Studies On The Carbon-supported Pd-based Cathodic Catalysts For Direct Methanol Fuel Cells

Platinum has been widely used as the catalyst for the oxygen reduction reaction (ORR) in direct methanol fuel cells (DMFC), but its high price and poor methanol tolerance ability have urged the development of non-platinic materials. Pd has similar valence shell electronic configuration and lattice constant to platinum. It is less expensive and relatively abundant compared to Pt. More importantly, the Pd catalyst is highly methanol tolerant. However, its catalytic activity still needs to be further improved. In this dissertation, the catalytic activity of Pd was improved by addition of non-metal phosphorus and metal tin, making them promising alternative materials for the ORR. The main works of the dissertation are as follows:1) A PdP/C catalyst was synthesized by the sodium hypophosphite reduction method and exhibited high electrocatalytic activity for the ORR. The onset potential and the half-wave potential for the ORR at the PdP/C catalyst electrode are 50mV and 85mV more positive than those at the Pd/C catalyst electrode. The limiting current density at the PdP/C catalyst electrode comes up to the level of the commercial Pt/C. The physicochemical and electrochemical characterization of the PdP/C catalyst showed that the high electrocatalytic activity of the PdP/C catalyst can be attributed to the smaller Pd particle size, the lower relative crystallinity, and the formation of the PdP alloy.The method of adding nonmetal to the Pd catalyst can not only improve the electrocatalytic activity but also avoid the metal leaking to the strong acid electrolyte under electrochemical oxidative conditions.2) A PdSn/C catalyst was synthesized by the conventional impregnation method and exhibited higher electrocatalytic activity than Pd/C catalyst for the ORR. The physicochemical and electrochemical characterization of the PdSn/C catalyst showed that the higher electrocatalytic activity of the PdSn/C catalyst can be attributed to the formation of PdSn alloy: the addition of Sn results in lower relative crystallinity of the catalyst; on the other hand, the d-band center of Pd on the surface shifts to a lower-energy position modified by Sn. This work further confirmed the significance of the theory on Electronic structure in designing better catalysts.