Study On Metal Oxides For Proton Exchange Membrane Fuel Cell Catalysts

Proton exchange membrane fuel cells can be as a promising portable power due to the high specific energy, environmentally friendly and low-temperature operation. However, the commercialization of PEMFC is hindered due to high cost and short life. The dissertation summarized the research status of the PEMFC catalysts, and then the tin oxide and niobium oxide are employed to enhance the catalytic activity and electrochemical stability for PEMFC catalysts.The lattice parameter, particle size and micro-morphology of the catalysts were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The valence and composition of metal elements on the catalyst surface was characterized by X-ray photoelectron spectroscopy (XPS). The catalytic activity and electrochemical stability of the catalysts was characterized by cyclic voltammetry, linear sweep curves and amperometric curves for the catalytic activity and electrochemical stability.Firstlyt, the SnO₂ hybrid support was synthesized using hydrolysis method, and then the Pt/SnO₂/C catalyst was prepared by ethylene glycol (EG) method, forming unique Pt/SnO₂/C triple junction structures. Oxygen reduction reaction (ORR) indicated that when the Pt/SnO₂ molar ratio was 5:5, Pt/SnO₂/C had high ORR activity of 15.6 A·g-1 Pt, which was 1.5 times that of Pt/C (10.6 A·g-1 Pt). The accelerated durability test (ADT) showed that the durability of the Pt/SnO₂/C catalyst was 1.0 times higher than that of Pt/C. XPS confirmed the electron was transfered from Sn to Pt at the Pt/SnO₂/C triple junctions, and the electron density of Pt was increased, which can effectively weak oxygen-Pt bond and promote the ORR kinetics. Moreover, the Pt nanoparticles at the Pt/SnO₂/C triple junctions can be effectively anchored by SnO₂ and C simultaneously, forming stable triangle structure, which was the reason for the enhanced stability. To decrease Pt loadings, the PtSn/SnO₂/C catalyst (12.4 mass%Pt) was prepared using EG and H2O hybrid phase, successfully constructing PtSn/SnO₂/C triple junction structure. Electrochemical measurements showed that the PtSn/SnO₂/C catalyst had 90% higher catalystic activity towards ethanol oxidation and higher CO-tolerance than that of Pt/C catalyst. The two reasons were proposed: the synergy effect between Pt and SnO₂ at the PtSn/SnO₂/C triple junctions, and the PtSn alloy effect.Then Nb₂O₅ was studied as cathode catalyst support of PEMFCs. Firstlyt, the Nb(OH)5/C material was obtained by hydrolysis method, then Nb₂O₅/C support was prepared from Nb(OH)5/C by calcining for 5 h at 700°C. XRD confirmed the formation of Nb₂O₅. The Pt/Nb₂O₅/C catalyst was prepared using EG method. Compared with Pt/C, the Pt/Nb₂O₅/C catalyst exhibited lowly catalystic activity towards oxygen reduction, which was attributed to the weak electron effect between Pt and Nb₂O₅, and empty d orbital and low conductivity of Nb₂O₅. The electron effect between Pt and Nb₂O₅ by heat treanment can be enhanced, which was contributed to the higher ORR activity of Pt/Nb₂O₅/C catalyst.NbOx/C-HT mixture support, containing a part of NbO₂ with the high conductivity, was obtained by calcining and reducing Nb₂O₅/C at high temperature. Then Pt/(NbOx/C-HT) was prepared using EG method. TEM showed that Pt nanoparticles with average particle size ².2 nm were highly dispersed on the niobium oxide surface in Pt/(NbOx/C-HT) catalyst. The result of ORR showed that the oxygen reduction mass activity of the Pt/(NbOx/C-HT) catalyst reached 1.3 times that of Pt/C. XPS confirmed that the electron was transfered from Nb to Pt at the interface between Pt and NbO₂ , and the electron density of Pt was increased, which can effectively weak adsorption strength of O₂ on Pt active sites and promote the ORR rate. Therefore, NbO₂-decorated Pt/C as the cathode catalyst has definite research value. Finally, the mixture of NbO₂ and carbon was used as catalyst supports. Then the effect of the NbO₂ percentage on the Pt/NbO₂/C catalyst was studied. The result of TEM showed that Pt nanoparticles were uniformly dispersed on the NbO₂ surface. The oxygen reduction activity of the Pt/NbO₂/C catalyst with 40% NbO₂ reached 1.65 times that of Pt/C, and the durability was improved 60%, which was also attributed to the electron ransfer from Nb to Pt at the interface between Pt and NbO₂.