On Sep 6, 2007, at 7:23 PM, Jones Beene wrote:
Wiki has a pretty good article on electron affinity:
http://en.wikipedia.org/wiki/Electron_affinity
Here is the thought (I have not checked to see if this
is a "re-invention" of someone else's idea) - take two
electrodes and a working medium, and hydrogen is the
only working medium that fits into this concept very
well (73 kj/mol)...
- such that one electrode has a much lower electron
affinity than does the H2 (zinc works well ~0) and the
other has a much higher (gold plated copper works here
~223).
Jones, this looks to me like it might be a pretty good idea on first
take, but it would take some serious engineering, especially due to
the low voltage that would be produced.
Here are some top of the head thoughts, which can of course include
errors.
One problem may be the tendency for many of the possible electron
donor candidates to form hydrides, and thus decay and slough off in
powdered form and/or reduce the surface area available for donating,
as well as the electron affinity due to large numbers of hydrogen
atoms already adjacent to each potential donor. I think phosphorous
at above 277 deg. C may be a good transporter candidate - but that
would take a solar concentrator to drive it, which might still be OK.
There again, a problem might be the tendency of the donor metals to
form phosphides and thus reduce their effectiveness. I expect in any
case an alloy would have to be used for a donor metal and it might
take a lot of research to find just the right one. The transporter
can be a molecule, and that is probably the best choice. The best
candidate molecule would probably be a dipole, so steam (which can
also exist at normal temperatures if at less than atmospheric
pressure) is probably right up there in the top candidates. In fact,
steam is well known for its static electric effects around differing
metals. Stainless steel might make a good donor metal for a steam
based electron transporter.
The gap between donor and acceptor metals should be made as small as
possible, possibly by using a fine dielectric powder as a separator,
or by using computer chip type construction techniques. The smaller
the gap the higher the current level that will be supported, all else
being constant.
No doubt about it on the acceptor side, gold is ideal.
I would think a choice of donor, transporter, and acceptor should
also be consistent with the electronegativity chart:
http://en.wikipedia.org/wiki/Electronegativity
In any event, electronegativity considerations may be important if
battery charging is involved. A significant problem is that an
electron transport cell of this kind is heat driven. The transport
molecule must be driven kinetically from the donor surface to the
acceptor surface against an electric field. The peak voltage the
cell can produce, as well as its current to voltage ratio, is
determined by operating temperature of that gap, and the field across
it. The peak voltage is limited by amount of the particles having
energies in the Boltzmann tail used/required in the transport of
electron carrying particles between surfaces.
There is the need to place a bunch of cells in series, because the
operating voltage of the gap is limited by the thermal energies of
the molecules, e.g. about 26 millivolts at 300 K. The series
achievement would likely have to engineered at a micro level, because
it would take about 1000 cells in series to generate 26 volts or so.
The main difficulty with achieving series operation is the fact you
have to transition current from the receiver metal back to the donor
metal at each stage, i.e. to make electrical connections between
cells, so therefore you have to climb right back up the energy hill
again at the metal to metal interfaces, losing what you gained. What
you are left with is the thermal energy that kicked the electrons
across the barrier in the first place, against the operating
potential of the cell, and which thus highly limits the cell voltage
and current. If some kind of energy extraction scheme can be
employed in the process of getting the electrons to transition from
the acceptor metal back to the donor metal through the return
circuit, then a lot more energy can be had. This trick might be
carried out using the difference between electron affinity, which
drives the thermal transport cell and electronegativity, which drives
an electrolysis cell. A solid state electrolyte can be used for this
purpose, even at a very small scale.
You need a source of energy to convert - like
focused sunlight onto the back side of the zinc. The
other electrode is finned and air-cooled. The zinc
emitter can be a Zn plated bimetal, so that there is
already a small thermoelectric effect.
Query: will a flow of hydrogen between the heat source
and a heat sink create an efficient flow of electrical
energy on the electrodes?
The molecules that transport electrons successfully from donor to
acceptor reduce the thermal energy of both electrodes. They leave the
donor as fast particles, thus taking heat from the donor, and arrive
at the acceptor cold. This, by convection and conduction, ultimately
drives the temperature of the whole system down. The heat energy has
to be replaced by solar or other sources.
One possibility is to use energy derived from, say battery charging,
to maintain a small potential bias across the gap between the donor
and acceptor electrodes. This then avoids the limits on charge
transport placed by the requirement to kinetically transport
electrons across the potential of the gap. In this way the energy
derived from the system is due whatever energy can be derived from
the difference between the two electron affinities plus whatever can
be gained from the electronegativity differences. In other words,
any energy coming out of this kind of biased gap system is then tied
to what use can be made of the resulting flow of electrons into the
acceptor metal, because no potential exists across the gap from which
to extract energy due to the current across the gap.
This flow can be accentuated
by using a Stirling engine configuration, and
especially the so-called lamina or thermoacoustic
Stirling, where the hot and cold ends, respectively
use the metals optimized for electron affinity and so
have electrical leads. The current then adds to the
mechanical energy, giving higher efficiency.
A sterling engine would just take heat away from this process I would
think. Better to insulate the box so as to keep the heat available
for ion transport.
That's one set of opinions for you.
Horace Heffner
http://www.mtaonline.net/~hheffner/
