The Study Of Highly Dispersed And Durable Pt-based Catalysts For Low-temperature Fuel Cell

Proton exchange membrane fuel cell(PEMFC) is considered to be the most potential fuel cell for its high energy conversion efficiency, environmental benign, high power density, safety, portability and convenience in operation. However, the poor durability, high cost and low Pt utilization of electocatalyst have been recently recognized as the most important issues to be addressed before the commercialization of PEMFCs. At present, carbon-supported Pt nanoparticles are still the most widely used catalysts in PEMFCs. The poor durability of the Pt/C catalyst is reflected by a fast and significant loss of platinum electrochemical surface area due to carbon support corrosion and Pt aggregation/ dissolution. In this theses, a series of high performance catalysts with high durability and Pt utilization are prepared through modification of Pt catalyst or carbon support with oxide, binary oxide and perfluorosulfonic acid(Nafion) materials.Firstly, a high durability Pt based electrocatalyst, in which carbon black support was modified by decorating a tin and silicon binary oxide layer on the carbon black, was prepared successfully. Here, Sn O2 as a promoter and Si O2 as a stabilizer. Transmission electron microscopy revealed that the binary oxide formed a thin layer on the surface of the carbon support particles. The catalyst exhibited significantly enhanced performance towards the oxygen reduction reaction(ORR): at 0.9 V(vs. RHE), Its ORR current density was 1.5 times higher than that of a commercial JM Pt/C catalyst with the same Pt loading. Furthermore, it showed excellent durability, After 8000 cyclic voltammetry cycles in 0.1 M H2SO4 solution, the electrochemically active surface area was almost unchanged and the ORR half-wave potential shifted by only 10 m V. We attribute the catalyst’s high activity and durability to the binary oxide coating the surface of the carbon nanoparticles. We suggest it may promoted by tin oxide and thereby enhancing the catalytic activity and preventing the Pt nanoparticles from aggregating and carbon support from corrosion. The high ORR performance and excellent stability of this catalyst make it promising for use in practical fuel cell applications.Further, we prepared a binary oxide modified Pt/C catalyst by directly decorating a commercial Tanaka Pt/C catalyst with a binary oxide of silicon and tin. The decorated catalyst shows significantly enhanced durability: after 5000 cycles, its ECSA and ORR activity decay by only 30% and 28.4%, respectively, compare with 75% and 99.9% for undecorated Pt/C under the same conditions. This improvement is attributable to the decoration, which preventing the Pt nanoparticles from aggregating, and synergistic effecting between the Pt active component and the binary oxide. Interestingly, the decorated catalyst also exhibits excellent self-humidification properties. Under non-humidified operating conditions, a membrane electrode assembly(MEA) with a decorated Pt/C catalyst anode is discharged at 0.6V for 8 h with only slight current density decay, whereas serious decay are observed in a MEA with a non-decorated Pt/C anode. We characterize the catalyst using XRD, SEM/TEM, and XPS. The better wettability of the MEA containing the decorated Pt/C catalyst is confirmed by contact angle measurements. This study suggests a facile and effective way to enhance the activity and durability of a Pt/C catalyst and the self-humidification performance of PEM fuel cells.Thirdly, we further tried to decorate a commercial Pt/C catalyst with a binary oxide of In and Sn(ITO), which is prepared by coprecipitation method. Transmission electron microscopy revealed that ITO particles are highly dispersed on the carbon black as sub-nanometer when the content of ITO is less than 20%. XRD and XPS analysis reveal that Sn enter the lattice of the indium oxide instead of part of the In, to form a ITO structure. The ITO modified catalyst exhibits excellent electrochemical stability and acid resistance, after long-time and high potential CV testing, the catalyst which decorated by ITO shows an excellent electrochemical stability. ITO prevents carbon support from corrosion by highly dispersed on it.Fourthly, we prepared a high performance platinum catalyst, in which we used a perfluorosulfonic acid(nafion) functionalized carbon black as support. The catalyst is extensively characterized by infrared spectroscopy(IR), transmission electron microscopy(TEM) and X-ray diffraction(XRD). The TEM image shows that the active components are highly dispersed on the carbon black with a particle size of 1.8nm. The catalyst shows improved activity towards the oxygen reduction reaction(ORR), which may results from the high dispersion of active component caused by the increase of electrochemically accessible surface areas, ion channel and easier charge-transfer at polymer/electrolyte interfaces, allowing higher dispersion and utilization of the deposited Pt nanoparticles.