High-Temperature Synthesized PGM-Free Oxygen Reduction Reaction Catalyst

Abstract

Platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) electrocatalysts have demonstrated substantial improvements in electrochemical cell and fuel cell performance in the last decade. However, the durability of these catalysts is still insufficient for the implementation of these catalysts in polymer electrolyte fuel cells (PEFCs), particularly in automotive power systems. Recently, carbon oxidation has been reported as a main cause for the destruction of ORR active sites in heat-treated metal-nitrogen-carbon (M-N-C) PGM-free catalysts (1). This result suggests that in order to improve their durability, M-N-C catalysts should be synthesized at as high a temperature as possible to maximize the graphitic content thereby making the catalyst more oxidation resistant. An optimum heat-treatment temperature for the best ORR activity of M-N-C catalysts depends on many factors, including the synthesis approach, type of precursors used, and the precursor ratio. One more factor to account for, based on the reasoning above, is the need to maximize the heat-treatment temperature without sacrificing catalyst activity. In this work we have found that for the high-temperature synthesis of active M-N-C catalysts, the most important factors are the selection of the nitrogen precursor, adjustment of the precursor ratio, and the choice of the synthesis route. Without fine-tuning of the synthesis conditions, the ORR activity of the PGM-free catalysts usually decreases when heat-treated above 900° C. By selecting an appropriate synthesis route and catalyst precursors we have been able to perform synthesis at 1100° C and obtain catalysts that show high RDE activity (E½ of 0.81 V vs. RHE, Figure 1). In this presentation, we will summarize the electrochemical and fuel cell performance of these catalysts, with particular emphasis on their durability. Acknowledgement Financial support of this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. Microscopy research was supported through a user project at ORNL’s Center for Nanophase Materials Sciences, which is an Office of Science User Facility. Reference Choi Chang Hyuck, Claudio Baldizzone, Jan-Philipp Grote, Anna K. Schuppert, Frédéric Jaouen, and Karl J. J. Mayrhofer, Stability of Fe-N-C Catalysts in Acidic Medium Studied by Operando Spectroscopy. Angew. Chem. Int. Ed. 54, 1 (2015). Figure 1