Sonoelectrochemical Production of Nanosized Fuel Cell Electrocatalysts

For the first time, this study describes the sonoelectrochemical synthesis of Pt nanoparticles from solutions containing Pt salt, ionomer, solvent and water in the presence of ultrasound (20kHz, ~100W.cm^-2) using a specially designed/constructed Glassy Carbon (GC) electrode acting as a Vibrating Working Electrode (VWE) [Fig.1]. From early studies, it was found that Pt mean nanoparticle sizes could be produced in the range [11 - 15nm] in the absence of surfactants and solvents [1,2]. In this investigation (undertook at the Birmingham Centre for Hydrogen and Fuel Cell Research laboratories, UK), the syntheses of Pt nanoparticles were performed galvanostatically at (298±1)K, at various ultrasonic and current pulses using several time managements and at a high cathode efficiency.  Physical characterisations by TEM, SEM, XRD and SAED showed that Pt mean grain sizes and cluster sizes of up to ~20nm and ~100nn were obtained respectively. Non-ultrasonicated (silent) but high-shear mixed catalyst inks made of Pt/C(Vulcan XC-72R)/Nafion® were prepared using the method described by Curnick et al. [3,4]. Electrochemical characterisation (Cyclic and RDE voltammetry) data of the as-prepared catalyst inks showed low ECSA values (<~35 m².g_Pt^-1). This finding was mainly attributed to: (i) the fairly ‘large’ Pt nanoparticle/cluster sizes, (ii) the ionomer acting as a ‘surfactant’ encapsulating the Pt nanoparticles thus preventing local electron transfer and (iii) poor dispersion of Pt nanoparticles/homogeneisation of the ink caused by the high-shear mixer (19,000rpm).  However it was observed that the ionomer prevented Pt nanoparticles aggregation. It was suggested that the Nafion® content may play an important role on the activity of the Pt nanoparticles as previously observed [3,4]. It also possible that ultrasonication affects the composition and morphology of the electroanalyte/Pt nanoparticles/ionomer, mainly due to cavitation and sonolysis phenomena as recently shown by Pollet and Goh [5].

References

[1] V. Zin, B.G. Pollet, M. Dabala, Sonoelectrochemical (20 kHz) Production of Platinum Nanoparticles from Aqueous Solutions, Electrochem. Acta 54 (2009) 7201-7206

[2] B.G. Pollet, The Use of Ultrasound for the Fabrication of Fuel Cell Materials, Int. J. Hydrogen Energy 35(21) (2010) 11986-12004

[3] O.J. Curnick, P.M. Mendes, B.G. Pollet, Enhanced Durability of a Pt/C Electrocatalyst Derived from Nafion®-Stabilised Colloidal Platinum Nanoparticles, Electrochem. Comm. 12(8) (2010) 1017-20

[4] O.J. Curnick, B.G. Pollet, P.M. Mendes, Nafion®-stabilised Pt/C Electrocatalysts with Efficient Catalyst Layer Ionomer Distribution for Proton Exchange Membrane Fuel Cells, RSC Advances 2 (22), 8368-8374

[5] B.G. Pollet, J.T.E Goh, The Importance of Ultrasonic Parameters in the Preparation of Fuel Cell Catalyst Inks, Electrochim. Acta, in press