Zinc electrode

A zinc electrode consists of an electrode made of metallic zinc in an electrolyte . Zinc electrodes are an important part of many galvanic cells and the corresponding batteries , for example the alkaline manganese cells . In addition, zinc electrodes are required for galvanizing . Suitable zinc electrolytes are used in electroplating for this purpose. Finally, zinc electrodes are used as a sacrificial anode in corrosion protection , e.g. B. on ships or boat propellers.

DC voltage sources (galvanic cells or batteries) with zinc electrode

In galvanic cells, the zinc electrode is the active material on the negative side , which is converted to zinc salts when the cell is discharged . All electrolytes practically used in batteries were and are watery. They can be acidic, neutral or alkaline: For example, the Daniell elements used in the 19th century contained sulfuric acid as an electrolyte on the zinc electrode, while most zinc electrodes used today are immersed in an alkaline electrolyte, e.g. B. in the alkaline manganese cells .

The most important galvanic cells with zinc electrodes are:

• Cells of the type of zinc-manganese dioxide cell , d. H. those with manganese dioxide . These include the historical Leclanché element , the zinc chloride cell , which was frequently used until around 1980, and the zinc-carbon cell , and finally the modern alkaline-manganese cells .
• the mercury oxide-zinc battery with mercury oxide HgO and the Clark normal element with mercury (I) sulfate Hg ₂ SO ₄ .
• the voltaic ash column presented in London in 1800 - the first battery ever. Furthermore, other historical cells such as the Grove element , the Bunsen element with carbon electrode, the Daniell element and the Gravity-Daniell elements with copper sulfate CuSO ₄ , and the Edison-Lalande element with copper oxide CuO.
• the chromic acid element ,
• as rechargeable cells the nickel-zinc accumulator , the zinc-bromine accumulator and the silver-zinc accumulator ,
• the silver oxide-zinc battery ,
• and often as a hearing aid battery inserted zinc-air batteries .

Finishes and manufacturing

Lockwood element with a specially shaped zinc electrode

The first zinc electrodes in the voltaic column were zinc flakes. Little by little, larger foils or massive plates made of zinc were also used. In the second half of the 19th century, more complex shapes made of solid zinc were sometimes used, e.g. B. the wheel-like shape with spokes in the upper half of the cell in the picture shown. Zinc electrodes are often designed as a zinc cup, which can also serve as a container for the cell; such containers can e.g. B. can be made of zinc sheet by multi-stage deep drawing.

In many modern cells, the zinc electrodes are made from pressed zinc powder. Rechargeable zinc electrodes can be easily made by applying zinc oxide paste to a grid; the actual zinc electrode is then created during the first charge.

Amalgamation

To ensure that a battery can be stored before use, the zinc electrode must not be corroded as it does not supply any current. The easiest way to do this - as has been known since around 1840 - is to amalgamate the zinc electrode . The electrochemical reason for the prevention of corrosion lies in the high overvoltage of hydrogen on mercury and in a higher overvoltage on amalgam compared to zinc. This will get the response

${\displaystyle \mathrm {Zn+2H_{2}O\longrightarrow Zn(OH)_{2}+H_{2}} }$

so difficult that the service life of the electrode is extended. Due to the toxicity of mercury and its amalgams, the use of amalgamated zinc was increasingly restricted: In the Federal Republic of Germany, battery manufacturers and the main association of German retailers signed a voluntary self-commitment on September 9, 1988. In addition to regulating disposal, it also stipulated that the mercury content in the alkaline-manganese batteries should be below 0.1% in three stages. should be reduced - in 1988 it was still 0.8% on average. However, the commitment that came into effect on April 1, 1989 was not very effective. The Battery Ordinance came into force on March 27, 1998, the European End of Life Battery Directive in 2006 and the Battery Act in 2009 .

Zinc as a sacrificial anode

Zinc sacrificial anodes are sold in various forms made from solid zinc. For example, there are screw pins, screw-on rings or bars made of zinc as accessories for boats and their engines. Zinc is mainly used in salt water, since magnesium is even more suitable for fresh water .

Reaction at the electrode, electrode potential and capacitance

The metallic zinc of zinc electrodes is oxidized during the discharge of the corresponding cell or during galvanic zinc plating. ^i.e. converted to zinc ions (Zn ²⁺ ). These always dissolve when galvanizing; in batteries they can also be deposited on the electrode as solid zinc salts.

${\displaystyle \mathrm {Zn\longrightarrow Zn^{2+}+2\ e^{-}} }$

Oxidation of zinc

In galvanic cells, the zinc forms the negative pole of the cell, since the electron release is 'voluntary', i. H. under energy release. In galvanic zinc deposition, the reaction is forced by an external voltage, whereby the zinc electrode is connected to the positive pole of the external voltage source, so that electrons are withdrawn from the zinc. The standard potential of a zinc electrode in acidic solutions is −0.763 V compared to the standard hydrogen electrode (SHE) . In alkaline solutions, zinc hydroxide Zn (OH) ₂ or zincates are formed . According to the Pourbaix diagram of zinc , the electrode potential depends on the pH value. In acidic solutions the following applies according to the Nernst equation for the dependence on the zinc concentration c (Zn ²⁺ ): ${\displaystyle E}$

${\displaystyle E=E^{\circ }+{\frac {RT}{z_{\mathrm {e} }F}}\ln {c(\mathrm {Zn} ^{2+})}}$

At room temperature, the number of charges z _e = +2 gives the value of the electrode slope 29 mV and thus the equation

${\displaystyle E=-0{,}763\,\mathrm {V} +0{,}029\,\mathrm {V} \log {c(\mathrm {Zn} ^{2+})}}$

In neutral and alkaline solutions, zinc hydroxide is formed accordingly

${\displaystyle \mathrm {Zn+2\ H_{2}O\longrightarrow Zn(OH)_{2}+2\ H^{+}+2\ e^{-}} }$

Accordingly, the potential depends on the pH value ; at room temperature it applies to pH values ​​greater than 5.61 (with c (Zn ²⁺ ) = 1 mol / l):

${\displaystyle E=-0{,}431\,\mathrm {V} -0{,}059\,\mathrm {V} \cdot \mathrm {pH} }$

The very negative potentials of the zinc electrode explained above enable batteries to be built which deliver a relatively high voltage with an aqueous electrolyte. In addition, zinc has a high specific charge (capacity) of 820 Ah / kg and thus, in addition to the favorable potential position, also has another advantage when used in batteries.

Individual evidence

1. ↑ Lucien F. Trueb, Paul Rüetschi: Batteries and accumulators: Mobile energy sources for today and tomorrow . Springer-Verlag, Berlin Heidelberg 1998, ISBN 978-3-540-62997-9 , p. 36 , doi : 10.1007 / 978-3-642-58741-2 .
2. ^ Julie Paradis, Daniel Bélanger, Jean-Yves Huot: Batteries for the 21st Century . Ed .: Wendy R. Cieslak, KM Abraham, Electrochemical Society Battery Division, Electrochemical Society Corrosion Division, Electrochemical Society Energy Technology Division (=  Electrochemical Society Proceedings . Volume 98 , no. 15 ). The Electrochemical Society, Pennington 1999, ISBN 1-56677-208-7 , Effect of Zinc Electrode Porosity and Composition on its Electrochemical Behavior in Alkaline Solutions, pp. 84 ( limited preview in Google Book Search [accessed November 26, 2015]).
3. ↑ Joseph Daniel IVAD, Jean-Yves Huot: Proceedings of the Symposium on Rechargeable Zinc Batteries . Commemorating the 100th Birthday of AN Frumkin. Ed .: Alvin J. Salkind, Frank McLarnon, Vladimir Sergeevich Bagot︠s︡kiĭ (=  Electrochemical Society Proceedings . Volume 95 , no. 14 ). The Electrochemical Society, Pennington January 1, 1996, OCLC 35280367 , Mercury and Lead-free Rechargeable Zn / MnO ₂ Cells, p. 109–120 ( limited preview in Google Book Search [accessed November 26, 2015]).
4. ↑ Written questions, answer of Parliamentary State Secretary Grüner of September 23, 1988 (PDF) German Bundestag, p. 33
5. ↑ ^{a b} Jan Christian Fenster: Zinc corrosion in alkaline aqueous solutions . Inaugural dissertation to obtain the doctoral degree of the mathematical and natural science faculty of the Heinrich Heine University Düsseldorf. Ed .: Faculty of Mathematics and Natural Sciences at Heinrich Heine University Düsseldorf (=  scientific theses dissertation ). April 2009, DNB  994974434/34 , 2 Basics - 2.1 Pourbaix diagram, p. 3-8 .