A simple, efficient, fluorine‐free synthesis method of MXene/Ti3C2Tx anode through molten salt etching for sodium‐ion batteries

MXenes are mentioned in many applications due to their unique properties. However, the traditional etching method has a lengthy synthesis time, dangerous process, and high cost. Molten salt etching is not only short in time but also safe and simple, laying a good foundation for industrialization. Here, we compare the traditional F‐containing etching method with the molten salt etching method. Transmission electron microscopy elemental mapping images and X‐ray photoelectron spectroscopy show that the Ti3C2Tx surface end of traditional etching is terminated by –F, while the Ti3C2Tx surface end of molten salt etching is terminated by –Cl. Finally, the sodium‐ion batteries are fabricated and the performance difference of the three etching methods is compared. The results show that the capacity of 102.1 mAh g–1 can still be reached when the molten salt etching MXene material returns to 0.1 A g–1 after the current density of 5 A g–1. After 500 cycles at 1 A g–1, there is no significant loss of capacity and the Coulomb efficiency is close to 100%. This work describes that molten salt etching MXene has comparable sodium storage capacity to conventional F‐containing etched MXene, making it a potential candidate for the production of large‐scale sodium‐ion batteries.

2][3][4] SIBs offer comparable working potential and energy density to LIBs.However, the larger radius of sodium ions (Na + : 1.02 Å) compared to lithium ions (Li + : 0.76 Å) can lead to significant structural expansion of electrodes and sluggish redox kinetics during the sodium/desodium reaction. 5,6These factors contribute to suboptimal cycle durability and lower rate performance in SIBs.In this context, it is crucial to investigate an appropriate anode material that can offer exceptional rate performance and efficient sodium storage capabilities.Currently, several types of negative electrode materials have been developed for SIBs, including intercalated materials, 7 transformed materials, 8 alloy materials, 9 and organic materials. 102][13][14] These materials are obtained through the selective etching of the "A" layer from the parent MAX phase, where "M" represents transition metals such as Ti, V, Nb, and others, "A" represents elements primarily from groups 13-16 (e.g., Al and Si), and "X" represents carbon and/or nitrogen. 15,169][20] Subsequently, an alternative method involving a milder HF-like reaction using a LiF+hydrochloric acid (HCl) mixture [21][22][23][24] was also developed.Along this line, other methods include the use of a mixture of HCl and fluoride salts (such as LiF) or ammonium bifluoride (NH 4 )HF 2 to generate a weaker "HF."However, these methods often require hours to days to achieve MXene synthesis, leading to reduced production efficiency and increased costs.Most importantly, the notable drawbacks of HF-based methods are the inherent hazards associated with using HF, which pose significant challenges in terms of experimental safety, scalability, and the potential for undesired MXene oxidation.Therefore, there is a pressing need for an efficient strategy that is simple, universally applicable, and prioritizes safety to achieve successful MXene etching.
In recent years, Li et al. 25 have introduced a Lewis acidic molten salt etching method as an alternative to traditional HF-based solvents for MXene synthesis.It is worth noting that etching MXene with a surficial "-F" can result in insufficient ion diffusion, leading to suboptimal performance of energy storage devices. 26,27owever, this limitation can be overcome by employing the molten salt method.What particularly excites us is the contrasting sodium storage properties between molten salt etching MXene and the conventional aqueous solutions containing F ions.9][30][31] However, to the best of our knowledge, limited research has been conducted to investigate the differences in properties between F ion-free etched MXene and MXene obtained through traditional F-containing ion etching methods.
In this study, we propose a novel, simple, and safe strategy for directly obtaining MXene through Lewis acidic molten salt etching.Additionally, we prepared HF and LiF+HCl as traditional F-containing ion etching methods for comparison.We conducted a systematic analysis of the structural differences and electrochemical properties of the MXenes treated with different etching methods.Our results demonstrate that MXene materials with similar properties can be obtained through both F ion-free etching and traditional F-containing etching.This study holds significant guiding significance for understanding MXene materials obtained through different etching methods.

| MXene synthesized via molten saltshielded synthesis method
First, 1 g of Ti 3 AlC 2 MAX phase was mixed with NaCl (3 g) and KCl (3 g) salts.The mixture was milled for 15 min using a mortar and pestle.The resulting powder was then uniaxially pressed into pieces using a steel mold with a diameter of 20 mm.The sample pieces were placed in a corundum crucible and covered with an inorganic salt mixture of NaCl (8 g) and KCl (8 g).The crucible was then placed in a muffle furnace without inert gas protection and heated to 700°C at a rate of 10°C min −1 .It was held at this temperature for 40 min before cooling the furnace to room temperature.The resulting MXene product was washed three times with deionized water to remove inorganic salts, followed by washing with 100 mL of 0.5 M (NH 4 ) 2 S 2 O 8 solution to remove the Cu elemental substance from the Ti 3 C 2 T x /Cu mixture.The resulting solution was rinsed more than three times with deionized water and then suctioned.Finally, the Ti 3 C 2 T x MXene powder was vacuum dried at 40°C for 8 h.

| MXene synthesized via HF etching method
Typically, 1 g of Ti 3 AlC 2 powder was slowly added to 20 mL HF solution.The mixture was kept at 40°C for 24 h while stirring with a magnetic stirrer at 400 rpm.The resulting solid precipitate was washed several times with deionized water and centrifuge at 3500 rpm for 3 min in each cycle until the pH of the supernatant is neutral.The resulting powder was then vacuum dried at 40°C for 12 h.

| MXene synthesized via LiF+HCl mixture etching method
Typically, 1.6 g of LiF was dissolved in a mixed solution of 17.5 mL HCl (12 M) and 2.5 mL of deionized water.Then 1 g of Ti 3 AlC 2 powder was slowly added.The mixture was kept at 40°C for 24 h while stirring with a magnetic stirrer at 400 rpm.The resulting solid precipitate was washed several times with deionized water and centrifuged at 3500 rpm for 3 min in each cycle until the pH of the supernatant became neutral.The resulting powder was then vacuum dried at 40°C for 12 h.

| Electrochemical characterizations
All electrochemical tests were performed using CR2032 coin cells.The working electrodes were prepared in the following manner: MXene Powders, Ketjen Black, and polyvinylidene difluoride were mixed in a weight ratio of 7:2:1.The mixture was then dispersed in N-methyl-2-pyrrolidone and coated on Cu foil using the scraper method to prepare the slurry.After drying overnight under vacuum at 100°C, a disc with a diameter of 12 mm was cut as the working electrode.The loading mass of electrodes was 1 (±0.1)mg cm -2 based on active materials.Na foils and Whatman glass fiber (GF/D) were used as the counter electrode and separator, respectively.A 1 M NaPF 6 dissolved in diethylene glycol dimethyl ether (DIGLYME) was used as the electrolyte.The coin cells were assembled in an argon-filled glovebox, with H 2 O and O 2 levels of <0.01 ppm.

| RESULTS AND DISCUSSION
Figure 1 depicts the fabrication process of multilayer MXene.It was synthesized through Lewis acidic molten salt reaction method (the detailed technological process is discussed in Supporting Information: Figure S1).Briefly speaking, it involves the CuCl 2 molten saltshielding method, where MAX is blended with NaCl and KCl to form flakes that are placed in a corundum crucible.Next, the NaCl/KCl/CuCl 2 mixture is used to cover the flakes for a duration of 10 min.The crucible is then placed in a muffle furnace under an air atmosphere and heated at 700°C.During this process, the Al etching reaction occurs, as shown in the following reaction mechanism display: (1) (2) (3) The Cu 2+ ions are reduced from the molten salt, and the temperature is maintained for 40 min before naturally cooling to room temperature.The sample is then cleaned with deionized water and ammonium persulfate to eliminate solid salts and copper particles, and finally vacuum dried to obtain the MXene material.In the traditional etching route with HF as an etching agent, the MAX powder is gradually added to the HF solution and magnetically stirred for 24 h or more at a constant temperature of 40°C.This process has been the most commonly used method for MXene etching since its development in 2011.However, in recent years, the LiF+HCl scheme has emerged as a safer alternative to the concentrated HF solution etching method.The detailed procedures for all three methods are presented in Section 2 of this paper, and a comprehensive flowchart is provided in Supporting Information: Figure S1.The CuCl 2 molten salt-shielding method is faster and more effective than the two traditional synthesis methods (HF and LiF+HCl).Therefore, in this study, we will henceforth refer to the molten salt-shielding method as the CuCl 2 method.The method of HF and LiF with HCl as the etching agent will be simplified and referred to as the HF and LiF methods, respectively.
Figure 2A depicts the XRD patterns of the Ti 3 AlC 2 powders and samples gained through different methods.
The distinctive peaks of Ti 3 AlC 2 are observed at 9.5°, 19.2°, 34.0°, 39.0°, and 41.8°, corresponding to the (002), (004), ( 101), (104), and (105) lattice planes, respectively.In comparison to the Ti 3 AlC 2 precursor, the intense peak at 39.0°, which corresponds to the (104) plane of Al layers, disappears in CuCl 2 or HF method.However, it only weakens after the LiF method, indicating that the MAX phase is not completely etched by the LiF method.Furthermore, in the CuCl 2 method, an additional (002) peak is observed at 8.06°, indicating an increased interlayer distance from 9.29 to 10.97 Å compared to the original MAX.On the other hand, the HF method exhibits a layer spacing of 9.7 Å.As demonstrated above, the CuCl 2 method offers a highly efficient approach to remove the Al layer in a shorter time and achieve a relatively larger layer spacing of MXene.In contrast, the HF method represents a more hazardous and laborious route to obtain MXene with a lower layer spacing.The LiF method resulted in the lowest purity product, but it exhibited the largest interlayer spacing of MXene.This can be attributed to the electrostatic repulsion between lithium ions on the surface of MXene after lithium-ion intercalation.Additionally, the XRD patterns of Ti 3 C 2 T x -CuCl 2 (Supporting Information: Figure S2) were compared between Cu and CuCl 2 .Based on a thorough analysis, it can be concluded that the MXene obtained through the CuCl 2 method does not contain Cu or CuCl 2 .
To further investigate the structural changes resulting from the etching of Ti 3 AlC 2 , we examined the morphological and structural transformation.SEM images of Ti 3 AlC 2 powders before and after the reaction with CuCl 2 , HF, and LiF are shown in Figure 2B-E 2G). 32The appearance of Ti-Cl bonds is evident from the 2p 3/2 peak (198.9 eV) and 2p 1/2 peak (200.6 eV) in the Cl 2p spectrum (Supporting Information: Figure S3), 33  molten salts (Equation 2).Furthermore, the Ti-O bond is observed at 530.5 eV in the O 1s spectrum (Figure 2H).The presence of -O surface groups is primarily attributed to the introduction of water molecules during the etching process (Equations 3 and 4).The XPS spectra of HF and LiF MXene, as well as a detailed discussion, are presented in Supporting Information: Figures S4 and S5.
The CuCl 2 method has been identified as the most effective, resulting in fully exfoliated MXene with a larger interlayer spacing, which is beneficial for sodium-ion transport and enhances storage performance.This effectiveness can be attributed to the abundant intercalation of Cl atoms between the layers and their occupation on top of the Ti atoms, which acts as a protective layer against oxidation.In contrast, the LiF and HF methods are controlled by the intercalation of lithium and fluorine atoms, respectively.To further investigate the detailed microstructure of the elements intercalated into Ti 3 C 2 T x MXene layers, TEM was conducted.TEM elemental mapping images were utilized to detect the presence of Cl and F. The HAADF-STEM elemental mapping in Figure 3G-I reveals the homogeneous distribution of Cl and F on the interlayer matrix, indicating that the methods employed in this study achieved both the etching of the MAX precursor and the uniform decoration of various elements on the MXene substrate.The three methods successfully yielded Ti 3 C 2 T x MXene with the characteristic accordion-like structure, as confirmed by the HRTEM images.The HRTEM images clearly displayed the layered arrangement of Ti 3 C 2 T x MXene, and the interlayer spacing observed in Figure 3D,E was similar and consistent with the corresponding XRD pattern result of (002) as shown in Figure 2A.Moreover, the HRTEM image of MXene prepared by the LiF method exhibited the largest interlayer spacing, which can be attributed to the electrostatic repulsion between lithium ions on the surface of MXene, as mentioned earlier (Figure 3F).This observation was consistent with the corresponding XRD result of (002) with a spacing of 13.71 Å, providing further evidence for the successful exfoliation of MXene through the LiF method.In conclusion, all three etching methods for Ti 3 AlC 2 MAX were effective, and the CuCl 2 method proved to be the most efficient approach for achieving exfoliated MXene with relatively larger interlayer spacing.
To evaluate the performance of sodium-ion storage, we prepared CR2032 coin-type half-cells using Ti 3 C 2 T x as the working electrode and Na foil as the counter electrode in a 1 M NaPF 6 /DIGLYME electrolyte (as described in Section 2). Figure 4A-C  (oxidation/reduction at 2.35/2.215][36][37] Moreover, a pair of redox peaks around 0.1 V is attributed to the Na + insertion/extraction process in MXene.Subsequently, the CV profiles in the subsequent cycles show a high degree of similarity to the first cycle.Additionally, Ti 3 C 2 T x MXene exhibits a distinctive pseudocapacitive-shaped CV curve, indicating enhanced Na + -ion migration within the electrode.The electrochemical performance of the MXene anode is further evaluated through galvanostatic charge-discharge (GCD) measurements at a current density of 0.05 A g −1 , as depicted in Figure 4D-F.The electrode demonstrates a prolonged sodiation slope region ranging from 1.0 to 0.1 V, followed by a desodiation slope region from 0 to 3 V.These observations align well with the pseudocapacitive-shaped CV curves.Notably, the GCD curves of the electrodes exhibit a remarkable overlap in the subsequent two cycles, indicating excellent electrochemical repeatability.Supporting Information: Figure S6 illustrates the MXene anode's performance, where the CuCl 2 electrode achieves a satisfactory initial reversible capacity of 168.9 mAh g −1 .In comparison, the HCl and LiF electrodes exhibit similar electrochemical performance, delivering a lower specific capacity of 152.8 and 130.3 mAh g −1 , respectively.Additionally, all three methods result in similar initial Coulombic efficiency (ICE) values after CuCl 2 , HF, and LiF treatments.The formation of a thin and compact SEI film contributes to this irreversible capacity.Detailed information on the first charge and discharge capacities of the three electrodes and their ICE values can be found in Supporting Information: Table S1.
The CV profiles of the CuCl 2 , HCl, and LiF electrodes at a scan rate of 0.1 mV s -1 are shown in Supporting Information: Figure S7.These profiles exhibit rectangular and highly symmetric shapes within the potential range of 0.01-2 V, without the presence of visible redox peaks.This suggests a pseudocapacitive Na + storage mechanism, which is consistent with the previously reported molten salt-derived MXenes. 25Furthermore, electrochemical impedance spectroscopy measurements were conducted to investigate the excellent reaction dynamics of the prepared electrodes, as shown in Figure 4G.The Nyquist diagrams typically exhibit low resistance at high frequencies.In particular, the semicircle observed in the medium-frequency region corresponds to the charge transfer resistance (R ct ) of Na + ions.Conversely, the sloping line observed in the lowfrequency region can be attributed to the diffusion of Na + ions within the electrodes. 38The CuCl 2 MXene electrode demonstrates a steeper slope line in comparison to the HF and LiF MXene electrodes, indicating enhanced Na + ion diffusion in a fresh cell.Moreover, all electrodes exhibit a minimal charge transfer resistance (R ct ), indicating improved efficiency of charge transfer.The rate performance of the three MXene-based electrodes at different current densities, ranging from 0.05 to 5 A g -1 , is illustrated in Figure 4H.As observed, the CuCl 2 MXene electrode represents reversible capacities of 165.5, 111.7, 95.4,82.6, 74.2, 68.8, and 57.5 mAh g -1 at current densities of 0.05, 0.1, 0.2, 0.5, 1, 2, and 5 A g -1 , respectively.Even when the current density returns to 0.1 A g -1 , the electrode still exhibits a capacity of 102.1 mAh g -1 .This capacity is only slightly lower than that of the HF and LiF MXene electrodes, indicating that the MXene material maintains consistent capacity after undergoing all three etching methods.However, the CuCl 2 MXene electrode, which has a rich Cl surface group with lower conductivity (Supporting Information: Figure S8), shows a slightly lower capacity compared to the other methods.Nevertheless, all MXene electrodes prepared using the three etching methods exhibit minimal capacity fading when the current density returns from 5 to 0.1 A g -1 , demonstrating their uniform charge-discharge super rate ability and the excellent stability of the MXene material.Furthermore, Figure 4I effectively illustrates the long-term cycling performance of the electrodes, specifically at a current density of 1 A g -1 .Notably, all curves consistently maintain a remarkable reversible capacity of approximately 100 mAh g -1 throughout 500 cycles, showcasing negligible capacity degradation.Additionally, the Coulombic efficiencies consistently approach 100% during the cycling tests.The corresponding GCD plot in Supporting Information: Figure S9 confirms the intriguing observation that all electrodes display an uplifting capacity behavior, which can be attributed to both the conventional activation process and the pillaring effect.The latter phenomenon refers to the enlargement of the MXene interlayer spacing caused by the significant volume expansion of the interbedded second phase of active materials during cycling.Notably, this effect is more pronounced in the electrodes treated with the CuCl 2 method compared to the other methods.In summary, the CuCl 2 method not only offers a highly efficient approach for obtaining MXene material but also demonstrates comparable performance in sodium energy storage, showcasing a more stable long cycling behavior enhanced by the pillar effect.This stands in contrast to the hazardous and labor-intensive nature of the HF and LiF methods.
To investigate the reaction kinetics of CuCl 2 during Na + insertion/extraction, CV profiles were collected at various scan rates ranging from 0.1 to 1 mV s -1 .These profiles were then analyzed to assess sodium-ion diffusion.The anodic peak is referred to as Peak 1, while the corresponding cathodic peak is labeled as Peak 2. By utilizing the relationship between scan rate and peak current, the capacitive and diffusion-controlled contributions can be quantitatively determined using the following equation: Here, v represents the scan rate and i represents the response current at a certain potential.The value of b = 1 signifies the capacitive-controlled process, while b = 0.5 indicates a diffusion-controlled process.The CV curve of CuCl 2 at various scan rates ranging from 0.1 to 1 mV s -1 is depicted in Figure 5A.Remarkably, both the anodic and cathodic peak currents of the prepared electrode exhibit a linear relationship with the square root of the scan rates, as illustrated in Figure 5B,C.The calculated b values for Peak 1 (reduction) and Peak 2 (oxidation) of CuCl 2 MXene are 1.02 and 0.77, respectively.These values suggest that the sodium-ion storage in CuCl2 MXene is predominantly governed by pseudocapacitive behaviors.In general, the capacitive contribution ratio can be divided into two components, as expressed by the following equation.
In this equation, the terms k 1 v and k 2 v 0.5 represent the current contributions of capacitive behavior and diffusion behavior, respectively. 39,40The k 1 and k 2 values can be determined by the equation and CV curves at various scan rates.At a sweep rate of 0.1 mV s -1 , the capacitance contribution to the total capacity of the MXene anode prepared using the CuCl 2 method amounts to 55.0% (highlighted region), as shown in Figure 5D.Remarkably, this contribution increases to 80.4% (Figure 5E,F) when the scan rate is enhanced to 1 mV s -1 .These results clearly indicate the significant involvement of pseudocapacitive ion storage, particularly at higher scan rates.Furthermore, we conducted CV tests at various scan rates for MXene synthesized using two other methods.The obtained data allowed us to generate a surface pseudocapacitive percentage plot, as depicted in Supporting Information: The primary objective of this study was not to optimize power and energy densities using CuCl 2 , HF, and LiF etching methods but rather to identify a readily available method for MXene synthesis without compromising capacity and rate capability in sodium energy storage.This was achieved by comparing different MXene preparation techniques and evaluating the resulting electrochemical properties.Encouragingly, electrodes prepared using CuCl 2 , HF, and LiF methods exhibited comparable electrochemical characteristics, with CuCl 2 demonstrating particular ease of preparation.These promising findings have motivated us to explore further optimization of these methods with low toxicity, aiming to enhance the electrochemical performance not only for Na + ions but also for other cations.We anticipate rapid advancements in the utilization of CuCl 2 -derived MXene, which holds substantial technological significance in the field of sodium energy storage.Our presented method provides an alternative approach for obtaining MXene materials, offering comparable properties to those produced through traditional methods, specifically for application in the anode of sodium energy storage.

| CONCLUSION
In conclusion, we have successfully developed a simple and low-toxicity strategy for fabricating MXene as anodes for SIBs by employing CuCl 2 as a Lewis acid in molten salt etching.This approach minimizes MXene damage under air atmosphere, enhances experimental safety, and efficiently utilizes the molten salt etching product.As a result, MXene with a rich Cl surface group is obtained, which effectively prevents oxidation of Ti in MXene by oxygen.XRD, SEM, and TEM analyses, along with element mapping measurements, confirm the complete exfoliation of the MAX phase into MXene using the CuCl 2 method.Notably, the CuCl 2 method yields a relatively larger interlayer spacing of 10.97 Å compared to the HF and LiF methods.Furthermore, electrochemical evaluations of the CuCl 2 , HF, and LiF methods demonstrate excellent Na + and electronic migration/ transfer kinetics, along with stable long-term sodiation/ desodiation processes for all MXene anodes.Notably, the MXene obtained through the CuCl 2 method exhibits a more pronounced pillar effect in long-term cycling compared to the other methods.This highlights the potential of the CuCl 2 method for large-scale production in SIBs, as it enables MXene with sodium storage capabilities comparable to those achieved through traditional methods.Our findings demonstrate that the Lewis acidic molten salt etching route presents a new and promising approach for MXene fabrication as anodes in SIBs.This work not only inspires further research in this field but also simplifies the production process for MXene materials on a large scale, ultimately facilitating their commercial application in SIBs.
, respectively.As shown in Figure 2C,D, MXene-Cu and MXene-HF exhibit an accordion-like structure, indicating successful etching and removal of the Al phase from the original Ti 3 AlC 2 MAX.However, in the case of LiF treatment, the layers of MXene remained stacked, as depicted in the SEM image in Figure 2E.Nevertheless, the XRD pattern confirms that the LiF method partially exfoliated the MAX phase, although it did not complete the transformation into MXene.X-ray photoelectron spectroscopy (XPS) was utilized to investigate the chemical composition of Ti 3 C 2 T x samples.The Ti 2p spectrum exhibited distinct peaks corresponding to Ti-C (II) (455.7/461.7 eV), Ti-Cl (457.7/463.6 eV), and Ti-O (458.3/464.2eV) species, as shown in Figure 2F.The presence of Ti-C(II) spectrum indicates the TiC 6 structure of Ti 3 C 2 T x MXene, which is further confirmed by the Ti-C bond observed at 283.0 eV in the C 1s spectrum (Figure indicating that Ti 3 C 2 MXene can be terminated by the -Cl group during the etching of CuCl 2 F I G U R E 1 Schematic diagram of the molten salt-shielding synthesis method.
illustrates the cyclic voltammetry (CV) curves of the Ti 3 C 2 T x electrode at a scan rate of 0.1 mV/s for the first three cycles after treatment with CuCl 2 , HF, and LiF, respectively.In all three cases, a broad main peak is observed at around 0.5 V during the first cathodic scan, corresponding to the formation of a solid electrolyte interphase (SEI) film.During the first anodic scan, shown in Figure 4A, three peaks are observed around 0.73, 1.03, and 1.65 V, which correspond to the multistep extraction of Na + ions.In the subsequent cyclic curve, HF showed pairs of redox peaks F I G U R E 3 TEM images of (A) CuCl 2 , (B) HF, and (C) LiF.HRTEM images of CuCl 2 , HF, and LiF (D-F); and (G) HAADF-STEM image and corresponding element mapping images of CuCl 2 .Elemental mapping images of (H) HF and (I) LiF.HAADF-STEM, high-angle annular dark-field scanning transmission electron microscopy; HF, hydrofluoric acid; HRTEM, high-resolution transmission electron microscopy; TEM, transmission electron microscopy.

F I G U R E 4
Initial cyclic voltammetry profiles at a scan rate of 0.1 mV s -1 and galvanostatic charge-discharge curves at a current density of 0.05 A g -1 for (A, D) CuCl 2 , (B, E) HF, and (C, F) LiF.(G) electrochemical impedance measurements of the CuCl 2 , HF, and LiF MXene at 0.01-10 5 Hz.(H) Rate performance of CuCl 2 , HF, and LiF MXene.(F) Cycling performance of CuCl 2 , HF, and LiF electrodes at a current density of 1 A g -1 .HF, hydrofluoric acid.
Figure S9.Additionally, Supporting Information: Figures S11-S13 present the pseudocapacitive contribution diagrams of MXene obtained through the three different methods, showcasing the results for each sweep speed.

F
I G U R E 5 Pseudocapacitance behavior of CuCl 2 .(A) CV curves at different scan rates.(B, C) relationship between log(I) and log(V); percentage of surface pseudocapacitance at (D) 0.6 and (E) 1.0 mV s -1 with scan rate; (F) percentage of surface pseudocapacitance at different scan rates.CV, cyclic voltammetry.