Page 70 - Handbook of Battery Materials

36 2 Practical Batteries

Figure 2.14 Various Ni–Cd batteries.
2.4
Nickel–MH Batteries

Nickel–MH batteries contain a nickel electrode similar to that used in
nickel–cadmium batteries as the positive electrode, and a hydrogen-absorbing
alloy for the negative electrode. This has made the development of a
hydrogen-absorbing alloy electrode important.
Hydrogen-absorbing alloy can reversibly absorb and desorb a large amount of
hydrogen. Hydrogen gas is rapidly absorbed in the gas phase, then desorbed on
the alloy (gas-solid reaction). In the electrode reaction, the alloy electro-chemically
absorbs and desorbs hydrogen in an alkaline solution (electrochemical reaction):
Positive electrode reaction:
Discharge
NiOOH + H 2 O + e −−−−−−→ Ni(OH) 2 + OH − 0.52 V vs NHE (2.7)
−
←−−−−−−
Charge
Negative electrode reaction:
Discharge
MH + OH −−−−−−→ M + H 2 O + e − −0.80 V vs NHE (2.8)
−
←−−−−−−
Charge
Overall battery reaction:
Discharge
NiOOH + MH −−−−−−→ Ni(OH) 2 + M 1.32 V (2.9)
←−−−−−−
Charge
where M = hydrogen-absorbing alloy and MH = metal hydride.
Figure 2.15 shows a typical mechanism of the charge–discharge reaction. During
charging, the electrolytic reaction of water causes the hydrogen, which is present
in atomic form on the surface of the hydrogen-absorbing alloy in the negative
electrode, to disperse into and be absorbed by the alloy (discharge reaction). During
discharge, the absorbed hydrogen reacts with hydroxide ions at the surface of
the hydrogen-absorbing alloy to become water once again (charge reaction). In
other words, the active material of the electrode reaction is hydrogen, and the
hydrogen-absorbing alloy acts as a storage medium for the active material.