Abstract
Mixed mass‐transport and kinetic control of sodium ion reduction at porous, inert electrodes on sodium beta′ alumina solid electrolyte (BASE) ceramic in a high‐temperature electrochemical cell has been observed and modeled.The high ionic conductivity of BASE and the reversibility of the liquid sodium/BASE anodic half‐cell led to assignment of potential‐dependent (nonohmic) resistances to kinetic and mass‐transport processes associated with the porous electrode. The morphology of these electrodes and typical sodium gas pressures are consistent with Knudsen, or free‐molecular, flow through the electrode. Equations for effusion of a gas through a cylindrical pore and equivalent pressure for a condensed phase evaporating irreversibly describe the pressure of sodium gas in the porous electrode. These equations and the current‐overpotential equation combine to yield equations for the cell's current‐voltage curve and its apparent charge‐transfer resistance in terms of rate and transport parameters. Inclusion of ohmic resistance losses including electrode sheet resistance via an iterative finite elements method results in a fairly complete model for the dc response of alkali metal thermoelectric converter cells.
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