One‐Dimensional π–d Conjugated Coordination Polymer for Electrochromic Energy Storage Device with Exceptionally High Performance

Abstract The rational design of previously unidentified materials that could realize excellent electrochemical‐controlled optical and charge storage properties simultaneously, are especially desirable and useful for fabricating smart multifunctional devices. Here, a facile synthesis of a 1D π–d conjugated coordination polymer (Ni‐BTA) is reported, consisting of metal (Ni)‐containing nodes and organic linkers (1,2,4,5‐benzenetetramine), which could be easily grown on various substrates via a scalable chemical bath deposition method. The resulting Ni‐BTA film exhibits superior performances for both electrochromic and energy storage functions, such as large optical modulation (61.3%), high coloration efficiency (223.6 cm2 C−1), and high gravimetric capacity (168.1 mAh g−1). In particular, the Ni‐BTA film can maintain its electrochemical recharge‐ability and electrochromic properties even after 10 000 electrochemical cycles demonstrating excellent durability. Moreover, a smart energy storage indicator is demonstrated in which the energy storage states can be visually recognized in real time. The excellent electrochromic and charge storage performances of Ni‐BTA films present a great promise for Ni‐BTA nanowires to be used as practical electrode materials in various applications such as electrochromic devices, energy storage cells, and multifunctional smart windows.

Typical synthesis of Ni-BTA film by CBD route. The pre-cleaned FTO glass was used as the transparent conductive substrate and the nonconductive side was covered with polyimide tape to prevent Ni-BTA deposition on this side. The substrate was vertically supported on the wall of an open bath container. The solution for CBD reaction was prepared by mixing a solution of 484.2 mg (2.04 mmol) of NiCl 2 ·6H 2 O in 30 ml of DI water and a solution of 384 mg (1.35 mmol) of BTA·4HCl in 210 ml of water. Thereafter, 6.9 ml of concentrated NH 4 OH was added to the mixture under a vigorous stirring. The reaction continued for 4 h under stirring at ambient conditions. Finally, the Ni-BTA films were obtained by removing the polyimide tape, and then washed with DI water, ethyl alcohol, respectively, and dried under room temperature for 6 h.
Solid-state device assembly. The solid-state electrochromic device was assembled by employing Ni-BTA nanowires film as the electrochromic layer, sprayed TiO 2 nanoparticles film as the ion storage layer, 1M KOH/ polyvinyl alcohol (PVA, 10wt%) as the solid electrolyte and VHB clear mounting tape (4010, 3 M) as the spacer, respectively. Ultimately, the solid-state electrochromic was encapsulated via epoxy.
Sample Characterization. The crystalline structure of the Ni-BTA powder was investigated by X-ray diffraction (XRD, Bruker D8 Advance) technique with Cu-Kα-radiation (λ=1.541874 Å). TOPAS 6 was used to undergo Pawley fit on the PXRD data. In order to maintain a stable refinement and to minimise parameter correlation, the unit cell, background and peak shape parameters were refined separately. Microstructure and morphology of Ni-BTA nanowires film on FTO glass were observed with a field emission-scanning electron microscope (FESEM, JEOL 7600F) at 5.0 kV and an atomic force microscope (AFM, Asylum Research).
Transmission electron microscopy (TEM) was performed on a JEOL JEM 2010 microscope operated at 200 kV accelerating voltage to observe the genuine microstructural informantion. X-ray pair distribution function (XPDF) data was collected at the I15-1 beamline at the Diamond Light Source, UK (λ = 0.161669 Å). Samples with small amount for the XPDF was loaded into a glass capillary with a diameter of 0.76 mm. Data on the sample, empty capillary and instrument data were collected. Background, container scattering, compton scattering, multiple scattering and absorption corrections were processed with the GudrunX program to achieve a Q = 22 Å -1 . [s1, s2] Thanks go to the beamline staff for completing the work as part of a rapid access call EE. X-ray absorption fine structure (XAFS) measurements were performed at 8C nano-probe XAFS beamline (BL8C) of Pohang Light Source (PLS-II) in the 3.0 GeV storage ring, with a ring current of 250 mA. The X-ray beam was monochromated by a Si(111) double crystal where the beam intensity was reduced by 20% to eliminate the higher-order harmonics. The x-ray beam was then delivered to a secondary source aperture where the beam size was adjusted to be 0.5 mm (v) × 1 mm (h). The Ni K edge XAFS measurements on the initial, colored, and bleached thin films were measured at room temperature in fluorescence mode with standard 45° geometry and calibration was done using Ni foil.
Due to the low concentration of Ni a four element Si drift detector was used to monitor the fluorescence x-rays and dead time corrections were taken into account.
Typically, 3-4 scans were averaged for an improved signal-to-noise ratio of the initial and bleached samples, however, because colored samples are prone to oxidization (over time) only one scan was obtained immediately after electrochemical cycling. All the obtained spectra were processed using Demeter[1] package. Keeping the signal to noise ratio of the data in mind extended x-ray absorption fine structure (EXAFS) analysis was only performed on the initial system (Ni-BTA) with Fourier-transform range of 3.5 -12 Å -1 using a Hanning window applied between 1.2 Å and 2.7 Å. The amplitude reduction factor (S o 2 ) was obtained by fitting the Ni metal. Photoelectron spectroscopy (XPS) was carried out on PHI Quantara II Scan X-Ray Microscope with monochromatic Al Kα irradiation (1486.6 eV, beam size is 100 μm in diameter). To confirm the electrochromic and energy storage reaction mechanism, Raman and fourier transform infrared spectroscopy (FTIR) were performed using a confocal Raman spectroscopy at 488 nm laser line (WITec, alpha300 SR) and a GX FTIR spectrometer (PerkinElmer Inc., Waltham, MA, USA), respectively.
Gas-adsorption measurements were conducted on a Tristar II 3020 analyzer at 77 K.
The specific surface area and pore volume were analyzed through a Brunauer-Emmett-Teller (BET) using N 2 gas and Barrett-Joyner-Halenda (BJH) analysis methods, respectively. Thermogravimetric analyses (TGA) was measured by a TA Q500 system. package [s3] and GROMOS96 force fields. [s4] The detail simulation information of force filed parameters of molecules, the structure information of simulation system, and simulation conditions were described in the following. The structures of molecules were obtained from DFT calculation within unit cells. The cutoff distance of 12 Å was utilized for short-range non-bonded interactions and the PME approach was used for long-range electrostatic forces. The system was subjected to a steepest descent energy-minimization, then to further thermalization at 300 K by NVT ensemble.

DOS Calculations of Geometric and Electronic Structures. The geometric and
electronic structures calculations of molecules were calculated at the density functional theory (DFT) plane-wave level utilizing the Vienna ab initio simulation package (VASP) [s5, s6] with the projected augmented wave method (PAW). [s7] The generalized gradient approximation (GGA) was used in these calculations. The calculations were carried out with the (9 × 9 × 9) Monkhorst-Pack k-points using a 400 eV cutoff energy for the molecules.   narrow scans Ni 2p core-level spectra, and (c) highly resolved N 1s spectra.            EXAFS oscillations (k3 χ(k)) are shown in the inset. The Fourier transforms represent raw data without correcting for phase shifts.   Figure S19 Electrochemical stability and coulombic efficiency of Ni-BTA nanowires film on FTO glass during a long term cycling of 10000 galvanostatic charge/discharge cycles at a current density of 12.5 A g -1 .