Resolving Active Sites and Porosity in PGM-Free Catalysts By Electron Microscopy

Abstract

Electrodes based on platinum group metal-free (PGM-free) catalysts are under development as an alternative to costly Pt-based cathodes for automotive fuel cell systems.[1,2] Novel PGM-free catalysts with much improved activity and durability must be developed in order to become competitive with their PGM nanoparticle counterparts for transportation applications. Analytical characterization by transmission electron microscopy plays an important role in identifying electrochemically active sites at the atomic scale as well as to exploring electrode morphology and porosity at the microscale. In this work, a hybrid PGM-free catalyst developed at Los Alamos National Laboratory was studied by multiscale and multidimensional electron microscopy methods. This promising catalyst, branded CM-PANI-Fe-C, has been synthesized by heat-treating two nitrogen precursors, polyaniline (PANI) and cyanamide (CM). The catalyst is comprised of fibrous carbonaceous agglomerates intermixed with few-layer graphene sheets. Low voltage aberration-corrected scanning transmission electron microscopy (STEM) and atomic resolution electron energy loss spectroscopy (EELS) were utilized to identify single Fe atoms coordinated with nitrogen within the few-layer graphene sheets (Fig. 1a,b). These atomic features have been identified by modeling as a possible active site which exhibits high four-electron selectivity. The thick electrode structures required to accommodate high non-PGM loadings also play a critical role in fuel cell performance. Hierarchal pore structures (spanning the micro to macro) are necessary for maintaining high mass transport through the thick electrodes. Complementary imaging and compositional mapping by energy dispersive X-ray spectroscopy (EDS) has been utilized to study the bulk electrode structures, as shown in Fig. 1c. These analytical studies will be extended to understand the role of different process parameters on catalyst and electrode structures at multiple length scales. References G. Wu, K. L. More, C. M. Johnston, P. Zelenay, Science 332 (2011) 443.Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, Energy Environ. Sci. 4 (2011) 3167. Acknowledgements Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE), and through a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. Figure 1