Development of Platinum-Niobium as a CO Tolerant Catalyst for PEFC Operating with CO Contaminated Hydrogen

Traditional Polymer Electrolyte Fuel Cells (PEFC) are fed with hydrogen in the anode and oxygen in the cathode. This type of fuel cell is especially attractive because of the high efficiency at low temperature. However, the use of reformate hydrogen compromise cell performance due to the presence of traces of carbon monoxide (CO). This contaminant is adsorbed on the surface of Pt, blocking the sites necessary for the hydrogen oxidation reaction. In this context, to improve the CO tolerance of fuel cell anodes, several bimetallic Pt-based catalysts have been proposed. [1] Some studies reported in the literature have suggested the use of niobium (Nb) as the second element in bimetallic catalysts. [2] Thus, this study aims at analyzing the effect of the addition of Nb to Pt in order to develop a CO tolerant catalyst. Pt₃Nb/C catalyst was prepared using a modified impregnation method, [1] physically and morphologically characterized and tested electrochemically in a single fuel cell operating on the traditional Nafion^® membranes at 85 ^oC.

Figure 1 shows the ionic currents signal m/z = 44 obtained by on-line differential electrochemical mass spectrometry (DEMS) experiments, corresponding to CO₂ formation. Lower CO oxidation overpotential is observed for the material with Nb, which is a consequence of the facilitated CO oxidation on Pt₃Nb/C catalyst. Figures 2 and 3 show the results corresponding to performance of a single fuel cell operating on H₂ or H₂ + 100 ppm CO. As it can be seen, the Pt₃Nb/C catalyst outperforms Pt/C 20%. The high CO tolerance of this material is usually explained by two distinct mechanisms: the bifunctional and electronic mechanisms. In the first, the Nb act as a source of oxygenated species, required for the electro-oxidation of CO to CO₂ at overpotentials lower than that for pure Pt. The electronic effect can be discussed in terms of the changes of the position of the Pt 5d-band center induced by the presence of the second metal that modifies the CO adsorption energy, reducing the CO coverage on Pt, and leaving more free Pt sites available for H₂ electro-oxidation. These effects will be discussed using on-line DEMS measurements and in situ X-ray absorption spectroscopy (XAS) analyses.

ACKNOWLEDGEMENTS

Authors thank FAPESP, CNPq and CAPES, Brazil, for the financial support.

REFERENCES

[1] Pereira, L. G. S., Paganin, V. A., Ticianelli, E. A. Electrochimica Acta, 54, 192 (2009).  

[2] Rocha, T. A., Ibanhi, F., Colmati, F., Linares, J. J., Paganin, V. A., Gonzalez, E. R. Journal of Applied Electrochemistry, 43, 817 (2013).