By Lubova Dziojeva
The concept of artificial photosynthesis was introduced more than a century ago by Giacomo Ciamician in a Science paper titled the “The photochemistry of the future” (Ciamician, 1912). He insisted on the importance of developing technologies that would move humanity away from the complete dependence on fossil fuels. Technologies that use sustainable resources are already developed. The Fischer −Tropsch (FT) cycle allows combination of H2 (obtained from the splitting of water) with CO (from reduction of CO2) to produce fuels. However, the hydrogen for FT is most commonly obtained from coal or natural gas making the process carbon positive. Large-scale solar farms could meet the demand for sustainable energy, especially with their cost being lower than that of thermal and nuclear power plants (Schmidt et al., 2019). However, there is no efficient storage solution for the energy generated by solar panels. Artificial photosynthesis would be able to meet both challenges by being carbon negative and providing a source of solar fuel.
Major progress has been made by a Harvard group of researchers led by Professor Daniel G. Nocera. Their approach centres on the concept of a Sustainocene. The Sustainocene theory was first introduced by a group of Australian researchers (Faunce, 2012) who suggested that it is impossible to view sustainability only in terms of ecological integrity if societal imbalances and poverty exist. Therefore, Professor Nocera’s team is working on ways to create a decentralised energy and solar fuel as well as fertiliser generation system. Their technology allows the use of sunlight, air and any water source as the only starting material.
Photosynthesis involves several stages. The primary steps consist of the absorption of sunlight at the oxygen-evolving complex (OEC) of photosystem II (PSII) to oxidise water to oxygen. The electrons produced as the byproduct of OEC reaction are transferred to ferredoxin of photosystem I. Catalysed by ferredoxin-NADP+ reductase they are used to reduce NADP+ to NADPH. A synthetic material to replace this reaction would have to capture a solar photon to generate a wireless current that is transferred to catalysts. Catalysis of water-splitting reaction and the subsequent fuel-formation would have to be performed at benign conditions.
Professor Nocera’s lab has produced an artificial leaf and a bionic leaf. The artificial leaf consists of a thin silicone plate covered with a catalyst coating. The artificial leaf can use a wider absorption spectrum compared to natural photosynthesis due to the use of materials such as amorphous silicon and germanium embedded in silicone (Nocera, 2012). Due to the use of a larger available spectrum of solar energy artificial photosynthesis has reached efficiencies that are 10 times that of the natural equivalent (Dogutan & Nocera, 2019). Catalyst coating consists of a ternary alloy (NiMoZn) and cobalt-phosphate cluster (Co-OEC) which both have self-healing properties (Nocera, 2012). This light-absorbing material can be placed in water and will direct solar energy to water-splitting at benign conditions. Hydrogen generated can be collected to fuel cells. However, modern infrastructure is built on the use of fuels therefore the application of fuel cells is not wide unless the underlying framework of distribution is changed. The bionic leaf is an extension of the artificial leaf that can convert sunlight, air and water into usable fuels and fertilizers.
The artificial leaf replaces photosystem II, the electron-relay part of the photosynthetic membrane and photosystem I. The bionic leaf uses the artificial leaf to generate the hydrogen and follows this by generation of fuel via an H2-oxidising autotrophic bacterium creating a hybrid inorganic-biological system. An adjustment had to be made due to the generation of reactive oxygen species by NiMoZn catalyst, which would damage bacterial DNA. Furthermore, Ni seeped into the solution from NiMoZn alloy inhibiting bacterial growth. Self-healing Cobalt-phosphorus alloy was used instead (Liu et al., 2016). Ralstonia eutropha was genetically edited to overexpress hydrogenases in the outer membrane to increase the consumption of H2 by the bacterium. Hydrogenase catalysed the reduction of NADP+ to NADPH which entered the Calvin cycle. Calvin cycle uses carbon dioxide from the atmosphere. 3-Phospho-glycerate produced as part of the cycle can be converted to acetyl-coenzyme A, biomass, polyhydroxybutyrate (PHB). PHB is a polyester polymer that is of interest as a biodegradable plastic. The range of possible end-products can be expanded by genetic engineering of R. eutropha to redirect the acetyl-CoA to the synthesis of isopropanol (Torella et al., 2015). The efficiency of conversion of solar energy to biomass obtained was 10%, 10 times higher than that found in the fastest-growing plants (Liu et al., 2016).
Some estimates indicate that food supply would have to double by 2050 to meet the increased demand of the 10 billion population (Assembly, 2009). Advances in agricultural technology could account for a large proportion of the demand increase, but this will be associated with higher fertiliser use. Fertiliser production is an energy-intensive and fossil fuel-reliant process. Currently, the Haber-Bosch method of producing ammonia for fertiliser use is responsible for 3-5% of world natural gas use and 1-2% of world’s energy use (Smith, Hill & Torrente-Murciano, 2020). Another problem that arises is that the higher agricultural demand is prevalent in the developing world, in areas without the distribution network nor the factories for the large-scale production of agricultural chemicals. The use of artificial leaf in combination with a nitrogen-fixing bacterium such as Xanthobacter autotrophicus will produce ammonia (Liu et al., 2017). Nitrogen-fixation is an energy-intensive process in biological systems and therefore it is controlled by downregulation pathways. Artificial leaf within a bioreactor will create an energy-rich environment using solar energy that will favour nitrogen-fixation by microorganisms producing biofertiliser. As well as being renewable and carbon-negative, biofertilisers also have lower nitrogen runoff rates (Liu et al., 2017). Nitrogen runoffs are associated with harmful algal blooms (Davidson et al., 2012).
The application of artificial and bionic leaves technologies will allow decentralised production of fuel and fertilizers. Artificial photosynthesis is an advantageous method of energy production in terms of environmental integrity as well as providing an access to fuel and fertiliser in less developed areas. Artificial photosynthesis could be part of the solution to address social imbalances as well as the climate crisis.
Assembly, U. G. (2009) Food Production Must Double by 2050 to Meet Demand From World’s Growing Population. Press Release, October. 9 2009.
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