Exchange Current Density of SOFC Electrodes: Theoretical Relations and Partial Pressure Dependencies Rate-Determined by Electrochemical Reactions

As a theoretical consideration on electrode defect chemistry, general relations of exchange current density quantitatively representing Solid Oxide Fuel Cell (SOFC) electrode performance are systematically derived as a function of gas partial pressures, equilibrium constants of adsorption and dissociation reactions on electrode surfaces, and electrochemical reaction rate constants for possible elemental reactions at the cathode and the anode, in the case that an electrochemical reaction is the rate-determining electrode reaction. Simplified expressions are also derived, under the condition that one kind of neutral or charged adsorbed species is predominant at the electrode, to derive gas partial pressure dependence of exchange current density for given rate-determining electrochemical reactions. Importance of considering elementary steps is highlighted to derive rate equations and to clarify various dependencies. Partial pressure dependencies of the exchange current density are compiled and discussed by simulating normalized exchange current density values for given partial pressures. The applicability and limitation of the Butler-Volmer type expressions of exchange current density for SOFC electrodes are carefully discussed.