Preparation Of MOFs Based Electrocatalyst And Their Application In Oxygen Reduction Reaction

Oxygen reduction reaction（ORR）,the cathodic reaction of fuel cells,limited the development of fuel cells due to its sluggish kinetic process.Platinum-based electrocatalysts can effectively catalyze the ORR process.However,Platinum is costly and can easily be CO poisoned.The development of highly efficient non-precious metal catalysts（M-N_x,M-S_x and M-O_x,etc.）has attracted people’s attention.Metal-organic frameworks（MOFs）,with the merits of well-defined structure,adjustable topology and large specific surface area,is a kind of long-range ordered and highly crystalline porous materials formed by the interconnection of metal ions and organic ligands.Since MOFs are formed by coordination reaction of metal with N,S,O,etc.,they are excellent precursors for the preparation of carbon-supported M-N_x,M-S_x and M-O_x based catalysts.ORR occurs at the three-phase interface of solid-liquid-gas,and fully exposed active sites,fast mass transfer and charge transfer rates are needed.In this paper,with aim of low cost and simple synthetic process,we chose MOFs as the research object and improved their ORR performance by constructing defects and hierarchy structure.Three types of non-noble metal catalysts with high ORR activity were designed and prepared.The main research contents include:（1）Ni/Co-catecholates（Ni/Co-CAT）with higher specific surface area,hierarchically pore channels and structural defects were prepared by rapid,environmental and energy saving ultrasonic method.More active sites were exposed and the mass transfer rate was accelerated.Compared with the traditional hydrothermal method,the half-wave potential of Ni-CAT and Co-CAT prepared by ultrasonic method was-0.196 V and-0.116 V（vs.Ag/Ag Cl）,respectively.The potential was more positive than that of hydrothermal method and showing good electrochemical stability in neutral solution.Co-CAT is only 32 m V lower than commercial Pt/C.（2）The multilevel hierarchy electrocatalyst（h-ZIF-800）composed of three-dimensional porous carbon matrix and low-dimensional building blocks of nanotubes was constructed.h-ZIF-800 possed hierarchical porous structure,open accessible active sites and lower charge transfer resistance.At the same time,the micropores and carbon nanotubes catalyzed by the Co element exhibited the characteristics of oxygen oxyphilicity and hydrophobicity,which prevented the catalyst from being flooded by the electrolyte and maintained a continuous channel for transporting O₂.In the ORR test,it showed outstanding activity with half-wave potential of-0.083 V,26 m V positive than the commercial Pt/C and excellent stability.（3）Fe-N_x electrocatalysts with carbon nanotubes and Fe nanoparticles embedded in carbon nanosheets were synthesized by stirring calcination method.The structure exposed more active sites,reactants could sufficiently reach the active sites centers to react in the 2D plane.The carbon shell coated with Fe nanoparticles effectively prevented the dissolution of Fe nanoparticles,thus ensuring the durable electrocatalytic activity and stability.The optimized catalysts exhibited excellent electrocatalytic activity(acid:E_1/2=0.528 V,basic:E_1/2=-0.099 V)and excellent stability under both acid and base conditions.