On Nov 23, 2009, at 2:48 AM, Michel Jullian wrote:
See: http://sci-toys.com/scitoys/scitoys/echem/fuel_cell/
fuel_cell.html
I had no idea an ultraclean rechargeable battery could be done so
simply!
Supplies:
<<- One foot of platinum coated nickel wire, or pure platinum wire.
Since this is not a common household item, we carry platinum coated
nickel wire in our catalog.
- A popsickle stick or similar small piece of wood or plastic.
- A 9 volt battery clip.
- A 9 volt battery.
- Some transparent sticky tape.
- A glass of water.
- A volt meter.>>
It seems to me a small amount of lye would help the reaction along.
No matter, the intent is apparently not to create a working cell,
i.e. generate power, it is merely to generate a voltage.
I see they sell the wire for $14.41 plus shipping. A bulk source for
wire and mesh might be:
http://www.gerarddaniel.com/
H2 and O2 are produced by short electrolysis runs, after which the
bubbles clinging to the electrodes are catalytically recombined by the
electrode surface material (platinum) to generate electricity :)
1/ The article features nice "explanations" of how it works, but how
does it _really_ work? In particular, in the generating (fuel cell)
phase, they don't say what makes the positive hydrogen ions climb
"uphill" from the negative electrode to the positive one, anyone can
explain this miracle? ;-)
2/ It seems to me a much higher capacity (and perhaps even practical)
rechargeable battery could be made by using a hydrogen
absorbing/desorbing material e.g. Pd for the negative electrode, and
by making gaseous oxygen available at the anode. Storing the latter is
not required of course, O2 from the air is fine... maybe a floating
support which would keep a grid or flat serpentine shaped positive
electrode at the surface of the water or just below?
Michel
The explanation looks bogus to me. I think the cell works by
reversible reactions, not recombination.
Bockris states that conduction in an electrochemical cell in the
volume between the interface layers is almost entirely due to
concentration gradients. That is because almost all the potential
drop is in the interface layers themselves. The E field in the bulk
of the cell is very small.
I expect the cell actually operates by creating even *more* bubbles,
not consuming the gas already there in the form of bubbles.
In the course of the brief electrolysis by battery, the volume of
water around the anode is filled with H3O+ ions, and the volume
around the cathode is filled with OH- ions. This can actually be
viewed by use of a dilute electrolyte, plus a pH indicator like
phenolphthalein, which is colorless in acidic electrolytes, and pink
in basic solutions. To do this first add the (liquid)
phenolphthalein to distilled water. To view the creation and
migration of OH- ions: before connecting the battery add a little bit
of hydrochloric acid to the water, and stir until it just turns
pink. When the battery is connected the volume around the cathode (-
electrode) will turn clear. HCl can be obtained from some bathroom
tile cleaners, which are simply hydrochloric acid, HCl. To view the
creation and migration of H3O+ ions: before connecting the battery
add a little bit of lye to the water, and stir. When the battery is
connected the volume around the anode (+ electrode) will turn pink.
It can take a little fooling around with concentrations to get the
effect to work quickly and dramatically. The diffusion occurs slowly
but at a clearly visible pace.
You can demonstrate the reversibility of the reactions by reversing
the battery. Note, however, that the diffusion occurs in a somewhat
random manner. It doesn't typically blossom out in a perfectly
spherical or cylindrical manner (depending on the electrode shape).
Reversing the reaction is thus not a perfect process either. I tried
some of this decades ago in a feeble attempt to make a display
technology. I got a nice red stream of ions coming from a copper
anode in a basic solution.
In any case I doubt it is actually recombination that causes the
potential at the electrodes. It is the presence of the high
concentration of ions in solution that makes the residual potential
when the battery is disconnected. The H3O+ ions take on electrons
through the wire originally releasing hydrogen at the site where the
hydrogen was generated, the anode, thus making *more* hydrogen
bubbles. Similarly, the OH- ions donate electrons to make H2O2 and
*more* O2 at the site where O2 was generated prior.
The meter is probably a 10 megohm meter, meaning registering the 2 V
potential requires generating 0.2 microamps of current, and thus 0.4
microwatts of power. Not much of a fuel cell!
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
