A La2O3/MXene composite electrode for supercapacitors with improved capacitance and cycling performance

ABSTRACT Developing efficient electrode materials is a key towards high power electrochemical energy storage devices. Two-dimensional (2D) MXene shows excellent conductivity and electrochemical performance among other materials. However, the restacking of MXene layers may degrade their specific capacity and cycling performance. Considering this challenge, here we have designed a composite made of 2D MXene nanosheets and lanthanum oxide (La2O3) nanoparticles to overcome the limitations. The bifunctionality of La2O3 nanoparticles prevents the restacking of MXene layers and enhances the electrochemical properties of the electrode due to its good Faradic characteristics. The specific capacitance of the La2O3/MXene composite electrode is 366 F/g at 1 A/g, which is 4.5 and 3 times higher than those of the individual La2O3 and MXene. The composite electrode displays a capacitance retention of 96% after 1,000 cycles, which is due to synergistic effects between the two components and indicates the potential of La2O3/MXene composite for supercapacitors.


Introduction
The development of clean, portable, and efficient energy storage systems is essential as a consequence of rapid industrialization and the explosive growth of energy demands. The world is currently undergoing a severe energy crisis as a result of our overwhelming reliance on fossil fuels and other non-renewable energy sources [1][2][3][4][5]. Due to their intermittent nature, renewable energy sources are difficult to utilize because, when incorporated into electric grids, they cannot produce stable electricity continuously [6][7][8][9][10][11]. In order to reduce the rising demand for power globally, energy storage devices are important. These technologies not only satisfy the requirements for energy consumption in various fields but also additionally provide a sustainable and environmentally friendly alternative that can complement conventional energy sources [12,13]. In this regard, supercapacitors (SCs) and batteries play key roles among the various forms of energy storage technologies. The dominance of batteries in the market is owing to their high energy density; however, batteries show limited power output and require longterm use of energy [14,15]. Alternatively, SCs exhibit better performance in terms of their higher power density, larger specific capacitance, greater efficiency, and longer lifetime. Considering their above-mentioned features and their central role among energy storage devices, lots of efforts are ongoing to fabricate new electrode materials to further enhance SCs performance.
2D materials are promising electrode materials for high-performance SCs and other applications because of their intriguing properties, which include large specific surface areas, atomically thin 2D nature, mechanical flexibility, and physical and chemical properties [16][17][18][19][20][21]. MXenes refer to transition metal carbides, nitrides, and carbonitrides, belonging to the 2D material family that have drawn a lot of interest in SCs because of their excellent conductivity and hydrophilic nature [22][23][24]. Although MXenes exhibit excellent electrochemical properties, pending challenges, such as low capacitance, unreacted microstructures, and restacking of their layers, need to be addressed. To address these challenges, different strategies such as doping, surface modification, morphology control, and their hybrid composites with transition metal oxides (TMOs) have been applied to enhance the performance of MXene-based SCs [25].
In particular, significant progress has been accomplished in improving the electrochemical performance of SCs by integrating TMOs with MXenes [26]. However, the low electrical conductivity of TMOs hinders their application for high-performance SCs [27]. On the other hand, rare earth metal oxides have been explored for many potential applications because of their unique properties such as 4f electron configuration, trivalent oxidation state (+3), large intermolecular distance (8.45-8.70 Å), and anion-exchangeability [28]. Among various rare earth metal oxides, lanthanum oxide (La 2 O 3 ) is considered as one of the most prominent candidates and has been utilized for many applications because of its low cost and excellent redox reversibility. La 2 O 3 also exhibits good electrochemical properties such as excellent pseudocapacitance properties because of the coexistence of La 2+ and La 3+ during the charging and discharging processes, which enhances the redox reactions [29]. However, La 2 O 3 material suffers from one major limitation, i.e., low electrical conductivity due to its wide band gap. Previous studies have shown that the SC performance of La 2 O 3 materials can be improved by the formation of composites. For instance, Zhang et al. have synthesized MnO 2 /La 2 O 3 composites by the hydrothermal method. The MnO 2 / La 2 O 3 electrode material achieved a specific capacitance of 245 F/g at 0.3 A/g, which is larger than that of the individual La 2 O 3 -based electrode [30]. In another report, Sankar et al. designed a composite of La 2 O 3 nanoparticles with rGO (La 2 O 3 /rGO), and the composite electrode material achieved a high energy density of 80 Wh kg −1 and a high power density of 2250 W kg −1 [31]. However, the achieved specific capacitance is still unsatisfactory for SCs applications and needs further improvements. Therefore, it is important to make La 2 O 3 /MXene composite with enhanced electrochemical performance for SCs.
In this work, we have designed an electrode material based on La 2 O 3 /MXene composite for the SC with improved capacitance and cycling performance by the synergistic effects between La 2 O 3 and MXene. The La 2 O 3 /MXene composite shows high capacitance and stability when compared with MXene or La 2 O 3 alone. The insertion of La 2 O 3 (NPs) prevents MXene nanosheets from restacking, facilitating fast electron transfer and ion diffusion in the composite. The La 2 O 3 /MXene composite displays good electrochemical properties, including a high capacitance of 366 F/g at 1 A/g and excellent cyclic stability with a capacitance retention of 96% after 1,000 cycles. Our results suggest the effectiveness of such a strategy in improving the SC performance of 2D MXene materials.

Preparation of MXene
The MXene nanosheets were produced by etching Al atoms from MAX structure (Ti 3 AlC 2 ) powder following the method reported in the literature [32][33][34]. Briefly, 0.5 g Ti 3 AlC 2 powder was added to the 10 mL of (40%) concentrated HF solution and stirred magnetically at 250 rpm for 24 h at room temperature. The resulting product was then centrifuged at 5,000 rpm and washed with deionized (DI) water to obtain MXene layer precipitates until the pH of the mixture reached 6. The resultant aqueous dispersions were subsequently vacuum-filtered using a PTFE membrane, and the obtained filtrate of MXene was subjected to freeze-drying for 24 h.
After adding 0.2 g of MXene powder to 15 mL DMSO, the mixture was magnetically stirred for 24 h at room temperature. The final product was centrifuged at 4,500 rpm for 15 min after being washed with DI water. To delaminate MXene flakes, the suspended scattered precipitate was ultrasonically treated for 1 h. The product was then centrifuged at 4,500 rpm for 30 min and heated in an oven at 70°C for 5 h.

Synthesis of La 2 O 3 NPs
The La 2 O 3 NPs were synthesized by following the reported method [35]. First, 25 mL 0.1 M La(NO 3 ) 3 aqueous solution was added into an aqueous solution of sodium hydroxide (25 mL, 1.0 M) to make a final volume of 50 mL. The obtained solution was then kept for stirring and refluxed at a moderate temperature for 2 h by using a magnetic stirrer. After stirring, the sample was centrifuged, washed with distilled water, and then dried at room temperature. The final product was then ground using mortar and pestle and transferred to calcinate at 300°C for 2 h.

Synthesis of the La 2 O 3 /MXene composite
The La 2 O 3 /MXene composite was made by combining 900 mg of La 2 O 3 and 100 mg of MXene powder in 25 mL DI water and ultrasonically processed for 1 h. The resulting solution was then dried in a vacuum oven for 12 h at 75°C. The obtained nanostructures were ground to make a fine powder using a mortar and pestle [36].

Electrode fabrication
To investigate the electrochemical performance of SCs, three types of electrodes made of MXene, La 2 O 3 NPs, and La 2 O 3 /MXene composite materials were fabricated. The preparation procedure of electrodes was described in the literature [36]. The active material (MXene, La 2 O 3 NPs, and La 2 O 3 /MXene composite) was added to acetylene black and used as a binder in a mortar and pestle in an 8:1:1 ratio. To produce a thick slurry, a few drops of ethanol were added to the amalgam product that had been formed. The asprepared slurry was then pasted on the single conducting side of a carbon cloth with an area of 1 × 2 cm 2 . The electrodes were then subjected to drying in the oven at 60°C. The weight of the applied material loading on the electrode surface is about 1-1.3 mg.

Material characterization
The phase and structural analyses of La 2 O 3 , MXene, and La 2 O 3 /MXene composites were performed by X-ray diffraction (XRD, JDX-352-JEOL, Tokyo, Japan, using Cu-Kα rays). The morphology and microstructure of the samples were characterized by scanning electron microscopy (SEM; JEOL SM6490, and Hitachi SU8010, 20 kV). Raman spectra were collected (LabRAM HR800, Kyoto, Japan) with a continuouswave, frequency-doubled ND:YAG laser with a wavelength of 532 nm. The electrochemical measurements, such as charge transfer, C sp , and resistance, were done using cyclic voltammetry (CV), galvanic chargedischarge (GCD), and electrochemical impedance spectroscopy (EIS) techniques (Autolab PGSTAT12, USA). A three-electrode system with 0.1 M KOH as an electrolyte was used to perform the above mentioned measurements. MXene, La 2 O 3 , and La 2 O 3 /MXene were used as the working electrodes, platinum wire served as the counter electrodes, and Ag/AgCl served as the reference electrode.

Results and discussion
We used a two-step co-precipitation method to prepare the La 2 O 3 /MXene composite (see details in the Experimental section). The combination of 2D MXene and La 2 O 3 NPs may achieve synergistic effects in which MXene offers high electrical transport and a large surface area, and the electrochemically active La 2 O 3 prevents MXene from restacking. Therefore, MXene and La 2 O 3 may reinforce the performance of SCs through fast ion transportation and increased ion accessibility due to large interlayer spacing. The schematic of the synthesis of La 2 O 3 /MXene composite is shown in Figure 1(a). We used Raman spectroscopy to study the quality of the material and its structure. A comparison of MXene, La 2 O 3, and La 2 O 3 /MXene can be seen in Figure 1(b). The four Raman peaks at 155, 265, 416, and 610 cm −1 belong to MXene. These peaks are considered as a key feature of Ti 3 C 2 and matched the literature [37,38]. Furthermore, the broadening of Raman peaks in the MXene spectrum indicates a decrement in the structural order due to the etching of Al atoms during the exfoliation process. The peaks at 300-700 cm −1 are typical bands related to the Ti-C bonding. For La 2 O 3 , three main peaks at 285, 345, and 449 cm −1 are assigned to the typical bands of the La 2 O 3 structure and match the literature [39,40] Figure 2(a). After HF treatment of MAX, the elimination of Al atoms results in the exfoliation of layered MXene into 2D sheets, as illustrated in Figure 2(b). The accordion-like obtained structure of MXene was possibly due to an exothermic reaction between MAX and HF [41,42]. By further comparing the obtained results with the literature, it was confirmed that the 2D MXene nanosheets were synthesized successfully after HF treatment [43][44][45][46][47]. The statistical analysis of the La 2 O 3 NPs (Figure 2(c)) shows that the La 2 O 3 NPs are non-uniform in size with an average size of ~47 nm ( Figure S1). The formation of the La 2 O 3 /MXene composite is confirmed by SEM images, as shown in Figure 2   The phase of the as-synthesized MAX, MXene, and the La 2 O 3 /MXene composite was evaluated by XRD. The XRD patterns of pure Ti 3 AlC 2 powder can be seen in Figure 3(a). For Ti 3 AlC 2 , a sharp characteristic (104) peak at 39° can be observed, which indicates the presence of Al atoms, along with other peaks at 41.7° (105), 36° (102), and 33.9° (100) showing the TiC impurity. The obtained XRD peaks of the Ti 3 AlC 2 were indexed following the literature [48]. The XRD pattern of pure MXene (Figure 3(b)) shows a much decreased intensity of the characteristic (104) peak at 39° compared with MAX phase, due to the elimination of Al atoms from MAX sheets (Ti 3 AlC 2 ) by using HF etchant. The XRD analysis further confirms that the characteristic peak of MAX becomes weaker, and broader as the HF reaction with MAX powder progresses, which results from a gradual decrement in the degree of the orderliness of planes. Figure 3(c) shows the magnified XRD pattern with a backward shift of (002) peak. One possible reason for this change may be the increase in the c-lattice parameter (LPc), where LPc = d spacing + thickness of 2D sheet, that ascribes the layered structure of MXene [49]. The XRD comparison of the La 2 O 3 , MXene, and La 2 O 3 /MXene composites is shown in Figure 3 The charge transfer rate at the electrode interface and the electrolyte are the key parameters to evaluate the electrochemical performance of SCs, which can be measured by CV. The cathodic and anodic peaks of MXene, La 2 O 3 , and the La 2 O 3 /MXene compositebased electrodes can be seen in Figure 4(a,b). Here, the cathode peaks are also known as reduction peaks in the negative current region, while the anodic oxidation peaks correspond to the positive current region. The variations in the CV loop areas at different scan rates of 10, 25, 50, 75, 100, and 125 mV/s were investigated under the same potential window ranges from 0 V to 0.8 V as illustrated in Figure 4(a). The results revealed that the areas of CV loops increased as the scan rates increased in all electrode materials. The possible reason is the large current that passes through the circuit at a large scan rate, ultimately increasing the loop area and C sp value [52]. Furthermore, to analyze the variation of capacitance of MXene, La 2 O 3 , and La 2 O 3 /MXene-based composite, CV measurements were performed at a scan rate of 10 mV/s. The nonrectangular shapes of CV curves are clear evidence of Faradic behaviors exhibited by the electrode material during the whole C sp . The CV loop area of the composite-based electrode was greater than that of MXene and La 2 O 3 alone, showing a large C sp of La 2 O 3 /MXene composite-based electrode, as seen in Figure 4(b). The variations in the loop's area might be attributed to the excellent conductivity and large surface area offered by the La 2 O 3 /MXene-based composite. The highly conductive MXene offered high charge carriers resulting in a decrement in internal resistance. Moreover, the large C sp of the composite was achieved due to the insertion of La 2 O 3 NPs in MXene, which provides improved Faradic behavior along with the electrical double-layer capacitance of MXene layers. Furthermore, the results obtained from CV loops of La 2 O 3 and MXene at various scan rates can be seen in Figure S2 in the Supporting Information. The following formula can calculate the specific capacitance of the samples.
Here, C sp is the specific capacitance, ∆V is the voltage window, v is the scan rate, and ∫Idv is the area of the CV curve. Using the above formula, the calculated C sp values of La 2 O 3 , MXene, and La 2 O 3 /MXene composite-based electrodes are 71, 125, and 298 F/g, respectively, at a scan rate of 10 mV/s. The La 2 O 3 /MXene composite electrode also showed excellent mechanical and structural stability with a capacitance retention of around 96% at 1,000 cycles ( Figure S3). Furthermore, capacitance by CV loops at different scan rates was measured, and the bar plot of La 2 O 3 , MXene, and La 2 O 3 /MXene composite showing the best performance of composite-based electrode ( Figure S4). To further explore the electrochemical performance of the as-prepared electrode materials, GCD measurements which can evaluate the C sp of SCs were performed at a current density of 1 A/g. Furthermore, variable current densities ranging from 1 A/g to 5 A/g were applied to investigate the effect of variable current densities on C sp. Increasing the current density decreases the area during the charge-discharge process, as shown in Figure 4(c). This effect is due to the redox reaction slowing down at high current density. In addition, at low current density, the electrolytic ions not only have a high probability of being adsorbed on the electrode surface but also have a high chance of penetrating inside the electrode. By further enhancing the current density value, the penetrating chances of ion charges reduce, and they can only reside on the outer surface of the electrode, which decreases the C sp value of electrode materials [53]. C sp of as-prepared samples is calculated by using the following formula [54] Here, I/m is the current density (A/g), ∫Vdt is the area under the discharge curve, and ΔV is the potential window. The comparison of the GCD profiles of MXene, La 2 O 3 , and La 2 O 3 /MXene composite further revealed that the C sp value of the composite is greater than that of the other two electrodes (Figure 4d). The  Figure 4(f) reveals that the C sp decreases gradually with increasing the current density. The electrochemical performance of La 2 O 3 /MXene composite-based electrodes is better than some of the reported electrodes using rare earth oxide or MXenebased composite material, as illustrated in Table 1. The calculated GCD analysis of La 2 O 3 and MXene can be seen in Figure S5. EIS was performed to evaluate the electrical resistance and charge transport properties of the electrode materials. EIS provides information about the internal charge transfer resistance of electrodes and the resistance between electrodes and electrolytes. Figure 5 shows the EIS curves of the MXene, La 2 O 3 , and La 2 O 3 /MXene composite under the frequency range of 100 kHz to 0.1 Hz with a Nyquist plot. Here, Zʹ and Zʹʹ correspond to the real and imaginary parts of impedances. The EIS result shows a linear line at a low frequency and a semicircle at a high frequency. The change in the shape and diameter of these semicircles depends on the transfer of charges at the electrode materials and ion's movements [65,66]. The obtained charge transfer resistance of the La 2 O 3 , MXene, and La 2 O 3 /MXene composite were 1.7, 0.5, and 1.5 Ω, respectively, as given in Table  S1. The composite material showed higher charge transfer resistance than MXene, which is attributed to the addition of semiconducting La 2 O 3 NPs ( Figure 5). The La 2 O 3 /MXene composite-based electrode exhibits good electrochemical performance because of combining highly conductive MXene with La 2 O 3 NPs. The comparative study of C sp obtained from CV, GCD, and charge transfer resistance is shown in Table S1.

Conclusion
In

Disclosure statement
No potential conflict of interest was reported by the author(s).