Floquet instability of a large density ratio liquid-gas coaxial jet with periodic fluctuation

By numerical simulation of basic flow, this paper extends Floquet stability analysis of interracial flow with periodic fluctuation into large density ratio range. Stability of a liquid aluminum jet in a coaxial nitrogen stream with velocity fluctuation is investigated by Chebyshev collocation method and the Floquet theory. Parametric resonance of the jet and the influences of different physical parameters on the instability are discussed. The results show qualitative agreement with the available experimental data.

Strong electronic coupling of CoNi and N-doped-carbon for efficient urea-assisted H2 production at a large current density

Exploiting efficient urea oxidation reaction(UOR)and hydrogen evolution reaction(HER)catalysts are significant for energy-saving H2 production through urea-assisted water electrolysis,but it is still challenging.Herein,carbon-encapsulated CoNi coupled with CoNiMoO(CoNi@CN-CoNiMoO)is prepared by solvothermal method and calcination to enhance the activity/stability of urea-assisted water electrolysis at large current density.It exhibits good activity for UOR(E10/1,000=1.29/1.40 V)and HER(E-10/-1000=-45/-245 mV)in 1.0 M KOH+0.5 M urea solution.For the UOR||HER system,CoNi@CN-CoNiMoO only needs 1.58 V at 500 mA cm-2 and shows good stability.Density functional theory calculation suggests that the strong electronic interaction at the interface between NiCo alloy and N-doping-carbon layers can optimize the adsorption/desorption energy of UOR/HER intermediates and accelerate the water dissociation,which can expedite urea decomposition and Volmer step,thus increasing the UOR and HER activity,respectively.This work provides a new solution to design UOR/HER catalysts for H2 production through urea-assisted water electrolysis.

THE NONLINEAR BEHAVIOR OF INTERFACE BETWEEN TWO-PHASE SHEAR FLOW WITH LARGE DENSITY RATIOS

The Navier-Stokes equations for the two-dimensional incompressible flow are used to investigate the effects of the Reynolds number and the Weber number on the behavior of interface between liquid gas shear flow. In the present study, the density ratios are fixed at approximately 10^0-10^3. The interface between the two phases is resolved using the level-set approach. The Reynolds number and the Weber number, based on the gas, are selected as 400-10000 and 40-5000, respectively. In the past, simulations reappeared the amplitude of interface growth predicted by viscous Orr-Sommerfeld linear theory, verifying the applicability and accuracy of the numerical method over a wide range of density and viscosity ratios; now, the simulations show that the nonlinear development of ligament elongated structures and resulted in the subsequent breakup of the heavier fluid into drops.

A multicomponent multiphase lattice Boltzmann model with large liquid–gas density ratios for simulations of wetting phenomena

A multicomponent multiphase（MCMP） pseudopotential lattice Boltzmann（LB） model with large liquid–gas density ratios is proposed for simulating the wetting phenomena. In the proposed model, two layers of neighboring nodes are adopted to calculate the fluid–fluid cohesion force with higher isotropy order. In addition, the different-time-step method is employed to calculate the processes of particle propagation and collision for the two fluid components with a large pseudoparticle mass contrast. It is found that the spurious current is remarkably reduced by employing the higher isotropy order calculation of the fluid–fluid cohesion force. The maximum spurious current appearing at the phase interfaces is evidently influenced by the magnitudes of fluid–fluid and fluid–solid interaction strengths, but weakly affected by the time step ratio.The density ratio analyses show that the liquid–gas density ratio is dependent on both the fluid–fluid interaction strength and the time step ratio. For the liquid–gas flow simulations without solid phase, the maximum liquid–gas density ratio achieved by the present model is higher than 1000：1. However, the obtainable maximum liquid–gas density ratio in the solid–liquid–gas system is lower. Wetting phenomena of droplets contacting smooth/rough solid surfaces and the dynamic process of liquid movement in a capillary tube are simulated to validate the proposed model in different solid–liquid–gas coexisting systems. It is shown that the simulated intrinsic contact angles of droplets on smooth surfaces are in good agreement with those predicted by the constructed LB formula that is related to Young＇s equation. The apparent contact angles of droplets on rough surfaces compare reasonably well with the predictions of Cassie＇s law. For the simulation of liquid movement in a capillary tube, the linear relation between the liquid–gas interface position and simulation time is observed, which is identical to the analytical prediction. The simulation results regarding the wetting phenomena of droplets on smooth/rough surfaces and the dynamic process of liquid movement in the capillary tube demonstrate the quantitative capability of the proposed model.

Controllable synthesis of a self-assembled ultralow Ru,Ni-doped Fe_(2)O_(3) lily as a bifunctional electrocatalyst for large-current-density alkaline seawater electrolysis

Highly efficient and stable bifunctional electrocatalysts that can be used for large-current-density electrolysis of alkaline seawater are highly desirable for carbon-neutral economies,but their facile and controllable synthesis remains a challenge.Here,self-assembled ultralow Ru,Ni-doped Fe_(2)O_(3) with a lily shaped morphology was synthesized on iron foam(RuNi-Fe_(2)O_(3)/IF)via a facile one-step hydrothermal process,in which the intact lily shaped RuNi-Fe_(2)O_(3)/IF was obtained by adjusting the ratio of Ru/Ni.Benefitting from the Ru/Ni chemical substitution,the as-synthesized RuNi-Fe_(2)O_(3)/IF can act as free-standing dual-function electrodes that are applied to electrocatalysis for the hydrogen evolution(HER)and oxygen evolution reactions(OER)in 1.0 mol L^(-1) KOH,requiring an overpotential of 75.0 mV to drive 100 mA cm^(-2) for HER and 329.0 mV for OER.Moreover,the overall water splitting catalyzed by RuNi-Fe_(2)O_(3)/IF only demands ultralow cell voltages of 1.66 and 1.73 V to drive 100 mA cm^(-2) in 1.0 mol L^(-1) KOH and 1.0 mol L^(-1) KOH seawater electrolytes,respectively.The electrodes show remarkable long-term durability,maintaining current densities exceeding 100 mA cm^(-2) for more than 100 h and thus outperforming the two-electrode system composed of noble catalysts.This work provides an efficient,economical method to synthesize self-standing bifunctional electrodes for large-current-density alkaline seawater electrolysis,which is of significant importance for ecological protection and energy exploitation.

STABILITY OF VISCOUS SHOCK WAVES FOR THE ONE-DIMENSIONAL COMPRESSIBLE NAVIER-STOKES EQUATIONS WITH DENSITY-DEPENDENT VISCOSITY

We study the large-time behavior toward viscous shock waves to the Cauchy problem of the one-dimensional compressible isentropic Navier-Stokes equations with density- dependent viscosity. The nonlinear stability of the viscous shock waves is shown for certain class of large initial perturbation with integral zero which can allow the initial density to have large oscillation. Our analysis relies upon the technique developed by Kanel~ and the continuation argument.

Self-supported Fe_(x)Ni_(1-x)MoO_(4) with synergistic morphology and composition for efficient overall water splitting at large current density

Developing the high activity,low cost and robust large-current-density-based electrocatalysts is of great significance for the industrial electrolytic water splitting.However,the current range of most reported materials is small,which makes it difficult for them to play their roles in practical applications.Here,a self-supported amorphous FexNi1-xMo O4/IF treated with ammonium fluoride (AF_(0.1)-FNMO/IF) is synthesized by one-step hydrothermal method.With the help of NH4F,AF_(0.1)-FNMO/IF exhibits a vertically cross-linked nanosheet with spherical structure.Electrochemical measurement shows that AF_(0.1)-FNMO/IF affords a large current density ordeal and only need low overpotentials of 289 and 345 m V to reach a current response of 500 m A/cm ^(2)for oxygen evolution reaction and hydrogen evolution reaction,respectively,together with long-time stability (both at 500,1000 and 2000 m A/cm ^(2)) in 1.0 mol/L KOH solution.Using it as bifunctional catalyst for overall water splitting,the current densities of 100,500,1000 and1500 m A/cm ^(2)are achieved at a cell voltage of 1.71,1.88,1.94 and 1.97 V with excellent durability,which is much better than that of most published electrodes.The work provides valuable insight for designing higher activity nickel iron-based molybdate catalysts with large current density.

Te-doped Fe_(3)O_(4) flower enabling low overpotential cycling of Li-CO_(2) batteries at high current density

Li-CO_(2)batteries(LCBs)suffer from high overpotentials caused by sluggish CO_(2)reaction kinetics.This work designs a Te-doped Fe_(3)O_(4)(Te-Fe_(3)O_(4))flower-like microsphere catalyst to lower the overpotential and improve the reversibility of LCBs.Experimental results reveal that Te doping modifies the electronic structure of Fe_(3)O_(4)and reduces the overpotential.The stable Te-O bond between Te and C_(2)O^(2-)_(4)could effectively inhibit the dispro-portionation reaction of the latter,enabling the Te-Fe_(3)O_(4)cathodes to exhibit a remarkable capacity(9485 mAh g^(-1))and a long cycling life(155 cycles)with an overpotential of 1.21 V and an energy efficiency of about 80%at a high current density(2000 mA g^(-1)).Through the interaction between Te and Li_(2)C_(2)O_(4)to inhibit the dispro-portionation reaction,this work successfully achieves long-term cycling of LCBs with low overpotential at high current density.

Three-Phase Heterojunction NiMo-Based Nano-Needle for Water Splitting at Industrial Alkaline Condition

Constructing heterojunction is an effective strategy to develop high-performance non-preciousmetal-based catalysts for electrochemical water splitting(WS).Herein,we design and prepare an N-doped-carbon-encapsulated Ni/MoO_(2) nano-needle with three-phase heterojunction(Ni/MoO_(2)@CN)for accelerating the WS under industrial alkaline condition.Density functional theory calculations reveal that the electrons are redistributed at the three-phase heterojunction interface,which optimizes the adsorption energy of H-and O-containing intermediates to obtain the best ΔG_(H*) for hydrogen evolution reaction(HER)and decrease the ΔG value of ratedetermining step for oxygen evolution reaction(OER),thus enhancing the HER/OER catalytic activity.Electrochemical results confirm that Ni/MoO_(2)@CN exhibits good activity for HER(ƞ_(-10)=33 mV,ƞ_(-1000)=267 mV)and OER(ƞ_(10)=250 mV,ƞ_(1000)=420 mV).It shows a low potential of 1.86 V at 1000 mA cm^(−2) for WS in 6.0 M KOH solution at 60℃ and can steadily operate for 330 h.This good HER/OER performance can be attributed to the three-phase heterojunction with high intrinsic activity and the self-supporting nano-needle with more active sites,faster mass diffusion,and bubbles release.This work provides a unique idea for designing high efficiency catalytic materials for WS.

Trimetallic oxyhydroxides as active sites for large-current-density alkaline oxygen evolution and overall water splitting

Earth-abundant electrocatalysts for large-current-density water splitting under alkaline condition are desirable.Oxygen evolution reaction,which is a bottleneck of the overall water splitting,faces the problems of complicated reconstruction and deficiency in rational design of active sites.Herein,we report a series of transition metal chalcogenides for alkaline OER.Among them,FeCoNi(S)displayed a low overpotential of 293 m V to deliver a current density of 500 m A cm^(-2),which is in the top level of non-precious metal based OER electrocatalysts.A combination of(ex)in situ characterizations and DFT calculation shows that Ni(Fe,Co)trimetallic oxyhydroxides were the active sites for highly-efficient OER.Furthermore,for FeCoNi(S),when used as a bifunctional catalyst for water splitting,it only required a cell voltage of 1.84 V to deliver~500 m A cm^(-2) with extraordinary long-term stability over 2000 h.This work provides the comprehension of high-efficiency,robust catalysts for OER and overall water splitting at large current densities in alkaline media.

Highly active bifunctional catalyst: Constructing FeWO_(4)-WO_(3) heterostructure for water and hydrazine oxidation at large current density

Developing high performance anode catalysts for oxygen evolution reaction (OER) and hydrazine oxidation reaction (HzOR) at large current density is an efficient pathway to produce hydrogen. Herein, we synthesize a FeWO_(4)-WO_(3) heterostructure catalyst growing on nickel foam (FeWO_(4)-WO_(3)/NF) by a combination of hydrothermal and calcination method. It shows good catalytic activity with ultralow potentials for OER (ζ_(10) = 1.43 V, ζ_(1.000) = 1.56 V) and HzOR (ζ_(10) = −0.034 V, ζ_(1.000) = 0.164 V). Moreover, there is little performance degradation after being tested for _(10)0 h at 1,000 (OER) and _(10)0 (HzOR) mA·cm−2, indicating good stability. The superior performance could be attributed to the wolframite structure and heterostructure: The former provides a high electrical conductivity to ensure the electronic transfer capability, and the later induces interfacial electron redistribution to enhance the intrinsic activity and stability. The work offers a brand-new way to prepare good performance catalysts for OER and HzOR, especially at large current density.

Interface engineering of NiSe_(2) nanowrinkles/Ni_(5)P_(4)nanorods for boosting urea oxidation reaction at large current densities

Deliberate modulation of the electronic structure via interface engineering is one of promising perspectives to build advanced catalysts for urea oxidation reaction(UOR)at high current densities.However,it still remains some challenges originating from the intrinsically sluggish UOR dynamics and the high energy barrier for urea adsorption.In response,we report the coupled NiSe_(2)nanowrinkles with Ni_(5)P_(4)nanorods heterogeneous structure onto Ni foam(denoted as NiSe_(2)@Ni_(5)P_(4)/NF)through successive phosphorization and selenization strategy,in which the produced closely contacted interface could provide high-flux electron transfer pathways.Theoretical findings decipher that the fast charge transfer takes place at the interfacial region from Ni_(5)P_(4)to NiSe_(2),which is conducive to optimizing adsorption energy of urea molecules.As expected,the well-designed NiSe_(2)@Ni_(5)P_(4)/NF only requires the low potential of 1.402 V at the current density of 500 mA·cm^(-2).More importantly,a small Tafel slope of 27.6 mV·dec^(-1),a high turnover frequency(TOF)value of 1.037 s^(-1)as well as the prolonged stability of 950 h at the current density of 100 mA·cm^(-2)are also achieved.This study enriches the understanding on the electronic structure modulation via interface engineering and offers bright prospect to design advanced UOR catalysts.

Double active sites promoting hydrogen evolution activity and stability of CoRuOH/Co_(2)P by rapid hydrolysis

Cobalt-based phosphides show excellent hydrogen evolution reaction(HER)performance,however,improving the intrinsic activity and stability of it in alkaline electrolyte still remains a challenge.Herein,CoRuOH/Co_(2)P/CF with heterojunction structure was developed by means of molten salt and rapid hydrolysis(30 s).The OH-from rapid surface hydrolysis of Co_(2)P as a hydrogen adsorption site can facilitate the formation of thin CoRuOH layer as a water dissociation site,which may bring out better synergistic effect for alkaline HER.Moreover,the covering of CoRuOH can improve the stability of Co_(2)P for HER.When drives at 100 mA/cm^(2),it only requires overpotential of 81 mV in 1.0 mol/L KOH(25℃).Even at higher current density(1000 mA/cm^(2)),CoRuOH/Co_(2)P/CF can also operate stability for at least 100 h.When coupling with NiFe-LDH/IF in a two-electrode system,the voltage of NiFe-LDH/IF(+)||CoRuOH/Co_(2)P/CF(-)at 1000 mA/cm^(2)is merely 1.77 V with 100 h,demonstrating great potential for water splitting.The implementation of this work provides a new strategy and reference for the further improvement of transition metal phosphides as HER electrocatalysts.

Doping-driven dual heterogeneous interfacial structures boosting the durability of industry-compatible water splitting at high current density

Developing highly stable electrocatalysts under industry-compatible current densities(>500 mA cm^(-2))in an anion-exchange membrane water electrolyzer(AEMWE)is an enormous challenge for water splitting.Herein,based on the results of density function theory calculations,a dual heterogeneous interfacial structured NiSe/Fe-Ni(OH)_(2)catalyst was subtly designed and successfully prepared by electrodepositing Fe-doped Ni(OH)_(2)on NiSe-loaded nickel foam(NF).Fe doping-driven heterogeneous structures in NiSe/Fe-Ni(OH)_(2)markedly boost catalytic activity and durability at industrially compatible current densities in single hydrogen and oxygen evolution reactions under alkaline conditions.In particular,NiSe/Fe-Ni(OH)_(2)shows a negligible performance loss at 600 mA cm^(-2)at least 1,000 h for overall water splitting,a distinguished long-term durability acting as AEMWE electrodes at 600 mA cm^(-2)and 1 A cm^(-2)at 85℃for at least 95 h.Owing to Fe doping-induced strong synergetic effect between Ni and Fe,dual heterostructure-promoted charge transfer and redistribution,abundant catalytic active sites,and improvement of stability and durability,a mechanism of Fe doping-driven heterogeneous interfacial structurepromoted catalytic performance was proposed.This study provides a successful example of theory-directed catalyst preparation and pioneers a creative strategy for industry-compatible water splitting at high current density.

AlN/GaN high electron mobility transistors on sapphire substrates for Ka band applications

We report the DC and RF characteristics of AlN/GaN high electron mobility transistors（HEMTs） with the gate length of 100 nm on sapphire substrates. The device exhibits a maximum drain current density of 1.29 A/mm and a peak transconductance of 440 m S/mm. A current gain cutoff frequency and a maximum oscillation frequency of 119 GHz and 155 GHz have been obtained, respectively. Furthermore, the large signal load pull characteristics of the AlN/GaN HEMTs were measured at 29 GHz. An output power density of 429 m W/mm has been demonstrated at a drain bias of 10 V. To the authors＇ best knowledge, this is the earliest demonstration of power density at the Ka band for Al N/Ga N HEMTs in the domestic, and also a high frequency of load-pull measurements for Al N/Ga N HEMTs.