Citation: Rizzuto, C.; Barberi, R.C.; Castriota, M. Development of Electrochromic Devices, Based on Polymeric Gel, for Energy Saving Applications. Polymers 2023, 15, 3347. https://doi.org/10.3390/ polym15163347 Academic Editor: Sixun Zheng Received: 30 June 2023 Revised: 28 July 2023 Accepted: 7 August 2023 Published: 9 August 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Development of Electrochromic Devices, Based on Polymeric Gel, for Energy Saving Applications Carmen Rizzuto 1 , Riccardo C. Barberi 1,2 and Marco Castriota 1,2, * 1 Department of Physics, University of Calabria Ponte Bucci, Cubo 33B, 87036 Rende, CS, Italy 2 CNR-Nanotec c/o Department of Physics, University of Calabria Ponte Bucci, Cubo 33B, 87036 Rende, CS, Italy * Correspondence: marco.castriota@fis.unical.it; Tel.: +39-0984-496141 Abstract: In this work, the implementation of an electrochromic device (10 cm × 10 cm in size) for energy saving applications has been presented. As electrochromic system has been used with an electrochromic solution (ECsol) made by ethyl viologen diperchlorate [EV(ClO 4 ) 2 ], 1,1 ′ -diethyl ferrocene (DEFc) and propylene carbonate (PC), as solvent. The final system has been obtained by mixing the ECsol, described above, with a polymeric system made by Bisphenol-A glycerolate (1 glycerol/phenol) diacrylate (BPA) and 2,2-Dimethoxy-2-phenylacetophenone (Irgacure 651) in a weight percentage equal to 60:40% w/w, respectively. Lithography has been used to make a spacer pattern with a thickness of about 15–20 μm between the two substrates. Micro-Raman spectroscopy confirmed the presence of the EV •+ as justified by the blue color of the electrochromic device in the ON state. Electrochemical and optical properties of the electrochromic device have been studied. The device shows reversible electrochromic behavior as confirmed by cyclic color variation due to the reduction and oxidation process of the EV 2+ /EV •+ couple. The electrochromic device shows a variation of the % transmittance in the visible region at 400 nm of 59.6% in the OFF state and 0.48% at 3.0 V. At 606 nm the transmittance in the bleached state is 84.58% in the OFF state and then decreases to 1.01% when it is fully colored at 3.0 V. In the NIR region at 890 nm, the device shows a transmittance of 74.3% in the OFF state and 23.7% at 3.0 V while at 1165 nm the values of the transmittance changed from 83.21% in the OFF state to 1.58% in the ON state at 3.0 V. The electrochromic device shows high values of CCR% and exhibits excellent values of CE in both visible and near-infrared regions when switched between OFF/ON states. In the NIR region at 890 nm, electrochromic devices can be used for the energy-saving of buildings with a promising CE of 120.9 cm 2 /C and 420.1 cm 2 /C at 1165 nm. Keywords: viologen; UV-Vis-NIR and Raman spectroscopies; cyclic voltammetry; color contrast ratio; coloration efficiency 1. Introduction The phenomenon of electrochromism is attracting much attention because of its vari- able applications starting from smart windows up to lower power display systems [1–3] using different kinds of available electrochromic materials: organic [4] and inorganic [5–7]. Organic electrochromic materials such as innovative conjugated polymers exhibit fast response time, contrast color, lower power consumption, and color versatility, and can be easily used in a wide range of multifunctional applications (flexible display, smart windows, camouflage, etc.) [8]. Moreover, these kinds of materials are sensitive to external stimuli, and this means that their properties can be modulated by light, electricity, solvent stimulation, etc. [9]. Recently, with the introduction of heteroatoms in the chain of the polymer, such as polythiophenes [10], and the use of the D-A-D method [11] it is possible to obtain greater control of the electrical properties of the materials improving electrochromic performances of the devices [12]. Due to its durability and low synthetic costs, Prussian blue offers fast transmittance switching properties with no memory effect that together Polymers 2023, 15, 3347. https://doi.org/10.3390/polym15163347 https://www.mdpi.com/journal/polymers