Study On Surface Functionalization Of Graphyne And Its Electrocatalytic Performance

As fossil fuel resources are depleted,energy consumption grows,environmental protections upsurge,and the demand for alternative and sustainable energy sources is rapidly growing.Fuel cells that directly convert chemical energy to electricity with high efficiency,high power density,and less pollution have been extensively studied to solve these energy issues.In fuel cells,the oxygen reduction reaction?ORR?plays a pivotal role in overall performance because it is a sluggish kinetic process.Thus,proper catalysts are needed to accelerate the ORR to improve the performance of fuel cells.So far,the most effective catalysts are based on platinum?Pt?.However,its limited reserve and high price limit large scale commercial applications of fuel cells.Moreover,the Pt-based electrodes also have some shortcomings such as low selectivity,poor durability and CO poisoning.So,developing cheap but efficient metal-free oxygen reduction reaction?ORR?electrocatalysts to substitute for Pt has been a hot issue in the energy and environment fields.Graphyne materials are potential candidates to fabricate low-cost but efficient metal-free oxygen reduction reaction?ORR?electrocatalysts.However,due to the coexistence of sp and sp² carbon atoms in graphyne,some factors playing important role in determining ORR activity have received little attention.In this paper,we systematically studied the effects of surface functional modification?doping,curling and vacancy defects?on the electronic properties and electrocatalytic performance of graphyne materials by density functional theory calculations.The aim is to provide theoretical basis for experimentally developing cheap and efficient catalysts.The main research content and conclusions are as follows:In the first chapter,the research background and significance of this thesis are summarized,including the development and application of fuel cells and their working principles.The application of graphyne materials and their functional modifications as fuel cell catalysts.Then,the purpose and significance of the current research and the problems to be solved are pointed out.In the second chapter,the density functional theory?DFT?and generalized gradient approximation?GGA?theory are described in detail.Then,we introduced several kinds of calculation software.In the third chapter,the ORR catalytic performance after Nitrogen doped graphdiyne?NGDY?was studied.The theoretically predicted overpotential(?_limit^ORR)of NGDY materials was0.442 V,which is comparable to that of Pt-based catalysts suggesting GDY is a candidate for non-expensive metal-free ORR catalyst.Results revealed that the good ORR performance of NGDY originates from the synergy of sp-N and sp²-N,which rules out the experimental proposal that sp-N doping is the dominating factor.Results further suggest that local positive charge is not a definite descriptor to predict ORR performance of GDY;instead ?G_O shows a better correlation with performance.Furthermore,the environment around the reaction site is critical for determining ORR performance.In the fourth chapter,the electrical properties and ORR catalytic properties of 2D graphene sheets curled to form 1D nanotubes were studied.The resulting?GyNTs are predicted to be excellent semiconductors with moderate band gaps ranging from 1.291 eV to 1.928 eV.In addition,the band gaps of zigzag?GyNTs and armchair?GyNTs show damped oscillatory behaviour,while those of C-?GyNTs do not show any chirality-or diameter-dependent oscillatory behaviour.It is revealed that the?2a,m?-?GyNTs,where a is positive integer,have nearly identical band gap values.When the graphyne sheet is rolled into nanotubes,the existence of curvature will have a certain effect on the activity of ORR.The results show the?5,0?-gGyNT would be excellent metal-free ORR catalysts.Its overpotential was computed to be 0.43 V and the number of active sites is up to 16.7%.Moreover,it is revealed that the curvature can tune the degree of exposure of p electrons of active site,thus tuning the ORR activity.In the fifth chapter,some different double-atom vacancies were introduced to?-graphyne?Gy?,?Gy,and?Gy.The results show the double-atom vacancy only leads to in-plane structural rearrangement for all three of the Gy systems.It was further revealed that the position of double-atom vacancy is a crucial factor in the manipulation of the electronic properties of?Gy and?Gy as compared with?Gy.After removing two adjacent carbon atoms,the softness of the carbon chain in the defect area increases,and the carbon chain with a higher flexible is more likely to deform when adsorbing the intermediate,thereby causing a certain effect on the catalytic activity.The calculated ?^ORR value of DAV1-?Gy is 0.435 V,indicating that the double-defect structure of graphyne is also an excellent non-metallic ORR catalyst.In addition,the prediction of catalytic activity by the descriptor-based method can also be applied to the double atom vacancy structure of graphyne,which provides another strategy for predicting and designing new catalysts.In the sixth chapter,we summarized the main conclusions and proposed the future studies.In summary,this paper systematically studies the electrical properties and oxygen reduction catalytic activity of graphyne after surface modification based on density functional theory.After surface modification?doping,curling,and vacancy defects?,the structure of graphyne will be deformed to a certain extent,and the electrical properties of the material will change accordingly.After the modification,the structure no longer has symmetry,and the charge of carbon atoms is no longer equivalent,which brings higher and more high catalytic activity reaction sites.We also found that after the modification of graphyne,the high-activity reaction site could not be accurately predicted by the positive charge alone due to the influence of the surrounding environment.In addition,the descriptor-based method to predict catalytic activity has been well applied in the three systems studied in this thesis,which is very important for further understanding the catalytic activity of graphyne catalysts,and also provides a new idea to design and develop energy materials.