Effect of alkali metal cations in alkaline iron battery electrodes

Rechargeable alkaline iron batteries (e.g. Ni-Fe and Fe-air) have been extensively studied recently as viable energy storage systems for renewable energy sources. However, inherent issues such as passivation of the iron and parasitic hydrogen evolution reaction (HER) on the electrode surface limit their full capability. Multiple approaches to improving iron electrode performance have been conducted, few of which focused on electrolyte composition. While alkali metal (AM) cations on the electrolyte do not directly participate in the electrochemical reactions, their intrinsic characteristics can dictate the performance of the electrode. Investigating the interface interactions and electrical double layer (EDL) structure can provide a deeper insight into the operation of iron electrodes in an alkaline solution. In this work, we investigated the effect of alkali metal cations (Li+, Na+, K+, Cs+) in the electrolyte solution in inhibiting passivation and HER on electrodeposited iron on carbon paper (Fe/CP) electrodes. The electrochemical measurements show that the iron redox and HER activities of the electrode increased with increasing cation size in the electrolyte. The non-covalent interactions between hydrated alkali metal cations and adsorbed OH species resulted to the formation of quasi-adsorbed clusters which can block active sites on the electrode surface. Furthermore, the concentration of these clusters decreases with increasing cation size which resulted to higher EDL capacitance and ECSA values of the electrode. The results of this work provide a better understanding of the surface reactions on iron electrodes and can help in developing novel techniques for suppressing passivation and parasitic HER on rechargeable alkaline iron batteries.