Two‐Dimensional Distribution of Electrochemical Reaction Rate in Porous Sulfur Electrodes of Sodium‐Sulfur Cells

Abstract

It is observed that the electrochemical reaction in the sulfur electrode of sodium‐sulfur cells progresses with not only radial but also axial nonuniformity. The axial nonuniformity seems to be caused by the dropping of the melt to the bottom of the sulfur electrode due to gravity. A two‐dimensional and two‐component (ionic and electronic) conduction model is derived to solve the problem numerically by use of the finite element method. The results of calculations confirm the assumption that the axially nonuniform distribution of the melt density causes the axial nonuniformity of the reaction rate. It is predicted that a modified sulfur electrode in which a mat of intermediate resistance is placed at the upper part of the electrode in addition to a thin, porous carbon mat of high resistance placed at the solid electrolyte surface is capable of making the reaction rate uniform. Experiments to measure the axial distribution of the reaction rate are carried out, and they confirm the nonuniform reaction in the axial direction.