Performance Comparison Between Iridium Oxide and Iridium-Ruthenium Oxide As Electro-Catalysts for PEM Electrolyzers

Water electrolysis is a well-established technology and one of the most widely used methods for producing high purity hydrogen. Among the different technologies available for water electrolysis, solid polymer electrolyte membrane (SPE)-based electrolyzers are one of the most promising, given their high efficiencies and suitable current density, even at low temperatures. Besides they present ecological cleanliness, considerably small mass-volume characteristics and a high degree of hydrogen purity [1-3].

The rate determining step in SPE water electrolysis is associated with the oxygen evolution reaction (OER), occurring at the anode. The most common electrocatalysts for the OER are noble metal oxides such as IrO₂, RuO₂, etc, given that exhibit high corrosion resistance and high activity for OER [4].

One of the main drawbacks of several IrO₂ preparation methods is the presence of chlorine contaminants as residues of chloride-based precursors. Chlorine species poisons the surface of the IrO₂ catalyst also causing contamination of the electrolyzer components, such as the polymer electrolyte, leading to loss of catalytic activity.

In this work, iridium oxide and iridium-ruthenium oxide  were synthesized by a sulphite complex route in order to avoid the above-mentioned problem. Membrane electrode assemblies (MEAs) were prepared with these catalysts and electrochemically tested for the oxygen evolution reaction in an SPE electrolyzer.

Iridium sodium sulfite was decomposed with H₂O₂ to form an amorphous iridium oxide; subsequently calcined at 450 ºC. In the case of iridium-ruthenium oxide, corresponding sulfites were mixed, decomposed and calcined in the same way, as previously described.

Physico-chemical characterization included X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). XRD was performed on the dry catalysts powders with a Philips X-Pert diffractometer equipped with a CuKa as radiation source. The diffraction patterns were fitted to Joint Committee on Powder Diffraction Standards (JCPDS). The morphology of the iridium and iridium-ruthenium oxide catalysts were investigated by Transmission Electron Microscopy in a Philips CM12 microscope.

For the MEA preparation, a Nafion 115 (Ion Power) membrane was used as the solid polymer electrolyte. The oxygen evolution catalysts based on iridium and iridium-ruthenium oxide were directly deposited onto one side of the Nafion 115 by a spray-coating technique. Inks were composed of aqueous dispersions of catalyst, deionized water, Nafion^Ò solution (5% Aldrich) and Ethanol (Carlo Erba); the anode catalyst loading was 2.5 mg·cm^{-2. A Ti mesh was used as backing layer. A commercial 30% Pt/Vulcan XC-72 (E-TEK, PEMEAS, Boston, USA) was used as the catalyst for the H}₂ evolution. The cathode electrode was prepared by directly mixing in an ultrasonic bath a suspension of Nafion ionomer in water with the catalyst powder. The obtained cathode paste was spread on carbon cloth backings (GDL ELAT from E-TEK) with a Pt loading of 0.6 mg·cm^-2. The ionomer content in both electrodes was 33 wt% in the catalytic layer after drying. MEAs (5 cm² geometrical area) were directly prepared in the cell housing by tightening at 9 N·m using a dynamometric wrench.

Electrochemical characterization included linear voltammetry, impedance spectroscopy and chrono-amperometric measurements. The electrochemical activity of these MEAs was analyzed in a temperature range from 25 °C to 80 °C.

The results have demonstrated promising performance and stability of these catalysts for their application in an SPE electrolyzer.

Acknowledgement

The authors acknowledge the financial support of the EU through the FCH JU ELECTROHYPEM Project. “The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2010-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement Electrohypem n. 300081”.

References

[1] Grigoriev S.A., Porembsky V.I., Fateev V.N. Pure hydrogen production by PEM electrolysis for hydrogen energy. International Journal of Hydrogen Energy 31 (2006) 171-175.

[2] Siracusano S., Baglio V., Di Blasi A., Briguglio N., Stassi A., Ornelas R., Trifoni E., Antonucci V., Arico A.S. Electrochemical characterization of single cell and short stack PEM electrolyzers based on a nanosized IrO₂

anode electrocatalyst. International Journal of Hydrogen Energy 35 (2010) 5558-5568.

[3] Slavcheva E., Radev I., Bliznakov S., Topalov G., Andreev P., Budevski E. Sputtered iridium oxide films as electrocatalysts for water splitting via PEM electrolysis. Electrochim Acta 52 (2007) 3889-3894.

[4] Hu J.M., Zhang J.Q., Cao C.N. Oxygen evolution reaction on IrO₂-based DSA-type electrodes: kinetics analysis of Tafel lines and EIS. International Journal of Hydrogen Energy 29 (2004) 791-797.