Nb as an influential element for increasing the CO tolerance of PEMFC catalysts

Platinum–niobium catalysts were prepared as candidates for CO tolerant anode catalysts for low and high temperature PEM fuel cells (PEMFCs). Three different compositions were prepared by the formic acid method, from platinum (hexachloroplatinic acid) and niobium (niobium chloride) precursors on Vulcan XC-72R carbon black. Deposition of the niobium was found to be quite difficult, and only a fraction of the desired composition was achieved. Mean particle sizes were all in the nanometric range, between 2 and 3 nm. Diffraction patterns display neither insertion of niobium within the crystalline structure of platinum, nor any crystalline phase associated to that material. Nevertheless, the presence of Nb displays a noticeable effect on the CO tolerance of the catalyst firstly revealed by a reduction of the CO stripping onset potential. Fuel cell results, operating with Nafion^® at low temperature (80 °C) and H₂ + 100 ppm of CO as fuel, and with H₃PO₄-doped ABPBI, at high temperature (150 °C) and H₂ + 20,000 ppm of CO, display an enhancement in the performance compared to pure platinum, so niobium may be an interesting material for increasing the tolerance to carbon monoxide in PEMFC. Finally, CO/O₂ polarisation curves display a decrease in the current density in the presence of Nb, confirming that the enhanced CO tolerance can be attributed to a strong electronic effect that weakens the Pt–CO adsorption strength.