Pd-Based Bimetallic and Trimetallic Catalyst for Direct Formic Acid Fuel Cells

Direct formic acid fuel cell (DFAFC) has generated great attraction in the field of energy systems, especially when it combines with carbon dioxide (CO₂) electrochemical reduction unit. This whole system is referred as regenerative direct formic acid fuel cell. In regenerative DFAFCs, formic acid is oxidized at the anode of fuel cell to produce electricity along with CO₂ and water (H₂O). These CO₂ and H₂O then can be reduced to formic acid through an electrochemical reduction process using renewable energy sources, such as solar energy or wind energy. Therefore, the CO₂-derived fuels hold interesting prospects for future energy systems based on non-fossil energy sources. In the regenerative DFAFC system, formic acid is used as the energy carrier and it is important to efficiently oxidize formic acid at the fuel cell’s anode to achieve a high system performance. Among the anode catalysts, Pd-based bimetallic or trimetallic catalysts have attracted a lot attention due to their better activity and stability than pure Pd toward formic acid oxidation [1-3]. In addition, the lower cost due to the use of less expensive transition metals makes these Pd-based bimetallic catalysts promising material for developing cheaper DFAFC.

In this study, we synthesize PdCu/C and PdNi/C alloys via metal salt reduction method. The nominal atomic ratio of Pd:Cu and Pd:Ni is 3:1. TEM images shows the particle sizes are about 5 nm, 7 nm, and 6 nm for Pd/C, PdCu/C and PdNi/C, respectively. Then, PdCu/C and PdNi/C are physically mixed by weight ratio of 1:1 and annealed at 400 °C in Ar. To make comparison, PdCu/C and PdNi/C are also annealed at the same condition separately. Cyclic voltammetry results indicate that, before annealing, PdCu/C shows the best activity toward formic acid oxidation (See Figure 1 (a)). When the samples are annealed at 400 °C, both the PdCu/C and physical mixture samples show the activity enhancements, while the activity of PdNi/C sample is not affected by the annealing process, as shown in Figure 1 (b). Based on the CV data, it is clear that the physical mixture sample is most affected by the annealing process as it shows the most activity enhancement. Consequently, after the annealing process, the physical mixture sample shows the best activity. For an example, at 0.1V (vs. Ag/AgCl), the annealed physical mixture sample shows the current density of 2.71 mA/cm², which is a factor of 1.5 higher than that of annealed PdCu/C sample. To investigate the effect of the annealing process to different samples and their electrochemical activity toward the formic acid oxidation, TEM and X-ray photoelectron spectroscopy are used.

References

1. Suo, Y. G., Hsing, I. M., Electrochimica Acta, 2011, 56 (5): p. 2174-2183.

2. Dai, L., Zou, S. Z., Journal of Power Sources, 2011, 196 (22): p. 9369-9372.

3. Du, C. Y., Chen, M., Wang, W. G., Yin, G. P., Acs Applied Materials & Interfaces, 2011, 3 (2): p. 105-109.