ALIGNED CNT FORESTS ON STAINLESS STEEL MESH FOR

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ALIGNED CNT FORESTS ON STAINLESS STEEL MESH FOR FLEXIBLE SUPERCAPACITOR ELECTRODE WITH HIGH CAPACITANCE AND POWER DENSITY Piyush Avasthi, Akash Kumar, and Viswanath Balakrishnan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02355 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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ACS Applied Nano Materials

ALIGNED CNT FORESTS ON STAINLESS STEEL MESH FOR FLEXIBLE

SUPERCAPACITOR

ELECTRODE

WITH

HIGH

CAPACITANCE AND POWER DENSITY Piyush Avasthi, Akash Kumar, Viswanath Balakrishnan* School of Engineering, Indian Institute of Technology Mandi, Himachal Pradesh, 175005, India * Corresponding author, E-mail: [email protected]

ABSTRACT Carbon nanotube based hybrid materials integrated with conducting stainless steel mesh is of great importance for developing electrochemically stable and mechanically flexible supercapacitors. We report sweet spot tunable CVD growth of vertically aligned carbon nanotube coated with TiO2 on stainless steel mesh with remarkable supercapacitor performance. Aligned CNT forest growth is observed at certain distance from the upstream inside the heating zone. We find that residence time of the carbon precursor has significant influence on controlling the density and height of vertically aligned carbon nanotube forest. When the residence time is increased by reducing the carrier gas flow rate from 590 sccm to 300 sccm, vertically aligned carbon nanotube forest height is increased significantly from ~9 µm to ~31 µm on stainless steel mesh. The developed vertically aligned CNT forest on stainless steel mesh found to be suitable for supercapacitor electrode but its superhydrophobic nature limits its energy storage performance. In order to tune its wettability for further improving the electrode performance, 3 nm TiO2 conformal coating is introduced on VACNT-SS mesh using atomic layer deposition. TiO2-VACNT hybrid shows superhydrophillic nature and able to achieve 16.24 mF/cm2 specific capacitance in particular current density of 1.67 mA/cm2. This CNT-TiO2 hybrid outperforms other CNT based supercapacitor electrodes and shows high power density of 1.18 mW/cm2. Around 99.7% capacitance was retained by the developed hybrid after 5000 charge discharge cycles. Supercapacitor measurements performed in flexible geometry and after severe ultrasonication to ensure the mechanical stability and flexibility of the developed TiO2 coated CNT forest on SS mesh.

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Keywords: Vertically aligned carbon nanotube, Stainless steel mesh, CVD growth, Tunable wettability, Flexible hybrid supercapacitor.

1. Introduction Carbon nanomaterials are well known for energy storage and conversion applications.1–3 In particular vertically aligned carbon nanotube (VACNT) forest on conducting substrates has been widely explored for diverse applications including thermal interface materials, interconnects, and supercapacitors. 4–12 When VACNT is grown on conducting substrates, it has several advantages over solution processed random CNT network in applications like supercapacitors, electrochemical and biological sensors.13–18 Solution processed CNT based electrode contain some polymeric binders which has detrimental effect on the performance. Moreover uniform alignment of the CNT is difficult to achieve in coating based electrode preparation methods hence the anisotropic properties of CNTs and surface area of electrode material generally get compromised compared to aligned CNT based electrodes as reported earlier.19,20 Solution processed methods also suffer from poor adhesion and high contact resistance at CNT-substrate interface. Direct growth of VACNT on conducting substrates provides good adhesion, mechanical integrity, better charge transport, less contact resistance and shows improved performance while retaining the vertical alignment.21–23 Various conducting substrates or metal buffer layers such as Au, Ag, Al, W, TiN and NiCr have been explored for CNT growth.13 While CNT forest growth with mm scale height is routinely achieved from fine catalyst particles deposited on SiO2/Si substrate with Al2O3 buffer layer, aligned CNT forest growth on conducting substrate is not well established. Among the several conducting substrates, stainless steel (SS) mesh has advantage of easy availability at low cost with more surface area and long durability. SS based supercapacitor might be useful in structural supercapacitors for electric vehicles although the density is higher than polymer based flexible substrates. Moreover growth of VACNT directly on SS mesh increases surface area as well as electrical conductivity. Since SS contains catalytic elements such as iron, it is possible to directly achieve CNT growth without additional catalyst. Although such direct VACNT growth on SS is documented earlier,

22,24–28

optimization of sweet spot growth of VACNT on SS mesh with mechanistic understanding is obscure. Such optimized CVD growth of VACNT on conducting SS mesh for supercapacitor application is highly demanding. Efforts have been made to improve its supercapacitor 2 ACS Paragon Plus Environment

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performance by various methods including doping, post growth treatments, defect generation, functionalization and hybrid approaches.

29–34

Most of these methods are using template for

growth or using rigorous post growth treatment for several hours which limits their practical usability. In present work, we demonstrate VACNT growth on SS mesh using chemical vapor deposition to enable development of low cost supercapacitor with high capacitance retention. Unlike earlier reports, we present tunable sweet spot positions for VACNT growth inside the heating zone for specific growth temperature and gas flow rate. Effect of carrier gas flow rate is carefully investigated and a statistical analysis of VACNT height as a function of distance in upstream heating zone at different gas flow rates is presented. In order to take advantage of high surface area and good electrical conducting VACNT on SS mesh, we demonstrate supercapacitor performance of as grown, binder free SS electrodes. In order to further improve its performance we adopted simple hybrid approach by coating 3 nm of TiO2 conformal coating using atomic layer deposition (ALD) which was completed in couple of minutes. The developed hybrid of TiO2 coated VACNT on SS mesh can serve as low cost, flexible hybrid supercapacitor material with high aerial capacitance, power density and capacitance retention for energy storage applications. Entire theme of the presented work is shown in Fig. 1 which schematically illustrates the overview of current work. In brief SS mesh was pretreated in acidic medium followed by CNT growth using CVD. Contact angle measurements revealed its hydrophobic nature. Further few nm TiO2 was conformally coated using atomic layer deposition to make it superhydrophillic. Finally its supercapacitor performance was evaluated and compared with bare VACNT on SS mesh.

2. Experimental 2.1. Substrate cleaning and pretreatment SS 304 grade wire mesh having 150 mesh per inch and wire diameter of 50 micron (commercially available) was used as substrate for CVD growth of VACNT. Prior to the substrate pretreatment the wire mesh was cleaned ultrasonically in acetone for 15 minutes followed by etching in 35% HCL for 30 minutes. Acid treated steel mesh was cleaned in acetone before loading into CVD system. 3 ACS Paragon Plus Environment


2.2. CVD growth of VACNT on stainless steel mesh Pretreated SS mesh was loaded into quartz tube chamber (45mm inner diameter) of CVD system (Microphase, Japan) having three independently controlled heating zones. Around 35 cm length of the tube remains inside the heating zone. Quartz tube was purged with 700 sccm of Ar for 5 minutes before starting the recipe. The growth was carried out at 700 oC with pre heating at 850 oC

for 30 minutes. Acetylene was used as carbon precursor with flow rate of 45 sccm for 5

minutes followed by 15 minutes Ar flow. Ar was used as carrier gas with different flow rates of 300, 450 and 592 sccm. 2.3. Conformal coating of TiO2 3 nm of TiO2 was conformally coated using ALD (Cambridge Nanotech) at 150 oC temperature for 64 cycles having layer thickness of 0.47 Å per cycle. Tetrakis (dimethylamino) titanium (TDMAT) and H2O were used as precursors in alternative pulses. 2.4. Material characterizations Field emission scanning electron microscope (FESEM) analysis was performed using FEI-Nova NanoSEM 450 equipped with energy dispersive x-ray spectroscopy (EDS, AMETEK). Scanning transmission electron microscopy- high angle annular dark field (STEM-HAADF) imaging and STEM-EDS mapping was done using Tecnai G2 FEI operated at 200 kV. STEM sample was prepared using Gatan 3 mm disc punch. VACNT grown SS mesh was cut into 3 mm disc and directly loaded into single tilt holder. X-ray diffraction was recorded using Rigaku Smartlab xray diffractometer with Cu Kα radiation at scan speed of 0.5 degree/min. Raman spectrum was recorded using Horiba HR-Evolution spectrometer with 532 nm laser using 50x objective. Brunauer-Emmett-Teller (BET) surface area was measured under nitrogen atmosphere using Quantachrome Autosorb iQ station. Contact angle measurements were done on Ramѐ-Hart Goniometer model 250 using sessile drop method. De ionized water was used as solvent. 2.5. Electrochemical measurements Cyclic voltammetry (CV), Galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements were performed using electrochemical work station (Autolab, Metrohm) in 0.1 M Na2SO4 aqueous electrolyte. Three electrode cell setup has 4 ACS Paragon Plus Environment

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been used where VACNT grown on SS is directly used as working electrode, Platinum foil was used as counter electrode and Ag/AgCl electrode was used as reference electrode. Specific capacitance (Csp), energy density (E) and power density (P) were calculated from GCD curves using equations as follows

𝐶𝑆𝑃 =

𝐼 𝑥 ∆𝑡

1𝐶𝑠𝑝 𝑥 (∆𝑉)2

𝐸=8 𝑃=

(1)

∆𝑉 𝑥 𝐴

3600

𝐸𝑥3600

(2) (3)

∆𝑡

Where I is applied current, ∆𝑡 is discharging time, ∆𝑉 is potential window after deducting IR drop, A is area of electrode. EIS was carried out at 10 mV perturbation voltage in 100 kHz to 0.1 Hz frequency range.

3. Results and discussion 3.1 CVD growth of VACNT Fig. 2 (a) and (b) show scanning electron micrographs of large scale growth of VACNT on flexible SS mesh. SEM observations reveal uniform growth of VACNT over the large area of SS mesh. High magnification image shown in Fig. 2 (b) confirms the VACNT in the form of bundles covering the SS mesh 3 dimensionally. Fig. 2 (c) shows the flexible nature under mechanical deformations of as grown VACNT on SS mesh for small sized sample. Fig. 2 (d) shows the importance of VACNT CVD growth for supercapacitor application indicated by the difference in area enclosed by CV curves as compared with bare SS mesh. We extended similar growth over large area (1 meter length x 1 cm width) by introducing rolled SS mesh inside the optimized heating zone. Any small variation in growth condition resulted in decrease in uniform coverage and vertical alignment leading to mixed morphology of CNT network and aligned bundles. We carried out systematic experiments and analyzed the results with respect to position, SS length and flow rate etc. Fig. 3 (a) shows Raman spectrum of VACNT in which defect mediated D-band is having less intensity compared to the graphitic nature G-band. Id/Ig=0.65 5 ACS Paragon Plus Environment


indicates good structural quality of CVD grown

multiwalled CNT inferring from earlier

reports.35 The observed 2D band around 2700 cm-1 is arising due to in plane vibrations among carbon atoms. Fig. 3 (b) shows comparative x ray diffraction patterns of SS mesh before and after etching along with after VACNT growth on SS mesh. It is observed that after etching there is not much change in phases present in SS mesh. However, after VACNT growth significant decrease in the peak intensities is observed along with emergence of α-martensite phase. This might be due to crystallization at 850 oC of amorphous oxides formed during etching. Apart from γ-austenite and α-martensite a broad peak is observed after CNT growth signifying mutiwalled CNT.36 Fig. 3 (c) represents SEM of VACNT and corresponding EDS mapping is shown in Fig. 3 (d) which confirms the presence of carbon in the aligned tubes indicated though red map. Such vertical alignment also observed in STEM-HAADF imaging as shown in Fig. S-1. In order to investigate effect of etching and annealing at 850 oC on microstructure, FESEM images were recorded before etching, after etching and after annealing as shown in Fig. S-2. Typical polycrystalline nature of SS mesh is observed in Fig. S-2-a with definite grains separated by grain boundaries before etching but after etching microstructure of SS surface changes into porous morphology as presented in Fig. S-2-b. High magnification image is shown in the inset of Fig. S-2-b. After annealing at 850 oC for 30 minutes in presence of Ar (592 sccm), fine particle microstructure emerged leading to growth of VACNT as shown in Fig.S-2-c. This nano roughness provides CNT nucleation sites and similar surface reconstruction is reported earlier.37 We found that position of the sample inside the uniformly heated zone has great influence on CVD growth of CNT. Further investigations revealed that within the uniform temperature heat zone, certain sweet spot give rise to highly aligned CNT forest. Other areas in the same temperature zone but located away from the sweet spot are responsible for either network morphology or amorphous carbon deposition. Fig. 4 (A) shows schematic representation of the sample characterized through SEM as a function of distance with respect to upstream heating zone within the CVD furnace. The temperature of all the three heating zones is maintained at same value controlled by individual heating elements in each zone. Fig. 4 (B) shows photograph of SS mesh after CNT growth inside the entire heating zone of 35 cm. Fig. 4(a,c,e,g) shows the SEM micrographs of samples taken after CVD growth from 1 cm, 15 cm, 27 cm, 34 cm with respect to upstream heat zone respectively. The corresponding high magnification images are shown in Fig. 4 (b,d,f,h). Microstructural characterization in Fig. 4 reveals that first two 6 ACS Paragon Plus Environment

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upstream heat zones contain almost no CNT growth or less dense network growth whereas third heating zone is more favorable for VACNT growth as presented in Fig. 4 (e-h). We carried out further experiments in order to understand the growth kinetics with in the sweet spot (downstream heating zone). It was found that initial area in the third zone, give rise to both deposition of amorphous carbon and VACNT growth as shown in Fig. 5 a & b respectively. We observed that as the distance increases, vertical alignment, forest height and coverage of VACNT increases as evident from Fig. 5 (c-i). Most favorable and dense growth is found towards the extreme downstream end of the heat zone. It is possible that the observed distance dependent growth morphology changes are influenced by resident time of carbon precursor at substrate inside the heat zone. To investigate this aspect, we carried out another set of experiments in which we changed the carrier gas Ar flow rates. As we reduced the flow rate of the Ar from 592 sccm to 450 sccm, we found that sweet spot of the growth is shifted towards the upstream side. Further reduction in Ar flow rate to 300 sccm, the VACNT growth zone shifted more towards the upstream. It is also found that vertical alignment starts from the second heat zone itself and indicates the effect of residence time on the growth kinetics. We found that decreasing flow rate, increases the forest height. Corresponding SEM image is shown in Fig. 6 (a) which shows large area coverage VACNT forest with more height as compared to 592 sccm of Ar flow. Magnified region of Fig. 6 (a) is shown in Fig. 6 (b) where vertical alignment of the tubes and their tangling can be seen. Effect of flow rate on forest height and distance dependent growth kinetics results are summarized in Fig. 6 (c). It is inferred from the Fig. 6 (c) that at 300 sccm flow rate of Ar, we observed maximum forest height of around 31 µm at distance of 25 cm with respect to upstream heating zone. For 450 sccm flow rate, maximum forest height of ~ 10 µm is observed at distance of 28 cm while 592 sccm Ar gas flow rate resulted in maximum forest height of ~ 8 µm at distance of 32 to 34 cm from upstream. It can also be seen from the graph that formation of forest starts from 5 cm and finishes around 25 and 28 cm for 300 sccm flow rate and 450 sccm respectively whereas for 592 sccm flow rate, it starts around 25 cm and ends up at 34 cm. The observed results strengthen our proposed mechanism considering the effect of residence time of carbon precursor influenced by carrier gas flow rate towards the VACNT forest growth on SS mesh. Earlier reports also observed the presence of sweet spot in CVD growth of CNT forest but the reason for such sweet spot in CNT growth is illusive.38 We carried out experiment where we fixed the Ar flow as 300 sccm but doubled the acetylene flow from 45 sccm to 90 sccm and 7 ACS Paragon Plus Environment


found that no VACNT growth is resulted which is in contradiction with earlier report.38 Moreover amorphous carbon deposition is found in the upstream region where as carbon fibers found in the downstream region. Hence varying the precursor flow rate adds further complexity as it is directly related to carbon concentration instead of changing only residence time. For CNT growth using C2H2, Ar flow rate known to affect residence time as well as gas phase composition along the CVD tube. The observed dense VACNT growth at low Ar gas flow rate of 300 sccm can be corroborated to high residence time of C2H2 which results in high concentration of active radicals and intermediates that are needed for CNT growth. In order to investigate the surface area of VACNT grown on SS mesh, BET surface area for both SS mesh and VACNT was measured under nitrogen environment. Base SS mesh is having surface area ~ 128.7 m2/g whereas after VACNT growth surface area increases to 174.8 m2/g. This increased surface area is resulting from the dense VACNT grown on SS mesh which is relevant for supercapacitor application with combination of high surface area and good electrical conductivity 3.2 Electrochemical measurements Fig. 7 (a) shows comparative CV plots of SS mesh after VACNT growth at different scan rates. With increasing scan rate, peak current and area under CV curve increases without distorting rectangular curve shape indicating high rate capability. For example, the shape of the CV curve is perfectly rectangular even at high scan rate of 500 mV/s suggesting electrical double layer (EDL) behavior arising from VACNT with low equivalent series resistance (ESR).39 Fig. 7 (b) shows galvanostatic charge-discharge (GCD) plots of SS mesh after VACNT growth at different current densities. It is evident from the nonlinear GCD plot that both EDL and redox reactions are contributing for the capacitance. The redox contribution might be arising from the catalyst particles present in VACNT. As we increase current density, charge-discharge time decreases and cycle becomes fast. The calculated specific capacitance from GCD plot is 5.99 mF/cm2 at 1.67 mA/cm2 current density. Corresponding Ragone plot is shown in Fig. S-3 in which energy density versus power density is plotted at different current densities. It can be seen from the plot that as increasing power density, energy density remains almost constant from 0.1 mA/cm2 to 1 mA/cm2 current densities. The calculated value of energy density and power density found to be 0.19 µWh/cm2 and 202 µW/cm2 respectively at current density of 1.67 mA/cm2. CNT’s are 8 ACS Paragon Plus Environment

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known for its high cyclic stability up to 1 million cycles and hence we restricted our measurements up to 2000 cycles.40 Capacitive retention result is presented in Fig. S-4-a and cyclic stability of 1001th cycle to 1010th cycle is shown in inset of Fig. S-4 (a). Interestingly specific capacitance increased up to 140% after 2000 cycles as compared to 1st cycle. 1st cycle is counted after exempting five pre conditioning cycles. We believe this increase in capacitance is due to improved wettability and activation of electrode via opening of channels after repeated cycles. Such high capacitive retention up to 225% is already reported for reduced graphene oxide-multiwalled CNT composite.41 To support this argument, we performed contact angle measurement of VACNT on SS mesh after growth and found the contact angle is around 123O as shown in Fig. 7 (c). We repeated the measurements on the same sample at same location after soaking it into DI water for 4 hours followed by drying. We found that contact angle substantially decreased to 105O as shown in Fig. 7 (d). Reduction in contact angle suggests improved wettability of the sample due to increased interfacial interaction between VACNT and electrolyte arising from the bundling and channel formation in the VACNT. EIS generated Nyquist plot is also compared before and after 2000 cycles and presented in Fig. S-4-b. It is evident form the plot that in low frequency region there is drastic decrease of resistance after 2000 cycles and slope of the curve also increases with almost vertical line shape suggesting increase of capacitive behavior. High frequency region is shown in inset of Fig. S-4-b. After 2000 cycles Nyquist plot is fitted with electrical circuit as shown in Fig. S-5 and its equivalent circuit is shown in inset of Fig. S-5. It is inferred from the circuit that presence of two resistances and two constant phase elements (CPE) are playing role in electrochemical activity. Two resistance may arise due to solution/electrolytic resistance and charge transfer resistance. Ideal capacitive element was replaced by CPE to get good fit. 1

𝑍𝐶𝑃𝐸 = [𝑄(𝑗𝜔)𝑛]

(5)

Where Q is defined as frequency independent constant, ω is radial frequency, n is exponent representing correction factor which varies from -1 to 1. For n = 0, it behaves as pure resistor and for n = 1, pure capacitor and n = -1, pure inductor. Two CPE elements arise because of deviation from ideal capacitive behavior. 42 Presence of CPE might be due to porous nature of VACNT.43 In present case, values of n for both CPE reaches close to ~ 0.9 indicating capacitive nature.42 The values of the components are presented in Table ST-1. 9 ACS Paragon Plus Environment


GCD curves as a function of number of cycles are presented in Fig. S-6 (a) which shows increased charge discharge time per cycles with increase of number of cycles and led to increase capacitance as evident from capacitive retention plot. Further, Bode modulus plot shown in Fig. S-6 (b) suggests that in high frequency region there is not much difference in impedance before and after 2000 cycles. Drastic decrease in resistance is observed after 2000 cycles in low frequency region which might be due to increase in porosity of electrode material leading to easy access of internal pores to electrolyte ions. At low frequency, ions also have more time to adsorb and desorb. Fig. S-6 (c) represents Bode phase angle plot showing that after 2000 cycles phase angle almost reaches to 90o, indicating improved capacitive behavior41. This increase in capacitance after repeated cycles makes dense VACNT-SS electrode material a good candidate for practical applications. However the superhydrophobic nature of VACNT-SS limits its electrochemical performance. In order to improve performance of above discussed electrode, VACNT on SS was conformally coated with 3 nm of TiO2 using atomic layer deposition since TiO2 is well known for its tunable wettability.44,45 TiO2 coating of 3 nm was confirmed by high resolution transmission electron microscope image as shown in Fig. 8 (a). Corresponding high magnification image is shown in Fig. 8 (b). Elemental composition was confirmed qualitatively by scanning transmission electron microscope energy dispersive spectroscopy (STEM-EDS). STEM revealed the presence of carbon, titanium and oxygen as shown in Fig. 8 (c,d,e) respectively and corresponding HAADF image is shown in Fig. 8 (f) The developed hybrid of TiO2 coated VACNT was explored for supercapacitor testing. Cyclic voltammograms were recorded at different scan rates as shown in Fig. S-7-a. Fig. S-7-a shows the signature of pseudo capacitance since the shape of the curve is not rectangular and having small peaks. GCD were recorded at different current densities as shown in Fig. S-7-b. It is inferred from GCD plot that material shows outstanding specific capacitance and power density values i.e 16.24 mF/cm2 and 202 mW/cm2 at 1.67 mA/cm2 current density and 12.53 mF/cm2 and 1.18 mW/cm2 respectively at 10 mA/cm2. A comparative CV plot is presented in Fig. 9 (a) which highlights the increased charge storage of VACNT after coating with TiO2. A comparative charge discharge plot is also presented in Fig. 9 (b) which also indicates increased cyclic time. The nonlinearity of GCD curves again reveals the presence of pseudo capacitance. The calculated specific capacitance values from GCD curves show ~ 3 fold increase in the values at similar current density. Corresponding Nyquist plot is presented in Fig. 9 (c) which shows significant decrease in 10 ACS Paragon Plus Environment

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resistance. This decrease in resistance might be due to increased wettability after coating with TiO2 as shown in Fig. 9 (d). After coating with TiO2, we tune the wettability and demonstrate the transition from superhydrophobic to superhydrophilic surface. This increased wettability is responsible for better electrode electrolyte interfacial interaction which is contributing for very high supercapacitor performance. We also carried out experiments to check the mechanical stability and integrity for both VACNT and TiO2 coated VACNT. We did ultrasonication for 10 minutes in 0.1 M Na2SO4 and recorded CV before and after ultrasonication as shown in Fig. 10 (a), (b) for VACNT-SS mesh and TiO2 coated VACNT-SS mesh respectively. Ultrasonication removed some of the weakly adhered CNTs from SS mesh, most of the CNTs grown on SS retained well. It was found that there is not significant decrease in charge storage as reflected by area enclosed by CV curve after subjecting the electrode to severe ultrasonication for 10 minutes. In order to confirm the adhesion of CNT’s on SS mesh, we recorded FESEM image after ultrasonication and found that CNT’s are intact on the substrate as shown in Fig. S-8. We also did the flexibility test and recorded CV before and after folding the TiO2 coated VACNT-SS mesh as shown in Fig. 10 (c). It is inferred from the comparative CV that there is not significant change before and after folding. In order to study the electrochemical stability we recorded 5000 charge discharge cycles and presented in Fig. 10 (d). Reported TiO2 coated VACNT hybrid shows excellent capacitive retention of 99.7% even after 5000 repeated charge discharge cycles. Finally a comparative chart of specific capacitance is presented in Fig. 11 and corresponding table is presented in Table ST-2. The analysis shows that the reported values of current work are better than the previously reported values for CNT based electrode material. In addition, most of the reported methods used Si/SiO2 substrate for growth which requires complicated post transfer process or used multistep solution/template growth which are having other consequences on performance.29,30,46,47 Few reports of VACNT growth directly on conducting substrates either used post growth heat treatments for several hours to generate defects or used additional catalyst deposition step followed by low pressure CVD growth.31,48 Present work demonstrates VACNT growth on conducting SS mesh without using inherent catalyst in atmospheric CVD and has viable advantages for commercial applications. TiO2 coated CNT found to suffer from lower Coulombic efficiency at lower current densities. While several reasons such as parasitic reactions, poor electrical conductivity and ion diffusion are known to reduce Coulombic efficiency, low structural activation of electrode at low current density is mainly responsible for 11 ACS Paragon Plus Environment


poor Coulombic efficiency of porous structures.49–51 It is possible that TiO2 coating reduces the channel dimensions in CNT microstructure and limits the ionic transport which warrants further investigation. Nevertheless conformal coating of TiO2 plays significant role in improving the performance due to enhanced wettability. Choice of other materials such as MnO2 would be better suited for coating on CNT for improving the specific capacitance. However conformal coating of MnO2 is not generic for ALD due to precursor limitations and more importantly their wetting properties are unknown. It might be worth exploring the possibility of decorating MnO2 nanoparticles on TiO2 coated CNT forest which may combine both superhydrophilic nature and high specific capacitance. Hence the presented approach of tuning the contact angle of CNT forest by TiO2 coating opens up possibilities for engineering the CNT surfaces towards better supercapacitor performance. Due to improved performance of VACNT-TiO2 hybrid, current work paves a way for developing low cost flexible supercapacitors having high aerial capacitance and power density along with high cyclic stability.

4. Conclusions CVD growth of VACNT forest directly grown on stainless steel mesh has been demonstrated by optimizing residence time of precursors. As grown VACNT-SS electrode was tested for supercapacitor application and the developed electrode exhibits excellent capacitive retention after repeated cycles with low equivalent series resistance. Further 3 nm TiO2 coated VACNT shows ~ 3 fold increase in specific capacitance and 5 fold increase in energy density. This hybrid shows excellent power density at higher current density and possesses good mechanical stability for flexible applications. The investigated aspects of growth optimization of vertically aligned CNT forest on SS mesh and its hybrid with TiO2 along with their supercapacitor performance is of great importance for developing, electrochemically stable and mechanically flexible hybrid electrode material towards low cost energy storage applications. High surface area of aligned CNT on SS and introducing coating of transition metal oxides or conducting polymers in order to increase the energy density are in current demand which can fulfill the requirements to make low cost flexible supercapacitors having high energy density.


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Acknowledgements We acknowledge financial support (Grant No. DST/TMD/MES/2K17/51(G)) from Department of Science and Technology (DST, India). We thank Advanced Materials Research Centre (AMRC) at IIT Mandi for providing materials characterization facilities and Dr. Rahul Vasih for contact angle measurements and related discussions.

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Figures

Fig. 1 Schematic illustration of present work.



Fig. 2 (a & b) FESEM images of large scale growth of VACNT on SS mesh using CVD at different magnifications, (c) corresponding photograph showing the flexible nature under mechanical deformation of as grown VACNT on SS mesh, (d) Comparative supercapacitor performance after VACNT growth showing highlighting the importance of VACNT growth.


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Fig. 3 (a) Raman spectra of VACNT on SS mesh showing structural characteristics of the as grown samples (b) Comparative x-ray diffraction data before etching, after etching and after VACNT growth showing presence of austenite and martensite phases long with presence of broad hump of CNT. (c) SEM image of VACNT on SS mesh (d) corresponding EDS mapping confirming the presence of aligned CNT bundles from SS.



Fig. 4 (A) Schematic representation of CVD heating zones as a function of upstream distance (B) Optical image of large scale VACNT growth. (a,c,e,g) SEM images at different positions inside heating zone, (b,d,f,h) corresponding high magnification SEM images respectively.


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Fig. 5 (a-i) SEM images of distance dependent VACNT growth in the downstream zone at different magnifications where (f-i) represents the sweet spot growth of VACNT.



Fig. 6 (a) SEM image of VACNT growth after reducing the carrier gas flow rate to 300 sccm leading to increased VACNT height (b) corresponding high magnification images showing dense forest growth and alignment. (c) Statistical analysis displaying VACNT height changes at different Ar flow rates as a function of distance showing maximum height at least Ar flow rate value.


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Fig. 7 Electrochemical measurements showing (a) cyclic voltammograms for SS meshVACNT as a function of increasing scan rate showing nearly ideal EDL behaviour (b) galvanostatic charge-discharge curves for SS mesh-VACNT as a function of current densities (c) contact angle measurement before soaking VACNT in DI water (d) contact angle measurement after soaking VACNT in DI water.



Fig. 8 (a) HRTEM image of 3nm TiO2 coated VACNT (b) corresponding high magnification image showing conformal coating of TiO2, number of walls in CNT along with diameter of hollow tube (c,d,e) STEM-EDS mapping confirming the presence of carbon, titanium and oxygen, corresponding STEM-HAADF image (f).


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Fig. 9 (a) Comparative CV of uncoated and TiO2 coated VACNT showing the improved charged storage capacity reflected by enclosed area of the curves (b) comparative charge discharge curves showing increased cyclic time and nonlinear nature arising from pseudo capacitance (c) Comparative Nyquist plot showing decreased resistance after coating with TiO2 (d) Comparative contact angle measurement before and after TiO2 coating showing tuning of electrode wettability from super hydrophobic to super hydrophilic.



Fig. 10 Comparative CV (a) for VACNT-SS mesh before and after ultrasonication showing slight reduction in capacitance (b) for TiO2 coated VACNT-SS mesh before and after ultrasonication showing negligible change in capacitance (c) for TiO2 coated VACNT-SS mesh before and after zigzag folding showing negligible change in capacitance, corresponding photograph is shown in inset (d) Capacitive retention upto 5000 cycles showing good cyclic stability.


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Fig. 11 Comparative specific capacitance values with earlier reported literature for carbon based electrode materials showing improved performance of the present work.



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ALIGNED CNT FORESTS ON STAINLESS STEEL MESH FOR

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