Highly Selective Toward Four- or Two-Electrons ORR on Nitrogen Doped Carbon Nano Structures in Fuel Cells for Space Applications

Human space travel requires several technological developments that support fuel generation and the energy-efficient preservation of closed systems in microgravity spaceship environments [1]. Fuel cells are promising candidates for clean energy conversion for terrestrial and space applications. The overpotential required for the Oxygen Reduction Reaction (ORR) and the degradation of the electrocatalysts are the main factors that diminish practical application of fuel cells [2]. ORR in aqueous solutions occurs mainly by two pathways: the direct four-electron reduction pathway from O2 to H2O, and the two-electron reduction pathway from O2 to hydrogen peroxide (H2O2). In fuel cell processes, the four-electron direct pathway is highly preferred. The two-electron reduction pathway is used in industry for H2O2 production [3].

Carbon nanostructures have been previously used as catalyst due to high stability and surface area, high electrical conductivity for providing electrical pathways, and mesoporous structure for the facile diffusion of reactants and by-products. Studies have revealed that carbon nanostructures and nitrogen doped carbon structures show catalytic activity in ORR [5,6]. A metal-free mesoporous nitrogen-doped carbon catalyst showed a high electrocatalytic activity, durability and selectivity toward peroxide by electrochemical converting of O₂ in a non-corrosive neutral as well as in acidic reaction medium [7,8].

Accordingly, in order to evaluate the ORR electron transfer pathway on highly stable nitrogen doped carbon nanostructures, we developed an alternative post-synthesis nitrogen doping of Vulcan and CNOs. The doping process was developed by thermal treatment in atmospheric pressure, using dicyandiamide (DCDA) as nitrogen precursor. The operational parameter conditions of the thermal reactor were a reaction temperature 700 °C, 2 mL/min of total argon gas flow, and a composition of precursors 2:1 DCDA:Vulcan and DCDA:CNOs. Our research involves increase surface area of carbon pristine source and improve its electronic structure and understanding of the nitrogen intercalation process by possible pyrolytic–nucleophilic mechanism of N-C doping reaction. The structural properties of the NVulcan and NCNOs were investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy.

NVulcan and NCNOs electrochemical characterization revealed higher performance than Vulcan and CNOs, respectively, due to N doping. Nitrogen doping in the Vulcan and CNOs enhances the electronic conductivity and specific capacitance. An analysis of the rotating disk electrode (RDE) technique data was done to evaluate the ORR kinetics, including n-values which are related to the mechanism of oxidation, at the NVulcan and NCNOs, using the Koutechy-Levich (K-L) equation. The pH Effect on Oxygen Reduction Reaction over N-doped carbon nanostructures in O₂ saturated 0.2 M Na₂2SO₄ was evaluated by a scan rate of 10 mV/s at different rotation rates: 300, 700 1100, 1500, and 1900 rpm. Our results could be evidence that the two-electrons and four-electrons transfer pathway selectivity, depend on the supporting electrolyte, i.e., pH value and nature of electrolyte, and increases to 0.2M Na₂SO₄ according to neutral (2.5 electrons) > acidic (1.4 electrons) > alkaline (3.4 electrons). Initial fuel cell tests, utilizing oxygen and RO water, showed that NVulcan and NCNOs can generate 0.30 and 0.08 w/w% peroxide concentration, respectively. The system output current was 0.20 amps for NVulcan and 0.30 amps for NCNOs. These results suggested that NVulcan performs an extremely high selectivity toward a two-electron pathway reduction process, whereas NCNOs catalyzes a four-electron route. Therefore, our approach would be promising to control of four- or two-electrons route kinetics of ORR in fuel cells for space technologies, by the supporting electrolyte, nanocarbon source and nitrogen doped nanocarbon configurations.

• NASA Strategic Plan. National Aeronautics and Space Administration, 2018, at: https://www.nasa.gov/sites/default/files/atoms/files/nasa_2018_strategic_plan.pdf.

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