Molten Salt Assisted Synthesis Of Metal-Nitrogen-Carbon Electrocatalysts For Oxygen Reduction And Application

Oxygen reduction reaction?ORR?is an important reaction process that occured on the air electrodes of metal-air batteries and fuel cells.However,the 4 electrons kinetics process leads to large overpotential.Therefore,electrocatalysts are needed to improve discharge efficiency of these batteries.Although platinum-based noble metal catalysts have high electrocatalytic activity under acidic or alkaline conditions,they are not suitable for large-scale applications because of their high price and poor stability.Therefore,the development of low cost,high efficient and stable non-noble metal oxygen reduction electrocatalysts has become a hot topic all over the world.Among these non-noble metal ORR catalysts,metal-nitrogen doped carbons?M-N-C,M=Fe,Co,Ni,binary metals,etc.?have excellent ORR electrocatalytic performance in both acidic and alkaline solutions.Generally,M-N-C catalysts can be synthesized by annealing precursors containing metals,nitrogen and carbon or metal organic framework and followed by acid etching.However,M-N-C catalysts have the problem that metal particles are easy to be agglomerated and not easy to be washed off by acid,which leads to the decrease of surface area and the density of active sites,thus reducing the ORR activity.This dissertation was going to solve these problems mainly by zinc chloride molten salt assisted method.?1?Iron-polypyrrole precursor was obtained by in situ polymerization of ferric chloride and pyrrole monomers,followed by mixing with zinc chloride,high temperature annealing,and pickling to obtain micro-mesoporous iron-nitrogen-carbon materials(Fe-N-C_Zn).It was found that zinc chloride could inhibit the decomposition of ferric chloride,make it form a co-melting salt and attach to the surface of the precursor to form a zinc ferrate coating layer during annealing.The electrochemical measurements showed that the onset and half-wave potentials of the Fe-N-C_Zn were 0.98 V and 0.90 V vs.RHE?Reversible Hydrogen Electrode?,respectively,and the limiting current density could reach to 6.06 mA/cm² in oxygen-saturated 0.1 M KOH solution at 1600 rpm with mass loading of 0.6 mg/cm².i-t test showed that the current density coultd still maintain 93.2%of the initial current density after 18000s test.The zinc-air battery test showed that Fe-N-Z_Zn+RuO₂ electrode had better cycling stability and charge-discharge rate performance than Pt/C+RuO₂ electrode.After cycling at 5 mA/cm² for 200 cycles,the overpotential increased only 0.19 V.?2?The electrocatalytic activity and stability were further enhanced by combining of nanosization and iron,cobalt bimetallic doping.⁵0 nm monodisperse polypyrrole nanospheres were obtained by polymerization of pyrrole monomer in ferric chloride-polyvinyl alcohol solution.After flocculation and washing,the polypyrrole nanospheres were mixed with a certain amount of ferric chloride,cobalt chloride and zinc chloride,and then annealed and acid etched to gain iron-cobalt-nitrogen doped carbon hollow nanospheres?Fe₅Co₁NC?.The characterization showed that the surface area of Fe₅Co₁NC was as high as 1222 m²/g.Co doping not only could form iron and cobalt bimetallic sites with high ORR activity,but also could effectively enhance the graphitization degree and pyridinic-nitrogen,graphitic-nitrogen content of the carbon.The electrochemical measurements showed that the onset,half-wave potentials of 0.2 mg/cm² material were0.95 V and 0.84 V vs.RHE,respectively,and the limiting current density could reach to6.90 mA/cm² in oxygen-saturated 0.1 M KOH solution at 1600 rpm,indicating that the catalyst had high metal atom utilization and active site density.After 10000 seconds i-t test,it could still maintain 95.5%of the initial current density.The zinc-air battery test showed that Fe₅Co₁NC+RuO₂ electrode had very high cycling stability.At 5 mA/cm²and 10 mA/cm²,the potential difference of charge/discharge could be stabilized at 0.8 V and 0.9 V,respectively,and could be cycled continuously 115 h?690 cycles?.?3?For simplifying the preparation of bimetallic-nitrogen-doped carbon materials and increasing the yield,commercial melamine foam was selected as nitrogen sources and frameworks.After wetting and adsorbing a certain amount of ferric chloride,zinc chloride and polyvinyl alcohol sol,porous and conjoined spherical iron-zinc-nitrogen-doped carbon?Fe-Zn-N-C?could be obtained by drying,annealing and pickling of above precursor.It was found that zinc chloride not only could make pores and regulate the structure of the carbons,but also could be doped into carbon lattice,which was beneficial to the formation of tetra-nitrogen coordination zinc single atom active sites.The surface area of Fe-Zn-N-C was as high as 1515.3 m²/g.The electrochemical test results showed that the onset and half-wave potentials of Fe-Zn-N-C could reach to 0.96 V and 0.87 V vs.RHE with mass loading of 0.6 mg/cm²,and the limiting current density was 6.0mA/cm² at 1600 rpm in O₂-saturated 0.1 M KOH solution.After 18000s i-t test,it could still maintain 89.9%of the original current density.The zinc-air battery tests also indicated the high cycling stability of the Fe-Zn-N-C+RuO₂ electrode.