As we all know the Sun is a source of clean and inexhaustible energy, with the possibility to be the answer for the future of all our energy needs. However there’s just one problem, the Sun doesn’t always shine and containing its energy can prove to be difficult. Despite the difficulty researchers from the Paul Scherrer Institute, as well as the ETH Zurich have found a way to convert its thermal energy into carbon dioxide and water, which converts directly into high-energy fuels. This chemical process is developed around the basis of combining cerium oxide and rhodium in a new material, providing a big step forward in making the chemical storage of solar energy a reality.

We are already harnessing the Sun’s energy in many different ways: while photovoltaic cells help convert light rays into electricity, solar thermal installations do something a little different, they use the thermal energy from the Sun to heat fluids to higher temperatures. The second method, when implemented on a large-scale, is used in solar thermal power plants. These power plants use thousands of mirrors to focus its rays onto a boiler and as a result directly or through heat exchange, produces steam. This steam, which can reach and exceed temperatures of 500 °C, helps move turbines to produce electricity.

The Paul Scherrer Institute and the ETH Zurich researchers collaborated to co-develop the new alternative to this system. Instead of converting water and carbon dioxide into, what is known as, high-energy fuel, this new alternative converts it directly into synthetic fuel.

According to Alxneit, a chemist at the PSI’s Solar Technology Laboratory, converting the energy directly into synthetic fuel allows it to be stored easier than electricity, because the solar energy will be stored as a chemical bond. Alxneit noted they also use the heat to start some chemical processes that can only occur at temperatures exceeding 1000°C. These steps forward in solar technology will make it possible to achieve high temperatures using only the Sun’s light.

Alxneit bases his research on the thermo-chemical cycle principle. Thermal-chemical cycle is a term that involves the cyclical process of chemical conversion and the heat that is required, also referred to as thermal energy. Research that dates back a decade ago already demonstrated what could be done while converting low-energy substances, like water and carbon dioxide waste, into hydrogen and carbon monoxide, which are energy-rich materials. However, this seems to work in the presence of certain materials including: cerium oxide, which is a combination of cerium and oxygen. Cerium tends to lose some oxygen atoms when it oxides to temperatures above 1500°C, but at much lower temperatures it actually reacquires the oxygen atoms. When water and carbon dioxide is placed over activated surface, they release their oxygen atoms as well, as a result converting water into hydrogen and carbon dioxide into carbon monoxide. All of this is going on when cerium reacquires its oxygen atoms and prepares for the next cycle.

Gaseous or fluid hydrocarbons fuel, which includes methane, petrol and diesel, are formed through the creation of the hydrogen and carbon monoxide in this process. Such fuels can be used right away or stored in tanks and delivered later on through a natural gas grid.

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Solar-driven thermochemical cycle (Image credit : Energy & Environmental Science Journal)

In the past this fuel production process needed a second one called Fischer-Tropsch Synthesis. SOLAR-JET, an European research consortium, suggested a combination of the thermo-chemical cycle and the older Fischer-Tropsch Synthesis procedure.

According to Alxneit this will solve any type of storage problem. However, it will take quite a bit of technical effort to fully carry out the Fisher-Tropsch Synthesis as in addition to solar installation it also requires another separate industrial-scale technical plant.

Alxneit and his co-workers developed a material that enables direct production of the fuel from within one process. This dispensed with both, the Fischer-Tropsch and the second step. They achieved this by adding just a little bit of rhodium to some of the cerium oxide. Since rhodium is a catalyst that allows some chemical reactions, rhodium also allows reactions with hydrogen, carbon monoxide, and carbon dioxide. The catalyst is a crucial topic for the creation of these type of solar fuels. Also, according to his PhD-candidate, Lin, it was very challenging to keep it under control due to several conditions required for the chemical reactions to commence and to develop such a catalyst that is able to handle the extreme heat of the activation process at 1500°C. For example, the small rhodium islands present on the materials surface cannot distort (increase in size or disappear altogether) in any way during the cooling process. The produced fuels are either stored or directly used, but after that the cycle begins again as the cerium oxide reacquires oxygen atoms.

At the PSI and the ETH laboratories researchers used multiple methods of structure and gas analysis to find the effectiveness of cerium oxide reduction and methane production. According to Alxneit the process that they are using only produces a little piece of usable fuel, regardless it shows that their idea isn’t science fiction written on paper anymore.

Researchers used a high performance oven to keep things simplistic during their experiments as their energy source needed to be present at all times. According to Jeroen van Bokhoven, who is the head of the PSI’s Laboratory for Catalysis and Sustainable Chemistry and Professor for Heterogeneous Catalysis at the ETH, they gained insights into the stability of catalysts during these tests. They were able to carry out 59 quick cycles because they used high performance oven. Newly developed material survived the first endurance test, which shows how probable their process is. Team can now dedicate their time to optimize it even further so that one day it could be used on large scale industrial plants.