 Schematics of a nickel–hydrogen battery | |
| Specific energy | 55–75W·h/kg[1][2] |
|---|---|
| Energy density | ~60W·h/L[2] |
| Specific power | ~220 W/kg[3] |
| Charge/discharge efficiency | 85% |
| Cycle durability | >20,000cycles[4] |
Anickel–hydrogen battery (NiH2 or Ni–H2) is a rechargeable electrochemical power source based onnickel andhydrogen.[5] It differs from anickel–metal hydride (NiMH) battery by the use ofhydrogen in gaseous form, stored in a pressurizedcell at up to 1200 psi (82.7 bar) pressure.[6] The nickel–hydrogen battery was patented in the United States on February 25, 1971 byAlexandr Ilich Kloss,Vyacheslav Mikhailovic Sergeev andBoris Ioselevich Tsenter from the Soviet Union.[7]
NiH2 cells using 26%potassium hydroxide (KOH) as anelectrolyte have shown aservice life of 15 years or more at 80%depth of discharge (DOD)[8] Theenergy density is 75 Wh/kg, 60 Wh/dm3[2]specific power 220 W/kg.[3] Theopen-circuit voltage is 1.55 V, the average voltage during discharge is 1.25 V.[9]
While the energy density is only around one third as that of alithium battery, the distinctive virtue of the nickel–hydrogen battery is its long life: the cells handle more than 20,000charge cycles[4] with 85%energy efficiency and 100%faradaic efficiency.
NiH2rechargeable batteries possess properties which make them attractive for theenergy storage of electrical energy in satellites[10] andspace probes. For example, theISS,[11]Mercury Messenger,[12]Mars Odyssey[13] and theMars Global Surveyor[14] are equipped with nickel–hydrogen batteries. TheHubble Space Telescope, when its original batteries were changed in May 2009 more than 19 years after launch, led with the highest number ofcharge and discharge cycles of any NiH2[15] battery inlow Earth orbit.[16]
The development of the nickel hydrogen battery started in 1970 atComsat[17] and was used for the first time in 1977 aboard the U.S. Navy'sNavigation technology satellite-2 (NTS-2).[18] Currently, the major manufacturers of nickel–hydrogen batteries areEagle-Picher Technologies andJohnson 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 afuel 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.[dubious –discuss]
The cells have the disadvantage of relatively high self-discharge rate, i.e. chemical reduction of Ni(III) into Ni(II) in the cathode:
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.[1]
Compared with other rechargeable batteries, a nickel–hydrogen battery provides goodspecific 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 (individual pressure vessel) cell), and the high pressure required makes for high-cost pressure vessels.[1]
The positive electrode is made up of a drysintered[20] porous nickel plaque, which containsnickel hydroxide. The negativehydrogenelectrode utilises a teflon-bondedplatinum blackcatalyst at a loading of 7 mg/cm2 and the separator is knitzirconia cloth (ZYK-15 Zircar).[21][22]
The Hubble replacement batteries are produced with a wet slurry process where a binder agent and powdered metallic materials aremolded and heated to boil off the liquid.[23]
