Nanoporous Heteroatom-Doped Carbons Derived from Cotton Waste: Efficient Hydrazine Oxidation Electrocatalysts

In this work, cotton-based denim waste is successfully used as a precursor to synthesize nanoporous P- and N-co-doped carbon materials that can serve as efficient electrocatalysts for the hydrazine oxidation reaction (HzOR). In the synthesis, the cotton denim waste is mixed with H3PO4 in two different denim/H3PO4 (wt/vol) ratios, namely, 1:1 and 1:3, wherein H3PO4 serves as both an activating agent and a source of P dopant atoms while the indigo carmine dye present in denim serves as a source of N dopant atoms to the carbon materials. The resulting P- and N-co-doped carbon materials, named PNC1 and PNC3, respectively, are characterized by various analytical techniques. The XPS spectra show that PNC1 has ca. 2.17 atomic % P and 1.95 atomic % N whereas PNC3 has 2.54 atomic % P and 0.71 atomic % N. Pore analyses by N2 porosimetry indicate that PNC1 has a higher surface area (1582 m2 g–1) than PNC3 (486 m2 g–1), although the former has a lower mesopore volume (0.39 cm3 g–1) than the latter (0.58 cm3 g–1). The SEM images of the two materials also show some notable structural differences. The results overall indicate that the structures and compositions of the materials can be easily tailored by varying the ratio of denim/H3PO4 in the precursor. The electrocatalytic activities of PNC1 and PNC3 toward HzOR were then evaluated, and PNC3 is found to be a better electrocatalyst than PNC1. In a 100 mmol L–1 hydrazine solution in phosphate buffer saline (PBS) at pH 7.4, PNC3 electrocatalyzes the reaction at a lower peak potential (ca. 0.70 V vs RHE) than PNC1 (ca. 0.77 V vs RHE). Additionally, the current density obtained during HzOR over PNC3 is higher (by 1.29 times) than the one obtained over PNC1. Furthermore, the onset potential by which PNC3 electrocatalyzes HzOR (0.42 V vs RHE) is comparable to or better than the values reported for some of the best HzOR electrocatalysts in the literature. Besides their high electrocatalytic activity, the materials remain stable during electrocatalysis of HzOR