Preparation And Performance Study Of Pd-Based Cathode Catalyst For Direct Methanol Fuel Cell

Direct methanol fuel cells (DMFCs) have attracted more and more attentions because of their high energy density, low exhaust and low system complexity. DMFCs have great potential for development theoretically. However, there are some drawbacks blocking DMFCs'commercialization, such as the slow kinetics of catalyst,especially cathodic catalyst for oxygen reduction reaction (ORR) and the crossover of methanol result in a decrease in the efficiency of the system. Therefore, the improvement of the electrocatalytic performance to achieve high ORR activity, excellent methanol tolerance and outstanding stability is still important and necessary.In this study, Pd-based cathode catalysts doped with different elements to improve the ORR activity for DMFC were prepared and investigated. The catalysts were well characterized by X-ray diffraction (XRD), Electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) etc. The ORR activity, methanol tolerance ability and the stability of the prepared catalysts were investigated and evaluated in 0.1 M KOH saturated by oxygen by cyclic voltommagram (CV) and linear potential sweep with rotating disk electrode. The ORR mechanism and kinetic process with Pd-based catalysts were analyzed and discussed.Pd based catalysts doped with non-metal elements (phosphorous or boron) and with carbon nanotubes (CNTs) as support have been prepared. The results show that M-Pd/CNTs (M=P and B) nanoparticles possess a face-centered cubic (fcc) structure. It was revealed that catalysts doped with non-metal elements were uniformly dispersed on the CNTs support. Moreover, Pd cathode catalyst doped with phosphorus (P-Pd/CNTs) has higher ORR activity than Pd cathode catalyst doped with boron (B-Pd/CNTs) and Pd cathode catalyst without doping (Pd/CNTs). Meanwhile, the P-Pd/CNTs catalyst has better methanol tolerance than commercial Pt/C catalyst and excellent stability with 90% of its original ORR activity kept after 1000 CV scans. Based on the RDE and RRDE experiments data, ORR with P-Pd/CNTs catalyst is a 4-electron reduction reaction.TiO₂/CNTs was synthesized by sol-gel method. Novel catalysts Pd/TiO₂/CNTs with various molar rations of Pd to TiO₂ have been prepared by using ethylene glycol as reducing agent. Compared with Pd/CNTs catalyst, the result of electrochemical measurements show the Pd/TiO₂/CNTs catalyst has higher ORR activity, and its stability was better than that of Pd/CNTs. The RDE results showed that the ORR on Pd/TiO₂/CNTs catalyst has a 4-eletron mechanism .A green and facile synthesized method of RuO₂ nanoparticle was adopted to prepare RuO₂/CNTs, and then Pd was loaded on the obtained RuO₂/CNTs to fabricate Pd/RuO₂/CNTs catalyst. The electrochemical result show RuO₂/CNTs with highly dispersed RuO₂ nanoparticle has excellent methanol tolerance ability. It was found that the initial ORR activity with Pd/RuO₂/CNTs catalysts is a little inferior to that with Pd/CNTs. However, the methanol tolerance and stability of Pd/RuO₂/CNTs was much better than those of Pd/CNTs. The RDE results showed that RuO₂/CNTs and Pd/RuO₂/CNTs catalyst had a similar 4-eletron process for ORR.