Page 26 - Battery Reference Book

Reversible electrodes 1/11

in contact with the active materials of the plates. These during the charge is about 1.65 V, rising at the end to
are full of small pores in which' diffusion is very slow, 1.8 V, whereas during the discharge it falls gradually
so that the coincentration of the acid is greater during from 1.3 to 1.1 V. Hence the energy efficiency is only
the charge anld less during the discharge than in the about 60%.
bulk of the solution. This difference results in a loss
of efficiency. 1.3 Reversible electrodes
The current efficiency of the lead accumulator, Le.
Amount of current taken out The electrodes constituting a reversible cell are
reversible electrodes, and three chief types of such
Current efficien'cy = during discharge
Amount of current put in electrodes are known. The combination of any two
during charge reversible electrodes gives a reversible cell.
The first type of reversible electrode involves a
is high, about 94-96%, but the charging process takes metal (or a non-metal) in contact with a solution of its
place at a higher electromotive force than the dis- own ions, e.g. zinc in zinc sulphate solution, or copper
charge, so that more energy is required for the former. in copper sulphate solution, as in the Daniel1 cell.
The energy efficiency measured by
Electrodes of the first kind are reversible with respect
Energy obtained (Discharge voltage x Quantity to the ions of the electrode material, e.g. metal or non-
in discharge ~2 of electricity) metal; if the electrode material is a univalent metal or
Energy requirez = (Charge voltage x Quantity hydrogen, represented by M, the reaction which takes
to charge C of electricity) place at such an electrode, when the cell of which it
is comparatively low, at 75585%. is part operates, is
A further example of a rechargeable battery is the
nickel-iron cell. In the discharged state the negative M+M++e
plate of this cell is iron with hydrated ferrous oxide, where e indicates an electron, and M+ implies a
and the positive plate is nickel with hydrated nickel hydrated (or solvated) ion in solution. The direction
oxide. When charged, the ferrous oxide is reduced to of the reaction depends on the direction of flow of
iron, and the nickel oxide is oxidized to a hydrated current through the cell. If the electrode material is a
peroxide. The cell reaction may thus be represented by univalent non-metal A, the ions are negative and the
corresponding reaction is
(charge
FeO + 2Ni0 F======+ Fe + Ni2O3 A-+A+e
discharge
The three oxides are all hydrated to various extents, As will be seen later, the potentials of these elec-
but their exact compositions are unknown. In order trodes depend on the concentration (or activity) of the
to obtain plates having a sufficiently large capacity, reversible ions in the solution.
the oxides halve to be prepared by methods which Electrodes of the second type involve a metal and
give particularly finely divided and active products. a sparingly soluble salt of this metal in contact with a
They are pac:ked into nickel-plated steel containers, solution of a soluble salt of the same anion:
perforated by numerous small lholes - an arrangement M 1 MX(s) HX(so1n)
which gives exceptional mechanical strength. The elec-
trolyte is usuallly a 21% solution of potash, but since The electrode reaction in this case may be written as
hydroxyl ions do not enter into the cell reaction the
electromotive force (1.33-1.35 V) is nearly indepen- Mfs) +X- + MX(s)+ e
dent of the concentration. Actually, there is a differ-
ence between the amount of water combined with the the ion X being that in the solution of the soluble
oxides in the charged and discharged plates. Water is acid, e.g. HX. These electrodes behave as if they were
taken up and the alkali becomes more concentrated reversible with respect to the common anion (the ion
during the discharge, but water is given out during the X in this case).
charge. The electromotive force therefore depends to a Electrodes of the second type have been made with
small extent 011 the free energy of water in the solution, various insoluble halides (silver chloride, silver bro-
which in turn is determined by the concentration of the mide, silver iodide and mercurous chloride) and also
dissolved potaish. Actually 2.9mol of water are liber- with insoluble sulphates, oxalates, etc.
ated in the discharge reaction, as represented above, The third important type of reversible electrode con-
and the variation of the electromotive force between sists of an unattackable metal, e.g. gold or platinum,
1.0~ and 5.3~ potash is from 1.351 to 1.335V. The immersed in a solution containing both oxidized and
potential of the positive plate is +OS5 and that of the reduced states of an oxidation-reduction system, e.g.
negative plate -0.8 on the hydlrogen scale. Sn4+ and Sn2+; Fe3+ and Fez+; or Fe(CN)i- and
The current efficiency, about 82%, is considerably Fe(CN):-. The purpose of the unattackable metal is
lower than that of the lead accumulator. The voltage to act as a conductor to make electrical contact, just