Platinum black is a fine powder of platinum

Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 with good catalytic

Heterogeneous catalysis

In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, but also immiscible liquids, e.g. oil and water. The great majority of practical heterogeneous catalysts...

 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 electrode

Electrode

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

s for applications in electrochemistry

Electrochemistry

Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...

. 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 cell

Proton exchange membrane fuel cell

Proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells , are a type of fuel cell being developed for transport applications as well as for stationary fuel cell applications and portable fuel cell applications. Their distinguishing features include lower...

s. 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.

Process of platinization of platinum metal

Before platinization, the platinum surface is cleaned by immersion in aqua regia

Aqua regia

Aqua regia or aqua regis is a highly corrosive mixture of acids, fuming yellow or red solution, also called nitro-hydrochloric acid. The mixture is formed by freshly mixing concentrated nitric acid and hydrochloric acid, usually in a volume ratio of 1:3, respectively...

 (50% solution, i.e., 3 volumes of 12 mol/kg of HCl, 1 volume of 16 mol/kg HNO₃, 4 volumes of water).

Platinization is often conducted from water solution of 0.072 mol/kg of chloroplatinic acid and 0.00013 mol/kg of lead acetate

Lead acetate

Lead acetate can refer to:* Lead acetate , Pb4* Lead acetate , Pb2...

, 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 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. The reason for this is not perfectly clear, although several explanations have been proposed.