Bionic Photosynthesis: The future of fuel production?

2013)- is also quite fascinating and almost mimics the growth of a plant shoot - ‘as supersaturation occurs, carbon atoms precipitate out from the particle’, forming a nanotube that has a ‘dome-shaped end’ (Dai, H. 2002). (Dai,2002) introduced 3 methods in which CNTs may align themselves: ‘self-assembly by intratube van der Waals interactions’ of MWCNTs, ‘selfassembly by van der Waals Interactions with substrates’ of ‘suspended’ SWCNTs and ‘electric filed directed nanotube growth’ of SWCNTs where CNTs were grown on silicon nanowires (SiNW) (Yoshida, H. et al. 2007). Although the potential benefits to society CNTs can offer are voluminous, there are reasons why the applications suggested above have not been used. Some of these reasons include: the amount of CNTs, as “most mass-produced CNTs are highly defective, and high-quality CNTs are hard to produce in large quantity” (Aron, J. 2016);purity of the CNTs, as ‘purification difficulties are great because of the insolubility of CNT and the limitation of liquid chromatography’ (Jahanshahi, M. and Kiadihi, A.S. 2013); environmental risks, as CNTs have only been looked into for around 30 years, which means it is difficult to fully understand the impacts it can have of used in large amounts.

The idea of artificial growth of CNTs has been discussed, but there is also some research that does suggest the possibility of naturally-made CNTs given the right conditions - researchers have found ‘evidence of naturally occurring MWCNTs produced from Pinus oocarpa and Pinus pseudostrobus, following a forest wildfire’ as well as extractions of ice cores containing ‘carbon nanotubes and fullerene nano crystals’(Murr, L. E. et al. 2004, p.2). In the future, as we discover more evidence and have a deeper understanding of the natural formation of CNTs, we may be able to develop sustainable new ways of creating CNTs that may be sufficient for more commonplace use in our daily lives.

-Alice

Introduction

For over a century, scientists have been fascinated with artificially replicating the process of photosynthesis that takes place in living organisms. In 1912, Giacomo Ciamician, an Italian chemist wrote, “So far, human civilization has made use almost exclusively of fossil solar energy… Would it not be advantageous to make better use of radiant energy?” He went on to suggest that “Meshing plants with technology would turbocharge photosynthesis.” [1] It could be argued that Ciamician’s ambitions have since been achieved; we have discovered how to convert solar energy to electrical energy, through the use of solar panels. The typical consumer solar panel constructed from rigid silicon crystals generally has efficiency rates of between 15 to 20 per cent, although some panels such as the monocrystalline solar panel, have even higher levels of efficiency. [2] This

completely outstrips the theoretical maximum amount of sunlight that plants can convert to biomass: 4.5 per cent, and the actual average of 1 per cent. [3] However, it is still the case that we have not fully harnessed the potential of solar energy, because electricity cannot be constantly generated from photovoltaic cells (solar panels), as the sun does not continually shine on panels. Electricity generated from solar energy must be stored using batteries, so that it can be used later on. Storing electricity using batteries is inefficient and costly, not least because batteries degrade every charge cycle. [3] Plants, on the other hand, have a much greater capacity to store energy as they store it in chemical bonds rather than charged particles- in sugars such as starch and sucrose- which are fuels. It is difficult to artificially produce fuels from solar energy in the same way and replicate photosynthesis, partly because of the light dependent reaction that must take place, to allow this process to occur. In this reaction the photolysis (breakdown) of water by light, catalysed by the oxygen-evolving complex (a water splitting enzyme), forms protons (H+ ions), electrons and oxygen: H2O → 2H+ + 2e- + ½O2. So far, the scientist who has most successfully replicated this reaction and converted sunlight and carbon dioxide into fuel is Daniel Nocera, of Harvard University, through his creation of the artificial leaf and bionic leaf. What is the Artificial leaf? The artificial leaf, that Nocera originally pioneered at MIT between 2009 and 2010, is a thin wafer that has the power to split water. At its centre is a semiconducting silicon sheet, or a photovoltaic panel. When light shines on this, energy is converted into a flow of wireless energy within the wafer, thus no external circuit is required for the leaf to operate. [4] This powers the electrolysis process that can be used to split water. In one of the first methods that Nocera developed, the current generated runs through an indium tin oxide anode, which had been placed in a solution of cobalt and phosphate salts. Co2+ions are oxidated at the anode to form Co3+ , which then forms a precipitate with phosphate ions (cobalt phosphide). This precipitate forms a catalytic film over the anode, and the film pulls electrons from the water, leaving behind protons and oxygen atoms. Then, oxygen atoms are brought together at the surface of the anode and the gas bubbles to the surface, whilst cobalt gains electrons and falls into solution as Co2+. At the same time, phosphate anions carry protons (H+ ions) to a platinum cathode where they are reduced, to form hydrogen gas. [5] In newer versions of the model, an alloy (NiMoZn) is used in place of platinum to generate hydrogen. [6] What is the bionic leaf? In order to produce fuel with the hydrogen collected from the splitting of water molecules, Nocera partnered with colleague Pamela Silver, a bioengineer at Harvard, to create the bionic leaf (which also contains the technology of the artificial leaf) in 2015. The hydrogen produced by the artificial leaf is fed to pre-starved bacteria, Ralstonia eutropha, which are placed in the same jar as the water-splitting leaf. The R. eutropha have been modified to essentially switch from growth to panic mode resulting in the microbes consuming the hydrogen and carbon dioxide. Carbon dioxide is absorbed either from the atmosphere or pure

sources. [7] The bacteria can then synthesise biomass and fuels such as isopropanol, isobutanol and isopentanol. [8] Overall, the bionic leaf fuses two distinct fields: inorganic chemistry, to split water, and biology to convert hydrogen and carbon dioxide to fuel. This is what sets it apart from similar projects and allows for it operate to at roughly 10 per cent efficiency at producing alcohol fuels, when provided with pure CO2. [9] What are the benefits of this technology? What is most advantageous about the bionic leaf is that it could provide a net-zero method of producing combustible fuels. The carbon dioxide released during the combustion of fossil fuels can be re-used by the bionic leaf to build renewable fuels, in a similar manner to plants. Crucially, the leaf is made of cheap and readily available materials, and operates at a neutral pH, under no special conditions. It can simply be placed into tap water, exposed to sunlight and will produce hydrogen or other combustible fuels. Nocera believes that the technology of the bionic or artificial leaf could be easily adopted into everyday life, in many households. He envisions the technology bringing cleaner fuels to those disconnected from the energy grid, in developing nations. "This science you can do in your backyard. You don't need a multi-billion-dollar massive infrastructure.” [7] Conclusion The bionic leaf promises to provide a feasible alternative to the use of fossil fuels, through the reversal of the process of combustion. This has been achieved by replicating photosynthesis; using solar energy and to split water and convert the hydrogen produced and atmospheric carbon dioxide into fuel. Despite this, it is unlikely that this technology will be implemented imminently because is in its early stages of development. At the moment, the leaf cannot produce large volumes of fuel in a short time period, and so will not compete with fossil fuel prices for some time. Still, Nocera and Silver’s innovation offers hope for the future and a potential solution to reverse excessive and lethal carbon dioxide emissions. [7,8]

-Orla