TiO2 Nanomembranes Fabricated by Atomic Layer Deposition for Supercapacitor Electrode with Enhanced Capacitance
- 193 Downloads
TiO2 is a promising environment friendly, low cost, and high electrochemical performance material. However, impediments like high internal ion resistance and low electrical conductivity restrict its applications as electrode for supercapacitor. In the present work, atomic layer deposition was used to fabricate TiO2 nanomembranes (NMs) with accurately controlled thicknesses. The TiO2 NMs were then used as electrodes for high-performance pseudocapacitors. Experimental results demonstrated that the TiO2 NM with 100 ALD cycles had the highest capacitance of 2332 F/g at 1 A/g with energy density of 81 Wh/kg. The enhanced performance was ascribed to the large surface area and the interconnectivity in the case of ultra-thin and flexible NMs. Increased ALD cycles led to stiffer NMs and decreased capacitance. Moreover, one series of two supercapacitors can light up one light-emitting diode with a working voltage of ~ 1.5 V, sufficiently describing its application values.
KeywordsAtomic layer deposition TiO2 nanomembranes Electrode Supercapacitor
Atomic force microscopy
Atomic layer deposition
Electrochemical impedance spectroscopy
Scanning electron microscopy
Tetrakis dimethylamide titanium
X-ray photoelectron spectroscopy
X-ray diffraction spectrometer
With the maturation of energy storage technology , supercapacitors have received vast attention due to their high power density, fast charge-discharge rate, and good cycling performance [2, 3, 4]. Pseudocapacitor is an important class of supercapacitors, which can deliver attractive high capacitance and energy density compared with electrochemical supercapacitors [5, 6, 7]. In the past few decades, the transition metal oxides (e.g., RuO2 , MoO2 , MnO2 , Ni/NiO , Co3O4 , and TiO2 ) and hydroxides [14, 15, 16] were used as classic electrode materials for pseudocapacitors owing to low cost, low toxicity, multiple oxidation states , and great flexibility in structures and morphology. However, their thermal instability, impurity defects, and rate capability are usually limited by the inadequate conductivity to support fast electron transport required by high rates. In order to solve these problems, low-dimensional TiO2 structures (1D, 2D, 2D + 1D, and 3D) with high surface-to-volume ratio, good surface structure, great electrical and thermal stability, favorable energy band gap properties, and high dielectric constant have been engaged as promising electrode materials for supercapacitors [18, 19, 20, 21, 22]. Especially, we think that 2D nanomembrane (NM) structures with excellent flexibility should have great potential in electrode applications. The thickness control of nanomembrane is therefore crucial in fabricating functional devices in well-defined nanoworld . In addition, large-scale manufacturing of nanoscale materials is also crucial for practical applications . One may note that atomic layer deposition (ALD) is a captivating technique used to construct nanodevices [25, 26]. This powerful technique can deposit thin films layer by layer with accurate thickness control and can conformally cover 3D structures with high aspect ratio [27, 28, 29, 30], and the productively can thus be greatly enhanced. In the current work, we present the fabrication of 2D TiO2 NMs with different thicknesses by performing ALD on 3D porous polymer template with large surface area [31, 32]. Microstructural characterization elucidates that the crystal structure of NM is a mixture of anatase and rutile phases. Electrochemical characterizations demonstrate that the ultra-thin and flexible NMs have the enhanced performance due to the large surface area and the interconnectivity among the NMs. The improved ion transportation causes Faradaic reaction on the surface as well as in the bulk , resulting in increased capacitance and energy densities.
Fabrication of TiO2 NMs
TiO2 NMs with various thicknesses (100, 200, and 400 ALD cycles) were deposited on a commercially available polyurethane sponge by using ALD technique. Tetrakis dimethylamide titanium (TDMAT) and de-ionized (DI) water were used as precursors in the presence of nitrogen (N2) gas which served as both carrier and purge gases. The flow rate of the carrier gas was 20 sccm. A typical ALD sequence includes TDMAT pulse (200 ms), N2 purge (20,000 ms), H2O pulse (20 ms), and N2 purge (30,000 ms). The precursors used were purchased from J&K Scientific Ltd., China. The precursor conformally covered the three-dimensionally porous sponge, which led to promoted productivity due to the large surface area of the template . The TiO2-coated sponges were calcinated at 500 °C for 4 h in an O2 flow of 400 mL/min, and the template was completely removed. The resultant TiO2 NMs were crushed and cleaned in ethanol, hydrochloric acid (HCl), and DI water.
Preparation of Electrode
In order to fabricate high-performance supercapacitor, TiO2 NMs with 100, 200, and 400 ALD cycles were used as the active material and polytetrafluoroethylene (PTFE) was used as binder. The contents of TiO2 NMs and binder were 90 wt% and 10 wt%, respectively. A homogeneous TiO2 NMs slurry was obtained by mixing the NMs and binder with a small quantity of ethanol, and a milling process was engaged. The prepared uniform slurry was deposited onto the cleaned nickel foam and then the sample was degassed at 60 °C for 2 h in vacuum. In order to complete the electrode fabrication, the sample was pressed under 10 MPa pressure. The prepared TiO2 NMs electrode was soaked in 1 M KOH solution for 12 h to activate the electrode. The loading densities of active materials were about ~ 1.5 mg cm−2 for all electrodes. The mass of the TiO2 NMs on nickel foam was obtained by calculating the mass difference between the electrode and nickel foam .
The crystallographic structure of the TiO2 NMs was inspected by X-ray diffraction technique (XRD). The XRD patterns were recorded by using a Bruker D8A Advanced XRD with Cu Kα radiation (λ = 1.5405 Å). The morphology of TiO2 NMs was examined by scanning electron microscopy (SEM, Zeiss Sigma). The Raman spectra of the samples were carried out on a Horiba Scientific Raman spectrometer (λ = 514 nm). The elemental analysis and chemical state of the TiO2 NMs were obtained by using a PHI 5000C EACA X-ray photoelectron spectroscope (XPS), with C 1s peak at 284.6 eV as the standard signal. Atomic force microscopy (AFM, Dimension Edge, Bruker, USA) with tapping mode was used for surface topography of TiO2 NMs.
Three-electrode system was utilized to study the electrochemical properties of the TiO2 NMs working electrode where Ag/AgCl and platinum foil were acted as a reference electrode and counter electrode, respectively. The cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS) measurements were accomplished on a Chenhua CHI 660E electrochemical workstation at 25 °C in 1 M KOH aqueous solution. EIS results were obtained over the frequency range of 100 KHz to 1 Hz with an amplitude of 5 mV. The calculation methods of specific capacitances and energy/power densities are described in Additional file 1.
Results and Discussion
In summary, we have fabricated TiO2 NMs for electrodes of supercapacitor, and the electrochemical performance of the NMs was studied in detail. The TiO2 NM electrode demonstrates increased capacitance with deceased NM thickness. At a current density of 1 A/g, the specific capacitance of 2332 F/g is obtained for TiO2 NM with 100 ALD cycles, and the corresponding energy density is calculated to be 81 Wh/kg. The enhancement of the performance is mainly attributed to the fabrication strategy and the ultra-thin feature of NMs, because the large surface area and short diffusion path of NMs facilitate ion transport through electrode/electrolyte interface. The interconnectivity among the NMs also remarkably enhances the ion transportation in the electrode. We also demonstrate that two supercapacitors connected in series can power a LED, suggesting the application potential of TiO2 NMs supercapacitor. The current facile design opens the way to build NMs electrodes for next-generation wearable energy storage devices at low-cost. However, for practical applications of NM-based structures in future supercapacitors, further studies are required.
The authors are grateful to Dr. Yingchang Jiang, Zhao Zhe, Dr. Atif Zahoor, Dr. Shahid Rasool, Dr. Alexander A Solovev and Dr. David H. Gracias for their helpful discussion and guidance.
This work is supported by the Natural Science Foundation of China (Nos. U1632115 and 61805042), Science and Technology Commission of Shanghai Municipality (No. 17JC1401700), and the Changjiang Young Scholars Program of China. Part of the work is also supported by the National Key Technologies R&D Program of China (No. 2015ZX02102-003).
Availability of Data and Materials
The datasets generated during and/or analyzed during the current study are available from the corresponding author on request.
FN carried out the experiment, analyzed the data, and wrote the manuscript. SN helped in analyzing the data. YTZ and DRW helped in the sample characterization. JZ, YFM, and GSH provided the research directions and revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
- 8.Trasatt S, Buzzancai G (1971) Ruthenium dioxide: a new interesting electrode material. Solid state structure and electrochemical behaviour. J Electroanal Chem 29:A1-A5Google Scholar
- 13.Lu X, Wang G, Zhai T, Yu M, Gan J, Tong Y, Li Y (2012) Hydrogenated TiO2 Nanotube Arrays for Supercapacitors. Nano Lett 12:1690−1696.Google Scholar
- 23.Huang G, Mei YF (2018) Assembly and self-assembly of nanomembrane materials—from 2D to 3D. Small 14:1703665Google Scholar
- 24.Li XJ, Liu W, Wang J, Rozen I, He S, Chen C, Kim GH, Lee HJ, Lee H-B-R, Kwon S-H, Li LT, Li QL, Wang J, Mei YF (2017) Nanoconfined atomic layer deposition of TiO2/Pt nanotubes: toward ultrasmall highly efficient catalytic nanorockets. Adv Funct Mater 27:1700598Google Scholar
- 28.Ritala M, Leskela M. Handbook of thin film materials; Nalwa, H. S., Ed.; Academic Press: San Diego. 2001Google Scholar
- 51.Zhang Q, Xu WW, Sun J, Pan ZH, Zhao JX, Wang X, Zhang J, Man P, Guo JB, Zhou ZY, He B, Zhang ZX, Li QW, Zhang YG, Xu L, Yao Y (2017) Constructing ultrahigh-capacity zinc−nickel−cobalt oxide @ Ni(OH)2 Core−Shell nanowire arrays for high-performance coaxial fiber-shaped asymmetric supercapacitors. Nano Lett 17:7552–7560CrossRefGoogle Scholar
- 52.Zhi J, Zhao W, Lin TQ, Huang F (2017) Boosting supercapacitor performance of TiO2 nanobelts by efficient nitrogen doping. Chem Electro Chem 4:2328–2335Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.