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Solid oxide fuel cells have been investigated frequently with models predicated on linear electrical circuit elements, ignoring finer details of electrochemical transport. To gain better understanding of the operation of solid oxide fuel cells based on their underlying physics, the most simple system of fundamental equations, consistent with solid oxide fuel cell thermodynamics, has been constructed. The system includes the continuity equation for mass/charge transport, the Poisson equation relating electrostatic potential and charge density, and interface flux expressions reflecting activated state processes. The driving force of all transport processes is the electrochemical potential gradient of mobile species. This potential includes entropic interaction and electrostatic energies. Gas phase mass transport was not considered in this work. The equation system governed calculations simulating various electrical and electro-chemical measurement experiments, specifically: equilibrium open-circuit cell voltage measurement, cell voltage measurement with increasing dc current density, and electrochemical impedance spectroscopy. The 1-D solid oxide fuel cell system investigated is composed of two dense mixed-conducting electrodes and a dense purely ion-conducting electrolyte. Relaxing the system from initial non-equilibrium, the calculated equilibrium cell voltage agreed O(10 −9%) with the theoretical Nernst voltage. Also, non-thermodynamic and non-material parameters did not affect this agreement, validating the thermodynamics of the equation system. The effects of kinetic and geometric variables on the behavior of the investigated system were clearly observed in both dc current density and electrochemical impedance simulations. As the perturbation amplitude was increased, the onset of nonlinear impedance response was seen. |
