Viologen-Immobilized 2D Polymer Film Enabling Highly Efficient Electrochromic Device for Solar-Powered Smart Window

On-Water Surface Synthesis of Two-Dimensional Polymer Membranes for Sustainable Energy Devices

ConspectusIon-selective membranes are key components for sustainable energy devices, including osmotic power generators, electrolyzers, fuel cells, and batteries. These membranes facilitate the flow of desired ions (permeability) while efficiently blocking unwanted ions (selectivity), which forms the basis for energy conversion and storage technologies. To improve the performance of energy devices, the pursuit of high-quality membranes has garnered substantial interest, which has led to the exploration of numerous candidates, such as polymeric membranes (e.g., polyamide and polyelectrolyte), laminar membranes (e.g., transition metal carbide (MXene) and graphene oxide (GO)) and nanoporous 2D membranes (e.g., single-layer MoS2 and porous graphene). Despite impressive progress, the trade-off effect between ion permeability and selectivity remains a major scientific and technological challenge for these membranes, impeding the efficiency and stability of the resulting energy devices.Two-dimensional polymers (2DPs), which represent monolayer to few-layer covalent organic frameworks (COFs) with periodicity in two directions, have emerged as a new candidate for ion-selective membranes. The crystalline 2DP membranes (2DPMs) are typically fabricated either by bulk crystal exfoliation followed by filtration or by direct interfacial synthesis. Recently, the development of surfactant-monolayer-assisted interfacial synthesis (SMAIS) method by our group has been pivotal, enabling the synthesis of various highly crystalline and large-area 2DPMs with tunable thicknesses (1 to 100 nm) and large crystalline domain sizes (up to 120 μm2). Compared to other membranes, 2DPMs exhibit well-defined one-dimensional (1D) channels, customizable surface charge, ultrahigh porosity, and ultrathin thickness, enabling them to overcome the permeability-selectivity trade-off challenge. Leveraging these attributes, 2DPMs have established their critical roles in diverse energy devices, including osmotic power generators and metal ion batteries, opening the door for next-generation technology aimed at sustainability with a low carbon footprint.In this Account, we review our achievements in synthesizing 2DPMs through the SMAIS method and highlight their selective-ion-transport properties and applications in sustainable energy devices. We initially provide an overview of the SMAIS method for producing highly crystalline 2DPMs by utilizing the programmable assembly and enhanced reactivity/selectivity on the water surface. Subsequently, we discuss the critical structural parameters of 2DPMs, including pore sizes, charged sites, crystallinity, and thickness, to elucidate their roles in selective ion transport. Furthermore, we present the burgeoning landscape of energy device applications for 2DPMs, including their use in osmotic power generators and as electrode coating in metal ion batteries. Finally, we conclude persistent challenges and future prospects encountered in synthetic chemistry, material science, and energy device applications within this rapidly evolving field.

pH-Controllable Redox Responsive Amphiphilic Viologens for Switchable Emulsions

A pH and redox dual responsive amphiphilic viologen is synthesized, which can be reversibly transformed among the zwitterionic (SVa), monovalent anionic (SV+), and divalent anionic (SVH2+) forms upon pH variation, exhibiting pH-controllable redox responsive properties. Switchable Pickering emulsions with different droplet size and viscosity are prepared by the mixture of hydrophilic silica nanoparticles and the viologens (SV+ or SVH2+) at acidic conditions, while such combination yielded an oil-in-dispersion emulsion at neutral pH value. Not only can rapid reversible demulsification/stabilization of the Pickering emulsions be achieved by redox reactions, but the rate of redox-demulsification can also be controlled by pH trigger. The dual-responsive amphiphilic viologens have potential applications in developing intelligent colloid materials and molecular logic systems.

Heterocycle- and Amine-Free Electrochromic and Electrofluorochromic Molecules for Energy-Saving See-Through Smart Windows and Displays

Electrochromic (EC) smart windows are an elegant alternative to dusty curtains, blinds, and traditional dimming devices. The EC energy storage smart windows and displays received remarkable attention in the optoelectronic industry as they hold promise for high energy efficiency, low power consumption, reversibility, and swift response to stimuli. However, achieving these properties remains challenging. Moreover, most EC molecules do not exhibit electrofluorochromism, which is highly essential for smart displays because its EC property can modulate the solar heat entering the building, and its electrofluorochromic (EFC) aspects can create lighting during the night. In this work, a structure-property relationship is utilized to develop new electrochromes that can store the injected charge, and these molecules indeed exhibit electrofluorochromism. The compounds are synthesized from tetrabenzofluorene with two aromatic acceptor units, and avoids the use of widely studied heterocycles and amine derivatives. The electrochromes switches from yellow to dark hue in solution, solid, and gel state. The compounds display exceptional electrochemical stability and reversibility in 1000 cycles and capacity retention of 93-100 % in 300 charging-discharging cycles. The proof-of-concept device fabrication of the self-dimming EC smart window presented here demonstrates that it can furnish visual comfort, modulate transmitted light and glare, and reduce energy usage.

Potential Gradient-Driven Dual-Functional Electrochromic and Electrochemical Device Based on a Shared Electrode Design

The integration of electrochromic devices and energy storage systems in wearable electronics is highly desirable yet challenging, because self-powered electrochromic devices often require an open system design for continuous replenishment of the strong oxidants to enable the coloring/bleaching processes. A self-powered electrochromic device has been developed with a close configuration by integrating a Zn/MnO2 ionic battery into the Prussian blue (PB)-based electrochromic system. Zn and MnO2 electrodes, as dual shared electrodes, the former one can reduce the PB electrode to the Prussian white (PW) electrode and serves as the anode in the battery; the latter electrode can oxidize the PW electrode to its initial state and acts as the cathode in the battery. The bleaching/coloring processes are driven by the gradient potential between Zn/PB and PW/MnO2 electrodes. The as-prepared Zn||PB||MnO2 system demonstrates superior electrochromic performance, including excellent optical contrast (80.6%), fast self-bleaching/coloring speed (2.0/3.2 s for bleaching/coloring), and long-term self-powered electrochromic cycles. An air-working Zn||PB||MnO2 device is also developed with a 70.3% optical contrast, fast switching speed (2.2/4.8 s for bleaching/coloring), and over 80 self-bleaching/coloring cycles. Furthermore, the closed nature enables the fabrication of various flexible electrochromic devices, exhibiting great potentials for the next-generation wearable electrochromic devices.

Viologen-based solution-processable ionic porous polymers for electrochromic applications

Electrochromic porous thin films are promising for applications in smart windows and energy-efficient optical displays. However, their generally poor processing ability and excessive processing times remain grand challenges. Herein, we report the design and convenient synthesis of core-altered N-arylated viologens with aldehyde groups (πV-CHO) as new building blocks to prepare soluble, viologen-embedded ionic porous polymers. We also demonstrate that these polymers can be easily solution-processed by drop-coating to fabricate high-quality electrochromic films with tunable optoelectronic properties in a cost-effective fashion. The prepared films exhibit excellent electrochromic performance, including a low driving voltage (1.2-1.4 V), fast switching times (0.8-1.7 s), great coloration efficiency (73-268 cm2 C-1), remarkably high optical contrast up to 95.6%, long cycling stability, and tunable oxidation and reduction colors. This work sheds important light on a new molecular engineering approach to produce redox-active polymers with combined properties of intrinsic porosity, reversible and tunable redox activity, and solution processability. This provides the materials with an inherently broad utility in a variety of electrochemical devices for energy storage, sensors, and electronic applications.

Continuous Ultrathin Zwitterionic Covalent Organic Framework Membrane Via Surface-Initiated Polymerization Toward Superior Water Retention

Efficient construction of proton transport channels in proton exchange membranes maintaining conductivity under varied humidity is critical for the development of fuel cells. Covalent organic frameworks (COFs) hold great potential in providing precise and fast ion transport channels. However, the preparation of continuous free-standing COF membranes retaining their inherent structural advantages to realize excellent proton conduction performance is a major challenge. Herein, a zwitterionic COF material bearing positive ammonium ions and negative sulphonic acid ions is developed. Free-standing COF membrane with adjustable thickness is constructed via surface-initiated polymerization of COF monomers. The porosity, continuity, and stability of the membranes are demonstrated via the transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM) characterization. The rigidity of the COF structure avoids swelling in aqueous solution, which improves the chemical stability of the proton exchange membranes and improves the performance stability. In the higher humidity range (50-90%), the prepared zwitterionic COF membrane exhibits superior capability in retaining the conductivity compared to COF membrane merely bearing sulphonic acid group. The established strategy shows the potential for the application of zwitterionic COF in the proton exchange membrane fuel cells.

2D Conjugated Polymer Thin Films for Organic Electronics: Opportunities and Challenges

2D conjugated polymers (2DCPs) possess extended in-plane π-conjugated lattice and out-of-plane π-π stacking, which results in enhanced electronic performance and potentially unique band structures. These properties, along with predesignability, well-defined channels, easy postmodification, and order structure attract extensive attention from material science to organic electronics. In this review, the recent advance in the interfacial synthesis and conductivity tuning strategies of 2DCP thin films, as well as their application in organic electronics is summarized. Furthermore, it is shown that, by combining topology structure design and targeted conductivity adjustment, researchers have fabricated 2DCP thin films with predesigned active groups, highly ordered structures, and enhanced conductivity. These films exhibit great potential for various thin-film organic electronics, such as organic transistors, memristors, electrochromism, chemiresistors, and photodetectors. Finally, the future research directions and perspectives of 2DCPs are discussed in terms of the interfacial synthetic design and structure engineering for the fabrication of fully conjugated 2DCP thin films, as well as the functional manipulation of conductivity to advance their applications in future organic electronics.

C-Rich Carbon Nitride Conjugated Polymer Enabling Ion-Migration-Induced Precise Electrochromic Display

The development of electrochromic (EC) displays has been in the challenge of displaying precise patterns, such as characters or high-resolution images of small size. High-performance EC materials as well as efficient, precise-display strategies are still urgent. To enable a microfactor-guided strategy for highly precise display, I3-/I- ion-migration-induced localized electrochromism is developed in an EC device based on the C-rich polymeric carbon nitride (CPCN). The CPCN material with an extended conjugated backbone of individual aromatic nuclei and heptazine rings has been reported possessing remarkable photorechargeable performance. Owing to the self-charging behavior, the CPCN exhibits color switching by the interfacial charge recombination with I3- ions in electrolyte and serves as the EC material with a coloration efficiency of 210.2 cm2 C-1 and an optical contrast of 48.6%. Material synthesis, electrode preparation, device design and fabrication, mechanism analysis, and performance evaluation of the CPCN-based EC display device are described.

Cross-linking reactions in Langmuir monolayers of specially designed aminolipids - a toolbox for the customized production of amphiphilic nanosheets

Synthetic amino lipids, already known as highly efficient gene therapy tool, are used in a novel way to create cross-linked stable one-molecule-thin films envisioned for future (bio)-materials applications. The films are prepared as Langmuir monolayers at the air/water interface and cross-linked 'in situ' via dynamic imine chemistry. The cross-linking process and the film characteristics are monitored by various surface-sensitive techniques such as grazing incidence X-ray diffraction, X-ray reflectivity, and infrared reflection-absorption spectroscopy. After transfer onto carbon grids, the cross-linked films are investigated by transmission and scanning electron microscopy. The obtained micrographs display mechanically self-supported nanosheets with area dimensions over several micrometers and, thus, an undeniable visual proof of successful cross-linking. The cross-linking process at the air/water interface allows to obtain Janus-faced sheets with a hydrophobic side characterized by aliphatic alkyl chains and a hydrophilic side characterized by nucleophilic groups like amines, hydroxyl groups and imine.

Development of Electrochromic Devices, Based on Polymeric Gel, for Energy Saving Applications

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(ClO4)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 EV2+/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 cm2/C and 420.1 cm2/C at 1165 nm.

Survey of Recent Advances in Molecular Fluorophores, Unconjugated Polymers, and Emerging Functional Materials Designed for Electrofluorochromic Use

In this review, recent advances that exploit the intrinsic emission of organic materials for reversibly modulating their intensity with applied potential are surveyed. Key design strategies that have been adopted during the past five years for developing such electrofluorochromic materials are presented, focusing on molecular fluorophores that are coupled with redox-active moieties, intrinsically electroactive molecular fluorophores, and unconjugated emissive organic polymers. The structural effects, main challenges, and strides toward addressing the limitations of emerging fluorescent materials that are electrochemically responsive are surveyed, along with how these can be adapted for their use in electrofluorochromic devices.

One-Dimensional Chain Viologen-Based Lanthanide Multistimulus-Responsive Materials with Photochromism, Photoluminescence, Photomagnetism, and Ammonia/Amine Vapor Sensing

In this work, a series of novel multistimulus-responsive lanthanide coordination polymers {[LnL(H₂O)₄]Cl₃·3H₂O}_n (Ln = Dy, Tb, Eu) constructed using a dicarboxylic acid viologen derivative L (L = N,N'-4,4'-bipyridiniodipropionate) and LnCl₃·6H₂O were prepared. All materials showed positive responses to UV light, and the photochromic phenomena accompanied by significant photoquenching of photoluminescence could be observed through a photoelectron transfer mechanism. Strikingly, the Dy analogue displayed photomagnetic behavior, as well as responded positively to small molecules of inorganic ammonia/organic amines. Furthermore, the good photoresponsive and ammonia/amine vapor-responsive properties of the Dy-based material were further fulfilled in dual-function papers involving erasable inkless printing and visual amine detection applications. This work aims to advance the development of multistimulus-responsive multifunctional materials incorporating viologen derivates and versatile lanthanide ions and further enriches the research in this field.

Deep-Learning Pipeline for Statistical Quantification of Amorphous Two-Dimensional Materials

Aberration-corrected transmission electron microscopy enables imaging of two-dimensional (2D) materials with atomic resolution. However, dissecting the short-range-ordered structures in radiation-sensitive and amorphous 2D materials remains a significant challenge due to low atomic contrast and laborious manual evaluation. Here, we imaged carbon-based 2D materials with strong contrast, which is enabled by chromatic and spherical aberration correction at a low acceleration voltage. By constructing a deep-learning pipeline, atomic registry in amorphous 2D materials can be precisely determined, providing access to a full spectrum of quantitative data sets, including bond length/angle distribution, pair distribution function, and real-space polygon mapping. Accurate segmentation of micropores and surface contamination, together with robustness against background inhomogeneity, guaranteed the quantification validity in complex experimental images. The automated image analysis provides quantitative metrics with high efficiency and throughput, which may shed light on the structural understanding of short-range-ordered structures. In addition, the convolutional neural network can be readily generalized to crystalline materials, allowing for automatic defect identification and strain mapping.

Recent Advances in Graphene-Based Nanocomposites for Ammonia Detection

The increasing demand to mitigate the alarming effects of the emission of ammonia (NH3) on human health and the environment has highlighted the growing attention to the design of reliable and effective sensing technologies using novel materials and unique nanocomposites with tunable functionalities. Among the state-of-the-art ammonia detection materials, graphene-based polymeric nanocomposites have gained significant attention. Despite the ever-increasing number of publications on graphene-based polymeric nanocomposites for ammonia detection, various understandings and information regarding the process, mechanisms, and new material components have not been fully explored. Therefore, this review summarises the recent progress of graphene-based polymeric nanocomposites for ammonia detection. A comprehensive discussion is provided on the various gas sensor designs, including chemiresistive, Quartz Crystal Microbalance (QCM), and Field-Effect Transistor (FET), as well as gas sensors utilising the graphene-based polymer nanocomposites, in addition to highlighting the pros and cons of graphene to enhance the performance of gas sensors. Moreover, the various techniques used to fabricate graphene-based nanocomposites and the numerous polymer electrolytes (e.g., conductive polymeric electrolytes), the ion transport models, and the fabrication and detection mechanisms of ammonia are critically addressed. Finally, a brief outlook on the significant progress, future opportunities, and challenges of graphene-based polymer nanocomposites for the application of ammonia detection are presented.

Emerging Electrochromic Materials and Devices for Future Displays

With the rapid development of optoelectronic fields, electrochromic (EC) materials and devices have received remarkable attention and have shown attractive potential for use in emerging wearable and portable electronics, electronic papers/billboards, see-through displays, and other new-generation displays, due to the advantages of low power consumption, easy viewing, flexibility, stretchability, etc. Despite continuous progress in related fields, determining how to make electrochromics truly meet the requirements of mature displays (e.g., ideal overall performance) has been a long-term problem. Therefore, the commercialization of relevant high-quality products is still in its infancy. In this review, we will focus on the progress in emerging EC materials and devices for potential displays, including two mainstream EC display prototypes (segmented displays and pixel displays) and their commercial applications. Among these topics, the related materials/devices, EC performance, construction approaches, and processing techniques are comprehensively disscussed and reviewed. We also outline the current barriers with possible solutions and discuss the future of this field.

Optimal acceleration voltage for near-atomic resolution imaging of layer-stacked 2D polymer thin films

Despite superb instrumental resolution in modern transmission electron microscopes (TEM), high-resolution imaging of organic two-dimensional (2D) materials is a formidable task. Here, we present that the appropriate selection of the incident electron energy plays a crucial role in reducing the gap between achievable resolution in the image and the instrumental limit. Among a broad range of electron acceleration voltages (300 kV, 200 kV, 120 kV, and 80 kV) tested, we found that the highest resolution in the HRTEM image is achieved at 120 kV, which is 1.9 Å. In two imine-based 2D polymer thin films, unexpected molecular interstitial defects were unraveled. Their structural nature is identified with the aid of quantum mechanical calculations. Furthermore, the increased image resolution and enhanced image contrast at 120 kV enabled the detection of functional groups at the pore interfaces. The experimental setup has also been employed for an amorphous organic 2D material.

High-resolution imaging of organic 2D materials using transmission electron microscopes is challenging. Here, the authors find the optimal electron acceleration voltage, and demonstrate 1.9 Å resolution, enabling detection of interstitial defects and functional groups in 2D polymer thin films.

Dynamic Refractive Index-Matching for Adaptive Thermoresponsive Smart Windows

Thermoresponsive smart windows (TRSWs) take great advantages in energy-efficient buildings and on-demand devices owing to their self-adaptiveness and external energy consumption-free nature. Currently used TRSWs largely rely on thermal-induced phase transitions in single-material systems, however, the intrinsic characteristics of which may not be suited for practical window utilization, such as poor luminous transparency and fixed critical temperature (T_c ). Herein, an adaptive TRSW based on dynamic refractive index (RI) matching between two phases is demonstrated, which is facilely fabricated by embedding ethylene glycol solution microdroplets into polydimethylsiloxane (PDMS) via a one-step emulsification approach, realizing a smart temperature response in PDMS. The TRSW presents high transparency (≈92%) and bidirectional transparency-temperature response (≈20% at 73 °C, ≈40% at 8 °C). Moreover, the RI dispersion generates a unique effect of wavelength selectivity with temperature. Notably, the effective optical-temperature response with variable T_c could be tuned over a wide range of 13-68 °C by adjusting the EGS concentration. The proposed strategy with dynamic RI matching allows TRSW construction to extend beyond phase transitional materials and greatly broadens the applicable scope of TRSWs, which is promising in the fields of smart optical devices such as smart windows, anti-counterfeiting, optical switches, and optical selection.