Enhanced oxygen reduction reaction performance of Co@N–C derived from metal-organic frameworks ZIF-67 via a continuous microchannel reactor

Traditional methods of preparing metal-organic frameworks(MOFs)compounds have the disadvantages such as poor dispersion,inefficient and discontinuous process.In this work,microchannel reactor is used to prepare MOFs-derived zeolite-imidazole material via flash nanoprecipitation to form ZIF-67+PEI(FNP),which reduces the MOF synthesis time down to millisecond time interval while keeping the synthesized ZIF-67+PEI(FNP)highly dispersed.The Co@N–C(FNP)catalyst obtained by flash nanoprecipitation and carbonization has a higher Co content and thus more active sites for oxygen reduction reaction than the Co@N–C(DM)catalyst prepared by direct mixing method.Electrochemical tests show that the Co@N–C(FNP)catalyst prepared by this method has excellent oxygen reduction performance,good methanol resistance and high stability.The onset potential and half-wave potential of Co@N–C(FNP)are 0.92 VRHE and 0.83 VRHE,respectively,which are higher than that of Co@N–C(DM)(Eonset=0.90 VRHEand E1/2=0.83VRHE).Moreover,the Zn-air battery assembled with Co@N–C(FNP)as the cathode catalyst has high open circuit voltage,high power density and large specific capacity.The performance of these batteries has been comparable to that of Pt/C assembled batteries.Density functional theory(DFT)calculations confirm that the Co(220)crystal plane present in Co@N–C(FNP)have stronger adsorption energy than that of Co(111)crystal plane in Co@N–C(DM),leading to better electrocatalytic performance of the former.

Improved oxygen reduction reaction via a partially oxidized Co-CoO catalyst on N-doped carbon synthesized by a facile sand-bath method

High active and durable non-noble metal electrocatalysts are urgently developed to satisfy the high performance oxygen reduction reaction(ORR). We successfully synthesized Co-CoOx anchored on nitrogen-doped carbon via a facile sand-bath method(SBM), i.e., Co-CoOx/N-C(SBM). The as-obtained Co-CoOx/N-C(SBM) exhibited overwhelming superiorities to Co-CoO/N-C prepared by conventional heat treatment(CHT), particularly in electrochemical performance of ORR. Although Co-CoOx/N-C(SBM)showed smaller specific surface area of 276.8 m^2/g than that of 939.5 m^2/g from Co-CoO/N-C(CHT), the Co-CoOx/N-C(SBM) performed larger pore diameter and more Co_3O_4 active component resulting in better ORR performance in 0.1 mol/L KOH solution. The Co-CoO_x/N-C(SBM) delivered onset potential of 0.91 V vs. RHE, mid-wave potential of 0.85 V vs. RHE and limited current density of 5.46 mA/cm^2 much better than those of the Co-CoO/N-C(CHT). Furthermore, Co-CoOx/N-C(SBM) showed greater stability and better methanol tolerance superior to the commercial 20 wt% Pt/C.

Overwhelming electrochemical oxygen reduction reaction of zinc-nitrogen-carbon from biomass resource chitosan via a facile carbon bath method

Developing high efficiency and low cost electrocatalysts is critical for the enhancement of oxygen reduction reaction(ORR),which is the fundamental for the development and commercialization of renewable energy conversion technology.Herein,zinc-nitrogen-carbon(Zn-N-C)was prepared by using biomass resource chitosan via a facile carbon bath method.The obtained Zn-N-C delivered a high specific surface area(794.7 cm^2/g)together with pore volume(0.49 cm^3/g).During the electrochemical evaluation of oxygen reduction reaction(ORR),Zn-N-C displayed high activity for ORR with an onset pote ntial E0=0.96 VRHE and a half wave potential E1/2=0.86 VRHE,which were more positive than those of the comme rcial 20 wt%Pt/C benchmark catalyst(E0=0.96 VRHE and E1/2=0.81 VRHE).In addition,the ZnN-C catalyst also had a better stability and methanol tolerance than those of the Pt/C catalyst.

Tuning bandstructure of folded MoS_(2) through fluid dynamics

The variation of interlayer coupling can greatly affect the bandstructure of few layered transition metal dichalcogenides(TMDs),for instance,transition of indirect-to-direct bandgap and vice versa,which is correlated with the charge carrier and optical density.However,methods that can modulate the coupling strength in a controllable way are still lacking.Here,we report a fluidic dynamic strategy to tune the interlayer coupling of folded bi-layer MoS_(2).By controlling the flow direction and particle size of the fluid,mono-layer MoS_(2)can be folded into bi-layer with a controlled folding direction for designated twist angles as well as tunable interlayer coupling.Compared with normally folded bi-layer MoS_(2),the photoluminescence(PL)peak of the direct-bandgap transition for folded bi-layer MoS_(2)by fluid flow is weakened accompanied with the re-appearance of indirect-bandgap transition peak.Besides,the fluid flow creates a clear trajectory on the folded MoS_(2),exhibiting various degrees of interlayer coupling along it.Field-effect transistors(FETs)were further fabricated on tunably coupled folded-bi-layers,proving that the bandstructure and electrical property is strongly correlated with the degree of interlayer coupling.This fluidic dynamic strategy can be extended to other TMDs on any substrate,and together with its excellent capability in controlled interlayer coupling,it will provide a new way for the development of TMDs optoelectronics.