Theoretical and Computational Investigations into Electrocatalytic Oxygen Reduction Reaction on Carbon-based Nanomaterials

Author:Li Yu Hang

Supervisor:peng feng


Degree Year:2018





Proton exchange membrane fuel cells,as one of new energy technologies,can directly convert the chemical energy of hydrogen,methanol and other fuels into the electric energy.Such a conversion is simple,high-efficient and environmentally friendly.However,at present,this technology has only been applied in the hydrogen fuel cell vehicles at a model level,and in the distributed kilowatt power generation at a experimental level.It has not been widely commercialized all over the world.One of the main reasons is that the cathodic oxygen reduction reaction(ORR)requires to be catalyzed by a relatively large amount of platinum group metal.The platinum is expensive and in the limited reserve.Besides,the stability of platinum-based catalyst at a high current density is relatively poor.Nowadays,Carbon-based nanomaterials,as a kind of metal-free catalysts,have been reported to possess a nice ORR activity,but the corresponding catalytic mechanism is still ambiguous.Moreover,the ORR activity in acidic electrolyte needs to be further enhanced.In addition to applications,ORR is substantial for the fundamental electrochemistry.In this dissertation,density functional theory(DFT)calculations were employed as the primary method,supplemented partially with molecular dynamics simulation and experimental methods to comprehensively investigate the structural properties of carbon-based nanomaterials and the origin of ORR catalytic activity.The theoretical insights help to guide the research and the development of carbon-based ORR catalysts.The primary contents in this dissertation are as follows.(1)Nitrogen-doped carbon nanotubes(NCNTs)exhibit an ORR activity comparable to the commercial Pt/C catalyst in alkaline electrolyte,but are less active in acid.The electronic structure and the surface solvent potential field distribution of NCNTs were studied by DFT calculation.It was found that the hydronium H3O+was stably located at 4-5?from the surface of the electrode under the combined electrostatic interaction produced by the NCNTs surface and the solvent.The intermediates and transition states during oxygen reduction reaction(ORR),hydrogen peroxide reduction reaction(HPRR)and hydrogen peroxide disproportionation(HPD)were designed and calculated.The results show that the energy barriers corresponding to the optimal four-electron ORR path,the two-electron ORR,the HPRR and the HPD are 0.729 eV,0.313 eV,0.690 eV and 0.434 eV,respectively.It indicates that O2 on the NCNT surface is easily reduced to H2O2.In addition,the rate determining steps of all reactions point to the process associated with the reduction and desorption of OH species due to the strong interaction between the carbon surface and the OH group,leading to a higher energy for the cleavage of the C-O bond.In summary,in acid,NCNTs catalyzes the ORR via a mixed reaction mechanism.The main feature is that the O2 molecule is reduced to OH and then further consumed by the HPD process.According to this mechanism,the electron transfer number of the overall ORR should be about 3.The experimental LSV test result is 3.086,which verifies the rationality of such a mixed mechanism.(2)It is essential to investigate the characteristics of oxygen molecule adsorption,activation and the interfacial electron transfer,in order to understand the natural role of nitrogen doping.The introduction of pyridinic nitrogen in the carbon material bulk phase leads to edge carbon at the pore inevitably,which causes a synthetic effect to change the spin density of carbon atoms.Since the O2 molecule is paramagnetic,the calculation results reveal a spin-dependent O2 adsorption behavior,where the carbon atom with high spin density is favorable for the adsorption of O2.From the insight into the frontier molecular orbital of O2,it is found that the energy of the lowest unoccupied molecular orbital(LUMO)is shifted as a function of the external electrostatic potential and the internal vibration.When O2 stands in the position with an external electrostatic potential of about+4 V,the LUMO will be reduced to a reasonable level,at which the electron transfer from electrode Fermi level to O2 LUMO occurs.The external electrostatic potential is primarily contributed by solvent dipoles and electrolyte ions in the electric double layer.Also,the provided electrostatic potential varies as the electrode surface configuration changes.Comparing the pure graphene with the nitrogen-doped graphene,calculation results indicate that the nitrogen doping causes the quantitative decrease in the difference between the electrode Fermi level and the O2 LUMO energy,enhancing the intensity of interfacial electron transfer.In addition,the diversity roles between the graphitic nitrogen and the pyridinic nitrogen are studies via the band structure calculation.If the substrate supports metal loadings,the graphitic nitrogen acts as an electron-rich impurity,making the carbon material work as an electron donor;the pyridinic nitrogen acts as an electron-deficient impurity,making the carbon material work as an electron acceptor.When both nitrogen species survive together,the specific electron transfer behavior depends on the ratio of pyridinic nitrogen to graphitic nitrogen in the carbon material.(3)The oxidation peak of superoxide anion,produced by one-electron reduction of O2,is observed distinctly in the CV test in acetonitrile.The equilibrium electrode potential for O2/O2-is-1.22 V(vs Ag/Ag+),and the corresponding equilibrium electron energy is-3.78 eV.Thus,the computational model can be established as a system including O2 and one H2O molecule.Based on CCSD(T)calculation,the potential energy surfaces are evaluated for electron transfer processes from[O2...H2O]0to[O2...H2O]-1and from[O2...H2O]-1to[OOH...OH]-2.In the equilibrium state,the thermodynamic potential difference of the first step is-3.754 eV,and the kinetic energy barrier is 0.17 eV.However,the thermodynamic potential difference of the second step is-2.912 eV,and the kinetic energy barrier is 0.40 eV.Accordingly,the electrode catalyst should provide the following functions:promoting the elongation of the O-O bond in O2 and stabilizing the intermediate OOH,in order to regulate the thermodynamics and kinetics for the second electron transfer process.The intermediates O2-,OOH and HO2 is in a relatively weak interaction with the surface of graphene without heteroatoms,which can not change the intrinsic reaction path of ORR.After nitrogen doping,the equilibrium position and the open degree of O2 and O2-stretching vibration potential energy curves tends to be altered.Besides,the oxygen-containing intermediates can chemically adsorbed on the surface,affecting the potential energy surface corresponding to proton coupled electron transfer.Eventually,O2-can be stabilized on the nitrogen doped graphene surface and be protonated to form OOH via proton transfer rather than electron transfer,reducing the probability of HO2-generation.