Daphuk_up (Hive Bee)
03-03-04 18:08
No 492846
      Some NiMH battery info     

Found this at: http://data.energizer.com/batteryinfo/application_manuals/nickel_metal_hydride.htm

Electrochemistry

The electrochemistry of the nickel-metal hydride cell is generally represented by the following charge and discharge reactions:

Charge

At the negative electrode, in the presence of the alloy and with an electrical potential applied, the water in the electrolyte is decomposed into hydrogen atoms, which are absorbed into the alloy, and hydroxyl ions as indicated below.

Alloy + H2O + e` => Alloy (H) + OH`

At the positive electrode, the charge reaction is based on the oxidation of nickel hydroxide just as it is in the nickel-cadmium couple.

Ni(OH)2 + OH` => NiOOH + H2O + e`

Discharge

At the negative electrode, the hydrogen is desorbed and combines with a hydroxyl ion to form water while also contributing an electron to the circuit.

Alloy (H) + OH` => Alloy + H2O + e`

At the positive electrode, nickel oxyhydroxide is reduced to its lower valence state, nickel hydroxide.

NiOOH + H2O + e` => Ni(OH)2 + OH`



Cell Components

Nickel-metal hydride cells, with the exception of the negative electrode, use the same general types of components as the sealed nickel-cadmium cell.



Negative Electrode

The basic concept of the nickel-metal hydride cell negative electrode emanated from research on the storage of hydrogen for use as an alternative energy source in the 1970s. Certain metallic alloys were observed to form hydrides that could capture (and release) hydrogen in volumes up to nearly a thousand times their own volume. By careful selection of the alloy constituents and proportions, the thermodynamics could be balanced to permit the absorption and release process to proceed at room temperatures and pressures. The general result is shown schematically in Figure 2 where the much smaller hydrogen atom is shown absorbed into the interstices of a bimetallic alloy crystal structure.

Two general classes of metallic alloys have been identified as possessing characteristics desirable for battery cell use. These are rare earth/nickel alloys generally based around LaNi5 (the so-called AB5 class of alloys) and alloys consisting primarily of titanium and zirconium (designated as AB2 alloys). In both cases, some fraction of the base metals is often replaced with other metallic elements. The AB5 formulation appears to offer the best set of features for commercial nickel-metal hydride cell applications.

The metal hydride electrode has a theoretical capacity approximately 40 percent higher than the cadmium electrode in a nickel-cadmium couple. As a result, nickel-metal hydride cells provide energy densities that are 20-40 percent higher than the equivalent nickel-cadmium cell.



Positive Electrode

The nickel-metal hydride positive electrode design draws heavily on experience with nickel-cadmium electrodes. Electrodes that are economical and rugged exhibiting excellent high-rate performance, long cycle life, and good capacity include pasted and sintered-type positive electrodes.

The balance between the positive and negative electrodes is adjusted so that the cell is always positive-limited as illustrated in Figure 3. This means that the negative electrode possesses a greater capacity than the positive. The positive will reach full capacity first as the cell is charged. It then will generate oxygen gas that diffuses to the negative electrode where it is recombined. This oxygen cycle is a highly efficient way of handling moderate overcharge currents.



Electrolyte

The electrolyte used in the nickel-metal hydride cell is alkaline, a dilute solution of potassium hydroxide containing other minor constituents to enhance cell performance.



Separator

The baseline material for the separator, which provides electrical isolation between the electrodes while still allowing efficient ionic diffusion between them, is a nylon blend similar to that currently used in many nickel-cadmium cells.



Cell Construction

The nickel-metal hydride couple lends itself to the wound construction shown in Figure 4, which is similar to that used by present-day cylindrical nickel-cadmium cells. The basic components consist of the positive and negative electrodes insulated by separators. The sandwiched electrodes are wound together and inserted into a metallic can that is sealed after injection of a small amount of electrolyte.

In variation of this design, nickel-metal hydride cells are also being produced in prismatic versions such as that illustrated in Figure 5. The prismatic cells may fit more easily into volume-critical applications.

Searching for non-catalytic methods of double bond hydrogenation, asked the same noob question as others, and found this info.

Note that at no point is a Nickel Hydride formed.  (Can't find a reference, not sure such a chemical species exists in a stable form.)  The alloy could be maybe used for something, but from the diagram at the site the batteries look like a pain in the ass to dissasemble.

Drug Chemists are Ta to a good Sm
 
 
 
 
    gsus
(Newbee)
03-03-04 22:19
No 492880
      battery compositions     

i personally find the old Zn/MnO2 batts most useful, but others prefer the Li's. its only a matter of time before MSDS's and other ingredient lists are classified. anyways, more on NiMH's will be found (for what its worth)here:
http://data.energizer.com
http://www.rayovac.com/customer/msds/NiMH.pdf
http://www.sanyo.com/batteries/pdfs/nimh.pdf
http://www.power-sonic.com/msdsnm.pdf
  hit the 'product safety datasheets' at energizer. their's are the most complicated. the others are very simple.