Transition-Metal Nanoparticles (Fe, Co, or Ni) Encapsulated in N-Doped Carbon as Catalysts for the Oxygen Reduction Reaction and as Gas Sensors of Volatile Organic Compounds

A chemical vapor-deposition-based pyrolysis method is used for synthesizing uniformly nitrogen-doped carbon frameworks nucleated through any of the three different transition-metal (Fe, Co, and Ni) nanoparticles. Although a fixed amount of nitrogen precursor was used during the synthesis of all the samples, the samples showed varying nitrogen concentrations in the carbon framework. The N-doping in graphitic carbon shells hugely enhanced the amount of defects, which boosted the surface area and porosity, and unveiled the active sites necessary for catalytic reaction. Among all the samples used for studying oxygen reduction reaction (ORR), the sample with Ni-embedded N-doped carbon framework exhibits the highest half-wave potential (E (1/2)) of similar to 0.886 V versus reversible hydrogen electrode (RHE) in an aqueous 0.1 M KOH solution, in comparison with the other two samples containing Fe or Co, and also with the commercially available Pt/C sample (similar to 0.854 V). This indicates that our samples can be used as efficient oxygen adsorbers under atmospheric conditions. Utilizing this advantage, we have successfully showcased our samples as sensors for the detection of various volatile organic compound (VOC) gases at room temperature. The VOCs that we used for this study include ethanol, propane-2-ol, acetone, toluene, hexane, and cyclohexane gases. Among all of the samples, the sample containing cobalt nanoparticles displayed the highest response for gas sensing. Furthermore, the response toward ethanol sensing is the highest in comparison to the other VOCs. Additionally, the response toward ethanol was examined at different relative humidity levels of 11-95% to ensure the applicability of our samples in real-time scenarios. Therefore, our samples are promising candidates for important applications, such as efficient energy conversion and gas sensing.