The news-making super-iron battery is detailed in a Science Magazine report: "Energetic Iron(VI) Chemistry: The Super-Iron Battery". by Stuart Licht *, Baohui Wang, Susanta Ghosh
(Department of Chemistry and Institute of Catalysis Science, Technion--Israel Institute of Technology, Haifa 32000, Israel. * To whom correspondence should be addressed. E-mail: chrlicht@techunix.technion.ac.il )
Early in the report, the authors indicate the applicability of their work to metal hydride batteries! As follows,
"Of growing importance are rechargeable (secondary) batteries such as metal hydride (MH) batteries (5), which this year have increased the commercial electric car range to 250 km per charge. In consumer electronics, primary, rather than secondary, batteries dominate. Capacity, power, cost, and safety factors have led to the annual global use of approximately 6 ΕΎ 10^10 alkaline or dry batteries, which use electrochemical storage based on a Zn anode, an aqueous electrolyte, and a MnO2 cathode, and which constitute the vast majority of consumer batteries. Despite the need for safe, inexpensive, higher capacity electrical storage, the aqueous MnO2/Zn battery has been a dominant primary battery chemistry for over a century. Contemporary alkaline and MH batteries have two common features: Their storage capacity is largely cathode limited and both use a KOH electrolyte.
We report a new class of batteries, referred to as super-iron batteries, which contain a cathode that uses a common material (Fe) but in an unusual (greater than 3) valence state. Although they contain the same Zn anode and electrolyte as conventional alkaline batteries, the super-iron batteries provide >50% more energy capacity. In addition, the Fe(VI) chemistry is rechargeable, is based on abundant starting materials, has a relatively environmentally benign discharge product, and appears to be compatible with the anode of either the primary alkaline or secondary MH batteries."
Graphs in the report show that the ">50%" energy increase at low discharge rates becomes 200% at high discharge rates (relative to the new, high-rate alkaline cells) -- that is, three times greater discharge life is obtained. However, the report does not quantitatively indicate how much improvement might be obtained for MH batteries utilizing super-iron cathodes.
Actually, the super-iron is used in the form of various salts, and BaFeO4 gave the best high-discharge rate performance, despite having a lower total energy capacity than for K2FeO4. In all cases, the tests were done with simple cathode mixtures (i.e., not optimized by additives):
"The BaFeO4, although of lower capacity than K2FeO4, discharges a higher fraction of this charge at higher current densities (Fig. 1 legend). Both Fe(VI) materials were used as synthesized. As exemplified by the history of MnO2 optimization, Fe(VI) coulombic efficiency will be further affected by additives other than graphite, and control of packing, electrolyte, and particle size (14). The average discharge potential of the K2FeO4 cathode of 1.58 V versus Zn is 24% greater than the average for the equivalent MnO2 cell (1.27 V), both determined to 90% depth of discharge (Fig. 1). Combined with the increased charge capacity, this potential also leads to a further increase in comparative energy capacity."
They discuss the instability issue at considerable length. Here are a couple of short, less technical quotes:
"The view of Fe(VI) species as intrinsically unstable is not correct. For example, an excess of K2FeO4 in contact with a concentrated KOH solution has a calculated stability of many years. Veprek-Siska and Ettel demonstrated that at elevated temperatures, the rapid decomposition of Fe(VI) is due to trace catalysis by Ni(II) and concluded "the rate of non-catalyzed decomposition is immeasurably low" (15)."
and
"Extension of the data collected over a few months to predict behavior in 10 years is risky, but based on the measured solution-phase Fe(VI) decomposition rate, after 10 years there will be less than a 10% loss of 1 g K2FeO4 in contact with 1 ml concentrated KOH solution."
Now for the most interesting discussion (to us ENER addicts, at least):
"Conventional MH batteries, with a capacity up to 95 Wúhours/kg (5), compared to 40 Wúhours/kg for NiCd, have advanced to where further energy storage improvements are largely limited by the cell's heavy nickel oxyhydroxide (NiOOH) cathode. This situation is analogous to the limitations of MnO2 for primary Zn batteries and is accentuated by the lower limiting NiOOH cathode capacity of ~290 mAúhours/g. These important secondary (rechargeable) batteries use alkaline KOH electrolyte.
We have also probed the reversible nature of Fe(VI) chemistry. An Fe(VI) charge-limited open-cell experiment provides fundamental evidence that the Fe(VI) cathode is significantly rechargeable. The cell has been discharged to 75% cathode capacity depth of discharge (DOD) for several cycles and more than 400 cycles at 30% DOD. The cell consists of an excess of MH anode (35 mAúhours, removed from a GP 35BVH button cell), and a limiting Fe(VI) cathode (9 mAúhours based on 406 mAúhours/g K2FeO4, using a Teflon mesh pressed over the K2FeO4 mix), in excess 12 M KOH electrolyte. The cell potential varies from 1.3 V (open circuit) to 1.1 V (at 5 mA/cm2) and is cycled at 2.5 mA charge and 1 mA discharge. This cell has a characteristic voltage similar to the conventional MH battery because of the similarity of the Fe(VI) potential to the 0.5 V formal potential of NiOOH."
The relative lack of data suggests that the authors have experimented much less with MH batteries than Zn-alkaline batteries. In particular, they do not state a value for the expected increases in energy storage. So, we should not yet draw much in the way of conclusions re super-iron and MH battery technology. However, the reference (5) about ENER's technology developments, and the reference to MH batteries in autos early in the report, both suggest that Licht might be in contact with Ovshinsky.
Ray |
