solar energy liquid fuel

Harnessing the power of the sun: Using solar energy to produce carbon-free liquid fuel 


Solar energy has emerged as a critical solution in the global fight against climate change. While much progress has been made in the production of electricity from renewable energy sources, the production of chemical fuels from renewable sources remains a significant challenge. The Sun-To-X project is a consortium of nine partners that aims to use solar energy to produce non-toxic, energy-dense, carbon-free liquid fuel that can be used in transport and energy storage. 

Funded by the European Union’s Horizon 2020, the project has three main objectives: building an efficient prototype device for solar hydrogen production, storing this hydrogen in the form of HydroSil through a thermochemical process, and demonstrating the use of HydroSil for reductive depolymerization of waste plastics. 


The science behind the project 

To produce hydrogen, Sun-To-X uses a photoelectrochemical approach that combines light absorption and electrodes into a single component: the semiconductor photoelectrode. While most research into photoelectrochemical technologies has focused on the use of liquid water as a feedstock, Sun-To-X uses ambient humidity as an alternative. The key difference between using liquid and gas phase water sources is the use of porous photoelectrodes to allow humidity to enter the device. Additionally, a water-absorbing solid electrolyte such as Nafion is required for the gas phase reaction to bring the humidity into contact with the photoelectrode.  

The Sun-To-X project aims to develop a 10% solar-to-hydrogen efficiency device. Challenges in the scalability and stability of photoelectrochemical systems mean that the technology readiness level of these tandem systems is currently three, which means functional in laboratory setup. The project aims to increase the technology readiness level to five, which means demonstration in a relevant environment. 


The results so far… 

One of the key results of the Sun-To-X project is the development of transparent gas diffusion layers by a scalable preparation technique, as recently published in Advanced Materials. The first quartz fibres were formed by blending a commercial quartz wool, which was then pressed into a wafer with a porosity of around 90% and annealed to fuse the quartz fibres. A coating of fluorine-doped tin oxide was then applied through a chemical vapor deposition process. 

The project offers a new value chain for chemical energy storage. Solar energy is used to produce hydrogen from ambient humidity or rain as a water feedstock. This hydrogen is then reacted through a thermochemical process with a recyclable silicon-oxide-based precursor to form HydroSil – a carbon-free, non-toxic, energy-dense liquid fuel that can be directly applicable in the transport and energy sectors. The HydroSil molecule is stable for more than a year, making it suitable for long-term storage of renewable energy. 

In addition, HydroSil has another application in the reductive depolymerisation of waste plastic towards the development of a circular economy. For all the processes in this value chain, the consortium has focused on the use of abundant materials to minimise its environmental impact. 


The Sun-To-X project aims to contribute to the European Union and Mission Innovation goals for economic development and the enhancement of energy security through the construction of a sustainable energy system. The development of green production methods of chemical fuels is critical to buffering intermittent energies such as wind and solar, supporting the national or international transport of energy, and providing energy to remote or decentralised locations. The Sun-To-X project is an excellent example of how solar energy can be used to produce carbon-free liquid fuel, which can contribute to the global fight against climate change. 

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