Composite Fe3O4-MXene-Carbon Nanotube Electrodes for Supercapacitors Prepared Using the New Colloidal Method

MXenes, such as Ti3C2Tx, are promising materials for electrodes of supercapacitors (SCs). Colloidal techniques have potential for the fabrication of advanced Ti3C2Tx composites with high areal capacitance (CS). This paper reports the fabrication of Ti3C2TX-Fe3O4-multiwalled carbon nanotube (CNT) electrodes, which show CS of 5.52 F cm−2 in the negative potential range in 0.5 M Na2SO4 electrolyte. Good capacitive performance is achieved at a mass loading of 35 mg cm−2 due to the use of Celestine blue (CB) as a co-dispersant for individual materials. The mechanisms of CB adsorption on Ti3C2TX, Fe3O4, and CNTs and their electrostatic co-dispersion are discussed. The comparison of the capacitive behavior of Ti3C2TX-Fe3O4-CNT electrodes with Ti3C2TX-CNT and Fe3O4-CNT electrodes for the same active mass, electrode thickness and CNT content reveals a synergistic effect of the individual capacitive materials, which is observed due to the use of CB. The high CS of Ti3C2TX-Fe3O4-CNT composites makes them promising materials for application in negative electrodes of asymmetric SC devices.


Introduction
Ti 3 C 2 T x belongs to the family of MXene-type materials, which are of great technological interest for applications in electrodes of SCs [1][2][3]. The interest in Ti 3 C 2 T x is attributed to the high capacitance and low electrical resistivity of this material. The promising capacitive properties of Ti 3 C 2 T x result from its high surface area and the redox active nature of surface functional groups. Enhanced capacitive properties were obtained for Ti 3 C 2 T x composites, containing different conductive additives, such as graphene [4], acetylene black [5], and carbon black [6] and for nitrogen-doped Ti 3 C 2 T x [7][8][9]. Moreover, advanced Ti 3 C 2 T x composites were developed, containing other components, such as ZnO [10], MnO 2 [11], TiO 2 [12], and Mn 3 O 4 [13]. Investigations revealed the stable cycling behavior of Ti 3 C 2 T x composites [14][15][16][17][18][19].
The progress in applications of SC devices will depend on the ability to fabricate efficient electrodes and devices with high C S , which can be achieved at high AM loadings. Another important benefit of high AM electrodes is their low ratio of the mass of electrochemically inactive components to the AMs. With the goal to increase energy-power characteristics, there is a growing trend in devices that operate in enlarged voltage windows. Of particular importance are environmentally friendly neutral electrolytes, such as normalized by the electrode area or mass of the active material, respectively, were obtained from the CV or GCD data, and complex C S * components (C S ' and C S ") were calculated from the EIS testing results obtained at a signal of 5 mV, as described in [41]. JSM-7000F microscope (JEOL, Peabody, MA, USA) was used for SEM investigations. Figure 1A,B shows SEM images of Ti 3 C 2 T x particles used in this investigation. The particles exhibit an accordion-like structure, which is beneficial for electrolyte access to the material. However, some small pores may not be accessible by the electrolyte. It is in this regard that the investigations of other pseudocapacitive materials did not show correlation between BET surface area and capacitance [44][45][46][47]. The SEM images of Ti 3 C 2 T X -Fe 3 O 4 -CNT composites ( Figure 1C,D) show that Ti 3 C 2 T X particles were covered with Fe 3 O 4 and CNT.

Results and Discussion
ials 2021, 14, x FOR PEER REVIEW 3 of 11 obtained from the CV or GCD data, and complex CS* components (CS' and CS") were calculated from the EIS testing results obtained at a signal of 5 mV, as described in [41]. JSM-7000F microscope (JEOL, Peabody, MA, USA) was used for SEM investigations. Figure 1A,B shows SEM images of Ti3C2Tx particles used in this investigation. The particles exhibit an accordion-like structure, which is beneficial for electrolyte access to the material. However, some small pores may not be accessible by the electrolyte. It is in this regard that the investigations of other pseudocapacitive materials did not show correlation between BET surface area and capacitance [44][45][46][47]. The SEM images of Ti3C2TX-Fe3O4-CNT composites ( Figure 1C,D) show that Ti3C2TX particles were covered with Fe3O4 and CNT. Ti3C2Tx particles were used for the fabrication of composite Ti3C2Tx-Fe3O4-CNT electrodes. Pure Ti3C2Tx-CNT and Fe3O4-CNT electrodes were also fabricated and tested for comparison. The X-ray diffraction patterns of the composite Ti3C2Tx-CNT, Fe3O4-CNT, and Ti3C2Tx-Fe3O4-CNT materials presented in the Supplementary Information ( Figure S1) show diffraction peaks of the individual components. All the electrodes contained 20% CNTs as conductive additives. In this investigation, CNTs were used as conductive additives for capacitive Fe3O4 [48][49][50] and Ti3C2Tx [1][2][3] materials. Previous investigations highlighted the need for the fabrication of electrodes with high AMs and enhanced ratio of the AM to the mass of current collector and other passive components [41]. Commonly used so far are activated carbon (AC) commercial supercapacitors with high AM [41,51] of about 10 mg•cm −2 . Another important parameter is electrode thickness [52]. It has been demonstrated that significant uncertainty in supercapacitor metrics stems from reporting gravimetric capacitance of thick electrodes with low packing density [51]. In such electrodes, empty space is filled by an electrolyte, thereby increasing the weight of the device without adding capacitance. However, such electrodes show enhanced AM normalized capacitance due to enhanced access of the electrolyte to the active materials [51]. Investigations showed Ti 3 C 2 T x particles were used for the fabrication of composite Ti 3 C 2 T x -Fe 3 O 4 -CNT electrodes. Pure Ti 3 C 2 T x -CNT and Fe 3 O 4 -CNT electrodes were also fabricated and tested for comparison. The X-ray diffraction patterns of the composite Ti 3 C 2 T x -CNT, Fe 3 O 4 -CNT, and Ti 3 C 2 T x -Fe 3 O 4 -CNT materials presented in the Supplementary Information ( Figure S1) show diffraction peaks of the individual components. All the electrodes contained 20% CNTs as conductive additives. In this investigation, CNTs were used as conductive additives for capacitive Fe 3 O 4 [48][49][50] and Ti 3 C 2 T x [1-3] materials. Previous investigations highlighted the need for the fabrication of electrodes with high AMs and enhanced ratio of the AM to the mass of current collector and other passive components [41]. Commonly used so far are activated carbon (AC) commercial supercapacitors with high AM [41,51] of about 10 mg·cm −2 . Another important parameter is electrode thickness [52]. It has been demonstrated that significant uncertainty in supercapacitor metrics stems from reporting gravimetric capacitance of thick electrodes with low packing density [51]. In such electrodes, empty space is filled by an electrolyte, thereby increasing the weight of the device without adding capacitance. However, such electrodes show enhanced AM normalized capacitance due to enhanced access of the electrolyte to the active materials [51]. Inves-tigations showed that electrodes must be of comparable thickness for the comparison of their performance [53]. It is important to note that AC has a relatively low density and typical thickness of AC electrodes with active mass of 10 mg·cm −2 is about 0.6 mm [54]. In our investigation, the thickness of all the investigated electrodes was 0.38 mm and AM loading was 35 mg·cm −2 . The higher AM of the fabricated electrodes, compared to that of AC electrodes, resulted from higher density of Ti 3 C 2 T x and Fe 3 O 4 materials used in this investigation. The high AM loading was beneficial for increasing the ratio of AM -to the total mass, which includes not only AM, but also mass of current collectors, electrolyte and other components.The ability to achieve high capacitance using electrodes with high AM and low impedance is critical for the development of advanced electrodes.

Results and Discussion
In this investigation, CB was used as a dispersant for Ti 3 C 2 T x , Fe 3 O 4 and CNTs. CB has generated significant interest as an advanced dispersant for the fabrication of composites for supercapacitors and other applications [55][56][57]. Sedimentation tests showed good colloidal stability of the Ti 3 C 2 T x , Fe 3 O 4 and CNT suspensions, prepared using CB. It is important to note that the chemical structure of CB contains a catechol ligand, which facilitates CB adsorption on inorganic materials by complexation of metal atoms on the material surface [58]. Such interactions of CB with Ti atoms on the Ti 3 C 2 T x surface or Fe atoms on the Fe 3 O 4 surface facilitated CB adsorption. The polyaromatic structure of CB allowed for its adsorption on CNTs and the adsorption mechanism of CB involved π-π interactions with side walls of CNTs [59]. The adsorbed cationic CB allowed for electrostatic dispersion of Ti 3 C 2 T x , Fe 3 O 4 and CNT and facilitated their enhanced mixing. Co-dispersion of Ti 3 C 2 T x with CNTs and Fe 3 O 4 with CNTs allowed for good performance of Ti 3 C 2 T x -CNT and Fe 3 O 4 -CNT electrodes at high AM loadings. Figure 2 shows capacitive performances of Ti 3 C 2 T x -CNT and Fe 3 O 4 -CNT electrodes. Cyclic voltammetry (CV) studies showed nearly rectangular shape CVs for Ti 3 C 2 T x -CNT electrodes and C S = 1.96 F·cm −2 at 2 mV s −1 . The obtained C S was significantly higher than literature data for Ti 3 C 2 T x based electrodes, discussed in the Introduction. The capacitance retention at 100 mV·s −1 was 23.5%. Relatively high capacitances were also achieved using Fe 3 O 4 -CNT electrodes. The highest C S = 4.42 F·cm −2 was attained at 2 mV·s −1 . The use of CB as a co-dispersant allowed for higher capacitance of the Fe 3 O 4 -CNT electrodes compared to the previous results [43] for the Fe 3 O 4 -CNT electrodes, containing functionalized CNTs. The capacitance retention at 100 mV s −1 was 14.9%. The capacitive properties of Fe 3 O 4 -CNT composites resulted from the double layer charging mechanism of Fe 3 O 4 and CNTs and pseudocapacitive mechanism of Fe 3 O 4 , attributed to Fe 2+ /Fe 3+ redox couple [48][49][50]. that electrodes must be of comparable thickness for the comparison of their performance [53]. It is important to note that AC has a relatively low density and typical thickness of AC electrodes with active mass of 10 mg•cm −2 is about 0.6 mm [54]. In our investigation, the thickness of all the investigated electrodes was 0.38 mm and AM loading was 35 mg•cm −2 .
The higher AM of the fabricated electrodes, compared to that of AC electrodes, resulted from higher density of Ti3C2Tx and Fe3O4 materials used in this investigation. The high AM loading was beneficial for increasing the ratio of AM -to the total mass, which includes not only AM, but also mass of current collectors, electrolyte and other components.The ability to achieve high capacitance using electrodes with high AM and low impedance is critical for the development of advanced electrodes. In this investigation, CB was used as a dispersant for Ti3C2Tx, Fe3O4 and CNTs. CB has generated significant interest as an advanced dispersant for the fabrication of composites for supercapacitors and other applications [55][56][57]. Sedimentation tests showed good colloidal stability of the Ti3C2Tx, Fe3O4 and CNT suspensions, prepared using CB. It is important to note that the chemical structure of CB contains a catechol ligand, which facilitates CB adsorption on inorganic materials by complexation of metal atoms on the material surface [58]. Such interactions of CB with Ti atoms on the Ti3C2Tx surface or Fe atoms on the Fe3O4 surface facilitated CB adsorption. The polyaromatic structure of CB allowed for its adsorption on CNTs and the adsorption mechanism of CB involved π-π interactions with side walls of CNTs [59]. The adsorbed cationic CB allowed for electrostatic dispersion of Ti3C2Tx, Fe3O4 and CNT and facilitated their enhanced mixing. Co-dispersion of Ti3C2Tx with CNTs and Fe3O4 with CNTs allowed for good performance of Ti3C2Tx-CNT and Fe3O4-CNT electrodes at high AM loadings. Figure 2 shows capacitive performances of Ti3C2Tx-CNT and Fe3O4-CNT electrodes. Cyclic voltammetry (CV) studies showed nearly rectangular shape CVs for Ti3C2Tx-CNT electrodes and CS = 1.96 F•cm −2 at 2 mV s −1 . The obtained CS was significantly higher than literature data for Ti3C2Tx based electrodes, discussed in the Introduction. The capacitance retention at 100 mV•s −1 was 23.5%. Relatively high capacitances were also achieved using Fe3O4-CNT electrodes. The highest CS = 4.42 F•cm −2 was attained at 2 mV•s −1 . The use of CB as a co-dispersant allowed for higher capacitance of the Fe3O4-CNT electrodes compared to the previous results [43] for the Fe3O4-CNT electrodes, containing functionalized CNTs. The capacitance retention at 100 mV s −1 was 14.9%. The capacitive properties of Fe3O4-CNT composites resulted from the double layer charging mechanism of Fe3O4 and CNTs and pseudocapacitive mechanism of Fe3O4, attributed to Fe 2+ /Fe 3+ redox couple [48][49][50].   [42]. The low electrical resistance is an important factor controlling capacitive performance of electrodes. The differential capacitance CS' derived from the EIS data at 5 mV signal  mance of electrodes. The differential capacitance C S ' derived from the EIS data at 5 mV signal amplitude was inferior to the integral C S calculated for potential span of 0.8 V. The discrepancy can be attributed to different parameters, such as charge-discharge time, electrode potential and limited accessibility of some redox sites at low voltages. The electrodes showed relatively high relaxation frequencies [60,61], corresponding to C S " maxima. amplitude was inferior to the integral CS calculated for potential span of 0.8 V. The discrepancy can be attributed to different parameters, such as charge-discharge time, electrode potential and limited accessibility of some redox sites at low voltages. The electrodes showed relatively high relaxation frequencies [60,61], corresponding to CS" maxima.  Figure 4A,B shows charge-discharge behavior of the Ti3C2TX-CNT and Fe3O4-CNT electrodes. The electrodes showed nearly triangular symmetric GCD profile. The capacitances were calculated from the GCD data and are presented in Figure 4C. CS reduced from 2.05 to 1.40 F•cm −2 and from 3.41 to 2.5 F•cm −2 , for Ti3C2TX-CNT and Fe3O4-CNT electrodes, respectively, in the current range 3-35 mA•cm −2 . The GCD data showed good capacitance retention with increasing current density. This investigation revealed a synergistic effect of Ti3C2TX, CNT and Fe3O4, which allowed for enhanced capacitance of the composite Ti3C2TX-Fe3O4-CNT electrodes, compared to the capacitances of Ti3C2TX-CNT and Fe3O4-CNT electrodes at the same AM, electrode thickness and CNT content. The use of CB as a dispersant was critical to achieve enhanced capacitance. The effect of CB is evident from the comparison of testing results for two composites, prepared at different experimental conditions, as was described in the Materials and Methods section. Ti3C2TX-(Fe3O4-CNT) electrodes were prepared by precipitation of Fe3O4 in the presence of CNTs dispersed with CB, followed by washing drying and mixing with Ti3C2TX. In contrast Ti3C2TX-Fe3O4-CNT electrodes were prepared by precipitation of Fe3O4 in the presence of co-dispersed Ti3C2TX and CNTs.  Figure 4A,B shows charge-discharge behavior of the Ti 3 C 2 T X -CNT and Fe 3 O 4 -CNT electrodes. The electrodes showed nearly triangular symmetric GCD profile. The capacitances were calculated from the GCD data and are presented in Figure 4C. C S reduced from 2.05 to 1.40 F·cm −2 and from 3.41 to 2.5 F·cm −2 , for Ti 3 C 2 T X -CNT and Fe 3 O 4 -CNT electrodes, respectively, in the current range 3-35 mA·cm −2 . The GCD data showed good capacitance retention with increasing current density. amplitude was inferior to the integral CS calculated for potential span of 0.8 V. The discrepancy can be attributed to different parameters, such as charge-discharge time, electrode potential and limited accessibility of some redox sites at low voltages. The electrodes showed relatively high relaxation frequencies [60,61], corresponding to CS" maxima.  Figure 4A,B shows charge-discharge behavior of the Ti3C2TX-CNT and Fe3O4-CNT electrodes. The electrodes showed nearly triangular symmetric GCD profile. The capacitances were calculated from the GCD data and are presented in Figure 4C. CS reduced from 2.05 to 1.40 F•cm −2 and from 3.41 to 2.5 F•cm −2 , for Ti3C2TX-CNT and Fe3O4-CNT electrodes, respectively, in the current range 3-35 mA•cm −2 . The GCD data showed good capacitance retention with increasing current density. This investigation revealed a synergistic effect of Ti3C2TX, CNT and Fe3O4, which allowed for enhanced capacitance of the composite Ti3C2TX-Fe3O4-CNT electrodes, compared to the capacitances of Ti3C2TX-CNT and Fe3O4-CNT electrodes at the same AM, electrode thickness and CNT content. The use of CB as a dispersant was critical to achieve enhanced capacitance. The effect of CB is evident from the comparison of testing results for two composites, prepared at different experimental conditions, as was described in the Materials and Methods section. Ti3C2TX-(Fe3O4-CNT) electrodes were prepared by precipitation of Fe3O4 in the presence of CNTs dispersed with CB, followed by washing drying and mixing with Ti3C2TX. In contrast Ti3C2TX-Fe3O4-CNT electrodes were prepared by precipitation of Fe3O4 in the presence of co-dispersed Ti3C2TX and CNTs. This investigation revealed a synergistic effect of Ti 3 C 2 T X , CNT and Fe 3 O 4 , which allowed for enhanced capacitance of the composite Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes, compared to the capacitances of Ti 3 C 2 T X -CNT and Fe 3 O 4 -CNT electrodes at the same AM, electrode thickness and CNT content. The use of CB as a dispersant was critical to achieve enhanced capacitance. The effect of CB is evident from the comparison of testing results for two composites, prepared at different experimental conditions, as was described in the Materials and Methods section. Ti 3 C 2 T X -(Fe 3 O 4 -CNT) electrodes were prepared by precipitation of Fe 3 O 4 in the presence of CNTs dispersed with CB, followed by washing drying and mixing with Ti 3 C 2 T X . In contrast Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes were prepared by precipitation of Fe 3 O 4 in the presence of co-dispersed Ti 3 C 2 T X and CNTs.
CV testing results showed significantly larger CV areas for Ti 3 C 2 T X -Fe 3 O 4 -CNT, compared to Ti 3 C 2 T X -(Fe 3 O 4 -CNT) electrodes ( Figure 5A,B). This resulted in higher capacitance of the Ti 3 C 2 T X -Fe 3 O 4 -CNT and indicated the influence of CB dispersant used for the preparation of the composites on the properties of the electrodes. The highest capacitances of 5.52 and 3.90 F·cm −2 were obtained for Ti 3 C 2 T X -Fe 3 O 4 -CNT and Ti 3 C 2 T X -(Fe 3 O 4 -CNT) electrodes, respectively, at 2 mV·s −1 . In order to analyze the charge storage properties of the electrodes, a parameter b was calculated from the following equation [62,63].
where i is a current, ν-scan rate and a is a parameter. Parameter b was found to be 0.68 for the Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes ( Supplementary Information, Figure S2). It is known that b = 1 for purely double-layer capacitive mechanism and b = 0.5 for battery-type materials. The electrodes with 0.5 < b < 1 combine capacitive and battery properties. According to [62], the battery-type charge storage mechanism is dominant for electrodes with 0.5 < b < 0.8. Therefore, the Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes show mixed double-layer capacitive and battery-type properties with a dominant battery-type charge storage mechanism. CV testing results showed significantly larger CV areas for Ti3C2TX-Fe3O4-CNT, compared to Ti3C2TX-(Fe3O4-CNT) electrodes ( Figure 5A,B). This resulted in higher capacitance of the Ti3C2TX-Fe3O4-CNT and indicated the influence of CB dispersant used for the preparation of the composites on the properties of the electrodes. The highest capacitances of 5.52 and 3.90 F•cm −2 were obtained for Ti3C2TX-Fe3O4-CNT and Ti3C2TX-(Fe3O4-CNT) electrodes, respectively, at 2 mV•s −1 . In order to analyze the charge storage properties of the electrodes, a parameter b was calculated from the following equation [62,63].
where i is a current, ν-scan rate and a is a parameter. Parameter b was found to be 0.68 for the Ti3C2TX-Fe3O4-CNT electrodes ( Supplementary Information, Figure S2). It is known that b = 1 for purely double-layer capacitive mechanism and b = 0.5 for battery-type materials. The electrodes with 0.5 < b < 1 combine capacitive and battery properties. According to [62], the battery-type charge storage mechanism is dominant for electrodes with 0.5 < b < 0.8. Therefore, the Ti3C2TX-Fe3O4-CNT electrodes show mixed double-layer capacitive and battery-type properties with a dominant battery-type charge storage mechanism. EIS studies ( Figure 6) revealed lower resistance, higher capacitance and higher relaxation frequency of Ti3C2TX-Fe3O4-CNT electrodes, compared to Ti3C2TX-(Fe3O4-CNT) electrodes. GCD data showed nearly triangular symmetric charge-discharge curves, with longer charge and discharge times for Ti3C2TX-Fe3O4-CNT electrodes, compared to Ti3C2TX-(Fe3O4-CNT) at the same current densities ( Figure 7A,B). The longer charge/discharge times indicated higher capacitances. The capacitances were calculated from the GCD data and presented in Figure 7C at different current densities. CS reduced from 4.35 to 3.33 F•cm −2 and from 3.46 to 2.58 F•cm −2 for Ti3C2TX-Fe3O4-CNT and Ti3C2TX-(Fe3O4-CNT) composites, respectively, with current increase from 3 to 35 mA•cm −2 .
The analysis of capacitances, measured using CV, EIS and GCD techniques showed that the capacitances of the Ti3C2TX-Fe3O4-CNT electrodes are higher than the capacitances of the Ti3C2Tx-CNT and Fe3O4-CNT electrodes. Therefore, the experimental results of this work showed a synergistic effect of the individual capacitive materials. The comparison of the data for Ti3C2TX-Fe3O4-CNT and Ti3C2TX-(Fe3O4-CNT) electrodes and literature data of the previous investigations for Ti3C2TX [42] and Fe3O4 electrodes [43] showed the beneficial effect of co-dispersion of the individual components, which was achieved using CB as a dispersant. The ability to achieve high CS of 5.52 F•cm −2 in the negative potential range in Na2SO4 is beneficial for the preparation of asymmetric SC. Ti3C2TX-Fe3O4-CNT electrodes showed relatively high CS, compared to other anode materials [41]. The comparison with CS for other Ti3C2TX-based electrodes in Na2SO4 electrolyte (Supplementary Information, Table S1) showed significant improvement in CS. The capacitance of the negative electrodes is usually lower than that of positive electrodes. Advanced positive electrodes, based on MnO2, Mn3O4, and BiMn2O5 have been developed with capacitance of about 5-8 EIS studies ( Figure 6) revealed lower resistance, higher capacitance and higher relaxation frequency of Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes, compared to Ti 3 C 2 T X -(Fe 3 O 4 -CNT) electrodes. GCD data showed nearly triangular symmetric charge-discharge curves, with longer charge and discharge times for Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes, compared to Ti 3 C 2 T X -(Fe 3 O 4 -CNT) at the same current densities ( Figure 7A,B). The longer charge/discharge times indicated higher capacitances. The capacitances were calculated from the GCD data and presented in Figure 7C at different current densities. C S reduced from 4.35 to 3.33 F·cm −2 and from 3.46 to 2.58 F·cm −2 for Ti 3 C 2 T X -Fe 3 O 4 -CNT and Ti 3 C 2 T X -(Fe 3 O 4 -CNT) composites, respectively, with current increase from 3 to 35 mA·cm −2 .
CNTs is comparable with capacitances of advanced positive electrodes. The Ti3C2TX-Fe3O4-CNT electrodes showed a slight CS increase for the first 400 cycles and remained nearly constant after this initial increase (Figure 8). A similar increase was observed in the literature for other materials and was attributed to microstructure changes during initial cycling [64,65]. In contrast, the capacitance of the Ti3C2TX-CNT and Fe3O4-CNT electrodes decreased after cycling (Figure 8).   F cm in the positive potential range [41]. Therefore, the capacitance of Ti3C2TX-Fe3O4-CNTs is comparable with capacitances of advanced positive electrodes. The Ti3C2TX-Fe3O4-CNT electrodes showed a slight CS increase for the first 400 cycles and remained nearly constant after this initial increase (Figure 8). A similar increase was observed in the literature for other materials and was attributed to microstructure changes during initial cycling [64,65]. In contrast, the capacitance of the Ti3C2TX-CNT and Fe3O4-CNT electrodes decreased after cycling (Figure 8).   The analysis of capacitances, measured using CV, EIS and GCD techniques showed that the capacitances of the Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes are higher than the capacitances of the Ti 3 C 2 T x -CNT and Fe 3 O 4 -CNT electrodes. Therefore, the experimental results of this work showed a synergistic effect of the individual capacitive materials. The comparison of the data for Ti 3 C 2 T X -Fe 3 O 4 -CNT and Ti 3 C 2 T X -(Fe 3 O 4 -CNT) electrodes and literature data of the previous investigations for Ti 3 C 2 T X [42] and Fe 3 O 4 electrodes [43] showed the beneficial effect of co-dispersion of the individual components, which was achieved using CB as a dispersant. The ability to achieve high C S of 5.52 F·cm −2 in the negative potential range in Na 2 SO 4 is beneficial for the preparation of asymmetric SC. Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes showed relatively high C S , compared to other anode materials [41]. The comparison with C S for other Ti 3 C 2 T X -based electrodes in Na 2 SO 4 electrolyte (Supplementary Information, Table S1) showed significant improvement in C S . The capacitance of the negative electrodes is usually lower than that of positive electrodes. Advanced positive electrodes, based on MnO 2 , Mn 3 O 4 , and BiMn 2 O 5 have been developed with capacitance of about 5-8 F cm −2 in the positive potential range [41]. Therefore, the capacitance of Ti 3 C 2 T X -Fe 3 O 4 -CNTs is comparable with capacitances of advanced positive electrodes. The Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes showed a slight C S increase for the first 400 cycles and remained nearly constant after this initial increase (Figure 8). A similar increase was observed in the literature for other materials and was attributed to microstructure changes during initial cycling [64,65]. In contrast, the capacitance of the Ti 3 C 2 T X -CNT and Fe 3 O 4 -CNT electrodes decreased after cycling (Figure 8).

Conclusions
Ti3C2TX-Fe3O4-CNT electrodes have been developed, which showed CS of 5.52 F•cm −2 in the negative potential range in 0.5 M Na2SO4 electrolyte. Such electrodes are promising for applications in asymmetric supercapacitor devices due to the high capacitance, which is comparable with the capacitance of advanced positive electrodes. The use of CB as an advanced co-dispersant allowed for the fabrication of Ti3C2TX-Fe3O4-CNT electrodes, which showed good capacitive performance at high AM loadings. The comparison of capacitive behavior of Ti3C2TX-Fe3O4-CNT electrodes with Ti3C2TX-CNT and Fe3O4-CNT electrodes with the same AM, thickness and CNT content revealed a synergistic effect of the individual capacitive materials.

Supplementary Materials:
The following are available online at www.mdpi.com/xxx/s1. Figure S1: X-ray diffraction patterns of (a) Ti3C2TX-CNT and (b) Fe3O4-CNT and (c) Ti3C2TX-Fe3O4-CNT composites, Figure S2: Current (i) versus scan rate (ν) dependence in a logarithmic scale used for the calculation of parameter b for Ti3C2TX-Fe3O4-CNT electrodes from the equation [1] Table S1: Characteristics of Ti3C2Tx-based electrodes with high active mass in Na2SO4 electrolyte.

Conclusions
Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes have been developed, which showed C S of 5.52 F·cm −2 in the negative potential range in 0.5 M Na 2 SO 4 electrolyte. Such electrodes are promising for applications in asymmetric supercapacitor devices due to the high capacitance, which is comparable with the capacitance of advanced positive electrodes. The use of CB as an advanced co-dispersant allowed for the fabrication of Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes, which showed good capacitive performance at high AM loadings. The comparison of capacitive behavior of Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes with Ti 3 C 2 T X -CNT and Fe 3 O 4 -CNT electrodes with the same AM, thickness and CNT content revealed a synergistic effect of the individual capacitive materials.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/ma14112930/s1. Figure S1: X-ray diffraction patterns of (a) Ti 3 C 2 T X -CNT and (b) Fe 3 O 4 -CNT and (c) Ti 3 C 2 T X -Fe 3 O 4 -CNT composites, Figure S2: Current (i) versus scan rate (ν) dependence in a logarithmic scale used for the calculation of parameter b for Ti 3 C 2 T X -Fe 3 O 4 -CNT electrodes from the equation [1] i = aν b , Table S1: Characteristics of Ti 3 C 2 T x -based electrodes with high active mass in Na 2 SO 4 electrolyte.