Accumulation of Inert Impurities in a Polymer Electrolyte Fuel Cell System with Anode Recirculation and Periodic Purge: A Simple Analytical Model

Christopher L Gardner,Paran J Sarma,Erik Kjeang

doi:10.3390/en15061968

Abstract

Anode recirculation with periodic purge is commonly used in polymer electrolyte fuel cell systems to control the accumulation of nitrogen, water, and other impurities that are present in the fuel or diffuse through the membrane from the cathode compartment. In this work, we develop a simple, generalized analytical model that simulates the time dependence of the accumulation of inert impurities in the anode compartment of such a system. It is shown that, when there is transport out of the anode chamber, the inert species is expected to accumulate exponentially until equilibrium is reached when the rate of inert entering the anode in the fuel supply and/or via crossover from the cathode is balanced by the rate of leakage and/or crossover to the cathode. The model is validated using recently published experimental data for the accumulation of N2, CH4, and CO2 in a recirculated system. The results show that nitrogen accumulation needs to be taken into account to properly adjust system parameters such as purge rate, purge volume, and recirculation rate. The use of this generalized analytical model is intended to aid the selection of these system parameters to optimize performance in the presence of inerts.

Highlights

To optimize fuel efficiency and to limit the accumulation of fuel impurities, a polymer electrolyte fuel cell (PEFC) is often operated in dead-ended anode (DEA) mode with either a small bleed or periodic purge [1,2]
Sufficient fuel velocity often cannot be maintained in a PEFC operating in DEA mode with small bleed because most of the hydrogen is consumed along the channel and the fuel flow velocity is low at the end of the flow field
We examine the accumulation of inert species in the anode chamber of a recirculated PEFC in the specific cases that (a) there is no loss from the anode chamber between purge periods and (b) there is physical leakage out of the anode chamber as well as diffusion of the species through the membrane between purge periods

Summary

Introduction

To optimize fuel efficiency and to limit the accumulation of fuel impurities, a polymer electrolyte fuel cell (PEFC) is often operated in dead-ended anode (DEA) mode with either a small bleed or periodic purge [1,2]. Sufficient fuel velocity often cannot be maintained in a PEFC operating in DEA mode with small bleed because most of the hydrogen is consumed along the channel and the fuel flow velocity is low at the end of the flow field In this situation, water accumulates resulting in fuel starvation over a portion of the anode. To overcome the problem of water accumulation, recirculation of the anode fuel stream is a commonly used technique to increase fuel velocity through the gas distribution plate while maintaining high fuel utilization—in some cases as high as 99%. These high fuel utilization rates, cause significant enrichment of any impurities (such as N2 , CH4 , CO, and CO2 ) in the fuel stream as well as impurities (such as N2 and CO2 ) that can diffuse across the membrane from the cathode

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Conclusion