A Quantitative Tunneling/Desorption Model for the Exchange Current at the Porous Electrode/Beta"- Alumina/Alkali Metal Gas Three Phase Zone at 700–1300K

The exchange current observed at porous metal electrodes on sodium or potassium beta"-alumina solid electrolytes in alkali metal vapor is quantitatively modeled with a multi-step process with good agreement with experimental results. No empirically adjusted parameters were used, although some physical parameters have poor precision. Steps include: (1) diffusion of Na+ ions to the reaction site; (2) stretching of the Na+ ionic bond with the β3"-alumina surface to reach a configuration suitable for accepting an electron to form a surface bound Na0 atom; (3) electron tunneling from the Mo electrode to the ions; and (4) desorption of Na0 atoms weakly bound at the reaction site on the β"-alumina surface; (5) electron tunneling between Na0 or Na+ on the defect block and (6) Na0 and/or Na+ mobility on the spinel block surface may extend the reaction area substantially.The rate is increasingly dominated by the region close to the three-phase boundary as temperature increases and the rate near the three-phase boundary increases fastest, because desorption has a higher energy than reorganization. At high temperatures, surface diffusion of Na+ ions from the defect block edges to the spinel block edges is responsible for an increase in the total effective reaction zone area near the three-phase boundary.