Performance and thermodynamic properties of Na-Sn and Na-Pb molten alloy electrodes for alkali metal thermoelectric converter (AMTEC)

Abstract

An alkali metal thermoelectric converter (AMTEC) testing cell was set up, and run with molten sodium-tin (Na-Sn) and sodium-lead (Na-Pb) alloy cathodes. The Na activity, the partial molar enthalpy and partial molar entropy of sodium in molten Na-Sn and Na-Pb alloys have been determined, using a Na concentration cell: Na(1) I beta″-alumina Na-Me(1), where Me= Sn or Pb. The thermodynamic results of these investigations agree with those of other authors. The electric performance of these Na-Me alloy electrodes of different Na concentration and temperatures is described, measuring current-voltage characteristics and a.c. impedance in the AMTEC test cell. The power density of the AMTEC cell with molten alloy cathodes decreases with increasing Na concentration, with the Na concentrations in molten alloys varying from 0.5 to 15 mol%. Maximum power densities of 0.21 to 0.15 W cm⁻² at 700°C for Na-Sn molten electrodes, and 0.30 to 0.15 W cm⁻² for Na-Pb molten electrodes have been obtained. The a.c. impedance data demonstrated that the molten alloy electrodes have a smaller cell resistance, 0.3–0.35 S2 cm⁻² at 700°C after 10–20 h. Comparison with the sputtered thin, porous film electrodes, showed that the contact resistance between electrode and surface of beta″-alumina plays an important role on enhancing cell power density. At 700°C the power density of an AMTEC cell with the molten Na-Pb alloy electrode can be raised to values of about 0.2 W cm eat current densities of 0.8 A cm⁻², but at cell voltages not exceeding 0.2V. A model for the theoretical efficiency of the AMTEC cell with molten Na metal electrodes is also presented.