Nowadays, hydrogen is being used as an energetic vector for saving excess of renewable energy. The most used techniques to generate hydrogen are thermochemical looping’s, electrolysers and hydrocarbons reforming. However, all these techniques have several drawbacks, namely the high temperatures needed, the use of sophisticated machinery and the long operation times required for the hydrogen production. Recently, the possibility to generate green hydrogen using electric energy, as microwave radiation, has been reported. This process uses metallic oxides as catalysts, e.g. CeO₂, and it occurs in a reactor at temperatures lower than 250 ºC in less than five minutes.
The hydrogen production mechanism happens in two steps. First, the material is irradiated with a microwave electromagnetic field, producing the reduction of the material with the concomitant release of oxygen. This radiation is able to stabilize a higher amount of oxygen vacancies in the fluorite structure at lower temperatures than the conventional radiative processes, as it is the case of thermosolar (>1000 ºC). When the microwaves are turned off in the presence of water, the material splits the H₂O molecule, therefore liberating a stream of molecular hydrogen and filling its oxygen vacancies. One example of this process is noted in the material Ce_0.8Gd_0.2O₂.
The release of oxygen is accompanied by an increase in the material electrical conductivity. Besides, it has been observed that a different conductivity behaviour can be inferred depending on the irradiated microwave power. If the microwave radiation power is lower than the activation energy (P_TH), the material conductivity behaves similar to conventional heating process. On the other hand, for a microwave radiation power higher than the PTH, the material undergoes a sudden spike in the conductivity. This rise is mainly ascribed to an increase of the electronic conductivity. Ionic conductivity can be tuned by doping the ceria lattice with aliovalent cations. In the specific case of ceria doped with trivalent lanthanides, ionic conductivity is predominant over the whole temperature range. This depends on the lattice parameter, when this value is close to that of pure ceria, there is a lower binding energy between the oxygen vacancies and the trivalent dopants, resulting in a low migration energy that facilitates the movement of oxygen ions through the ionic lattice. In this work, we have synthesized and characterized a set of lanthanide-doped ceria, e.g. Ce_0.9M_0.1O₂ (M = La, Y, Yb, Sm, Nd, Er, and Gd), in order to study how the electromagnetic field interacts with the material, leading to material reduction. For this purpose, optical techniques and dielectric measurements have been used to monitor the process at each step. Finally, in order to increase the amount of hydrogen produced, the microwave-assisted water splitting process has been studied in each of them.