Multifunctional MXene–Fe₃O₄–Carbon Nanotube Composite Electrodes for High Active Mass Asymmetric Supercapacitors

Abstract

Ti₃C₂T_x–Fe₃O₄–carbon nanotube composites were prepared for electrochemical energy storage in the negative electrodes of supercapacitors. The electrodes show a remarkably high areal capacitance of 6.59 F cm⁻² in a neutral Na₂SO₄ electrolyte, which was obtained by the development of advanced nanofabrication strategies and due to the synergistic effect of the individual components. Enhanced capacitance was achieved using the in-situ synthesis method for the Fe₃O₄ nanoparticles. The superparamagnetic behavior of the Fe₃O₄ nanoparticles facilitated the fabrication of electrodes with a reduced binder content. Good mixing of the components was achieved using a celestine blue co-dispersant, which adsorbed on the inorganic components and carbon nanotubes and facilitated their co-dispersion and mixing. The capacitive behavior was optimized by the variation of the electrode composition and mass loading in a range of 30–45 mg cm⁻². An asymmetric device was proposed and fabricated, which contained a Ti₃C₂T_x–Fe₃O₄–carbon nanotube negative electrode and a polypyrrole–carbon nanotube positive electrode for operation in an Na₂SO₄ electrolyte. The asymmetric supercapacitor device demonstrated high areal capacitance and excellent power-density characteristics in an enlarged voltage window of 1.6 V. This investigation opens a new avenue for the synthesis and design of MXene-based asymmetric supercapacitors for future energy storage devices.

1. Introduction

Advanced energy storage technology is of significance today, due to the energy crises challenge worldwide [1,2,3,4,5]. MXenes have been extensively investigated as new energy storage materials [6,7,8] since their discovery in 2011 [9]. Due to their large surface area, good conductivity and rich surface chemistry, MXenes have demonstrated great potential for the development of high performance pseudocapacitors [10,11,12,13]. As the most widely studied material among the various types of MXenes, Ti₃C₂T_x with the accordion-like particle shape contains alternation of Ti and C layers with a number of functional terminations, such as -O, -F and -OH [9].

Many efforts have been made to enhance the electrochemical performance of Ti₃C₂T_x in composites with polymers [14,15,16,17], oxides [18,19,20], hydroxides [21,22] and carbon-based materials [23,24,25]. These studies showed that combining Ti₃C₂T_x with other capacitive materials is a promising strategy for the development of advanced supercapacitor electrodes.

Fe-based materials, such as FeOOH, Fe₂O₃ and Fe₃O₄, exhibit high capacitance in a relatively large negative potential range [26,27,28,29,30]. Therefore, they have great potential to be considered as candidates for composites with MXene with enhanced electrochemical performance. Ma et al. [31] employed Fe₂O₃ nanoparticles intercalating flexible MXene hybrid paper and achieved an ultrahigh volumetric capacitance of 2607 F cm⁻³ as well as excellent cycling performance. In another research, a freestanding Ti₃C₂/FeOOH hybrid film was designed in which amorphous FeOOH quantum dots acted as pillars to prevent restacking of MXene sheets [32]. A flexible electrode was fabricated by few-layered Ti₃C₂T_x MXenes coupling with Fe₂O₃ nanorod arrays and grown on carbon cloth, which presented a remarkable areal capacitance of 725 mF cm⁻² at a current density of 1 mA cm⁻² [33]. It has been proven that MXene can provide iron oxides/hydroxides with efficient ion/electron transport pathways, so the electronic conductivity of the composites can be enhanced [34,35,36].

Although the tremendous efforts have been made to develop the advanced MXene electrodes with excellent electrochemical performance, very limited research paid attention to the significance of achieving high active mass loading (AML) of electrodes for supercapacitors [37,38,39,40,41,42]. For practical applications, the AML is required to be at least 10 to 20 mg cm⁻² [43,44,45]. High AML is of great importance to reduce the relative mass of current collectors and other in-active components in the total mass of electrodes and devices. Designing of supercapacitors with high active mass loading will be beneficial to meet the demand of manufacturing. However, with increasing AML, the diffusion of electrolyte in the electrode and the conductivity will be drastically decreased. As the result, low capacitance and poor rate performance can be expected.

In this research, we fabricated MXene-Fe₃O₄-CNT anodes with high AML in the range of 30-45 mg cm⁻², optimized their composition and performance and application for the fabrication of asymmetric supercapacitor devices. Fe₃O₄ was selected because of its high theoretical capacitance(2299 F g⁻¹), earth abundance and low price [46]. Moreover, it exhibits advanced ferrimagnetic properties, which can be controlled by its particle size and synthetic strategy [47,48]. So Fe₃O₄ can impart magnetic properties to the composite materials. This offers benefits of reduced binder content in the composite electrodes and opens the gate for the fabrication of electrodes with advanced functionality. The successful synthesis of composite materials is based on the enhanced co-dispersion and improved mixing of different components, which can be attributed to the application of efficient dispersing agent celestine blue and advanced in-situ synthesis method. We analyzed the influence of electrode composition on electrochemical performance. The fabricated negative electrode showed high areal capacitance of 6.59 F cm⁻² at a sweep rate of 2 mV s⁻¹ at 40 mg cm⁻² AML. Combining negative MXene-Fe₃O₄-carbon nanotube with positive polypyrrole (PPy)-MCNT electrode, we fabricated advanced asymmetric supercapacitor device working in enlarged voltage window of 1.6 V and showing high power-density characteristics.

2. Materials and Methods

2.1. Starting Materials

FeCl₂·4H₂O, FeCl₃·6H₂O, NH₄OH, celestine blue (CB), pyrrole (Py), pyrocatechol violet (PCV), ammonium persulfate (APS), sodium sulphate (Na₂SO₄) and poly (vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (PVBAA, 65 kDa) were ordered from Millipore Sigma company. Multiwalled carbon nanotubes (MCNT, diameter = 13 nm, length = 1–2 μm) were purchased from Bayer Corporation. Ti₃C₂T_x (BET surface area 5.03 m² g⁻¹) was supplied by Laizhou Kai Kai Ceramic Materials Co., Ltd., Weifang, China. Commercial Ni foam (surface area 2.01 m² g⁻¹, Vale) with 95% porosity was used as a current collector.

2.2. Synthesis of Composite Materials

In method 1, the synthesis procedure for Fe₃O₄ was similar to that described in a previous investigation [49]. Fe₃O₄ particles were precipitated from a solution containing Fe²⁺ and Fe³⁺ in a molar ratio of 1:2 at a temperature of 50 °C by adding 2 M NH₄OH for pH adjustment to pH = 9. After washing, vacuum filtration and drying in a furnace overnight at 60 °C, the powders were mixed with Ti₃C₂T_x and MCNT in a mass ratio of 6:3:1 of Ti₃C₂T_x:Fe₃O₄:MCNT, and then dispersed in water using CB dispersant under probe sonication for 5 min.

Method 2 involved the in-situ synthesis of composites by the precipitation of Fe₃O₄ in the presence of co-dispersed Ti₃C₂T_x and MCNT. The suspensions of Ti₃C₂T_x and MCNT were co-dispersed by CB under probe sonication, then combined with the solution containing Fe²⁺ and Fe³⁺ in a ratio of 1:2. The pH adjustment was achieved by injection of 2 M NH₄OH dropwise to 9, keeping the temperature at 50 °C. This was followed by washing, vacuum filtrating and drying processes; the powders were then ready for electrode preparation. The mass ratios between Ti₃C₂T_x, Fe₃O₄ and MCNT were 60:30:10, 55:30:15 and 55:25:20, and the obtained materials were named as 60Ti₃C₂T_x–30Fe₃O₄–10MCNT, 55Ti₃C₂T_x–30Fe₃O₄–15MCNT and 55Ti₃C₂T_x–25Fe₃O₄–20MCNT, respectively.

The synthesis of polypyrrole(PPy)–MCNT composites followed a previous work [50]. In a typical procedure, 24 µL of pyrrole monomer was injected into the well-dispersed MCNT suspension for magnetic stirring for 5 h under 4 °C. The polymerization was achieved by addition of APS as the oxidant by a mass ratio APS:Py = 1:1. PCV was applied as a dopant for the polymerization of pyrrole and as a dispersant for MCNT, and the molar ratio was PCV:Py = 1:10. The precipitate was washed, vacuum filtrated and dried at room temperature.

2.3. Fabrication of Electrodes

The obtained materials were grinded into fine powders and used for preparation of slurries in ethanol containing 3% PVBAA binder, then impregnated into a commercial nickel foam current collector. The mass of active materials of electrodes was 30–45 mg cm⁻².

2.4. Characterization

The Quantum Design Magnetic Properties Measurement System was used to perform the magnetic measurements. The morphology study was performed by scanning electron spectroscopy (SEM) by JEOL JSM-7000F and transmission electron microscopy (TEM) by Talos 200X. EIS and CV were conducted by AMETEK 2273 potentiostat (Princeton). GCD was tested by Biologic AMP 300 potentiostat. For electrochemical testing, a 0.5 M Na₂SO₄ solution was applied as an electrolyte, and a Pt gauze and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. For CV and GCD testing, the negative electrodes were tested in the potential range from −1.1 to −0.3 V. The potential range from −0.4 to +0.5 V was applied for positive electrode testing. The device was tested in a voltage window of 1.6 V. The mass- and areal-normalized capacitances (C_m and C_s, respectively) were calculated from the CV and GCD data according to the Equations (1)–(4):

$$\mathrm{For} \mathrm{CV},\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathrm{Cm}=\frac{\int IdV}{2m\mathsf{\nu }\mathsf{\Delta }V}$$

(1)

$$\mathrm{Cs}=\frac{\int IdV}{2A\mathsf{\nu }\mathsf{\Delta }V}$$

(2)

$$\mathrm{For} \mathrm{GCD},\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathrm{Cm}=\frac{\int Idt}{m\mathsf{\Delta }V}$$

(3)

$$\mathrm{Cs}=\frac{\int Idt}{A\mathsf{\Delta }V}$$

(4)

where I denotes current, V is potential, m is electrode active mass, $\mathsf{\nu }$ stands for scan rate, A represents the area of electrode, $\mathsf{\Delta }V$ is the potential range, and t is the discharge time.

EIS measurements of complex impedance Z* were performed at 5 mV AC signal and components of complex capacitance C* were obtained according to the Equations (5)–(8):

Z* = Z′ − iZ″

(5)

C* = C′ − iC″

(6)

$${\mathrm{C}}^{\prime }=\frac{{\mathrm{Z}}^{″}}{\omega {\left|\mathrm{Z}\right|}^2}$$

(7)

$${\mathrm{C}}^{\prime }=\frac{{\mathrm{Z}}^{\prime }}{\omega {\left|Z\right|}^2}$$

(8)

where $\omega =2\pi f,$ $f$ is frequency.

3. Results and Discussions

Fe₃O₄ is a ferrimagnetic material. Magnetic measurement of as-prepared Fe₃O₄ indicated its superparamagnetic behavior at room temperature (Figure 1). Such behaviour resulted from the use of small Fe₃O₄ particles. Magnetic particles facilitated the fabrication of the electrodes with reduced content of non-conductive binder. Moreover, magnetic particles added advanced functionality to the composite electrodes.

The TEM images in Figure 2 show that the typical size of the as-synthesized Fe₃O₄ particles was about 10–15 nm, typically smaller than the superparamagnetic critical size of ~25 nm [47]. The small particle size explains the superparamagnetic behavior of Fe₃O₄. It is beneficial for particle dispersion and the fabrication of composites.

The electrode material prepared by method 1 was tested for its electrochemical performance in the potential range from −1.1 to −0.3 V. The testing results were plotted as shown in Figure 3. The CV graph in Figure 3A shows a nearly rectangular shape. The scan rate was increased from 2 to 100 mV s⁻¹ and the capacitance was found to decrease (Figure 3B) from 2.42 to 0.55 F cm⁻². The capacitance retention at a high scan rate of 100 mV s⁻¹ (R₁₀₀) was calculated to be 22.7%. The resistance of the electrode (Figure 3C), corresponding to the real part of Z*, was found to be 2.69 Ohms at a frequency of 10 mHz. The relatively low resistance achieved at 35 mg cm⁻² mass loading indicated good conducting properties of the individual components. The C* dispersion of the relaxation type (Figure 3D) was observed and the relaxation frequency was 0.05 Hz, so a 20 s relaxation time constant could be calculated according to τ = 1/f from the C″ data. The charge and discharge curves were linear at 3–35 mA cm⁻² current densities, showing good charge and discharge performance (Figure 3E). The capacitance was calculated as 2.42 F cm⁻² at 3 mA cm⁻² current density and decreased to 1.75 F cm⁻² when the current density increased to 35 mA cm⁻², as shown in Figure 3F.

Method 2 offered the benefits of improved mixing of the components due to the in-situ synthesis of Fe₃O₄. It was demonstrated that the 60Ti₃C₂T_x–30Fe₃O₄–10MCNT electrodes, prepared by method 2, showed notably higher specific capacitance, compared to the electrodes of the same composition, prepared by method 1. Moreover, the 55Ti₃C₂T_x–30Fe₃O₄–15MCNT and 55Ti₃C₂T_x–25Fe₃O₄–20MCNT electrodes, were prepared by method 2 and tested for comparison. CB was applied as a co-dispersant for Ti₃C₂T_x and MCNT. The adsorption of the CB molecules on MCNT can be promoted by the polyaromatic structure of CB [51]. Moreover, the catechol group of CB facilitated its adsorption on Ti₃C₂T_x and Fe₃O₄ by creating bidentate chelating or bridging bonds with the Ti and Fe atoms on the material surfaces. The morphologies of the materials prepared by method 2 were studied by SEM and presented in Figure 4. It can be seen that MCNT and Fe₃O₄ were formed between the open layers of Ti₃C₂T_x and on the Ti₃C₂T_x surface. This resulted in improved mixing of the electrode components.

The 60Ti₃C₂T_x–30Fe₃O₄–10MCNT, 55Ti₃C₂T_x–30Fe₃O₄–15MCNT and 55Ti₃C₂T_x–25Fe₃O₄–20MCNT, electrodes with an AML of 35 mg cm⁻² fabricated by method 2 were studied for their electrochemical performance. Figure 5 shows the CV and EIS results. The largest CV area was acquired for the 60Ti₃C₂T_x–30Fe₃O₄–10MCNT electrode, indicating the highest capacitance. The capacitances at 2 mV s⁻¹ scan rate were calculated to be 5.44, 4.92 and 4.62 F cm⁻² for the 60Ti₃C₂T_x–30Fe₃O₄–10MCNT, 55Ti₃C₂T_x–30Fe₃O₄–15MCNT and 55Ti₃C₂T_x–25Fe₃O₄–20MCNT electrodes, respectively. The equivalent circuit (Figure S1) developed for the high AML electrodes was used for the impedance spectroscopy [52]. The R_t values were calculated to be 0.65, 0.62 and 0.93 for the 60Ti₃C₂T_x–30Fe₃O₄–10MCNT, 55Ti₃C₂T_x–30Fe₃O₄–15MCNT and 55Ti₃C₂T_x–25Fe₃O₄–20MCNT electrodes, respectively. The relatively low impedance was achieved at high AML of 35 mg cm⁻², which indicated low resistance and high capacitance. The Bode modulus are provided in Figure S2. In Figure 5D,E, the relaxation type dispersion was observed for these electrodes.

The pseudocapacitive behaviour of Ti₃C₂T_x is related to the oxidation state change between Ti⁴⁺/Ti³⁺, following the bonding and de-bonding of the oxygen group [53], as described by Equation (9). The electrochemical reaction of Fe₃O₄ can be attributed to the redox reaction between Fe³⁺/Fe²⁺, accompanying with the intercalation of Na⁺ ions [54], as shown in Equation (10).

Ti₃C₂O_X(OH)_yF_z + δNa⁺ + δe⁻ ↔ Na_δTi₃C₂O_X(OH)_yF_z

(9)

Fe₃O₄ + δNa⁺ + δe⁻ ↔ Na_δFe₃O₄

(10)

The contribution of surface and intercalation capacitances to the total energy storage by the composite electrodes was analyzed using the Dunn’s method [55,56,57]. A critical parameter b was applied to indicate the energy storage mechanism: for battery-type materials with diffusion-controlled capacitance, b = 0.5, and for purely double-layer capacitors with surface-controlled behaviour, b = 1 [58]. For materials with 0.5 < b < 0.8, the battery-type energy storage was dominant [59]. The b-value of as-prepared composites were determined to be 0.60, 0.56 and 0.62 for the 60Ti₃C₂T_x–30Fe₃O₄–10MCNT, 55Ti₃C₂T_x–30Fe₃O₄–15MCNT and 55Ti₃C₂T_x–25Fe₃O₄–20MCNT electrodes, respectively (Figure S3). Therefore, the composite electrodes showed a mixed energy storage mechanism with a dominant battery-type behaviour.

Figure 6 demonstrates the GCD results of the composite electrodes. The linear shapes of all the curves at different current densities indicated good pseudocapacitive performance. The longest charge–discharge time was obtained for the 60Ti₃C₂T_x–30Fe₃O₄–10MCNT sample, due to its highest capacitance. The decrease of capacitance was observed for all the electrodes when current was increased, as shown in Figure 6D. Based on the testing results above, the electrode of 60Ti₃C₂T_x–30Fe₃O₄–10MCNT showed the best performance with the highest capacitance and relatively low impedance at a high mass loading.

The active mass is another important factor of supercapacitor electrodes. Recent research emphasized the significance of high active mass loading of electrodes for practical applications [51]. With the increasing AML of an electrode, the performance can be drastically hindered due to the inefficient diffusion of electrolyte ions. To optimize the pseudocapacitive performance, electrodes with different AML from 30 to 45 mg cm⁻² of 60Ti₃C₂T_x–30Fe₃O₄–10MCNT were fabricated and tested. The testing results were compared, as shown in Figure 5 (a) and Figure 7. As shown in Figure 7A, the largest CV area was found for the electrode at 40 mg cm⁻² mass loading, indicating its highest capacitance. The areal capacitances calculated from the CV data were found to be 4.18 F cm⁻², 5.44 F cm⁻², 6.59 F cm⁻² and 5.89 F cm⁻², for mass loadings at 30, 35, 40 and 45 mg cm⁻², respectively at scan rate of 2 mV s⁻¹. From the EIS results shown in Figure 5C–E (a) and Figure 7C–E, the lowest resistance related to Z′ and the highest real capacitance C′ were achieved for the 40 mg cm⁻² electrode.

The GCD results shown in Figure 6A and Figure 8 are consistent with the CV and EIS data. Good pseudocapacitive performance was proven by the triangular-shaped curves. The longest discharge time was obtained for the electrode at 40 mg cm⁻² mass loading, which is the result of its highest capacitance.

Fabrication of an advanced asymmetric supercapacitor requires a high-performance positive electrode exhibiting comparable capacitance in an overlapping voltage window with the negative electrode. The PPy–MCNT electrode was tested in a −0.4–+0.5 V potential range and showed a capacitance of 4.65 F cm⁻² at scan rate of 2 mV s⁻¹. The rectangular-shaped CV curves, low impedance achieved at high mass loading and nearly linear GCD curves showed in Figure 9 confirmed the good capacitive properties of the PPy–MCNT positive electrode and as a result allowed the design of an asymmetric device.

The as-assembled asymmetric supercapacitor device contained 60Ti₃C₂T_x–30Fe₃O₄–10MCNT-40 mg as a negative electrode and PPy–MCNT as a positive electrode. The charge and mass balances were achieved as described in reference [51]. Briefly, the mass of the cathode and anode should follow the charge conservation principle:

Q = m + C + V + = m − C − V−

(11)

where m+ and m− are the active mass of the cathode and anode, C+ and C− represent the capacitance of the cathode and anode, and V+ and V− are the potential range of the cathode and anode, respectively.

As shown in Figure 10, both the positive and negative electrodes showed good capacitive behavior in the overlapping potential windows, which is of significant importance for the successful development of an asymmetric device.

The as-fabricated asymmetric supercapacitor was tested in an enlarged voltage window of 1.6 V. An areal capacitance as high as 1.96 F cm⁻² was obtained at a 2 mV s⁻¹ scan rate from the CV (Figure 11A,B) and 1.49 F cm⁻² was obtained at 10 mA cm⁻² current density from the GCD (Figure 12A,B). The total resistance for the device at high active mass loading (Figure 11C) depended on the resistances, related to the ion diffusion in the pores of both the electrodes and a membrane. The Cs′ value was obtained as 0.77 F cm⁻² at the frequency of 0.01 Hz (Figure 11D).

The Ragone plot in Figure 12C showed that good power-energy characteristics were achieved in this investigation. The highest power density obtained from the device was 223.17 W m⁻² (305.53 W kg⁻¹) and the highest energy density was determined as 4.34 Wh m⁻² (5.94 Wh kg⁻¹). Cycling behaviour studies showed that the cell maintained 80% capacitance after 1000 CV cycles at scan rate of 50 mV s⁻¹ (Figure 12D). It is suggested that the reduction of capacitance during cycling was not related to reduction of capacitance of the negative electrode. The investigation of cycling behavior of the negative electrode (Supplementary information, Figure S4) showed that the capacitance increased during 1000 cycles with a capacitance retention of 112%. The suggested mechanism of the capacitance increase was ascribed to microstructure changes of the electrode during cycling, as was observed in other investigations [60]. Therefore, the cycling performance of the device could be enhanced by the improvement of cycling stability of PPy–MCNT electrode.

4. Conclusions

Ti₃C₂T_x–Fe₃O₄–MCNT electrodes were prepared for applications in the anodes of asymmetric supercapacitors with high capacitance and high loading. As-prepared Fe₃O₄ in the Ti₃C₂T_x–Fe₃O₄–MCNT composite material showed superparamagnetic properties due to the small particle size. The use of Fe₃O₄ imparted magnetic properties to the composite and allowed for the fabrication of an anode with low binder content, which was the key to achieving the high loading feature. The in-situ synthesis method allowed for significant improvement in the capacitance. The capacitive behavior of the electrodes prepared by the in-situ synthesis was optimized by variation of the composition and AML. The catecholate type co-dispersant with polyaromatic structure is beneficial for adsorption on MCNT, at the same time it can form bidentate chelating or bridging bonding on the Ti₃C₂T_x and Fe₃O₄ surface. The strong adsorption and efficient co-dispersion facilitated the in-situ synthesis approach. The composite material containing 60% Ti₃C₂T_x, 30% Fe₃O₄ and 10% MCNT showed a remarkably high capacitance of 6.59 F cm⁻² at a scan rate of 2 mV s⁻¹ and low impedance at a high mass loading of 40 mg cm⁻². The key factors for achieving such high capacitance were the use of the in-situ synthesis method and celestine blue as a co-dispersant, reduced binder content and optimization of the composition and mass loading. The outstanding capacitive properties of composites render them a potential candidate for negative electrodes for asymmetric supercapacitors. The advanced asymmetric supercapacitor with a negative electrode of 60Ti₃C₂T_x–30Fe₃O₄–10MCNT and a positive electrode of PPy–MCNT was fabricated, which exhibited superior electrochemical performance in an enlarged voltage window of 1.6 V. The optimal MXene-based electrode and advanced asymmetric supercapacitor showed great potential for future applications of commercial energy storage devices.