Investigation of Model N-C and Fe-N-C Oxygen Reduction Catalysts Under in Situ Conditions

Abstract

Developing commercially competitive polymer electrolyte membrane fuel cells (PEMFCs), requires reducing their cost, of which the most expensive component is the platinum group metal (PGM) oxygen reduction reaction (ORR) catalyst. Several approaches to cost reduction of the ORR catalyst have been investigated, including reduced Pt loading by creating novel Pt nanostructures and bimetallic catalysts, and replacing PGM catalysts with earth-abundant, inexpensive materials. High surface area carbons modified with nitrogen (N-C) and iron (Fe-N-C) are a promising PGM catalyst alternative. However, due to the highly heterogeneous nature of these catalysts the true nature of their most active site(s) is still debated despite decades of research. [1,2] It is crucial to identify what species are contributing to the ORR in order to elucidate synthesis-property-performance correlations, which can guide the rational design of high performance PGM-free ORR electrocatalysts. Characterizing Fe-N-C ORR catalysts with techniques such as scanning transmission electron microscopy (STEM), energy dispersive x-ray spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS) provides valuable structural and chemical information. Combination of these techniques is an effective approach for synthesis-property correlations, however such ex situ techniques have significant limitations imposed by vacuum requirements. Observation of adsorbates, intermediates, and products during ORR steps are possible when using in situ characterization, providing further insight into active species. Ambient pressure XPS (AP-XPS) and x-ray absorption spectroscopy (XAS) have been used to show changes in N and Fe species in a humidified O2 environment, at an elevated temperature, and with applied potential. [3,4] The best performing catalysts reported in the literature typically require multi-step preparation methods and by nature are quite heterogeneous. Development of model catalyst materials with controlled morphology and speciation reduces the parameter space that must be considered in elucidating synthesis-property-performance correlations. Analysis of these model N-C and Fe-N-C materials with advanced in situ characterization methods can significantly improve interpretation of the data from more heterogeneous materials, yielding direct evidence of a chemical species’ participation in the ORR. In this work, N-C nanospheres with high graphitic content and micro-porosity were first synthesized by a solvothermal treatment of resorcinol, formaldehyde, and ethylenediamine, and a subsequent pyrolysis under flowing nitrogen. Modification of the N-C nanospheres with Fe was accomplished via wet-impregnation of various Fe precursors followed by a second N2 pyrolysis. By varying the volume of ethylenediamine, the Fe precursor, and the pyrolysis temperatures, a set of N-C and Fe-N-C nanospheres with diverse properties were produced. The changes in materials with synthesis conditions were evaluated using STEM-EDS and XPS, demonstrating control over N and Fe speciation and quantity. Select N-C and Fe-N-C nanospheres were then characterized with AP-XPS, and in the case of Fe-N-C nanospheres, in situ XAS. Measurements of XP spectra and the Fe L2,3 edge were conducted at 100 mTorr of O2 and 200 mTorr of 1:1 O2:H2O at temperatures of 60 °C and 80 °C. Resulting spectra for model N-C and Fe-N-C nanospheres are shown, highlighting the changes in N 1s features (Figure 1a-b) from AP-XPS, and change in Fe valency from XAS (Figure 1c). By understanding the ORR on these model Fe-N-C nanospheres, synthesis-property-performance conclusions can be drawn, guiding the development of highly active Fe-N-C catalysts. (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) Mamtani, K.; Singh, D.; Tian, J.; Millet, J. M. M.; Miller, J. T.; Co, A. C.; Ozkan, U. S. Evolution of N-Coordinated Iron-Carbon (FeNC) Catalysts and Their Oxygen Reduction (ORR) Performance in Acidic Media at Various Stages of Catalyst Synthesis: An Attempt at Benchmarking. Catal. Letters 2016, 146, 1749–1770. (3) Artyushkova, K.; Matanovic, I.; Halevi, B.; Atanassov, P. Oxygen Binding to Active Sites of Fe–N–C ORR Electrocatalysts Observed by Ambient-Pressure XPS. J. Phys. Chem. C 2017, acs.jpcc.6b11721. (4) Jia, Q.; Ramaswamy, N.; Hafiz, H.; Tylus, U.; Strickland, K.; Wu, G.; Barbiellini, B.; Bansil, A.; Holby, E. F.; Zelenay, P.; et al. Experimental Observation of Redox-Induced Fe-N Switching Behavior as a Determinant Role for Oxygen Reduction Activity. ACS Nano 2015, 9, 12496–12505. Figure 1