Free‐Standing α‐MoO3/Ti3C2 MXene Hybrid Electrode in Water‐in‐Salt Electrolytes

While transition‐metal oxides such as α‐MoO3 provide high capacity, their use is limited by modest electronic conductivity and electrochemical instability in aqueous electrolytes. Two‐dimensional (2D) MXenes, offer metallic conductivity, but their capacitance is limited in aqueous electrolytes. Insertion of partially solvated cations into Ti3C2 MXene from lithium‐based water‐in‐salt (WIS) electrolytes enables charge storage at positive potentials, allowing a wider potential window and higher capacitance. Herein, we demonstrate that α‐MoO3/Ti3C2 hybrids combine the high capacity of α‐MoO3 and conductivity of Ti3C2 in WIS (19.8 m LiCl) electrolyte in a wide 1.8 V voltage window. Cyclic voltammograms reveal multiple redox peaks from α‐MoO3 in addition to the well‐separated peaks of Ti3C2 in the hybrid electrode. This leads to a higher specific charge and a higher rate capability compared to a carbon and binder containing α‐MoO3 electrode. These results demonstrate that the addition of MXene to less conductive oxides eliminates the need for conductive carbon additives and binders, leads to a larger amount of charge stored, and increases redox capacity at higher rates. In addition, MXene encapsulated α‐MoO3 showed improved electrochemical stability, which was attributed to the suppressed dissolution of α‐MoO3. The work suggests that oxide/MXene hybrids are promising for energy storage.


Introduction
Electrochemical capacitors (ECs) or supercapacitors are important energy storage devices that complement rechargeable batteries and are usually characterized by long lifetimes and high power. [1][2][3] Their energy storage performance can be further enhanced by employing new electrode materials and architectures with improved properties and by using redesigned electrolytes. [4] While a significant amount of research has focused on the incorporation of new materials, [2,5] studying their behavior in different electrolytes is equally important. charge storage at positive potentials due to a widened voltage window. [1] Nonetheless, the mixed double-layer/faradaic charge storage capacity of highly conductive MXenes is somewhat limited compared to oxides, where the charge storage mechanism is governed by faradaic redox reactions. [24] However, the low electronic conductivity of oxides such as MnO 2 , V 2 O 5 , and MoO 3 , limits their rate performance. [25] Among these oxides, several modifications of MoO 3 have been investigated for energy storage. [26][27][28] Particularly, α-MoO 3 has gained wide attention thanks to its high theoretical capacity of 279 mAh g −1 (1005 C g −1 ). [29] The high oxidation state (+VI) of molybdenum in α-MoO 3 enables multiple reduction processes which are accompanied by the transfer of multiple electrons, with high specific capacity. [30] While α-MoO 3 is an attractive energy storage material, it is unstable in aqueous electrolytes. The absence of an SEI layer allows easy cointercalation of water molecules, which may lead to dissolution. [30,31] Moreover, the low intrinsic conductivity of α-MoO 3 leads to electrode degradation, which makes this material unsuitable for high-power devices. [32] So far, several strategies, such as coating with carbon [33] or polymers, [34] electrochemical deposition, [30] utilizing porous substrates, [35] and poly(vinyl alcohol) (PVA) based quasi-solid-state electrolytes [32] have been employed to suppress α-MoO 3 dissolution and improve its stability in aqueous electrolytes. However, a simple and straightforward approach to enable α-MoO 3 for high-rate devices is still desired.
We anticipated that the integration of α-MoO 3 with Ti 3 C 2 MXene would be an effective way to improve the electronic conductivity and stability of the hybrid electrodes and build high-rate supercapacitor devices. In the literature, several strategies for preparing oxide/MXene hybrids by hydrothermal [36] or solvothermal [37,38] treatment have been documented. In many cases, oxidation of MXene has been observed, [39,40] which may result in unwanted variations in the electrochemical performance. Moreover, the preparation of slurry electrodes requires binders and conductive additives, which increase the overall inactive mass. Therefore, developing a simple-yet-effective approach of hybrid electrode preparation without affecting the properties of the components is important. Ideally, this hybrid material would allow the fabrication of carbon-and binder-free electrodes for energy storage devices.
Herein, we demonstrate a straightforward technique of roomtemperature mixing of α-MoO 3 and Ti 3 C 2 MXene to prepare binder- less, free-standing α-MoO 3 /Ti 3 C 2 hybrid electrodes ( Figure 1). The remarkable mechanical properties of Ti 3 C 2 allow for easy preparation of free-standing films with α-MoO 3 and flexible energy storage devices. The hybrid α-MoO 3 /Ti 3 C 2 electrode shows multiple redox peaks of α-MoO 3 in addition to the well-separated peaks of Ti 3 C 2 in 19.8 m LiCl. The widened potential window of 1.8 V enhances the charge storage properties and couples with improved electrochemical stability and rate performance. These observations highlight the important role of MXene in achieving redox capacity at higher rates and suppressing the dissolution of oxides.

Morphological and Structural Characterization
Scanning electron microscopy (SEM) imaging revealed that α-MoO 3 sample obtained after the hydrothermal reaction exhibited rod-like morphology with diameters between 200-500 nm and lengths of ∼3-10 μm (Figure 2a). The X-ray diffraction (XRD) patterns and SEM images of Ti 3 AlC 2 shown in Figure S1a-c, Supporting Information are consistent with the literature [19] and the Ti 3 C 2 shows a wavy rugged surface with wrinkles due to the restacking of MXene sheets ( Figure S2, Supporting Information), while the cross-sectional image of freestanding 2D Ti 3 C 2 film exhibits a thin stacked morphology (Figure 2b). In contrast, the surface morphology of the 50:50 electrode reveals α-MoO 3 nanobelts embedded between MXene sheets throughout the film (Figure 2c). The random distribution of nanobelts between the MXene sheets improves their interfacial contact and helps in reducing the restacking of MXene sheets, which is beneficial for ion accessibility. [44] The cross-sectional SEM image of the 50:50 sample further confirms a uniform distribution of α-MoO 3 nanobelts between MXene sheets (Figure 2d). XRD analysis revealed that the white powder of α-MoO 3 has orthorhombic structure (space group of Pbnm) and can be indexed with JCPDS #05-0508. The intense diffraction peaks (020), (040), and (060) suggest a preferred orientation of these α-MoO 3 nanobelts lying flat on the a-c plane. [41] XRD of the MXene reveals a pure Ti 3 C 2 phase without MAX residues, which is also consistent with the previous reports. [19] The 50:50 sample exhibits the dominant (002) diffraction peak of MXene and (020), (040), and (060) peaks corresponding to α-MoO 3 , which are slightly shifted to smaller 2θ angles due to unknown reasons ( Figure 2e). Raman spectrum of the 50:50 sample further shows the characteristics of both α-MoO 3 [45] and Ti 3 C 2 MXene [46] as demonstrated in Figure 2f. Anatase TiO 2 with 144 cm −1 shift was not observed in the 50:50 sample, indicating Ti 3 C 2 MXene was not oxidized by α-MoO 3.
[46] were observed in a wide potential range of −1.1 to +1.2 V (vs Ag) without any HER or OER. These redox peaks indicate high specific charges/capacities. While these redox peaks, characteristic of α-MoO 3 phase, were present in both, cathodic and anodic scans, they were not fully reversible or some of the peaks were partially reversible, as shown in Figure S3, Supporting Information. The exact reason for the occurrence of these peaks is not known, however, possible reasons could be the transition of Mo between different oxidation states. [30] Interestingly, these peaks start disappearing at scan rates above 1 mV s −1 and completely vanish at 5-10 mV s −1 , possibly due to diffusion limitations ( Figure S4, Supporting Information). This observation could explain the poor rate capability of α-MoO 3 caused by low conductivity. [25,30] The CV curves of Ti 3 C 2 electrode at The surface-controlled partial chargetransfer mechanism associated with these well-separated peaks was explained by the process of desolvation-free insertion/extraction of Li + ions and confirmed by in situ XRD and atomic force microscopy (AFM). [1] After testing Ti 3 C 2 , the CV responses of all hybrid electrodes (α-MoO 3 /Ti 3 C 2 ratios of 90:10, 80:20, 70:30, 60:40, 50:50, and 10:90) were recorded ( Figure S6, Supporting Information). Interestingly, 90:10 and 80:20 electrodes showed improved but similar electrochemical activity under negative potentials, whereas the wellseparated peaks of MXene were also observed under positive potentials ( Figure S6a,b, Supporting Information). However, the peaks related to MXenes were not pronounced in the 70:30 electrode, but an enhanced charge storage was observed throughout the potential window ( and Figure S6e, Supporting Information) exhibit multiple α-MoO 3 redox peaks in addition to the Ti 3 C 2 peaks. Moreover, upon testing the 50:50 electrode at higher rates, a better and more stable current response was observed. Due to this, the 50:50 electrode was considered as the optimum composition, which demonstrated both, large charge storage and high-rate redox capacity. We also tested the 10:90 electrode and obtained very stable responses at higher scan rates due to the ample amount of MXene, but the charge storage efficiency was significantly lower compared to other hybrid electrodes due to insufficient amount of high-capacity α-MoO 3 ( Figure S6f, Supporting Information). The CV curves of all eight electrodes at 0.2 mV s −1 are presented in Figure S7, Supporting Information. The α-MoO 3 electrode clearly shows very high charge storage capacity. Moreover, the high electronic conductivity of MXene compensates for the low α-MoO 3 conductivity leading to the α-MoO 3 peaks being more pronounced in the hybrid electrode, compared to a carbon-containing less-conductive α-MoO 3 electrode ( Figure 3d). As shown in Figure 3e, while α-MoO 3 provides high specific charge values of~704 C g −1 at 0.2 mV s −1 , its rate performance is poor as it delivers only~70 C g −1 at 5 mV s −1 , approximately 10% of the 0.2 mV s −1 value. On the other side, Ti 3 C 2 MXene stores a charge of~166 C g −1 at 0.2 mV s −1 , but maintains~70 C g −1 at a scan rate of 100 mV s −1 . The 70:30 electrode delivered the highest specific charge value; however, its rate efficiency was relatively poor (<20% at 10 mV s −1 compared to 0.2 mV s −1 ), possibly due to a larger content of α-MoO 3 . In contrast, the rate performance of the 50:50 electrode was better than all other hybrid electrodes ( Figure S8, Supporting Information). Moreover, it showed the next best charge storage ability after the 70:30 electrode with a specific charge value of 401 C g −1 at 0.2 mV s −1 and~166 C g −1 at 100 mV s −1 , which is nearly 42% retention. The rate performance of α-MoO 3 , Ti 3 C 2 , and 50:50 electrodes shown in Figure S9, Supporting Information indicates that the introduction of MXene into the α-MoO 3 resulted in rateefficient hybrid electrodes.

Electrochemical Investigation
A similar electrochemical behavior of Ti 3 C 2 and 50:50 electrodes was also observed in saturated LiBr solution, illustrating the versatility of the proposed concept, which could possibly be generalized to other oxide/MXene systems ( Figure S10a,b, Supporting Information). Cycling stability of the α-MoO 3 electrode at 1 mV s −1 shows a quick drop in capacity possibly due to dissolution, and a catastrophic failure after 785 cycles, whereas the 50:50 electrode exhibited an improved stability due to the presence of MXene (Figure 3f). Ti 3 C 2 MXene retains 94% of its initial capacity after 10 000 cycles, as demonstrated previously. [1] The phase change of α-MoO 3 could be a possible reason of the initial decrease in the charge retention up to 200 cycles, which then becomes gradual up to 600 cycles, and stabilizes with a nearly constant value until 2000 cycles. To verify the phase change phenomenon, we recorded SEM images of the α-MoO 3 electrode after 50 cycles and observed that the α-MoO 3 nanobelts have transformed into amoeboid shape, which suggests a possible phase transformation and/or dissolution ( Figure S11, Supporting Information). While the actual mechanism of suppressed dissolution of α-MoO 3 by MXene needs to be investigated in a separate study, the phenomenon can be understood from the fact that MXene nanoflakes wrap around α-MoO 3 nanobelts as shown in the SEM images of the 50:50 electrode in Figure 2c,d, without allowing excess electrolyte to reach the α-MoO 3 surface, only a limited α-MoO 3 /electrolyte contact is possible through the exposed nanobelt edges. The XRD pattern of the 50:50 electrode indicates the preferred orientation of nanobelts, suggesting the formation of a tight oxide/MXene heterointerface. In such electrode configurations, ions from the electrolyte still have the access to intercalate into the α-MoO 3 structure, however, the limited amount of solvent is not sufficient to cause dissolution of α-MoO 3 , observed in the case of the α-MoO 3 slurry electrodes. Therefore, MXene encapsulation could be a powerful approach to mitigate capacity decay due to oxide dissolution.
Galvanostatic charge-discharge (GCD) curves for α-MoO 3 , Ti 3 C 2 , and 50:50 electrodes were recorded at different current densities. Two noticeable plateaus in Ti 3 C 2 electrode are in line with the well-separated peaks that were observed in CV ( Figure S12a, Supporting Information). Interestingly, α-MoO 3 exhibits many plateaus at low current densities, however, at higher current densities these plateaus disappear possibly because of diffusion limitations.
Moreover, a continuous drop in charging/discharging time, with the number of cycles at a constant current density, was observed due to poor stability of the electrode ( Figure S12b, Supporting Information). In fact, the charging/discharging responses at higher current densities were not even visible for the α-MoO 3 electrode, because of which they have been shown separately in Figure S12c, Supporting Information. The 50:50 electrode demonstrates symmetric and stable charge-discharge response with increasing number of cycles, which highlights the important role of Ti 3 C 2 in suppressing the dissolution and improving the electrochemical stability of α-MoO 3 ( Figure S12d, Supporting Information). The first three charge/discharge cycles for Ti 3 C 2 , α-MoO 3 , and 50:50 electrodes are shown in Figure 4a-c, indicating that the Ti 3 C 2 addition leads to stability enhancement of α-MoO 3 in the hybrid rate-efficient electrode.
Further, these 50:50 electrodes were used to prepare a symmetric supercapacitor. A schematic of the symmetric cell, which was assembled in the plastic Swagelok cell using Celgard as a separator, is given in Figure 5a. Glassy carbon electrodes were used as current collectors. Before assembling this symmetric cell, charge balancing was performed by balancing the masses on both sides. For this, CV curves of 50:50 electrode were recorded at four different scan rates in the threeelectrode configurations in the full (−1 to +0.8 V), negative (0 to −1 V vs Ag), as well as positive potential window (0 to +0.8 V vs Ag). The average ratio of charge stored at four different scan rates in the negative window to the positive window was found to be 2.54, so for the charge balancing in the symmetric cell, the mass of positive electrode was made 2.54 times higher ( Figure S13a-d, Supporting Information). After charge balancing, the CV curves were recorded in the 2-electrode configuration and a potential window of 1.4 V was achieved (Figure 5b). At low scan rates, two pairs of peaks were observed in the cathodic as well as anodic scans, however, these peaks disappeared at higher scan rates. Galvanostatic charge/discharge curves at different current densities (1-5 A g −1 ) were highly symmetrical with good Coulombic efficiency (Figure 5c). The calculated amount of cell-specific charges at different current densities show a good fulldevice performance (Figure 5d), which resulted in an energy density of 10.77 Wh kg −1 and power density of 1332.7 W kg −1 . Furthermore, a remarkably high cycling stability was observed for the full cell, as it retained~76% of its initial capacity after 20 k cycles with nearly 100% Coulombic efficiency (Figure 5e). This observation was further confirmed by comparing the stable CV responses at 1st, 10 000th, and 20 000th cycles, without any significant distortion in CV curve shapes or current responses (Figure 5f).
As a proof-of-concept, a symmetrical two-cell device was made. Two single cells were fabricated with 80 mg 50:50 hybrid electrodes, saturated LiCl aqueous electrolyte, a Celgard 3501 porous separator, and carbon foil tabs (Figure 6a). A single cell had a maximum operating voltage of 1.4 V. To obtain a sufficient powering time for a red LED, two cells were placed into series to obtain a maximum operating voltage for the device of 2.8 V. The devices were then placed into series with the LED and charged at constant voltage for 15 min before unhooking the power supply from the device. The operating voltage and current were measured along with time that the LED was lit (Figure 6b). Without a voltage regulator, the LED acts as a constantresistance resistor, however, the device is able to keep the LED lit for Energy Environ. Mater. 2023, 6, e12516 5 of 9 over 5 min (Figure 6c). The electrodes and device are also able to operate under mechanical deformation as seen in Video S1, Supporting Information. This demonstrates the possibility of using oxide/MXene hybrid electrodes in flexible hybrid electronics. Table S1, Supporting Information shows supercapacitor performance of some similar oxides/ MXenes electrodes.

Conclusions
Binder-free, mechanically robust, and flexible free-standing hybrid electrodes made of α-MoO 3 / Ti 3 C 2 MXene with different ratios of individual components were prepared by room-temperature mixing followed by vacuum filtration. These hybrid electrodes, along with free-standing Ti 3 C 2 and α-MoO 3 slurry-based electrodes, were electrochemically investigated in saturated LiCl (19.8 m) water-in-salt electrolyte. The results reveal that the introduction of highly conductive MXene significantly improves the rate performance and cyclability of α-MoO 3 , while maintaining a significant capacity. Furthermore, a symmetrical cell of optimized 50:50 electrodes was constructed, exhibiting a good charge-storage ability with cyclic stability over 20 000 cycles with nearly 100% Coulombic efficiency. As a proof of concept, these optimized 50:50 electrodes were then used to construct flexible devices (with 2 cells connected in series) to light-up a red LED (1.8 V) for 5 min after 15 min of charging. The work concludes that the approach of introducing highly conductive MXene to less-conductive oxides, in absence of any other  conductive additives or binders, is a promising strategy for achieving high redox capacity at higher rates, which can be extended to various oxides and other less conductive materials. Embedding α-MoO 3 nanobelts between the MXene nanosheets not only expands the potential window compared to the MXene electrode and improves the electronic conductivity compared to the oxide electrode, but also leads to improved electrochemical stability of α-MoO 3 , which was attributed to the suppressed dissolution enabled by the MXene encapsulation. The proposed method of preparing flexible hybrid electrodes eliminates the need for binders or additives and can be explored for a variety of applications. Overall, the present work demonstrates a simple-yet-effective approach of preparing hybrid electrodes and maximizing their electrochemical properties in water-in-salt electrolytes, while eliminating the associated shortcomings of individual materials.

Experimental Section
Characterization: X-ray diffraction (XRD) patterns were recorded by a powder diffractometer (Rigaku Smart Lab, USA) with CuK α radiation (λ = 1.54Å) at a step size of 0.03°with 0.5 s holding time. Raman spectra were recorded with a Renishaw InVia Raman microscope using LEICA CTR6000 setup with a 633 nm laser and 1800 lines mm −1 grating at 10% laser power and a 63× (NA = 0.7 objective). Spectra were acquired with a dwell time of 60 s with three accumulations. The morphology and structure of materials were observed with scanning electron microscopy (SEM; Zeiss Supra 50VP).
Synthesis of α-MoO 3 : Mo (99.95%, −100 mesh, Alfa Aesar) and hydrogen peroxide (30 wt.% H 2 O 2 Alfa Aesar) were used for the synthesis of α-MoO 3 . All the chemicals were used as received without further purification. For the synthesis of α-MoO 3, Mo powder (metal base) was suspended in 2 mL of water followed by dropwise addition of hydrogen peroxide under continuous stirring as the temperature of the reaction was set to 60°C. Shortly after all the Mo powder was dissolved, and after a few hours a yellow precipitate formed which was then vacuum filtered. This precursor was hydrothermally treated in 12 mL of water at 220°C for 24 h. The product of the hydrothermal treatment was filtered and washed with copious amount of water. [41,42] Synthesis of Ti 3 AlC 2 MAX phase, Ti 3 C 2 MXene, and its free-standing film: Ti (99.5%, −325 mesh, Alfa Aesar), Al (99.5%, −325 mesh, Alfa Aesar), and TiC (99.5%, 2 μm, Alfa Aesar) powders were used for MAX phase synthesis. Lithium chloride (LiCl, 99%, Acros Organics), hydrofluoric acid (HF, 48.5-51%, Acros Organics), and hydrochloric acid (HCl, 36.5-38%, Fisher Chemical) were used for the synthesis and delamination of Ti 3 C 2 . Note, for MAX/MXene synthesis, it is vital to follow all safety precautions, especially for HF handling, and it is recommended to follow safe synthesis protocols developed for MXenes. [43] Ti 3 AlC 2 was synthesized by first mixing a 100 g mixture of TiC:Ti:Alf (2:1.25:2.2 atomic ratio). [18] The powders were ball milled in a 2:1 weight ratio with 5 mm alumina for 18 h. The mixture was then placed into a furnace (Carbolite Gero) with Ar continually flowing at 200 SCCM. The furnace was heated to 1400°C for 2 h at a heating/cooling rate of 3°C min −1 . The powders were then drilled with a TiNcoated bit, washed with HCl overnight to remove all intermetallic and metallic impurities, and then sieved to below 45 μm.
To synthesize Ti 3 C 2 , 3 g of the MAX powder was slowly added to a mixed-acid etchant (HF:HCl:H 2 O volume ratio (ml) of 6:36:18) at 35°C and 300 rpm. The reaction proceeded for 24 h. Afterward, the MXene/etchant mixture was transferred to centrifugation tubes and was repeatedly washed by centrifugation at 2550 rcf for 5 min using 15 MΩ deionized (DI) water. After each centrifugation, the acidic supernatant was decanted, with fresh DI water added and the process was repeated. This was continued until the supernatant was neutral (pH > 6).
Afterward, the multilayer MXene was delaminated using LiCl. 3 g LiCl was added to 60 ml DI water at 35°C and 300 rpm, the multilayer MXene (3 g) was added to this mixture and stirred for 18 h. Afterward, the mixture was placed in 150 mL centrifuge tubes filled with fresh DI water and centrifuged at 2550 rcf for 15 min cycles. This process was repeated throughout the entire collection process. When the supernatant was clear, the supernatant was discarded. When the supernatant was black, the supernatant was instead collected. In both cases, fresh DI water was added, and the sediment was fully redispersed. This was repeated until the supernatant transitioned from clear to black, back to clear. The delaminated Ti 3 C 2 was then concentrated by centrifugation (20 816 rcf) for 10 min. The clear supernatant was discarded, then the overall concentration was adjusted to be 1 mg mL −1 . The free-standing film of Ti 3 C 2 was prepared by vacuum filtration of delaminated Ti 3 C 2 solution.
Synthesis of α-MoO 3 /Ti 3 C 2 MXene hybrid free-standing films: Before preparing the hybrid free-standing films, 1 mg mL −1 stock suspensions of both α-MoO 3 and Ti 3 C 2 were prepared. The α-MoO 3 /Ti 3 C 2 hybrid free-standing films were fabricated by a facile room temperature mixing of appropriate amounts of α-MoO 3 (1 mg mL −1 ) and Ti 3 C 2 (1 mg mL −1 ) according to their (weight to volume, w/v) ratios. Six different mixed suspensions with α-MoO 3 :Ti 3 C 2 ratios of 90:10, 80:20, 70:30, 60:40, 50:50, and 10:90 were prepared by room temperature stirring for half an hour, and free-standing films were then obtained by vacuum-assisted filtration of these mixed suspensions. The α-MoO 3 /Ti 3 C 2 hybrids with various α-MoO 3 : Ti 3 C 2 ratios of X:Y are simply called X:Y sample/electrode throughout the manuscript.
Electrochemical measurements: α-MoO 3 , Ti 3 C 2 , and α-MoO 3 /Ti 3 C 2 were employed as the working electrodes in a Swagelok cell, while an Ag wire and an activated carbon (AC, YP50F) film were used as the reference and counter electrodes, respectively. Glassy carbon electrodes were used as current collectors, even though the hybrid electrode had sufficient conductivity to operate without a current collector. AC films were prepared by mixing 5 wt% polytetrafluoroethylene (60 wt% in water, Sigma Aldrich) and 95 wt% of YP50F (Kuraray, Japan). The above slurry was rolled into~100 μm thick films followed by drying in a vacuum oven at 70°C for 12 h. These AC films were used as counter electrodes for the three-electrode measurements. The vacuum filtered films of Ti 3 C 2, and hybrids were used as electrodes directly, while the α-MoO 3 electrode was fabricated by preparing a slurry of the active material (α-MoO 3 ), carbon black (SUPER C65), and polytetrafluoroethylene (Millipore-Sigma) in ethanol in a weight ratio of 85:10:5. The slurry was then rolled into a film. 3 mm disk-shaped electrodes were then punched from this film, weighed, and used as working electrodes. In the three-electrode assembly, Celgard (25 μm thick 3501 polypropylene membrane) was used as a separator between the working electrode and AC counter electrode.
All measurements were done at room temperature on a VMP3 electrochemical workstation (BioLogic, France) using CV and galvanostatic charge-discharge (GCD) techniques. CVs were recorded at different scan rates in the potential window of −1.1 to +1.2 V (vs. Ag) for α-MoO 3 , −0.8 to +0.8 V (vs Ag) for Ti 3 C 2 , and −1 to +0.8 V (vs Ag) for different combinations of α-MoO 3 /Ti 3 C 2 electrodes. The Ag wire was used as reference electrode because the precipitation occurs in the KCl solution of the Ag/AgCl after cycling. The three-and two-electrode measurements were performed in 19.8 m LiCl electrolyte. The mass loading for each electrode is mentioned explicitly in the corresponding figures.
The specific charge (C) was calculated from the anodic scan of CV curve: where i is the current changed by time t, and m is the mass of the active material in the working electrode for three-electrode cell and is the mass of active material in both electrodes for a two-electrode cell. The energy density (E) of the full cell was estimated based on Equation (3): where voltage V and current i are the function of time t, and m is mass of both electrodes. The power density was calculated from Equation (4): where E is the energy density and Δt is the discharge time.

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For proof-of-concept, a device was made with two cells in series. Each cell used electrodes with an α-MoO 3 :Ti 3 C 2 ratio of 50:50 for both, the anode and the cathode. The device was charged to 2.8 V under constant voltage for 15 min using a 30 V/10 A DC Power Supply PS-3010DF. Upon disconnecting from the power supply, the output was measured using two channels. One channel was put into series with the device and measured the operating current using chronoamperometry. A second channel was placed in parallel to measure the operating voltage using open circuit voltage.