Most of the energy that fuels our lives comes from plants. Whether it is a fossil fuel that was formed hundreds of millions of years ago or the food we eat, all carbon-borne energy has its ultimate origins in plant photosynthesis.
By burning fossil fuels (the fossilised remains of ancient plants) and releasing the carbon stored within into the atmosphere, we are warming the earth, with potentially devastating consequences.
Photosynthesis, put simply, uses the energy in sunlight to convert water and carbon dioxide into plant structures. It also creates an energy store.
Unsurprisingly, given our need to quickly move away from fossil fuel use, researchers world-wide are frantically investigating methods of turning plant material into more useful forms of fuel. One way is to mimic the process plants use to make energy, through artificial photosynthesis.
And a recent discovery may be a big step in generating new, climate-friendly fuels.
Beating plants at their own game
Plant-based photosynthesis is actually fairly inefficient in terms of gathering and storing solar energy. It’s unlikely that the energy needs of the world could be supplied solely from plant sources such as biofuels — there just isn’t enough space to grow enough plants.
Another approach is to understand the fundamental processes that take place in photosynthesis and mimic them in an industrial setting — a concept known as artificial photosynthesis.
If artificial photosynthesis can be developed that is more efficient than plant-based photosynthesis, a large fraction of our fuel needs could conceivably be supplied from these “solar fuel” factories. They could develop wherever sunshine and water are plentiful, and the atmosphere supplies the rest.
But artificial photosynthesis is still a work in progress. Researchers have developed various approaches to tackle the challenges of making artificial photosynthesis work on a large-scale.
As always, the devil is in the details: by delving deep into how natural photosynthesis works we can see that plants have evolved a multi-step process.
How photosynthesis works
First, the energy from light needs to be harvested. In plants, this is done by a light-harvesting complex called antennae molecules found in the chloroplasts in a leaf.
Then, plants use the harvested energy from light to create free energy in the form of electrons and split water molecules to gain oxygen molecules. This reaction is “catalysed” by a manganese oxide compound.
These electrons are then passed to a different site and given a further energy boost from another photon of sunlight. This generates new carbon molecules from carbon dioxide.
The key to this process are the catalysts. These catalysts have to be able to absorb light and free up energy in the form of electrons, and pass them on to split carbon dioxide molecules.
A combination of materials is clearly required to carry out these different tasks. And to work effectively the different materials need to sit in very close contact with one another, so all of this needs to be shrunk down to the size of a nano-particle.
Getting closer to the answer
Our group recently made significant advances, in the form of a new catalyst developed by researcher Dr Haitao Li in the Monash laboratories of the ARC Centre of Excellence for Electromaterials Science (ACES). The discovery was published recently in Advanced Energy Materials.
Dr Li took tiny copper oxide spheres (Cu2O) (about 1 micrometre across, or a thousandth of a millimetre) and attached tiny carbon dots (about 2 nanometres across) at random points across its surface.
The copper oxide very effectively absorbs light to free up energy in the form of electrons, as well as attaching CO2 to its surface. The carbon dots help split the water molecules.
Once exposed to sunlight the catalyst steadily absorbs and converts CO2. The product of this process is methanol — an extremely useful liquid fuel that could be used to run cars, heat homes, or generate electricity.
Carbon capture — the capture of CO2 from power station flues — could accelerate the development of artificial photosynthesis technology. At the moment, there is no application for this CO2 and so it must be “sequestered” in geological formations, at significant economic cost.
Since artificial photosynthesis could make direct use of this concentrated CO2, it will make carbon capture technology more economic. This CO2 would be the perfect feed-stock for the high efficiency artificial photosynthesis process.
Approaches to artificial photosynthesis are being investigated in a variety of directions in research groups around the world and there is a growing movement, led by Prof Tom Faunce at ANU, to try to coordinate these efforts at a high level to accelerate our progress (see also here).