Relationship of Chemical Composition, Crystalline Structure, and Morphology to ORR Performance in Transition Metal-Nitrogen-Carbon (MNC) Catalysts
Authors: Workman Michael J , Serov Alexey , Dzara Michael J. , Ngo Chilan , Gordon Jonathan , McKinney Samuel , Pylypenko Svitlana , Atanassov Plamen , Artyushkova Kateryna 
Published: February 2020

Research into alternative catalysts for the oxygen reduction reaction in PEMFCs continues to advance due to ongoing efforts to understand structure-to-property relationships in these materials. Though much work has been done on platinum group metal free (PGM-free) catalysts for the ORR, there is still a need to develop tangible relationships between synthesis methods, morphology, chemistry, and performance of these materials. One catalyst that has shown promise for both performance and durability is synthesized from precursors of iron salts and nicarbazin (Fe-NCB) using the sacrificial support method (SSM).1 Though synthesis procedures for this catalyst have been well optimized, there has not been a systematic study of structure-to-property relationships and how they relate to synthesis parameters.

In addition to fundamental properties of the catalyst itself, knowledge of how the material integrates into the electrode in an operational fuel cell is critical. To address this issue, both electrochemical testing and data from an operational fuel cell are required. Further, to be able to understand interactions between individual catalyst particles, and their interactions with ionomer in the catalyst layer, analysis of the catalyst must include measurement of chemistry and morphology both for the bulk and the surface.

To elucidate relationships between morphology, chemistry, and performance, a multivariate study on synthesis parameters and outcomes will be presented. TEM-EDS maps will be acquired and analyzed to examine nanoscale elemental distributions. Concentrations of atomically dispersed Fe with respect to particle and carbon plane edges, as well as the presence of Fe nanoparticles will be addressed. Correlations between location specific relative concentrations of Fe and N will be explored for correlations with performance, morphology, peroxide generation, and potentials for the Fe+2-Fe+3 transition. To address bulk carbon crystallite structure including stacking order, layer strain, and lateral crystallite size, a curve fitting algorithm will be applied to XRD spectra of these materials.2 Graphitic content and strain analyses will be related to both TEM observations and XPS chemical analysis. Morphology characterization will be completed by pore size analysis with nitrogen isotherms, catalyst surface analysis using the discrete wavelet transform, and particle size distributions. Relationships between catalyst structure from sub-nanometer to micron scale, elemental distributions within individual catalyst particles, electrochemical performance, and fuel cell performance will be elucidated and presented.

1. Serov, A.; Artyushkova, K.; Niangar, E.; Wang, C.; Dale, N.; Jaouen, F.; Sougrati, M.-T.; Jia, Q.; Mukerjee, S.; Atanassov, P. Nano-structured non-platinum catalysts for automotive fuel cell application. Nano Energy 2015, 16, 293-300.

2. Shi, H.; Reimers, J.; Dahn, J. Structure-refinement program for disordered carbons. Journal of applied crystallography 1993, 26, 827-836.

Figure 1