Preparation And Investigation Of Ti₄O₇for Support Of Pt Catalyst For Fuel Cell

Electrocatalyst stability has been recently recognized as one of the keychallenges that must be addressed before the commercialization of proton exchangemembrane fuel cell （PEMFC）. At present, the most widely used catalyst system is Ptor PtM in the form of small nanoparticles supported on amorphous carbon-particle（Pt/C or PtM/C）. However, due to the weak cohesion between the catalyst and carbonsupport, the catalyst nanoparticles readily move and agglomerate together. In addition,the electrochemical corrosion of the carbon support causes agglomeration andsintering of the Pt particles, which seriously influences the stability of catalysts.Therefore it is of vital importance for the development of novel robust non-carbonsupport materials which are oxidation and corrosion resistance in severe PEMFCcircumstances. Research in these areas has become hot topics in fuel cell community.In the thesis, conductive ceramic Ti₄O₇was taken as research subjects. A series ofTi₄O₇with different sizes and morphologies were prepared. Thereafter, the Ti₄O₇, withspecific surface area of26m2/g was used as support to synthesize the Pt/Ti₄O₇catalyst. The physical properties and performance of Pt/Ti₄O₇for oxygen-reductionreaction （ORR） were investigated systematically.Firstly, the thermodynamic consideration of the Ti-O system was carried out.Based on the phase stability diagram of the Ti-O and Ti-C-O, the possible synthesisconditions of Ti₄O₇were adopted. The thermal and electrochemical stability of Ti₄O₇were investigated by the thermogravimetric （TG） method and electrochemicalaccelerated durability tests （ADT）, respectively. The results showed that the Ti₄O₇hadthe equivalent thermal stability compared with the commercial carbon supportXC-72R. There was no obvious change below500oC. Both were stable and satisfiedthe demands for PEMFC. However, there were big differences between the Ti₄O₇andXC-72R after5000cycles of ADTs. Severe corrosion was observed in XC-72R.However, there was almost no change for Ti₄O₇.Secondly, different routes of synthesis were investigated, one step andmulti-steps syntheses were adopted. One step titania reduction methods included thecarbon thermal reduction, hydrogen thermal reduction and the polyvinyl alcohol （PVA）thermal decomposition and reduction processes. These synthesis processes were easyto operate, but the Ti₄O₇products were sintering with large grain sizes, they can notprovide the large surface areas for the active components. The multi-step methodsinclude3steps: first, preparing the titania-silica composite oxide as the precursor;second, hydrogen reduction of the precursor; third, etching the silica and getting theTi₄O₇products. It may increase the difficulty of the reduction, but the final products of Ti₄O₇are with large surface areas. The specific surface area of Ti₄O₇whichobtained from hydrogen reduction of the silica covered titania nanotubes（TiO2NT@SiO2） was26m2/g. The specific surface area of pineal shaped Ti₄O₇products could reach140m2/g, when it was obtained from hydrogen reduction of thetitania-silica co-precipitation precursor. These high surface areas are extremely largerthan those obtained from one step method. This study also provides a new approach tosynthesize nano-structured ceramic materials for catalyst support.Thirdly, Pt loaded on the fiber-like nanostructured Ti₄O₇（NS-Ti₄O₇）. Theelectrocatalytic properties of the Pt/NS-Ti₄O₇were investigated. The initialelectrocatalytic activity of Pt/NS-Ti₄O₇was22.33A/g. It is almost twice of the one ofPt/XC-72R （10.30A/g）. After3000cycles of ADTs, the electrocatalytic activity of Pt/NS-Ti₄O₇was still13.31A/g. It is30times higher than that of Pt/XC-72R （0.45A/g）.It shows the excellent stability of Pt/NS-Ti₄O₇. The transmission electron microscopy（TEM）, high resolution transmission electron microscopy （HRTEM）, X-rayphotoelectron spectroscopy （XPS） were conducted to investigate the mechanism ofthe enhanced electrocatalytic stability. The superior durability of Pt/NS-Ti₄O₇isattributed to the high stability of Ti₄O₇as well as the strong metal-support interactionbetween Pt and Ti₄O₇.