Boron and nitrogen co-doped porous carbon and its enhanced properties as supercapacitor

Abstract

Boron and nitrogen co-doped porous carbons (BNCs) were prepared through a facile procedure using citric acid, boric acid and nitrogen as C, B and N precursors, respectively. The BNC samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and nitrogen sorption at 77 K. Cyclic voltammetry and galvanostatic charge/discharge experiments were adopted to investigate their electrochemical behaviors. The BNC-9 and BNC-15 samples with high specific surface areas of 894 and 726 m² g⁻¹ showed the large specific capacitance up to 268 and 173 F g⁻¹, respectively, with the current of 0.1 A g⁻¹. When the current was set as 1 A g⁻¹, the energy densities were 3.8 and 3.0 Wh kg⁻¹ and the power densities were 165 and 201 W kg⁻¹ for BNC-9 and BNC-15, respectively. Thus, BNC-15 is more suitable to apply in high-power-demanded occasion, while BNC-9 tends to store more energy.

Introduction

Electrical double-layer capacitors (EDLCs), based on charge stored and released along the double layer which forms at the electrode/electrolyte interface, have attracted a great attention in recent years, since they possessed a high power density, quick charge/discharge rate and long cycle-life [1], [2], [3]. Carbon materials can be promising candidates for supercapacitor applications because of their chemical stability, low-cost, fine conductivity and kinds of existing forms [2]. Porous carbon materials are interesting due to their high surface area which could be of great importance for high specific capacitance. Various methods were adopted to synthesize porous carbons, including traditional methods [4] such as chemical activation, physical activation, and combination of the physical and chemical activation processes [5], [6], [7], carbide-derived carbons [8], [9], and newly developed template-synthesized mesoporous/microporous carbons. It is known that the capacitance of carbon materials is closely related to their pore structure and texture. Recently, Gogotsi et al. [8], [10] investigated the relationship between the pore sizes of the carbon electrodes and ion sizes of the electrolyte for electrical double-layer capacitors (EDLCs). Their results are in conflict with the traditional attitude toward inaccessibility of small pores to solvated ions and indicate that the micropores largely contribute to the capacitance. Cheng et al. [11] synthesized a kind of porous carbon (HPGC), which combines macroporous cores, mesoporous walls and micropores. The special hierarchical structures lead to fine capacitance retention at a high sweep rate, e.g., the HPGC have a superior frequency response. Besides the influences from the pore structure, the texture of materials can also change the capacitor behaviors. In order to enhance the capacitance, heteroatoms were doped into carbon materials, which not only strengthen the wettability of the interface between electrolyte and electrodes, but also introduce pseudocapacitive effects. Recently, various porous carbon materials, including nitrogen-enriched carbons were massively synthesized. Béguin et al. [12] reported zeolite-templated synthesis of microporous nitrogen-doped carbon, whose specific capacitance was larger than that of nitrogen-free porous carbon with the similar specific surface area and median pore diameters. Zhao et al. [13] obtained nitrogen contained porous carbon by the pyrolysis and carbonization of melamine–formaldehyde resin. Their results showed that the moderate nitrogen content can enhance the surface wettability and reduce the resistance.

Several works have been reported on boron and nitrogen doping carbon materials applied in the fuel cell and Li ion battery [14], [15]. And it was reported that the boron doping may improve the specific capacitance per surface area for the multi-walled carbon nanotubes [16]. But their experimental conditions are strict because ultrahigh temperature was necessary for diffusion of the boron atoms into carbon frameworks. Some of the approaches needed moiety-free environment to prevent the boron and carbon precursor from hydrolysis [17]. Besides, our boron dopant quantities are higher than that of the reported materials [14], [16], [17]. In this study, we reported a facile approach to prepare boron and nitrogen co-doped porous carbons through carbonization of the gel containing boron and carbon precursors. The obtained boron and nitrogen enriched carbon materials (denoted as BNC) showed prominent capacitances. These synthetic process avoided toxic carbon precursors such as acetonitrile, formaldehyde, etc. Furthermore, this method has the advantage over the template-synthesized carbons which involving awkward and costly preparation of mesoporous silica or zeolites. The texture and pore structure of the materials can be tuned, whose influences on the capacitive behavior will be discussed in this paper.