Niobium pentoxide nanoparticles @ multi-walled carbon nanotubes and activated carbon composite material as electrodes for electrochemical capacitors

Energy Storage Materials 22 (2019) 311–322

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Niobium pentoxide nanoparticles @ multi-walled carbon nanotubes and activated carbon composite material as electrodes for electrochemical capacitors Rafael Vicentini a, Willian Nunes a, Bruno G.A. Freitas a, Leonardo M. Da Silva b, **, Davi Marcelo Soares a, Reinaldo Cesar a, Cristiane B. Rodella c, Hudson Zanin a, * a Center for Innovation on New Energies, Advanced Energy Storage Division, Carbon Sci-Tech Labs, School of Electrical and Computer Engineering, University of Campinas, Av Albert Einstein 400, Campinas, SP, 13083-852, Brazil b Department of Chemistry, Federal University of Jequitinhonha and Mucuri's Valley, Rodovia MGT 367, Km 583, 5000, Alto da Jacuba, 39, 100-000, Diamantina, MG, Brazil c Brazilian Synchrotron Light Laboratory (LNLS), Brazilian Center for Research in Energy and Materials (CNPEM), Campinas, S~ ao Paulo, 13083-970, Brazil

A R T I C L E I N F O

A B S T R A C T

Keywords: Niobium pentoxide Carbon–carbon scaffold Energy storage Pseudocapacitance operando Raman study

We report here on a novel method to produce nanostructured porous carbon composite electrodes decorated with niobium pentoxide nanoparticles. The carbon support is composed of multiwalled carbon nanotubes and activated carbon composite material. Nanotubes work simultaneously as binder and additive for activated carbon supported on the nickel-aluminide current collector. The hydrated niobium pentoxide nanoparticles attached on the carbon surface improved the charge storage process, introducing Faradaic reactions (pseudocapacitance) to the storage mechanism. The pseudocapacitive process involving the niobium pentoxide was stable since the oxide nanoparticles partially covered with a porous carbon layer were not deactivated. This approach improved the electrical conductivity and chemical stability and also avoided reaggregation and deactivation of niobium pentoxide nanoparticles by passivation. The electrochemical performance of the symmetric coin cell using an aqueous lithium sulfate solution was evaluated by cyclic voltammetry, galvanostatic (re)charge/discharge curves, and the electrochemical impedance spectroscopy techniques. In short, our results showed that the composite material has good electrochemical properties, including high specific capacitance (~220 F g
1), long lifespan (more than 200 thousand cycles), and high energy (0.11–6.5 kW kg
1) and power (3.1–6.1 Wh kg
1) densities for the applied gravimetric currents in the range of 0.5–30 A g
1. We additionally performed in situ Raman analyses (operando studies) using the composite electrode under dynamic potential conditions. We observed reversible shift on the D band position, and the intensity of the Raman signal decreased during cycling due to SO2
4 adsorption.

1. Introduction Pseudocapacitors (PCs) are a class of electrochemical capacitors (ECs) that combine electrostatic and ultra-fast Faradaic energy storage processes. PCs are one of the most attractive fields of ECs because they possess the excellent, well-known characteristics associated with electrical double-layer capacitors (EDLCs), but they also exhibit Faradaic reactions like batteries. As a result, PCs has intrinsically high energy storage capabilities, which is one of the drawbacks for EDLCs that severely restrict their applications in several different technological processes. In this scenery, PCs are hybrid systems between ELDCs and

batteries being the best or the worse of them both. The significant features involving the PCs are: (i) low ohmic equivalent series resistance (RESR), enabling high load currents and very high specific power; (ii) charge and discharge in seconds with no end-of-charge termination needed; (iii) they can be (re-)charged/discharged with a prolonged lifespan, and (iv) high coulombic efficiency [1–3]. On the other hand, PCs have the following drawbacks: (i) the specific energy is lower than that verified for batteries; (ii) an almost linear voltage discharge curve is commonly verified which could prevent using the full energy stored, and (iii) in some cases a high self-discharge is verified [2,3]. To overcome these limitations keeping typical EDLCs advantages,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L.M. Da Silva), [email protected] (H. Zanin). https://doi.org/10.1016/j.ensm.2019.08.007 Received 9 May 2019; Received in revised form 7 August 2019; Accepted 7 August 2019 Available online 10 August 2019 2405-8297/© 2019 Elsevier B.V. All rights reserved.

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i.e., a very high aggregation between the individual components was obtained for the composite. Before synthesis, 5.0 cm  7.0 cm rectangular aluminum foils were etched by hydrochloric acid (1:4 (v/v)) solution for 30 s to increase surface roughness and enhance the adherence (e.g., anchoring process). Afterward, a nickel thin-film was electrodeposited onto the aluminum foil using a 2.0 M Ni(NO3)2 aqueous solution (conditions: 14.3 mA cm
2 at 60  C for 1.0 min). In this process, we electrodeposited a nickel layer of 5 μm on each side of the aluminum foils (e.g., 15 μm thick). After the electrodeposition, 0.1 mg cm
2 of AC was uniformly distributed on the Ni-coated Al and then inserted into a homemade CVD furnace, where the temperature increased from the room temperature to 650  C at a rate of 65  C min
1 under a nitrogen atmosphere. Al and Ni both on separated phases combined forming one phase material composite, consolidating an alloy (e.g., Ni3Al), which is extremely stable on neutral aqueous media. In addition, this alloy permits to accomplish the welding process in the case of manufacture of the device on a great scale. The role of the Ni3Al current collector was presented elsewhere by us [7]. For further experimental details, we recommend assessing this published work. At 650  C, 10,000 sccm of a nitrogen flow was used to drag the vapor of the carbon source composed of 43.6 wt% of camphor (C10H16O) dissolved in ethanol (C2H5OH) and containing 2 wt% of Ni(NO3)2. That is our standard recipe for the MWCNT growth. After this process, electrodes were cooled down to room temperature for 90 min under N2 atmosphere. At room temperature, electrode foils were shaken upside down to remove any loosely bounded particle and then cut on disc-shape of 1.0 cm of diameter (geometric area of 1.93 cm2) for assembling the symmetric coin cell. The as-prepared electrodes were shakeout to remove the loosely bounded particles. In the end, the samples of the carbon-based composite material exhibited an average thickness of 10  1 μm. We henceforward refer to the carbon-carbon composite anchored on the nickel aluminide current collector as the AC-CNT support electrode.

pseudocapacitive components must be inserted on the electrode materials. For instance, by incorporating transition metal oxides (TMOs) on the electrode material, redox reactions are also included in the electrode system, significantly increasing the energy stored at the electrode/solution interface. However, most of the TMOs suffers from poor conductivity and easy deactivation due to passivation and/or metal dissolution [4]. An abundant TMO is niobium pentoxide (Nb2O5), which may lead to a high energy density of ~20 Wh kg
1 [5]. However, this material in its pure state exhibits poor electric conductivity (σ~3  10
6 S cm
1) [4] and low chemical stability, thus limiting its practical applications. Therefore, a good strategy for the improvement of the electrode material characteristics is the combination of Nb2O5 with some carbon materials (e.g., AC and CNTs) to obtain composite electrodes. As a further achievement regarding the composite electrodes, we do not employ organic binders, keeping ohmic resistance of the device to a very low level. Most of the literature reports have studied supercapacitive properties of Nb2O5 on aprotic electrolytes, where voltage ranges of 2–3 V can be achieved [6]. However, organic electrolytes impose some drawbacks, since they are expensive, flammable, toxic, ecological unfriendly, and requires a complicated assembling process. In this work, we investigated Nb2O5-based composite electrodes on aqueous-based electrochemical capacitors and evidenced redox activity of Nb2O5 nanoparticles, resulting from protonation and de-protonation processes due to a strong interaction of the oxide (e.g., niobic acid) with water. Therefore, the purpose of this work is to produce Nb2O5based electrodes, which are very stable in aqueous electrolyte. To do that, we deagglomerated and well distributed Nb2O5 nanoparticles by electrospraying them onto high surface area carbon scaffolds and then annealed the composite to obtain a strong chemical interaction between these components. This fabrication process partially covers the niobium pentoxide nanoparticles with a thin mesopore carbon layer, thus improving the electrical conductivity of the electrode material and avoiding reaggregation of the nanoparticles. The composite electrodes were very stable in aqueous electrolyte allowing us to study them by conventional in situ electrochemical and operando (Raman and XRD) techniques, i.e., the aging process of the electrode materials on cell devices was monitored up to 250 thousand cycles in aqueous electrolyte and using a large voltage window of 1.8 V. It is worth mentioning that in most of the literature reports the niobium-based electrodes did not run more than 10 thousand cycles using aprotic media. The carbon scaffold used in this work was composed of highly defective multiwalled carbon nanotubes (MWCNTs) and activated carbon (AC) composite anchored onto the nickel-aluminide alloy current collectors, which is very stable in aqueous electrolytes [7]. Interesting findings were obtained in the current work since the Nb2O5-based composite electrodes exhibited a very large voltage window of 1.8 V in aqueous solution, good specific capacitance, and very high stability during the long-term charge-discharge galvanostatic tests. The contributions of the individual constituents of the composite to the overall specific capacitance was discriminated, being confirmed the significant electrochemical contribution given by Nb2O5 to the charge storage process. In addition, experiments accomplished using dry oxide (e.g., in the absence of water) and in the presence of water revealed the interaction of the former with the latter results in important acidic-base properties that increased the conductivity of the hydrated oxide.

2.1.2. The Nb2O5 @ AC-CNT composite AC-CNT electrodes were electrosprayed by Nb2O5 nanoparticles as described in Fig. 1(a). This method consisted of dispersing the agglomerated nanoparticles in a liquid to form a colloidal suspension, which was dry-sprayed to promote the nanoparticle deagglomeration on the carbon surface (target). Nb2O5 powder of analytical grade was donated by CBMM Co. (Brazil). A high electric field was applied, and electrical force drives the liquid atomization, making the droplets to undergo to a fraction of the Rayleigh limit [8] forming a Taylor cone [9]. The liquid solution was then disintegrated into droplets, which tend to be apart until the solution is in the gaseous phase and nanoparticles deagglomerate because of the electrostatic repulsion. Fig. 1(a) illustrates the configuration known as a simple nozzle for direct spraying used in the present work. This homemade experimental setup is simple, requiring a high voltage source, a plastic syringe, a needle, and an insulating box. For the application, the solution was placed inside a 22G 1 1/4 metallic needle, located 3.0 cm above and at the center of the samples, aiming to obtain a better uniformity of the composite film. An 11 kV power source was connected to the syringe, which showed sufficient to ionize the droplets. The samples located below the metal tip were placed on a grounded metal sheet in order to guarantee that the ionized particles would reach them due to the electric field lines generated. The concentration of the electrosprayed solution was 0.05 M of Nb2O5 dissolved in methanol. The reason for choosing methanol over other solvents lies in its low boiling point and low viscosity. Other liquids that are more viscous than methanol, such as water or cyclohexane, may cause undesirable solvent deposition onto the substrate [10]. The flow of the solution was set at 1.0 μL s
1, and the application was performed during 3.0 s. After the application, the samples were annealed using a furnace from room temperature up to 650  C under a 1000 sccm N2 flux

2. Experimental section 2.1. Synthesis of the electrodes 2.1.1. The carbon-carbon composite The composite electrode material containing AC and MWCNT was prepared by modifying the aluminum foil during the MWCNT growth. MWCNTs worked simultaneously as a binder and additive for the ACbased electrodes. In this sense, MWCNTs were synthesized directly onto the AC placed on nickel-coated aluminum foils binding all of them, 312

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Fig. 1. Schematic representation of (a) electrode preparation, (b) coin cell assembly, and (c) Raman operando characterization setup.

spectrometer (Thermo Scientific). Spectrum deconvolutions were performed by Casa XPS (LNNano). Crystal phase analyses were performed using a PANanalytical X'Pert PRO Materials Research Diffractometer equipped with a cobalt X-ray tube (Custom wavelength of 1.789 Å). In order to obtain good signal for the diffractograms, the electrodes were sonicated during 5 min in ethanol to remove the active material adhered on the substrate. After this process, the material was dried in air atmosphere.

for 1.0 h. We henceforward refer as Nb2O5@AC-CNT electrode for the C-Nb2O5 composite anchored on the nickel-aluminide current collector. After all, the average weights of the individual components present in the composite are 0.2 mg of AC, 0.2 mg of MWCNT, and 0.1 mg of Nb2O5. 2.2. Ex-situ materials characterization Samples morphology and nanostructure were characterized by scanning electron microscopy (Dual FIB: FEI Nanolab 200) and highresolution transmission electron microscopy (JEOL 2100 MSC). The samples were investigated using a Renishaw inVia Raman spectrometer through a 514.5 nm excitation wavelength using a 20 objective lens. Raman shift was calibrated using a monocrystalline diamond and spectra were measured at room temperature (24  C). Spectra analyses were accomplished by proper subtraction of the baseline with deconvolution using Lorentzian and Gaussians functions employing the Fityk software [11]. Normalization was carried out using the most intense peak. The surface chemistry of the samples was analyzed with the XPS technique using K-alpha X-ray radiation using a photoelectron

2.3. Electrochemical measurements The electrochemical measurements involving the Nb2O5@AC-CNT electrodes housed in a symmetric coin cell, using the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques, were performed using an Ametek Versa STAT 4 potentiostat-galvanostat. A model 2032-coin cell containing a cellulosic paper (separator) soaked with a 1.0 M Li2SO4 aqueous solution was used throughout, as shown in Fig. 1(b). The CV and GCD curves were measured at 10
1000 mV s
1 and 0.5–30 A g
1 ranges, 313

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3. Results and discussion

respectively. The EIS analysis was carried out in the frequency range of 100 kHz to 10 mHz using a sinusoidal perturbation with an amplitude of 10 mV (peak-to-peak). Experimental findings were validated by applying the Kramers-Kr€ onig tests. The specific (gravimetric) capacitance was calculated from the discharge curve using Eq. (1) [1,2]: Cspecific ¼

I

1  Fg m dV dt

3.1. Materials characterization The surface morphology and structure analyzed by SEM & TEM of the as-grown Nb2O5@AC-CNT material evidenced the AC and MWCNT decorated with the niobium pentoxide nanoparticles. We can observe in Fig. 2(a–b) the SEM micrographs with different magnifications of 800 for top view analysis and closer look with 40,000 used to identify the CNT and Nb2O5 nanoparticles. From the top view perspective, we may observe AC large particles from 10 up to 80 μm in length and 10–30 μm in width. From close, we may see that AC particles were covered by CNT and Nb2O5 nanoparticles, which were fairly well distributed all over the AC surface. The TEM micrographs demonstrated that MWCNTs have diameters ranging from 10 to 25 nm and a turbostratic structure. The Nb2O5 nanoparticle diameters ranged from 10 to 30 nm. A mesoporous thin-film carbon layer partially covered the outer surface regions of Nb2O5 nanoparticles with 1–2 nm in thickness (Fig. 2(g)). For our best understanding, carbon encapsulates Nb2O5 nanoparticles by a diffusion-related process induced by the thermal treatment during the synthesis, i.e., the smaller oxide particles due to the high thermal energy are progressively inserted the into the macropores of the carbon material. In addition, the thermal mobility induces the formation of dangling carbon bonds on the top surface of MWCNTs, thus producing a porous carbon layer onto the Nb2O5 nanoparticles, encapsulating them. In fact, carbon materials strongly bind to the oxygen present in the metal oxide (Nb2O5) and porous carbons [12,13]. Raman spectrum took from Nb2O5@AC-CNT composite is contrasted with Raman spectra took from Nb2O5, AC, and MWCNT materials (see Fig. 3(a)). Before the peak identification, we may observe that Nb2O5@AC-CNT is a combination of Nb2O5, AC, and MWCNT. Raman shift signal for Nb2O5 is below 1000 cm
1, and for carbon, it is above this value. A typical Raman peak of Nb2O5 is verified at 992 cm
1, which is assigned as longitudinal optic modes [14]. Five peaks were observed for the as-received Nb2O5. The band at 178 cm
1 can be assigned to the external modes while that at 279 cm
1 was assigned to the triply degenerated vibration mode in the T3u region [14]. The bands verified at 629 and 679 cm
1 are due to the transverse optic modes [15]. For carbon-based structures (AC-CNT), the Raman spectrum is also a combination of AC and MWCNT Raman spectra. On first order, the main and common peaks both for AC and MWCNT are due to the G and D bands, which are associated with in-plane vibration modes [16–18]. The G band is assigned to the E2g Raman active phonons at the Brillouin zone center [17–19]. The appearance of the D band involves an intervalley double resonance process around the K and K’ points in the first Brillouin zone and requires defects for its activation in the Raman scattering process [17–20]. Additionally to D and G bands, in the case of AC, we have two bands centered at 1200 cm
1 and 1500 cm
1 denoted as D1 & D2, respectively. D1 & D2 bands are due to sp2 clusters of fourfold coordinated bonds [21], assigned to amorphous domains [22]. Dippel et al. [23] suggested that the ~1200 cm
1 band are related to sp2-sp3 bonds or C–C and C¼C stretching vibrations, while Cuesta et al. [24] and Jawhari et al. [25] suggested that the band at ~1500 cm
1 is related to amorphous carbon such as organic molecules or carbon bonded to functional groups. On the second order, we observed for MWCNT and the composite material a band centered at 2700 cm
1 which is designated as G’ or 2D and related to the second harmonic of the D band [20]. All these Raman spectra confirm that AC, MWCNT, and Nb2O5 are present in the Nb2O5@AC-CNT composite, i.e., the synthesis process has combined the materials instead of replacing them for another. Although Raman spectrum identifies chemical bonds on Nb2O5, there is a lack of information accounting for the crystalline phase of Nb2O5 nanoparticles. In order to identify the crystal phase of Nb2O5, the XRD diffraction was performed on the composite materials and the patterns are presented

(1)

where I is the discharge (cathodic) current, m is the mass of the active (composite) material present on both electrodes, and dV/dt is the slope of the discharge curve. The ratio of discharge and charge times obtained in the GCD experiments yielded the coulombic efficiency η of the supercapacitor device [1, 2]:

η¼

tdischarge  100% tcharge

(2)

The average specific energy (E) and power (P) were calculated using Eqs. (3) and (4), respectively [1,2]: E¼

 CV 2 103
 Wh kg
1 2 3600

(3)

P¼

 E
Wh kg
1 t

(4)

where t is the discharge time. To evaluate the volumetric capacitance, first, the volume of both electrodes needs to be calculated: Vol ¼ A  ð2  thicknessÞ where A ¼ 1.92 cm2, thickness ¼ 0.001 cm. The volumetric capacitance (CVol) was calculated according to the following equation: CVolumetric ¼

 I
F:cm
3 Vol dV dt

2.4. Operando Raman characterization Raman spectra were taken using a Renishaw inVia spectrometer by applying a 633 nm excitation wavelength, which was focused as a single spot with the aid of a 50 objective lens. Thus, all measurements were performed at the same place/point. Fig. 1(c) shows the schematic representation of the experimental setup, evidencing the electrochemical cell and the Raman microscope lens. The cell was composed of two electrodes separated by a cellulosic mesoporous membrane soaked with a neutral aqueous electrolyte (e.g., 1.0 M of Li2SO4). An EL-CELL opto in-situ Raman device was used throughout. All Raman spectra were taken during a cyclic voltammetry experiment acomplished at the scan rate of 0.1 mV s
1 and covering the voltage range of 0–1 V. Electrochemical experiments were carried out using a model VersaSTAT 3 potentiostat from Ametek. The cell open circuit voltage (OCV) was ~50 μV. The Raman spectra were taken during the voltammetric curve using steps of 200 mV and the acquisition time and accumulation number were 60 s and 1, respectively. The laser power was 1% of the total power (42 μW) to avoid damage of the electrodes. The analyzed region of the sample (site) was carefully chosen and seted as the origin by the software to avoid moving. In addition, after each spectrum acquisition, a visual confirmation of the laser incidence on the site was performed. We were constantly checking the electrode surface to observe any damage or drying of the electrolyte. 314

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Fig. 2. (a & b) SEM micrographs took from Nb2O5@AC-CNT material anchored onto the Ni3Al current collector; and (c–h) TEM micrographs took from Nb2O5@ACCNT powder.

Fig. 3. (a) Raman spectra from (i) Ni3Al:AC-CNT, (ii) Nb2O5, and (iii) Nb2O5@AC-CNT materials; (b) XRD diffraction pattern from (i) Ni3Al:AC-CNT, (ii) Nb2O5 and (iii) Nb2O5@AC-CNT materials; (c) XPS spectra from Nb2O5@AC-CNT material on C1s, O1s, and Nb3d regions.

alloy (see the details in the experimental section). From data showed in Fig. 3(b), we can correctly identify the formation of the Ni3Al alloy. In fact, the peaks at 48.56 , 58.39 , 52.03 , and 59.34 were assigned to the Ni3Al ordered face-centered cubic crystal structures referring to the (1 0 0), (1 1 0), (1 1 1), and (2 0 0) hkl-planes [26], all in agreement with the collection code 58039 from ICSD database and the literature [27].

in Fig. 3(b). We separated the materials to show their individual XRD patterns. In order to identify the signal referring to the Nb2O5 nanoparticles, we performed first the XRD diffraction for the Ni3Al:AC-CNT support. After that, we registered the spectrum for Nb2O5 and Nb2O5@AC-CNT samples. In the case of Nb2O5@AC-CNT, the samples were first sonicated in ethanol solution to remove them from the metallic 315

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The diffraction pattern of Nb2O5@AC-CNT in the absence of Ni3Al evidenced two different polymorphic crystalline phases of niobium pentoxide, namely, the Nb2O5 monoclinic phase of spatial group P2 (PDF# 00-016-0053) and the Nb2O5 orthorhombic phase of spatial group Pbam (PDF# 00-030-0873). These two phases are denoted as known as TT-Nb2O5 and T-Nb2O5, respectively [28]. Monoclinic and orthorhombic phases have 0.37 nm and 0.39 nm of interplanar spaces, respectively. As can be seen, the Nb2O5 nanoparticles exhibited a very strong signal, which covers the signal from carbon and NixAl. As expected, the AC was amorphous and, therefore, the diffractogram did not show crystalline phases. In addition, we did not verify changes in the crystalline phase of the as-received Nb2O5 and heated samples. This corroborates the idea that the heat treatment only forms the core-shell structure, as evidenced in Fig. 2(g). The elements and chemical states of the Nb2O5@AC-CNT composite have been studied through XPS analysis. As shown in Fig. 3(c), the (i) C1s, (ii) O1s, and (iii) Nb3d signals are found with their corresponding binding energies. The XPS spectrum in Figure (i) reveals the C1s carbon region. The main peak at 284.7 eV can be assigned to the C¼C bond, evidencing the simultaneous presence of sigma and π (296.3 eV) bonds due to the graphene structure present in the MWCNTs. Furthermore, peaks corresponding to higher binding energies such as 288.7, 290.8, and 293.5 eV correspond to the presence of oxygen linked to carbon (e.g., C
O, C¼O, and –COOH). These carbon-carbon hybridizations, Nb2O5 structure, bonding, and vibrations, are all in agreement with the Raman findings. Besides the C1s signal, Fig. 3(c) (ii) shows the spectrum of the O1s region, which was resolved into different bands, one centered at 532.2 eV and assigned to the –C¼O surface group, and the other centered at 530.9 eV and attributed to the carboxyl functional group present in the carbon material [29]. Fig. 3(c) (iii) presents symmetric peaks centered at 207.9 eV and 210.7 eV, with similar FWHM in the Nb3d region, which were attributed to the doublet Nb3d5/2 and Nb3d3/2 core levels of Nb5þ, respectively. This feature is typical of niobium oxides, particularly for the Nb2O5 [29] present in the composite. These results provide direct evidence of the composite composition and chemical bonds. 3.2. Electrochemical characterization studies 3.2.1. Analysis of the voltammetric curves We investigated the electrochemical behavior of the two-electrode symmetrical cell filled with a 1.0 M Li2SO4 aqueous solution and assembled using different pairs of electrodes. Fig. 4(a) and (b) show the voltammetric curves registered at different scan rates for the voltage intervals of 1.0 V (e.g., AC-CNT) and 1.8 V (e.g., Nb2O5@AC-CNT), respectively. As seen, quasi-rectangular voltammetric curves were verified in all cases after application of a very high scan rate of 1000 mV s
1, implying in very good capacitance retention and low equivalent series resistance (RESR). These findings are in agreement with good accessibility of the electrolyte species inside the porous electrode structure and good electrode wettability, as well as a high conductivity of the composite electrode material. Therefore, the use of MWCNTs and AC in conjunction with the niobium oxide nanoparticles resulted in a composite electrode material with good properties for the charge and energy storage processes. As can be seen, the aqueous-based symmetric coin cell assembled using the Nb2O5@AC-CNT composite electrodes can achieve a very high voltage of 1.8 V in neutral solutions, i.e., no gas evolution due to water splitting was verified. Specific capacitance values of ~105 F g
1 and 232 F g
1, both obtained at ν ¼ 10 mV s
1, were verified for the AC-CNT and Nb2O5@AC-CNT electrodes, respectively. The large working potential window observed in this work is very interesting for the energy storage proposals using supercapacitors. From the fundamental viewpoint, it was verified by us that the large potentials achieved in the pseudocapacitive potential interval are a consequence of the existence of very low exchange current densities accounting for the hydrogen and

Fig. 4. Voltammograms took at different scan rates (5–1000 mV s
1) for the symmetrical coin cell assembled with (a) AC-CNT and (b) Nb2O5@AC-CNT electrodes. For both cases the Ni3Al alloy was used as the current collector. (c) Voltammograms took for the symmetric coin cell housing the Nb2O5@AC-CNT electrodes after cycling from 500 up to 250,000 cycles. Electrolyte: 1.0 M Li2SO4.

oxygen evolution reactions. Since these gas-evolving reactions are highly irreversible due to the kinetic restrictions, the onset of these reactions can be verified very far from the predicted thermodynamic potential. In light of the Tafel's analysis, our composite electrode is a very poor electrocatalyst for the water-splitting reaction, thus strongly inhibiting its occurrence at significant (observable) rates. It is worth mentioning that our hermetically closed coin cell was operated for 250 thousand cycles without verification of an increase of the internal pressure caused by the presence of hydrogen and/or oxygen gases. Classic examples of poor electrocatalysts for these gas-evolving reactions comprise some carbons and SnO2 which exhibit very poor electrocatalytic activities (e.g., very low exchange current density) for the hydrogen and oxygen evolution reactions, respectively [2]. Therefore, compared to MWCNTs, AC, or a combination of them, Nb2O5@AC-CNT exhibited the largest working 316

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(e.g., the acid-base properties of the hydrated oxide nanoparticles). On the contrary, the intercalation of Liþ ions in aqueous solutions is only possible for some particular cases (e.g., LiMn2O4) where the host metal oxide is stable in concentrated Liþ solutions, and the metal intercalation take priority over the Hþ intercalation [31]. Also, the presence of ‘sharp peaks’ in the voltammetric curves is almost a pre-requisite to identify the intercalation of Liþ ions [32,33]. Therefore, we can rule out the contribution of the Liþ intercalation process to the strong pseudocapacitance verified in the present work. It is worth mentioning that very similar experimental findings were previously reported by Idress et al. [6], who verified that T-Nb2O5 electrodes immersed in 1.0 M Na2SO4 and 1.0 M Li2SO4 aqueous electrolytes yielded the same voltammetric profiles, i.e., the intercalation process involving lithium (e.g., Nb2O5 þ xLiþ þ xe
↔ LixNb2O5) did not occur on aqueous media. This is the reason that in the case of Nb2O5 electrodes the literature reports is mainly concerned with the study of the intercalation of Liþ ions in aprotic media [34–37]. Fig. 4(c) shows the voltammetric profiles as a function of the cyclability of the cell voltage (e.g., a long-term cyclability test). As can be verified, quasi-rectangular (“mirror-like”) voltammetric profiles were observed even after 200,000 cycles, i.e., very good retention of the electrochemical properties was verified for the Nb2O5@AC-CNT composite electrodes. After 250,000 cycles of charge-discharge, the overall capacitance decreased by about 30%, and the coin cell was considered deactivated. It is important to emphasize that the vast majority of the literature reports commonly considers for the long-term stability tests only a moderate number of cycles (e.g., 10,000 cycles), thus leading to unsatisfactory results. In our case, contrasting CV and GCD (Fig. S3), we observed that from 200 k to 250 k cycles the cell started to fail due to the appearance of large ohmic components (e.g., RESR). The reason for the cell deactivation (failure) after 200 k cycles verified at 1.8 V is the passivation of the metallic components of the coin cell, especially the current collector. The occurrence of this passivation process was monitored using the EIS technique where significant changes were verified in the medium-to-high frequencies in agreement with this type of process [2].

potential window due to its very poor electrocatalytic activity for the water-splitting reaction in neutral solutions (see the Supporting Information). We included Fig. S1 for contrasting the CV results obtained for the electrochemical coin cell assembled with different constituents of the composite. In this sense, Fig. S1 contrasts CV from Ni3Al:CNT, Ni3Al:ACCNT, Ni3Al:Nb2O5, Ni3Al, and Nb2O5@AC-CNT, at 100 mV s
1 in order to identify the capacitance contribution of the individual components of the composite on the overall electrochemistry response. It was verified that the Ni3Al current collector represents a negligible contribution to the charge storage process while the specific capacitance due to CNTs was improved in the presence of AC. In addition, CVs confirmed the electrochemical activity exhibited by Nb2O5 in aqueous solution due to its strong interaction with water where protons from the electrolyte can be injected/ejected during the charge/discharge processes according to the general ‘protonic condenser’ behavior (Trasatti's definition) which is commonly verified for several different transition metal oxides in aqueous solutions [2]. Hydrated niobium oxide (Nb2O5⋅H2O; niobic acidic) is capable of dissociating the water present on his surface. Thus during the Faradaic process (e.g., Nb(IV)/Nb(V)) the local electroneutrality is ensured by intercalation/deintercalation of the hydrated protons (2NbO2 þ H2O ↔ Nb2O5 þ 2Hþ þ 2e
) (see further discussion in this work). From the above considerations, the presence of deagglomerated Nb2O5 nanoparticles partially encapsulated by a thin mesoporous carbon layer considerably improved the charge storage process occurring at the electrode/solution interface, i.e., the pseudocapacitance due to the solidstate Faradaic reactions involving the Nb(IV)/Nb(V) redox couple strongly increased the overall capacitance composed of electrostatic and Faradaic contributions (see further discussion). Similar findings were previously reported in the literature [6]. It is worth mentioning that we cannot discriminate the capacitive and Faradaic (e.g., mass-controlled) events occurring during the chargedischarge processes for the composite electrodes since our voltammetric findings performed as a function of the scan rate (data not shown) revealed the traditional linear relationship (i ¼ Cν) that represents both the true capacitance as well as the pseudocapacitance accounting for the solid-state redox transition reaction when the Faradaic process is highly reversible [2]. It is worth noting that the redox peaks commonly verified on neutral aqueous solutions at ~0.6 and ~1.1 V/SCE for the T-Nb2O5 electrodes housed in a conventional three-electrode cell [6] were not observed in this work. This behavior can be understood considering that the voltammetric profiles obtained for the electrodes assembled in a coin cell (e.g., two-electrode system) are commonly distinct from their counterparts verified using a conventional three-electrode cell since in the former case the cell voltage is simultaneously distributed over the positive and negative electrodes. Also, the specific (gravimetric) capacitance of symmetric two-electrode cell is ¼ of that obtained for the same material acting as the working electrode in a three-electrode cell. According to Pourbaix [30], the following relationship gives the thermodynamic equilibrium for the different niobium species: 2NbO2 þ H2O ↔ Nb2O5 þ 2Hþ þ 2e
,

3.2.2. Analysis of the galvanostatic charge-discharge curves Fig. 5 shows the electrochemical data obtained for the symmetric coin cell using the Nb2O5@AC-CNT electrodes. Fig. 5(a) shows the galvanostatic charge-discharge (GCD) curves for the Nb2O5@AC-CNT electrodes housed in the coin cell as a function of the voltage range. As seen, triangular profiles were obtained in accordance to the expected behavior for pseudocapacitors, i.e., in the case of a “battery-like” behavior, the GCD curves are commonly characterized by a plateau due to small change of the voltage during charge discharge. We observed no water splitting up to 1.8 V. Fig. 5(b and c) show the discharge curves at different voltage ranges as a function of the applied gravimetric current. Typical triangular GCD curves were also verified in this study, revealing that the use of a voltage range of 1.8 V can be quite beneficial for the energy storage process (see further discussion). It was verified that the coulombic efficiency was ~99% for the 1.0–1.8 V interval (see Fig. 5(d)). After this interval, the efficiency of the device decreased to ~93% at 2.0 V. The specific capacitance, energy, and power are presented in Fig. 5(e). A maximum specific capacitance of ~220.48 F g
1 was verified at 0.5 A g
1 for the voltage range of 1.8 V. On the contrary, a lower value of ~112.45 F g
1 for 0.5 A g
1 and 1.0 V. From 0.5 to 30 A g
1 the capacitance retentions were 71% and 52% at 1.0 V and 1.8 V, respectively. Despite the lower capacitance retention, the use of a 1.8 V voltage window is quite beneficial for the energy storage purposes since the energy stored in a supercapacitor depends on the square of its voltage (see further discussion). It is worth mentioning that the maximum specific capacitance verified for the substrate alone (e.g., AC-CNT) was ~100 F g
1, i.e., the presence of the niobium oxide considerably improved the overall capacitance of the composite material. A comprehensive analysis of an electrochemical capacitor using symmetrical ACCNT electrodes may be found elsewhere [7].

(5)

where E0 ¼
0.289 V
0.060  pH. Therefore, it is straightforward to demonstrate that for an experimental electrode potential value of ca. 0.6 V/SCE that the relationship a(Nb(V))/a(Nb(IV))2 ¼ 6  1049 holds for the ratio of the chemical activities (a) of the redox species. Therefore, the redox equilibrium in neutral solutions is strongly displaced to the right in favor of the Nb2O5 conferring very high stability for this oxide. In light of the above analysis, one can argue that the redox activity involving the Nb2O5 nanoparticles, induced by application of an external electric field, results in the pseudocapacitive behavior through a doping and de-doping process due to a strong interaction of the oxide with water

317

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Fig. 5. Electrochemical data obtained for the symmetric coin cell using the Nb2O5@AC-CNT electrodes: (a) Galvanostatic charge-discharge (GCD) curves obtained for a gravimetric current of 5 A g
1 as a function of the cell voltage; (b) GCD at 1.0 V at different gravimetric currents; (c) GCD at 1.8 V at different gravimetric currents; (d) Data extracted from Fig.(a) revealing the coulombic efficiency as a function of the voltage; (e) Data extracted from Figs.(b) and (c) showing the specific capacitance, energy, and power for the electrodes of the coin cell as a function of the gravimetric current, and (f) Ragone plot for the symmetric coin cell calculated for the different cell voltages (e.g., 1.0 V and 1.8 V). Electrolyte: 1.0 M Li2SO4.

energy storage process is superior to that verified for other energy storage devices, such as batteries, and are equal to ion-lithium capacitors. Furthermore, the present findings are in greatly concurrent with that previously reported in the literature regarded with the use of aqueous electrolytes [7,39–47]. Therefore, the advantages of the designed electrode architecture for the composite electrodes are the formation of highly stable electrode material that sustains a wide capacitive voltage range in aqueous solutions allied to a very high resistance to wear during the long-term cyclability testes accomplished using 250 thousand cycles. On the contrary, most of the literature reports used less than 20 thousand cycles for evaluation of the stability of the Nb2O5-based electrodes in organic electrolytes.

It was verified for the symmetric coin cell operating at 1.0 V values for the specific power and energy in the ranges of 0.06–3.6 kW kg
1 and 0.63–0.96 Wh kg
1, respectively, for the applied gravimetric currents in the range of 0.5–30 A g
1. On the other hand, for the symmetric coin cell operating at 1.8 V, the corresponding values of these parameters were of 0.11–6.5 kW kg
1 (specific power) and 3.1–6.1 Wh kg
1 (specific energy). Therefore, a significant improvement (e.g., 324%) of the energy storage capabilities was obtained at 1.8 V. For all cases, the energy and power density was divided by 4, as suggested by Gogotsi and Simon [38]. To summarize the above findings, we present a Ragone plot in Fig. 5(f). As can be seen, the specific power and energy values of our coin cell are situated in the superior right part of the graph as is indicated by the red frame rectangle. The position of this frame on the Ragone confirms that the overall performance exhibited by our coin cell for the 318

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low frequencies that coth(x1/2)/(x1/2) ffi 1/3 þ 1/x. Therefore, the transfer function at low frequencies is given by the following equation:

3.2.3. Analysis of the impedance data Fig. 6(a) shows the complex-plane plot took at 0.5 V for the symmetric coin cell housing the Nb2O5@AC-CNT electrodes. As can be seen, the impedance plot obtained before the cyclability tests was characterized by a capacitive straight line at low-frequencies followed by a negligible depressed semicircle observed at very high frequencies. In addition, at high frequencies, there is an inclined line with a phase angle of 64 . In principle, these findings indicate the occurrence of negligible passivation of the metallic substrate and the presence of a porous electrode structure which is theoretically predicted by a phase angle of 45 in the case of evenly distributed identical cylindrical pores [48,49]. The fact that the profiles of the complex-plane plot, before (as-grown) and after 550,000 cycles of charge-discharge, are practically the same indicate great stability of the coin cell against wear due to the chemical dissolution of the metallic components and/or corrosion of the metallic current collector. As previously described by us [40,41], the transfer function (ZPDL) representing the impedance behavior of a coin cell composed of symmetric porous electrodes can be expressed as follows: ZPDL ¼ RESR þ

Rp BðjωÞ1=2

!

  coth BðjωÞ1=2

 ZPDL ¼ RESR þ R
j

1 ωCp

 (7)

where R* ¼ Rp/3. The overall ohmic resistance R (¼ RESR þ R*) is obtained by extrapolation of the almost vertical capacitive line on the real axis (Z/). This transfer function can be represented by a circuit containing the resistance R connected in series with the overall capacitance Cp. In practice, the RESR value obtained from extrapolation at high frequencies is subtracted to obtain the value of R*. Due to the frequency dispersion phenomenon inherent to solid electrodes, the capacitor Cp was replaced by a constant phase element (e.g., ZCPE ¼ 1/Y0(jω)n) in order to obtain a good simulation. Accordingly, at high frequencies, one has that coth(x1/2) ffi 1, and the transfer function is given as follows: ZPDL ¼ RESR þ

(6)

Rp BðjωÞ1=2

¼ RESR þ

1 Y0ðHFÞ ðjωÞ1=2

(8)

pffiffiffi where Y0ðHFÞ ¼ 2π np r 3=2 Cdl 1=2 =ρ1=2 and B ¼ Rp  Y0(HF). Thus, the transfer function can be represented by a circuit containing the RESR connected in series with the impedance represented by Y0(HF). At least in principle, Y0(HF) includes information about the pores (e.g., np and r). However, the parameters r and np cannot be determined separately using the numerical simulation/fitting procedure since only the product npr3/2 can be obtained in practice. Fig. 6(b) shows the values of the different parameters extracted from the impedance analysis. As can be seen, the use of a high voltage (1.8 V) caused rapid degradation of the coin cell while the application of a lower voltage (1.0 V) resulted in a very high lifespan for the coin cell. The RESR values determined at 1.0 V were practically independent of the cycle number (e.g., GCD experiments) indicating high stability of the

where B ¼ ð2ρl2 Cdl =rÞ1=2 and Cp ¼ 2πnprlCdl. The parameters r and l are the radius and the pore length, respectively, ρ is the specific electrolyte resistivity, Rp is the electrolyte resistance within the pore, np is the number of identical cylindrical pores, Cdl is the capacitance of the electrical double-layer referring to a single pore and Cp is the overall capacitance corresponding to the assembly of pores. RESR is the equivalent series resistance. As previously discussed [40,41], the impedance analysis based on the high- and low-frequency limits, excluding the medium-frequency region, avoids the undesirable theoretical complications inherent to the fact that in real cases the pores are not perfectly cylindrical and not evenly distributed on the surface [50]. In light of these considerations, one has at

Fig. 6. EIS plots of the Nb2O5@AC-CNT electrodes in 1.0 M Li2SO4 electrolyte before and after the long-term cycling process. The impedance data were obtained at (a) 0.5 V. (b) Impedance data obtained at 1.0 V and 1.8 V whose values were extracted by applying the porous electrode model. Cyclability tests were conducted using the galvanostatic charge-discharge (GCD) method carried out at 50 A g
1. 319

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unknowns in the porous electrode model. A more pronounced change in B values was verified at 1.8 V.

composite electrodes, i.e., the electrode wear and/or passivation of the current collectors were strongly inhibited in the present case. On the contrary, in the case of the 1.8 V, a sudden increase of the RESR values was observed after 100,000 cycles. As already mentioned in this work, the sudden increase of the RESR values is caused by the passivation of the metallic components of the coin cell, especially the current collector. The occurrence of this passivation process can be verified by inspection of the impedance findings obtained for the medium-to-high frequencies, which are characteristic of this type of process [48]. As can be seen, the other impedance parameters followed a similar trend as a function of the cycle number. As expected, the specific capacitance determined at 1.8 V and represented by Y0(LF)(∝ CEDL) was higher in comparison to the that found at 1.0 V. Average pseudocapacitances in the range of ~32–64 F sn
1 g
1 were verified at 1.0 V, while values in the range of ~96–128 F sn
1 g
1 were observed at 1.8 V. The analysis of Rp revealed a nonsystematic variation in the range of 6–19 mΩ g for the coin cell polarized at 1.0 V. By contrast, the coin cell polarized at 1.8 V showed an increase of Rp from 11 to 30 mΩ g, revealing that the electrolyte penetration inside the porous electrode structure depends on the cycle number and the applied voltage. The impedance data obtained at high frequencies for the coin cell polarized at 1.0 V showed the Y0(HF) values increased from 144 to 432 Ω
1 sn
1 g
1 as a function of the cycle number. Considering from the theoretical viewpoint that Y0(HF) ∝ npr1/2Cdl/ρ1/2, it is possible to argue by assuming that np, r, and ρ are constant that the prolonged chargedischarge process increased the pore capacitance (Cdl) enclosed by the pore walls. As can be seen, the increase in Y0(HF) values can be more pronounced at 1.8 V. The parameter B determined at 1.0 V revealed a nonsystematic increase from 0.7 to 3.1 sn as a function of the cycle number. In principle, this behavior can indicates a change in the pore geometry (r and/or l) as a function of the charge-discharge cycles. However, as already mentioned, this type of analysis involving the parameters l and r is not straightforward since there is one equation and two

3.2.4. Analysis of the operando Raman study Fig. 7 shows the Raman spectra took from Nb2O5@AC-CNT on coin cell under dynamic conditions and their respective analyses. Fig. 7(a) presents a voltammetric curve took just before the Raman operando analyses. These curves were taken at 100 mV s
1 using different voltage intervals ranging from 0 to 1.8 V, thus evidencing good electrochemical responses of the system. Fig. 7(b) shows the operando Raman spectra captured during the CV experiment at 1.0 mV s
1 using a 1.0 M Li2SO4 aqueous solution. The CVs are shown in Fig. 7(c). The applied voltage for each spectrum and the CV current are indicated by the blue arrow between Fig. 7(b) and (c). We separate our analyses on the Nb2O5, electrolyte, and MWCNT Raman signature regions. From the analysis of these findings, we did not observe any new band formed in the Nb2O5 region. This suggests that new phases referring to Nb2O5 are not being formed under polarization conditions of the Nb2O5@AC-CNT composite electrode. In order to better investigate that point, operando XRD diffraction analyses using synchrotron light was performed on Nb2O5@AC-CNT using specified cell (Fig. S4), evidencing that Nb2O5.nH2O nanoparticles @ MWCNT have amorphous form when hydrated, as was previously shown by Nakajima et al. [51]. Fig. 7(d) presents data from the electrolyte on the left-hand side and MWCNT on the right-hand side. We can observe in Fig. 7(e) the intensity increase of sulfate peak centered at 990 cm
1. We also observe reversible shifts on the D band position from 1329 to 1334 cm
1 as well as from 1334 to 1329 cm
1 range along of the CV experiments. We understand that these reversible D band shifts cause reversible changes on double resonance process, which caused the appearance of the D band peak. For our best understanding, the applied voltage caused charge-transfer and adsorption processes at the electrode/electrolyte interface [52,53], resulting in an unbalance on the Brillouin zone. In the case of Fig. 7(f), we observed a considerable reduction on D band intensity, which we attributed to blocking on MWCNT signal

Fig. 7. (a) Voltammetric curves obtained for the Nb2O5@AC-CNT composite at 100 mV s
1 before the in situ Raman analyses. (b) operando Raman spectra registered for the symmetric cell composed of Nb2O5@AC-CNT during the cyclic voltammetry experiments accomplished at 1.0 mV s
1 using the voltage window of 0–1.8 V. (d) First-order Raman spectra of the electrode evidencing the band at 990 cm
1 and the D, G, and D0 bands. (e) The Raman band at 990 cm
1 as a function of the applied potential. (fa) Plot of the intensity and position of the D band as a function of the cell potential. 320

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due to very slow CV scan rate applied for appropriated measurement. In other words, we believe this reduction is related to adsorption of Liþ and SO2
4 on the top surface of carbon regions of the composite electrode, covering edges, where most of the defects are located [16,20]. By polarizing the working electrode, the ions are attracted to the carbon edges. The edge domains present on carbon structures are well-known to propitiate fast interaction with the electrolyte species. By covering the edges of carbon regions with Liþ and SO2
4 , the Raman signal ascribed to the D band decreased while the Raman band attributed to SO2
4 increased (Fig. 7(e)). We repeated the experiment on the 101th and 1001th cycles, and the behavior is pretty much the same.

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4. Conclusions We report in this work a novel method for fabrication of nanostructured porous electrodes composed of niobium pentoxide nanoparticles dispersed on multiwalled carbon nanotubes and activated carbon without the use of binders and/or additives. It was verified that most of the Nb2O5 nanoparticles were partially covered by a carbon mesoporous thin film which quickly drains the current, thus avoiding the passivation process. The Nb2O5@AC-CNT composite resulted in a high voltage range of 1.8 V in neutral solutions since the presence of Nb2O5 strongly inhibited the water-splitting reaction. A maximum specific capacitance of 232 F g
1 was verified for the composite electrodes. This superior specific capacitance is in agreement with the presence of a Faradaic process involving the adsorption/desorption of hydrated protons during the oxidation/reduction of the Nb-sites. It was verified that the electrochemical properties remained stable even after 200 thousand charge/discharge cycles, while the impedance analysis revealed that the coin cell was deactivated after 250 thousand cycles due to a dominance of the ohmic components. The in-situ Raman characterization studies (operando studies) did not reveal any formation of new bands accounting for chemical changes occurring in the Nb2O5 structure in neutral aqueous solutions. In addition, the operando XRD diffraction studies showed that in the presence of water the hydrated oxide (Nb2O5.nH2O) nanoparticles @ MWCNT are amorphous. Acknowledgements The authors are very grateful to LNNano/CNPEM for SEM, HRTEM (especially Jefferson Bettini), XPS and XPD beamline at LNLS/CNPEM support and staff, to CCS Nano/FEEC for SEM support, and to the financial support from the Brazilian funding agencies CNPq (301486/ 2016-6), FAPESP (2014/02163-7, 2017/11958-1) and CAPES. The authors gratefully acknowledge support from Shell and the strategic importance of the support given by ANP (Brazil's National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. L.M. Da Silva wishes to thank “Fundaç~ao do Amparo a Pesquisa do Estado de Minas Gerais – FAPEMIG” and National Council for Scientific and Technological Development – CNPq (PQ-2 grant). Authors are also grateful to CBMM Brazilian miner for Nb2O5 powder donation. Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.ensm.2019.08.007. References [1] I. Buchmann, BU-209: How Does a Supercapacitor Work? Batter. Univ., 2018. [2] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, first ed., Springer US, New York, 1999. [3] F. Beguin, E. Frąckowiak, Supercapacitors : Materials, Systems, and Applications, Wiley-VCH, 2013. [4] G. Li, X. Wang, X. Ma, Nb2O5-carbon core-shell nanocomposite as anode material for lithium ion battery, J. Energy Chem. 22 (2013) 357–362, https://doi.org/ 10.1016/S2095-4956(13)60045-5. 321

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