Preparation and Electrocatalytic Properties of One-Dimensional Transition Metal Chalcogenide Nanomaterials

Author:Li Geng

Supervisor:hou shu en zhao ling


Degree Year:2019





With the energy crisis and environmental pollution becoming increasingly serious,hydrogen energy has become a key concern for people as an efficient and clean energy source.Realizing the commercial application of hydrogen energy,cheap and large-scale producing hydrogen has become its most basic challenges.Producing hydrogen by water splitting is one of the simplest and most effective methods in the current hydrogen producing technologies.Because of high overpotential and slow reaction kinetics of water splitting,the actual efficiency of water splitting is low,which limits its rapid development.Therefore,researching efficient and stable electrocatalysts has become an objective need for the development of water splitting technology.So far,the state-of-the-art electrocatalysts for hydrogen evolution reaction(HER)are Pt-based compounds,whereas the ruthenium,iridium and their oxides are highly efficient for oxygen evolution reaction(OER).However,the high cost and scarcity of noble metal-based catalysts seriously restrict their application in the field of water splitting.Therefore,it is of great significance to explore highly efficient and stable non-noble metal based catalysts.Among those non-precious metal based electrocatalysts,transition metal(Fe,Co,Ni,etc.)based catalysts have attracted much attention due to their abundant content,unique electronic structure and good electrochemical activity.In the past decade,one-dimensional(1D)nanomaterials and nanostructures have been substantially investigated in the field of water splitting due to their high electrochemically active area,fast charge transport,effcient mass transport of reactant species,and effective release of gas produced.It was found that one-dimensional nanomaterials exhibited lower overpotential and faster reaction kinetics as electrocatalysts,showing great potential in water splitting.Aiming at the problems of high cost and low activity of electrocatalysts,a series of one-dimensional transition metal chalcogenide nanocatalysts were designed and synthesized in this paper.By optimizing the morphology and structure of the materials,the catalytic activity of catalysts was improved,and the structure-activity relationship was discussed.The main research works are as follows:(1)Perovskite oxide with large specific surface area and high electrical conductivity were designed and found to be effective in increasing their OER activity and reaction kinetics.Carbon dots(CQDs)modified Ba0.5Sr0.5Co0.8Fe0.2O3-δ(BSCF-NFs)nanofibers(CQDs@BSCF-NFs)were prepared by electrospinning and electrophoresis.It has been found that CQDs and nanofibers could effectively improve the electrical conductivity and specific surface area of catalysts,then increasing the OER activity and reaction kinetics of catalysts.The electrochemical test results showed that CQDs@BSCF-NFs only required an overpotential of 350 mV to deliver the current density of 10 mA·cm-2,which was much lower than 380 mV of BSCF-NFs and 420 mV of bulk BSCF.CQDs could adjust the specific surface area,the concentration of surface oxygen vacancy and work function of BSCF-NFs to improve the electrocatalytic performance of catalysts.(2)The nanofiber catalysts with rich pores and uniform distribution of active materials were designed and found to be effective in improving their bifunctional catalytic activity.Combined with the characteristics of metal organic framework and nanofibers,S,N co-doped carbon coated Co9S8 nanofibers(Co9S8@S,NC-NFs)were prepared by electrospinning and high temperature sulfurization.Compared with bulk Co9S8@S,NC(specific surface area was 113.04 m2·g-1),the specific surface area of Co9S8@S,NC-NFs were greatly improved,increasing to 326.47 m2·g-1,which provided larger active area for electrochemical reaction.At the same time,Co-Nx structure and structural defects of S,N co-doping C structure provided new active sites for electrochemical reaction.The performance of catalysts was improved due to the morphology and structural defects.The results showed that Co9S8@S,NC-NFs exhibited favorable bifunctional catalytic activity under alkaline conditions.At a current density of 10 mA·cm-2,Co9S8@S,NC-NFs exhibited lower overpotentials of 300 and 108 mV for OER and HER,respectively.Applying it to the overall water splitting system,Co9S8@S,NC-NFs||Co9S8@S,NC-NFs could deliver current density of 10 mA·cm-22 by applying just 1.54 V and stably catalytic water decomposition up to 50 hours,showing highly efficient activity and stability for water splitting in strong alkaline medium.(3)The carbon nanofiber materials were designed and found to be effective in improving the stability and activity of catalysts.Porous nitrogen-doped carbon-coated NiSe2 nanofibers(NiSe2@NC-NFs)catalysts were prepared by electrospinning and one-step sintering.The results showed that nitrogen-doped carbon in nanofibers increased the structural defects and specific surface area of catalysts,then effectively enhanced the electrochemical active area and reaction kinetics of catalysts.NiSe2@NC-NFs exhibited good bifunctional catalytic activity in 1 M KOH.The results showed that NiSe2@NC-NFs only required overpotentials of 200 and 77.9 mV to drive a current density of 10 mA·cm-22 in strong alkaline media for OER and HER,respectively.Two-electrode system with NiSe2@NC-NFs as cathode and anode(NiSe2@NC-NFs||NiSe2@NC-NFs)exhibited high efficiency and stability of performance for water-splitting in alkaline media.NiSe2@NC-NFs||NiSe2@NC-NFs could afford the cell voltage of 1.57 V to deliver the current density of 10 mA·cm-2,and it could stably catalytic water decomposition up to 50 hours in alkaline medium.(4)The heterostructure containing a metal conductor was designed and found to be effective in improving the reaction kinetics and activity of catalysts.Boron-nitrogen co-doped graphite-coated Co/CoSe2 heterostructure nanotube catalysts(Co/CoSe2@B,NG-NTs)were prepared by simple high-temperature-pyrolysis.The results showed that heterostructure had a significant effect on the bifunctional catalytic activity of catalysts.One-dimensional nanotube structure effectively increased the specific surface area of catalysts,which provided larger electrochemical active area for catalysts.Graphite and B,N doped carbon structure greatly improved the conductivity of catalysts and enhanced the electrochemical reaction kinetics.Moreover,the charge transfer between Co and CoSe2 regulated the electronic structure of active materials and optimized the function of CoSe2,and then enhanced the performance of catalysts.The catalytic activity of catalysts were improved due to the morphology and structure.The results showed that Co/CoSe2@B,NG-NTs exhibited high bifunctional catalytic activity in alkaline electrolytes.In 1 M KOH solution,to deliver current density of 10mA·cm-2,Co/CoSe2@B,NG-NTs only required overpotentials of 230 and 70 mV for OER and HER,and the overpotentials were much lower than the latest reported non-precious metal based catalysts.Two-electrode system with Co/CoSe2@B,NG-NTs as cathode and anode(Co/CoSe2@B,NG-NTs||Co/CoSe2@B,NG-NTs)exhibited high efficiency and stabilityofperformanceforwater-splittinginalkalinemedia.Co/CoSe2@B,NG-NTs||Co/CoSe2@B,NG-NTs could deliver current density of 10mA·cm-22 by applying just 1.57 V in 1 M KOH,and it could maintain stable work for water splitting up to 50 hours.