Atomically dispersed Mn-Nx catalysts derived from Mn-hexamine coordination frameworks for oxygen reduction reaction

Metal-organic frameworks recently have been burgeoning and used as precursors to obtain various metal-nitrogen-carbon catalysts for oxygen reduction reaction(ORR).Although rarely studied,Mn-N-C is a promising catalyst for ORR due to its weak Fenton reaction activity and strong graphitization catalysis.Here,we developed a facile strategy for anchoring the atomically dispersed nitrogen-coordinated single Mn sites on carbon nanosheets(MnNCS)from an Mn-hexamine coordination framework.The atomically dispersed Mn-N4 sites were dispersed on ultrathin carbon nanosheets with a hierarchically porous structure.The optimized MnNCS displayed an excellent ORR performance in half-cells(0.89 V vs.reversible hydrogen electrode(RHE)in base and 0.76 V vs.RHE in acid in half-wave potential)and Zn-air batteries(233 mW cm^(−2)in peak power density),along with significantly enhanced stability.Density functional theory calculations further corroborated that the Mn-N4-C1_(2)site has favorable adsorption of*OH as the rate-determining step.These findings demonstrate that the metal-hexamine coordination framework can be used as a model system for the rational design of highly active atomic metal catalysts for energy applications.

Flow-visualization and numerical investigation on the optimum design of cavitating jet nozzle

Cavitating jet is widely used in drilling,rock cutting and ocean re source exploitation because of its stro ng erosion ability.The analysis of the relationship between the flow characteristics and the structure of cavitating jet nozzle is critical.Here,we utilized 3 D printed technology and high-speed photography to design visualization experime nts to analyse the impact of the variation of resonator and throat size of the organ-pipe self-resonating cavitating nozzles on the cavitation characteristics through image processing.The velocity field,pressure field and vapor volume fraction injected by the nozzle were taken as the objective functions to study the influence of different structural parameters on the cavitation effect based on FLUENT 19.0 software,and the results were compared with the experimental results.The results show that increasing the length and diameter of the resonator contributes to the occurrence of cavitation and the structure stability of the flow field.However,excessive size affects self-resonant of the nozzle and makes it difficult to form resonance effect.In this study,the optimal values of nozzle throat length and divergent angle are twice the throat diameter and 40°,respectively.This research provides an integrated research method to study the optimization of self-resonating nozzle and cavitating jet characteristics.

Numerical Study on Heat Transfer Characteristic of the Plate-Fin Microchannel Heat Sink for Water-Based Thermal Management of CPU Chip

For effective water-based thermal management of high heat generating CPU chip,a series of numerical simulation has been conducted to study the effects of heat flux,fin height and flow rate on convective thermal performance of the plate-fin microchannel heat sinks.The characteristics of heat transfer and flow resistance have been quantificationally discussed and JF factor is employed to evaluate the comprehensive efficiency of convective heat transfer of microchannel heat sink.Results show that the increase in fin height and flow rate of cooling water is helpful to decrease the maximum temperature of CPU chip.Large flow rate and heat flux and short fin height are benefit to improve Nusselt number Nu,but they lead to large resistance coefficient fRe simultaneously.Analysis of JF factor shows that the microchannel with short fins shows better convective thermal performance when the thermal power of the CPU chip is small.The fins should be heightened when the CPU is operating at higher thermal power.The employment of JF factor in the present work shows its pertinence and convenience in the application of design of microchannel heat sink.

Construction of 2D/2D Ti_(3)C_(2)T_(x)MXene/CdS heterojunction with photothermal effect for efficient photocatalytic hydrogen production

The insensitivity of semiconductors to visible and infrared light is a key constraint on the utilization of light energy in photocatalytic reactions.Constructing photocatalysts with full-spectrum absorption through surface engineering is an effective approach to fully harnessing light energy in semiconductor materials.Herein,a novel stable Ti_(3)C_(2)T_(x)MXene/CdS heterojunction catalyst is obtained by in-situ epitaxial growth of two-dimensional(2D)CdS nanosheets on 2D MXene interface via a solvothermal method.The exceptional light absorption properties of MXene confer outstanding full-spectrum driven photocat-alytic hydrogen evolution capability upon the heterogeneous catalyst.The unique 2D/2D structure effectively mitigated the recombination of photogenerated carriers,enhancing the photocatalytic performance of the catalyst.Moreover,the composite catalyst exhibits a significantly higher surface temperature of 80.4℃under visible light irradiation at an intensity of 0.1 W/cm^(2),which is 1.84 times higher than that of CdS.Under irradiation of visible and near infrared light,the composite catalyst with photothermal ef-fect demonstrates a remarkable hydrogen evolution rate of 65.4 mmol g^(-1)h^(-1),which is 7.2 times higher than that of CdS catalyst.This study introduces a novel approach for constructing full-spectrum absorption catalysts and expands the application of the photothermal effect in photocatalytic hydrogen evolution research.

Integration of a thermochemical energy system driven by solar energy and biomass for natural gas and power production

Energy systems with multi-energy product outputs driven by renewable energy sources are becoming increasingly popular.To satisfy the diversification of energy use forms in China,this study proposes a new thermochemical energy system driven by solar energy and biomass for natural gas and power production.In this system,syngas from solar-driven biomass gasification is used to synthesize natural gas,whereas the unreacted syngas is burned directly in a combined cycle for power generation.To adjust the production capacity of the system,a shift reaction was used to change the H_(2)/CO ratio in the syngas.The biomass gasification model was experimentally verified,and the thermodynamic performance of the system was studied numerically.The results showed that the production rate of natural gas,with a heat value of 714.88 k J/mol,was approximately 0.306 m^(3)-SNG/kg-bio,and the primary energy efficiency was 47%.The new system showed a good energy-saving potential of 15.29%.Parametric analysis indicated that an increase in the gasification temperature led to a reduction in the natural gas production and an increase in the power output of the system,with a maximum energy efficiency of 66.72%at gasification temperature of 1050°C.With an increase in the syngas share entering the transfer reactor,the natural gas production rate and energy efficiency of the system were improved with an optimum share of approximately 0.55,thereby facilitating the development and optimization of operation strategies.This study provides a promising way to increase the share of renewable energy instead of fossil fuels.

Constructed TiO_(2)/WO_(3)heterojunction with strengthened nano-trees structure for highly stable electrochromic energy storage device

Tungsten trioxide(WO_(3))has been widely regarded as a prospective bifunctional material due to its electrochromic and pseudocapacitive properties,while still facing the dilemma of inadequate cycle stability and trapping-induced degradation.Here,inspired by the trees-strengthening approach,a unique titanium dioxide(TiO_(2))nanorod arrays strengthened WO_(3)nano-trees(TWNTs)heterojunction was rationally designed and constructed.In sharp contrast to the transmittance modulation(ΔT)attenuation of primary WO_(3)nano-trees during cycling,the TWNTs film showed not only excellent electrochromic performance but also fascinating cycle stability(77.35%retention of the initialΔT after 10,000 cycles).Besides,the trapping-induced degradation could be easily rejuvenated by a potentiostatic de-trapping process.An electrochromic energy storage device(EESD)was further assembled based on the TWNTs film to deliver excellentΔT(up to 79.5%at 633 nm),fast switching speed(tc/tb=1.9 s/14.8 s),extremely high coloration efficiency value(443.4 cm^(2)·C^(−1)),and long-term cycle stability(over 10,000 charge/discharge cycles).This innovative study provided in-depth insights into the electrochromism nature and a significant step in the realization of stable electrochromic-energy storage application,paving the way for multifunctional smart windows as well as next-generation optoelectronic devices.

Evaluation and development of a predictive model for conjugate phase change heat transfer of energy storage system partially filled with porous media

The present study proposes a predictive model to explore the effect of partially filled porous media on the con-jugate heat transfer characteristic of phase change material(PCM)with interfacial coupling conditions between pure fluid region and porous region.The enthalpy-porosity method,local thermal non-equilibrium model and Darcy-Forchheimer law are comprehensively considered to describe the convective heat transfer process in porous media.The modified model is then validated by benchmark data provided by particle image velocimetry(PIV)ex-periments.The phase change behavior,heat transfer efficiency and energy storage performance are numerically investigated for different partial porous filling strategies in terms of filling content,position,height of porous foam and inclination angles of cavity.The results indicate that due to the resistance in porous region,the shear stress exerted by the main vortex(natural convection)in pure fluid region and the momentum transferred,a secondary vortex phenomenon appears in the porous region near the fluid/porous interface.Moreover,such dis-continuity of permeability and fluid-to-porous thermal conductivity results in the cusp of phase change interface at the horizontal fluid/porous boundary.Among four partial porous filling cases,the lower porous filling one has more desirable heat transfer performance,and the 3/4H lower porous filling configuration is the best solution for optimization of the latent heat thermal energy storage(LHTES)systems.For tilted cavity,the increase of inclination angle positively affects the heat transfer efficiency as well as the energy storage rate of the LHTES system,where the performance of 3/4H lower porous filling configuration is further highlighted.

Solution of Smoluchowski coagulation equation for Brownian motion with TEMOM

The particle number density in the Smoluchowski coagulation equation usually cannot be solved as a whole,and it can be decomposed into the following two functions by similarity transformation:one is a function of time(the particle k-th moments),and the other is a function of dimensionless volume(self-preserving size distribution).In this paper,a simple iterative direct numerical simulation(iDNS)is proposed to obtain the similarity solution of the Smoluchowski coagulation equation for Brownian motion from the asymptotic solution of the k-th order moment,which has been solved with the Taylor-series expansion method of moment(TEMOM)in our previous work.The convergence and accuracy of the numerical method are first verified by comparison with previous results about Brownian coagulation in the literature,and then the method is extended to the field of Brownian agglomeration over the entire size range.The results show that the difference between the lognormal function and the self-preserving size distribution is significant.Moreover,the thermodynamic constraint of the algebraic mean volume is also investigated.In short,the asymptotic solution of the TEMOM and the self-preserving size distribution form a one-to-one mapping relationship;thus,a complete method to solve the Smoluchowski coagulation equation asymptotically is established.