Only the main "families" will be described in this article. Two-step cycles Two-step thermochemical cycles, often involving metal oxides, can be divided into two categories depending on the nature of the reaction: volatile and non-volatile. Volatile cycles utilize metal species that sublime during the reduction of the metal oxides, and non-volatile cycles can be further categorized into stoichiometric cycles and non-stoichiometric cycles. During the reduction half-cycle of the stochiometric cycle, the metal oxide is reduced and forms a new metal oxide with different oxidation states (Fe3O4 → 3FeO + 1/2 O2); a non-stochiometric cycle's reduction of the metal oxide will produce vacancies, often oxygen vacancies, but the crystal structure remains stable and only a portion of the metal atoms change their oxidation state (CeO2 → CeO2-δ + δ/2 O2). Non-stoichiometric cycles with CeO2 The non-stoichiometric cycles with CeO2 can be describes with the following reactions: Reduction reaction: CeO2 → CeO2-δ + δ/2 O2 Oxidation reaction: CeO2-δ + δ H2O → CeO2 + δ H2 The reduction occurs when CeO2, or ceria, is exposed to a inert atmosphere at around 1500 °C to 1600 °C, and hydrogen release occurs at 800 °C during hydrolysis when it is subjected to an atmosphere containing water vapor. One advantage of ceria over iron oxide lies in its higher melting point, which allows it to sustain higher temperature during reduction cycle. In addition, ceria's ionic conductivity allows oxygen atoms to diffuse through its structure several orders of magnitude faster than Fe ions can diffuse through iron oxide. Consequently, the redox reactions of ceria can occur at occur at a larger length scale, making it an ideal candidate for thermochemical reactor testing. Ceria-based thermochemical reactor has been created and tested as early as 2010, and viability of cycling was corroborated under realistic solar concentrating conditions. One disadvantage that limits ceria's application is its relatively lower oxygen storage capability. Non-stoichiometric cycles with perovskite The non-stoichiometric cycles with a perovskite ABO3 can be describes with the following reactions: Reduction reaction: ABO3 → ABO3-δ + δ/2 O2 Oxidation reaction: ABO3-δ + δ H2O → ABO3 + δ H2 The reduction thermodynamics of perovskite makes it more favorable during the reduction half-cycle, during which more oxygen is produced; however, the oxidation thermodynamics proves less suitable, and sometimes perovskite is not completely oxidized. The two atomic sites, A and B, offer more doping possibilities and a much larger potential for different