It looks like the metal-metal junction can be nicely engineered.
http://en.wikipedia.org/wiki/Standard_electrode_potential_%28data_page
%29
http://www.mpoweruk.com/chemistries.htm
http://en.wikipedia.org/wiki/Electron_affinity
Some numbers:
El. S.E.P Elec.AFF.
Au -0.60 223
Zn -0.76 0
Pb -0.36 35
Ag +0.80 126
Cu +0.16 119
It appears in this case it is possible to have your cake and eat it
too. There is no direct correlation between electron affinity and
standard electrode potential. That is to say, the metal to metal
junction can actually add energy to the process, especially in an H2O
or H2O plus H gas transport environment. Some cases:
Battery - Zinc-Copper
El. S.E.P Elec.AFF.
Zn -0.76 0
Cu +0.16 119
Junction electron current: Zn-->Cu
Gap electron current: Zn-->Cu (Not good)
Dry Pile - Zinc-Silver
==================
El. S.E.P Elec.AFF.
Zn -0.76 0
Ag +0.80 126
Junction electron current: Zn-->Ag
Gap electron current: Zn-->Ag (Not Good)
Zinc-gold
==================
El. S.E.P Elec.AFF.
Zn -0.76 0
Au -0.60 223
Junction electron current: Zn-->Au
Gap electron current: Zn-->Au (Not good)
Lead-gold
==================
Pb -0.36 35
Au -0.60 223
Junction electron current: Au-->Pb
Gap electron current: Pb-->Au (OK)
It is of interest that electrons in Zn-Ag battery flow from the Zn
electrode to the Ag electrode. The bias across the metal to metal
junction is such that electrons gain a potential going from zinc to
silver. This is in *opposition* to the way the electrons flow in a
battery. It is of further interest that zinc is a hole conductor.
It acts like a p-type semiconductor at a junction with electron
conductors, which then act like n-type conductors. The metal to
metal interface thus should form a depletion region and thus a
barrier potential. See Figure 2. There are plus charges on the n-
region side and - charges on the p-region zinc side of the barrier.
Electrons have a fight uphill energy-wise going from the n-type
conductor to the p-type zinc.
electron donor
zzzzzzzzzzzzzzzzz
- - - - - - - - - Interface
+ + + + + + + + + Depletion Region
aaaaaaaaaaaaaaaaa
electron acceptor
^
| Gap
e-
Transport
electron donor
zzzzzzzzzzzzzzzzz
- - - - - - - - - Interface
+ + + + + + + + + Depletion Region
aaaaaaaaaaaaaaaaa
electron acceptor
Key:
zz - Zinc electrode
aa - Silver electrode
++ - Plus charge adjacent to depletion region
-- - Minus charge adjacent to depletion region
Fig. 2 - Diagram of Dry Pile Mechanics
We can clearly see that, provided the electron affinity model
describes the operation of the dry pile, that it is not ideally
engineered. The metal-to-metal interface loses energy gained in the
electron transport. This can be overcome by using a dielectric
between the zinc and acceptor metal, and operating in pulse mode. The
leakage current of the dielectric then must be engineered such that
the system recovers prior to the next pulse.
The now clear alternative to this is to use for the electron donor
lead or similar metal with standard electrode potential above that of
the acceptor metal or semiconductor. A lead-transporter-gold cell is
looking pretty good at this point.
One problem is cost of the gold or platinum used for the electron
acceptor. This can be avoided by gold (or platinum) plating or
deposition on both sides of any metal foil, and then depositing lead
on one side of that foil as the donor side of the foil. All that is
needed then is a means of separating the foils to make the gaps. This
can be accomplished by coating one side of the foil with a porous
dielectric separating material or closely spaced powder granules.
Horace Heffner
http://www.mtaonline.net/~hheffner/
