Horace Heffner wrote:
It is of interest that electrons in Zn-Ag battery flow from the Zn
electrode to the Ag electrode.
This is to be expected from the Wiki chart:
http://en.wikipedia.org/wiki/Electron_affinity
... which can be confusing since other references have it stated in
different ways. As you mention, there is no direct correlation between
electron affinity and standard electrode potential, or band gap, or
conductivity. In fact there is occasionally a reverse correlation, but
that would be dependent on the transport mechanism.
On this Wiki chart the electron-affinity, E sub(ea), of an atom or
molecule is the energy required to *detach an electron* from a singly
charged negative ion, i.e., the energy change for the process, so that a
higher number means more energy is required to detach the electron.
... and ERGO the lowest numbers (like Zn = 0) are the better donors
(since less energy is required) and the higher numbers (like Au = 223)
are the better receptors - even though Au, if you look at it in terms of
electrical conductivity is superior. Zinc is a quandary because it has
so many phases and can switch roles unexpectedly.
From my POV (stressing transport mobility more than you are) - it seems
that the ideal situation would be to frame H2 which serves as the highly
mobile transporter (H2 = 73) with a non-hydriding donor of the lowest
possible binding energy with a receptor of the highest energy - so that
the H2 is intermediate between them; and consequently if negative
affinity is available for the donor, then so much the better - even
though that term usually does refer to the standard electrode potential
and not to "electron affinity" per se (as in the Wiki table).
... but for the acceptor there is a clear choice -gold- and Au is the
best choice since it can be plated in a thin coating over any metal, and
is generally non-hydriding at low temperature.
Gold hydrides are stable, surprisingly, but formation of them does not
normally take place at STP:
http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=b604404b&JournalCode=DT
However, one thing we have not stressed until your last post, but could
be very relevant is how to engineer an extremely low loss (thin ?)
tunneling layer - the so-called "leaky" layer... No prior dry pile
construction has ever done that properly, IMHO. Most of these dry pile
designs came along before the advent of semiconductors, and perhaps
there are lessons from that technology to be taken backwards in time, so
to speak. Few in the public realize that one of the few commercial
niches where QM has actually been utilized to engineer something of real
usefullness, is in the the CPU.
Jones
