Preparation And Performance Of Cathodic Oxygen Reduction Catalysts For Pt Free And Low Pt Fuel Cells Based On Conductive Polymers

Proton exchange membrane fuel cells（PEMFCs）are important green and efficient energy conversion devices,capable of direct conversion of chemical energy to electric power by supporting chemical anodic（such as H₂ or alcohols oxidation）and cathodic（e.g.,O₂ reduction）reactions.However,the sluggish kinetics and the large overpotentials during the oxygen reduction reactions（ORRs）on the cathodes dramatically hinder their usages.Currently,carbon-supported Pt-based catalysts are the most efficient for ORRs in both alkaline and acidic solutions.Nevertheless,expensiveness,scarcity,poor stability,and cross-effects limit their widespread industrial applications.Thus,developing strongly electro-active catalysts containing less Pt or without Pt with high durability needs to be addressed to resolve above drawbacks.First,in this paper,two-dimensional layered materials titanium carbide（MXene）and transition metal oxide cerium oxide（Ce O₂）nanoparticles were used as templates for preparing conductive polypyrrole（PPy）and polyaniline（PANI）through an in-situ polymerization method.Then the binary carbon-based platinum-free catalysts doped uniformly with nitrogen were synthesized by pyrolysis of PPy-surrounding MXene and PANI-encapsulated Ce O₂ NPs respectively under Ar₂ atmosphere.The resulted binary catalysts exhibit enhanced activities and stabilities for ORR performance,which is comparable to that of commercial Pt/C.Then low noble Pt-,and Pd-based ternary catalysts were synthesized by adding small amount of Pt、Pd sources respectively during the preparation of the above-mentioned Ce O₂/nitrogen-doped carbon binary catalyst precursor,followed by high-temperature carbonization.Compared with commercial 20%Pt/C,the formed low Pt-based ternary catalyst exhibits an enhanced ORR activities,and what is important is that the Pt content in the catalytic material is only one fifth of the former.Further the synthesized low Pd content supported Ce O₂/nitrogen-doped carbon catalyst was employed for assembling Zn-air batteries device.The work of this thesis has made a useful exploration for the preparation of high activity and high stability platinum-free and low-platinum oxygen reduction catalysts,and provided a certain method and practical basis.More details are listed as follows:（1）Noble Metal-free catalysts attracted much attention as promising candidates for Pt-based catalyst replacement to advance applications related to oxygen reduction reaction（ORR）,which is critical for large-scale renewable energy storage and conversion.Herein,this work focused on a synthesis of noble metal-free ORR electrocatalysts consisting of porous N-rich carbon/MXene,which was obtained using very conductive and reactive Ti₃C₂MXene and polypyrrole（PPy）as a C and N source.The electrocatalyst exhibited excellent electrocatalytic activity and stability with an onset and a half-wave potentials equal to 0.85 and 0.71 V,respectively.Results obtained in this work demonstrate how to design efficient noble metal-free ORR electrocatalysts applicable to other chemical systems.（2）This work developed a highly porous N-doped carbon catalyst embedded with Ce O₂ nanoparticles（NPs）.This composite catalyst was synthesized by pyrolysis of PANI-encapsulated Ce O₂ NPs（Ce O₂/PANI）combined with in situ polymerization.The resulting catalyst exhibited an outstanding ORR performance with 0.94 and 0.81 m V onset and positive half-wave potentials,respectively,and 5.52 m A cm^-2diffusion-limited current density.The catalyst also demonstrated excellent stability.These exceptional characteristics indicate that a synergy of the N-doped carbon and Ce O₂ NPs provide a novel strategy of fabrication of novel ORR catalysts without using noble-metals for applications related to fuel cells and metal-air batteries.（3）This study reports an high-efficient ternary ORR electrocatalyst containing N-doped carbon/Ce O₂-supported platinum catalyst（Pt/N-C/Ce O₂）with low Pt content,which is fabricated by a facile two-step procedure using polyaniline（PANI）as an N source.The doped chloroplatinic anions are reduced to form evenly dispersed Pt nanoparticles（NPs）incorporated with the N-doped carbon matrix.A combination of Pt NPs with exposed highly active（111）facets,Ce O_2,and active pyridinic N contributed to the enhanced ORR activity and stability in alkaline solutions.The activity of our Pt_0.1/Ce O₂/N-C catalyst containing only 5.6 wt%of Pt reaches 238.46 m A mg _Pt^-1,which is four times higher than that of the commercial 20%Pt/C catalyst(58.8 m A mg _Pt^-1).These findings reveal an important new strategy for designing high-yield and cost-effective Pt-based ORR catalysts in simple and easy way.（4）Simple and convenient two-step method was used to prepare the carbon-based ORR ternary catalyst based on Pd-Pd₄S.In the process of in-situ polymerization of polyaniline,ammonium persulfate serves as an oxidant and provides sulfur source for the synthesis of Pd₄S.In the process of polymerization,S₂O₈^2-and Pd Cl₄^2-are simultaneously doped into the polymer matrix to obtain catalytic precursors.Then,the final product obtained by carbonization under the protection of atmosphere is recorded as Pd-Pd₄S/Ce O₂/N-C.Electrochemical tests in alkaline electrolyte showed that the half-wave potential of Pd-Pd-Pd₄S/Ce O₂/N-C was only 60 m V different from that of commercial 20%Pt/C,and showed excellent catalytic performance in the performance test of zinc air battery.