(169f) Self-Powered Integrated Circuits Via Sol-Gel Based Methanol Fuel Cell

Recent advancements in portable electronics have created a need for a new, higher-density, lower-cost, and longer-life energy source. As demand for micro scale devices increase, such as for wireless sensors, so does the need for an energy source designed for low power systems. Direct methanol fuel cells (DMFC) have received attention as a power source for low power devices because DMFCs can be operated at atmospheric temperature and pressure and have the highest energy density of all convenient fuels. Integration of fuel cell directly into electronic devices could provide the highest energy density electronic device. The implementation of a low power DMFC has several challenges, including lowering methanol permeability through a polymer-based electrolyte, orientation-independent operation, and development of a carbon dioxide vent from the fuel tank.

Improving the methanol conversion efficiency is the most significant challenge to achieve long-life. Several energy loss mechanisms exist. Energy loss occurs due to a voltage drop, iR (where i is the current and R is the resistance membrane resistance) across the proton exchange membrane (PEM). In the case of low current fuel cells, the membrane conductivity can be significantly lower than high power cells since the current will be lower. Methanol permeability through the PEM is a critical issue because it results in fuel loss and lowers the effectiveness of oxygen reduction reaction at the cathode. Crossover through a Nafion membrane is a considerable problem for low-power, long-life fuel cells. Low-power cells can tolerate a modest loss in conductivity, resulting in higher iR drop compared to high-current fuel cells, in order to achieve lower methanol permeability for extended life and higher overall fuel conversion efficiency.

A sol-gel fabrication method for glass membranes which is potentially compatible with silicon devices has been explored which allows control of both the composition and structure of the glass. It is also less expensive and more rapid than vacuum processes. The sol gel process involves two reactions: hydrolysis and condensation using alkoxides as a precursor. In this process, hydrolysis and condensation takes place via 3 steps: i) nucleophilic substitution of the end group, ii) proton transfer from the attacking molecule to an alkoxide (within the transition state), and iii) removal of the end group as alcohol or water. The characteristics and properties of the inorganic silica network are related to a number of sol-gel parameters. The parameters that affect the rate of hydrolysis and condensation reactions include: pH, mixing temperature, reaction time, precursor concentration, H2O/Si mole ratio (R), curing temperature, and reaction time.

In this study, sulfonic acid-functionalized glass membranes have been synthesized using functionalized alkoxy silanes via sol-gel chemistry. The ionic conductivity and methanol permeability of glass membranes have been studied as a function of the sol-gel components and processing conditions. The sol-gel parameters have been optimized to form conductive and ductile PEMs in low-power DMFCs. Compliant Pt/C-SiO2 composite electrodes were prepared and used to form a membrane electrode assembly (MEA) on the glass membranes.