Sunlight accounts for the largest energy input onto the earth’s surface, providing more energy in one hour than all of the energy presently consumed by mankind in one year. While the conversion of solar energy into electricity is well developed, there is a strong need also for storing solar energy in form of fuels. Fuels presently account by far for the largest part of our energy needs and are required for example for most transportation purposes and for adjusting the daily and seasonal changes in solar irradiation to the often counter cyclic demands.
More than 2 billion years ago autotrophic cyanobacteria evolved the most efficient method for utilizing and storing solar energy: they catalyze the light-driven splitting of the abundant water into O2, protons and electrons. Nature uses these protons and electrons mostly for CO2 reduction leading to biomass formation. In plants, the overall energy efficiency of this process is in general of order of 1 %. While this number does not sound impressive, one needs to remember that this includes all maintenance, self-repair and reproduction processes.
The primary processes of photosynthesis are highly efficient. If one finds a way to combine the high-energy electrons produced by light-induced water-splitting with protons to form molecular hydrogen, overall energy efficiencies of 10-20% are feasible. This may be achieved either in genetically modified cyanobacterial or algae systems, or within fully synthetic devices that mimic important features of the natural processes. Burning of photo-catalytically produced H2 returns water; this closes the reaction cycle and making solar hydrogen a renewable fuel.
In biological systems the present strategies involve directing as many as possible electrons from PSII directly to either hydrogenases or nitrogenases for the production of H2. Alternatively, pathways for the production of other solar fuels and valuable products are engineered into the microorganisms.
In artificial photosynthesis the goal is to understand and utilize the principles that biology developed within fully synthetic systems. The main processes involved include the harvesting of sun light, its conversion into positive and negative charges, and utilizing these charges for the oxidation of water and the synthesis of a fuel. The advantage over the biological approach is that higher energy efficiencies can be expected. In recent years much progress was achieved with regard to developing individual components, and even some complete devices were reported that mostly relied on earth abundant, non-toxic materials, which is an important requirement for future mass implementation. Present challenges include improvement of the energy efficiency and stability of such systems, and the reduction of the costs for their production.
Artificial solar fuels devices can rely either on homogeneous or heterogeneous catalysis. For homogeneous systems water-soluble molecular catalysts are required that combine all above named steps. Heterogeneous devices can represent suspensions of semiconductors, possibly decorated with catalysts, or artificial leaf devices in which a light absorber and charge generator is decorated on the positive side with a water-splitting catalysts, and on the negative side with a proton reduction catalysts. The light absorber may be a semiconductor of suitable bandwidth (or a combination thereof in multi junction solar cells), or for example be based on organic solar cells that may include carbon nano-materials. As catalysts both inorganic oxides and coordination complexes coupled to surfaces are presently explored.
A third approach is the combination of solar cells with electrolyzers containing electrodes made of earth-abundant catalysts. These are principally the same as in the artificial leaf approach and therefore such devices are presently developed both in the solar cell as well as the artificial photosynthesis communities.