Coupled Electrocatalysis and Gas Phase Diffusion in a Stabilized‐Zirconia Tubular Flow Oxygen Pump

Abstract

A tubular electrochemical reactor constructed from a 10 mole percent electrolyte tube with porous Au electrodes on its inner and outer surfaces was operated as an oxygen pump in the temperature range 650°–850°C. Gas mixtures with different content were circulated through the inside cathodic wall of the tube, where was electrochemically reduced and pumped out through the outer anodic wall of the tube to an annular chamber. The current‐overpotential behavior of the cell was found to be dominated by catalytic rate processes at the cathode. The electrochemical kinetics can be described by a modified Butler‐Volmer expression. The behavior of the tubular flow reactor in the integral mode of operation was studied by numerically solving the convective‐diffusion equation of change for molecules in the gas phase, incorporating both radial and axial diffusion and electrochemical reaction on the tube wall. The finite‐difference method of successive over‐relaxation was used to solve the governing mass‐transfer equation. When the axial change in concentration at the tube wall is accounted for, the computed integrated currents agree very well with the experimentally measured values, allowing the prediction of total ionic current and fractional reactant conversion for a given set of flow and physical variables. Under certain conditions, axial diffusion is found to be quite pronounced, with concentration disturbances penetrating the region well upstream of the inlet to the reaction zone , and significantly decreasing the concentrations at from values in the entering feed stream.