Performance degradation and recovery of YSZ membrane under sulphuric acid thermal decomposition

The effect of sulphur dioxide (SO2) present in the cathode feed stream on the
performance of a 8 mole% yttria-stabilized zirconia/platinum (8YSZ/Pt) cell was
studied. SO2 concentration of 30% in oxygen (5ml/min) and helium (30ml/min) was
exposed to the cell applying an external dc voltage of 1.5V at 850˚C. Experiments showed
that the cell performance had a sharp degradation (type I degradation) followed by a
gradual degradation (type II degradation) inferred from current and oxygen permeate
flux changes. Type II degradation may be related to formation of sulphide on cathode of
membrane cell during the SO2 exposure period. Strong but short dc voltage pulses could
reduce significantly membrane performance degradation impacting upon both charge
transfer and mass transfer properties. In this work, the optimum voltage pulse for
achieving the most stable membrane performance in SO2 exposure time was 10V, 0.08s
length. High pulse frequency was helpful for achieving a high and stable membrane
performance SO2 exposure tests lasting up to 2 hours. An oxygen separation process
based on a YSZ tubular membrane was constructed and initial proof of concept
experiments performed. Performance degradation by sulphur poisoning and recovery by
voltage pulse of YSZ tubular membrane were also observed with this system. However,
the oxygen permeation flux of the YSZ tubular membrane was lower than that of YSZ
planar membrane, which may be related to YSZ thickness and the gas flow pattern.
Initial experiments to integrate the membrane system into the thermal decomposition
of H2SO4 to produce SO2 and O2 were carried out in a quartz tube reactor filled with an
alumina supported Pt catalyst. A promising and feasible method of monitoring oxygen
in-situ to determine acid conversion was proposed. Future work should concentrate on
improvement of YSZ tubular membrane configuration and deeper integration of
membrane operation and acid decomposition.