Study On Electrocatalysts Modified By Organics For Direct Alcohol Fuel Cells

Direct alcohol fuel cell (DAFC) possesses the wide application in the portable equipment, electric car and field power etc., due to the free (or low)-pollution, abundant sources, high energy efficiency, the easy storage and transportation of the fuel. However, the low electrochemical activity and high cost of electrocatalysts are still one of the key issues hindering the commercial application of DAFC. Therefore, to improve the electrocatalytic activity and to decrease the loading mass of noble metals are effective routes to the commercial application of DAFC. In this dissertation, organic material (such as dye, Nafion, fatty amine, aromatic amine) modified electrocatalysts or catalyst supports in DAFC have been evaluated. Their micrographs, structure, properties and applications have been investigated by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), cyclic voltammetry (CV), chronoamperometry and polarization method, etc. The main points of this dissertation are summarized as follows:(1) Nafion (Nf) - thionine (Th) ion-pair as dispersant and second catalyst was introduced into the preparation of Pt catalyst for methanol electrooxidation and the resulted catalyst supported on carbon (NfThPt/C) was investigated by electrochemical methods. For the same loading mass of catalyst, the peak current density of methanol oxidation on the NfThPt/C/graphite electrode is about 7.25 and 3.04 times as high as that on the E-TEK Pt/C/graphite and E-TEK PtRu/C/graphite electrodes, respectively. Moreover, NfThPt/C catalyst shows excellent anti-poisoning ability and long-term cycle stability. The electrocatalytic properties of Pt for methanol electrooxidation are improved obviously by Nf-Th ion-pair.(2) Organic dye neutral red (NR) was introduced in the anodic electrocatalyst system for methanol oxidation and the resulting electrode was investigated by CV, EIS and polarization method. For the same loading mass of Pt catalyst, 1.25 times larger exchange current density, 1.83 times higher specific activity and better long-term cycle stability can be obtained at Pt/NR/graphite electrode, as compared with the electrode without NR.(3) As the support of Pt and PtRu catalysts for ethanol electrooxidation, the ethylene diamine (ED)-grafted carbon nanotubes (CNTs) were prepared by chemical synthesis method and characterized by FTIR. The morphology and elemental composition of the PtRu/ED/CNTs and Pt/ED/CNTs catalysts were characterized by TEM and EDS, respectively. TEM results demonstrate that the ED-grafted CNTs are beneficial to loading PtRu and Pt electrocatalysts with well dispersion and small particle size. On the other hand, the electrocatalytic properties of the PtRu/ED/CNTs and Pt/ED/CNTs catalysts were characterized by CV and chronoamperometry. Under the same loading mass of catalyst, the electrocatalytic activities of both PtRu/CNTs/graphite and Pt/CNTs/graphite electrodes are enhanced obviously by the introduction of ED. The electrocatalytic activity of the Pt/ED/CNTs/graphite electrode is even higher than that of the PtRu/CNTs/graphite electrode. These results indicate that the ED/CNTs are the promising catalyst support for ethanol electrooxidation.(4) The conductive polymer poly(neutral red) polymerized on the graphite electrode (PNR/graphite) was used as the catalyst support for catalytic oxidation of ethanol in acidic solution and investigated by electrochemical methods. The Pt nanoparticles loaded on the surface of the PNR/graphite electrode exhibit higher electrocatalytic activity for ethanol oxidation in comparison with the Pt supported on the graphite electrode. With the equivalent loading mass of Pt catalyst, the specific activity (SA) at peak a of the Pt/PNR/graphite electrode where PNR was polymerized for 10 cycles in 5.0×10?4 M NR + 0.50 M H2SO4 solution is 3478.00 A C-1 and about 2.20 times as high as that of the Pt/graphite electrode (1582.74 A C-1). The corresponding results imply that the electrochemical performance (high electrocatalytic activity, better long-term cycle stability) of Pt catalyst for ethanol oxidation is improved by the assistance of PNR, which may enhance the immobilization of Pt nanoparticles and reduce the COads poisoning on the Pt surface.(5) Poly(ο-phenylenediamine)/carbon nanotubes (PoPD/CNTs) composite film has been prepared and investigated as the support of Pt nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). The results from SEM and CV indicate that Pt nanoparticles have been highly dispersed on the PoPD/CNTs composite film and exhibit improved electrocatalytic activity for ORR in acid medium. The specific electrocatalytic activity (SA) of the Pt/PoPD/CNTs loaded on the glass carbon (GC) rotating disk electrode (Pt/PoPD/CNTs/GC electrode), in which PoPD was polymerized in 5.0×10?2 Mο-phenylenediamine (oPD) + 0.20 M Na2SO4 solution at pH 1, is 524.20 A C-1 and about 2.12 times higher than that of the Pt/GC electrode. The kinetic behavior of ORR on the Pt/PoPD/CNTs/GC electrode was also investigated by linear sweep voltammetry (LSV). The results indicate that a 4e-reduction of O2 is dominant on the Pt/PoPD/CNTs/GC electrode in air-saturated 0.10 M H2SO4. Additionally, the electrocatalytic performance of Pt catalyst for ORR is improved obviously by the PoPD/CNTs composite film.(6) N-hydroxyphthalimide-carbon nanotubes (NHPI-CNTs) composite as the catalyst support was introduced and the electrocatalytic properties of the Pt/NHPI-CNTs/GC electrode toward the dioxygen reduction reaction (ORR) have been investigated by CV and LSV. From CV, the peak current density of ORR on the Pt/NHPI-CNTs/GC electrode is about 1.64 times higher than that on the Pt/GC electrode. The kinetic parameters demonstrate that a 4e-reduction of O2 to H2O is dominant on the Pt/NHPI-CNTs/GC electrode during the ORR process.