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Water balance and multiplicity in a polymer electrolyte membrane fuel cell
Author(s) -
Chia EeSunn J.,
Benziger Jay B.,
Kevrekidis Ioannis G.
Publication year - 2004
Publication title -
aiche journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.958
H-Index - 167
eISSN - 1547-5905
pISSN - 0001-1541
DOI - 10.1002/aic.10190
Subject(s) - chemical reaction engineering , fuel cells , library science , dept , chemistry , engineering , chemical engineering , computer science , stereochemistry , organic chemistry , catalysis
Polymer electrolyte membrane fuel cells (PEM-FC) have received widespread attention as an alternative power source (EGG Benziger et al., 2004); this reactor design, which bypasses the additional complexity of twoand three-dimensional (3-D) integral reactors (Springer et al., 1991; Bernardi and Verbrugge, 1992; Fuller and Newman, 1993; Janssen, 2001), focuses on reaction/transport dynamic coupling under well-defined conditions. Ignition/extinction phenomena and multiple steady states were reproducibly demonstrated by Moxley et al. (2003). These experiments demonstrate that the relation of proton transport with water activity in the PEM membrane underpins the observed dynamical phenomena. Water ionizes and shields stationary anions in the membrane, which causes proton transport to increase by orders of magnitude. In this article we present and analyze a remarkable analogy between water balance in the differential PEM-FC and energy balance in the classical exothermic stirred tank reactor. This is accomplished through a simplified model that embodies what we believe to be the essential physics controlling ignition in a PEM fuel cell. Water, the reaction product in the PEM-FC, autocatalytically accelerates the reaction rate by enhancing proton transport through the PEM. This is analogous to the Arrhenius temperature-based rate acceleration due to the heat produced by an exothermic reaction. We modify the established textbook analysis of heat autocatalyticity in a CSTR (van Heerden, 1953; Aris, 1965; Perlmutter, 1972; Uppal et al., 1974; Schmitz, 1975) to present water management autocatalyticity in a stirred tank reactor PEM-FC. Steady states arise at the intersection of a (linear) water removal curve and a (sigmoidal) water production curve. Having established the autocatalyticity analogy between the exothermic CSTR (Aris, 1965), and what we will now call the STR-PEM, we can use modeling tools for the dynamic/parametric analysis of chemical reactors (continuation, singularity theory, multiplicity criteria, numerical stability and bifurcation analysis) to explore the STR-PEM dynamic and parametric operation (Doedel, 1981; Farr and Aris, 1986; Balakotaiah and Luss, 1982a,b). The fuel cell literature contains extensive anecdotal reports that PEM cells only operate when sufficient water is present in the membrane. Our analysis helps elucidate the role of critical initial membrane water content for ignition; the same tools can help quantify critical inlet stream humidity.