Carbon supported palladium-platinum catalyst for oxygen reduction reaction in high temperature proton exchange membrane fuel cell

Abstract

Platinum (Pt) is the most commonly adopted electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells (PEMFCs) due to its noteworthy features. However, for PEMFCs to have wide practical applications and become commercially viable, the challenging issue of the high catalyst cost, resulting from the exclusive conventional practice of platinum based catalysts should be addressed. Therefore, a study of the palladium (Pd) as a partial substitution to the platinum on carbon (C) has been conducted in high temperature PEMFCs. A series of metal electrocatalyst (Pt/C, Pd/C, and 10-40 wt% Pt-Pd/C) were synthesized via chemical reduction method and their characteristics have been observed by cyclic voltammetry, linear sweep voltammetry, field emission scanning electron microscope and energy dispersion x-ray, Fourier transform infrared spectroscopy, x-ray diffraction and nitrogen-physisorption. Among all, 30 wt% Pt-Pd/C, gave a promising 0.9 Wcm-2 power density at 170 °C. 30 wt% Pt-Pd/C was then nominated to further enhance the catalyst layer. Polybenzimidazole (PBI) was added onto the catalyst layer of 30 wt% Pt-Pd/C in order to inrease the porosity and facilitate the transport of oxygen in the catalyst layer due to their hydrophobic properties. The PBI ratio was varied towards 30 wt% Pt-Pd/C (PBI: 30 wt% Pt-Pd/C; 1:99, 3:97, 5:95 and 9:91). Short-term durability for all catalysts was conducted from 24-96 hr revealed that the impedance curves of 5:95 catalysts showed the slowest performance decay of the membrane electrode assembly (MEA). Hence, this result indicated that the decay of the catalyst could be prevented by appropriate PBI loading as well as increasing the lifetime of the MEA. The 5:95 MEA delivered a peak power density of 1.30 Wcm-2, corresponding to an overall Pt utilization 0.02 mgPt/cm. At 170 °C, the MEA cathodic catalyst utilization was 65 kW/gPt. This is 1.5 times higher than the Pt-utilization efficiency of a reference fuel cell prepared using commercial catalyst layer, which emphasizes the enhancement that was mainly attributing by the Pd substitution and PBI ionomer in the catalyst. All the result indicated in this study strongly motivate the application of combining suitable ratio of PBI binder in an optimum metal loading catalyst. This combination would produce a low resistance MEA in order to compensate an encouraging power density.