Observation and modeling of the reduction of hematite particles to metal in alkaline solution by electrolysis

The electrochemical conversion in alkaline solutions of single hematite particles, weighting between 3 mg and 1 g, could be observed at macro- and microscopic scales, thanks to a dedicated experimental configuration. The results presented here reveal a very specific reaction mode at both scales, notably with a layer of metal progressing on the surface of the solid particle during the electrochemical reduction. In addition, conversion in the volume of the solid was observed. Mössbauer and chemical analysis of the iron metal content reveals a high Faradaic efficiency at the various stage of the conversion, and confirm magnetite phase as a key intermediate. Microscopic scale observations of hematite/magnetite conversion are presented, showing the solid-state feature of this reaction at low temperature. These observations together with the quantitative results lead to the proposal of a simple electrochemical model allowing the current variation during the electroreduction of hematite particles to iron metal to be predicted. The model provides the value for the current density related to the hematite reduction to iron, regardless of the shape or the size of the original particles.