Source: https://www.wesrch.com/energy/paper-details/pdf-TR1L02N7LQVUF-harnessing-solar-energy-for-the-production-of-clean-fuels
Timestamp: 2019-04-19 06:54:49+00:00

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These three research themes will overlap, and all will exploit fundamental research elucidating the precise molecular mechanisms involved in the splitting of water into hydrogen and oxygen in photosynthesis by both plants and bacteria. This process, which evolved 2.5 billion years ago, created the conditions for animal life by converting atmospheric carbon dioxide into carbohydrates, and also produced all the fossil fuels, which humans are turning back into carbon dioxide at an increasing rate, threatening catastrophic environmental effects. The same process now holds our salvation again. Although the principal products of photosynthesis in plants and bacteria are carbohydrates, certain algae and cyanobacteria can produce hydrogen directly from water using sunlight, providing a basis for genetic modification to increase yields, and for the creation of suitable artificial systems. Furthermore, photosynthesis is also capable of generating other chemicals currently made industrially, such as nitrates, and other high value compounds for chemical industry. The European research program will therefore also seek to develop systems for converting solar energy directly into such chemicals with much greater efficiency, offering the prospect not just of producing unlimited energy, but also fixing atmospheric carbon dioxide to bring concentrations back down to pre-industrial levels as part of the overall thrust for clean renewable energy. There are considerable challenges, with the first being to mimic the functioning of natural photosynthetic systems, particularly photosystem II, the enzyme complex in the leaves of plants that splits water into hydrogen and oxygen via a catalyst comprising four manganese atoms along with some calcium. Significant progress has been made recently on this front. Participants at the ESF's brainstorming conference, describe the solar fuels project as the quest for building the "artificial leaf". There is growing conviction in Europe and elsewhere that by 2050 a large proportion of our fuels will come from such "artificial leaves" and that there is no time to lose starting the crucial enabling research, in order to gain technology leadership in this important future key technology.
By the year 2050 even the most conservative estimates predict that the world's energy requirements will more than double. Energy is a prerequisite to economic stability and its supply at an affordable cost for the both the developed and developing world can not be guaranteed. For a large part of the global population fulfilling very basic energy intensive needs as food, desalinated water and housing will not be achieved within the current energy scenarios for the coming decades. In addition, improving education and public health critically depends on getting access to affordable energy. Even more important, the current energy system is far from sustainable. This problem is made all the more critical for Europe because its ecological footprint is over double than what the continent can regenerate. In other words, it now takes more than two years to regenerate what we use in a single year, and even on a global scale we have already a serious backlog replacing our current unsustainable linear competitive economy by a sustainable cyclic adaptive economy by about 23%. Most urgently, continued increased use of fossil fuels to satisfy the extra energy demand brings unacceptable environmental consequences with the increased concentrations of CO2 in the atmosphere that this produces. The biosphere can recycle 4-7 gigaton of CO2 yearly, and we currently produce around 25 gigaton per year. In addition, energy savings rates in Europe and other developed economies have slowed since 1990, as has the decline in CO2 emissions relative to GDP. Despite all measures the CO2 emission will still grow to ~32 gigaton yearly by 2030, mostly from power stations generating electricity and fuels for transportation. This shows the enormity of the problem: contemplated measures are by far insufficient to reduce the ecological overshoot. How can we provide the energy that mankind requires and at the same time reduce greenhouse gas emissions? Filling the energy gap without disruptions of the supply while reducing energy poverty in developing countries and without a dangerous continued increase in atmospheric CO2 destabilizing the climate represents the most serious challenge to the continued existence of man on planet Earth. It is almost impossible to overemphasize the disastrous combined effects that running out of inexpensive energy supplies and the catastrophic rises in atmospheric CO2 will have on both the developed and the developing world. Current political problems pale into insignificance by comparison and governments worldwide are pondering about decisive actions to turn the tide.
Figure 1. The enormity of the problem is made clear by simulations of the IPCC, which predict that long after CO2 emissions are reduced atmospheric concentrations will continu to grow for 100300 years before they stabilize, while the surface air temperature continues to rise slowly for a century or more. Thermal expansion and melting of ice sheets continues to contribute to sea-level rise for many centuries. Adapted from: Climate Change 2001: Synthesis Report ­Summary for Policymakers, Intergovernmental Panel on Climate Change (IPCC).
nuclear fusion. This technology may become important in the future, however, it has yet to come to fruition. The other alternative is to use solar energy. Enough solar energy reaches the surface of the Earth every hour to supply the whole world's annual energy requirements. How can we convert enough of this incident solar energy into usable forms so that we can maintain our way of life? Solar energy represents a continuous and maintenance free clean power source of more than 100,000 TW. Harnessing solar energy for the production of a mix of clean fuels for continuous use without the risk of interruption of the energy supply is a major challenge. There exists at present only a single conversion technology with prospects for long term large-scale use: photovoltaics. In a photovoltaic cell, photons are absorbed and converted to electrical energy. This leads to electric currents that have to be used immediately. The current stops when the light goes out. The advantage of this system is that there are no energy storage losses, but it is insufficient for a continuous and reliable energy supply because of inadequate reserve margins. For almost 3 billion years, Nature has found fantastic solutions to convert solar energy to produce fuels (See the appendix for the scientific background). In a variety of photosynthetic processes, light energy is dynamically stored as efficient as thermodynamically possible. First, it is stored in a light antenna that allows for rapid transfer of the solar energy to reaction centers that produce electric charges of opposite polarity and then an electrochemical membrane potential. Afterwards, this is fed into a variety of chemical processes that form a dynamic reserve embedded in the energy distribution network of the cell where it is used for maintenance, growth and maturation of organisms (See Figure 2). Plants and green eubacteria use light energy to split water into oxygen and a form of hydrogen in an enzyme complex called photosystem II. Ultimately this hydrogen powers all life on Earth. The main waste product of photosynthesis is oxygen. Together with fuels, this waste product is used to drive cyclic processes that sustain life on Earth in a renewable manner. In recent years structural biology has provided profound insight into the structure and operational mechanisms of the molecular machinery of the photosynthetic apparatus. This sets the stage for a technology push in the coming years. Now is the time to dedicate ourselves to solving this very important problem. Scientists, energy companies and government organizations are contemplating about how to build up significant research efforts into potentially important directions to fulfill the basic research needs that will lead to the necessary breakthroughs in renewable technologies. The pace of technology development is the key to making the energy system more economically, socially and environmentally sustainable. The financial system has the capacity to fund the required investments, but it will need active steering into the right direction. Doing nothing is not an option. We require the sort of financial and dedicated commitment to tackle this problem that was shown in sending a man to the moon or in the Manhattan Project. The clock is already ticking. Action is needed now if there is to be a solution within the next 30-40 years.
Traditional organic chemistry based production methods may not fulfill the requirements with respect to costs and sustainability, in which case the building materials would have to be produced in cell factories. Because of the large area and quantity needed, the marginal costs involved in collecting the solar energy will be the limiting factor in the end. Nature solves this by self-assembly, selfrepair, self-reproduction and by starting from a molecular absorber with a high extinction coefficient. The main technological challenge in artificial photosynthesis is then to learn how to exploit these processes that make photosynthesis affordable and viable in the living world. Considering the massive fundamental research effort that is currently being built up in the life sciences and nanosciences, it is timely to implement a grid-based research program directed towards resolving fundamental research needs and performing technology development for future energy supply and conversion by artificial photosynthesis. With such a concentrated effort, exploration, implementation, validation and up-scaling of artificial photosynthesis can be realized well within the required time scale of 25-30 years, in a bottom-up process guided by clear visions.
production (PRD 10). However, current energy harvesting from biomass has a low efficiency due to intrinsic free energy losses in cellular metabolism. Development of systems and synthetic biology approaches will lead to the engineering of photosynthetic cell factories that are designed for optimal fuel production (PRD 11). Extraction of the ideas behind the molecular machinery used for the harnessing and converting solar energy in photosynthetic organisms will lead to the engineering of new devices, at the forefront of chemistry, biology and physics (PRD 12). Ultimately, high efficiency can only be obtained through the combination of different energy conversion modules designed to extract all the exploitable energy content of incident solar radiation. This is outlined in PRD 13, where heat by-products of solar-to-fuel conversion devices are used in an integrated solar hydrogen tower. Throughout the research that will lead us to clean fuel, the functioning of integrated devices or components must be tested, requiring novel techniques and methodologies to probe functionalities on a nano- and meso-scale. These new techniques and instruments are summarized in PRD 14. The development of novel systems with the desired solar to fuel conversion properties requires in silico evolutionary computational strategies to speed up the design of functionally active superstructures (PRD 15).
Figure 3. Schematic picture of an integrated artificial device for production of hydrogen and methanol.
Nature has provided a proof of principle that large-scale water splitting and hydrogen formation can be driven by solar energy. Therefore nature provides a blueprint for the development of new technologies to produce fuels in an environmentally safe way. Deciphering the molecular basis of these reactions is an essential basis for the development of biomimetic and biotechnological systems for solar-driven hydrogen production.
All problems of light-driven water splitting and hydrogen formation have been solved in nature, but crucial aspects of the evolutionary successful solution are not understood. The significant progress obtained in the past still is an insufficient basis for technological developments. This holds in particular for the light-driven water splitting in Photosystem II and dihydrogen formation in hydrogenases. This PRD is aiming at elucidation of the multistep catalytic processes of water splitting and dihydrogen formation in natural systems.
Research on the functions of the natural system constitutes the basis for technological systems for solar hydrogen production in two ways: 1) Elucidation of the function of natural systems facilitates biomimetic approaches. The natural systems involve a protein matrix carrying organic (e. g. pigments) cofactors and catalytic metal complexes. The protein environment also tunes the chemical properties of the functional cofactors and organizes the substrate/products access to/from the active site. The natural systems likely represent suitable blueprint for the catalytic site and the smart matrix of artificial systems. The self-assembly of the biological water-splitting metal site may also serve as a guide for synthesis/assembly in artificial systems. 2) Molecular-level understanding of the related enzymology, required for cellular approaches to light-driven fuel formation.
2) Elucidation of the chemical basis of how biological systems convert electrons and protons to molecular hydrogen. The structure of some hydrogenases is known to great detail already. Nevertheless the challenge remains to reveal the full mechanism by characterizing the involved intermediates by spectroscopy and crystallography. For future applications in biomimetic systems, it is important to address the following aspects: (i) By which means some hydrogenases become oxygen insensitive or at least tolerant? (ii) How can the H2 production rate be increased and the uptake rate decreased? Further systematic studies into the structure and function of hydrogenases from organisms living under various conditions is required to fully understand the role of the protein (including channels) and the different active sites during catalysis. Investigations also need to be directed towards coupling of the electron and proton flow from photosynthesis (PSII or PSI) to hydrogenases. Nitrogenases also evolve H2 and detailed knowledge about this enzyme may also hold important clues for producing H2 from protons and electrons produced by water splitting.
10 years: - Elucidation of the basic catalytic mechanism of PSII water splitting and hydrogenase function interfaced with research on (i) biomimetic systems and (ii) cellular biological systems. - Elucidation the role of the protein matrix in directed proton transfer, control of water accessibility and providing stability to the catalytic site. - Gaining understanding and control of the synthesis/assembly of the active metal site. Subsequent 10-20 years: Intensified transfer of the blueprint knowledge to parallel technological initiatives.
The incorporation of a stable light antenna will substantially increase the efficiency of fuelproducing solar cells.
The proof of principle of constructing artificial antennae has amply been given. The challenges are to make cost-effective antennae with high absorption cross-section in a broad wavelength region that are easily assembled into the proper size, can be correctly positioned with respect to the other subunits of the solar device, are sufficiently photostable in the presence of oxygen and charge separators and can be (self)repaired or replaced. Furthermore, methods need to be developed to characterize and optimize functioning and organization of a supramolecular antenna on a nanoscale (see PRD 12 and 14).
The construction of antennae with a broad absorption cross-section and broad wavelength coverage can be established within 5 years. Being able to repair photodamaged antennae, to assemble these at the proper sizes and co-assemble them with charge-separation units (see PRD 12) should be feasible within 10 years.
Achieving efficient primary conversion of solar energy by fast long-lived charge separation will have a significant impact on the efficiency of energy conversion and storage in solar-tofuel converting devices.
Fast and long-lived charge separation need to be established in a bio-inspired structure, coupling the charge separating device to an antenna and coupling proton to electron transfer. Furthermore, the role of the protein environment in charge separation processes needs to be elucidated to utilize electrostatic fields for functioning and stability of the artificial chargeseparating constructs. Progress will critically depend on novel analysis and theory methods (PRD 14 and 15).
5 years: Reaction centres suitable for operation in bio-inspired solar-to-fuel cells. 10 years: Coupling of reaction centres to an antenna (see also PRD 1) and further optimization. 15-20 years: Coupling to catalytic units, and integration in a solar-to-fuel device (see PRD 12).
The design and synthesis of multielectron catalysts for fuel production from abundant substrates (water, CO2) involves the discovery of new catalysts for water-splitting, hydrogen production and for CO2 reduction.
Design principles for the catalysts that are needed are currently lacking, and implementing additional requirements of cost and availability will require radically new concepts. Inspiration from natural systems provides the sole starting point, but it is not necessarily representative of the ideal system from an operational perspective. The chemical nature of the catalyst should allow a high conversion rate, high turn-over number, and be composed of cheap and environmental friendly materials. Amongst promising candidates are complexes based on first row transition metals and nanoparticles. Generation of hydrogen-rich fuels based on carbonaceous (CO2) or nitrous (N2) feedstock will require the development of alternate catalytic systems. While platinum is an excellent catalyst material for H2 production, its availability and cost prevent its use on large scale and long term and its usefulness for other systems has not been demonstrated. Alternative molecular catalysts can already be achieved with different systems but the efficiency and durability are low. Theoretical methods allowing novel evolutionary in silico design approaches will help to optimize the design prior to experimental implementation.
A catalytic centre that will exploit the accumulated solar energy to drive a multi-electron reaction is a prerequisite to achieve catalytic fuel production driven by solar energy.
per photon) needed for the catalytic reaction must be accumulated and stored long enough to allow the completion of the catalytic cycle.
H2 catalysts already exist and will continue to be improved. New catalysts, including metalfree catalysts should become available over the next 10 years. Water-splitting with molecular catalysts requires a breakthrough, which must also occur within the next 5-15 years.
Today's main way to produce chemical fuels by use of solar energy uses photovoltaic solar cells in combination with electrolyzers. Considerable effort is devoted in classical photovoltaics to develop so-called 3rd generation solar cells achieving either solar efficiencies above 30 % or costs far below those of today. Here principles taken from Nature can lead to new solutions finally even allowing for a combination of the two goals.
sensitizing entities for DSCs in order to extend the charge creating monolayer and thereby allowing for larger electrode structure and/or thinner porous electrodes so that solid hole-conductors can also give the required efficiencies while allowing for more stable modules, and even completely new types of multiple band gap solar cells utilizing directional energy transfer into different parts of accordingly structured PV devices.
Solutions are expected to be developed mainly from research on biological self-organization and on photosynthetic antenna-systems and reaction centres. Further down in time even selfadjustment and self-repair might be used in these devices.
Raising the efficiency in Third Generation Solar Cells to above 30, in multiple band gap cells even close to 50 % might be possible. In DSCs the described solutions could lead to a major improvement in module stability (to above 10 years) and efficiency (above 10 %).
Artificial structures mimicking biological processes or long time stable biologically produced systems need to be developed and introduced into solar cell production schemes. Antenna systems applicable in DSCs need to be found and tested, the control of nanostructures in biology needs to be understood and translated to materials formation for PV application. Also the understanding of directed energy transfer must be improved and artificial, preferably inorganic structures for use in PV need to be developed.
In order to introduce such solutions into the production within the next 10 years allowing for a considerable contribution to solving our energy problem within the next 20 years prototypes based on our current knowledge should be available within the next 5 years. Once new structures are produced in larger scale the introduction of improved versions should be possible faster.
Novel approaches to system assembly of nanoscale components benefit by learning the selfassembly, self-adjustment and self-repair features of biological systems, including fault tolerance.
Rapid development in nanoscience and the revolution in biomedical research combine to create an unusual opportunity for advances in basic materials research for solar energy. The individual microscopic steps of solar energy utilization each take place on the nanometer scale. As a consequence, developing the ability to pattern and control matter on this length scale generates unusual opportunities to create new materials for solar energy. Smart materials is a new area which already has led to the first engineering applications in the development of high-tech applications airplane and automotive sectors on a micro- to meso-scale, while applications on a nanoscale have not yet been realized. For the development of PV devices, smart materials could include concepts like selfadjustment to various environmental conditions such as light intensities, temperatures etc. In addition such materials could be defect-tolerant and include self-repair for increasing the life time of energy conversion devices.
The major hurdle for massive implementation of PV is the high production costs, both for established silicon-based PV and emerging DSC technologies. The best way to overcome this hurdle will be to implement self-assembly, self-repair and defect tolerance principles from nature since these will not only reduce the cost but also have an impact on reliability.
5-10 years: Resolving self-assembly principles. 10-15 years: Proof of principle for self-assembling and self-repairing direct solar-to-fuel concepts. 20 years: Prototyping of novel devices using smart materials.
Fast changes either in energy supply (clouds changing the solar intensity) or demand (especially in small decentralized electricity grids) could be buffered by integrated energy storage in PV systems.
Energy storing photoelectrochemical systems have been developed already in the seventies. Overall solar efficiencies of about 10 % were achieved with low stability. Since then there has been little progress and no further breakthroughs. Thus far PV productivity is well below the threshold of ~ 20% that is generally considered an acceptable practical limit for a fluctuating power source. This will change when PV becomes a major source of energy. New concepts could now be explored inspired by biology, combining electrical charge separation with electrochemical storage processes in a smooth and reversible interconversion where energy is temporarily stored in a membrane ion potential. A disadvantage is a water based system and this requires direction of research into dedicated solid nanostructures with low energy barriers and good stability.
Energy-storing PV devices will reduce the total area necessary to cover a certain electricity demand (higher overall efficiency). Further, they will lead to scalability of the full system.
Photoelectrochemical systems using new PV materials and highly efficient catalysts need to be further developed. Potential redox-couples need to be explored for temporary storage. Nonlinearities in biological system charge separation processes need to be explored for intermediate storage in ion gradients. Further, the possibility to derive a super capacitor concept from here, need to be investigated, as well as the possibility to charge a redox battery by these processes.
Development of an ionic or redox couple buffering system will take 5-10 years and prototyping 10-15 years. Practical implementation will be 20 years onward.
Direct Solar Water Splitting has already in the seventies been "the stuff, that photochemists dreams are made of" (Lord George Porter, 1975). Considerable progress has been made but today's systems are by far too expensive. Major scientific improvements are needed in order to apply these techniques in commercial systems.
A key problem in electrolyzers in general, and in an even much more pronounced way in photoelectrochemical systems, is the catalysis needed for water splitting, especially for the oxygen production. The current state of the art uses Pt and Pt/Ru catalysts in quite high concentrations which need to be reduced by about a factor of 10 or preferably replaced by non-noble metals in order to become commercially interesting. This requires well-defined catalysts of controlled structures not yet available. Here a biomimetic catalyst or catalyst formation might bring essential breakthroughs. But also for the materials used as semiconductors significant improvements are needed to replace today's extremely expensive structures. Also here nanostructured devices are already under investigation.
Direct solar water splitting can in principle be of considerably higher efficiency than an electrolyzer-coupled photovoltaic system. Besides a solar hydrogen formation here also the reduction of nitrogen (N2) from air would be of great benefit since the formed ammonia would be a very convenient fuel especially for mobile applications. Since the proof of concept (10% efficient photoelectrochemical system were built in the National Renewable Energy Laboratory) was already done, here new materials, structures and catalysts are necessary mainly to improve the price and the life time.
The main challenge is to achieve photoelectrochemical fuel production with efficiencies >10 % at costs of below 2 /kg. This requires completely new materials, structures and catalysts. Biomimetic approaches might be of great help.
2-3 years: Upscaling of existing solid state/electrolysis systems to technical scale and for the use of less expensive preparation techniques. 5-10 years: Proof of concept of direct photoelectrochemical water splitting and to achieve an efficiency of more than 3%. 10-20 years: Achieve an efficiency > 10%.
Microalgae and cyanobacteria are the most promising organisms for conversion of solar energy into CO2 neutral biofuels such as biodiesel and for direct production of H2 from water. Strong research emphasis should be put on molecular biology, bioinformatics, systems biology as well as on metabolic and bioreactor engineering to select suitable micro-organisms, to improve their capability of biofuel production from sunlight and to design functional and profitable photobioreactor or pond systems.
The rapid development of clean fuels for the future is a critically important global challenge for three main reasons. First, new fuels are needed to supplement and ultimately replace depleting oil reserves. Second, fuels capable of zero-CO2 emissions are needed to slow the impact of global warming. Third, sunlight must be the ultimate energy source and water as a source of electrons, since both are available in unlimited amounts. Emphasis will be put on: 1) biological CO2 mitigation processes driven by sunlight and leading to the production of hydrocarbon biofuels and 2) solar powered bio-H2 production processes based on water-splitting photosynthetic micro organisms, green algae and cyanobacteria. 1) CO2 to hydrocarbons Research on water-based photosynthetic micro-organisms (primarily microalgae and cyanobacteria) has the potential to meet all of the above goals: microalgae have shown, in experimental plants, photosynthetic efficiencies of up to 5%, in the absence of any genetic optimization. They can accumulate large quantities of triacylglycerols (the starting point for biodiesel). Unlike land plants, whose CO2-fixation capacity gets saturated at low CO2 concentrations, microalgae and cyanobacteria thrive on concentrated CO2 sources, such as flue gas from power plants. Many can grow on marine water, a plentiful resource and they are able to utilize N- and P-rich wastewater as a nutrient source. 2) Water to H2 Hydrogen (H2) is considered to be one of the most promising clean fuels for the future. Advances in hydrogen fuel cell technology and the fact that the oxidation of H2 produces only H2O, increase its attractiveness. Yet, despite the many positive aspects of a future hydrogen economy, its viability is completely dependent upon the development of cost-effective sustainable large-scale H2 production systems, to replace the processes of steam reformation of natural gas, petroleum refining, and coal gasification. Emphasis is made here on direct (H2O H2) methods of solar powered H2 production using water-splitting micro organisms (microalgae and cyanobacteria).
Currently the global energy consumption rate is ~13TW-yr and is predicted to increase to 46TW-yr by 2100, suggesting that easily accessible reserves will be largely depleted by the end of the century. Although the rate of fossil fuel depletion is surprisingly fast, a much more rapid switch to zero-CO2 emissions fuels appears to be needed, to control the effects of global warming. Anthropogenic emissions of CO2 (total 7gtc p.a.) are only a fraction of the carbon recycled by terrestrial and marine photosynthesis (210 gtc carbon p.a.). Photosynthesis is the only viable alternative to geological sequestration for depleting CO2 from the atmosphere. CO2 neutral conversion of light to biofuels like H2O to H2 or CO2 to hydrocarbons will have important impacts on the successful replacement of fossil fuels. This includes a reduction of atmospheric CO2, the independence from imported energy sources and the development of huge novel markets. Land-derived biofuels are only partially able to meet this demand. In contrast, photosynthetic water splitting micro organisms ideally combine light collection, concentrated CO2 fixation and biofuel/bioH2 production in a single highly efficient cell system. In addition, their ability to grow in aquatic environments including sea eliminates potential competition with food producing agriculture.
Anaerobic photosynthetic bacteria have the capacity to evolve significant amounts of H2 powered by sunlight. This H2 can in principle be produced either by hydrogenases or nitrogenases. The bacteria use reductant derived either from solar powered reactions or the oxidation of organic compounds to drive H2 production. The concept of this work package is to use sunlight and waste organic materials, for example sewage, agricultural or animal wastes, as electron donors to support the growth of bacteria and their subsequent generation of H2. The procedures/experiments described in this work package will therefore not only produce solar-powered H2 but have the added environmental benefit of treating and removing waste products in a carbon neutral process.
Successful completion of this work package will allow development of new technologies for solar-powered H2 production by anaerobic photosynthetic micro-organisms. This will provide significant amounts of solar-derived H2 as a carbon-neutral replacement for fossil fuels. Since every household, farm and town in Europe continually produces large amounts of sewage or other waste material there are huge reserves of the required feed stocks to support this solarpowered H2 production concept.
3) systems to selectively funnel reduction from the solar energy pathways and the oxidation of organic nutrient to the enzymes that produce H2 4) strategies to either recycle or sequester the carbon locked up in the microbial biomass produced during solar-powered H2 production 5) produce a working lab-based prototype for optimization with different microbes and feedstocks With this knowledge base, we can then take the following steps 1) develop platforms to either trap the H2 for future use or for direct conversion into other fuels 2) collaborate with engineers in order to scale up laboratory-based prototypes to the sizes needed to make this a scalable process of use to a range of users from single family farms to large towns Development of this concept will need the involvement of microbial ecologists, biochemists, biophysicists, system biologists, computational and structural biologists. Once the basic underpinning research has been established, engineers, materials chemists and other physical scientists will be needed to establish the required new technologies to bring the concept of anaerobic solar-powered H2 production a reality.
The basic research underpinning this concept should be complete in 5-10 years. As this basic research comes to fruition, the development of useable systems for anaerobic, solar-powered H2 production will begin and continue for the following 10 years. We expect these systems will continually be modified as new information on the biology of the process is obtained and more efficient platforms are developed.
Fundamental research into systems biology will lead to novel routes for solar to fuel conversion and direct application in cell factories based on minimal life systems. Photosynthetic cell factories will use sunlight to directly produce fuels or biomass, or high value compounds starting from polysaccharides.
Current energy harvesting from biomass has a low efficiency due to intrinsic free energy losses at various levels in cellular metabolism. Nevertheless, this process has the attractive feature of automatic self-reproduction, so that for the long term energy budget it is not necessary to invest energy in renewing the photosynthetic apparatus. This is in marked contrast to PV cell production. It is a huge but timely scientific challenge to engineer from scratch a self-reproducing biological solar energy converter with maximized efficiency. A combination of systems-biology- and synthetic-biology approaches will be required for this. This should be a long-term concerted effort in which geneticists, physiologists, molecular biologists, biochemists, physicists, and computational experts should join forces to maximize the translation of atomic-level understanding to high yield solar to fuel conversion and coupling it with downstream production processes. This approach will require understanding of the living cell as a self-regulating, self-ordering, self-maintaining and adaptive entity and will eventually lead to a merger of life science and materials science. Life is based on interplay between diversification at the bottom, through random mutations, and selection from the top (i.e. at the organism level). Currently the basic understanding is still lacking regarding how to implement a smart matrix that provides the appropriate selection criteria for development, (genetic) adaptation and evolution into artificial systems with the desired properties. Bacteria generally live in ecosystems where tasks often are distributed over a multitude of species, and the steady state is maintained on a multi-organism level with high metabolic energy cost, in part because of the fluctuating environmental conditions. Alternatively, bacteria have become organelles like chloroplasts, embedded in a host matrix (eukaryotic plant cell) with lower metabolic costs. Several cyanobacterial genomes have been sequenced already at an early stage in the genomics revolution, since 1996, and the investigation of their systems biology properties is progressing well. Chlamydomonas reinhardtii, a recently sequenced single cell green alga, has been put forward as the "cell factory of the future" and biotechnology has produced variants that accumulate high quantities of polysaccharides. Time has come to design an "artificial organism" at the drawing table, including the minimum of pathways that are required to produce an on-board solar energy to fuel system to drive cell-factories in an efficient way. Probably the most suitable fuel is here polysaccharide that can fuel useful chemical conversion pathways taken from a variety of cell factory systems.
A major challenge is to provide comprehensive insight into the systems biology of relevant photosynthetic organisms to provide a sound basis for the de novo design of photosynthetic cell factories. Major hurdles will be to understand and implement: (i) Minimally metabolizing smart matrices and (ii) Proper (genetic) adaptation modules, for guiding development and evolution towards the desired function. This requires development of methods for de-novo design of minimal life units, including physiological and genetic adaptation for evolutionary optimization strategies, systems design, specification of biological and biophysical mechanisms leading to the desired properties, and analyses of the control and sensitivity in the designed systems. This should lead to the development of minimal self-reproducing units that can be applied in a variety of production processes, including biomass and bioenergy production. As an alternative, the design of an artificial chloroplast-type entity (i.e. without selfreproduction capability) embedded in a host matrix could be considered. This may further improve the efficiency. The overall success of this approach will also depend on the inclusion of methods that allow for an efficient harvesting of end products, such as spontaneous aggregation of products, selfsedimentation at the end of the (artificial) life cycle and facile drying of sedimented biomass.
5 years: photosynthetic cell factory producing polysaccharide reserves; 7 years: proof of principle for optimized minimal organism for biomass and/or fuel production; 10 years: artificial fuel producing organelle in a host matrix and 5 cell factory prototypes; 14 years: prototype minimal organisms; 15 years: implementation of cell factory production schemes and production with minimal organisms; 18years: prototype artificial organelle in host; 30 years: production with artificial organelles in hosts.
Paradigms addressing the integration of functionality at the molecular level are needed to combine independently-developed functional components into operational devices.
unit size, potentially relaxing bottlenecks associated to reduced charge carrier mobility and low absorption cross-section.
The attainment of the research goals set forth within will enable the construction of nextgeneration fuel and/or energy producing eco-devices that can be easily prepared and inexpensively assembled. Individual sub-units of end-of-life cycle devices can be recovered and recycled by molecular disassembly. The integration of biological components in hybrid synthetic devices will be possible using tailored nanostructured electrodes or individual molecular connectors.
Wide-based knowledge on the large-scale assembly of molecular units into functional devices is lacking and inspiration from natural systems should be sought. In parallel synthetic methodologies adapted for the preparation of hybrid inorganic-organic and bio-synthetic systems are needed to tackle large, complex architectures. New approaches to nano-structured materials (electrodes and membranes) must be developed for large-scale applications. Theory and modelling is essential to make rapid progress.
Suitable nano-structured materials are under development and should be available within a relatively short timeframe (2-5 years). Coupling of antenna systems with functional reaction centres would need to be developed within 8-10 years. This would be necessary to envisage the large-scale integration of operational catalytic assemblies into self-organized nanoscale devices to be completed within 20 years.
The objective here is to lead the renewable energy market with an innovative technology that does not consume fuel resources or produce greenhouse gas emissions. Glass heliostat antennas will concentrate the light in a macroscopic analogue of the PSI/PSII water splitting enzyme installed in a solar tower and operating at high temperature.
In PV there is a drive towards the 3rd generation to overcome thermalization losses. PV produces electrons in a conductor, and it will be difficult there to thermalize electrons and regain the energy. Therefore PV researchers look for methods to overcome the thermalization losses at the absorber stage. With fuel an opportunity opens up since it is possible to perform the fuel conversion at high temperature with a concentrator and do the thermalization afterwards using the gas that comes out and extract the energy by conventional means. The efficiency of photovoltaics in general decreases with temperature while it increases with concentrating the light intensity. The efficiency of utilizing heat produced by inevitable losses strongly increases with temperature. There exists an interesting temperature range between about 600 and 800 °C which is limited on the lower side by the availability of stable (ceramic) conductors and on the high side by the availability of stable steel parts. In addition there is already a lot of scientific activity exploring the possibility of energy conversion in high temperature fuel cells, while high temperature solar energy conversion in concentrating power stations (solar tower or solar dish systems) also exists. Finally there is already research to produce dye sensitized inorganic mimics of biological water splitting centres at a nanoscale. Very large scale PV solar tower systems have been installed in France and Spain in Europe as well as in Israel, the US and Russia. The price of electricity produced in such plants is currently about tenfold lower than by photovoltaics. Crossing the border between these two research communities will lead to novel concepts for producing both electricity and fuel in a large scale hybrid energy conversion concept by analogy with the organizational principles in photosynthesis.
Considerable efficiency gain towards 70-80% overall efficiency of one of the cheapest technologies for solar electricity generation already being fully competitive with today's electricity market price. The Solar Hydrogen Tower project can potentially change the world's energy markets forever, and possibly, reduce the effects of global warming. The high efficiency will minimize the land area that will be necessary for solar energy assimilation.
Implementation of a ceramic mimic of PSII/PSI to produce fuel in addition to operating thermal power plants to produce electricity. Since still all waste heat would be available for electricity generation as in a conventional solar thermal plant, the addition of such a system might even double the efficiency of the system directly leading to much lower electricity production costs. To achieve such an improvement is another main challenge.
range) may take place in 20 years, projecting 30 years for the first commercial system (0.5-2 GW).
Figure 5. Solar Tower plant in Almeria, Spain.
To obtain information about geometrical and functional structure of nanoscale solar energy converting devices and related catalysts on various lengths (nm-cm) and time scales (fs-s).
The integration process of the functional components will result in a multitude of nanoscale devices of great variety and displaying a plethora of very different properties. Hence, a detailed characterization of the features of the integrated systems at the fundamental physical level is crucial. We must understand the sequence of molecular events leading from the photon to the fuel, why certain events proceed with a relatively low efficiency, where the losses in the system occur etc. To study such highly complex and dynamic nanoscale structures at the molecular level demands the development of novel and adaptation of existing techniques to reveal information about their functional architecture. This approach includes also new methodologies for modeling, simulation, and quantum chemical calculations. Next to conventional high-resolution microscopies such as AFM and electron microscopy the development of high-resolution (spatial, temporal) spectroscopies is essential. For example: AFM affinity microscopy, Magnetic Resonance Force Microscopy (MRFM), multi-pulse, multi-resonance NMR and EPR, multi-pulse ultra-fast optical spectroscopy, high-resolution fluorescence microscopy, and surface spectroscopies (SERS, Plasmons, XAS etc.). Methods have to be developed to manipulate and control the structures by applying chemical and physical techniques to optimise function. Good examples are optical tweezers to manipulate the supramolecular organisation, the exchange of cofactors to tailor specific functions. Since development of a nanoscale solar energy converting device will require the characterization of many alternatives it is essential to elucidate a small number of principal functional parameters to be used for screening and combinatorial approaches.
New technologies for characterization of functional architectures and operation of nanoscale solar energy converting devices at the molecular level will enable and facilitate construction, characterisation and manipulation of robust solar energy converting devices.
Development and adaptation of new techniques is expected to take 5-10 years and characterization of novel nanoscale solar energy converting devices will take 5-20 years.
In biology, evolutionary design proceeds in an interplay between diversification at the bottom and selection from the top. The objective is to develop new computational tools able to mimic this process and predict in silico the properties and function of nanoscale structures to assist and drive the design of nanodevices for solar energy conversion.
A promising strategy for future breakthroughs in solar energy conversion is the design of highly efficient, artificial, molecular-level energy conversion machines exploiting the principles of natural photosynthesis. Progress in the design and synthesis of nanodevices for artificial photosynthesis needs the concomitant development of computational modeling methods that are able to predict new nanosized materials with specific target properties, with a particular emphasis on nanostructures with desired energy conversion and catalytic properties. Photosystems I and II, as well as other enzymes can provide inspiration for the development of artificial nanodevices. For instance, the primary photochemistry of photosynthesis, where the chlorophyll molecule is excited by sunlight and the energy produced helps to break down a water molecule resulting in O2 evolution and release of protons, can be translated into artificial devices of various kinds. These two examples (as well as most of the organic systems) belong to the "Soft Condensed Matter" category. In other words, their non-bonded energy is comparable to their entropy and thermal fluctuations play an important role in their (inherently) statistical description. The processes that are relevant to solar energy conversion may involve systems with thousands of atoms and different time scales ranging from ps to ms. Hence, even the simplest models must incorporate closely coupled quantum and atomistic/mesoscopic/macroscopic levels. It is therefore crucial to extend the size range and time scale of existing methods to deal with these multi-scale processes. The long-term goal here is to use such hybrid quantum-classical methods (such as Density Functional (DFT) based or QM-MM simulations), as predictive tools, which starting from a wide range of simultaneous desired properties as inputs can yield materials arrangements as outputs. To achieve this goal we need to develop an appropriate interface between these methods and evolutionary algorithms currently used in computer science to find solution to optimization and search problems.
New computational tools able to predict nanostructures with desired target functions and properties will enable us to enormously focus the search for bio-inspired structures to only those that are potentially active and promising for their high efficiency.
1) Finding the appropriate compromise between the complexity of the biological system and a simplified model structure taking into account the crucial elements of the active site. 2) To sample parameter space in an economical way to find the most rapid convergence to desired systems and to overcome the time scale limitations in the simulations. 3) Improve the accuracy and efficiency in computing excited-state potential energy surfaces. 4) Develop new computational strategies, specifically, grid-based computing methods for they provide seamless and scalable access to wide-area distributed resources and they are particularly suitable in the implementation of evolutionary algorithms. 5) Develop a rational compound design technique which would allow to avoid screening of the high-dimensional chemical space spanned by all the possible combinations and configurations of electrons and nuclei and to perform a gradual optimization of the chemical structure of a compound using grand-canonical density functional methods. 6) Develop an adaptive technique which would let us study large systems: a part of the system (e.g. next to the electrodes) can be treated quantum mechanically, the nearest layer atomistically, and the bulk of the system has a coarse-grained description.
Development of novel computational tools with predictive power in combination with evolutionary modeling is expected to take 5-10 years. In 10-20 years we expect to be able to perform evolutionary modeling of a 10-nm sized macromolecular structure.
Aerobic/Anaerobic Occurring or living only in the presence of oxygen/ occurring or living when oxygen is absent. Photosynthetic pigments that occur in various bacteria. Imitating, copying, or learning from biological systems. Part of a planet's outer shell -- including air, land, surface rocks and water -- within which life occurs, and which biotic processes alter or transform. From the broadest geophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere (rocks), hydrosphere (water), and atmosphere (air). Literally "hydrates of carbon," chemical compounds which act as the primary biological means of storing or consuming energy; other forms being via fat and protein. Relatively complex carbohydrates are known as polysaccharides. Substance (Greek: , catalyt s) that accelerates the rate or ease of a chemical reaction (called catalysis) without itself being changed at the end of the chemical reaction. Catalysts participate in reactions but are neither reactants nor products of the reaction they catalyse, except for autocatalysis where the reaction product is itself the catalyst for that reaction. Green photosynthetic pigment found in plants, algae, and cyanobacteria. Organelles found in plant cells and eukaryotic algae that conduct photosynthesis. Chloroplasts capture light energy from the sun to produce the free energy stored in ATP and NADPH. Processes that remove the excess salt and other minerals from water in order to obtain fresh water suitable for consumption or irrigation. Measure of the 'load' imposed by a given population on nature. It represents the land area necessary to sustain current levels of resource consumption and waste discharge by that population. Consumption of resources that goes beyond the surplus resources that can be consumed by humans without damaging the ecosystems. Device in which an electric current splits water into hydrogen and oxygen.
is known as Avogadro's number and is approximately 6.0221415 10 . Nanoscale Organelle Of the order of 10 -9 meters (nanometers) in size. Discrete structure of a cell having specialized functions. An organelle is to the cell what an organ is to the body (hence the name organelle, the suffix -elle being a diminutive). Photon Conversion Efficiency. Efficiency of converting sunlight into stored energy.
photovoltaic energy, solar thermal energy, wind power, lowhead hydro power, geothermal energy, landfill and minebased methane gas, energy from waste and sustainable biomass energy. Self-Assembly The process where components spontaneously organize or assemble into more complex objects without a central control mechanism or external assistance. The process takes place through random movements of the molecules and formation of weak chemical bonds between surfaces with complementary shapes. Any chemical system that exists at a higher level of complexity than individual molecules (for example, multienzyme complexes, organelles and membranes). 1012 Watt.
The support of the ESF European Science Foundation, Strasbourg, Robert Bosch AG, Stuttgart, the Max-Planck-Institut für Bioanorganische Chemie, Mülheim an der Ruhr and the Leiden Institute of Chemistry, Leiden University for sponsoring the workshop and for the preparation of the White Paper is gratefully acknowledged. Frank Niele (Shell) is thanked for a critical assessment of the introduction segment at an early stage in the writing process. Part of the appendix is adapted from a background report on solar energy conversion prepared by an international panel of scientists at a US Department of Energy workshop.
Photosynthetic organisms are ubiquitous on the surface of the Earth and, in fact, responsible for the development and sustenance of all life on the planet. Among these many different classes of photosynthetic organisms many varied types of light-harvesting and electron transport systems are used. However, they all use the same basic pattern whereby the light energy is initially absorbed (and concentrated) by an antenna system and this energy is then transferred to a specialized reaction centre, where that captured energy is transduced into useful chemical energy. This allows photosynthesis to operate efficiently over a wide dynamic range of light intensities. Photosynthesis can be divided into oxygenic (O2 producing) photosynthesis carried out by cyanobacteria and plants, and non-oxygenic photosynthesis (e.g. purple bacteria, green-sulphur bacteria). Oxygenic organisms harness solar energy to extract the H+ and e- from H2 O, required for CO2 fixation. Non-oxygenic organisms cannot generate the necessary oxidizing potential, to oxidize H2 O and therefore extract H+ and e- from alternative substrates (e.g. H2S). Under normal conditions both processes use the derived H+ and e- for synthesis of ATP, NAD(P)H and ultimately CO2 fixation, to produce carbohydrates such as starch and glycogen which can be considered to be H+ and e- `stores' that can be used for the CO2 neutral production of fuels.
Figure 1. Compartimentalization of light harvesting and charge separation (schematic).
Purple non-sulphur bacteria contain the most studied photosynthetic apparatus. It consists of two light-harvesting pigment-protein complexes (LH1 and LH2) and a single type of reaction centre (RC). Both of these pigment-protein complexes are membrane bound and utilize bacteriochlorins and carotenoids.4, 5 Green sulphur bacteria use large aggregates (up to 10,000) of Chl c molecules that are contained in specialized vesicles called chlorosomes. The energy harvested by these chlorosomes is transferred via the FMO complex to the membrane bound reaction center.6 Cyanobacteria use large peripheral phycobilisomes as their major light-harvesting system. The phycobilisomes funnel absorbed energy down into the membrane and supply excitation energy to both photosystem I (PSI) or the photosystem II (PSII).7 Photosystem I (PSI) contains ~100 chlorophyll a and 12-16 -carotene molecules. PSI couples electron transport to an electrochemical membrane potential and produces NADPH for carbon fixation.8 In contrast, PSII contains 36 Chl a, two pheophytin a and about seven -carotene molecules. Uniquely, the PSII reaction centre can split water to produce oxygen. The next section will focus on the structure and dynamics of light harvesting and photoconversion within the purple bacterial photosynthetic apparatus and within PSI and PSII of plants and cyanobacteria where we have detailed X-ray crystal structures.
with kf the radiative constant and r0 the half-radius of the Förster process. Since the length scale of exciton migration in the antenna is large, the exciton lifetimes should be long enough to allow for photons striking any part of the antenna to reach the RC. Once excitons are trapped in the RC, the process of charge separation should take place faster than back-transfer to the antenna units. This is achieved by a relatively large antenna-RC distance versus a short distance for the redox pigments in the RC. Furthermore, surrounding of the RC's by assemblies of antenna pigments gives a spatial arrangement in which energy transfer is optimized by multiple entries to the RC. The combination of constraints (spatial distance antenna-RC and multiple entries) is achieved by circular-like arrangements of antenna pigments around the RC's.
An extremely important design feature of the photosynthetic complexes is their built-in mechanism against photo-oxidative damage. In shade, light is efficiently harvested in photosynthesis. However, in full sunlight, much of the energy absorbed is not needed and there are vitally important switches to specific antenna states, which safely dissipate the excess energy as heat. This way protection is given against the potential photo-damage of the photosynthetic membrane. Carotenoids (Car) have a keyrole in this photo-protective mechanism. They are highly unsaturated long-chain polyenes that function in a photoprotective capacity by directly quenching the chlorophyll excited triplet state, which could otherwise sensitize formation of singlet oxygen. Furthermore, they act as accessory lightharvesting pigments by absorbing light energy in the visible spectrum unavailable to chlorophylls and stabilize the (non-covalently assembled) structures of the photocomplex assemblies. Carotenoids have a strong excitonic coupling with the light-harvesting pigments to achieve efficient energy transfer. In plants ­with a ratio Car:Chl about 1:6­ the carotenoids are capable of 100% quenching of the Chl triplets. This means that each Car is capable to quench on average six Chls, requiring a very specific positioning of the carotenoids in the photocomplexes, which has been optimized by the protein environment.
Splitting of water into protons and oxygen is energetically demanding and chemically difficult. The manganese-containing, water splitting catalytic site in natural photosystem II (PSII) complexes performs this reaction at close to thermodynamically limited efficiency (< 0.2 V overvoltage), at a high turnover rate (~ 103 s-1), and under mild external but low effective internal pH conditions. The catalytic reaction involves highly oxidative chemistry. The D1-protein binds the majority of the cofactors involved in light-driven charge transfer reactions of PSII, including the primary electron donor P680 and the Mn cluster at which the water-splitting reaction occurs. During operation the D1protein is damaged, most likely due to singlet oxygen and/or oxygen radicals formed during the watersplitting process. As a consequence, the reaction centre of PSII has the most dramatic self-repairing system within photosynthesis. The vulnerable D1 protein is removed from the complex from time to time (about 30-60 minutes in an illuminated leaf!) and replaced by a newly synthesized D1-protein. The operational details of this self-repair process are not yet fully resolved.
Figure 2. Photosynthetic unit of purple bacteria. (a) Modeled structure of light harvesting units 1 and 2 (LHI and LHII) and the reaction center (RC). The structures of LH-II and the RC are from the crystal structures, and LHI is simulated by analogy to LHII. (from Hu et al., 1998) (b) Dynamics of energy transfer processes between bacteriochlophylls B850 and B800 in LH2, B875 in LH1, and the special pair P870 in the RC. 2 (c) AFM image of arrangement of LH2 (highlighted in green) and LH1-RC (red) complexes in the bacterial membrane. The bright spots are the LH1-RC complex and the inset in the second panel is a model using the known crystal structures to reproduce the image.3 (d) Energies and time-scales of electron transfer within the reaction center.
which may participate as either a direct or "virtual" intermediate state in the electron transfer (see Figure 2d). The electron then moves to QA in ~200 ps and is subsequently transferred to QB in ~200 s.22 Following a second photochemical reduction, QBH2 moves away from the RC through a break in the LH1 ring, a process likely regulated by a small alpha-helical protein, PufX.20 As observed by atomic force microscopy (AFM), the LH1-RC complexes form small linear arrays that are interconnected by groups of 10-20 LH2 complexes (Figure 2c).3 This architecture allows for efficient EET between LH2 complexes and from any LH2 to several LH1-RC complexes. Once deposited in an LH1 complex, the close proximity of other LH1s allows for efficient migration of the excitation should the initially associated RC be already in use.
Photosystem I (PSI) functions to produce NADPH that is used to reduce carbon dioxide in the reactions of the Calvin cycle. When cooperating with PSII, it uses the energy of light to transfer electrons from plastocyanin or soluble cytochrome c6 to ferredoxin and eventually to NADP+.23 In an alternative pathway, the electrons from ferredoxin are transferred back to plastocyanin via the cytochrome b6f complex. This cyclic electron transport, which does not require the input of free energy by PSII, results in a transmembrane electrochemical gradient that can be used to produce ATP. NADPH and ATP are used to reduce CO2 to carbohydrates in the subsequent dark reactions. 24 An Xray structure of PSI has been obtained at 2.5  resolution25 (see Figure 3a). The PSI core is a large pigment-protein complex consisting of 11-13 protein subunits. The largest two subunits, PsaA and PsaB, comprise a heterodimer which binds the majority of the reaction centre co-factors and core antenna pigments. The reaction centre is bound in between PsaA and PsaB along the local pseudo-C2 axis. The components of the reaction centre are as follows: P700 (a dimer containing one Chl a and one Chl a' molecule8) where charge separation is initiated, Ao (a Chl a molecule), A1 (a phylloquinone), and the three [4Fe4S] clusters Fx, FA, and FB (see Figure 3b). While the exact electronic structure of the P700 special pair is still unknown, the redox potential of P700* of ~ -1.2 V makes it the one of the most powerful reducing agents found in Nature. After photoexcitation, P700 transfers an electron to Ao within 1-3 ps. Subsequent electron transfer from Ao-· to A1 occurs within 20-50 ps.25 Reoxidation of A1-· via Fx shows biphasic kinetics with 1/2 = 20 ns and 150 ns and is the source of much debate. The kinetics of electron transfer from Fx to the terminal iron sulphur clusters FA and FB are also under review since the rates are faster than for electron transfer from A1 to Fx. Once the electron is transferred from Fx, through FA, to the terminal iron sulphur cluster FB, it reaches the soluble electron carrier ferrodoxin ( 1/2 = 500 ns) and leaves the site for use in the reduction of NADP+. Photosystem I contains an integral antenna system consisting of about 90 Chl a molecules and 22 carotenoids. The antenna pigments can be divided into three regions, one where the antenna pigments surround the inner core, and two peripheral regions where chlorophylls form layers on the stromal and luminal sides of the membrane. The average RC Chl to antenna Chl distance is larger than 20 , however two antenna Chl molecules are located within 14  of the closest RC pigment and may serve as a functional bridge between other antenna and the RC. Included in the peripheral regions are the chlorophylls which absorb at longer wavelength that P700 (so called `red' chlorophylls). The redshifted absorption is caused by exciton interactions, electronic interactions with polypeptides and changes in the dielectric constant of the surrounding protein environment. Excitation on the antenna pigments results in a rapid equilibrium distribution of the energy among the antenna chlorophylls with a 4­8 ps lifetime.25 The rate of energy transfer from the antenna system to P700 or `trapping time' varies between (20 and 35 ps)-1 and depends on the organism and the antenna size.
Figure 3. Photosystems I (PSI) and II (PSII) (from Grotjohann et al., 2004). (a) Comparison of the protein structures and cofactor arrangement of each photosystem. The major proteins that flank the reaction centers are shown in orange (PSI) and silver (PSII). Smaller protein subunits are depicted in yellow. (b) Arrangement of the redox cofactors within the reaction center of each photosystem. Chlorophylls are shown in green; pheophytins in yellow; plastoquinones in lime; coordinating histidines in red. The 4Fe4S clusters are shown above the PSI quinines in yellow and blue and the oxygen evolving complex is shown in purple and red below the tyrosine in PSII.
Photosystem II (PSII) catalyzes one of the most thermodynamically demanding reactions in biology: the conversion of light energy into a redox couple capable of oxidizing water.26 As a by-product, O2 is released into the atmosphere. The crystal structure of PSII reveals details with a resolution of 3.5 . The PSII core complexes consist of 19 proteins. The central region shows striking similarities to the protein structure of the bacterial reaction centre (see Figure 3a).27 The RC complex of PSII contains 6 Chl a molecules, 2 -carotene molecules and 2 pheophytin a molecules and is bound to a heterodimeric protein core formed by subunits D1 (a.k.a PsbA) and D2 (a.k.a. PsbD, see Figure 3b). Unlike the photosystem I P700 analogue, P680 is not a `special pair' of strongly interacting molecules as the exciton coupling of P680 is far weaker.28 Instead, the four inner-most chlorophyll a molecules are coupled so that the initially local excited state becomes delocalized over the reaction centre chlorophylls in 100-500 fs. The intermediate charge transfer state, formed with an intrinsic rate constant of 1.5 ps, is attributed uniquely to Chl1+·-Pheo1-·.29 Electron transfer proceeds further to the first plastoquinone QA within 200 ps. QA-· then doubly reduces the secondary quinone acceptor, QB, with the possible involvement of a non-heme iron located on the pseudo-C2 axis with time constant of 0.2-0.4 ms and 0.6-0.8 ms for the first and second reductions respectively. After receiving two protons, QB- then leaves its binding pocket as a plastoquinol molecule. The plastoquinol then diffuses out of the protein to be oxidized by cytochrome b6f.
Figure 4. Electron storage and fuel production using oxygenic photosynthesis. Two consecutive light-driven reactions (h ) in the photosystems PS I and PS II transfer electrons via plastoquinon(PQ) to ferredoxin (Fd). Subsequently the electrons can be used to reduce protons (H+) and produce fuel, either by CO2 fixation via NADPH into starch or by hydrogenase/nitrogenase into H2 (adapted from 1).
Figure 5. A proposed mechanism for the final step of the state cycle when the dioxygen bond of O2 is formed that is of possible interest for design of artificial photosynthesis components. The very high oxidation state of the Mn-cluster, particularly the Mn ion outside the Mn3Ca2+O4 cubane, leads to a high electron deficient oxo (= 0) after deprotonation of water molecules during the S-stae cycle or oxyl radical (-0·) which facilitates a nucleophilic attack on an oxygen of a second water molecule located in the coordination sphere of Ca2+. The arrows indicate direction of movement of electrons (Barber 2006).
The oxidized primary donor P680+· is among the strongest oxidants generated in biological systems and has a redox potential estimated to be 1.3 V versus the standard hydrogen electrode. The first P680+· reduction by a redox-active tyrosine residue occurs on a time scale of 20-40 ns. The tyrosine residue then oxidizes one of the Mn ions contained within a cluster of four in the oxygen evolving center (OEC). For each reduction of P680+· to P680 one oxidizing equivalent is generated. As P680 undergoes further photoinduced oxidation cycles, the (Mn)4 cluster accumulates oxidizing equivalents and the time for re-reduction of P680 increases from tens of ns to hundreds of ns. After four oxidizing equivalents are accumulated by the (Mn)4 cluster, one oxygen molecule is produced, and the cycle begins again. The recently determined structure of PSII has provided insights into the organization of the 4 Mn ions and Ca2+ which make up the catalytic center of the oxygen evolving complex and the protein side chains which surround it including those involved in directly in ligating the metals (see Figure 5).
Figure 6. The basic mechanisms of the photosynthetic solar cell. A chlorophyll molecule is connected to a reservoir at ambient temperature T ~ 300 K, and emitting heat at a rate Iqout, is excited due to the heat flow Iqin from the solar reservoir at temperature T 5800 K with a rate g to an excited state separated by an energy h from the ground state. The excited chlorophyll state either decays with loss rate 1/ or produces an electron-hole pair with net charge separation rate I and free energy = e h.
with If, Ib the forward and backward reaction rates.
For instance, in the PSII reaction centre light excitation energy localizes on one or several chlorophyll molecules P*, Chl* (See Figure 3). The enzyme is able to keep its overall catalytic turnover efficiency at > 90% yield in a cascade of reversible electron transfer reactions. The overall time scale is set by the initial fluorescence life time of the P* of ~ 10-9 s and charge separation gets gradually slower as the probability for recombination becomes smaller for the later intermediates. In this way energy dissipation is used to optimize catalytic turnover with a maximum rate of 103-104 s-1. Almost nothing is wasted since the dissipating enzyme operates very close to the thermodynamic limit imposed by the second law of thermodynamics for linear non-equilibrium processes. The highest yields and efficiencies are attainable when energy is used as soon as it is assimilated, avoiding intermediate storage, for instance by generating and using electricity, by running catalytic converters at a nanoscale directly from the photoconverter, and storing the energy in a redox couple, by coupling a chemical cycle directly to a photoconverter like in the chloroplast, by connecting the chloroplast directly to respiration like in the plant, or by using light driven cell factories for production of food and chemical feedstock.
Figure 7. Thermodynamic scheme for the reaction centre of PSII. The stabilization required to allow the reactions with water and with plastoquinone is obtained by a large energy dissipation in the form of heat and by the high energy level of the primary radical pair, P+ I-.
Table 1 Thermodynamic limits for a dynamic reserve. In practice molecular dyes have an excited state life time of ~10-9 s, the theoretical maximum storage time is thirty orders of magnitude longer and the efficiency limit for storage on human time scales is 0.5-0.4.
with high efficiency. Each half reaction requires four photons and together they lead to the net production of solid (i.e. without back pressure) carbohydrate fuel. This task is performed by the Calvin cycle that runs in the chloroplast organelle, which relies for its maintenance and reproduction on its embedding in the higher organism environment of the green plant. In general chemical cycles perform optimally when they run in a small compartment. In vivo 9­10 photons are required for reactions (9) and (8), under the most favourable conditions, with low irradiation flux. An energy input of 1760 kJ is required per mole of O2 produced, which is approximately four times more than the standard free energy change of 467 kJ/mol, and corresponds with a maximum efficiency of 27% of absorbed light at 700 nm.37 This reduces by 700/400 for 400 nm illumination, to 15%. The mean optimal efficiency between 400 nm and 700 nm thus reduces to ca. 21% of the absorbed light. This is equal to a conversion efficiency of 9% of the total solar spectrum, assuming only the spectrum between 400­700 nm is used. In comparison, commercial multi-junction Silicon photovoltaic cells absorb above 1.1 eV from the visible through to the near infrared region and are rated at ~14% over the entire solar spectrum, but without storage of the electrons that are produced. In photosynthesis, much of the energy that is stored is subsequently used to support cellular processes and therefore only a very minor fraction is stored as biomass. 38, 39 The starting point for all biological solar driven H2 production methods is the water splitting process (7). Algal H2 production was reported as early as the 1930s. 40 H2 production rates can be increased by temporally separating the water oxidation from the H2 production process, catalyzed by hydrogenase.41 However, this process has the down side that H2 is only produced ~50% of the time and it uses 6 H2 quanta of light per H2 produced. It would therefore be more efficient to use a direct H2 O continuous conversion process using 4 quanta at low sulphur concentrations, in which PSII generates O2 at a rate just below that at which it is consumed by respiration. For comparison, typically four Si PV cells can be connected in series to an electrolyzer to produce hydrogen in an operational configuration with overvoltage, and around 16 photons are used to drive the four-electron water splitting reaction. However, since the Si cells operate with a band gap of 1100 nm they collect ~40% more photons than photosynthesizers and this contributes to the overall efficiency of the device. The energy produced in photosynthesis has to be sufficient to drive the desired catalytic reaction for maintaining the chemical (fuel) reserve and its downstream utilization. Photosynthesis is able to produce intermediate solid fuel with very good quantum yield, since ~10 photons are needed in practice to drive reactions (7) and (8) at low light intensity. An isolated photosynthetic system can sustain a catalytic conversion rate of 103-104 s-1 with ~300 chlorophyll molecules in the spectrum of solar irradiance.42 Catalysis occurs on the time scale of self-diffusion of molecules, ~10-3 s. Fecatalysts derived from hydrogenases may be attractive for use in bulk catalytic converters since the proton diffusion is very rapid and the natural enzyme can catalyze the reduction of protons to form hydrogen at a rate of more than 25,000 s-1, among the highest of any known enzyme.
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