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 A
rechargeable metal-air battery cell might be of flow battery
design or not. A typical metal-air reaction involves a cell with
an air-cathode, an electrode where Oxygen is available on one
side, and the electrolyte (an aqueous Potassium Hydroxide
solution is typical) is on
the other. The Oxygen reacts at the air cathode with the
electrolyte to form Hydroxides (OH- ions), which may then react
with the ionic metal to form, for example, elemental Zinc.
The drawing here is just one of many potential metal-air
designs, using an organic acid as electrolyte and an ionic
membrane.
The reactant metal, Zinc for example, is plated or accumulates
on an electrode in a Zinc-Air cell during the charging phase,
and is turned back into Zinc ions in solution during the
discharge phase.
Metal Air cells have at least the potential of virtually
unlimited cyclability. In addition, metal air cells may
have a high energy density, and, for Zinc-Air cells, may be
constructed of relatively benign, inexpensive materials, and
could potentially be the safest of large scale batteries.
The main detriment to Metal Air batteries is a somewhat lower
efficiency, and the issue of loss of water through the air
cathode.
Several companies are in development on utility-scale metal-air
batteries, but no company presently is manufacturing metal-air
batteries on a utility scale.
Unlimited cyclability, good energy density and inexpensive,
relatively safe materials? Why don't we have these today?
Several issues, but primarily the problems of making an
effective air cathode. In 1993 Sandia National Laboratories
performed a very comprehensive study on Zinc Air batteries.
This study was focused primarily on the use of Zinc Air
batteries in electric vehicles, and many of the issues were
specific to this application. However, the main
outstanding issue was the air cathode.. how to create an air
cathode that would efficiently catalyze the hydroxide reaction
and not corrode or degrade over time.
An important note is that IBM announced in August of 2009 a
major effort to develop a Lithium-Air battery, which has the
potential of producing 'serious' electric vehicle batteries,
batteries with an energy density sufficient to power a car for
400 miles. Because of the high reactivity of Lithium
metal, IBM will have to create an air membrane that passes
Oxygen freely while denying even a trace of water vapor.
While it is unlikely that this will produce a cost effective
utility scale battery, it is an effort worth following.
In addition, several companies are working on Zinc-Air batteries
where the Zinc anodes, once oxidized, are mechanically removed
and replaced, and then reprocessed at some remote factory.
However, the biggest new potential technology in Metal-Air
batteries is being funded by the DOE to Arizona State spin off
Fluidic Energy. Called Metal-Air Ionic Liquid or MAIL batteries,
the advance here is the potential use of Ionic Liquids as
electrolyte. Imagine transparent shampoo, a viscous liquid that
doesn't evaporate, and conducts electricity well. Because
Ionic liquids won't evaporate, the air cathode or air membrane
that is required in a Metal-Air battery won't dry out over time,
reducing capacity. In addition, the use of Ionic liquids
may allow different types of air cathodes to be used.
Another major benefit of MAIL batteries is that the Ionic Liquid
electrolyte won't electrolysize into Hydrogen and Oxygen.
In any battery using aqueous electrolytes, the materials used
are limited by the fact that an electrode voltage in excess of
1.23 volts will begin to electrolysize Hydrogen out of the
water. Higher-energy metals than, say, Zinc can't be used
because of this issue. By using an electrolyte of Ionic
Liquids, it may be possible to use a these higher-energy metals,
and therefore build a battery of much higher energy density,
potentially an order of magnitude higher than Lithium Ion.
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