Solar Thermal Fuels Research Articles

Fluorination effects on photoisomerization dynamics of azobenzene

The photoisomerization dynamics of azobenzene (Azo-H) and its modulation by electron-withdrawing groups (EWGs) are important for designing photophysical applications, such as artificial muscles, liquid crystal displays, and solar thermal fuels. In this study, the electron-withdrawal kinetics of fluorination and the tailoring of the photoisomerization dynamics of azobenzene were explored using theoretical multiscale simulations. First-principle calculations provide insight into the mechanical aspects of photoisomerization, whereas molecular dynamics simulations offer a time-dependent perspective. Fluorinated azobenzene (Azo-F) exhibits superior photoisomerization responsiveness owing to changes in its molecular conformation and energy states induced by the EWG. These thermodynamic advantages were influenced by the stabilization of the molecular orbitals near the diazene groups. Despite the thermodynamic advantages of Azo-F, heavy fluorine atoms reduce the actuating force and work capacity owing to their increased inertia. This study provides a comprehensive understanding of photoisomerization kinetics modulated by the local fluorination of Azo-H.

Visible‐Light Responsive Ionic Columnar Self‐Assemblies Exhibiting Fast Photothermal Energy Conversion at Room‐Temperature

AbstractA visible‐light responsive ionic liquid crystalline (LC) system based on gallic ester and tetra‐ortho‐substituted azobenzene is designed and synthesized. All the prepared derivatives exhibit columnar hexagonal mesophases at room temperature either in their pristine form or on cooling from isotropic states. The strategic design of these molecules allows for rapid photoisomerization, which is further aided by the dynamic LC phase. These systems attains photostationary states (PSSs) within 5 min of charging and discharging. The maximum Z conversion on charging in the solid‐state at room‐temperature is ≈80% with a highest temperature rise of 9.1 °C on discharging. This new molecular design enables the efficient working of solar thermal fuels (STFs) even in low sunlight intensity conditions, potentially promoting widespread adoption for mitigating global energy demands.

Photochromic Carbon Nanomaterials: An Emerging Class of Light-Driven Hybrid Functional Materials.

Photochromic molecules have remarkable potential in memory and optical devices, as well as in driving and manipulating molecular motors or actuators and many other systems using light. When photochromic molecules are introduced into carbon nanomaterials (CNMs), the resulting hybrids provide unique advantages and create new functions that can be employed in specific applications and devices. This review highlights the recent developments in diverse photochromic CNMs. Photochromic molecules and CNMs are also introduced. The fundamentals of different photochromic CNMs are discussed, including design principles and the types of interactions between CNMs and photochromic molecules via covalent interactions and non-covalent bonding such as π-π stacking, amphiphilic, electrostatic, and hydrogen bonding. Then the properties of photochromic CNMs, e.g., in photopatterning, fluorescence modulation, actuation, and photoinduced surface-relief gratings, and their applications in energy storage (solar thermal fuels, photothermal batteries, and supercapacitors), nanoelectronics (transistors, molecular junctions, photo-switchable conductance, and photoinduced electron transfer), sensors, and bioimaging are highlighted. Finally, an outlook on the challenges and opportunities in the future of photochromic CNMs is presented. This review discusses a vibrant interdisciplinary research field and is expected to stimulate further developments in nanoscience, advanced nanotechnology, intelligently responsive materials, and devices.

Isomer‐Dependent Melting Behavior of Low Molar Mass Azobenzene Derivatives: Observation of a Higher Melting Z‐Isomer

AbstractAzobenzene compounds are putative solar thermal fuels (STF) due to the excellent photostability and structural control of isomerization rates. Azobenzenes, in which both Z‐ and E‐isomers are liquid at room temperature, are promising candidates for STF flow technology. A literature survey of melting points led to the synthesis and isomer separation of ortho‐ and meta‐monosubstituted azobenzenes with fluoro, methyl, ethyl, trifluoromethyl and methoxy substituents and several dimethyl substituted azobenzenes. Four of the compounds are liquid azobenzenes with higher specific energy than literature work with higher molar mass, liquid compounds. Eight of the compounds unexpectedly displayed a higher melting point for the Z‐isomer which is rarely observed. Intermolecular close contacts in the crystal lattice of the Z‐isomer are the main factor responsible for the higher melting temperatures.

Spatiotemporal Utilization of Latent Heat in Erythritol‐based Phase Change Materials as Solar Thermal Fuels

AbstractSolar thermal fuels (STFs) have been particularly concerned as sustainable future energy due to their impressive ability to store solar energy in chemical bonds and controllably release thermal energy. However, currently studied STFs mainly focus on molecule‐based materials with high photochemical activity, toxicity, and compromised features, which greatly restricts their applications in practical scenarios of solar energy utilization. Herein, we present a novel erythritol‐based composite phase change material (PCM) as a new type of STFs with an outstanding capability to store solar energy as latent heat in its stable supercooling state and release thermal energy as needed. This composite PCM with stored thermal energy can be maintained stably at room temperature and subsequently release latent heat as high as 224.9 J/g during the crystallization process triggered by thermal stimuli. Remarkably, solar energy can be converted into latent heat stored in the composite PCM over months. Through mechanical stimulations, the released latent heat can increase the temperature of the composite up to 91 °C. This work presents a new concept of using spatiotemporal storage and release of latent heat in PCMs for solar energy utilization, making it a potential candidate as STFs for developing future clean energy techniques.

Spatiotemporal Utilization of Latent Heat in Erythritol-based Phase Change Materials as Solar Thermal Fuels.

Solar thermal fuels (STFs) have been particularly concerned as sustainable future energy due to their impressive ability to store solar energy in chemical bonds and controllably release thermal energy. However, currently studied STFs mainly focus on molecule-based materials with high photochemical activity, toxicity, and compromised features, which greatly restricts their applications in practical scenarios of solar energy utilization. Herein, we present a novel erythritol-based composite phase change material (PCM) as a new type of STFs with an outstanding capability to store solar energy as latent heat in its stable supercooling state and release thermal energy as needed. This composite PCM with stored thermal energy can be maintained stably at room temperature and subsequently release latent heat as high as 224.9 J/g during the crystallization process triggered by thermal stimuli. Remarkably, solar energy can be converted into latent heat stored in the composite PCM over months. Through mechanical stimulations, the released latent heat can increase the temperature of the composite up to 91 °C. This work presents a new concept of using spatiotemporal storage and release of latent heat in PCMs for solar energy utilization, making it a potential candidate as STFs for developing future clean energy techniques.

Solar Thermal Energy Storage Systems Based on Discotic Nematic Liquid Crystals That Can Efficiently Charge and Discharge below 0 °C

AbstractSolid‐state solar thermal fuels (SSTFs) serve as efficient means of storing solar energy as chemical potential energy in a closed loop system and releasing it as heat on‐demand. An ideal SSTF requires photoswitchability in visible‐region without any external heating as well as extended storage times. However, existing systems often rely on ultraviolet (UV) light or heating for photoswitching, or suffer from low storage times. Addressing this, a novel strategy is presented to obtain visible‐light responsive SSTFs designed to operate effectively at room‐temperature or sub‐zero temperatures by innovatively integrating a tetra‐ortho‐fluoro/chloro azobenzene arm in triphenylene based liquid crystal (LC) moiety. The resulting compounds exhibit discotic nematic (ND) mesophases till −6.5 °C. These compounds exhibit excellent photocyclability, photostability, and sufficient half‐lives of cis states. Achieving up to 77% charging under sunlight with a bandpass filter and 62.4% without it, these systems uniquely demonstrate efficient chargeability and dischargeability at sub‐zero temperatures. Upon discharging, temperature rise of up to 6.5 and 29.5 °C occur at room‐temperature (25 °C) and sub‐zero temperatures (around −6 to −7 °C), respectively. This efficacy is attributed to less‐ordered ND phases providing conformational freedom for photoisomerization at low temperatures.

Visible light activated dendrimers for solar thermal energy storage and release below 0 °C

Molecular solar thermal (MOST) fuels offer a closed-cycle and renewable energy storage strategy that can harvest photons within the chemical conformations and release heat on demand through reversible isomerization of molecular photoswitches.

High free volume polymers of intrinsic microporosity for efficient solar thermal fuels with enhanced energy storage capacity

Anchoring photoswitches on high free volume polymers is an emerging strategy to prepare high-performance solar thermal fuels (STFs), however, it still lacks a deep understanding about the relationship between free volume and energy storage performance. Herein, a bottom-to-up strategy was developed to prepare azobenzene-based STFs based on polymers of intrinsic microporosity (Azo-PIMs), in which free volumes could be changed by controlling the feeding ratio of co-monomers. Azo-PIM-1 and Azo-PIM-2 were prepared to have 7.41% and 19.07% azo grafting densities, respectively. The fractional free volume (FFV) and specific surface areas of Azo-PIM-1 are 0.17 and 552.8 m2 g−1, respectively, which are higher than that of Azo-PIM-2 (0.15 and 335.5 m2 g−1) due to the lower azo grafting density. The energy densities (EDg) of Azo-PIM-1 and Azo-PIM-2 in cis isomers are 13.39 J g−1 and 36.75 J g−1, and the corresponding storage enthalpy (ΔH) are 88.53 kJ mol−1 and 103. 35 kJ mol−1, respectively. Their ΔH are higher than that of the azo monomer of 83.76 kJ mol−1, demonstrating that PIMs as templates can efficiently increase the stored energy per azo moiety. Meanwhile, the higher ΔH of Azo-PIM-2 than that of Azo-PIM-1 indicates that the reduced free volume can enhance the energy storage capacity by prohibiting the rotation of azo photoisomerization. This work provides a basis for understanding the role of free volume on the energy storage performance of polymer-based STFs.

Water-Soluble Azobenzene-Based Solar Thermal Fuels with Improved Long-Term Energy Storage and Energy Density

Water-Soluble Azobenzene-Based Solar Thermal Fuels with Improved Long-Term Energy Storage and Energy Density

Visible-Light Responsive Azobenzene and Cholesterol Based Liquid Crystals as Efficient Solid-State Solar–Thermal Fuels

Visible-Light Responsive Azobenzene and Cholesterol Based Liquid Crystals as Efficient Solid-State Solar–Thermal Fuels

Enhancing solar thermal storage properties of azobenzenes with conductive polymer: Electropolymerization of carbazole containing photoactive cyanoazobenzene derivative

Solar thermal fuels (STFs) have recently drawn attention as a promising strategy for utilization of solar energy that is a clean and renewable source. Especially solid state solar thermal fuels have high potential to be used in thermal applications. However, up to date, it is seen that the solar thermal storage capacity in the polymeric state is lower than in the corresponding monomers. Herein, for the first time, photoactive azobenzene and electroactive carbazole containing monomer (Cz-AzoCN) was designed and thermal storage properties of the monomer and the corresponding polymer obtained by electropolymerization were investigated. This study showed that polycarbazoles containing azobenzene pendant groups can be an effective thermal storage platform due to their easy functionality, high light absorption and rigid conjugated chain structure resembling the structure of rigid templates decorated with closely packed photochromic units. Additionally, electropolymerization is an easy and rapid method for polymer synthesis, producing polymer in the form of film that is ready for irradiation. The gravimetric energy storage density of p(Cz-AzoCN) was calculated as 145.12 j g−1 which was 128 % higher than that of monomer. This is due to boosted light harvesting of conjugated polycarbazole backbone as an organic photosensitizer that providing effective solar thermal storage.

Understanding Solid-State Photochemical Energy Storage in Polymers with Azobenzene Side Groups.

Solar thermal fuel (STF) materials store energy through light-induced changes in the structures of photoactive molecular groups, and the stored energy is released as heat when the system undergoes reconversion to the ground-state structure. Solid-state STF devices could be useful for a range of applications; however, the light-induced structural changes required for energy storage are often limited or prevented by dense molecular packing in condensed phases. Recently, polymers have been proposed as effective solid-state STF platforms, as they can offer the bulk properties of solid materials while retaining the molecular-level free volume and/or mobility to enable local structural changes in photoresponsive groups. Light-induced energy storage and macroscopic heat release have been demonstrated for polymers with photoisomerizable azobenzene side groups. However, the mechanism of energy storage and the link between the polymer structure, energy density and storage duration has not yet been explored in detail. In this work, we present a systematic study of methacrylate- and acrylate-based polymers with azobenzene side groups to establish the mechanism of energy storage and release and the factors affecting energy density and reconversion kinetics. For polymers with directly attached azobenzene side groups, the energy storage properties are in line with previous work on similar systems, and the photoisomerization and reconversion properties of the azobenzene side groups mirror those of molecular azobenzene. However, the inclusion of an alkyl linker between the azobenzene side group and the backbone significantly increases the photoswitching efficiency, giving almost quantitative conversion to the Z isomeric state. The presence of the alkyl linker also reduces the glass transition temperature and leads to faster spontaneous thermal reconversion to the E isomeric form, but in all cases, half-lives of more than 4 days are observed in the solid state, which provides scope for applications requiring daily energy storage-release cycles. The maximum gravimetric energy density observed is 143 J g-1, which represents an increase of up to 44% compared to polymers with directly attached azobenzene moieties.

Improvement of Solar Thermal Fuels by Anchoring Azobenzene to Polymers of Intrinsic Microporosity

Improvement of Solar Thermal Fuels by Anchoring Azobenzene to Polymers of Intrinsic Microporosity

Flexible wearable fabrics for solar thermal energy storage and release in on-demand environments

Efficient solar thermal energy storage and release via molecular solar thermal (MOST) fuels is essential to meet the ever-increasing global energy demands. However, most reported MOSTs still face some challenges, such as energy storage relying on ultraviolet (UV) light and solvent, and applications limited to room temperature. Herein, we propose a novel flexible wearable fabric consisting of azobenzene-containing dendrimers, polydopamine, and cotton fabric, which not only can efficiently store photon of ultraviolet, green, red light and sunlight energy but also enables applications in low- or room-temperature solvent-free environments. These remarkable properties are attributed to the red-shift of the n-π* band of the σ-fluoroazobenzene, the low glass-transition temperature of dendrimer fuel, and the photothermal effect of polydopamine. The storage energy density of the wearable fabric can reach 0.05 MJ kg−1 (18.2 kJ mol−1) accompanied by a storage half-life of up to approximately one month. Blue light-triggered heat release from wearable fabrics can increase the temperature by 11.1–12.3 °C, showing excellent results in room-temperature wrist guards and low-temperature body-warming applications. This work paves the way for the development of wearable fabrics for solar thermal energy storage and release in on-demand environments such as sunlight, solvent-free, and low temperature.

Photoresponse and electrochemical behaviour of azobenzene anchored graphene oxide for energy storage application

Solar energy is considered to be a renewable source of energy that is intrinsically more sustainable than fossil fuels. Developing photoactive hybrid materials to store solar energy has recently received much attention. Herein, a photoactive molecule-graphene oxide hybrid was synthesized and characterized systematically. The solar energy storage performances of the hybrids were studied using various photophysical studies. The energy density and power density of the hybrid materials were 47 WhKg−1 and 156.6 WKg-1 respectively which showed 3 fold higher than the pristine compound. The photoelectrochemical behaviour of the hybrid were also studied using Cyclic voltammetry and Electrochemical impedance spectroscopy (EIS). Results showed the electrochemical performances can be varied due to their changing conformations from trans-to-cis isomerization. This work enables the research community in developing a promising material for solar thermal fuels as well as in energy storage devices.

Visible-Light-Photomeltable Azobenzenes as Solar Thermal Fuels

Photomelting of azobenzene is a phenomenon that is typically achieved by UV-light irradiation. However, many applications based on azobenzene photomelting, including the solar thermal fuel (STF) application, require responsiveness to visible light. Therefore, in this study, we designed and synthesized several azobenzene derivatives that melt under visible-light irradiation and explored their potential application as STFs. By combining molecular design elements that endow azobenzene with visible-light-responsiveness (ortho-halogen) and photomeltability (para-alkoxy), we successfully synthesized four compounds that exhibit visible-light photomelting properties. These compounds also exhibit a long cis-isomer lifetime, which makes them good candidates for STFs in the future.

Solar–Thermal Fuels and the Role of Carbon Nanomaterials: A Perspective with Emphasis on the Azobenzene System

Molecular switches have long been investigated aiming at developing sustainable and renewable energy-based technologies. Among molecular photoswitches, azobenzene (AB) has been the quintessential template for solar–thermal fuels (STFs), acting as “molecular batteries”, storing and discharging energy by converting between its trans- and cis-isomers upon absorption of UV light. Nevertheless, key properties for device performance such as stored energy, energy density, stability of the “charged” metastable isomer, or the control of energy release are highly influenced by a mediating interface. This has motivated the study of metal and carbon nanomaterials as templates in hybrid STFs. The latter have been shown to be especially promising owing to their unique electronic properties, topological tailoring, and abundant sourcing. This review discusses recent advances that explore enhancements on several molecular photoswitch systems, with a special emphasis on the AB system, associated with changes in the molecular substituents and the development of hybrid systems, involving mainly carbon nanotemplates, such as reduced graphene oxide, graphene, carbon nanotubes, and fullerenes. The energy density, τ1/2, grafting density, and charge transfer of the AB and the hybrid adsorbed AB system on carbon nanomaterials are discussed. Key facets of the design of hybrid STF devices are discussed, along with a perspective on the aspects that merit further investigation toward the development of efficient STFs.

Photoliquefaction and phase transition of m-bisazobenzenes give molecular solar thermal fuels with a high energy density.

A series of m-bisazobenzene chromophores modified with various alkoxy substituents (1; methoxy, 2; ethoxy, 3; butoxy, 4; neopentyloxy) were developed for solvent-free molecular solar thermal fuels (STFs). Compounds (E,E)-1-3 in the crystalline thin film state exhibited photoliquefaction, the first example of photo-liquefiable m-bisazobenzenes. Meanwhile, (E,E)-4 did not show photoliquefaction due to the pronounced rigidity of the interdigitated molecular packing indicated by X-ray crystallography. The m-bisazobenzenes 1-4 exhibited twice the Z-to-E isomerization enthalpy compared to monoazobenzene derivatives, and the latent heat associated with the liquid-solid phase change further enhanced their heat storage capacity. To observe both exothermic Z-to-E isomerization and crystallization in a single heat-up process, the temperature increase of differential scanning calorimetry (DSC) must occur at a rate that does not deviate from thermodynamic equilibrium. Bisazobenzene 1 showed an unprecedented gravimetric heat storage capacity of 392 J g-1 that exceeds previous records for well-defined molecular STFs.

Photoliquefiable Azobenzene Surfactants toward Solar Thermal Fuels that Upgrade Photon Energy Storage via Molecular Design.

Photoresponsive phase change materials (PPCMs) are capable of storing photon and heat energy simultaneously and releasing the stored energy as heat in a controllable way. While, the azobenzene-based PPCMs exhibit a contradiction between gravimetric energy storage density and photoinduced phase change. Here, a type of azobenzene surfactants with balance between molecular free volume and intermolecular interaction is designed in molecular level, which can address the coharvest of photon energy and low-grade heat energy at room temperature. Such PPCMs gain the total gravimetric energy density up to 131.18 J g-1 by charging solid sample and 160.50 J g-1 by charging solution. Notably, the molar isomerization enthalpy upgrades by a factor of up to 2.4 compared to azobenzene. The working mechanism is explained by the computational studies. All the stored energy can release out as heat under Vis light, causing a fast surface temperature rise. This study demonstrates a new molecular designing strategy for developing azobenzene-based PPCMs with high gravimetric energy density by improving the photon energy storage.