Electrocatalyst for Direct Alcohol Fuel Cells

Direct alcohol fuel cells (DAFC) offer a number of advantages compared to other fuel cell solutions or batteries. So the liquid fuel allows for easy handling and high energy storage densities. At the same time, the system design can be kept simple and thus offers a certain robustness. Direct methanol fuel cells converting (DMFC) using low temperature proton exchange membrane technology have achieved a high level of maturity and have already entered some special markets like battery chargers for portable applications and recreational vehicles, back-up power systems or material handling vehicles. Two facts are currently hindering a wider use DMFC, high costs due to the high usage of platinum based catalysts and the reluctance of many potential users to employ methanol as fuel. In this contribution catalyst developments with respect to two different concepts to address these issues will be presented. The first is the use of alkaline anion exchange membrane fuel cells in order to avoid or reduce the use of platinum. The second concept is the use of ethanol as alternative non-toxic fuel with even higher energy density.

Because of the less corrosive effect of the alkaline environment less noble metals can be used as catalysts. In particular palladium has shown high activity for the oxidation of alcohols under these conditions [1]. In order to further enhance both, the activity of the catalysts and the CO₂ current efficiencies Pd binary [2] and ternary alloys were investigated.  As will be shown, the addition of nickel or silver as second metal is able to enhance the catalytic activity of palladium for the methanol as well as the ethylene glycol oxidation reaction. Thereby, the addition of silver also enhances the CCE of the methanol oxidation reaction, whereas the addition of nickel enhances the CCE of the ethylene glycol oxidation reaction. In both cases the addition of tin or ruthenium as third element to form ternary catalysts causes a further significant increase of the performance; so the combination of palladium and nickel with either tin or ruthenium leads to high CCEs and high faradic currents for the ethylene glycol oxidation at significantly lower potentials.

Fig. 1: CCE of the methanol oxidation at Pd_xM/C  catalysts (upper) and the ethylene glycol oxidation at Pd_xM_yNi_y/C catalysts (lower).

As has been reported, the alkaline environment is not suitable to enhance the complete oxidation of ethanol. In contrary, the high availability of OH fosters the formation of acetic acid or acetate which are dead end products. In order to enhance the catalytic activity of platinum based catalysts in an acidic environment the addition of tin is known to be useful [3]. However, no positive effect with respect to the complete oxidation of ethanol has been reported so far. On the other hand side positive results on the addition of rhodium on the CCE of the ethanol oxidation have been reported by different groups [4, 5]. In this contribution results of a study of the effect of rhodium and/or tin addition to platinum electrodes on the ethanol oxidation in acidic environment at ambient temperature will be reported. It will be shown that the addition of either metal causes an enhancement of the catalytic activity. Furthermore it will be shown that the addition of rhodium is able to increase the CCE of the ethanol oxidation even at ambient temperature. Using a recently developed set-up for DEMS like measurements under HT-PEMFC relevant conditions [6], the ethanol oxidation at Pt/C and PtRh/C catalysts was also studied in the vapour phase at 150 °C. It will be shown that CCE under these conditions is much higher exceeding 65% for both catalysts. The particular effect of the addition of rhodium under these conditions was found to be that it mitigates the self-poisoning of the catalysts by ethanol which allows for achieving high CCEs also for technical relevant ethanol concentration of 5 mol l^-1.

[1] D. Bayer, C. Cremers, H. Baltruschat, J. Tubke, ECS Transactions, 41 (2011) 1669-1680.

[2] T. Jurzinsky, C. Cremers, K. Pinkwart, J. Tübke, ECS Transactions, 58 (2013) 633-636.

[3] C. Lamy, S. Rousseau, E.M. Belgsir, C. Coutanceau, J.M. Leger, Proc. 54th Annual ISE Meeting, Sao Pedro, BRAZIL, Sep 01-05, 2003.

[4] F.H.B. Lima, E.R. Gonzalez, Electrochimica Acta, 53 (2008) 2963-2971.

[5] A. Kowal, M. Li, M. Shao, K. Sasaki, M.B. Vukmirovic, J. Zhang, N.S. Marinkovic, P. Liu, A.I. Frenkel, R.R. Adzic, Nature Materials, 8 (2009) 325-330.

[6] C. Niether, M.S. Rau, C. Cremers, D.J. Jones, K. Pinkwart, J. Tübke, Journal of Electroanalytical Chemistry, 747 (2015) 97-103.