(Invited) Structure-to-Property Correlations for Pemfcs Electrocatalysts Based on Chemistry and Morphology of Catalysts and Catalyst Layers

Abstract

Among the different Fuel Cell technologies, one is based on the implementation of Polymer Electrolyte Membrane (PEMFC). In this tutorial we will discuss methodologies for extracting chemical structural data related to active sites of electrocatalysts from X-ray Photoelectron Spectroscopy and morphological data related to transport characteristics from focused ion beam/scanning electron microscopy (FIB-SEM) sectioning. Significant research effort has been directed towards replacement of the platinum by materials mainly consisting of metal–nitrogen–carbon (MNC) network. Heteroatomic polymeric precursors; metal macrocyclic compounds; metal salt and nitrogen/carbon organic molecules have been reported as the source for the formation of Me-N-C networks. Development and optimization of non-PGM electrocatalysts consisting of an MNC framework are hindered by the complex nature of the materials, a partial understanding of the reaction mechanisms and precise chemistry of the active site or sites. The lack of this knowledge hinders the rational design of performance optimization strategies. Moreover, the large-scale deployment of PEMFCs based on non-precious metal based catalysts is facing a major challenge: increase of the lifetime. One way to improve durability is to maintain the chemical integrity and pore structure, and hence, the effectiveness of the catalyst layer (CL) over the lifetime of the fuel cell. The activity and durability of MNC catalyst layers are directly related to the interaction of catalyst with ionomer and ionomer dispersion. Chemical properties such as composition and morphological properties of the catalyst such as porosity, roughness, and texture, will affect the interaction between the catalyst and ionomer. The pore structure is critical to the transport of oxygen to active sites and removal of water. The change in pore-structure induced by the chemical changes introduced during fuel cell operation has to be understood to design non-PGM electrocatalyst with highest possible lifetime. XPS is the workhorse of surface spectroscopic techniques for the analysis of catalyst structure due to its ability to determine surface oxidation states and chemical environment with a resolution that allows discrimination of chemical species and identification of the surface moieties. The link between features detected in the XPS spectrum and defect geometries can be challenging, and the derivation of detailed structure-to-property relationship from XPS observations alone can prove to be difficult. Identification of the structure and binding site of species in the proximity of the catalyst surface can be accomplished using shifts in binding energies. We will discuss computational and experimental approaches for deriving an accurate structural understanding of catalysts and catalyst layers. [1-4] Application of XPS as post-mortem analysis for understanding degradation and failure mechanism will be also discussed. Application of FIB-SEM and methodology to extract useful metric representations of the morphology of catalysts layers such as specific surface area, total porosity, connectivity of pores and others will be presented. [5, 6] The post-mortem analysis of tested catalyst layers will probe the evolution of morphological parameters as a result of degradation. By a combination of spectroscopic and morphological analyses and MEA testing we will introduce structural metrics reflecting the properties of catalyst layer that can be used as adequate prognostic measures of activity and durability. 1. Serov, A., et al., Nano-structured Non-Platinum Catalysts for Automotive Fuel Cell Application. Nano Energy, 2015. 16: p. 293-300. 2. Kabir, S., et al., Experimental and Computational Identification of Graphitic-N Moiety Present in Self-Supported Electrocatalysts. Surface and Interface Analysis, 2015. submitted. 3. Kabir, S., et al., Computational and experimental evidence for a new TM-N3/C moiety family in non-PGM electrocatalysts. Physical Chemistry Chemical Physics, 2015. 17(27): p. 17785-17789. 4. Artyushkova, K., et al., Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal-Nitrogen-Carbon Electrocatalysts.The Journal of Physical Chemistry C, 2015. 5. Çeçen, A., et al., 3-D Microstructure Analysis of Fuel Cell Materials: Spatial Distributions of Tortuosity, Void Size and Diffusivity. Journal of The Electrochemical Society, 2012. 159(3): p. B299-B307. 6. Ziegler, C., S. Thiele, and R. Zengerle, Direct three-dimensional reconstruction of a nanoporous catalyst layer for a polymer electrolyte fuel cell. Journal of Power Sources, 2011. 196(4): p. 2094-2097. Figure 1