Flood mitigation strategies and diagnostic techniques for polymer electrolyte fuel cells [electronic resource]
Keywords: Mechanical Engineering

A persistent challenge in polymer electrolyte membrane fuel cell (PEFC) systems is water management. Membrane humidity must be maximized to ensure good ionic conductivity, while excess product water must be removed to prevent electrode, gas diffusion layer, and flow field channel flooding. This challenge is often addressed by using fully humidified inlet gas streams and incorporating serpentine channel cathode flow fields for air delivery. Although successful in achieving stable performance, the high pressure drops required by such flow fields result in large parasitic power losses (as high as 35% of stack power) and increased system complexity. In the present work, we aim to minimize this loss with novel cathode designs which enable stable, flood-free performance, in parallel channel flow fields. We first present a detailed study of an active water management system for PEFCs, which uses a hydrophilic, porous carbon cathode flow field as a water transport wick, coupled with an electroosmotic (EO) pump for water removal. We characterize in-plane transport issues and power distributions using a three by three segmented PEFC design. Transient and steady state data provide insight into the dynamics and spatial distribution of flooding and flood-recovery processes. Segment-specific polarization curves reveal that the combination of a wick and an EO pump can effect a steady state, uniform current distribution for a parallel channel cathode flow field, even at low air stoichiometries ([alpha] = 1.5). Wicks are often an integral part of fluid capacitance and transport in many systems, including not only fuel cells, but also heat pipes, microfluidic chips, and lateral flow chemical assays on cellulose paper. In the second section, we explore porous polymer monoliths as a new wick material and wick fabrication method applicable to fuel cells and potentially other systems. Polymer monolith chemistries, long used for high surface-to-volume ratio separation and filtering media in analytical chemistry, offer tremendous flexibility in resulting monolith pore-structure, chemical composition, and surface chemistry (including wettability). We leverage this flexibility to design, fabricate (including casting), and characterize integrated hydrophilic porous monoliths, with the aim of achieving high permeability wick materials. In the third section, we integrate these porous polymer wicks into a metal cathode flow field which is similar to many state-of-the-art architectures (e.g., stamped metal or injection molded). This wick is in situ polymerized to conform to and coat cathode flow field channel walls, thereby spatially defining regions for water and air transport. At the very low air stoichiometry of 1.15, our system delivers a peak power density of 0.68 Wcm-2. This represents a 62% increase in peak power over a control case. The open channel and manifold geometries are identical for both cases and we demonstrate near identical inlet-to-outlet cathode pressure drops at all fuel cell operating points. We therefore show significant performance enhancement without introducing additional parasitic losses.