Thesis Abstracts 2002

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Degradation And Reliability Modelling Of Polymer Electrolyte Membrane (pem) Fuel Cells

By: Mike Fowler, PhD

Supervisors: Dr. John C. Amphlett and Dr. Brant A. Peppley

Abstract

To date there has been very little reliability or end of life analysis conducted for proton exchange membrane (PEM) fuel cell systems. Voltage degradation as a fuel cell ages is a widely observed phenomenon, but little systematic information has been reported, nor has this phenomenon been included in electrochemical models. This work documents and classifies the failure modes that can be experienced in PEM fuel cells. A test station was adapted for the long term operation of a single, 50 cm2, internally hydrated, PEM fuel cell. An endurance test was conducted to age the cell under normal operating conditions for 1350 hours, at which time membrane failure was experienced.

Changes in the polarization curve predicted by the Generalized Steady State Electrochemical Degradation Model (GSSEDM) are demonstrated from the data for the performance of typical PEM fuel cell hardware. This work develops and applies the generalized steady state electrochemical model for a PEM cell, and introduces two new terms to account for membrane electrode assembly (MEA) ageing, specifically the ageing of the MEA materials. One term is based on the concept that the water carrying capacity of the membrane deteriorates with time-in-service. The second term involves intrinsic rate constants associated with the reactions on the anode and cathode side, and the changes in catalytic activity due to catalyst degradation. The resulting model is largely mechanistic with most terms being derived from theory or including coefficients that have a theoretical basis, but also includes empirical parameters to deal with the changing performance. The value of such a generic model to predict or correlate PEM fuel cell stack voltages over the life of the fuel cell is demonstrated in this work. From the experimental data a membrane conductivity degradation rate was determined, and the value for l_DR was found to be -0.0007 hr^-1. A term was introduced for degradation rate of the fuel cell due the changes in apparent catalytic activity. The proposed apparent activity ageing rate (k_DR) is -0.055 µVK/hr. The overall voltage degradation rate of 11 µVK/hr is consistent with other experimentally observed rates under reasonably controlled laboratory conditions and near continuous operation.