The Nickel Metal Hydride (NiMH) battery represents an evolution from the Nickel Hydrogen (NiH2) cell. Originally used in aerospace applications, the NiH2 cell had an advantage in having a large cycle life as well as a decent specific energy. The NiH2 cell however had the disadvantage that it had a poor volumetric efficiency requiring tanks of hydrogen gas, and was expensive due to the use of platinum as a catalyst. The NiMH battery was invented using a similar metal hydride hydrogen storage system as an electrode, whereby the system would fit into a single cell.
Compared to Nickel Cadmium (NiCd), NiMH batteries have numerous advantages. NiMH batteries run at the same voltage, 1.2 volts, as NiCd batteries despite having a higher specific energy. NiMH are also more environmentally friendly than NiCd as they do not use cadmium.
Cobalt’s role in NiMH is in the nickel electrode. During charge, the cobalt is oxidised to cobalt hydroxide. Cobalt remains in the Co3+ form during discharge providing a reserve capacity to the MH electrode in doing so. On average, Ni-MH batteries contain around 4% cobalt.
NiMH batteries are often found in similar applications to lithium ion batteries whereby a high specific energy is required such as in power tools and even in some hybrid vehicles. Although NiMH batteries are cheaper than lithium ion batteries, they have a lower specific energy resulting in newer technologies mostly utilising lithium ion. Other disadvantages of NiMH include a high self-discharge (around 50% greater than NiCd) and a degradation of performance if stored at elevated temperatures.