. . - Advances in Stndies of Electrode Kinetics and Mass Transport in AMTEC Cells

Previous work reported from JPL has included characterization of eleetrode kinetics and alkali atom transport from electrodes including MO) W, WRhX, WPtX(Mn), in sodium AMTEC cells and vapor exposure cells; and Mo in potassium vapor exposure cells. [Williams, cl al., 1990a, 1991, 1992] These studies were generally performed in cells with small area electrodes (about 1 to 5 cm2), and device geometry had little effect on transport. Alkali metal diffusion coefficients through these electrodes have been characterized, and approximate surface diffusion coefficients were derived in cases of activated transport. A basic model of eketrode kinetics and transport at the alkali metal vapor/ porous metal electrode/ alkali beta’’-alumina solid electrolyte (BASE) three phase boundary has been proposed which accounts for electrochemical reaction rates with a collision frequency near the three phase boundary and tunneling from the porous electrode partial] y covered with adsorbed alkali metal atoms. [Williams, a al., 1990b, 1992] The small electrode effeet in AMTEC cells has been discussed in several papers, but quantitative investigations have described only the overall effect and the important contribution of eketrolyte resistance. [Underwood, e/ al., 1991] The quantitative characterization of transport losses in cells with large area electrodes has been limited to simulations of large area eleetrode effects, or characterimtion of transport losses from large area electrodes with significant longitudinal temperature gradients, [Webcr, et al., 1992] This paper describes new investigations of electrochemical kinetics and transport, with four 14.4 cm2 Wpt~ electrodes, including the influenee of electrode size on the mass transport loss in the AMTEC cell. These electrodes exhibit very slow sintering, as well as excellent sodium transport properties, making them attractive candidates for AMTEC power conversion use. [Ryan, et al., 1991, 1992] However, the facile sodium transport in WP~ electrodes makes characterization of the transport process difficult,