The development and fabrication of miniaturized direct methanol fuel cells and thin-film lithium ion battery hybrid system for portable applications

In this work, a hybrid power module comprising of a direct methanol fuel cell (DMFC) and a Li-ion battery has been proposed for low power applications. The challenges associated with low power and small DMFCs were investigated and the performance of commercial Li-ion batteries was evaluated. At low current demand (or low power), methanol leakage through the proton exchange membrane (PEM) reduces the efficiency of a DMFC. Consequently, a proton conducting methanol barrier layer is needed. Since volume restrictions make it infeasible to use balance of plant components (active fuel channels) in DMFCs, passive fuel delivery systems are the only available option. When using a passive fuel delivery system, a CO2 vent is required to release the pressure built in the fuel tank of these DMFCs. In this work, the above challenges in advancing the performance of a low power DMFC have been addressed. Finally, Li-ion batteries have been evaluated for low self-discharge and high voltage efficiency.;Phospho-silica glass (PSG) was formed as a methanol resistant PEM. It was demonstrated that phosphorus doping in the silica matrix increased the ionic conductivity of the glass membrane. The doping increased the number of non-bridging type bonds (P-OH) in the silica matrix, which in turn increased the acidic-OH groups that allow proton conduction. It was also observed that the plasma deposition condition of the glass membrane could be altered to achieve a desired ionic conductivity in the membrane. At low temperature and high RF power, the PSG films demonstrated high ionic conductivity and structural integrity. When compared to PEMs with pure Nafion membranes, the PEMs fabricated with Nafion-PSG hybrid membranes not only showed a reduction in the methanol permeability but also an improvement in the DMFC performance.;A CO2 vent was fabricated from poly (dimethyl siloxane) (PDMS) and poly (1-trimethyl silyl propyne) (PTMSP) base polymers. It was observed that the vent membrane had higher permeability coefficient for CO2 than methanol. Addition of hydrophobic additives like 1, 6-divinylperfluorohexane to the base polymer further enhanced the selectivity of the vent for CO 2 transport. When operated at high temperatures, the rate of CO 2 flux increased, which in turn increased its permeability coefficient. However, the sorption of methanol molecules limited their transport through the polymer backbone and, as a result, their permeability coefficient decreased with temperature. The experimental results validated the theoretical model developed for estimating the vent efficiency. It was observed that at high selectivity (alpha of 9.2), the vent was 95% efficient.;The use of Li-ion battery in a power module as a secondary energy storage device results in a non-traditional duty cycle. This unique duty cycle requires the battery to be active for a very short span of time compared to its dormant or sleep stage. Under such a load, the effects of self-discharge and voltage loss were evaluated for Panasonic coin cells and thin film LiPON cells. It was observed that the thin film battery outperformed the others in terms of low energy loss. Nonetheless, the performance of small Panasonic coin cells with vanadium oxide cathode was comparable at low discharge rates of less than 0.01% depth of discharge. Lastly, it was also observed that the batteries have stable cycles at low discharge rates.