Laser-architected MXene composite for photoenhanced microsupercapacitor

Abstract

Photoenhanced energy storage devices represent an emerging technology that integrates solar energy harvesting with efficient electrochemical storage. However, achieving seamless and efficient coupling between light utilization and high-performance charge storage within a single, integrated platform remains a major challenge. To address this challenge, this study develops laser-architected MXene-graphene composites that unify high-performance energy storage with photoenhancement. The reduced graphene oxide (rGO) and partially oxidize MXene composite induced by a laser redox process exhibits a hierarchical architecture with outstanding conductivity and surface area. This synergy enables supercapacitors with photoenhanced capacitance, delivering a 228% boost under illumination and record-breaking metrics in capacitance, 2591.75 farads per cubic centimeter (1455.21 farads gram), energy density, 0.518 watt-hours per cubic centimeter (0.345 watt-hours gram), and power density, 320.35 watts per cubic centimeter (180.31 watts gram). Our findings offer a promising route toward integrated, photoenhanced energy systems, advancing the vision of efficient and sustainable power technologies.

A laser-architected MXene-graphene composite with record-breaking photoenhanced energy storage performance is developed.

INTRODUCTION

In today’s rapidly industrializing world, the energy crisis has become a substantial barrier to further progress. Addressing the dual threats of resource depletion and environmental pollution requires the advancement of clean and renewable energy technologies (1–3). Among various approaches, electrochemical energy storage systems are particularly promising, powering everything from portable electronic devices to electrified transportation and multifunctional components. Supercapacitors, in particular, have gained attention for their high power density, long cycle life, and remarkable flexibility. Concurrently, solar energy represents an abundant and sustainable power source, yet its intermittent nature underscores the urgent need for efficient storage strategies. Bridging this gap, integrated systems capable of high-efficiency solar energy conversion and storage are essential for enabling next-generation energy infrastructure (4, 5).

Recent studies have demonstrated the effective enhancement of ion transport through the direct coupling of solar energy into the photogenerated carrier and electric field dynamics of nanomaterials via photosensitive materials (6). The generation of charge carriers using photosensitizers to augment the capacitive performance of energy storage materials has progressively emerged as an effective research strategy (7–9). However, the divergent functionalities of photosensitive and electrochemical energy storage materials typically necessitate distinct material systems to achieve photoenhanced capacitance. MXene, a leading electrochemical energy storage material, has garnered substantial attention over the past decade (10). Our prior work used laser-induced selective oxidation to generate the photosensitive material titanium dioxide (TiO₂) in situ within MXene. This technique proves particularly effective within composite two-dimensional material systems, providing additional nucleation sites for TiO₂ formation (11, 12). Nevertheless, achieving integrated design of photosensitive and electrochemical energy storage functionalities within such materials, coupled with precise control over composite composition and morphology to enhance capacitive storage performance, remains a substantial challenge.

In this study, inspired by natural layered and three-dimensional flower-like architectures, we designed an integrated MXene/TiO₂–reduced graphene oxide (rGO) composite material (LfMT). Synergistic control over the material’s microstructure and composition was achieved through femtosecond laser temporal shaping and parameter modulation. This integrated photosensitive/energy storage material system provides abundant light-trapping sites for the TiO₂ photosensitizer, markedly enhancing its solar spectral response. Graphene oxide (GO) acts as both a catalyst and substrate carrier during laser-induced modification of MXene, undergoing photoinduced reduction to form rGO. This process further augments the composite’s capacitive characteristics and electrical conductivity. The three-dimensional composite architecture substantially increases the specific surface area, furnishes abundant active sites, and facilitates efficient charge and ion transport. When deployed as an electrode, the LfMT composite exhibits exceptional capacitive and photocatalytic performance. Under illumination, its electrochemical energy storage capacity is substantially enhanced, enabling stable energy output to meet persistent power delivery requirements.

RESULTS

Laser-induced various flower-like LfMT composite

In conventional laser liquid-phase ablation, continuous or long-pulse laser systems typically generate substantial thermal effects, thereby facilitating thermally driven chemical reactions in the target materials. In contrast, femtosecond lasers, with their ultrashort pulse duration (100 fs) and extremely high instantaneous power (10¹⁴ W), induce pronounced nonequilibrium effects in solutions, producing a large number of free electrons. In our setup (fig. S1A), a single-pulse Gaussian laser is transformed via a Michelson interferometer into a double-pulse femtosecond laser with tunable pulse delay (10 ps). This configuration enables real-time monitoring of sample processing with precise control over the dispersion of mixed MXene and GO nanosheets. The entire procedure is fully automated, ensuring high accuracy and reproducibility. As illustrated in Fig. 1 (A and C), when the MXene-GO mixture is exposed to shaped femtosecond laser pulses, rapid and intense plasma formation is initiated within the liquid medium. This plasma, generated under highly nonequilibrium conditions near the focal region, contains abundant reactive species. Simultaneously, GO is photothermally reduced to rGO, while MXene undergoes partial photoinduced oxidation, forming oxidized nanoparticles. The application of dual-pulse laser irradiation enhances the ionization process, intensifying the plasma effect, which facilitates the morphological reshaping and controlled oxidation of MXene nanosheets, alongside the complete reduction of GO. Through systematic variation of laser irradiation parameters, including power (5 to 40 mW) and pulse delay (0 to 30 ps), we achieved precise control over MXene morphology (fig. S1B), enabling the creation of floral structures with diverse configurations.

Fig. 1. Schematic diagram of the formation process and the morphology of LfMT/rGO composite materials.

(A) Reaction process of the shaped femtosecond laser with MXene and GO in a liquid-phase environment. (B) Photos of the different flowers in nature. (C) Schematic diagram of the laser-induced synthesis of LfMTs. (D) SEM images of different morphologies of LfMTs synthesized in our experiment. In accordance with varying laser types and parameters, the resulting LfMT is designated accordingly. The hybrid material synthesized through a single femtosecond laser operating at an energy level of 10 mW is denoted as s-LfMT. A double-pulse femtosecond laser with a pulse delay of 10 ps is implemented. The induced material distribution, classified on the basis of distinct laser power (10, 20, and 30 mW), is labeled as d-LfMT/10, d-LfMT/20, and d-LfMT/30, respectively. (E) BET surface area of the MXene/GO and as-prepared LfMT composite. (F) Comparison of electrical conductivities of MXene/GO and LfMTs with different bulk densities. (G) The volumetric capacitance and the ability to enhance energy storage under light are compared with other similar work. (H) Energy and power densities of the d-LfMT/20 composite MSCs compared with other capacitors.

As shown in Fig. 1B, we draw inspiration from natural floral morphologies, such as daisies, chrysanthemums, dandelions, and seagrass flowers, which feature elongated petals and branched structures that optimize surface area for efficient sunlight absorption and water collection. Similarly, by fine-tuning femtosecond laser parameters, we can produce LfMT structures that mimic these natural forms (Fig. 1D). This biomimetic structure substantially increases the material’s specific surface area, providing abundant sites for ions and charges and ensuring thorough contact with the electrolyte. The Brunauer-Emmett-Teller (BET) surface area analysis (Fig. 1E) shows that the d-LfMT/20 composite achieves the highest surface area at 726.88 m² g⁻¹, higher than the 37.52 m² g⁻¹ of GO/MXene. The pore-size distribution curve indicates that the dominant mesopores range from 6.5 to 13.2 nm, with most pores between 7.39 and 7.94 nm (fig. S2). The conductivity of different materials (Fig. 1F) varies with density, with d-LfMT/20 composites achieving a high conductivity of 97,181 S m⁻¹, maximizing the electrochemical potential of the composite.

We evaluated the composite as an electrode by comparing its electrochemical energy storage performance under illuminated and dark conditions. The results demonstrated outstanding characteristics, including an exceptionally high volumetric capacitance and a 228% enhancement in energy storage capacity upon light irradiation (Fig. 1G). This capability surpasses similar works (13–21), with light-assisted conditions boosting the volumetric capacitance of our microsupercapacitors (MSCs) to 2591.3 F cm⁻³. A Ragone plot (Fig. 1H) comparing the energy density and power density of d-LfMT/20 composite MSCs with other energy storage devices shows a remarkable energy density of 0.458 watt-hours (Wh) cm⁻³ and a power density of 283.5 W cm³, several orders of magnitude higher than other supercapacitors (13, 14, 17, 21–27).

Characterization and electrochemical simulation of laser-induced LfMT composites

We used COMSOL Multiphysics to model and simulate the electric field and potential distributions of different electrode materials under a fixed electrolyte environment. The simulations reveal that LfMT composites exhibit stronger electric fields and higher potential distributions compared to the pristine material, particularly at the edges of the flower-like structures (Fig. 2A). Among them, the d-LfMT/20 composite shows the highest overall electric field intensity and potential distribution, indicating superior electrochemical performance (Fig. 2, B and C). These results suggest that the hierarchical flower-like architecture plays a key role in enhancing the functionality of the electrode composites.

Fig. 2. Formation mechanism and characterization of LfMTs.

(A) Simulated potential and electric field distributions of different LfMTs by the COMSOL soft. Electric field (B) and potential (C) distribution at edge interfaces of different materials. (D) Large-area distribution of LfMT composites; the illustration shows the size distribution of flower-like structures. (E) SEM image of the single LfMT and the surface element energy of the LfMT. (F) TEM and the magnified high-resolution TEM image of LfMT composites. (G) Raman spectra of GO/MXene, s-LfMT, d-LfMT/10, d-LfMT/20, and d-LfMT/30. a.u., arbitrary units. Ti 2p (H) and O 1s (I) peaks observed in the XPS analysis of d-LfMT/20.

To further investigate the formation mechanism of this structure, we conducted pump-probe measurements to capture plasma plume dynamics during the laser ablation process, as shown in fig. S3 (A and B). Transient morphological changes of vapor bubbles at the gas-liquid interface were observed, including distinct stages of bubble formation, expansion, stabilization, contraction, springback, and collapse. As illustrated in fig. S3C, we used an objective lens and a telephoto imaging system to project the plasma plume onto an intensified charge-coupled device camera, enabling high-speed imaging with a minimum time resolution of 2 ns. This configuration allowed us to monitor the rapid plasma evolution and material synthesis dynamics, which ultimately lead to the emergence of the bionic flower-like LfMT composite structure.

To comprehensively characterize the nanoflower structures generated via laser liquid-phase ablation, we used a combination of analytical techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). For a comprehensive analysis of the nanoflower structures created through laser liquid-phase ablation, we used SEM, TEM, Raman spectroscopy, XRD, and XPS techniques. Figure 2D shows the distribution of the nanoflowers, with sizes ranging from 300 to 900 nm. Nanoflowers around 600 nm are the most abundant, accounting for ~34% of the population. Figure 2E reveals the elemental composition of a single flower-like structure, showing high peaks for Ti and C, with less oxygen, indicating that the structure is primarily MXene with minor oxidation. Figure 2F presents a TEM image of a complete flower profile, with high-resolution TEM images revealing titanium dioxide nanoparticles growing within Ti₃C₂ layers and rGO. The lattice fringes correspond to the (002) crystal faces of rGO, the (103) plane of Ti₃C₂, and the (101) plane of TiO₂ (28, 29). Raman spectra (Fig. 2G) of the composites, induced by different laser parameters, show distinct TiO₂ peaks at around 150° and 401° while retaining some original MXene peaks (30). A markedly lower D/G intensity ratio was observed in pristine GO compared to laser-treated composites, indicating effective laser-induced reduction, as shown in fig. S4A. XRD patterns confirmed the coexistence of graphene, MXene, and TiO₂ in the composites (31), whereas full-range XPS spectra (fig. S4, B and C) revealed the elemental composition of C, O, and Ti (32). In situ Raman (fig. S4D) and XPS analyses (fig. S4E) were conducted under photocharging (0 → 1.2 V) and discharging (1.2 → 0 V) conditions to evaluate the electrochemical response of the LfMT electrodes. The Raman spectra showed stable peak positions and enhanced intensity under illumination, indicating structural integrity and increased surface polarizability. Corresponding XPS spectra revealed stronger O 1s and Ti 2p signals under light, particularly at high voltages, suggesting photoinduced surface oxidation and enhanced ion interactions. These results collectively confirm the chemical and structural stability of the LfMT electrode during operation and highlight the role of photoassisted effects in boosting interfacial activity and overall device performance (33, 34).

The XPS spectra of Ti 2p and O 1s for d-LfMT/20 composites reveal the chemical states of titanium and oxygen, providing evidence of laser-induced oxidation and TiO₂ formation, as shown in Fig. 2 (H and I). The Ti 2p spectrum reveals changes in the ratio of Ti─C (455.1 and 461.2 eV) and Ti─O (458.5 and 464.4 eV) peaks, indicating the composition and content ratio of the material (35). The O 1s spectrum allows us to assess the O─C (532.3 eV) bond and observe changes in the Ti─O bond. Detailed XPS analysis of the Ti 2p, O 1s, and C 1s regions revealed distinct bonding characteristics, including C─C (284.8 eV) and C─O (286.4 eV) components, offering insights into the chemical environment. These results indicate effective reduction of GO, attributed to laser-induced photochemical and photothermal effects (36), as illustrated in fig. S5. Table S1 summarizes the overall proportion of C and O in the XPS spectrum, showing that the C─C bond content increases while the O─C bond proportion decreases after laser induction. These findings confirm the substantial reduction of GO by the femtosecond laser, which photochemically breaks the oxygen bond while preserving the stable C─C bond. In addition, the increasing Ti─O bond proportion in the composite product compared to the original GO/MXene material signifies the oxidation of MXene during laser processing, leading to the formation of titanium oxide. A detailed analysis of the Ti 2p XPS spectra reveals the progressive oxidation of titanium in LfMT composites with increasing laser energy (fig. S6). Compared to the pristine GO/MXene, laser-treated samples show a decrease in Ti─C and Ti²⁺ signals, alongside a marked increase in Ti⁴⁺ content, indicating the formation and dominance of TiO₂. Quantitative results (table S2) confirm that d-LfMT/20 exhibits the highest TiO₂ content and lowest unstable TiO fraction, reflecting enhanced oxidation and stability of titanium species in the composite (37, 38).

Energy storage and photocatalytic performance of LfMT composite–based microdevices

We developed a microdevice by coating a flexible polyethylene terephthalate (PET) substrate with an LfMT composite material, using a polyvinyl alcohol (PVA)/H₂SO₄ gel electrolyte as the separator. This configuration creates a sandwich structure for MSCs. The electrochemical storage mechanism of LfMT composites acting as electrodes involves efficient charge accumulation and transfer processes, as depicted in Fig. 3A. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements of LfMT-MSCs exhibit well-defined triangular and rectangular profiles (fig. S7, A and B), indicative of excellent capacitive behavior. A summary of areal capacitance values from CV and GCD data is provided in Fig. 3B, where the d-LfMT/20 variant achieves the highest capacitance of 188 mF cm⁻², outperforming pristine GO/MXene composites and other laser-processed materials. GCD and CV curves of d-LfMT/20 MSCs across various current densities and scan rates demonstrate stable electrochemical behavior and rate adaptability (fig. S8, A to C). Both areal and volumetric capacitances were evaluated at different scan rates, with d-LfMT/20-MSC achieving a maximum volumetric capacitance of 1250 F cm⁻³ and a maximum areal capacitance of 375 mF cm⁻² (Fig. 3C). We also determined the specific capacitance of the LfMT-based MSC at various scan rates, reaching a maximum value of 751.2 F g⁻¹. In addition, the corresponding gravimetric energy and power densities were calculated, yielding peak values of 0.1502 Wh g⁻¹ and 81.07 W g⁻¹, respectively, as shown in fig. S9. Figure 3D illustrates the equivalent series resistance (ESR) curve under varying laser energies, revealing that d-LfMT/20 has an internal resistance of only 0.025 ohms cm⁻², lower than that of d-LfMT/10 and d-LfMT/30-MSC, confirming its superior electrochemical performance. Figure 3E shows the cycling stability of d-LfMT/20-MSC, retaining ~95.3% of its performance after 12,000 cycles. The similarity between the GCD curves of the first and last cycles underscores the device’s excellent stability, making d-LfMT/20 highly suitable for MSCs with outstanding performance. To ensure that the performance comparison is scientifically meaningful and practically valuable, we have included in Fig. 3F a selection of ultrathin (thickness ranging from 0.8 to 20 μm) graphene- and MXene-based MSCs for comparison with our d-LfMT/20 MSCs in terms of areal capacitance. It can be clearly observed that the performance of our fabricated device substantially outperforms those of the referenced works. For ultrathin energy storage devices, it is generally challenging to achieve high mass loading of electrode materials. Therefore, achieving high areal capacitance is of great practical importance for their real-world application. Table S3 summarizes and compares the capacitance, energy density, and power density of this work with those reported in related studies, highlighting the comprehensive superiority of our device performance (39–53).

Fig. 3. Electrochemical and photocatalytic performances of the resulting LfMT.

(A) Schematic diagram of LfMT composite electrodes for the MSCs. (B) Areal capacitance of MXene/GO and different LfMT-MSCs according the CV curves and GCD profiles. (C) Areal capacitance and volumetric capacitance of MSC under different scan rates. (D) Nyquist plots of the different LfMT-MSCs. (E) Cycle life of the d-LfMT/20-MSC. Inset: Two GCD curves of the MSC before and after 12,000 cycles under the voltage windows of 1.2 V. (F) Comparison of the areal capacitance of similar MXene or graphene-based MSCs. (G) Schematic diagram of the mechanism of using LfMT materials in photocatalysis. (H) Comparison of absorption spectra in the 200- to 800-nm range after different treatments of samples. (I) Kubelka-Munk function (Ahν)^1/2 of MXene/GO and different LfMTs. (J) Valence band XPS spectra of different LfMTs. (K) EPR of different LfMTs. (L) Instantaneous photocurrent curves of samples under different treatments at 0-V potential.

Figure 3G demonstrates the application of the d-LfMT/20 composite material in photocatalysis. The catalytic performance of semiconductor-based photocatalysts is primarily governed by their optical properties and charge carrier dynamics. Among these, the optical bandgap is a critical parameter that defines the material’s ability to absorb photons and can be quantitatively evaluated through ultraviolet-visible (UV-vis) absorption spectroscopy. Femtosecond laser modulation enables precise tuning of nanomaterial optical characteristics, which can be confirmed via spectral analysis. As shown in Fig. 3H, the absorption spectra (300 to 800 nm) of differently treated samples reveal that temporally shaped femtosecond laser irradiation notably enhances the visible-light absorption of MXene/GO composites. The d-LfMT/20 composite exhibits strong photon absorption across the 200- to 800-nm range, likely due to increased TiO₂ content. Using the Kubelka-Munk function, the optical absorption edge of d-LfMT/20 extends to 768 nm (0.77 eV), enhancing visible light absorption and increasing photon utilization efficiency (Fig. 3I). The fluorescence intensity of d-LfMT/20 is markedly lower than that of pristine MXene/GO, suggesting more efficient charge carrier separation, as evidenced in fig. S10A. Linear sweep voltammetry results (fig. S10B) show that the d-LfMT/20 composite has a lower onset potential and higher photocurrent density than other samples. Valence band XPS (Fig. 3J) measured the valence band position of TiO₂, revealing a shift from 3.06 to 2.06 eV, which facilitates electron excitation. With a reduced optical bandgap of 0.77 eV, the conduction band shifts downward by 1.17 eV, enabling the material to absorb about 90% of solar photon energy, highlighting its superior light absorption and carrier mobility. Electron paramagnetic resonance (EPR) spectroscopy detected an EPR signal at g = 2.003, indicating the presence of Ti³⁺ ions (Fig. 3K). This confirms that laser ablation successfully introduced Ti³⁺ ions into the TiO₂ lattice, creating oxygen vacancies that enhance carrier mobility. Figure 3L shows the instantaneous photocurrent curves of samples under various treatments at 0 V (versus Pb/Hg), with d-LfMT/20 achieving a high photocurrent density of 45 μA cm⁻² under sunlight, ~9 times greater than the pristine sample, indicating superior charge separation and transport efficiency during the photocatalytic reaction. The practical application potential of our materials was further verified by the photocatalytic degradation of methylene blue. As shown in fig. S10C, the absorption intensity of methylene blue nearly reached zero within 60 min of irradiation, indicating almost complete degradation (η = 93.49%) of the dye by d-LfMT/20.

Photoassisted enhancement of rechargeable electrochemical energy storage

Figure 4A illustrates the schematic of the enhanced electrochemical energy storage mechanism of the d-LfMT/20 electrode under light-assisted conditions. The integration of TiO₂ nanoparticles and rGO layers within the MXene matrix effectively narrows the optical bandgap to 0.77 eV, thereby overcoming the intrinsic wide bandgap limitation of TiO₂ (3.02 eV). The narrowed optical bandgap substantially extend the carrier excitation wavelength into the near-infrared region, substantially enhances the proportion of the solar spectrum available for photon absorption and carrier generation. In addition, MXene and rGO serve as efficient charge transport channels, facilitating rapid photocarrier transfer within TiO₂. Under illumination, the MXene/TiO₂/rGO hybrid generates abundant photogenerated electron-hole pairs, which are captured by electrolyte anions, thereby promoting enhanced cation storage at the electrode interface. Figure 4B illustrates the photocycle and constant current discharge processes of the hybrid material under various discharge current densities in darkness, ranging from 0.22 to 0.80 mA cm⁻². Following a photoassisted charge lasting 260 s, discharge time decreases as current density increases. The d-LfMT/20 MSC exhibits superior capacitance performance under illumination, with discharge time extending by 206% compared to nonilluminated conditions (Fig. 4C). We also examined the CV curves of the MSCs under illuminated and nonilluminated conditions at different scan rates (fig. S11A). Figure 4D and fig. S11B summarize the comparison of capacitance at various scan rates, demonstrating that light-assisted energy storage is enhanced by 228%, with the areal and gravimetric capacitance of the MSCs reaching 717.6 mF cm⁻²and 1455.21 F g⁻¹.

Fig. 4. Mechanism analysis and experiment of photoassisted electrochemical energy storage enhancement.

(A) Schematic diagram of the representation and the charging mechanism of the electrode. VB, valence band; CB, conduction band. (B) Photocharge for 260 s and galvanostatic discharge in darkness at various discharging current densities for d-LfMT/20 light-assisted MSCs. (C) GCD curves for the d-LfMT/20 light-assisted supercapacitors at different current densities in darkness (dashed curves) and light (solid curves). (D) Areal capacitances against various current densities for the d-LfMT/20 light-assisted MSCs with and without light. (E) Energy and power densities of the d-LfMT/20 light-assisted MSCs compared with other similar capacitors. (F) Photocharged and discharged at 1 mA cm² under the dark and illuminated conditions. (G) Photos of a high-power bulb powered by d-LfMT/20 light-assisted MSCs with and without light. (H) Optimized structure of the pristine MXene/GO and d-LfMT/20 composite. (I) Calculated DOS of the d-LfMT/20 composite.

A Ragone plot (Fig. 4E) compares the energy density and power density of d-LfMT/20 MSCs under illuminated and nonilluminated conditions with other energy storage devices. Under illumination, our MSCs show the improvement in both energy and power density, outperforming other similar MSCs (38–40, 54–56). Another Ragone plot is presented in fig. S11C, exhibiting outstanding gravimetric energy and power densities (0.345 Wh g⁻¹ and 180.31 W g⁻¹) under light-enhanced conditions, which underscore the overall high-performance energy storage capability of the device (57–65). Figure 4F illustrates that our microdevice can simultaneously harvest light and store energy, enabling direct charging using light. It can operate continuously in light mode, with a charging voltage reaching up to 1200 mV and sustaining discharge for 1100 s at 600 mV, highlighting its improved performance over operation in darkness. Photoassisted energy storage devices provide a promising approach for sustainable solar energy applications, with great potential for rechargeable supercapacitors. Nyquist plots comparing the d-LfMT/20 MSC under illuminated and dark conditions reveal differences in charge transfer resistance, indicating light-enhanced electrochemical kinetics (fig. S11D). The smaller semicircle in the high-frequency region indicates a faster charge transfer rate, attributed to the photogenerated electrons produced under constant light, which enhance conductivity and facilitate the charge transfer reaction. Successive GCD curves of the d-LfMT/20 MSC were obtained at 8 mA cm⁻² using a chopped light on/off mode (fig. S11E). The capacity retention of the MSC decreases rapidly when the light is turned off and continues to decline in darkness. We also tested the cycling durability of the LfMT-based MSC under light-enhanced conditions. After 12,000 charge-discharge cycles, the device retained an impressive 96.81% of its initial capacitance (fig. S11F), surpassing the 95.3% retention observed in the dark. This indicates that illumination promotes a more complete expression of the material’s intrinsic capacitive properties. Conversely, capacity retention improves when the light is turned on. When two devices are connected in series, they can successfully illuminate a high-power bulb, with the bulb showing enhanced brightness under light illumination (Fig. 4G). The LfMT-MSCs fabricated on PET substrates exhibit excellent electrochemical stability under various deformation states. As shown in fig. S12, the CV curves and capacitance retention remain nearly unchanged under bending (up to 180°) and twisting (up to 720°), with all retention values exceeding 97%, confirming the device’s remarkable mechanical flexibility and reliability.

To verify the compositional stability and long-term energy storage performance of the LfMT electrode material, XRD and XPS characterizations were conducted on the electrodes after 0 and 12,000 charge-discharge cycles, as well as after 0 and 30 days of storage (fig. S13, A to C). The nearly identical characteristic peaks confirm the excellent structural and chemical stability of the material after extended cycling and storage. In addition, we measured the electrical conductivity of LfMTs at different storage durations, along with the corresponding CV and GCD curves of the LfMT electrodes (fig. S13, D to F). The results show that, even after 30 days, both the conductivity and capacitive performance of the LfMT electrodes remain nearly unchanged compared to the freshly prepared samples, demonstrating the remarkable electrochemical stability of the material. SEM characterization was performed on the LfMT material after 12,000 electrochemical cycles (fig. S13G), and clear flower-like structures were still observed on the surface, further confirming the structural stability of the composite. To evaluate the repeatability of the fabrication process, 10 d-LfMT/20 MSCs were fabricated under identical conditions (fig. S13H), and their electrochemical performances were tested under both illuminated and dark conditions (fig. S13I). It can be observed that the performance values of all samples cluster closely around the average line, indicating good consistency across the dataset.

The crystal structure of MXene/GO and d-LfMT/20 was optimized, as shown in Fig. 4H. The rGO component (yellow region) gains electrons, whereas MXene (blue region) loses electrons. Unlike the pre–laser irradiation state, which involved a simple physical mixture without a heterojunction system and no electron transfer between the two materials, the current system facilitates efficient electron transfer from Ti atoms to graphene. The density of states (DOS) results (Fig. 4I) reveal that Ti atoms predominantly contribute to the region near the Fermi level, indicating that charge carriers within the system are mainly provided by Ti atoms. Compared to the nonheterojunction configuration, the heterojunction system has a greater number of charge carriers and offers more efficient electron transfer pathways. When light irradiates the positive electrode, electron-hole pairs are generated, and the photogenerated electrons transfer from d-LfMT/20 to the external circuit, leaving behind photogenerated holes. As a result, Ti atoms on the surface of d-LfMT/20 attract positively charged H+ ions. The voltage of the d-LfMT/20 MSC is determined by the potential difference between SO₄²⁻ ions adsorbed on the positive electrode and H⁺ ions adsorbed on the negative electrode. The presence of photoinduced charge carriers enables the recombination of residual photogenerated holes with electrons from the external circuit, thereby enhancing charge transport dynamics and ion exchange throughout the electrochemical storage process. This synergistic effect substantially boosts the overall energy storage performance.

DISCUSSION

In summary, we have successfully engineered laser-induced various flower-like LfMT composites with tunable morphologies. These materials function as integrated, light-responsive high-performance electrodes, exhibiting outstanding capacitance and an ultranarrow optical bandgap that substantially enhances their photocatalytic activity. The hierarchical architecture facilitates efficient transport of photogenerated charge carriers and provides abundant electrochemically active sites. Under illumination, the built-in electric field formed at the heterojunction interface effectively promotes charge separation and carrier migration. The resulting electron-hole pairs can recombine with electrons from the external circuit, enabling a light-assisted enhancement of electrochemical energy storage. The resulting supercapacitor demonstrates a remarkable 228% increase in capacitance under light, along with exceptionally high energy and power densities. By incorporating light as an auxiliary energy source, we realize a photoenhanced, integrated microenergy storage device with excellent practical performance. This strategy not only enables efficient utilization of ambient solar energy but also establishes a promising framework for the advancement of high-efficiency photoelectrochemical energy storage technologies.

MATERIALS AND METHODS

LfMT composite materials

MXene (Ti₃C₂T_x) nanosheets (50 mg ml⁻¹, 100 to 200 nm) and GO dispersion (50 mg ml⁻¹, 200 to 500 nm) were purchased from XFNANO Materials Tech Co. Ltd. (Nanjing/Jiangsu, China). Twenty milliliters of each dispersion was thoroughly mixed and then transferred into a 2 cm–by–2 cm glass container. The focused femtosecond laser beam was directed at the center of the mixed dispersion to induce ablation. After 10 min of laser ablation, the treated dispersion was extracted using a pipette for subsequent performance testing and material characterization. The laser-induced composite material was synthesized under ambient pressure and room temperature.

LfMT-based device fabrication

Before electrochemical measurements, transparent electrodes are coated on a flexible PET substrate treated with oxygen plasma under vacuum for 20 min. The ablated composite solution was extracted using a dropper. An ultrathin transparent PET substrate (1 cm by 1 cm/~0.064 g) was used to evaluate the mass loading of the transparent electrode material. The LfMT composite electrode material was uniformly coated onto the substrate, and the weight of the substrate was measured before and after coating using a high-precision balance. The difference in weight corresponds to the effective mass loading of the electrode material. This method allows us to accurately control the amount of material applied, thereby ensuring precise mass loading of the electrode. The PVA/H₂SO₄ gel electrolyte for LfMT-based MSCs was synthesized as follows: 2 g of PVA was added to 20 ml of deionized water and stirred vigorously at 85°C until a clear, homogeneous solution was obtained. Subsequently, 6 g of concentrated sulfuric acid (97 wt %) was added to the PVA solution and stirred at room temperature for 1 hour to ensure complete dissolution. The resulting gel electrolyte was then drop-cast onto the electrode surface prepared on PET substrates. Two electrodes were assembled face-to-face and gently pressed together. The assembled device was left to air-dry for 24 hours, allowing the gel to solidify, thereby forming an integrated flexible MSC.

Shaped femtosecond laser

Laser parameters: Femtosecond laser pulses (wavelength: 800 nm, pulse width: 35 fs, and repetition rate: 1 kHz) were generated using a Spectra-Physics titanium-sapphire oscillator (Tsunami) and a regenerative amplifier system (Spitfire Ace). Pulse delay parameters: Gradient dual-pulse delays ranging from 0 to 30 ps were obtained by introducing an optical path difference using a Michelson interferometer. Power parameters: The average power was measured using a Newport laser power meter (model 919P-003-10) and set at gradients of 5, 10, 20, 30, and 40 mW. Single-pulse energy (E_pulse) was calculated from the average power (P_avg) and repetition frequency (f_rep) using the equation

$$E_{\mathit{\text{pulse}}}=P_{\mathit{\text{avg}}}/f_{\mathit{\text{rep}}}$$ (1)

yielding corresponding pulse energy gradients of 5, 10, 20, 30, and 40 μJ. Motion parameters: Sample translation was controlled using a German Physik Instrumente six-axis positioning stage (model M-840.5DG) with a scanning rate of 500 μm s⁻¹. Focusing optics parameters: The temporally shaped Gaussian laser beam was transformed into a spatiotemporally shaped pulsed beam through the synergistic action of axicons (cone angles: 2° to 3°) and a high–numerical aperture (NA) focusing objective (Olympus 20×, NA = 0.45).

Characterization of LfMT composites

The SEM images were obtained by Hitachi SEM at Tsinghua University. XPS analysis was performed by the ESCALAB 250Xi spectrometer. Raman spectrum was collected by Via-reflex spectrometer, and the excitation laser line was 532 nm. XRD patterns are performed using D8 Advance (Bruker) and CuKα radiation. UV-vis absorption spectrum and photoluminescence characteristics were measured by an Agilent Cary 5000 spectrophotometer. TEM images were obtained using JEM-2100. The UV-vis spectra were measured using an Agilent Cary-5000 spectrophotometer in the 200- to 800-nm range. The EPR spectra were recorded on a Bruker E500 system at room temperature.

Electrochemical characterization of the LfMT devices

CV is performed at different sweep rates, and GCD tests are performed on LfMT-MSCs with a potential window of 1.2 V. The area capacitance (mF cm⁻²) of each electrode is derived from the CV and GCD tests by Eqs. 2 and 3, respectively, as follows

$$C=\frac{1}{\mathrm{\vartheta }\times V}\int \begin{array}{l}\mathit{Vf}\\ \mathit{Vi}\end{array}I\left(V\right)\mathit{dV}$$ (2)

where I, ϑ, and V represent the current applied, scanning rate, and voltage (Vf and Vi are the final voltage and initial voltage, respectively)

$$C=\frac{I}{(-\mathit{dV}/\mathit{dt})}$$ (3)

where I is the discharge current and dV/dt is the slope of the discharge curve. The energy density of a supercapacitor (Wh cm⁻³) is calculated according to the following formula

$$E_{\mathit{\text{cell}}}=C_{\mathit{\text{cell}}}\mathrm{\Delta }{E}^2/(2\times 3600)$$ (4)

where ∆E indicates the operating voltage window. Therefore, the power density of the resulting supercapacitor (W cm⁻³) can be calculated by the following formula

$$P_{\mathit{\text{cell}}}=E_{\mathit{\text{cell}}}\times 3600/t$$ (5)

where t represents the discharge time (t = ∆V/ϑ).

Photocatalytic tests of the LfMT devices

Photocatalytic tests of the LfMT devices were analyzed in a three-electrode configuration. Sun conditions (AM 1.5 G, 100 mW cm⁻²) were provided by a 300-W xenon lamp (Perfect Light PLS-SXE 300C). The potential of the working electrode was controlled using a CHI-670E electrochemical workstation. Transient photocurrent responses of the samples were recorded at 0 V (vs. Ag/AgCl).

Computational details

All the calculations are performed in the framework of the density functional theory (DFT) with the projector augmented plane-wave method, as implemented in the Vienna ab initio simulation package. The generalized gradient approximation proposed by Perdew, Burke, and Ernzerhof is selected for the exchange-correlation potential. The long-range van der Waals interaction is described by the DFT-D3 approach. The cutoff energy for plane wave is set to 500 eV. The energy criterion is set to 10⁻⁶ eV in iterative solution of the Kohn-Sham equation. A vacuum layer of 15 Å is added perpendicular to the sheet to avoid artificial interaction between periodic images. The K-mesh resolved in real space is 0.04 2π/Å. All the structures are relaxed until the residual forces on the atoms have declined to less than 0.03 eV/Å.

Supplementary Materials

This PDF file includes:

Figs. S1 to S13

Tables S1 to S3