Effect of Sample Configuration on AMTEC Electrode/Electrolyte Characteristics Measurements in a Sodium Exposure Test Cell

Abstract

An experimental investigation was conducted to determine the effect of sample configuration on the alkali metal thermal-toelectric converter ͑AMTEC͒ electrode/electrolyte characteristics measurements in a sodium exposure test cell. Two electrode/ electrolyte configurations ͑tube and disk͒ were tested to determine if there was a difference in the characteristics determined from the measurements. It was demonstrated that the configuration of the sample had little effect ͑ϳ15%͒ on the measurements at typical AMTEC operating temperatures. Electrochemical impedance spectroscopy and controlled potential current-voltage curve techniques were used to determine these characteristics.The alkali metal thermal-to-electric converter ͑AMTEC͒ is a static energy conversion device that converts thermal energy to electricity. The energy conversion is based on the unique properties of the ceramic ␤Љ-alumina solid electrolyte ͑BASE͒, which conducts sodium ions through it while preventing both electron conduction and neutral sodium transport. 1 The performance of AMTEC depends on the performance of its electrodes. 2 In order to quantify the performance of AMTEC electrodes and compare different electrodes, two parameters are typically used. The morphology factor, G, is the measure of impedance to neutral sodium transport from the electrode/electrolyte interface to the lowpressure vapor space 3where P Na is the sodium vapor pressure, T Na is the temperature of sodium source, T el is the temperature of the electrode, M is the molecular weight of sodium, F is the Faraday constant, R is the gas constant, and j lim is the limiting current. The normalized exchange current density, B, is the measure of the resistance to the charge transfer at the electrode/electrolyte interface. B is normalized to the sodium collision rate and reaction rate at unit activity of sodium 4where R act is the apparent charge-transfer resistance, and all other terms are as defined for Eq. 1. One of the experiments used to evaluate the performance of AMTEC electrodes is the sodium exposure test cell ͑SETC͒. It was originally designed to perform lifetime studies and materials investigations of AMTEC components including brazes, sodium containment, current collector ͑typically molybdenum mesh͒, electrode, and electrolyte. The SETC has become the tool researchers use to quantify and compare electrode performance without building a complete AMTEC cell. The potential of the SETC for accurately determining the electrode parameters B and G is essential for predicting AMTEC electrode performance and lifetime. 5 In order to obtain accurate results and correlate the data obtained from SETC experiments with data from other AMTEC experiments, it is very important to identify and resolve inconsistencies in the measurement of SETC data. A possible major source of inconsistency in the measurements is the electrode/electrolyte configuration. 6 Samples used in the SETC have anode and cathode bands deposited on the outside of the electrolyte, both exposed to the same sodium vapor spa...