To achieve EU goal of climate-neutrality by 2050, the development of strategies that allow the large-scale implementation of technologies for energy generation with zero green-house emissions are necessary. Hydrogen offers a higher energy density compared to conventional fuels and it can be obtained through several sustainable processes such as photochemical and electrolysis. With the aim of practical implementation, in addition to electrolysis, thermochemical water splitting has been in the sight.

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.
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.
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.
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.  


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.

Characteristic Non porous Porous materials
Sintering +++ ++
Kinetics + +++
Heating homogeneity + +++
Mass transfer ++ +++

Reactivity solid-gas

+ +++
Pressure drops +++ +
Reactant and products difussion + +++


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.

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