Integrating Non-Platinum Group Metal Catalysts into Membrane Electrode Assemblies: Chemical Interactions and Morphology of the Catalytic Layer

As an alternative to expensive Pt-based electrocatalysts, metal-nitrogen-carbon (M-N-C) and nitrogen-carbon composites based on the carbon network have been studied extensively in recent years.^{[1, 2]} There is still no clear understanding of the reaction mechanisms and well-defined chemistry of the active site/s. The lack of this knowledge limits the rational design for performance and synthesis optimization strategies. The makeup and structure of the active site/s of the non-PGM ORR electrocatalysts, including geometry (coordination) and chemistry (composition and oxidation state) remain contentious even after 50 years of research.

The most realistic method of estimating catalytic performance and durability of non-PGM catalysts is testing within a Membrane Electrode Assembly (MEA). There is a complex interplay between catalyst and ionomer within an MEA affecting its overall performance. Effect of Nafion on the catalyst layer (CL) durability on non-PGM containing systems is under-investigated and is of critical importance in advancing the technology. There is a missing link between durability and activity parameters as well as chemical and morphological changes that occur inside the catalyst layer during the oxygen reduction reaction in a fuel cell.

The relationship between activity and the amount and types of species (surface moieties) present in M-N-C materials is usually established through spectroscopic correlations with such analytical methods as XPS, XANES, XPES, TOF-SIMS, Mossbauer spectroscopy and others. 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 majority of the studies reported so far focus on building these structure-to-property correlations between the chemical signature of the catalyst itself. There is an emerging agreement that Fe and N functionalities, displayed on the surface if the carbonaceous substrate/support, govern ORR activity. The types of nitrogen and iron-nitrogen functionalities that are present in these materials are well understood as presented in Figure 1.  These consist of in-plane defects such as graphitic N and N-coordinated to three or four nitrogens and a multitude of possible edge sites such as pyridinic, pyrrolic, quaternary and Fe-N₂/Fe-N sites. In catalyst layers, however, the makeup of the active site may be affected by the interactions with ionomer. Negatively charged sulfonate group of nafion (Figure 2) may have a preferential tendency towards binding with certain nitrogen groups. Figure 3 shows high resolution N 1s spectra for M-N-C electrocatalyst synthesized from imidazole family of nitrogen containing precursors from its powder form and the catalyst layer. Due to the interaction of surface groups on the catalyst surface with sulfonate groups of Nafion, the rearrangement in high binding energy range is observed. Higher relative amount of peaks between 401-403 eV in catalyst layers is not due to higher amounts of pyrrolic, N⁺ and graphitic carbon, but is due to the shift in the position of pyridinic and nitrogen coordinated to iron peaks due to interaction with ionomer.

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. In order to address these complicating issues in obtaining accurate information on N speciation in M-N-C ORR catalysts, especially in presence of ionomer in catalyst layer, we will perform DFT calculations of binding energy shifts of N 1s spectra and use the information obtained from DFT calculations as input into curve-fitting of XPS spectra. In addition, we will use DFT calculation to evaluate the strength of the interaction between different types of nitrogen and sulfonate groups.

By combination of theoretical calculations, spectroscopic analyses and MEA testing for a set of different nitrogen precursors will we report on a set of structural metrics reflecting the properties of catalyst layer that can be used as adequate prognostic measures of activity and durability.

References:

[1] A. A. Serov, M. Min, G. Chai, S. Han, S. J. Seo, Y. Park, H. Kim, C. J Appl Electrochem, 39 (2009) 1509–1516.

[2] A. Serov, A. Aziznia, P. H. Benhangi, K. Artyushkova, P. Atanassov, E. Gyenge J. Mater. Chem. A 1 (2013) 14384-14391.