Molecular Solar Thermal Energy Research Articles

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.

Engineering of Norbornadiene/Quadricyclane Photoswitches for Molecular Solar Thermal Energy Storage Applications.

ConspectusRenewable energy resources are mostly intermittent and not evenly distributed geographically; for this reason, the development of new technologies for energy storage is in high demand.Molecules that undergo photoinduced isomerization reactions that are capable of absorbing light, storing it as chemical energy, and releasing it as thermal energy on demand are referred to as molecular solar thermal energy storage (MOST) or solar thermal fuels (STF). Such molecules offer a promising solution for solar energy storage applications. Different molecular systems have been investigated for MOST applications, such as norbornadienes, azobenzenes, stilbenes, ruthenium derivatives, anthracenes, and dihydroazulenes. The polycyclic strained molecule norbornadiene (NBD), which photoconverts to quadricyclane (QC), is of great interest because it has a high energy storage density and the potential to store energy for a very long time. Unsubstituted norbornadiene has some limitations in this regard, such as poor solar spectrum match and low quantum yield. In the past decade, our group has developed and tested new NBD systems with improved characteristics. Moreover, we have demonstrated their function in laboratory-scale test devices for solar energy harnessing, storage, and release.This Account describes the most impactful recent findings on how to engineer key properties of the NBD/QC system (photochemistry, energy storage, heat release, stability, and synthesis) as well as examples of test devices for solar energy capture and heat release. While it was known that introducing donor–acceptor groups allows for a red-shifted absorption that better matches the solar spectrum, we managed to introduce donor and acceptor groups with very low molecular weight, which allowed for an unprecedented solar spectrum match combined with high energy density. Strategic steric hindrance in some of these systems dramatically increases the storage time of the photoisomer QC, and dimeric systems have independent energies barriers that lead to an improved solar spectrum match, prolonged storage times, and higher energy densities. These discoveries offer a toolbox of possible chemical modifications that can be used to tune the properties of NBD/QC systems and make them suitable for the desired applications, which can be useful for anyone wanting to take on the challenge of designing efficient MOST systems.Several test devices have been built, for example, a hybrid MOST device that stores sunlight energy and heat water at the same time. Moreover, we developed a device for monitoring catalyzed QC to NBD conversion resulting in the possibility to quantify a significant macroscopic heat generation. Finally, we tested different formulations of polymeric composites that can absorb light during the day and release the energy as heat during the night for possible use in future window coating applications. These lab-scale realizations are formative and contribute to pushing the field forward toward the real-life application of MOST systems.

Photo- and Collision-Induced Isomerization of a Charge-Tagged Norbornadiene-Quadricyclane System.

Molecular photoswitches based on the norbornadiene–quadricylane (NBD–QC) couple have been proposed as key elements of molecular solar thermal energy storage schemes. To characterize the intrinsic properties of such systems, reversible isomerization of a charge-tagged NBD–QC carboxylate couple is investigated in a tandem ion mobility mass spectrometer, using light to induce intramolecular [2 + 2] cycloaddition of NBD carboxylate to form the QC carboxylate and driving the back reaction with molecular collisions. The NBD carboxylate photoisomerization action spectrum recorded by monitoring the QC carboxylate photoisomer extends from 290 to 360 nm with a maximum at 315 nm, and in the longer wavelength region resembles the NBD carboxylate absorption spectrum recorded in solution. Key structural and photochemical properties of the NBD–QC carboxylate system, including the gas-phase absorption spectrum and the energy storage capacity, are determined through computational studies using density functional theory.

Donor-Acceptor Substituted Benzo-, Naphtho- and Phenanthro-Fused Norbornadienes

The photochromic norbornadiene/quadricyclane (NBD/QC) couple has found interest as a molecular solar thermal energy (MOST) system for storage of solar energy. To increase the energy difference between the two isomers, we present here the synthesis of a selection of benzo-fused NBD derivatives that contain an aromatic unit, benzene, naphthalene or phenanthrene, fused to one of the NBD double bonds, while the carbon atoms of the other double bond are functionalized with donor and acceptor groups. The synthesis protocols involve functionalization of benzo-fused NBDs with bromo/chloro substituents followed by a subjection of these intermediates to a cyanation reaction (introducing a cyano acceptor group) followed by a Sonogashira coupling (introducing an arylethynyl donor group, -C≡CC6H4NMe2 or -C≡CC6H4OMe). While the derivatives have good absorption properties in the visible region (redshifted relative to parent system) in the context of MOST applications, they lack the ability to undergo NBD-to-QC photoisomerization, even in the presence of a photosensitizer. It seems that loss of aromaticity of the fused aromatics is too significant to allow photoisomerization to occur. The concept of destroying aromaticity of a neighboring moiety as a way to enhance the energy density of the NBD/QC couple thus needs further structural modifications, in the quest for optimum MOST systems.

Molecular solar thermal energy storage properties of photochromic molecules physisorbed onto nanoparticles

In this study, we present the changes in molecular solar thermal energy storage properties of the 3-amino-substituted-dihydroazulene/vinylheptafulvene photo- and thermochromic system induced by physisorption of the system onto different nanoparticles. The calculations are carried out with a combined quantum mechanical/molecular mechanical method where the molecules are described by density functional theory. The nanoparticles are represented using molecular mechanics.We find that both the energy storage capability and back reaction barrier for the system can be influenced by the nanoparticles. The influence depends on the type of nanoparticle, the orientation of the molecular system, and the molecule-cluster separation.

Intermolecular London Dispersion Interactions of Azobenzene Switches for Tuning Molecular Solar Thermal Energy Storage Systems.

The performance of molecular solar thermal energy storage systems (MOST) depends amongst others on the amount of energy stored. Azobenzenes have been investigated as high-potential materials for MOST applications. In the present study it could be shown that intermolecular attractive London dispersion interactions stabilize the (E)-isomer in bisazobenzene that is linked by different alkyl bridges. Differential scanning calorimetry (DSC) measurements revealed, that this interaction leads to an increased storage energy per azo-unit of more than 3 kcal/mol compared to the parent azobenzene. The origin of this effect has been supported by computation as well as X-ray analysis. In the solid state structure attractive London dispersion interactions between the C-H of the alkyl bridge and the π-system of the azobenzene could be clearly assigned. This concept will be highly useful in designing more effective MOST systems in the future.

Surface chemistry of 2,3-dibromosubstituted norbornadiene/quadricyclane as molecular solar thermal energy storage system on Ni(111).

Dwindling fossil fuels force humanity to search for new energy production routes. Besides energy generation, its storage is a crucial aspect. One promising approach is to store energy from the sun chemically in strained organic molecules, so-called molecular solar thermal (MOST) systems, which can release the stored energy catalytically. A prototypical MOST system is norbornadiene/quadricyclane (NBD/QC) whose energy release and surface chemistry need to be understood. Besides important key parameters such as molecular weight, endergonic reaction profiles, and sufficient quantum yields, the position of the absorption onset of NBD is crucial to cover preferably a large range of sunlight's spectrum. For this purpose, one typically derivatizes NBD with electron-donating and/or electron-accepting substituents. To keep the model system simple enough to be investigated with photoemission techniques, we introduced bromine atoms at the 2,3-position of both compounds. We study the adsorption behavior, energy release, and surface chemistry on Ni(111) using high-resolution X-ray photoelectron spectroscopy (HR-XPS), UV photoelectron spectroscopy, and density functional theory calculations. Both Br2-NBD and Br2-QC partially dissociate on the surface at ∼120 K, with Br2-QC being more stable. Several stable adsorption geometries for intact and dissociated species were calculated, and the most stable structures are determined for both molecules. By temperature-programmed HR-XPS, we were able to observe the conversion of Br2-QC to Br2-NBD in situ at 170 K. The decomposition of Br2-NBD starts at 190 K when C-Br bond cleavage occurs and benzene and methylidene are formed. For Br2-QC, the cleavage already occurs at 130 K when cycloreversion to Br2-NBD sets in.

Applications of Photoswitches in the Storage of Solar Energy

AbstractPhotoswitches are organic or organometallic chromophores that undergo a reversible chemical transformation upon absorption of light. Among the most commonly studied photoswitches are stilbenes and azobenzenes, capable of efficient interconversion between cis and trans isomers. When one isomer is significantly less thermodynamically stable than the other, photoisomerization of the stable to the metastable isomer converts a fraction of the absorbed photon energy into excess free energy (chemical potential). If the metastable isomer is sufficiently inert at room temperature, its photoconversion provides a means of storing solar energy, which is recovered by triggering heat‐releasing thermal conversion of the metastable to the stable isomer. In other words, such a photoswitch acts as a battery that captures solar energy, stores it as chemical potential and releases it on demand as heat. This process is known as molecular solar thermal energy storage or a molecular solar thermal battery. Unlike the more established conventional solar thermal storage, which uses sunlight to heat, melt or vaporize material, molecular solar thermal energy storage does not require thermal insulation to prevent discharge but relies on the kinetic activation barrier separating the two isomers. Unlike solar‐to‐chemical energy conversion by photosplitting of H2O or photoreduction of CO2, which comprise open‐system cycles, photoswitches are thermodynamically closed storage media. Successful deployment of molecular solar thermal energy storage requires new photoswitches that combine a seemingly contradictory set of molecular parameters: a large difference in the free energies of the two isomers separated by a large kinetic barrier; a high quantum yield of photogeneration of the metastable isomer that itself is either photochemically inactive or transparent to sunlight; highly selective isomerizations that allow many charge/discharge cycles without accumulation of side‐products even at high discharge temperatures. While the optimal photoswitch for molecular solar thermal energy storage remains to be invented, a large body of empirical observations acquired in the past decade provides several potentially valuable starting points for such a search.

Molecular Solar Thermal Energy Storage Systems with Long Discharge Times Based on the Dihydroazulene/Vinylheptafulvene Couple

Molecular solar thermal energy storage (MOST) systems based on photochromic molecules that undergo photoisomerization to high‐energy isomers are attractive for storage of solar energy in a closed‐energy cycle. One challenge is to control the discharge time of the high‐energy isomer. Here we show that incorporation of a strong acceptor substituent in the seven‐membered ring of the dihydroazulene/vinylheptafulvene (DHA/VHF) couple increases the half‐life of the energy‐releasing VHF‐to‐DHA back‐reaction from hours to more than a day in a polar solvent. For some derivatives, the absorption maximum of the photo‐active DHA is also significantly redshifted, thereby better matching the solar spectrum. Synthetic protocols and kinetics studies are presented together with a computational study of the energy densities of the systems and excitation spectra. The computations show that the increased lifetime of the high‐energy isomer is counter‐balanced by a lower energy storage capacity in vacuo than for the parent system, but a slightly higher energy density than for the parent system in a polar solvent.

Controlled Catalytic Energy Release of the Norbornadiene/Quadricyclane Molecular Solar Thermal Energy Storage System on Ni(111)

We have investigated the surface chemistry of the molecular solar thermal energy storage system of the valence isomer pair norbornadiene (NBD)/quadricyclane (QC) on Ni(111). Our multimethod approach includes UV-photoelectron spectroscopy (UPS), high-resolution X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS), and density functional theory (DFT) calculations. The NBD/QC system holds the potential to be utilized in future energy storage technologies due to its comparably high gravimetric energy storage density, and the release of energy in a catalytic and sustainable cycle. UPS shows molecular adsorption of both compounds at 120 K, as is also predicted by DFT. NEXAFS and DFT suggest an adsorption geometry of NBD with both double bonds binding to the surface (η2:η2). For QC, no preference is found, and both the η2:η2 and the η2:η1 adsorption geometry are stable. The conversion of QC to NBD is thermally activated. From UPS, a reaction temperature of ∼175 K is determin...

Molecular solar thermal energy storage in photoswitch oligomers increases energy densities and storage times

Molecular photoswitches can be used for solar thermal energy storage by photoisomerization into high-energy, meta-stable isomers; we present a molecular design strategy leading to photoswitches with high energy densities and long storage times. High measured energy densities of up to 559 kJ kg−1 (155 Wh kg−1), long storage lifetimes up to 48.5 days, and high quantum yields of conversion of up to 94% per subunit are demonstrated in norbornadiene/quadricyclane (NBD/QC) photo-/thermoswitch couples incorporated into dimeric and trimeric structures. By changing the linker unit between the NBD units, we can at the same time fine-tune light-harvesting and energy densities of the dimers and trimers so that they exceed those of their monomeric analogs. These new oligomers thereby meet several of the criteria to be met for an optimum molecule to ultimately enter actual devices being able to undergo closed cycles of solar light-harvesting, energy storage, and heat release.

Evaluating Dihydroazulene/Vinylheptafulvene Photoswitches for Solar Energy Storage Applications

AbstractInvited for this month′s cover are the groups of Kasper Moth‐Poulsen at Chalmers University of Technology and Mogens Brøndsted Nielsen at University of Copenhagen. The cover image shows a conceptual graphic of molecular solar thermal energy storage system (MOST) for outdoor facilities. The Full Paper itself is available at 10.1002/cssc.201700679.

Evaluating Dihydroazulene/Vinylheptafulvene Photoswitches for Solar Energy Storage Applications.

Efficient solar energy storage is a key challenge in striving toward a sustainable future. For this reason, molecules capable of solar energy storage and release through valence isomerization, for so‐called molecular solar thermal energy storage (MOST), have been investigated. Energy storage by photoconversion of the dihydroazulene/vinylheptafulvene (DHA/VHF) photothermal couple has been evaluated. The robust nature of this system is determined through multiple energy storage and release cycles at elevated temperatures in three different solvents. In a nonpolar solvent such as toluene, the DHA/VHF system can be cycled more than 70 times with less than 0.01 % degradation per cycle. Moreover, the [Cu(CH3CN)4]PF6‐catalyzed conversion of VHF into DHA was demonstrated in a flow reactor. The performance of the DHA/VHF couple was also evaluated in prototype photoconversion devices, both in the laboratory by using a flow chip under simulated sunlight and under outdoor conditions by using a parabolic mirror. Device experiments demonstrated a solar energy storage efficiency of up to 0.13 % in the chip device and up to 0.02 % in the parabolic collector. Avenues for future improvements and optimization of the system are also discussed.

Exploring the potential of a hybrid device combining solar water heating and molecular solar thermal energy storage

A hybrid solar energy system consisting of a molecular solar thermal energy storage system (MOST) combined with a solar water heating system (SWH) is presented.

Tuning the photochemical properties of the fulvalene-tetracarbonyl-diruthenium system.

In a Molecular Solar-Thermal Energy Storage (MOST) system, solar energy is converted to chemical energy using a compound that undergoes reversible endothermic photoisomerization. The high-energy photoisomer can later be converted back to the parent compound and the excess energy is released as heat. One of the most studied MOST systems is based on fulvalene-tetracarbonyl-diruthenium, and this paper demonstrates, for the first time, the possibility to tune the photochemical properties of this system by positive steric hindrance working on the fulvalene unit.

Exploring the potential of fulvalene dimetals as platforms for molecular solar thermal energy storage: computations, syntheses, structures, kinetics, and catalysis.

A study of the scope and limitations of varying the ligand framework around the dinuclear core of FvRu2 in its function as a molecular solar thermal energy storage framework is presented. It includes DFT calculations probing the effect of substituents, other metals, and CO exchange for other ligands on ΔHstorage . Experimentally, the system is shown to be robust in as much as it tolerates a number of variations, except for the identity of the metal and certain substitution patterns. Failures include 1,1',3,3'-tetra-tert-butyl (4), 1,2,2',3'-tetraphenyl (9), diiron (28), diosmium (24), mixed iron-ruthenium (27), dimolybdenum (29), and ditungsten (30) derivatives. An extensive screen of potential catalysts for the thermal reversal identified AgNO3 -SiO2 as a good candidate, although catalyst decomposition remains a challenge.

Diaryl-substituted norbornadienes with red-shifted absorption for molecular solar thermal energy storage

Red-shifting the absorption of norbornadienes (NBDs), into the visible region, enables the photo-isomerization of NBDs to quadricyclanes (QCs) to be driven by sunlight. This is necessary in order to utilize the NBD-QC system for molecular solar thermal (MOST) energy storage. Reported here is a study on five diaryl-substituted norbornadienes. The introduced aryl-groups induce a significant red-shift of the UV/vis absorption spectrum of the norbornadienes, and device experiments using a solar-simulator set-up demonstrate the potential use of these compounds for MOST energy storage.