The Role of Chemistry and Morphology of Carbon in Oxygen Reduction Reaction. Structure-to-Property Studies (2015)

Development and optimization of electrocatalysts framework is 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. In Pt-based catalysis, carbon-based support is used to ensure electronic conductivity between the electrocatalyst and the current collector. Surface chemistry and structural morphology of support, catalyst and other components of catalyst layers are critical for mass transport properties, water and heat management and subsequently electrocatalytic activity and durability. As an alternative to expensive Pt-based electrocatalysts, metal-nitrogen-carbon (MNC) and nitrogen-carbon composites based on the graphene network have been studied extensively in recent years. There is still a partial understanding of the reaction mechanisms and precise chemistry of the active site or sites. There are two main hypotheses, one claiming that nitrogen functionalities on carbon-based support are directly responsible for their ORR activity and the second suggesting that nitrogen groups serve as the coordinating environment for metal where the ORR happens. Candidate structures participating in ORR consist of multitudes of nitrogen defect motifs in the carbon matrix of different degrees of graphitization with metal incorporated as metal particles as well as those linked to nitrogen defects in a variety of possible architectures. In this report, the role of carbon chemistry and morphology in durability of Pt-based electrocatalysts and activity of MNC electrocatalysts is studied by multi-technique approach combining surface chemical analysis, morphological image processing and electrochemical testing. Presented research combines the strength of several analytical and computational material sciences methods. Surface chemistry of individual building blocks is characterized by XPS. Morphology of membrane electrode assemblies (MEAs) tested under different accelerated stress test (AST) conditions is characterized by FIB-SEM. Figure 1 shows 3D volumes reconstructed from low surface area carbon (LSAC)-supported Pt electrocatalysts at the beginning of life and after two AST protocols. Increase of pores at higher upper potential limit is evident. Morphological 3D metrics such as texture parameters, roughness, porosity, connectivity as measured by Euler number and other were extracted from 3D FIB-SEM datasets. For Pt-LSAC catalysts, area porosity stays the same while connectivity decreases, while for high surface area supported catalyst, the porosity increases along with loss in connectivity between particles. Correlation of structure and catalytic activity and durability of different families of electrocatalysts for oxygen reduction is achieved through the application of principal component analysis (PCA) to spectroscopic, macroscopic and microscopic data combined with results of electrochemical measurements on set of samples with range of electrochemical activities. Figure 1