Preparation And Study Of High Performance Fuel Cell Pt-Catalysts Based On New Support Materials

As an eco-friendly and clean energy technology for transportation vehicles, fuel cells（FCs） have attracted worldwide attention due to their high energy density, fuel portability, zero emissions, relatively low operating temperature and can start rapidly. Despite enormous progress in FCs, the commercialization of FCs is still impeded by certain problems, such as limited efficiency of catalysts, low tolerance to carbon monoxide（CO）, unsatisfactory electrochemistry stability and durability, and high cost of precious metal catalysts. It still remains one of the core challenges toward high performance FCs commercialization that development of excellent catalysts with maximum catalytic activity, such as CO tolerance and wonderful durability while minimum loading of platinum（Pt）. As crucial components of FCs catalysts, support materials are of great importance to the catalyst activity and fuel cell performances. Thus, finding an ideal support material is one of the effective strategies to solve the fuel cell commercialization problem as well as an important issue in the research of fuel cells.In this dissertation, through the screening, designing, optimization of catalyst support, catalysts were prepared and their electrochemical properties were studied. Feasibility discussionon preparation of the new-generation high-performance catalyst was also provided. We innovatively utilized various implementation methods to prepare and functionalize the graphene nanoribbons, carbon nanofibers and further realized its applications as FCs catalyst support materials. The detailed research works are listed as follows:1) Graphene nanoribbons（GNRs） were first used as a novel support material for Pt nanoparticles（NPs）. Compared with Pt/MWCNT, Pt/GNR hybrids show much larger electrochemically active surface area（ECSA: 74.1 m2/g）, higher electrochemical stability, and better CO tolerance（If/Ib: 1.14） towards electro-oxidation of methanol.2) Through electrospun and carbonization process, Polyacrylonitrile（PAN） based carbon nanofibers（CNFs） were synthesized and used as support material for Pt NPs. Compared with the commercial catalyst Pt/XC72 R, Pt/CNF hybrids exhibit higher Pt NPs dispersion, higher electrochemical activity（ECSA: 57.6 m2/g） and better CO tolerance（If/Ib: 0.86） due to the higher N doping. Further more, after 1500 CV cycles, the ECSA still keeps 76.4%（ESCA1500/ESCAinitial）, suggesting its better durability. Considering the facile preparation, it provides the feasibility for optimization of the CNFs to further improve the catalyst performance.3) AgNO₃ and silver nanowires（Ag NWs） were used as nano-material for CNFs to prepare composite material AgNP-CNFs and AgNW-CNFs via electrospinning and the subsequent carbonation. The composite material AgNP-CNFs and AgNW-CNFs were used as support materials to prepare catalyst Pt/AgNP-CNF and Pt/AgNW-CNF. The present Ag NPs evidently improved the graphitized structure of CNFs（ID/IG was reduced to 1.84 from 3.7） and the dispersibility of Pt NPs. The bimetallic between Pt NPs and Ag NPs of Pt/AgNP-CNF remarkablely improved the anti-CO-toxic of Pt. However, if metal salt was addied in PAN, the carbon material would be produced lots of nanopores, which interrupt the continuity of the electronic conduction, resulting the electrochemical activity（ECSA: 58.3 m2/g） of Pt/Ag-CNF was not significantly improved. Although AgNWs can improve the graphite structure and the electronic conductivity of CNFs, but the dispersibility of Ag NWs in CNFs is very poor, leading to not uniformly Pt NPs, thereby affecting the catalytic activity and electrochemical stability of the catalyst. Although the exploration of taking AgNO₃ and Ag NWs as nano-additives for PAN to synthetise composite CNFs support for Pt NPs is not ideal, but it provides a guideline for the future exploration: that is not only raising the graphite structure of the carrier, but also improving the dispersibility of catalyst.4) MWCNT and GNRs were used as nanofillers in CNFs separately for the preparation of composite M（G）-CNFs nanofibers. Using GNRs as nanofillers can effectively enhance graphitization（ID/IG: 2.4） and electronic conductivity of carbon nanofibers（CNFs）. Furthermore, the residual oxygenated groups on carbonized GNRs and GNR itself are helpful to improve the dispersion as well as CO tolerance of Pt NPs as the surface defects. Compared with Pt/CNF（Pt on pristine CNFs）and Pt/M-CNF（Pt on MWCNTs embedded CNFs）, Pt/G-CNF hybrids exhibit significantly improved ECSA（110.7 m2 g-1）, remarkably enhanced peak current density for methanol oxidation（534.2 mA mg-1）, better CO tolerance（If/Ib: 0.99）, more excellent durability and higher electrochemical stability, testifying G-CNFs are promising support materials for higher performance catalysts towards fuel cells.5) Via a facile and effective spontaneous deposition of MnO2 in KMnO₄ acid solutionand the subsequently heat treatment, MnO decorated electrospun CNFs（MnO-CNFs） with significantly enhanced graphitic structure（ID/IG: 1.02） and homogeneous surfaces defects were prepared. Compared with Pt/CNF hybrids, the resulting catalyst Pt/MnO2-CNF own 4-fold increased ECSA（137.5 m2 g-1）, 3-fold enhanced peak current density for methanol oxidation（930 mA mg-1）, remarkably improved CO tolerance（If/Ib: 1.12）, and durability（ESCA1500/ESCAinitial: 77%） due to the synergetic effect of their improved graphitization, more uniform distribution of Pt NPs, self N-doping as well as the bifunctional effect between Pt NPs and the redeposition of MnO2.6) MnO-CNFs were used to support PtAu alloy NPs for methanol electro-oxidation through a facile in-situ chemical co-reduction. Catalyst PtAu/MnO2-CNF owns many advantages, such as good dispersity, high alloying degree andsmaller size. The ECSA and If/Ib of PtAu/MnO2-CNF hybrids were calculated to 154.6 m2 g-1 and 1.53 separately, which is higher than Pt/MnO2-CNF and PtAu/CNF, suggesting the improved electrochemical activity and better CO tolerance, which can be ascribed to the synergistic effect between the PtAu alloy NPs, catalyzation with transition metals, N doping and highly dispersed catalyst NPs.The outstanding performance of Pt/MnO2-CNF and PtAu/MnO2-CNF hybrids suggests MnO-CNFs are promising support materials for high performance electrocatalysts for fuel cells.