The Bionic Leaf: Renewable Fuel Through Artificial Photosynthesis to Power the World and Address the Fossil Fuel Dilemma

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Introduction

Fossil fuels are non-renewable hydrocarbon-containing materials occurring within Earth’s crust, and currently account for approximately 82% of the world’s primary energy source (Ritchie 2015). However, in the last 200 years excessive usage by humanity has caused increasing shortages in the availability of fossil fuels, hence prompting research into other forms of renewable energy. One of the most promising developments in the search for renewable energy has been the Artificial leaf. The Artificial leaf, also known as the Bionic leaf, is a silicon-based device that utilises the sun and water to generate useable and environmentally beneficial fuels (Cottingham 2017). When exposed to sunlight; it carries out the same processes of photosynthesis by splitting water molecules into hydrogen and oxygen to combine with CO₂ and form beneficial sugars (Biello, 2016).  Through the continual development of complex chemical models and theory surrounding the Artificial leaf, it’s potential benefits to society can be explored and implemented on a worldwide scale in the future.

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Chemical Background

The artificial leaf is a device that manipulates traditional photosynthesis to generate fuel from solar energy and water (cottingham 2017). Photosynthesis is an endothermic reaction as it consumes energy from the sun, further converting it to functional chemical energy. Water molecules during photosynthesis are photo-oxidised through the absorption of light energy through a green pigment; chlorophyll, discharging oxygen and protons. Furthermore, the resulting protons are then used for the generation of hydrogen. Carbon dioxide is then utilised from the surrounding air to create intermediate glucose molecules that can be utilised for the growth of the plant by consuming the previously formed protons and oxygen (Lambers 2018). Illustrated below in equation 1, is the chemical equation for photosynthesis:

$6C{O}_{2\left(g\right)}+6H_2{O}_{\left(l\right)}\to C_6{H}_{12}{O_6}_{\left(\mathit{aq}\right)}+{O_2}_{\left(g\right)}$

Equation 1: Chemical equation of photosynthesis

Two half reactions of oxidation and reduction occur during the photosynthetic reaction transpiring within the Artificial leaf. These half reactions are essential to producing fuels. By mimicking the photosynthetic activity of a plant, it pairs the water splitting catalyst; the oxygen-evolving-complex enzyme, with the bacteria Ralstonia eutropha. This consumes hydrogen and converts the carbon dioxide in the atmosphere into efficient alcohol based fuels (Cottingham 2017). The artificial leaf also encompasses an energy absorbing photovoltaic panel made from silicone, that sources its energy from the sun. Both sides of this panel consist of a layer of  cobalt-phosphorus alloy catalysts (Meduna, 2018) to power the chemical reaction by utilising solar energy to promote the splitting of the water molecules into hydrogen and oxygen molecules (Saint n.d). As a result, a separation of protons and electrons is triggered, which are then apprehended on the chip and recombined to form hydrogen gas that be stored for subsequent tests or utilised for the instantaneous production of electricity (Mian n.d). The configuration and progression of the Artificial leaf is illustrated In figure 1. The formation of hydrogen gas is caused by a reduction of $H^{+}$

ions and occurs at the cathode. Considered as the simplest solar fuel to synthesise (Y.Kuang 2019), Hydrogen only required the relocation of two electrons to two protons. However, the reduction of hydrogen must occur stepwise, with formation of an intermediate hydride ion as a product of the reaction (Meng 2019). This is reaction is illustrated below by equation 3:

Figure 2: Process of the Artificial leaf (Wired 2014)

$2e^{–}+2H^{+}\rightleftharpoons H^{+}+H^{–}\rightleftharpoons H_2$

Equation 3: Reduction of H⁺ ions at the cathode to form H₂

The oxidation of water molecules occurs at the anode. The previously referenced oxygen-evolving complex performs this reaction by gathering electrons and distributing them to the water molecules, resulting in the production of molecular oxygen and protons. This half equation is expressed below in equation 4.

Equation 4: Oxidation of water molecules at the anode

By combining the half equations that occur at the anode and the cathode, the overall reaction showing the materialisation of hydrogen fuel and oxygen occurring in the Artificial leaf is represented by the equation below in figure 5:

Equation 5: Formation of hydrogen and oxygen from water

Science as a human endevour

In 2015, the original model of the Artificial leaf with newly developed catalysts was tested. This included the nickel-molybdenum-zinc alloy catalyst, which was used in preliminary studies concerning the function of  the artificial leaf under simple conditions and exhibited increased stability over preceding catalyst designs (Torella 2015). In laboratory studies, the authors; Dr. David Nocera and Pamela Silver; demonstrated that a prototype version of the artificial leaf could operate continuously for at least forty-five hours exclusive of a drop-in functionality (Y.Reece 2011). This prototype also allowed for the conversion efficiency of water to biomass to be approximately 1%; which is comparable to that of natural photosynthesis. Using this knowledge, the first fully purposeful artificial leaf, dubbed the ‘Bionic Leaf 1.0’ was developed in late 2015.

After development of the ‘Bionic Leaf 1.0”, critical defects were perceived in its design; most notably with the nickel-molybdenum-zinc alloy catalyst. This was because the original catalyst, produced excessive levels of reactive oxygen species; which are toxic for the bacteria species present within the cell (Nocera 2016). Because of this, scientists were required use higher voltages, which reduced the system’s efficiency. In 2016, Nocera and colleagues published a paper in the journal Science announcing a newly developed system containing an alternate catalyst consisting of a cobalt and phosphorus alloy (Sciencedaily 2017). This particular alteration and development to the previous model not only left the bacteria unharmed, but also allowed for lower voltages to be administered to the cell, hence resulting in increased levels of efficiency (smdp 2019). This new model of the artificial leaf was dubbed “The Bionic leaf 2.0”, as it was a vast improvement over the previous model.

Research currently being conducted by Harvard biologist Pamela Silver also includes the development of an alternate version of the Artificial leaf that utilises genetically engineered variants of the bacteria Ralstonia eutropha that can be synthesised to form specific products (Nature 2017). Instead of the formation of generic biomass, this new method allows for the formation of specific of alcohols, which can be used as fuel. For example, by using a specific genetically engineered variant of the bacteria R. Eutropha, a team from Harvard University made isopropanol ( $C_3{H}_{8}O$

), an alcohol molecule that can be used as fuel like ethanol or gasoline and can be easily separated from water with salt (Khan 2016). They also were able to create other alcohol-based fuels such as; Isobutanol and Isopentanol along with PHB, a precursor to bioplastic (Khan 2016). Silver and colleagues state that the device was able to achieve an average efficiency of approximately 10% in converting sunlight into alcohol fuels (colon 2017.).

Dr. Nocera’s team also developed a strain of the bacteria that was resistant to reactive oxygen molecules (Smdp 2019). This however, was not beneficial for the team as they had already developed the cobalt and phosphorus alloy catalyst that solved the problem. As a result, Dr. Nocera suggested that this development of oxygen resistant bacteria could be useful for other teams who also wanted integrate and experiment with microbe-based biofuel techonlogy but have concerns surrounding the manifestation and consequences of potentially toxic chemicals. This proposed development would resolve concerns surrounding problematic bacterial samples and subsequently result in improved efficiency of the process, causing it to become more profitable and marketable in the future.  In response to what additional developments to artificial leaf technology were coming next, Dr. Nocera stated that “the next step is to try to use this process to develop nitrogen-based chemicals that could be used in artificial earth friendly variants of fertiliser” (Khan 2016).

Potential Impact

In early 2018, Dr. Nocera detailed in a press release that him and his team were continuing their research, with a predominant focus on outsourcing this technology to the developing world (Harvard Gazette 2018). As a result, distribution of the bionic leaf is expected to be concentrated predominantly around lesser developed countries (Mian 2015). This proposal was due to Nocera’s initial design, permitting for an estimated one to three bottles of water to yield enough energy to power a single household 8in less-developed regions of the world. This would result in an increase in general wellbeing for those in third world or underprivileged countries and hence likely stimulate economic growth in those areas (colon 2015). The utility of the artificial leaf also makes hydrogen a renewable energy source due to the abundance of sunlight and water on Earth. Increased prevalence of the artificial leaf in society would also reduce reliance on finite energy resources such as fossil fuels. As the bi-products of the artificial leaf are considered environmentally friendly, this would likely reduce global pollution and as a result, help slow the enhanced greenhouse gas effect and its associated widespread impact on the environment (i.e. extreme weather events, droughts, rise in sea levels), agriculture (i.e. declining crop yields) and health (i.e. increase heat strokes). With the artificial leaf, individuals can locally produce their own energy and can be independent apart from an electricity grid. This offers a significant advantage in that hydrogen energy could be produced almost continuously anywhere and at any time. Artificially photosynthesised fuel is also a carbon-neutral source of energy, which could be used for transportation or powering houses. Due to these reasons, a commercially viable artificial leaf would potentially resolve numerous key challenges currently plaguing clean energy as it would allow for direct and inexpensive storage of solar energy while producing a carbon-neutral fuel that could alter the transportation sector, even offering a way to make long-distance air travel environmentally sustainable (Liu 2016).

However, current models of the Artificial leaf still have limitations that prevent it from being implemented on a worldwide scale. Due to the current cost of the bionic leaf, it is currently unable to compete with fossil fuel prices and contains limitations that fossil fuels currently do not possess (colon, 2016). Fossil fuels are currently much more accessible and more efficient in proving energy, and hence would likely be favoured by majority of the public situated in technologically advanced and high socio-economic areas. Notable concerns surrounding the safety of storing hydrogen fuels due to its high risk of flammability also limit the current practical implementation and usage the artificial leaf (Britannica 2015).

Conclusion

Due to high carbon emissions and scarcity of non-renewable fuel reserves caused by increased burning of fossil fuels in the last 200 years, development of new innovative forms of renewable energy such as the artificial leaf have become a necessity. By researching the development of the artificial leaf, it was concluded that implementation of artificial leaf technology on a global scale, would likely improve the lives of hundreds of thousands of individuals in third world countries by providing efficient electricity sources to those secluded from the electricity grid. The artificial leaf’s usage is also predicted cause a reduction of the enhanced greenhouse effect, which ideally, would directly result in a decline in global warming, and hence improve the health of the biosphere, and thus the wellbeing of society. However, when compared to fossil fuels, current models of the artificial leaf provide inefficient energy, hence limiting its current implementation into all area’s society.

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