MICROSTRUCTURAL ENGINEERING OF ELECTRODES FOR SOLID OXIDE CELLS VIA INFILTRATION AND FREEZE TAPE CASTING

The development of renewable energy sources is nowadays becoming a priority in response to climate change. However, because of the intermittent nature of the natural power sources, new technologies able to balance the energy fluctuations between production and demand are also required. In this context, solid oxide cells are gaining interest due to their high operating efficiency, fuel flexibility and operational reversibility. In particular, the latter aspect offers the opportunity of combining a fuel cell and an electrolyzer in a single energy system being able to work alternatively between the two operating modes. Despite their promising operational advantages, the final commercialization of these devices has still to take place due to technological issues which prevent their competitiveness on the market. The performance optimization of solid oxide cells can be approached on different scales. This work focuses on the button cell level, dealing with experimental and modelling activities for the synthesis of high performing electrocatalysts and the microstructural optimization of the electrodes. The research activities performed on the μ-scale are expected to significantly impact the electrochemical performance of the single cells, which represent the operational core of solid oxide cells stacks. Specifically, electrochemical and microstructural optimizations carried out on the button cell level (e.g. identification of best performing or more stable electrocatalysts and innovative cell architectures) can drastically improve the performance and degradation rate of the stacks after upscaling. In this context, particular attention was dedicated to manufacturing processes able to improve the design of the-state-of-the-art composite electrodes by boosting gas diffusion and enhancing the amount of triple phase boundary sites. In particular, the infiltration and freeze tape casting shaping routes have been studied to manufacture nanostructured graded porous electrodes. Initially, the research activities focused on the synthesis and electrochemical characterization of a cobalt-free electrocatalyst (i.e. La1-xSrxCuyFe1-yO3-δ) for the air electrode. Two different electrode architectures were manufactured to study the effect of the electrode microstructure and electrocatalyst distribution on the electrochemical activity and operational stability over time. Specifically, in one electrode configuration the electrocatalyst was synthesized in-situ by the infiltration of a metal precursor solution inside a porous ionic-conducting scaffold made out of Sm doped-ceria. In the second electrode architecture a La1-xSrxCuyFe1-yO3-δ paste was slurry coated over a Sm doped-ceria supporting electrolyte. The electrochemical characterization of the electrodes, carried out by electrochemical impedance spectroscopy, highlighted the high electrochemical activity of La1-xSrxCuyFe1-yO3-δ as air electrode in fuel cell mode (polarization resistance equal to 0.0153 Ω∙cm2 at 700 °C at open circuit voltage) but insufficient structural stability over time. The impact of the electrode microstructure on the electrochemical performance of button solid oxide cells was investigated in depth by the use of the freeze tape casting shaping route. In particular, the latter was used to manufacture hierarchical porous backbones of yttrium-stabilized zirconia and gadolinium-doped ceria with the purpose of boosting the gas mass transfer in the electrodes. The unique morphology of the freeze tape cast backbones was investigated by X-ray micro- and nano-holotomography. Experimental activities have been coupled to the construction of modelling frameworks (both microstructural and electrochemical) to optimize the electrode microstructure and predict the performance gain which can be obtained by the use of graded porous scaffolds. Specifically, a 2D electrochemical model, which simulated the performance of a nickel-yttrium-stabilized zirconia fuel electrode, highlighted the reduction of the concentration overvoltage by 41.8% and 87.4% in the functional and diffusion layers (at 0.7 V and 750 °C) when using the hierarchical electrode architecture. Nanostructured graded porous fuel electrodes were subsequently manufactured coupling the impregnation and freeze tape casting techniques. Specifically, freeze tape cast backbones of yttrium-stabilized zirconia and gadolinium-doped ceria were repeatedly infiltrated with a nickel nitrate solution and, subsequently, electrochemically characterized. The best electrochemical behavior was shown by nickel-impregnated scaffolds of gadolinium-doped ceria, due to its mixed ionic-electronic conductivity in reducing atmosphere (polarization resistance equal to 0.823 Ω∙cm2 at 750 °C at open circuit voltage). It has been observed that the electrochemical properties of the tested fuel electrodes are significantly affected by the process parameters of the experimental infiltration and morphological characteristics of the scaffolds. The identification of theoretical guidelines which could lead the experimental process towards the desired microstructural properties was then found necessary to complete the design optimization of the electrodes. In particular, a stochastic numerical framework was developed with the purpose of retrieving the best experimental conditions to manufacture an optimized nanostructured electrode. The microstructural properties of the freeze tape cast electrodes were computed using their 3D renderings obtained by X-ray tomography. The infiltration algorithm was then used to identify the optimal electrocatalyst volume fraction, according to the selected nanoparticles size, that could maximize the electrode performance. Eventually, the electrochemical performance of a conventional nickel-yttrium-stabilized zirconia composite fuel electrode and a nickel-impregnated yttrium-stabilized zirconia freeze tape cast electrode were compared by means of a simplified 1D electrochemical model. At first, the framework was calibrated on the current-voltage characteristics and impedance data of the composite. After its validation, the microstructural properties of the composite cermet were replaced by those of the nanostructured freeze tape cast electrodes using, as inputs, the values extracted from the analyses of the 3D reconstructions. A substantial operational advantage was estimated when using the nanostructured freeze tape cast electrode architecture, which reduced by more than by 50% the polarization resistance of the cermet. However, the impact of the tortuosity factor and percolation fraction of the electrocatalyst resulted to provide a non-negligible contribution. Specifically, a large colimitation provided by the poor transport of the ionic and electronic species was evidenced when the percolation of the nickel phase did not reach 90%.