Highly Active Non-PGM Catalysts Prepared from Metal-Organic Frameworks

Abstract

Proton exchange membrane fuel cells (PEMFCs) represent the future powertrain for the automotive application due to their high efficiency, high power density, and emission of only water from the vehicle. Technology advancements are needed to enable the widespread commercialization of fuel cell-powered vehicles, including increased durability and decreased cost. Platinum is currently the electrocatalyst of choice for PEMFCs, contributing a significant fraction of the overall fuel cell stack cost [1]. There is an imperative need to develop inexpensive, non-platinum group metal (PGM) material as alternative catalysts. At Argonne National Laboratory, we are developing new methods of preparing non-PGM materials as potential substitutes for platinum cathode electrocatalysts for PEMFCs. Our approaches focus on transition metal and nitrogen-doped carbon composites prepared from highly porous organic precursors as electrocatalysts for the oxygen reduction reaction (ORR). One such method involves the use of metal-organic frameworks (MOFs) as sacrificial templates [2-4]. For example, we reported recently a one-pot solid-state synthesis technique to prepare Fe-doped zeolitic imidazolate frameworks (ZIFs), a sub-class of MOFs, which are subsequently pyrolyzed to yield highly-active ORR catalysts [4]. Although the one-pot synthesis method has been successfully demonstrated, we found that the catalyst activity could be substantially improved by optimizing the processing conditions of individual steps from synthesis, thermal activation, to post-treatment. For example, a major factor influencing catalytic activity are the heat treatment conditions. Optimizing pyrolysis time has proven to dramatically enhance ORR activity. Increased catalyst conductivity is another factor contributing to catalyst performance improvement. Furthermore, we also found that MOF synthesis conditions can significantly influence precursor structure and the resulting ORR activity of the catalyst. The improved catalyst activity has to be ultimately demonstrated in membrane-electrode assemblies (MEAs) of fuel cells. The engineering process of preparing MEAs using MOF-based non-PGM catalyst is, to certain degree, difference from that of conventional carbon-supported catalysts. Parameters such as the catalyst loading, perfluorosulfonic acid ionomer to catalyst ratio, affect the catalyst performance at the fuel cell level. In this presentation, we will also describe our effert in optimizing the MEA fabrication process. These improvements at both catalyst and MEA levels have yielded impressive ORR activity when tested in a fuel cell system, moving towards the performance targets set by U.S. DOE for the automotive application. The work performed at Argonne National Laboratory is supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office.