Mixed ionic and electronic conducting electrode studies for an alkali metal thermal to electric converter

摘要

This research focuses on preparation, kinetics, and performance studies of mixedionic and electronic conducting electrodes (MIEE) applied in an alkali metal thermal toelectric converter (AMTEC). Two types of MIEE, metal/sodium titanate and metal/??-alumina were investigated, using Ni, Cu, Co and W as the metal components. Pure metalelectrodes (PME) were also studied, including Ta, Ni, Nb, Ir, W and MoRe electrodes.The stability of MIEE/???-alumina solid electrolyte (BASE) interface was studied interms of the chemical potential of Na-Al-Ti-O system at 1100K (typical AMTECoperating temperature). Ni metal was compatible with sodium titanate and BASE anddisplayed the best initial performance among all tested PMEs. Ni/sodium titanateelectrodes with 4/1 mass ratios of metal/ceramic performed best among all testedelectrodes. Scanning Electron Microscope (SEM) observations showed that grainagglomeration, which is the main mechanism for electrode degradation, occurred in alltested electrodes. Ceramic components were able to effectively limit the growth ofmetal grains and resulted in a long lifetime for MIEEs. Ni particles in the MIEE formed a network microstructure that was close to the theoretical morphology of the idealelectrode. A model based on percolation theory was constructed to interpret and predictthe performance of MIEEs.The electrode kinetics was studied and a theoretical expression for the interfaceimpedance was derived for both PME and MIEE, using electrochemical impedancespectroscopy (EIS). The conductivity of the Na2Ti3O7 and Na2Ti6O13 mixture wasmeasured. The average activation energy for the bulk conductivity was 0.87ev. Finally,theoretical analysis clarified that the transfer coefficient ? value change would cause atmost a few percent change in the electrode performance parameter B.