Theoretical Investigation Of Proton Transfer Mechanisms And Electric Double Layer At Rutile Metal Oxide/Electrolyte Interfaces

Global climate warming and environmental pollution caused by the emission of fossil fuel gas,as well as people’s growing energy demand,have promoted the development of renewable and clean energy.Electrochemical devices,i.e.,lithium ion batteries,supercapacitors,fuel cells and solar cells,have emerged as promising energy storage and conversion systems owing to their green,pollution-free and zero-emission,and sustain up to thousands of charge and discharge cycles.The common feature of these electrochemical devices is that they consist of two electrodes and an electrolyte solution.The chemical reactions and energy conversion or storage occurs at the electrode/electrolyte interface,thus the interfaces are highly electrified under working condition.The net interfacial charge is compensated by counter-ions from electrolyte,which builds up the so-called electrical double layer（EDL）.The surface chemistry of metal oxides electrode is more involved than that of metal electrode,due to the surface protonic charge of metal oxide depend on the deprotonated or protonated of surface groups according to the pH of the electrolyte.The structure of EDLs dictates the chemistry and the physics of electrified interfaces,and the differential capacitance is the key property for characterizing EDLs.In this thesis,we selected the metal oxide electrode/electrolyte interface as the research project,the proton transfer mechanism and electric double layer at the oxide interface are studied.The main findings are summarized as follows:1.Water-mediated proton hopping mechanisms at the SnO₂（110）/H₂O interface from ab initio deep potential molecular dynamics.We simulate and extend the time scale of the trajectory to 1 nanosecond（ns）from ab initio deep potential molecular dynamics（DPMD）based on the 100 ps density functional theory data,to investigate the structural and dynamical properties of the interfacial groups at SnO₂（110）/H₂O interface.It is found three pathways of proton transfer reaction among the interfacial species,i.e.,proton transfer from terminal adsorbed water Sn5cOH₂ to bridge oxygen site Sn2Obr directly（surface-PT）or via a mediated solvent water molecule（mediated-PT）,and proton transfer between terminal Sn5cOH₂ and Sn5cOH（adlayer-PT）,due to the characteristic dissociative water adsorption.The average degree of dissociation of adsorbed water a is estimated as～0.63.Furthermore,by analyzing the orientations of water molecules at the interface,we find three orientation peaks distributed among adsorbed water molecules,which are 38°,60° and 87°.We also calculated the free energies of three types of proton transfer reactions,and we found the solvent plays as a mediation is helpful for interfacial PT reaction,which in turn leads to the lowest energy barrier,compared to that of the monolayer of adsorbed water interfaces.In addition,the temperature dependence（from 280 K to 430 K）of SnO₂（110）/H₂O interface also studied with the same deep potential.We found the melting point of bulk water at SnO₂（110）/H₂O interface is 318 K using PBE functional with D3 dispersion correction at NVT condition.As the temperature increases,the a increases first below 318 K and then goes to equilibrium of～0.62.2.Finite-field molecular dynamics simulation of proton hopping electrochemical interfaces:the cause of electrified SnO₂（110）.We construct the electrified SnO₂（110）/NaCl interfaces using the finite-finite AIMD simulations.It is found that the direction of dissociated proton transfer is opposite to that of the electric field in the EDL at low pH while the same at high pH.Because of this contrast,the degree of water dissociation increases with the pH and the free energy of water dissociation decreases with the pH.Based on the above observations,we quantitatively show that the dipole of interfacial adsorbed groups,i.e.,water molecule,hydroxyl and proton,at electrified SnO₂（110）/NaCl interfaces significantly modulates the double layer potential.We thus derive a theoretical model to obtain the relation between capacitance and interface polarization,and show an intriguing asymmetric distribution of the differential capacitance of water at the Helmholtz EDL.3.Origin of asymmetric electrical double layers at electrified oxide/electrolyte interfaces.At the metal oxide surfaces,water molecules have preferred orientations due to the chemisorption,where water dipoles point to the solution phase.This surface effect can be captured by introducing an auxiliary field which is not due to the proton charge but an intrinsic property of metal oxide surfaces.This offset due to the specific orientation of adsorbed water at the point of zero charge is the origin for the asymmetric EDLs at meta oxide-electrolyte interfaces.We also find that the dissociative water adsorption prefers the inner sphere binding of counter-ions,which in turn leads to a higher Helmholtz capacitance,compared to that of the nondissociative case at the interfaces.This work provides a molecular interpretation of asymmetric EDLs seen experimentally in a range of metal oxides/hydroxides.4.Comparison of proton transfer mechanism at the RuO₂（110）/H₂O interface and SnO₂（110）/H₂O interface.We first calculate the acidity constants of RuscOH₂ and Ru2ObrH⁺at RuO₂（110）/H₂O interface,which are 5.2 and 4.1,respectively.The PZC of RuO₂（110）interface is found as 4.7,which is consistent with the experimental values of 4.0～6.0.Furthermore,we extend the time scale of RuO₂（110）/H₂O interface to 1 ns from DPMD based on the 30 ps density functional theory data.It is also found that there are three pathways of proton transfer reaction at the RuO₂（110）interface:surface-PT,mediated-PTand adlayer-PT,which is the same as SnO₂（110）/H₂O interface.By analyzing the dipole orientation of water molecules at the interface,we find the main dipole peak distribute at 60°,which is very different from the three peaks of dipole orientation at the SnO₂（110）/H₂O interface.The degree of dissociation of adsorbed water at the RuO₂（110）is～0.49,which is smaller than that of SnO₂（110）interface～0.63.By calculating the free energy of proton transfer reactions,we found that the lowest energy barrier at RuO₂ is adlayer-PT,the highest energy barrier is surface-PT,while the lowest energy barrier of SnO₂ interface is mediated-PT.Therefore,the orientation of interfacial water molecules affects the pathway of proton transfer reaction at the metal oxide interface.