Tuesday, 8 December 2015

Taking a Leaf out of Nature’s Book

Mimicking photosynthesis may be the key to unlocking a future energy scene dominated by renewables. But nature’s simple process still holds many secrets. In light of the high-level Cefic breakfast debate on advanced materials and energy challenges that took place at the 7th European Innovation Summit, we asked science writer Ben Skuse to delve into how breakthroughs in materials may help resign fossil fuels to the past through the development of novel technologies and perhaps – eventually – artificial leaves.

The development of novel processes using waste carbon dioxide - up to and including the ultimate goal of artificial photosynthesis - feature in the SusChem Innovation and Research Agenda

Photosynthesis is a wonder of nature. It transforms energy from the light that the Sun bathes the Earth in to energy‐rich sugars. Simply put, it takes carbon dioxide and water, and converts them to glucose and oxygen.
There are two stages to this process. The first – water splitting – converts water into oxygen and a protein. In the next step, the protein reacts with CO2 to produce biomass. So far, scientists have only managed to master the former, splitting water using electrolytic processes to create hydrogen gas instead of biomass. But even on its own this feat was a huge achievement, paving the way for hydrogen fuel cell vehicles being actively commercialised today by the likes of Daimler and Toyota, and for the power industry taking hydrogen energy storage seriously as an option to deal with intermittent renewable power generation.

Hydrogen has some limitations
While hydrogen has one of the highest energy densities of any fuel, it is also the lightest of all elements. This means its storage requires very large volumes or very high pressures, resulting in issues of safety. Furthermore, the high cost of developing infrastructure and the energy intensity of the water splitting process offer sceptics a strong argument that hydrogen may not be the future for energy storage or the automotive industry.

“Hydrogen has some limitations,” confirms Sophie Wilmet, Cefic Innovation Manager. Sophie believes CO2 conversion technologies might provide a good alternative for large-scale storage of renewable energy using existing infrastructure. “CO2 can be used to address the energy storage challenge brought about by the rise in renewables, as well as for alternative fuels for transport.”

Carbon as a resource
Although not using direct photoconversion of CO2, a number of technologies are being actively explored to transform CO2 from a reviled waste product to a useful resource, as Sophie explains: “From CO2 you can produce basic and added-value chemicals”.

For example, a process co-developed by RWTH Aachen University and Covestro, formerly Bayer MaterialScience, has led to the construction of a plant that will be opened in 2016 in Dormagen, Germany, capable of producing up to 5000 metric tons per year of polyols, a polyurethane intermediate. About 20% of the content of the polyols will be from waste CO2 captured from a nearby ammonia plant, with the final material a flexible foam for mattresses.

Another innovator is Icelandic company Carbon Recycling International (CRI), whose renewable methanol reduces carbon emissions by more than 90% compared to fossil fuels. The fuel is produced from CO2 and hydrogen that comes from renewable sources of electricity. The world's first liquid renewable transport fuel production facility from non-biological sources of energy, CRI has a 4000 metric ton per year production capacity.

Further novel ideas include using large volumes of waste CO2 from industrial processes to produce syngas (BASF);  converting waste gases from iron and steel mills into ethanol and other important chemicals, such as acetic acid, acetone, isopropanol, n-butanol or 2,3 butanediol (Siemens/LanzaTech); and creating a closed carbon cycle using renewable energy, CO2 and water to provide sustainable fuels for vehicles and decentralised electricity generation (sunfire).

Mimicking nature
Capable of absorbing CO2 at the very low concentrations (400 parts per million) found in the air, absorbing energy from low-photon count sunlight, and photosynthetic cell self-repair, the ‘technology’ within plants is far more advanced than anything devised by humankind so far.
However, with aeons to perfect the technique, it comes as something of a surprise that energy conversion in plants is not actually particularly efficient: “For most plants the photosynthetic and storage efficiency is an average of 1%,” explains Dr Junwang Tang, Reader in Energy from University College London, UK.  Why is photosynthesis so inefficient? “The natural process is capable of utilising 100% of photons but green plants give up that potential to protect themselves – nature doesn’t need so much energy.”

As a result, if society were to mimic photosynthesis unaltered, there would not be enough land on Earth to cycle the carbon required for a sustainable future. Instead, researchers are aiming to enhance the process from a number of angles. “We have learnt how nature stores CO2 and we have realised that we can probably do better,” exclaims Junwang.

Direct photoconversion
A major roadblock in developing such technology is finding photocatalysts that can absorb as much of the solar spectrum as possible while still being efficient. As plants only use a fraction of the visible range, great potential lies in the untapped electromagnetic spectrum, so photocatalysts that respond to different regions are being investigated. Other researchers are exploring doping, nanomaterials and co-catalyst surface-loading to improve the photocatalytic response of promising materials.

However, with numerous other hurdles to climb before real-world application, Sophie expects there to be a long wait before artificial leaves are realised: “It still requires development in terms of new concepts, designs of photoelectrodes and integration of the system,” she explains. “For Cefic, it’s part of our overall long-term strategy, but more like a second- or third-generation technology that will not have impact by 2020.”

Even though tangible impact from direct photoconversion seems a long way off, Europe’s competitors are keen to advance the state of the art now, with a number of multi-million Euro projects funded in Japan, a Joint Centre for Artificial Photosynthesis set up in the US and well-funded initiatives in many other parts of the world.

As a result, Junwang believes Europe’s highly able yet currently fragmented and small community of scientists working in the area needs to be brought together: “Europe is very strong in fundamental understanding of artificial and natural photosynthesis, but countries like Japan, USA and China are investing heavily in this technology through well-funded projects. If we don’t invest more – just like has happened with graphene – other countries will heavily patent the field.”

The Cefic breakfast debate
The Cefic breakfast debate took place at the 7th European Innovation Summit in the European Parliament on 8 December. The event was hosted by Jerzy Buzek, MEP and covered the wide-ranging topic of 'Advanced Materials and breakthrough opportunities for the energy transition’.

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