Abstract
The development of direct formate fuel cells encounters important obstacles related to the sluggish oxygen reduction reaction (ORR) and low tolerance to formate ions in Pt-based cathodes. In this study, electrocatalysts formed by earth-abundant elements were synthesized, and their activity and selectivity for the ORR were tested in alkaline electrolyte. The results showed that carbon-encapsulated iron-cobalt alloy nanoparticles and carbon-supported metal nitrides, characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD), do not present significant activity for the ORR, showing the same half-wave potential of Vulcan carbon. Contrarily, nitrogen-doped carbon, synthesized using imidazole as the nitrogen source, showed an increase in the half-wave potential, evidencing an influential role of nitrogen in the ORR electrocatalysis. The synthesis with the combination of Vulcan, imidazole, and iron or cobalt precursors resulted in the formation of nitrogen-coordinated iron (or cobalt) moieties, inserted in a carbon matrix, as revealed by X-ray absorption spectroscopy (XAS). Steady-state polarization curves for the ORR evidenced a synergistic effect between Fe and Co when these two metals were included in the synthesis (FeCo-N-C material), showing higher activity and higher limiting current density than the materials prepared only with Fe or Co. The FeCo-N-C material presented not only the highest activity for the ORR (approaching that of the state-of-the-art Pt/C) but also high tolerance to the presence of formate ions in the electrolyte. In addition, measurements with FeCo-N-C in the cathode of an passive air-breathing direct formate fuel cells, (natural diffusion of formate), showed peak power densities of 15.5 and 10.5 mW cm−2 using hydroxide and carbonate-based electrolytes, respectively, and high stability over 120 h of operation.
Highlights
The utilization of more efficient energy storage or conversion devices such as fuel cells, batteries, and photo-electrochemical systems are decisive for the development of a sustainable society and may help to decrease the carbon dioxide emissions to the atmosphere [1]
Considering that some of these structures were reported to be active for the oxygen reduction reaction (ORR), especially in alkaline media [23,24,37,38], the acid treatment was conducted in order to remove most of these secondary structures
One of the main challenges involved in the commercialization of fuel cells is to develop electrocatalysts for ORR that are tolerant to the presence of the fuel in the cathode
Summary
The utilization of more efficient energy storage or conversion devices such as fuel cells, batteries, and photo-electrochemical systems are decisive for the development of a sustainable society and may help to decrease the carbon dioxide emissions to the atmosphere [1]. This is an intrinsic difficulty to understand the structures that govern the activity of the ORR in this kind of materials It is reported the synthesis, characterization, and the electrocatalytic activity of electrocatalysts constituted of nitrogen-coordinated iron and/or cobalt embedded on carbon matrix (Fe, Co or FeCo-N-C) for the ORR, in alkaline media. Some nanostructures, commonly produced during the pyrolysis step, were synthesized, and their activities for the ORR were compared to that of the FeCo-N-C material It is presented the results for the ORR electrocatalysis in the presence of formate ions, in different electrolytes with different pHs, and the performance and stability of the FeCo-N-C material as the cathode electrocatalyst in passive air-breathing direct formate fuel cells
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