Gad. It still isn't right! Corrections below. I have vertigo at
the moment and can't think straight. I've actually done half of this
experiment, though decades ago, and it is interesting how the
concentration gradient wanders, it doesn't follow what you would
expect for any kind of E field.
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 preferentially filled with H3O+ ions, as
the OH- ions release their electrons and form O2 and H2O2, and the
volume around the *cathode* is filled with OH- ions as the H3O+ ions
present at the cathode surface are electrolyzed. 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. Connect the battery. To view
the creation and migration of OH- ions: add a little bit of boric
acid to the water, and stir. Repeat the process until you can see
the electrolyte turns pink in the vicinity the *cathode* (-
electrode) once the electrolyte settles down. Boric acid was chosen
because it is commonly available from pharmacies. To view the
creation and migration of H3O+ ions add a little bit of lye to the
water and stir. Repeat the process until you can see the electrolyte
is pink, but when the electrolyte settles down you can see the volume
around the *anode* (+ electrode) gradually turing clear. 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!
It would be interesting to run the current for a while until a
significant concentration gradient can be visualized, and then
disconnect the battery to see what effect the current generated
through the meter has on the visible gradients.
Note that the concentration gradients of H3O+ and OH- particles does
not necessarily require an E field to maintain them, provided there
are other radicals in the electrolyte. Salt buffers can be used to
increase conductivity without driving Ph to extremes. The presence of
additional radicals can balance the charges to neutral, or to match
the E field in the electrolyte. For example, if the electrolyte is
NaOH, the Na+ can redistribute to neutralize the charges. If boric
acid is used, the B(OH)4- radical will balance the charges.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
