Characterization and evaluation of Fe–N–C electrocatalysts for oxygen reduction directly synthesized by reactive spray deposition technology

Abstract

A novel method for synthesizing iron–nitrogen–carbon (Fe–N–C) electrocatalysts using a modified flame spray pyrolysis technique called reactive spray deposition technology (RSDT) is described. The physicochemical properties of the RSDT-synthesized catalyst are quantified through a series of experiments including nitrogen adsorption, electron microscopy, thermogravimetry, spectroscopy and electrochemical analysis with the overall aim of exploring opportunities to use RSDT as a single-step, scalable alternative to multi-step, energy-intensive furnace-based methods for synthesizing PGM-free electrocatalysts for oxygen reduction. The Fe–N–C is synthesized by pyrolyzing a liquid solution precursor mixture under oxygen lean conditions without the use of additional support material or heat treatment steps. Properties of critical importance to the performance of the Fe–N–C catalyst are discussed in detail with respect to properties of similar catalysts synthesized by multi-step methods reported in the literature. Materials characterization shows evidence of ORR active Fe–Nx sites, a high fraction of pyridinic nitrogen and carbon-encapsulated iron-rich particles. The existence of undesired amorphous carbon mixed with the catalytically active material is also observed and may require process development in future to remove. Rotating disk electrode analysis in alkaline media of the RSDT-synthesized Fe–N–C confirmed catalytic activity toward oxygen reduction, which is shown to follow a two-step reaction mechanism. While the activity of the Fe–N–C catalyst is lower than that of commercial Pt/C, it shows superior stability with a decrease in half-wave potential of only 5 mV after 4000 cycles in alkaline media, encouraging further investigation of this alternative flame-based synthesis route.