Theoretical and Experimental Study of Solid Oxide Fuel Cell (SOFC) Using Impedance Spectra

Solid oxide fuel cell (SOFC) is a promising alternative energy source, with its advantages of high operating efficiency, fuel flexibility, low emissions and relatively low cost. However, there are several challenges concerning the SOFC research. Little is known about the complex interfacial electrochemistry and thermochemistry, and it is also difficult to diagnose problems and optimize cell performance. Therefore, physics-based models are needed to better understand the underlying mechanisms of SOFCs. This research work addressed two important aspects of the numerical modeling of SOFCs: the multicomponent gas diffusion in porous electrode at the anode and the heterogeneous electrocatalysis of oxygen reduction reaction (ORR) at the cathode. First, anode was diagnosed to be mainly controlled by multicomponent gas diffusion inside the anode bulk (supporting) layer, and the Dusty Gas model is identified as an appropriate model to describe the gas diffusion resistance extracted from no bias AC impedance. Anode-supported SOFCs with Ni-yttria-stabilized zirconia (YSZ) anode were used to study the multicomponent gas transport in porous electrodes. A fuel gas mixture of H2-H2O-N2 was fed to the anode and AC impedance data were measured at 800oC by varying hydrogen partial pressure at both no bias and a current of 300 mA. Impedance data were also collected at no bias at three different temperatures (800oC, 850oC and 900oC). For the first time, three models were used to analytically derive the diffusion resistance (Rb), which was then compared to the values extracted from experimental impedance data. The Dusty Gas model yields the best predictions and the tortuosity