Secondary batteries - nickel systems : nickel–metal hydride : metal hydrides: Metal Hydrides

Many scientists and even policy-makers are nowadays openly speculating about the future hydrogen economy for which it is believed that hydrogen-driven fuel cells are going to play a dominant role in transportation, thereby replacing conventional and inefficient internal combustion engines. To enable such beneficial economy it has been argued that safe and efficient storage of hydrogen gas is one of the crucial aspects to be solved. However, it has generally not been realized that our society has already entered the hydrogen economy a few decades ago with the development of nickel-metal hydride (Ni-MH) batteries. Advantageously, this battery system is indeed based on the smallest energy carrying chemical particle shuttling between the two electrodes. Storage of hydrogen in the negative electrode of Ni-MH batteries has been accomplished efficiently, which is safe and at low pressures in the form of metal hydrides (MH). Since its discovery, this battery type has become widely accepted to power our portable electronics, due to their high storage capacity, excellent rate capability, and environmental friendliness. Its popularity is currently further boosted resulting from the large-scale introduction in hybrid electrical vehicles (HEVs) for which Ni-MH batteries are exclusively used and it is to be expected that this expansion is further amplified in the near future into the direction of plug-in (hybrid) electrical vehicles (P(H)EV). This battery system can, therefore, be considered as the first commercial success toward a fully developed hydrogen era. In this article the basic electrochemical principles underlying Ni-MH operation are first outlined and then the materials research, which has enabled this successful battery system, is reviewed. Finally, more recent developments in materials research, which may lead to a new generation of high-energy density Ni-MH batteries in the near future, are described.