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[Display omitted] • Highly exposed isolated FeN 5 sites are embedded in N-doped carbon frameworks. • FeN 5 exhibits a maximum FE CO of 98% and over 95% from −0.4 to −1.0 V vs. RHE. • The axial coordinated N modulates the d -band center and facilitates CO desorption. • FeN 4 and axial coordinated N act synergistically to boost CO 2 -to-CO conversion. Electrochemical CO 2 conversion offers a sustainable approach to alleviate the energy crisis but remains a long-standing challenge. Herein, atomically dispersed FeN 5 sites anchored on N-doped carbon matrix templated by zeolitic imidazole framework-8 (ZIF-8) are facilely synthesized for efficient catalyzing CO 2 reduction. The FeN 5 single-atom catalyst (Fe-SA/ZIF) presents the Faradaic efficiency of 98% at −0.7 V vs. RHE and over 95% in a wide potential range from −0.4 to −1.0 V vs. RHE, surpassing most of the reported single-atom catalysts for CO 2 RR. Further density functional theory (DFT) calculations reveal that the outstanding activity of FeN 5 sites mainly originates from the lowered d -band center modulated by the out-of-plane coordinated pyridinic N, which reduces the CO adsorption energy from −1.71 to −1.49 eV compared to FeN 4 moieties. Thus, the synergistic effect between FeN 4 sites and the coordinated pyridine offers new insight in designing outstanding electrocatalysts for multiple applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hollow ZIF-8 in micron accumulated continuously across membrane to construct transport highway. • The real CO 2 transport distance was reduced. • In-situ polymerization intensifies the interface affinity between ZIF-8 and PEO. The hollow filler has been verified to effectively enhance the CO 2 permeability of mixed matrix membrane (MMMs) by constructing low-resistance transport pathway. However, the in-continuity of hollow fillers across the membrane results in insufficient CO 2 permeability. In this work, the micron-sized hollow ZIF-8 (Mic-H-ZIF-8) were designed and proposed to accumulate continuously across the membrane to construct the transport highway for CO 2. Importantly, the in-situ polymerization was utilized to achieve continuous accumulation of the Mic-H-ZIF-8 fillers across the membrane. The Mic-H-ZIF-8 presents cavity size of 4 μm and shell thickness of 100 nm, and three or four Mic-H-ZIF-8 microspheres are aligned sequentially across the membrane, in which the effective gas transport distance is decreased from 13 μm to the range of 0.8–3 μm. Consequently, the CO 2 permeability of 5 wt% Mic-H-ZIF-8/PEO MMM is 224.6 % and 63.5 % higher than those of pure PEO membrane and nanosized hollow ZIF-8/PEO MMM at 0.1 MPa. Meanwhile, the CO 2 /N 2 selectivity is still as high as 70.0. This CO 2 separation performance is far exceeding the Robeson upper bound in 2008, and beyond the McKeown upper bound in 2019 as the feed pressure increasing, confirming the significance of continuous transport highway constructed by micron-sized hollow MOFs on improving CO 2 separation of MMMs. [ABSTRACT FROM AUTHOR]

Facilitated transport membrane with fixed and mobile carriers is essential yet challenging to improve the CO 2 capture. Herein, a facilitated transport membrane with continuous pathway was constructed by the in-situ interlinkage of mobile aniline with the fixed carriers distributing along electrospun polyacrylonitrile (PAN) nanofibers. In this membrane, fixed amine carriers are introduced through the alkalinous hydrolysis of PAN fibers (HPAN), and are distributed along the HPAN fibers to construct a long-range continuous transport pathway for CO 2. The mobile aniline carriers are introduced into the interfibrous voids of the HPAN nanofibers mat (NFM) by in-situ polymerization of Poly (ethylene glycol) diacrylate (PEGDA), which interlink the fixed amine carriers to achieve the facilitated transport pathway continuous completely. And the fixed amine and mobile aniline carriers are demonstrated to co-exist in the An/PEO@HPAN membrane (NFCM) by the FTIR and XPS results. Meanwhile, the XRD and DSC results indicate that the introduction of aniline increases the free volume of PEO matrix, which is beneficial for enhancing CO 2 permeation. Therefore, the 10 wt% An/PEO@HPAN-3h NFCM presents 90.81 % and 13.15 % increases in CO 2 permeability and CO 2 /N 2 selectivity under mixed gas at 0.3 MPa in contrast with the membrane without mobile aniline carriers. Under mixed gas conditions, this NFCM exhibits CO 2 permeability of 202.93 Barrer and CO 2 /N 2 selectivity of 83.89 at 0.1 MPa, reaching the 2019 McKeown upper bound. [Display omitted] • Fixed amine carriers are distributed along the HPAN nanofibers. • Mobile aniline carriers are introduced with in-situ polymerization of PEGDA. • Aniline interlinks the long-range continuous transport pathway in short range. • Continuous facilitated transport pathway improves CO 2 permeability. [ABSTRACT FROM AUTHOR]

[Display omitted] • A series of Co SACs were synthesized with tunable Co-N 5- x C x (x = 1, 2, 3) sites. • Engineering the hybrid N and C coordination greatly affect the CO 2 RR performance. • FE CO boosted from 54 to 76 and 92% at −0.8 V with increased ratio of Co-C. • Co-C promotes the electron donation and strengthen binding interaction with *COOH. Single-atom catalysts (SACs) with well-defined active sites provide an efficient route for catalyzing CO 2 reduction reaction (CO 2 RR). Although enormous attention has been focused on metal-N x moieties, understanding the effect of metal-C coordination and engineering of the hybrid metal-N/C sites have rarely been reported. Herein, we fabricated Co SACs with tunable isolated Co-N 5-x C x (x = 1, 2, 3) sites supported on porous carbon frameworks. Benefiting from the difference in electronegativity of the coordinated N and C atoms, the CO Faradaic efficiency (FE CO) enhanced considerably from 54% to 76% and 92% at −0.8 V vs. RHE for Co-N 4 C 1 , Co-N 3 C 2 , and Co-N 2 C 3 , respectively. Further density functional theory (DFT) calculations uncover that the increased ratio of Co-C coordination induced electron enrichment of Co atoms and upper-shift the d -band center to near Fermi level, and thus favorably promotes the electron-donating ability of Co centers and strengthens the adsorption of *COOH. [ABSTRACT FROM AUTHOR]

Table of contents for mechanism model of Cu-N 3 and Cu-N 4 moieties. • Highly exposed isolated Cu-N x sites were embedded in microporous carbon frameworks. • CO 2 -to-CO conversion was boosted with tailored Cu–N coordination environment. • Cu-N 4 exhibited a maximum FE CO of 98% and over 90% from −0.6 to −1.1 V vs. RHE. • Edge-hosted Cu-N 4 was identified as the accurate configuration of active site by DFT calculations. Although considerable progress has been achieved by Cu nanoparticles for catalyzing CO 2 reduction reaction (CO2RR), Cu single atom catalysts (Cu SACs) are generally suffered from inferior performance to that of widely investigated Fe, Co, Ni SACs. This phenomenon mainly ascribes to the lack of effective geometry and electronic engineering of copper active center from an atomic level. Herein, highly exposed atomically dispersed Cu-N x (x denotes Cu–N coordination number) sites anchored on 3D porous carbon matrix are successfully synthesized through facile one step thermal activation, and Cu-N 4 sites exhibit boosted activity and selectivity compared to its nearly inert Cu-N 3 counterparts. Aided by density functional theory (DFT) calculations, the edge-hosted Cu-N 4 moieties are revealed as key active sites for efficient CO generation via optimized local coordination environment and electronic properties, which strongly interact with *COOH intermediate and facilitate the desorption of *CO. As a result, Cu-N 4 catalyst achieves high CO Faradaic efficiency (FE CO) of over 90% from −0.6 to −1.1 V vs. RHE with a maximum value of 98%, surpassing the previously reported Cu SACs for CO 2 -to-CO conversion. This work provides new insight into proper Cu SACs design and fundamental mechanism understanding to boost CO2RR. [ABSTRACT FROM AUTHOR]

• In-situ position of ILs into cages of ZIF-8 was proposed to tune cage size. • CO 2 -philic cages constructed by ILs improve CO 2 adsorption. • Molecular simulation reveals increased CO 2 adsorption by increasing ILs in cages. • CO 2 /N 2 selectivity of MMMs is increased due to pore screening and CO 2 adsorption. The regulation on cage size and CO 2 -philic environment of metal–organic frameworks (MOFs) is essential to improve CO 2 capture. In this work, [Bmim][Tf 2 N] was proposed to position into cages of ZIF-8 to achieve the selective permeation of CO 2 by pore screening and CO 2 adsorption. [Bmim][Tf 2 N] was introduced into cages of ZIF-8 by "adsorption-infiltration" strategy. Due to the strong interaction between [Tf 2 N]− and ZIF-8, the cages present a CO 2 -philic environment, which preferentially attracts CO 2 to enter these cages, ensuring the dissolution selectivity of CO 2. Besides, the proper cage size tuned by [Bmim][Tf 2 N] provides CO 2 with a faster transport channel due to its smaller average kinetic diameter than N 2 , guaranteeing the diffusion selectivity of CO 2. Consequently, the CO 2 separation in [Bmim][Tf 2 N]@ZIF-8/Pebax MMMs is intensified. With increasing the [Bmim][Tf 2 N] content in cages of ZIF-8, the pore volume by BET results decreases and the CO 2 adsorption from molecular simulation increases. In response, the CO 2 /N 2 selectivity of [Bmim][Tf 2 N]@ZIF-8/Pebax MMMs is enhanced constantly. Meanwhile, the CO 2 permeability is declined due to the narrowed cage size. All of these results demonstrate the significance of pore screening and CO 2 adsorption induced by [Bmim][Tf 2 N]. Specifically, [Bmim][Tf 2 N] 8 @ZIF-8/Pebax MMM at filler loading of 15 wt% exhibits the CO 2 permeability of 108.3 Barrer and the CO 2 /N 2 selectivity of 86.7, exceeding the 2008 Robeson upper bound. Especially the CO 2 /N 2 selectivity locates at a higher level than those of most reported works, indicating that appropriate tuning of the physicochemical properties within the nanocages of MOFs is significative to improve CO 2 capture. [ABSTRACT FROM AUTHOR]

Pore size adjustment has been considered to be a successful strategy for enhancing the sieving of MOF-based mixed matrix membranes (MMMs). However, the increase in CO 2 /N 2 selectivity usually comes at the expense of permeability. Herein, the regulating on the pore size of polystyrene-modified hollow NH 2 -MIL-101 (PHNM) is achieved by controlling the confined dissolving of PSA polymer chain. During the etching of PSA template, the dissolved PSA polymer chains diffused outward through the pores and intergranular space of NH 2 -MIL-101 under the concentration difference. As a consequence, the pores of NH 2 -MIL-101 were unavoidably constrained by the partly dissolved PSA segments, achieving the regulation on the pore size. Meanwhile, there are also some PSA segments adhering to the surface of NH 2 -MIL-101 to form a PSA coating layer. In this unique PHNM, the reduced pore size endows it with better CO 2 /N 2 selectivity than NH 2 -MIL-101. Meanwhile, the high interface compatibility provided by surface-modified PSA segments and the enhanced CO 2 affinity of PHNM resulting from amino groups also synergistically improve the CO 2 /N 2 selectivity. Additionally, the hollow core offers gas molecules a low-resistance transfer channel, improving the permeability of CO 2. The hollow size of PHNM was adjusted from 33 to 57 nm by controlling the etching time of PSA, and the pore size of PHNM was regulated from 0.58 to 0.83 nm by controlling the confined dissolving process of PSA. Thus, the PHNM-based MMMs exhibit high CO 2 permeability as well as high CO 2 /N 2 selectivity. Compared with NH 2 -MIL-10/Pebax MMM, the MMM with PHNM-2 loading of 10 wt% shows 30% and 21% enhancements in the CO 2 permeability (226.3 Barrer) and CO 2 /N 2 selectivity (71.3), respectively, far exceeding the Robeson upper bound (2008). These results reveal that the PHNM based MMMs express a great potential in CO 2 separation. [Display omitted] • Confined dissolving of PSA polymer chain is proposed to tune pore size. • Pore size of MIL-101 is regulated from 0.58 to 0.83 nm. • Surface-modified PSA and amino groups intensify interface compatibility. • Hollow structure constructs low-resistance region to improve CO 2 permeability. [ABSTRACT FROM AUTHOR]

Reducing the effective pore size of MOFs is necessary to improve gas screening of mixed matrix membrane (MMMs). However, continuous and precise regulation on pore size of MOFs is still challenging. In this work, dopamine (DA) monomers are proposed to polymerize in situ into pore cages of UIO-66, achieving the tuning of effective pore size for CO 2 screening. Meanwhile, the abundant hydroxyl and amino groups in polydopamine (PDA) offer high affinity with CO 2 to improve the selective permeation of CO 2. Furthermore, additional PDA existing outside UIO-66 at longer polymerization time can intensify the interface compatibility to promote CO 2 /N 2 selectivity. Through increasing PDA content in the cage by prolonging polymerization time, the effective pore size and total pore volume of UIO-66 are decreased by 13.2% and 59.2%, respectively, compared with pure UIO-66. Correspondingly, PDA@UIO-66/Pebax MMM presents greatly improved CO 2 separation performances, especially the CO 2 /N 2 selectivity, compared with UIO-66/Pebax MMM. Specifically, both the permeability and selectivity of CO 2 increase with the PDA content into the cages increasing, which is attributed to the synergy of reduced pore size and abundant CO 2 -philic groups in PDA. The PDA 10.3 @UIO-66/Pebax MMM shows the best CO 2 /N 2 selectivity of 70.55 and CO 2 permeability of 94.56 Barrer, 29.2% and 27.9% higher than those of UIO-66/Pebax MMM, respectively. The proposed PDA@UIO-66 based MMMs exhibit superior CO 2 separation performance, especially high CO 2 /N 2 selectivity compared with most reported UIO-66 based MMMs. Together with the good long-term stability, the proposed regulating strategy on effective pore engineering of MOFs is promising and competitive in CO 2 screening of MOFs based MMM. [Display omitted] • Dopamine is polymerized in situ into pore cages of UIO-66 to tune pore size. • Pore size of UIO-66 is decreased gradually with PDA content in pore cage. • Abundant CO 2 -philic groups in PDA improve CO 2 selective permeation further. • PDA outside UIO-66 intensifies the interface compatibility. [ABSTRACT FROM AUTHOR]

Layered double hydroxide (LDH) bearing numerous hydroxyl groups shows high affinity with CO 2 and is promising in mixed matrix membranes (MMMs) based CO 2 capture. However, its non-porous structure limits the further improvement of CO 2 permeability. In this work, 3D hollow CoNi-LDH nanocages were developed to construct low-resistance and CO 2 -philic transport channel in MMM by in-situ etching of ZIF-67 template. The resultant material exhibits hollow structure with cavity size of about 500 nm, mesopores and micropores from LDH stacking and ZIF-67 template. The outer layered CoNi-LDH nanosheets can attract more CO 2 molecules by their abundant hydroxyl groups, resulting in an improvement in dissolution selectivity of CO 2 over N 2. Meanwhile, the tortuous gas diffusion paths originated from 2D nanosheet stacking can further enhance the CO 2 selectivity over N 2. Moreover, the hollow nanocages provide CO 2 molecules with fast transport path, resulting in high CO 2 permeability. As a result, compared with pure Pebax membrane and 2D LDH/Pebax MMM, all the 3D hollow CoNi-LDH/Pebax MMMs present enhanced CO 2 permeability. The 3D hollow CoNi-LDH-2h/Pebax MMM performs the highest CO 2 permeability of 135.49 Barrer, which are increased by 141.95% and 71.50% than those of the above two membranes, respectively. The selectivity of 3D hollow CoNi-LDH-2h/Pebax MMM is about 26.30% higher than that of the pure Pebax membrane. The present membrane presents comparable CO 2 capture capacity in contrast with most reported Pebax based MMMs and surpasses the Robeson upper bond. Together with the good long-term stability, the 3D hollow CoNi-LDH nanocages are believed to be a promising candidate for MMMs to improve CO 2 capture. [Display omitted] • MMM with low resistance and CO 2 -philic transport channel was constructed. • 3D Hollow CoNi-LDH nanocages were developed by etching ZIF-67 template. • Hollow and porous structure ensures the high CO 2 permeability. • Abundant hydroxyl groups and tortuous diffusion paths ensure high selectivity. [ABSTRACT FROM AUTHOR]

Metal organic frameworks (MOFs) based mixed matrix membrane (MMM) for CO 2 capture shows insufficient CO 2 permeability due to random disperse of MOFs in form of individual particle. Therefore, in this work, a continuous MOFs transport pathway for CO 2 with low resistance was constructed in the MMM by assistance of electrospun nanofiber. Specifically, PAN electrospun nanofiber mat (NFM) was utilized as skeleton to obtain continuous ZIF-8 nanofibers. Then flexible polyethylene glycol (PEG) molecules were photo-polymerized in situ into the inner-voids within ZIF-8@PAN NFM to form dense membrane. As a result, ZIF-8 particles are distributed in PEO matrix continuously, leading to a low-resistance transport pathway for gas molecules in MMM, which is proved by the cross-sectional SEM images. Furthermore, the higher Tg of ZIF-8@PAN-x/PEO MMM by DSC also indicates the good interfacial compatibility between ZIF-8 and PEO. Compared with pure PEO membrane, all the ZIF-8@PAN-x/PEO MMMs present both enhanced CO 2 permeability and CO 2 /N 2 selectivity. The enhanced CO 2 permeability is attributed to the continuous ZIF-8 transport pathway, and the improved selectivity is ascribed to the size screening from nanofibers overlapping and stacking and the tortuous transport pathway. Importantly, compared with ZIF-8/PEO MMM, in which ZIF-8 particles are randomly dispersed into PEO matrix, the ZIF-8@PAN-4/PEO MMM with the same ZIF-8 loading of about 16.6 wt% presents enhanced CO 2 permeability and CO 2 /N 2 selectivity, which are increased by 52.4% and 44.3%, respectively. This is due to the continuous ZIF-8 transport pathway and good interfacial compatibility, which is also confirmed by the result of gas diffusivity and solubility coefficients. Together with the outstanding mechanical strength and stability, the proposed MMM with continuous ZIF-8 transport pathway paves a new way to improve CO 2 permeability. [Display omitted] • MMM with continuous MOFs pathway was constructed to capture CO 2. • PAN nanofibers were used to induce MOFs continuous in long-range. • MMM was obtained by in-situ polymerization of PEG within MOFs NFM. • Both high permeability and selectivity were achieved by continuous gas pathway. [ABSTRACT FROM AUTHOR]

The electrospun nanofiber mat (NFM) with 3D networks has advantages of high porosity and large specific surface area, but its porous structure limits the application in gas membrane separation. Therefore, in this work, a novel and defect-free nanofibers interpenetrating composite membrane is proposed for CO 2 /N 2 separation. And it is constructed by in-situ polymerization of low-molecular-weight polyethylene glycol (PEG) into the inner-voids within PAN NFM. In this nanofiber composite membrane (NFCM), PAN nanofibers interpenetrate into PEO matrix to form a "reinforced-concrete" structure with good interfacial compatibility between them, which has been demonstrated by SEM, EDS, ATR-FTIR and C1s XPS characterizations. Particularly, the PAN nanofiber acts as "Rebar" framework, while the void-sealing polymer PEO serves as "Concrete". They are bonded into a whole through adhesive force to bear the external force together, resulting in elevated mechanical properties than the pure PEO membrane. Furthermore, the CO 2 permeability of each PEO/PAN NFCM shows a minor decease in contrast with corresponding pure PEO membrane due to the barriers of PAN nanofibers to CO 2 molecules. However, the CO 2 /N 2 selectivity of each NFCM has been increased by 13–15 in contrast with its corresponding pure PEO membrane. This enhancement can be explained by the size screening from PAN nanofibers overlapping and stacking and the tortuous transport pathway along the impermeable PAN nanofibers. As a result, the 3:7/PAN NFCM exhibits a CO 2 permeability of 343 Barrer and a CO 2 /N 2 selectivity of 65.4, exceeding the Robeson upper bond of 2008. Together with the excellent mechanical properties, the proposed nanofibers interpenetrating composite membrane displays a bright prospect for CO 2 capture. [Display omitted] • Nanofibers interpenetrated composite membrane was proposed to capture CO 2. • The NFCM mimicks "reinforced-concrete" to be mechanically robust. • In-situ polymerization of PEG is proposed to seal the inner-voids within PAN NFM. • Overlap and stack of nanofibers provide size screening and tortuous transport pathway. [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNTs) with 1D gas transport channel has great prospect in MMMs based CO 2 separation. However, the insufficient compatibility with polymer and inner diameter of CNTs limit further improvement of their CO 2 separation performances. In this work, the unique ZIF-8 hollow nanotubes (HNTs) were proposed to construct high-speed gas transmission channel in MMMs by a three-step method (electrospinning-calcination-hydrothermal), aiming at concurrently improving the gas permeability and selectivity. The hollow inner cavity provides long-range and low-resistance gas transmission channels to ensure high gas permeability. At the same time, the ZIF-8 crystals composing the tube wall can not only screen for gas molecules pair, but also strengthen the affinity between the filler and polymer matrix by its organic ligands, thus the gas selectivity can be raised. Compared with ZIF-8 nanoparticles and ZnO HNTs, ZIF-8 HNTs based MMMs present competitive CO 2 separation. When the loading of ZIF-8 HNTs is 5 wt%, the gas separation performance shows the best, in which the CO 2 permeability is 147 Barrer and the CO 2 /N 2 selectivity is 68, approaching to the 2008 Robeson upper limit and surpassing most reported CNTs/Pebax based membranes. Therefore, the proposed ZIF-8 HNTs are promising for MMMs based CO 2 separation. [Display omitted] • ZIF-8 HNTs were designed by three steps of electrospinning-calcination- hydrothermal. • ZIF-8 HNTs were proposed to construct high-speed gas transmission channel in MMMs. • CO 2 permeability of ZIF-8 HNTs MMMs is 147 Barrer, 63% higher than that of pure ZIF-8. • CO 2 /N 2 selectivity of ZIF-8 HNTs MMMs is 68, 15% higher than that of pure ZIF-8. • CO 2 permeability and selectivity of ZIF-8 HNTs/Pebax surpassed most reported CNTs/Pebax. [ABSTRACT FROM AUTHOR]

[Display omitted] • PSA modified hollow ZIF-8 were designed by a hard template-incomplete etching strategy. • The hollow core of PHZ builds a low-resistance CO 2 transport freeway. • The outside PSA layer intensifies the interface affinity between ZIF-8 and Pebax. The trade-off between gas permeability and selectivity represents a huge challenge for membrane separation. In this work, we design unique polystyrene-acrylate (PSA) modified hollow ZIF-8 (PHZ) nanospheres via a facile hard template-incomplete etching strategy as fillers for mixed matrix membranes (MMMs). In the vesicles shaped PHZ, the hollow core builds a low-resistance CO 2 transport freeway, leading to improved CO 2 permeability. Meanwhile, the outside PSA layer intensifies the interface affinity between ZIF-8 and Pebax through molecular chains entanglements and hydrogen bonding, enhancing membrane selectivity. Through controlling the PSA etching process, both the hollow core with tunable size and the outside PSA layer with adjustable thickness are achieved. The 10 wt% PHZ-2/Pebax MMMs present the best performance with a CO 2 permeability of 172.4 Barrer and CO 2 /N 2 selectivity of 87.9. The higher CO 2 /N 2 selectivity proves that the PSA layer on the PHZ has better compatibility with Pebax. The higher CO 2 permeability is attributed to the low-resistance hollow core. The gas separation performance of PHZ-based MMMs not only well exceeded the 2008 Robeson upper-bond line but also was superior to those of other published MOF-based MMMs. The concept of vesicle-shaped PHZ paves a way to concurrently enhance both the gas permeability and selectivity of MMMs. [ABSTRACT FROM AUTHOR]