Electrochemical and Spectroscopic Characterization of Non-Precious Metal Fe-N-C ORR Catalysts Synthesized By Direct Flame Spray Pyrolysis
Authors: Poozhikunnath Abhinav , Yu Haoran , Bonville Leonard J. , Maric Radenka 
Published: February 2020

The electrocatalyst is one of the major components of low temperature Proton Exchange Membrane (PEM) fuel cells, and despite being the focus of research and development for several decades, platinum nanoparticles supported on high surface area carbon remain the state-of-the-art for both acid and alkali based fuel cells. A major obstacle to the commercialization of PEM fuel cells is the cost associated with the use of expensive Pt group electrocatalysts on both the anode and cathode. DOE projections estimate that in high volume production, the cost of platinum alone could account for about 50% of the total cost of fuel cell power systems [1, 2]. There have been many attempts to alleviate this problem by reducing platinum loading on both electrodes. However, progress in reducing stack cost by reducing the platinum content has been offset by the increasing global price and limited availability of platinum. In addition, due to the sluggish oxygen reduction kinetics, the platinum loading on the cathode side when using pure hydrogen as a fuel source, is at least 3-5 times more than the loading on the anode side. Consequently, there is considerable interest in developing Pt-free Oxygen Reduction Reaction (ORR) catalysts that are cheaper to manufacture and more readily available than their Pt-based counterparts [3].

Nitrogen doped graphitic carbon is a promising candidate for Pt-free electrocatalysts. The incorporation of transition metals such as Fe or Co results in the formation of a metal-nitrogen coordination (M-Nx, where x is usually 2 or 4) which has been found to greatly enhance activity of non-precious metal catalysts towards oxygen reduction. M-N-C catalysts are typically synthesized through the pyrolysis of metal, nitrogen and carbon containing precursors in an inert or ammonia atmosphere between 800-1000°C followed by additional acid leaching and heat treatment steps [4-6]. The performance and durability of the catalyst under fuel cell operation conditions is dependent on the pore size distribution and active site density, both of which are significantly influenced by the synthesis process.

Fe-N-C catalysts were synthesized for this study by a modified flame based deposition technique called Reactive Spray Deposition Technology (RSDT). The catalyst material was produced through the incomplete combustion of an atomized spray composed of anhydrous FeCl3 and cyanamide dissolved in a xylene-methanol-propane precursor mix. A nitrogen-gas sheath was also employed around the flame to reduce the entrainment of air into the flame. The oxygen lean conditions in the flame resulted in the formation of carbonaceous material due to the incomplete combustion of the organic precursors. XRD and Raman results indicated the presence of partially graphitic carbon with a disordered structure. The RSDT synthesis technique resulted in a morphology with mesoporous and macroporous features, as evidenced by SEM imaging and BET measurements (figures 1a and 1b). TGA analysis of the catalyst revealed the presence of amorphous carbon and soot mixed in with the graphitic material, suggesting further optimization of the synthesis process may be required. XPS analysis was performed to analyze the nature of the N-C bonding and detect the presence of the catalytically active Fe-Nx moieties (figure 1c). Electrochemical analysis of the catalyst will be conducted using Rotating Disk Electrode to measure the ORR activity and stability in acid medium. MEAs prepared from the Fe-N-C catalyst will be used to study the performance and durability under actual fuel cell operation conditions.

Figure 1: (a) Secondary electron SEM image, and (b) BET analysis results of Fe-N-C catalyst synthesized by RSDT. (c) High resolution XPS scan of RSDT synthesized catalyst showing N1s peak along with table listing fraction of area under fitted peaks.

References

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Figure 1