CNT/MnO2 Supercapacitor Electrodes on Flexible Graphite Current Conductor Foil

Carbon nanotube (CNT) has attracted much attention as suitable electrode material for supercapacitors. There are four major design considerations among others, for CNT-based supercapacitors. First, most fabrication approaches reported so far involve coating commercially available CNT powder onto current collectors, often using less conductive binder materials, making the electrical contact between electrode and current collector non-ohmic. Such processes would increase the equivalent series resistance (ESR) and lower the power density of the supercapacitors. Therefore it is desirable to grow CNTs directly on current collectors. Second, CNT has limited specific capacitance, insufficient for extremely high power demand. Thus it should be combined with materials of high pseudocapacitance such as transition metal oxides and/or conductive polymers. The composite material has to be nano-structured in order to take advantage of large surface area of CNTs and high pseudocapacitance of the other material(s). Third, the current collector needs to be flexible to fit various cell configurations. Last, the current collector, as well as the electrodes, needs to be chemically stable and resistant to harsh chemical and electrochemical environment within the cells. For these considerations we have carefully designed a supercapacitor system utilizing nanostructured CNT/MnO2 hybrid electrodes on highly flexible graphite current collector. Vertically aligned CNTs were grown directly on the current collector using hot filament-assisted CVD (HFCVD). The height, diameter, and density of CNTs were fine tuned during the growth process for optimal infiltration of MnO2 nanoparticles. A typical thin film of CNT “forest” is shown in Fig. 1. MnO2 nanoparticles were prepared and applied onto CNT films using a unique dripping process developed in our lab. By carefully controlling the length, inter-tube spacing of CNTs, as well as the size and quantity of MnO2 nanoparticles, a uniform CNT/MnO2 composite thin film electrode can be obtained (Fig. 2). The control of CNT density is critical to the electrode performance. Extremely low density leads to low surface area of CNTs per electrode, while very densely packed CNTs make the infiltration of MnO2 nanoparticles difficult. As the result, these nanoparticles would build up on top of CNT film thus can not contribute to the capacitor performance efficiently (Fig. 3). The quantity of MnO2 nanoparticles also needs to be precisely controlled. Excessive amount of MnO2 would make the nanoparticles precipitate from the CNT network (Fig. 4) which contribute little to the specific energy density of the capacitance. Fig. 5 shows the change in capacitance of an electrode with increased amount of MnO2. It is can be seen that the capacitance increased initially with MnO2 nanoparticles applied onto CNT film but began to drop after reaching the highest value due to built-up of MnO2. With the unique process we were able to prepare CNT/MnO2 hybrid electrodes with areal capacitance ~300-400 times that of pure CNT electrode (Fig. 6). The thin graphite current collectors used in this work are very flexible thus can be applied in various supercapacitor cell configurations including cylindrical, prismatic, and coin-type.