Re: Porous graphite for electrodes


"John Savage" <rookswood@xxxxxxxxxxxxxxxx> wrote in message
news:061231000124234.31Dec06$rookswood@xxxxxxxxxxxxxxxx
Fred Kasner <fkasner@xxxxxxxxxxxxx> writes:
Gordon wrote:
I am looking for a source of porous graphite such as could be
used as non-reactive electrodes in saline solutions. I need the
very large surface to volume ration that porous or fibrous carbon
could provide. There is no need for high electrical conductivity,
but I do need the large surface area that such material should
provide.

Break apart a zinc based standard dry cell. The central electrode is a
porous carbon rod. You can clean it then bake it to remove adsorbed
impurities.

Coincidently, a week back I did exactly that, incl. heating the rods over
a gas flame then wiping off some black ooze until they came clean. I then
used the rods for electrolysis of acidified water. After about 7 hours
(and collecting half a litre of H2) a significant amount of black powder
had accumulated in a circle underneath the oxygen electrode, though none
had come off the hydrogen electrode. So it seems that the more-minute
bubbles liberated at the oxygen electrode are more energetic at eroding
particles of carbon off the rod.

These rods don't seem to be graphite; would they be manufactured from
compressed coke?

The subject of this post contains the word 'graphite', a certain
cristalline form of carbon that has lubricating properties because the
crystals adhere in one plane only, so they form sheets that slide over each
other. There exists no porous form and the material is too soft to maintain
a fixed shape, not suitable for electrodes of any kind.
The suggestion to use the carbon rod of a Leclanche element or dry cell is a
very good one: that is a porous electrode, manufactured from carbon and some
clay, sintered at high temperature. As an electrolysis electrode it does
release particles because of the porous structure, there is no chemical bond
between carbon and baked clay. If you want an electrode that stays clean,
cover it with fine fabrics, cotton wool, filtration paper, permeable
plastic or microfiber, used in modern air filters.


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