An integrated electrochromic supercapacitor based on nanostructured Er-containing titania using an Er(III)-doped polyoxotitanate cage

Abstract

The novel heterometallic polyoxotitanate cage [Ti₈O₇(OEt)₂₁Er] can be used as a single-source precursor for the formation of nanostructured Er-containing titania materials (Er@TiO₂). Based on the electrochromic properties and lithium ion storage capacity of Er@TiO₂, an integrated bifunctional EC supercapacitor has been designed.

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With optical properties being adjustable upon applying external voltages, electrochromic (EC) devices have attracted significant attention for potential applications, such as smart mirrors and windows, displays, sensors, memory devices, active optical filters and camouflage.¹ Another class of promising future devices is supercapacitors (electrochemical capacitors) which have applications in energy storage due to their superior power density, high rate ability and long cycle lifetime.² Recently, highly integrated energy systems have sparked worldwide interest with multifunctional performance,³ among which the EC supercapacitor is one of the most significant systems for portable electronic devices and utilizing renewable energy.⁴ Integrating the electrochromism functionality with an energy storage ability into one supercapacitor device is very attractive for a range of applications. Such devices could be used, for example, in energy-storage smart windows, which can store energy by charging the window and adjusting the lighting and heating of a building, but also for sensing variations in the level of stored energy.

Titania (titanium dioxide, TiO₂) based materials are abundant, environmentally-benign semiconductors, which have attracted considerable attention in photocatalysis, sensors, energy storage and EC devices.⁵ Due to their chemical and structural stability during reversible lithium ion (Li⁺) intercalation/deintercalation, these materials are also promising candidates for application in supercapacitors.⁶ Incorporation of metal ion dopants into titania, which could introduce additional states to their electronic transitions, is an important method to adjust the optical and electrochemical properties of titania. Earlier studies have suggested that it is very difficult to dope ions homogeneously into the titania matrix through conventional mechanical or chemical methods.⁷ By contrast, metal-doped polyoxotitanate (POT) cages of molecular [Ti_xO_y(OR)_z(M)_n], which bear well-defined inorganic Ti_xO_y cores and organically soluble fragments, might function as single-source precursors for formation of stoichiometrically controllable metal-containing titania.⁸ In our previous study, Eu- and Ce-containing titania materials were prepared through Eu(III)-doped POT and Ce(III)-doped POT cages.⁹ Herein, we report new nanostructured Er-containing titania materials (Er@TiO₂) derived from a novel Er(III)-doped POT cage [Ti₈O₇(OEt)₂₁Er] (1). Based on the Li⁺ storage capacity and EC properties of Er@TiO₂, an integrated EC supercapacitor was designed, which exhibits excellent performances.

Solvothermal reaction of Ti(OEt)₄ with ErCl₃ in dry ethanol yielded large pink crystals of Er(III)-doped POT cage 1 after workup (Fig. 1b and ESI†). A single-crystal X-ray diffraction (XRD) study revealed that 1 has a “berry shaped” ErTi₈O₂₈ core (Fig. 1a), in which the eight-coordinate (dodecahedral) Er(III) ion is located at the base, being bonded to two μ₄-O and two μ₃-O atoms from the Ti₈O₇ core as well as four EtO-ligands. All eight of the Ti atoms of 1 have octahedral geometries. The structure of 1 is very similar to that found for the Ce^III derivative [Ti₈O₇(HOEt)(OEt)₂₁Ce],^9a the only difference being the additional coordination of a neutral EtOH ligand to the lanthanide ion (resulting in a nine-coordinate Ce^III ion).

Fig. 1 (a) Solid-state structure of cage 1, H-atoms have been omitted for clarity (blue = O, red = Ti, pink = Er,); (b) optical image of crystals of cage 1.

Like a range of other reported POT cages, crystals of 1 are easily hydrolyzed in ambient air, readily soluble in dichloromethane (Fig. S1†) and decomposed when heating to high temperatures. Based on these properties, we employed 1 as a single-source precursor to fabricate the Er@TiO₂ material. Calcination of crystals of 1 at 500 °C in air gives a pink solid of Er@TiO₂-A (Fig. 2a), while spraying a dichloromethane solution of 1 followed by evaporation and sintering at 500 °C gives a pale pink solid of Er@TiO₂-B (Fig. 2b). As shown in Fig. S2,† powder XRD indicates that both Er@TiO₂-A and Er@TiO₂-B are mainly composed of a mixture of anatase TiO₂ and amorphous phases. Scanning electron microscopy (SEM) analysis reveals that the morphology of Er@TiO₂-A is that of microparticle agglomerates (Fig. 2c and S3a†), while Er@TiO₂-B contains abundant bubble-like pores which are presumably formed during the evaporation of dichloromethane (Fig. 2d and S3b†). Transmission electron microscopy (TEM) images (Fig. 2e and f) indicate that both Er@TiO₂-A and Er@TiO₂-B contain disordered pores on the nanoscale. HRTEM images and SAED patterns (Fig. 2g and h) further prove that these two materials consist of anatase and amorphous TiO₂. X-ray photoelectron spectroscopy (XPS) reveals that Er is present as Er(III) in both Er@TiO₂-A (Fig. S5†) and Er@TiO₂-B (Fig. S7†). Energy dispersive X-ray spectroscopy (EDS) reveals Ti : Er molar ratios of ca. 8 : 1 in both Er@TiO₂-A (Fig. S8 and Table S3†) and Er@TiO₂-B (Fig. S9 and Table S4†), which is the same as that in the precursor 1.

Fig. 2 (a) Optical image of Er@TiO₂-A, (b) optical image of Er@TiO₂-B, (c) SEM image of Er@TiO₂-A, (d) SEM image of Er@TiO₂-B, (e) TEM image of Er@TiO₂-A, (f) TEM image of Er@TiO₂-B, (g) HRTEM image and SAED pattern (insert) of Er@TiO₂-A, (h) HRTEM image and SAED pattern (insert) of Er@TiO₂-B.

As shown in Fig. 3a, type IV¹⁰ nitrogen sorption isotherms of Er@TiO₂-A and Er@TiO₂-B exhibit capillary condensation steps at relative pressures of 0.6–1.0 and 0.3–1.0, respectively, suggesting the existence of hierarchical pore structures of micro- to mesopore sizes. The Brunauer–Emmett–Teller specific surface area of Er@TiO₂-A is around 27.70 m² g⁻¹, with the average pore size width of 6.79 nm, while these values are around 71.73 m² g⁻¹ and 1.91 nm for Er@TiO₂-B. The pore-size distribution in both samples, calculated from the desorption branch of the isotherm using the Barrett–Joyner–Halenda method, spans the range from 1.5 to 30 nm (Fig. 3b), and such hierarchical structures are expected to enhance their electrochemical performance.¹¹ Compared with Er@TiO₂-A, Er@TiO₂-B has a much larger surface area and narrower pore size, pointing to a higher energy storage capacity.

Fig. 3 (a) Nitrogen adsorption and desorption isotherms. (b) Pore size distributions of Er@TiO₂-A and Er@TiO₂-B materials; (c) charge and discharge curves of Er@TiO₂-A and Er@TiO₂-B and anatase electrodes at 100 mA g⁻¹; (d) rate capability at various current densities of Er@TiO₂-A, Er@TiO₂-B and anatase electrodes.

The electrochemical lithium storage performances of Er@TiO₂-A, Er@TiO₂-B and anatase nanoparticles as the control group were evaluated by using lithium half-cells (ESI†). As shown in Fig. 3c, voltage plateaus are observed during the discharge and charge at 1.7 and 2.0 V in the charge–discharge curves of the anatase electrode,^5c,d but no significant voltage plateau can be observed in the curves of Er@TiO₂-A and Er@TiO₂-B electrodes. This phenomenon, which indicates that Er@TiO₂ is potentially usable in a Li-ion supercapacitor, is presumably due to the introduction of Er(III) into the TiO₂ matrix. Like many other metal doped TiO₂ materials, the band gap of Er@TiO₂ is reduced via Er(III) doping so the redox potential of Er@TiO₂ is likely to be fundamentally affected. The doping is also likely to reduce the rate of hole–electron recombination, which will lead to more charge carriers being available in the system. In addition, interfacial properties (such as the rate of electron transfer), and the microstructure of the interface may also be influenced by Er(III) doping. As shown in Fig. 3d, the energy storage capacity of Er@TiO₂-B is superior to that of Er@TiO₂-A and anatase electrodes, the cell of Er@TiO₂-B delivered a specific capacity of approximately 119 mA h g⁻¹ at a current density of 50 mA g⁻¹, and it retained 58.5 mA h g⁻¹ by increasing the current density to 250 mA g⁻¹. Thus, Er@TiO₂-B is further studied as a component of the EC supercapacitor below.

Thin films of Er@TiO₂-B were spray-cast onto indium tin oxide (ITO) glass slides (ESI†). As shown in the SEM images in Fig. 4a, the Er@TiO₂-B film is around 1 μm in thickness, which is composed of porous nanoparticles. This film can not only absorb near infrared (NIR) and visible light,¹² but also exhibit significant EC behavior. Fig. 4b shows the visible and NIR transmission spectra of an Er@TiO₂-B film in the coloured and bleached states in 1.0 M LiClO₄/propylene carbonate (PC) solution. Er@TiO₂-B is a cathodically-colouring material with the colour changed from blue-black (colored state) at −1.4 V to off-white (bleached state) at 1.0 V, which corresponds to the intercalation/deintercalation of Li⁺ into/out of the Er@TiO₂-B film (Fig. S12†).

Fig. 4 (a) SEM images of the cross-section and surface of the Er@TiO₂-B film; (b) visible and NIR transmittance spectra of the Er@TiO₂-B film in different potentials; (c) CV curves (voltage vs. Ag/AgCl) of the Er@TiO₂-B film at scan rates from 20 mV s⁻¹ to 200 mV s⁻¹; (d) EC switching response of the Er@TiO₂-B film monitored at 900 nm with a residence time of 100 s.

The cyclic voltammogram (CV) curves of the Er@TiO₂-B film are measured at various scan rates ranging from 20 to 200 mV s⁻¹. Generally, for an ideal electric double-layer capacitor CV curves appear nearly rectangular, while large redox current waves indicate a battery-like pseudocapacitance arising from the faradaic process. As illustrated in Fig. 4c, a set of redox waves are observed between −0.75 and −1.5 V, which is consistent with the latter case. During this redox process, the conversion between Ti⁴⁺ and Ti³⁺ is accompanied by Li⁺ insertion/extraction. The current density increases with the increasing scan rate, indicating an excellent electrochemical performance. Fig. 4d shows the EC switching response of the Er@TiO₂-B film at the near NIR wavelength (900 nm). It reveals a 15.26% optical contrast and the switching time was 16.3 s for coloration and 17.7 s for bleaching (Fig. S13†), which is sufficient for an electrode of the EC supercapacitor.

Recently, we reported a series of conjugated polymer as EC materials which exhibit a rapid switching response and display multicolors.¹³ Among them, poly(4,4′,4′′-tris[4-(2-bithienyl)phenyl]amine) (PTBTPA), possessing a noticeable electrochromism with reversible colour changes and high optical contrast, is an anodically colouring material.^13b The colour of the PTBTPA film changes from orange in the reduced state (0 V) to dark blue in the oxidized state (1.2 V), in accordance with the intercalation/deintercalation of perchlorate ions (ClO₄⁻) into/out of the PTBTPA film. By taking advantage of the excellent performance of ion intercalation/deintercalation of our anodically and cathodically coloring materials, we therefore assembled a sandwich-like EC supercapacitor for both energy storage and electrochromism by stacking an Er@TiO₂-B film and a PTBTPA film with a LiClO₄-PMMA (PMMA is polymethyl methacrylate) gel as the electrolyte (ESI†). As illustrated in Scheme 1, when the EC supercapacitor is being charged, Li⁺ ions in the electrolyte are intercalated into the anode of the Er@TiO₂-B film; for charge balance, ClO₄⁻ ions are simultaneously intercalated into the cathode of the PTBTPA film. During the discharge process, the movement of Li⁺ and ClO₄⁻ ions is opposite which are deintercalated out of the Er@TiO₂-B and PTBTPA films.

Scheme 1 Illustration of the charge and discharge process of the as-prepared EC supercapacitor.

At its rest state, the as-prepared EC supercapacitor is in yellow (Fig. 5a). The color is changed to dark gray when the EC supercapacitor is charged above 1.3 V (Fig. 5b) and bleached (Fig. 5c) when it is discharged below 0.3 V. Fig. 5d illustrates the transmission spectra of the EC supercapacitor in the charged and discharged states, which indicates that the charged EC supercapacitor is more absorptive, decreasing the transmittance of incident light in the visible range. The energy storage properties of the EC supercapacitor were characterized by electrochemical measurements. Fig. 5e shows galvanostatic charge–discharge curves at different current densities. The non-ideal triangular shape of the curves is consistent with a typical characteristic of pseudo-capacitance arising from the redox process. Table S5† lists the areal capacitances calculated based on the galvanostatic charge–discharge curves. It shows that the device can deliver an areal capacitance of 0.020 F cm⁻² at a current density of 0.5 mA cm⁻² and 0.016 F cm⁻² at a current density of 1.0 mA cm⁻². These results indicate that the integration of the EC supercapacitor with nanostructured Er-containing titania is feasible and efficient, which is worth further research for practical application.

Fig. 5 (a) Camera picture of an as-prepared EC supercapacitor; (b) and (c) pictures of an as-prepared EC supercapacitor at its charged and discharged states; (d) transmission spectra of the EC supercapacitor in the charged and discharged states; (e) galvanostatic charge–discharge curves at different current densities.

In summary, an integrated bifunctional EC supercapacitor was designed based on nanostructured Er-containing titania, Er@TiO₂-B due to its excellent EC properties and lithium ion storage capacity. Er@TiO₂-B is derived from an Er(III)-doped polyoxotitanate cage [Ti₈O₇(OEt)₂₁Er] (1). This EC supercapacitor can reversibly change its colour during the charge–discharge process, which can potentially be used for sensing variations in the level of stored energy and in energy-storage smart windows.

Acknowledgements

We thank the National Natural Science Foundation of China (NSFC, No. 21501148, 21405114 and 21573160), the China Postdoctoral Science Foundation (CPSF-2015M570075), and the Zhejiang Provincial Natural Science Foundation of China (No. LQ15E030002 and LY15E030006).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, crystal data and analytical characterization of 1; preparation and characterization of Er@TiO₂-A and Er@TiO₂-B. CCDC 1437540. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qi00114a

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