Currently, advanced processes are being developed to increase the market potential of hydrogen fuel cells, with thermochemical cycles being amongst the most studied. The Fuel Cells and Hydrogen Joint Undertaking is a Public Private Partnership between the European Commission and the European industry and researchers aiming to accelerate the market introduction of fuel cells and hydrogen energy technologies. The production of hydrogen through the splitting of water using concentrated solar light has been receiving support for a number of years as a major pathway for producing green hydrogen.

The EU-funded SOL2HY2 (Solar to hydrogen hybrid cycles) project set out to study the solar-powered hybrid sulfur cycle. The cycle is one of the most promising and economically competitive approaches to generating hydrogen, using thermal energy.

The hybrid sulfur cycle is a two-step water-splitting process for breaking water molecules into hydrogen and oxygen. Based on sulfur oxidation and reduction, it is classified as a hybrid thermochemical cycle because it uses an electrochemical reaction for one of the two steps and a high-temperature thermochemical reaction for the other one.

Initial work on the electrolysis step in the hybrid sulfur cycle was conducted by the nuclear industry. This is because nuclear heat sources were more economical than solar energy. However, recent technologies are unlikely to achieve hydrogen costs below EUR 3 to 3.50 per kilogramme.

Consortium members have integrated solar power sources with a new low-energy process linking production of hydrogen and sulfuric acid. The simplified structure, extra revenues from acid sales and co-use of existing plants may drop hydrogen production costs by 50 % compared to conventional process designs.

SOL2HY2 researchers optimised the cyclical process for a high share of renewable resources, reduced capital costs and product stream balance. The team developed a new strategy for sulfur-dioxide depolarised electrolysers, and demonstrated its many advantages over conventional processes.

The consortium designed an electrolyser that operates at room temperature by manufacturing and testing new coatings concepts that do not contain platinum or palladium. Furthermore, for the high-temperature step of the process, a solar reactor using a cheap and stable catalyst was developed and demonstrated on-sun.

These results will lead to reduced hydrogen production costs. This cost-effective and eco-friendly hydrogen production method may find application in the fuel cell, energy production and transport industries.