In a new study published in the journal Nano Energy, researchers from Forschungszentrum Jülich in Germany and Oak Ridge National Laboratory (ORNL) provide in-depth insight into the electrochemically induced surface reaction processes on iron anodes in concentrated alkaline electrolyte in iron-air batteries.
Using in-situ electrochemical atomic force microscopy (in-situ EC-AFM) at the Center for Nanophase Materials Sciences at ORNL, they were able to observe how deposits of iron hydroxide particles (Fe(OH)2) form at the iron electrode under conditions similar to those prevalent during charging and discharging. A deeper understanding of the charging and discharging reactions is viewed as the key for the further development of this type of rechargeable battery to market maturity.
Iron–air batteries, originally proposed in the 1970s, have theoretical energy densities of more than 1,200 Wh/kg; by comparison, present-day lithium-ion batteries come in at about 600 Wh/kg, and even less (350 Wh/kg) if the weight of the cell casing is taken into account.
When it comes to volumetric energy density, iron–air batteries could perform even better with 9,700 Wh/l—almost five times higher than today’s lithium-ion batteries (2,000 Wh/l). Although lithium–air batteries, which are technically considerably more difficult and complicated to realize, can have gravimetric energy densities of up to 11,400 Wh/kg, even these advanced cells theoretically have “only” 6,000 Wh/l. Iron–air batteries are thus particularly interesting for a multitude of mobile applications in which space requirements play a large role. In addition, their main constituent—iron—is an abundant and therefore cheap material.
However, due to technological challenges such as catalyst corrosion, self-discharge and hydrogen evolution during battery charging, research into metal–air batteries was abandoned in the 1980s for a long time. The past few years, however, have seen a rapid increase in research interest.
Iron–air batteries draw their energy from a reaction of iron with oxygen. In this process, the iron oxidizes almost exactly as it would during the rusting process. The oxygen required for the reaction can be drawn from the surrounding air so that it does not need to be stored in the battery. These material savings are the reason for the high energy densities achieved by metal–air batteries.
Different from lithium-ion technology, the electrochemistry of iron is confined to surface reactions which result in the formation of a redox-layer on the iron electrode surface in alkaline electrolyte. The redox-layer on iron is mostly referred to as the passivating layer or passive film since the redox-reaction products accumulate on the electrode and eventually prevent further electrochemical reactions of the metallic iron surface beneath. However, the material constituting the redox-layer is not inactive and affects the redox-behavior of the electrode. The effects of the redox-layer on the electrochemistry of iron are, first, the contribution of secondary electrochemical reactions and, second, the moderation of the available electrode surface area.
Due to its tremendous technological importance regarding the protection against corrosion, the passivation of iron has been a subject to numerous physical and chemical studies. As a result of these studies, different models for the structure and the composition of the redox-layer have been proposed. However, although being interesting from a fundamental and an applied point of view, to date, only a few groups have investigated the evolution of the redox-layer in alkaline media and even less have reported in-situ microscopy studies in concentrated electrolytes due to the aggressive measurement conditions.
Accordingly, till present, research with respect to the redox-behavior of iron misses an in-depth understanding for the evolution of the redox-layer in concentrated alkaline electrolyte which is, in fact, deeply required for corrosion science and the improvement of iron-based batteries which would typically use 6-8M KOH. The topography and the growth mechanism of the redox-layer determine the available surface area and are jointly responsible for the corrosion behavior of iron as well as for the discharge capacity of iron-air batteries.—Weinrich et al.
The insights obtained by the researchers create a new basis for improving the properties of the battery in a targeted manner.
|Changes to the electrode surface over the course of four charging/discharging (redox) cycles. Copyright: Forschungszentrum Jülich / H. Weinrich Click to enlarge.|
The deposits do not decrease the power of the battery. On the contrary, since the nanoporous layer increases the active surface area of the electrode, it contributes to a small increase in capacity after each charging and discharging cycle. Thanks to the investigations, the researchers have for the first time obtained a complete picture of this layer growth.
It was previously assumed that the deposition is reversed during charging. But this is obviously not the case.— Dr. Hermann Tempel from Jülich’s Institute of Energy and Climate Research (IEK-9)
Furthermore, a direct link was verified for the first time between the layer formation at the electrode surface and the electrochemical reactions.
There is, however, still a long way to go until market maturity. Although isolated electrodes made of iron can be operated without major power losses for several thousand cycles in laboratory experiments, complete iron–air batteries, which use an air electrode as the opposite pole, have only lasted 20 to 30 cycles so far.
The results were obtained within the scope of a project on high-temperature and energy materials, which was funded by the German Federal Ministry of Education and Research. It was made possible through a cooperation agreement between ORNL and Forschungszentrum Jülich. Both establishments have been collaborating closely on various scientific areas since 2008.
Henning Weinrich, Jérémy Come, Hermann Tempel, Hans Kungl, Rüdiger-A. Eichel, Nina Balke (2017) “Understanding the nanoscale redox-behavior of iron-anodes for rechargeable iron-air batteries” Nano Energy 41 doi: 10.1016/j.nanoen.2017.10.023