Researchers from Clemson University and Idaho National Laboratory in the United States have created a new material that can produce hydrogen and carbon monoxide solely from heat, without using electricity or fossil fuels. Heat can be sourced from concentrated solar power, a nuclear reactor or even recycled industrial heat.
This material is a complex metal oxide with a crystalline structure and an unusual chemical composition: Ca₀.₂Gd₀.₂La₀.₂Pr₀.₂Sr₀.₂Mn₀.₆Al₀.₄O₃. It is a so-called high-entropy perovskite, in which five elements – calcium, strontium, gadolinium, lanthanum and praseodymium – are uniformly distributed across a single lattice site. Two more elements, manganese and aluminum, provide chemical reactivity and structural strength. This combination ensures that the material is resistant to overheating and destruction, even at extreme temperatures above 1,300°C.
The most important feature of the new oxide is its ability to repeatedly participate in a thermochemical cycle, during which thermal energy is converted into chemical energy. The cycle consists of two stages. First, the material is heated to 1,350°C in an argon atmosphere, as a result of which it loses some of its oxygen, or discharges. After it gets cooled to 1,000°C, water vapor or carbon dioxide is fed into the atmosphere. During reduction, the oxide removes oxygen from these molecules and releases hydrogen or carbon monoxide. This is how high-temperature heat gets directly converted into fuel.
While perovskites have long been considered promising catalysts for these processes, earlier prototypes degraded quickly and required excessively high temperatures. The new material has demonstrated immense stability and efficiency. During testing, it withstood ten full cycles without loss of activity and provided an average yield of 320 micromoles of hydrogen per gram of material, which is one and a half to two times higher than the best industrial analogues and six times higher than the cerium oxides previously used. With the use of CO₂, the carbon monoxide yield was 420 micromoles per gram, three times higher than the previous top performers.
Even with a water vapor concentration reduced to 5%, the catalyst maintained stability and high productivity. The reactions were fast: a full reduction and oxidation cycle took less than half an hour.
The secret to this stability lies in the material’s structure. High entropy, or the multitude of equally possible combinations of atoms, makes the lattice thermodynamically stable. It does not sinter, delaminate or lose shape even with repeated heating and cooling cycles. Meanwhile, the microporous structure ensures rapid gas exchange and accelerates reactions.
This solution paves the way for new types of solar thermochemical systems that will be able to produce hydrogen and carbon monoxide directly from water and CO₂ using only concentrated heat. Moreover, all the elements that make up the oxide are widespread and abundant, which makes large-scale production of this catalyst feasible.
