Solid Oxide Fuel Cells (SOFC) are energy devices that can convert the chemical energy of a fuel directly into electrical energy. This gives them very high electrical efficiencies, which can rise up to 80% when using its high-quality waste heat in turbines. They can work using different fuels, like hydrogen, natural gas, synthesis gas, ethanol, methanol, etc. However, in order to play a ron into the energy production chain, operating temperature should decrease to the range of 500-700 ºC, without reducing the generated electric power.
The conventional SOFC are based on the oxygen ion conduction of the electrolyte, which separates the fuel combustion reaction in the electrochemical half-reactions, thereby generating electrical energy. By lowering the operating temperature in this type of SOFC, with thin electrolytes (or membranes) and hydrogen as fuel, the main operation limitation focuses on the oxygen activation and reduction that occurs at the electrode so-called cathode. On the other hand, usage of other carbon-based fuels is incompatible with the actually used anode materials.
It is therefore necessary to develop new cathodes with improved electrocatalytic properties for oxygen reduction at lower temperatures, whose thermo-mechanical properties are compatible with the other components of the cell, and to obtain anodes capable of working with carbon-based fuels.
The combination of several lanthanides and barium in the perovskite structure (LalPrpSmsBab)0.58Sr0.4Fe0.8Co0.2O3 has generated compounds with electrode polarization resistances lower than the state-of-the-art La0.6Sr0.4Fe0.8Co0.2O3 cathode in the 450-650 °C temperature range. The improvement in oxygen activation and diffusion of these materials has been associated with cooperative processes derived from the combination of two or three elements in the same position of the crystal structure. The results confirm that the increase of oxygen surface exchange and oxygen diffusion are responsible for the increase in the electrocatalysis of these cathodes with multiple elements. The compositions La0.2175Pr0.2175Ba0.145Sr0.4Fe0.8Co0.2O3 or Pr0.435Ba0.145Sr0.4Fe0.8Co0.2O3 offering polarization resistance values of 80 and 190 mOhm•cm2 at 650 º C respectively, are proposed as promising cathodes for solid oxide fuel cells operating at intermediate temperatures.
The electrochemical studies carried out in crystal structures based on swedenborgites MBaCo3ZnO7, with Y, Er and Tb in the M position, revealed an activity for oxygen activation comparable to the La0.6Sr0.4Fe0.8Co0.2O3 cathode in the 500-650 ºC range: electrode polarization resistance values obtained at 650 ºC for TbBaCo3ZnO7 and YBaCo3ZnO7 compounds were 0.46 i and.29 Ohm•cm2 respectively. Better thermal compatibility of cathodes with the rest of the cell materials could be achieved by means of using MBaCo3ZnO7-based compounds. Specifically, for the TbBaCo3ZnO7 compound, the thermal expansion coefficient was estimated as 9.45•10-6 K-1 in the range 25-900 °C, almost half that of the La0.6Sr0.4Fe0.8Co0.2O3 compound. Furthermore, chemical compatibility of the TbBaCo3ZnO7 compound with the Ce0.8Gd0.2O1.9 electrolyte material and its CO2-tolerance make this material a good candidate as solid oxide fuel cell cathode, with a 1 mOhm•cm2•h-1 degradation.
The electrocatalytic methane activation is achieved with La1-xSrxCr1-yNiyO3 perovskites. These compounds have shown to be structurally stable under reducing conditions and after redox cycling. The nickel content which minimizes the polarization resistance of these electrodes is close to 10% mol. Thus, the composition La0.85Sr0.15Cr0.9Ni0.1O3 has shown the lowest polarization resistance and activation energy values in the 650-900 °C range, both under hydrogen and methane. The formation of metallic nickel nanoparticles on the surface of the anode is responsible for the improvement of the surface reactions for fuel activation. The decrease in the initial reduction temperature in compounds with a low content of nickel allows to obtain a greater dispersion of nanoparticles smaller in size that improve the surface processes. The possible coke formation on the nickel active sites, which would limit the anode operation, could be eliminated with the regeneration through an oxidation process. This re-oxidation of the electrode permits the metallic nickel nanoparticles to be re-incorporated into the crystal structure. Additionally, methane steam reforming has been performed using this type of electrocatalytic and regenerative compounds, achieving 30% methane conversion at 900 ºC.