Researchers at Binghamton University, State University of New York have created a micro-scale biological solar cell that generates a higher. This contribution discusses why we should consider developing artificial photosynthesis with the tandem approach followed by the Dutch BioSolar Cells. Title, The BioSolar Cells project: sustainable energy from photosynthesis. Author (s), Klein Lankhorst, R.M.. Source, Wageningen: Project Office BioSolar Cells.
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This contribution discusses why we should consider developing artificial photosynthesis with the tandem approach followed by the Dutch BioSolar Cells consortium, a current operational paradigm for a global artificial photosynthesis project. We weigh the advantages and disadvantages of a tandem converter against other approaches, including biomass.
Owing to the low density of solar energy per unit area, artificial photosynthetic systems must operate at high efficiency to minimize the xells or sea area required. In particular, tandem converters are a much better option than biomass for densely populated countries and use two photons per electron extracted from water as the raw material into chemical conversion to hydrogen, or carbon-based fuel when CO 2 is also used.
A principal challenge is to forge materials for quantitative conversion of photons to chemical products within the physical limitation of an biksolar potential of ca 2. When going from electric charge in the tandem to hydrogen and back to electricity, only the energy equivalent to 1.
A critical step is then to learn from nature how to use the remaining difference of ca 1. Probably the only way to achieve this is by using bioinspired responsive matrices that have quantum—classical pathways for a coherent conversion of photons to fuels, similar to what has been achieved by natural selection in evolution.
Making a field of interpretation for biosolar cells
In appendix A for the expert, we derive a propagator that describes how catalytic reactions can proceed coherently by a convergence of time scales of quantum electron dynamics and classical nuclear dynamics. We propose that synergy gains by such processes form a basis for further progress towards high efficiency and yield for a global project on artificial photosynthesis.
Finally, we look at artificial photosynthesis research in The Netherlands and use this as an example of how an interdisciplinary approach is beneficial to artificial photosynthesis research. We conclude with some of the potential societal consequences of a large-scale roll out of artificial photosynthesis. However, these resources will dwindle in the foreseeable future.
Also, burning fossil fuels leads to emissions of large quantities of carbon dioxide CO 2which is one of the major greenhouse gases causing global warming. Furthermore, fossil fuels are not evenly distributed around the world, leading to political tensions and potential problems with energy supply in countries that rely on imported fossil fuel. These arguments make sustainable, low carbon energy supplies one of the most pressing challenges facing society.
There are ample renewable energy celps on the planet for supplying mankind’s increasing demand. The largest of celps sources is solar energy. The conversion of energy from the Sun is therefore an obvious place to turn to when seeking alternative energy sources.
There are a number of technologies for converting sunlight into electricity; the most common being photovoltaic cells. However, electricity is not readily stored, which means that the production of electricity has to be balanced with the demand at night time or during the winter season and it is not a practical energy source for applications such as long-distance air and sea transport. Thus, rather than stopping at the light capturing and charge separation steps of photosynthesis, there is increasing drive to mimic the processes of photosynthesis for fuel production.
Despite the growing momentum of research in this field, artificial photosynthesis remains largely unknown in energy and climate change policy documents [ 2 ].
As well as providing a mechanism for bringing together scientists working on artificial photosynthesis, a global consortium on artificial photosynthesis may serve to raise the visibility of this subject [ 3 ]. The most compelling argument for a global artificial photosynthesis derives from the sheer size of the energy system.
Such an effort can only be deployed on a truly global level. The yield of artificial photosynthesis relates to the surface and higher efficiency means less surface. In this paper, we indicate current directions in the development of artificial photosynthesis devices that would fit in a global initiative, and challenges that need to be overcome in forging responsive matrix materials for quantitative conversion of photons to chemical products with high efficiency.
In addition, we show how the Dutch BioSolar Cells consortium can be considered as a template for a global consortium on artificial photosynthesis. In the end, we reflect on the role of a global consortium and potential societal consequences of artificial photosynthesis. Photosynthesis is the chemical process by which plants, algae and some bacteria store energy from the Sun in the form of carbohydrates that act as fuels.
The four main steps of photosynthesis are light harvesting, charge separation, water oxidation and fuel production [ 5 — 7 ]. In light harvesting, antenna molecules, mostly chlorophyll but also carotenes, absorb sunlight and transfer the energy among themselves and eventually through to the reaction centre where charge separation takes place.
In this way, energy from sunlight is used to separate positive and negative charges from each other. The positive charges are used to oxidize water. The electrons are transferred biosolae cytochrome b 6 f and mobile electron carriers to photosystem I where they are excited again and used to produce carbohydrate fuel.
A schematic diagram of what happens in photosynthesis is shown in figure 1. A schematic diagram of natural photosynthesis showing light absorption, charge separation, water oxidation and fuel production. The path of the yellow line indicates the approximate energy of the electrons in analogy to the Z -scheme. In recent years, progress has been made in elucidating the structures of many of the proteins involved in photosynthesis. This, in turn, helps us to understand and replicate their functions.
Online version in colour. Four photons are required to drive each of dells half reactions. Thus, eight photons are required biosllar the total chemical reaction. As four electrons are carried over and eight photons are used, the process proceeds with two photons per electron.
Nature uses two photosystems in tandem to drive the two chemical reactions of water splitting and fuel production. The reactions occur in proportion to the number of photons absorbed. Contrary to common belief, natural photosynthesis is not determined by insolation, the total amount of solar radiation energy that is collected per unit of time, but by the total light sum, the number of photons from the blue to red — nm part of the spectrum that is collected per unit of time.
Although parts of the natural photosynthetic process are highly efficient, the overall solar-to-carbohydrate efficiency is low. Thus, unmodified natural photosynthesis cannot serve mankind’s purposes for fuel production, but can be used as a blueprint eclls efficient artificial photosynthesis.
The incoming photon flux, energy and electron transfer, and catalysis, operate on very different time, energy and length scales [ 89 ]. This puts design limits on how components should be matched for the most efficient solar-to-fuel conversion [ 8 ] operating close to the theoretical limits on solar energy conversion [ 10 ].
If we are going to make the best possible use of the incoming sunlight for fuel production with two photons per electron, it makes sense to capture as many photons of sunlight as possible.
The photosynthetic apparatus in plants absorbs light around nm. This means that plants use only half of the incoming photons. In comparison, silicon solar cells absorb light at around nm and therefore absorb more photons [ 11 ].
Nature uses two photosystems in tandem to drive the two chemical reactions of water oxidation and CO 2 reduction. The same can be done with an artificial device. A weakness in the natural system is that the two photosystems absorb light of approximately the same energy, so the two systems are competing for the same photons while the infrared photons remain unused.
In an artificial system, we can do this differently: In this way, the number of photons of sunlight that is ce,ls by our system is maximized. Furthermore, the cut-off wavelengths are better matched to the electrochemical work [ 11 ]. Optimal matching is obtained with cut-off wavelengths of and nm [ 12 ]. Thus, many biosolwr are now aiming to produce tandem devices that have two absorbers to make the best possible use of the incoming light to drive water splitting and fuel production with two photons per electron.
A schematic of a tandem artificial photosynthesis device is shown in figure 2 along with its light-absorbing properties. Just as in natural photosynthesis, artificial photosynthesis occurs in four steps: Schematic of a tandem artificial photosynthetic device a and its light-absorbing properties b. This device operates in a fashion analogous to natural photosynthesis.
As in figure 1the approximate energy of the electrons as they pass through the device is shown. The tandem is in balance when both halves receive the same number of photons. An optimal use of the sunlight is reached with cut-off wavelengths of biiosolar and nm.
Making a field of interpretation for biosolar cells – Leiden University
Natural photosynthesis has an efficient strategy for achieving light harvesting and charge separation: Also, secondary absorbers such as carotenes absorb light in regions of fells spectrum where chlorophyll does not absorb biosolad. In this way, the absorption cross section of the reaction centre is increased.
For artificial photosynthesis, strategies similar to that found in nature can be employed, where multiple absorbers that exhibit complementary absorption profiles and are capable of efficient excitation-energy transfer are used as light harvesters and the excitation energy is transferred to an artificial reaction centre [ 13 — 15 ].
For the tandem device described above, light is absorbed on biosoar fuel production side by a photosensitizer that, when excited, has sufficient reduction potential to inject an electron into the fuel production catalyst.
On the water oxidation side, the optically excited photosensitizer has sufficient reduction potential to inject an electron into the fuel reduction side and fill the hole there, and subsequently has sufficient oxidation potential to oxidize the water oxidizing catalyst. The sensitizers may be made from organic molecules.
The simplest form of this is a donor—acceptor diad [ 16 ], but more complex structures are commonly used to prevent charge recombination [ 17 ].
Alternatively, semiconductors, as in solar cells, may be used. These materials can efficiently absorb sunlight and separate charges and are stable with extended exposure to sunlight. Their disadvantage is their limited flexibility, which means that various techniques of doping, nanostructuring and coating are needed to give them the desired properties.
Finally, molecular light harvesters can be used in combination with semiconductor charge separators, in a concept similar to that used in dye-sensitized solar cells [ 1819 ]. Recent advances in the determination of the structure of the photosystem II protein complex [ 20 ] shows the oxygen-evolving centre found in oxygenic photosynthesis has a cluster of four manganese atoms and a calcium atom surrounded by protein. This enzyme stores oxidizing potential in a series of one-electron steps and then splits water into oxygen, electrons and protons.
It performs this process at low bjosolar and at a very rapid rate [ 2122 ].
The BioSolar Cells project : sustainable energy from photosynthesis
As water oxidation involves multi-electron chemistry, water-splitting catalysts invariably include one or more transition metals in their structures. Some success has been achieved with water oxidation catalysts based on expensive metals such as iridium [ 23 — 25 ] and ruthenium [ bioxolar27 ]. However, these are not practical for very large-scale use.
Natural photosynthesis demonstrates that catalysts based on abundant, inexpensive elements are possible. Such catalysts are more desirable. The development of artificial water oxidation catalysts that employ abundant elements is a very active research field, but an inexpensive, robust and efficient catalyst has yet to be discovered.
There have been promising results with catalysts based on cobalt [ 28 — 31 ] and other cheap transition metals [ 32 — 35 ], but their efficiency and durability need to be further improved before they are suitable for larger scale use. Different solar fuels can be envisaged as products of artificial photosynthesis. There are two main types of fuels: Hydrogen hiosolar a natural choice of fuel when water is the raw material.
To make hydrogen, the protons that result from the splitting of water need to be reduced to produce molecular hydrogen:. The easiest way of doing this is on the surface of a noble metal such as platinum.
But this approach is too expensive to commercialize on a large scale.