Design Of Efficient Two-dimensional Electrochemical Catalysts Using Doping And Loading Of Transition Metals

Energy is an important material basis for mankind and natural society.Although traditional energy sources(such as coal,oil,natural gas and so on)bring about convenience to human beings,they exist obvious shortcomings:non-renewable and pollution of environment.Along with the continuous development of society and economy,the demand for energy is also growing rapidly.Therefore,using new clean and renewable energy to replace the consumption of traditional fossil energy is a hot topic in current theoretical and experimental research.Oxygen evolution reactions(OER),as anodic reactions of electrocatalytic water splitting,play a vital role in many energy storage and conversion systems,and it considered to be the efficiency bottleneck of energy storage and conversion.The oxidation of water molecules to oxygen molecules requires the continuous transfer of four electrons,which makes the reaction kinetics of OER face a huge challenge:the binding energy of all reaction intermediates should be optimized on the catalyst surface to minimize overpotential.Currently,the most advanced OER catalysts in alkaline media are iridium oxides(IrO2)and ruthenium oxides(RuO2).Although the catalytic of IrO2 and RuO2 is excellent,the material cost is high and the long-term stability is poor,which greatly limits the industrial development of OER.Transition metal layered double hydroxide(LDHs)are a two-dimensional material with a sandwich structure.The middle layer is an atomic layer of mixed divalent and trivalent metal cations,and the upper and lower layers are hydroxide anion layers.Recently,NiFe LDHs have been reported by many research groups to have great OER catalytic activity,and its catalytic efficiency can be adjusted by doping with other transitional metals.Although the existing experimental synthesis has improved the OER catalytic performance of LDHs,the active site of the reaction and the internal mechanism of the catalytic enhancement are still unclear.Therefore,it is necessary to further explore the structure activity relationship of doped NiFe LDHs in OER reaction by means of theoretical and experimental research.In this dissertation,transition metals(Cr,Mn and Co)doped NiFe LDHs with different molar ratios were designed by hydrothermal method,and the electrocatalytic oxygen evolution reaction performances of the synthesized samples were tested by a three-electrode system.The experimental results showed that the catalytic activity of different kinds of transition metal doped NiFe LDHs for OER in 1 M KOH solution was Cr-NiFe LDHs>Co-NiFe LDHs>Mn-NiFe LDHs>NiFe LDHs.Among them,the overpotential of Cr doped NiFe LDHs(molar ratio of Cr:Ni:Fe is 0.1:3.0:0.9)at the current density of 10 mA-cm-2 was 340 mV,and the corresponding Tafel slope was 59.3 mV·dec-1.In addition,first principle calculations were used to investigate the catalytic performance enhancement mechanism of transition metal doped NiFe LDHs for OER.The results show that the enhanced OER activity can be attributed to the change of the electronic structure of NiFe LDHs caused by the transition metal atom doping,which leads to the increase of the bonding strength of the reaction intermediate(O*).On the other hand,ammonia is an important chemical raw material.The hydrogenation of nitrogen to ammonia is an important cornerstone of modern chemical industry,and has far reaching significance for the development of human society.At present,the industrial synthesis of ammonia is mainly according to addition reaction of hydrogen and nitrogen under the conditions of high temperature,high pressure and catalyst.The conditions of this reaction process are harsh,and the reaction efficiency of a single cycle is too low.Under this background,biological enzyme nitrogen fixation,photocatalysis and electrocatalysis nitrogen reduction reactions(NRR)emerge as the times require.Electrocatalytic nitrogen fixation is considered to be the most promising method due to this reaction can be motivated at room temperature and ordinary pressure.The technical difficulty of electrocatalytic nitrogen fixation lies in the competition between nitrogen molecular activation and hydrogen evolution reaction.The investigation shows that the chemical combination of single atom and two-dimensional nanomaterials can regulate the electronic structure of two-dimensional materials and the chemical environment of single metal atom,which can improve the catalytic selectivity,stability and energy conversion efficiency of NRR.As an important member of transition metal chalcogenides,VS2 has a graphene-like structure,low preparation cost and abundant resources.In this work,two-dimensional VS2 material was synthesized by hydrothermal method,and then Fe1/VS2(Fe single atom was supported on VS2)catalyst was obtained by impregnation method.The results demonstrated that the Faraday efficiency of Fe1/VS2 electrocatalytic nitrogen reduction reaction was 17.95%at a voltage of-0.5 V vs.RHE,and the corresponding ammonia production rate was 4.93 ug.h-1.mg-1,with great stability.