Research On New Highly Stable Supports For Proton Exchange Membrane Fuel Cells

With the advantages of high energy conversion efficiency,high power density and zero environmental pollution,proton exchange membrane fuel cell（PEMFC）is considered as one of the most promising clean energy sources for transportation,stationary power plants,submarines,spacecraft and other applications in the 21st century.However,while recent advances in fundamentals and applications have propelled PEMFC to become a mature technology,its long-term durability remains inadequate,hindering its large-scale commercialization.In proton exchange membrane fuel cell systems,there is a loss of cell performance after long-term operation due to degradation of the materials contained in the membrane electrode assembly（MEA）,which is mainly related to the degradation of the electrocatalytic materials（high specific surface area carbon-loaded Pt-based nanoparticles）.The corrosion of the carbon support,i.e.the electrochemical oxidation of carbon,not only directly causes the reduction of the electrode catalyst activity（shedding and agglomeration of Pt nanoparticles）,but the resulting oxygen diffusion problem will lead to a more serious degradation of the cell performance.Consequently,the corrosion of carbon supports caused during the operation of PEMFC system is one of the important technical challenges that need to be overcome to improve the fuel cell lifetime.In order to solve the corrosion problem of the catalyst support,the majority of researchers have not only developed new carbon materials such as carbon nanomaterials（CNT and CNF）,modified graphite,mesoporous carbon materials and carbon aerogels for fuel cell catalyst supports,but also successively some non-carbon materials that replace traditional carbon-based carriers have been explored,including metal nitrides and carbides,metal oxides,and conductive polymers.In view of above,starting from development of new type of high-stability supports,this thesis designs and develops the following two carrier materials:using the excellent properties of Zeolitic Imidazolate Frameworks（ZIFs）to prepare porous carbon materials and V/Nb nitride non-carbon supports with good stability in high-acid solutions,and their electrochemical stability was investigated.Mainly include the following:（1）Study on the controllable and batch preparation of ZIF-8-derived carbon.ZIF-8precursors of different particle sizes（100 nm-1μm）were prepared in a controlled manner by controlling the feeding ratio,solvent dosage and reaction temperature in order to obtain different sizes of the derived carbon materials.The methanol solvent used can be easily recovered,and the precursors of fixed size can be prepared in batch by scaling up the feeding ratio and solvent dosage with other factors.The derivatized carbon material has superb structural stability,and the structure is ordered and uniform in size after high-temperature carbonization,maintaining its unique dodecahedral cubic structure.In addition,the derived carbon retains the excellent qualities of high specific surface area and porous structure of the precursor,which is conducive to the uniform dispersion and stabilization of Pt nanoparticles on its surface.It was found that increasing the carbonization temperature could enhance the physicochemical properties of the derived carbon,among which Carbon-1000℃ has better electrical conductivity and specific surface area,and Pt/Carbon-1000℃ also exhibits optimal electrochemical activity and good cycling durability.（2）Study of the relationship between graphitization degree and stability of ZIF-8 derived carbon.It is generally accepted that amorphous carbon can readily react with oxygen in the cathodic environment of fuel cells to form surface oxygen-containing groups and subsequently further evolution to generate CO₂.In view of the stronger resistance of graphitic carbon to electrochemical oxidation,we introduced transition metal M（M=Co,Ni,Fe,Mn）to participate in the carbonization process of ZIF-8 precursors and obtained the derivative carbon with a high degree of order（degree of graphitization）carbon material M-GC,where Co has a significant advantage in forming the highest degree of graphitization at the optimal temperature and duration of the carbonization process.Compared to the conventional preparation of graphitized carbon up to 3000℃,graphitized derivative carbon can be obtained at 1000℃ at low cost under transition metal catalysis.Durability tests showed that the graphitized derivatized carbon-loaded Pt catalyst（Pt/Co-GC）had superior electrochemical stability than the Vulcan amorphous carbon-loaded Pt catalyst（commercial Pt/C）,with half-wave potential losses of 19 m V and 105m V for Pt/Co-GC and commercial Pt/C,respectively,after durability tests at 0.6-1.0 V potential;and at The results indicate that increasing the graphitization of the carrier can effectively enhance the stability of the catalysts.（3）Design synthesis and performance study of a new corrosion-resistant nitride(V_0.5Nb_0.5N).A V/Nb-based nitride was prepared and a ternary highly acid-resistant V_0.5Nb_0.5N non-carbon carrier was successfully synthesized using a complexation-nitration strategy.V_0.5Nb_0.5N retains the advantages of excellent electrical conductivity of VN and high specific surface and stability of Nb_0.8N,and Pt nanoparticles are most uniformly dispersed on its surface and exhibit optimal electrochemical activity.In addition,the half-wave potential loss after durability tests at 0.6-1.0 V working potential and 1.0-1.5 V high potential was only 13 m V and10 m V,and this ternary nitride loaded Pt catalyst has excellent electrochemical stability.This nitride as a carrier avoids the carbon corrosion problem and has potential application value.