Extracting Model Parameters and Paradigms from Neutron Imaging of Dead-ended Anode Operation

In a PEMFC, feeding dry hydrogen into a dead-ended anode (DEA), reduces the overall system cost, weight and volume due to reduced need for a hydrogen-grade humidification and recirculation subsystems, but requires purging to remove the accumulated water and inert gas. Although the DEA method of operation might be undesirable due to its associated high spatial variability it provides a unique perspective on the evolution of the water accumulation in the anode. Sections of the channel nearest the inlets are significantly drier than those nearest the outlet as shown in the neutron imaging of a 53 cm2 PEMFC. This method allows in-situ visualization of distinct patterns, including water front propagation along the channels. In this paper we utilize neutron imaging of the liquid water distributions and a previously developed PDE model of liquid water flow in the GDL to (a) identify a range of numerical values for the immobile saturation limit, (b) propose a gravity-driven liquid flow in the channels, and (c) derive the two-phase GDL boundary conditions associated with the presence of liquid water in the channel. NOMENCLATURE Vl GDL liquid water volume Vp GDL open pore volume W mass flux N molar flux ∗Address all correspondence to this author. Email: [email protected] p pressure T temperature R universal gas constant A f c fuel cell active area s GDL liquid water saturation sim immobile saturation limit ε GDL porosity c concentration Dv water vapor diffusivity Mv water vapor molar mass ρl liquid water density νmix gas mixture kinematic viscosity λH2 hydrogen excess ratio = (Utilization) −1 i current density tw water thickness INTRODUCTION Feeding dry hydrogen to the anode of a Proton Exchange Membrane Fuel Cell (PEMFC) reduces the overall system cost, weight and volume due to reduced need for a hydrogen-grade humidification subsystem. However, when operating the anode with a dry hydrogen feed, it is commonly understood that the portion of the channel nearest the inlet will be significantly drier than the channel portion nearest the outlet. This condition may be problematic at higher current density, since the proton carrying capacity of the membrane depends on membrane hydration. A dry membrane is ineffective at transporting ions and hence pro1 Copyright c © 2009 by ASME Proceedings of ASME 2009 Seventh International Fuel Cell Science, Engineering and Technology Conference FuelCell2009 June 8-10, 2009 Newport Beach, California, USA Downloaded 22 Sep 2010 to 141.212.134.235. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm duces more heat, which further dries the membrane. Due to the slow gas flow velocity on the anode, and the low water vapor carrying capacity of the hydrogen gas stream, water removal from the anode is more difficult than the cathode. Numerous articles on liquid water droplet formation and removal from the surface of the Gas Diffusion Layer (GDL) have been published [1–4]. Neutron imaging work clearly illustrates the difficulty of liquid water removal [5]. A neutron image shown in Fig. 1, also clearly indicates the dry-inlet, wet-outlet phenomenon. Excessive amounts of water in the anode channel can cause a reversible degradation in cell voltage. This excess water can also lead to H2 starvation and carbon corrosion if additional conditions, such as high potential, occur simultaneously [6, 7].