Green hydrogen production through low temperature thermochemical water splitting cycles based on non-ordered and ordered macroscopic structures of La0.6Sr0.4Co0.2Fe0.8O3±δ perovskite

Abstract

The production of green hydrogen is one of the main targets of current energy and environmental policies. In this context, thermochemical water splitting is one of the potential methodologies that enable its production. This process is based on the thermal reduction of a metal oxide, followed by its re-oxidation with water releasing hydrogen. The main problem of this process, which hinders its full-scale application, is that reducing the metal oxide usually requires very high temperatures (>1500 degrees C). To decrease this reduction temperature, non-stoichiometric oxides such as perovskites have been proposed. In a previous work, the authors have presented La1-xSrxMeO3 +/-delta (x = 0.2-0.4; Me = Mn, Fe and Co) perovskites as active materials decreasing the operation temperature to 1400 degrees C. However, those perovskites showed a significant lack of stability upon cycling, limiting their use in a future scale-up of the process. In this work, we present a multi-substituted perovskite type A(1-x)A'xB1-yB'O-y(3 +/-delta) (La0.6Sr0.4Co0.2Fe0.8O3 +/-delta, named LSCF) as redox material with increasing stability and remarkable activity in the hydrogen production cycles even at temperatures below 1000 degrees C. This material was synthesised by reactive grinding as a green synthesis method optimising the variables of the process. Three reduction temperatures for the thermochemical water splitting were evaluated in the range 800-1200 degrees C at the same oxidation temperature of 800 degrees C. LSCF perovskite has been used in powder form with a H-2 production of 5.22 cm(3)STP/g(material)cycle when the reduction was performed at 800 degrees C and 6.83 cm(3)STP/g(material)cycle when this reduction step was performed at 1000 degrees C. Afterwards, the LSCF was shaped into two different macroporous structures looking for a potential scaling-up of the process: reticulated porous ceramic structure (RPC) and a ceramic monolith structure with straight and well-ordered channels in which the perovskite forms a thin layer over the internal channels surface. The macroscopic structures exhibited good activity and stability working isothermally at 800 degrees C under N-2 atmosphere, reaching H-2 productions higher than 10 cm(3)STP/g(material)cycle. Particularly, the monolithic structure, characterised by its open macroporosity improves the heat transfer phenomena and the contact between the gas-phase and the perovskite, obtaining a stable hydrogen production under isothermal conditions of 17 cm(3)STP/g(material)cycle at 800 degrees C. That could be increased up to 32.5 cm(3)STP/g(material)cycle when the reduction step of the thermochemical water splitting is performed at 1000 degrees C. To the best of our knowledge, this is the higher value obtained for hydrogen production by a perovskite in this application at this reaction conditions. These results confirm the LSCF as a potential material for green hydrogen production by low-temperature thermochemical cycles.