Platinum black

Platinum black (Pt black) is a fine powder of platinum with good catalytic properties. The name of platinum black is due to its black color.

Platinum black is widely used as a thin film covering solid platinum metal, forming platinum electrodes for applications in electrochemistry. The process of covering platinum electrodes with such a layer of platinum black is called "platinization of platinum". The platinized platinum has a true surface area much higher than the geometrical surface area of the electrode and, therefore, exhibits catalytic action superior to that of shiny platinum.

Platinum black powder is used as a catalyst in proton exchange membrane fuel cells. In common practice, the 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 optimize the uniformity of the coating, electrical conductivity, and in the case of application to the membrane, to prevent dehydration of the membrane during the application.

Manufacturing of platinum black powder

Platinum black powder can be manufactured from ammonium chloroplatinate by heating at 500 °C in molten sodium nitrate for 30 minutes, followed by pouring the melt into water, boiling, washing, and reduction of the brown powder (believed to be platinum dioxide) with gaseous hydrogen to platinum black. Mills, A., "Porous Platinum Morphologies: Platinised, Sponge and Black", Platinum Metals Review, 51, 1, Jan 2007 http://www.platinummetalsreview.com/pdf/52-pmr-jan07.pdf ]

Process of platinization of platinum metal

Before platinization, the platinum surface is cleaned by immersion in aqua regia (50% solution, i.e., 3 volumes of 12 mol/kg of HCl, 1 volume of 16 mol/kg HNO₃, 4 volumes of water)D.T. Sawyer, A. Sobkowiak, J.L. Roberts, Jr., "Electrochemistry for Chemists, 2nd edition", John Wiley and Sons, Inc., 1995.] .

Platinization is often conducted from water solution of 0.072 mol/kg of chloroplatinic acid and 0.00013 mol/kg of lead acetate, at a current density of 30 mA/cm² for up to 10 minutes. The process evolves chlorine at the anode; the interaction of the chlorine with the cathode is prevented by employing a suitable separation (e.g., a glass frit).

Another author recommends electroplating with the current density of 5 mA/cm² while reversing the polarity every 30 s for 15 minutes.

After platinization, the electrode should be rinsed and stored in distilled water. The electrode loses its catalytic properties on prolonged exposure to air.

Platinum metal sponge

Platinum sponge is yet another form of platinum metal with a developed surface area, distinct from platinum black and platinized platinum. Platinum sponge can be obtained by strongly heating ammonium chloroplatinate. It has a gray to black colour, while its catalytic properties vary depending on the the specifics of the manufacturing.

Potential of platinized platinum versus shiny platinum

In hydrogen saturated hydrochloric acid, the shiny platinum electrode is observed to assume positive potential versus that of platinum black at zero net current (+ 340 mV at room temperature). With the temperature increasing to 70 °C, the difference in potentials dropped to zero. [D.J.Ives, G.J. Janz, "Reference Electrodes, Theory and Practice", Academic Press, 1961, p.88.] The reason for this is not perfectly clear, although several explanations have been proposed.

ee also

*Standard hydrogen electrode
*Overpotential

References