Catalysis of the Oxygen Reduction Reaction: Thin Layer Sonoelectrochemistry

The oxygen reduction reaction (ORR) is a multi-electron and -proton transfer mechanism. In acidic solution, the slow step is typically taken as the first electron transfer.

 O₂ + e + H⁺ ⇌HO₂

Even with platinum electrocatalysts, ORR kinetics impose parasitic losses of ~30 % in proton exchange membrane (PEM) H₂|O₂ fuel cells. Because of poor O₂ reduction kinetics, H₂is generated by steam reforming of natural gas rather than by water electrolysis.

Typically, catalysis is achieved by chemical manipulation of the catalyst materials. Physical means, such as increased temperature and pressure, can also increase rates.  One means to increase temperature and pressure is irradiation of a fluid with ultrasound so as to induce cavitation.  Acoustic energy is carried by a wave that first compresses the fluid, then rarefies the fluid to form microscopic voids, and then compresses the fluid to collapse the voids. On collapse at the thin interface between the fluid and void, extreme temperature (T > 5000 K) and pressure (P > 10³ atm) are generated.¹

Ultrasound has been applied to electrochemical systems with the first studies undertaken by Coury, et. al.² In these studies, cavitation was generated in a bulk electrolyte where electrodes were placed at a distance from a sonic horn or other ultrasound source. Large transport enhancements were observed, but effects on heterogeneous reactions were not observed. Bubbles introduced noise in the voltammetry and the distance between the ultrasound source and the electrodes allowed energy to dissipate. See Scheme 1.

Here, the ORR is investigated by sonoelectrochemistry undertaken in a thin layer electrochemical cell,^3,4where:

• Acoustic energy attenuates less in the thin layer and so impacts the electrode processes..
• Acoustic energy is reflected back into the condensed phases rather than pass into the less dense air phase.
• Frank cavitation is not observed and not necessary to the enhanced rate of the ORR.
• ORR rate is increased to the mass transport limit.

As shown in Figure 1, on sonication in a thin layer, O₂reduction proceeds at the mass transport limited rate. The experimental set up is shown in Figure 2.

In general, sonication in a thin layer increases interfacial rates without significant impact on transport^3,4 and provides a general, physical means to effect catalysis.

References and Acknowledgments: Support of the National Science Foundation and the University of Iowa is gratefully acknowledged.

[1] K.S. Suslick, Y. Didenko, M.M.  Fang, T. Hyeon, K.J. Kolbeck, M.M. Mdleleni, and M. Wong, Acoustic Cavitation, Phil. Trans. Royal Soc. A 1999.

[2] C.R.S Hagan R. S. and L.A. Coury Jr. Anal. Chem., 1994, 66(3), 399–405.

[3] Chester G. Duda, Ph.D. dissertation, University of Iowa, 2012.

[4] J. Leddy, C.G. Duda, and W.J. Leddy III, U.S. Patent application, 2013.