| Proton exchange membrane fuel cells(PEMFCs)are widely used in mobile communications,navigation,aerospace and military fields due to their low operating temperature,high efficiency and no pollution.However,oxygen reduction reaction(ORR)occurred at the cathode is sluggish,and a catalyst is needed to accelerate the kinetics process.The noble metal Pt-based catalysts demonstrate excellent catalytic activity,but the low reserves,high price,limited atomic utilization efficiency,and poor stability,inhibit their wide application.Therefore,developing low-Pt and non-Pt catalysts,and meanwhile improving their activity and stability are the key to the current research.For this purpose,three works were carried out in this thesis:(1)The core-shell structured materials constructed by Fe/Fe3C cores and nitrogen doped carbon shells represent a type of promising non-precious oxygen reduction reaction catalysts due to well-established active sites at the interface positions.However,the traditional liquid phase polymerization route for preparing such materials normally leads to a compact macropore-deficient structure with randomly dispersed metallic nanoparticles,being not good for mass transfer and formation of high-density dispersion of active sites.Herein,we report an“in-situ solid phase polymerization strategy”in which a frozen block containing uniformly dispersed oligomers is firstly achieved by combining a well-controlled hydrothermal reaction and a subsequent liquid nitrogen-facilitated fast solidification.During the following freeze-dry process,the oligomers in-situ polymerize into 3D highly cross-linked network in the confined space of ice block which not only effectively avoids the direct stack of polymerized intermediates,but also prevents the agglomeration of metallic nanoparticles.The finally obtained monodisperse Fe/Fe3C nanoparticles embedded nitrogen-doped carbon aerogel catalyst,in ORR,delivers an ultrahigh activity as the half-wave potential and the kinetic current density at 0.9 V reach 0.919 V and 7.83 m A cm-2 respectively in an alkaline solution.Using this route,a range of aerogel materials loaded with well-dispersed metallic nanoparticles can be synthesized and used in the fields such as catalysis,environment,and energy etc.(2)In the conventional preparation of electrocatalysts for ORR,direct deposition of active metallic nanoparticles into porous channels frequently leads to narrowed mass transfer channels,and relatively weak adhesion of the nanoparticles on the pore-walls.Herein,we developed a dual-template method in which CTAB-templated mesopores allow confined formation of uniform 2.39 nm Pt3Co nanoparticles,while the larger mesoporous channels(2?9 nm)made by Si O2 are solely used to expedite mass transfer.Particularly,as the soft template CTAB can be assembled around hard template Si O2 to form an interesting bimodal-pore structure,in the final product Pt3Co/C-O,active Pt3Co nanoparticles are located around the mass transfer channels rather than inside them.Compared to the conventional material with Pt3Co positioned inside the channels,Pt3Co/C-O displays?29%enhancement of the mass transfer efficiency.In H2-O2membrane electrode assembly(MEA),Pt3Co/C-O gives a peak power density of 1.33 W cm-2 under an ultralow cathodic loading of 0.06 mg Pt cm-2,representing a very promising low-Pt loading catalyst.Meanwhile,the confining effect of mesopores well facilitates the remaining of the small-sizes of the nanoparticles during continuous electrocatalytic reaction,thus significantly boosting the durability of the catalyst.Based on this study,a range of catalysts with readily-accessible active sites and highly-efficient mass transfer channels can be prepared for various applications.(3)The thickness of the catalyst layer in MEA largely affects mass transfer efficiency.The thinner the catalytic layer is,the higher the mass transfer efficiency will be.The thickness of the catalytic layer depends on Pt loading on the carbon support,and a higher Pt loading would lead to a thinner catalyst layer under same Pt loading of the MEA.Typical alloy catalysts have 20 wt%Pt loading on carbon support,while at a higher loading,the metallic nanoparticles frequently agglomerate into larger sizes which further induces decrease of the catalytic activity.In this work,H-Pt Ni/C catalyst with average Pt Ni particle size of 3.55 nm and Pt loading as high as 33 wt%was prepared by the dual-template method.In H2-O2 fuel cell,the peak power density of H-Pt Ni/C reaches 1.51 W cm-2 under cathodic loading of 0.100 mg Pt cm-2,being 21%higher than the value of 1.25 W cm-2 generated by the low Pt loading catalyst L-Pt Ni/C(13.3 wt%),which can be ascribed to well-remained small sizes of the alloy nanoparticles and reduced thickness of the catalyst layer in the former.In addition,the same method can be used to prepare Pt Fe alloy catalyst with a high-Pt loading of 36 wt%.Thus the dual-template method exhibits good universality and can be widely used for preparation of monodispersed high-Pt loading catalysts. |
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