Two-step thermochemical cycles can offer low energy losses and competitive efficiencies for H₂ production compared to methane reforming. The process requires as inputs reusable active redox materials, e.g. metal oxides, thermal energy, and water to produce H₂ and O₂ at temperatures from 500 °C to 2000 °C, which can be supplied through solar concentrators and waste heat from industrial processes.

The first step of the process involves the metal oxide reduction to activate the material and release oxygen from its lattice. In the second step, the material is oxidized taking oxygen from water steam and producing hydrogen. (Figure 1).

Figure 1.  Reaction pathways in two-step thermochemical water splitting for H₂ production.

The first thermal reduction is highly endothermic occurring at high temperature, while the water splitting process is slightly exothermic occurring at lower temperature. The first step requires temperatures higher than 1200ºC⁴ and involves the formation of oxygen vacancies in the metal oxides during the thermal reduction. The reduction of the metal oxide in the first step can be partial or complete, leading to the obtention of an oxygen deficient oxide or a metal, which in both cases react with water steam in the second step to regenerate the oxide (Figure 2).

Figure 2.  Reaction pathways in two-step thermochemical water splitting for H₂ production.

Factors considered for two-step redox thermochemical cycles materials design

The active redox materials hold the main role in the thermochemical water splitting process, and understanding the correlations between the electronic, crystalline, and microstructural properties of the materials with their performance its crucial not only for the design of more efficient and stable systems, but also for analysing and overcoming the current material limitations.

Figure 3. Factors influencing the materials performance for H₂ production from thermochemical water splitting.

Abundance and cost

For achieving sustainable and cost-effective scalable process, robust earth-abundant based materials showing high performance and cyclability for H₂ production are required.

Oxygen exchange capacity

The amount of oxygen released in the reduction step depends on the oxygen exchange capacity of the material and it influences the maximum amount of fuel produced in the oxidation step. The oxygen exchange capacity is defined by the non-stoichiometry (d).

The goal is to have materials with high oxygen exchange capacities and high stability over a large number of cycles⁶.

Volatile – non-volatile

The cycles based on metal oxides can be categorized in volatile (e.g. ZnO/Zn) or non-volatile (e.g. Fe₃O₄/FeO), depending if the oxide undergoes solid-gas transition, or remains in solid phase during the reactions. The oxygen released and the fuel produced in volatile systems is higher compared to the non-volatile. However, after the reduction step the material should be quenched to prevent recombination⁷.

Stoichiometric – non-stoichiometric

Meanwhile, the non-volatile systems can be classified in stoichiometric (e.g. Fe₃O₄) and non-stoichiometric oxides (CeO₂), and the thermodynamic and kinetic properties of these oxides are affected by doping or substitution by anions and cations. In a general way, the stoichiometric oxides exhibit higher oxygen release, but their disadvantage is a slow kinetic and low structural and chemical stability, which hinders the performance of the material and limits the amount of fuel produced⁷.

Crystalline structure

Materials that offer stable crystalline structure with tuneable composition are highly attractive since they offer the possibility of  doping and oxygen vacancy formation with no phase transformation after the redox reactions. The most studied families include perovskites, spinels, and pyrochlores, among others⁸.

Redox activity

The redox capacity is highly influenced by the electronic structure of the material, and cations in the crystalline structure play a determinant role during the redox reactions. For instance, materials with cerium, iron, manganese, lanthanum, and other transition metals have shown high reducibility. However, an ideal material must exhibit appropriate thermodynamic and kinetic properties for both reduction and oxidation steps, which is still a challenge since not all the materials show fast oxidation kinetics⁵.  

Doping

The effect of doping on the properties of metal oxides for thermochemical water splitting has been studied in deep. For instance, it has been proved that substituting cerium by divalent, trivalent and tetravalent dopants of smaller radius is beneficial to promote higher amount of oxygen released.  Meanwhile, a dopant with higher valence and smaller ionic radius favors the reduction extent of doped ceria, modifying the M-O bonds in the crystalline structure, which are more easily breakable in these conditions⁹.

Surface area and pore size

Materials exhibiting microporous structure benefit the oxidation reaction, due to the high surface area, while materials with macroporous structure, with pores in the range of millimetres, favour homogeneous heating⁵. Also, it has been shown that the use of nanoparticles enhances the H₂ production due to higher surface exposed area, improving the reaction kinetics, the heat and mass transfer and the overall reaction rate. Table 1 summarizes the advantages of porous materials for thermochemical water splitting.

Table 1. Advantages of porous materials for thermochemical water splitting.

Perspectives

Nowadays, the main limitations regarding the active materials for thermochemical water splitting are associated with the high reduction temperature, and the low H₂ production and cyclability of the materials, due to undesired processes including sintering, low reactivity, and secondary phases formation. For this reason, in CIC energiGUNE, we are working in the design and development of new and competitive materials that can offer stable H₂ production and large lifetime as prospects for their use in large-scale H₂ production.