| Direct alcohol fuel cells(DAFCs)have received great attention due to their higher power density,easier storage and transport than hydrogen fuel cells.The synthesis of highly active Pdbased nanocatalysts with well organized-structure and low price is desired for DAFCs commercialization.An excellent catalyst should play an important role in inhibiting the formation of CO-intermediate species and accelerate the kinetics of methanol oxidation reaction(MOR)and ethanol oxidation reaction(EOR)processes.Pd-based catalysts offer such attributes due to their unique structure and electronic properties over bulk counterparts.However,the main challenges of Pd-based catalyst development are the low stability and high cost of the catalyst.At present,the best way to improve the stability and reduce the cost,is to introduce another metal(such as Sn,Ag,Cd,etc.)to the Pd nanoparticles(NPs)in order to provide more OHads during the MOR and EOR processes.In this thesis,the Pd-based nanocatalysts toward MOR and EOR are investigated from three aspects:the structural perspective,effective elements and effective supports.The specific conclusions are as follows:First:PdSn/f-C nanocatalysts with different Pd/Sn ratios were prepared via the impregnation reduction method.The functionalization of carbon black XC72 was done to smoothen the surface and increase the active sites of the support.The PdSn/f-C nanocatalysts with high stability were successfully prepared on a f-C support by impregnation reduction of metal precursors.TEM results showed that the f-C with uniform thickness was successfully made,and the Pd,Sn nanoparticles with small particle size(5.5 nm)were dispersed uniformly on the f-C to form the PdSn/f-C nanocatalysts.The electrochemical performance showed that the prepared PdSn/f-C nanocatalysts have high activity and long-term stability.Their oxidation activity and stability to ethanol was 6.5 and 7.5 times higher than that of the Pd/C(JM)catalyst,respectively.Second:Various PdAg/MWCNT nanocatalysts were successfully developed by a facile onestep room temperature synthesis method towards MOR and EOR.The support was successfully functionalized by a solution of HNO3 and H2SO4 to smooth the surface and increase the active sites.Morphological characterization demonstrated that the PdAg/MWCNT nanocatalysts have well-organized structure,uniform dispersion with small particle size and high surface area.The PdAg/MWCNT showed an excellent activity,stability and anti-poisoning capability.The catalytic activity of the Pd3.5Ag/MWCNTs was 6.5 times(methanol)and 7.5 times(ethanol)higher than that of the Pd/C(JM).Moreover,the PdAg/MWCNTs have much higher anti-poisoning resistance than the Pd/C(JM),due to the synergetic effect between the influence of Ag and the MWCNT support,as well as abundant surface-active sites.Third:A room temperature seed mediated growth strategy was successfully developed to prepare ternary Pd@CdAg core-shell as advanced novel non-Pt anode nanocatalysts for EOR in an alkaline electrolyte.The introduction of oxophilic Cd and Ag metals into Pd nanocatalysts can reduce the adsorption energy of OHads on the Pd@CdAg nanocatalysts and inhibit the COads on the Pd surface.Morphological characterization demonstrated that the as-synthesized Pd@CdAg nanocatalysts have well-organized core-shell nanostructure with Pd NPs as the core and CdAg alloy as the shell.Electrochemical results indicated that the core-shell Pd@CdAg catalyst exhibited remarkable electrochemical activity(2996 mA mgPd-1),excellent CO tolerance and longterm durability over the Pd@Cd(1285.8mA mgPd-1),Pd@Ag(2422.9 mA mgPd-1)and Pd/C(JM)(477.4 mA mePd-1)nanocatalysts.The excellent catalytic activity and stability of the Pd@Cd1Ag1 nanocatalyst can be associated with their big electrochemical active surface area,positive influence of CdAg NPs on Pd NPs and the unique well-organized core-shell nanostructure.These results suggested that the as-synthesized Pd@CdAg core-shell could serve as advanced catalysts towards alkaline DEFCs. |
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