- Email: [email protected]
TiO2 nanotube-based aqueous lithium ion capacitors with high energy density
Journal Pre-proof Boron-doped graphene/TiO2 nanotube-based aqueous lithium ion capacitors with high energy density Jiepei Gao, Guojun Qiu, Hongji Li, Mingji Li, Cuiping Li, Lirong Qian, Baohe Yang PII:
S0013-4686(19)32046-8
DOI:
https://doi.org/10.1016/j.electacta.2019.135175
Reference:
EA 135175
To appear in:
Electrochimica Acta
Received Date: 2 September 2019 Revised Date:
24 October 2019
Accepted Date: 28 October 2019
Please cite this article as: J. Gao, G. Qiu, H. Li, M. Li, C. Li, L. Qian, B. Yang, Boron-doped graphene/ TiO2 nanotube-based aqueous lithium ion capacitors with high energy density, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135175. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphical Abstract
Boron-doped graphene/TiO2 nanotube-based aqueous lithium ion capacitors with high energy density Jiepei Gao a, Guojun Qiu a, *, Hongji Li a, *, Mingji Li b, *, Cuiping Li b, Lirong Qian b, Baohe Yang b a
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of
Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P.R. China b
Tianjin Key Laboratory of Film Electronic and Communication Devices, School of Electrical and
Electronic Engineering, Tianjin University of Technology, Tianjin 300384, P.R. China
*Corresponding Authors. Tel.: +86 022 60214259 E-mail: [email protected] (G. Qiu); [email protected] (H. Li); [email protected] (M. Li).
1
Abstract Graphene and its derivatives have gained tremendous research interest for energy storage because of their high capacitance and structural stability. However, the synthesis of graphene nanosheets with high electrochemical activity remains a great challenge. Herein, boron-doped graphene (BG)/TiO2 nanotube array/Ti multilayer films were constructed by growing BG sheets on the TiO2 nanotubes. The high electrochemical activity of BG sheets and electronic transfer capability of TiO2 nanotubes led to good structural stability and energy-storage capability. The as-fabricated capacitors with two BG/TiO2/Ti electrodes in a CMC/LiCl gel electrolyte could realize a wide operational voltage of 2.6 V and deliver a high energy density of 221.8 Wh·kg–1 at 5.98 kW·kg–1, which still remains 102.4 Wh·kg–1 as the power density increased to 35.1 kW·kg–1. These devices show very high electrochemical lifespans, with approximately 91.3% retention after 10000 charge/discharge cycles.
Keywords: Boron-doped graphene; TiO2 nanotubes; Lithium-ion capacitors; High energy density
2
1. Introduction Lithium-ion capacitors (LICs) operate on the principle of synergistic interactions between faradaic (lithium insertion/extraction) and electric double layer processes, which is an effective strategy for simultaneously increasing energy density and power density [1-3]. Most known LICs are constructed using organic electrolytes [4-7]. The safety concern of organic electrolytes cannot be ignored, so we envision that aqueous electrolytes can be used to construct LICs instead [8, 9]. An active material supports the insertion/extraction of Li+ ions while maintaining the fast charge/discharge advantages of the electric double layer process, which is the key to constructing an aqueous LIC. The strategy of widening the potential window and increasing the specific capacitance can substantially increase the energy density of the aqueous capacitors [10-12]. Here, we seek to increase the potential window using TiO2 nanotubes and doped graphene, increasing the specific capacitance using the pseudo-capacitance behavior of lithium ions. The advantages of TiO2 nanostructures include their reversible redox reaction with lithium ions (TiO2 + xLi+ + xe- ↔ LixTiO2), small volume effect, low cost, and environmentally-benign nature [13, 14]. In particular, one-dimensional TiO2 nanotubes form an array that can be erected on the current collector, which not only improves the transfer properties of vector ions and electrons, but also greatly improves the support and buffering ability for repeated intercalation/deintercalation of lithium ions [15, 16]. Therefore, we decided to design a new active material based the skeleton of TiO2 nanotube arrays and hope to further increase their surface area, electrochemical activity, and conductivity. We investigated core-shell structures such as MoS2@TiO2 hybrid nanostructures [17], Ag/Au/polypyrrole (PPy) nanowires [18], PPy/carbon nanotubes [19], and Zn/CuCrO2 nanowires [20]; doping materials include boron-doped graphene [21], nitrogen-doped carbon dots [22], B/N 3
co-doped graphene nanotubes [23], and C-doped g-C3N4 [24]. The advantage of core-shell structures is that the roles of the shell and the core are very clear and are often synergistic. The charge polarization of heteroatoms can adjust the electron density between the heteroatoms and adjacent skeletal atoms, and is an example of pseudocapacitive behavior [25, 26]. Therefore, we propose a strategy for preparing an LIC by preparing a boron-doped graphene (BG)/TiO2 nanotube array hybrid structure (BG/TiO2) as the active material of an electrode. For many years, BG has been the focus of theoretical research, especially its p-type conductivity which is considered to be a crucial property for the development of graphene-based electronic devices [27, 28]. The substitution of boron in the graphene lattice affects the electron cloud distribution on the graphene surface, which increases the charge amount at the doping position. Boron atoms doped into graphene adsorb Li ions and promote the intercalation reaction of Li ions to the enhancement of electrochemical activity [29]. Furthermore, dye cells [30], Li ion batteries [31], electrochemical capacitors [32], and other energy storage devices with BG exhibit excellent energy storage performance. In this paper, BG sheets were grown on an anodic oxidation-processed TiO2 nanotube array by chemical vapor deposition (CVD) to yield BG/TiO2 nanotube-array electrodes. CVD can control the morphology of graphene and oxygen contamination, which improve the conductivity of TiO2 nanotubes by carbonizing them. The BG/TiO2/Ti electrode was prepared for the first time in this study using a novel CVD technique. By investigating the influence of key process parameters on the morphology and structure of the BG layer and the electrochemical performance of the BG/TiO2/Ti electrode, an aqueous LIC was constructed and its feasibility for applied use was determined. 2. Experimental 4
2.1. Preparation of boron-doped graphene/TiO2 nanotube (BG/TiO2) electrodes Titanium (Ti) sheets (20 mm × 10 mm × 1 mm) were sanded with sandpaper, and then ultrasonicated in ultrapure water, ethanol, and ultrapure water for 15 min each and dried. An ammonium fluoride (NH4F) solution having a concentration of 0.25 M was prepared by mixing ultrapure water and glycerol (at a volume ratio of 1:3) as a solvent. In order to prepare TiO2 nanotubes by anodization, an electrolysis system was constructed using an ITECH DC source meter (ITECH Electronic Co., Ltd), a Ti sheet as an anode and a Pt sheet as a cathode in the above electrolyte. The voltage was set to 19.9 V, the magnetic stirring speed was set to 700 rpm, and the anodization time was 2 h. Next, this system was annealed at 500 °C for 3 h in a vacuum tube furnace (SK-G03123K), yielding a TiO2 nanotube array electrode (TiO2/Ti or TiO2). Boron-doped graphene (BG) was deposited on the TiO2/Ti substrate using an electron-assisted hot-filament plasma chemical vapor deposition (EA-HF CVD) system. Five lengths of 18.5 cm were suspended from the filament frame at equal intervals, and the chamber pressure was pumped below 1 Pa. Methane (CH4) at a flow rate of 18 sccm and hydrogen (H2) at a flow rate of 300 sccm were introduced to maintain the chamber pressure at 5000 Pa, the filament current was controlled at around 115 A and held for 40 min to carbonize the filament. Six TiO2/Ti substrates were placed on the sample stage in the reaction chamber and evacuated to < 1 Pa. A boron-source gas, methane (CH4), and hydrogen (H2) were introduced into the chamber until the chamber pressure rose to 400 Pa. The flow rates of boron-source gas, CH4, and H2 were 50, 10, and 40 sccm, respectively. Among these, the carrier gas of the boron source was H2, which carries the liquid boron source as a trimethyl borate/ethanol mixed solution (volume ratio of 3:1) into the CVD reaction chamber. The filament power was then turned on and the filament current adjusted to 115 A, with the spacing between the 5
sample and filament spacing of 1 cm. Finally, the DC bias supply was turned on to heat the sample platform, applying a bias voltage of 26 V and a bias current of 4 A. The chamber pressure was maintained at 400 Pa, the substrate temperature was 900 °C, and the reaction time was controlled from 20 s to 10 min. 2.2 Characterization The microscopic morphology and microstructure characteristics of the samples were characterized by field emission scanning electron microscopy (FE-SEM; MERLIN Compact, Carl Zeiss, Germany) and transmission electron microscopy (TEM; JEM-2100F, JEOL, Japan). Analysis of the valence-bond structure, phase structure, and elemental composition was carried out using a Raman spectrometer (LabRAM HR Evolution, Horiba Scientific, Japan), an X-ray diffractometer (XRD; D/max-2500/PC, Rigaku, Japan), an X-ray photoelectron spectrometer (XPS; Escalab 250Xi, Thermo Scientific, USA), and elemental mapping images collected using an energy dispersive X-ray (EDX) line and a TEM instrument. The electrochemical properties of the electrodes and devices were analyzed using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD). The electrochemical performance of the electrodes was tested by a three-electrode system consisting of sample, Pt, and Ag/AgCl electrodes. A 4.7 M LiCl aqueous solution was used for the electrolyte solution. The two-electrode system was used to test the energy storage performance of the device. The LIC devices were fabricated using two BG/TiO2/Ti electrodes, with a membrane (areal density: 30 g·m–2) as the separator and carboxymethylcellulose sodium (CMC)/LiCl gel as the electrolyte (CMC: 0.16 g·mL–1, LiCl: 4.7 M, excluded volume effect). All of the electrochemical performances were tested on an electrochemical workstation (CHI1140C, Chenhua Co., China). The specific capacitances of the 6
electrode and devices can be calculated from the CV and GCD curves according to the following equations:
Cm(or A) =
Cm(or A) =
i∆t for CV, m(or A)( ∆V -iR)
∫
V2
V1
(1)
idV
2m(or A)∆V
for GCD.
(2)
Moreover, the energy and power densities of the devices were calculated according to the following equations: Em =
1 C ( ∆ V)2 , 2 m
(3)
Pm =
Em , ∆t
(4)
where Cm (or A) (F·g–1, or mF·cm–2), Em (Wh·kg–1), Pm (W·kg–1), I (A), △t (s), m (g), A (cm2), △V (V), iR (V), and ν (V·s–1) are the specific capacitance, energy density, power density, discharge current, discharge time, total mass of active materials in the electrodes, surface area, potential window, IR drop, and scan rate, respectively. 3. Results and discussion 3.1. Fabrication and characterization of BG/TiO2 electrodes Fig. 1a schematically presents the fabrication procedure of BG/TiO2 nanotube array/Ti electrodes (BG/TiO2). In general, TiO2 could be reduced to TiC in CH4 and H2 in accordance with the following reactions: CH4 = C + 2H2, TiO2 + C + 2H2 = TiC + 2H2O, TiO2 + C= TiO + CO, and TiO2 + 2C = TiC + 2CO. Under the high temperature and high pressure conditions in this experiment, these reactions proceed spontaneously. We employed TiO2 nanotubes prepared by electrochemical anodization as the substrates for the in situ growth of BG. The TiO2 nanotubes formed a densely 7
packed array on the Ti substrate, and the TiO2/Ti films were directly placed into the CVD reaction chamber. After the CH4 was introduced into the CVD reaction chamber, the partial TiO2 phase was converted into a TiC phase, and a graphene domain formed there. The TiO2 nanotube array controlled the growth direction of the BG sheets, and the height of the BG layer on top of the TiO2 nanotube array increased rapidly with prolonging the growth time of BG. After growing for 20 s, the TiO2 nanotubes converted into BG/(TiO2+TiC) nanotubes (still referred to as BG/TiO2), and the sample changed from yellowish to black (Fig. 1b). We then assembled an aqueous LIC device using two BG/TiO2 electrodes (Fig. 1b and c). We confirmed the growth of graphene on the TiO2 nanotubes by comparing TEM and high-resolution TEM (HRTEM) images of the TiO2 nanotubes and BG/TiO2 nanotubes (Fig. 2a–f). After destroying the graphene sheets grown vertically on the TiO2 nanotube surface, the graphene domains at the time of nucleation on the tube wall can be observed (Fig. 2d). The inverse fast Fourier transform (iFFT) and fast Fourier transform (FFT) patterns of region I show a well-defined carbon six-membered ring atomic arrangement (insets in Fig. 2e). In addition, the layer count of graphene was 2–7 layers, which was further verified by TEM at the folding edges of the graphene in region II (Fig. 2f). In particular, the presence of B was further confirmed by combining the dark field TEM image and the EDX elemental mapping as shown in Fig. 2g; the prepared graphene was BG sheets. Extensive experimental efforts have been made to finely control the height of BG layers to optimize charge transport for LICs. This was realized by using a constant CH4 flow rate of 18 sccm for growth times of 20 s, 40 s, 1 min, 5 min, and 10 min. In this respect, time-dependent CVD growth is effective for tuning the height of the BG layers (Fig. 3). The surface SEM images show that the size of the BG sheets is rapidly increased as the growth time of BG is extended from 20 s to 8
10 min (Fig. 3a–f). In the cross-sectional SEM image of the samples, the nanotubes have an average length of less than 1 µm, and the tubular morphology was preserved after the CVD process (Fig. 3g– i). XPS spectra were used to analyze the chemical state of our samples. Fig. 4a shows the survey XPS spectra recorded for BG/TiO2 samples with a BG growth time of 20 s. The growth time of the TiO2 nanotubes was extended from 30 min to 2 h, and the chemical state of the C 1s in the samples varied significantly over time (Fig. 4b). The C 1s main component of the three BG/TiO2 samples is the sp2 hybrid C-C bond at 284.8 eV [31]. In addition, a peak was present at 286.3 eV corresponding to the C-O band, which is related to the adsorption of oxygenates on the exposed faces and sides of BG sheets [33]. When the TiO2 nanotubes were grown for 30 min, the C 1s spectrum exhibited two additional peaks at 281.9 and 288.6 eV, which coincided with the Ti-C bonds of TixCy and C-O-B bonds of BG, respectively [34-36]. As the growth time of the TiO2 nanotubes was extended to 1.5 h, the Ti-C peak was still present, but the C-O-B bond virtually disappeared. The one peak in the B 1s spectrum of TiO2 nanotubes grown for 30 min represented the chemical environment of boron atoms bonded to oxygen at 192.1 eV (-BCO2) in BG (Fig. 4c). When the growth time was extended to more than 1.5 h, the B 1s band was shifted to approximately 189.1 eV. The peak having a binding energy of 189.1 eV can be associated to sp2 C-B bonds present in graphene, which is indicative of the formation of substitutional bond atoms [33]. Fig. 4d shows O 1s spectra of the BG/TiO2 samples. When the growth time of the TiO2 nanotubes is 30 min, it is mainly composed of Ti4+-O bonds (at 530.2 eV) and Ti3+-O bonds (at 531.1 eV) in the TiO2 layer [16, 37]. When the growth times of the TiO2 layer are 1.5 and 2 h, the O 1s spectra contains three binding energy peaks at 530.2, 531.2, and 532.9 eV. The extra peak at 532.9 eV which corresponds to the C=O or C-OH bonds in the BG 9
surface, indicating that BG is easily formed. The Ti 2p spectra of BG/TiO2 layers are shown in Fig. 4e. Two peaks located at 458.6 and 464.3 eV, which are assigned to Ti 2P3/2 and Ti 2P1/2, respectively, indicating the presence of Ti4+ chemical state in TiO2; and Ti 2P3/2 at 454.9 eV and Ti 2P1/2 at 461.1 eV correspond to TiC [7, 38-40]. While another peak at 456.6 eV imply the formation Ti3+ after boron-doping [13, 41]. Clearly, after the TiO2 growth time was extended from 30 min to 2 h, the Ti-O bond was weakened while the Ti-C and Ti-B bonds were enhanced. The TiO2 nanotubes grown for 2 h are easily carbonized as a substrate, and the boron atoms are more likely to replace the carbon atoms in the TiC lattice. We analyzed the composition and defect structure of BG by comparing the Raman spectra of four samples (Fig. 5a). The characteristic bands in the Raman spectrum of TiO2 are concentrated below 1000 cm–1. Raman spectra of the BG/TiO2 films exhibit a characteristic G band (1580 cm–1) and 2D band (2700 cm–1) of graphene, along with a D band (1350 cm–1). Our characterization results confirmed the feasibility for growing highly crystalline BG layers on the TiO2 nanotubes by CVD. First, BG was successfully grown for 20 s to 10 min, and it completely covered the tubular substrate. Second, the D/G intensity ratios decreased from 2.12 to 0.79 with increased BG growth times from 20 s to 600 s, indicating that the distribution density of graphene sheets can be controlled by changing the growth time. Third, the ID/ID’ intensity ratios (peak height ratio) of the BG/TiO2 samples with BG grown for 20 s, 5 min, and 10 min were 11.3, 3.82, and 5 respectively, showing the prevalence of vacancy defects rather than boundary-like defects [42, 43]. The D’ band is generated as an E2g phono during the intra-valley double-resonance process, and such defects in BG/TiO2 samples mainly originate from the heterogeneous boron in BG. Finally, the 2D/G intensity ratios of the BG/TiO2 samples ranged from 0.6 to 1.57, suggesting that the growth time did not change the quality 10
of the graphene; BG layers are empirically evaluated to be mainly composed of few layers and multilayers of graphene. The XRD patterns of the TiO2/Ti, BG/TiO2/Ti, and BG/Ti films are shown in Fig. 5b. We constructed an anatase TiO2 nanotube array (PDF#894921) on the Ti substrate. After growing BG layer for 20 s, a diffraction peak of graphene appeared at 26°, which was assigned to the (002) graphite plane (PDF#802283). In addition, several weak characteristic peaks of TiC also appeared. Comparing the XRD patterns of the BG/TiO2/Ti and BG/Ti films, the former characteristic peak of the TiO2 phase was weakened, the peak intensity of TiC was weaker than the latter, and the intensity of Ti peak was higher than the latter. The TiO2 layer of BG/TiO2 itself transformed into the TiC phase with characteristic peaks of the (111) phase at 36.35°, (200) phase at 42°, (220) phase at 60.92°, and (311) phases at 72.7° (PDF#021179), indicating reduced carbonization of the Ti current collector. This not only improves the conductivity of the nanotube array, but also effectively suppresses the embrittlement of the Ti current collector. Based on analyses of their morphology, structure, and composition, the prepared BG/TiO2/Ti electrodes can be used as capacitor electrodes. Because the conductivity of the TiO2 nanotube layer is improved by carbonization and BG growth, and the BG sheets have a high electrochemically-active surface area, the multilayer film electrode should exhibit excellent electrochemical performance. 3.2. Electrochemical evaluation of the BG/TiO2 electrodes In order to optimize the electrode material, the electrochemical performances of BG/TiO2 electrodes with different BG growth times were compared in a three-electrode cell (Fig. 6), and then the capacitance characteristics were confirmed again by constructing symmetrical LICs (Fig. S1). Fig. 6a and b shows the CV curves of the TiO2 and BG/TiO2 electrodes collected at a scan rate of 100 11
mV·s–1 with a potential window ranging from 0 to 0.8 V and -1.6 to 0 V. After comparing the potential windows and specific capacitances of different electrodes, the BG/TiO2 electrode with a growth time of 20 s exhibited the best electrochemical performance (Fig. 6c). Fig. 6d and e shows the GCD curves of the TiO2 and BG/TiO2 electrodes collected at a current density of 5 A·g–1 over a potential window of -1.1 to 0.5 V, and at a current density of 0.5 A·g–1 over a potential window of 0 to 1 V. The GCD curves of the electrodes also further verified the conclusions drawn from the CV curves that the specific capacitance of the BG/TiO2 electrode with BG growth for 20 s was significantly higher than that of the other electrodes (Fig. 6f). The synergetic effect between BG and the TiO2 nanotube arrays enhanced the capacitance performance of the BG/TiO2 electrode (Fig. S2). When these electrodes are used to construct symmetrical LIC devices, the operating voltage exceeds 2.6 V, and the LIC with BG/TiO2 electrodes for a growth time of 20 s has a larger specific capacitance than the other devices. The specific capacitance value of the 20 s BG/TiO2 electrode-based LIC device reached 180.8 F·g–1 at a current density of 10 A·g–1 (Fig. S1). 3.3. Electrochemical evaluation of the BG/TiO2 electrode with BG growth time of 20 s A three-electrode measuring system was used to evaluate the electrochemical performance of the BG/TiO2 electrode with BG growth times of 20 s in a 4.7 M LiCl aqueous solution. We first investigated the potential window of the electrode in the positive/negative potential range (Fig. 7a, b). The potential range without hydrogen evolution and oxygen evolution is -1.5 to 1.1 V. In this potential range, the CV curves were acquired by changing the scan rate from 1 to 1000 mV·s–1, as shown in Fig. 7c. At a scan rate of 100 mV·s–1, the specific capacitance of the electrode as a negative electrode reached 377.3 F·g–1; in the range of the potential window containing positive and negative potentials, the specific capacitance also exceeded 238.7 F·g–1 (Fig. 7d). In the potential 12
window of -1.5 to 1 V, the specific capacitance of the electrode reached 809.3 F·g–1 at a low scan rate of 1 mV·s–1; after the scan rate was increased to 1000 mV·s–1, the specific capacitance also reached a high value of 185 F·g–1 (Fig. 7e). Second, we recorded GCD curves in different potential windows of -1.1 to 0.5 V and 0 to 1 V at different current densities, as shown in Figs. 7f and 7g. In the negative potential range, pseudocapacitive characteristics are exhibited due to the insertion/deintercalation of Li+ ions; in the positive potential range, the electrode exhibits a clear electric double layer capacitance behavior. These are consistent with the CV curve characteristics (Fig. 7a–c). The BG/TiO2 electrode delivered high specific capacitances of 361.7 F·g–1 at a current density of 5 A·g–1 and 155.9 F·g–1 at a current density of 30 A·g–1. Additionally, the long-term cycling performance of the electrode at 1 A·g–1 is shown in Fig. 7i. Capacitance retention of 98.8% of the initial capacitance was observed after 10000 cycles, denoting the good long-term cycling stability of the electrode. 3.4. Electrochemical evaluation of the BG/TiO2 electrode-based LIC devices LIC devices were fabricated using two BG/TiO2 electrodes (BG: 20 s) with CMC/LiCl gel as the electrolyte, as illustrated in Fig. 1c. Fig. S3a records a series of CV curves with varying voltage windows to determine the optimum operating voltage window of the LICs. The current response of Li+ ions and the specific capacitance of the devices increased with increasing operating voltage from 2 V to 3 V (Fig. S3b). Among them, the highest (0–3 V) and stable (0–2.6 V) voltage ranges were selected to investigate the scan rate-specific capacitance dependence by recording the CV curves (Fig. 8a). The specific capacitance of the LIC device calculated from the CV curves was 91.0 F·g–1 (based on the mass of active materials on the two electrodes) at a scan rate of 20 mV·s–1 and retained 30.5 F·g–1 at a scan rate of 1000 mV·s–1, indicating an acceptable rate performance (Fig. 8b). From 13
the perspective of low internal resistance and high stability of the LIC device, the GCD curves were tested at various current densities with a voltage range of 0–2.5 V (Fig. 8c). Notably, the specific capacitance of our LIC device was calculated to be as high as 279.3 F·g–1 at a current density of 5 A·g–1 and was maintained at 134.7 F·g–1 at a current density of 30 A·g–1, indicating good rate capability (Fig. 8d). A high energy density of 221.8 Wh·kg–1 was achieved at a power density of 5.98 kW·kg–1 and remained at 102.4 Wh·kg–1 at a high power density of 35.1 kW·kg–1 (Fig. 9a). These data considerably exceed previously-reported information for most types of hybrid capacitors, including CTAB-Sn(IV)@Ti3C2//AC
(239.5
Wh·kg–1/10.8
kW·kg–1)
[44],
Li3VO4/N-doped
carbon
nanowires-based LIC (136.4 Wh·kg–1/532 W·kg–1) [45], BTNT//AC Na-ion hybrid capacitor (68 Wh·kg–1/625 W·kg–1; 23 Wh·kg–1/7.5 kW·kg–1) [41], GCNT electrode-based LIC (29 Wh·kg–1/20.5 W·kg–1) [6], LS900//ACSF(25 Wh·kg–1/156 W·kg–1) [5], Co9S8/α-MnS@N-C@MoS2//Ni doped graphene (64.2 Wh·kg–1/729.2 W·kg–1; 23.5 Wh·kg–1/11.3 kW·kg–1) [46], Li2MnO3 SSC (35 Wh·kg– 1
/495.2 W·kg–1) [47], CG@SF//CG hybrid capacitors (121 Wh·kg–1/0.2 kW·kg–1; 60.1 Wh·kg–1/18
kW·kg–1) [48], and Na0.5MnO2//Fe3O4@C (81 Wh·kg–1/647 W·kg–1) [49]. As depicted in Fig. 9b, an LIC device (effective area: 1 cm2) can light up 49 red light-emitting diodes (LEDs) for more than 6 s after charging for 20 s. Nearly 91.3% of the initial specific capacitance is retained after 10000 cycles for the LIC device based on BG/TiO2 electrodes with a BG growth time of 20 s (Fig. 9c), while that of the device based on BG/TiO2 electrodes with BG growth time of 10 min is 108.1% (Fig. 9d). All of these results demonstrate that the working potential window and energy storage capacity can be easily tailored. 4. Conclusions 14
In summary, boron-doping and ultrathin graphene sheets were vertically grown on TiO2 nanotube arrays to prepare BG/TiO2/Ti electrodes. To the best of our knowledge, this is the first report on the preparation of BG/TiO2 nanotubes via CVD methods. The development of such BG/TiO2 nanostructures improves the conductivity of the TiO2 nanotubes, and challenges the process of controlling the distribution of BG sheets. This system provides a large active area surface and suitable charge transferring channels for electrolyte ions. Lithium ion capacitors prepared with BG/TiO2/Ti electrodes show high energy density (221.8 Wh·kg–1 at 5.98 kW·kg–1) and power density (high power density of 35.1 kW·kg–1 at 102.4 Wh·kg–1), excellent cycling stability (91.3% retention after 10000 cycles). More importantly, the BG/TiO2/Ti electrodes are produced by anodizing, annealing, and CVD processes, each of which has high operability and good repeatability, enabling industrial manufacturing. These BG/TiO2/Ti electrodes, in principle, can be used for a wide range of other energy and environmental-related systems such as batteries, fuel cells, catalysis, and water treatment. Acknowledgements This work was supported by the National key R&D program of China (No. 2016YFB0402700) and the Natural Science Foundation of Tianjin City (No. 17JCZDJC32600) Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/xxxx.
15
References [1] W. Ahn, D.U. Lee, G. Li, K. Feng, X. Wang, A. Yu, G. Lui, Z. Chen, Highly oriented graphene sponge electrode for ultra high energy density lithium ion hybrid capacitors, ACS Appl. Mater. Interfaces 8 (2016) 25297-25305. [2] J. Ajuria, M. Arnaiz, C. Botas, D. Carriazo, R. Mysyk, T. Rojo, A.V. Talyzin, E. Goikolea, Graphene-based lithium ion capacitor with high gravimetric energy and power densities, J. Power Sources 363 (2017) 422-427. [3] H. Wang, C. Zhu, D. Chao, Q. Yan, H.J. Fan, Nonaqueous hybrid lithium-ion and sodium-ion capacitors, Adv. Mater. 29 (2017) 1702093. [4] M.-S. Park, Y.-G. Lim, J.-W. Park, J.-S. Kim, J.-W. Lee, J.H. Kim, S.X. Dou, Y.-J. Kim, Li2RuO3 as an additive for high-energy lithium-ion capacitors, J. Phys. Chem. C 117 (2013) 11471-11478. [5] M. Halim, G. Liu, R.E.A. Ardhi, C. Hudaya, O. Wijaya, S.-H. Lee, A.Y. Kim, J.K. Lee, Pseudocapacitive characteristics of low-carbon silicon oxycarbide for lithium-ion capacitors, ACS Appl. Mater. Interfaces 9 (2017) 20566-20576. [6] R.V. Salvatierra, D. Zakhidov, J. Sha, N.D. Kim, S.-K. Lee, A.-R.O. Raji, N. Zhao, J.M. Tour, Graphene carbon nanotube carpets grown using binary catalysts for high-performance lithium-ion capacitors, Acs Nano 11 (2017) 2724-2733. [7] F.M. Auxilia, J. Jang, K. Jang, H. Song, M.H. Ham, Au@TiO2/reduced graphene oxide nanocomposites for lithium-ion capacitors, Chem. Eng. J. 362 (2019) 136-143. [8] W. Raza, F. Ali, N. Raza, Y. Luo, K.-H. Kim, J. Yang, S. Kumar, A. Mehmood, E.E. Kwon, Recent advancements in supercapacitor technology, Nano Energy 52 (2018) 441-473. [9] L. Zhao, H. Li, M. Li, S. Xu, C. Li, C. Qu, L. Zhang, B. Yang, Lithium-ion storage capacitors 16
achieved by CVD graphene/TaC/Ta-wires and carbon hollow spheres, Appl. Energy 162 (2016) 197-206. [10] B. Evanko, S.W. Boettcher, S.J. Yoo, G.D. Stucky, Redox-enhanced electrochemical capacitors: status, opportunity, and best practices for performance evaluation, ACS Energy Lett. 2 (2017) 2581-2590. [11] J. Krummacher, A. Balducci, Al(TFSI)3 as a conducting salt for high-voltage electrochemical double-layer capacitors, Chem. Mater. 30 (2018) 4857-4863. [12] Z.-H. Huang, Y. Song, X.-X. Liu, Boosting operating voltage of vanadium oxide-based symmetric aqueous supercapacitor to 2 V, Chem. Eng. J. 358 (2019) 1529-1538. [13] A. Byeon, M. Boota, M. Beidaghi, K.V. Aken, J.W. Lee, Y. Gogotsi, Effect of hydrogenation on performance of TiO2(B) nanowire for lithium ion capacitors, Electrochem. Commun. 60 (2015) 199-203. [14] J.-Y. Liao, D. Higgins, G. Lui, V. Chabot, X. Xiao, Z. Chen, Multifunctional TiO2–C/MnO2 core–double-shell nanowire arrays as high-performance 3D electrodes for lithium ion batteries, Nano Lett. 13 (2013) 5467-5473. [15] Y.F. Yuan, F. Chen, S.M. Yin, L.N. Wang, M. Zhu, J.L. Yang, Y.C. Wu, S.Y. Guo, Foam-like, 3-dimension mesoporous N-doped carbon-assembling TiO2 nanoparticles (P25) as high-performance anode material for lithium-ion batteries, J. Power Sources 420 (2019) 38-45. [16] J. Li, Y. Li, Q. Lan, Z. Yang, X.-J. Lv, Multiple phase N-doped TiO2 nanotubes/TiN/graphene nanocomposites for high rate lithium ion batteries at low temperature, J. Power Sources 423 (2019) 166-173. [17] X. Xu, Z.Y. Fan, S.J. Ding, D.M. Yu, Y.P. Du, Fabrication of MoS2 nanosheet@TiO2 nanotube 17
hybrid nanostructures for lithium storage, Nanoscale 6 (2014) 5245-5250. [18] H. Moon, H. Lee, J. Kwon, Y.D. Suh, D.K. Kim, I. Ha, J. Yeo, S. Hong, S.H. Ko, Ag/Au/polypyrrole core-shell nanowire network for transparent, stretchable and flexible supercapacitor in wearable energy devices, Sci Rep 7 (2017) 41981. [19] Y. Su, I. Zhitomirsky, Asymmetric electrochemical supercapacitor, based on polypyrrole coated carbon nanotube electrodes, Appl. Energy 153 (2015) 48-55. [20] T. Cossuet, J. Resende, L. Rapenne, O. Chaix-Pluchery, C. Jimenez, G. Renou, A.J. Pearson, R.L.Z. Hoye, D. Blanc-Pelissier, N.D. Nguyen, E. Appert, D. Munoz-Rojas, V. Consonni, J.L. Deschanvres, ZnO/CuCrO2 core-shell nanowire heterostructures for self-powered UV photodetectors with fast response, Adv. Funct. Mater. 28 (2018) 1803142. [21] S. Agnoli, M. Favaro, Doping graphene with boron: a review of synthesis methods, physicochemical characterization, and emerging applications, J. Mater. Chem. A 4 (2016) 5002-5025. [22] S.S.J. Xavier, G. Siva, J. Annaraj, A.R. Kim, D.J. Yoo, G.G. Kumar, Sensitive and selective turn-off-on fluorescence detection of Hg2+ and cysteine using nitrogen doped carbon nanodots derived from citron and urine, Sens. Actuators B: Chem 259 (2018) 1133-1143. [23] H. Tabassum, W.H. Guo, W. Meng, A. Mahmood, R. Zhao, Q.F. Wang, R.Q. Zou, Metal-organic frameworks derived cobalt phosphide architecture encapsulated into B/N Co-doped graphene nanotubes for all pH value electrochemical hydrogen evolution, Adv. Energy Mater. 7 (2017) 1601671. [24] M.A. Mohamed, M.F. M. Zain, L. Jeffery Minggu, M.B. Kassim, N.A. Saidina Amin, W.N. W. Salleh, M.N.I. Salehmin, M.F. Md Nasir, Z.A. Mohd Hir, Constructing bio-templated 3D porous 18
microtubular C-doped g-C3N4 with tunable band structure and enhanced charge carrier separation, Appl. Catal. B 236 (2018) 265-279. [25] M.S. Lee, H.-J. Choi, J.-B. Baek, D.W. Chang, Simple solution-based synthesis of pyridinic-rich nitrogen-doped graphene nanoplatelets for supercapacitors, Appl. Energy 195 (2017) 1071-1078. [26] Q. Geng, G. Huang, Y. Liu, Y. Li, L. Liu, X. Yang, Q. Wang, C. Zhang, Facile synthesis of B/N co-doped 2D porous carbon nanosheets derived from ammonium humate for supercapacitor electrodes, Electrochim. Acta 298 (2019) 1-13. [27] H.R. Jiang, T.S. Zhao, L. Shi, P. Tan, L. An, First-principles study of nitrogen-, boron-doped graphene and Co-doped graphene as the potential catalysts in nonaqueous Li-O2 batteries, J. Phys. Chem. C 120 (2016) 6612-6618. [28] B.V. Senkoyskiy, D.Y. Usachov, A.V. Fedorov, T. Marangoni, D. Haberer, C. Tresca, G. Profeta, V. Caciuc, S. Tsukamoto, N. Atodiresei, N. Ehlen, C. Chen, J. Avila, M.C. Asensio, A.Y. Varykhalov, A. Nefedov, C. Woll, T.K. Kim, M. Hoesch, F.R. Fischer, A. Gruneis, Boron-doped graphene nanoribbons: electronic structure and Raman fingerprint, Acs Nano 12 (2018) 7571-7582. [29] X.L. Wang, Z. Zeng, H. Ahn, G.X. Wang, First-principles study on the enhancement of lithium storage capacity in boron doped graphene, Appl. Phys. Lett. 95 (2009) 183103. [30] W.D. Tennyson, M. Tian, A.B. Papandrew, C.M. Rouleau, A.A. Puretzky, B.T. Sneed, K.L. More, G.M. Veith, G. Duscher, T.A. Zawodzinski, D.B. Geohegan, Bottom up synthesis of boron-doped graphene for stable intermediate temperature fuel cell electrodes, Carbon 123 (2017) 605-615. [31] M. Sahoo, S. Ramaprabhu, One-pot environment-friendly synthesis of boron doped graphene-SnO2 for anodic performance in Li ion battery, Carbon 127 (2018) 627-635. 19
[32] D. Cui, H. Li, M. Li, C. Li, L. Qian, B. Zhou, B. Yang, Boron-Doped Graphene Directly Grown on Boron-Doped Diamond for High-Voltage Aqueous Supercapacitors, ACS Appl. Energy Mater. 2 (2019) 1526-1536. [33] M. Cattelan, S. Agnoli, M. Favaro, D. Garoli, F. Romanato, M. Meneghetti, A. Barinov, P. Dudin, G. Granozzi, Microscopic view on a chemical vapor deposition route to boron-doped graphene nanostructures, Chem. Mater. 25 (2013) 1490-1495. [34] X. Cui, L. Meng, X. Zhang, X. Wang, J. Shi, Heterogeneous atoms-doped titanium carbide as a precious metal-free electrocatalyst for oxygen reduction reaction, Electrochim. Acta 295 (2019) 384-392. [35] B. Tang, H. Chen, Y. He, Z. Wang, J. Zhang, J. Wang, Influence from defects of three-dimensional graphene network on photocatalytic performance of composite photocatalyst, Compos. Sci. Technol. 150 (2017) 54-64. [36] L.L. Zhang, Q.C. He, S.Z. Huang, J.L. Zhu, J.L. Key, P.K. Shen, A novel boron and nitrogen co-doped three-dimensional porous graphene sheet framework as high performance Li-ion battery anode material, Inorg. Chem. Commun. 96 (2018) 159-164. [37] H. Zhang, Y. Jiang, Z. Qi, X. Zhong, Y. Yu, Sulfur doped ultra-thin anatase TiO2 nanosheets/graphene nanocomposite for high-performance pseudocapacitive sodium storage, Energy Storage Mater. 12 (2018) 37-43. [38] L. Cheng, D. Qiao, P. Zhao, Y. He, W. Sun, H. Yu, Z. Jiao, Template-free synthesis of mesoporous succulents-like TiO2/graphene aerogel composites for lithium-ion batteries, Electrochim. Acta 300 (2019) 417-425. [39] Z. Cui, J. Yao, T. Mei, S. Zhou, B. Hou, J. Li, J. Li, J. Wang, J. Qian, X. Wang, Strong lithium 20
polysulfides chemical trapping of TiC-TiO2/S composite for long-cycle lithium-sulfur batteries, Electrochim. Acta 298 (2019) 43-51. [40] Y.-T. Weng, H.-A. Pan, N.-L. Wu, G.Z. Chen, Titanium carbide nanocube core induced interfacial growth of crystalline polypyrrole/polyvinyl alcohol lamellar shell for wide-temperature range supercapacitors, J. Power Sources 274 (2015) 1118-1125. [41] B. Babu, S.G. Ullattil, R. Prasannachandran, J. Kavil, P. Periyat, M.M. Shaijumon, Ti3+ induced brown TiO2 nanotubes for high performance sodium-ion hybrid capacitors, ACS Sustainable Chemistry & Engineering 6 (2018) 5401-5412. [42] J. Jiang, R. Pachter, F. Mehmood, A.E. Islam, B. Maruyama, J.J. Boeckl, A Raman spectroscopy signature for characterizing defective single-layer graphene: Defect-induced I(D)/I(D′) intensity ratio by theoretical analysis, Carbon 90 (2015) 53-62. [43] S.C. O’Hern, M.S.H. Boutilier, J.-C. Idrobo, Y. Song, J. Kong, T. Laoui, M. Atieh, R. Karnik, Selective ionic transport through tunable subnanometer pores in single-layer graphene membranes, Nano Lett. 14 (2014) 1234-1241. [44] J. Luo, W. Zhang, H. Yuan, C. Jin, L. Zhang, H. Huang, C. Liang, Y. Xia, J. Zhang, Y. Gan, X. Tao, Pillared structure design of MXene with ultralarge interlayer spacing for high-performance lithium-ion capacitors, Acs Nano 11 (2017) 2459-2469. [45] L. Shen, H. Lv, S. Chen, P. Kopold, P.A. van Aken, X. Wu, J. Maier, Y. Yu, Peapod-like Li3VO4/N-doped carbon nanowires with pseudocapacitive properties as advanced materials for high-energy lithium-ion capacitors, Adv. Mater. 29 (2017) 1700142. [46] S. Kandula, K.R. Shrestha, N.H. Kim, J.H. Lee, Fabrication of a 3D hierarchical sandwich Co9S8/α-MnS@N–C@MoS2 nanowire architectures as advanced electrode material for high 21
performance hybrid supercapacitors, Small 14 (2018) 1800291. [47] W. Xu, Z. Jiang, Q. Yang, W. Huo, M.S. Javed, Y. Li, L. Huang, X. Gu, C. Hu, Approaching the lithium-manganese oxides' energy storage limit with Li2MnO3 nanorods for high-performance supercapacitor, Nano Energy 43 (2018) 168-176. [48] E. Kim, H. Kim, B.-J. Park, Y.-H. Han, J.H. Park, J. Cho, S.-S. Lee, J.G. Son, Etching-assisted crumpled graphene wrapped spiky iron oxide particles for high-performance Li-ion hybrid supercapacitor, Small 14 (2018) 1704209. [49] N. Jabeen, A. Hussain, Q. Xia, S. Sun, J. Zhu, H. Xia, High-performance 2.6 V aqueous asymmetric supercapacitors based on in situ formed Na0.5MnO2 nanosheet assembled nanowall arrays, Adv. Mater. 29 (2017) 1700804.
22
Figure captions Fig. 1. (a) Schematic of the stepwise fabrication of the BG/TiO2 electrode. (b) Photos of Ti, TiO2/Ti, BG/TiO2/Ti, and BG/TiO2-based LIC. (c) Schematic of BG/TiO2-based LIC. Fig. 2. TEM and HRTEM images of TiO2 (a, b), destroyed BG/TiO2 nanotubes (c, d), and normal BG/TiO2 nanotubes (e, f). Inset shows FFT, iFFT, and atomic structure corresponding to the region marked I in (e). (g) Dark field TEM and elemental mapping analysis of BG/TiO2. The growth time of the BG layers is 20 s. Fig. 3. Surface SEM images of TiO2 (a) and BG/TiO2 with BG grown for 20 s (b), 40 s (c), 1 min (d), 5 min (e), and 10 min (f). Cross-section SEM images of TiO2 (g) and BG/TiO2 with BG grown for 20 s (h) and 10 min (i). Fig. 4. Survey XPS (a) and core XPS spectra of C 1s (b), B 1s (c), O 1s (d), and Ti 2p (e), recorded for BG/TiO2/Ti electrodes with a growth time of 20 s in the BG layer. From top to bottom, the growth time of the TiO2 nanotubes was 0.5, 1.5, and 2 h. Fig. 5. (a) Raman spectra of TiO2 and BG/TiO2 electrodes grown for 20 s, 5 min, and 10 min. (b) XRD patterns of TiO2, BG/ TiO2 with BG grown for 20 and 10 min, and BG/Ti with BG grown for 10 min. Fig. 6. Comparison of electrochemical performance of TiO2 and various BG/TiO2 electrodes in a three-electrode configuration. (a, b) CV curves collected at a scan rate of 100 mV·s–1. (c) Specific capacitance form to the CV curves as a function of the BG growth times. (d) GCD curves collected at a current density of 5 A·g–1. (e) GCD curves measured at a current density of 0.5 A·g–1. (f) Specific capacitance from to GCD curves as a function of BG growth time. 23
Fig. 7. Electrochemical performance of BG/TiO2 electrode with BG growth time of 20 s. (a–c) CV curves in various potential windows at a scan rate of 100 mV·s–1 and in a potential window of -1.5– 1.1 V at various scan rates. (d) Specific capacitance as a function of potential window. (e) Specific capacitance as function of scan rate. (f, g) GCD curves at various current densities. (h) Specific capacitance as a function of current density. (i) Cycling performance in the potential window of 0–1 V at a current density of 1 A·g–1. Fig. 8. Electrochemical properties of BG/TiO2-LIC device. (a) CV curves at various scan rates of 5 to 1000 mV·s–1. (b) Specific capacitance as a function of scan rate. (c) GCD curves at various current densities of 5 to 30 A·g–1. (d) Specific capacitance as a function of current density. Fig. 9. (a) Ragone plots of the BG/TiO2-based LIC device and recently reported values for comparison. (b) Photograph of 49 LED powered by a BG/TiO2-based LIC. (c, d) The long-term cycling performance of the as-assembled LIC devices with growth times of 20 s (c) and 10 min (d).
24
Fig. 1. (a) Schematic of the stepwise fabrication of the BG/TiO2 electrode. (b) Photos of Ti, TiO2/Ti, BG/TiO2/Ti, and BG/TiO2-based LIC. (c) Schematic of BG/TiO2-based LIC.
1/9
Fig. 2. TEM and HRTEM images of TiO2 (a, b), destroyed BG/TiO2 nanotubes (c, d), and normal BG/TiO2 nanotubes (e, f). Inset shows FFT, iFFT, and atomic structure corresponding to the region marked I in (e). (g) Dark field TEM and elemental mapping analysis of BG/TiO2. The growth time of the BG layers is 20 s.
2/9
Fig. 3. Surface SEM images of TiO2 (a) and BG/TiO2 with BG grown for 20 s (b), 40 s (c), 1 min (d), 5 min (e), and 10 min (f). Cross-section SEM images of TiO2 (g) and BG/TiO2 with BG grown for 20 s (h) and 10 min (i).
3/9
Fig. 4. Survey XPS (a) and core XPS spectra of C 1s (b), B 1s (c), O 1s (d), and Ti 2p (e), recorded for BG/TiO2/Ti electrodes with a growth time of 20 s in the BG layer. From top to bottom, the growth time of the TiO2 nanotubes was 0.5, 1.5, and 2 h.
4/9
Fig. 5. (a) Raman spectra of TiO2 and BG/TiO2 electrodes grown for 20 s, 5 min, and 10 min. (b) XRD patterns of TiO2, BG/ TiO2 with BG grown for 20 and 10 min, and BG/Ti with BG grown for 10 min.
5/9
Fig. 6. Comparison of electrochemical performance of TiO2 and various BG/TiO2 electrodes in a three-electrode configuration. (a, b) CV curves collected at a scan rate of 100 mV·s–1. (c) Specific capacitance form to the CV curves as a function of the BG growth times. (d) GCD curves collected at a current density of 5 A·g–1. (e) GCD curves measured at a current density of 0.5 A·g–1. (f) Specific capacitance from to GCD curves as a function of BG growth time.
6/9
Fig. 7. Electrochemical performance of BG/TiO2 electrode with BG growth time of 20 s. (a–c) CV curves in various potential windows at a scan rate of 100 mV·s–1 and in a potential window of -1.5– 1.1 V at various scan rates. (d) Specific capacitance as a function of potential window. (e) Specific capacitance as function of scan rate. (f, g) GCD curves at various current densities. (h) Specific capacitance as a function of current density. (i) Cycling performance in the potential window of 0–1 V at a current density of 1 A·g–1.
7/9
Fig. 8. Electrochemical properties of BG/TiO2-LIC device. (a) CV curves at various scan rates of 5 to 1000 mV·s–1. (b) Specific capacitance as a function of scan rate. (c) GCD curves at various current densities of 5 to 30 A·g–1. (d) Specific capacitance as a function of current density.
8/9
Fig. 9. (a) Ragone plots of the BG/TiO2-based LIC device and recently reported values for comparison. (b) Photograph of 49 LED powered by a BG/TiO2-based LIC. (c, d) The long-term cycling performance of the as-assembled LIC devices with growth times of 20 s (c) and 10 min (d).
9/9
Highlights Boron-doped graphene was grown on TiO2 nanotubes (BG/TiO2) by in situ CVD. A BG/TiO2-based aqueous lithium ion capacitor with operating voltage of 2.6 V was constructed. A specific capacitance of 134.7 F·g–1 at a current density of 30 A·g−1 is achieved. The capacitor shows a high energy density of 221.8 Wh·kg–1 at 5.98 kW·kg–1.
Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Boron-doped graphene/TiO2 nanotube-based aqueous lithium ion capacitors with high energy density”.
Hongji Li School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China Tel.: 0086-022-60214259 E-mail address: [email protected]
S0013-4686(19)32046-8
DOI:
https://doi.org/10.1016/j.electacta.2019.135175
Reference:
EA 135175
To appear in:
Electrochimica Acta
Received Date: 2 September 2019 Revised Date:
24 October 2019
Accepted Date: 28 October 2019
Please cite this article as: J. Gao, G. Qiu, H. Li, M. Li, C. Li, L. Qian, B. Yang, Boron-doped graphene/ TiO2 nanotube-based aqueous lithium ion capacitors with high energy density, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135175. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphical Abstract
Boron-doped graphene/TiO2 nanotube-based aqueous lithium ion capacitors with high energy density Jiepei Gao a, Guojun Qiu a, *, Hongji Li a, *, Mingji Li b, *, Cuiping Li b, Lirong Qian b, Baohe Yang b a
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of
Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P.R. China b
Tianjin Key Laboratory of Film Electronic and Communication Devices, School of Electrical and
Electronic Engineering, Tianjin University of Technology, Tianjin 300384, P.R. China
*Corresponding Authors. Tel.: +86 022 60214259 E-mail: [email protected] (G. Qiu); [email protected] (H. Li); [email protected] (M. Li).
1
Abstract Graphene and its derivatives have gained tremendous research interest for energy storage because of their high capacitance and structural stability. However, the synthesis of graphene nanosheets with high electrochemical activity remains a great challenge. Herein, boron-doped graphene (BG)/TiO2 nanotube array/Ti multilayer films were constructed by growing BG sheets on the TiO2 nanotubes. The high electrochemical activity of BG sheets and electronic transfer capability of TiO2 nanotubes led to good structural stability and energy-storage capability. The as-fabricated capacitors with two BG/TiO2/Ti electrodes in a CMC/LiCl gel electrolyte could realize a wide operational voltage of 2.6 V and deliver a high energy density of 221.8 Wh·kg–1 at 5.98 kW·kg–1, which still remains 102.4 Wh·kg–1 as the power density increased to 35.1 kW·kg–1. These devices show very high electrochemical lifespans, with approximately 91.3% retention after 10000 charge/discharge cycles.
Keywords: Boron-doped graphene; TiO2 nanotubes; Lithium-ion capacitors; High energy density
2
1. Introduction Lithium-ion capacitors (LICs) operate on the principle of synergistic interactions between faradaic (lithium insertion/extraction) and electric double layer processes, which is an effective strategy for simultaneously increasing energy density and power density [1-3]. Most known LICs are constructed using organic electrolytes [4-7]. The safety concern of organic electrolytes cannot be ignored, so we envision that aqueous electrolytes can be used to construct LICs instead [8, 9]. An active material supports the insertion/extraction of Li+ ions while maintaining the fast charge/discharge advantages of the electric double layer process, which is the key to constructing an aqueous LIC. The strategy of widening the potential window and increasing the specific capacitance can substantially increase the energy density of the aqueous capacitors [10-12]. Here, we seek to increase the potential window using TiO2 nanotubes and doped graphene, increasing the specific capacitance using the pseudo-capacitance behavior of lithium ions. The advantages of TiO2 nanostructures include their reversible redox reaction with lithium ions (TiO2 + xLi+ + xe- ↔ LixTiO2), small volume effect, low cost, and environmentally-benign nature [13, 14]. In particular, one-dimensional TiO2 nanotubes form an array that can be erected on the current collector, which not only improves the transfer properties of vector ions and electrons, but also greatly improves the support and buffering ability for repeated intercalation/deintercalation of lithium ions [15, 16]. Therefore, we decided to design a new active material based the skeleton of TiO2 nanotube arrays and hope to further increase their surface area, electrochemical activity, and conductivity. We investigated core-shell structures such as MoS2@TiO2 hybrid nanostructures [17], Ag/Au/polypyrrole (PPy) nanowires [18], PPy/carbon nanotubes [19], and Zn/CuCrO2 nanowires [20]; doping materials include boron-doped graphene [21], nitrogen-doped carbon dots [22], B/N 3
co-doped graphene nanotubes [23], and C-doped g-C3N4 [24]. The advantage of core-shell structures is that the roles of the shell and the core are very clear and are often synergistic. The charge polarization of heteroatoms can adjust the electron density between the heteroatoms and adjacent skeletal atoms, and is an example of pseudocapacitive behavior [25, 26]. Therefore, we propose a strategy for preparing an LIC by preparing a boron-doped graphene (BG)/TiO2 nanotube array hybrid structure (BG/TiO2) as the active material of an electrode. For many years, BG has been the focus of theoretical research, especially its p-type conductivity which is considered to be a crucial property for the development of graphene-based electronic devices [27, 28]. The substitution of boron in the graphene lattice affects the electron cloud distribution on the graphene surface, which increases the charge amount at the doping position. Boron atoms doped into graphene adsorb Li ions and promote the intercalation reaction of Li ions to the enhancement of electrochemical activity [29]. Furthermore, dye cells [30], Li ion batteries [31], electrochemical capacitors [32], and other energy storage devices with BG exhibit excellent energy storage performance. In this paper, BG sheets were grown on an anodic oxidation-processed TiO2 nanotube array by chemical vapor deposition (CVD) to yield BG/TiO2 nanotube-array electrodes. CVD can control the morphology of graphene and oxygen contamination, which improve the conductivity of TiO2 nanotubes by carbonizing them. The BG/TiO2/Ti electrode was prepared for the first time in this study using a novel CVD technique. By investigating the influence of key process parameters on the morphology and structure of the BG layer and the electrochemical performance of the BG/TiO2/Ti electrode, an aqueous LIC was constructed and its feasibility for applied use was determined. 2. Experimental 4
2.1. Preparation of boron-doped graphene/TiO2 nanotube (BG/TiO2) electrodes Titanium (Ti) sheets (20 mm × 10 mm × 1 mm) were sanded with sandpaper, and then ultrasonicated in ultrapure water, ethanol, and ultrapure water for 15 min each and dried. An ammonium fluoride (NH4F) solution having a concentration of 0.25 M was prepared by mixing ultrapure water and glycerol (at a volume ratio of 1:3) as a solvent. In order to prepare TiO2 nanotubes by anodization, an electrolysis system was constructed using an ITECH DC source meter (ITECH Electronic Co., Ltd), a Ti sheet as an anode and a Pt sheet as a cathode in the above electrolyte. The voltage was set to 19.9 V, the magnetic stirring speed was set to 700 rpm, and the anodization time was 2 h. Next, this system was annealed at 500 °C for 3 h in a vacuum tube furnace (SK-G03123K), yielding a TiO2 nanotube array electrode (TiO2/Ti or TiO2). Boron-doped graphene (BG) was deposited on the TiO2/Ti substrate using an electron-assisted hot-filament plasma chemical vapor deposition (EA-HF CVD) system. Five lengths of 18.5 cm were suspended from the filament frame at equal intervals, and the chamber pressure was pumped below 1 Pa. Methane (CH4) at a flow rate of 18 sccm and hydrogen (H2) at a flow rate of 300 sccm were introduced to maintain the chamber pressure at 5000 Pa, the filament current was controlled at around 115 A and held for 40 min to carbonize the filament. Six TiO2/Ti substrates were placed on the sample stage in the reaction chamber and evacuated to < 1 Pa. A boron-source gas, methane (CH4), and hydrogen (H2) were introduced into the chamber until the chamber pressure rose to 400 Pa. The flow rates of boron-source gas, CH4, and H2 were 50, 10, and 40 sccm, respectively. Among these, the carrier gas of the boron source was H2, which carries the liquid boron source as a trimethyl borate/ethanol mixed solution (volume ratio of 3:1) into the CVD reaction chamber. The filament power was then turned on and the filament current adjusted to 115 A, with the spacing between the 5
sample and filament spacing of 1 cm. Finally, the DC bias supply was turned on to heat the sample platform, applying a bias voltage of 26 V and a bias current of 4 A. The chamber pressure was maintained at 400 Pa, the substrate temperature was 900 °C, and the reaction time was controlled from 20 s to 10 min. 2.2 Characterization The microscopic morphology and microstructure characteristics of the samples were characterized by field emission scanning electron microscopy (FE-SEM; MERLIN Compact, Carl Zeiss, Germany) and transmission electron microscopy (TEM; JEM-2100F, JEOL, Japan). Analysis of the valence-bond structure, phase structure, and elemental composition was carried out using a Raman spectrometer (LabRAM HR Evolution, Horiba Scientific, Japan), an X-ray diffractometer (XRD; D/max-2500/PC, Rigaku, Japan), an X-ray photoelectron spectrometer (XPS; Escalab 250Xi, Thermo Scientific, USA), and elemental mapping images collected using an energy dispersive X-ray (EDX) line and a TEM instrument. The electrochemical properties of the electrodes and devices were analyzed using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD). The electrochemical performance of the electrodes was tested by a three-electrode system consisting of sample, Pt, and Ag/AgCl electrodes. A 4.7 M LiCl aqueous solution was used for the electrolyte solution. The two-electrode system was used to test the energy storage performance of the device. The LIC devices were fabricated using two BG/TiO2/Ti electrodes, with a membrane (areal density: 30 g·m–2) as the separator and carboxymethylcellulose sodium (CMC)/LiCl gel as the electrolyte (CMC: 0.16 g·mL–1, LiCl: 4.7 M, excluded volume effect). All of the electrochemical performances were tested on an electrochemical workstation (CHI1140C, Chenhua Co., China). The specific capacitances of the 6
electrode and devices can be calculated from the CV and GCD curves according to the following equations:
Cm(or A) =
Cm(or A) =
i∆t for CV, m(or A)( ∆V -iR)
∫
V2
V1
(1)
idV
2m(or A)∆V
for GCD.
(2)
Moreover, the energy and power densities of the devices were calculated according to the following equations: Em =
1 C ( ∆ V)2 , 2 m
(3)
Pm =
Em , ∆t
(4)
where Cm (or A) (F·g–1, or mF·cm–2), Em (Wh·kg–1), Pm (W·kg–1), I (A), △t (s), m (g), A (cm2), △V (V), iR (V), and ν (V·s–1) are the specific capacitance, energy density, power density, discharge current, discharge time, total mass of active materials in the electrodes, surface area, potential window, IR drop, and scan rate, respectively. 3. Results and discussion 3.1. Fabrication and characterization of BG/TiO2 electrodes Fig. 1a schematically presents the fabrication procedure of BG/TiO2 nanotube array/Ti electrodes (BG/TiO2). In general, TiO2 could be reduced to TiC in CH4 and H2 in accordance with the following reactions: CH4 = C + 2H2, TiO2 + C + 2H2 = TiC + 2H2O, TiO2 + C= TiO + CO, and TiO2 + 2C = TiC + 2CO. Under the high temperature and high pressure conditions in this experiment, these reactions proceed spontaneously. We employed TiO2 nanotubes prepared by electrochemical anodization as the substrates for the in situ growth of BG. The TiO2 nanotubes formed a densely 7
packed array on the Ti substrate, and the TiO2/Ti films were directly placed into the CVD reaction chamber. After the CH4 was introduced into the CVD reaction chamber, the partial TiO2 phase was converted into a TiC phase, and a graphene domain formed there. The TiO2 nanotube array controlled the growth direction of the BG sheets, and the height of the BG layer on top of the TiO2 nanotube array increased rapidly with prolonging the growth time of BG. After growing for 20 s, the TiO2 nanotubes converted into BG/(TiO2+TiC) nanotubes (still referred to as BG/TiO2), and the sample changed from yellowish to black (Fig. 1b). We then assembled an aqueous LIC device using two BG/TiO2 electrodes (Fig. 1b and c). We confirmed the growth of graphene on the TiO2 nanotubes by comparing TEM and high-resolution TEM (HRTEM) images of the TiO2 nanotubes and BG/TiO2 nanotubes (Fig. 2a–f). After destroying the graphene sheets grown vertically on the TiO2 nanotube surface, the graphene domains at the time of nucleation on the tube wall can be observed (Fig. 2d). The inverse fast Fourier transform (iFFT) and fast Fourier transform (FFT) patterns of region I show a well-defined carbon six-membered ring atomic arrangement (insets in Fig. 2e). In addition, the layer count of graphene was 2–7 layers, which was further verified by TEM at the folding edges of the graphene in region II (Fig. 2f). In particular, the presence of B was further confirmed by combining the dark field TEM image and the EDX elemental mapping as shown in Fig. 2g; the prepared graphene was BG sheets. Extensive experimental efforts have been made to finely control the height of BG layers to optimize charge transport for LICs. This was realized by using a constant CH4 flow rate of 18 sccm for growth times of 20 s, 40 s, 1 min, 5 min, and 10 min. In this respect, time-dependent CVD growth is effective for tuning the height of the BG layers (Fig. 3). The surface SEM images show that the size of the BG sheets is rapidly increased as the growth time of BG is extended from 20 s to 8
10 min (Fig. 3a–f). In the cross-sectional SEM image of the samples, the nanotubes have an average length of less than 1 µm, and the tubular morphology was preserved after the CVD process (Fig. 3g– i). XPS spectra were used to analyze the chemical state of our samples. Fig. 4a shows the survey XPS spectra recorded for BG/TiO2 samples with a BG growth time of 20 s. The growth time of the TiO2 nanotubes was extended from 30 min to 2 h, and the chemical state of the C 1s in the samples varied significantly over time (Fig. 4b). The C 1s main component of the three BG/TiO2 samples is the sp2 hybrid C-C bond at 284.8 eV [31]. In addition, a peak was present at 286.3 eV corresponding to the C-O band, which is related to the adsorption of oxygenates on the exposed faces and sides of BG sheets [33]. When the TiO2 nanotubes were grown for 30 min, the C 1s spectrum exhibited two additional peaks at 281.9 and 288.6 eV, which coincided with the Ti-C bonds of TixCy and C-O-B bonds of BG, respectively [34-36]. As the growth time of the TiO2 nanotubes was extended to 1.5 h, the Ti-C peak was still present, but the C-O-B bond virtually disappeared. The one peak in the B 1s spectrum of TiO2 nanotubes grown for 30 min represented the chemical environment of boron atoms bonded to oxygen at 192.1 eV (-BCO2) in BG (Fig. 4c). When the growth time was extended to more than 1.5 h, the B 1s band was shifted to approximately 189.1 eV. The peak having a binding energy of 189.1 eV can be associated to sp2 C-B bonds present in graphene, which is indicative of the formation of substitutional bond atoms [33]. Fig. 4d shows O 1s spectra of the BG/TiO2 samples. When the growth time of the TiO2 nanotubes is 30 min, it is mainly composed of Ti4+-O bonds (at 530.2 eV) and Ti3+-O bonds (at 531.1 eV) in the TiO2 layer [16, 37]. When the growth times of the TiO2 layer are 1.5 and 2 h, the O 1s spectra contains three binding energy peaks at 530.2, 531.2, and 532.9 eV. The extra peak at 532.9 eV which corresponds to the C=O or C-OH bonds in the BG 9
surface, indicating that BG is easily formed. The Ti 2p spectra of BG/TiO2 layers are shown in Fig. 4e. Two peaks located at 458.6 and 464.3 eV, which are assigned to Ti 2P3/2 and Ti 2P1/2, respectively, indicating the presence of Ti4+ chemical state in TiO2; and Ti 2P3/2 at 454.9 eV and Ti 2P1/2 at 461.1 eV correspond to TiC [7, 38-40]. While another peak at 456.6 eV imply the formation Ti3+ after boron-doping [13, 41]. Clearly, after the TiO2 growth time was extended from 30 min to 2 h, the Ti-O bond was weakened while the Ti-C and Ti-B bonds were enhanced. The TiO2 nanotubes grown for 2 h are easily carbonized as a substrate, and the boron atoms are more likely to replace the carbon atoms in the TiC lattice. We analyzed the composition and defect structure of BG by comparing the Raman spectra of four samples (Fig. 5a). The characteristic bands in the Raman spectrum of TiO2 are concentrated below 1000 cm–1. Raman spectra of the BG/TiO2 films exhibit a characteristic G band (1580 cm–1) and 2D band (2700 cm–1) of graphene, along with a D band (1350 cm–1). Our characterization results confirmed the feasibility for growing highly crystalline BG layers on the TiO2 nanotubes by CVD. First, BG was successfully grown for 20 s to 10 min, and it completely covered the tubular substrate. Second, the D/G intensity ratios decreased from 2.12 to 0.79 with increased BG growth times from 20 s to 600 s, indicating that the distribution density of graphene sheets can be controlled by changing the growth time. Third, the ID/ID’ intensity ratios (peak height ratio) of the BG/TiO2 samples with BG grown for 20 s, 5 min, and 10 min were 11.3, 3.82, and 5 respectively, showing the prevalence of vacancy defects rather than boundary-like defects [42, 43]. The D’ band is generated as an E2g phono during the intra-valley double-resonance process, and such defects in BG/TiO2 samples mainly originate from the heterogeneous boron in BG. Finally, the 2D/G intensity ratios of the BG/TiO2 samples ranged from 0.6 to 1.57, suggesting that the growth time did not change the quality 10
of the graphene; BG layers are empirically evaluated to be mainly composed of few layers and multilayers of graphene. The XRD patterns of the TiO2/Ti, BG/TiO2/Ti, and BG/Ti films are shown in Fig. 5b. We constructed an anatase TiO2 nanotube array (PDF#894921) on the Ti substrate. After growing BG layer for 20 s, a diffraction peak of graphene appeared at 26°, which was assigned to the (002) graphite plane (PDF#802283). In addition, several weak characteristic peaks of TiC also appeared. Comparing the XRD patterns of the BG/TiO2/Ti and BG/Ti films, the former characteristic peak of the TiO2 phase was weakened, the peak intensity of TiC was weaker than the latter, and the intensity of Ti peak was higher than the latter. The TiO2 layer of BG/TiO2 itself transformed into the TiC phase with characteristic peaks of the (111) phase at 36.35°, (200) phase at 42°, (220) phase at 60.92°, and (311) phases at 72.7° (PDF#021179), indicating reduced carbonization of the Ti current collector. This not only improves the conductivity of the nanotube array, but also effectively suppresses the embrittlement of the Ti current collector. Based on analyses of their morphology, structure, and composition, the prepared BG/TiO2/Ti electrodes can be used as capacitor electrodes. Because the conductivity of the TiO2 nanotube layer is improved by carbonization and BG growth, and the BG sheets have a high electrochemically-active surface area, the multilayer film electrode should exhibit excellent electrochemical performance. 3.2. Electrochemical evaluation of the BG/TiO2 electrodes In order to optimize the electrode material, the electrochemical performances of BG/TiO2 electrodes with different BG growth times were compared in a three-electrode cell (Fig. 6), and then the capacitance characteristics were confirmed again by constructing symmetrical LICs (Fig. S1). Fig. 6a and b shows the CV curves of the TiO2 and BG/TiO2 electrodes collected at a scan rate of 100 11
mV·s–1 with a potential window ranging from 0 to 0.8 V and -1.6 to 0 V. After comparing the potential windows and specific capacitances of different electrodes, the BG/TiO2 electrode with a growth time of 20 s exhibited the best electrochemical performance (Fig. 6c). Fig. 6d and e shows the GCD curves of the TiO2 and BG/TiO2 electrodes collected at a current density of 5 A·g–1 over a potential window of -1.1 to 0.5 V, and at a current density of 0.5 A·g–1 over a potential window of 0 to 1 V. The GCD curves of the electrodes also further verified the conclusions drawn from the CV curves that the specific capacitance of the BG/TiO2 electrode with BG growth for 20 s was significantly higher than that of the other electrodes (Fig. 6f). The synergetic effect between BG and the TiO2 nanotube arrays enhanced the capacitance performance of the BG/TiO2 electrode (Fig. S2). When these electrodes are used to construct symmetrical LIC devices, the operating voltage exceeds 2.6 V, and the LIC with BG/TiO2 electrodes for a growth time of 20 s has a larger specific capacitance than the other devices. The specific capacitance value of the 20 s BG/TiO2 electrode-based LIC device reached 180.8 F·g–1 at a current density of 10 A·g–1 (Fig. S1). 3.3. Electrochemical evaluation of the BG/TiO2 electrode with BG growth time of 20 s A three-electrode measuring system was used to evaluate the electrochemical performance of the BG/TiO2 electrode with BG growth times of 20 s in a 4.7 M LiCl aqueous solution. We first investigated the potential window of the electrode in the positive/negative potential range (Fig. 7a, b). The potential range without hydrogen evolution and oxygen evolution is -1.5 to 1.1 V. In this potential range, the CV curves were acquired by changing the scan rate from 1 to 1000 mV·s–1, as shown in Fig. 7c. At a scan rate of 100 mV·s–1, the specific capacitance of the electrode as a negative electrode reached 377.3 F·g–1; in the range of the potential window containing positive and negative potentials, the specific capacitance also exceeded 238.7 F·g–1 (Fig. 7d). In the potential 12
window of -1.5 to 1 V, the specific capacitance of the electrode reached 809.3 F·g–1 at a low scan rate of 1 mV·s–1; after the scan rate was increased to 1000 mV·s–1, the specific capacitance also reached a high value of 185 F·g–1 (Fig. 7e). Second, we recorded GCD curves in different potential windows of -1.1 to 0.5 V and 0 to 1 V at different current densities, as shown in Figs. 7f and 7g. In the negative potential range, pseudocapacitive characteristics are exhibited due to the insertion/deintercalation of Li+ ions; in the positive potential range, the electrode exhibits a clear electric double layer capacitance behavior. These are consistent with the CV curve characteristics (Fig. 7a–c). The BG/TiO2 electrode delivered high specific capacitances of 361.7 F·g–1 at a current density of 5 A·g–1 and 155.9 F·g–1 at a current density of 30 A·g–1. Additionally, the long-term cycling performance of the electrode at 1 A·g–1 is shown in Fig. 7i. Capacitance retention of 98.8% of the initial capacitance was observed after 10000 cycles, denoting the good long-term cycling stability of the electrode. 3.4. Electrochemical evaluation of the BG/TiO2 electrode-based LIC devices LIC devices were fabricated using two BG/TiO2 electrodes (BG: 20 s) with CMC/LiCl gel as the electrolyte, as illustrated in Fig. 1c. Fig. S3a records a series of CV curves with varying voltage windows to determine the optimum operating voltage window of the LICs. The current response of Li+ ions and the specific capacitance of the devices increased with increasing operating voltage from 2 V to 3 V (Fig. S3b). Among them, the highest (0–3 V) and stable (0–2.6 V) voltage ranges were selected to investigate the scan rate-specific capacitance dependence by recording the CV curves (Fig. 8a). The specific capacitance of the LIC device calculated from the CV curves was 91.0 F·g–1 (based on the mass of active materials on the two electrodes) at a scan rate of 20 mV·s–1 and retained 30.5 F·g–1 at a scan rate of 1000 mV·s–1, indicating an acceptable rate performance (Fig. 8b). From 13
the perspective of low internal resistance and high stability of the LIC device, the GCD curves were tested at various current densities with a voltage range of 0–2.5 V (Fig. 8c). Notably, the specific capacitance of our LIC device was calculated to be as high as 279.3 F·g–1 at a current density of 5 A·g–1 and was maintained at 134.7 F·g–1 at a current density of 30 A·g–1, indicating good rate capability (Fig. 8d). A high energy density of 221.8 Wh·kg–1 was achieved at a power density of 5.98 kW·kg–1 and remained at 102.4 Wh·kg–1 at a high power density of 35.1 kW·kg–1 (Fig. 9a). These data considerably exceed previously-reported information for most types of hybrid capacitors, including CTAB-Sn(IV)@Ti3C2//AC
(239.5
Wh·kg–1/10.8
kW·kg–1)
[44],
Li3VO4/N-doped
carbon
nanowires-based LIC (136.4 Wh·kg–1/532 W·kg–1) [45], BTNT//AC Na-ion hybrid capacitor (68 Wh·kg–1/625 W·kg–1; 23 Wh·kg–1/7.5 kW·kg–1) [41], GCNT electrode-based LIC (29 Wh·kg–1/20.5 W·kg–1) [6], LS900//ACSF(25 Wh·kg–1/156 W·kg–1) [5], Co9S8/α-MnS@N-C@MoS2//Ni doped graphene (64.2 Wh·kg–1/729.2 W·kg–1; 23.5 Wh·kg–1/11.3 kW·kg–1) [46], Li2MnO3 SSC (35 Wh·kg– 1
/495.2 W·kg–1) [47], CG@SF//CG hybrid capacitors (121 Wh·kg–1/0.2 kW·kg–1; 60.1 Wh·kg–1/18
kW·kg–1) [48], and Na0.5MnO2//Fe3O4@C (81 Wh·kg–1/647 W·kg–1) [49]. As depicted in Fig. 9b, an LIC device (effective area: 1 cm2) can light up 49 red light-emitting diodes (LEDs) for more than 6 s after charging for 20 s. Nearly 91.3% of the initial specific capacitance is retained after 10000 cycles for the LIC device based on BG/TiO2 electrodes with a BG growth time of 20 s (Fig. 9c), while that of the device based on BG/TiO2 electrodes with BG growth time of 10 min is 108.1% (Fig. 9d). All of these results demonstrate that the working potential window and energy storage capacity can be easily tailored. 4. Conclusions 14
In summary, boron-doping and ultrathin graphene sheets were vertically grown on TiO2 nanotube arrays to prepare BG/TiO2/Ti electrodes. To the best of our knowledge, this is the first report on the preparation of BG/TiO2 nanotubes via CVD methods. The development of such BG/TiO2 nanostructures improves the conductivity of the TiO2 nanotubes, and challenges the process of controlling the distribution of BG sheets. This system provides a large active area surface and suitable charge transferring channels for electrolyte ions. Lithium ion capacitors prepared with BG/TiO2/Ti electrodes show high energy density (221.8 Wh·kg–1 at 5.98 kW·kg–1) and power density (high power density of 35.1 kW·kg–1 at 102.4 Wh·kg–1), excellent cycling stability (91.3% retention after 10000 cycles). More importantly, the BG/TiO2/Ti electrodes are produced by anodizing, annealing, and CVD processes, each of which has high operability and good repeatability, enabling industrial manufacturing. These BG/TiO2/Ti electrodes, in principle, can be used for a wide range of other energy and environmental-related systems such as batteries, fuel cells, catalysis, and water treatment. Acknowledgements This work was supported by the National key R&D program of China (No. 2016YFB0402700) and the Natural Science Foundation of Tianjin City (No. 17JCZDJC32600) Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/xxxx.
15
References [1] W. Ahn, D.U. Lee, G. Li, K. Feng, X. Wang, A. Yu, G. Lui, Z. Chen, Highly oriented graphene sponge electrode for ultra high energy density lithium ion hybrid capacitors, ACS Appl. Mater. Interfaces 8 (2016) 25297-25305. [2] J. Ajuria, M. Arnaiz, C. Botas, D. Carriazo, R. Mysyk, T. Rojo, A.V. Talyzin, E. Goikolea, Graphene-based lithium ion capacitor with high gravimetric energy and power densities, J. Power Sources 363 (2017) 422-427. [3] H. Wang, C. Zhu, D. Chao, Q. Yan, H.J. Fan, Nonaqueous hybrid lithium-ion and sodium-ion capacitors, Adv. Mater. 29 (2017) 1702093. [4] M.-S. Park, Y.-G. Lim, J.-W. Park, J.-S. Kim, J.-W. Lee, J.H. Kim, S.X. Dou, Y.-J. Kim, Li2RuO3 as an additive for high-energy lithium-ion capacitors, J. Phys. Chem. C 117 (2013) 11471-11478. [5] M. Halim, G. Liu, R.E.A. Ardhi, C. Hudaya, O. Wijaya, S.-H. Lee, A.Y. Kim, J.K. Lee, Pseudocapacitive characteristics of low-carbon silicon oxycarbide for lithium-ion capacitors, ACS Appl. Mater. Interfaces 9 (2017) 20566-20576. [6] R.V. Salvatierra, D. Zakhidov, J. Sha, N.D. Kim, S.-K. Lee, A.-R.O. Raji, N. Zhao, J.M. Tour, Graphene carbon nanotube carpets grown using binary catalysts for high-performance lithium-ion capacitors, Acs Nano 11 (2017) 2724-2733. [7] F.M. Auxilia, J. Jang, K. Jang, H. Song, M.H. Ham, Au@TiO2/reduced graphene oxide nanocomposites for lithium-ion capacitors, Chem. Eng. J. 362 (2019) 136-143. [8] W. Raza, F. Ali, N. Raza, Y. Luo, K.-H. Kim, J. Yang, S. Kumar, A. Mehmood, E.E. Kwon, Recent advancements in supercapacitor technology, Nano Energy 52 (2018) 441-473. [9] L. Zhao, H. Li, M. Li, S. Xu, C. Li, C. Qu, L. Zhang, B. Yang, Lithium-ion storage capacitors 16
achieved by CVD graphene/TaC/Ta-wires and carbon hollow spheres, Appl. Energy 162 (2016) 197-206. [10] B. Evanko, S.W. Boettcher, S.J. Yoo, G.D. Stucky, Redox-enhanced electrochemical capacitors: status, opportunity, and best practices for performance evaluation, ACS Energy Lett. 2 (2017) 2581-2590. [11] J. Krummacher, A. Balducci, Al(TFSI)3 as a conducting salt for high-voltage electrochemical double-layer capacitors, Chem. Mater. 30 (2018) 4857-4863. [12] Z.-H. Huang, Y. Song, X.-X. Liu, Boosting operating voltage of vanadium oxide-based symmetric aqueous supercapacitor to 2 V, Chem. Eng. J. 358 (2019) 1529-1538. [13] A. Byeon, M. Boota, M. Beidaghi, K.V. Aken, J.W. Lee, Y. Gogotsi, Effect of hydrogenation on performance of TiO2(B) nanowire for lithium ion capacitors, Electrochem. Commun. 60 (2015) 199-203. [14] J.-Y. Liao, D. Higgins, G. Lui, V. Chabot, X. Xiao, Z. Chen, Multifunctional TiO2–C/MnO2 core–double-shell nanowire arrays as high-performance 3D electrodes for lithium ion batteries, Nano Lett. 13 (2013) 5467-5473. [15] Y.F. Yuan, F. Chen, S.M. Yin, L.N. Wang, M. Zhu, J.L. Yang, Y.C. Wu, S.Y. Guo, Foam-like, 3-dimension mesoporous N-doped carbon-assembling TiO2 nanoparticles (P25) as high-performance anode material for lithium-ion batteries, J. Power Sources 420 (2019) 38-45. [16] J. Li, Y. Li, Q. Lan, Z. Yang, X.-J. Lv, Multiple phase N-doped TiO2 nanotubes/TiN/graphene nanocomposites for high rate lithium ion batteries at low temperature, J. Power Sources 423 (2019) 166-173. [17] X. Xu, Z.Y. Fan, S.J. Ding, D.M. Yu, Y.P. Du, Fabrication of MoS2 nanosheet@TiO2 nanotube 17
hybrid nanostructures for lithium storage, Nanoscale 6 (2014) 5245-5250. [18] H. Moon, H. Lee, J. Kwon, Y.D. Suh, D.K. Kim, I. Ha, J. Yeo, S. Hong, S.H. Ko, Ag/Au/polypyrrole core-shell nanowire network for transparent, stretchable and flexible supercapacitor in wearable energy devices, Sci Rep 7 (2017) 41981. [19] Y. Su, I. Zhitomirsky, Asymmetric electrochemical supercapacitor, based on polypyrrole coated carbon nanotube electrodes, Appl. Energy 153 (2015) 48-55. [20] T. Cossuet, J. Resende, L. Rapenne, O. Chaix-Pluchery, C. Jimenez, G. Renou, A.J. Pearson, R.L.Z. Hoye, D. Blanc-Pelissier, N.D. Nguyen, E. Appert, D. Munoz-Rojas, V. Consonni, J.L. Deschanvres, ZnO/CuCrO2 core-shell nanowire heterostructures for self-powered UV photodetectors with fast response, Adv. Funct. Mater. 28 (2018) 1803142. [21] S. Agnoli, M. Favaro, Doping graphene with boron: a review of synthesis methods, physicochemical characterization, and emerging applications, J. Mater. Chem. A 4 (2016) 5002-5025. [22] S.S.J. Xavier, G. Siva, J. Annaraj, A.R. Kim, D.J. Yoo, G.G. Kumar, Sensitive and selective turn-off-on fluorescence detection of Hg2+ and cysteine using nitrogen doped carbon nanodots derived from citron and urine, Sens. Actuators B: Chem 259 (2018) 1133-1143. [23] H. Tabassum, W.H. Guo, W. Meng, A. Mahmood, R. Zhao, Q.F. Wang, R.Q. Zou, Metal-organic frameworks derived cobalt phosphide architecture encapsulated into B/N Co-doped graphene nanotubes for all pH value electrochemical hydrogen evolution, Adv. Energy Mater. 7 (2017) 1601671. [24] M.A. Mohamed, M.F. M. Zain, L. Jeffery Minggu, M.B. Kassim, N.A. Saidina Amin, W.N. W. Salleh, M.N.I. Salehmin, M.F. Md Nasir, Z.A. Mohd Hir, Constructing bio-templated 3D porous 18
microtubular C-doped g-C3N4 with tunable band structure and enhanced charge carrier separation, Appl. Catal. B 236 (2018) 265-279. [25] M.S. Lee, H.-J. Choi, J.-B. Baek, D.W. Chang, Simple solution-based synthesis of pyridinic-rich nitrogen-doped graphene nanoplatelets for supercapacitors, Appl. Energy 195 (2017) 1071-1078. [26] Q. Geng, G. Huang, Y. Liu, Y. Li, L. Liu, X. Yang, Q. Wang, C. Zhang, Facile synthesis of B/N co-doped 2D porous carbon nanosheets derived from ammonium humate for supercapacitor electrodes, Electrochim. Acta 298 (2019) 1-13. [27] H.R. Jiang, T.S. Zhao, L. Shi, P. Tan, L. An, First-principles study of nitrogen-, boron-doped graphene and Co-doped graphene as the potential catalysts in nonaqueous Li-O2 batteries, J. Phys. Chem. C 120 (2016) 6612-6618. [28] B.V. Senkoyskiy, D.Y. Usachov, A.V. Fedorov, T. Marangoni, D. Haberer, C. Tresca, G. Profeta, V. Caciuc, S. Tsukamoto, N. Atodiresei, N. Ehlen, C. Chen, J. Avila, M.C. Asensio, A.Y. Varykhalov, A. Nefedov, C. Woll, T.K. Kim, M. Hoesch, F.R. Fischer, A. Gruneis, Boron-doped graphene nanoribbons: electronic structure and Raman fingerprint, Acs Nano 12 (2018) 7571-7582. [29] X.L. Wang, Z. Zeng, H. Ahn, G.X. Wang, First-principles study on the enhancement of lithium storage capacity in boron doped graphene, Appl. Phys. Lett. 95 (2009) 183103. [30] W.D. Tennyson, M. Tian, A.B. Papandrew, C.M. Rouleau, A.A. Puretzky, B.T. Sneed, K.L. More, G.M. Veith, G. Duscher, T.A. Zawodzinski, D.B. Geohegan, Bottom up synthesis of boron-doped graphene for stable intermediate temperature fuel cell electrodes, Carbon 123 (2017) 605-615. [31] M. Sahoo, S. Ramaprabhu, One-pot environment-friendly synthesis of boron doped graphene-SnO2 for anodic performance in Li ion battery, Carbon 127 (2018) 627-635. 19
[32] D. Cui, H. Li, M. Li, C. Li, L. Qian, B. Zhou, B. Yang, Boron-Doped Graphene Directly Grown on Boron-Doped Diamond for High-Voltage Aqueous Supercapacitors, ACS Appl. Energy Mater. 2 (2019) 1526-1536. [33] M. Cattelan, S. Agnoli, M. Favaro, D. Garoli, F. Romanato, M. Meneghetti, A. Barinov, P. Dudin, G. Granozzi, Microscopic view on a chemical vapor deposition route to boron-doped graphene nanostructures, Chem. Mater. 25 (2013) 1490-1495. [34] X. Cui, L. Meng, X. Zhang, X. Wang, J. Shi, Heterogeneous atoms-doped titanium carbide as a precious metal-free electrocatalyst for oxygen reduction reaction, Electrochim. Acta 295 (2019) 384-392. [35] B. Tang, H. Chen, Y. He, Z. Wang, J. Zhang, J. Wang, Influence from defects of three-dimensional graphene network on photocatalytic performance of composite photocatalyst, Compos. Sci. Technol. 150 (2017) 54-64. [36] L.L. Zhang, Q.C. He, S.Z. Huang, J.L. Zhu, J.L. Key, P.K. Shen, A novel boron and nitrogen co-doped three-dimensional porous graphene sheet framework as high performance Li-ion battery anode material, Inorg. Chem. Commun. 96 (2018) 159-164. [37] H. Zhang, Y. Jiang, Z. Qi, X. Zhong, Y. Yu, Sulfur doped ultra-thin anatase TiO2 nanosheets/graphene nanocomposite for high-performance pseudocapacitive sodium storage, Energy Storage Mater. 12 (2018) 37-43. [38] L. Cheng, D. Qiao, P. Zhao, Y. He, W. Sun, H. Yu, Z. Jiao, Template-free synthesis of mesoporous succulents-like TiO2/graphene aerogel composites for lithium-ion batteries, Electrochim. Acta 300 (2019) 417-425. [39] Z. Cui, J. Yao, T. Mei, S. Zhou, B. Hou, J. Li, J. Li, J. Wang, J. Qian, X. Wang, Strong lithium 20
polysulfides chemical trapping of TiC-TiO2/S composite for long-cycle lithium-sulfur batteries, Electrochim. Acta 298 (2019) 43-51. [40] Y.-T. Weng, H.-A. Pan, N.-L. Wu, G.Z. Chen, Titanium carbide nanocube core induced interfacial growth of crystalline polypyrrole/polyvinyl alcohol lamellar shell for wide-temperature range supercapacitors, J. Power Sources 274 (2015) 1118-1125. [41] B. Babu, S.G. Ullattil, R. Prasannachandran, J. Kavil, P. Periyat, M.M. Shaijumon, Ti3+ induced brown TiO2 nanotubes for high performance sodium-ion hybrid capacitors, ACS Sustainable Chemistry & Engineering 6 (2018) 5401-5412. [42] J. Jiang, R. Pachter, F. Mehmood, A.E. Islam, B. Maruyama, J.J. Boeckl, A Raman spectroscopy signature for characterizing defective single-layer graphene: Defect-induced I(D)/I(D′) intensity ratio by theoretical analysis, Carbon 90 (2015) 53-62. [43] S.C. O’Hern, M.S.H. Boutilier, J.-C. Idrobo, Y. Song, J. Kong, T. Laoui, M. Atieh, R. Karnik, Selective ionic transport through tunable subnanometer pores in single-layer graphene membranes, Nano Lett. 14 (2014) 1234-1241. [44] J. Luo, W. Zhang, H. Yuan, C. Jin, L. Zhang, H. Huang, C. Liang, Y. Xia, J. Zhang, Y. Gan, X. Tao, Pillared structure design of MXene with ultralarge interlayer spacing for high-performance lithium-ion capacitors, Acs Nano 11 (2017) 2459-2469. [45] L. Shen, H. Lv, S. Chen, P. Kopold, P.A. van Aken, X. Wu, J. Maier, Y. Yu, Peapod-like Li3VO4/N-doped carbon nanowires with pseudocapacitive properties as advanced materials for high-energy lithium-ion capacitors, Adv. Mater. 29 (2017) 1700142. [46] S. Kandula, K.R. Shrestha, N.H. Kim, J.H. Lee, Fabrication of a 3D hierarchical sandwich Co9S8/α-MnS@N–C@MoS2 nanowire architectures as advanced electrode material for high 21
performance hybrid supercapacitors, Small 14 (2018) 1800291. [47] W. Xu, Z. Jiang, Q. Yang, W. Huo, M.S. Javed, Y. Li, L. Huang, X. Gu, C. Hu, Approaching the lithium-manganese oxides' energy storage limit with Li2MnO3 nanorods for high-performance supercapacitor, Nano Energy 43 (2018) 168-176. [48] E. Kim, H. Kim, B.-J. Park, Y.-H. Han, J.H. Park, J. Cho, S.-S. Lee, J.G. Son, Etching-assisted crumpled graphene wrapped spiky iron oxide particles for high-performance Li-ion hybrid supercapacitor, Small 14 (2018) 1704209. [49] N. Jabeen, A. Hussain, Q. Xia, S. Sun, J. Zhu, H. Xia, High-performance 2.6 V aqueous asymmetric supercapacitors based on in situ formed Na0.5MnO2 nanosheet assembled nanowall arrays, Adv. Mater. 29 (2017) 1700804.
22
Figure captions Fig. 1. (a) Schematic of the stepwise fabrication of the BG/TiO2 electrode. (b) Photos of Ti, TiO2/Ti, BG/TiO2/Ti, and BG/TiO2-based LIC. (c) Schematic of BG/TiO2-based LIC. Fig. 2. TEM and HRTEM images of TiO2 (a, b), destroyed BG/TiO2 nanotubes (c, d), and normal BG/TiO2 nanotubes (e, f). Inset shows FFT, iFFT, and atomic structure corresponding to the region marked I in (e). (g) Dark field TEM and elemental mapping analysis of BG/TiO2. The growth time of the BG layers is 20 s. Fig. 3. Surface SEM images of TiO2 (a) and BG/TiO2 with BG grown for 20 s (b), 40 s (c), 1 min (d), 5 min (e), and 10 min (f). Cross-section SEM images of TiO2 (g) and BG/TiO2 with BG grown for 20 s (h) and 10 min (i). Fig. 4. Survey XPS (a) and core XPS spectra of C 1s (b), B 1s (c), O 1s (d), and Ti 2p (e), recorded for BG/TiO2/Ti electrodes with a growth time of 20 s in the BG layer. From top to bottom, the growth time of the TiO2 nanotubes was 0.5, 1.5, and 2 h. Fig. 5. (a) Raman spectra of TiO2 and BG/TiO2 electrodes grown for 20 s, 5 min, and 10 min. (b) XRD patterns of TiO2, BG/ TiO2 with BG grown for 20 and 10 min, and BG/Ti with BG grown for 10 min. Fig. 6. Comparison of electrochemical performance of TiO2 and various BG/TiO2 electrodes in a three-electrode configuration. (a, b) CV curves collected at a scan rate of 100 mV·s–1. (c) Specific capacitance form to the CV curves as a function of the BG growth times. (d) GCD curves collected at a current density of 5 A·g–1. (e) GCD curves measured at a current density of 0.5 A·g–1. (f) Specific capacitance from to GCD curves as a function of BG growth time. 23
Fig. 7. Electrochemical performance of BG/TiO2 electrode with BG growth time of 20 s. (a–c) CV curves in various potential windows at a scan rate of 100 mV·s–1 and in a potential window of -1.5– 1.1 V at various scan rates. (d) Specific capacitance as a function of potential window. (e) Specific capacitance as function of scan rate. (f, g) GCD curves at various current densities. (h) Specific capacitance as a function of current density. (i) Cycling performance in the potential window of 0–1 V at a current density of 1 A·g–1. Fig. 8. Electrochemical properties of BG/TiO2-LIC device. (a) CV curves at various scan rates of 5 to 1000 mV·s–1. (b) Specific capacitance as a function of scan rate. (c) GCD curves at various current densities of 5 to 30 A·g–1. (d) Specific capacitance as a function of current density. Fig. 9. (a) Ragone plots of the BG/TiO2-based LIC device and recently reported values for comparison. (b) Photograph of 49 LED powered by a BG/TiO2-based LIC. (c, d) The long-term cycling performance of the as-assembled LIC devices with growth times of 20 s (c) and 10 min (d).
24
Fig. 1. (a) Schematic of the stepwise fabrication of the BG/TiO2 electrode. (b) Photos of Ti, TiO2/Ti, BG/TiO2/Ti, and BG/TiO2-based LIC. (c) Schematic of BG/TiO2-based LIC.
1/9
Fig. 2. TEM and HRTEM images of TiO2 (a, b), destroyed BG/TiO2 nanotubes (c, d), and normal BG/TiO2 nanotubes (e, f). Inset shows FFT, iFFT, and atomic structure corresponding to the region marked I in (e). (g) Dark field TEM and elemental mapping analysis of BG/TiO2. The growth time of the BG layers is 20 s.
2/9
Fig. 3. Surface SEM images of TiO2 (a) and BG/TiO2 with BG grown for 20 s (b), 40 s (c), 1 min (d), 5 min (e), and 10 min (f). Cross-section SEM images of TiO2 (g) and BG/TiO2 with BG grown for 20 s (h) and 10 min (i).
3/9
Fig. 4. Survey XPS (a) and core XPS spectra of C 1s (b), B 1s (c), O 1s (d), and Ti 2p (e), recorded for BG/TiO2/Ti electrodes with a growth time of 20 s in the BG layer. From top to bottom, the growth time of the TiO2 nanotubes was 0.5, 1.5, and 2 h.
4/9
Fig. 5. (a) Raman spectra of TiO2 and BG/TiO2 electrodes grown for 20 s, 5 min, and 10 min. (b) XRD patterns of TiO2, BG/ TiO2 with BG grown for 20 and 10 min, and BG/Ti with BG grown for 10 min.
5/9
Fig. 6. Comparison of electrochemical performance of TiO2 and various BG/TiO2 electrodes in a three-electrode configuration. (a, b) CV curves collected at a scan rate of 100 mV·s–1. (c) Specific capacitance form to the CV curves as a function of the BG growth times. (d) GCD curves collected at a current density of 5 A·g–1. (e) GCD curves measured at a current density of 0.5 A·g–1. (f) Specific capacitance from to GCD curves as a function of BG growth time.
6/9
Fig. 7. Electrochemical performance of BG/TiO2 electrode with BG growth time of 20 s. (a–c) CV curves in various potential windows at a scan rate of 100 mV·s–1 and in a potential window of -1.5– 1.1 V at various scan rates. (d) Specific capacitance as a function of potential window. (e) Specific capacitance as function of scan rate. (f, g) GCD curves at various current densities. (h) Specific capacitance as a function of current density. (i) Cycling performance in the potential window of 0–1 V at a current density of 1 A·g–1.
7/9
Fig. 8. Electrochemical properties of BG/TiO2-LIC device. (a) CV curves at various scan rates of 5 to 1000 mV·s–1. (b) Specific capacitance as a function of scan rate. (c) GCD curves at various current densities of 5 to 30 A·g–1. (d) Specific capacitance as a function of current density.
8/9
Fig. 9. (a) Ragone plots of the BG/TiO2-based LIC device and recently reported values for comparison. (b) Photograph of 49 LED powered by a BG/TiO2-based LIC. (c, d) The long-term cycling performance of the as-assembled LIC devices with growth times of 20 s (c) and 10 min (d).
9/9
Highlights Boron-doped graphene was grown on TiO2 nanotubes (BG/TiO2) by in situ CVD. A BG/TiO2-based aqueous lithium ion capacitor with operating voltage of 2.6 V was constructed. A specific capacitance of 134.7 F·g–1 at a current density of 30 A·g−1 is achieved. The capacitor shows a high energy density of 221.8 Wh·kg–1 at 5.98 kW·kg–1.
Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Boron-doped graphene/TiO2 nanotube-based aqueous lithium ion capacitors with high energy density”.
Hongji Li School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China Tel.: 0086-022-60214259 E-mail address: [email protected]