Advanced Microscopy of Non-Precious Metal Catalysts for Structure-Property-Performance Correlations

Abstract

It is necessary to develop renewable and environmentally benign energy systems to offset the impact of human development on the environment. Polymer electrolyte membrane fuel cells (PEMFCs) can be a transformative technology but they are costly, with the Pt-based catalyst contributing most to the price of a PEMFC.1,2 Developing earth-abundant non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR) can significantly reduce the cost of PEMFCs. Advances in the performance of NPMCs have been made, however the connection between chemical and morphological properties and electrochemical performance are still not well understood.3 ,4 Difficulties in elucidating the exact chemical environment and morphology of highly active catalysts is largely due to their heterogeneous nature.1,2 A particularly promising NPMC catalyst was synthesized from iron nitrate and nicarbazin precursors using a sacrificial support method.5 By varying several parameters in this synthesis process, a set of iron containing, nicarbazin derived (Fe-NCB) catalyst materials were prepared with slight variations in chemical and structural properties. In order to deconvolute the contribution of different properties to electrochemical performance, measured in rotating disk electrode (RRDE) and membrane electrode assemblies (MEAs) a study spanning multiple sophisticated characterization techniques was employed. Transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDS) mapping were used to investigate the morphology of Fe-NCB catalysts and the relative distribution of nitrogen and iron species within the porous carbon network. Figure 1 shows a high-angle annular dark field (HAADF) image and variation in N/Fe ratio as determined by area-averaged EDS for one of the investigated Fe-NCB catalysts. This data was correlated to surface elemental composition and chemical speciation information from X-ray photoelectron spectroscopy (XPS), and by combining these techniques with electrochemical performance, the contributions of various active sites were investigated. The effect of different levels of porosity on the distribution of Nafion® ionomer and its effects on Fe-NCB catalyst performance were also studied. While TEM/EDS analysis conducted in ultra-high vacuum yields important information about a material’s properties, it is limited as compared to the impact of characterization conducted in a realistic operating environment. To gain better understanding of composition-morphology-performance correlations, a series of Fe-NCB catalyst materials was investigated using an in-situ/in-operando TEM cell for electrochemical studies. In-situ/in-opreando observations of material changes under relevant conditions (in the presence of electrolyte, applied potential) add another layer of understanding to the material properties and provide direction for further improvements in the catalyst composition and structure.