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Artificial process that uses sunlight energy to drive chemical synthesis

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Artificial photosynthesis is a chemical process that biomimics the natural process of photosynthesis. The term artificial photosynthesis is used loosely, referring to any scheme for capturing and then storing energy from sunlight by producing a fuel, specifically a solar fuel.[1] An advantage of artificial photosynthesis would be that the solar energy could converted and stored. By contrast, using 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, but it has never been demonstrated in any practical sense. The economics of artificial photosynthesis are noncompetitive.[2]

Overview

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Numerous schemes have been described as artificial photosynthesis.

• Photocatalytic water splitting, the conversion of water into hydrogen and oxygen:

2 H2O &#; 2 H2 + O2 This scheme is the simplest form of artificial photosynthesis conceptually, but has not been demonstrated in any practicable way.

• Light-driven carbon dioxide reduction, the conversion water, carbon dioxide into carbon monoxide or organic compounds and oxygen. In the conceptually simplest manifestation, this process gives CO:

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2 CO2 &#; 2 CO + O2 Related processes give formic acid (HCO2H): 2 H2O + 2 CO2 &#; 2 HCO2H + O2 Variations might produce formaldehyde or, equivalently, carbohydrates: 2 H2O + CO2 &#; H2CO + O2 These processes replicate natural carbon fixation.

Natural (left) versus artificial photosynthesis (right)

Because of the socio-economic implications, artificial photosynthesis is very topical, despite the many challenges.[4][5][2][6] Ideally the only inputs to produce such solar fuels would be water, carbon dioxide, and sunlight. The only by-product would be oxygen,[5][2][7] by using direct processes.[8][9][10]

History

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Artificial photosynthesis was first anticipated by the Italian chemist Giacomo Ciamician during .[11] In a lecture that was later published in Science[12] 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".[13]

During the late s, Akira Fujishima discovered the photocatalytic properties of titanium dioxide, the so-called Honda-Fujishima effect, which could be used for hydrolysis.[14]

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 .[15][16] 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.

Since the 's, much has been learned about catalysts hydrogen evolution reaction[17] and oxygen evolution reaction. Unfortunately, no practical system has been demonstrated despite intense efforts.[18][19]

Since the 's, much has been learned about catalysts hydrogen evolution reaction and oxygen evolution reaction.[20] Unfortunately, no practical system has been demonstrated despite intense efforts.[21]

Catalysis

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Catalytic triad

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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.

Some concepts for artificial photosynthesis consist of distinct components,[22] which are inspired by natural photosynthesis:[23][24]

• 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.

These processes could be replicated by a triad assembly, which could oxidize water at one catalyst, reduce protons at another, and have a photosensitizer molecule to power the whole system[25]

Catalysts

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Some catalysts for solar fuel cells are envisioned to produce hydrogen.[23]

1) 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). 2) 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.[26]

Selected catalysts

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Many catalysts have been evaluated for both the O2-evolution and the reductive sides of the process. Those listed below, which includes both oxidizer and reducers, are not practical, but illustrative:

[36] 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.[37]

• Metal-Organic Framework (MOF)-based materials have been shown to be a highly promising candidate for water oxidation with first row transition metals.

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  The stability and tunability of this system is projected to be highly beneficial for future development.

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Catalyst stability

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Catalysts for artificial photosynthesis are expected to effect turn over numbers in the millions. Catalysts often corrode in water, especially when irradiated. Thus, they may be less stable than photovoltaics over long periods of time. Hydrogen catalysts are very sensitive to oxygen, being inactivated or degraded in its presence; also, photodamage may occur over time.[23][41]

Research centers

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Sweden

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The Swedish Consortium for Artificial Photosynthesis, the first of its kind, was established during as a collaboration between groups of three universities, Lund, Uppsala and Stockholm, being presently active around Lund and the Ångström Laboratories in Uppsala.[42] The consortium was built with a multidisciplinary approach to focus on learning from natural photosynthesis and applying this knowledge in biomimetic systems.[24]

During , the United States Department of Energy established the Joint Center for Artificial Photosynthesis,[43] with the aim of finding a cost-effective method to produce fuels using only sunlight, water, and carbon-dioxide as inputs.[44]

Japan

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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".[45] 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.[46][47]

Global

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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.[48] Conferences on this theme have been held at Lord Howe Island during ,[49] at Chicheley Hall in the UK in [50] and at Canberra and Lord Howe island during .[51]

Various components

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Hydrogen catalysts

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Hydrogen is the simplest solar fuel. Its formation involves only the transference of two electrons to two protons:

2 e&#; + 2 H+ &#; H2

The hydrogenase enzymes effect this conversion[23][52][53]

Dirhodium photocatalyst[54] and cobalt catalysts.[23][55]

Water-oxidizing catalysts

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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 K).[10]

The exact structure of the oxygen-evolving complex has been hard to determine experimentally.[56] As of , the most detailed model was from a 1.9 Å resolution crystal structure of photosystem II.[57] 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.[58]

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.[23] 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.[59]

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.[9]

Photosensitizers

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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.[23] Other noble metal-containing complexes used include ones with platinum, rhodium and iridium.[23]

Metal-free organic complexes have also been successfully employed as photosensitizers. Examples include eosin Y and rose bengal.[23] Pyrrole rings such as porphyrins have also been used in coating nanomaterials or semiconductors for both homogeneous and heterogeneous catalysis.[9][36]

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 () describes one such antenna that uses zeolite L as a host for organic dyes, to mimic plant's light collecting systems.[60] 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.[61] The resulting material may be interfaced to an external device via a stopcock intermediate.[62][63]

Carbon dioxide reduction catalysts

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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.[64] 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.[65] The simplest product from CO2 reduction is carbon monoxide (CO), but for fuel development, further reduction is needed (for example, to multi-carbon products), and a key step also needing further development is the transfer of hydride anions to CO.[65]

Photobiological production of fuels

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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 produce hydrogen naturally.[66] Algae biofuels such as butanol and methanol have been produced at various scales. This method has benefited from the development of synthetic biology,[66][67][68] Diverse biofuels have been developed, e.g., acetic acid from carbon dioxide using "cyborg bacteria".[69]

Some solar cells are capable of splitting water into oxygen and hydrogen, approximately ten times more efficient than natural photosynthesis.[70][71] Sun Catalytix, the startup based on the artificial leaf, stated that it will not be scaling up the prototype as the device offers few savings over other ways to make hydrogen from sunlight.[72]

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.[73]

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.[66]

Several carbon-based biofuels have also been produced using cyanobacteria, such as 1-butanol.[74]

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.[66][74] Another topic being developed is the optimization of photobioreactors for commercial application.[75]

Food production

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Researchers have achieved controlled growth of diverse foods in the dark . It may become a way to increase energy efficiency of food production and reduce its environmental impacts.[76][77] However, it is unclear if food production mechanisms based on the experimental process are viable and can be scaled.

Some advantages, disadvantages, and efficiency

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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.[78] In reality, the efficiency of photosynthesis is much lower and is usually below 1%, with some exceptions such as sugarcane in tropical climate.[79] In contrast, the highest reported efficiency for artificial photosynthesis lab prototypes is 22.4%.[80] However, plants are efficient in using CO2 at atmospheric concentrations, something that artificial catalysts still cannot perform.[81]

See also

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References

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UIC student and study co-author Rohan Sartape with an artificial leaf developed in the laboratory of Meenesh Singh, assistant professor of chemical engineering at the College of Engineering. (Photo: Jim Young/UIC)

Engineers at the University of Illinois Chicago have built a cost-effective artificial leaf that can capture carbon dioxide at rates 100 times better than current systems. Unlike other carbon capture systems, which work in labs with pure carbon dioxide from pressurized tanks, this artificial leaf works in the real world. It captures carbon dioxide from more diluted sources, like air and flue gas produced by coal-fired power plants, and releases it for use as fuel and other materials. 

&#;Our artificial leaf system can be deployed outside the lab, where it has the potential to play a significant role in reducing greenhouse gases in the atmosphere thanks to its high rate of carbon capture, relatively low cost and moderate energy, even when compared to the best lab-based systems,&#; said Meenesh Singh, assistant professor of chemical engineering in the UIC College of Engineering and corresponding author on the paper. 

Using a&#;previously reported theoretical concept, the scientists modified a standard artificial leaf system with inexpensive materials to include a water gradient &#; a dry side and a wet side &#; across an electrically charged membrane. 

On the dry side, an organic solvent attaches to available carbon dioxide to produce a concentration of bicarbonate, or baking soda, on the membrane. As bicarbonate builds, these negatively charged ions are pulled across the membrane toward a positively charged electrode in a water-based solution on the membrane&#;s wet side. The liquid solution dissolves the bicarbonate back into carbon dioxide, so it can be released and harnessed for fuel or other uses. 

The electrical charge is used to speed up the transfer of bicarbonate across the membrane. 

Illustration of a carbon capture process designed by UIC College of Engineering scientists. Carbon dioxide from air or flue gas is absorbed by a dry organic solution to form bicarbonate ions, which migrate across a membrane and are dissolved in a liquid solution to concentrated CO2. Carbon atoms are shown in red, oxygen atoms are shown in blue and hydrogen atoms are shown in white. (Credit: Aditya Prajapati/UIC)

When they tested the system, which is small enough to fit in a backpack, the UIC scientists found that it had a very high flux &#; a rate of carbon capture compared with the surface area required for the reactions &#; of 3.3 millimoles per hour per 4 square centimeters. This is more than 100 times better than other systems, even though only a moderate amount of electricity (0.4 KJ/hour) was needed to power the reaction, less than the amount of energy needed for a 1 watt LED lightbulb. They calculated the cost at $145 per ton of carbon dioxide, which is in line with recommendations from the Department of Energy that cost should not exceed around $200 per ton. 

&#;It&#;s particularly exciting that this real-world application of an electrodialysis-driven artificial leaf had a high flux with a small, modular surface area,&#; Singh said. &#;This means that it has the potential to be stackable, the modules can be added or subtracted to more perfectly fit the need and affordably used in homes and classrooms, not just among profitable industrial organizations. A small module of the size of a home humidifier can remove greater than 1 kilogram of CO2 per day, and four industrial electrodialysis stacks can capture greater than 300 kilograms of CO2 per hour from flue gas.&#; 

The UIC scientists report on the design of their artificial leaf and the results of their experiments in &#;Migration-assisted, moisture gradient process for ultrafast, continuous CO2 capture from dilute sources at ambient conditions,&#; which is published in Energy & Environmental Science. &#; 

The research is funded by a grant (DE-SC-) from the U.S. Department of Energy. A patent application titled &#;Artificial photosynthetic systems for integrated carbon capture and conversion&#; has been filed by the Office of Technology Management at UIC. 

Co-authors of the paper from UIC, Argonne National Laboratory, Oklahoma State University and Braskem are Aditya Prajapati, Rohan Sartape, Tomas Rojas, Naveen Dandu, Pratik Dhakal, Amey Thorat,&#;Jiahan Xie, Ivan Bessa, Miguel Galante,&#;Marcio Andrade, Robert Somich, Marcio Rebouças, Gus Hutras, Nathalia Diniz, Anh Ngo and Jindal Shah. 

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