(This article is sponsored by The Boston Group)

The 26 January 2005 issue of MIT’s campus newspaper, Tech Talk, recently reported on the past quarter-century’s 25 most important inventions.  Advanced batteries, viz. the Nickel Metal Hydride (NiMH) and Lithium ion rechargeable batteries, were ranked at number 15.  The Hybrid car, which the paper noted was made possible by the NiMH battery, was ranked at number 16. 

Besides electric and hybrid automobiles, the NiMH battery has found extensive use in cell phones, power tools, computers, and other consumer electronics.  The advanced batteries are fast replacing the decades older Nickel-Cadmium (Ni-Cd) batteries.  The NiMH battery does not suffer from the memory limitation of Ni-Cd batteries that forces the consumer to nearly completely discharge the battery prior to recharge. The NiMH battery can be recharged at will.

The NiMH battery is nearly identical in construction to the Ni-Cd battery.  The nickel hydroxide positive electrode is separated from the negative electrode by an aqueous electrolyte.  The only difference between the batteries is the material from which the negative electrode is fabricated.  While the Ni-Cd battery uses the environmentally hazardous metal Cd, the NiMH battery uses a specially processed rare earth-transition metal (RETM) alloy based on the LaNi5 (La= lanthanum) composition. 

The hydrogen storage capacity of this alloy at near ambient temperature and pressure is huge. More hydrogen can be stored in a given volume of the RETM alloy compared to an equivalent volume of liquid hydrogen, which is only obtainable at cryogenic temperatures. The special alloy processing is the key technology that ensures long battery life and a commercially viable product.

The overall cell reaction is as follows:

Charge (forward reaction)

    Ni(OH)2  +  RETM              =              NiOOH  +  RETM-H
               
Discharge (reverse reaction)

RETM is the hydrogen absorbing alloy material, and RETM-H the alloy in its hydrided (or charged) condition.   The forward reaction represents the battery charge cycle in which energy is stored within the battery.  The backward reaction represents the discharge cycle in which the stored energy is recovered.  The battery function requires the storage and transport of hydrogen from one electrode to the other.

For charging, the electrodes are connected to an external power source.  Each hydrogen atom liberated at the positive nickel hydroxide electrode releases an electron, forming an ion.  The resulting hydrogen ion (proton) is transported through the electrolyte to the negative electrode, where it gains back an electron to form a neutral hydrogen atom available for absorption into the RETM negative electrode. 

The backward or reverse reaction represents the battery discharge cycle that occurs when an external load is attached to the battery terminals, allowing the current (electrons) to flow through the circuit in the reverse direction.  The hydrided RETM alloy (RETM-H) desorbs its stored hydrogen, which is transported back as an ion to the positive electrode.  The NiOOH at the positive electrode is reverted to Ni(OH)2 as it picks-up this desorbed hydrogen.

The NiMH battery can be assembled either discharged or charged.  Where the battery is discharged, the Ni(OH)2 positive electrode is assembled with the unhydrided RETM negative electrode.  The battery may be charged later with an external power source.

Assembling the charged battery appears the preferred route.  The NiOOH positive electrode is assembled with the RETM negative electrode, which is hydrided by subjecting the unsealed assembly to hydrogen gas.  After sealing, the battery is rendered ready for use.

Alloy hydriding via the gas or the electrochemical routes produces identical results.  The electrochemical potential (E) of the hydrogen-absorbing electrode is uniquely related the hydrogen gas pressure: E = RT/nF ln PH2, (R = Universal Gas constant; T = absolute temperature; F = Faraday constant; and n = 2). Thus, gas absorption can be studied through electrochemical means, and vice versa.  In both cases, a neutral hydrogen atom is presented to the metal alloy for absorption.  (The hydrogen gas molecule dissociates at the alloy surface, and yields neutral hydrogen atoms.) 

Each year, well over a billion NiMH batteries are manufactured worldwide.  The NiMH battery presents the classic case of serendipity giving birth to a major technology that affects us on a daily basis.  The NiMH battery alloy was discovered by researchers engaged in developing strong RETM permanent magnets.  The RETM magnets, discovered in the late 1960s, were a major success story of the time and now constitute a billion dollar industry.  The development of high strength RETM permanent magnets and its role in NiMH battery alloy development constitute subjects for other articles.

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