materials

Article Composite Fe3O4-MXene- Electrodes for Prepared Using the New Colloidal Method

Wenyu Liang and Igor Zhitomirsky *

Department of and Engineering, McMaster University, Hamilton, ON L8S 4L7, Canada; [email protected] * Correspondence: [email protected]

Abstract: 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 (CS). This paper reports the fabrication of Ti3C2TX-Fe3O4-multiwalled carbon −2 nanotube (CNT) electrodes, which show CS of 5.52 F cm in the negative potential range in 0.5 M −2 Na2SO4 electrolyte. Good capacitive performance is achieved at a mass loading of 35 mg cm 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.   Keywords: iron oxide; MXene; ; electrode; dispersion; composite; carbon nanotube Citation: Liang, W.; Zhitomirsky, I.

Composite Fe3O4-MXene-Carbon Nanotube Electrodes for Supercapacitors Prepared Using the 1. Introduction New Colloidal Method. Materials Ti3C2Tx belongs to the family of MXene-type materials, which are of great technologi- 2021, 14, 2930. https://doi.org/ cal interest for applications in electrodes of SCs [1–3]. The interest in Ti3C2Tx is attributed to 10.3390/ma14112930 the high capacitance and low electrical resistivity of this material. The promising capacitive properties of Ti3C2Tx result from its high surface area and the redox active nature of surface Academic Editor: Suzy Surblé functional groups. Enhanced capacitive properties were obtained for Ti3C2Tx compos- ites, containing different conductive additives, such as [4], acetylene black [5], Received: 30 April 2021 and carbon black [6] and for nitrogen-doped Ti C T [7–9]. Moreover, advanced Ti C T Accepted: 26 May 2021 3 2 x 3 2 x Published: 29 May 2021 composites were developed, containing other components, such as ZnO [10], MnO2 [11], TiO2 [12], and Mn3O4 [13]. Investigations revealed the stable cycling behavior of Ti3C2Tx

Publisher’s Note: MDPI stays neutral composites [14–19]. with regard to jurisdictional claims in High specific capacitance (Cm) normalized by active mass (AM) was reported for −2 published maps and institutional affil- composite electrodes [9,12,14,20–28] with relatively low AMs, typically below 8 mg·cm . −2 iations. The CS of such electrodes was below 1 F·cm . Capacitive properties of Ti3C2Tx compos- ites were tested in various electrolytes, such as HCl [29], H2SO4 [27,30,31], KOH [12,32], KCl [33], K2SO4 [34], Na2SO4 [34], Li2SO4 [34], and other electrolytes [35,36]. Ti3C2Tx- based electrodes were utilized for the fabrication of symmetric SCs, containing two similar Ti C T -based electrodes, with maximum operation voltages in the range of Copyright: © 2021 by the authors. 3 2 x Licensee MDPI, Basel, Switzerland. 0.4–1.2 V [9,31,37,38]. This article is an open access article The progress in applications of SC devices will depend on the ability to fabricate distributed under the terms and efficient electrodes and devices with high CS, which can be achieved at high AM loadings. conditions of the Creative Commons Another important benefit of high AM electrodes is their low ratio of the mass of elec- Attribution (CC BY) license (https:// trochemically inactive components to the AMs. With the goal to increase energy–power creativecommons.org/licenses/by/ characteristics, there is a growing trend in devices that operate in enlarged voltage win- 4.0/). dows. Of particular importance are environmentally friendly neutral electrolytes, such as

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Na2SO4, which facilitate the design of asymmetric aqueous cells with voltage windows above 1.2 V. −2 Ti3C2Tx-based electrodes with AMs of 1–3 mg cm were analyzed in Na2SO4 elec- trolyte [34,39,40] and relatively high Cm were obtained at such low AM loadings. Therefore, the development of electrodes with higher AMs can potentially result in high CS. However, it is challenging [41] to achieve high CS owing to the electrolyte diffusion limitations and high electrical resistance at high AMs. The increase in AM to the level of 20 mg·cm−2 −2 allowed the design of composites [42] with CS of 1.087 F·cm at the galvanostatic charging conditions of 1 mA·cm−2 and 0.783 F·cm−2 at potential sweep conditions of 1 mV·s−1. Such electrodes [42] were utilized for symmetric Ti3C2Tx SC. The objective of this study was to form Fe3O4-Ti3C2Tx-CNT electrodes for SCs. The use of CB as a co-dispersant allowed the fabrication of electrodes, which showed good electrochemical performance at AM of 35 mg·cm−2. CB allowed adsorption on individual materials and their dispersion due its polyaromatic structure, containing a chelating catechol ligand and electric charge. The experimental data of this investigation showed that CS of −2 5.52 F·cm can be achieved in the negative potential range in 0.5 M Na2SO4 electrolyte due to the use of advanced co-dispersant and a synergistic effect of the individual components.

2. Materials and Methods

Celestine blue (CB), FeCl3·6H2O, FeCl2·4H2O, NH4OH, Na2SO4, co-polymer of vinyl butyral, vinyl acetate and vinyl alcohol (PVBAA, 65 kDa) were purchased from Millipore Sigma, Burlington, MA, USA. The diameter and length of CNT (multiwalled, Bayer Corp. Whippany, NJ, USA) were 13 nm and 1–2 µm, respectively. Ti3C2Tx was purchased from Laizhou Kai Kai Ceramic Materials Co., Ltd., Laizhou, China. Fe3O4 was prepared as described in by a chemical precipitation method [43] from solutions of FeCl2 and FeCl3, containing dispersed CNT or co-dispersed CNT and Ti3C2Tx. In contrast to the previous investigation [43], pristine CNT were used. In this approach, CNT and Ti3C2Tx were dispersed or co-dispersed using CB as a surfactant. For the fabrication of Fe3O4-CNT electrodes, the synthesis of Fe3O4 was performed in the presence of CNT, dispersed using CB. For the fabrication of Ti3C2Tx-CNT electrodes, Ti3C2Tx was co-dispersed with CNT in water using CB as a co-dispersant. Active materials (AM) for Ti3C2Tx-Fe3O4-CNT electrodes were prepared by precipitating Fe3O4 in the presence of co-dispersed Ti3C2Tx and CNT. The amount of the CB dispersant in the suspension was 15% of the total mass of Ti3C2Tx, Fe3O4 and CNT. After filtration, obtained AM were washed with water and in order to remove non-adsorbed dispersant and dried in air. In order to analyze the effect of CB, AM for Ti3C2Tx-(Fe3O4-CNT) electrodes were prepared by fabrication of Fe3O4-CNT powder, as described above, and its mixing with Ti3C2Tx. The Ti3C2Tx/Fe3O4 mass ratio was 5:3 in the Ti3C2Tx-(Fe3O4-CNT) and Ti3C2Tx-Fe3O4-CNT electrodes. The mass ratio of CNT to the mass of active materials, such as Fe3O4 in Fe3O4-CNT, Ti3C2Tx in Ti3C2Tx-CNT, Fe3O4 and Ti3C2Tx (total) in Ti3C2Tx-(Fe3O4-CNT) and Ti3C2Tx-Fe3O4-CNT was 1:4. Obtained powders were used for the fabrication of slurries in ethanol for the impreg- nation of commercial Ni foam current collectors (95% porosity, Vale, Rio de Janeiro, Brazil). The slurries contained dissolved PVBAA binder. The mass of the binder was 3% of the total mass of the active material (AM). The total AM of impregnated material after drying was 35 mg cm−2, which included 3% PVBAA binder. All of the impregnated Ni foams were pressed using a calendering machine in order to obtain a final electrode thickness of 0.38 mm. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) studies were performed using a potentiostat (PARSTAT 2273, AMETEK, Berwyn, PA, USA). Gal- vanostatic charge discharge (GCD) was conducted using a Biologic AMP 300 potentiostat. The capacitive behavior of the electrodes was tested in an aqueous 0.5 M Na2SO4 solution. Pt gauze was utilized as a counter electrode, and a saturated calomel electrode (SCE) was 2 used as a reference. The area of the working electrode was 1 cm . CS and Cm, Materials 2021, 14, 2930 3 of 11

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normalized by the electrode area or mass of the active material, respectively, were obtained

obtained fromfrom the theCV CVor GCD or GCD data, data, and andcomplex complex CS* components CS* components (CS’ and (CS ’C andS”) were CS”) cal- were calculated culated fromfrom the EIS the testing EIS testing results results obtained obtained at a signal at a signalof 5 mV, of 5as mV, described as described in [41]. in JSM [41-]. JSM-7000F 7000F microscmicroscopeope (JEOL, (JEOL, Peabody Peabody,, MA, USA MA,) USA)was used was for used SEM for investigations. SEM investigations. 3. Results and Discussion 3. Results and Discussion Figure1A,B shows SEM images of Ti 3C2Tx particles used in this investigation. The Figure 1A,B shows SEM images of Ti3C2Tx particles used in this investigation. The par- particles exhibit an accordion-like structure, which is beneficial for electrolyte access to the ticles exhibit an accordion-like structure, which is beneficial for electrolyte access to the ma- material. However, some small pores may not be accessible by the electrolyte. It is in this terial. However, some small pores may not be accessible by the electrolyte. It is in this regard regard that the investigations of other pseudocapacitive materials did not show correlation that the investigations of other pseudocapacitive materials did not show correlation be- between BET surface area and capacitance [44–47]. The SEM images of Ti3C2TX-Fe3O4-CNT tween BET surface area and capacitance [44–47]. The SEM images of Ti3C2TX-Fe3O4-CNT composites (Figure1C,D) show that Ti 3C2TX particles were covered with Fe3O4 and CNT. composites (Figure 1C,D) show that Ti3C2TX particles were covered with Fe3O4 and CNT.

Figure 1. SEM images at different magnifications of (A,B) as-received Ti3C2Tx and (C,D) Ti3C2TX- Figure 1. SEM images at different magnifications of (A,B) as-received Ti3C2Tx and (C,D) Ti3C2TX-Fe3O4-CNT. Fe3O4-CNT.

Ti3C2Tx particles were used for the fabrication of composite Ti3C2Tx-Fe3O4-CNT elec- 3 2 x 3 2 x 3 4 Ti C T particlestrodes. Pure were Ti used3C2T xfor-CNT the andfabrication Fe3O4-CNT of composite electrodes Ti wereC T - alsoFe O fabricated-CNT elec- and tested for trodes. Pure comparison.Ti3C2Tx-CNT Theand X-rayFe3O4- diffractionCNT electrodes patterns were of also the compositefabricated Tiand3C 2testedTx-CNT, for Fe3O4-CNT, comparison. andThe TiX-3rayC2T diffractionx-Fe3O4-CNT patterns materials of the presented composite in Ti the3C Supplementary2Tx-CNT, Fe3O4- InformationCNT, and (Figure S1) Ti3C2Tx-Fe3O4show-CNT diffractionmaterials peakspresented of the in individualthe Supplementary components. Information All the electrodes (Figure S1) contained 20% show diffractionCNTs peaks as conductive of the individual additives. components. In this investigation, All the electrodes CNTs were contained used as20% conductive ad- CNTs as conductiveditives foradditives. capacitive In this Fe3 Oinvestigation4 [48–50] and, CNT Ti3Cs2 wereTx [1 –used3] materials. as conductive Previous addi- investigations tives for capacitivehighlighted Fe3O4 the[48– need50] and for Ti the3C fabrication2Tx [1–3] materials of electrodes. Previous with investigations high AMs and high- enhanced ratio of lighted the needthe AMfor the to fabrication the mass of of current electrodes collector with andhigh otherAMs passiveand enhanced components ratio of [ 41the]. Commonly AM to the massused of so current far are collector activated and carbon other (AC) passive commercial components supercapacitors [41]. Commonly with high used AM [41,51] of so far are activatedabout 10 carbon mg·cm (AC)−2. Another commercial important supercapacitors parameter iswith electrode high AM thickness [41,51] [ 52of]. It has been about 10 mgdemonstrated∙cm−2. Anotherthat important significant parameter uncertainty is electrode in supercapacitor thickness [ metrics52]. It has stems been from reporting demonstratedgravimetric that significant capacitance uncertainty of thick in supercapacitor electrodes with metrics low packing stems from density reporting [51]. In such elec- gravimetric capacitancetrodes, empty of thick space electrodes is filled bywith an low electrolyte, packing therebydensity [ increasing51]. In such the electrodes, weight of the device empty space withoutis filled by adding an electrolyte, capacitance. thereby However, increasing such the electrodes weight of show the device enhanced without AM normalized adding capacitance.capacitance However, due tosuch enhanced electrodes access show of enhanced the electrolyte AM normalized to the active capacitance materials [51]. Inves- due to enhanced access of the electrolyte to the active materials [51]. Investigations showed

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thattigations electrodes showed must that be electrodesof comparable must thickness be of comparable for the comparison thickness forof their the comparison performance of [53their]. It performanceis important to [53 note]. It that is important AC has a rel toatively note that low AC density has a and relatively typical lowthickness density of AC and −2 electrodestypical thickness with active of AC mass electrodes of 10 mg with∙cm− active2 is about mass 0.6 of mm 10 mg[54·]cm. In ouris aboutinvestigation 0.6 mm, [the54]. thicknessIn our investigation, of all the investigated the thickness electrodes of all the was investigated 0.38 mm and electrodes AM loading was 0.38was mm35 mg and∙cm AM−2. −2 Theloading higher was AM 35 of mg the·cm fabricated. The higher electrodes, AM of compared the fabricated to that electrodes, of AC electrodes, compared toresulted that of fromAC electrodes,higher density resulted of Ti from3C2Tx higher and Fe density3O4 materials of Ti3C used2Tx and in this Fe3O investigation.4 materials used The in high this AMinvestigation. loading was The beneficial high AM for loading increasing was beneficialthe ratio of for AM increasing - to the the total ratio mass, of AM which - to in- the cludestotal mass, not onl whichy AM, includes but also not only mass AM, of current but also collectors, mass of current electrolyte collectors, and otherelectrolyte compo- and nents.Theother components.The ability to achieve ability high to capacitance achieve high using capacitance electrodes using with electrodes high AM with and highlow im- AM pedanceand low is impedance critical for isthe critical development for the development of advanced ofelectrodes advanced. electrodes. In this investigation, CB was used as a dispersant for Ti C T , Fe O and CNTs. In this investigation, CB was used as a dispersant for Ti3C2Tx, 3Fe23Ox4 and3 CNT4 s. CB has generatedCB has generated significant significantinterest as an interest advanced as an dispersant advanced for dispersant the fabrication for theof composites fabrication for of supercapacitorscomposites for supercapacitorsand other applications and other [55– applications57]. Sedimentation [55–57]. tests Sedimentation showed good tests colloidal showed good colloidal stability of the Ti3C2Tx, Fe3O4 and CNT suspensions, prepared using CB. It stability of the Ti3C2Tx, Fe3O4 and CNT suspensions, prepared using CB. It is important to note is important to note that the chemical structure of CB contains a catechol ligand, which that the chemical structure of CB contains a catechol ligand, which facilitates CB adsorption facilitates CB adsorption on inorganic materials by complexation of metal atoms on the on inorganic materials by complexation of metal atoms on the material surface [58]. Such in- material surface [58]. Such interactions of CB with Ti atoms on the Ti3C2Tx surface or teractions of CB with Ti atoms on the Ti3C2Tx surface or Fe atoms on the Fe3O4 surface facili- Fe atoms on the Fe O surface facilitated CB adsorption. The polyaromatic structure of tated CB adsorption.3 The4 polyaromatic structure of CB allowed for its adsorption on CNTs CB allowed for its adsorption on CNTs and the adsorption mechanism of CB involved and the adsorption mechanism of CB involved π-π interactions with side walls of CNTs [59]. π-π interactions with side walls of CNTs [59]. The adsorbed cationic CB allowed for The adsorbed cationic CB allowed for electrostatic dispersion of Ti3C2Tx, Fe3O4 and CNT and electrostatic dispersion of Ti3C2Tx, Fe3O4 and CNT and facilitated their enhanced mixing. facilitated their enhanced mixing. Co-dispersion of Ti3C2Tx with CNTs and Fe3O4 with CNTs Co-dispersion of Ti3C2Tx with CNTs and Fe3O4 with CNTs allowed for good performance allowed for good performance of Ti3C2Tx-CNT and Fe3O4-CNT electrodes at high AM load- of Ti C Tx-CNT and Fe O -CNT electrodes at high AM loadings. ings. 3 2 3 4 Figure2 shows capacitive performances of Ti 3C2Tx-CNT and Fe3O4-CNT electrodes. Figure 2 shows capacitive performances of Ti3C2Tx-CNT and Fe3O4-CNT electrodes. Cyclic voltammetry (CV) studies showed nearly rectangular shape CVs for Ti3C2Tx-CNT Cyclic voltammetry (CV) studies−2 showed nearly−1 rectangular shape CVs for Ti3C2Tx-CNT electrodes and CS = 1.96 F·cm at 2 mV s . The obtained CS was significantly higher electrodes and CS = 1.96 F∙cm−2 at 2 mV s−1. The obtained CS was significantly higher than than literature data for Ti3C2Tx based electrodes, discussed in the Introduction. The capaci- literaturetance retention data for at Ti 1003C2 mVTx based·s−1 was electrodes, 23.5%. Relatively discussed highin the capacitances Introduction. were The also capacitance achieved retention at 100 mV∙s−1 was 23.5%. Relatively high capacitances−2 were also achieved using−1 using Fe3O4-CNT electrodes. The highest CS = 4.42 F·cm was attained at 2 mV·s . Fe3O4-CNT electrodes. The highest CS = 4.42 F∙cm−2 was attained at 2 mV∙s−1. The use of CB The use of CB as a co-dispersant allowed for higher capacitance of the Fe3O4-CNT elec- as a co-dispersant allowed for higher capacitance of the Fe3O4-CNT electrodes compared trodes compared to the previous results [43] for the Fe3O4-CNT electrodes, containing tofunctionalized the previous results CNTs. [ The43] for capacitance the Fe3O4- retentionCNT electrodes, at 100 mV containing s−1 was functionalized 14.9%. The capacitive CNTs. The capacitance retention at 100 mV s−1 was 14.9%. The capacitive properties of Fe3O4- properties of Fe3O4-CNT composites resulted from the double layer charging mechanism 3 4 2+ 3+ CNTof Fe composites3O4 and CNTs resulted and pseudocapacitivefrom the double layer mechanism charging of mechanism Fe3O4, attributed of Fe O to and Fe CNT/Fes andredox pseudocapacitive couple [48–50]. mechanism of Fe3O4, attributed to Fe2+/Fe3+ redox couple [48–50].

−11 FigureFigure 2. 2. (A(A,B,B) )Cyclic Cyclic voltammetry voltammetry data data at at (a) (a) 2, 2, (b) 5 and (c) 10 mV s , ,((CC) )capacitances capacitances for for ( (((AA,C,C)) (a) (a))) Ti Ti33CC22TTXX−CNT-CNT and and 3 4 ((((BB,C,C)) (b) (b))) Fe Fe3OO−4CNT-CNT electrodes. electrodes.

FigureFigure 3 shows EIS data forfor thethe TiTi33C2TTxx-CNT-CNT and and Fe Fe33OO4-4CNT-CNT electrodes. electrodes. The The Nyquist Nyquist plotplot of of complex complex impedance impedance revealed revealed lower lower resistance resistance,, R = RZ’ =, compared Z’, compared to the to literature the literature data [42].data The [42 ].low The electrical low electrical resistance resistance is an important is an important factor controlling factor controlling capacitive capacitive performance perfor- of electrodes. The differential capacitance CS’ derived from the EIS data at 5 mV signal

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amplitudemance of electrodes.was inferior The to the differential integral C capacitanceS calculated for CS ’potential derived fromspan of the 0.8 EIS V. dataThe discrep- at 5 mV amplitude was inferior to the integral CS calculated for potential span of 0.8 V. The discrep- signal amplitude was inferior to the integral CS calculated for potential span of 0.8 V. The ancyancy can can be be attributed attributed to to different different parameters parameters, such, such as as charge charge–discharge–discharge time, time, electrode electrode po- po- tentialdiscrepancy and limited can be accessibility attributedto ofdifferent some redox parameters, sites at low such voltages. as charge–discharge The electrodes time, showed elec- trodetential potential and limited and limitedaccessibility accessibility of some of redox some sites redox at low sites voltages. at low voltages. The electrodes The electrodes showed relatively high relaxation frequencies [60,61], corresponding to CS” maxima. relatively high relaxation frequencies [60,61], corresponding to CS” maxima. showed relatively high relaxation frequencies [60,61], corresponding to CS” maxima.

Figure 3. (A) Nyquist Z” vs. Z’ graph for EIS data, (B) CS’ and (C) CS”, derived from the EIS data for (a) Ti3C2TX−CNT and FigureFigure 3. 3.( A(A)) Nyquist Nyquist Z” Z” vs. vs. Z’ Z’ graph graph for for EIS EIS data, data (,B (B)C) CS’S’ and and ( C(C)C) CS”,S”, derived derived from from the the EIS EIS data data for for (a) (a Ti) Ti3C3C2T2TXX-CNT−CNT and and (b) Fe3O4−CNT electrodes. 3 4 (b)(b) Fe Fe3OO4-CNT−CNT electrodes. electrodes.

Figure 4A,B shows charge-discharge behavior of the Ti3C2TX-CNT and Fe3O4-CNT FigureFigure4 A,B4A,B shows shows charge-discharge charge-discharge behavior behavior of of the the Ti 3TiC32CT2XT-CNTX-CNT and and Fe Fe3O3O4-CNT4-CNT electrodes.electrodes.electrodes. The TheThe electrodes electrodeselectrodes showed showedshowed nearly nearlynearly triangular triangulartriangular symmetric symmetric GCD GCD profile. profile.profile. The The capaci- capaci- capac- tances were calculated from the GCD data and are presented in Figure 4C. CS reduced itancestances were calculatedcalculated fromfrom thethe GCDGCD datadata andand areare presentedpresented inin FigureFigure4 C.4C. C CS Sreduced reduced from 2.05 to 1.40 F∙cm−2− and2 from 3.41 to 2.5 F∙cm−2, for−2 Ti3C2TX-CNT and Fe3O4-CNT elec- fromfrom 2.052.05 toto 1.401.40 FF·∙cmcm−2 andand from from 3.41 3.41 to to 2.5 2.5 F F∙cm·cm−2, for, for Ti3 TiC23TCX2-TCNTX-CNT and and Fe3O Fe4-3CNTO4-CNT elec- −2 − trodes,electrodes,trodes, respectively, respectively, respectively, in in the inthe thecurrent current current range range range 3 –335– 3–3535 mA mA mA∙cm∙cm·cm−. 2The. The2. TheGCD GCD GCD data data data showed showed showed good good good ca- ca- pacitancecapacitancepacitance retention retention retention with with with increasing increasing increasing current current current density density density.. .

FigureFigure 4. 4. GalvanostaticGalvanostatic charge charge–discharge–discharge curves curves of of(A ()TiA)Ti3C32CTX2−TCNTX-CNT,, (B) ( BFe)3 FeO43−OCNT4-CNT at (a) at (a)3, (b) 3, (b)5 (c) 5 7, (c) (d) 7, (d)10 (e) 10 20 (e) and 20 and (f) 35 (f) Figure 4. Galvanostatic−2 charge–discharge curves of (A)Ti3C2TX−CNT, (B) Fe3O4−CNT at (a) 3, (b) 5 (c) 7, (d) 10 (e) 20 and (f) 35 mA35 mA∙cm·−cm2, (C) capacitances,(C) capacitances derived derived from GCD from tests GCD for tests (a) Ti for3C (a)2TX- TiCNT3C2 TandX-CNT (b) Fe and3O4- (b)CNT Fe 3electrodes.O4-CNT electrodes. mA∙cm−2, (C) capacitances derived from GCD tests for (a) Ti3C2TX-CNT and (b) Fe3O4-CNT electrodes.

ThisThis investigation investigation revealed revealed a asynergistic synergistic effect effect of of Ti Ti3C32CT2XT, XCNT, CNT and and Fe3 FeO43,O which4, which al- This investigation revealed a synergistic effect of Ti3C2TX, CNT and Fe3O4, which al- lowedallowed for for enhanced enhanced capacitance capacitance of of the the composite composite Ti33C2TTXX-Fe-Fe3O3O4-4CNT-CNT electrodes, electrodes, com- com- paredlowed to for the enhanced capacitances capacitance of Ti C ofT -CNT the composite and Fe O Ti-CNT3C2TX-Fe electrodes3O4-CNT at electrodes, the same AM, com- pared to the capacitances of Ti3C32TX2-CNTX and Fe3O4-3CNT4 electrodes at the same AM, elec- electrodepared to the thickness capacitances and CNT of Ti content.3C2TX-CNT The useand ofFe CB3O4- asCNT a dispersant electrodes was at the critical same to AM, achieve elec- trodetrode thickness thickness and and CNT CNT content. content. The The use use of of CB CB as as a adispersant dispersant was was critical critical to to achieve achieve enhancedenhanced capacitance. capacitance. The The effect effect of of CB CB is is evident evident from from the the comparison comparison of of testing testing results results forenhanced two composites, capacitance. prepared The effect at different of CB is experimentalevident from the conditions, comparison as was of testing described results in forfor two two composites, composites, prepared prepared at at different different ex experimentalperimental conditions, conditions, as as was was described described in in the the the Materials and Methods section. Ti3C2TX-(Fe3O4-CNT) electrodes were prepared by Materials and Methods section. Ti3C2TX-(Fe3O4-CNT) electrodes were prepared by precip- Materials and Methods section. Ti3C2TX-(Fe3O4-CNT) electrodes were prepared by precip- precipitation of Fe3O4 in the presence of CNTs dispersed with CB, followed by washing itation of Fe3O4 in the presence of CNTs dispersed with CB, followed by washing drying itation of Fe3O4 in the presence of CNTs dispersed with CB, followed by washing drying drying and mixing with Ti3C2TX. In contrast Ti3C2TX-Fe3O4-CNT electrodes were prepared and mixing with Ti3C2TX. In contrast Ti3C2TX-Fe3O4-CNT electrodes were prepared by pre- 3 2 X 3 2 X 3 4 byand precipitation mixing with of Ti FeC3OT4.in In thecontrast presence Ti C ofT co-dispersed-Fe O -CNT electrodes Ti3C2TX and were CNTs. prepared by pre- cipitation of Fe3O4 in the presence of co-dispersed Ti3C2TX and CNTs. cipitation of Fe3O4 in the presence of co-dispersed Ti3C2TX and CNTs.

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CVCV testing testing results results showed showed significantly significantly larger CV areas for TiTi33C22TX--FeFe33OO4-4CNT,-CNT, com- com- paredpared to to Ti Ti3CC22TTXX-(Fe-(Fe3O3O4-4CNT)-CNT) electrodes electrodes (Figure (Figure 55A,B).A,B). This This resulted resulted in higher capacitance ofof the the Ti Ti3CC22TTXX-Fe-Fe3O3O4-4CNT-CNT and and indicated indicated the the influence influence of of CB CB dispersant dispersant used for the prepa-prep- arationration ofof thethe compositescomposites onon thethe propertiesproperties ofof thethe electrodes.electrodes. The highest capacitances of of · −2 5.525.52 and and 3.90 3.90 F F∙cmcm−2 werewere obtained obtained for for Ti3 TiC32TCX2-TFeX3-FeO4-3CNTO4-CNT and andTi3C Ti2T3XC-(Fe2TX3O-(Fe4-CNT)3O4-CNT) elec- −1 trodes,electrodes, respectively, respectively, at 2 atmV 2∙ mVs−1. ·Ins order. In order to analyze to analyze the charge the charge storage storage properties properties of the of electrodes,the electrodes, a parameter a parameter b was b wascalculated calculated from from the following the following equation equation [62,63 [62]., 63]. i =i = a νaνb b (1)(1)

wherewhere ii isis a a current, current, νν——scanscan rate rate and and a a is is a a parameter. parameter. Parameter Parameter b b was was found found to to be be 0.68 0.68 for for the Ti3C2TX-Fe3O4-CNT electrodes (Supplementary Information, Figure S2). It is known that b the Ti3C2TX-Fe3O4-CNT electrodes (Supplementary Information, Figure S2). It is known that =b 1 = for 1 for purely purely double double-layer-layer capacitive capacitive mechanism mechanism and and b b = = 0.5 0.5 for for battery battery-type-type materials. materials. The The electrodeselectrodes with with 0.5 0.5 < < b b < < 1 1 combine combine capacitive capacitive and and battery battery properties. properties. According According to to [62] [62],, t thehe batterybattery-type-type charge charge storage storage mechanism mechanism is isdominant dominant for for electrodes electrodes with with 0.5 0.5< b < 0. b8. < Therefore, 0.8. There- the Ti3C2TX-Fe3O4-CNT electrodes show mixed double-layer capacitive and battery-type prop- fore, the Ti3C2TX-Fe3O4-CNT electrodes show mixed double-layer capacitive and battery-type ertiesproperties with a with dominant a dominant battery battery-type-type charge charge storage storage mechanism. mechanism.

−1 −1 FigureFigure 5. 5. ((AA,B,B) )Cyclic Cyclic voltammetry voltammetry data data at at(a) (a) 2, 2,(b) (b) 5 and 5 and (c) (c) 10 10mV mV∙s ·,s (C),( capacitancesC) capacitances for (( forA,C (()A (a)),C) Ti (a))3C2 TiTX3−C(Fe2T3XO-(Fe4-CNT)3O4- and ((B,C) (b)) Ti3C2TX-Fe3O4−CNT electrodes. CNT) and ((B,C) (b)) Ti3C2TX-Fe3O4-CNT electrodes.

EISEIS studies studies ( (FigureFigure 6) revealed lower resistance, higher capacitance and higher higher relax- relax- ationation frequency frequency of of Ti Ti3C3C2T2XT-FeX-Fe3O34-OCNT4-CNT electrodes, electrodes, compared compared to Ti to3C Ti2T3XC-(Fe2TX3O-(Fe4-CNT)3O4-CNT) elec- trodes.electrodes. GCD GCD data showeddata showed nearly nearly triangular triangular symmetric symmetric charge charge–discharge–discharge curves, curves, with longerwith longer charge charge and and discharge discharge times times for for Ti Ti3C3C2T2XT-XFe-Fe3O34O-CNT4-CNT electrodes, electrodes, compared compared to to TiTi33CC22TTX-X(Fe-(Fe3O3O4-CNT)4-CNT) at the at same the samecurrent current densities densities (Figure 7 (FigureA,B). The7A,B). longer The charge/dis- longer chargecharge/discharge times indicated times higher indicated capacitances. higher capacitances. The capacitances The capacitances were calculated were calculatedfrom the GCDfrom data the GCD and presented data and presentedin Figure 7 inC Figureat different7C at differentcurrent densities. current densities.CS reduced C Sfromreduced 4.35 −2 −2 tofrom 3.33 4.35 F∙cm to−2 3.33 and F from·cm 3.46and to from 2.58 3.46F∙cm to−2 2.58for Ti F·3cmC2TX-Fefor3O Ti43-CNTC2TX and-Fe3 OTi43-CNTC2TX-(Fe and3O Ti4-3CNT)C2TX- −2 composites(Fe3O4-CNT), respectively, composites, with respectively, current increase with current from increase3 to 35 mA from∙cm 3−2 to. 35 mA·cm . 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 bene- ficial 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 elec- trodes showed relatively high CS, compared to other anode materials [41]. The comparison with CS for other Ti3C2TX-based electrodes in Na2SO4 electrolyte (Supplementary Infor- mation, 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

MaterialsMaterials 2021 2021, 14,, 14x FOR, x FOR PEER PEER REVIEW REVIEW 7 of7 of11 11

−2 F cmF cm−2 in inthe the positive positive potential potential range range [41]. [41]. Therefore, Therefore, the the capacitance capacitance of ofTi 3TiC23TCX2-TFeX-Fe3O34O- 4- CNTCNTs iss is comparable comparable with with capacitances capacitances of of advanced advanced positive positive electrodes. electrodes. The The Ti 3 TiC23TCX2-TX- FeFe3O34O-CNT4-CNT electrodes electrodes showed showed a slighta slight C S CincreaseS increase for for the the first first 400 400 cycles cycles and and remained remained nearlynearly constant constant after after this this initial initial increase increase (Figure (Figure 8). 8 A). similarA similar increase increase was was observed observed in inthe the literatureliterature for for other other materials materials and and was was attributed attributed to tomicrostructure microstructure changes changes during during initial initial Materials 2021, 14, 2930 7 of 11 cyclingcycling [64,65 [64,65]. In]. Incontrast, contrast, the the capacitance capacitance of ofthe the Ti 3TiC23TCX2-T CNTX- CNT and and Fe Fe3O34O-CNT4-CNT electrodes electrodes decreaseddecreased after after cycling cycling (Figure (Figure 8) .8 ).

S S 3 2 X FigureFigureFigure 6. 6. (6.A ()(A Nyquist) Nyquist Z” Z Z”vs” .vs vs.Z’. Z’graph Z’ graph graph for for EIS for EIS data, EIS data, ( data,(B (,(CB),) ((C, B)()B,,C )( B)),C)S ’C ( Band’)C and (SC’ )( and CC)S ”C, ( Cderived”,)C derivedS”, from derived from the the fromEIS EIS data the data for EIS for (a) data (a) Ti 3TiC for2TCX (a)−T − (Fe3O4−CNT) and (b) Ti3C2TX−Fe3O4−CNT electrodes. (FeTi3OC42−TCNT)X-(Fe 3andO4-CNT) (b) Ti3 andC2TX (b)−Fe Ti3O3C4−2CNTTX− Feelectrodes.3O4-CNT electrodes.

−2 Figure 7. GCD curves for (A) Ti3C2TX−Fe3O4−CNT), (B) Ti3C2TX−Fe3O4−CNT at (a) 3, (b) 5 (c) 7, (d) 10 (e) 20 and (f) 35 mA∙cm−2 , FigureFigure 7. 7.GCDGCD curves curves for ( forA) Ti (A3C)2 TiTX3−CFe2T3OX-Fe4−CNT),3O4-CNT), (B) Ti3C (B2T)X Ti−Fe3C3O2T4−XCNT-Fe3O at4 -CNT(a) 3, (b) at 5 (a) (c) 3,7, (b)(d) 10 5 (c)(e) 7,20 (d)and 10 (f) (e)35 mA 20 and∙cm (f), (C) capacitances−2 versus current density, calculated from GCD data for (a) Ti3C2TX−(Fe3O4−CNT) and (b) Ti3C2TX−Fe3O4−CNT. (C35) capacitances mA·cm ,( versusC) capacitances current density, versus calculated current density, from GCD calculated data for from(a) Ti GCD3C2TX− data(Fe3O for4−CNT) (a) Ti and3C2 T(b)X-(Fe Ti3C3O2T4X-CNT)−Fe3O4− andCNT. (b) Ti3C2TX-Fe3O4-CNT.

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 benefi- cial effect of co-dispersion of the individual components, which was achieved using CB as a −2 dispersant. The ability to achieve high CS of 5.52 F·cm 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 −2 MnO2, Mn3O4, and BiMn2O5 have been developed with capacitance of about 5–8 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 (Figure8). 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 (Figure8). Materials 2021, 14, 2930 8 of 11 Materials 2021, 14, x FOR PEER REVIEW 8 of 11

FigureFigure 8. 8. CapacitanceCapacitance retention retention for for(a) Ti (a)3C Ti2T3XC−CNT,2TX-CNT, (b) Fe (b)3O4− FeCNT3O4 -CNTand (c) and Ti3C (c)2TX− TiFe3C3O2T4−XCNT-Fe3O 4- electrodes.CNT electrodes.

4.4. Conclusions Conclusions −2 Ti3C2TX-Fe3O4-CNT electrodes have been developed, which showed CS of 5.52 F·cm −2 Ti3C2TX-Fe3O4-CNT electrodes have been developed, which showed CS of 5.52 F∙cm in the negative potential range in 0.5 M Na2SO4 electrolyte. Such electrodes are promising 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 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 is comparable with the capacitance of advanced positive electrodes. The use of CB as an an advanced co-dispersant allowed for the fabrication of Ti3C2TX-Fe3O4-CNT electrodes, advanced co-dispersant allowed for the fabrication of Ti3C2TX-Fe3O4-CNT electrodes, which showed good capacitive performance at high AM loadings. The comparison of which showed good capacitive performance at high AM loadings. The comparison of ca- capacitive behavior of Ti3C2TX-Fe3O4-CNT electrodes with Ti3C2TX-CNT and Fe3O4-CNT pacitive behavior of Ti3C2TX-Fe3O4-CNT electrodes with Ti3C2TX-CNT and Fe3O4-CNT elec- electrodes with the same AM, thickness and CNT content revealed a synergistic effect of trodes with the same AM, thickness and CNT content revealed a synergistic effect of the the individual capacitive materials. individual capacitive materials. Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1. Figure S1: .3390/ma14112930/s1. Figure S1: X-ray diffraction patterns of (a) Ti3C2TX-CNT and (b) Fe3O4-CNT X-ray diffraction patterns of (a) Ti3C2TX-CNT and (b) Fe3O4-CNT and (c) Ti3C2TX-Fe3O4-CNT compo- and (c) Ti C T -Fe O -CNT composites, Figure S2: Current (i) versus scan rate (ν) dependence in a sites, Figure3 2S2:X Current3 4 (i) versus scan rate (ν) dependence in a logarithmic scale used for the cal- logarithmic scale used for the calculation of parameter b for Ti3C2TX-Fe3O4-CNT electrodesb from the culation of parameterb b for Ti3C2TX-Fe3O4-CNT electrodes from the equation [1] i = aν , Table S1: equation [1] i = aν , Table S1: Characteristics of Ti3C2Tx-based electrodes with high active mass in Characteristics of Ti3C2Tx-based electrodes with high active mass in Na2SO4 electrolyte. Na2SO4 electrolyte. Author Contributions: Conceptualization, W.L. and I.Z.; methodology, W.L.; validation, W.L.; for- malAuthor analysis, Contributions: W.L.; investigation,Conceptualization, W.L.; resources, W.L. and I.Z.; I.Z.; data methodology, curation, W.L.; W.L.; writing validation,—original W.L.; formal draft preparation,analysis, W.L.; W.L. investigation, and I.Z.; writing W.L.;— resources,review and I.Z.; editing, data W.L. curation, and W.L.;I.Z.; supervision, writing—original I.Z.; project draft administration,preparation, W.L. I.Z.; and funding I.Z.; writing—review acquisition, I.Z. andAll authors editing, have W.L. read and I.Z.;and supervision,agreed to the I.Z.; published project versionadministration, of the manuscript I.Z.; funding. acquisition, I.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Natural Sciences and Engineering Research Council of Funding: Canada, grantThis number research RGPIN was funded-2018-04014. by the Natural Sciences and Engineering Research Council of Canada, grant number RGPIN-2018-04014. Institutional Review Board Statement: Not Applicable. Institutional Review Board Statement: Not Applicable. Informed Consent Statement: Not Applicable. Informed Consent Statement: Not Applicable. Data Availability Statement: The data presented in this study are available in: “Composite Fe3O4- MXeneData Availability-carbon nanotube Statement: electrodesThe data for supercapacitors presented in this prepared study are by available new colloidal in: “Composite method”. Fe3O4- MXene-carbon nanotube electrodes for supercapacitors prepared by new colloidal method”. Acknowledgments: SEM investigations were performed at the Canadian Centre for Electron Mi- croscopy.Acknowledgments: SEM investigations were performed at the Canadian Centre for Electron Microscopy. ConflictsConflicts of Interest: The a authorsuthors declare no conflict conflicts of of interest. interest. References References 1. Anasori, B.; Lukatskaya, M.R.; Gogotsi, Y. 2D metal carbides and nitrides (MXenes) for . Nat. Rev. Mater. 2017, 2, 1. Anasori, B.; Lukatskaya, M.R.; Gogotsi, Y. 2D metal carbides and nitrides (MXenes) for energy storage. Nat. Rev. Mater. 2017, 2, 16098. [CrossRef] 16098, doi:10.1038/natrevmats.2016.98. 2. Xu, S.; Wei, G.; Li, J.; Han, W.; Gogotsi, Y. Flexible MXene–graphene electrodes with high volumetric capacitance for integrated 2. Xu, S.; Wei, G.; Li, J.; Han, W.; Gogotsi, Y. Flexible MXene–graphene electrodes with high volumetric capacitance for integrated co-cathode energy conversion/storage devices. J. Mater. Chem. A 2017, 5, 17442–17451. 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