Investigation of the properties influencing the deactivation of iron electrodes in iron-air batteries

Iron-air batteries hold the potential to be a key technology for energy storage, thanks to their energy density, low cost, safety and abundance of their materials. In order to scale the technology up and optimize the cell formulations, it is key to obtain a clear understanding of how the physical-chemical properties of the electrode influence their electrochemical behaviour, in particular, the capacity loss. In this work, we propose for the first time mathematical correlations between textural and crystallographic properties of iron electrodes and their electrochemical stability. By adjusting synthesis parameters, we were able to tune pore size and volume, surface area and crystal size of iron oxides, and found that stability is highly correlated to both surface area and pore size. Large surface area and small average pore size provide electrodes with enhanced stability. We hypothesize that the cause for deactivation is the passivation of the electrodes ascribed to the formation of a non-conductive, non-reactive iron (II) hydroxide layer during discharge, which then cannot be reduced to iron again. We validate this hypothesis with electrochemical impedance spectroscopy studies, which show that, in the more stable electrodes, the charge transfer resistance in the Fe(OH)2 to Fe reduction does not significantly change after cycling, contrary to the behaviour of the less stable electrodes, corroborating our hypothesis. Furthermore, the electrode with the best properties was cycled 100 times, retaining almost 75% of its initial capacity at the end of the 100 cycles. These results are highly relevant for the future design and operation of iron-air batteries.