Electrochemically Produced High Rate, High Capacity Iron Electrodes for Use in Iron-Air Batteries

Iron-based alkaline battery systems such as iron-nickel or iron-air batteries exhibit the potential for large scale stationary energy storage due to the high abundancy of iron and its low cost.<br/>Iron electrodes have a high lifespan and are insensitive to overcharging, partial and deep discharge. High energy densities, which come close that of today's lithium-ion batteries, can be achieved. In an iron-based alkaline battery system, it is assumed that iron is oxidized to iron hydroxide during the first discharge step, which correspondents to a capacity of 960 mAh/g_Fe. This can then be further oxidized to magnetite with a theoretical capacity of up to 1280 mAh/g_Fe. There are however two limiting factors in conventional iron electrodes. Typical iron electrodes that are manufactured on the basis of iron powder, have experimentally determined capacities that are in the range of only 100 - 300 mAh/g_Fe, which is therefore significantly lower than the theoretical capacity. It is assumed that the low experimental capacity is related to a buildup of a passivating layer and thus hinders the charge/discharge reaction. However, it has not yet been clearly proven what exactly leads to this passivation and how it can be mitigated. Another bottleneck of iron electrodes is the low discharge rate, which makes it inferior compared to other types of batteries.<br/>To overcome these limitations, we have developed a thin film iron electrode, that delivers high capacities of 500 mAh/g_Fe at a discharge rate of 1C. The coating was first carried out on planar substrates, to investigate how different deposition parameters such as e.g. pH value, precursors/additives, deposition time, current density, substrate material, etc. affect the morphology and adhesion of the iron layers as well as the electrochemical properties, in particular the iron-related capacity to be achieved. These properties were examined with methods of SEM / EDX, chronoamperometry, chronopotentiometry and cyclic voltammetry. In addition, material changes of the electrodes depending on the state of charge and cycle were analyzed in order to identify the cause of the as aforementioned low capacities achieved in practice with typical iron electrodes. For this purpose, scanning electron microscopy/EDX, FTIR, X-ray diffractometry and Raman spectroscopy were used to examine material phases.<br/>The produced planar electrodes have shown good performance. But to increase the active mass to substrate ratio, electrodes have to be 3 dimensionally structured. For this purpose, iron has been deposited onto 3-dimensionally structured carbon layers. The challenge in fabricating these types of electrodes was to obtain a homogenous coating, which goes deep into the material without clogging the pores. Similar to the case of planar electrodes, this has been achieved through a variation of deposition parameters.