Synthesis And Applications Of Carbon-based Transition Metal Nanosheet Materials In Energy Conversion

Due to the new concept of sustainable development,the development of new energy conversion system becomes the trend of future energy structure.The crucial application of energy conversion such as hydrogen and oxygen reactions in electrolysis of water(HER,OER),oxygen reduction reaction in fuel cells,and hydrogen evolution in photoelectrochemical water splitting,are often limited by their slow reaction dynamics.The core issue is to develop catalysts that are inexpensive,highly reactive and stable,while the equipment used in these devices is also important for improving the energy conversion efficiency.Nobel-metal catalysts such as Pt,Ir,Ru.etc.exhibit excellent performance in energy conversion reactions,but the small reserves on the earth severely restrict their large-scale practical applications.Therefore,the rational design and development of low-cost,high activity,and good stability transition metal catalysts as a substitute has become a hot topic of current research.However,the shortcoming of transition metal catalysts is liable to form oxides in the reaction,resulting in poor conductivity and high energy free energy of reaction intermediates,also severely limit their development.Carbon materials are considered to be an ideal catalyst support material because of their superior conductivity and large specific surface area.Therefore,in order to improve the electrochemical reaction rate in each energy conversion reaction,this paper relies on the design of carbon-based transition metal two-dimensional(2D)material to expose more active sites,improve the conductivity,and optimizes the morphology and composition to maximize the response to dynamicas of different reactions of energy conversion.(1)Aiming at the poor electrocatalytic oxygen evolution performance of Co(OH)2 nanosheets,a N-doped carbon nanosheets structure modified by in-situ formation of Co nanoparticles in Co(OH)2 nanosheets was investigated.The in-situ formation of the 2D carbon nanosheets greatly improves the conductivity,and the successful incorporation of N changes the electronic structure of the carbon material.Thereafter,after etching a portion of Co nanoparticles exposed on the nanosheet by acid,a large number of pore structure are left on the sheets,which is very advantageous for exposing more active sites.The composite exhibits excellent performance in an electrocatalytic oxygen production reaction with an overpotential of 10 mA/cm2 in 1 M KoH of only 270 mV and a Tafel slope of 73 mV/dec.The material exhibits excellent HER performance,which reaches an overpotential of 162 mV at-10 mA/cm2 in 1 M KOH.The catalyst also exhibits the excellent overall water splitting performance with a voltage of 1.68 V to reach the 10 mA/cm2,and has the good stability performance.(2)Based on the protective effect of N-rich metal phthalocyanine compounds on the morphology of two-dimensional nanosheets,the morphology and properties of the products after interaction with amorphous Co-B nanosheets were further explored.After a high temperature reaction at 800?,NiCo alloy nanoparticles were embedded in B,N co-doped carbon nanosheet material(NiCo@BNC-800).The double-heteroatoms doping in two-dimensional carbon material greatly enhance the conductivity of the catalyst.At the same time,the synergistic effect of NiCo alloy nanoparticles and B,N co-doped carbon materials reduces the adsorption free energy of ORR reaction intermediates.Therefore,NiCo@BNC-800 exhibits excellent ORR performance with an onset potential of 1.03 V,a half-wave potential of 0.85 V and a limiting current of-5.9 mA/cm2.Meanwhile,it also presents excellent oER performance with an overpotential of 10 mA/cm in 1 M KOH of only 230 mV and a Tafel slope of74 mV/dec,it also shows the good stability.(3)Due to the synergistic effect of double transition metals plays an important role in improving the kinetics of catalytic reaction,a method for synthesizing NiCoB catalyst(NiCoB-GO)directly on GO surface was explored based on the synthesis method of amorphous Co-B nanosheets.Compared to NiCoB,which is a self-assembled sheet-like superposed structure,the introduction of GO largely limits their stack growth.It has been found that the specifie surface area of the amorphous NiCoB-GO catalyst is significantly larger than that of NiCoB,which is significant for increasing the contact area of the catalyst and the electrolyte,and exposing more active sites.NiCoB-GO exhibits excellent hydrogen production performance in neutral conditions.A low overpotential of 110 mV is needed to reach the-10 mA/cm2 in 1 M Na2SO4,and it also shows the good stability.After that,based on the characteristics of amorphous NiCoB-GO nanosheet catalyst which is easy to form good interface and excellent hydrogen production in neutral conditions,the Cu2O/C/NiCoB-GO hetero-junction photocathode with good interface is successfully prepared.The preparation Process of this photocathode is as follows:a carbon film with a thickness of 20-30 nm was firstly deposited on the surface of Cu2O by chemical vapor deposition(CVD),and the photoelectrode(Cu2O/C)exhibits excellent optical properties.Laterly,the amorphous NiCoB-GO nanosheet catalyst was spun onto the surface of Cu2O/C.Due to the formation of good interface and the synergistic effect of the carbon film and NiCoB-GO provides a transport route for photo-induced electrons inside Cu2O,the separation of electron-holes is effectively promoted ultimately.The composite photocathode has a photocurrent value of-2.9 mA/cm2 at 0 V vs.RHE and an onset potential of 0.63 V,also the stability is improved.In order to develop more efficient catalysts for various energy conversion reactions,this thesis focus on the design of highly conductive carbon-based transition metal two-dimensional nanosheets materials composite structure to expose more active sites and improve the conductivity,and investigate the structure-activity relationship of energy conversion reactions.This paper provides a useful reference of synthesis of catalysts for other energy conversion such as CO2 reduction and N2 reduction,and provides a new idea for the good interface of semiconductor/catalyst in photovoltaic system.