Theoretical Study and Design of Electrocatalysts Based on Two-Dimensional Materials

Author:Ling Chong Yi

Supervisor:wang jin lan li qi


Degree Year:2018





Electrochemical energy conversion,including water splitting,CO2 reduction,N2 fixation and so on,may achieve the goal of sustainable and green production of fuels and chemicals.Stable,high-efficient and low-cost electrocatalysts play a key role in these energy conversion technologies.Although great efforts have been paid in the development of electrocatalysts during the past decades,none of them meet the requirement of practical utilization.Owing to their advantages of large surface areas,efficient charge transfer,excellent mechanical flexibility,low-cost and easy to modulate,layered two-dimensional materials have stood out from various kinds of catalysts.Besides,with the continuous enhancement of the performance of computer as well as the rapid development of relevant theory and numerical algorithm,first-principles calculation has played an increasingly important role in scientific research,including the screening and design of electrocatalysts.In this dissertation,first-principles calculations were employed to investigate and design highly active electrocatalysts for energy conversion based on two-dimensional materials.The main results are summarized as below:(1)MoS2 nanowires for HER with ultra-high performance and active site density.MoS2 is a promising alternative to Pt for hydrogen evolution reaction.However,low density of active sites and relatively high energy barrier(Ea)greatly limit its practical application.Through a careful analysis of the bonding characteristics,we found that the high Ea originated from the long H-H distance(dH-H)between two neighboring adsorbed H atoms(over 3.0?)and the newly synthesized MoS2 NW@Au(755)contains a very short dH-H value(about 1.5?)at its Mo edges.As a result,the calculated Ea of H2 evolution through the Tafel mechanism is only 0.49 eV,making the Volmer-Tafel mechanism operative and the Tafel slope can be as low as 30 mV/dec.This barrier is not only much lower than that of other MoS2 based catalysts,but even lower than that of the best HER catalyst,Pt.Besides,the ultra-narrow width endows this NWs a high active site density.Through substitution of the Au(755)substrate with non-noble metals,such as Ni(755)and Cu(755),the activity can be maintained.(2)Screening and design of highly active MXenes for HER based on a simple descriptor.MXenes is a new class of two-dimensional materials,which has large surface area and excellent thermodynamic stability.During the synthesis process,H2 evolution can always be observed.These evidences demonstrate that MXenes may be promising catalysts for HER,which however,has never been reported.Through the computation,Ti2CO2 and W2CO are found to be highly active for HER.Based on the molecular orbital theory,we propose that charge transfer may server as a simple activity descriptor for HER.The reliability of the descriptor is first approved by the calculation results of 10 single-metal MXenes.TiVCO2 is then efficiently selected out from 90 kinds of bi-metal MXenes as a potential HER catalyst with high activity.Moreover,for systems with relative high binding strength with H,such as V2CO2,the introduction of metal atom promoters to the surface is proposed to be an efficient way to modulate their catalytic performance.The binding strength between H atom and surface O atoms will be weakened due to the charge transfer from the promoters to surface O atoms and the HER activity will be increased subsequently.(3)Design of single atom,multifunctional catalysts based onβ12-BM.Single-atom catalysts(SACs)can make full use of metal atoms and yet entail high selectivity and activity.Meanwhile,multifunctional catalysts can enable higher performance while lowering the cost than separate unifunctional catalysts.Supported single-atom bifunctional catalysts are therefore of great economic interest and scientific importance.Due to the special structure of boron monolayers(BMs),i.e.,composed of electron-deficient hexagonal holes and electron-rich trigonal holes,BMs are promising supports for SACs.Meanwhile,our theoretical work indicates that BMs may possess high intrinsic activity for HER.Based on these,we design a single atom,bifunctional catalyst(Ni1/β12-BM)for overall water splitting and a single atom,multifunctional catalyst for one-pot CO2 capture,activation and conversion(V1/β12-BM).Ni1/β12-BM is highly active for both HER and OER,with overpotentials of 0.06and 0.40 V,respectively.A viable experimental route for the synthesis of Ni1/β12-BM SAC is demonstrated from computer simulation.Substantial charge transfer between V1/β12-BM and CO2 triggers the activation of CO2 into anionic CO2–,which can be efficiently hydrogenated into CH3OH with a rather low rate-determining barrier of 1.04 eV.Moreover,the adsorption of H2O molecules can make the reaction intermediates closer to the hydrogen source owing to the steric hindrance,which plays a key role in lowering the reaction barrier.(4)High-throughput screening and rational design of catalysts for N2 fixation.Electrocatalytic or photocatalytic N2 redution can achieve the green and sustainable production of NH3.During this process,efficienct catalysts play a crucial role,which remains a great challenge.Through a careful analysis of the relative stability of all the reaction intermediates during the NRR process,hydrogenation of N2 and NH2 are found to be the key steps that determine the overall performance of the catalysts.On this basis,a general,efficient and accurate two-step strategy for the screening of catalysts for N2 reduction.A high-throughput screening is further performed among nitrogen-doped graphene supported single atom catalysts and 10 promising candidates are extracted out from 540 systems.Most strikingly,single W atom embedded in graphene with three C atoms coordination(W1C3)exhibits the best performance with a record low onset potential of 0.25 V.Moreover,on the basis of a concept of electron―acceptance-donation‖,a metal-free single atom catalyst,namely,boron(B)atom decorated on graphitic-carbon nitride(B/g-C3N4),for the reduction of N2 is proposed by using extensive first-principles calculations.Our results reveal that gas phase N2 can be efficiently reduced into NH3 on B/g-C3N4 through the enzymatic mechanism with a record low onset potential(0.20 V).