Micro-nanostructural designs of bifunctional electrocatalysts for metal-air batteries

Water-based rechargeable metal-air batteries play an important role in the storage and conversion of renewable electric energy.However,the sluggish kinetics of the oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)have limited the practical application of rechargeable metal-air batteries.Most of reviews were focused on single functional electrocatalysts while few on bifunctional electrocatalysts.It is indispensable but challenging to design a bifunctional electrocatalyst that is active and stable to the two reactions.Recently,attempts to develop high active bifunctional electrocatalysts for both ORR and OER increase rapidly.Much work is focused on the micro-nano design of advanced structures to improve the performance of bifunctional electrocatalyst.Transition-metal materials,carbon materials and composite materials,and the methods developed to prepare micro-nano structures,such as electrochemical methods,chemical vapor deposition,hydrothermal methods and template methods are reported in literatures.Additionally,many strategies,such as adjustments of electronic structures,oxygen defects,metal-oxygen bonds,interfacial strain,nano composites,heteroatom doping etc.,have been used extensively to design bifunctional electrocatalysts.To well understand the achievements in the recent literatures,this review focuses on the micro-nano structural design of materials,and the related methods and strategies are classed into two groups for the improvement of intrinsic and apparent activities.The fine adjustment of nano structures and an in-depth understanding of the reaction mechanism are also discussed briefly.

Electrochemical reduction of CO_2 in solid oxide electrolysis cells

The effort on electrochemical reduction of COto useful chemicals using the renewable energy to drive the process is growing fast recently. In this review, we introduce the recent progresses on the electrochemical reduction of COin solid oxide electrolysis cells（SOECs）. At high temperature, only CO is produced with high current densities and Faradic efficiency while the reactor is complicated and a better sealing technique is urgently needed. The typical electrolytes such as zirconia-based oxides, ceria-based oxides and lanthanum gallates-based oxides, anodes and cathodes are introduced in this review, and the cathode materials, such as conventional metal–ceramics（cermets）, mixed ionic and electronic conductors（MIECs） are discussed in detail. In the future, to gain more value-added products, the electrolyte, cathode and anode materials should be developed to allow SOECs to be operated at temperature range of 573–873 K. At those temperatures, SOECs may combine the advantages of the low temperature system and the high temperature system to produce various products with high current densities.

Detrimental phase evolution triggered by Ni in perovskite-type cathodes for CO2 electroreduction

Perovskite oxides are popular as cathode materials of solid oxide electrolysis cells, because of their good redox stability and high resistance to coke formation.Unexpectedly, a negative effect of Ni doping is found on Sr2Fe(1.5-x)NixMo(0.5)O(x = 0, 0.05, 0.1, 0.2) cathode for pure CO2 electroreduction at 800 ℃, although Ni is highly active for CO2 electroreduction.The CO2 electroreduction performance degrades with the increase of Ni doping amount.Various characterization techniques are used to disclose the negative effect.Ni doping decreases the perovskite stability under electroreduction conditions, Fe and Ni cations in the B-site are reduced to metal nanoparticles and SrCO3 forms on the surface of the perovskite.The phase instability results from the weaker Ni–O bond.Although the Fe-Ni nanoparticles are in favor of the CO2 electroreduction, too much SrCO3 and carbon deposition block the charge transfer and diffusion of oxygenous species on the cathode surface.

Boosting the oxygen evolution reaction through migrating active sites from the bulk to surface of perovskite oxides

The oxygen evolution reaction (OER) dominates the efficiency of electrocatalytic water splitting owing to its sluggish kinetics.Perovskite oxides (ABO_(3)) have emerged as promising candidates to accelerate the OER process owing to their high intrinsic activities and tailorable properties.Fe ions in perovskite oxides have been proved to be a highly catalytic element for OER,while some Fe-based perovskites such as SrTi_(0.8)Fe_(0.2)O_(3-δ)(STF) and La_(0.66)Ti_(0.8)Fe_(0.2)O_(3-δ)(LTF) exhibit inferior OER activity.Yet the essential reason is still unclear and the effective method to promote the activity of such perovskite is also lacking.Herein,an in-situ exsolution strategy was proposed to boost the OER by migrating Fe from the bulk to the surface.Significantly enhanced OER activity was achieved on STF and LTF perovskites with surfacedecorated oxygen vacancies and Fe nanoparticles.In addition,theoretical calculation confirmed that the oxygen vacancies and Fe nanoparticle on surface could lower the overpotential of OER by facilitating the adsorption of OH^(-).From this study,migration of the active elements in perovskite is found to be an effective strategy to increase the quantity and activity of active sites,providing new insights and understanding for designing efficient OER catalysts.

Porous carbon layers wrapped CoFe alloy for ultrastable Zn-Air batteries exceeding 20,000 charging-discharging cycles

Developing high active and stable bifunctional electrocatalysts towards oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)is essential for the development of rechargeable Zn-air batteries.Herein,a facile strategy to synthesize the porous carbon layers wrapped CoFe alloy(C/CoFe)through the pyrolysis of a homogeneous mixture containing Co,Fe ions and N-doped carbon quantum dots(Ndoped CQDs)was reported.The prepared carbon layers with multi-level pore structures provides more active sites and optimizes the homogeneity of the electron and mass transport.In addition,the carbon layers,which is doped by Co/Fe/N atoms,is responsible for high ORR activity,while the CoFe alloy plays a vital role in OER performance.The as-synthesized catalyst exhibits an excellent bifunctionality for electrochemical oxygen reactions,which is comparable to the commercial Pt/C and IrO2 benchmarks.Owing to the carbon layers protects CoFe alloy nanoparticles from the harsh environment,the rechargeable Znair battery with the C/CoFe catalyst delivers excellent stability during 20,000 charging-discharging cycles.

Rational design of CO_(2)electroreduction cathode via in situ electrochemical phase transition

CO_(2)electroreduction reaction(CO_(2)RR),combined with solid oxide electrolysis cells(SOECs),is a feasible technology for the storage of renewable electric energy,while its development is limited by the catalytic activity and stability on cathodes.Here,a novel garnet oxide(Gd_(3)Fe_(5)O_(12))cathode is designed,where the garnet oxide is converted to perovskite oxide and iron via in situ electrochemical phase transition during CO_(2)electroreduction,resulting in high activity with Faradaic efficiency close to 100%and great stability over 1000 h galvanostatic test.A variety of experimental characterizations and density functional theory calculations indicate that in situ exsolved Fe clusters can effectively enhance the adsorption energies of intermediates and lowering the CO_(2)dissociation barriers.Microkinetic modelling confirms that CO_(2)RR goes through a dissociative adsorption mechanism and the electronic transfer for CO_(2)dissociation is the rate-determining step.

MOF glass foam: a blueprint to porous membrane scaling-up

In comparison with conventional energy-intensive separation technologies, such as distillation and crystallization,membrane separation is labelled as an advanced technology with low energy consumption, a small carbon footprint,continuous operation, etc. Metal-organic frameworks(MOFs) are a burgeoning category of crystalline materials consisting of metal ions/clusters coordinated with organic ligands.

Metal-organic framework nanosheets: a class of glamorous low-dimensional materials with distinct structural and chemical natures

Metal-organic framework nanosheets have gained great attention because of the diversified structures,tunable chemical functionalities,large surface area and ultrathin thickness.In this review,we introduce the recent progress in the favorable applications for catalysis,sensing,energy storage and gas separation,which has significantly addressed the advantages of the nanosheets.A summary of nanosheet fabrication approaches is put forward to establish a comprehension on the origin of the MOF nanosheets.And at last but not the least,we present the concerns on the challenges and opportunities of these materials from our perspectives.

Partial oxidation of methane to syngas in a mixed-conducting oxygen permeable membrane reactor

Mixed-conducting oxygen permeable membranes represent a class of novel ceramic membranes, which exhibit mixed oxygen ionic and electronic conductivities. At high temperatures, oxygen can permeate through the membrane from the high to low oxygen pressure side under an oxygen concentration gradient. Theoretically, the permselectivity of oxygen is 100%. Recently, anovel mixed-conducting membrane-Ba0.5Sr0.5Coo.8Feo.2O3-δ has been developed, which showsextremely high oxygen permeability and promising stability. Furthermore, the reactor made with such membranes was successfully applied to the partial oxidation of methane to syngas reaction using air as the oxygen source, which realized the coupling of the separation of oxygen from air and the partial oxidation of membrane reaction in one process. At 850℃, methane conversion 】 88%, CO selectivity 】97% and oxygen permeation rate of about 7.8 mL/(cm2 ·min) were obtained.

Fast formation of NaA zeolite membrane in the microwave field

NaA zeolite membrane was successfully synthesized on the porous α-Al2O3 support by microwave heating. The synthesis of NaA zeolite membrane in the microwave field only needs 15 min and the synthesis time is 10 times shorter than that by conventional heating. SEM characterization indicates that the zeolite crystals in the NaA zeolite membrane synthesized by microwave heating are uniform in size; the membrane thickness is about 4 μm and is thinner than that of the NaA zeolite membrane synthesized by conventional heating. Gas permeation studies indicate that the permeances of the NaA zeolite membrane synthesized by microwave heating are 3-4 times higher than those of the NaA zeolite membrane synthesized by conventional heating, while their permselectivities are comparable.

Metal-organic framework-based CO2 capture: from precise material design to high-efficiency membranes

A low-carbon economy calls for CO capture technologies.Membrane separations represent an energy-efficient and environment-friendly process compared with distillations and solvent absorptions.Metal-organic frameworks(MOFs),as a novel type of porous materials,are being generated at a rapid and growing pace,which provide more opportunities for high-efficiency CO capture.In this review,we illustrate a conceptional framework from material design and membrane separation application for CO capture,and emphasize two importance themes,namely(i)design and modification of CO-philic MOF materials that targets secondary building units,pore structure,topology and hybridization and(ii)construction of crack-free membranes through chemical epitaxy growth of active building blocks,interfacial assembly,ultrathin two-dimensional nanosheet assembly and mixed-matrix integration strategies,which would give rise to the most promising membrane performances for CO capture,and be expected to overcome the bottleneck of permeability-selectivity limitations.

Perovskite-type B-site Bi-doped ceramic membranes for oxygen separation

Novel mixed conducting oxides, B-site Bi-doped perovskites were exploited and synthesized. Cubic perovskite structures were formed for BaBi0.2COyFe0.8-yO3-δ (y≤0.4) and BaBixCo0.2Fe0.8-xO3-δ (x=0.1 -0.5). The materials exhibited considerable high oxygen permeability at high temperature. The oxygen permeation flux of BaBi0.2Co0.35Fe0.45O3-δmembrane reached about0.77×106 mol/cm2 ·s under an air/helium oxygen partial pressure gradient at 900 ℃, which was much higher than that of other bismuth-contained mixed conducting membranes. The permeation fluxes of the materials increased with the increase of cobalt content, but no apparent simple relationship was found with the bismuth content. The materials also demonstrated excellent reversibility of oxygen adsorption and desorption. Stable time-related oxygen permeation fluxes were found for BaBi0.2Co0.

Metal-Organic Framework Membranes and Membrane Reactors:Versatile Separations and Intensified Processes

Metal-organic frameworks are an emerging and fascinating category of porous solids that can be self-assembled with metal-based cations linked by organic molecules.The unique features of MOFs in porosity(or surface areas),together with their diversity for chemical components and architectures,make MOFs attractive candidates in many applications.MOF membranes represent a long-term endeavor to convert MOF crystals in the lab to potentially industry-available commodities,which,as a promising alternative to distillation,provide a bright future for energy-efficient separation technologies closely related with chemicals,the environment,and energy.The membrane reactor shows a typical intensified process strategy by combining the catalytic reaction with the membrane separation in one unit.This review highlights the recent process of MOF-based membranes and the importance of MOF-based membrane reactors in relative intensified chemical processes.

ZIF-L membrane with a membrane-interlocked-support composite architecture for H/CO separation

Metal–organic framework(MOF)membranes hold great promise in energy-efficient chemical separations.The outstanding challenges of the microstructural design stem from(1)thinning of membranes to immensely reduce the mass-transfer resistance(for high permeances);(2)tuning of orientation to optimize the selective transport of gas molecules,and(3)reinforcement of intercrystalline structure to subside leakage through defective gaps(for high selectivity).Here,we propose the ZIF-L membrane that is completely confined into the voids of the alumina support through an interfacial assembly process,producing an appealing membrane-interlocked-support(MIS)composite architecture that meets the requirements of the microstructural design of MOF membranes.Consequently,the membranes show average H2 permeances of above 4000 GPU and H_(2)/CO_(2) separation factor(SF)of above 200,representing record-high separation performances of ZIF-L membranes and falling into the industrial target zone(H_(2) permeance>1000 GPU and H_(2)/CO_(2) SF>60).Furthermore,the ZIF-L membrane possessing the MIS composite architecture that is established with alumina particles as scaffolds shows mechanical stability,scraped repeatedly by a piece of silicon rubber causing no selectivity loss.

Modification strategies for metal-organic frameworks targeting at mem brane-based gas separations

Metal-organic frameworks(MOFs)hold great promises as membrane candidates for highly efficient separation applications,benefiting from the diversified structures,high surface areas and adjustable chemical functionalities.However,non-selective defects and framework flexibility are two main concerns which would attenuate the ultimate separation performance and stability.Modification helps to orientationally optimize the gas adsorption and diffusion behaviors via manipulation towards framework chemical components,aperture sizes,nanocages,and intercrystalline/intracrystalline defects,consequently promoting membrane separation performance and membrane stability.In view of recent progresses of modification on MOF-based membranes,two categories of modification strategies were summarized,namely post-synthetic modification and in situ modification.And the merits and demerits are elucidated.Furthermore,challenges and opportunities for the current modification strategies were discussed from our perspectives,with an expectation to provide guidelines to the future development of MOF-based membranes which were aspired to reach the commercially attractive performance region.

Investigation of novel zir-conium based perovskite-type mixed conducting membranes for oxygen separation

A series of BaZr0.2Co0.8-χFeχO3-σ materials for oxygen separation were synthesized through a citric and EDTA acid combined complexing method, and their crystal structures, oxygen permeabilities, sintering and sealing abilities were investigated. The results showed that the cubic perovskite structure was formed for materials in the composition range investigated. Oxygen permeation flux and stability, as well as sintering and sealing abilities of the synthesized materials were increased or improved apparently due to the introduction of zirconium. For example, the oxygen permeation flux reached 0.8 mL/min · cm2 when χ = 0.2 or 0.3 at 950℃, and a stable time-related oxygen permeation flux was found for the BaZr0.2Co0.3Fe0.5O3-σ membrane at 800℃.

Improved hydrogen separation performance of asymmetric oxygen transport membranes by grooving in the porous support layer

Hydrogen separation through oxygen transport membranes(OTMs)has attracted much attention.Asymmetric membranes with thin dense layers provide low bulk diffusion resistances and high overall hydrogen separation performances.However,the resistance in the porous support layer(PSL)limits the overall separation performance significantly.Engineering the structure of the PSL is an appropriate way to enable fast gas transport and increase the separation performance.There is no relevant research on studying the influence of the PSL on hydrogen separation performance so far.Herein,we prepared Ce0.85Sm0.15O1.925–Sm0.6Sr0.4Cr0.3Fe0.7O3-δ(SDC-SSCF)asymmetric membranes with straight grooves in PSL by tape-casting and laser grooving.A~30%improvement in the hydrogen separation rate was achieved by grooving in the PSLs.It indicates that the grooves may reduce the concentration polarization resistance in PSL for the hydrogen separation process.This work provides a straight evidence on optimizing the structures of PSL for improving the hydrogen separation performance of the membrane reactors.

Separation of butane isomer by tubular silicalite-1 zeolite membrane

The permeation properties of n-butane, i-butane and n-butane/i-butane mixture （n-butane 24.3% （molar ratio）, i-butane 75.7%） through a tubular silicalite1 zeolite membrane were studied at 298 and 473 K respectively. The permselectivities of n-butane and i-butane under pressure difference of 0.06 MPa at 298 and 473 K were 16.3 and 7.4 respectively. The separation factors of n-butane/i-butane mixture were between 2.0 and 2.5 at 298 and 473 K. At 298 K, the permeances of n-butane in the mixture were lower than those of single component while the permeances of i-butane in the mixture were almost the same as those of single component. At 473 K, the permeances of n-butane and i-butane in the mxiture were decreased compared with those of the single component, and the permeance of n-butane decreased more rapidly than that of i-butane.