Rational Design of Non-PGM ORR Electro-catalyst and Exploration on ORR Pathway

Author:Zhang Xiao Hua

Supervisor:shi jian lin


Degree Year:2018





PEMFCs(Proton Exchange Membrane Fuel Cells)are clean and highly efficient with only water and electricity as the final product,and could be applied to various fields such as automobile industry,unmanned aerial vehicles and etc.While the sluggish kinetics of ORR(oxygen reduction reaction)on the cathode requires large amounts of platinum to serve as ORR electro-catalysts,which severely limits the commercialization of PEMFCs.Among all the candidates,M-Nx/C(M=Fe,Co)type electro-catalyst is the most promising alternative to platinum.Our research is mainly focused on three parts:design and synthesis of highly active M-Nx/C electro-catalysts by dispersing M-Nx active sites,determining the location and density of M-Nx active sites and exploration on ORR pathway by modulating the surface state of the electro-catalysts.(1)As Co-N coordination bonds of Co-Nx active sites would break up when pyrolysis at high temperature,which leads to much lower density of Co-Nx active sites.To circumvent this phenomenon,we adopted the polymer network to disperse Co-Nx active sites and maintain the initial density of Co-Nx active sites.The as-prepared electro-catalyst exhibited reasonable ORR activity and excellent stability.Furthermore,we have determined the region where Co and N species co-distributed as the region where Co-Nx active sites situate by evaluating the distribution of Co and N species respectively,which provides the possibility for visual analyzing the distribution of Co-Nx active sites.Besides,the different response of ORR activity in acidic and alkaline media in the presence of probe molecules indicated relatively lower density of Co-Nx active sites,further validating that higher ORR activity in acidic medium is originated from higher density of Co-Nx active sites.(2)To further increase the density of Co-Nx active sites,we adopted zinc ion(Zn2+)to disperse Co-Nx active sites at atomic scale.By controlling the amount of Zn2+,we could synthesize Co-Nx/C electro-catalysts with various densities of Co-Nx active sites.The mass of cobalt ion could be calculated by subtracting the mass of metallic cobalt phase from the total mass of cobalt species.When Co/Zn approaches 15/85,ORR activity reaches the optimal.Furthermore,by introducing probe molecules to modulate oxygen adsorption behavior on the catalyst’s surface,hydrogen peroxide significant participation in ORR has been verified.Considering hydrogen peroxide reduction reaction activity,the high over-potential and sluggish kinetics of ORR is derived from the incomplete reduction of hydrogen peroxide.By differentiating the value of ring current,we have obtained the production or consumption rate of H2O2.It is confirmed that both the as-prepared Co-Nx/C and 20 wt%Pt/C catalyze ORR via two successive steps.The H2O2 yield is the result of competition between the desorption of H2O2 from the catalyst’s surface into the electrolyte and the reduction of H2O2 on the surface.Thus in order to minimize H2O2 generation during ORR,the catalyst and its surface should be reasonably designed to effectively adsorb hydrogen peroxide species,and more importantly,subsequently be able to efficiently reduce it into water.(3)To further understand the influence of the density of active sites on ORR pathway,we evaluated the ORR activity of Pt and CoNC at different oxygen partial pressures and obtained the relatively density of active sites of Pt and CoNC compared with the number of the surface adsorbed oxygen molecules.When oxygen partial pressure is lower,ORR activities of both Pt and CoNC are higher with larger kinetic current and lower hydrogen peroxide yield.The response of ORR acticity of Pt to oxygen partial pressure is more obvious than that of CoNC.In the presence of introduced probe molecules,their influence on ORR activity is much smaller when oxygen partial pressure is lower for Pt;While their influence on ORR activity is much larger when oxygen partial pressure is lower for CoNC.The different response to oxygen partial pressure between Pt and CoNC demonstrates the discrepancy between the densities of active sites of Pt and CoNC.