Preparation And Bifunctional Electrocatalytic Properties Of Cobalt Nitrogen/phosphorus Doped Porous Carbon Materials Based On Metal-organic Framework

Nowadays,fuel cells,metal-air batteries and water splitting as the electrochemical energy conversion and storage devices have been paid intense attention in renewable energy system with the constant increase of clean and sustainable energy demand,which have excellent conversion efficiency and high energy capacity.The application of these advanced technologies in daily life mainly depends on a series of electrochemical reactions,such as the hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）,hydrogenation reaction（HOR）and oxygen reduction reaction（ORR）.However the sluggish kinetics of those half-reaction hinder the efficiency of the electrochemical energy conversion and storage devices,therefor the effective electrocatalysts are needed to improve the catalytic efficiency.Until now,Pt,Ir,Ru-based alloys and their oxides are still well-known as the ORR/HER and OER catalysts.Because of the exorbitant price and scarce reserves,developing a cost-efficient and high-stability catalytic materials is also a big challenge for their large-scale application.Metal-organic frameworks（MOFs）are a class of 3D porous materials that consist of metal ions/clusters and organic ligands.Thanks to the distinctive structural flexibility and tuneable porosity for MOFs,MOF-derived ORR/OER/HER materials can be widely used in electrochemical energy conversion.MOF-derived materials are easily to be adjusted in catalytic activity and durability by changing the design and preparation methods and controlling the preparation conditions of MOFs.In this paper,a novel metal nitrogen/phosphorus-doped heteroatom porous carbon materials were prepared using Bio-MOF-11 by changing the carbonization conditions and doping,and they have excellent catalytic activities as the ORR/OER and HER electrocatalysts.The main research content is as follows:（1）Bio-MOF-11 was successfully prepared though solvothermal method using cobalt acetate tetrahydrate as a metal salt,adenine as a ligand,and N’N-dimethylformamide（DMF）as a solvent by adjusting the solvothermal reaction conditions,such as the ratio of metal to ligand,the hydrothermal reaction temperature and time.The results of XRD,TG and SEM showed that Bio-MOF-11 had the octahedral morphology with uniform size,monodisperse and good thermal stability.（2）N-rich Bio-MOF-11 was used as the precursor to prepare electrocatalytic materials.Cobalt-based,nitrogen-doped porous carbon materials with in situ grown CNTs（Co-N/PC@CNT-Ts）were obtained through the pyrolysis of Bio-MOF-11 at different temperatures（600,700,800 ^oC）,which have both high catalytic activity and stability for ORR/OER.And Co-N/PC@CNT-700 has the highest catalytic activity,which affords a low reversible overvoltage（bifunctional performance parameter）of0.862 V between ORR and OER.According to the characterization results,we can concluded:First,there are the highest amounts of CNTs on the surface of Co-N/PC@CNT-700,which not only improves the specific surface area of the material,but also accelerates the electrons transfer during electrocatalysis.In addition,the increased exposure of active centers（Co N_x）in Co-N/PC@CNT-700 can improve the electrochemical activity.The synergistic effect of uniform Co N_x active sites embedded in graphitized carbon and unique in situ grown CNT structure is attributed to the bifunctional electrocatalytic performance.（3）The simple and scalable method was reported to prepare Co-Co P@N/C-CNT with two-step of subsequent pyrolysis and low temperature phosphidation at different atmospheres based on Bio-MOF-11 precursor.Cobalt N-doped carbon with in situ carbon nanotubes（Co@N/C-CNT）was easily prepared by the pyrolysis of Bio-MOF-11 at Ar.Phosphoric Co-Co P@N/C-CNT inherited the morphology and porosity of Co@N/C-CNT.Importantly,Co-Co P@N/C-CNT shows a significantly enhanced bifunctional catalytic activity towards HER and OER.Remarkable,its onset potential of 66.1 m V for HER and 1.43 V vs.RHE for OER,and the Tafel values were66 and 87 m V dec^-1,respectively.Compared with the MOF-derived Co@N/C-CNT and Co P,the more excellent performance of Co-Co P@N/C-CNT mainly depends on heteroatom P/N-doped,the special structure of in-situ N/C-CNTs and porous structure.Their synergistic effect not only provide active sites（Co N_x,Co P）,improve electrochemical active surface area but also accelerate electronic conductivity.