Abstract
As one of the most promising platinum group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR), Fe–N–C catalysts with a high density of FeNx moieties integrated in a highly graphitic carbon matrix with a proper porous structure have attracted extensive attention to combine the high activity, high stability and high accessibility of active sites. Herein, we investigated a ZnCl2/NaCl eutectic salts-assisted ionothermal carbonization method (ICM) to synthesize Fe–N–C catalysts with tailored porous structure, high specific surface area and a high degree of graphitization. However, it was found to be challenging to anchor a high density of FeNx sites onto highly graphitized carbon. Iron precursors with preexisting Fe–N coordination were required to form FeNx sites in the nitrogen-doped carbon with a high degree of graphitization, while individual Fe and N precursors led to a Fe–N–C catalyst with poor-ORR activity. This provides valuable insights into the synthesis-structure relationship. Moreover, the FeNx moieties were identified as the major active sites in acidic conditions, while both FeNx sites and Fe2O3 were found to be active in alkaline medium.
Highlights
The oxygen reduction reaction (ORR) is a cornerstone of various electrochemical energy storage and conversion devices [1,2,3], including metal–air batteries and fuel cells
The significant amount of Pt needed at the cathode side to accelerate the sluggish ORR kinetics remains a long-standing impediment for the widespread commercialization of Proton exchange membrane fuel cell (PEMFC) and hydroxide exchange membrane fuel cells (HEMFCs) [4,5]
The catalysts obtained after pyrolysis and water-wash
Summary
The oxygen reduction reaction (ORR) is a cornerstone of various electrochemical energy storage and conversion devices [1,2,3], including metal–air batteries and fuel cells. Proton exchange membrane fuel cell (PEMFC) is a key technology to provide clean and sustainable energy conversion with potential applications in portable electronics, electric vehicles, and residential power generation. Hydroxide exchange membrane fuel cells (HEMFCs) have attracted significant attention due to the tremendous progress in developing highly conductive and stable membranes and ionomers against hydroxide attack [4]. The significant amount of Pt needed at the cathode side to accelerate the sluggish ORR kinetics remains a long-standing impediment for the widespread commercialization of PEMFCs and HEMFCs [4,5]. Extensive efforts have been devoted to reducing or eliminating the use of precious metals at the cathode side in the past decades [6,7,8,9,10]. Fe(Co)–N–C catalysts represent the most promising precious metal-free (PGM-free) catalysts, with their ORR activity approaching or Catalysts 2019, 9, 144; doi:10.3390/catal9020144 www.mdpi.com/journal/catalysts
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