Study On Structural Design And Activity Regulation Of New Electrocatalyst

The excessive dependence and consumption of traditional fossil fuels have led to a series of major issues that seriously endanger human survival,such as environmental pollution and global warming.Therefore,it is imperative to develop clean energy,reduce fossil energy consumption,and build a green and sustainable energy system.In this process,new energy electrocatalytic conversion technologies using water and hydrogen as the media play an important role.In this dissertation,a series of research work has been carried out on the development of efficient and stable catalysts,which are the core issues involved in electrochemical reactions（hydrogen evolution reaction HER,oxygen evolution reaction OER,oxygen reduction reaction ORR）.And a series of catalysts have been studied and developed using the method which closely combining theoretical calculation and experimental research,with a core idea of structural design to increase the number of active sites and the regulation strategies to improve intrinsic activity.1.Using the density functional theory（DFT）calculations,strain effects and ligand effects on the HER performance of different Ir metal alloys were investigated.In this work,we found that the alloy structure of Ir₃Ni show good intrinsic hydrogen evolution activity and could be further improved to the optimal activity by adjusting compressive strain.After that,a three-dimensional nanoframe structure was selected by taking the advantages in strain formation and stability.Through the selective etching of phase separated Ir-Ni rhombohedral dodecahedral nanocrystals,high-quality ultra-fine（sub 1nm）Ir-Ni nanoframeworks were obtained.This framework structure not only achieves the ultimate exposure of Ir₃Ni active sites(ECSA:123.4 m²/g_Ir^-1),but also generates and stabilizes a large number of local strain region through its own structural characteristics during the synthesis process.It exhibits excellent HER activity in both acidic and alkaline media,exhibiting ultra-low overpotential(13 m V and 19 m V,at 10 m A cm^-2)and high mass activity(9.93 mg_Ir^-1 and 2.11 mg_Ir^-1,at an overpotential of 30 m V).2.For designing the oxygen evolution reactions catalysts,density functional theory（DFT）calculations were used to investigate the oxygen surface treated Ni₂P catalyst and the oxygen vacancy formation Ni₃Fe-hydroxide catalyst.The results showed that:A.For the structure of Ni₂P catalyst,the O adsorption and O substituted Ni₂P had better OER electrocatalytic intrinsic activity,with their OER overpotential values of 0.27 V and 0.45V,respectively,which were significantly lower than that of original Ni₂P（0.96 V）.In addition,the catalytic activity of O adsorption and O substituted Ni₂P nanosheets could be also well explained using a modified p-band center theoretical model.B.The modification strategy of generating oxygen vacancies on the surface of Ni₃Fe-hydroxides can adjust the the d-band center positions of the surface Fe and Ni sites and optimize the adsorption strength of each intermediate in the OER reaction,thus improving the OER activity.In addition,the Ni Fe-hydroxide catalyst with oxygen vacancies was experimentally synthesized by hydrothermal method.It has excellent OER experimental activity,with an overpotential of only 214 m V at 10 m A cm^-2,which is superior to most of the newly reported OER catalysts.3.Based on the theoretical calculations and experimental research,two strategies for improving the ORR activity and stability of existing Fe based carbon nitrogen monoatomic catalysts（Fe-N-C）have been proposed.In this dissertation,we found that:A.By constructing molecular heterostructures of Fe and Co,the magnetic moment of the metal center can be adjusted to optimize its adsorption strength with ORR intermediates,which greatly improves the intrinsic ORR activity.Moreover,in the experiments,we developed a"two-step specific-adsorption strategy"to synthesize Fe Co MHs catalysts,and used the in situ rotation in AC-HAADF-STEM characterization combining with EELS testing,to characterize the possible formation of the Fe Co diatomic structure,and then confirming the formation of Fe,Co molecular heterostructures.In alkaline electrolytes,the synthesized Fe Co-MHs exhibit excellent catalytic activity for ORR(E_1/2=0.95 V),indicating that Fe Co-MHs is one of the best non-noble metal ORR catalysts.At the same time,as a cathode catalyst,Fe Co-MHs gives a 319.73 m W cm^-2 in ZAB which can also run stably for over 1300 hours at 10 m A cm^-2,demonstrating its potential commercial application value.B.It could be also found that the construction of Fe,Mn dual-atom sites can neutralize the positive charges on the adjacent C atoms,and thus inhibiting the oxidation of the adjacent C site,improving the stability of the supports.Moreover,the formation of Fe-Mn dual-atom sites can also reduce the negative effects of C oxidation on the reduction of catalytic activity at the Fe sites and the strength of the Fe-N bond,thereby significantly improving the stability of the active sites of the catalyst.In the experiments,we developed a coating-pyrolysis synthesis strategy to prepare Fe Mn-N-C bimetallic catalysts,and confirmed the synthesis by physical characterization such as STEM,XAS,and XPS on Fe-NC@Mn-NC The catalyst prefers the advantageous structure of Fe-Mn-N₆ with high activity and stability.Compared to Fe-NC,the Fe-NC@Mn-NC catalyst showed significantly enhanced activity and stability in both RRDE and fuel cell tests.4.Exploring the structural properties of nanomaterials is of great significance for constructing composite interface catalytic reactions with multi-field coupling.The stability,electronic band structure,and optical absorption properties of 2D MX and Janus M₂XY monolayer structures were studied using density functional theory.We found that Janus M₂XY and the pristine MX monolayer structures have negative formation energies,indicating that they can be synthesized on thermodynamics views.At the same time,both MX and M₂XY monolayer structures exhibit suitable band gap that exceed the minimum band gap requirement（1.23 e V）required for photocatalytic water splitting.However,all MX monolayers are indirect bandgap materials,while Ga₂STe,Ga₂Se Te,In₂STe,and In₂Se Te monolayers constructed through Jansu structures are direct bandgap materials.The mechanism for this transition were induced by the valence band maximum at theΓpoint,being composed of the px and py orbitals of the M and Y atoms in M₂XTe instead of the pz orbitals of the M and X atoms in the MX and other M₂XY monolayers.Moreover,the pristine MX and Janus M₂XY single-layer structures a considerable absorption coefficient in the visible light region（～3×10⁴/cm）.After that,we also studied the formation of multilayer structures for Ga S and Ga₂SSe,and found that the formation of this multilayer structure is also conducive to the reduction of material band gaps and the improvement of absorption coefficient.