Bone char-derived metal-free N- and S-co-doped nanoporous carbon and its efficient electrocatalytic activity for hydrazine oxidation

Bone char (BC) was successfully used, for the first time, both as a self-template/a pore-former and a precursor of heteroatoms (N and S atoms) during carbonization of sucrose, allowing for the synthesis of nanoporous N- and S-co-doped carbon (NSC) material possessing high surface area and excellent electrocatalytic activity. BC's ability to help with the formation of nanopores in the carbon material was indirectly confirmed by making a control material, denoted as pyrolyzed sucrose or PS, under the same condition but without including BC in the reaction media. N₂ gas porosimetry showed that NSC had a very large BET surface area (1108 m² g⁻¹), which is about 60% higher than that of PS (443 m² g⁻¹). Comparison of the SEM images of the two materials also indicated some differences in their textural and morphological features. XPS analysis showed that NSC had a higher content of S (2.29%) than PS (0.21%) and that the S atoms were distributed mostly in the form of thiophenic moieties (32.3% for the PS and 59.2% for the NSC). Although some of the S groups were originated from sulfuric acid, which was used for the dehydration of sucrose during the synthesis of the materials, this result indicated that BC was the major source of the S dopant atoms in NSC as well as the major reason for the formation of thiophenic groups in this material. Furthermore, while PS's structure did not have N dopants, NSC's lattice had about 1.39% of N dopant atoms that existed in the form of pyridinic, pyrrolic and graphitic groups and that were also originated from BC. X-ray diffraction and Raman spectroscopy revealed that NSC's lattice had a higher density of defects than PS. Owing to its high surface area and optimal density of heteroatom dopant groups and defect sites, NSC exhibited excellent electrocatalytic activity toward the hydrazine oxidation reaction (HzOR), or the lowest overpotential ever reported for this reaction, along with a high current density. Besides making it among the most efficient electrocatalysts for HzOR, its electrocatalytic performance can make this metal-free material a good alternative to the conventional metal-based electrocatalysts that are commonly used in HzOR-based fuel cells.