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Synthesis and Electrochemical Characterization of Niobium Pentoxide/Carbon Nanotubes Composites

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In the last few years, a range of energy-storage device applications has expanded from portable electronics to large-scale energy storage systems, including renewable energy storage and electric transportation [1,2]. To fulfill the energy and power density requirements for energy applications, many researchers have given attention to the synthesis of nanomaterials based on oxides due to their chemical, physical, optical, and electronic properties [1–4]. In this way, niobium pentoxide (Nb2O5) is a promising candidate because of its semiconductor properties with a band gap of ~3.4 eV, n-type, low toxicity, surface acidity, and good chemical and thermal stabilities [3,5–8]. It has been demonstrated that Nb2O5 can deliver high power through a mainly pseudocapacitive reaction of Li-ion (Li+), which could occur not only at the surface but also in the bulk of the Nb2O5 nanocrystals in non-aqueous Li+ electrolyte [2,9].

Moreover, the intercalation pseudo-capacitive behavior was highly dependent upon the presence of a crystalline structure, where amorphous and pseudo-hexagonal (TT-Nb2O5) exhibited lower specific capacitance values than the orthorhombic (T-Nb2O5) phase, however, nanoparticles aggregation is inevitable due to the high calcination temperature (>600 °C) for the orthorhombic phase formation of Nb2O5 [10–17]. Nonetheless, the application of Nb2O5 has been hampered by low bulk electrical conductivity (~3.4×10-6 at 300 K) and by the difficult control of the ideal crystal structure [3,10,11]. Therefore, when T-Nb2O5 nanocrystals were fabricated into a relatively thick electrode, the power performance would be limited due to the impaired mobility of electrons. A possible effective method to improve these adversities in Nb2O5 is through of surface modifications (e.g., carbon coating), which could expose more redox active nanoparticles to the electrolyte as well as greatly improve the electronic conductivity [1,3,11–13].

It must be noted that the introduction of multiwalled carbon nanotubes (MWCNTs) networks can enhance electron transport of the Nb2O5 and further improve the rate capability. Physical mixing of MWCNTs and Nb2O5 nanoparticles can fabricate composite with improved conductivity, but this type of mixing fails to fulfill a good interfacial interaction between the MWCNTs and Nb2O5 [3]. Wherefore, soft-chemistry methods like oxidant-peroxo method matched with hydrothermal treatment and heating by microwaves could be a promising alternative for niobium synthesis, mainly because it is performed at low temperatures which avoids the elimination of the hydroxyl groups in as-formed surfaces resulting in a material with high surface area and greater number of acid sites [6,18,19]. In addition, carbon materials have a good microwave radiation absorption, which facilitates the interaction with other particles [20].

The use of niobium oxides for energy storage devices is already well known and exploited [21] but its performance is only obtained by a hybrid supercapacitor based on lithium intercalation processes [21] where high values of energy and power density are obtained. These intercalation processes require the application of potentials between 1 to 3 V vs. Li/Li+, a careful and humidity-free assembly scheme and non-aqueous electrolytes [16]. In this work, niobium pentoxide with different structures and morphologies were grown onto the surface of carbon nanotubes were synthesized by MHS. Moreover, we evaluated some electrochemical properties of electrodes with high mass loading on Ni foam current collectors (Fig. 1) using aqueous electrolyte, which brings the material closer to real and practical applications. Methods The acid functionalization on the MWCNTs was performed to create functional groups through a controlled functionalization of the nanotube walls, which allowed the anchoring and growth of the Nb2O5 nanoparticles. This functionalization is accomplished by exposing the MWCNTs in a concentrated HNO3/H2SO4 (1:3) as described by Goyanes, et al. 2007 [22]. Nb2O5 nanoparticles were synthesized by the microwave-assisted hydrothermal method followed by a heat treatment.

The sample preparation started with the dispersion of the 50 mg of functionalized MWCNTs and 100 mg of SDS (1:2) in 100 mL of distilled H2O in an ultrasonic bath. After dispersing, 2 g of (NH4[NbO(C2O4)2(H2O)2]·nH2O) and 1.6 mL of H2O2 were added under ultrasound frequency. The solution was ultrasonicated continuously for 30 min and then poured into a Teflon vessel reaching about 50% of its volume, the autoclave was sealed and heated in the microwave equipment (Electrolux, MEF41, Brazil). The synthesis was performed with amendments according to Marins, et al., 2017 [23], where the temperature was set at 160 °C during 30 min, under a constant pressure of approximately 6 kgf cm-2 using 2.45 GHz microwave radiation with a maximum output power of 1500 W. Temperature and pressure during synthesis were monitored and controlled with the aid of a shielded thermocouple inserted directly into the vessel and with a pressure transducer sensor connected to the vessel [5]. After cooling at room temperature, the obtained powders were washed and centrifuged at 5000 rpm six times with distilled H2O to remove unreacted reagents and surfactant. The washed powder was over dried with no air circulation at 50 °C for 48 h. This sample was identified as CNT+TT-Nb2O5.

To obtain the samples under heat treatment, part of the powder was heated in a furnace (FT-1200 1Z, Fortelab, Brazil) at 400 °C for 120 min and 650 °C for 180 min with a heating rate of 10 °C/min under nitrogen atmosphere aiming to get the orthorhombic phase [12,22]. After cooling at room temperature, the powders were stored in an oven with no air circulation at 50 °C and identified as CNT+T-Nb2O5. PVB solution in ethanol was added to the mixture in the electrode preparation. The PVB content in the slurry was 3% of the total mass of Nb2O5 and MWCNTs to each sample. The obtained slurry was used for the impregnation of Ni foam current collectors with area of 1 cm2. The active mass loading was 32 mg cm-2.

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