A schematic diagram of the assembly (Figure 1) shows the components of the coin-cell (20 mm diameter and 3.2 mm thickness) SCs. Three SS, 18Cr9Ni from RECORD, discs pairs were used for VACNFs growth which also worked as current collectors. The diameter and thickness of the discs in the pair was different. A standard coin cell disc punching machine (Xiamen Tmaxcn Inc.)1 mm thick and 16 mm diameter was used on one side and a thin 200 μm thick and 14 mm diameter on the other side. The lower thickness of the current collector was used on one side due to the limited total thickness possible (3.2 mm) and the smaller diameter was used to prevent any short circuit due to possible separator misalignment during assembly. The 200 μm disc was cut very carefully to avoid any kind of wrinkle that would create problems for uniform VACNF growth. The discs were first cleaned using standard cleaning 1 (SC1) to remove any organic contaminants. The palladium (Pd) catalyst layer required to grow the CNFs was deposited in two ways. On one SS pair a 10 nm thick Pd catalyst film was deposited by electron beam evaporation and it was named as Cell1. On second pair the Pd catalyst was deposited in the form of nanoparticles by spin coating a polyvinylpyrrolidone/Pd (9:1) solution (details of preparation in [16]) and named as Cell2. The VACNFs were grown directly on the current collectors using the DC-PECVD method as described previously [17, 18] and conducted for 2 hours at 550oC using ammonia (NH3) and acetylene (C2H2) gases. The produced VACNFs were analysed by scanning electron microscopy (SEM) performed by using a JEOL JSM-6301F. The SEM images were taken at 40o tilt angle enabling the measurement of the length of the VACNFs.

The coin-cells were assembled and sealed inside a vacuum glove box(Xiamen Tmaxcn Inc.)under argon atmosphere (O2 <5 ppm, H2O <1 ppm) by using a mechanical coin cell crimper (Xiamen Tmaxcn Inc.). A VACNF electrode on a 200 μm thick current collector was put at the bottom and then 20 μL of an electrolyte of 1 M LiPF6 in EC/DMC (1:1) was poured on the VACNFs using a micropipette. The separator (Celgard) was placed on the electrode by another 20 μL of electrolyte and a second electrode with the VACNFs facing the separator. A spring (to guarantee a good contact) was placed on the top electrode and finally the cell was assembled by putting the top cap on the spring. A reference coin-cell was made containing only SS current collectors and named as Cell3. All the measurements were made two days after assembly to allow the electrolyte to properly and equally wet the electrodes.

Figure 1.a) Schematic diagram of the coin cell assembly (Xiamen Tmaxcn Inc.)and b) photograph of an assembled VACNF SC coin-cell.Int. J. Electrochem. Sci., Vol. 12, 2017 6656

The electrochemical performance of the coin-cells was evaluated in a two electrodes set-up by cyclic voltammetry(CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques using a Gamry Instruments Reference 3000 potentiostat equipped with the software Framework 6.33. The capacitance was measured using CV, in which the voltage at the working electrode was swept to a set potential and back to the initial voltage at a certain voltage scan rate, and the corresponding current at the working electrode was plotted vs. the voltage. The capacitance was calculated by integrating the charge and discharge curves. For an ideal SC the CV has a rectangular shape illustrating immediate change in current direction on reversing the sweep voltage. The rate capability i.e. the capacitance retention at higher voltage scan rates, was evaluated by measuring the capacitance at scan rates ranging from 20-200 mV/s. The IR drop, the energy loss or initial voltage drop due to the intrinsic resistance ofbatteryelectrolyte(Xiamen Tmaxcn Inc.)and electrode materials [19], and the cycle life were both evaluated by GCD in which the capacitor is charged to a set potential and then discharged using a constant current - here from 0 to 3.5 V and back to 0 V using a 1 A/g current density. Finally, the ESR and the capacitance behavior at different frequencies were measured by electrochemical impedance spectroscopy (EIS); a small AC potential is applied and the corresponding AC current signal is recorded – here 10 mV is applied in the frequency range 10 mHz to 1 MHz under open circuit voltage (OCV) conditions