ENERGY


more than two photons per electron to absorb the whole solar spectrum. And despite the desirability of having a single material that performs all the necessary tasks, from absorption and charge separation to catalysis, no inorganic material capable of doing so has yet been discovered. Nanotechnology holds the promise of designing nanostructure subsystems that mimic the various parts of the natural photosynthesis. But progress in this area


is slow. Molecular-inorganic


hybrids combine the advantages of both


molecular and inorganic


systems. A semiconductor or an organic pigment molecule on a semiconductor surface absorbs the light; charge separation then takes place within the semiconductor and the charge is injected directly into optimised molecular catalysts attached to the surface. This promising approach has yielded a number of experimental devices.


Hybrid systems Semi-synthetic systems also involve a hybrid of biological and artificial components. For example, a biological component that harvests light and splits water could be modified chemically or by synthetic biology and tethered to a suitable scaffold, which would go some way to creating a response matrix. This system could then be linked to a hydrogen-producing enzyme or a catalyst to produce a fuel with a high-energy density. Another idea is to interface chlorophyll molecules with semi-artificial components that perform the rest of the photosynthesis process. This approach is still in development, especially as it is not yet clear whether biological components can be made sufficiently robust to operate outside their natural environments. Gerald Manbeck and chemists


from the US Department of Energy’s (DOE) Brookhaven National Laboratory and Virginia Tech have


taken a different tack. They have designed two photocatalysts that incorporate individual components specialised for light absorption, charge separation, or catalysis into a single ‘supramolecule’. In both molecular systems, multiple light-harvesting centres made of ruthenium (Ru) metal ions are connected to a single catalytic centre made of rhodium (Rh) metal ions through a bridging molecule that promotes electron transfer from the Ru centres to the Rh catalyst, where hydrogen is produced. ‘[Our work] is for the reduction half of water-splitting only,’ Manbeck explains. ‘For these studies, the electrons originate from the oxidation of a sacrificial reductant, dimethylaniline, which is oxidised by the Ru excited state. Integration with a water oxidation catalyst is clearly needed for practical use.’ One of the drawbacks they


had to overcome is managing the synchronicity of the various subsystems within the supramolecule. This is because the various processes required to generate the hydrogen production occur at different rates. Failure to synchronise them means the separated charges, the negatively charged light-excited electron and positive ‘hole’ left behind after the excited molecule absorbs light energy, have the chance to recombine – reducing efficiency and generating waste heat. Another complication is that two


electrons are needed to produce each hydrogen molecule. For catalysis to happen, the system must be able to hold the first electron long enough for the second to arrive. Manbeck and his team minimise this by building their supramolecules with multiple light absorbers that work independently, thereby increasing the probability of using each electron productively and improving functionality under low light conditions. In addition, to achieve maximum


efficiency, the Rh catalyst must be sufficiently low energy to accept the electrons from the Ru light absorbers when the absorbers are exposed to light.


In another approach, Fernando


Uribe-Romo and his team at the University of Central Florida, US, have made metal-organic frameworks (MOFs) that promote CO2 from the atmosphere.


capture


‘The MOFs [we make] are a


type of “coordination polymer”,’ says Uribe-Romo; they are formed by polymerisation of an organic monomer, like terephthalic acid, with a sub-nano size crystalline and porous metal-oxide cluster. He likens their structures to that of a honeycomb, but with much smaller holes and where CO2


molecules fly


around instead of bees. ‘Our MOFs use titanium as the metal oxide cluster, so we can combine synergistically the light harvesting ability of the terephthalate-based monomer with the catalytic activity of titanium.’ Currently, Uribe-Romo’s team is


investigating ways to absorb colours of light with lower energy than blue, to induce artificial photosynthesis with a wider spectrum of light. ‘Additionally, we seek to improve


the rates and quantum yields, as the first generation of MOFs we presented have very small rates and yields, so there is plenty of research to pursue,’ Uribe-Romo continues. Finally, a team led by Daniel


Nocera of Harvard University, US, has produced hybrid artificial photosynthetic systems that can store solar energy and chemically reduce CO2


. They combined a biocompatible


Earth-abundant inorganic catalyst system to split water into molecular hydrogen and oxygen. When grown in contact with these catalysts, the bacterium Ralstonia eutropha consumed the H2


and synthesised


alcohol based fuels. This ‘artificial leaf 2’ is a development of a less efficient artificial leaf developed by Nocera’s team in 2012. The synthesis of fuels or chemicals from low CO2 in the presence of O2


with a hybrid


system is a major development. The system is believed to be scalable to industrial level because it has a CO2 reduction energy efficiency of around 50%. It can produce bacterial biomass or liquid fuel alcohols, in the process removing 180g of CO2


/kWhour of


electricity. ‘We have actually done the whole thing and it works – carbon dioxide + water + sunlight to make biomass and fuels,’ says Nocera. With the other work in the pipeline, the human race seems on track to crack one of Nature’s greatest secrets and cheap renewable energy will finally be within our grasp.


concentration


09 | 2017 35


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