Abstract
Detailed insight into electrochemical reaction mechanisms and rate limiting steps is crucial for targeted optimization of solid oxide fuel cell (SOFC) electrodes, especially for new materials and processing techniques, such as Ni/Gd-doped ceria (GDC) cermet anodes in metal-supported cells. Here, we present a comprehensive model that describes the impedance of porous cermet electrodes according to a transmission line circuit. We exemplify the validity of the model on electrolyte-supported symmetrical model cells with two equal Ni/Ce0.9Gd0.1O1.95-δ anodes. These anodes exhibit a remarkably low polarization resistance of less than 0.1 Ωcm2 at 750 °C and OCV, and metal-supported cells with equally prepared anodes achieve excellent power density of >2 W/cm2 at 700 °C. With the transmission line impedance model, it is possible to separate and quantify the individual contributions to the polarization resistance, such as oxygen ion transport across the YSZ-GDC interface, ionic conductivity within the porous anode, oxygen exchange at the GDC surface and gas phase diffusion. Furthermore, we show that the fitted parameters consistently scale with variation of electrode geometry, temperature and atmosphere. Since the fitted parameters are representative for materials properties, we can also relate our results to model studies on the ion conductivity, oxygen stoichiometry and surface catalytic properties of Gd-doped ceria and obtain very good quantitative agreement. With this detailed insight into reaction mechanisms, we can explain the excellent performance of the anode as a combination of materials properties of GDC and the unusual microstructure that is a consequence of the reductive sintering procedure, which is required for anodes in metal-supported cells.
Highlights
Metal-supported solid oxide fuel cells (MSCs) offer a variety of advantages over anode, or electrolyte supported cells, such as lower material costs, easier stacking [1,2], mechanical robustness and fast heating rates
Power densities above 2 W/cm2 [11] at 700 ◦ C were demonstrated for metal-supported fuel cells with Ni/Ce0.9 Gd0.1 O1.95-δ (Ni/GDC, or gadolinium-doped ceria) cermet anodes prepared to those used in this study
We only present experimental results for Ni/Gd0.1 Ce0.9 O1.95-δ (Ni/GDC) anodes, the presented equivalent circuit model is applicable to other porous cermet and single-phase mixed ion and electric conductor (MIEC) electrodes
Summary
Metal-supported solid oxide fuel cells (MSCs) offer a variety of advantages over anode, or electrolyte supported cells, such as lower material costs, easier stacking (e.g., by welding) [1,2], mechanical robustness and fast heating rates These properties, together with the potential for high power densities (>2 A/cm2 ) proven on cell level tests [3,4], render this cell type most suited for mobile applications [3,5,6,7,8,9]. Power densities above 2 W/cm2 [11] at 700 ◦ C were demonstrated for metal-supported fuel cells with Ni/Ce0.9 Gd0.1 O1.95-δ (Ni/GDC, or gadolinium-doped ceria) cermet anodes prepared to those used in this study For these cells, electrolyte and cathode processing had to be thoroughly optimized [3,4,7,11,12]. MSCs with PrOx cathode and Ni/Sm-doped ceria anode catalysts infiltrated into porous zirconia backbone exhibited reasonably high performance [13]
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