4 January 2009
Fabrication of Multi-walled Carbon Nanotube Buckypaper and its Application to Batteries
Fabrication of high quality Multi-walled carbon nanotube (MWNT) buckypaper was very difficult because they are much shorter than the single-walled carbon nanotubes (SWNTs), making the uniform dispersion very difficult. Consequently the MWNT buckypapers so produced were thick and uneven, which have been proven to be the main cause of their poor properties. We are now able to produce high quality thin MWNT buckypapers with diameters up to 70 mm, and thickness down to 50 ?m. Our approach for making MWNT buckypapers is totally additive-free and hence the inherent properties of MWNTs, such as thermal and electrical conductivity, are expected to be preserved in the buckypaper. This simple and effective approach implies a dramatic reduction in manufacturing cost, as well as in raw material price since MWNTs, rather than SWNTs, are used. The MWNT buckypapers produced in our laboratory are significantly better than that prepared by the frit compression approach. The following figures show a 35mm buckypaper. As can been seen from the figures, the buckypaper has good uniform surface finishing. It is flat and highly flexible, which are ideal for thin film applications.
Buckypapers can be used as filter membranes to trap micro-particles in air or fluid. They can be used as electromechanical actuators in artificial muscles. They also can be used as next-generation electronics and displays, or to protect electronic circuits and devices from electromagnetic interference. Another superior property of buckypaper is its resistance to fire. The inherent properties of individual carbon nanotube such as good thermal conductivity may also be employed to develop effective heat sinks that would allow the onboard electronic equipment to disperse heat more efficiently, consequently advances to the miniaturisation of the devices. Studies in our group have shown that the electrochemical performances of thin film electrodes with buckypaper substrate have been improved drastically as compared with those on other kind of substrates.
Improving the High Rate Discharge and Self-discharge Characteristics of Ni/MH Batteries
In order to compete with Li ion batteries in the field of electric tools and hybrid electric vehicles, Ni/MH batteries require substantial improvement on their ability to discharge at high rates (HRD) and resistance to self-discharge, especially when tested at high temperature. However, there two requirements seems to be contradict to each other. Our group investigated the effect of electrolyte and MmNi5-based alloy electrodes on high-rate discharge capacity by cross-examining different combinations of metal hydride electrode and electrolyte systems. It was found that self-discharge and HRD capacity of Ni-MH battery is mostly affected by the type of electrolyte, then the type of MH electrode. After long term charging/discharging, there was a significant amount of Al and Mg dissolved from the alloy electrodes into the electrolyte. This contributed to the degradation of the negative electrodes. Thus by altering the composition of the electrolyte to EO, the capacity remained in the batteries can largely be improved. The HRD can also be improved by using another modified electrolyte EM with different composition, as can be seen from figure b, the electrode in electrode EM still retained a very high efficiency of 55% even at a discharge rate of 9C, at a relatively high temperature of 333K. There was no loss of capacity for electrodes in electrolyte EM, even after 3 days of high temperature storage.
This work was financially supported by the International Science Linkage Grant, Department of Education, Science & Training