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Electrochemical borohydride oxidation on nanodispersed ruthenium supported on high surface area carbon (Ru/C) is investigated by means of cyclic voltammetry, CO-stripping, rotating disk electrode experiments and chronoamperometry as well as direct borohydride fuel cell tests in a mixed electrolyte approach. All experimental results of Ru/C are compared to a commercially available Pt/C catalyst. Ru/C shows high Coulombic efficiency with a complete eight electron transfer at low anodic potentials. The electrochemical active potential window is −0.05–0.30 V vs RHE with higher activity compared to Pt/C in diffusion limited regions. In situ fuel cell tests show superior behavior in current density areas below 20 mA cm–2.

Electrochemical borohydride oxidation reaction, intermediates and reaction products are studied by cyclic voltammetry with rotating disk electrodes, chronoamperometry coupled with 11B NMR spectroscopy and in-situ FTIR measurements. Depending on the electrode potential of a Pd/C catalyst on a rotating disk electrode the number of exchanged electrons varies between four and eight. By employing chronoamperometry the boron species are analyzed during the ongoing reaction with NMR. During all experiments no boron products other than BO2− are detected. Incomplete borohydride consumption during the chronoamperometry studies show that the palladium surface is poisoned by an intermediate in the low potential region. In-situ FTIR measurements confirmed that there is no interaction with BH2-species which are acting as catalyst poison.

The Alkaline Fuel Cells (AFC) have been well developed in the 1960s for on-board power supply for Apollo missions to the moon. Later, in 1990s, the Polymer Electrolyte Membrane Fuel Cell (PEMFC) replaced the AFC due to their advantages such as a lighter and more compact system attributed to solid membranes as electrolyte. However, AFC systems have now reached new attention due to the necessity of using inexpensive non-precious metal catalysts, which is one of the greatest advantages of alkaline media. In this work, precious metal-free oxide catalysts are investigated regarding their performance and long-term stability. Electrodes containing these precious metal-free catalysts are designed, manufactured and in-situ characterized in AFC half-cell tests and single-cell measurements. The main focus lies on increasing the long-term stability of cathodes in alkaline media. Cathodes are manufactured and tested in half-cells for 10,000 h. Tests are performed at different operating conditions: gases (O2, synthetic air, and ambient air), gas flow and temperature are varied. The influence of the operating conditions on the degradation rate and the stability of the cathodes are evaluated accordingly. Furthermore, single cell tests are conducted with non-noble metal anodes, thereby achieving a precious metal-free AFC. Especially the activation processes are investigated. In order to preserve cathode performance, the anode activation is performed separately. Afterwards, the two electrodes are assembled in a custom-made test cell and characterized in fuel cell operation. For comparison of the performance of this precious metal-free AFC, single cells with platinum based catalysts are operated in parallel test cells. The results of the test series will be presented at the meeting.

The direct borohydride fuel cell (DBFC) is gaining more and more interest in recent years because of high energy density and the easy handling of liquid fuels. The oxidation of the borohydride anions at the anode delivers up to eight electrons per molecule at a theoretical potential of -1.24 V vs. SHE [1,2]: BH4 - + 8OH- → BO2 - + 6H2O + 8e- In combination with an oxygen cathode and an alkaline membrane or electrolyte an electrochemical cell with a nominal voltage of 1.64 V is formed. The electrochemical oxidation is investigated on various noble metal catalysts such as Pt/C, Au/C and Pd/C and bimetallic alloys thereof with first row transition metals. The electrochemical properties of the catalysts are characterized by cyclic voltammetry in alkaline electrolyte containing NaBH4 in standard three electrode configuration with a rotating disc electrode (RDE) [3]. Figure 1 : Cyclic voltammograms of Pt/C with 5mM NaBH4 on a RDE, anodic sweeps at 200, 400, 600, 900, 1200, 1600 and 2000 rpm. The borohydride oxidation on Pt/C starts at around -50 mV vs. RHE getting diffusion limited at 200 mV vs. RHE with a complete eight electron transfer. Due to diffusion limitation data processing according to Levich and Koutecky-Levich is possible. Figure 2 : Levich plot of Pt/C catalyst at 0.50 V vs. RHE (data from Fig. 1) Figure 3 : Koutecky-Levich plot of Pt/C catalyst at 0.50 V vs. RHE (data from Fig. 1) Levich and Koutecky-Levich (Fig. 2 and 3) analysis show an eight electron transfer reaction at a potential of 0.5 V vs. RHE in the diffusion controlled region and as well at 0.01 V vs. RHE in the kinetic controlled region. Although platinum shows excellent catalytic activity towards borohydride electrooxidation, the main side reaction, namely the hydrolysis reaction is also catalyzed. Besides the loss of fuel, the formation of gaseous hydrogen can harm the electrode due to mechanical stress. This issue can be addressed by replacing platinum with less active catalysts. Gold based electrocatalysts for example do not catalyze the hydrolysis side reaction. On the roadway towards a practicable application it is necessary to improve the selectivity at the anode catalyst system. Acknowledgements Support by NAWI Graz and financial support by the Austrian Federal Ministry of Transport, Innovation and Technology (BMVIT) and The Austrian Research Promotion Agency (FFG) through the program a3plus and the IEA research cooperation is gratefully acknowledged. References [1] E. Gyenge, Electrochim. Acta 49 (2004) 965. [2] D. Finkelstein, N. Mota, J. Phys. Chem. C 113 (2009) 19700. [3] M. Chatenet, M.B. Molina-Concha, N. El-Kissi, G. Parrour, J.-P. Diard, Electrochim. Acta 54 (2009) 4426. Figure 1

The direct borohydride fuel cell (DBFC) is one of several promising direct liquid fuel cells due to easy fuel handling and high energy storage density [1]. DBFCs are still in an early stage of development and further improvements of anode catalyst materials and MEA design are necessary for widespread application. Herein we present an extensive study of Palladium based catalysts for the anode reaction of DBFCs (see reaction 1) including several electrochemical experiments supported by 11B-NMR and FTIR measurements. BH4 - + 8OH- → BO2 - + 6H2O + 8e- (1) (E0 = -1.24 V vs. SHE) Palladium based electrocatalysts nanodispersed on high surface area carbon are prepared by reduction of precursor salts in an aqueous dispersion of the support material. The catalyst is characterized by transition electron microscopy (TEM). Rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) experiments with subsequent analysis of reaction products by 11B-NMR are conducted. Fig. 1 shows a typical result of Pd/C on a RDE with a borohydride concentration of 5 mM. Additionally mechanistic aspects on Palladium are investigated employing in-situ FTIR spectroscopy [2]. By combining all results obtained from the experiments mentioned above a substantial proposition of ongoing mechanisms of BH4 --electrooxidation on Palladium is presented. [1] J. Ma, N. a. Choudhury, Y. Sahai, Renew. Sustain. Energy Rev. 14 (2010) 183. [2] B.M. Concha, M. Chatenet, C. Coutanceau, F. Hahn, Electrochem. Commun. 11 (2009) 223. Caption Figure 1: RDE experiments of Pd/C catalyst at rotation rates of 0-2000 rpm (anodic sweeps) in 1M NaOH with 5 mM NaBH4, insert: according Levich-plot at 0.830 V. Figure 1