Characterization of Flame Synthesized Metal and Nitrogen Doped Nanocarbons for Oxygen Reduction Reaction

Abstract

Carbon based electrocatalysts have shown demonstrable potential to replace platinum-based Oxygen Reduction Reaction (ORR) catalysts in low temperature fuel cells [1, 2]. The presence of a graphitic nano-shell surrounding the primary carbon particle and the formation of disordered sites in graphene through doping by nitrogen atoms, have been associated with activity towards the reduction of oxygen [3]. In addition, the introduction of transition metal atoms results in the formation of M-Nx moieties and carbon encapsulated metal-rich nanoparticles, drastically increasing the ORR activity [4]. It has also been reported that the synthesis process significantly affects the catalytic activity and performance in fuel cell electrodes of the doped carbon catalysts [1]. Therefore, the influence of the synthesis process and the effect of dopant atoms on the morphological and physico-chemical characteristics of metal, nitrogen doped carbon deserves a more detailed analysis in order to understand the correlation between process parameters and catalytic activity. This study looks at the morphological, chemical, structural and electrochemical properties of doped nanocarbons with special emphasis on Fe, N doped carbon (Fe-N-C) electrocatalysts. A novel flame-based synthesis technique, capable of generating partially graphitized carbon black, with a primary particle size ranging from 10 to 15 nm is also discussed in relation to the catalyst synthesis process (figure 1 a-c). Metal and nitrogen doped nanocarbon ORR catalysts were synthesized by a modified flame spray pyrolysis-based deposition technique called Reactive Spray Deposition Technology (RSDT) [5]. The carbonaceous material was synthesized directly from atomized organic solvents and precursors by pyrolysis at flame temperatures exceeding 1500°C. A sheath of nitrogen gas surrounding the flame and controlled oxygen flow rates were utilized to control the flame equivalence ratio and ensure incomplete combustion of the organic material resulting in the formation of carbonaceous material. The levels nitrogen and metal doping were controlled by selecting the concentrations of nitrogen and metal precursors during RSDT deposition. The structure of the carbon was analyzed using a combination of X-Ray Diffraction (XRD), Raman spectroscopy and high resolution Transmission Electron Microscopy (TEM) as shown in figure 1 d-f. XRD and Raman results indicated the presence of partially graphitized carbon [6] with highly disordered structure resulting from the incorporation of the dopant atoms into the graphene layers, which was correlated to an increase in ORR activity. Thermogravimetric Analysis (TGA) revealed the presence of amorphous and perhaps highly functionalized carbon resulting from the partial decomposition of organic precursors, suggesting further optimization of the synthesis process is required. X-Ray Photoelectron Spectroscopy (XPS) was used to analyze the near surface composition, while the electrochemical activity towards ORR was measured by Rotating Disk Electrode (RDE) in alkaline electrolyte. The effect of dopant atoms, including the influence of different transition metal atoms, on the physical and chemical characteristics of carbon particles is also discussed. Figure 1: (a) Photograph of RSDT deposition process for synthesizing doped nanocarbons for catalytic applications, (b) schematic and (c) BF TEM image showing structure of Fe, N doped carbon aggregates. The chemical and structural characterization of the primary particles using (d) XPS and (e) HRTEM, and (f) Raman spectroscopy, highlighting the influence of dopant elements Fe and N in controlling the structure of RSDT synthesized carbon.