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What is a Nickel Hydrogen Battery?
Nickel Hydrogen Battery

Nickel Hydrogen Battery - Example and example in use

NASA
The nickel hydrogen battery was used for the first time in 1977 aboard the U.S. Navy's Navigation technology satellite-2 (NTS-2).

The Nickel-Hydrogen battery can be considered a hybrid between the nickel-cadmium battery and the fuel cell. The cadmium electrode was replaced with a hydrogen gas electrode. This battery is visually much different from the Nickel-Cadmium battery, because the cell is a pressure vessel, which must contain over one thousand pounds per square inch (psi) of hydrogen gas. It is significantly lighter than nickel-cadmium, but is more difficult to package, much like a crate of eggs.

Nickel-hydrogen batteries are sometimes confused with Nickel-Metal Hydride batteries, the batteries commonly found in cell phones and laptops. Nickel-hydrogen, as well as nickel-cadmium batteries use the same electrolyte, a solution of potassium hydroxide, which is commonly called lye.

Incentives for developing nickel/metal hydride (Ni-MH) batteries comes from pressing health and environmental concerns to find replacements for the nickel/cadmium rechargeable batteries. Due to worker's safety requirements, processing of cadmium for batteries in the U.S. is already in the process of being phased out. Furthermore, environmental legislation for the 1990's and the 21st century will most likely make it imperative to curtail the use of cadmium in batteries for consumer use. In spite of these pressures, next to the lead-acid battery, the nickel/cadmium battery still has the largest share of the rechargeable battery market. Further incentives for researching hydrogen-based batteries comes from the general belief that hydrogen and electricity will displace and eventually replace a significant fraction of the energy-carrying contributions of fossil-fuel resources, becoming the foundation for a sustainable energy system based on renewable sources. Finally, there is considerable interest in the development of Ni-MH batteries for electric vehicles and hybrid vehicles.

The nickel/metal hydride battery operates in concentrated KOH (potassium hydroxide) electrolyte. The electrode reactions in a nickel/metal hydride battery are as follows:

Cathode (+): NiOOH + H2O + e- Ni(OH)2 + OH- (1)

Anode (-): (1/x) MHx + OH- (1/x) M + H2O + e- (2)

Overall: (1/x) MHx + NiOOH (1/x) M + Ni(OH)2 (3)

The KOH electrolyte can only transport the OH- ions and, to balance the charge transport, electrons must circulate through the external load. The nickel oxy-hydroxide electrode (equation 1) has been extensively researched and characterized, and its application has been widely demonstrated for both terrestrial and aerospace applications. Most of the current research in Ni/Metal Hydride batteries has involved improving the performance of the metal hydride anode. Specifically, this requires the development of a hydride electrode with the following characteristics: (1) long cycle life, (2) high capacity, (3) high rate of charge and discharge at constant voltage, and (4) retention capacity.
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