Ti3+ Induced Brown TiO2 Nanotubes for High Performance Sodium

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Ti3+ induced Brown TiO2 Nanotubes for High Performance Sodium Ion Hybrid Capacitors Binson Babu, Sanjay Gopal Ullattil, Ranjith Prasannachandran, Jithesh Kavil, Pradeepan Periyat, and Manikoth M Shaijumon ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00236 • Publication Date (Web): 17 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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ACS Sustainable Chemistry & Engineering

Ti3+ induced Brown TiO2 Nanotubes for High Performance Sodium Ion Hybrid Capacitors Binson Babu,a† Sanjay Gopal Ullattil,b† Ranjith Prasannachandran,a Jithesh Kavil,b Pradeepan Periyatb,c*and Manikoth M. Shaijumona* a

School of Physics, Indian Institute of Science Education and Research

Thiruvananthapuram, Maruthamala PO, Vithura, Thiruvananthapuram, Kerala, 695551, India. b

Department of Chemistry, University of Calicut, Kerala, India-673 635.

c

Department of Chemistry, Central University of Kerala, India-671314. † *

These authors have contributed equally.

[email protected], [email protected], [email protected]

ABSTRACT Sodium ion hybrid capacitors are attracting great attention and are emerging as promising energy storage devices, with their remarkable footsteps in energy and power densities. However, development of efficient electrode materials that result in minimum trade-off between their energy and power densities, and that allow long-term cycling stability, still remains a challenge for realizing their full potential as an alternate energy storage system for commercial applications. Herein, for the first time, we study the sodium-ion intercalation pseudocapacitance behavior of brown TiO2 nanotubes for their application as efficient anode material for Na-ion hybrid capacitor. We synthesized semi-crystalline and crystalline anatase brown TiO2 nanotubes, aggregated in a flower-like morphology, through hydrothermal route, and performed detailed electrochemical studies. The kinetic studies reveal that semicrystalline brown TiO2 exhibit Na-ion intercalation pseudocapacitive behavior with 57% of capacitive storage at 1.0 mV s-1 whereas crystalline brown TiO2 is more faradaic in nature. Further, hybrid Na-ion capacitors are fabricated with brown TiO2 materials as anode and activated carbon as cathode, and the fabricated device showed excellent electrochemical

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performance with a high energy density of ~ 68 Wh kg-1 and a high power density of ~12.5 kW kg-1 and with good cycling stability up to 10,000 cycles with ~ 80% capacitve retention. The obtained results represent a promising approach towards developing efficient electrodes for hybrid Na-ion capacitors.

Keywords: energy storage; semi-crystalline brown TiO2 nanotubes; pseudocapacitance; hybrid sodium ion capacitor.

Introduction Energy is one of the biggest challenges of this century and with the development of renewable energy sources, tremendous efforts are being devoted to design efficient energy storage systems. Electrochemical storage of energy by using batteries and supercapacitors is a mature technology, however, still needs improvement in terms of energy and power densities and long-term cycling stability.1-3 The concept of hybrid Li/Na-ion capacitors that combine the virtue of faradaic and double layer processes is quite promising and result in improved electrochemical performance in terms of both energy and power densities. Amatucci et al.,4 integrated the high surface area carbonaceous material as cathode and the crystalline Liintercalating material as anode in Li-based non-aqueous electrolyte. Here the cathode provides high power density by virtue of the double layer capacitive behaviour, while the intercalation behavior of the anode results in higher energy density. Such a hybrid energy storage device achieves higher energy density than supercapacitor and higher power density than LIB, thus placing it at the upper right position in the Ragone plot.5-7 With the on-going thrust on sodium chemistry, owing to their huge abundance over lithium and similar electrochemical property with that of lithium,8-10 the hybrid ion capacitor based on Na-ion intercalation/redox anode electrode with double layer capacitive cathode is


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gaining increasing interest among the energy storage research community.11-16 Several sodium-based oxide materials have been initially studied for Na-ion battery, expecting a direct translation of lithium chemistry. However, developing efficient sodium storage materials for the development of high performance Na-ion capacitor systems still remains a challenge. Though hard carbonaceous materials are reported to be a decent anode candidate, its random structure inhibits the Na-insertion capacity at high rates.17 Several conversion as well as alloying electrodes, for example, tin and antimony based materials, have been explored as possible Na-ion anode material, owing to the interest based on their high initial capacity. Poor cyclability due to their huge volume expansion upon repeated chargedischarge cycles still remains a key issue.18-20 With very low volume expansion of the lattice, titanium based compounds, on the other hand, exhibit much improved cycling stability and are being considered as promising negative electrodes for sodium ion batteries. Though various polymorphs such as anatase, rutile, brookite and amorphous titania have been explored,21-22 the low electron mobility (~10-12 S cm-1) originating from the wide band gap (~3.2 eV) and low sodium ion diffusivity in crystalline bulk TiO2 hinder their wide usage.23 Controlling the TiO2 particle size to nano dimension shortens the Na-diffusion path, and hence increases the ionic diffusivity.24 Recently Ti3+ induced TiO2 materials, named ‘brown or black TiO2’ emerged as a high conductive polymorph with wide area absorption and are being studied as a potential candidates not only in the area of photocatalysis and dye sensitized solar cells, but also for electrochemical energy storage.25-28 The enhanced conductivity arises due to the colossal bandgap narrowing, which is attributed to the introduction of Ti3+ during TiO2 reduction by foreign donor atoms or the oxygen deficiencies or defects created in the nanostructures.29 Herein, for the first time, we study the sodium-ion intercalation pseudocapacitance behavior of brown TiO2 nanotubes for their application as efficient anode material for Na-ion


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hybrid capacitor. Brown TiO2 nanotubes with semi-crystalline nature (BTNT) and dark brown colored highly crystalline TiO2 nanotubes (DBTNT) with flower-like morphology are obtained through hydrothermal route. A comparative study of the electrochemical performance of semi-crystalline and crystalline anatase brown TiO2 has been conducted with respect to Na-Metal and the kinetic behavior of each material is studied. Hybrid Na-ion capacitors are fabricated with semi-crystalline and crystalline brown TiO2 nanotubes as anode and activated carbon as cathode, and the devices showed high energy and power densities with excellent cycling stability. With its high conductivity, porous nature and high pseudocapacitive Na-ion intercalation behavior, brown TiO2 represents a potential candidate for Na-ion hybrid capacitors.

EXPERIMENTAL Materials synthesis and characterization Titanium (IV) butoxide, 97% (Sigma-Aldrich), isopropanol, extrapure AR (Merck), hydrogen peroxide, 30% (Merck), manganese acetate tetrahydrate, extrapure, AR (Merck), sodium hydroxide pellets (Merck), hydrochloric acid, 35% (Merck), were all used as received without further purification. Deionized water was used in all the experiments. 0.4 M Titanium (IV) butoxide (6.8 g) was mixed with 50 ml isopropanol and stirred for 10 min at 600 rpm. 0.02 M Mn (II) acetate tetrahydrate (0.2451 g in 50 ml H2O2) was added quickly to the above solution with continuous stirring. Stirring was continued for further 30 min after the addition of the dopant. This step has facilitated hydroxylation and Mn (II) doping simultaneously. The dark brown sol formed was kept in a Teflon lined autoclave at 150 °C for 12 h. The brown anatase titania solid was obtained after drying at 80 °C. For the preparation of brown TiO2 nanotubes, 3 g of the brown anatase TiO2 sample was dispersed in 10 M NaOH solution (32 g of NaOH pellets in 80 ml water), which was stirred


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continuously for 2 h and the solution was kept in a Teflon lined autoclave at 150 °C for 14 h. The solid sample obtained was washed once using 100 ml of 1M HCl solution at an initial pH ~0.6 - 0.7 for 1 h followed by five times washing using 100 ml of pure water. The solution pH remained in between ~8 - 9 after each wash followed by drying at 80 °C, leading to the formation of brown TiO2 nanotubes with semi-crystalline phase (BTNT), and upon further annealing at 500 °C results in the formation of dark brown crystalline anatase TiO2 (DBTNT). The Brown TiO2 samples were characterized using various analytic and spectroscopic techniques including powder X-ray diffraction (XRD) (Empyrean, PANalytical XRD instrument with Cu-Kα radiation, λ = 1.54 Å), Raman spectroscopy (LabRAM HR, HORIBA JOBINYVON Raman spectrophotometer with 532 nm), FT-IR spectroscopy (IR Prestige-21, FT-IR SHIMADZU), UV-visible absorption spectroscopy (Jasco-V-550 UV-visible spectrophotometer), Thermogravimetric analysis (TGA) (SDT Q800), X-ray Photoelectron Spectroscopy (Scienta Omicron XPS, Al-Kα source ~1400eV), Scanning electron microscopy (Nova NanoSEM 450, FEI) and Transmission electron microscopy (Tecnai G2 TF20 200 kV). The Brunauer- Emmet-Teller (BET) surface area measurements were done by using Micromeritics 3-Flex surface characterization analyser and the N2 adsorptiondesorption measurements were performed at 77 K up to a maximum relative pressure of 1 bar.

Electrochemical characterization The Brown TiO2 electrodes were prepared by mixing the active material, Super P (Alfa Aesar) and carboxymethyl cellulose (CMC) binder (Sigma-Aldrich) in a weight ratio of 80:10:10 in dilute citric acid solution and the slurry was uniformly coated on a 12 mm diameter stainless steel current collector, with an active material mass loading density of ~


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2.3 mg/cm2. Activated carbon electrodes were prepared by making a thick slurry with 80 wt% active material, 10 wt% acetylene black and 10 wt% polytetrafluroethylene (PTFE, 60 wt% in H2O, Sigma) in ethanol, which was rolled into a uniform film and pressed onto stainless steel mesh (Alfa Aesar), with an active material mass loading of ~5.8 mg/cm2. The electrodes were dried in vacuum at 120 °C for 4 h. The anodic and cathodic part was evaluated individually with metallic sodium as counter and reference electrode in a non-aqueous electrolyte solution of 1 M NaPF6 in propylene carbonate (PC), separated by glass microfiber filter separator (Whatman, UK). The device fabrication was carried out in a standard CR 2032 coin cell assembled inside the argon-filled glove box (MBRAUN, Germany) (H2O < 0.1 ppm, O2 < 0.1 ppm). Na-ion full cell hybrid capacitor was assembled with Brown TiO2 electrode as anode and the activated carbon (AC) (purchased from Active Char pvt. ltd.) as cathode, with optimized mass loading, by using same organic electrolyte. All the electrochemical characterizations such as cyclic voltammetry (CV), galvanostatic chargedischarge and electrochemical impedance spectroscopy (EIS) were performed in a multichannel electrochemical workstation (Biologic, VMP3).

RESULTS AND DISCUSSION Brown TiO2 nanotubes (BTNT) have been synthesized through hydrothermal technique. The XRD pattern (Figure 1a) for as-prepared BTNT (inset of figure 1a), shows only two anatase crystalline diffraction peaks (101) and (200), indicating the semi-crystalline nature, compared to the annealed material (DBTNT) which is dark brown in colour. This shows pure anatase phase of TiO2.

25-26, 30-31

The peaks observed in DBTNT at 25.310, 37.170, 38.230, 38.630,

47.970, 54.390, 55.000, 62.930, 69.370, 70.180, 75.410 and 82.840 corresponds to (101), (103), (004), (112), (200), (105), (211), (204), (116), (220), (215) and (224) planes which represent the Tetragonal anatase TiO2 crystal system with a space group of I41/amd (JCPDS 01-075-


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2550) and is compared with commercially available white anatase TiO2 (W-TiO2) (SigmaAldrich). The Raman spectra analysis (Figure 1b) of both prepared samples show broadening of peaks and shifting of Eg mode towards higher wave number, compared to that of commercial pristine white anatase TiO2, indicative of lattice defects associated with oxygen vacancies.32 During hydrothermal synthesis the presence of Mn2+ reduces Ti4+ to Ti3+, inducing oxygen vacancies in the lattice sites of TiO2. This results in colossal decrease in the band gap energy from ~ 3.0 eV (white TiO2) to ~ 1.86 eV (semi crystalline BTNT) and ~ 1.68 eV (crystalline DBTNT), which are calculated using Tauc plot (Figure 1c) derived from UV-absorption spectra (inset of Figure 1c), resulting in enhanced conducting nature of the brown TiO2 materials. FT-IR spectrum recorded for the sample (Figure 1d) shows a peak around 466 cm-1 that is attributed to the presence of Ti-O stretching bond, confirming the formation of TiO2. 33-34 The sharp peaks at 3390 cm-1 and 1618 cm-1 are due to the hydroxyl stretching and bending frequencies, respectively.35 A small peak observed at 924 cm-1 is due to the δ - (Ti-OH) deformation, indicating a slightly deformed crystal lattice. 36,23


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Figure 1. (a) Powder XRD pattern (b) Raman spectra (c) Tau plot derived from UV-absorbance spectra (inset) and (d) FTIR spectra of BTNT, DBTNT and commercial pristine white anatase TiO2.

The X-ray Photoelectron spectroscopy is a powerful characterization tool for quantitative elemental analysis, which can be used to find the lattice oxygen vacancies in Brown TiO2. Figure 2a shows the typical wide XPS survey spectra recorded for semi-crystalline BTNT and crystalline DBTNT. The de-convoluted XPS spectrum for Ti 2p (Figure 2b) shows characteristic peaks of Ti3+ located at 456.2 eV and 461.8 eV, corresponding to Ti3+ 2p3/2 and Ti3+ 2p1/2, as shoulder peaks of Ti4+ 2p3/2 and Ti4+ 2p1/2, located at 458.2 eV and 463.9 eV, respectively for BTNT. For crystalline DBTNT, the observed peaks of Ti3+ 2p3/2 at 456.4 eV and Ti3+ 2p1/2 at 462.0 eV, as shoulder peaks of Ti4+ 2p3/2 at 458.6 eV and Ti4+ 2p1/2 at 464.2


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eV, are recognized as signature features of highly conductive Ti3+ induced defective TiO2. 3031, 37-38

The generation of oxygen vacancies at the lattice sites due to Mn2+ doping induces

Ti3+, which could be quantified by considering the electrostatic neutrality.39 The ratio of Ti3+/Ti4+ is ~14.5% for BTNT and ~ 14.2% for DBTNT, which are calculated from the % concentration of Ti3+ and Ti4+ from the XPS spectra. For maintaining the electrostatic balance in TiO2, each oxygen vacancy is accompanied with two Ti3+ and the concentration of oxygen deficiencies is found to be ~ 3.17% for BTNT and ~ 3.11% for DBTNT. The de-convoluted O 1s spectra at 531.1 eV for BTNT and 531.3 eV for DBTNT are due to the oxygen deficiency induced by the reduction environment during the hydrothermal synthesis,38 while the observed peak at 532.7 eV corresponds to surface adsorbed oxygen. The presence of slight amount of Mn-dopant is visible at 640.0 eV (Mn2p3/2) and 652.32 eV (Mn2p1/2) in Mn 2p spectra for both samples (Figure 2d).


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Figure 2. (a) Wide area XPS survey spectra of BTNT and DBTNT, (b-d) high resolution XPS spectra for Ti 2p, O 1s and Mn 2p, respectively, recorded for the same samples.

The as synthesized brown TiO2 (BTNT) exhibits an interesting morphology where the nanotubes are assembled together forming a flower like shape with an average diameter in the micrometer range (Figure 3a). The TEM images (Figure 3b and c) clearly depict the presence of nanotubes at the crossroads of two micro flowers. The nanotubes with 5 mV s-1), the diffusion limitation happens which is analogous to the two regions in Figure 5c. This large deviation in peak shifts and the rate limitation at larger scan rates (charging time < 600 s) arises from numerous factors like ohmic resistance of the active material, solid-electrolyte interphase resistance, Na-ion diffusion limitation and polarization, resulting in the requirement of overpotential to deliver higher currents.49 For quantitative analysis of the capacitive nature of both the samples, the current at each potential ‘𝑖 𝑉 ’ is assumed to have contributions from non-faradaic ‘𝑘! 𝑣’ and faradaic ‘𝑘! 𝑣 !/! ’ processes.50

ie,

𝑖 𝑉 = 𝑘! 𝑣 + 𝑘! 𝑣 !/!

(3)

By rearranging, we get

! ! !

!!

= 𝑘! 𝑣 !/! + 𝑘!

(4)

where 𝑘! and 𝑘! are the capacitive and faradaic coefficients, determined from the ‘slope’ and ‘y-intercept’, respectively, of the linear plot (Figure S5 c and d) generated from (4). By fitting the ‘𝑘-values’ in (3), we get the capacitive and faradaic current contributions. Figure 5e and f illustrates a capacitive contribution of ~ 57% (shaded area) at 1.0 mV s-1 for semi-crystalline


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Figure 5. Cyclic voltammograms measured at different scan rates from 0.1-5.0 mV s-1 for (a) semi-

crystalline BTNT and (b) crystalline DBTNT vs. Na/Na+, (c) plot showing specific peak current vs. sweep rates ranging from 0.1-1000 mV s-1, (d) curve shows the relation between percentage of intercalated Na-ions and 𝑣 !!/! . Cyclic Voltammogram of (e) semi-crystalline BTNT and (f)

crystalline DBTNT at 1.0 mV s-1 clearly indicating the capacitive current (shaded region) from total current, comparison of total charge storage in (g) semi-crystalline BTNT and (h) crystalline

DBTNT at different scan rates, clearly indicating the respective contribution to the total capacity from sodium intercalation and non-Faradaic capacitance.

BTNT and ~ 47% (shaded area) for crystalline DBTNT and is found to increase with scan rate. At a scan rate of 5 mV s-1, ~83% of the total specific capacity is contributed by capacitive storage for semi-crystalline BTNT, while crystalline DBTNT possesses ~ 69%. From the above qualitative and quantitative analysis, both the samples exhibit good kinetics but the semi-crystalline BTNT shows more capacitive nature due to its less crystalline nature with almost double the BET surface area than crystalline DBTNT. With good rate capability and capacitive retention, both materials show promising characteristics as good anode material for high performance hybrid Na-ion capacitor. However, due to high capacitive charge storage behavior, the semi-crystalline BTNT is expected to be more favorable than crystalline DBTNT. An asymmetric hybrid Na-ion capacitor is assembled by taking BTNT and DBNT as anode and coconut derived activated carbon (AC) with high BET surface area of ~ 1600 m2/g as cathode, respectively, by using 1M NaPF6 in propylene carbonate (PC) as the electrolyte. For the full cell fabrication, the electrode mass loading has been optimized so as to attain maximum energy density. Figure 6 a, b represents the cyclic voltammogram (CV) and galvanostatic charge-discharge voltage profiles of semi-crystalline BTNT, crystalline DBTNT and activated carbon (AC) electrodes, respectively, in a potential


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range of 0-3 V and 3 - 4.5 V vs. Na-metal. The CVs indicate good pseudocapacitive behavior, reulting from the intercalation of Na-ions in both brown TiO2

nanotubes and a near

rectangular geometry, resulting from the adsorption of PF6ˉ anions on the surface of activated cabon. At a constant current density of 100 mA g-1, individual specific capacities of 90 mAh g-1 for semi-crystalline BTNT and 100 mAh g-1 for crystalline DBTNT and 40 mAh g-1 for activated carbon electrode vs. Na-metal are obtained.

Figure 6. (a) Typical charge- discharge voltage profiles and (b) Cyclic voltammograms of semi-

crystalline BTNT, crystalline DBTNT and activated carbon, vs. Na/Na+ in half-cell configuration.

Hybrid Na-ion capacitor is fabricated with an optimum mass ratio of 1:2.25 for BTNT//AC and 1:2.5 for DBTNT//AC and the electrochemical studies are conducted in a potential range of 1 - 4 V. Apart from the typical hybrid nature, the CV of BTNT //AC full cell hybrid device (Figure 7a) resembles more of a double layer behavior, because of the fast psuedocapacitive nature of semicrystalline BTNT, compared to DBTNT//AC which is more more hybrid in nature (Figure 7b). Figure 7c, d shows the galvanostatic charge-discharge (CD) profile of BTNT //AC and DBTNT//AC hybrid capacitor. During charging the anions, PF6ˉ from the electrolyte, are adsorbed on the surface of activated carbon, whereas simultaneous pseudocapacitive intercalation of Na-ions occurs at the TiO2 electrode and the reverse process


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occurs during discharge. Figure 7e, f represents excellent rate capability of both hybrid devices with good capacity retention even after cycling at higher current density of 5 A g-1. Figure 7g shows the specific capacitance (F/g) of both hybrid device at different current densities (from CD) and scan rates (from CV), reflecting the rate capability of both devices. From galvanostatic charge-discharge profiles, the specific power density (𝑃! ) and specific energy density (𝐸! ) of the hybrid device are calculated by using the relations:,

𝑃! =

! ×! !

(5)

and 𝐸! = 𝑃! ×𝑡

where 𝑉 =

!!"# !!!"# !

(6)

, 𝑉!"# and 𝑉!"# being the initial and final discharge voltages, 𝐼 is the

applied current, 𝑀 is the total active mass of both electrodes (~ 6.8 mg for BTNT//AC and ~ 5.8 mg for DBTNT//AC) and 𝑡 is the time taken for discharge.51 A high energy density of ~ 68 Wh kg-1 is obtained at a power density of 625 W kg-1 for BTNT //AC and the device shows an energy density of ~ 23 Wh kg-1 even at 7.5 kW kg-1. But the DBTNT//AC full cell hybrid device shows only a maximum energy density of ~ 39 Wh kg-1 at 625 W kg-1 and ~ 18 Wh kg-1 for 7.5 kW kg-1. The fabricated BTNT //AC hybrid Na-ion capacitor prevails many of the reported Na-ion hybrid capacitors like Nb2O5 ns//PSC,12 N-TiO2//AC,52 Na-TNT//Graphite,53 NTO@CNT//PSC,54 MoS2-C//Graphite,55 V2O5-CNT//AC,56 Na-TNT//AC,57 G-NTP//GNS,14 Na2Ti2O4(OH)2//RK,

16

Nb2O5@C//rGO-50,13 C-NVP//CDC,58 as indicated in Ragone plot

(Figure 7h).


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Figure 7. Cyclic voltammograms at various scan rate of (a) BTNT//AC and (b) DBTNT//AC Na-ion hybrid capacitor (full cell) tested between 1 and 4.0 V, Charge/discharge cycling profiles of (c) BTNT//AC and (d) DBTNT//AC Na-ion hybrid capacitor at different constant current densities, rate capability plots of (e) BTNT//AC and (f) DBTNT//AC Na-ion hybrid capacitor device at different current densities with coulombic efficiencies, (g) specific capacitance (F/g) of both devices at different scan rate and current densities, (h) Ragone plots


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obtained for the Na-ion hybrid capacitors from present work are compared with reported Naion hybrid devices.

Cyclability is a major driving factor for the practical implementation of the hybrid supercapacitor technology. The fabricated BTNT//AC hybrid Na-ion capacitor exhibits excellent capacitive retention of ~80% even after 10,000 cycles with 100% coulombic efficiency at 1.0 A g-1 (Figure 8a) better than DBTNT//AC hybrid capacitor which exhibits a capacitive retention of ~ 62% even after 10,000 cycles. Further, electrochemical impedance spectroscopy (EIS) measurements are perfomed on BTNT //AC (Figure 8b) and DBTNT//AC (Figure 8c) Na-ion hybrid capacitor devices and the Nyquist plots are recorded before and after cycling the devices. Only a small shift is observed for the total equivalent series resistance (ESR) measured

before and after cycling of BTNT//AC than DBTNT//AC.

Equivalent circuit generated by Z-fit technique is shown as inset of Figure 8b and c. For DBTNT//AC hybrid capacitor, the equivalent circuit contain a warburg coefficient element (w) having a waburg coefficient of ~ 51 Ω s-1/2, an indication of diffusion limitation in the device that is not included in the equivalent circuit of BTNT//AC hybrid capacitor, further reflecting on its better stability. The electrochemical performances of hybrid devices could be further improved by making appropriate carbon composites of the anode material. Postmortem studies of the electrode after 10,000 cycles were carried out and the XRD (Figure 8d) and SEM images (Figure S6)

of the cycled brown TiO2 electrode revealed no major

structural deterioration, except for few cracks as seen in the SEM images (Figure S6). The electrochemical analysis of both full cell hybrid device shows that the semcrystalline BTNT is more favorable anode material than crystalline DBTNT due to its high capacitive nature which is attributed to its slight amorphous nature and high surface area.


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Figure 8. (a) Cycling performance as well as coulombic efficiency of BTNT//AC and DBTNT//AC Na-ion hybrid device at 1.0 A g-1. Nyquist plots obtained before and after cycling with fitted curves for (b) BTNT//AC and (c) DBTNT//AC Na-ion hybrid capacitor are shown with corresponding equivalent circuits as insets in respective figures. (d) XRD of semicrystalline BTNT and crystalline DBTNT electrodes before and after 10,000 cycles.

CONCLUSION To summarize, we combined the potential of brown TiO2 material as anode along with high performance activated porous carbon as cathode in a Na-ion hybrid capacitor. The hydrothermally synthesized semi-crystalline and pure crystalline anatase brown TiO2 nanotubes aggregated in a flower like morphology with a high surface area, exhibited good


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electrochemical properties as efficient pseudocapacitive Na-intercalation material. The semicrystalline BTNT exhibits more capacitive behavior contributing to more than half the total storage capacity (~57% at 1.0 mV s-1) whereas the crystalline DBTNT exhibits ~ 53% faradaic in nature. The Na-ion hybrid capacitor fabricated with both brown TiO2 along with activated carbon (AC) shows that the semi-crystalline BTNT is more suitable anode for hybrid capacitor. The fabricated BTNT//AC hybrid device exhibits a maximum energy density of ~ 68 Wh kg-1 and a maximum power density of ~12.5 kW kg-1, with an excellent capacitive retention of ~ 80% even after 10,000 cycles, thus making the highly conductive brown TiO2 as a promising material for sodium ion hybrid capacitors. ASSOCIATED CONTENT Supporting Information. Additional data including N2 adsorption-desorption isotherm and pore volume distribution of BTNT and DBTNT; Thermogravimmetric Analysis (TGA) spectrum of samples; HR-SEM image and Raman Spectra of fully discharged samples; Cyclic voltammogram and i/ 𝑣 ½ vs. 𝑣 ½ curves of both BTNT and DBTNT samples; SEM images of BTNT and DBTNT electrodes for before and after cycling. AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected], [email protected] ACKNOWLEDGEMENTS MMS gratefully acknowledge financial support from Technology Mission Division, Department of Science & Technology, Govt. of India [DST/TMD/MES/2k16/114]. PP and SGU are thankful to UGC for the startup grant and BSR fellowship. REFERENCES 1. Dunn, B.; Kamath, H.; Tarascon, J.-M., Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334 (6058), 928-935.


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Table of Contents


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Synopsis Semi-crystalline brown TiO2 nanotubes aggregate in flower like morphology, which are studied as efficient anode material for Na-ion hybrid capacitor.


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Ti3+ Induced Brown TiO2 Nanotubes for High Performance Sodium

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