Liangliang
Zhu
a,
Connor Kang
Nuo Peh
a,
Ting
Zhu
a,
Yee-Fun
Lim
b and
Ghim Wei
Ho
*abc
aDepartment of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583. E-mail: elehgw@nus.edu.sg
bInstitute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602
cEngineering Science Programme, National University of Singapore, 9 Engineering Drive 1, Singapore 117575
First published on 4th April 2017
A hybrid composite that unifies multiple structural attributes is the key for improved functional device performance. Moreover, hierarchical and porous assembly of dissimilar constituent elements at various length scales and dimensionality is highly appropriate for mass transfer and surface/interfacial-dominated reactions. Here, we demonstrate solution processed 2D-on-2D hierarchical architecture of nanobelt core–nanosheets shell consisting of dissimilar transition metal composite. The defining benefits of such hetero-structured material design are short ion diffusion pathway of a thin 2D nanobelt core that is interfaced to large surface area and porous 2D nanosheets. Accordingly, an integrated energy system that delivers multiple functionalities, i.e. supercapacitive charge storage and electrochromic optical modulation, is investigated due to its attractive complementary energy storage and energy conservation capabilities. Consequently, bifunctional electrochromism–supercapacitive properties are demonstrated where desirable optical transmittance modulation and coloration efficiency properties in parallel with favorable pseudocapacitive storage and specific capacitance properties are simultaneously realized. Such an integrated energy system is anticipated to have a broad range of smart applications in buildings, automobiles and other emerging electronics needs.
Electrochemical-based phenomena, i.e. supercapacitive charge storage and electrochromic optical modulation, have garnered immense attention due to their corresponding attractive energy storage and energy conservation capabilities.14,15 In prior works, electrochromic materials were mostly studied with external powering batteries without considering deployment or reutilization of the energy stored in these electrochromic materials for useful purposes.16,17 However, with increasing interest in integrated energy systems that can deliver multiple functionalities, it is appealing to adopt electrochromism and supercapacitive capabilities into an integrated system. The associated mechanisms are highly compatible in operation since some of the electrochemically active materials exhibit charge insertion/extraction or chemical reduction/oxidation processes that are accompanied by reversible change of colors.18,19 Despite rapid advancement in the independent research on supercapacitor and electrochromic nanomaterials, there is limited work on completely solution-processed 2D hetero-structured transition metal materials with hierarchical porous architecture that exhibit complementary capacitive storage and chromogenic capabilities. Presumably the challenges lie in the difficulties in controlling different growth mechanisms and in regulating growth dynamics of disparate material systems into an appropriate design framework.
Herein, we synthesize a 2D-on-2D hierarchical architecture of nanobelt core–nanosheets shell of disparate transition metal composite based completely on the solution processing method. The growth of 2D Ni(OH)2 nanosheets on free-standing 2D MoO3 nanobelts manifests in a 3D hierarchical architecture of numerous nanosheets that extend radially outward. The defining advantages of such hetero-structured material design are short ion diffusion pathway of the 2D thin nanobelt core that interfaces with large surface area and interconnected porous 2D nanosheets.20 Accordingly, bifunctional electrochromism–supercapacitive application is demonstrated based on the inexpensive electrochemical active transition metal hydroxide/oxide hierarchical structure. The porous core–shell nanostructures of Ni(OH)2/MoO3 display desirable electrochromic optical modulation and coloration efficiency properties alongside favorable pseudocapacitive storage and specific capacitance properties. The high rate capacity for complementary energy storage and optical modulation capabilities can be attributed to the beneficial structural features of the hybrid metal oxide–metal hydroxide composition and nanobelt backbone core that supports large surface area electrochemically active nanosheets. To this end, bifunctional electrochromic and supercapacitive materials are anticipated to have a broad range of energy applications in smart windows, displays, automobiles and various intelligent electronics.
Fig. 1 (a) Schematic illustration of synthesis of 2D-on-2D MoO3/Ni(OH)2. (b–d) SEM images of MoO3 nanobelts. (e–g) SEM images of MoO3/Ni(OH)2. |
The crystal structures of MoO3 nanobelts and MoO3/Ni(OH)2 were also investigated by XRD and the spectra are shown in Fig. 2g. XRD perfectly assigned to orthorhombic α-MoO3 (JCPDS card no. 05-0508) in accordance with the results of TEM analysis, which is the conventional thermodynamically stable phase. The absence of noticeable impurity peaks suggests the high purity of the α-MoO3 product. The stronger intensities of the (020), (040), and (060) diffraction peaks suggest highly anisotropic growth of the nanostructures.26 The structure of α-MoO3 is shown in Fig. 2g (inset, upper), which is constituted of [MoO6] octahedra, with sharing edges and corners, resulting in zigzag chains and unique layer parallel to the (010) plane structure, presenting open channels for H+ ion intercalation.29 After growth of Ni(OH)2 nanosheets, the enhanced peaks located at 33° and 59° can be indexed to (110) and (300) crystal planes of α-Ni(OH)2 (JCPDS card no. 22-0444), the crystalline structure of which is illustrated in Fig. 2g (inset, lower). The XRD spectrum of pure Ni(OH)2 nanosheets (morphology and structure are illustrated in Fig. S1 and S2†) synthesized in the absence of MoO3 nanobelts under the same conditions further indicates the α-Ni(OH)2 phase, as shown in Fig. S3.†30 The EDX spectra of both MoO3 nanobelts and MoO3/Ni(OH)2 in Fig. 2h show C, O and Mo peaks at 0.26 keV, 0.52 keV, 2.02 keV and 2.30 keV, corresponding to the composition of MoO3 nanobelts. Ni peaks are seen at 0.85, 7.47 and 8.25 keV in MoO3/Ni(OH)2, indicating the growth of Ni(OH)2 nanosheets.23,31 The chemical compositions of MoO3 nanobelts and MoO3/Ni(OH)2 were also confirmed by FT-IR spectra, shown in Fig. S4.† The bands appearing at approximately 3600–3000, 1700–1400, 1380, 1300–1000 and 1000–800 cm−1 can be assigned to the characteristic vibrations of O–H stretching, O–H bending, NO3− anions, C–O stretching and Mo–O2 stretching, Mo–O–Mo bridge stretching.32 The peak at 610 cm−1 can be assigned to the Ni–O, O–Ni–O, and Ni–O–Ni vibrations.30
XPS analysis was adopted to determine the chemical states of the MoO3 nanobelts and MoO3/Ni(OH)2, particularly for Mo, O and Ni. The wide scan spectrum in Fig. 3a shows the binding energy peaks at 285.0 and 530.1 eV, which are attributed to C 1s and O 1s, respectively. Binding energies of 232.6 and 235.8 eV are indicative of Mo 3d5/2 and Mo 3d3/2, with the Mo 3d spectrum (Fig. 3b) corresponding to the Mo6+.27 Meanwhile, a slight shift and FWHM broadening of the peaks of Mo 3d and O 1s for MoO3/Ni(OH)2 could be ascribed to the growth of Ni(OH)2 nanosheets. A shoulder peak appearing at 532.6 eV (Fig. 3c, MoO3/Ni(OH)2) is attributed to the interstitial oxygen or the chemisorbed oxygen atoms on the surface of MoO3.33 The Ni 2p peak is shown in Fig. 3d. The Ni 2p3/2 peak is located at 855.6 eV with a shakeup satellite peak at about 861.2 eV and Ni 2p1/2 at 873.2 eV with a shakeup satellite peak at about 879.2 eV, which correspond to NiII.34,35 The gap between Ni 2p3/2 and Ni 2p1/2 is about 17.6 eV, which is in agreement with previous reports.36 N2 adsorption–desorption measurements obtained from MoO3/Ni(OH)2 and pure Ni(OH)2 nanosheets gave specific surface areas of 152.6 and 148.2 m2 g−1 and a similar pore size distribution of ∼2 nm, as shown in Fig. 3e and f, respectively.
The as-prepared MoO3/Ni(OH)2 and pure Ni(OH)2 nanosheets were evaluated as electrodes for supercapacitors. Fig. 4a shows the cyclic voltammetry (CV) curves obtained from different scan rates (1–20 mV s−1) with a voltage window of 0–0.6 V (vs. SCE) for pure Ni(OH)2 nanosheets and MoO3/Ni(OH)2. A pair of redox peaks is observed in each curve suggesting that measured capacitance is mainly based on the pseudocapacitive mechanism. The intensities of the redox peaks increase and the cathodic peak is shifted to the positive direction while the anodic peak is shifted to the negative direction with scan rate, further confirming the pseudocapacitive property of the material.37,38 The average specific capacitances of the samples can be calculated from the CV curves based on the following equation:39
(1) |
The average specific capacitances of the two samples calculated from different scan rates are displayed in Fig. 4a and b. Specific capacitances of 1510, 1455, 1142, 736, and 358 F g−1 can be calculated at scan rates of 1, 2, 5, 10 and 20 mV s−1 for MoO3/Ni(OH)2, which are higher than those of pure Ni(OH)2 except at the rate of 20 mV s−1 (Fig. 4c). The specific capacitances were also determined by galvanostatic charge–discharge (GCD) measurements at various current densities at room temperature and calculated by the following equation:40
(2) |
Furthermore, the cyclic stability of the electrode materials is another key issue for practical application; therefore, the GCD test was performed to evaluate the stability of the electrode at a current density of 5 A g−1. As shown in Fig. S6,† the electrode exhibits good long-term cyclic stability, maintaining 80% of its original capacitance (a decrease from 1150 to 921 F g−1) after 3000 cycles. The electrochemical impedance spectroscopy (EIS) analyses of MoO3/Ni(OH)2 and pure Ni(OH)2 nanosheets in the frequency range from 0.01 to 100 kHz are shown as Nyquist plots in Fig. 4f. The equivalent series resistance was only 1.71 Ω in the high-frequency region, which is lower than that of pure Ni(OH)2 nanosheets. The semicircles crossing high and mid-frequencies are attributed to the charge-transfer resistance, which has been enlarged and shown in the inset of Fig. 4f. The lower equivalent series resistance together with the smaller radius of charge-transfer resistance can generally indicate a lower impedance and better electric conductivity for MoO3/Ni(OH)2, which could lead to a better electrochemical performance.
α-Ni(OH)2 is an excellent anodic electrochromic material and can change color upon OH− ion insertion due to its isostructure with hydrotalcite and constituents of hydroxyl-deficient host and interlayer species, such as anions and H2O molecules.41 The electrochromic reaction of Ni(OH)2 is a reversible redox reaction in which the reduced form is nearly transparent while the oxidized material is dark brown, which is summarized in the following equation:42,43
[Ni(OH)2 + OH−]bleached ↔ [NiOOH + H2O + e−]colored | (3) |
The as-prepared MoO3/Ni(OH)2 also exhibits electrochromic property due to the shell of α-Ni(OH)2 nanosheets and the transmittance spectra of the sample coated on FTO glass recorded at ±1, 1.5 and 2 V are shown in Fig. 5a. The good reversibility of the bleaching/coloration and a stable transmittance change (ΔT) of 7.6, 13.5 and 26.1% for ±1, 1.5 and 2 V at a wavelength of 500 nm could be verified. Also, the bleaching/coloration responses occur almost instantaneously. Digital photographs of reversible bleached and colored states of the MoO3/Ni(OH)2 coating are shown in Fig. 5a, insets. The corresponding coloration efficiency is calculated to be 34.8 cm2 C−1 (Fig. 5b), which is comparable with that of Ni(OH)2/NiO reported previously.43,44 This coloration phenomenon could be ascribed to the charge storage behavior of the electrode observed during the oxidation of the α-Ni(OH)2 layer. It is also believed that the system can store charge by the conversion of α-Ni(OH)2 into the NiOOH counterpart which finally acts as a color center for the respective electrochromic behavior.45
The electrochromism is shown to be driven by an as-prepared supercapacitor (Fig. S7†) which is a more efficient power source due to its high capacity, fast charging, and excellent cycling stability (Fig. 5c). An asymmetric supercapacitor cell, using a slurry of MoO3/Ni(OH)2 packed into full cells and CNTs as counter electrodes, was fabricated and then connected to power the electrochromic reaction on the FTO glass (Fig. 5d). The “Singapore Lion” pattern of MoO3/Ni(OH)2 was coated in advance. Notably, the pattern is almost transparent under the bleached state but discernibly displayed after being powered by the supercapacitor cell (Fig. 5e and S8†). The display evolved from a clear coated glass that allows high transmittance of the printed words to selective dark black patterned coloration. The corresponding transmittance spectra are shown in Fig. 5f, revealing a similar transmittance between the coated and bare FTO glass in the bleached state. Whereas in the colored state, a broad absorption band in the visible range and a corresponding transparency decrease from 80% to 56% at a wavelength of 500 nm were observed, making it a promising, inexpensive optical modulation material.
Given the electrochromic process happens when the charge injects or ejects, the electrochromic process can be integrated with charge storage devices.14 Here we demonstrate the combination of electrochromism and energy storage, achieving the advantages of both in these two associated effects. The combination has two aspects (Fig. 6a). Firstly, the electrochromic device was made using 3 pieces of FTO glass coated with a thicker layer of MoO3/Ni(OH)2 and connected in order to store enough energy for subsequent application (Fig. 6d). After applying a potential of +2 V, electrochromism took place and the glasses turned black (Fig. 6e). The corresponding transmittance spectra in Fig. 6b show a transmittance level of 60% in the bleached state and 16% in the colored state. The transmittance contrast increased because of the higher loading of MoO3/Ni(OH)2. During the electrochromic reaction process, the charging performance of the MoO3/Ni(OH)2/FTO glass was investigated at different current densities of 2, 5, 10 and 20 mA. A higher current density leads to a faster charging rate and higher resultant voltage, as shown in Fig. 6c. Thereafter, the energy stored in the electrochromic devices was utilized. The three charged electrochromic FTO substrates are connected in series to form a closed circuit to power an LED (Fig. 6f). The electrochromic device could easily light up a commercial green LED after being charged (Fig. 6f, inset). Our newly designed 2D-on-2D architecture not only has a good electrochromic property, but also features further energy storage properties, which may be used to power electronic devices, such as mobile phones and sensors, in the future.
Footnote |
† Electronic supplementary information (ESI) available: Additional SEM, TEM, XRD, FTIR and photographs. Specific capacitances and cycling performances of the material. See DOI: 10.1039/c7ta01858d |
This journal is © The Royal Society of Chemistry 2017 |