Micromachined silicon structures for free-convection PEM fuel cells

Abstract

This paper presents details on the design, fabrication, testing and modeling of micromachined gas diffusion media (GDM) for micro proton exchange membrane (PEM) fuel cell applications. Two-tiered mesh structures were thru-etched into silicon wafers and subsequently assembled with membrane electrode assemblies (MEAs) and tested with hydrogen fuel and ambient air as the oxidizer. These silicon structures doubled as gas diffusion layers and supports for thermally evaporated gold current collection layers that mated with commercially available MEAs of the catalyst-on-membrane variety. In general, the cell V–I performance curves approached that of conventional GDM-based free-convection cells for current densities less than 75 mA cm−2 on an iR-free basis. At higher current densities, the cell's operability became less stable as product water flooded the micromesh structures as evidenced by stereoscopic images during cell operation. Single- and two-phase flow modelings of the fuel cell operating in free-convection mode were also developed and the simulations support the experimental results that water accumulation significantly reduces the maximum current density achievable for such micro fuel cells. Improved water management approaches are proposed.