(26b) A Hybrid Micro Fuel Cell - Thin Fillm Lithium Ion Power Source

Wireless sensors and other micro-scale systems for biomedical, military, space, and consumer use demand small form factors, extended energy supply life, stand-alone operation and full integration. Unfortunately, conventional lithium ion (Li-ion), nickel cadmium and nickel-metal hydride, when conforming to micro-scale dimensions, cannot provide the energy density required for long operational life and the end result is a tiny chip powered from a bulky off-chip battery.

One plausible solution is to continually scavenge thermal, solar, and/or kinetic energy from the environment and use it to supply the system. Its effectiveness, however, is suspect due to its intermittent power levels, asynchronous nature and cumbersome harvesting requirements are difficult to overcome. Therefore, exploring ways to design and manage an environmentally independent hybrid sourcing technology to concurrently supply high power and energy is important.

Liquid feed direct methanol fuel cells are poised to provide the required dimensions and high energy density characteristics for wireless and other micro-scale systems. Concentrated aqueous methanol solutions ( > 10 M) have an intrinsic energy density nearly 10 times higher than state-of-the-art Li-ion cells and are environmentally ?green?.

The design and performance of a hybrid micro direct methanol fuel cell ? Li-ion battery energy and power management solution will be presented and discussed. The presentation will focus on discussion at the component level, where each component is designed with the goal of achieving high efficiency, extended lifetime, and small form factor.

The presentation will also focus on issues related to constructing passive micro-scale devices with a focus on minimizing losses in order to increase both fuel utilization and device life. Individual components to be discussed include: Nafion®-free conductive glasses, novel glass composite catalyst structures, fuel delivery, CO2 venting, etc. Also, possible Li-ion battery chemistries will be discussed with respect to their parasitic losses (self discharge) and load discharge characteristics. Finally, we will discuss the power management circuitry and full system integration.

Acknowledgement

The authors would like to thank the Test Resource Management Center (TRMC) Test and Evaluation/Science and Technology (T&E/S&T) Program for their support. This work is funded by the T&E/S&T Program through the Naval Undersea Warfare Center, Newport, RI, contract number N66604-06-C-2330.