Platinum black

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Platinum black, also known as platinum bog or platinum black, is a fine powder made from elemental platinum with good heterogeneous catalytic properties. The name of the platinum black is historical and was chosen because of its black color.

Platinum black is often used as a thin film for solid platinum metal, such as platinum electrodes for electrochemical applications . The process of coating platinum electrodes with such a layer of platinum black is called platinizing of platinum . The platinum-plated platinum has a catalytically active surface that is significantly larger than the geometric area of ​​the electrode. In addition, compared to smooth platinum, there is a lower overvoltage with regard to hydrogen and oxygen on platinum-plated platinum electrodes.

Platinum black is used as a catalyst in fuel cells . For this purpose, platinum black is either sprayed or hot pressed onto the membrane or gas diffusion layer. A suspension of platinum black and carbon powder in ethanol- water solutions serves to avoid the uniformity of the coating, the electrical conductivity and, in the case of application to the membrane, the membrane from drying out during application.

Production of platinum black powder

Platinum black powder is made from ammonium chloroplatinate by heating it to 500 ° C in molten sodium nitrate for 30 minutes, quenching the melt in water, then boiling, washing and finally reducing the brown powder, probably platinum dioxide , with hydrogen to platinum black. Synthesis by reduction of platinum chloride solution with formaldehyde and sodium hydroxide solution is also possible, with phosphorus (V) oxide being washed out and the precipitate being vacuum-dried.

Platinum plating of platinum metal

Platinizing electrodes increases their surface area. This creates the prerequisites for a precision measurement of the ohmic resistance of an electrolyte, since an increase in the double-layer capacitance goes hand in hand with the enlargement of the electrode surface. This reduces capacitance effects when measuring with alternating voltage. Before platinum plating , the platinum surface is cleaned by immersing it in aqua regia . Platinization is carried out by electrochemical reduction of hexachloridoplatinic acid in aqueous solution and a little lead (II) acetate at a current density of about 30 mA / cm 2 for up to 10 minutes. During the process, chlorine develops on the anode. After platinization, the electrode is rinsed and stored in distilled water. The electrode loses its catalytic properties on prolonged exposure to air. Hamann describes the platinizing process as

"Precipitation of platinum in finely divided form on a metal surface"

- CH Hamann, W. Vielstich : Elektrochemie

This is based on 10 percent hexachloridoplatinic acid and electrolysis currents around 10 mA / cm². Under these circumstances, the surface of the electrode to be platinized can be increased by a factor of 1000.

Individual evidence

  1. ↑ a b Platinschwarz - Lexicon of Chemistry. In: Spektrum.de. Spektrum Akademischer Verlag, Heidelberg, 1998, accessed on January 5, 2020 .
  2. ↑ a b c D. A. Skoog, JJ Leary: Instrumental Analytics . Springer Verlag, 1996, ISBN 3-540-60450-2 , pp. 504 .
  3. ^ A. Mills: "Porous Platinum Morphologies: Platinized, Sponge and Black" , Platinum Metals Review , 51, 1, Jan 2007 (pdf; 410 kB)
  4. ↑ a b C.H. Hamann, W. Vielstich: Elektrochemie . 4th edition. Wiley-VCH, Weinheim, 2005, ISBN 978-3-527-31068-5 , pp. 15 .