Electric Vehicles in Hybrid Configuration - Hydrogen Fuel Cells for Road Vehicles - page 149

MH þ OH ! M þ H 2 O þ e

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during charge reduction of water determines hydrogen adsorption into the lattice of
the inter-metallic alloy to form the metal hydride.
The overall discharge cell reaction can be written as:

NiOOH þ MH ! Ni O ð 2 þ M ð 5 : 17 Þ

to which a cell voltage of 1.32 V at room temperature is associated. Unlike
other nickel based batteries the charge-discharge mechanism in a Ni-MH
system is not based on anodic dissolution-precipitation, but employs the proton
transfer in homogeneous solid phase between nickel hydroxide and hydrogen
storage alloy.
The Ni-MH batteries started their existence since the beginning of the last
century and their performance, especially in terms of capacity, have been
improving dramatically. Today nickel-metal hydride batteries are considered a
better solution for electric vehicle applications than nickel-cadmium batteries,
because of their better performance without the toxicity issues of cadmium
[ 18 ]. They have a specific energy greater than 50 Wh/kg and a specific power of
200 W/kg. Moreover, these batteries tolerate over 600 full charge/discharge
cycles to 80% depth of discharge and they can be recharged in a quite fast way
to 80% of the full capacity. Their cells tolerate both overcharge and over-
discharge, because of the reactions of gas recombination inside the cells. The
disadvantages of Nickel-metal hydride batteries are low power, fast self-dis-
charge, and sensibility to temperature. More recent versions of this type of
batteries present cooling by air or liquid systems to reduce their size and obtain a
faster recharge. Moreover, they are rather more expensive than lead batteries in
automobile applications.

5.3.2.5 Sodium-Nickel Chloride (Zebra) Batteries

The last type of nickel based battery here considered is the so-called sodium-
nickel chloride or Zebra battery, firstly developed in 80s in Pretoria, South
Africa (Zebra stands for ZEolite Battery Research Africa). The anode is made of
liquid sodium, the electrolyte is based on sodium ion conducting b-alumina and
the cathode is constituted by nickel chloride. This is flooded with liquid NaAlCl 4
which acts as a secondary electrolyte, i.e., its function is to enhance the transport
of sodium ions from the solid nickel chloride to and from the alumina electrolyte
[ 19 ]. They work at high temperature (157C is the temperature necessary to have
sodium in its molten state, but the better performance is obtained in the range
250-350C) and operate with the following discharge semi- reactions:at the
anode:

Na ! Na þ þ e

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