Solar chemical

Last updated December 28, 2024

Solar chemical refers to a number of possible processes that harness solar energy by absorbing sunlight in a chemical reaction. The idea is conceptually similar to photosynthesis in plants, which converts solar energy into the chemical bonds of glucose molecules, but without using living organisms, which is why it is also called artificial photosynthesis.^[1]

A promising approach is to use focused sunlight to provide the energy needed to split water into its constituent hydrogen and oxygen in the presence of a metallic catalyst such as zinc. This is normally done in a two-step process so that hydrogen and oxygen are not produced in the same chamber, which creates an explosion hazard. Another approach involves taking the hydrogen created in this process and combining it with carbon dioxide to create methane. The benefit of this approach is that there is an established infrastructure for transporting and burning methane for power generation, which is not true for hydrogen. One main drawback to both of these approaches is common to most methods of energy storage: adding an extra step between energy collection and electricity production drastically decreases the efficiency of the overall process.

Background

As early as 1909, the dimerization of anthracene into dianthracene was investigated as a means of storing solar energy, as well as the photodimerization of the naphthalene series.^[2] In the 1970s and 1980s a fuel had been made from another reversible chemical, the norbornadiene to quadricyclane transformation cycle, which failed because the reversal process had a low potential. Ruthenium-based molecules were also attempted but dismissed because ruthenium is both rare and too heavy of a material.^[3] In the past decade,^{[ as of? ]} a new hybrid nanostructure was theorized as a new approach to the previously known concept of solar energy storage.

Chemical storage

Photodimerization is the light induced formation of dimers and photoisomerization is the light induced formation of isomers. While photodimerization stores the energy from sunlight in new chemical bonds, photoisomerization stores solar energy by reorienting existing chemical bonds into a higher energy configuration.

In order for an isomer to store energy then, it must be metastable as shown above. This results in a trade-off between the stability of the fuel isomer and how much energy must be put in to reverse the reaction when it is time to use the fuel. The isomer stores energy as strain energy in its bonds. The more strained the bonds are the more energy they can store, but the less stable the molecule is. The activation energy, Ea, is used to characterize how easy or hard it is for the reaction to proceed. If the activation energy is too small the fuel will tend to spontaneously move to the more stable state, providing limited usefulness as a storage medium. However, if the activation energy is very large, the energy expended to extract the energy from the fuel will effectively reduce the amount of energy that the fuel can store. Finding a useful molecule for a solar fuel requires finding the proper balance between the yield, the light absorption of the molecule, the stability of the molecule in the metastable state, and how many times the molecule can be cycled without degrading.

Various ketones, azepines and norbornadienes among other compounds, such as azobenzene and its derivatives, have been investigated as potential energy storing isomers.^[4] The norbornadiene-quadricyclane couple and its derivatives have been extensively investigated for solar energy storage processes. Norbornadiene is converted to quadricyclane using energy extracted from sunlight, and the controlled release of the strain energy stored in quadricyclane (about 110 kJ/mole) as it relaxes back to norbornadiene allows the energy to be extracted again for use later.

Research into both the azobenzene and norbonadiene-quadricyclane systems was abandoned in the 1980s as unpractical due to problems with degradation, instability, low energy density, and cost.^[5] With recent advances in computing power though, there has been renewed interest in finding materials for solar thermal fuels. In 2011, researchers at Massachusetts Institute of Technology (MIT) used time-dependent density functional theory, which models systems at an atomic level, to design a system composed of azobenzene molecules bonded to carbon nanotube (CNT) templates. The CNT substrates will allow customizable interactions between neighboring molecules which greatly helps in fine tuning the properties of the fuel, for example an increase in the amount of energy stored.^[3] Through experimental procedures, researchers were able to get the first proof of principle that the hybrid nanostructure works as a functional thermal fuel. Azobenzenes have the advantage of absorbing wavelengths that are very abundant in sunlight. When this happens, the molecule transforms from a trans-isomer to a cis-isomer which has a higher energy state of about 0.6 eV.^[5] To bring the molecule back down to its original state, i.e. release the energy it had collected, there are a few options. The first is to apply heat but that is associated with a cost which, relative to the amount of heat that will be produced from the release, is not cost-efficient. The second, more effective option is to use a catalyst that lowers the thermal barrier and allows the heat to be released, almost like a switch.^[6] The transition back from cis to trans can also be triggered by blue visible light.

This system provides an energy density comparable to lithium-ion batteries, while simultaneously increasing the stability of the activated fuel from several minutes to more than a year and allowing for large numbers of cycles without significant degradation.^[3] Further research on greater improvement is being done by examining different possible combinations of substrates and photoactive molecules.

Applications

There are a wide variety of both potential and current applications for solar chemical fuels. One significant benefit of the technology is its scalability. Since the energy can be stored and then later converted to heat when needed, it is ideal for smaller on-the-go units. Such units range from portable stoves or small personal heaters that can be charged in the sun to providing medical sanitation in off-grid areas. There are also plans to use the system developed at MIT as a window de-icing system in automobiles. It also has the ability to be scaled up and heat larger homes or buildings or even bodies of water. A solar thermal fuel would ideally be able to cycle indefinitely without degradation, making it ideal for larger-scale implementations that generally would need more replacements of other forms of storage.

Related Research Articles

Energy is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat and light. Energy is a conserved quantity—the law of conservation of energy states that energy can be converted in form, but not created or destroyed. The unit of measurement for energy in the International System of Units (SI) is the joule (J).

<span class="mw-page-title-main">Energy storage</span> Captured energy for later usage

Energy storage is the capture of energy produced at one time for use at a later time to reduce imbalances between energy demand and energy production. A device that stores energy is generally called an accumulator or battery. Energy comes in multiple forms including radiation, chemical, gravitational potential, electrical potential, electricity, elevated temperature, latent heat and kinetic. Energy storage involves converting energy from forms that are difficult to store to more conveniently or economically storable forms.

Solar energy is the radiant energy from the Sun's light and heat, which can be harnessed using a range of technologies such as solar electricity, solar thermal energy and solar architecture. It is an essential source of renewable energy, and its technologies are broadly characterized as either passive solar or active solar depending on how they capture and distribute solar energy or convert it into solar power. Active solar techniques include the use of photovoltaic systems, concentrated solar power, and solar water heating to harness the energy. Passive solar techniques include designing a building for better daylighting, selecting materials with favorable thermal mass or light-dispersing properties, and organize spaces that naturally circulate air.

Liquid hydrogen (H₂(l)) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H₂ form.

Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet, visible (400–750 nm), or infrared radiation (750–2500 nm).

Azobenzene is a photoswitchable chemical compound composed of two phenyl rings linked by a N=N double bond. It is the simplest example of an aryl azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of similar compounds. These azo compounds are considered as derivatives of diazene (diimide), and are sometimes referred to as 'diazenes'. The diazenes absorb light strongly and are common dyes. Different classes of azo dyes exist, most notably the ones substituted with heteroaryl rings.

In chemistry, photoisomerization is a form of isomerization induced by photoexcitation. Both reversible and irreversible photoisomerizations are known for photoswitchable compounds. The term "photoisomerization" usually, however, refers to a reversible process.

Energy transformation, also known as energy conversion, is the process of changing energy from one form to another. In physics, energy is a quantity that provides the capacity to perform work or moving or provides heat. In addition to being converted, according to the law of conservation of energy, energy is transferable to a different location or object, but it cannot be created or destroyed.

In physics, energy density is the quotient between the amount of energy stored in a given system or contained in a given region of space and the volume of the system or region considered. Often only the useful or extractable energy is measured. It is sometimes confused with stored energy per unit mass, which is called specific energy or gravimetric energy density.

Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen:

Thermal decomposition, or thermolysis, is a chemical decomposition of a substance caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition. If decomposition is sufficiently exothermic, a positive feedback loop is created producing thermal runaway and possibly an explosion or other chemical reaction. Thermal decomposition is a chemical reaction where heat is a reactant. Since heat is a reactant, these reactions are endothermic meaning that the reaction requires thermal energy to break the chemical bonds in the molecule.

Thermal energy storage (TES) is the storage of thermal energy for later reuse. Employing widely different technologies, it allows surplus thermal energy to be stored for hours, days, or months. Scale both of storage and use vary from small to large – from individual processes to district, town, or region. Usage examples are the balancing of energy demand between daytime and nighttime, storing summer heat for winter heating, or winter cold for summer cooling. Storage media include water or ice-slush tanks, masses of native earth or bedrock accessed with heat exchangers by means of boreholes, deep aquifers contained between impermeable strata; shallow, lined pits filled with gravel and water and insulated at the top, as well as eutectic solutions and phase-change materials.

<span class="mw-page-title-main">Photochromism</span> Reversible chemical transformation by absorption of electromagnetic radiation

Photochromism is the reversible change of color upon exposure to light. It is a transformation of a chemical species (photoswitch) between two forms by the absorption of electromagnetic radiation (photoisomerization), where the two forms have different absorption spectra.

<span class="mw-page-title-main">Norbornadiene</span> Chemical compound

Norbornadiene is an organic compound and a bicyclic hydrocarbon. Norbornadiene is of interest as a metal-binding ligand, whose complexes are useful for homogeneous catalysis. It has been intensively studied owing to its high reactivity and distinctive structural property of being a diene that cannot isomerize. Norbornadiene is also a useful dienophile in Diels-Alder reactions.

Quadricyclane is a strained, multi-cyclic hydrocarbon with the formula CH₂(CH)₆. A white volatile colorless liquid, it is highly strained molecule (78.7 kcal/mol). Isomerization of quadricyclane proceeds slowly at low temperatures. Because of quadricyclane's strained structure and thermal stability, it has been studied extensively.

A photoswitch is a type of molecule that can change its structural geometry and chemical properties upon irradiation with electromagnetic radiation. Although often used interchangeably with the term molecular machine, a switch does not perform work upon a change in its shape whereas a machine does. However, photochromic compounds are the necessary building blocks for light driven molecular motors and machines. Upon irradiation with light, photoisomerization about double bonds in the molecule can lead to changes in the cis- or trans- configuration. These photochromic molecules are being considered for a range of applications.

The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few.

Photoelectrochemical processes are processes in photoelectrochemistry; they usually involve transforming light into other forms of energy. These processes apply to photochemistry, optically pumped lasers, sensitized solar cells, luminescence, and photochromism.

A solar fuel is a synthetic fuel produced using solar energy, through photochemical, photobiological, electrochemical, or thermochemical methods. Sunlight is the primary energy source, with its radiant energy being transduced to chemical energy stored in bonds, typically by reducing protons to hydrogen, or carbon dioxide to organic compounds.

References

↑ Magnuson, A; et al. (2009). "Biomimetic and Microbial Approaches to Solar Fuel Generation". Accounts of Chemical Research. 42 (12): 1899–1908. doi:10.1021/ar900127h. PMID 19757805.
↑ Bolton, James (1977). Solar Power and Fuels. Academic Press, Inc. ISBN 978-0-12-112350-5., p. 235-237
1 2 3 Kolpak, Alexie; Jeffrey Grossman (2011). "Azobenzene-Functionalized Carbon Nanotubes As High-Energy Density Solar Thermal Fuels". Nano Letters. 11 (8): 3156–3162. Bibcode:2011NanoL..11.3156K. doi:10.1021/nl201357n. PMID 21688811.
↑ Bolton, James (1977). Solar Power and Fuels. Academic Press, Inc. ISBN 978-0-12-112350-5., p. 238-240
1 2 Durgan, E.; Jeffrey Grossman (4 March 2013). "Photoswitchable molecular rings for solar-thermal energy storage". Journal of Physical Chemistry Letters. 4 (6): 854–860. CiteSeerX 10.1.1.707.1787 . doi:10.1021/jz301877n. PMID 26291346.
↑ "Materials Processing Center" . Retrieved 2017-08-09.

External links

ANU Thermochemical energy storage system - Australian National University, Canberra.
Laboratory for Solar Technology - Paul Scherrer Institute, Villigen, Switzerland.
Power & Energy Magazine, March 2004 article on Paul Scherrer Institute work
Solar Chemistry Project Plataforma Solar de Almería, Spain,
Isracast - Israel,
Hydrogen Solar- UK.

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[1] Magnuson, A; et al. (2009). "Biomimetic and Microbial Approaches to Solar Fuel Generation". Accounts of Chemical Research. 42 (12): 1899–1908. doi:10.1021/ar900127h. PMID 19757805.

[2] Bolton, James (1977). Solar Power and Fuels. Academic Press, Inc. ISBN 978-0-12-112350-5., p. 235-237

[:0-3] 1 2 3 Kolpak, Alexie; Jeffrey Grossman (2011). "Azobenzene-Functionalized Carbon Nanotubes As High-Energy Density Solar Thermal Fuels". Nano Letters. 11 (8): 3156–3162. Bibcode:2011NanoL..11.3156K. doi:10.1021/nl201357n. PMID 21688811.

[4] Bolton, James (1977). Solar Power and Fuels. Academic Press, Inc. ISBN 978-0-12-112350-5., p. 238-240

[Durgan_854–860-5] 1 2 Durgan, E.; Jeffrey Grossman (4 March 2013). "Photoswitchable molecular rings for solar-thermal energy storage". Journal of Physical Chemistry Letters. 4 (6): 854–860. CiteSeerX 10.1.1.707.1787 . doi:10.1021/jz301877n. PMID 26291346.

[6] "Materials Processing Center" . Retrieved 2017-08-09.

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