Zinc electrodes

When the reaction between zinc and copper(II) sulphate was carried out in the form of an electrochemical cell (p. 94), a potential difference between the copper and zinc electrodes was noted. This potential resulted from the differing tendencies of the two metals to form ions. An equilibrium is established when any metal is placed in a solution of its ions. 

The impressed current method with metal oxide-coated niobium anodes is usually employed for internal protection (see Section 7.2.3). In smaller tanks, galvanic anodes of zinc can also be used. Potential control should be provided to avoid unacceptably negative potentials. Pure zinc electrodes serve as monitoring and control electrodes in exposed areas which have to be anodically cleaned in the course of operation. Ag-AgCl electrodes are used to check these reference electrodes. 

To exploit the energy produced in this reaction, the half reactions are separated. The oxidation reaction is carried out at a zinc electrode (Zn Zir + 2 electrons) and the reduction reaction is carried out at a copper electrode (Cu"" + 2 electrons Cu metal). Electrons flow through a metal wire from the oxidizing electrode (anode) to the reducing electrode (cathode), creating electric current that can be harnessed, for example, to light a tungsten bulb. 

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Offshore, both Ag/AgCl and metallic zinc electrodes are used for potential measurements and are also employed for current density surveys undertaken on the offshore platforms and pipelines, as discussed below. It has been found beneficial for offshore applications to install together an electrode of each type on a structure, one acting as a function check on the other. ... 

As the above half-reactions proceed, a surplus of positive ions (Zn2+) tends to build up around the zinc electrode. The region around the copper electrode tends to become deficient in positive ions as Cu2+ ions are consumed. To maintain electrical neutrality, cations must move toward the copper cathode or, alternatively, anions must move toward the zinc anode. In practice, both migrations occur. 

The platinum electrode does not participata in the reaction. The zinc electrode does. 

When reference is made to the electrode potential of a zinc electrode, it is the e.m.f. of the cell ... 

At the zinc electrode, zinc ions pass into solution, leaving an equivalent negative charge on the metal. Copper ions are deposited at the copper electrode, rendering it positively charged. By completing the external circuit, the current (electrons) passes from the zinc to the copper. The chemical reactions in the cell are as follows ... 

A salt bridge serves as an ionconducting connection between the two half-cells. When the external circuit is closed, the oxidation reaction starts with the dissolution of the zinc electrode and the formation of zinc ions in half-cell I. In half-cell II copper ions are reduced and metallic copper is deposited. The sulfate ions remain unchanged in the aqueous solution. The overall cell reaction consists of an electron transfer between zinc and copper ions ... 

The initial voltage of an alkaline-manganese dioxide battery is about 1,5 V. Alkaline-manganese batteries use a concentrated alkaline aqueous solution (typically in the range of 30-45 % potassium hydroxide) for electrolyte. In this concentrated electrolyte, the zinc electrode reaction proceeds, but if the concentration of the alkaline solution is low, then the zinc tends to passivate. 

The zinc electrode is probably the most widely used metallic negative. The material is relatively cheap, has a good electrochemical equivalent (820 Ah/kg), and shows high open-circuit voltages (OCVs) in most systems (Table 1). 

It is so universally applied that it may be found in combination with metal oxide cathodes (e.g., HgO, AgO, NiOOH, Mn02), with catalytically active oxygen electrodes, and with inert cathodes using aqueous halide or ferricyanide solutions as active materials ("zinc-flow" or "redox" batteries). The cell (battery) sizes vary from small button cells for hearing aids or watches up to kilowatt-hour modules for electric vehicles (electrotraction). Primary and storage batteries exist in all categories except that of flow-batteries, where only storage types are found. Acidic, neutral, and alkaline electrolytes are used as well. The (simplified) half-cell reaction for the zinc electrode is the same in all electrolytes ... 

This reaction may be followed by others (complex formation and/or precipitation) which are independent of the electrode potential but determined by the nature and concentration of the electrolyte. It is impossible to discuss all the problems relating to zinc electrodes without looking at the electrolyte system and the kind of cell operation (primary or rechargeable). The only way to cover all the possible combinations is by another mode of characterization or categorization, which is used in the subsequent sections ... 

The consequences of shape change are densification and loss of electrode porosity, increased current density caused by loss of zinc surface area, and finally earlier passivation. Two different forms of pasi-vation can stop the discharge of a zinc electrode before the active material is exhausted. "Spontaneous" passivation occurs... 

Another complication had to be matched when the zinc electrode was made reversible in a battery with unstirred electrolyte or an electrolyte gel, dendritic growth of the electrolytically deposited metal takes place. The formation of dendrites cannot be fully suppressed by the use of current collectors with large surface areas (grids, wire fabrics). However, by using improved separators combined in multi layer arrangements, the danger of short-circuiting is reduced. 

The reason for this limited cycle life is the high solubility of the zinc electrode in alkaline electrolyte the zincate ions formed are deposited again during the subsequent charging in the form of dendrites, i.e., of fernlike crystals. They grow in the direction of the counterelectrode and finally cause shorts. 

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