Porous structured niobium pentoxide/carbon complex for lithium-ion intercalation pseudocapacitors
Graphical abstract
Introduction
Due to people’s concerns about energy issues, more and more attention has been paid to supercapacitors. Supercapacitors have high power supply and relative long cycle life, which make them as supplementary to batteries and a newly promising class of energy storage devices [1], [2], [3], [4], [5]. Of many kinds of sueprcapacitors, attention on Li ion capacitors (LICs) has been escalating due to their high energy density since proposed in 2001 [6], [7]. Benefits are not only more energy density from Faradaic lithium-intercalation electrodes, but also high power density and excellent cycling stability from non-Faradaic capacitive electrodes [8]. These high performance lithium ion hybrid electrochemical supercapacitors (Li-HEC) posses higher power supply than the rechargeable battery and higher energy capacitance than supercapacitors [6], [9], [10], [11], [12], [13], [14], [15], [16]. Therefore, Li-HEC is the most promising device which could bridge the gap between the Li-ion batteries and electric double-layer capacitors (EDLCs) and turn out to be an ultimate power supply in the near future [7], [17].
Dunn et al. [18] have found a new class of pseudocapacitor materials arising from the reversible Li+ intercalation into orthorhombic phase (designated as T-Nb2O5). The Li+ intercalation could not only happen at the surface, but also in the bulk of crystals, and the overall kinetics are not limited by diffusion [19]. Intercalation and extraction process can be described by the following equation [20]:
When x = 2, theoretical capacitance is acquired, up to 360 F g−1 [18], [21]. T-Nb2O5 has the unique open channels to facilitate Li+ transport [22]. No kinetic limit from the solid-state diffusion is found in the crystal of T-Nb2O5 during the bulk intercalation process [23], [24]. T-Nb2O5 is a promising pseudocapacitor material due to its higher theoretical capacitance, an excellent rate performance and a stable cyclic life. However, there are some limitations imposed on the application of Nb2O5, for example, its low conductivity. Nb2O5 is a semiconductor with a bandgap of ∼3.4 eV, and poor conductivity (3 × 10−6 S cm−1) [25], [26], [27]. Many contributions have been devoted to overcome the influences of its poor conductivity. One method is the modification of morphology of the material to have a facile access to the electrolyte. Kong et al. [28] synthesized the mesoporous Nb2O5 from a cellulose nanocrystal template. Kong synthesized Nb2O5 core–shell microspheres with high rate performance [29]. Wei et al. synthesized Nb2O5 nanobelts with thickness and width in nanometers [30]. Le Viet et al. synthesized Nb2O5 nanofiber by electrospinning [31]. Liu et al. [32] synthesized Nb2O5 nanosheets through hydrothermal reaction. The other is the combination of Nb2O5 with conductive carbon materials, such as carbide-derived carbon (CDC) [33], carbon [34], [35] and graphene [25], [36], [37], etc. Kong et al. [29] synthesized the carbon wrapped Nb2O5 nanoparticles. Wang et al. [38] synthesized the composite of Nb2O5 grown on carbon nanotubes (CNTs). Wang et al. [39] tried to physically mix the Nb2O5 nanoparticles and CNTs during the electrode slurry preparation. The introduction of CNTs could enable fast electron transport and effectively improve rate capability. However, the physical mixing is more like increasing the amount of conductive agent, which could not have a quite good contact between CNTs and Nb2O5. Besides, carbon materials are more likely to aggregate during the hybrid process. Additional functioning steps are needed to get a well dispersion of carbon materials in composites [17]. Hence challenge still remains that developing a general and effective chemical approach to construct conductive T-Nb2O5/C composites.
The multi-dimensional carbon material has been considered a promising candidate to combine with Nb2O5 to obtain high active material mass loading, sufficient electronic conductivity and direct exposure of the active sites to the electrolyte. Recently, Long et al. [19]reported a free-standing T-Nb2O5/graphene composites with ultrahigh gravimetric/volumetric capacitance of 625.5 F g−1 and 961.8 F cm−3 at 1 mV s−1, respectively. Lee, et al. [35] prepared the T-Nb2O5/CNCs nanocrystals with the core-shell structure, which exhibited a reversible specific capacity of ∼180 mAh g−1 at 0.05 A g−1. Ordered mesoporous phenolic resins from the reaction of resorcinol with formaldehyde are widely used as support materials in the field of catalysis, electrodes and adsorbents [40]. There are many synthesis methods, like evaporation-induced self-assembly (EISA), a two-phase system based pathway and hydrothermal [41], [42], [43]. Phenolic resins via a soft-templated hydrothermal method are of convenience compared with time-consuming EISA. Briefly, traditional hard template strategy involves synthesis of hard template, repetitive filling of the pores with carbon precursor, pyrolysis at a high temperature, and removal of the hard template. Due to the multi steps involved and low yield of the carbon, this method is complicated with no commercial value. The other attracting strategy is the soft template approach, which helps to synthesize phenolic resins with nano-scale properties. The added template controls the polymerization of phenolic resin and helps to form nano-scale pores and tunnels, which provides the active sites for materials.
Herein, we introduce a facile one-step soft-templated hydrothermal synthesis of the composites of Nb2O5/carbon. All the precursors and the template were put into the autoclave and the composites were synthesized after carbonation in nitrogen. Nb2O5 nanoparticles are intimately loaded upon the carbon materials, which helps enhance the conductivity of Nb2O5. The electrochemical tests show that the composite has a quite good performance as supercapacitor electrodes. The in situ hybrid needs no pre-functionalization for carbon materials before the combination. The straightforward synthesis approach is able to obtain the porous Nb2O5/C composites, which greatly improve the utilization of active T-Nb2O5 nanocrystals. The Nb2O5/C has a high specific capacitance, 387 F g−1 at 0.1 A g−1 and 210 F g−1 at 5 A g−1. It also exhibits superior cycling performance with no obvious drop after 1000 cycles at 5 A g−1.
Section snippets
Chemicals
Niobium(V) oxalate hydrate (Nb(HC2O4)5·xH2O) were purchased from Alfa Aesar. Soft template poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer Pluronic F127 (PEO106-PPO70-PEO106, Mw = 12600) was got from Sigma-Aldrich. Other chemicals were bought from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification.
Preparation of Nb2O5/C samples
Nb2O5/C composite was synthesized by a templated hydrothermal synthesis as follow: 3.78 g F127, 0.62 g niobium oxalate
Material characterization
The synthesis procedures of the samples and the possible mechanism are demonstrated in Fig. 1.
The formation of nano particles of Nb2O5 occurs firstly during the hydrothermal process. The addition of Pluronic F127 does help to lower the diameters of Nb2O5 particles [44], [45]. As the hydrothermal reaction continues, the growth of Nb2O5 and polymerization of phenolic resins happens at the same time. The polymers fill the intervals between the Nb2O5 particles and make 3-dimentional connection
Conclusions
This work presents a facile synthesis method of the Nb2O5/C composite based on one-pot hydrothermal process. The formation of Nb2O5 nanoparticles and carbon precursors occurs at the same time during the hydrothermal condition. After the calcination in nitrogen, the phase transformation of Nb2O5 particles and carbonization of resin happens to obtain the Nb2O5/C composite. Its highly porous nature is beneficial to the ion transport and the load of active material. The composite has a good rate
Acknowledgements
We are grateful to the support from Shanghai Science & Technology Committee Project (11JC1404400, 14DZ0500700) and Shanghai Energy-Saving Centre of Heat-Exchange-System (Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power).
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