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Encyclopedia of Physical Science and Technology EN010b-481 July 14, 2001 18:45

484 Noble Metals (Chemistry)

to improve the reflective quality. Rhodium is also used the cell is approximately two years. The Ag/Cd cell has an
with platinum in the molten glass industry, in spinnerets energy density close to that of the Ag/Zn system but with
in the fiber industry, and with iridium in high-temperature the advantage that it is nonmagnetic. This cell has a 95%
thermocouples. efficiency of charge acceptance, but battery performance
◦
declines below 0 C and above 40 C. This cell is used
◦
in appliances, power tools, and satellites, where nonmag-
7. Osmium
netic properties are important, and to power submarine
Osmium has limited usage due to the formation of toxic simulator target drones. The Ag/Fe cell has the advantage
volatile oxide. It is used when a very hard and wear resis- of a high discharge rate capability with the cycling char-
tant alloy is required (e.g., spindle pivots). acteristics of the iron electrode. The discharge voltage is
approximately 1.1 V. The energy density is similar to that
of the Ag/Zn system, but it can be recharged up to 500
8. Ruthenium
times without performance changes. Applications include
Ruthenium is not as industrially important as other noble emergency power; telecommunications systems in teth-
metals, but it is used as RuO 2 to coat the dimensionally ered balloons; and small button cells used in hearing aids,
stable titanium anodes used in the production of chlorine, calculators, and electric razors. The Ag/H cell is in the re-
chlorate, and caustic, as well as oxygen in water electrol- search and development stage, but it shows promise. The
ysis. Ruthenium has also found some use as a binder for Ag/Zn and Ag/Cd systems have higher power/weight or
high-temperature cemented carbides. unit volume ratings than the lead–acid, Ni/Zn, and Ni/Cd
systems, but the silver systems are more expensive.
E. Batteries
F. Electrodes
Noble metal usage in batteries is essentially limited to sil-
ver which includes both primary and secondary batteries. The noble metals are used in the construction of anodes
Primary batteries, sold in a charged state and not in- because they will not dissolve as some materials have a
tended for recharge, use Ag 2 O and AgO as cathodic ma- tendency to do. They also assist in the reduction of over-
terials. The primary silver oxide/zinc cell has a high en- potential at the interface between the electrode and the
ergy output per unit weight and a fairly constant voltage solution.
during discharge, but due to its high cost, applications are Platinum can be used by itself or as a coating for ti-
limited mainly to wet cell uses with the military. Com- tanium or tantalum to produce anodes that are useful in
mercial uses of minature dry cells include hearing aids, the production of sodium hypochlorite, dimethylsebacate,
watches, and photoelectric exposure devices. The silver and tetraalkylthioramdisulfide. Anodes of Ti–Pt and Ti–
oxide cell is a good reference voltage source. Open circuit Pt–Ir are used for cathodic protection and sodium chlo-
voltage is 1.85 V for AgO/Zn and 1.60 V for Ag 2 O/Zn. rate production. Titanium by itself will build up a layer
Discharge voltage is ∼1.5 V in each case. Commercial of surface oxide that is nonconducting; platinum or Pt–Ir
cells use Ag 2 O, but an AgO cell is being developed be- prevents this from occurring by giving additional stabil-
cause its electrical capacity per gram would be twice that ity to the system. Platinum and Ti–Pt anodes are used in
of the Ag 2 O cell. Primary lithium batteries are known with noble metal plating solutions. A titanium electrode coated
a variety of silver oxidizing agents (e.g., Ag 2 CrO 4 , AgF, with 70Pt–30Ir is used in chlorine and sodium chlorate
andAgCl).Theadvantagesofthesecellsarethelowequiv- production because of its high current efficiency and low
alent weight of lithium and its excellent reducing ability overpotential for chlorine evolution and high overpoten-
for making a high-energy-density battery. tial for oxygen evolution. The iridium is present as the
Rechargeable secondary batteries using silver are the dioxide and it is the species responsible for the improved
Ag/Zn, Ag/Cd, Ag/Fe, and an experimental Ag/H system. overpotential. Anodes of Ti–Ru are used in the chloroal-
The most widely known secondary battery, lead–acid, is kali industry. The electrode is composed of a titanium
not direct competition for the silver cells. The Ni/Cd sys- core with a Ru 2 O 3 and RuO 2 conductive coating to pro-
tem has the majority of this specialty battery market. The tect the system. Electrodes composed of Ta 2 O 5 with an
Ag/Zn battery currently has the highest energy density of IrO 2 coating have been proposed for use in oxygen gener-
the rechargeable systems. The cell has a 95–100% effi- ation. Anodes that are coated with platinum are stable in
ciency in charge acceptance and one of the flattest voltage mercury cells and have a lifetime of 3–5 yr in a chlorine
curves of any practical battery. Life is approximately 10– cell and 2–4 yr in a sodium chlorate cell.
30 cycles for high rate of discharge and approximately In general, precious metal electrodes or precious
100–300 cycles for low rate of discharge. The wet life of metal coated electrodes are used in batteries, fuel cells,