Solid oxide cells (SOCs)  are devised to become a key technology in an energy scenario grounded on the widespread use of renewable sources, finding applications into several industrial sectors (grid balancing, seasonal renewable energy storage, synthetic e-fuels) due to their large fuel flexibility and conversion efficiency. The state-of-the-art Solid Oxide Cells, SOCs, still suffer from issues related to long-term chemical and mechanical instabilities due to various degradation processes driven by both the high operating temperature (700-850 °C) and polarization. These degradations, which have a strong impact on the SOCS performances, limit their lifetime hindering the industrial deployment of the technology.
All the electrochemical reactions taking place on the active sites of the electrodes involve mass transfer processes, which play a key role on their performance when working in both fuel cell (SOFC) and electrolysis (SOEC) modes. Within this context, in the last years, the efforts towards the development of graded porous electrodes have been growing due to their inherent unique morphological features which are expected to boost mass transfer within the gas diffusion channels.
This work focuses on the manufacturing process, electrochemical and morphological characterizations of hierarchical porous fuel electrodes for middle - low temperature SOCs applications. The studied innovative electrodes are produced by combining the freeze tape casting technique to shape the ion-conducting scaffold and the impregnation method for the decoration of the backbone walls with the electrocatalyst. The graded porous scaffolds are made out of 8 mol% Yttria Stabilized Zirconia (8YSZ) and 10 mol% Gadolinium Doped Ceria (GDC10) while Ni nanoparticles are added by infiltration as electrocatalyst.
Symmetrical electrolyte-supported cells were experimentally tested at different operating temperatures and hydrogen partial pressures and their performance was investigated by electrochemical impedance spectroscopy (EIS). The differential relaxation time (DRT) analysis and the equivalent circuit (EC) modelling were also carried out to support the interpretation of the EIS data and improve the understanding of the limiting processes hindering the cell electrochemical performance. A promising polarization resistance, Rp, of 0.821 Ω*cm² was obtained at open circuit voltage (OCV) at 750 °C for a 550 μm thick Ni-GDC10 electrode.
The microstructure of an 8YSZ freeze tape cast backbone was reconstructed by X-Ray microtomography and synchrotron X-Ray nano-holotomography for two regions of the hierarchical scaffold. In-house codes implemented in MATLAB R2022b were employed to extract microstructural properties such as volume fraction, interfacial specific surface area, particle size distribution and tortuosity factor.
The raw reconstruction of the backbone was also employed to build a synthetic microstructure featuring the experimental scaffold architecture decorated with electrocatalyst nanoparticles. The generation of a synthetic solid phase simulating the distribution of Ni nanoparticles aims at the identification of key parameters to engineer an electrode with optimized electrochemical properties, thus providing insightful indications to the impregnation manufacturing process, (e.g. required electrocatalyst volumetric percentage for sufficient electronic percolation, density of active TPBls, etc.).