Study Of Pd/TiO₂/Ti And Pt/CeO₂ Composite Catalysts Synthesized Electrochemically

Fuel cell converting chemical energy directly into electrical energy with high efficiency and low emission of pollutants may help to reduce environmental impact. Fuel cell is a practical answer to the world's pressing need for clean and efficient power. For decades, Pt-based catalysts are widely used in catalysis of oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). However, the drawbacks with use of platinum, limited supply, high cost and easily poisoned by trace of carbon monoxide limit its use. Thus, a great effort has been devoted to development of new fuel cell electro-catalysts in order to increase Pt activity and reduce platinum consumption. At the same time, platinum-free catalysts have been highly esteemed.Pd/TiO₂ as an important platinum-free catalyst was synthesized and investigated. Firstly, highly dense, well ordered vertically TiO₂ nanotubes arrays as catalyst support were fabricated by anodic oxidation on a pure titanium sheet and factors influencing TiO₂/Ti nanotubes arrays are discussed. The optimum TiO₂ nanotubes were obtained by pulse anodic oxidation instead of direct current anodic oxidation in the electrolyte of 1 M H₂SO₄ + 0.5 wt % HF for 20 min. SEM images show the prepared TiO₂ nanotubes have an inner diameter of 70 nm, an outer diameter of 100 nm, and a length of 400 nm. Secondly, to reduce the consumption of Pt, Pd, as the same group element, is supported on the TiO₂/Ti by modulated pulse current electrodeposition after sensitization and activation. Pd/TiO₂/Ti exhibits excellent catalytic activity to ORR. In addition, Ni assistant Pd/TiO₂/Ti catalyst was also synthesized and characterized. Ni-Pd/TiO₂/Ti catalyst performs an enhanced catalysis to the ORR in the base line of Pd/TiO₂/Ti.Meanwhile, CeO₂ modified Pt electrode, Pt/CeO₂, was fabricated to increase the activity of Pt. Firstly, Cerium dioxide with forms of nanoparticles, nanorods, and nanowires were prepared via an electrochemically synthesized route. CeO₂ morphology was controlled by changing strength and direction of imposed electrical field. The properties of synthesized CeO₂ were characterized by field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). The obtained CeO₂ nanoparticles with a polycrystalline face-centered cubic phase have a uniform size ranging from 15 to 50nm. The CeO₂ nanorods are composed of many tiny interconnected nanocrystallites at various orientations and CeO₂ nanowires display the outer diameter of about 30-40 nm and lengths up to 1μm. A possible formation mechanism has been suggested to explain the formation of CeO₂ nanowires. Secondly, Pt/CeO₂ electrodes were used to catalysis of methanol oxidation and oxygen reduction reactions. The results show methanol electrooxidation and oxygen reduction on Pt/CeO₂ electrode are superior to that on pure Pt electrode.