Final Technical Report: Effects of Impurities on Fuel Cell Performance and Durability

Abstract

EXECUTIVE SUMMARYThe main objectives of this project were to investigate the effect of a series of potential impurities on fuel cell operation and on the particular components of the fuel cell MEA, to propose (where possible) mechanism(s) by which these impurities affected fuel cell performance, and to suggest strategies for minimizing these impurity effects. This project was carried out primarily at Clemson University and the Savannah River National Lab. Fuel cell investigations were done at SRNL while all investigations of the MEA components were carried out at Clemson. Meetings took place weekly at Clemson University and at SRNL with the respective project participants. Joint meetings between researchers at Clemson and SRNL took place quarterly, on average. In addition, frequent communications among project participants took place via e-mail and telephone discussions. On other occasions the researchers met with the technical staff of John Deere for discussions. At least one member of our team routinely participated in the DOE-Sponsored Joint Hydrogen Quality Task Force Meetings (usually conducted monthly).The nature and concentrations of impurities investigated and their impact on fuel cell performance and individual components are given in the table below. The negative effect on Pt/C was to decrease hydrogen surface coverage and hydrogen activation at fuel cell conditions. The negative effect on Nafion components was to decrease proton conductivity, primarily by replacing/reacting with the protons on the Bronsted acid sites of the Nafion.Even though already well known as fuel cell poisons, the effects of CO and NH 3 were studied in great detail early on in the project in order to develop methodology for evaluating poisoning effects in general, to help establish reproducibility of results among a number of laboratories in the U.S. investigating impurity effects, and to help establish lower limit standards for impurities during hydrogen production for fuel cell utilization.New methodologies developed included (1) a means to measure hydrogen surface concentration on the Pt catalyst (HDSAP) before and after exposure to impurities, (2) a way to predict conductivity of a Nafion membranes exposed to impurities using a characteristic acid catalyzed reaction (methanol esterification of acetic acid), and, more importantly, (3) application of the latter technique to predict conductivity on Nafion in the catalyst layer of the MEA. H 2 -D 2 exchange was found to be suitable for predicting hydrogen activation of Pt catalysts.Using a combination of standard catalyst characterization techniques, a structure sensitive catalytic reaction technique (cyclopropane hydrogenolysis), and reaction and transport modeling led to the finding that in the catalyst layer (essentially Nafion/Pt/C) the following best describes the structure. Pt is highly dispersed on the C support, but primarily in the meso-and macropores. The Nafion (ca. 30 wt%) resides primarily on the external surface of the C support where it blocks significant numbers ...