Final Report - Advanced MEA's for Enhanced Operating Conditions, Amenable to High Volume Manufacture

摘要

This report summarizes the work completed under a 3M/DOE contract directed at advancing the key fuel cell (FC) components most critical for overcoming the polymer electrolyte membrane fuel cell (PEMFC) performance, durability & cost barriers. This contract focused on the development of advanced ion exchange membranes & electrocatalysts for PEMFCs that will enable operation under ever more demanding automotive operating conditions & the use high volume compatible processes for their manufacture. Higher performing & more durable electrocatalysts must be developed for PEMFCs to meet the power density & lifetime hours required for FC vehicles. At the same time the amount of expensive Pt catalyst must be reduced to lower the MEA costs. While these two properties are met, the catalyst must be made resistant to multiple degradation mechanisms to reach necessary operating lifetimes. In this report, we present the work focused on the development of a completely new approach to PEMFC electrocatalyts, called nanostructured thin film (NSTF) catalysts. The carbon black supports are eliminated with this new approach which eliminates the carbon corrosion issue. The thin film nature of the catalyst significantly improves its robustness against dissolution & grain growth, preserving the surface area. Also, the activity of the NSTF for oxygen reduction is improved by over 500% compared to dispersed Pt catalyts. Finally, the process for fabricating the NSTF catalysts is consistent with high volume roll-good manufacturing & extremely flexible towards the introduction of new catalyst compositions & structures. This report documents the work done to develop new multi-element NSTF catalysts with properties that exceed pure Pt, that are optimized for use with the membranes discussed below, & advance the state-of-the-art towards meeting the DOE 2010 targets for PEMFC electrocatalysts. The work completed advances the understanding of the NSTF catalyst technology, identifies new NSTF-ternary catalyst materials for higher performance, documents enhanced durability under multiple types of accelerated tests by factors of 10x to 50x over conventional catalysts, & demonstrates their performance & durability in large area MEA FC stack tests. The PEMFC ion exchange membrane is the other key functioning FC component on which work was completed. While improvements have been made to standard PFSA type membranes, they still require humidification to achieve adequate proton conductivity & so their use at elevated temperatures & drier operating conditions is limited. Membranes with increased durability & conductivity under hotter, drier conditions allow the use of FC's in many applications, particularly automotive. Towards this goal, 2 approaches were pursued in the work reported here. The first part was designed for immediate application at drier conditions & operating temperatures between 85C and 120C, focused on the development of a membrane based on a low equivalent weight (EW), perfluorinated sulfonic acid (PFSA) ionomer for good ionic conductivity at low humidification, & the use of stabilizing additives for improved oxidative stability. The ionomer used was developed at 3M & has a shorter acid containing side-chain than the Nafion? ionomer. This ionomer also has a higher Tα & higher modulus than that of a Nafion? membrane of the same EW, allowing lower EW ionomers to be prepared with very good mechanical properties. In addition, more than 50 stabilizing additives were evaluated in ex-situ, Fenton’s tests & more than 10 of these were incorporated into membranes & evaluated in accelerated FC tests. This work led to thin (25-30 micron) cast membranes with substantially improved conductivity & durability under simulated automotive conditions, compared to membranes currently available. The 2nd body of membrane work was focused on developing & characterizing 3 approaches for making new PEM's for operation under hot (>120C) & dry (dew point <80C) FC conditions: inorganic materials with enhanced proton conductivity, polymer matrices swollen with low molecular weight fluorinated acids & proton conducting ionic liquids. New materials developed show the promise of the development of new membranes with even better characteristics under demanding FC operating conditions, further improving the efficiency & viability of FC systems.