Enhanced Electrocatalytic Oxygen Reduction Performance of Differently Optimized S,N Heteroatom Dual-Doped Carbon-Encapsulated Iron Carbide–Carbon (Fe3C@C-SN) Nanostructures

In this study, we present a pyrolytically derived iron-based nonprecious metal catalyst (NPMC), Fe3C embedded in heteroatom (S,N)-codoped carbon matrix, and explored it as a potential NPMC for oxygen reduction in alkaline media. The as-prepared catalysts are well characterized for their structure, crystallite size, morphology, different bonding states of the dopants, and defect levels in the carbon matrix. The optimization is performed for ideal reaction temperature and dopant amounts in Fe3C@C nanostructures. From the electrochemical study, it is found that among the different variants, the sample prepared at a temperature of 800 °C with 20 wt % dopant, i.e., Fe3C@C-SN/25-800, shows a more positive onset potential (Eonset) of 0.844 V (vs reversible hydrogen electrode (RHE)) and a low half-wave potential (E1/2) value of 0.670 V. It also shows good long-term oxygen reduction reaction (ORR) stability and methanol tolerance in a 0.1 M KOH aqueous electrolyte. The measurement of intrinsic parameters, double-layer capacitance (Cdl), and charge transfer resistance (RCT) values validate the current–voltage profile of the samples. The major active sites are identified as Fe–Nx and Nx–C in the nanostructures. Fe3C@C-SN/25-800 also exhibits considerable oxygen evolution reaction (OER) activity among its variants and requires a potential difference (ΔE = E1/2(ORR) – EJ=10 mA cm–2 (OER)) of 0.980 V for overall oxygen electrochemistry. The best electrocatalytic activity can be attributed to the combination of several factors, namely, chosen reaction temperature, dopant concentration, better graphitization, and the presence of a high amount of heteroatoms suitably aligned in the carbon matrix (pyridinic-N, thiophenic-S, etc.) that synergistically enhance the overall performance.