The prototype of an artificial tree: the parabolic mirror focuses the rays of the Sun on the device in which water is split into hydrogen and oxygen. | Image: LRESE@EPFL and Sohhytec

Have you heard of using artificial trees to tackle global warming? This technology is currently at the prototype and pilot-project stages, but research-developed devices can already mimic the natural photosynthesis of plants using artificial materials. The approach is to use sunlight and an abundant raw material such as water and/or CO₂ to produce hydrogen and hydrocarbons, some of which could be used not only as fuels, but also as raw materials for the chemical and pharmaceutical industries.

In recent decades, research into artificial photosynthesis has focused on separating water into hydrogen and oxygen – the first step in nature’s photosynthesis – which works quite well according to Sophia Haussener, the director of the Laboratory for Renewable Energy Science and Engineering at EPFL. “The production of solar hydrogen is already a success”, she says. She is also a co-founder of the start-up Sohhytec, which has patented an artificial tree with an “integrated photo-electrochemical” device. What this actually refers to is a seven-metre-wide parabolic mirror on the EPFL campus that focuses the sun’s rays onto a device, while water is pumped into the heart of the installation. The combined action of the heat and the production of electric charges induced by the light leads to the fission of the water molecule and to the efficient production of hydrogen.

“For the coming decades, green hydrogen looks very promising for powering the transport sector (for medium and long distances) as well as for covering the specific needs of the chemical and pharmaceutical industries. Synthetic fuels may play a role in the complete decarbonisation of the Swiss economy by 2050”, says David Parra of the Chair of Energy Efficiency within the Institute of Environmental Sciences (ISE) at the University of Geneva.

Carbon, the key element

In the future, Haussener will extend her research to CO₂ reduction – the second stage of natural photosynthesis – which remains a real challenge. “This approach, which results in many different products – whose separation is problematic – is still in its infancy”, she says. “Splitting water into hydrogen and oxygen is a simpler step, but hydrogen is gaseous under normal conditions and hence difficult to store. On the other hand, extending this reaction to the reduction of CO₂ may produce hydrocarbons, potentially in liquid form, which are easier to store”.

This is a promising approach to providing a steady supply of renewable carbon sources. Carbon is the key element in most chemicals, fuels and materials used in everyday life. Today, our needs are largely met by fossil resources. But this is a reality that is incompatible with a world of low CO₂ emissions. “Climate change is real. We must urgently do everything in our power to contribute to a more sustainable society. Plants are teaching us how to do this”, says Raffaella Buonsanti, who heads the Laboratory of Nanochemistry for Energy (LNCE) at EPFL Valais Wallis. Her challenge is “to develop nanoparticles capable of specifically converting CO₂ into the desired product”. It should be noted that there are changes to the physical and chemical properties of materials when reduced to the nanometric scale. They become better catalysts due to a higher surface-to-volume ratio. Buonsanti makes catalytic nanocrystals from particles immersed in a solvent, an approach that enables high-precision control of their composition, size and shape. But how do these properties affect the ability of copper nanocrystals, for example, to reduce CO₂ electrochemically to methane or ethylene? Well, there lies the challenge of her research: studying the link between morphology and catalytic selectivity. She hopes to contribute “within 10 years to an efficient, selective and stable device that can recycle CO₂ while storing renewable energy”.

Target: energy efficiency of at least 10 percent

According to Kevin Sivula, a professor and the director of the Laboratory for Molecular Engineering of Optoelectronic Nanomaterials at EPFL, “we are not trying to imitate exactly the mechanisms of a natural leaf. Given the low conversion efficiency of natural photosynthesis, we are aiming for alternative mechanisms that could be considerably more efficient. Our approach is to use semiconductor materials, which are known to capture and convert solar energy, and to design them for reactions for solar-fuel production”.

Trees convert sunlight, water and CO₂ into sugars and then into hydrocarbons with an energy efficiency of less than one percent, and this takes a long time. “It is therefore important to find technical solutions with higher levels of efficiency. For artificial photosynthesis to become economically interesting, it seems the minimum target would be the ten percent threshold. This would mean that smaller areas would be needed to produce the same amount of fuel or hydrocarbons as trees”, says Haussener.