Functionally Decorated Carbon Nanotube Networks for Energy Storage in Supercapacitors

A novel approach has been developed for the fabrication of Fe3O4 decorated multiwalled carbon nanotubes (MWCNT) for energy storage in negative electrodes of electrochemical supercapacitors. Synthesis of Fe3O4 was performed in the presence of MWCNT, dispersed using various cationic and anionic polyaromatic dispersants. The comparison of experimental results obtained using different dispersants provided an insight into the influence of the chemical structure of the dispersant molecules on the microstructure of the Fe3O4-MWCNT materials. It was found that positively charged groups and chelating catechol ligands of the dispersants facilitated the formation of Fe3O4 decorated MWCNT with low agglomeration. The Fe3O4 -MWCNT materials, prepared using different dispersants were used for the fabrication of electrodes with mass loading of 40 mg cm−2. The highest capacitance was obtained in 0.5 M Na2SO4 electrolyte for Fe3O4 decorated MWCNT prepared using cationic celestine blue dye as a dispersant. Improved cyclic voltammetry profile was obtained using FeOOH as an additive. Asymmetric devices were fabricated and tested based on the Fe3O4 decorated MWCNT negative electrodes and MnO2-MWCNT positive electrodes.


INTRODUCTION
Electrochemical supercapacitors are currently under intensive investigation for energy storage and capacitive water purification applications Zhao and Zheng, 2015;Ding et al., 2020). Research is focused on the development of new materials and electrolytes (Brousse and Bélanger, 2003;Shi and Zhitomirsky, 2010;Li et al., 2019), fabrication of nanoparticles of active materials (Luo et al., 2016;Silva et al., 2018), design and modeling of composite electrodes and devices (Pavaskar et al., 2018;Xing et al., 2019). New strategies have been designed for the fabrication of carbon based electrodes with enhanced capacitive performance (Salinas Torres et al., 2019). Significant interest has been generated in application of carbon nanotubes (CNT) for the fabrication of composite electrode materials (Lu et al., 2019). The interest in CNT applications for supercapacitors is related to high electronic conductivity and high surface area of CNT.
It was recognized that CNT can be decorated with other functional materials (Pan et al., 2015;Hao et al., 2016). With a desire to fabricate new and advanced devices, there is growing interest in the development of new techniques for the fabrication of functionally decorated CNT, such as laser ablation (Imbrogno et al., 2017), atomic layer deposition (Ding et al., 2018), polymerization (Zhu et al., 2014), magnetron sputtering (Wei et al., 2014), electrodeposition (Chitturi et al., 2016), and chemical precipitation (Hong et al., 2013). Decorated CNT are of particular interest for energy storage and generation devices, such as supercapacitors, batteries, and fuel cells. The use of functionally decorated CNT allowed for the fabrication of advanced batteries with enhanced capacity (Hong et al., 2013;Wei et al., 2014), good cyclic stability (Chitturi et al., 2016) and improved conductivity (Wang et al., 2010). It has been reported that CNT can be decorated with catalysts for advanced application in fuel cells (Sonkar et al., 2017). Significant advances have been achieved in applications of functionally decorated CNT for photovoltaic devices (Mathew et al., 2011;Tai et al., 2014). It has previously been shown that carbon nanotubes can be decorated with oxides (Ojha et al., 2019), polypyrrole (Su and Zhitomirsky, 2015) and activated carbon  for energy storage applications in supercapacitors.
Previous studies highlighted the need in the development of efficient manufacturing techniques for the fabrication of functionally decorated CNT. Such techniques need to be based on the careful selection of dispersing agents, which must be well adsorbed on the CNT surface. Good dispersion of CNT is critical for their decoration with functional materials. The use of networks of decorated CNT is promising for the fabrication of advanced supercapacitor electrodes with high active mass.
The objective of this investigation was the fabrication of Fe 3 O 4 decorated CNT for application in negative electrodes of asymmetric supercapacitors. Following this objective we investigated anionic and cationic aromatic dispersants for the dispersion of CNT. The results presented below indicated that chemical precipitation of Fe 3 O 4 in the presence of CNT allowed for the fabrication of Fe 3 O 4 decorated CNT. Testing results provided an insight into the influence of electric charge and chelating groups of the dispersants on the Fe 3 O 4 formation on the CNT surface. The fibrous networks of decorated CNT were used for the fabrication of negative electrodes with high active mass of 40 mg cm −2 . The capacitive behavior was linked to dispersant structure. Finally, we fabricated and tested an asymmetric supercapacitor device.
For decoration of the MWCNT by Fe 3 O 4 nanoparticles, the synthesis of the nanoparticles was performed in the presence of dispersed MWCNT. In this procedure, 1 g L −1 of MWCNT were dispersed in DI water by ultrasonication for 15 min and then 0.25 g L −1 dispersant was added and suspension was ultrasonicated again for 15 min. A stoichiometric mixture of Fe 2+ and Fe 3+ salts dissolved in DI water was added to the suspension of MWCNT in order to obtain the mass ratio of Fe 3 O 4 to MWCNT equal to 1.5. The pH was adjusted to 9 by the addition of 1 M NH 4 OH. The obtained suspension was ultrasonicated for 40 min and then filtrated. The materials were washed with DI water and dried overnight. The magnetite decorated MWCNT, prepared using CB, PV, AA, and CP were Transmission electron microscopy was performed (dark field STEM) using a JEOL 2010F field emission microscope. X-ray diffraction (XRD) analysis was performed using the Bruker D8 Discover instrument comprising Davinci diffractometer and Co-Kα radiation. Particle size distribution was done using dynamic light scattering (DLS) model (DelsaMax Pro-Beckman Coulter). The analysis was carried out based on testing of 4 sets of each material with a concentration of 0.4 mg L −1 in DI, where every set consisted of 10 acquisition.
Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) studies of single electrodes and asymmetric device were performed in 0.5 L of 0.5 M Na 2 SO 4 electrolyte as described in prior investigations (Shi and Zhitomirsky, 2013;Zhu et al., 2014;Chen et al., 2017). EIS measurements were carried out in the frequency range of 10 mHz−100 kHz with a sinusoidal signal of 10 mV. The components of complex capacitance (C ′ s and C ′′ s) were calculated from the EIS data as C ′ s = Z ′′ /ω|Z| 2 A and C ′′ s = Z ′ /ω|Z| 2 A, where ω = 2πf and f is frequency. Galvanostatic charge-discharge of individual electrodes at different current densities was performed using Biologic VMP 300 potentiostat. The charge-discharge behavior of the asymmetric device was analyzed using battery analyzers BST8-MA and BST8-3 (MTI Corporation, USA).

RESULTS AND DISCUSSION
Figures 1A-D shows chemical structures of aromatic dispersants used for dispersion of MWCNT. The polyaromatic structure of the dispersants was beneficial (Ata et al., 2018) for their adsorption on MWCNT. The adsorption mechanism involved π-π interactions. The adsorbed dispersants imparted a positive charge (CB, AA) or a negative charge (PV, CP) to MWCNT. The small size, electric charge and good adsorption of the dispersants on MWCNT facilitated MWCNT dispersion by unzipping mechanism (Ata et al., 2018; Figure 1E) and allowed for the fabrication of stable suspensions. Figures 1F,G illustrates different types of interactions of the dispersants with Fe 3 O 4 particles. The precipitation of Fe 3 O 4 was achieved at pH = 9, which is above the isoelectric point (pH = 6.5) (Parks, 1965) of this material. Therefore, the Fe 3 O 4 particles were negatively charged. The negative charge of Fe 3 O 4 resulted in electrostatic repulsion of Fe 3 O 4 and anionic PV or CP, adsorbed on MWCNT ( Figure 1F). In contrast, electrostatic attraction existed between Fe 3 O 4 and cationic CB or AA molecules, adsorbed on MWCNT ( Figure 1G). Previous investigations (Ata et al., 2014) showed that molecules, containing a catechol group, strongly adsorbed on inorganic particles, and facilitated their efficient dispersion. Therefore, CB and PV can be adsorbed on Fe 3 O 4 particles by catecholate type bonding. Figure 1F shows bonding of PV to the Fe atom on the particle surface. A similar mechanism can be suggested for CB bonding. It is important to note that phenolic molecules containing single OH groups, such as CP, show poor adsorption on inorganic molecules (Ata et al., 2014). In contrast, molecules from the catechol family, such as PV, containing two adjacent OH groups, show very strong bonding to the inorganic particles (Ata et al., 2014).
X-ray diffraction studies confirmed the formation of pure Fe 3 O 4 by precipitation from mixed Fe 2+ and Fe 3+ salt solutions ( Figure S1). The materials prepared by precipitation from the same solutions, containing dispersed MWCNT, showed Xray diffraction peaks of Fe 3 O 4 and MWCNT (Figure 2). Peak broadening resulted from the small particle size of Fe 3 O 4 . TEM analysis revealed influence of the dispersants on the material morphologies. On the other hand, the chelating bonding of the catechol groups of PV promoted Fe 3 O 4 formation on the MWCNT surface. The chelating bonding was a dominating mechanism, which allowed for the formation of decorated MWCNT. However, the TEM studies of M-PV-MWCNT also revealed the formation of small agglomerates of the Fe 3 O 4 particles as it is shown in Figure 3F. The analysis of the TEM images    The effect of different dispersing agents has also been studied using DLS analysis (Figure 4). The size distribution of the aggregates in all samples is bimodal with different degree of polydispersity. The first peak at ∼100 nm is present in all four dispersions with different intensities. The second peak was observed in the range of 10 3 -10 4 nm. The intensity and polydispersity of the second peak are smallest for M-CB-MWCNT. The polydispersity of M-PV-MWCNT is broader than that of M-CB-MWCNT. The analysis of the DLS data for M-AA-MWCNT and M-CP-MWCNT revealed significant increase in the relative intensity of the second peak, compared to the intensity of the first peak. The second peak for M-AA-MWCNT and M-CP-MWCNT shifted to larger radius numbers, compared to the second peak for M-CB-MWCNT. The increase in relative intensity of the second peak for M-AA-MWCNT and M-CP-MWCNT and peak shifts indicated an increasing number of agglomerates and increase in the agglomerate size of the particles. This is in agreement with the TEM data and indicates the beneficial effect of the catechol groups of CB and PV for the formation of decorated MWCNT and reduction of Fe 3 O 4 agglomeration. It is important to note that the decoration of MWCNT resulted from the dispersant mediated adsorption. In this approach the changes in MWCNT structure, resulting from chemical reactions and high temperature treatment (Xing et al., 2015(Xing et al., , 2017(Xing et al., , 2019   However, the CV for M-CB-MWCNT showed redox peaks and deviated significantly from the box shape. Significant reduction of the charge and discharge currents was observed in the potential range of −0.4 to −0.7 V. The observed redox peak can result from the redoxactive properties of CB (Noorbakhsh et al., 2008). Such CV shape is detrimental for the fabrication of asymmetric cells. In order to improve the CV profile, the M-CB-MWCNT was combined with FeOOH, which shows good charge storage properties in the range of −0.4 to −0.7 V (Chen et al., 2018). Therefore, M-FH-CB-MWCNT was prepared and tested. This material showed improved CV shape, compared to M-CB-MWCNT. The capacitance of 5.76 F cm −2 (144 F g −1 ) was obtained at a scan rate of 2 mV s −1 . Figure 6 shows frequency dependences of the components of differential AC capacitance, calculated from the impedance data. M-CB-MWCNT showed the highest real part of capacitance C ′ S at low frequencies, compared to other materials. However, the M-FH-CB-MWCNT electrodes showed lower C ′′ S , which indicated lower energy losses. The slightly higher relaxation frequency, corresponding to maximum of C ′′ S , indicated better performance. The M-CP-MWCNT electrodes, showed lower C ′ S and lower relaxation frequency, compared to other materials. The capacitive behavior of the electrodes has also been analyzed by chronopotentiometry. Figure 7 compares galvanostatic charge-discharge curves for different electrodes and capacitances, calculated from the discharge data at different current densities. The charge-discharge curves were of nearly triangular shape. The highest capacitance at 3 mA  M-FH-CB-MWCNT electrodes were used for the fabrication of the asymmetric supercapacitor cells. One of the problems related to the fabrication of asymmetric devices with Na 2 SO 4 electrolyte is related to lower capacitance of negative electrodes, compared to the capacitance of advanced positive electrodes, such as MnO 2 -MWCNT (Chen et al., 2017). However, M-FH-CB-MWCNT electrodes showed relatively high capacitance and the asymmetric device was fabricated using 40 mg cm −2 M-FH-CB-MWCNT negative electrode and 35 mg cm −2 MnO 2 -MWCNT positive electrode. It is known that aqueous asymmetric devices based on Fe 3 O 4 negative electrodes and MnO 2 positive electrodes (Brousse and Bélanger, 2003) can operate in a voltage window of 1.8. Water decomposition can be avoided due to overpotential for oxygen evolution. Figure 8A shows CVs for the device in a voltage window of 1.6 V. The current increased with increasing scan rate. As expected, the capacitance C S of the device was about 50% of the capacitance of the individual electrodes ( Figure 8B). The device showed relatively low impedance and relatively high relaxation frequency (Figures 8C-E). The asymmetric devices showed ideal triangular shape charge-discharge curves (Figure 9A, inset). The capacitance slightly decreased ( Figure 9A) with current density in the range of 3-50 mA cm −2 . The capacitance retention after 1,000 cycles was 75% ( Figure 9B). The Coulombic efficiency remained about 100% during cycling. The device had an energy density of about 1 mWh cm −2 which showed relatively small variations with increasing current density in the range of 3-50 mA cm −2 (Figure 9C). Two cells, connected in series, allowed for the powering of eleven 20 mA LEDs, as it is shown in Figure 9D.  M-FH-CB-MWCNT showed good capacitive behavior at mass loading of 40 mg cm −2 . Asymmetric devices were fabricated, containing M-FH-CB-MWCNT negative electrodes and MnO 2 -MWCNT positive electrodes, which showed promising capacitance and power-energy characteristics.

DATA AVAILABILITY STATEMENT
The datasets generated for this study are available on request to the corresponding author.

AUTHOR CONTRIBUTIONS
MN conducted synthesis of materials, performed materials characterization and electrochemical testing, analysis of the data and contributed to the writing of the manuscript. RS contributed to DLS and electron microscopy testing, data analysis, and writing of the manuscript. IZ contributed to the development of new dispersants, cell design. IP and IZ contributed to the data analysis and writing of the manuscript.

FUNDING
This work was supported by the Natural Sciences and Engineering Research Council of Canada.