Cerium oxide could be key to liquid solar fuel

Materials World magazine
,
1 Sep 2016

A new process for producing fuels from water and carbon dioxide could create a way to store solar energy, as Simon Frost reports. 

How can we store solar energy? To end our reliance on fossil fuels, that is one of the questions scientists want to answer most, and one solution could lie in using it to convert water and carbon dioxide into gaseous and liquid fuels. Researchers at Switzerland’s Paul Scherrer Institute (PSI) and ETH Zurich have found a way to create hydrocarbons from CO2 and H20 through interaction with cerium oxide, using thermal energy – albeit at temperatures currently unachievable with solar power.  

Their study is based on thermochemical cycling – heating cerium oxide (CeO2) to around 2,000K causes it to lose oxygen atoms, which the material seeks to reacquire when cooled. When water and carbon dioxide are directed over this ‘activated’ cerium surface, some of their oxygen atoms are stolen, converting the H2O into hydrogen (H2), and CO2 into carbon monoxide (CO) – and the cerium is re-oxidised, restoring the conditions to restart the process. 

PSI chemist Ivo Alxneit explained to Materials World, ‘Our main goal was to find a catalyst that can survive the extremely high temperatures required for the partial reduction of cerium oxide and produce a fuel that can be considered a final product from CO2 and H2O. Rhodium, producing methane (CH4), is the first success for now.’ The cerium-rhodium compound they examined allowed the chemists to produce small amounts of methane gas, which Alxneit describes as ‘a renewable fuel with, nominally, zero CO2 emission.’ 

Skipping steps

Stuart Irvine, Director of the Centre for Solar Energy Research, UK, and Chair of the IOM3 Energy Materials Group, told Materials World, ‘The novelty of this approach is combining the generation of H2 and CO with the formation of hydrocarbons via catalysis using rhodium. This would considerably simplify the production of liquid fuels from solar energy.’ 

A one-step process could theoretically remove the need for Fischer-Tropsch synthesis, which is used for converting carbon monoxide gas and hydrogen gas into liquid hydrocarbons, requiring its own industrial-scale plant and considerable expertise to run it. 

Irvine noted that the study differs from most work into solar fuel production, which tends to focus on hydrogen production. ‘What I find particularly attractive here is the potential production of a synthetic diesel from solar energy that could be used in transport. That accounts for 25% of energy use and carbon emissions, so effectively recycling CO2 is attractive,’ he said. 

Early days

The research is still in its infancy – only small amounts of methane have been produced so far, and no liquid fuel as yet. Solar power cannot yet provide the heat required, either – during their experiments, the researchers used a high-performance oven to test their process. 

‘Presently, concentrated solar power (CSP) technology is implemented at scales up to around 20MWth. These power stations use concentrated solar radiation to bring heat-transfer fluids, such as steam, molten salt or thermal oil to temperatures of around 1,000K, and then use conventional steam turbines to generate electricity,’ Alxneit explained. ‘At present, no facility exists to reach temperatures around 2,000K on a MW scale – however, concepts that should make the required temperatures achievable on a multi-MW scale exist, such as using concentrating heliostates and secondary concentrators.’  

Irvine summarised that ‘This is an exciting new development, but the high temperature needed for the process is a challenge that might limit its potential.’ Increasing temperature capabilities is, however, an obvious priority for research into concentrated solar power technology. In the meantime, Alxneit’s co-researchers at ETH Zurich, led by Professor Jeroen van Bokhoven, are focusing on funding research into improving the yield and variety of fuels produced by the process, chiefly by experimenting with different catalysts. 

‘Obtaining a liquid fuel instead of methane would make the process even more attractive, as this would address the transportation sector,’ said Alxneit. ‘We do not yet know enough about the reaction mechanism to carry out targeted research into improving the yield, but we know that changing the ratio of CO2 to H20 influences it.’ 

There’s a long way to go yet, but this research could mark solar diesel’s emergence from the realm of science fiction.

To read the open-access paper First demonstration of direct hydrocarbon fuel production from water and carbon dioxide by solar-driven thermochemical cycles using rhodium–ceria in the journal Energy & Environmental Science, visit rsc.li/2aRdNIv