Density functional theory study of active sites and reaction mechanism of ORR on Pt surfaces under anhydrous conditions

Identifying active sites and catalytic mechanism of the oxygen reduction reaction under anhydrous conditions are crucial for the development of next generation proton exchange membrane fuel cells(PEMFCs)operated at a temperature>100℃.Here,by employing density functional theory calculations,we studied ORR on flat and stepped Pt(111)surfaces with both(110)and(100)type of steps.We found that,in contrast to ORR under hydrous conditions,(111)terrace sites are not active for ORR under anhydrous conditions,because of weakened binding of ORR intermediates induced by O*accumulation on the surface.On the other hand,step edges,which are generally not active for ORR under hydrous conditions,are predicted to be the active sites for ORR under anhydrous conditions.Among them,(110)type step edge with a unique configuration of accumulated O stabilizes O_(2)adsorption and facilitates O_(2)dissociation,which lead an overpotential<0.4 V.To improve ORR catalysts in high-temperature PEMFCs,it is desirable to maximize(110)step edge sites that present between two(111)facets of nanoparticles.

Modification of short-range repulsive interactions in ReaxFF reactive force field for Fe–Ni–Al alloy

The short-range repulsive interactions of any force field must be modified to be applicable for high energy atomic collisions because of extremely far from equilibrium state when used in molecular dynamics(MD)simulations.In this work,the short-range repulsive interaction of a reactive force field(ReaxFF),describing Fe-Ni-Al alloy system,is well modified by adding a tabulated function form based on Ziegler-Biersack-Littmark(ZBL)potential.The modified interaction covers three ranges,including short range,smooth range,and primordial range.The short range is totally predominated by ZBL potential.The primordial range means the interactions in this range is the as-is ReaxFF with no changes.The smooth range links the short-range ZBL and primordial-range ReaxFF potentials with a taper function.Both energies and forces are guaranteed to be continuous,and qualified to the consistent requirement in LAMMPS.This modified force field is applicable for simulations of energetic particle bombardments and reproducing point defects'booming and recombination effectively.

基于声学散射透镜的低成本光声断层成像方法

光声断层成像需要使用换能器阵列及多通道系统,硬件成本较高,制约了其广泛应用.为降低成本,设计了一种基于声学散射透镜的光声断层成像方法,利用声学多重散射过程中的多路径效应达到增大探测阵列等效数值孔径的作用,从而减少信号探测过程所需换能器数量.通过数值模拟实验,首先分析了散射透镜格林函数的精确测量方法,随后利用四个散射透镜共同作用实现了高质量光声断层成像,并对影响成像质量的相关因素进行了具体研究.提出的散射透镜无须复杂制作工艺或特殊材料,系统只需四个超声换能器即可实现快速成像,可降低光声断层成像系统的硬件成本.

A strategy to improve the electrochemical performance of Ni-rich positive electrodes:Na/F-co-doped LiNi_(0.6)Mn_(0.2)Co_(0.2)O_(2)

Ni-rich layered lithium transition metal oxides LiNi_xMn_yCo_zO_(2)(1-y-z≥0.6)are promising candidates for cathode materials,but their practical applications are hindered by high-voltage instability and fast capacity fading.Using density functional theory calculations,we demonstrate that Na-,F-doping,and Na/F-co-doping can stabilize the structure and result into a higher open circuit voltage than pristine LiNi_(0.6)Mn_(0.2)Co_(0.2)O_(2)(NMC622)during the charging process,which may attain greater discharge capacity.F doping may inhibit the diffusion of Li ions at the beginning and end of charging;Na doping may improve Li ion diffusion due to the increase in Li layer spacing,consistent with prior experiments.Na/F-codoping into NMC622 promotes rate performance and reduces irreversible phase transitions for two reasons:(i)a synergistic effect between Na and F can effectively restrain the Ni/Li mixing and then enhances the mobility of Li ions and(ii)Ni/Li mixing hinders the Ni ions to migrate into Li layers and thus,stabilizes the structure.This study proposes that a layer cathode material with high electrochemical performance can be achieved via rational dopant modification,which is a promising strategy for designing efficient Li ion batteries.

Ambient formaldehyde concentrations in summer in 30 Chinese cities and impacts on air cleaning of built environment

Formaldehyde is an important carcinogen commonly found indoors.Its indoor sources have been intensively in-vestigated.But study on outdoor formaldehyde concentration,which is potentially an important source to indoors,remains scarce.This study attempts to characterize temporal and spatial distribution of the atmospheric formalde-hyde concentration in Chinese cities.Diurnal variation of ambient formaldehyde was examined in 6 cities and peak hours were identified between 12:00 pm and 3:00 pm.Consequently,outdoor formaldehyde concentrations were measured in the peak hours in 30 cities during the summer months of Jul.-Aug.,2022.The formaldehyde concentrations in the peak hours fell into a range of 0.005-0.087 mg/m^(3)(median value is 0.027 mg/m^(3)),87.7%of which have exceeded the chronic reference exposure criteria of 0.009 mg/m^(3) set by Office of Environmental Health Hazard Assessment.Health risk analysis suggests that exposure to ambient formaldehyde could cause a median carcinogenic risk of 1.9×10^(−5)(3.17×10^(−6)-6.13×10^(−5)),higher than threshold limit of 10−6.Pearson correlation analysis of the 30 cities shows that ambient summertime atmospheric formaldehyde concentrations of the city are positively correlated with its Gross Domestic Product(r=0.48).We also found that the outdoor formaldehyde concentrations in urban areas(median:0.017 mg/m^(3))is slightly higher than those in suburban areas(median:0.013 mg/m^(3)).Results here prove that outdoor formaldehyde is ubiquitous in Chinese cities and reduces effectiveness of ventilation in dilution indoor concentrations.Neglecting it would underestimate air cleaner capacity needed by a factor of about two.It should be accounted for in health analysis and air quality engineering control design of built environment in the future.

Engineering the high-entropy phase of Pt-Au-Cu nanowire for electrocatalytic hydrogen evolution

Hydrogen economy,as the most promising alternative energy system,relies on the hydrogen production through sustainable water splitting which in turn relies on the high efficiency electrocatalysts.PtAuCu A1-phase alloy has been predicted to be a promising electrocatalyst for the hydrogen evolution.As such preferred phase of Pt-Au-Cu is not thermodynamically favored,herein,we stabilize PtAuCu alloy by engineering the high-entropy phase in the form of nanowire.Density functional theory(DFT)calculations indicate that,in comparison with the ordered phase and segregated phases with discrete hydrogen binding energy,the high-entropy phase provides a diverse combination of site composition to continuously tune the hydrogen binding energy,and thus generate a series of highly active sites for the hydrogen evolution.Reflecting the theoretical prediction,electrochemical tests show that the A1-phase PtAuCu nanowire significantly outperforms its nanoparticle counterpart with phase segregation,toward the electrocatalysis of hydrogen evolution,offering one of the best hydrogen evolution electrocatalysts.