> Battery - Zinc-Copper > El. S.E.P Elec.AFF. > Zn -0.76 0 > Cu +0.16 119
The Zn Cu cell voltage is 1.1V, corresponding to a Cu half reaction standard electrode potential of +0.34V (0.34-(-0.76)=1.1), not +0.16V cf http://en.wikipedia.org/wiki/Galvanic_cell. You must have picked the wrong Cu half reaction. Also note that the standard electrode potentials are only valid in aqueous solution. BTW something puzzles me about those standard electrode potentials. They are relative to the Standard hydrogen electrode http://en.wikipedia.org/wiki/Standard_hydrogen_electrode , whose _absolute_ potential is about 4.5V (other sources give a more precise value of 4.44V): ------------------------------------------------------ standard hydrogen electrode (abbreviated SHE), also called normal hydrogen electrode (NHE), is a redox electrode which is placed in the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be 4.4V to 4.6V, but to form a basis for comparison with all other electrode reactions, Hydrogen's standard electrode potential (E0) is declared to be zero. Potentials of any other electrodes are compared with the standard hydrogen electrode. Hydrogen electrode is based on the redox half cell: 2H+(aq) + 2e- -> H2(g) ------------------------------------------------------ It would seem sensible to conclude from the above that, near zero current and in Standard conditions, the Zn negative terminal of the Zn/Cu battery is at 4.44-0.76=3.68V above the bulk of the electrolyte, while the Cu positive terminal is at 4.44+0.34=4.78V above the electrolyte (salt bridge voltage drop is zero cf http://en.wikipedia.org/wiki/Galvanic_cell ), so that the potential difference between the Cu and the Zn is indeed 4.78-3.68=1.1V. The problem with this conclusion is, if _both_ electrodes are _above_ electrolyte potential, then the hydrated ions of their double layers should be negative at both electrodes, which we know is not true (they are positive at the negative electrode). I must be missing something obvious, any help welcome. Michel ----- Original Message ----- From: "Horace Heffner" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Monday, September 10, 2007 12:55 AM Subject: Re: [Vo]:Energy conversion via Electron affinity > 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/ > > >
