Electrochemical Oxidation of Borohydride for Direct Fuel Cells

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.