Design, fabrication, and analysis of chemical hydride-based micro fuel cells

Abstract

The performance of portable electronic devices (e.g. cell phones, tablets, laptops) is limited by the energy density of the batteries that power them. Extensive and ongoing research to develop better power sources to replace batteries has largely failed to deliver a viable replacement. The current research focuses on developing and characterizing a AA form factor micro fuel cell (MFC) which achieves higher energy density than conventional batteries and MFCs. The ultimate goal of this research is to develop a MFC technology that is a viable replacement for primary and secondary batteries in a broad range of consumer portable devices, enabling substantially longer run time, enhanced performance, and the ability to operate independently of the electrical grid. The AA MFC described herein is a novel integration of a water-recycling hydrogen-air proton exchange membrane (PEM) fuel cell and a chemical hydride-based hydrogen generator. Hydrogen is produced in the hydrogen generator via hydrolysis reaction between a solid chemical hydride and water vapor. Hydrogen and oxygen from air react at the fuel cell, generating electrical energy, water vapor, and waste heat. Water vapor is recovered directly through the fuel cell to generate additional hydrogen, thus the hydrogen generation process can be considered "water-less" in that the water required for the hydrogen generator is derived from the fuel cell and ambient air. The hydrogen generation rate is regulated by a passive (zero power consumption) pneumatic diaphragm valve, which controls diffusive transport of water vapor from the fuel cell to the chemical hydride based on the pressure difference between the interior of the MFC and the ambient environment. A two-cell planar PEM fuel cell (series connected, 1.5V nominal output) was developed and evaluated over a range of conditions. Power density of 180mW/cm^2 was demonstrate with hydrogen fuel, and a total power of 150mW was demonstrated with fuel, meeting the performance targets for the AA MFC. Several hydride fuel chemistries were evaluated for reaction rate and practical energy density in a vapor-hydrolysis reactor, and lithium aluminum hydride was selected for use in the AA MFC. Further experiments exploring the impact of hydride particle size and void fraction on reaction rate and yield were conducted, and an improved fuel formulation (5µm particle size, 32% void fraction) was selected.A pneumatic diaphragm valve was developed and evaluated for open and closed conductance, hysteresis, and repeatability. The valve met its performance targets for each, and achieved a leak rate (proportional to closed conductance) that was >10X smaller than is required for stability. Fully integrated AA MFCs were evaluated under constant current (20mA) and constant potential (1.2V) discharge tests, using three fuel formulations (crushed pellets, polydisperse powder, monodisperse powder). First generation MFCs using crushed pellets demonstrated 532 Whr/l and low reaction yield (60%) resulting from unreacted hydride in the core of the larger fuel particles. First generation MFCs using polydisperse powdered fuel demonstrated higher energy density (688Whr/l ) but still had low reaction yield (64%) resulting from unreacted fuel in the core of the larger fuel particles. Second generation MFCs utilizing an improved fuel formulation (5µm monodisperse particles, 32% void fraction) demonstrated higher average power, reaction yield, and total extracted energy vs. the first generation devices. The highest performing second generation MFC demonstrated a reaction yield of 93% and 1003Whr/l, the highest ever reported in a MFC, and equivalent to ~3X greater than conventional AA alkaline batteries.