Characteristics

Nickel Hydrogen batteries can be considered as hybrid batteries combining NiCad battery and fuel cell technologies, using pressurised hydrogen in place of the cadmium electrode. They were developed for aerospace applications and are still the number one energy storage system in many satellite projects.

Nickel Hydrogen cells employ the same nickel hydroxide Ni(OH)2 for the positive electrode (cathode) as Nickel Cadmium cells and the same alkaline potassium hydroxide (KOH) for the electrolyte. The negative electrode (anode), however, uses hydrogen in gaseous form as the active material instead of cadmium. In some ways, the anode current collector resembles a fuel cell electrode with a surface of Platinum or Palladium. It does not store hydrogen and is not transformed but simply acts as a catalytic surface which provides or absorbs electrons. In the discharged state, hydrogen is absorbed by the Nickel hydroxide.

Since the Hydrogen remains in gaseous form within the cell when the cell is charged, the case has to withstand very high pressures of up to about 7 X 106 Pascals (70 bar). Because of this, Nickel Hydrogen cells must be hermetically sealed.

The electrode reactions in a nickel/metal hydride battery are as follows:

Cathode (+) : NiOOH + H2 O + 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)

Advantages

• Light weight and high gravimetric energy density.
• High cycle life (50,000 cycles) and calendar life (15 years)
• Can be deep discharged.
• Abuse tolerant.
• Maintenance free.

• Because Hydrogen is generated during charging and absorbed during discharging, the gas pressure can be used as a simple measure of the State of Charge (SOC)

   

Shortcomings

• Low volumetric energy density due to gaseous Hydrogen
• High self-discharge rate
• Needs high pressure storage and exotic materials

• Limited applications due to high cost.

   

Applications

• Aerospace

• Used to power the Hubble Space Telescope

   

Costs

Very expensive.