Platinum black
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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 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.
Additionally Platinum black is used in Proton Exchange Membrane fuel cells as a catalyst. In common practice the Platinum black is either sprayed on or hot pressed onto the membrane or gas diffusion layer. A solution of alcohol, water, Platinum black and carbon powder 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.
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[edit] Process of platinization
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)[1].
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)[1].
After platinization, the electrode should be rinsed and stored in distilled water. The electrode loses its catalytic properties on prolonged exposure to air.
[edit] Potential of Pt black 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. [2] The reason for this is not perfectly clear, although several explanations have been proposed.
