Nickel–hydrogen battery

Lore & Background

The development of the nickel–hydrogen battery started in 1970 at Comsat and was used for the first time in 1977 aboard the U.S. Navy's Navigation Technology Satellite-2 (NTS-2). The nickel–hydrogen battery was patented in the United States on February 25, 1971 by Alexandr Ilich Kloss, Vyacheslav Mikhailovic Sergeev and Boris Ioselevich Tsenter from the Soviet Union. Currently, the major manufacturers are Eagle-Picher Technologies and Johnson Controls, Inc.

The nickel–hydrogen battery combines the positive nickel electrode of a nickel–cadmium battery and the negative electrode, including the catalyst and gas diffusion elements, of a fuel cell. During discharge, hydrogen contained in the pressure vessel is oxidized into water while the nickel oxyhydroxide electrode is reduced to nickel hydroxide. Water is consumed at the nickel electrode and produced at the hydrogen electrode, so the concentration of the potassium hydroxide electrolyte does not change.

As the battery discharges, the hydrogen pressure drops, providing a reliable state of charge indicator. In one communication satellite battery, the pressure at full charge was over 500 pounds/square inch (3.4 MPa), dropping to only about 15 PSI (0.1 MPa) at full discharge. If the cell is over-charged, the oxygen produced at the nickel electrode reacts with the hydrogen present in the cell and forms water; as a consequence the cells can withstand overcharging as long as the heat generated can be dissipated.

The cells have the disadvantage of relatively high self-discharge rate, i.e. chemical reduction of Ni(III) into Ni(II) in the cathode: NiOOH + 1/2H2 → Ni(OH)2, which is proportional to the pressure of hydrogen in the cell; in some designs, 50% of the capacity can be lost after only a few days' storage. Self-discharge is less at lower temperature.

Compared with other rechargeable batteries, a nickel–hydrogen battery provides good specific energy of 55–60 watt-hours/kg, and very long cycle life (40,000 cycles at 40% DOD) and operating life (> 15 years) in satellite applications. The cells can tolerate overcharging and accidental polarity reversal, and the hydrogen pressure in the cell provides a good indication of the state of charge. However, the gaseous nature of hydrogen means that the volume efficiency is relatively low (60-100 Wh/L for an IPV cell), and the high pressure required makes for high-cost pressure vessels.

Reader's Guide

The nickel–hydrogen battery has been notably used in space applications. For example, the Mercury Messenger, Mars Odyssey and the Mars Global Surveyor are equipped with nickel–hydrogen batteries. The Hubble Space Telescope, when its original batteries were changed in May 2009 more than 19 years after launch, led with the highest number of charge and discharge cycles of any NiH2 battery in low Earth orbit.

The ISS formerly had nickel-hydrogen batteries until 2019, where they were replaced with lithium-ion batteries. NiH2 cells using 26% potassium hydroxide (KOH) as an electrolyte have shown a service life of 15 years or more at 80% depth of discharge (DOD). While the energy density is only around one third as that of a lithium battery, the distinctive virtue of the nickel–hydrogen battery is its long life: the cells handle more than 20,000 charge cycles with 85% energy efficiency and 100% faradaic efficiency. The positive electrode is made up of a dry sintered porous nickel plaque, which contains nickel hydroxide.

The negative hydrogen electrode utilises a teflon-bonded platinum black catalyst at a loading of 7 mg/cm2 and the separator is knit zirconia cloth (ZYK-15 Zircar). The Hubble replacement batteries are produced with a wet slurry process where a binder agent and powdered metallic materials are molded and heated to boil off the liquid. Several designs exist: individual pressure vessel (IPV), common pressure vessel (CPV), single pressure vessel (SPV), bipolar design, dependent pressure vessel (DPV), and common/dependent pressure vessel (C/DPV).