Preparation Of Black Phosphorus-based Materials And Study On Their Electrocatalytic Properties For Ammonia Synthesis

With the rapid growth of global population and the continuous expansion of economic scale,the environmental pollution caused by the massive consumption of fossil fuels has become increasingly serious,and the issue of future energy security has attracted widespread attention.Develop sustainable,fossil-free ways to develop renewable energy and conversion technologies that can reduce CO₂ emissions and meet the growing energy needs of human society.Electrocatalysis plays a key role in clean energy conversion technology due to its advantages of simple operation and high energy conversion efficiency.It is one of the feasible ways to develop new energy technologies to convert small molecular compounds in the atmosphere(e.g.,N₂,CO₂,etc.)into high value-added chemicals(e.g.,ammonia,CO,hydrocarbons,etc.)through coupling with renewable energy.Electrocatalysts play a key role in these energy conversion technologies by increasing the speed,efficiency and selectivity of the chemical transformations involved.However,the development and design of highly efficient electrocatalysts is still the biggest challenge to improve catalytic performance and make clean energy technology widely applied.Appropriate nano-structured electrocatalysts can enhance the interaction between catalysts and small molecule compounds of reactants,promote the adsorption and activation of small molecule of reactants,reduce the energy barrier of chemical reactions,and finally achieve more efficient conversion of clean energy.The aim of this thesis is to design and synthesize a highly efficient and stable catalyst for electrocatalytic reduction of N₂.Based on the good performance of black phosphorus for electrocatalytic synthesis of ammonia,four new and efficient catalysts for electrocatalytic synthesis of ammonia were prepared based on black phosphorus.The catalytic activity of each catalyst was evaluated by various testing methods.It lays a theoretical and experimental foundation for the preparation and application of black phosphorus-based materials.The main research contents are as follows:(1)Reducing the size of black phosphorus can expose more reactive sites of electrocatalytic synthesis of ammonia,which can improve the catalytic performance.At room temperature and pressure,Faraday efficiency of 13.02%and ammonia production rate of 36.91?g h^-1 mg^-1_cat.were achieved under-0.5 V in 0.01 M HCl solution.The electrical cycle test shows that BPQDs has good stability and selectivity.(2)Doping is an effective method to regulate the electronic structure and stability of catalysts.The electronic structure was adjusted by doping copper ions to enhance the electrocatalytic performance.Electrochemical test results showed that the addition of copper effectively improved the performance of BPQDs electrocatalytic synthesis of ammonia.Under room temperature and pressure,the ammonia yield of electrocatalytic synthesis of ammonia was 48.34?g h^-1 mg^-1_cat.and Faraday efficiency was as high as 14.6%.The cyclic stability test showed that Cu-BPQDs had good stability,because copper ions formed phosphorous copper bonds with phosphorus atoms on BPQDs,which inhibited the reaction between BPQDs and oxygen.At the same time,copper ions as the active center of catalysis improved the activity and stability of electrocatalytic synthesis of ammonia.The density functional theory calculation shows that the control step of copper doped BPQDs electrocatalytic synthesis of ammonia is from*NN to*NNH.The addition of copper ions reduces the reaction energy barrier in the control step of BPQDs electrocatalytic synthesis of ammonia,which enables the reaction to reduce nitrogen at a lower reaction energy and effectively improves the catalytic performance of BPQDs.(3)Recombination with other catalysts is also one of the methods to enhance the catalytic activity of the catalyst.Although Cu-BPQDs have larger specific surface area and more active NRR sites,they have better electrocatalytic activity.However,Cu-BPQDs tend to aggregate,which inevitably leads to loss of active sites and poor electrical conductivity.In order to further improve the performance of Cu-BPQDs electrocatalytic synthesis of ammonia.Cu-BPQDs and MWCNTs with good electrical conductivity were prepared by ultrasonic composite Cu-BPQDs/MWCNTs.Electrochemical test results show that Cu-BPQDs loaded into MWCNTs can effectively improve the electrocatalytic ammonia synthesis performance of single Cu-BPQDs.Under room temperature and pressure,in 0.01M HCl electrolyte,when the applied voltage is-0.4V,the Faraday efficiency of ammonia synthesis is 16.3%,and the ammonia yield is increased to 52.83?g h^-1 mg^-1_cat..Cyclic stability tests show that Cu-BPQDs/MWCNTs composites have good stability and selectivity.(4)Although Cu-BPQDs loaded into MWCNTs improved their dispersion and conductivity,the performance of electrocatalytic synthesis of ammonia was enhanced.However,due to the low or no activity of MWCNTs in the electrocatalytic synthesis of ammonia,the improvement of the performance of monolithic catalysts for the electrocatalytic synthesis of ammonia is limited.In order to solve this problem,we loaded Cu-BPQDs onto Ti₃C₂ nanosheets with electrocatalytic activity for nitrogen,which not only improved the conductivity and stability of Cu-BPQDs,but also prevented their agglomeration and released more active sites.At the same time,the coupling effect of Cu-BPQDs formed at the interface,thus further enhanced the activity of electrocatalytic ammonia synthesis.The results of electrochemical test show that Cu-BPQDs/Ti₃C₂ has good electrocatalytic performance.Under room temperature and pressure,the ammonia yield of electrocatalytic synthesis of ammonia is 54.31?g h^-1 mg^-1_cat..Faraday efficiency is 18.1%.The cyclic stability test shows that Cu-BPQDs/Ti₃C₂ has good stability,which is due to the formation of P-O-Ti bond after Cu-BPQDs and Ti₃C₂ compound,which improves the conductivity and stability of Cu-BPQDs.