Thin films of ceramic or ceramic-metal (cermet) electrodes with appropriate structure and morphology significantly affect performance of solid oxide fuel cells (SOFCs. Thus, electrode parameters such as thickness, porosity and tortuosity dictate the values of gaseous effective diffusivities that eventually determine the extent of concentration polarization losses in these high temperature ceramic fuels cells¹. In addition, electrode thickness is also a factor dictating concentration polarization losses in working fuel cells which should be as low as possible. Furthermore, it was established that, even total ohmic cell polarization in ceramic fuel cells based on conventional yttria stabilized zirconia (YSZ) electrolyte is significantly affected by electrode microstructure to a greater extent than by electrolyte ohmic resistance. This becomes more evident at low electrolyte thicknesses (i.e. <20 μm) and is related to contiguity factors between metal particles in the fabricated cermets².

On the other hand, nanostructured thin film electrodes have shown great potential for operation in fuel cells due to their enhanced conversion efficiencies compared to their bulk materials counterparts essentially leading to low polarization losses³. In these structures, controlling the electrode film thickness, porosity and tortuosity is of crucial importance as these dictate the magnitude of effective gas diffusivities of reactants and products and the subsequent overall polarization losses⁴. Therefore, for a given set of chosen materials, the fabrication method adopted for the ceramic fuel cell has to be optimized for the above electrode parameters for optimum performance.

In general, innovative fabrication methods with the potential of producing nanostructural components in ceramic fuel cells and possessing inherently the capacity of controlling electrode microstructural characteristics such as porosity, pore size, thickness and uniformity have not given much attention in the literature compared to materials research⁵. The major advantages of these techniques over the conventional ceramic techniques such as tape casting, are their potential on producing very thin layers of components (i.e. < 1 μm) and the reduction of the subsequent sintering temperatures due to their more active precursor components. Another advantage lies in their capacity of producing nanocomposite electrodes that offer enhanced electrochemical performance⁶.

The spray pyrolysis (SP) technique belongs to this family of deposition methods⁷ and involves spraying of a solution of metal salts onto a heated substrate. It is robust, easily up scalable and with easily controllable processing parameters. It offers the advantage of accurate control of stoichiometry at droplet level and possess the potential of incorporating the sintering step in the process. It has been used for fabrication of a large number of electroceramic materials and progress in solid oxide fuel cell component fabrication has been reviewed recently⁵. The technique belongs to the family of molecular precursor deposition methods in which the film is formed on a support via a physical and/or chemical process by appropriately tuning the characteristics of the physicochemical processes involved during deposition in order to achieve the desired functional characteristics.

In the present work we focus on the optimization of the technique for fabricating electrodic films. In particular, results are presented of the application of air pressurized spray pyrolysis using aqueous solutions for the production of film cermets of Cu-CeO₂, and La_0.75Sr_0.25MnO₃ (LSM) onto dense YSZ substrates. The use of aqueous solutions reduce significantly the cost and the risk factors to personnel health and to the environment. Cu-CeO₂ constitutes a potential anode that combines the functionalities of copper ( an electronic conductor) and a good oxidation catalyst (i.e. CeO₂) for potential hydrocarbon fuels fed into a SOFC while LSM is the typical state of the art cathode used with a YSZ electrolyte⁸. The films are characterized by X-ray diffraction and scanning electron microscopy (SEM).

References

¹W. He et al., Journal of Power Sources, 195, 532 (2010)

²F. Zhao, A. V. Virkar, Journal of Power Sources 141, 79 (2005)

³W. Hea et al., , Nano Energy 1, 828, (2012)

⁴W. He , J. B. Goodenough, Journal of Power Sources 251, 108 (2014)

⁵N. E. Kiratzis, Ionics 22,751 (2016)

⁶S.J. Litzelman et al., Fuel Cells 08, 294 (2008)

⁷E. Papastergiades et al., Ionics 15, 545 (2009)

⁸N.E.Kiratzis et al., Journal of Electroceramics 24, 270 (2010)