Bypassing Formation of Oxide Intermediate via Chemical Vapor Deposition for the Synthesis of an Mn-N-C Catalyst with Improved ORR Activity
Thomas Stracensky, Li Jiao, Qiang Sun, Ershuai Liu, Fan Yang, Sichen Zhong, David A. Cullen, Deborah J. Myers, A. Jeremy Kropf, Qingying Jia, Sanjeev Mukerjee, Hui Xu
Abstract
High Resolution Image Download MS PowerPoint Slide A significant barrier to the commercialization of proton exchange membrane fuel cells (PEMFCs) is the high cost of the platinum-based oxygen reduction reaction (ORR) cathode electrocatalysts. One viable solution is to replace platinum with a platinum-group metal (PGM) free catalyst with comparable activity and durability. However, PGM-free catalyst development is burdened by a lack of understanding of the active site formation mechanism during the requisite high-temperature synthesis step, thus making rational catalyst design challenging. Herein we demonstrate in-temperature X-ray absorption spectroscopy (XAS) to unravel the mechanism of site evolution during pyrolysis for a manganese-based catalyst. We show the transformation from an initial state of manganese oxides (MnO x ) at room temperature, to the emergence of manganese-nitrogen (MnN 4 ) site beginning at 750 °C, with its continued evolution up to the maximum temperature of 1000 °C. The competition between the MnO x and MnN 4 is identified as the primary factor governing the formation of MnN 4 sites during pyrolysis. This knowledge led us to use a chemical vapor deposition (CVD) method to produce MnN 4 sites to bypass the evolution route involving the MnO x intermediates. The Mn-N-C catalyst synthesized via CVD shows improved ORR activity over the Mn-N-C synthesized via traditional synthesis by the pyrolysis of a mixture of Mn, N, and C precursors.