Zinc-air Battery Electrocatalysts Based On Binary Transition Metal Nitrogen Carbon Materials Based On MOFs

Zinc-air batteries(ZABs)are renewable energy conversion devices with the advantages of environmental friendliness,abundant manufacturing resources,simple and convenient operation and high energy density.They also have good application prospects in reducing environmental pollution and replacing fossil energy.However,the slow kinetics of oxygen reduction and evolution reactions(ORR and OER)corresponding to the discharge and charge processes of the air electrode seriously hinder the development of batteries.Although noble metal catalysts can accelerate the kinetic processes of ORR and OER,they have disadvantages such as high cost and scarcity of fabrication resources,which limit the large-scale use of zinc-air batteries.Therefore,the development of highly active and stable non-precious metal catalysts to accelerate the kinetics of ORR and OER and reduce the overall overpotential during battery operation is a current research hotspot.However,the surface free energy of single-metal catalysts is large,which makes the overall activity of the catalysts low,which is far from the current practical application requirements and needs to be further improved.Bimetallic catalysts can effectively avoid the above problems due to the synergistic effect between different metals.Non-precious metal catalysts represented by transition metals have been widely studied due to their good catalytic performance,low price and abundant reserves.Among many widely studied transition metal catalysts,M-N-C-type catalysts represented by Fe-NCs are characterized by their highly unsaturated coordination environment,small size effect,high intrinsic activity,and good interaction between metals and nitrogen-carbon substrates and other advantages are considered to be the most promising alternatives to noble metal catalysts.However,the surface free energy of single-metal catalysts is large,which makes the overall activity of the catalysts low,which is far from the current practical application requirements and needs to be further improved.Bimetallic catalysts can effectively avoid the above problems due to the synergistic effect between different metals.In this paper,binary transition metal carbonitride catalysts were prepared by combining transition metals with MOFs-derived carbon supports,and their microscopic morphology,structural composition and electrochemical performance were investigated.The specific work is as follows:(1)The porous carbon material(PNCH)obtained by the high-temperature carbonization of metal-organic frameworks compound ZIF-8 was used as the carrier,and nitrogen doped porous carbon polyhedrals supported Fe and Ni dual single-atomic catalysts(FeNi-DSAs/PNCH)were successfully prepared by a simple and rapid micro wave-as sis ted adsorption and subsequent annealing process.The existence of single atoms and the uniform dispersion of Fe and Ni dual single-atoms on PNCH in the catalyst were identified by SEM,TEM,HAADF-STEM,XRD,XPS and BET,and the electrochemical catalytical performances towards ORR including electrochemical activity,stability,methanol resistance,anti-poisoning ability,etc,were tested.At the same time,the materials were used as the catalyst of the air electrode for assembling a zinc-air batteries,and the battery performance was further tested.The above physical characterization and electrochemical tests show that the FeNi-DSAs/PNCH catalyst has excellent ORR catalytic activity(the half-wave potential E 1/2 is 0.89 V)and good stability(after continuous operation for 26000 s,the current density still maintains 96%of the original current density).In addition,the zinc-air batteries assembled with FeNi-DSAs/PNCH can produce a specific capacity of 802.18 mAh gzn-1,which is very close to the theoretical value of 820 mAh gzn-1 and far superior to that of the Pt/C assembled zinc-air batteries(664.78 mAh gzn-1).This work not only provides a simple method to prepare bimetallic single-atom catalysts,but also provides a feasible strategy to enhance catalytic activity by tuning the electronic structure of catalytically active sites in various catalysts.(2)By adding ferrous sulfate and cobalt chloride in the process for synthesizing two-dimensional metal-organic frameworks compound Zn-ZIF,two dimensional trimetallic MOFs nanosheets(ZnFeCo-ZIF)was synthesized by standing at room temperature.Then,FeCo alloy nanoparticles-doped porous nitrogen carbon nanotube catalysts(FeCo@NC-CNTs)were obtained by high temperature calcination and hydrochloric acid etching.The microscopic morphology and structure of the prepared materials were analyzed by SEM,TEM,HAADF-STEM,XRD,XPS and BET.The electrochemical tests related to ORR and OER were carried out,including electrochemical activity,stability,methanol resistance,etc.At the same time,the material was used as an air electrode for zinc-air batteries assembly,and the battery performance was further tested.The above physical characterizations and electrochemical tests show that the FeCo@NC-CNTs catalyst has excellent electrocatalytic performance,the half-wave potential E1/2 of ORR is 0.87 V,and the overpotential η10 of OER is 400 mV.In addition,the zinc-air batteries assembled with FeCo@NC-CNTs can produce a specific capacity of 741.06 mAh gZn-1,and the assembled batteries have good rechargeability.