A Novel Architecture for Micro Fuel Cells S. Moghaddam *,** , E. Pengwang * , K. Lin ** , R. Masel ** , and M. Shannon * * Department of Mechanical Science and Engineering at University of Illinois at Urbana-Champaign, Urbana, IL, USA, saeedmog@uiuc.edu ** Department of Chemical and Biomolecular Engineering at University of Illinois at Urbana-Champaign Urbana, IL, USA ABSTRACT Development of a fully integrated millimeter-scale (3×3×1 mm 3 ) fuel cell with a completely passive control mechanism is reported in this study. Fabrication of this unique power source was enabled through development of a novel self-regulating micro-hydrogen generator. The hydrogen generator stops generating hydrogen automatically when hydrogen is not consumed, enabling the micro fuel cell to operate passively, similar to a battery. The implemented passive control mechanism occupies only 0.5% of the total device volume. The first generation of this device delivered an energy density of 254 W-hr/L. Subsequent generations of this device can potentially reach 1000 W-hr/L. Keywords: micro power generation, micro fuel cell, hydrogen generation, proton exchange membrane, microvalve 1 INTRODUCTION The ever-increasing power demands and miniaturization of portable electronics, micro-sensors and actuators, and emerging technologies such as cognitive arthropods have created a significant interest in development of micro fuel cells. In terms of energy density, metal hydrides (e.g. NaBH 4 ), methanol, and most hydrocarbon fuels have an energy density up to an order of magnitude higher than the competitive battery technologies. Micro fuel cells, however, can potentially outperform the batteries only if their fuel to device volume ratio can be maximized and the power consumption of their auxiliary systems to regulate fuel delivery and power control is significantly reduced. While fabrication of small-scale membrane electrode assembly (MEA) is widely reported in literature [1-3], shrinking the size of the auxiliary systems (pump, valves, sensors, distribution components, and power and control electronics for these components) has remained a challenge. While this might be somewhat feasible in centimeter-scale fuel cells, fitting all the auxiliary components within a few cubic millimeters volume is quite a challenge. Despite the advancements in fuel cell components and fabrication processing, there has been very little progress made on micro fuel cell system integration. Integrated micro-fuel cell architectures suggested in literature (e.g. in [4-7]) are scaled-downed versions of large-scale systems with numerous auxiliary components. These components can be much larger than the membrane electrode assembly (MEA), greatly reducing the overall device energy density. In addition, they consume power, which reduces available output power from the micro fuel cell for a further reduction of the device energy density. Additionally, auxiliary components normally require numerous microfabrication steps and have integration difficulties that can result in higher production costs and added complexity of micro fuel cells operation. In this study, we report development of a fully integrated millimeter-scale fuel cell with on-board fuel and control system. Fabrication of this power source was enabled through development of a new mechanism for controlling hydrogen generation rate inside the device. The control mechanism stops generating hydrogen automatically when H 2 is not consumed by the fuel cell. The volume of the control mechanism is less than 50 nL (approximately 0.5% of the device volume) and requires no energy input. This technology has enabled fabrication of the first fully integrated millimeter-scale fuel cell that operates much like a battery. This technology can also be implemented in centimeter-scale micro fuel cells to enhance their energy density and reliability and reduce their complexity and cost. 2 OPERATION PRINCIPLE Figure 1 shows a 3D schematic cross section of the device that consists of four layers including; 1) water reservoir, 2) membrane, 3) hydride reactor, and 4) MEA. During the device operation, water enters the narrow space between the bottom of the reservoir and the membrane (cf. Figure 2) through a gate at the bottom of the water reservoir. Capillary forces within the membrane holes keep the water from flowing into the hydride reactor. As depicted in Figure 2, water vapor diffuses into the hydride reactor. Hydrogen is generated when water vapor reacts with the hydride. The generated hydrogen then leaves the hydride reactor through a porous wall at the bottom of the reactor and reaches the MEA. If hydrogen is not used by the MEA (i.e. open circuit mode), pressure builds up inside the hydride reactor. Since the membrane is designed to deflect at a pressure less than the capillary forces within the membrane holes, it deflects and plugs the water gate thereby stopping the outflow of water from the reservoir. Clean Technology 2009, www.ct-si.org, ISBN 978-1-4398-1787-2 151