Electrochromic Degradation Research Articles

Mechanistic Insights into Anion-Induced Electrochromism of Ru(II)-Based Metallo-Supramolecular Polymer.

The metallo-supramolecular polymer (MSP) is considered one of the most promising electrodes for electrochromic devices due to its intrinsically stable redox properties. Nevertheless, despite extensive work focusing on improving the electrochromic and electrochemical properties of MSPs, little experimental evidence exists from in-depth investigations on the anion-induced electrochromism of MSPs. Herein, Ru-based metallo-supramolecular polymer (polyRu) films with excellent electrochromic performance were fabricated through a novel electrochemical deposition method, and the electrochromic mechanism was further understood. The polyRu films possess fast reaction kinetics with a short switching time of 4.0 s (colored) and 2.8 s (bleached) and highly reversible redox properties due to the resulting impacts on the capacitive behaviors (containing surface, near-surface and intercalation pseudo-capacitance) of the perchlorate ions in the electrochromic process. Moreover, the electrochromic degradation of the polyRu films is considered to stem from the numerous nanopores in the film induced by ClO4- transport and the exchange of counter anions from Cl- to ClO4-. In addition, a physical model, revealing the transport of conduction ions and the evolution of the structure and properties of the polyRu film during the electrochromic process, is presented. It is observed that the charge balance of Ru3+ and Ru2+, achieved through the adsorption/desorption of ClO4- on the film, provides electrochromic and electrochemical reversibility to the polyRu film under positive/negative bias. Correspondingly, a transformation from polyRu·(Cl-)2n to polyRu·(ClO4-)x(Cl-)2n-x in the polyRu film is induced by a counter anion exchange from Cl- to ClO4-. Revealing the detailed perchlorate ion transfer kinetics and electrochromic mechanism in this film can offer new insights into the application of metallo-supramolecular polymers in electrochromic devices.

Uncovering Phase Transformation, Morphological Evolution, and Nanoscale Color Heterogeneity in Tungsten Oxide Electrochromic Materials

The electrode–electrolyte interfacial chemistry determines the durability of electrochromic devices. The long‐term cycling stability of transition metal (TM) oxide electrochromic films in the acidic electrolyte is impoverished, which is a major obstacle towards deep market penetration through sustainable and cost-effective smart windows. The research interest in nanostructuring TM oxide films to improve electrochromic performance has grown rapidly. However, there are few fundamental studies that investigate the mechanism of electrochromic degradation. In this work, we revisit the classical tungsten trioxide (WO3) materials as the platform to uncover the previously unknown interrelationship between morphological evolution, phase transformation, nanoscale color heterogeneity, and durability in these electrochromic materials. Through synchrotron spectroscopic, microscopic, and imaging characterizations, we experimentally reveal that tungsten dissolution/redeposition and proton-trapping are the two fundamental origins for performance degradation. The WO3–electrolyte interface is reshaped due to the metal dissolution and redeposition during electrochemical cycling. The recurrent redeposition process provokes the in situ crystal growth, which ultimately leads to the phase transformation from the semicrystalline WO3 to a nanoflake-shaped, proton-trapped tungsten trioxide dihydrate (H x WO3·2H2O). Besides, the heterogeneous proton trapping during long-term electrochemical cycling brings about the charge heterogeneity of thin-film electrodes, which further creates a significant degree of nanoscale color heterogeneity. The interaction between phase transformation, morphological evolution, and nanoscale color heterogeneity results in degraded cycling durability, including inferior optical modulation, switching kinetics, coulombic efficiency, and bleached-state transparency. Our work highlights that the high interfacial reactivity near the electrode surface may be the underlying mechanism for undesired bulk structural changes of electrochromic materials. The overwhelming morphological changes reported here may raise concerns about the legitimacy of a large body of literature that attributes the electrochromic performance solely to initial nanostructure design. More fundamental studies are needed to probe and control the interfacial chemistry in electrochromic and other electrochemical devices.

In situ electrochromic efficiency of a nickel oxide thin film: origin of electrochemical process and electrochromic degradation

Schematically deciphering the electrochemical mechanism of the electrochromic NiOx thin film based on the Li+-ion electrolytes.

Charge-transfer kinetics and cyclic properties of inorganic all-solid-state electrochromic device with remarkably improved optical memory

Optical memory effect plays a critical role in energy-saving coefficient of electrochromic devices (ECDs). Due to the finite electron-blocking capacity of ions electrolyte layer and the weak interfacial energy between the ions electrolyte layer and electrochromic layers, typically five-layer inorganic all-solid-state ECDs generally suffer from poor optical memory. A seven-layer inorganic ECD with significantly improved optical memory is designed by means of the embedment of Ta2O5 layers between ions conductive layer and electrochromic layers. Compared with the typical five-layer ECD-1 (ITO/NiOx/LiNbO3/WO3/ITO), the seven-layer ECD-2 with the structure of ITO/NiOx/Ta2O5/LiNbO3/Ta2O5/WO3/ITO presents much less leakage current, larger optical modulation and higher coloration efficiency during electrochemical cycling. The further analyses reveal that the embedded Ta2O5 layers greatly increase the energy barrier between NiOx, WO3 and LiNbO3, imposing restrictions on the transfer kinetics of electrons contributing to the leakage current and enlarging the threshold potential available to ECD-2, which gives rise to a high coloration efficiency up to 98.0cm2C−1. Furthermore, the seven-layer ECD-2 maintains a stable optical modulation of approximately 52.5% for over 10,000 cycles, even though there is a compromise in response characteristic due to the Li+-ion trapping in electrochromic layers, which provides a further insight into the electrochromic degradation of inorganic all-solid-state complementary ECDs.

The trade-off between electrochromic stability and contrast of a thiophene—Quinoxaline copolymer

The stability of organic electrochromic devices is a crucial issue for their applications. However, until now the degradation mechanism of electrochromic materials are still not fully understood especially for electrochromic conjugated polymers (ECPs). To improve device stability, intensive investigation on the degradation mechanism of ECPs is urgently needed. Here we report our study on the electrochromic degradation in a thiophene–quinoxaline copolymer: poly [2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1). The results of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectra (UPS) and UV–vis transmission spectra reveal that there are three main factors during the electrochromic degradation of TQ1. The first one is anion (ClO4−) irreversibly deep trapped, while the second is peroxidation of the thiophene group in TQ1. Both factors reduce the conductivity and electrochromism of TQ1. The third is structural relaxation resulting lager conjugated system of TQ1 molecules in film, which is gradually developed during 400 cycling of CV at a narrow potential range (0–1V). When a potential range 0–0.7V is applied, all three factors are prohibited, no electrochromism degradation is observed anymore, although the contrast becomes smaller. Our investigation systematically discloses the degradation mechanism during the electrochemistry processing of a ECP (TQ1), demonstrating the significance of trade-off between the electrochromic stability and contrast of the ECP.

The electrochromic behavior of Bi2O3 thin film prepared by sol–gel technique

Bi2O3 is a polymorphic material with δ phase most known for its highest oxide ion conductivity. In this work, Bi2O3 thin films were prepared by sol–gel technique with annealing temperature of 450 and 550 °C. Electrochromic performances of the films were inspected. It discovered that the color state of the Bi2O3 thin films switched between black and light yellow transparent when applying a voltage of −2 ~ 2 V. Electrochromic degradation was further observed in the thin films. However, steadier electrochromic efficiency was found in the film annealed at higher temperature. This could be related to the larger δ-Bi2O3 phase fraction which may contribute to higher conductivity and show positive electrochromic effect. Bi2O3 is a polymorphic material with δ phase most known for its highest oxide ion conductivity. In this work, Bi2O3 thin films were prepared by sol–gel technique with annealing temperature of 450 and 550 °C. Electrochromic (EC) performances of the films were inspected. It discovered that the color state of the Bi2O3 thin films switched between black and light yellow transparent when applying a voltage of −2 ~ 2 V. EC degradation was further observed in the thin films. However, steadier EC efficiency was found in the film annealed at higher temperature. This could be related to the larger δ-Bi2O3 phase fraction which may contribute to higher conductivity and give a positive EC result.

Eliminating Electrochromic Degradation in Amorphous TiO2 through Li-Ion Detrapping.

The quest for superior and low-cost electrochromic (EC) thin films, for applications in smart windows, remains strong because of their large importance for energy-efficient buildings. Although the development of new EC materials for improved devices is important, diminishing or reversing degradation is another key issue, and electrical rejuvenation of degraded EC materials can offer new opportunities. Here we demonstrate that cathodically coloring EC thin films of TiO2, which normally suffer from ion-trapping-induced degradation of charge capacity and optical modulation upon extensive electrochemical cycling, can recover their initial EC performance by a rejuvenation procedure involving Li(+) ion detrapping. Thus, the initial performance can be regained, and refreshed TiO2 films exhibit the same degradation features as as-deposited films upon prolonged electrochemical cycling. The rejuvenation was carried out by using either galvanostatic or potentiostatic treatments. Our study may open avenues to explore the recovery not only of EC materials and devices based on those but also for other ion-exchange-based devices.

Study of the electrical-conductivity kinetics of KTP crystals used in the modulators of solid-state lasers

This paper presents the results of studies of the kinetics of the conduction current in the pulsed regime of high-resistance KTP crystals with electrical resistivity (1–5)×1010 Ω cm, grown from solutions in a phosphate melt by a modified Czochralski method. It is established that, when the electric field is switched on, there is slow growth of the conduction-current pulse, whose leading edge is about 5.2 ms wide. In the electrooptic modulators of Nd:YAG lasers controlled by a driver with pulsed bias, the bias-pulse width usually lies in the range 50–300 μs. The conduction current through high-resistance KTP crystals does not exceed 100 nA in such a regime, and, as shown by our experiments on lifetime, there are virtually no limits on the use of modulators based on KTP crystals associated with their electrochromic degradation.

Electrochromic degradation in nickel oxide thin film: A self-discharge and dissolution phenomenon

Nickel oxide thin films known as optical active counter electrode in electrochromic devices were grown by Pulsed Laser Deposition (PLD) at room temperature (RT) under a 10 −1 mbar oxygen pressure. From intense electrochemical characterizations of as-deposited NiO electrodes in alkaline medium, a mechanism taking into account the three typical steps of their cycling life, namely the activation period, the steady state and the degradation period, is proposed. The reversible color change from transparent to brownish, generally ascribed to the Ni(II)/Ni(III) couple in the literature, was clearly identified to the electrochemically active nickel hydroxide/oxy-hydroxide phases. The reason lies in a spontaneous chemical conversion of NiO into Ni(OH) 2 when the film is immersed in KOH medium. Afterwards, once cycling is performed, there is in competition with the electrochemical process, a self-discharge phenomenon associated to a partial dissolution of the thin-film oxidized phases. This second process becomes predominant on cycling leading to a progressive degradation of the electrochromic performances.