Artificial process that uses sunlight energy to drive chemical synthesis
Artificial photosynthesis is a
chemical process that
biomimics the natural process of
photosynthesis. The term artificial photosynthesis is used loosely, refer to any scheme for capturing and storing energy from sunlight by producing a fuel, specifically a
solar fuel.[1] An advantage of artificial photosynthesis is that the solar energy can be immediately 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, which could be used for transportation or homes. The economics of artificial photosynthesis are not competitive.[2]
Overview
Numerous schemes have been described as artificial photosynthesis.
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.
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
Artificial photosynthesis was first anticipated by the Italian chemist
Giacomo Ciamician during 1912.[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]
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.[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.
Catalysis
Catalytic triad
The organization of the components for artificial photosynthesis is crucial. [17] Natural photosynthesis can be divided in three steps:[18][19]
Light-harvesting complexes in bacteria and plants capture photons and transduce them into electrons, injecting them into the photosynthetic chain.
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[20]
Catalysts
Catalysis is a major focus of research. Two classes of catalysts are generally recognized for the construction of solar fuel cells for hydrogen production:[18]
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.[21]
Selected catalysts
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.
Cd 1-xZn xS photocatalysts. Cd 1-xZn xSsolid solution catalyze hydrogen production from water splitting under sunlight irradiation.[22]
Inexpensive iron carbonyl complexes catalyze hydrogen evolution.[28][29] Gold electrode covered with layers of
indium phosphide to which iron carbonyl compounds to achieve photoelectrochemical hydrogen production.[30]
[31] 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.[32]
Metal-Organic Framework (MOF)-based materials have been shown to be a highly promising candidate for water oxidation with first row transition metals.[33][34] The stability and tunability of this system is projected to be highly beneficial for future development.[35]
Catalyst stability
Catalysts for artificial photosynthesis are expected to effect
turn over numbers in the millions. Catalysts often corrode in water, especially wen 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.[18][36]
Research centers
Sweden
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.[37] The consortium was built with a
multidisciplinary approach to focus on learning from natural photosynthesis and applying this knowledge in biomimetic systems.[19]
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."[40] 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.[41][42]
Global
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.[43]
Conferences on this theme have been held at Lord Howe Island during 2011,[44] at Chicheley Hall in the UK in 2014[45] and at Canberra and Lord Howe island during 2016.[46]
Various components
Hydrogen catalysts
Hydrogen is the simplest solar fuel. Its formation involves only the transference of two electrons to two protons:
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).[10]
The exact structure of the oxygen-evolving complex has been hard to determine experimentally.[51] As of 2011, the most detailed model was from a 1.9 Å resolution crystal structure of photosystem II.[52] 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.[53]
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.[18] 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.[54]
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-
dopedCo3O4),
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
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.
Metal-free organic complexes have also been successfully employed as photosensitizers. Examples include
eosin Y and
rose bengal.[18] Pyrrole rings such as porphyrins have also been used in coating
nanomaterials or
semiconductors for both homogeneous and heterogeneous catalysis.[9][31]
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.[55] 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.[56] The resulting material may be interfaced to an external device via a stopcock intermediate.[57][58]
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.[59] 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 metalpolyphosphine 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.[60] 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.[60]
Photobiological production of fuels
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.[61]Algae biofuels such as
butanol and
methanol have been produced at various scales. This method has benefited from the development of
synthetic biology,[61][62][63] Diverse biofuels have been developed, e.g., acetic acid from carbon dioxide using "cyborg bacteria".[64]
Some solar cells are capable of splitting water into oxygen and hydrogen, approximately ten times more efficient than natural photosynthesis.[65][66] 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.[67]
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.[68]
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.[61]
Several carbon-based biofuels have also been produced using cyanobacteria, such as 1-butanol.[69]
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.[61][69] Another topic being developed is the optimization of
photobioreactors for commercial application.[70]
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.[73] In reality, the efficiency of photosynthesis is much lower and is usually below 1%, with some exceptions such as
sugarcane in tropical climate.[74] In contrast, the highest reported efficiency for artificial photosynthesis lab prototypes is 22.4%.[75] However, plants are efficient in using CO2 at atmospheric concentrations, something that artificial catalysts still cannot perform.[76]
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