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June 15, 2022

Nickel-Hydrogen Battery Chemistry FAQ

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How do you think a long-lived satellite such as the Hubble Space Telescope can recharge its cells thousands of times a year and not have its batteries wear out? They do it with this ancient but amazing technology, with cells that actually store the anode as hydrogen gas.

Nickel/Hydrogen cells

Anode:Hydrogen Gas

Cathode: Nickel oxyhydroxide

Electrolyte: Potassium hydroxide

Applications:Space satellites that require long cycle life, over 40,000 cycles. Nickel/Hydrogen batteries have a high self-discharge rate, something like 80% a month, which isn't a problem for satellite applications.

The NiH2 cell is a welded pressure vessel. It has a high specific energy, 60WH/kg, long life, can tolerate overcharge and cell reversal, but has a low volumetric energy density, 50 WH/liter.

Here are the cell reactions:

Location Reactions Voltage
Anode ½H2 + OH- —> H2O + e- 0.83
Cathode NiOOH + H2O + e- —> Ni(OH)2 + OH- 0.52
Overall NiOOH + ½H2 —> Ni(OH)2 1.35

In order to get the hydrogen gas into solution a Teflon-bonded platinum black catalyst is used, similar to that used in fuel cells. This platinum electrode has the added advantage that it can recombine oxygen with hydrogen extremely fast. Since the only bad chemical reaction during over charge is the creation of oxygen at the positive electrode this means that the Nickel/Hydrogen battery is impossible to overcharge (though there may be a thermal runaway problem if the excess heat isn't dissipated.) A similar reaction keeps any damage from being done if the cell is reverse-charged.

The battery weight for a 10kW satellite is about 350 kg, or 770 lbs.

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