Effect of Li ions on reduction of Fe oxides in aqueous alkaline solution

It has long been known that the performance of the Fe negative electrode in Fe/Ni or Fe/Air batteries is improved by the presence of lithium ions in the electrolyte. This work therefore investigated quantitatively the effect of Li+ on the reduction of some FeII and FeIII oxides, as possible intermediates of Fe reduction/oxidation cycles, by comparing the extent of the reduction in pure 6.0 M KOH with that in lithiated alkaline media. Fe oxides were studied as pressed powder samples embedded in Ni foam. It was found that, in 6.0 M KOH, only FeO is electrolytically reduced at room temperature, whereas Fe2O3 and, to an even greater extent Fe3O4, are very recalcitrant to the reduction process. The situation dramatically changes in, for instance, 4.0 M KOH + 2.0 LiOH electrolyte, in which the extent of the reduction, even at room temperature, becomes significant for all compounds. This behaviour is very probably due to the reduction of Li+ within the oxide lattice to produce LixFeyOz intercalation-compound intermediates, which are then reduced to metallic Fe and Li hydroxide.