We have been able to produce highly robust supercap films with excellent cycle performance by electrodeposition of simple Mn(II) salts in the presence of an organic stabiliser (polyethyleneimine). The resulting films can now be rubbed or scratched with considerable force before delimitation.
Fig. 1 : Black trace: chronoamperometry method: 0.65 V vs. NHE, duration: 60s, then cathodic deposition method: 2 mA/cm2, duration: 350 s. Red trace: chronoamperometry method: 0.65 V vs. NHE, duration: 60s, then cathodic deposition method: 2 mA/cm2, duration: 450 s.
A typical deposition procedure was as follows: MnCl2•4H2O (5 mM) and polyethyleneimine (0.8 g/litre) in 5% ethanol/water were mixed and allowed to age for a few hours until a brown solution had developed. Oxidative deposits were made on nickel foil (area = 1 cm2) by a combined potentiostatic / galvanostatic method. This required first a potentiostatic deposition at low potential (+0.45 V vs. NHE) for a short time (60 s), to give a very thin, robust, ultra-fine MnOx layer on the Ni foil with high surface area. After this, galvanostatic deposition at 2 mA cm-2 for 4 minutes at room temperature produced a thick manganese dioxide film on top of the ultra-fine seed layer deposited potentiostatically. The capacitance of films was initially gauged by cyclic voltammetry at varying scan rates (see Figure 1), employing the formula: i × ν = C, where i is the steady-state current density, ν is the voltage scan rate and C is the capacitance. Cyclic voltammetry has the advantage that it is a fast and effective way to screen many different materials and deposition conditions for activity. By this rapid screening technique, we were able to obtain capacitances on the order of 120 mFcm-2 for these manganese dioxide supercapacitors (see Figure 1). The films also demonstrate excellent stability with repeated cycling (see Figure 2).
Fig. 2 (left): multi-cycle charge-discharge experiments at 1 mAcm-2 on a manganese dioxide film on a nickel substrate. Figure 2 (right): Single charge-discharge curves for manganese dioxide films of various thickness, versus load current. Deposition conditions: Current density: 2 mA / cm2, duration: 350s Ag/AgCl as reference, Pt mesh as counter electrode, electrolyte: 0.25 M Na2SO4 aqueous solution. CV window: 0 - 0.8 V vs. Ag/AgCl.