Electrodes and Sensor Technology

Electrodes and Sensor Technology

Electrochemical processes are chemical methods using electric current. Today they are utilized in the chemical industries for large-scale syntheses, and also for advanced power technologies, as well as for ordinary sources of current. When consuming energy to enforce a reaction, electrochemical processes need an external power source: In the electrolysis electrodes mediate the current flow through the - mostly liquid - reaction medium. Due to their corrosion resistance and their electrical properties, the platinum group metals (PGM's); platinum, iridium, ruthenium, and also silver, play a very important part in the manufacture of electrodes on a large scale. Heraeus manufactures suitable compounds and solutions as core products, which serve to coat or to produce such electrodes. Moreover, Heraeus produces dispersed metals (“Blacks”) for coating of electrochemical sensors.

Anodes coated with oxides of iridium and of ruthenium are used for industrial processes in very aggressive media and for strongly oxidating products like the chlorine-alkaline electrolysis. These anodes are usually called Dimensionally Stable Anodes (DSA). This coating improves the electrical properties of the anodes, and makes them resistant to passivation and abrasion.

For coating, precious metal compounds are mostly applied in the form of their solutions. They are decomposed pyrolytically, and so the metals are strongly connected with the surface material.

Compound CAS Number Formula Metal Cont. approx. Color
Ruthenium(III) chlorid hydrate 14898-67-0 RuCl3 · n H2O
40 % brownish black
Ruthenium(III) chloride solution 14898-67-0 RuCl3 up to 20 % brown
Trinitratonitrosylruthenium(II) 34513-98-9 [Ru(NO3)3(NO)] 30 % reddish brown
Trinitratonitrosylruthenium(II) solution 34513-98-9 [Ru(NO3)3(NO)] up to 15 % claret
Dihydrogen hexachloroiridate(IV) hydrate 16941-92-7 H2[IrCl6] · n H2O 43 % black
Dihydrogen hexachloroiridate(IV) solution, “Chloroiridic Acid”; “CIA” 16941-92-7 H2[IrCl6] up to 25 % brown
Iridium(III) chloride hydrate 14996-61-3 IrCl3 · n H2O 54 % black
Iridium(III) chloride hydrate, Type G 14996-61-3 IrCl3 · n H2O 54 % green
Iridium(IV) chloride hydrate 10025-97-5 IrCl4 · n H2O 53 % black
Iridium(III) chloride solution 10025-83-9 IrCl3 up to 10 % dark brown

Fuel cells generate electric power by the direct and controlled chemical conversion of high-energy reactants. For example, industry is striving for the controlled reaction of the "fuel" hydrogen H2 with oxygen O2 to water H2O. Therefore, fuel cells are of particular interest to automotive and communication technologies and to hermetically sealed systems like spacecrafts.

For some fuel cells, platinum crystallites and alloys, which are deposited on e.g. carbon modifications, serve as catalytically active electrodes. Other PGM's are being tested, too.

Those Pt-based materials also used in polymer electrolyte membrane (PEM) electrolysis for the generation of H2, the energy source of the future. Moreover, Iridium-based materials are simultaneously applied for the necessary water splitting into O2.

Compound CAS Number Formula Metal Cont. approx. Color
Dihydrogen hexachloroplatinate(IV) hydrate “Chloroplatinic Acid“, “CPA” 26023-84-7 H2[PtCl6] · n H2O 40 % orange
Platinum black 7440-06-4 Pt 90-100 % black
Pt nanoparticles on Carbon 7440-06-4 Pt/C 10-40 % black
Iridium black 7439-88-5 Ir 98 % black
Iridium(IV)oxide 12030-49-8 IrO2 85-86 % black
Iridium-Ruthenium-Oxides IrxRu1-xO2 (0 < x < 1) 76-86 % black
Ruthenium(III) chlorid hydrate 14898-67-0 RuCl3 · n H2O 40 % brownish black
Platinum oxalate blue

Electrochemical sensors serve to detect toxic gases (e.g. CO). For this application various carriers are coated with precious metals.

CAS Number Formula Metal Cont. approx. Color
Platinum black, Pt 7440-06-4 Pt 90-100 % black
Iridium black 7439-88-5 Ir 98 % black
Platinum/Ruthenium black 7440-06-4 /7440-18-8 Pt/Ru 25/75 % black
Gold black 7440-57-5 Au 97 % brown