, accounting for its inferior CV performance. A particularly noteworthy observation is shown in Figure 5c at −0.3 V, where the untreated noP-H₂SO₄ device exhibits the smallest Warburg slope among all Ti₃C₂T_x supercapacitors in this study, indicating that its interfacial behavior is closest to that of an ideal electric double-layer capacitor. This finding contrasts sharply with the large slope observed for noP-AA using the AA electrolyte. It is then inferred that while the pristine Ti₃C₂T_x surface possibly provides a highly efficient ion diffusion pathway in a simple proton (H⁺) ion system, it is not conducive to ion transport when larger and more complex active species like AA are introduced. This underscores the critical role of nitrogen functionalities or microstructures, introduced by moderate nitrogen plasma treatment, in enhancing the diffusion efficiency of specific active species. Nevertheless, this optimization effect strongly depends on the electrochemical environment. Taken together, the analysis reconfirms that a nitrogen plasma treatment duration of 5 min to 10 min can be considered the optimal time window for Ti₃C₂T_x, as it strikes a balance between generating sufficient active sites and preserving the integrity of the conductive substrate. However, its practical benefits still need further verification for different application systems.

To further clarify the effect of nitrogen plasma treatment on the charge storage mechanism of Ti₃C₂T_x supercapacitors, the b values were calculated by fitting the logarithmic relationship between scan rate and peak current. A b value closer to 1 indicates more surface-controlled behavior, whereas a b value closer to 0.5 reflects more diffusion-controlled processes. Here, “surface-controlled” refers to the dominance of near-surface charge storage kinetics (i ∝ v^b), rather than requiring rapid long-range diffusion of AA into deep MXene interlayers. Although AA is bulkier than protons, its contribution in this system is mainly interfacial/surface-mediated, and moderate N₂-plasma activation lowers the interfacial electron transfer barrier by introducing polar terminations/active sites, thereby strengthening the surface kinetic contribution reflected by higher b values. The b values of noP-H₂SO₄, noP-AA, N5P-AA, N10P-AA, and N15P-AA were 0.71, 0.74, 0.81, 0.79, and 0.73, respectively. All are within the range of 0.7 to 0.8, indicating that their charge storage involves both surface- and diffusion-controlled contributions, with surface effects being dominant. The lowest b value of noP-H₂SO₄ suggests a greater reliance on proton diffusion at the electrode surface and between layers. The use of AA raises the b value to 0.74 (noP-AA), reflecting the contribution from the surface redox reactions of AA. Nitrogen plasma treatment further increases the b values to 0.81 (N5P-AA) and 0.79 (N10P-AA), demonstrating that nitrogen functional groups enhance surface polarity and electrolyte wettability, and generate more active sites, thereby strengthening surface-controlled behavior. In contrast, prolonged treatment reduces the b value to 0.73 (N15P-AA), likely due to structural degradation and defect accumulation from over-etching, which impede ion/electron transport and shift the mechanism toward diffusion control. Overall, AA combined with moderate nitrogen plasma treatment can effectively reinforce the surface-controlled properties of Ti₃C₂T_x electrodes, while excessive plasma treatment damages their electrochemical activity.

The b-value trend aligns well with the Warburg analysis of mass transport. Therefore, b value mainly reflects kinetic control near the interface, whereas the Warburg slope reflects mass transport limitation. noP-H₂SO₄ exhibits the smallest Warburg slope, indicating faster ion replenishment and minimal diffusion resistance in the protonic electrolyte. By contrast, noP-AA exhibits a larger b value and a markedly larger Warburg slope, reflecting hindered ion transport caused by the bulkier and more complex AA molecules. The large b values of N5P-AA and N10P-AA combine with their lowest Warburg slopes, confirming that nitrogen plasma treatment improves wettability and ion transport efficiency by introducing nitrogen functionalities and tailoring surface structure, thus increasing the contribution from surface-controlled processes. Conversely, N15P-AA exhibits a reduced b value and a larger Warburg slope, indicating accumulated defects and transport limitations that deteriorate its electrochemical performance. These results jointly provide quantitative evidence that moderate nitrogen plasma treatment can optimize the kinetic control mechanism of Ti₃C₂T_x electrodes, reaffirming its importance in achieving more efficient, surface-dominated charge storage.

XPS analysis was also conducted to examine the C 1s, O 1s, F 1s, and Ti 2p spectra, aiming to elucidate the evolution of surface chemistry in Ti₃C₂T_x electrodes before and after nitrogen plasma treatment, and compare with the aforementioned electrochemical results for verification. As displayed in Figure 6a, the pristine Ti₃C₂T_x exhibits mainly two characteristic peaks at approximately 282.0 eV (C–Ti–T_x) and 284.8 eV (C–C/C–H), indicating structural integrity with negligible contributions from C–O or C=O bonds [21]. As shown in Figure 6b, after electrochemical cyclings in AA, both the electrodes of noP-AA and N10P-AA exhibit CF_x peaks (290.0 eV to 298.0 eV) and intensified C–O (~286.9 eV) and C=O (~288.8 eV) signals, demonstrating surface oxidation and the participation of fluorine species in reactions during the cyclings [21]. A comparison of the C–Ti–T_x peaks reveals the highest intensity for pristine Ti₃C₂T_x and the lowest intensity for N10P-AA, which can be attributed to high-energy ion bombardment during plasma treatment that disrupted Ti–C bonds, generated more adventitious carbon (C–C/C–H), and possibly induced passivation layer formation or altered oxidation kinetics, thereby suppressing excessive oxidation. As shown in Figure 6c, nitrogen plasma treatment also introduces new carbonaceous structures on the electrode surface of N10P-AA. These changes are highly consistent with the aforementioned electrochemical results, that is, the passivation layer in N10P-AA protected its electrode surface. It enabled higher conductivity and rate capability, particularly at high scan rates. As shown in Figure 6d, the F 1s spectra exhibit a dominant peak at approximately 685.0 eV corresponding to C–Ti–F_x bonds and a weak peak at 686.5 eV to 688.7 eV assigned to fluorine impurities [21]. As seen in Figure 6e,f, after electrochemical cyclings in AA, the impurity peak is markedly attenuated, suggesting the reductive effect of AA removes unstable fluorine species. Notably, Figure 6f shows that the 10-min nitrogen plasma treatment effectively eliminates fluorine-containing impurities on the electrode surface of N10P-AA, helps reconstruct the surface, and reduces surface defects.

As displayed in Figure 7a–c, the O 1s spectra comprising the peaks of TiO₂ (529.9 eV), C–Ti–O_x (530.8 eV), C–Ti–OH_x (531.9 eV), and H₂O (533.4 eV), provide more insights into the evolution of surface functional groups [21]. Figure 7a reveals that the pristine Ti₃C₂T_x contains more TiO₂ on its surface. As shown in Figure 7b, after electrochemical cyclings in AA, the peak intensity of TiO₂ on the electrode surface of noP-AA weakens while that of C–Ti–O_x slightly increases, indicating that AA, as a strong reducing agent, removes part of the unstable oxide layer and generates more stable hydroxyl terminal groups (C–Ti–OH_x and –OH). As revealed in Figure 7c, the TiO₂ signal intensity on the electrode surface of N10P-AA is even smaller after 10-min nitrogen plasma treatment, and the C–Ti–OH_x signal intensity becomes relatively larger, strongly indicating that nitrogen plasma promotes surface hydroxylation and mitigates excessive Ti oxidation/passivation during cycling. The hydroxyl terminations impart excellent hydrophilicity and provide more adsorption and proton-buffering sites, contributing to enhanced capacitance and ion diffusion. The findings from Figure 6d–f and Figure 7a–c correlate well with those from CV, EIS, and Warburg analysis, confirming that the Ti₃C₂T_x electrode of N10P-AA possesses more stable and electrochemically favorable surface functional groups.

Figure 7d–f presents the Ti 2p spectra of Ti₃C₂T_x under different conditions. As seen in Figure 7d, the pristine material exhibits four pairs of feature peaks corresponding to C–Ti–O, C–Ti–OF, C–Ti–F, and TiO₂, indicating that its surface has been partially oxidized during synthesis and exposure [21]. As displayed in Figure 7e, the peak intensity markedly decreases after electrochemical cyclings in AA, reaffirming the reductive role of AA in suppressing the formation of TiO₂. Meanwhile, the ratios of C–Ti–F and C–Ti–O increase, reflecting the redistribution of surface terminations. As shown in Figure 7f, after 10-min nitrogen plasma treatment to achieve N10P-AA, the peak intensity of TiO₂ increases instead, suggesting that the plasma introduces more oxygen-containing groups. Compared with the electrode material in noP-AA, that in N10P-AA exhibits more prominent C–Ti–O and C–Ti–OF peaks, indicating that nitrogen plasma promotes oxygen affinity and also reinforces re-oxidation tendency on the surface. These results highlight the competing effect of AA and plasma: AA tends to suppress the formation of TiO₂, whereas nitrogen plasma inversely promotes the formation of TiO₂ and related oxygen-containing functional groups. The interaction between the two largely determines the chemical composition and stability of the Ti₃C₂T_x surface. To avoid ambiguity, it should be emphasized that XPS peak intensities mainly reflect the relative distribution of surface species within the probing depth, rather than a monotonic change in the absolute amount of a single phase. After electrochemical cycling in AA, the reducing nature of AA can attenuate unstable oxide-rich features, whereas N₂ plasma exposure can simultaneously promote the redistribution of surface terminations and expose additional Ti sites. Therefore, the increased Ti–O/TiO₂-related contribution observed in Ti 2p spectrum after plasma treatment is best interpreted as a reorganization toward oxygen-containing terminations (e.g., Ti–O/Ti–OH/Ti–OF), rather than the growth of a thick, insulating TiO₂-rich passivation layer. In this context, the apparently different Ti–O/TiO₂-related contributions in the O 1s and Ti 2p spectra can be reconciled within a unified framework of termination/oxidation-state reorganization, rather than contradictory oxidation behaviors.

Looking at Figure 6 and Figure 7, the C 1s, F 1s, O 1s, and Ti 2p spectra obtained from different conditions show no significant chemical shifts in their respective peak positions, implying that the bonding environment and chemical states of Ti₃C₂T_x remain relatively stable. The effects of nitrogen plasma and using AA are primarily associated with regulating surface properties by changing the relative proportions of surface functional groups rather than altering the intrinsic bulk structure of Ti₃C₂T_x. The enhanced performance of N10P-AA over other devices is attributed to adjusting the number and distribution of surface functional groups rather than the difference in the intrinsic chemical state. The XPS results provide a surface chemistry foundation [21], strongly supporting the aforementioned conclusions from electrochemical analysis. They clearly reveal that nitrogen plasma treatment (particularly 10 min) can effectively inhibit excessive oxidation of the Ti₃C₂T_x electrode during electrochemical cyclings while preserving the conductive framework. This protective effect is consistent with reduced R_CT and mitigated diffusion limitation for N10P-AA in the AA electrolyte, resulting in a larger b value and a smaller Warburg slope, and thus the overall best electrochemical performance among the AA-based devices. In contrast, noP-AA, which has not been treated with nitrogen plasma, suffers from severe oxidation and surface passivation, which impede both electron and ion transport, leading to inferior electrochemical performance.

4. Conclusions

This study demonstrated that nitrogen plasma modification effectively optimized the electrochemical performance of multilayer Ti₃C₂T_x electrodes and supercapacitors. Two electrolytes, 3 M H₂SO₄ and 500 μM AA, were compared. The results showed that AA significantly broadened the stable potential window to approximately 0.7 V, while the H₂SO₄ solution only limited it to approximately 0.14 V. The Ti₃C₂T_x electrode treated with nitrogen plasma for 10 min exhibited the best energy storage performance, achieving an ideal balance between introducing active sites and maintaining the integrity of the conductive network. The N10P-AA supercapacitor exhibited consistently small R_CT in the AA electrolyte across the investigated bias range and achieved the best balance of R_s and R_CT among the AA-based devices, demonstrating better energy storage characteristics. At medium to high scan rates (100 mV·s⁻¹ to 1000 mV·s⁻¹), the electrode of N10P-AA outperformed the untreated electrode, maintaining the largest CV curve area and rapid ion response, demonstrating higher rate capability. Furthermore, even at an ultra-high scan rate (10,000 mV·s⁻¹), N10P-AA maintained a relatively higher coulombic efficiency (>72%), demonstrating that this method indeed improved the reversibility during rapid charge and discharge cycles. However, prolonged plasma treatment time (for N15P-AA) could cause structural damage and conductivity loss due to over-etching, resulting in performance degradation.

Mechanism analysis further revealed the reasons for performance improvement: (1) Nitrogen plasma treatment introduced nitrogen-containing functional groups and surface defects, enhancing interface polarization, wettability, and conductivity. The EIS results showed that the R_CT in N10P-AA was effectively reduced over the entire bias voltage range, compared to the untreated devices. The b values ranging from 0.7 to 0.8 indicated that the charge storage of all devices involved both surface-controlled and diffusion-controlled contributions, with the surface effect being dominant. N10P-AA exhibited a combination of a higher b value (0.79) and a relatively minor Warburg slope, suggesting that its rapid surface reactions and the effective ion replenishment to the interface could avoid diffusion bottlenecks. (2) Surface chemical analysis (XPS) results confirmed that the 10-min nitrogen plasma treatment effectively suppressed excessive oxidation during electrochemical cyclings, promoted the formation of hydroxylated surface functional groups (such as C–Ti–OH_x) beneficial for electrochemical reactions, and removed surface fluorine impurities, providing a more stable and electrochemically active surface. In summary, moderate nitrogen plasma modification offers a feasible and optimized strategy for future high-efficiency electrochemical energy storage and potential biosensing applications.