Study on gas–liquid flow characteristics in stirred tank with dual-impeller based on CFD-PBM coupled model

Study on gas–liquid flow in stirred tank with two combinations of dual-impeller(six-bent-bladed turbine(6BT)+six-inclined-blade down-pumping turbine(6 ITD),the six-bent-bladed turbine(6BT)+six-inclinedblade up-pumping turbine(6ITU))was conducted using computational fluid dynamics(CFD)and population balance model(PBM)(CFD-PBM)coupled model.The local bubble size was captured by particle image velocimetry(PIV)measurement.The gas holdup,bubble size distribution and gas–liquid interfacial area were explored at different conditions through numerical simulation.The results showed that the 4 mm bubbles accounted for the largest proportion of 33%at the gas flow rates Q=0.76 m^(3)·h^(-1) and 22%at Q=1.52 m^(3)·h^(-1) for combined impeller of 6BT+6ITU,while the bubbles of 4.7 mm and 5.5 mm were the largest proportion for 6BT+6ITD combination,i.e.25%at Q=0.76 m^(3)·h^(-1) and 22%at Q=1.52 m^(3)·h^(-1),respectively,which indicated that 6BT+6ITU could reduce bubble size effectively and promote gas dispersion.In addition,the gas holdup around impellers was increased obviously with the speed compared with gas flow rate.So it was concluded that 6ITU impeller could be more conductive to the bubble dispersion with more uniform bubble size,which embodied the advantages of 6BT+6ITU combination in gas–liquid mixing.

Arranging Electronic Localization of PtCu Nanoalloys to Stimulate Improved Oxygen Electroreduction for High-Performance Fuel Cells

Developing exceptionally durable and efficient oxygen reduction reaction(ORR)catalysts is of paramount importance to the widespread commercialization of proton exchange membrane fuel cells(PEMFCs)but is still challenging.Herein,PtCu nanoalloys rooted on nitrogen-doped carbon nanosheets(PtCuNC-700)with fully exposed PtCu nanoalloys and strong metal–support interaction were developed.Benefiting from its structural and compositional merits,PtCuNC-700 showcases superior ORR activity and stability with a specific activity of 1.05mA cm^(−2)and mass activity of 0.45 A mgPt^(−1),4.2-fold and 3.7-fold higher than Pt/C(0.25 mA cm^(−2)and 0.12 A mgPt^(−1)),respectively.Moreover,PtCuNC-700 exhibits first-class performance in H2/air PEMFC assessment and delivers a peak power density of 929.7 mW cm^(−2)and excellent cycling stability up to 30,000 cycles.Theoretical calculations disclose that the synergistic effect of alloying Pt with Cu combined with the strong interaction between PtCu nanoalloys and nitrogen-doped carbon nanosheets can effectively modify the local electron configuration and density of states of Pt sites approaching the Fermi level.Hence,the PtCu-alloy catalysts realized here diminish the energy barrier for ORR and accelerate their reaction kinetics.This work provides a reliable and effective approach to boost the activity and stability of Pt alloy-based ORR electrocatalysts in PEMFCs.

Breaking the Scaling Relationship Limit:From Single-Atom to Dual-Atom Catalysts

Recent decades have witnessed the rapid development of catalytic science,especially after Taylor and Armstrong proposed the notion of the“active site”in 1925.By optimizing reaction paths and reducing the activation energies of reactions,catalysts appear in more than 90%of chemical production reactions,involving homogeneous catalysis,heterogeneous catalysis,and enzyme catalysis.Because of the 100%efficiency of active atom utilization and the adjustable microenvironment of metal centers,single-atom catalysts(SACs)shine in various catalytic fields for enhancing the rate,conversion,and selectivity of chemical reactions.Nevertheless,a solo active site determines a fixed adsorption mode,and the adsorption energies of intermediates from multistep reactions linking with a solo metal site are related to each other.For a specific multistep reaction,it is almost impossible to optimally adjust the adsorption of every intermediate on the solo site simultaneously.This phenomenon is termed the scaling relationship limit(SRL)and is an unavoidable obstacle in the development of pure SACs.Dual-atom catalysts(DACs),perfectly inheriting the advantages of SACs,can exhibit better catalytic performance than simple SACs and thus have gradually gained researchers’attention.Depending on the dual-metal structure,dual-metal sites(DMSs)in DACs can be divided into two separated heterometal sites,two linked homometal sites,and two linked heterometal sites.Two separated heterometal sites prescribe a distance limit between two metal sites for electron interaction.Currently,the active origins of DACs can be summarized in the following three points:(1)electronic effect,in which only one metal center serves as active site and the other plays an electronic regulatory role;(2)synergistic effect,in which two metal centers separately catalyze different core steps to improve catalytic performance together;(3)adsorption effect,in which offering additional sites changes the adsorption structures to break the SRL based on SACs.Among the three active origins,optimizing the adsorption structure of intermediates upon DMSs is one of the most effective technologies to boost the catalytic property of DACs on the basis of SACs.To date,few contributions have focused on the development of DACs in various heterogeneous catalysis environments,including O_(2) reduction reaction,O_(2) evolution reaction,H_(2) evolution reaction,CO_(2) reduction reaction,N_(2) reduction reaction,and other conversion reactions.In this Account,a summary of recent progress regarding DACs in heterogeneous catalysis will be presented.First,the evolution of DACs from an unpopular discovery to research hot spot is illustrated through a timeline.In the next section,the DACs are divided into three categories,and the potential active origins of DACs are revealed by comparison with SACs.In addition,the techniques for constructing DACs are systematically summarized,including preparation of carbonous,pyrolysis-free,noncarbon-supported,and complex-type DACs.Furthermore,the underlying active origins of DACs in specific energy-and environment-related reactions are introduced in detail with assistance of theoretical calculations.Finally,we affirm the contribution of DACs to catalysis,particularly heterogeneous electrocatalysis,and provide an outlook regarding the development direction for DACs by discussing the major challenges.It is anticipated that this Account can inspire researchers to propel the advance of DACs.

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N_(5)Site for Enhanced Electrocatalytic Reaction

Metal porphyrins are star molecules that possess welldefined coordination metal centers for versatile catalytic reactions.However,most previous work has focused on the correlations between in-plane symmetric configuration of metal-N_(4)sites and their catalytic performance.Addressing the catalytic contribution of additional axial coordination to such symmetric configuration remains a challenge.Theoretical calculations revealed that axially anchoring an extra pyridine on the tetra-coordinated cobalt porphyrin(Co-N4)to construct penta-coordinated cobalt porphyrin(Co-N_(5))renders cobalt a higher electron density,thereby favoring the rate-determining O_(2)adsorption/activation and reducing the oxygen electroreduction barrier.Therefore,a well-defined Co-N_(5)site is rationally introduced into the azo-linked polymer framework for a fundamental structure-catalytic performance correlation study.As-prepared Co-N_(5)catalyst exhibits a 26 mV positive shift in half-wave potential compared with the pyridine-free Co-N_(4)counterpart,discloses a markedly higher power density(141.4 mW cm^(−2)),and possesses better long-term durability(over 160 h cycles)in a Zn-air battery.Moreover,such a Co-N_(5)catalyst also showcases potential applications for CO_(2)reduction with high CO_(2)-to-CO conversion faradic efficiency and better selectivity than the Co-N_(4)counterpart because coordination of the fifth pyridine evokes electronic localization that suppresses a competitive side reaction.This work proves the positive electrocatalytic contribution of axial penta-coordination on well-defined metalporphyrin-based catalysts and offers atomic understanding of the structure-performance correlation on single atom catalysts for future catalyst design.

Molecular crowding agents engineered to make bioinspired electrolytes for high-voltage aqueous supercapacitors

The development of low-cost and eco-friendly aqueous electrolytes with a wide voltage window is the key to achieving safe high energy density supercapacitors(SCs).In this work,a molecular crowding electrolyte is prepared by simulating the crowded environment in living cells.Ion transport in the molecular crowding electrolyte can be effectively improved via reducing the molecular weight of the crowding agent,polyethylene glycol(PEG).The results show that PEG with a molecular weight of 200(PEG200)can significantly improve ionic conductivity while maintaining a wide voltage window.These advantages enable commercial activated carbon-based SCs to work at 2.5 V with high energy density,outstanding rate performance and good stability for more than 10,000 cycles.On this basis,three series of molecular crowding electrolytes using sodium perchlorate,lithium perchlorate,and sodium trifluoromethanesulfonate as salts are developed,demonstrating the versatility of PEG200 for wide-voltage aqueous electrolytes.