Friedemann Call

Materials-Related Aspects of Thermochemical Water and Carbon Dioxide Splitting: A Review

Thermochemical multistep water- and CO2-splitting processes are promising options to face future energy problems. Particularly, the possible incorporation of solar power makes these processes sustainable and environmentally attractive since only water, CO2 and solar power are used; the concentrated solar energy is converted into storable and transportable fuels. One of the major barriers to technological success is the identification of suitable active materials like catalysts and redox materials exhibiting satisfactory durability, reactivity and efficiencies. Moreover, materials play an important role in the construction of key components and for the implementation in commercial solar plants. The most promising thermochemical water- and CO2-splitting processes are being described and discussed with respect to further development and future potential. The main materials-related challenges of those processes are being analyzed. Technical approaches and development progress in terms of solving them are addressed and assessed in this review.

Perovskite oxides for application in thermochemical air separation and oxygen storage

Perovskites AMO3−δ are ideal for thermochemical air separation due to their oxygen nonstoichiometry δ, which can be varied by changing the temperature and oxygen partial pressure. We show in this work how materials can be selected for chemical looping air separation from thermodynamic considerations and present thermogravimetric experiments carried out on (Ca,Sr) ferrites and manganites, and doped variants, all synthesized via a citric acid auto-combustion method. SrFe0.95Cu0.05O3−δ and Ca0.8Sr0.2MnO3−δ show the best gravimetric oxygen storage capacity of all tested materials at T < 1200 °C. The redox reactions are completed in <1 min in air and are highly reversible. A significant re-oxidation reaction of reduced samples was observed at temperatures as low as 250 °C at an oxygen partial pressure of 0.16 bar. We studied phase formation via XRD and lattice expansion during reduction via in situ XRD experiments. The objective is to validate the potential and boundary conditions of such materials to pave the way for competitive air separation based on thermochemical cycling.