Synthesis and characterization of iron and nitrogen co-doped carbon catalyst for oxygen reduction

Abstract

Global pollution and global warming (climatic change) predominantly result from over dependence on fossil fuels. This has been exacerbated by increasing demand for energy driven by rising global population and industrialization. Renewable energy sources can reduce the environmental impact of fossil fuels and other non-environmental systems, thus drawing global interest. Due to their low CO2 emissions, fuel cells and metal-air batteries are preferred energy systems to replace fossil fuels. But the slow Oxygen Reduction Reaction and Oxygen Evolution Reaction in fuel cells and metal air batteries during discharge and charging make these technologies difficult. For fuel cells and metal air batteries to advance, efficient and cost-effective electrocatalysts are needed for the ORR half-reaction. This study developed an efficient and cost-effective iron and nitrogen co-doped carbon electrocatalyst as an alternative to platinum-based ORR catalysts. The catalyst was prepared pyrolysing caffeine and egg albumen as the nitrogen-rich biomass source, graphite powder as a support matrix, and Iron (III) chloride hexahydrate as the Fe source and pyrolyzing at 850 oC. The catalysts, OSS1 for egg albumen and OSS2 for caffeine, were characterised using X-ray diffraction and SEM with EDX. The synthesised catalysts' electrocatalytic characteristics were examined using Rotating Disc Electrode in oxygen saturated in 0.1 M KOH in a three-electrode cell configuration with silver electrode as the reference electrode. The catalyst prepared with caffeine as the nitrogen source (OSS2) had the most promising ORR activity, with a higher kinetic current of 2.189 mA cm-2 at -0.466 V and a selectivity of oxygen reduction to water via hydrogen period pathway (n = 3.24) predominantly via the 4-electron pathway, Tafel slope of 114.650 mVdec-1, and exchange current density of 0.459 mAcm-2. OSS1 had a slightly lower kinetic current of 1.609 mA cm-2 at -0.433 V. Finally, Graphitic Carbon showed a reduced kinetic current of 1.007 mA cm-2 at -0.290 V, selectivity of oxygen reduction to water via peroxide pathway (n = 2.38), Tafel slope of 95.920 mVdec-1, and exchange current density of 0.354 mAcm-2. The electron transfer number of the catalysts differed significantly (p = 0.001), indicating that the nitrogen supply significantly affects ORR activity. Thus, the results demonstrate the impact of biomass-derived nitrogen precursor characteristics on Fe-N-C catalyst for oxygen reduction.