Proton exchange membrane (PEM) water electrolysis represents one of numerous environmentally friendly but at present economically unattractive technologies. Iridium is commonly used as oxygen evolution catalyst (OER) catalyst [1], but is highly priced and one of the rarest elements in the earth crust. Several approaches have been proposed to overcome the challenges associated with the precious metal OER catalyst such as reducing the particle size thus increasing the surface to mass ratio [2], using electroceramic supports [3] or enhancing the phase structure [4]. We present a variety of highly active nanostructured iridium based catalysts for anodes of proton exchange membrane (PEM) electrolysis: IrO_x-Ir, IrPt, IrRuO_x, IrV, and Ir/Ti₄O₇. Clusters of nanosized crystallites, Fig. 1a, are obtained by reducing surfactant stabilized IrCl₃ in water free conditions [5]. The catalysts shows up to five-fold higher activity towards OER than commercial Ir-black, Fig 1c. The improved kinetics of the catalysts are reflected in the high performance of the PEM electrolyzer (1 mg_Ir cm^-2), showing an unparalleled low over-potential and so far negligible degradation. Our results demonstrate that this enhancement cannot be only attributed to increased surface area but rather to the ligand effect and low coordinate sites resulting in a high turnover frequency (TOF). The catalysts developed herein set a benchmark and pave a strategy for the development of ultra-low loading catalyst layers for PEM electrolysis.

References

[1]         M. Carmo, D.L. Fritz, J. Mergel, D. Stolten, Int. J. Hydrogen Energy 38 (2013) 4901.

[2]         E. a. Paoli, F. Masini, R. Frydendal, D. Deiana, C. Schlaup, M. Malizia, T.W. Hansen, S. Horch, I.E.L. Stephens, I. Chorkendorff, Chem. Sci. 6 (2015) 190.

[3]         H.-S. Oh, H.N. Nong, T. Reier, M. Gliech, P. Strasser, Chem. Sci. 00 (2015) 1.

[4]         M.S. Smith, Rodney D. L. Prévot, R.D. Fagan, Z. Zhang, P.A. Sedach, M.K.J. Siu, S. Trudel, C.P. Berlinguette, Sci. Express (2015).

[5]         P. Lettenmeier, L. Wang, U. Golla-Schindler, P. Gazdzicki, N. Cañas, M. Handl, R. Hiesgen, S.S. Hosseiny, A.S. Gago, A.K. Friedrich, Angew. Chemie (2015) In press.