Would require scrubbed extremely clean CO2, because {oxygen, NO, sulphates, water vapour} would poison the solution.

BaCl₂ (barium chloride) melts at about 963°C, but they were running at a higher temp:

  At 1000 °C, this voltage was above the decomposition potential of BaO (2.21 V) but below that of BaCl2 (3.414 V), allowing electrochemical regeneration of the reactive medium without decomposition of the solvent salt