Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1980-05-01
1983-09-06
Edmundson, F.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204290F, 204293, 2524253, C25B 1106
Patent
active
044028156
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of preparing active electrodes and in particular to such electrodes having improved efficiency and/or stability and the use thereof in electrochemical cells.
An electrochemical cell is a device which has as basic components at least one anode and one cathode and an electrolyte. The cell may use electrical energy to achieve a chemical reaction such as the oxidation or reduction of a chemical compound as in an electrolytic cell. Alternatively, it can convert inherent chemical energy in a conventional fuel into low voltage direct current electrical energy as in a fuel cell. The electrodes, particularly the cathode, in such a cell may be of relatively inexpensive material such as iron or nickel. However, electrodes of such material tend to have low activity. These problems may be overcome to a degree by using electrodes made with active precious metals such as platinum. These precious metals may be used as catalytic coatings on the surface of an electrode core of inexpensive material. Such catalyst coatings are termed electrocatalysts. The level of precious metal required for high activity and stability generally leads to high costs.
The above problems are particularly acute in electrochemical cells having a hydrogen electrode. Such electrochemical cells are used for several purposes such as for example, the electrolysis of water to produce hydrogen and oxygen, in chlorine cells in which brine is electrolysed and in fuel cells which generate power by the oxidation of hydrogen. Of these processes, the electrolysis of water is used on an industrial scale for producing high purity hydrogen.
In the case of the production of hydrogen and oxygen by the electrolysis of water, water is decomposed into its elements when a current, eg a direct current, is passed between a pair of electrodes immersed in a suitable aqueous electrolyte. In order to obtain the gases evolved in a pure and safe condition, an ion-permeable membrane or diaphragm is placed between the electrodes to prevent the gases mixing. The basic elements of this cell are thus two electrodes, a diaphragm and a suitable electrolyte which is preferably an alkaline electrolyte such as an aqueous solution of sodium hydroxide or potassium hydroxide due to their high conductivity and relatively low corrosivity.
In this case, the voltage, V, applied across the electrodes can be divided into three components, the decomposition voltage of water, E.sub.d, the overvoltage at the electrodes, E.sub.o, and the Ohmic loss in the inter-electrode gap which is the product of the cell current, I, and the electrical resistance (including the membrane resistance) of this gap, R.
Thus V=E.sub.d +E.sub.o +IR.
At 25.degree. C. and at a pressure of one atmosphere, the reversible decomposition voltage of water is 1.23 V. However, in practice cells operate at voltages of 1.8 to 2.2 V, as a result, inter alia, of activation overvoltage.
Activation overvoltage results from the slowness of the reactions at the electrode surface and varies with the metal of the electrode and its surface condition. It may be reduced by operating at elevated temperatures and/or by using improved electrocatalysts, but increases with the current density of the electrode reaction. The use of cathodes containing precious metal electrocatalysts such as platinum, for example, does achieve a reduction in activation overvoltage. However, the technical advantage to be obtained by the use of such precious metal electrocatalysts is substantially offset by the expense. The use of mixed cobalt/molybdenum oxide as electrocatalyst has also been suggested. Such an electrode can be made by painting a nickel gauze with a mixed cobalt/molybdenum oxide electrocatalyst bonded with polytetrafluoroethylene (PTFE) followed by curing under hydrogen at 300.degree. C. for 2 hours, which initially has an electrode potential, versus a reversible hydrogen electrode (RHE), of -182 mV at a current of 1000 mA/cm.sup.2 and 70.degree. C. The curing temperature is normally maintained at or below 300.d
REFERENCES:
patent: 3291714 (1966-12-01), Hall et al.
patent: 3759842 (1973-09-01), Bianchi et al.
patent: 3833357 (1974-09-01), Bianchi et al.
patent: 4152240 (1979-05-01), Kuo
Brown David E.
Mahmood Mahmood N.
Turner Alan K.
Wood Dermott
Edmundson F.
The British Petroleum Company p.l.c.
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