Subject: Re: Aerogel

From: Paul Dietz <[email-masked]>
Date: Thu, 8 Jun 1995 10:15:30 -0500

[email-masked] (Stan Bischof) wrote:
> David Tashima (tash) wrote:
>  : This is true, but it is conceivable to coat the surfaces of the bubbles
with
>  : conductive material in such a way as to create a substantial capacitance.

>  Considering that this is probably a closed cell material and
>  you need _two_ electrodes for a capacitor it is not at all
>  obvious how you would go about this!

>  Any even conceptual suggestions about how you might do this?

>  If it's an open cell design then making a battery electrode becomes
>  pretty obvious, but even for open cell I don't see an obvious
>  way to create two opposed electrodes.

Aerogels are open cell.

You'd make a capacitor with them the same way you make an
electrolytic capacitor.  One electrode is the aerogel, the other
is the electrolyte itself.  The electric field is across the
electrolytic "double layer" at the interface between the two.
This double layer is very thin, about one Debye length (a couple
of atomic radii).

The aerogel in question would likely be a carbon aerogel, formed
by pyrolysis of an organic polymer aerogel.  Carbon aerogels are
conductive.  Work has been done on these in California, at Livermore,
I think.

One interesting application is capacitive deionization.  The idea
here is to pass water containing some ionic species through the
aerogel.  An electric charge is applied.  This attracts ions to
the interface to neutralize the charge.  The surface area of the
aerogel is so large that large quantities of ions can be immobilized
this way, enough to desalinate seawater.  The ions are flushed later
at higher concentration in a purge stream by removing the electric
field.  Projections are that this could be five times more energy
efficient at deionizing seawater than reverse osmosis, and even better
on brackish water.  Other applications are removal of trace metal
pollutants, water softening without sodium, and removal of radioactive
contaminants in nuclear reactor waste water.

	Paul