Photocatalytic water splitting

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Photocatalytic_water_splitting

Photocatalytic water splitting is a process that uses photocatalysis for the dissociation of water (H₂O) into hydrogen (H
₂) and oxygen (O
₂). Only light energy (photons), water, and a catalyst(s) are needed, since this is what naturally occurs in natural photosynthetic oxygen production and CO₂ fixation.

Hydrogen fuel production using water and light (photocatalytic water splitting), instead of petroleum, is an important renewable energy strategy.

Concepts

2 mol H₂O is split into 1 mol O
₂ and 2 mol H
₂ using light in the process shown below.

${\displaystyle \begin{array}{c}E>1.23\text{eV}\\ 2{\text{H}}_2^{}\text{O}\stackrel{-\rightharpoonup }{\leftharpoondown -}2{\text{H}}_2^{}+{\text{O}}_2^{}\\ (E\text{: photon energy.})\end{array}}$

The process of water-splitting is a highly endothermic process (ΔH > 0). Water splitting occurs naturally in photosynthesis when the energy of four photons is absorbed and converted into chemical energy through a complex biochemical pathway (Dolai's or Kok's S-state diagrams).

O–H bond homolysis in water requires energy of 6.5 - 6.9 eV (UV photon). Infrared light has sufficient energy to mediate water splitting because it technically has enough energy for the net reaction. However, it does not have enough energy to mediate the elementary reactions leading to the various intermediates involved in water splitting (this is why there is still water on Earth). Nature overcomes this challenge by absorbing four visible photons. In the laboratory, this challenge is typically overcome by coupling the hydrogen production reaction with a sacrificial reductant other than water.

Materials used in photocatalytic water splitting fulfill the band requirements and typically have dopants and/or co-catalysts added to optimize their performance. A sample semiconductor with the proper band structure is titanium dioxide (TiO
₂) and is typically used with a co-catalyst such as platinum (Pt) to increase the rate of H
₂ production. A major problem in photocatalytic water splitting is photocatalyst decomposition and corrosion.

Method of evaluation

Photocatalysts must conform to several key principles in order to be considered effective at water splitting. A key principle is that H
₂ and O
₂ evolution should occur in a stoichiometric 2:1 ratio; significant deviation could be due to a flaw in the experimental setup and/or a side reaction, neither of which indicate a reliable photocatalyst for water splitting. The prime measure of photocatalyst effectiveness is quantum yield (QY), which is:

QY (%) = (Photochemical reaction rate) / (Photon absorption rate) × 100%

To assist in comparison, the rate of gas evolution can also be used. A photocatalyst that has a high quantum yield and gives a high rate of gas evolution is a better catalyst.

The other important factor for a photocatalyst is the range of light that is effective for operation. For example, a photocatalyst is more desirable to use visible photons than UV photons.

Photocatalysts

The solar-to-hydrogen (STH) efficiency of photocatalytic water splitting, however, has remained very low. Here we have developed a strategy to achieve a high STH efficiency of 9.2 per cent using pure water, concentrated solar light and an indium gallium nitride photocatalyst. The success of this strategy originates from the synergistic effects of promoting forward hydrogen–oxygen evolution and inhibiting the reverse hydrogen–oxygen recombination by operating at an optimal reaction temperature (about 70 degrees Celsius), which can be directly achieved by harvesting the previously wasted infrared light in sunlight. Moreover, this temperature-dependent strategy also leads to an STH efficiency of about 7 per cent from widely available tap water and sea water and an STH efficiency of 6.2 per cent in a large-scale photocatalytic water-splitting system with a natural solar light capacity of 257 watts. Our study offers a practical approach to produce hydrogen fuel efficiently from natural solar light and water, overcoming the efficiency bottleneck of solar hydrogen production.

NaTaO
₃:La

NaTaO
₃:La yielded the highest water splitting rate of photocatalysts without using sacrificial reagents. This ultraviolet-based photocatalyst was reported to show water splitting rates of 9.7 mmol/h and a quantum yield of 56%. The nanostep structure of the material promotes water splitting as edges functioned as H
₂ production sites and the grooves functioned as O
₂ production sites. Addition of NiO particles as co-catalysts assisted in H
₂ production; this step used an impregnation method with an aqueous solution of Ni(NO
₃)
₂•6H
₂O and evaporated the solution in the presence of the photocatalyst. NaTaO
₃ has a conduction band higher than that of NiO, so photo-generated electrons are more easily transferred to the conduction band of NiO for H
₂ evolution.

K
₃Ta
₃B
₂O
₁₂

K
₃Ta
₃B
₂O
₁₂ is another catalyst solely activated by UV and above light. It does not have the performance or quantum yield of NaTaO
₃:La. However, it can split water without the assistance of co-catalysts and gives a quantum yield of 6.5%, along with a water splitting rate of 1.21 mmol/h. This ability is due to the pillared structure of the photocatalyst, which involves TaO
₆ pillars connected by BO
₃ triangle units. Loading with NiO did not assist the photocatalyst due to the highly active H
₂ evolution sites.

(Ga
_.82Zn
_.18)(N
_.82O
_.18)

(Ga
_.82Zn
_.18)(N
_.82O
_.18) had the highest quantum yield in visible light for visible light-based photocatalysts that do not utilize sacrificial reagents as of October 2008. The photocatalyst featured a quantum yield of 5.9% and a water splitting rate of 0.4 mmol/h. Tuning the catalyst was done by increasing calcination temperatures for the final step in synthesizing the catalyst. Temperatures up to 600 °C helped to reduce the number of defects, while temperatures above 700 °C destroyed the local structure around zinc atoms and were thus undesirable. The treatment ultimately reduced the amount of surface Zn and O defects, which normally function as recombination sites, thus limiting photocatalytic activity. The catalyst was then loaded with Rh
_2-yCr
_yO
₃ at a rate of 2.5 wt% Rh and 2 wt% Cr for better performance.

Molecular catalysts

Proton reduction catalysts based on earth-abundant elements carry out one side of the water-splitting half-reaction.

A mole of octahedral nickel(II) complex, [Ni(bztpen)]²⁺ (bztpen = N-benzyl-N,N’,N’-tris(pyridine-2-ylmethyl)ethylenediamine) produced 308,000 moles of hydrogen over 60 hours of electrolysis with an applied potential of -1.25 V vs. standard hydrogen electrode.

Cobalt-based photocatalysts have been reported, including tris(bipyridine) cobalt(II), compounds of cobalt ligated to certain cyclic polyamines, and some cobaloximes.

In 2014 researchers announced an approach that connected a chromophore to part of a larger organic ring that surrounded a cobalt atom. The process is less efficient than a platinum catalyst although cobalt is less expensive, potentially reducing costs. The process uses one of two supramolecular assemblies based on Co(II)-templated coordination of Ru(bpy)+32 (bpy = 2,2′-bipyridyl) analogues as photosensitizers and electron donors to a cobaloxime macrocycle. The Co(II) centers of both assemblies are high spin, in contrast to most previously described cobaloximes. Transient absorption optical spectroscopies indicate that charge recombination occurs through multiple ligand states within the photosensitizer modules.

Bismuth vanadate

Bismuth vanadate is a visible-light-driven photocatalyst with a bandgap of 2.4 eV. BV have demonstrated efficiencies of 5.2% for flat thin films and 8.2% for core-shell WO₃@BiVO₄ nanorods with thin absorbers.

Bismuth oxides

Bismuth oxides are characterized by visible light absorption properties, just like vanadates.

Tungsten diselenide (WSe₂)

Tungsten diselenide has photocatalytic properties that might be a key to more efficient electrolysis.

III-V semiconductor systems

Systems based on III-V semiconductors, such as InGaP, enable solar-to-hydrogen efficiencies of up to 14%. Challenges include long-term stability and cost.

2D semiconductor systems

2-dimensional semiconductors such as MoS
₂ are actively researched as potential photocatalysts.

Aluminum‐based metal-organic frameworks (MOF)

An aluminum‐based metal-organic framework (MOF) made from 2‐aminoterephthalate can be modified by incorporating Ni²⁺ cations into the pores through coordination with the amino groups. Molybdenum disulfide

Porous organic polymers (POPs)

Organic semiconductor photocatalysts, in particular porous organic polymers (POPs), attracted attention due to their low cost, low toxicity, and tunable light absorption vs inorganic counterparts. They display hHigh porosity, low density, diverse composition, facile functionalization, high chemical/thermal stability, as well as high surface areas. Efficient conversion of hydrophobic polymers into hydrophilic polymer nano-dots (Pdots) increased polymer-water interfacial contact, which significantly improved performance.

Ansa-Titanocene(III/IV) Triflate Complexes

Beweries, et. al., developed a light-driven "losed cycle of water splitting using ansa-titanocene(III/IV) triflate complexes".

Indium gallium nitride

An Indium gallium nitride (InxGa1-xN) photocatalyst achieved a solar-to-hydrogen efficiency of 9.2% from pure water and concentrated sunlight. The effiency is due to the synergistic effects of promoting hydrogen–oxygen evolution and inhibiting recombination by operating at an optimal reaction temperature (~70 degrees C), powered by harvesting previously wasted infrared light. An STH efficiency of about 7% was realized from tap water and seawater and efficiency of 6.2% in a larger-scale system with a solar light capacity of 257 watts.

Sacrificial reagents

Cd
_1-xZn
_xS

Solid solutions Cd
_1-xZn
_xS with different Zn concentration (0.2 < x < 0.35) have been investigated in the production of hydrogen from aqueous solutions containing ${\displaystyle {\text{SO}}_3^{2-}/{\text{S}}^{2-}}$ as sacrificial reagents under visible light. Textural, structural and surface catalyst properties were determined by N
₂ adsorption isotherms, UV–vis spectroscopy, SEM and XRD and related to the activity results in hydrogen production from water splitting under visible light. It was reported that the crystallinity and energy band structure of the Cd
_1-xZn
_xS solid solutions depend on their Zn atomic concentration. The hydrogen production rate increased gradually as Zn concentration on photocatalysts increased from 0.2 to 0.3. The subsequent increase in the Zn fraction up to 0.35 reduced production. Variation in photoactivity was analyzed for changes in crystallinity, level of the conduction band and light absorption ability of Cd
_1-xZn
_xS solid solutions derived from their Zn atomic concentration.

Solar fuel

From Wikipedia, the free encyclopedia

A solar fuel is a synthetic chemical fuel produced from photovoltaic solar energy. Solar fuels can be produced through photochemical (i.e. activation of certain chemical reactions by photons), photobiological (i.e., artificial photosynthesis), and electrochemical reactions (i.e. using the electricity from solar panels to drive a chemical reaction).

Solar fuels can also be produced by thermochemical reactions (i.e., through the use of solar heat supplied by concentrated solar thermal energy to drive a chemical reaction).

Light is used as an energy source, with solar energy being transduced to chemical energy, typically by reducing protons to hydrogen, or carbon dioxide to organic compounds.

A solar fuel can be produced and stored for later use, when sunlight is not available, making it an alternative to fossil fuels and batteries. Examples of such fuels are hydrogen, ammonia, and hydrazine. Diverse photocatalysts are being developed to carry these reactions in a sustainable, environmentally friendly way.

Overview

The world's dependence on the declining reserves of fossil fuels poses not only environmental problems but also geopolitical ones. Solar fuels, in particular hydrogen, are viewed as an alternative source of energy for replacing fossil fuels especially where storage is essential. Electricity can be produced directly from sunlight through photovoltaics, but this form of energy is rather inefficient to store compared to hydrogen. A solar fuel can be produced when and where sunlight is available, and stored and transported for later usage. This makes it much more convenient, because it can be used in situations where direct sunlight is not available.

The most widely researched solar fuels are hydrogen, because the only product of using this fuel is water, and products of photochemical carbon dioxide reduction, which are more conventional fuels like methane and propane. Upcoming research also involves ammonia and related substances (i.e. hydrazine). These can address the challenges that come with hydrogen, by being a more compact and safer way of storing hydrogen. Direct ammonia fuel cells are also being researched.

Solar fuels can be produced via direct or indirect processes. Direct processes harness the energy in sunlight to produce a fuel without intermediary energy conversions. Solar thermochemistry uses the heat of the sun directly to heat a receiver adjacent to the solar reactor where the thermochemical process is performed. In contrast, indirect processes have solar energy converted to another form of energy first (such as biomass or electricity) that can then be used to produce a fuel. Indirect processes have been easier to implement but have the disadvantage of being less efficient than the direct method. Therefore, direct methods should be considered more interesting than their less efficient counterparts. New research therefore focusses more on this direct conversion, but also in fuels that can be used immediately to balance the power grid.

Hydrogen production

See also: Hydrogen production

Photoelectrochemical

See also: Photoelectrolysis of water

 

A sample of a photoelectric cell in a lab environment. Catalysts are added to the cell, which is submerged in water and illuminated by simulated sunlight. The bubbles seen are oxygen (forming on the front of the cell) and hydrogen (forming on the back of the cell).

In a solar photoelectrochemical process, hydrogen can be produced by electrolysis. To use sunlight in this process, a photoelectrochemical cell can be used, where one photosensitized electrode converts light into an electric current that is then used for water splitting. One such type of cell is the dye-sensitized solar cell. This is an indirect process, since it produces electricity that then is used to form hydrogen. Another indirect process using sunlight is conversion of biomass to biofuel using photosynthetic organisms; however, most of the energy harvested by photosynthesis is used in life-sustaining processes and therefore lost for energy use.

A semiconductor can also be used as the photosensitizer. When a semiconductor is hit by a photon with an energy higher than the bandgap, an electron is excited to the conduction band and a hole is created in the valence band. Due to band bending, the electrons and holes move to the surface, where these charges are used to split the water molecules. Many different materials have been tested, but none so far have shown the requirements for practical application.

Photochemical

See also: Photocatalytic water splitting

In a photochemical process, the sunlight is directly used to split water into hydrogen and oxygen. Because the absorption spectrum of water does not overlap with the emission spectrum of the sun, direct dissociation of water cannot take place; a photosensitizer needs to be used. Several such catalysts have been developed as proof of concept, but not yet scaled up for commercial use; nevertheless, their relative simplicity gives the advantage of potential lower cost and increased energy conversion efficiency. One such proof of concept is the "artificial leaf" developed by Nocera and coworkers: a combination of metal oxide-based catalysts and a semiconductor solar cell produces hydrogen upon illumination, with oxygen as the only byproduct.

Photobiological

In a photobiological process, the hydrogen is produced using photosynthetic microorganisms (green microalgae and cyanobacteria) in photobioreactors. Some of these organisms produce hydrogen upon switching culture conditions; for example, Chlamydomonas reinhardtii produces hydrogen anaerobically under sulfur deprivation, that is, when cells are moved from one growth medium to another that does not contain sulfur, and are grown without access to atmospheric oxygen. Another approach was to abolish activity of the hydrogen-oxidizing (uptake) hydrogenase enzyme in the diazotrophic cyanobacterium Nostoc punctiforme, so that it would not consume hydrogen that is naturally produced by the nitrogenase enzyme in nitrogen-fixing conditions. This N. punctiforme mutant could then produce hydrogen when illuminated with visible light.

Another mutant Cyanobacteria, Synechocystis, is using genes of the bacteria Rubrivivax gelatinosus CBS to produce hydrogen. The CBS bacteria produce hydrogen through the oxidation of carbon monoxide. Researchers are working to implement these genes into the Synechocystis. If these genes can be applied, it will take some effort to overcome the problems of oxygen inhibition in the production of hydrogen, but it is estimated that this process can potentially yield as much as 10% solar energy capture. This makes photobiological research a very exciting and promising branch of the hydrogen production explorations. Still the problems of overcoming the short-term nature of algal hydrogen production are many and research is in the early stages. However, this research provides a viable way to industrialize these renewable and environmental friendly processes.

Thermochemical

See also: Thermochemical cycle

In the solar thermochemical process, water is split into hydrogen and oxygen using direct solar heat, rather than electricity, inside a high temperature solar reactor which receives highly concentrated solar flux from a solar field of heliostats that focus the highly concentrated sunlight into the reactor.

The two most promising routes are the two step cerium oxide cycle and the copper chlorine hybrid cycle. For the cerium oxide cycle the first step is to strip the CeO₃ into Ce₂O₃ at more than 1400 °C. After the thermal reduction step to reduce the metal oxide, hydrogen is then produced through hydrolysis at around 800 °C. The copper chloride cycle requires a lower temperature (~500°C), which makes this process more efficient, but the cycle contains more steps and is also more complex than the cerium oxide cycle.

Because hydrogen manufacture requires continuous performance, the solar thermochemical process includes thermal energy storage. Another thermochemical method uses solar reforming of methane, a process that replicates traditional fossil fuel reforming process but substitutes solar heat.

In a November 2021 publication in Nature, Aldo Steinfeld of Swiss technological university ETH Zurich reported an artificial photosynthesis where carbon dioxide and water vapour absorbed from the air are passed over a cerium oxide catalyst heated by concentrated solar power to produce hydrogen and carbon monoxide, transformed through the Fischer-Tropsch process into complex hydrocarbons forming methanol, a liquid fuel. Scaling could produce the 414 billion L (414 million m³) of aviation fuel used in 2019 with a surface of 45,000 km² (17,000 sq mi): 0.5% of the Sahara Desert. One author, Philipp Furler, leads specialist Synhelion, which in 2022 was building a solar fuel production facility at Jülich, west of Cologne, before another one in Spain. Swiss airlines, part of the Lufthansa Group, should become its first customer in 2023.

Carbon dioxide reduction

See also: Electrochemical reduction of carbon dioxide, Photoelectrochemical reduction of carbon dioxide, Photochemical reduction of carbon dioxide, and Biofuel

Carbon dioxide (CO₂) can be reduced to carbon monoxide (CO) and other more reduced compounds, such as methane, using the appropriate photocatalysts. One early example was the use of Tris(bipyridine)ruthenium(II) chloride (Ru(bipy)₃Cl₂) and cobalt chloride (CoCl₂) for CO₂ reduction to CO. In recent years many new catalysts have been found to reduce CO₂ into CO, after which the CO could be used to make hydrocarbons using for example the Fischer-Tropsch process. The most promising system for the solar-powered reduction of CO₂ is the combination of a photovoltaic cell with an electrochemical cell (PV+EC).

For the photovoltaic cell the highly efficient GaInP/GaAs/Ge solar cell has been used, but many other series-connected and/or tandem (multi-junction) PV architectures can be employed to deliver the required voltage and current density to drive the CO₂ reduction reactions and provide reasonable product outflow. The solar cells/panels can be placed in direct contact with the electrolyzer(s), which can bring advantages in terms of system compactness and thermal management of both technologies, or separately for instance by placing the PV outdoors exposed to sunlight and the EC systems protected indoors.

The currently best performing electrochemical cell is the gas diffusion electrode (GED) flow cell. In which the CO₂ reacts on Ag nanoparticles to produce CO. Solar to CO efficiencies of up to 19% have been reached, with minimal loss in activity after 20h. 

CO can also be produced without a catalyst using microwave plasma driven dissociation of CO₂. This process is relatively efficient, with an electricity to CO efficiency of up to 50%, but with low conversion around 10%. These low conversions are not ideal, because CO and CO₂ are hard to separate at large scale in a efficient manner. The big upside of this process is that it can be turned off and on quite rapidly and does not use scarce materials. The (weakly ionised) plasma is produced using microwaves, these microwaves can accelerate the free electrons in the plasma. These electrons interact with the CO₂ which vibrationally excite the CO₂, this leads to dissociation of the CO₂ to CO. The excitation and dissociation happens fast enough that only a little bit of the energy is converted to heat, which keeps the efficiency high. The dissociation also produces an oxygen radical, which reacts with CO₂ to CO and O₂. 

Also in this case, the use of microorganisms has been explored. Using genetic engineering and synthetic biology techniques, parts of or whole biofuel-producing metabolic pathways can be introduced in photosynthetic organisms. One example is the production of 1-butanol in Synechococcus elongatus using enzymes from Clostridium acetobutylicum, Escherichia coli and Treponema denticola. One example of a large-scale research facility exploring this type of biofuel production is the AlgaePARC in the Wageningen University and Research Centre, Netherlands.

Ammonia and hydrazine production

Hydrogen rich substances as ammonia and hydrazine are great for storing hydrogen. This is due to their energy density, for ammonia at least 1.3 times that of liquid hydrogen. Hydrazine is almost twice as dense in energy compared to liquid hydrogen, however a downside is that dilution is required in the use of direct hydrazine fuel cells, which lowers the overall power one can get from this fuel cell. Besides the high volumetric density, ammonia and hydrous hydrazine have a low flammability, which makes it superior to hydrogen by lowering the storage and transportation costs.

Ammonia

Direct ammonia fuel cells are researched for this exact reason and new studies presented a new integrated solar-based ammonia synthesis and fuel cell. The solar base follows from excess solar power that is used to synthesize ammonia. This is done by using an ammonia electrolytic cell (AEC) in combination with a proton exchange membrane (PEM) fuel cell. When a dip in solar power occurs, a direct ammonia fuel cell kicks into action providing the lacking energy. This recent research (2020) is a clear example of efficient use of energy, which is essentially done by temporary storage and use of ammonia as a fuel. Storage of energy in ammonia does not degrade over time, which is the case with batteries and flywheels. This provides long-term energy storage. This compact form of energy has the additional advantage that excess energy can easily be transported to other locations. This needs to be done with high safety measures due to the toxicity of ammonia for humans. Further research needs to be done to complement this system with wind energy and hydro-power plants to create a hybrid system to limit the interruptions in power supply. It is necessary to also investigate on the economic performance of the proposed system. Some scientists envision a new ammonia economy that is almost the same as the oil industry, but with the enormous advantage of inexhaustible carbon-free power. This so called green ammonia is considered as a potential fuel for super large ships. South Korean shipbuilder DSME plans on commercializing these ships by 2025.

Hydrazine

Another way of storing energy is with the use of hydrazine. This molecule is related to ammonia and has the potential to be equally as useful as ammonia. It can be created from ammonia and hydrogen peroxide or via chlorine based oxidations. This makes it an even denser energy storing fuel. The downside of hydrazine is that it is very toxic and that it will react with oxygen quite violently. This makes it an ideal fuel for oxygen low area's such as space. Recent launched Iridium NEXT satellites have hydrazine as their source of energy. However toxic, this fuel has great potential, because safety measures can be increased sufficiently to safely transport and convert hydrazine back into hydrogen and ammonia. Researchers discovered a way to decompose hydrazine with a photo catalysis system that works over the entire visible-light region. This means that sunlight can not only be used to produce hydrazine, but also to produce hydrogen from this fuel. The decomposition of hydrazine is done with a p-n bilayer consisting of fullerene (C₆₀), also known as "buckeyballs" which is a n-type semiconductor and zinc phthalocyanine (ZnPc) which is a p-type semiconductor creating an organic photo catalysis system. This system uses visible light irradiation to excite electrons to the n-type semiconductor creating an electric current. The holes created in the p-type semiconductor are forced in the direction of the so called Nafion part of the device, which oxidizes hydrazine to nitrogen gas and dissolved hydrogen ions. This was done in the first compartment of the fuel cell. The hydrogen ions travel through a salt bridge to another compartment to be reduced to hydrogen gas by the electrons, gained by the interaction with light, from the first compartment. Thus creating hydrogen, which can be used in fuel cells. This promising studies shows that hydrazine is a solar fuel that has great potential to become very useful in the energy transition.

A different approach to hydrazine are the direct fuel cells. The concepts for these cells have been developed since the 1960s. Recent studies provide much better direct hydrazine fuel cells, for example with the use of hydrogen peroxide as an oxidant. Making the anode basic and the cathode acidic increased the power density a lot, showing high peaks of around 1 W/cm² at a temperature of 80 degrees Celsius. As mentioned earlier the main weakness of direct hydrazine fuel cells is the high toxicity of hydrazine and its derivatives. However hydrous hydrazine, which is a water-like liquid retains the high hydrogen density and can be stored and transported safely using the existing fuel infrastructure. Researchers also aim for self-powered fuel cells involving hydrazine. These fuel cells make use of hydrazine in two ways, namely as the fuel for a direct fuel cell and as the splitting target. This means that one only needs hydrazine to produce hydrogen with this fuel cell, so no external power is needed. This is done with the use of iron doped cobalt sulfide nanosheets. The doping with iron decreases the free-energy changes of hydrogen adsorption and hydrazine dehydrogenation. This method has a 20 hour stability and 98% Faradaic efficiency, which is comparable with the best reported claims of self-powered hydrogen generating cells.

Other applications

• Electrolysis of water for hydrogen production combined with solar photovoltaics using alkaline, PEM, and SOEC electrolyzers; This basic use of solar light generated electric power to separate water into hydrogen and oxygen has proven a little bit more efficient than for example hydrogen capture by steam reforming. The alkaline production technology of hydrogen has low costs and is considered mature. This has a consequence that the yield per unit of time is significantly higher than when using PEM technology. However, PEM technology has no corrosion issues and is more efficient, whereas alkaline production technology has the disadvantage of corrosion and worse efficiency. In addition to that, PEM technology has a fast start-up and simple maintenance. Though, in bulk production the alkaline hydrogen production technology is superior.
• Heliogen claims success in the use of solar heliostats used to direct sunlight to a tower, to reach temperatures over 1000°C in the production of hydrogen. Temperatures above 2500°C can thermochemically split water into hydrogen and oxygen without the use of electricity. This can be done using the heat of nuclear power plants or by adaptive solar mirror fields to redirect the sunlight to reach high temperatures needed for these thermochemical processes. However, this way of producing hydrogen is in its infancy and it has not yet been proven that this production hydrogen is profitable and efficient, because it has to compete with other, mature technologies.

Artificial photosynthesis

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Artificial_photosynthesis 

Artificial photosynthesis is a chemical process that biomimics the natural process of photosynthesis to convert sunlight, water, and carbon dioxide into carbohydrates and oxygen. The term artificial photosynthesis is commonly used to refer to any scheme for capturing and storing the energy from sunlight in the chemical bonds of a fuel (a solar fuel). Photocatalytic water splitting converts water into hydrogen and oxygen and is a major research topic of artificial photosynthesis. Light-driven carbon dioxide reduction is another process studied that replicates natural carbon fixation.

Research on this topic includes the design and assembly of devices for the direct production of solar fuels, photoelectrochemistry and its application in fuel cells, and the engineering of enzymes and photoautotrophic microorganisms for microbial biofuel and biohydrogen production from sunlight.

Overview

The photosynthetic reaction can be divided into two half-reactions of oxidation and reduction, both of which are essential to producing fuel. In plant photosynthesis, water molecules are photo-oxidized to release oxygen and protons. The second phase of plant photosynthesis (also known as the Calvin-Benson cycle) is a light-independent reaction that converts carbon dioxide into glucose (fuel). Researchers of artificial photosynthesis are developing photocatalysts that are able to perform both of these reactions. Furthermore, the protons resulting from water splitting can be used for hydrogen production. These catalysts must be able to react quickly and absorb a large percentage of the incident solar photons.

Natural (left) versus artificial photosynthesis (right)

Whereas photovoltaics can provide energy directly from sunlight, the inefficiency of fuel production from photovoltaic electricity (indirect process) and the fact that sunshine is not constant throughout the day sets a limit to its use. One way of using natural photosynthesis is for the production of a biofuel, which is an indirect process that suffers from low energy conversion efficiency (due to photosynthesis' own low efficiency in converting sunlight to biomass), the cost of harvesting and transporting the fuel, and conflicts due to the increasing need of land mass for food production. The purpose of artificial photosynthesis is to produce a fuel from sunlight that can be stored conveniently and used when sunlight is not available, by using direct processes, that is, to produce a solar fuel. With the development of catalysts able to reproduce the major parts of photosynthesis, the only inputs needed to produce clean energy would ultimately be water, carbon dioxide and sunlight. The only by-product would be oxygen, and production of a solar fuel has the potential to be cheaper than gasoline.

One process for the creation of a clean and affordable energy supply is the development of photocatalytic water splitting under solar light. This method of sustainable hydrogen production is a major objective for the development of alternative energy systems. It is also predicted to be one of the more, if not the most, efficient ways of obtaining hydrogen from water. The conversion of solar energy into hydrogen via a water-splitting process assisted by photosemiconductor catalysts is one of the most promising technologies in development. This process has the potential for large quantities of hydrogen to be generated in an ecologically sound manner. The conversion of solar energy into a clean fuel (H₂) under ambient conditions is one of the greatest challenges facing scientists in the twenty-first century.

Two methods are generally recognized for the construction of solar fuel cells for hydrogen production:

• A homogeneous system is one such that catalysts are not compartmentalized, that is, components are present in the same compartment. This means that hydrogen and oxygen are produced in the same location. This can be a drawback, since they compose an explosive mixture, demanding gas product separation. Also, all components must be active in approximately the same conditions (e.g., pH).
• A heterogeneous system has two separate electrodes, an anode and a cathode, making possible the separation of oxygen and hydrogen production. Furthermore, different components do not necessarily need to work in the same conditions. However, the increased complexity of these systems makes them harder to develop and more expensive.

Another area of research within artificial photosynthesis is the selection and manipulation of photosynthetic microorganisms, namely green microalgae and cyanobacteria, for the production of solar fuels. Many strains are able to produce hydrogen naturally, and scientists are working to improve them. Algae biofuels such as butanol and methanol are produced both at laboratory and commercial scales. This method has benefited from the development of synthetic biology, which is also being explored by the J. Craig Venter Institute to produce a synthetic organism capable of biofuel production. In 2017, an efficient process was developed to produce acetic acid from carbon dioxide using "cyborg bacteria".

History

Artificial photosynthesis was first anticipated by the Italian chemist Giacomo Ciamician during 1912. In a lecture that was later published in Science he proposed a switch from the use of fossil fuels to radiant energy provided by the sun and captured by technical photochemistry devices. In this switch he saw a possibility to lessen the difference between the rich north of Europe and poor south and ventured a guess that this switch from coal to solar energy would "not be harmful to the progress and to human happiness."

During the late 1960s, Akira Fujishima discovered the photocatalytic properties of titanium dioxide, the so-called Honda-Fujishima effect, which could be used for hydrolysis.

Visible light water splitting with a one piece multijunction semiconductor device (vs. UV light with titanium dioxide semiconductors) was first demonstrated and patented by William Ayers at Energy Conversion Devices during 1983. This group demonstrated water photolysis into hydrogen and oxygen, now referred to as an "artificial leaf" with a low cost, thin film amorphous silicon multijunction sheet immersed directly in water. Hydrogen evolved on the front amorphous silicon surface decorated with various catalysts while oxygen evolved from the back side metal substrate which also eliminated the hazard of mixed hydrogen/oxygen gas evolution. A polymer membrane above the immersed device provided a path for proton transport. The higher photovoltage available from the multijunction thin film device with visible light was a major advance over previous photolysis attempts with UV or other single junction semiconductor photoelectrodes. The group's patent also lists several other semiconductor multijunction compositions in addition to amorphous silicon.

The Swedish Consortium for Artificial Photosynthesis, the first of its kind, was established during 1994 as a collaboration between groups of three different universities, Lund, Uppsala and Stockholm, being presently active around Lund and the Ångström Laboratories in Uppsala. The consortium was built with a multidisciplinary approach to focus on learning from natural photosynthesis and applying this knowledge in biomimetic systems.

Research of artificial photosynthesis is experiencing a boom at the beginning of the 21st century. During 2000, Commonwealth Scientific and Industrial Research Organisation (CSIRO) researchers publicized their intent to emphasize carbon dioxide capture and its conversion to hydrocarbons. In 2003, the Brookhaven National Laboratory announced the discovery of an important intermediate part of the reduction of CO₂ to CO (the simplest possible carbon dioxide reduction reaction), which could result in better catalysts.

One of the disadvantages of artificial systems for water-splitting catalysts is their general reliance on scarce, expensive elements, such as ruthenium or rhenium. During 2008, with the funding of the United States Air Force Office of Scientific Research, MIT chemist and director of the Solar Revolution Project Daniel G. Nocera and postdoctoral fellow Matthew Kanan attempted to circumvent this problem by using a catalyst containing the cheaper and more abundant elements cobalt and phosphate. The catalyst was able to split water into oxygen and protons using sunlight, and could potentially be coupled to a hydrogen gas producing catalyst such as platinum. Furthermore, while the catalyst broke down during catalysis, it could self-repair. This experimental catalyst design was considered a major improvement by many researchers.

Whereas CO is the prime reduction product of CO₂, more complex carbon compounds are usually desired. During 2008, Andrew B. Bocarsly reported the direct conversion of carbon dioxide and water to methanol using solar energy in a very efficient photochemical cell.

While Nocera and coworkers had accomplished water splitting to oxygen and protons, a light-driven process to produce hydrogen is desirable. During 2009, the Leibniz Institute for Catalysis reported inexpensive iron carbonyl complexes able to do just that. During the same year, researchers at the University of East Anglia also used iron carbonyl compounds to achieve photoelectrochemical hydrogen production with 60% efficiency, this time using a gold electrode covered with layers of indium phosphide to which the iron complexes were linked. Both of these processes used a molecular approach, where discrete nanoparticles are responsible for catalysis.

During 2009, F. del Valle and K. Domen showed the effect of the thermal treatment in a closed atmosphere using Cd
_1-xZn
_xS photocatalysts. Cd
_1-xZn
_xS solid solution reports high activity in hydrogen production from water splitting under sunlight irradiation. A mixed heterogeneous/molecular approach by researchers at the University of California, Santa Cruz, during 2010, using both nitrogen-doped and cadmium selenide quantum dots-sensitized titanium dioxide nanoparticles and nanowires, also yielded photoproduced hydrogen.

Artificial photosynthesis remained an academic field for many years. However, in the beginning of 2009, Mitsubishi Chemical Holdings was reported to be developing its own artificial photosynthesis research by using sunlight, water and carbon dioxide to "create the carbon building blocks from which resins, plastics and fibers can be synthesized." This was confirmed with the establishment of the KAITEKI Institute later that year, with carbon dioxide reduction through artificial photosynthesis as one of the main goals.

During 2010, the United States Department of Energy established, as one of its Energy Innovation Hubs, the Joint Center for Artificial Photosynthesis. The mission of JCAP is to find a cost-effective method to produce fuels using only sunlight, water, and carbon-dioxide as inputs.  JCAP is managed by a team from the California Institute of Technology (Caltech), directed by Professor Nathan Lewis and brings together more than 120 scientists and engineers from Caltech and its main partner, Lawrence Berkeley National Laboratory. JCAP also draws on the expertise and capabilities of key partners from Stanford University, the University of California at Berkeley, UCSB, University of California, Irvine, and University of California at San Diego, and the Stanford Linear Accelerator.  Additionally, JCAP serves as a central hub for other solar fuels research teams across the United States, including 20 DOE Energy Frontier Research Center.  The program had a budget of $122M over five years, subject to Congressional appropriation

Also during 2010, a team directed by professor David Wendell at the University of Cincinnati successfully demonstrated photosynthesis in an artificial construct consisting of enzymes suspended in a foam housing.

During 2011, Daniel Nocera and his research team announced the creation of the first practical artificial leaf. In a speech at the 241st National Meeting of the American Chemical Society, Nocera described an advanced solar cell the size of a poker card capable of splitting water into oxygen and hydrogen, approximately ten times more efficient than natural photosynthesis. The cell is mostly made of inexpensive materials that are widely available, works under simple conditions, and shows increased stability over previous catalysts: in laboratory studies, the authors demonstrated that an artificial leaf prototype could operate continuously for at least forty-five hours without a drop in activity. In May 2012, Sun Catalytix, the startup based on Nocera's research, stated that it will not be scaling up the prototype as the device offers few savings over other ways to make hydrogen from sunlight. Leading experts in the field have supported a proposal for a Global Project on Artificial Photosynthesis as a combined energy security and climate change solution. Conferences on this theme have been held at Lord Howe Island during 2011, at Chicheley Hall in the UK in 2014 and at Canberra and Lord Howe island during 2016.

Current research

In energy terms, natural photosynthesis can be divided in three steps:

• Light-harvesting complexes in bacteria and plants capture photons and transduce them into electrons, injecting them into the photosynthetic chain.
• Proton-coupled electron transfer along several cofactors of the photosynthetic chain, causing local, spatial charge separation.
• Redox catalysis, which uses the aforementioned transferred electrons to oxidize water to dioxygen and protons; these protons can in some species be utilized for dihydrogen production.

A triad assembly, with a photosensitizer (P) linked in tandem to a water oxidation catalyst (D) and a hydrogen evolving catalyst (A). Electrons flow from D to A when catalysis occurs.

Using biomimetic approaches, artificial photosynthesis tries to construct systems doing the same type of processes. Ideally, a triad assembly could oxidize water with one catalyst, reduce protons with another and have a photosensitizer molecule to power the whole system. One of the simplest designs is where the photosensitizer is linked in tandem between a water oxidation catalyst and a hydrogen evolving catalyst:

• The photosensitizer transfers electrons to the hydrogen catalyst when hit by light, becoming oxidized in the process.
• This drives the water splitting catalyst to donate electrons to the photosensitizer. In a triad assembly, such a catalyst is often referred to as a donor. The oxidized donor is able to perform water oxidation.

The state of the triad with one catalyst oxidized on one end and the second one reduced on the other end of the triad is referred to as a charge separation, and is a driving force for further electron transfer, and consequently catalysis, to occur. The different components may be assembled in diverse ways, such as supramolecular complexes, compartmentalized cells, or linearly, covalently linked molecules.

Research into finding catalysts that can convert water, carbon dioxide, and sunlight to carbohydrates or hydrogen is a current, active field. By studying the natural oxygen-evolving complex (OEC), researchers have developed catalysts such as the "blue dimer" to mimic its function. However, these catalysts are still inefficient.

Photoelectrochemical cells that reduce carbon dioxide into carbon monoxide (CO), formic acid (HCOOH) and methanol (CH₃OH) are under development. Similar to natural photosynthesis, such artificial leaves can use a tandem of light absorbers for overall water splitting or CO₂ reduction. These integrated systems can be assembled on lightweight, flexible substrates, resulting in floating devices resembling lotus leaves.

Phycobilitproteins from algae are under development for renewable energy production.

Hydrogen catalysts

Hydrogen is the simplest solar fuel to synthesize, since it involves only the transference of two electrons to two protons. It must, however, be done stepwise, with formation of an intermediate hydride anion:

2 e⁻ + 2 H⁺ ⇌ H⁺ + H⁻ ⇌ H₂

The proton-to-hydrogen converting catalysts present in nature are hydrogenases. These are enzymes that can either reduce protons to molecular hydrogen or oxidize hydrogen to protons and electrons. Spectroscopic and crystallographic studies spanning several decades have resulted in a good understanding of both the structure and mechanism of hydrogenase catalysis. Using this information, several molecules mimicking the structure of the active site of both nickel-iron and iron-iron hydrogenases have been synthesized. Other catalysts are not structural mimics of hydrogenase but rather functional ones. Synthesized catalysts include structural H-cluster models, a dirhodium photocatalyst, and cobalt catalysts.

Water-oxidizing catalysts

Water oxidation is a more complex chemical reaction than proton reduction. In nature, the oxygen-evolving complex performs this reaction by accumulating reducing equivalents (electrons) in a manganese-calcium cluster within photosystem II (PS II), then delivering them to water molecules, with the resulting production of molecular oxygen and protons:

2 H₂O → O₂ + 4 H⁺ + 4e⁻

Without a catalyst (natural or artificial), this reaction is very endothermic, requiring high temperatures (at least 2500 K).

The exact structure of the oxygen-evolving complex has been hard to determine experimentally. As of 2011, the most detailed model was from a 1.9 Å resolution crystal structure of photosystem II. The complex is a cluster containing four manganese and one calcium ions, but the exact location and mechanism of water oxidation within the cluster is unknown. Nevertheless, bio-inspired manganese and manganese-calcium complexes have been synthesized, such as [Mn₄O₄] cubane-type clusters, some with catalytic activity.

Some ruthenium complexes, such as the dinuclear µ-oxo-bridged "blue dimer" (the first of its kind to be synthesized), are capable of light-driven water oxidation, thanks to being able to form high valence states. In this case, the ruthenium complex acts as both photosensitizer and catalyst. This complexes and other molecular catalysts still attract researchers in the field, having different advantages such as clear structure, active site, and easy to study mechanism. One of the main challenges to overcome is their short-term stability and their effective heterogenization for applications in artificial photosynthesis devices.

Many metal oxides have been found to have water oxidation catalytic activity, including ruthenium(IV) oxide (RuO₂), iridium(IV) oxide (IrO₂), cobalt oxides (including nickel-doped Co₃O₄), manganese oxide (including layered MnO₂ (birnessite), Mn₂O₃), and a mix of Mn₂O₃ with CaMn₂O₄. Oxides are easier to obtain than molecular catalysts, especially those from relatively abundant transition metals (cobalt and manganese), but suffer from low turnover frequency and slow electron transfer properties, and their mechanism of action is hard to decipher and, therefore, to adjust.

Recently Metal-Organic Framework (MOF)-based materials have been shown to be a highly promising candidate for water oxidation with first row transition metals. The stability and tunability of this system is projected to be highly beneficial for future development.

Photosensitizers

Structure of [Ru(bipy)₃]²⁺, a broadly used photosensitizer.

Nature uses pigments, mainly chlorophylls, to absorb a broad part of the visible spectrum. Artificial systems can use either one type of pigment with a broad absorption range or combine several pigments for the same purpose.

Ruthenium polypyridine complexes, in particular tris(bipyridine)ruthenium(II) and its derivatives, have been extensively used in hydrogen photoproduction due to their efficient visible light absorption and long-lived consequent metal-to-ligand charge transfer excited state, which makes the complexes strong reducing agents. Other noble metal-containing complexes used include ones with platinum, rhodium and iridium.

Metal-free organic complexes have also been successfully employed as photosensitizers. Examples include eosin Y and rose bengal. Pyrrole rings such as porphyrins have also been used in coating nanomaterials or semiconductors for both homogeneous and heterogeneous catalysis.

As part of current research efforts artificial photonic antenna systems are being studied to determine efficient and sustainable ways to collect light for artificial photosynthesis. Gion Calzaferri (2009) describes one such antenna that uses zeolite L as a host for organic dyes, to mimic plant's light collecting systems. The antenna is fabricated by inserting dye molecules into the channels of zeolite L. The insertion process, which takes place under vacuum and at high temperature conditions, is made possible by the cooperative vibrational motion of the zeolite framework and of the dye molecules. The resulting material may be interfaced to an external device via a stopcock intermediate.

Carbon dioxide reduction catalysts

In nature, carbon fixation is done by green plants using the enzyme RuBisCO as a part of the Calvin cycle. RuBisCO is a rather slow catalyst compared to the vast majority of other enzymes, incorporating only a few molecules of carbon dioxide into ribulose-1,5-bisphosphate per minute, but does so at atmospheric pressure and in mild, biological conditions. The resulting product is further reduced and eventually used in the synthesis of glucose, which in turn is a precursor to more complex carbohydrates, such as cellulose and starch. The process consumes energy in the form of ATP and NADPH.

Artificial CO₂ reduction for fuel production aims mostly at producing reduced carbon compounds from atmospheric CO₂. Some transition metal polyphosphine complexes have been developed for this end; however, they usually require previous concentration of CO₂ before use, and carriers (molecules that would fixate CO₂) that are both stable in aerobic conditions and able to concentrate CO₂ at atmospheric concentrations haven't been yet developed. The simplest product from CO₂ reduction is carbon monoxide (CO), but for fuel development, further reduction is needed, and a key step also needing further development is the transfer of hydride anions to CO.

Photobiological production of fuels

Some photoautotrophic microorganisms can, under certain conditions, produce hydrogen. Nitrogen-fixing microorganisms, such as filamentous cyanobacteria, possess the enzyme nitrogenase, responsible for conversion of atmospheric N₂ into ammonia; molecular hydrogen is a byproduct of this reaction, and is many times not released by the microorganism, but rather taken up by a hydrogen-oxidizing (uptake) hydrogenase. One way of forcing these organisms to produce hydrogen is then to annihilate uptake hydrogenase activity. This has been done on a strain of Nostoc punctiforme: one of the structural genes of the NiFe uptake hydrogenase was inactivated by insertional mutagenesis, and the mutant strain showed hydrogen evolution under illumination.

Many of these photoautotrophs also have bidirectional hydrogenases, which can produce hydrogen under certain conditions. However, other energy-demanding metabolic pathways can compete with the necessary electrons for proton reduction, decreasing the efficiency of the overall process; also, these hydrogenases are very sensitive to oxygen.

Several carbon-based biofuels have also been produced using cyanobacteria, such as 1-butanol.

Synthetic biology techniques are predicted to be useful for this topic. Microbiological and enzymatic engineering have the potential of improving enzyme efficiency and robustness, as well as constructing new biofuel-producing metabolic pathways in photoautotrophs that previously lack them, or improving on the existing ones. Another topic being developed is the optimization of photobioreactors for commercial application.

Food production

Researchers have achieved controlled growth of diverse foods in the dark via solar energy and electrocatalysis-based artificial photosynthesis. It may become a way to increase energy efficiency of food production and reduce its environmental impacts. However, it is unclear if food production mechanisms based on the experimental process are viable and can be scaled.

Employed research techniques

Research in artificial photosynthesis is necessarily a multidisciplinary topic, requiring a multitude of different expertise. Some techniques employed in making and investigating catalysts and solar cells include:

• Organic and inorganic chemical synthesis.
• Electrochemistry methods, such as photoelectrochemistry, cyclic voltammetry, electrochemical impedance spectroscopy, dielectric spectroscopy and bulk electrolysis.
• Spectroscopic methods:
  • fast techniques, such as time-resolved spectroscopy and ultrafast laser spectroscopy;
  • magnetic resonance spectroscopies, such as nuclear magnetic resonance, electron paramagnetic resonance;
  • X-ray spectroscopy methods, including x-ray absorption such as XANES and EXAFS, but also x-ray emission.
• Crystallography.
• Molecular biology, microbiology and synthetic biology methodologies.

Advantages, disadvantages, and efficiency

Advantages of solar fuel production through artificial photosynthesis include:

• The solar energy can be immediately converted and stored. In photovoltaic cells, sunlight is converted into electricity and then converted again into chemical energy for storage, with some necessary losses of energy associated with the second conversion.
• The byproducts of these reactions are environmentally friendly. Artificially photosynthesized fuel would be a carbon-neutral source of energy, which could be used for transportation or homes.

Disadvantages include:

• Materials used for artificial photosynthesis often corrode in water, so they may be less stable than photovoltaics over long periods of time. Most hydrogen catalysts are very sensitive to oxygen, being inactivated or degraded in its presence; also, photodamage may occur over time.
• The cost is not (yet) advantageous enough to compete with fossil fuels as a commercially viable source of energy.

A concern usually addressed in catalyst design is efficiency, in particular how much of the incident light can be used in a system in practice. This is comparable with photosynthetic efficiency, where light-to-chemical-energy conversion is measured. Photosynthetic organisms are able to collect about 50% of incident solar radiation, however the theoretical limit of photosynthetic efficiency is 4.6 and 6.0% for C3 and C4 plants respectively. In reality, the efficiency of photosynthesis is much lower and is usually below 1%, with some exceptions such as sugarcane in tropical climate. In contrast, the highest reported efficiency for artificial photosynthesis lab prototypes is 22.4%. However, plants are efficient in using CO₂ at atmospheric concentrations, something that artificial catalysts still cannot perform.