Molecular Solar Thermal Energy Storage Research Articles

Triplet-triplet annihilation mediated photon upconversion solar energy systems.

Solar energy harvesting is among the best solutions for a global transition toward carbon-neutral energy technologies. The existing solar energy harvesting technologies like photovoltaics (PV) and emerging molecular concepts such as solar fuels and molecular solar thermal energy storage (MOST) are rapidly developing. However, to realize their full potential, fundamental solar energy loss channels like photon transmission, recombination, and thermalization need to be addressed. Triplet-triplet annihilation mediated photon upconversion (TTA-UC) is emerging as a way to overcome losses due to the transmission of photons below the PV/chromophore band gap. However, there are several challenges related to the integration of efficient solid-state TTA-UC systems into efficient devices such as: wide band absorption, materials sustainability, and device architecture. In this article, we review existing work, identify and discuss challenges as well as present our perspective toward possible future directions.

Aza-bicyclooctadiene/tetracyclooctane couples as promising photoswitches for molecular solar thermal energy storage applications

The position of N in the aza-BOD/TCO photoswitching systems govern the thermochemical and photophysical properties for molecular solar thermal energy storage application.

Visible light activated energy storage in solid-state Azo-BF2 switches.

We present here a group of Azo-BF2 photoswitches that store and release energy in response to visible light irradiation. Unmodified Azo-BF2 switches have a planar structure with a large π-conjugation system, which hinders E-Z isomerization when in a compacted state. To address this challenge, we modified the switches with one or two aliphatic groups, which altered the intermolecular interactions and arrangement of the photochromes in the solid state. The derivative with two substituents exhibited a non-planar configuration that provided particularly large conformational freedom, allowing for efficient isomerization in the solid phase. Our discovery highlights the potential of using double aliphatic functionalization as a promising approach to facilitate solid-state switching of large aromatic photoswitches. This finding opens up new possibilities for exploring various photoswitch candidates for molecular solar thermal energy storage applications.

Fused Bis(hemi-indigo): Broad-Range Wavelength-Independent Photoswitches.

Wavelength-independent conversion of organic photoswitches in the photostationary state is a rare phenomenon that opens up a way for many practical applications. In this work, three fused bis(hemi-indigo) derivatives with different substitution patterns were synthesized and their photoswitching was investigated by optical spectroscopy, real-time NMR spectroscopy and TD-DFT calculations. We disclosed that the Z-E photoisomerization of the meta-bis(hemi-indigo) derivative was remarkably independent of the irradiation wavelength from UV up to yellow light. The wavelength-independent forward photoswitching together with the inhibited backward photoisomerization, high thermal stability of the photoinduced isomers as well as significant overlap between the photoswitch absorption and the solar spectrum allows to suggest bis(hemi-indigo) derivatives as promising candidates for molecular solar thermal energy storage (MOST) systems.

Photocontrolled Energy Storage in Azobispyrazoles with Exceptionally Large Light Penetration Depths.

Azobispyrazole, 4pzMe-5pzH, derivatives with small terminal substituents (Me, Et, i-Pr, and n-Pr) are reported to undergo facile reversible photoswitching in condensed phases at room temperature, exhibiting unprecedentedly large effective light penetration depths (1400 μm of UV at 365 nm and 1400 μm of visible light at 530 nm). These small photoswitches exhibit crystal-to-liquid phase transitions upon UV irradiation, which increases the overall energy storage density of this material beyond 300 J/g that is similar to the specific energy of commercial Na-ion batteries. The impact of heteroarene design, the presence of ortho methyl substituents, and the terminal functional groups is explored for both condensed-phase switching and energy storage. The design principles elucidated in this work will help to develop a wide variety of molecular solar thermal energy storage materials that operate in condensed phases.

Photon Energy Storage in Strained Cyclic Hydrazones: Emerging Molecular Solar Thermal Energy Storage Compounds

The generally small Gibbs free energy difference between the Z and E isomers of hydrazone photoswitches has so far precluded their use in photon energy storing applications. Here, we report on a series of cyclic and acyclic hydrazones, which possess varied degrees of ring strain and, hence, stability of E isomers. The photoinduced isomerization and concurrent phase transition of the cyclic hydrazones from a crystalline to a liquid phase result in the storage of a large quantity of energy, comparable to that of azobenzene derivatives. We demonstrate that the macrocyclic photochrome design in combination with phase transition is a promising strategy for molecular solar thermal energy storage applications.

Long‐Term Energy Storage Systems Based on the Dihydroazulene/Vinylheptafulvene Photo‐/Thermoswitch

AbstractThe dihydroazulene/vinylheptafulvene (DHA/VHF) couple presents a photo‐/thermoswitch that has attracted interest for the development of molecular solar thermal energy storage systems. Here we present the synthesis and optical properties as well as the switching properties of DHA derivatives incorporating alkyne and norbornadiene (NBD) substituents at position C2. The corresponding VHF isomers exhibited remarkably long lifetimes. Thus, the VHF‐to‐DHA back‐reaction half‐life for a derivative with a phenylethynyl substituent was 22 days in acetonitrile at room temperature, while the half‐life of a VHF reference compound having a phenyl substituent is around 3.5 h under the same conditions. This finding is particularly attractive in the quest for long‐term energy storage systems, considering limited enhancement of molecular weight and maintenance of good quantum yield photoisomerization. Moreover, from the thermal back‐reaction of an NBD compound, used as a precursor for the DHA‐NBD dyad, we estimated the radical‐stabilizing influence of a C(Me)=C(CN)2 substituent.

Prospects of Improving Molecular Solar Energy Storage of the Norbornadiene/Quadricyclane System through Bridgehead Modifications.

We have investigated novel bicyclic diene molecular solar thermal energy storage systems that presently are the ones with the highest predicted energy density. Using a variety of different ab initio quantum chemical methods, we report storage energies, absorption spectra, and reaction barriers for the release of stored energy for a series of bicyclic dienes. The bicyclic dienes are all constructed by modifying the bridgehead of the well-known norbornadiene/quadricyclane (NBD/QC) system. In conclusion, we find it promising that it is possible to significantly amplify the storage energy of the NBD/QC system without seriously compromising other crucial properties by introducing simple modifications to the bridgehead.

Photoisomerization kinetics of a novel photoswitchable film based on methyl red doped with sodium hexachloroplatinate hosted in polyethylene oxide

AbstractThis work proposes a novel dopant material to improve the energy storage in azo molecules. Organometallic material with platinum base atom (sodium hexachloroplatinate (IV) (Na2PtCl6)) was used as a dopant material in polyethylene oxide‐methyl red (PEO‐MR) films. The photoisomerization kinetics, the energy storage, and the electrical conductivity distribution of PEO‐(MR‐Na2PtCl6) films are investigated and studied. In addition, the effect of Na2PtCl6 on the chemical and crystal structure is also premeditated. It can be concluded that adding Na2PtCl6 to PEO‐MR film improves the energy storage in the azo molecules. Therefore, the addition of Na2PtCl6 into PEO‐MR films can be proposed for different applications, especially in molecular solar thermal energy storage media.

Thermo-optical performance of molecular solar thermal energy storage films

Due to their potential for solar energy harvesting and storage, molecular solar thermal energy storage (MOST) materials are receiving wide attention from both the research community and the public. MOST materials absorb photons and convert their energy to chemical energy, which is contained within the bonds of the MOST molecules. Depending on the molecular structure, these materials can store up to 1 MJ/kg, at ambient temperature and with storage times ranging from minutes to several years. This work is the first to thoroughly investigate the potential of MOST materials for the development of energy saving windows. To this end, the MOST molecules are integrated into thin, optically transparent films, which store solar energy during the daytime and release heat at a later point in time. A combined experimental and modeling approach is used to verify the system's basic functionality and identify key parameters. Multi-physics modeling and simulation were conducted to evaluate the interaction of MOST films with light, both monochromatic and the entire solar spectrum, as well as the corresponding dynamic energy storage. The model was experimentally verified by studying the optical response of thin MOST films containing norbornadiene derivatives as a functional system. We found that the MOST films act as excellent UV shield and can store up to 0.37 kWh/m2 for optimized MOST molecules. Further, this model allowed us to screen various material parameters and develop guidelines on how to optimize the performance of MOST window films.

Status and challenges for molecular solar thermal energy storage system based devices.

Molecular solar thermal energy storage systems (MOST) offer emission-free energy storage where solar power is stored via valence isomerization in molecular photoswitches. These photoswitchable molecules can later release the stored energy as heat on-demand. Such systems are emerging in recent years as a vibrant research field that is rapidly transitioning from basic research to applications. Since a major part of the attention is focused on molecular design and engineering, MOST-based device development has not been systematically summarized and introduced to a broad audience. This tutorial review will discuss the most commonly used and developed devices from a chemical engineering point of view. It is expected that future developers of MOST technology could be inspired by the existing devices, keeping in mind the summarized essential practical challenges towards large-scale implementations and more innovative applications.

High throughput screening of norbornadiene/quadricyclane derivates for molecular solar thermal energy storage.

We present a procedure for performing high throughput screening of molecular compounds for molecular solar thermal energy storage devices using extended tight binding (xTB) methods. In order to validate our approach, we performed screening of 3230 norbornadiene/quadricyclane (NBD/QC) derivatives in terms of storage energies, activation barriers and absorption of solar radiation using our approach, and compared it to high level density functional theory (DFT) and cluster perturbation (CP) theory calculations. Our comparisons show that the xTB screening framework correlates very well with DFT and CP theory in that it predicts the same relative trends in the studied parameters although the storage energies and thermal reaction barriers are significantly offset. Utilizing the screening methodology, we have been able to locate compounds that would either be excellent candidates or compounds that should not be considered further for molecular solar thermal energy storage devices. This methodology can readily be extended and applied to screening other molecular motifs for molecular solar energy storage.

Synthesis, characterization and computational evaluation of bicyclooctadienes towards molecular solar thermal energy storage.

Molecular solar-thermal energy storage (MOST) systems are based on photoswitches that reversibly convert solar energy into chemical energy. In this context, bicyclooctadienes (BODs) undergo a photoinduced transformation to the corresponding higher energy tetracyclooctanes (TCOs), but the photoswitch system has not until now been evaluated for MOST application, due to the short half-life of the TCO form and limited available synthetic methods. The BOD system degrades at higher temperature via a retro-Diels–Alder reaction, which complicates the synthesis of the compounds. We here report a cross-coupling reaction strategy that enables an efficient synthesis of a series of 4 new BOD compounds. We show that the BODs were able to switch to the corresponding tetracyclooctanes (TCOs) in a reversible way and can be cycled 645 times with only 0.01% degradation. Half-lives of the TCOs were measured, and we illustrate how the half-life could be engineered from seconds to minutes by molecular structure design. A density functional theory (DFT) based modelling framework was developed to access absorption spectra, thermal half-lives, and storage energies which were calculated to be 143–153 kJ mol−1 (0.47–0.51 MJ kg−1), up to 76% higher than for the corresponding norbornadiene. The combined computational and experimental findings provide a reliable way of designing future BOD/TCO systems with tailored properties.

Virtual screening of norbornadiene-based molecular solar thermal energy storage systems using a genetic algorithm.

We present a computational methodology for the screening of a chemical space of 1025 substituted norbornadiene molecules for promising kinetically stable molecular solar thermal (MOST) energy storage systems with high energy densities that absorb in the visible part of the solar spectrum. We use semiempirical tight-binding methods to construct a dataset of nearly 34000 molecules and train graph convolutional networks to predict energy densities, kinetic stability, and absorption spectra and then use the models together with a genetic algorithm to search the chemical space for promising MOST energy storage systems. We identify 15 kinetically stable molecules, five of which have energy densities greater than 0.45 MJ/kg, and the main conclusion of this study is that the largest energy density that can be obtained for a single norbornadiene moiety with the substituents considered here, while maintaining a long half-life and absorption in the visible spectrum, is around 0.55 MJ/kg.

Ortho-Substituted 2-Phenyldihydroazulene Photoswitches: Enhancing the Lifetime of the Photoisomer by ortho-Aryl Interactions.

Photochromic molecules are systems that undergo a photoisomerization to high-energy isomers and are attractive for the storage of solar energy in a closed-energy cycle, for example, in molecular solar thermal energy storage systems. One challenge is to control the discharge time of the high-energy isomer. Here, we show that different substituents in the ortho position of a phenyl ring at C-2 of dihydroazulene (DHA-Ph) significantly increase the half-life of the metastable vinylheptafulvene (VHF-Ph) photoisomer; thus, the energy-releasing VHF-to-DHA back-reaction rises from minutes to days in comparison to the corresponding para- and meta-substituted systems. Systems with two photochromic DHA-Ph units connected by a diacetylene bridge either at the para, meta and ortho positions and corresponding to a linear or to a cross-conjugated pathway between the two photochromes are also presented. Here, the ortho substitution was found to compromise the switching properties. Thus, irradiation of ortho-bridged DHA-DHA resulted in degradation, probably due to the proximity of the different functional groups that can give rise to side-reactions.

Wearable solar energy management based on visible solar thermal energy storage for full solar spectrum utilization

Efficient solar energy harvesting offers great potential for a global energy utilization beyond conventional fossil energy. However, current solar thermal approaches depend on sole molecular solar thermal energy storage (MOST) or photothermal materials (PTMs) by the rigid devices, leading to incomplete solar spectrum utilization and impossible wearability. Here we design the visible solar storage fabric (VSSF) with UV–Vis-NIR wavelengths utilization and wearable contrast photochromic display. This VSSF integrates the Azo-PCM@PS nanocapsule and Cs0.32WO3 nanoparticle, enabling the photochemistry thermophysics coupled energy storage and vis-NIR light harvesting, respectively. Rationally designed full solar spectrum utilization achieves high heat release (83 °C) energy efficiency of ∼4.8% under sunlight, which can protect human body from cold injury by a self-heating wrister. A photothermally-driven lifter based on VSSF is demonstrated to raise the targeted subject upon light triggering. Notably, the real-time energy state can be monitored by color change, in view of relationship establishment between color and energy density. This concept of wearable solar energy management provides the possibility of high solar energy efficiency by capturing sunlight through full solar spectrum.

Effect of Iodine Filler on Photoisomerization Kinetics of Photo-Switchable Thin Films Based on PEO-BDK-MR.

We report the effect of an iodine filler on photoisomerization kinetics of photo-switchable PEO-BDK-MR thin films. The kinetics of photoisomerization and time progression of PEO-BDK-MR/I2 nanocomposite thin films are investigated using UV-Vis, FTIR spectroscopies, and modified mathematical models developed using new analytical methods. Incorporating iodine filler into the PEO-BDK-MR polymeric matrix enhances the isomerization energy barrier and considerably increases the processing time. Our outcomes propose that enhanced photoisomerized and time processed (PEO-BDK-MR)/I2 thin films could be potential candidates for a variety of applications involving molecular solar thermal energy storage media.

Design of phase-transition molecular solar thermal energy storage compounds: compact molecules with high energy densities.

A series of compact azobenzene derivatives were investigated as phase-transition molecular solar thermal energy storage compounds that exhibit maximum energy storage densities around 300 J g-1. The relative size and polarity of the functional groups on azobenzene were manifested to significantly influence the phase of isomers and their energy storage capacity.

Promoting the thermal back reaction of vinylheptafulvene to dihydroazulene by physisorbtion on nanoparticles.

We investigate the effects of nanoparticles on molecular solar thermal energy storage systems and how one can tune chemical reactivities of a molecular photo- and thermoswitch by changing the nanoparticles. We have selected the dihydroazulene/vinylheptafulvene system to illustrate the effects of the nanoparticles on the chemical reactivities of the molecular photo- and thermoswitch. We have utilized the following nanoparticles: a TiO2 nanoparticle along with nanoparticles of gold, silver and copper. We calculate the rate constants for the release of the thermal energy utilizing a QM/MM method coupled to a transition state method. The molecular systems are described by density functional theory whereas the nanoparticles are given by molecular mechanics including electrostatic and polarization dynamics. In order to investigate whether the significant stabilization of the transitions state provided by the nanoparticles is general to the DHA/VHF system, we calculated the transition state rate constant of the parent- and 3-amino-substituted-DHA/VHF systems at 298.15 K in the four different orientations and at the three different separations. We observe that the transition state rate constant of the parent system is only increased as the cyano groups are oriented towards the nanoparticle while the presence of the nanoparticle actually impedes the reactions using the three other orientations. On the other hand, for the substituted system the nanoparticle generally leads to a significant increase in the rate of the reaction. We find that the nanoparticles can have a substantial effect on the calculated rate constants. We observe, depending on the nanoparticle and the molecular orientation, increases of the rate constants by a factor of 106. This illustrates the prospects of utilizing nanoparticles for controlling the release of the stored thermal energy.

New Insight on Photoisomerization Kinetics of Photo-Switchable Thin Films Based on Azobenzene/Graphene Hybrid Additives in Polyethylene Oxide

In this work, we reported a new insight on the kinetics of photoisomerization and time evolution of hybrid thin films considering the azo-dye methyl red (MR) incorporated with graphene accommodated in polyethylene oxide (PEO). The kinetics of photoisomerization and time-evolution of hybrid thin films were investigated using UV-Vis s and FTIR spectroscopies, as well as appropriate models developed with new analytical methods. The existence of azo-dye MR in the complex is crucial for the resource action of the trans cis cycles through UV-illumination Visible-illumination relaxations. The results of the UV–Vis and the FTIR investigations prove the cyclical trans cis-states. Consequently, PEO-(MR-Graphene) hybrid composite thin films can be introduced as possible applicants for photochromic molecular switches, light-gated transistors, and molecular solar thermal energy storage media.