Functionalized Carbon Nanotube and MnO2 Nanoflower Hybrid as an Electrode Material for Supercapacitor Application

Functionalized carbon nanotube (FCNT) and Manganese Oxide (MnO2) nanoflower hybrid material was synthesized using hydrothermal technique as a promising electrode material for supercapacitor applications. The morphological investigation revealed the formation of ‘nanoflower’ like structure of MnO2 connected with FCNT, thus paving an easy path for the conduction of electrons during the electrochemical mechanism. A significant improvement in capacitance properties was observed in the hybrid material, in which carbon nanotube acts as a conducting cylindrical path, while the major role of MnO2 was to store the charge, acting as an electrolyte reservoir leading to an overall improved electrochemical performance. The full cell electrochemical analysis of FCNT-MnO2 hybrid using 3 M potassium hydroxide (KOH) electrolyte indicated a specific capacitance of 359.53 F g−1, specific energy of 49.93 Wh kg−1 and maximum specific power of 898.84 W kg−1 at 5 mV s−1. The results show promise for the future of supercapacitor development based on hybrid electrode materials, where high specific energy can be achieved along with high specific power and long cycle life.

After the functionalization, the functional groups attached on the surface of nanotubes were identified using FTIR analysis as shown in the Figure S1. The intense peak at 3435.22 cm -1 refers to the O-H stretching vibrations of hydroxyl and carboxyl groups. The three other major peaks found in the FTIR spectrum are 1628.61 cm -1 , 1380.85 cm -1 , and 1101.10 cm -1 correspond to C=C, -OH, and C-O functional groups respectively. Other two minor peaks at 2917.57 cm -1 and 2845.97 cm -1 belong to C-H bonding of carboxyl groups and might be arising from the traces of water in the KBr disk used for the analysis which cannot be fully removed. The peak in the fingerprint region at 604.97 cm -1 represents a bending vibration of CH-bond vibrations indicating the surface functionalization by carboxylic group. The overall FTIR analysis indicate that the successful functionalization of carbon nanotubes [1][2][3][4].

S4.1. Raman analysis
The Raman spectrum shown in Figure S8 indicates the presence of major bands of CNT and MnO2. The D-band at 1347.12 cm -1 is the most prevalent among all the graphitic compounds, originated from defects and disorder in the carbon structure from the hybridized vibrational modes of the graphene edges. The G-band at ~1588.23 cm -1 originates from the stretching of C-C bonds representing sp 2 hybridization and graphitized carbon structure. The 2D-band at 2685.02 cm -1 represents the second-order two-phonon process, resulting from the interlayer interactions within the depths of graphene layers of the CNT [9]. The D-band to G-band ratio was found to be 1.56 which represents the defect dominated structure of FCNT. The two major bands at 561.21 cm -1 and 637.14 cm -1 represent the formation of MnO2 in which the peak at 637.14 cm -1 arises from the symmetric stretching vibrations of the MnO6 octahedron in the MnO2 compounds [10]. The band located at 561.21 cm -1 can be attributed to the ν3 (Mn-O) stretching vibration in the basal plane of MnO6 sheets [11]. This feature is particularly strong in birnessite compounds and related to the high rate of Mn(IV) in the birnessite family. Hence, hybrid FCNT-MnO2 has the characteristic peaks of both FCNT and MnO2 with defects dominated FCNT structure.

S4.2. Cell preparation
The electrochemical cell (full-cell or 2-terminal configuration) was prepared by preparing electrodes at the first stage. The electrode was prepared by taking 8:1:1 weight proportions of active material, carbon black, and binder (PVDF), and thoroughly grinding using a mortar-pestle, with the addition of NMP solvent. The slurry formed was coated on to 35µm thick graphite foil. The electrodes were vacuum dried overnight at a temperature of 90 °C. Graphite foil was chosen as a substrate as it doesn't corrode with KOH electrolyte, whereas metal foils like aluminum (Al) or copper (Cu) or stainless steel (SS) easily get corroded with KOH electrolyte for longer cycle tests. Graphite foil is lightweight, possess good electrical conductivity and less expensive compared to metal foils. The adhesion of the electrode material (FCNT-MNO2 hybrid) to Al, SS and Cu foils was weak resulted to easy peel off the electrode material from the substrate. However, the adhesion of FCNT-MNO2 hybrid electrode material to the Graphite foil substrate was strong enough to achieve a stable device structure. Graphite foil has the corrosion resistant property to most of the electrolytes which was another reason for choosing it in our study. The electrodes were weighed before keeping them in a cell for electrochemical analysis. Whatman filter paper wetted with 3M KOH electrolyte was sandwiched between the electrodes prepared and were finally sealed in a commercial Swagelok cell for electrochemical characterization.
CV is used with input ramp signal of potential 0 to 1 V. The CV curve is plotted between potential (x-axis) and current (y-axis) at different scan rates of 5, 10, 20, 50, 100, and 200 mV s -1 . Galvanostatic charge-discharge is used with a static input current. The GCD curve is generated between time (sec) and potential (V) at different current densities of 0.25, 0.5, 1, 2, and 5 A g -1 . This technique is also used for the cyclic performance analysis. The third technique used in this analysis is potentiostatic electrochemical impedance analysis (EIS) with an input sine wave of Vrms of 5 mV amplitude, applied frequency range between 0.1 Hz to 10 kHz, and with an initial voltage equals to open circuit voltage of the electrochemical cell just before the analysis.
From cyclic voltammetry: Specific capacitance: where C is the specific capacitance in F g -1 , Idv is the integral area under the CV curve in mA V -1 , V is the scan rate mV s -1 , ∆V is the potential window in 'V', and is the total mass of active material in the electrochemical cell in 'g'.
Specific energy: where is the specific capacitance obtained from Equation 1, and the specific energy, Es is measured in Wh kg -1 .
Specific power: where C is the specific capacitance obtained from Equation 1, and the specific power, Ps is measured in W kg -1 . From Galvanostatic charge discharge: The energy efficiency (ηE) and coulombic efficiency (ηC) are calculated using the following equation (4) and (5) Specific capacitance: where C is the specific capacitance in F g -1 , is the current applied in mA, ∆t is the time under discharge curve, ∆V is the potential window in 'Volt', and is the total mass of active material in the electrochemical cell in 'gm'.
The specific capacitance of single electrode can be obtained from Eq. (6), as Specific energy: where C is the specific capacitance obtained from Equation 7, ∆V is the potential window in 'Volt', and the specific energy, Es is measured in Wh kg -1 . Specific power: where E is the specific energy obtained from Equation 8, and ∆t is the time under discharge curve, and the specific power, Ps is measured in W kg -1 .