A Carbon-Neutral Future?
Humans have been heavily relying on fossil fuels, the result of solar energy accumulated through biological processes over millions of years, for centuries. Today we are experiencing an unprecedented increase in atmospheric carbon dioxide, resulting from this fossil fuel consumption, which is expected to result in dramatic climate change. Furthermore, we are simultaneously diminishing traditional fossil fuel stocks, and in search of new cost-competitive and environmentally friendly alternatives.
If only we could convert carbon dioxide into fuel, we could kill these two birds with one stone. Carbon dioxide conversion techniques – processes inspired by nature that mimic photosynthesis – offer a promising solution to these global issues.
For billions of years, there has been an equilibrium between carbon dioxide production by plants through photosynthesis, and carbon dioxide consumption by living organisms via the processes of respiration and fermentation. Essentially, living organisms live on ‘fresh’ solar energy supplied constantly through a natural carbon cycle, while we use mainly ‘fossilized’ energy from the past. This practice, if continued, will irreversibly deplete fossil fuels and hence the race is on to find viable alternatives.
While significant progress has already
been made in the development of
new ‘green’ energy sources such as
solar and wind, we still lack a process
similar to that plants use to convert
carbon dioxide into carbohydrates
Prof Geoffrey Maitland of Imperial College observes “it is still expected that 40% of world energy consumption will come from fossil fuels by 2040. However if even 50% of the carbon dioxide emitted by these fuels could be sucked out of the atmosphere and turned back into high-energy organic molecules, carbon dioxide levels would essentially stay where they are now.”
While significant progress has already been made in the development of new ‘green’ energy sources such as solar and wind, we still lack a process similar to that plants use to convert carbon dioxide into carbohydrates. Technologies for simple carbon dioxide conversion into fuel-products, however, are already available at industrial scale.
An example is methanol synthesis from carbon dioxide and hydrogen employing an appropriate catalyst. In this case, the energy for converting carbon dioxide is supplied by hydrogen and therefore the source of hydrogen, decides whether or not the process contributes to carbon dioxide emissions.
Alternatively, carbon dioxide can be converted directly through electrochemical processes. When a large enough voltage is applied to an aqueous solution containing carbon dioxide, the carbon dioxide is converted into various potential fuels or fuel generators such as formate, carbon monoxide and methane.
More recently a new technique inspired by the delicate chemistry taking place in plants has been proposed. An ‘artificial leaf’ converts solar energy into hydrogen, which in turn can convert carbon dioxide into fuel. If this process is successful, and can be implemented at scale and cost, it could resolve some of the issues surrounding intermittent solar electricity generation from ‘conventional’ solar cells and most other renewables, and form a crucial component of a carbon-neutral fuel cycle.
For traditional carbon conversion technologies, carbon capture and storage is a major component of the process. Although carbon dioxide is present in the atmosphere, even at a higher concentration than in the past few centuries, its presence is still in ppm (393.65 ppm1 in February 2012 at Mauna Loa, Hawaii). Direct extraction of carbon dioxide from the atmosphere is expensive, and ideally we should capture it at source.
Installing a compressor and a carbon dioxide tank directly at an exhaust pipe of a combustion engine would give rise to a mixture of carbon dioxide, water and oxygen – impractical for most conversion techniques. In a fuel cell, however, carbon dioxide and water form in different compartments and therefore pure carbon dioxide can be captured.
Technologies for carbon dioxide conversion include methanol synthesis from carbon dioxide with the energy for the conversion supplied by hydrogen, which in turn must be derived from some energy source. For example, water can be split into hydrogen and oxygen when a large enough voltage is applied. So, if this electricity is supplied from renewable sources, the methanol synthesis process can produce fuel from carbon dioxide in an emission-free manner.
How efficient is this thermochemical process? The efficiency of an electrochemical cell for hydrogen production is about 50%, which is appropriately reduced by the efficiency of the electricity supply – ending up with say 10% efficiency for solar power input. However, only 85% of the energy from the carbon dioxide conversion reaction can be stored in methanol, leaving an estimate of the maximum efficiency of the entire process at around 8%.
Why can’t we use hydrogen directly as a fuel? Hydrogen is an attractive fuel because it has very high specific energy, its only combustion product is water and it can be used in modified combustion engines without replacement fuel cells. However, it has a low energy density in uncompressed states, and the compression process is often risky and expensive.
Alternatively, carbon dioxide can be converted directly into fuel-products through electrochemical processes. Perhaps the easiest way is to do this in an aqueous solution containing carbon dioxide. When a large enough voltage is applied, carbon dioxide is converted into various products such as formate, carbon monoxide and methane. The first two chemicals are not common as fuels because they produce little energy when burnt, but could serve as reactants for other processes to produce better fuels.
If artificial photosynthesis systems could
take up around 10% of the sunlight
falling on them, we would only need to
cover 0.16% of the Earth’s surface to
achieve a global energy consumption
rate of 20TW – the predicted amount of
energy the world will need in 2030
For the electrochemical conversion of carbon dioxide, the required total voltage roughly triples that needed thermodynamically2. In other words, 33% is typical efficiency from the electrochemical cell. Again, a solar panel with 20% efficiency would diminish the maximum efficiency of this process to 6%.
These numbers are merely illustrative of the difficulty in converting renewable energy into fuels. The losses presented here do not represent the whole process and the numbers are likely to be smaller, and/or change comparatively, when efficiencies of auxiliary units of both processes are taken into account.
The 21st Century Leaf
Just like living leaves, the ‘artificial leaf’ is a cyclic system where the two basic steps of photosynthesis take place. The first one, consisting of the splitting of water into hydrogen and oxygen in the presence of sunlight, is a water electrolysis reaction at the cellular scale. The ‘clean’ hydrogen produced can then be directly used as fuel, or employed in carbon dioxide conversion schemes such as those outlined above.
The world’s first practical ‘artificial leaf’ was invented by MIT researchers last year: it is a solar cell composed of a thin sheet of semiconducting silicon coated with different catalysts on each side. It is placed into a water container and exposed to sunlight to generate bubbles of hydrogen on one side, and oxygen on the other.
Research in the photovoltaics area is growing rapidly, but producing cost-effective technology that can compete on price with fossil fuels is the major challenge. Unlike the other two main energy production systems powered by the sun - photovoltaics and solar thermal devices – hydrogen-producing artificial leaves could be produced cheaply. The device is composed entirely of inexpensive, abundant materials – mainly silicon, cobalt and nickel – and operates under simple conditions, that is, in ordinary water3.
If the leaf can do it, we
can do it even better
The leaf created by the MIT researchers can currently convert 2.5% of solar energy falling on it into hydrogen in its wireless form. When using electrical wires to connect the catalysts to the cell instead of linking them together, the efficiency increases to 4.7%4.
Hence, electricity generation from the water splitting reaction is promising, but it is not realistic that batteries alone could power cars or aircrafts. The next question is whether we can take a step further and develop cheap, reliable and efficient catalysts to make high-energy chemical bonds using sunlight.
Imperial College’s Artificial Leaf Project, spearheaded by the Energy Futures Lab, is working on the solar driven reduction of carbon dioxide to energy rich carbon based fuels. Approaches range from direct photochemical reduction of carbon dioxide and photovoltaics plus electrochemical carbon dioxide reduction to solar water photolysis coupled to the dark reduction of carbon dioxide by molecular hydrogen.
A Carbon-Neutral Future
Professor James Barber, leader of the Artificial Leaf Project, is optimistic. He argues that if artificial photosynthesis systems could take up around 10% of the sunlight falling on them, we would only need to cover 0.16% of the Earth’s surface to achieve a global energy consumption rate of 20TW – the predicted amount of energy the world will need in 20305.
He follows on that “incredible amounts of activity are already being put into the field of artificial photosynthesis … (but) it is very hard to predict how fast this new, cutting-edge technology encompassing solar research will be mastered into a workable system.”
“If the leaf can do it, we can do it even better”
Prof James Barber, Imperial College 2012
Palang Bumroongsakulsawat is a PhD student working on electrochemical conversion of carbon dioxide. He focuses on kinetics of carbon dioxide, and using semiconductors as electrodes to capture photons for the conversion of carbon dioxide in a single device.
 Hui, L. & Oloman, C. (2006) Development of a continuous reactor for the electro-reduction of carbon dioxide to formate – Part 1: Process variables. Journal of Applied Electrochemistry. 36(10): 1105-1115. American Chemical Society (2011) 27 March, Debut of the first practical “artificial leaf” [Press release]
 MIT News (2011) ‘Artificial leaf’ makes fuel from sunlight [online] Available at: <http://web.mit.edu/newsoffice/2011/artificial-leaf-0930.html> [Accessed 30 March 2012]
 Jha, A. (2009) Scientists explore how the humble leaf could power the planet. The Guardian, 11 August.