Elsevier

Electrochimica Acta

Volume 274, 1 June 2018, Pages 389-399
Electrochimica Acta

Structural engineering of N/S co-doped carbon material as high-performance electrode for supercapacitors

https://doi.org/10.1016/j.electacta.2018.04.084Get rights and content

Highlights

  • The structure engineering strategy with a rationally design is used to prepare the N/S co-doped ordered hierarchical carbon (NSHPC).

  • The obtained NSHPC possesses a high specific surface area of 2711.7 m2 g−1.

  • The capacitive performance of the electrodes is optimized by modifying the synthetic procedure and tuning the pores distribution.

  • The supercapacitor based on NSHPC-1:1 shows superior capacitance performance and excellent rate capability.

Abstract

Nitrogen and sulfur co-doped carbon (NSC) is considered as a promising electrode material for supercapacitors (SCs). However, the specific capacitance and the rate performance of NSC are not satisfied for practical applications due to its inappropriate structural properties. Herein, we report the structural engineering of NSC by optimizing pore structure to obtain an ordered hierarchical pore network. The as-prepared NSC possesses a high gravimetric capacitance of 322 F g-1 at the current density of 1.0 A g−1 and retains 206 F g−1 (64% of its capacitance at 1.0 A g−1) at the high current density of 100 A g−1. Moreover, it reveals an excellent cycling performance with ∼93.4% capacitance retention after 10000 cycles at the current density of 20 A g−1. An assembled symmetric supercapacitor (SC) shows a remarkable energy density of 11.4 Wh kg−1 and a high power density of 12.9 kW kg−1. The outstanding performances of NSC electrodes benefit from the efficient ion diffusion and fast charge transfer. This work paves a way to rationally design and engineer the structure of NSC for achieving both high rate capability and high specific capacitance for SCs applications.

Introduction

Supercapacitors (SCs) are one of the most advanced energy storage devices for portable devices, electric vehicles and stationary energy storage due to their high power density, fast charge/discharge rate, and long cycling life [1,2]. According to energy storage mechanisms, SCs can be categorized into pesudocapacitors and electrical double layer capacitors (EDLCs). For the pesudocapacitors, they store energy via reversible Faradaic reactions at the active material's surface, such as transition metal oxides [3,4], conductive polymers [5], transition metal sulfides [6,7], binary metal oxides [8], noble metal based materials [9] and metal organic frameworks [10] etc. However, pesudocapacitors based on metal derivatives and conductive polymers usually suffer from poor stability and rate capability, thus limiting their applications [11]. As an alternative, EDLCs that storage energy through electrostatic adsorption of ions at the electrode/electrolyte interface can charge-discharge very fast with excellent cycling stability [12]. Porous carbon materials, for example, activated carbon [13], carbon fibers [14], mesoporous carbon spheres [15], hollow carbon nanospheres [16], graphene [17] and carbon nanotubes [18] have been selected to construct EDLCs due to their good electrical conductivity, high specific surface area and excellent physicochemical stability [13]. Unfortunately, most carbon materials based EDLCs still have comparatively low specific capacitance (below 300 F g−1), leading to low energy density of the EDLCs [19,20]. Therefore, it is still necessary to develop high performance carbon electrodes for SCs applications.

In recent years, many efforts have been devoted to introduction of pseudocapacitance to porous carbons to improve their power and energy densities [21,22]. Particularly, multiple heteroatoms doping (such as N/S, N/O, N/B, N/P, P/S etc.) has been demonstrated as an effective way to both increase the pseudocapacitance and enhance electronic conductivity of porous carbons [[23], [24], [25], [26], [27]]. Among these works, N/S co-doping has been received special attentions due to its simple and effective doping strategies [21,23,[28], [29], [30], [31], [32], [33], [34]]. However, a common problem for N/S co-doped carbon (NSC) materials is their capacitance loss at high current densities when being used for SCs. Two reasons have been suggested to account for this phenomenon: (1) The ion diffusion is too sluggish within the micropores of NSC to maintain fast charge-discharge rate [[35], [36], [37]]; (2) The pseudocapacitive reaction of NSC might be reduced at high charge-discharge rates [21,29]. To resolve this problem, preparing NSC that possesses both high specific capacitance and excellent rate capability is highly desired.

In 2010, Black et al. reported that multiscale pores are favorable for charge accumulation by simulating the charge accumulation behavior of different pore shapes with a transmission line model [38]. Later, Ervin showed that the presence of multiscale pores can facilitate ion diffusion in a thick graphene film [39]. Both works emphasized that multiscale pores are favorable for ion diffusion and consequently deliver large capacitance at high current densities. To this end, in 2014, Xu et al. developed a holey graphene aerogel frameworks containing an inter-connected three-dimensional (3D) macroporous network with mesoporous and microporous graphene sheets. This electrode showed a remarkably gravimetric capacitance of 298 F g-1 at a current density of 1 A g−1, and a high gravimetric capacitance of 202 F g-1 at a current density of 100 A g−1 [40]. Similarly, Zhu et al. synthesized a 3D hierarchical porous carbon network with multiscale pores, delivering a high gravimetric capacitance of 320 F g-1 at 0.5 A g−1, and a gravimetric capacitance as high as 126 F g-1 at an ultrahigh current density of 200 A g−1 [41]. Very recently, Li et al. found that the microporous graphene aerogels with periodic macro-porosity can maintain 88.7% of the capacitance when current densities increased from 0.5 to 10 A g−1 [42]. More recently, Hu and co-workers reported the preparation of an ordered hierarchical porous carbon monolith by using natural wood. The inherited structure of abundant open channels with various pore widths along the growth direction of wood provides straight diffusion tunnels with low tortuosity for ions. The existed mesopores and nanopores endow the electrodes with a high capacitance of 3204 mF cm-2 at a current density of 1.0 mA cm−2, and 2800 mF cm−2 when the current density increased to 30 mA cm−2 (87.3% retention of the maximal capacitance) [43]. These reports strongly showed that fabricating a well-ordered porous structure that interconnected with hierarchical pores for carbon based electrodes should simultaneously achieve high gravimetric capacitance and excellent rate capability. However, to our best knowledge, hierarchically architectured NSC with rational pore structure has been rarely reported. Traditionally, KOH activation method was utilized to create hierarchical pore for carbon materials, while the resultant pores are disordered and the pore size distribution is un-controllable [[44], [45], [46], [47], [48]]. Therefore, rationally design and controllably engineer an ordered hierarchical pore structure for NSC is critically essential.

Herein, we report the structural engineering of NSC by optimizing the pore structure. The optimized NSHPC electrode reaches a large specific capacitance of 322 F g-1 at a current density of 1.0 A g−1 (larger than most of the previously reported NSC electrodes). Moreover, it possesses outstanding rate performance with retaining 206 F g-1 at a high current density of 100 A g−1 and shows an excellent cycling performance with ∼93.4% capacitance retention after charge-discharge for 10000 cycles at the current density of 20 A g−1. With the NSHPC electrodes, the assembled symmetric SC exhibits a high energy density of 11.4 Wh kg−1 and a high power density of 12.9 kW kg−1. The outstanding performance of NSHPC is attributed to its hierarchical porous structure (micro-, meso-, and macropores are favorable for sufficient adsorption and rapid transport of ions), and multiple heteroatoms doping effect (introducing additional pseudocapacitance). Specifically, the N/S co-doping is beneficial for achieving a large specific capacitance at relatively low current densities. The observed high capacitance of NSHPC at high current densities mainly contributed from the EDLC which is highly dependent on its ordered hierarchical pores. These findings might be important for exploring other multiple heteroatoms doped porous carbon materials that should be employed in developing high performance supercapacitors.

Section snippets

Synthesis of the carbon materials

The N/S co-doped hierarchical porous carbon (NSHPC) was synthesized by using a hard template method followed by KOH activation. Typically, chitosan (1.0 g, with purity over 99%, degree of deacetylation of 95%) was added into deionized water (35 mL) under vigorous magnetic stirring for 30 min. Then HCl (6 mL of 1 M) was dropwise into the turbid liquid under magnetically stirring. After that, 1.0 g trithiocyanuric acid (TTCA, Aladdin Industrial Corporation) and 1.0 g SiO2 spheres (particle size

Structure characterization

Fig. 1 illustrates the synthesis process of NSHPC. Chitosan can easily form a uniform film on the surface of SiO2 spheres due to the electrostatic attraction between inherently negative charged silica surface and protonated amine groups of chitosan polymers [[49], [50], [51]]. Meanwhile, TTCA can be homogeneously embedded in chitosan films, therefore providing N and S sources for the subsequent multiple heteroatoms doping. N/S co-doped carbon materials with embedded silica spheres were obtained

Conclusion

In conclusion, we have successfully fabricated a N/S co-doped carbon electrode with an ordered hierarchical pore structure. The unique structural features engineered by a rational manner significantly offer the NSHPC electrode with outstanding SCs performances. Firstly, NSHPC-1:1 has high a specific surface area of 2711.7 m2 g−1 that ensures a large number of charges can be stored via EDLC. Secondly, the interconnected multiscale pores network not only eases electrolyte infiltration, but also

Acknowledgment

Financial support from the National Natural Science Foundation of China (grant nos. 21575016) and from the National Program for Support of Top-notch Young Professionals is gratefully acknowledged.

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