Page 251 - Handbook of Battery Materials

8.3 Battery Anodes (‘Negatives’) 221

In principle, all these reactions may be run in pure water, but in order to obtain
sufficiently high currents, electrolytes with better conductivity (aqueous solutions
of KOH or NaOH or seawater [4]) are necessary. The latter is found only in naval
applications.
While in alkaline solutions, the problem of aluminum corrosion has to be
overcome by protecting the metal by adding inhibitors like zinc oxide [5], zincate
ions [6], a combination of zinc species with organic additives [7], or by replacing
water in the electrolyte with methanol [8]; the aluminum anode in seawater has to
be activated by breaking the passive surface layer with suitable etchants [9–12].
Although high-purity aluminum (up to 99.999%) is used in science and research
dealing with the main discharge reactions and battery design concepts, most
prototypes and batteries are made with aluminum alloys. Such materials are Alcan
AB, ERC 3–4, Alcan BDW [13, 14], or S1–S6 (binary aluminum–tin alloys) [15].
Continuous efforts are made to replace expensive aluminum alloys by cheaper
unalloyed Al qualities [16, 17] or to replace the normal sheet electrode by a
polymer/aluminum composite electrode [18]. A survey of widely used alloys has
been carried out by Zein el Abedin [19].

8.3.2
Cadmium (Cd)

Although one of the most common storage batteries is called the
‘Nickel/Cadmium’-system (‘NiCad’), correctly written (−) Cd/KOH/NiO(OH) (+),
cadmium is not usually used to form a battery anode. The same can be said with
regard to the silver/cadmium [(−) Cd/KOH/AgO (+)] and the ‘MerCad’-battery
[(−) Cd/KOH/HgO (+)]. The ‘metallic negative’ in these cases may be formed
starting with cadmium hydroxide incorporated in the pore system of a sintered
nickel plate or pressed upon a nickel-plated steel current collector (pocket plates),
which is subsequently converted to cadmium metal by electrochemical reduction
inside the cell (type AB2C2). This operation is performed by the purchaser on first
use of these (storage) batteries in accordance with the users manual by inserting
them into the appropriate charger for a certain time (e.g., overnight). Cadmium
hydroxide for anode formation usually contains some additives (e.g., iron, nickel,
graphite) and – in some cases – organic inhibitors and/or polymer binding agents
[20–25].
The other method is cadmium electrodeposition on a nickel-plated steel foil
(serving as current collector) using a plating bath containing acidified cadmium
sulfate (type AB2C3). In this case the user is supplied with a battery in charged
(‘ready for use’) condition.
The reversible anodic charge/discharge reaction is:

−
Cd(OH) + 2e ←→ Cd + 2OH − (8.5)
2
Because of the high hydrogen overvoltage of cadmium in the caustic electrolyte, no
amalgamation is needed.