Nickel-Metal Hydride (NiMH) Batteries

Nickel-metal hydride (NiMH) batteries currently are used in computers, medical equipment, and other applications. They have greater specific energy and specific power capabilities than lead-acid or nickel-cadmium batteries, but they are more expensive. The components are recyclable, so the main challenges with nickel-metal hydride batteries are their high cost, the high temperature they create during charging, the need to control hydrogen loss, their poor charge retention, and their low cell efficiency.

Metal hydrides have been developed for high hydrogen storage densities and can be incorporated directly as a negative electrode, with a nickel hydroxyoxide (NiOOH) positive electrode and a potassium/lithium hydroxide electrolyte. The electrolyte and positive electrode had been extensively developed for use in nickel-cadmium cells.

The electrochemical reaction is

During discharge, hydroxyl (OH-) ions are generated at the nickel hydroxyoxide positive electrode and consumed at the metal hydride negative electrode. The converse is true for water molecules, which means that the overall concentration of the electrolyte does not vary during chargingldischarging. There are local variations, and care must be taken to ensure that the flow of ions across the separator is high enough to prevent the electrolyte "drying out" locally.

The conductivity of the electrolyte remains constant through the chargeldischarge cycle because the concentration remains constant. In addition, there is no loss of structural material from the electrodes; thus, they do not change their electrical characteristics. These two details give the cell very stable voltage operating characteristics over almost the full range of charge and discharge.