Substantial Role of Nitrogen and Sulfur in Quaternary-Atom-Doped Multishelled Carbon Nanospheres for the Oxygen Evolution Reaction

The synergy between nitrogen (N) and sulfur (S) in quaternary heteroatom-doped carbons is rarely probed, although these elements can significantly alter the performance of the oxygen evolution reaction (OER). Herein, quaternary heteroatom (N, S, P, O)-doped multishelled carbon (NSPO-C) nanospheres are synthesized from heteroatom-containing poly(cyclotriphosphazene-codioxo-thiane) (PCD) polymer nanospheres. The contents of these quaternary heteroatoms were controlled via a facile carbonization process. The OER performance was tested, which was found to be related to the N and S contents, and the as-prepared NSPO-C-8 nanosphere anode with optimized contents of N (2.76 wt %) and S (1.52 wt %) showed a maximum OER activity, that is, it required a very low overpotential of 339 mV to obtain a current density of 10 mA cm^-2 with a low Tafel slope value (39.40 mV dec^-1), which is much lower than its conventional RuO₂ (401 mV), 20% Pt/C (566 mV), and PO-C nanosphere (452 mV) counterparts. Higher performance is attributed to the synergy between N and S in the NSPO-C nanospheres, which provides maximum exposure to electroactive sites, while special morphology ensures efficient pathways for fast charge transportation. These findings advocate that polyphosphazene-derived heteroatom-doped carbons are potential candidates to fabricate high-performance devices for water oxidation.