Metal hydride-nickel oxide cells,

The sealed nickel-metal hydride cell (more consistently metal hydride-nickel oxide cell) has a similar chemistry to the longer-established hydro-gen-nickel oxide cell considered in Chapter 9. In most respects (including OCV and performance characteristics), it is very similar to the sealed nickel-cadmium cell, but with hydrogen absorbed in a metal alloy as the active negative material in place of cadmium. The replacement of cadmium not only increases the energy density, but also produces a more environmentally friendly power source with less severe disposal problems. The nickel-metal hydride cell, however, has lower rate capability, poorer charge retention and is less tolerant of overcharge than the nickel-cadmium cell. 

The reaction at the positive is the same as that in the nickel-cadmium cell  

As in the nickel-cadmium cell, the electrolyte is concentrated potassium hydroxide. Depending on the metal alloy used, the emf has a value usually in the range 1.32-1.35 V, which turns out to be almost the same as that of the nickcl-cadmium cell. Note that the electrolyte composition is completely invariant during cycling. Unlike the situation with the nickel-cadmium cell, water is not involved in the cell reaction. 

Button, cylindrical and prismatic sealed cells are similar in design to the starved-electrolyte configuration of nickel-cadmium cells. A schematic diagram of a six-cell battery is shown in Fig. 6.11. Because of the slightly 

Nickel-melal hydride cells can be discharged at the 2 C rate (and in some cases at 4 C) and charged at 1 C. An AA-sized cell with a nominal capacity of over 1 Ah can thus be discharged at over 2 A and with a peak current of over 10 A. The energy density is highly dependent on rate, but for comparable conditions is 25% higher than an equivalent nickel-cadmium cell. Fig. 6.12 shows a comparison of the discharge characteristics of these two systems. 

---

The manufacture of secondary batteries based on aqueous electrolytes forms a major part of the world electrochemical industry. Of this sector, the lead-acid system (and in particular SLI power sources), as described in the last chapter, is by far the most important component, but secondary alkaline cells form a significant and distinct commercial market. They are more expensive, but are particularly suited for consumer products which have relatively low capacity requirements. They are also used where good low temperature characteristics, robustness and low maintenance are important, such as in aircraft applications. Until recently the secondary alkaline industry has been dominated by the cadmium-nickel oxide ( nickel-cadmium ) cell, but two new systems are making major inroads, and may eventually displace the cadmium-nickel oxide cell - at least in the sealed cell market. These are the so-called nickel-metal hydride cell and the rechargeable zinc-manganese dioxide cell. There are also a group of important but more specialized alkaline cell systems which are in use or are under further development for traction, submarine and other applications. 

Newton s second law, L0 nickel, 49, 665 nickel arsenide structure, 201 nickel surface, 189 nickel tetracarbonyl, 665 nickel-metal hydride cell, 520 NiMH cell, 520 nitrate ion, 69, 99 nitration, 745 nitric acid, 629 nitric oxide, 73, 629 oxidation, 549 nitride, 627 nitriding, 208 nitrite ion, 102 nitrogen, 120, 624 bonding in, 108 configuration, 35 photoelectron spectrum, 120... 

Fig. 6.11 Six-cell nickel-meial hydride batiery. 1. positive cap, connected to the nickel oxide electrode 2, can, connected to metal hydride electrode and serving as negative terminal 3. separator 4. cathode 5, anode 6. plastic battery case which contains interconnected cells and electronic management system. (By permission of Duracell.)...

Batteries contain several voltaic cells in series and are classified as primary (e.g., alkaline, mercury, and silver), secondary (e.g., lead-acid, nickel-metal hydride, and lithium-ion), or fuel cell. Supplying electricity to a rechargeable (secondary) battery reverses the redox reaction, forming more reactant for further use. Fuel cells generate a current through the controlled oxidation of a fuel such as H2. 

Accurate sorting relies on the identification of a number of different properties of a battery. These include the physical size and shape, the weight, the electromagnet properties and any surface identifiers such as colour or unique markings. These properties can be analysed in a number of different combinations in order to sort batteries into nickel cadmium, nickel metal hydride, lithium, lead acid, mercuric oxide, alkaline and zinc carbon batteries. Due to an voluntary marking initiative introduced by the european battery industry, it is now also possible to separate the alkaline and zinc carbon cells further into mercury free and mercury containing streams. 

EEI has used both commercially sintered silver powder with 1-mm-thick electrodes and sintered silver powder with in-house electrodes made from an ABj-type metal-hydride alloy (also known as lanthanum-nickel alloy) using rare earth material for initial laboratory cell test evaluations. Introduction of rare earth material improves the oxidation resistance during the alloy manufacturing process. The ABj-type metal-hydride alloy should be widely used in sealed Ni-MH batteries. The use of rare earth metal lanthanum will provide improved electrical performance, enhanced reliability, and ultra-high longevity for the sealed Ni-MH and Ag-MH battery systems. 

A variation of the nickel-metal hydride cell is the silver-metal hydride cell where the positive nickel oxyhydroxide electrode is replaced by a silver oxide electrode. The silver-metal hydride cell has the advantage of a higher-energy density and better high-rate performance, and may be useful in critical applications where its higher cost and shorter cycle life is acceptable. 

---