2022

Short summary

The EnStoreSW project aims at developing polymers based on thiophene derivatives for use both as capacitive and electrochromic materials, as well as their interdisciplinary investigation and testing in flexible energy storage smart windows, so as to reach TRL 4. For this purpose, in Stage 1 (year 2022) the synthesis of monomers and polymers based on ProDOT was carried out, with emphasis on the synthetic processes optimization and purification methods, as well as on their structural identification and physico-chemical investigation as isolated molecules (in solution). Thus, 3 monomers based on ProDOT with different substituents (type alkyl, ester or hydroxyl) were synthesized through different techniques that required the optimization of the experimental conditions. The monomers were purified by column chromatography and structurally characterized by spectral methods, such as FTIR, 1H and 13C-NMR, which confirmed their correct structure. Also, they were studied by UV-Vis absorption and fluorescence spectroscopy, with the aim to evaluate the main electronic transitions that occur in these monomers, but also the changes induced on the electronic transitions by the different anchored side substituents. Based on these monomers and a commercial monomer, 8 polymers based on ProDOT were obtained. The synthesis of polymers was carried out by different methods, such as electrochemical polymerization, oxidative polymerization, direct C-H arylation and Yamazaki-Higashi polycondensation. After optimizing the synthesis conditions, the polymers were isolated from the reaction medium and purified by specific methods, then confirmed from the structural point of view by FTIR, 1H and 13C-NMR spectroscopy, while their molecular weights were evaluated by gel permeation chromatography. The photo-optical behavior of the polymers was investigated by UV-Vis absorption and fluorescence spectra recorded in solution. These highlighted the presence of a main absorption band at wavelengths beyond 450 nm, with the exception of one polymer, which, due to a reduced conjugation, dispalyed a weak absorption at approximately 360 nm. By excitation with the wavelength corresponding to the main absorption maximum, all polymers showed fluorescence, with emission in the blue, yellow or orange-red spectral range, according to the structure of each polymer. All polymers showed oxidation processes in the cyclic voltammetry measurements, of reversible or irreversible character, depending on the structure of the monomeric unit. DFT molecular simulations showed that the polymers have planar, conjugated structures, with molecular orbitals delocalized mainly on the thiophene ring, excepting the polymer obtained by polycondensation, which has a twisted conformation and well-separated molecular orbitals. The simulation of the oxidized species (polarons/bipolarons) generation led to absorption spectra with new bands, which indicated that the present polymers have potential as electrochromic materials. The obtained results were analyzed and discussed in the stage report, and part of them were disseminated at 3 scientific events in the form of 2 oral communications and 1 invited lecture.

2023

Short summary

As a continuation of the activities carried out in Stage 1 of the EnStoreSW project, in Stage 2 (year 2023), our pursuit was to obtain electrodes based on polypropylenedioxythiophene (PProDOT), and to test them in the three-electrode cell configuration in order to demonstrate the polymer potential for energy storage smart window (ESSW) application. First, 2 flexible electrodes were obtained by embedding conductive silver nanowires into polyimide films, which showed excellent transparency and mechanical properties, as well as a good electrical conductivity, of up to 27 S/cm. Then, the ProDOT-based polymers reported in Stage 1 were deposited on both rigid and flexible surfaces, when 8 rigid electrodes and 8 flexible electrodes based on PProDOT as active layer were obtained after an extensive optimization processes of the experimental conditions for homogeneous films deposition. From the morphological point of view, some of these electrodes presented a porous structure, but in some cases, due to the dense packing of PProDOT conjugated chains, the films displayed a molecular organization into granules or particles. The obtained polymer films were investigated with regard to the photo-optical, thermal and wetting properties, being found that they display the main absorption band centered in the visible range, at a wavelength depending on the degree of conjugation and molecular planarization in solid state, a good thermal stability, in the range of 310°C-383°C, and a high hydrophobic character. The electrochemical characteristics of the electrodes modified with PProDOT evaluated by cyclic voltammetry and spectroelectrochemistry highlighted the ability of these polymers to change their color under an applied voltage, and also, to store electric charges. Thus, the specific areal capacitance of these polymers varied from tens of µF/cm² to values up to 4 mF/cm^2, depending on the scan rate, polymer structure and method used to prepare the polymer. The electrochromic effect experienced by the majority of the investigated polymers consisted in the discoloration of the opaque film from violet or cherry-red to transmissive green/blue, or vice versa, coloring from transmissive yellow to opaque green, with short color switching times, of seconds or even under 1s, and a bleaching/coloring efficiency of up to 696 cm²/C. The long-term stability and reversibility of the electrochromic response of the rigid and flexible electrodes based on PProDOT was evaluated by chronoamperometry based on the variation of transmittance corresponding to the oxidized and neutral species as a function of time, over up to 500 color switching cycles. The energy storage potential of the PProDOT-based electrodes was also demonstrated on the basis of galvanostatic charge-discharge tests, which highlighted that when fully charged, the PProDOT films change the color, and when discharged, they return to the original color, suggesting the potential of these polymers for use as electrode materials in ESSWs. The obtained results were systematized in the Stage 2 report, and part of them were disseminated through 10 participations (2 invited lectures, 6 oral presentations, 2 posters) at 4 scientific events and 3 scientific articles published/under evaluation in journals with FI> 3.