Nitrogen/sulfur co-doped porous carbon nanosheets and its electrochemical performance

Abstract

Porous carbon materials have aroused extensive interest in the field of energy conversion and storage due to their high surface area, regulatable pore structure, high electrical conductivity and stability, and good electrochemical activity. Nevertheless, granular porous carbons usually result in the relatively long electrolyte-diffusion pathway, which seriously limits the ions transport and then damage the electrochemical performance. Two-dimensional (2D) carbon materials can solve this problem because they can provide short electrolyte-diffusion channel and realize the fast electron transport. On the other hand, dual-heteroatom codoping has been confirmed to be quite an effective approach to improving the electrochemical performance of carbon materials. Therefore, a simple and efficient synthesis of co-doped 2D porous carbon materials is highly attractive.<br/>In this work, nitrogen/sulfur co-doped porous carbon nanosheets (NSPCNs) are prepared from methyl orange (MO) doped polypyrrole (PPy) nanotubes by a thermal-treating process in the presence of KOH under N<sub>2</sub> atmosphere. MO-doped PPy nanotubes are prepared through a self-degraded process by using MO-FeCl<sub>3</sub> complex as the template initiator. In the thermal process, the combination of the dedoping derived from the interaction between MO and KOH, the pyrolysis of PPy, and KOH activation results in the exfoliation of PPy nanotubes and the formation of NSPCNs. Scanning electron microscopy and transmission electron microscopy analyses demonstrate that as-prepared NSPCNs interconnect to form a hierarchical porous architecture containing micropores, mesopores, and macropores, which provides the three-dimensional interconnected channel for electrolyte diffusion with little hindrance. The N<sub>2</sub> sorption measurements indicate that NSPCNs have a high specific area of 1744.8 m<sup>2</sup>/g and volume of 1.01 cm<sup>3</sup>/g. The X-ray photoelectron spectroscopy measurements indicate that nitrogen and sulfur have been incorporated into the framework of the as-prepared carbon sample. The doped nitrogen is present in the form of pyridinic, pyrrolic, and quaternary state, and the doped sulfur appears in the form of C-S<sub><i>n</i></sub>-S and-SO<sub><i>n</i></sub>-configuration. The synergistic effect of co-doped nitrogen and sulfur promote the redistribution of spin and charge density, which can greatly enhance the surface wettability and increase the electrochemical active sites of carbon materials. These features endow as-prepared NSPCNs with excellent electrochemical properties. Electrochemcial impedance spectroscopic measurements indicate that the charge transfer resistance of NSPCN in polysulfide electrolyte is 11.2 Ω·cm<sup>2</sup>, suggesting a very high electrocatalytic activity of NSPCNs for regenerating the polysulfide electrolyte. Under the illumination of 100 mW/cm<sup>-2</sup>, the NSPCNs' electrode-based quantum dot-sensitized solar cell achieves a conversion efficiency of 4.30%, which is comparable to that of the PbS electrode-based cell. Furthermore, NSPCNs display excellent capacitive performance. In 6 M KOH aqueous electrolyte, NSPCNs achieve a high specific capacitance of 312.8 F/g at a current density of 0.4 A/g. Even the current density increases to 20 A/g, the NSPCNs still maintain a specific capacitance of 200.6 F/g, indicating a good rate performance. Therefore, the as-prepared NSPCNs can be used as the high-performance electrode materials for quantum-dot sensitized solar cells and supercapacitors.