Nitrogen-Doped Carbon-Encapsulated FeCo Nanoparticles As Non-Noble Metal Cathode Electrocatalyst for Direct Formate Fuel Cells
Authors: Galiote Nelson Alexandre , Oliveira Francisca Elenice , de Lima Fabio Henrique Barros 
Published: February 2020

The widespread development of high energy density storage and conversion devices as fuel cells is hampered by the oxygen catalyst electrode, for which, in most of the cases, has the dependence of high cost noble-metals such platinum and palladium for decreasing the overpotential. In this context, the alternative is developing materials based on more abundant elements. Herein, the synthesis and the electrocatalytic activity of electrocatalysts constituted by a nitrogen-doped carbon matrix and iron and cobalt atoms, represented by FeN/C, CoN/C and FeCoN/C were investigated. According to TEM images, for all synthesized materials, the presence of a significant amount of nanoparticles embedded or encapsulated by the carbon matrix was evidenced. The results of XRD showed the patterns of the respective oxides and metallic phases, and a peak related to the alloy Co7Fe3 phase for the bimetallic material. The FeCoN/C sample, submitted to the acidic treatment, presents a total disappearance of those XRD peaks associated to the oxide phases, and the TEM images still showed the presence of the encapsulated nanoparticles. A thermal treatment of this material in NH3 atmosphere, at 950 °C, was conducted to form Fe and Co nitrides, as evidenced by XRD, and, probably, to an increase in the total amount of the nitrogen doping atoms in the carbon layer. Additional experiments of in situ XANES (X-ray Absorption Near Edge Structure), in the Fe and Co K edges, and in 0.5 mol L-1 H2SO4 or 1.0 mol L-1 KOH eletrolyte, showed a stable position of the main absorption peak as a function of the electrochemical potential (in the ORR domain: 1.0 – 0.4 V vs. RHE). This indicate that the metallic nanoparticles are protected or not in contact with the electrolyte due to the presence of the covering N-doped carbon layer, accounting for its impressive stability. Potentiodynamic measurements of oxygen reduction, carried out in O2-saturated KOH 1.0 mol L-1 electrolyte, showed that the treated bimetallic electrocatalyst presented high electrocatalytic activity, with an onset potential of 0.94V vs. RHE; only 60 mV more negative than that of the state-of-the-art Pt/C (20 wt.%) electrocatalyst. Interestingly, the ORR polarization curve for the bimetallic material was a combination of the curves for the individual FeN/C and CoN/C electrocatalysts, showing a synergistic effect between Fe and Co atoms, for the ORR. In addition, the oxygen reduction reaction was conducted in the presence of formate ion dissolved in the electrolyte at different concentrations. Impressively, the FeCoN/C electrocatalyst was totally tolerant to formate, with identical ORR curves in the presence and in the absence of formate ions in the electrolyte. This fact shows that the N-doped carbon-encapsulated FeCo nanoparticle electrocatalyst is a promising candidate to substitute noble-metals electrodes in direct formate fuel cells, owing to its high stability and tolerance to the presence of the fuel in the electrolyte. Acknowledgements

The authors gratefully acknowledge financial support from FAPESP (2016/06197-9, 2013/16930-7, and 2016/13323-0) and CNPq.