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Solid Oxide Electrolysis Cells electrodes based on mesoporous materials

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Universitat de Barcelona. Facultat de Física

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[eng]  The need of substituting the current energetic model by a system based on clean Renewable Energy Sources (RES) have gained more importance in the last decades due to the environmental issues related to the use of fossil  fuels.  These energy sources are site-specific and intermittent, what makes essential the development of Energy Storage Systems (ESS) that allows the storage of the electricity generated by renewable energies.
Among the technologies under development for the storage of electrical  energy, Solid Oxide Electrolysis Cells (SOECs) have been proposed in the last decades  as a promising technology. Achieving efficiencies higher than 85%, SOEC technology is able to convert electrical energy into chemical energy through the reduction of H2O, CO2 or the combination of both; generating H2, CO or syngas (H2 +CO). The implementation of this technology based on renewable electrical energy, combined with fuel cells would allow closing the carbon cycle.
The work presented in this thesis has been devoted to enhance the performance of SOEC. The approach that is presented for that propose is based on the implementation of high surface area and thermally stable mesoporous metal oxide materials on the fabrication of SOEC electrodes. High performance and stability of the electrodes was expected during its characterization. Structural and electrochemical characterization techniques have been applied during the development of this thesis for this purpose.
The thesis is organized in eight chapters briefly described in the following:

Chapter 1 briefly analyses the current energy scenario presenting electrolysers as a promising technology for the storage of electrical energy. Besides, basic principles of SOECs operation and the state-of-the-art materials of SOECs are reviewed.
 
Chapter 2 describes all the experimental methods and techniques employed in this thesis for the synthesis and characterization of synthesised materials and fabricated cells.
Chapter 3 presents the results obtained from the structural characterization of the mesoporous materials and fabricated electrodes, revealing the successful implantation of the hard-template method for obtaining Sm0.2Ce0.8O1.9 (SDC), Ce0.8Gd0.2O1.9 (CGO) and NiO mesoporous powders, and the fabrication of SDC-SSC (Sm0.5Sr0.5CoO3-δ), CGO- LSCF (La0.6Sr0.4Co0.2Fe0.8O3) and NiO-SDC electrodes based on mesoporous materials. The attachment of the mesoporous scaffold for the fabrication of oxygen electrodes has been optimized at 900 °C.
Chapter 4 compares electrolyte- and fuel electrode-supported cell configurations  based on the same oxygen electrode. The electrochemical performance and the microstructural characterization of these cells are considered for that purpose. Showing a maximum current density of -0.83 and -0.81 A/cm2 on electrolysis and co- electrolysis modes respectively, fuel electrode-supported cells are considered more suitable for SOEC fabrication.
Chapter 5 presents a study focused on analysing the influence of the oxygen electrode interface on the SOEC performance. The electrochemical and microstructural characterization of barrier layers and oxygen electrodes fabricated applying different methods are discussed in this chapter. The combination of a barrier layer fabricated by Pulsed Laser Deposition (PLD) with an oxygen electrode based on mesoporous materials resulted on the injection of up to -1 A/cm2, what allows concluding that   this
interface microstructure is directed related with the best performing SOECs in this thesis.
Chapter 6 shows the performance of SOEC cells on co-electrolysis mode containing the optimized oxygen electrode, fabricated by infiltration of mesoporous scaffolds. The long-term stability of infiltrated mesoporous composites have been demonstrated during 1400 h, registering degradation rates of 2%/kh and <1%/kh when current densities of -0.5 A/cm2 and -0.75 A/cm2 are injected, respectively.
 
Chapter 7 shows results of the scale-up of the mesoporous-based electrodes for the fabrication of large area cells. Their electrochemical performance shows high fuel flexibility, injecting -0.82 A/cm2 on co-electrolysis mode; and long-term stability injecting -0.5 A/cm2 for 600 h.
The conclusions of this thesis are presented in Chapter 8.

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[spa]  Una de las principales desventajas de las fuentes de energías renovables es que producen energía eléctrica de forma discontinua. Los electrolizadores de alta temperatura basados en óxidos sólidos (SOEC) se presentan como una tecnología prometedora para el almacenamiento de energía eléctrica. Alcanzando eficiencias mayores de un 85%, los electrolizadores SOEC permite convertir energía eléctrica en energía química mediante la reducción de las moléculas de agua (H2O), dióxido de carbono (CO2), o la combinación de ambas; generándose hidrógeno (H2), monóxido de carbono (CO) o gas de síntesis (H2 +CO) como producto.
El trabajo que se presenta en esta tesis tiene como objetico mejorar el rendimiento de los electrolizadores SOEC mediante la utilización de óxidos metálicos mesoporosos, caracterizados por poseer alta área superficial y ser estables a altas temperaturas.
Esta tesis está organizada en ocho capítulos. Los capítulos 3, 4, 5, 6 y 7 presentan los resultados alcanzados:
El capítulo 3 presenta la caracterización estructural de los materiales mesoporosos y  de los electrodos fabricados. Además, la temperatura de adhesión del material mesoporoso ha sido optimizada y se ha fijado a 900 °C.
El capítulo 4 compara electrolizadores fabricados soportados por el electrodo de combustible y por el electrolito. Los resultados muestran que las densidades de corriente más altas fueron inyectadas en los electrolizadores soportados por el electrodo de combustible, considerándose esta configuración la más apropiada.
El capítulo 5 presenta la influencia de la microstructura de la intercara del electrodo de oxígeno   en   el   rendimiento   de   los   electrolizadores   SOEC.    La       caracterización
 
electroquímica, apoyada por la caracterización microestructural, ha demostrado que la máxima densidad de corriente ha sido inyectada por el electrolizador cuya barrera de difusión ha sido depositado por láser pulsado (PLD) y la capa funcional del electrodo de oxígeno mediante infiltración de materiales mesoporosos.
El capítulo 6 estudia el electrodo de oxígeno optimizado. Durante 1400 h de operación continua y caracterización microstructural, se ha demostrado la estabilidad de este electrodo.
Por último, el capítulo 7 muestra los resultados obtenidos del escalado de los electrodos mesoporosos en celdas de mayor área (25 cm2). La caracterización electroquímica muestra alta flexibilidad ante las composiciones de gases utilizadas, y estabilidad de los electrodos mesoporosos propuestos.

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Solid Oxide Electrolysis Cells electrodes based on mesoporous materials