Fabrication and Characterization of Functional Ceramic Films by Solution Spray Pyrolysis: Correlations with the Thermal Decomposition Characteristics of the Constituent Salts

Abstract

Thin films of ceramic or ceramic-metal (cermet) electrodes with appropriate structure and morphology are always sought in ceramic fuel cells and electrolyzers. These films can serve as either electrodes or electrolytes in solid oxide fuel cells (SOFCs) or electrolyzers (SOECs) requiring different morphological and structural characteristics for optimal function. Thus, electrode parameters such as thickness, porosity and tortuosity essentially determine the values of gaseous effective diffusivities and subsequently the extent of concentration polarization losses [1]. In addition, nanostructured thin film electrodes have shown enhanced conversion efficiencies compared to their bulk materials counterparts, essentially leading to low polarization losses [2].On the other hand, the electrolyte has to possess minimal porosity not allowing mixing between the anode and cathode compartments and adequately blocking electronic transport. Low thicknesses are desirable for minimization of the cell’s total ohmic resistance. However, at low electrolyte thicknesses, total ohmic cell polarization is affected by electrode microstructure to a greater extent than by electrolyte ohmic resistance due to contiguity factors between metal particles in the fabricated cermets and metal depletion in the anode functional layer [3].Generally, for a given set of chosen materials, all of the above microstructural requirements for electrodes and electrolytes have to be met by the adopted fabrication methods which have to be subsequently optimized with respect to their operational parameters. It is advantageous to resort to the so called innovative fabrication methods for the above, in particular for their functional components. The term refers to methods with the potential of producing nanostructural components and controlling in situ the film’s characteristics such as porosity, pore size, thickness and uniformity.The solution spray pyrolysis technique (SSP) belongs to this family of methods as it is based on molecular deposition and it involves spraying of a solution of suitable constituent metal salts onto a heated substrate [4].The technique is robust, environmentally friendly if water is used as solvent, easily upscalable and with easily controllable processing parameters. It also possesses the potential of producing very thin layers of components (i.e. < 1 μm) and therefore, reducing the subsequent sintering temperatures due to the more active precursor components of the constituent particles forming the final film.In a recent publication [4], we have shown that the technique can be optimized in terms of deposition temperature, time and concentration of the precursor salts in order to fabricate crack-free films of La0.75Sr0.25MnO3 (LSM) and CuO-CeO2 on dense YSZ substrates with thicknesses of the order of 0.65–3.5 μm. 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 electrolyte [5]. It was shown that lower deposition temperatures (but above the solvent boiling point) generally lead to more porous films and the precursor salt concentration is coupled with the optimal deposition temperature range and deposition time. In the present communication, we extend these findings in order to include films of Gadolinia (10mole%) stabilized Ceria (CGO10) on dense Zr0.92Y0.08O2 - x (YSZ) and La0.3Sr0.7FeO3 (LSF70) substrates. CGO10 is a typical solid electrolyte more suitable for intermediate temperature applications (i.e. 500-750°C) and it is amenable to being fabricated with lower porosities by SSP than YSZ [6]. Focus is on the thermal characteristics of the constituent salts with respect to correlations with the appropriate deposition temperatures as well as the effect of deposition time. The produced films are characterized by X-ray diffraction and scanning electron microscopy (SEM). CGO10 films were also characterized in terms of their electronic blocking capability in contact with the LSF70 substrates.