Issues Magazine

Back to Nature: Making Molecular Biofuels

By Warwick Hillier

As the environmental and supply problems of fossil fuels loom, chemical reactions harnessed by nature are inspiring efforts to derive energy by emulating photosynthesis.

Imagine a line of 100 W incandescent light bulbs side-by-side all the way around the Moon and back. Switch them on and you would consume about 1 TW of electricity. Today we use 15 times that amount (15 x 1015 W).

In Australia and in most other developed OECD countries the per capita energy load is an average ~6–7 kW per person. This load takes into account more than household consumption – it includes energy for transportation, to produce the food we eat, and the farming practices, industry and manufacturing we rely upon.

Energy use is not by any means evenly distributed among the world’s population. It is closely correlated with lifestyle and gross domestic product (GDP). Using GDP and world population levels it is possible to predict energy consumption.

This approach can be validated over the previous 100 years by factoring in past GDP and population numbers. It can also be scaled forward based on expected population numbers. Using this method, the estimates for 2050 are an energy demand of 28–35 TW.

Two central issues from this energy generation are a concern. First is how to provide the additional 15 TW of energy on top of today’s energy demand. The second problem is, given most of the current energy production on Earth is derived from fossil fuels, how can we move to new technologies and reduce our dependence on fossil fuels?

The insidious carbon dioxide pollution emitted to the atmosphere from fossil fuel combustion is a long-term problem. Carbon dioxide levels are already greater than the Earth has seen for 750,000 years, and are removed only slowly from the atmosphere. Every day 85 million barrels of oil are pumped out of the ground and burnt, and coal and gas are used on an equivalent scale. The annual consumption represents the combustion of about one million years of fossilised photosynthesis.

The effects have changed the composition of the atmosphere, and carbon dioxide-free energy solutions are imperative. Many options are on the table but the sheer scale of energy supply needs careful consideration.

Energy on Demand

In terms of sustainable energy supply, one energy source has vast capacity. The Sun provides in 1 hour all the energy we use in a whole year. The solar radiation arriving on the Earth’s surface has low density, so it has to be collected over large areas. This problem can be solved in the sparsely populated interiors of many continents.

The other problem is that the Sun shines for only about 12 hours per day, so the energy must be stored. Various solar fuel initiatives are working on the concept of converting solar energy to electricity and storing it.

Hydroelectricity generation begins with water storage in dams, and the potential energy of falling water generates electricity. However, the sheer quantity of water to make energy at the scale required is unfeasible.

An elegant storage solution lies with chemistry. The approach is to make a chemical bond such that the product of a reaction is more reactive than the initial materials used for the reaction.

Humans first experimented with chemical bonds when they made fire, but burning biomass, with its associated release of carbon, has contributed to the current climatic perils. Another reaction is needed.

The best way to proceed is to rely on something abundant and catalyse the formation of a higher energy product. The key is scalability; something that can work for the entire planet.

One such material is water, which can be energetically “split” into hydrogen and oxygen gas, which when combined are quite reactive. The storage issues of hydrogen are by no means solved, but an enormous amount of research is currently being directed at the hydrogen economy.

Molecular Biofuels

Some spectacular natural chemical reactions are based on abundant elements. They include the splitting of water, fixation of atmospheric nitrogen and the capture and conversion of carbon in the atmosphere into cellulose and wood.

These and other reactions are all catalysed by enzymes in living cells that operate at room temperature, neutral pH and atmospheric pressure. By comparison, most industrial catalytic reactions require energy input and operate at very high temperatures and pressures to facilitate energetically unfavourable chemical reactions.

Thermal water splitting is driven above 1100°C, yet the biological splitting of water is performed in living cells in complete silence and with benign conditions. This biological reaction is a unique solution for energy storage.

In nature, the splitting of water by light energy derived from the Sun is performed by an enzyme called photosystem II, which uses the element manganese. Remarkably, there is only one synthetic chemical equivalent of this manganese-based reaction, and it is nowhere near as efficient. This is unfortunate because the element is plentiful and inexpensive.

There are also commercial solutions based on platinum, but platinum is by no means abundant, and it is expensive.

Artificial Photosynthesis

The natural water-splitting reaction is a rather complex beast, which makes its coupling to energy storage rather challenging. We have taken a bio-inspired approach by bio-engineering a catalyst for splitting water.

The use of engineered proteins for artificial photosynthesis requires a great simplification of a real living plant cell. We based our development on building a protein that is produced in bacteria. In this way, complex catalysts are self-assembled using the machinery of a cell. This means we can grow the bacteria in a nutrient “broth” and make a lot of protein. The bacteria are harvested by centrifugation and we can purify the protein from the bacterial protein using a special affinity tag that will bind to a separation column. The protein can be modified in the lab in solution to contain different pigments for the photochemistry. With a photoactive protein that undergoes the storage of charge we have artificial photosynthesis.

The task is not complete but we have assembled a biological protein that is photoactive and undergoes reactions leading up to water-splitting. The trick for the natural reaction is that the light comes in stepwise.

An analogy is the “test your strength” sideshow – swing the big wooden mallet and drive a metal rod up to hit a bell. In a similar way the light energy comes swinging in and drives electrons up to make more reactive compounds.

To split water, however, you need to hit the bell four times. This means the energy has to be stored, and in the natural system the energy is stored by oxidising the metals. Our artificial protein is capable of capturing light energy and storing charge, but it seems to manage only one or two hits of the bell.

Part of the problem is that we have simplified things, so we are working to optimise the system. However, in developing this protein we first needed to demonstrate a proof-of-concept as this was the first attempt of such a project in the world. Future work will involve adding more storage capacity to the protein.

The current research is a promising strategy for developing artificial photosynthesis, something that has long been the subject of intrigue and speculation for biologists.

Applications of Artificial Photosynthesis

One possibility for energy capture is coupling the protein to a conductive surface. Other groups are using organic dye molecules to make solar panels out of windows. Our synthetic protein could also be used in this way to make electricity, but its stability would only probably be a few hours in full sunlight.

The natural photosynthetic machinery has repair mechanisms for fixing photo-damaged or “sunburnt” proteins. Our system does not yet contain a repair cycle, although in future proteins a built-in repair cycle may be possible.

In the longer term a catalyst for the splitting of water into oxygen and (using a second catalyst) hydrogen are the real needs for solar energy storage. One volume of water from an Olympic swimming pool split via catalysts to oxygen and hydrogen every 30 seconds represents the storage of about 1 TW of energy.

Catalysts based on abundant elements such as manganese could make cost-effective hydrogen generation possible. Our artificial catalyst – assuming that we are ultimately up to the task of copying nature – could be used in an electrode to make molecular fuels.

Past and Future

Some 40 years ago political interests spurned a race for the Moon that would ultimately see the assembly of the Saturn V spacecraft for the Apollo program. The technology used and developed at the time was remarkable. The whole endeavour was primarily undertaken by one country, but it brought together the best engineers, academics and manufacturing to assemble a remarkable project.

The crisis in climate and energy has been likened to an Apollo program in scale. Although not a burden that will be carried by one country, the energy ethos for the future will require an engineering approach. Given the perilous outcomes of unabated carbon dioxide emissions, world economies have to transform existing and build new energy infrastructure over the next 50 years.

Our bio-engineering strategy is to develop a molecular biofuel that copies the most efficient enzyme in biology. The efficiency of the reaction means that high percentages of electrical energy are stored directly as chemical energy, with only small energy losses.

Such a catalyst would be the grail of energy research.