(658a) Nano Tungsten Carbide Catalysts for Polymer Electrolyte Membrane Fuel Cells

The interstitial compounds of metal-C and metal-N, especially the early transition metal carbides have been of interest for catalysis since the pioneering work of Levy and Boudart,^[1] who showed that tungsten carbides display Pt like behavior in several catalytic reactions. The conventional synthesis of carbides is through high temperature carburization of metals, leading to a low surface area. One of the most facile routes to high-surface-area transition metal carbides can be attributed to Lee et al.,^[2]who developed a temperature-programmed reduction-carburization method that allows the formation of carbides from precursor oxides under a wide range of conditions. Unfortunately, the unstable meso-porous structure restricts the application of this material.

Here we report a simple and efficient preparation method for production of nano carbides dispersed in carbon by the combination of hydrothermal carbonization and temperature-programmed reduction-carburization. TEM and SEM confirm the presence of highly dispersed, uniform tungsten carbide nanoparticles of size around 5-10 nm. We have invented a novel composite membrane based on the proton exchange polymer that incorporates WC/C nanoparticles to provide self-hydrating functionality and reduction of gas crossover through the membrane. The nano WC/C catalyst is highly cost-effective and catalyzes H₂/O₂ to generate water during fuel cell operation. Specifically, the catalyst improves the proton conductivity of the membrane, while simultaneously reducing H₂ and O₂crossover through the membrane. The power density of the WC/C-Nafion membrane is 20% higher than that of the pure Nafion membrane. We have conducted a series of fuel cell durability tests operating at 90 °C and 35% relative humidity under open circuit voltage conditions for 100 hours. The results show that the durability of the WC/C-Nafion membrane surpasses that of both the pure recast Nafion membrane and a Pt-Nafion membrane. Using a dual-beam SEM microscope, we directly observe the tomography of fresh and used composite membranes and reveal the degradation mechanisms of the membrane.

References

[1] R. B. Levy, M. Boudart, Science 1973, 181, 547−549.

[2] J. S. Lee, S. T. Oyama, M. Boudart, J. Catal. 1987, 106, 125−133.