MOFs and POPs and more – new classes of highly efficient non-PGM and ultralow PGM fuel cell catalysts prepared from rationally designed porous precursors

H. Barkholtz, L. Chong, Z. Kaiser, D-J Liu
Argonne National Laboratory,
United States

Keywords: fuel cell, ORR catalyst, non-PGM, low-PGM

Summary:

Finding inexpensive and stable replacements for the platinum group metals (PGMs) has been the ultimate goal for proton exchange membrane fuel cell catalyst research. Among all the non-PGM candidates, transition metal doped nitrogen-carbon (TM-N-C) composites appear to be the most promising at present in promoting oxygen reduction reaction (ORR) at cathode. Since non-PGM catalysts are known to have lower turn-over frequency per catalytic site when compared to platinum, their active site densities must be substantially higher to deliver a comparable performance. At Argonne National Laboratory, we developed recently several new approaches using metal-organic frameworks (MOFs) and porous organic polymers (POPs) as the precursors to generate TM-N-C type of non-PGM catalyst. For example, we demonstrated that the “carbon support-free” non-PGM electrode catalysts can be prepared through zeolitic imidazolate framework (ZIF, a subclass of MOF), taking the advantages of its 3-dimensional metal-N4 coordination structure, high volumetric active site density and high surface area. Both single and binary MOF-based catalysts have been developed in our laboratory with excellent ORR activities. We also developed a method of preparing non-PGM catalyst using POPs containing high density, evenly distributed metallated N-coordination sites in the highly porous networks. POPs with both two-N coordinated and four-N coordinated transition metals were investigated which also showed excellent ORR activities after thermal activation. More recently, we have successfully incorporated the porous precursors into a nanofibrous network which improved not only the mass and charge transports, but also the catalyst durability. In this presentation, we will focus on the design, synthesis and activation strategies of several non-PGM and low-PGM catalysts and their physical/chemical properties obtained from various characterization techniques.