Accumulator Nickel-Iron Alkaline Cells

ACCUMULATOR NICKEL-IRON ALKALINE CELLS In this type of accumulator which was invented and developed by Edison, the active materials of the electrodes consist of the oxides of iron and nickel which are immersed in an electrolyte of potassium hydroxide.

Construction and Assembly.—The active material of the positive plates is nickel hydrate. As this material is only a poor conductor, its resistance is reduced by mixing with a better conducting mate rial. The means adopted is to fill a helical tube (fig. 23) made from finely perfo rated nickelled steel strip with alter nately-arranged thin layers of nickel hydrate, the positive active material, and fine metallic nickel flake. Each tube is about 4in. long and 4in. diameter, and contains over 300 of these highly-com pressed double layers. Each tube is sur rounded and reinforced externally by steel rings equidistantly spaced. The complete positive plate (fig. 24) consists of a light nickelled steel grid on which a double row of the unit tubes already de scribed is mounted. To prevent plate dis tortion, adjacent tubes are wound oppo sitely, a right-hand wound tube being mounted adjacent to a left-hand wound one, and so on.

The negative plates of the cell also consist of an assemblage of small units, but of oblong pocket form instead of being tubular. Iron oxide prepared in a special manner, and incorporated with a trace of mercury to prove the conductivity, is enclosed in flat steel pockets, which, like the tubes in the positive plate, are made of nickelled steel strip, finely perforated. These units are mounted in a light, punched, nickelled steel frame and subject to hydraulic pressure of 120 tons in order to prevent subsequent loosening and to corrugate the faces of the pockets. The assembled cell consists of a series of positive and negative plates as indicated in fig. 25. Contact between adjacent plates is prevented by hard rubber strips and pin insulators, while sheets of the same material prevent contact of the plates with the sides of the nickel plated steel container.

The terminal posts of the as sembled plates project through insulated and liquid-tight stuffing glands or boxes mounted in the welded-in top of the cell which is also provided with a combined filler cap and gravity gas escape non-return valve.

The electrolyte which is em ployed consists of a solution of potash in distilled water with a small percentage of lithia. The density of the electrolyte does not change to any large degree between charge and discharge. The proper level of the electro lyte in the cell is subsequently maintained by the additions of distilled water.

Electrolytic Reactions.

On first charging the cell,-the green nickel hydrate becomes more highly oxidized and becomes black to right represents discharging that from right to left the charging reaction) :— 2Ni Fez=±2Ni(OH)2+ Fe(011)2 The material in the positive plate is not homogeneous but con sists of mixtures or possibly a solid solution of the different oxides of nickel. On charging, the lower oxides of nickel are brought by the current to a higher degree of oxidation though not com pletely to on account of the instability of this compound. According to Foerster (Electrical World [London] 1922, p. 27o) the potential of the fully-charged plate corresponds to that of when these fall by 0.1 volt to a value at i8° C with a 2o% potas sium hydroxide solution, of 1.36 volt, corresponding to the oxide The change occurring during the operation of the cell is thus seen to consist only of a transference of OH' from the active mass of one electrode to the other. The net effect is thus the transfer of oxygen from one plate to the other. During discharge the electrolyte becomes more concentrated at the nickel or positive plate through the liberation of hydroxyl ions and the concentra tion falls by the same amount at the negative plate through the combination of hydroxyl ions. The total change of concentration, throughout the electrolyte is consequently only that due to the in complete hydration of the According to this effect the electrolyte becomes slightly more concentrated on discharge and more dilute on charge.

Voltage, Capacity and Efficiency.

The average working voltage of this cell is 1.2 volts at the normal or 5-hour rate of discharge. The behaviour during charge or discharge is shown in fig. 26. The capacity of the cell is affected only very little by the rate of discharge and amounts to from 7.5 to 12 amp. hours per lb. of assembled battery. The energy capacity amounts to from 8.5 to 15 watt hours per lb. of cell according to the size of cell.

in colour, while the iron oxide is reduced to a chemically pure form of iron. Various equations have been proposed to represent the chemical reactions occurring during the operation of the cell. That which most probably holds is the following (change from left An output of one h.p. hour can thus be obtained for a weight of 50-60 lb. At the normal rate of charge and discharge the ampere-hour efficiency as rated by the manufacturer is 8o%, and the watt-hour efficiency 6o%.

The advantages of the Edison over the lead accumulator are its longer life, lighter weight, greater robustness against vibra tion and its immunity from harm due to standing in a discharged condition, being overdischarged, short circuited or charged in the reverse direction. (J. N. P.) BIBLIOGRAPHY.—G. Plante, Recherches sur l'electricite (1879) ; GladBibliography.—G. Plante, Recherches sur l'electricite (1879) ; Glad- stone and Tribe, Chemistry of Secondary Batteries (1884) ; E. J. Wade, Secondary Batteries (i9oi) ; F. Dolezalek, The Theory of the Lead Accumulator (1906) ; G. W. Vinal, Storage Batteries (New York, 1924) ; M. Arendt, Storage Batteries (1928).