ABSTRACT
A nickel/metal hydride (Ni/MH) cell is basically composed of a hydrogen storagealloy/metalhydride(M/MH)negativeelectrode,anickeloxyhydroxide/nickel hydroxide(NiOOH/Ni(OH)2) positive electrode, an alkaline aqueous electrolyte solution,andaseparator,asillustratedinFigure7.1.Positiveandnegativeelectrode reactions and total cell reaction are represented as follows1:
( )Positive electrode Ni(OH) OH NiOOH H O e2 2+ = + + − − (7.1)
( )Negative electrode M H O e MH OH2+ + = + − −
(7.2)
( )Cell reaction Ni(OH) M NiOOH MH2 + = + (7.3)
7.1 Introduction .................................................................................................. 201 7.1.1Cell Reaction .................................................................................... 201 7.1.2History of Development of Ni/MH Batteries and Their Features ......203 7.1.3Applications ......................................................................................205 7.1.4Materials Development for High-Performance Ni/MH Batteries ......206
7.2 Technical Trends on the Development of Hydrogen Storage Alloys for the Negative Electrode ..................................................................................207 7.2.1Magnesium-Containing Hydrogen Storage Alloys...........................207 7.2.2Vanadium-Containing Alloys ........................................................... 214
7.3Technical Trends on the Development of a Positive Electrode .................... 219 7.4 Outlook for the Future .................................................................................. 223 References ..............................................................................................................224
During charging, at the positive electrode Ni(OH)2 is oxidized to NiOOH and at the negative electrode water is reduced to atomic hydrogen, followed by being absorbed in M to form MH. On the other hand, during discharging at the positive electrode NiOOH is reduced to Ni(OH)2 and at the negative electrode MH is oxidized to M. The cell reaction is apparently represented by a reversible transfer of hydrogen between both electrodes. It is notable that the total amount of water and hydroxide and the concentration of the electrolyte solution are unchangeable during charging and discharging because water does not apparently contribute to the cell reaction. This is different from the lead-acid cell and nickel-cadmium cell where the volume of water changes during charging or discharging. The unique cell reaction mechanism contributes to high capacity, high power, long cycle life, etc.
