Laser-induced MXene-functionalized graphene nanoarchitectonics-based microsupercapacitor for health monitoring application

Laser-Induced MXene-Functionalized Graphene Nanoarchitectonics-Based Microsupercapacitor for Health Monitoring Application Sujit Deshmukh, Kalyan Ghosh, Martin Pykal, Michal Otyepka, and Martin Pumera* Cite This: ACS Nano 2023, 17, 20537−20550 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Microsupercapacitors (micro-SCs) with mechanical flexibility have the potential to complement or even replace microbatteries in the portable electronics sector, particularly for portable biomonitoring devices. The real-time biomonitoring of the human body’s physical status using lightweight, flexible, and wearable micro-SCs is important to consider, but the main limitation is, however, the low energy density of micro-SCs as compared to microbatteries. Here using a temporally and spatially controlled picosecond pulsed laser, we developed high-energy-density micro-SCs integrated with a force sensing device to monitor a human body’s radial artery pulses. The photochemically synthesized spherical laser-induced MXene (Ti3C2Tx)-derived oxide nanoparticles uniformly attached to laser-induced graphene (LIG) act as active electrode materials for micro-SCs. The molecular dynamics simulations and detailed spectroscopic analysis reveal the synergistic interfacial interaction mechanism of Ti−O−C covalent bonding between MXene and LIG. The incorporation of MXene nanosheets improves the graphene sheet alignment and ion transport while minimizing selfrestacking. Furthermore, the micro-SCs based on a nano-MXene-LIG hybrid demonstrate high mechanical flexibility, durability, ultrahigh energy density (21.16 ×10−3mWh cm−2), and excellent capacitance (∼100 mF cm−2@ 10 mV s−1) with long cycle life (91% retention after 10 000 cycles). Such a single-step roll-to-roll highly reproducible manufacturing technique using a picosecond pulsed laser to induce MXene-derived spherical oxide nanoparticles (size of quantum dots) attached uniformly to laser-induced graphene for biomedical device fabrication is expected to find a wide range of applications. KEYWORDS: Laser-induced MXene, laser-induced graphene, covalent bonding, microsupercapacitor, biomonitoring device The booming of miniaturized portable and wearable electronics such as stretchable displays, 1 force-sensitive detectors (FSDs), 2 artificial electronics skin, 3 and wearable microsensors 4,5 have raised the demand of power sources (batteries and supercapacitors) that are capable of working in flexible deformation or to integrate with a variety of electronics devices. A key branch of such modern electronic sectors deals with health monitoring sensors that can collect real-time physiological and electrophysiological data from the human body. 6 However, microbatteries are still the devices of choice for this type of application despite their slow charge/ discharge processes and limited life cycle. 7 Microsupercapacitors (micro-SCs), especially with planar interdigitated structures, are promising alternatives to microbatteries due to their high power densities, longer lifetimes, and much faster charge/ discharge rates. 8,9 The main challenge in using micro-SC devices in modern electronics sectors is to increase the energy density to a level comparable to or even exceeding those of microbatteries without compromising the electrochemical properties. A general strategy to improve the energy density of SCs is to create porous conductive electrode materials with an adequate high packing density to maximize the utilization of the small size of micro-SCs. In this context, graphene sheets are ideal candidates because of their ultrahigh surface area (2630 m2g−1), excellent electrical conductivity, and rich surface chemistry. 10−12 To commercialize graphene, methodologies Received: August 6, 2023 Accepted: September 26, 2023 Published: October 4, 2023 Article www.acsnano.org © 2023 The Authors. Published by American Chemical Society 20537 https://doi.org/10.1021/acsnano.3c07319 ACS Nano 2023, 17, 20537−20550 This article is licensed under CC-BY 4.0 Downloaded via TECHL UNIV OF OSTRAVA on March 22, 2024 at 06:02:57 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. have been developed for producing graphene on a large scale and roll-to-roll compatible thin films without compromising the fundamental properties of graphene sheets. 13,14 Laser-induced photothermal conversion of a nonconductive carbon source into a conductive graphene structure has recently emerged as a rollto-roll compatible technique for producing laser-induced graphene (LIG). 15 This approach allows graphene to assemble into many intriguing structures such as one-dimensional fibers 16,17 and three-dimensional foams 18 as well as the production of desired patterns or geometries by adjusting laser settings. 19,20 Although LIG-based micro-SCs have large gravimetric capacitances, they are constrained by a poor volumetric performance. A probable explanation for this behavior is the strong intersheet π−πinteraction, which, although boosting the packing density, does not allow for high ion accessibility. The pioneering concept of laser processing graphene by El-Kady et al. for micro-SCs has delivered outstanding power output but fails to achieve the high areal capacitance (<5 mF cm−2) due to the stacking problem. 21 Because pure electrical double layers of graphene have limited capacitance, the stacking problem is strategically mitigated by combining LIG with highly electroactive materials and pseudocapacitive materials that have larger capacitance. 22,23 A new class of graphene-analogous materials, 2D transitionmetal carbides, and nitrides (known as MXene) have gained huge interest from researchers due to their 2D structure and customizable surface chemistry, which provide MXene with a plethora of exciting features such as ultrahigh metallic conductivity (15 100 S cm−1), strong hydrophilicity, and notable mechanical capabilities. 24,25 As a result, MXene has shown enormous promise in energy storage applications, such as SCs as well as lithiumand sodium-ion batteries. 26−29 Since the first discovery of 2D MXene (Ti3C2Tx) in 2011 by Gogotsi and colleagues, it has been the most used transition-metal carbide (TMC) for energy storage applications. 30 However, like other 2D materials, the inevitable agglomeration and layer-by-layer restacking due to the high van der Waals force severely reduce the electrochemically active sites and limit the permeability of electrolyte ions. 31 As a result, the dense MXene sheets suffer from low specific capacitance (100−300 F g−1) and poor cyclic stability, which need to improve further. 28,32 Thus, the intercalation of MXene nanosheets between the 3D network Figure 1. Scheme for the synthesis of the O-LIM-LIG hybrid using picosecond pulsed laser and morphological characterization. (a) Stacked Ti3C2TxMXene and delaminated Ti3C2TxMXene with increased interlayer spacing. Schematic model of Ti3C2TxMXene layers composed of Ti, C, and Txatoms. (b) Scheme of the microfabrication steps of the O-LIM-LIG hybrid. (c) Thermal oxidation of delaminated MXene due to appreciable IR energy absorption results in spherical LIM particles attached to LIG, as demonstrated by a defocused lasing mechanism. SEM top view of (d) stacked Ti3C2TxMXene, (e) delaminated Ti3C2TxMXene, (f) LIG, (g) LIM-LIG, and (h) the O-LIM-LIG hybrid showing the uniform distribution of spherical LIM across the LIG surface. (i) Elemental mapping images for the O-LIM-LIG revealing the presence of Ti, O, and C. Scale bar 0.5 μm. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.3c07319 ACS Nano 2023, 17, 20537−20550 20538 of LIG would be a very effective approach to inhibiting the selfrestacking of both graphene and MXene flakes. However, the process to fabricate MXenes in the form of ultrathin flakes/ particles with a few-nanometer thickness/diameter that can intercalate between the graphene network is a key challenge to overcome. Beyond the production of nanosized MXene, the ease with which they may be integrated into devices represents another key challenge in achieving the potential of MXene for applications. Herein we reported a versatile process to fabricate nanoMXene (Ti3C2Tx) functionalized and cross-linked with graphene platelets through Ti−O−C covalent bonding. The approach involves a single-step lasing process on an MXenecoated polyimide (PI) sheet by a diode-pumped Nd:YAG solid pulsed laser beam. When ablated by an Nd:YAG solid pulsed laser, the delaminated MXene sheet absorbs the IR energy and generates high temperature (over 2000 K) on the PI sheet within a rapid uptake time (submillisecond time scale) which causes the self-assembly and ordering of the C−C bond to form an MXenedecorated 3D graphene nanostructure. 33 It is known that Ti3C2Tx, for example, degrades at roughly 200 °C owing to surface group collapse, whereas Ti3C2is projected to endure up to 1000 °C. 33,34 Hence, laser irradiation first creates Ti−O−C bonding and π−πbridging at the MXene graphene interface, and then, it decays the Ti3C2Txsheets into Ti-rich oxide nanoparticles by thermal oxidation that are decorated over the 3D network of LIG. However, the MXene-functionalized LIG was hydrophobic in nature; therefore, we used a room-temperature oxygen plasma treatment to modify the underlying surface wettability of MXene-functionalized LIG, allowing better electrode−electrolyte interaction. Even the laser-processed MXene has now emerged as a promising material for optoelectronics, sensors, or microsupercapacitors. 35−37 The proposed approach is easily scalable, and devices are prepared on a wide scale while maintaining flexibility. All of the devices developed demonstrated energy densities equivalent to microbatteries while retaining good rate performance, cycle stability, and mechanical flexibility. Direct patterning of laser-induced nano-MXene intercalated graphene would enable roll-to-roll manufacturing of active electrode material for a host of applications from energy storage devices to biomonitoring. RESULTS AND DISCUSSION Synthesis and Characterization of MXene-Derived Oxide-Particle-Functionalized Graphene Sheets. The schematic diagram to prepare the MXene-decorated LIG is shown in Figure 1a,b. First, the delaminated MXene (Ti3C2Tx) was spin-coated (500 rpm; 60 s) on the flexible PI sheet followed by pulsed laser (Nd:YAG) writing on the MXene-coated PI sheet. Note that we have used here the defocused method (Figure 1c), resulting in multiple lases in a single run of the pulsed laser. This method allows us to simply adjust the laser’s spot size while maintaining consistent dot density. Lowering the substrate by ∼3 mm below the focal point increases the spot size resulting in multiple lasing on a given spot while maintaining the density of the laser spot constant. Because each spot may lase many times in a single laser pass, this defocused approach enhances processing speed. The sheet resistance of defocused LIG (∼20 Ωsq−1) is also lower as compared to LIG prepared at the focal length. In this process, an in situ decoration/bonding of the laser-induced MXene derive Ti-rich oxide nanoparticles (LIM) inside the 3D porous network of graphene was achieved. The laser process mechanism is further elaborated in the following discussion. When exposed to an IR laser pulse (Figure 1c), the MXenecoated PI sheet absorbs the IR energy and develops a high local temperature (over 2000 K) within a rapid uptake time (submillisecond). 33 This rapid initial uptake leads to the Figure 2. Confocal laser scanning microscope optical image and corresponding 2D and 3D false color profile of (a−c) LIG, (d−f) LIM-LIG, and (g−i) O-LIM-LIG. Scale bar ∼50 μm. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.3c07319 ACS Nano 2023, 17, 20537−20550 20539 formation of carbonized steam (O2, CO, CO2, CH4, N2) from the PI sheet resulting in a 3D porous interconnected structure of LIG (Figure 1f). On the other hand, the carbon source from the MXene explosively vaporizes, and metal ion oxidation takes place when the delaminated MXene nanosheets with abundant oxygen-containing functional groups absorb the IR energy. 33 This instantaneous high-temperature thermal oxidation therefore degrades the Ti3C2Txsheets into spherical Ti-rich oxide nanoparticles. However, before being employed as an SC electrode, it was activated further by utilizing oxygen (O2) plasma treatment. A comparative analysis of surface morphologies for the pristine MXene, delaminated MXene, LIG, and laser-processed MXene-graphene samples are discussed next. The successful delamination of Ti3C2TxMXene and LIMdecorated graphene electrodes was confirmed by scanning electron microscopy (SEM). The surface morphology of the pristine Ti3C2TxMXene is displayed in Figure 1d, confirming the multilayer stack of Ti3C2Txwith an accordion-like structure. After delamination with a strong oxidizing agent (viz. DMSO), the surface oxidation of Ti3C2leads the surface to become rougher (Figure 1e). Figure 1g illustrates the uniform distribution of spherical LIM on the LIG network (called: LIM-LIG), where the LIM particle size ranges from 1 μm to the subnanometer range. The surface morphology of O2plasmatreated LIM-LIG (called: O-LIM-LIG) is similar to that of LIMLIG (Figure 1h). The heterostructure with this sort of distribution of LIM over graphene not only is helpful for improved space utilization but also efficiently prevents graphene sheets from self-restacking. Subsequently, in order to corroborate the uniform distribution of titanium-rich spherical particles across the LIG network, an additional elemental mapping analysis is carried out. Elemental mapping images of the O-LIM-LIG (Figure 1i) and MXene (Supplementary Figure 1) were acquired by energydispersive X-ray spectroscopy (EDX) to see the surface element and to verify the surface decoration of LIM over the LIG network. These reveal the presence of C, O, and Ti in the OLIM-LIG hybrid, which is further verified by Raman, XRD, and XPS analysis. The cross-sectional SEM images (Supplementary Figure 2) reveal that the O-LIM-LIG has a thickness of ∼107 μm, which was taken into account to compute the volumetric capacitance of the micro-SC devices. Figure 2 displays the optical images of LIG, LIM-LIG, and OLIM-LIG captured by a confocal laser scanning microscope (CLSM). The topography (Figure 2a,d,g) is represented together with equivalent 2D (Figure 2b,e,h) and 3D (Figure Figure 3. Physical characterizations. (a) Raman spectra of delaminated Ti3C2TxMXene, LIG, LIM-LIG, and O-LIM-LIG films. Fitted Raman spectra were within the range of 1000−3000 cm−1. (b) XRD spectra of Ti3C2TxMXene, delaminated Ti3C2TxMXene, LIG, O-LIM-LIG, and LIM films. (c) Ti 2p, C 1s, and O 1s high-resolution XPS spectra of delaminated Ti3C2TxMXene and O-LIM-LIG. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.3c07319 ACS Nano 2023, 17, 20537−20550 20540 2c,f,i) false color image maps where distinct colors correspond to the various height profiles of the samples. The height profiling was used to estimate the average surface roughness (Rq) of the samples. The Rqvalues of LIG, LIM-LIG, and O-LIM-LIG were calculated as 7.7, 8.6, and 8.9 μm, respectively. The O-LIM-LIG surface becomes rougher with LIM insertion into the LIG surface and subsequent treatment with the O2plasma treatment. Note that the surface wettability is highly vulnerable to surface roughness, 38 and increasing surface roughness leads to increased ion-accessible surface area, which is one of the fundamental factors for improving electrochemical SC performance. Raman measurements were performed to determine the sorts of defects and growing disorders in the graphene network caused by the incorporation of MXene and the O2plasma treatment. As displayed in Figure 3a, the delaminated MXene exhibits bands around ∼258, ∼420, and ∼608 cm−1corresponding to the inplane Ti−C vibration (Egsymmetry of Ti3C2), in-plane vibration of O atoms (Egsymmetry) of hydrogen-terminated MXene Ti3C2(OH)2, and mixed contribution of out-of-plane Ti−C vibration (A1g symmetry of Ti3C2) and vibration of H atoms of Ti3C2(OH)2. 39,40 The band at ∼151 cm−1is the Eg vibrational mode of anatase TiO2formed due to the spontaneous oxidation of Ti atoms. 41 In the case of LIG’s Raman signal, three prominent peaks are visible (D ∼1340 cm−1, G∼1574 cm−1, and 2D ∼2673 cm−1), which is consistent with the previous report. 9 Interesting to note that after the laser process and oxidization by plasma treatment, the O-LIM-LIG exhibits three peaks at ∼151, ∼389, and ∼607 cm−1in addition to D, G, and 2D peaks. These are the characteristic Raman active modes of anatase TiO2particles with symmetries Eg(1), B1g(1), A1g, and Eg(3). The Raman results confirmed the formation of anatase TiO2phase from on the Ti3C2Txsurface during the lasing process. To further clarify the types of defects due to the formation of anatase Ti-rich oxide particles inside the LIG network, Raman spectra of LIG, LIM-LIG, and O-LIM-LIG are further fitted with the Lorentzian function (Figure 3a), and the extracted fitted parameters are listed in the Materials and Methods section. The single Lorentzian peak fitting of LIG’s 2D band with a full width at half-maximum of 86 cm−1confirms the presence of a few layers of graphene stacked along the caxis. Note that three significant changes are visible when comparing the Raman spectra of LIG with LIM-LIG and O-LIM-LIG; 2D band blueshift of ∼3 cm−1, increment of the ID/IGratio, and the presence of an asymmetric G peak or appearance of an additional D′peak at ∼1605 cm−1. The 2D band blueshift is caused by bond angle disorder and compressive stress at the LIG/MXene-derived oxide interfaces. 9 The ID/IGratio is maximum for O-LIM-LIG, indicating the MXene-derived anatase TiO2and oxygen functional groups have a strong impact on the in-plane sp2domain of LIG. Finally, the ID/ID′ ratio (ID/ID′LIM‑LIG ∼5.5, ID/ID′O‑LIM‑LIG ∼3.8) indicates the defects linked with vacancies (Supplementary Table 1). 42 X-ray diffraction (XRD) studies were performed (Figure 3b) to further validate the laser-induced transition of Ti3C2Txto MXene-derived anatase TiO2particles. The identified XRD peaks of delaminated Ti3C2TxMXene and LIG are consistent with the previous published reports. 9,43 However, the characteristic XRD peaks of MXene are not visible in the spectral analysis of O-LIM-LIG. Consider that delaminated MXene sheets undergo significant stratification and fragmentation by absorbing the laser energy, resulting in losing their 2D planar (002) stuck format and converting to Ti-rich oxide nanoparticles (as can be seen in Figure 1). However, it is worth noting that the transformation of Ti3C2Txto anatase MXene-derived TiO2 appears to be ambiguous based on these XRD results. To clarify this ambiguity, we conducted a controlled experiment in which a glass slide was coated with delaminated MXene and treated with a laser thereafter. It is interesting to note that the residual presence of Ti3C2Txstill existed in LIM but shifted toward a high angle (∼8.9°). Meanwhile, a couple of additional XRD peaks emerged corresponding to the (101) and (004) planes of anatase TiO2. Hence, the dual presence of both MXene and MXene-derived anatase TiO2nanoparticles is confirmed in the LIM samples. As evidenced from XRD and Raman, the surface composition of the O-LIM-LIG has altered significantly after the laser writing and oxidation due to plasma treatment. To further confirm the surface chemical state/composition, X-ray photoelectron spectroscopy (XPS) characterization was carried out. The survey spectra report (Supplementary Figure 3) indicates the presence of Ti, C, and O in the three samples. An additional F peak is evident in the pristine MXene film due to residual F−ions of the hydrofluoric solution used to etch the MAX phase. The atomic ratio of carbon to oxygen (C/O) is computed from the survey spectra, and we found that the C/O value is considerably reduced to 3.6 for the O-LIM-LIG compared to the LIG (C/O = 9.15). This implies that a significant number of oxygen functional groups were bound randomly either with planar sp2 hybridized benzene rings or with Ti atoms, which led to the higher sheet resistance of O-LIM-LIG as compared to other samples (see Supplementary Figure 8). The deconvolution of C 1s, O 1s, and Ti 2p clarifies it further (Figure 3C, Supplementary Figures 4 and 5). The binding energy values of the fitted peaks are listed in Supplementary Table 2. As illustrated in Figure 3C, several peaks are identified in the Ti 2p XPS spectrum of delaminated MXene, which is consistent with the previous findings. 43,44 For LIM-LIG and O-LIM-LIG films, only TiO2 binding energy peaks are evident, whereas the Ti−C binding energy peak is absent. The C 1s XPS spectrum of MXene consists of four obvious peaks designated as C−C, C−OH, C− O�C, and C−Ti−Txrespectively. After laser treatment, the peak attributed to C−Ti−Txdisappeared for LIM-LIG and OLIM-LIG. Furthermore, as shown in the O 1s spectrum, the intensity of the C−Ti−Oxpeak is decreased for O-LIM-LIG than the Ti3C2Tx, while a pronounced TiO2peak is noted. To reinforce this argument, we conducted a comparison of the Ti 2p XPS spectra (Supplementary Figure 6) for MXene, delaminated MXene, and LIM powder. In the case of MXene, distinct peaks corresponding to Ti−C (∼455 eV) and TiO2 (∼459 eV) were observed. Conversely, in the XPS spectrum of delaminated MXene, there was a decrease in the intensity of the Ti−C peak and an increase in the intensity of the TiO2peak. This suggests that the oxidation level of the Ti atoms has increased during the solution-based delamination process. Intriguingly, the LIM sample did not exhibit a Ti−C peak; only peaks corresponding to the binding energy of TiO2were evident. These results confirm the conversion of the Ti3C2Tx surface into MXene-derived TiO2nanoparticles, which is in agreement with the Raman measurements. The conversion of Ti3C2Txinto Ti-rich oxide nanoparticles and the induced oxygenated groups via plasma treatment have a significant influence on the underlying wettability of the O-LIMLIG hybrid. Supplementary Figure 7 displays the water contact angle (WCA) values of LIG (WCALIG ∼110°) and LIM-LIG (WCALIM‑LIG ∼94°), indicating that both are hydrophobic in ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.3c07319 ACS Nano 2023, 17, 20537−20550 20541 nature, while the O-LIM-LIG electrode becomes superhydrophilic (WCAO‑LIM‑LIG ∼0°). It could be correlated with the increased proportion of polar bonds C−O, C�O, and other oxygenated groups, 17 which make the graphene edges more favorable to interact with water molecules, and eventually, the droplet sinks into the porous network of O-LIM-LIG. Increased hydrophilicity leads to increased ion-accessible surface area, which is critical for micro-SC device performance. However, the Figure 4. Interfacial interaction mechanism between MXene and LIG. (a) Initial and final snapshots taken from molecular dynamics simulation executed with reactive force field (ReaxFF) showing the formation of an MXene-derived nanoparticle covalently bound to the graphene surface (the covalent bonds are highlighted by yellow circles) at the high temperature (2000 K) induced by the laser pulse. (b) Snapshots taken from various MD simulations with different initial structures using ReaxFF at 2000 K showing the formation of covalently bonded nanoparticles on periodic graphene surfaces. The inset shows a detail of the interlaced carbon network of polyaromatic hydrocarbon-like structures with aliphatic side chains (Ti and O atoms are omitted for clarity). Aromatic cycles are colored in yellow. Graphs showing the number of direct covalent bonds between the MXene nanoparticle and the graphene surface (the average value is calculated from the last 50 ps). Colors: C (orange), Ti (gray), and O (red). Figure 5. Electrochemical performances of individual electrodes in 1 M H2SO4aqueous electrolyte. (a) CV profiles of the O-LIM-LIG at different scan rates and the corresponding (b) areal and volumetric capacitance as a function of scan rate. A quasirectangular CV shape indicates efficient double-layer formation. (c) GCD profile comparison between LIG, LIM-LIG, and O-LIM-LIG at the current density of 2.6 mA cm−2. (d) GCD of O-LIM-LIG with varying current density. (e) Evolution of the areal and volumetric specific capacitance of O-LIM-LIG as a function of current density. (f) EIS plots of LIG, LIM-LIG, and O-LIM-LIG with a magnified EIS plot of O-LIM-LIG provided in the inset. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.3c07319 ACS Nano 2023, 17, 20537−20550 20542 sheet resistance is compromised for the O-LIM-LIG (see Supplementary Figure 8) as compared to other electrodes due to the presence of oxygenated groups. Molecular Dynamics Simulation. The formation of MXene-originated nanoparticles on LIG was studied by molecular dynamics (MD) simulations with a reactive force field ReaxFF (see Materials and Methods for details). 45 We adopted the developed ReaxFF force field parameters, which were successfully used to investigate the dynamics and structural changes of heterostructures involving Ti3C2MXenes. 46 We Figure 6. Electrochemical performance of individual micro-SCs in PVA-H2SO4gelled electrolyte. (a) Schematic fabrication process of the micro-SC device. Micro-SC device fabricated through laser writing on a delaminated Ti3C2Tx-coated PI sheet followed by solid-state gel electrolyte coating. (b) Comparative CV curves of LIG, LIM-LIG, and O-LIM-LIG film at a scan rate of 10 mV s−1. (c) CV profiles of the corresponding areal and volumetric capacitances as a function of scan rate. (d) Comparative GCD profiles of the LIG, LIM-LIG, and the corresponding O-LIM-LIG at a current density of 0.125 mA cm−2. (f) GCD profiles of O-LIM-LIG at different current densities and (g) corresponding areal and volumetric capacitance as a function of current densities. (h) Cyclic stability of O-LIM-LIG. The device retains ∼91% of its initial capacitance after 10 000 charge/discharge cycles. (i) CV profiles of O-LIM-LIG under different bending conditions. The electrochemical performances are unaffected by mechanical deformation. (j) Cyclic stability of O-LIM-LIG under the 180°bending condition. The bent device retains >85% of its initial capacitance after 8000 charge/discharge cycles. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.3c07319 ACS Nano 2023, 17, 20537−20550 20543 focused on formation, morphological changes, and interaction of MXene nanoparticles with the graphene surface with variable flexibility (fixed/artificially wrinkled graphene) at elevated temperatures (2000 K) induced by the laser pulse. Regardless of the input structure, spherical oval-shaped particles were formed, some CO molecules were released, and carbon atoms constituted structures resembling functionalized polyaromatic hydrocarbons. The nanoparticles composed of Ti and O atoms, which were interlaced with polycyclic aromatic (hydro)carbonlike molecules with aliphatic carbon side chains (inset of Figure 4b, Ti and O atoms are omitted for clarity). The formed nanoparticles conjugated via covalent bonds with the graphene, which displayed a tendency to wrap the nanoparticle enlarging the contact surface (Figure 4 and Supplementary Figure 9) of both systems. The results of MD simulations support the experimental observations indicating the tendency of MXene to form Ti/O-rich nanoparticles covalently bound to graphene. Flexible Energy Storage Devices. The introduction of nano-MXene into the 3D network of LIG results in a substantially enhanced porosity and roughness in the LIMLIG film as compared to LIG (Figures 1 and 2). Further O2 plasma treatment made LIM-LIG film superhydrophilic, which is beneficial for electrolyte ion accessibility inside the porous structure. Due to the distinctive and uniform decoration of nano-MXene/MXene-derived oxide (MDO) across the porous LIG, with the added benefit of superhydrophilicity, O-LIM-LIG was expected to serve as an innovative class of electrode material for SC application. The electrochemical performance was first evaluated in a three-electrode system. Supplementary Figure 10 represents the cyclic voltammetry (CV) profile of LIG, LIM-LIG, and O-LIMLIG at a scan rate of 20 mV s−1. The CV curves exhibit a typical quasirectangular shape without any distinct peaks within the voltage range from −0.2 to 0.8 V, indicating electrical doublelayer type charge storage. Notably, the O-LIM-LIG film shows a higher CV integration area as compared to the MXene and LIMLIG film. The CV curves of O-LIM-LIG SC at various scanning rates (1−100 mV s−1) emerged as almost quasirectangular shapes (Figure 5a and Supplementary Figure 11). Note that the O-LIM-LIG micro-SC delivered exceptional areal (CA∼291 mF cm−2at 5 mV s−1) and volumetric capacitance (CV∼27 F cm−3 at 5 mV s−1), both of which are higher than the most reported LIG and MXene-based micro-SCs. The scan rate variation of CA and CVis shown in Figure 5b with a capacitance retention of around 35% from 20 to 100 mV s−1.Figure 5c shows the constant current galvanostatic charge−discharge (GCD) curves of the LIG, LIM-LIG, and O-LIM-LIG at a current density of 2.6 mA cm−2. The longest discharge GCD profile reveals the superiority of the O-LIM-LIG over the LIG and LIM-LIG. The triangular GCD shape at different current densities (Figure 5d and Supplementary Figure 12) with a Coulombic efficiency of ∼97% reveals the excellent reversibility of the O-LIM-LIG SC without any noticeable voltage drop at the beginning of the discharge curve. Figure 5e illustrates the CV∼35 F cm−3and corresponding CA∼372 mF cm−2at ∼2 mA cm−2outperforming previously reported LIG and MXene-based microSCs. To further investigate the kinetics of ion transport of the LIMLIG films and pure LIG film, electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit voltage (OCV). As shown in Figure 5f, it is clear to observe that the Nyquist plots of LIG and LIM-LIG deliver similar types of graphs containing a steady proportional increase in the imaginary and real impedance, indicating a sluggish ion diffusion for these electrodes, whereas the O-LIM-LIG showed a steeper slope in the low-frequency region relative to the LIG and LIM-LIG films, which is consistent with the improved ion accessibility and transport due to the additional activation of the O2plasma treatment. Furthermore, the interfacial charge transfer resistance (Rct) of O-LIM-LIG (∼40 Ω) is evidently less than LIG (∼70 Ω) and LIM-LIG film (∼65 Ω), demonstrating the ionic conductivity of the O-LIM-LIG was improved following O2plasma treatment, which is consistent with the result of CV and GCD testing. Next, to prepare a solid-state flexible micro-SC device, poly(vinyl alcohol) (PVA)-H2SO4polymer gel electrolyte was casted on the interdigitated electrode (IDE) surface without any binder, separator, or any packaging material (see Figure 6a). All devices’ CV curves have a quasirectangular shape (Figure 6b), suggesting strong electrical double-layer (EDL) characteristics. The O-LIM-LIG exhibited the highest I−Vloop area among the three CV profiles, highlighting the superiority over pure LIG and LIM-LIG in terms of areal and volumetric capacitance value. The CV curves of the O-LIM-LIG micro-SC have an almost quasirectangular shape at a lower scan rate (1 to 10 mV s−1), indicating the capacitive behavior of the electrodes (Figure 6c). However, the nature of the CV curve became resistive with increasing scan rate (Supplementary Figure 13), demonstrating the increased effect of the internal resistance of the electrode at a high current density. This is further supported by the large intercept (∼260 Ω) on the axis in the Nyquist plot (Supplementary Figure 15). Nonetheless, its galvanostatic charge−discharge curves (Figure 6e,f) have a triangular shape with a Coulombic efficiency of ∼97%, indicative of the formation of efficient EDL with excellent reversibility and good charge propagation between the interdigitated electrodes. The longest discharge period in the triangular GCD profile at a current density of 0.125 mA cm−2(Figure 6e) further confirmed the superiority of the O-LIM-LIG over the LIG and LIM-LIG. We measured the specific capacitance over a wide range of GCD current density and CV scan rate, respectively. Notably, the OLIM-LIG micro-SC device delivered exceptional CA∼130 mF cm−2and CV∼12 F cm−3at a scan rate of 5 mV s−1(Figure 6d). Figure 6g illustrates that the volumetric capacitance of the micro-SC (Ccell/v normalized to the whole device volume) is ∼6.6 F cm−3at 0.5 mA cm−2, corresponding to the areal capacitance (Ccell/A) of ∼70 mF cm−2outperforming most reported LIG and MXene-based micro-SCs (Supplementary Table 3). Figure 6h shows that micro-SC retains ∼91% of its initial capacitance even after 10 000 charge−discharge cycles demonstrating its excellent electrochemical stability with long cycle life. The O-LIM-LIG micro-SC was further subjected to a mechanical bending test to see its adaptability to flexible and wearable electronics. Figure 6i shows that the micro-SC retains ∼100% capacitance compared to its flat state when severely bent. Furthermore, the flexibility endurance test of the micro-SC device was carried out by 8000 GCD cycles by keeping the device bent at 180°. Outstanding cyclic stability was recorded (Figure 6j) with capacitance retention of 85% after 8000 GCD cycles. The high mechanical flexibility of the O-LIM-LIG microSC makes it a viable candidate for flexible microelectronics. Meanwhile, EIS further explains the excellent capacitive performance of the O-LIM-LIG micro-SC (Supplementary Figure 15). EIS measurement shows that in the high-frequency region, O-LIM-LIG (∼260 Ω) has the smallest equivalent series ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.3c07319 ACS Nano 2023, 17, 20537−20550 20544 resistance as compared to LIM-LIG (>400 Ω) and LIG (>800 Ω) and displayed a larger slope in the low-frequency region. MXene inclusion minimizes the graphene layer stacking, resulting in a wide surface area with well-defined mesoporosity for effective electrolyte penetration and ion adsorption. Meanwhile, the presence of oxygen functional groups improves the polar interaction with the electrolyte solution. 7 The Ragone plot (Figure 7) further showcases the potential of the O-LIM-LIG interdigitated electrodes for high energy/power density micro-SCs. Our micro-SCs have an areal energy density of ∼21.2 μWh cm−2and a power density of 0.075 mW cm−2at a current density of 0.125 mA cm−2. By increasing the current density to 10-fold (1.25 mA cm−2), almost 45% of the energy density has been retained (∼9μWh cm−2), while the power density increases to 0.75 mW cm−2. The energy and power Figure 7. Ragone plot of the O-LIM-LIG micro-SC where energy and power densities are compared with the state-of-the-art LIG and MXenebased energy storage systems and commercially available supercapacitors. Volumetric energy and power densities of O-LIM-LIG micro-SC compared with commercial supercapacitors and a 4 V/500 μAh Li film battery. 7,21,54 Data for the Li battery are reproduced from ref 3. Data for the 2.75 V/44 mV activated carbon supercapacitor, 5.5 V/100 mF commercial supercapacitor, and 3 V/300 μF Al electrolytic capacitor are reproduced from refs 21 and 49. Figure 8. Application of micro-SC devices connected in series and parallel conditions. CV curves at 5 mV s−1and GCD profiles at 0.5 mA cm−2of O-LIM-LIG micro-SC devices connected in (a,c) series and (b,d) parallel. (e) Schematic of the series and parallel combinations of three microSC electrodes. (f) Photographs of commercial red, yellow, and green LEDs powered by micro-SC devices connected in series. (g) Schematic of powering the FSD to monitor the human body radial artery pulses using wearable micro-SC devices. (h) Force-dependent voltage response of FSD powered by serially connected micro-SC (micro-SC/FSD) and an external 5 V power supply (control). (i) Recorded live radial pulses of a human body around ∼80 BPM. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.3c07319 ACS Nano 2023, 17, 20537−20550 20545