Efficient proton conduction in porous and crystalline covalent-organic frameworks(COFs)

To attain the objectives of carbon peaking and carbon neutrality,the development of stable and highperformance ion-conducting materials holds enormous relevance in various energy storage and conversion devices.Particularly,crystalline porous materials possessing built-in ordered nanochannels exhibit remarkable superiority in comprehending the ion transfer mechanisms with precision.In this regard,covalent organic frameworks(COFs)are highly regarded as a promising alternative due to their preeminent structural tunability,accessible well-defined pores,and excellent thermal/chemical stability under hydrous/anhydrous conditions.By the availability of organic units and the diversity of topologies and connections,advances in COFs have been increasing rapidly over the last decade and they have emerged as a new field of proton-conducting materials.Therefore,a comprehensive summary and discussion are urgently needed to provide an"at a glance"understanding of the prospects and challenges in the development of proton-conducting COFs.In this review,we target a comprehensive review of COFs in the field of proton conductivity from the aspects of design strategies,the proton conducting mechanism/features,the relationships of structure-function,and the application of research.The relevant content of theoretical simulation,advanced structural characterizations,prospects,and challenges are also presented elaborately and critically.More importantly,we sincerely hope that this progress report will form a consistent view of this field and provide inspiration for future research.

Precisely manipulation of core composition of core-shell-type cobalt polyoxoniobates and proton conduction study

The development of core-shell nanoclusters with controllable composition is of utmost importance as the material properties depend on their constituent elements.However,precisely tuning their compositions at the atomic scale is not easily achieved because of the difficulty of using limited macroscopic synthetic methods for atomic-level modulation.In this work,we report an interesting example of precisely regulating the core composition of an inorganic core-shell-type cobalt polyoxoniobate[Co_(26)Nb_(36)O_(140)]^(32−)by controlling reaction conditions,in which the inner Co-core composition could be tune while retaining the outer Nb-shell composition of resulting product,leading to a series of isostructural species with a general formula of{Co_(26-n)Nb_(36+n)O_(140)}(n=0–2).These rare species not only can display good powder and single-crystal proton conductivities,but also might provide helpful and atomic-level insights into the syntheses,structures and composition modifications of inorganic amorphous core-shell heterometal oxide nanoparticles.

Boosting the proton conduction in a magnetic dysprosium-organic framework by introducing conjugate NH_(4)^(+)-NH_(3)pairs

Metal-organic frameworks(MOFs)with inherent porosity and suspended acidic groups are promising proton conducting materials in water or aqua-ammonia media.Herein we report a new lanthanide phosphonate,namely,Dy_(2)(amp_(2)H_(2))_(2)(mal)(H_(2)O)_(2)·5H2O(MDAF-6).It possesses a 3D open-framework structure,and shows a high NH_(3)adsorption capacity of 142.4 cm^(3)/g at P/P0=0.98 at 298 K due to acid-base interaction.Interestingly,the proton conductivity of MDAF-6-NH3 is enhanced by five orders of magnitude compared to MDAF-6 after 8.5 h exposure in saturated NH_(3)-H_(2)O vapor,indicating the importance of coexistent conjugate acid-base pairs of H_(3)O+-H_(2)O and NH_(4)^(+)-NH_(3)in promoting proton conduction.Magnetic studies of MDAF-6 revealed slow magnetization relaxation under zero dc field,characteristic of singlemolecule magnet behavior.This work provides not only a new multifunctional MOF material,but also a new strategy to improve proton conduction in aqua-ammonia medium.

Expanding the dimensionality of proton conduction enables ultrahigh anhydrous proton conductivity of phosphoric acid-doped covalent-organic frameworks

It is of great significance to develop high-temperature anhydrous proton conducting materials.Herein,we report a new strategy to significantly enhance the proton conductivity of covalent organic frameworks(COFs)through expanding the dimensionality of proton conduction.Three COF-based composites,COF-1@PA,COF-2@PA,and COF-3@PA(PA:phosphoric acid),are prepared by PA doping of three COFs with similar pore sizes but different amounts of hydrophilic groups.With the increase of hydrophilic groups,COFs can load more PA because of the enhanced hydrogen–bonding interactions between PA and the frameworks.powder X-ray diffraction(PXRD),scanning electron microscopy(SEM),and two-dimensional(2D)solid-state nuclear magnetic resonance(NMR)analyses show that PA can not only enter the channels of COF-3,but also insert into its 2D interlayers.This expands the proton conduction pathways from one-dimensional(1D)to three-dimensional(3D),which greatly improves the proton conductivity of COF-3.Meanwhile,the confinement effect of 1D channels and 2D layers of COF-3 also makes the hydrogen-bonded networks more orderly in COF-3@PA-30(30μL of PA loaded on COF-3).At 150℃,COF-3@PA-30 exhibits an ultrahigh anhydrous proton conductivity of 1.4 S·cm−1,which is a record of anhydrous proton conductivity reported to date.This work develops a new strategy for increasing the proton conductivity of 2D COF materials.

Ultrathin nanoporous metal electrodes facilitate high proton conduction for low-Pt PEMFCs

Design of catalyst layers(CLs)with high proton conductivity in membrane electrode assemblies(MEAs)is an important issue for proton exchange membrane fuel cells(PEMFCs).Herein,an ultrathin catalyst layer was constructed based on Pt-decorated nanoporous gold(NPG-Pt)with sub-Debye-length thickness for proton transfer.In the absence of ionomer incorporation in the CLs,these integrated carbon-free electrodes can deliver maximum mass-specific power density of 198.21 and 25.91 kW·gPt^(-1) when serving individually as the anode and cathode,at a Pt loading of 5.6 and 22.0 pg·cm^(-2),respectively,comparable to the best reported nano-catalysts for PEMFCs.In-depth quantitative experimental measurements and finite-element analyses indicate that improved proton conduction plays a critical role in activation,ohmic and mass transfer polarizations.

Dual-sided symmetric crystalline orientation of covalent organic framework membranes for unidirectional anhydrous proton conduction

Two-dimensional covalent organic frameworks(2D COFs)have sparkled wide-ranging research to explore proton-conducting materials.However,the powder-pressed pellets or continuous membranes of 2D COFs are composed of randomly arranged crystallites,which are undesirable for proton transport via a shortcut pathway.We report a controlled strategy for preparing a conformably oriented free-standing COF membrane to address the critical challenge.A monofunctional aldehyde precursor is used as a modulator to enhance reversible association and optimize growth orientation in an interfacial polymerization system.The preferred face-on alignment is achieved throughout the membrane from its flat side to the nanoflake-standing side and,in turn,generates the unidirectional pore channels for accommodating 1,2,4-triazole as proton carriers.The composite merges distinctive features including orientation,crystallinity,porosity,and mechanical strength into one system,exhibiting ultrafast and stable anhydrous proton conduction at high operating temperatures with low activation energy.Our findings offer an innovative strategy for the oriented crystallization of free-standing COF membranes for energy conversion applications.

Pillared-layer MOF Based on Template-directed Method: Synthesis, Structure and Proton Conduction Properties

By using Co(NH3)63+as the template,a new complex HNU-38 was synthesized under hydrothermal reaction through the adoption of H3BTC and Cl-as the ligands(H3BTC=1,3,5-benzenetricarboxylic acid).HNU-38 crystallized in the monoclinic system,P21/c space group with a=9.9696(3),b=17.0580(6),c=16.5263(6)?,β=100.400(2)o,Z=4,V=2764.31(16)?3,Mr=883.63,Dc=2.123 g/cm3,F(000)=1736,S=0.920,R=0.0358 and wR=0.0838(Ⅰ>2σ(Ⅰ)).In HNU-38,the Cd2+and BTC3-were linked together to form layers with Cl-serving as the pillar.It should be noted that(H2O)n chains were found in the channels and play a co-templating role along with the Co(NH3)63+cations in HNU-38,and the proton conduction properties were investigated.

Sulfonate-Functionalized Polyoxovanadate-Based Metal-Organic Polyhedra for Enhanced Proton Conduction via the Synergy of Linker and Metal Cluster Vertex

Metal-organic polyhedra(MOPs)have emerged as novel porous platforms for proton conduction,however,the concerted employment of both linker and metal cluster vertex is rarely applied for the fabrication of MOPs-based high conducting materials.Herein we report the synthesis of sulfonate-functionalized polyoxovanadate-based MOPs for enhanced proton conduction via the synergistic effect from linker and metal cluster node.MOPs 1 and 2 exhibit octahedral cage configuration constructed from{V_(5)O_(9)Cl}vertex and 5-sulfoisophthalate linker.Owing to the ordered packing of octahedral cages along three axes,3D interpenetrated open channels that are lined with high-density sulfonates are thus formed within 2.Coupled with the proton-conductive{V_(5)O_(9)Cl}vertexs as well as protonated counterions,an extensive H-bonded network is therefore generated for facile proton transfer.2 exhibits high proton conductivity of 3.02×10^(-2)S cm^(-1)at 65℃under 90%RH,recording the highest value for MOPs pellet sample.This value is enhanced~1order of magnitude compared with that of carboxylate-functionalized analogue 3,clearly illustrating the advantage of combining linker and metal cluster node for enhanced proton conduction.This work will further promote the exploitation of high proton conductive MOPs-based materials by the synergy design strategy.

Nanostructured Polymer Composite Electrolytes with Self-Assembled Polyoxometalate Networks for Proton Conduction

The key challenge for the use of polymer electrolytes is to realize a high ionic conductivity without scarifying their mechanical performance.Herein,we report a facile strategy to prepare a nanostructured polymer electrolyte with both high proton conductivity and high modulus,based on the electrostatic self-assembly of polyoxometalate cluster H_(3)PW_(12)O_(40)(PW)and comb copolymer poly(ether-etherketone)-grafted-poly(vinyl pyrrolidone)(PEEK-gPVP).The incorporation of protonic acid PW can enable the PEEK-g-PVP to be highly proton conductive and create flexible composite electrolyte membranes.Moreover,nanoscale phase separation between PEEK domains and PVP/PW domains spontaneously occurs in these membranes,forming a bicontinuous structure with three-dimensional(3D)-connected PW networks.Due to the dual role of PW networks as both proton transport pathways and mechanical enhancers,these membranes exhibit proton conductivities higher than 30 mS cm^(−1) and modulus over 4 GPa.Notably,the direct methanol fuel cells equipped with these membranes show good cell performance.Given the wide tunability of comb copolymers and polyoxometalates,this system can be extended to develop a variety of functional electrolyte materials,for example,the lithium-ion conductive electrolytes by using polyoxometalatebased lithium salts,which provides a promising platform to explore versatile electrolyte materials for energy and electronic applications.

Development and Challenges of Electrode Ionomers Used in the Catalyst Layer of Proton-Exchange Membrane Fuel Cells:A Review

The electrode ionomer plays a crucial role in the catalyst layer(CL) of a proton-exchange membrane fuel cell(PEMFC) and is closely associated with the proton conduction and gas transport properties,structural stability,and water management capability.In this review,we discuss the CL structural characteristics and highlight the latest advancements in ionomer material research.Additionally,we comprehensively introduce the design concepts and exceptional performances of porous electrode ionomers,elaborate on their structural properties and functions within the fuel cell CL,and investigate their effect on the CL microstructure and performance.Finally,we present a prospective evaluation of the developments in the electrode ionomer for fabricating CL,offering valuable insights for designing and synthesizing more efficient electrode ionomer materials.By addressing these facets,this review contributes to a comprehensive understanding of the role and potential of electrode ionomers for enhancing PEMFC performance.

A coordination compound featuring a supramolecular hydrogen-bonding network for proton conduction

A new coordination compound [Mg（L）（H2 O）5·H2 O]（NKU-109, H2 L=5-（4 H-1,2,4-triazol-4-yl）benzene-1,3-dicarboxylic acid） was solvothermally synthesized, featuring a supramolecular hydrogen-bonding network. A good proton conductivity of 5.87×10^-4S/cm was recorded at 70℃ and a relative humidity of75% in alternating current（AC） impedance experiment, which sheds a new light on the design of proton conduction materials based on coordination compounds.

Nafion-threaded MOF laminar membrane with efficient and stable transfer channels towards highly enhanced proton conduction

Porous laminar membranes hold great promise to realize ultrafast ion transfer if efficient and stable transfer channels are constructed in vertical direction.Here,metal-organic framework(MOF)nanosheets bearing imidazole molecules in the pores were designed as building blocks to assemble free-standing MOF laminar membrane.Then,Nafion chains were threaded into the pores induced by electrostatic attraction from imidazole molecules by slowly filtering dilute Nafion solution.We demonstrate that the threaded Nafion chains lock adjacent MOF nanosheets,affording highly enhanced structural stability to the resultant laminar membrane with almost no water swelling.Significantly,abundant acid-base pairs are formed in the pores along Nafion chains,working as efficient,continuous conduction pathways in vertical direction.Proton conductivities as high as 110 and 46 mS·cm^(-1)are obtained by this membrane under 100%and 40%relative humidity(RH),respectively,which are two orders of magnitude higher than that of pristine MOF membrane.The conductivity under low humidity(40%RH)is even over 2 times higher than that of commercial Nafion membrane,generating the maximum power density of 1,100 mW·cm^(-2)in hydrogen fuel cell(vs.291 mW·cm^(-2)of Nafion membrane).Besides,the influence of water state on proton transfer in confined space is investigated in detail.

Tricarboxy Ligand CuⅡ Metal-organic Framework with Magnetic and Proton Conduction Properties

Hydrothermal reaction of Cu(NO3)2·3 H2O and 4-(4-carboxyphenoxy)isophthalic acid(H3cpia)results in a two-dimensional coordination polymer based on paddle-wheel-like dinuclear clusters,namely[C60H54Cu4O39]n(1).Complex 1 is of triclinic system,space group P1 with a=10.7318(18),b=12.267(2),c=14.528(2)?,α=113.560(2)o,β=96.156(3)o,γ=103.552(3)o,V=1660.5(5)?3,Z=1,S=1.057,F(000)=842,R=0.0517 and wR=0.1426(I>2σ(I)).In this structure,tricarboxylate liagnds are partial deprotonation and potenial proton carriers.Strong antiferromagnetic interaction between CuⅡions exists within the Cu2 cluster and complex 1 exhibits relatively high proton conductivity(σ>1.23×10-5 S×cm-1)at 76%relative humidity(RH).

Precise modification of poly(aryl ether ketone sulfone)proton exchangemembranes with positively charged bismuth oxide clusters for high proton conduction performance

In the field of proton exchange membranes(PEMs),it is still a great challenge to explore new Nafion alternatives,maintaining the high proton conductivity and lowering the cost of practical application.In this work,a series of low sulfonated poly(aryl ether ketone sulfone)(SPAEKS)membranes hybridized by[Bi_(6)O_(5)(OH)_(3)]_(2)(NO_(3))10⋅6H_(2)O(H_(6)Bi_(12)O_(16))have been successfully fabricated.When the doping amount of H6Bi12O16 reaches 5 wt%,the DS15-Bi12-5 showing the best proton conductive ability and mechanical properties.The proton conductivity can achieve 72.8 mS⋅cm−1 at 80℃ and the tensile strength can reach 43.57 MPa.Confirmed by experimental data and activation energy(Ea)calculations,the existence of Bi cluster makes more hydrogen bonds,providing additional proton hopping sites and offers more proton transport vehicles,leading to a high proton conduction performance.This work proved that polyoxometalates(POMs)can replace the role of sulfonate groups in SPAEKS to a certain extent and work out the defects of high sulfonation,making a remarkable contribution to the practical application of low sulfonated SPAEKS.

Hydrogen production at intermediate temperatures with proton conducting ceramic cells:Electrocatalytic activity,durability and energy efficiency

Proton conducting ceramic cells(PCCs)are an attractive emerging technology operating in the intermediate temperature range of 500 to 700℃.In this work,we evaluate the production of hydrogen at intermediate temperatures by proton conducting ceramic cell electrolysis(PCCEL).We demonstrate a highperformance steam electrolysis owing to a composite positrode based on BaGd_(0.8)La_(0.2)Co_(2)O_(6-δ)(BGLC1082)and BaZr0.5Ce0.4Y0.1O3-δ(BZCY541).The high reliability of PCCEL is demonstrated for 1680 h at a current density as high as-0.8 A cm^(-2)close to the thermoneutral cell voltage at 600℃.The electrolysis cell showed a specific energy consumption ranging from 54 to 66 kW h kg^(-1)that is comparable to state-of-the-art low temperature electrolysis technologies,while showing hydrogen production rates systematically higher than commercial solid oxide ceramic cells(SOCs).Compared to SOCs,the results verified the higher performances of PCCs at the relevant operating temperatures,due to the lower activation energy for proton transfer comparing with oxygen ion conduction.However,because of the p-type electronic conduction in protonic ceramics,the energy conversion rate of PCCs is relatively lower in steam electrolysis.The faradaic efficiency of the PCC in electrolysis mode can be increased at lower operating temperatures and in endothermic conditions,making PCCEL a technology of choice to valorize high temperature waste heat from industrial processes into hydrogen.To increase the faradaic efficiency by optimizing the materials,the cell design,or the operating strategy is a key challenge to address for future developments of PCCEL in order to achieve even more superior techno-economic merits.

Silk nanofibril as nanobinder for preparing COF nanosheet-based proton exchange membrane

Two-dimensional covalent organic framework nanosheets(CONs)with ultrathin thickness and porous crystalline nature show substantial potential as novel membrane materials.However,bringing CONs materials into flexible membrane form is a monumental challenge due to the limitation of weak interactions among CONs.Herein,one-dimensional silk nanofibrils(SNFs)from silkworm cocoon are designed as the nanobinder to link sulfonated CON(SCON)into robust SCON-based membrane through vacuum-filtration method.Ultrathin and large lateral-sized SCONs are synthesized via bottom-up interface-confined synthesis approach.Benefiting from high length-diameter ratio of SNF and rich functional groups in both SNF and SCON,two-dimensional(2D)SCONs are effectively connected together by physical entanglement and strong H-bond interactions.The resultant SCON/SNF membrane displays dense structure,high mechanical integrity and good stability.Importantly,the rigid porous nanochannels of SCON,high-concentration-SO3H groups insides the pores and H-bonds at SCON-SNF interfaces impart SCON/SNF membrane high-rate proton transfer pathways.Consequently,a superior proton conductivity of 365 mS cm^(-1)is achieved at 80C and 100%RH by SCON/SNF membrane.This work offers a promising approach for connecting 2D CON materials into flexible membrane as high-performance solid electrolyte for hydrogen fuel cell and may be applied in membrane-related other fields.

Hydrothermal Synthesis,Crystal Structure and Proton Conductivity of a Pr–Ca Heterometal-organic Framework Generated by 1,2,4,5-Benzenetetracarboxylic Acid

A heterometal-organic framework {[Pr2Ca（betc）2（H2O）7]·H2O}n（1） was prepared by the hydrothermal reaction of 1,2,4,5-benzenetetracarboxylic acid（H4betc） with Pr（NO3）3 and CaCO3, and further characterized by single-crystal X-ray structural analysis, elemental analysis, IR, thermal gravimetric, and X-ray powder diffraction. Complex 1 crystallizes in triclinic, space group P1 with a = 7.3668（12）, b = 10.1726（14）, c = 11.2264（15） A, a = 100.404（2）, b = 106.113（3）, g = 109.158（3）o, V = 728.48（19） A3, Mr = 966.26, Z = 1, F（000） = 470, Dc = 2.203 g/cm3, m（Mo Kα） = 3.585 mm-1, the final R = 0.0195 and w R = 0.0470（I 〉 2s（I））. Complex 1 is a 3D network with pcu topology with 1D porosity and rich hydrogen-bonding interactions. The proton conductivity of complex 1 was also studied under ~97% relative humidity and the different temperature conditions.

Proton dynamics in phosphotungstic acid impregnated mesoporous silica proton exchange membrane materials

Phosphotungstic acid is an excellent proton conductor that can be incorporated into porous supports, and nanocomposite proton exchange membrane materials made from mesoporous silica impregnated with phosphotungstic acid have been suggested for use in fuels cells operating> 100 ℃. In this work, quasielastic neutron scattering was used to study proton self-diffusion in mesoporous disordered and P6 mm symmetry silica impregnated with two concentrations of phosphotungstic acid. Overall, the silica structure had a significantly greater effect on proton conduction and diffusion than phosphotungstic acid concentration, with higher proton conduction occurring for the P6 mm symmetry silica samples. Quasielastic neutron scattering revealed two populations of protons diffusing through each sample, and that proton conduction is limited by the slower of these populations, which diffuse via a jump-diffusion mechanism. Whilst the fundamental jump-diffusion mechanism by which these slower protons moved was found to be similar for both silica supports and phosphotungstic acid concentrations, the faster diffusion occurring in P6 mm structured silica arises from a lower residence time of protons moving between sites in the jump-diffusion model, suggesting a lower energy barrier.

Preparation and Conducting Behavior of Amphibious Organic/Inorganic Hybrid Proton Exchange Membranes Based on Benzyltetrazole

A series of novel amphibious organic/inorganic hybrid proton exchange membranes with H3PO4 doped which could be used under both wet and dry conditions was prepared through a sol-gel process based on acrylated triethoxysilane（A-TES） and benzyltetrazole-modified triethoxysilane（BT-TES）.The dual-curing approach including UV-curing and thermal curing was used to obtain the crosslinked membranes.Polyethylene glycol（400） diacrylate（PEGDA） was used as an oligomer to form the polymeric matrix.The molecular structures of precursors were characterized by 1 H,13 C and 29 Si NMR spectra.The thermogravimetric analysis（TGA） results show that the membranes exhibit acceptable thermal stability for their application at above 200 oC.The differential scanning calorimeter（DSC） determination indicates that the crosslinked membranes with the mass ratios of below 1.6 of BT-TES to A-TES and the same mass of H3PO4 doped as that of A-TES possess the-T g s,and the lowest T g（-28.9 ℃） exists for the membrane with double mass of H3PO4 doped as well.The high proton conductivity in a range of 9.4―17.3 mS/cm with the corresponding water uptake of 19.1%―32.8% of the membranes was detected at 90 oC under wet conditions.Meanwhile,the proton conductivity in a dry environment for the membrane with a mass ratio of 2.4 of BT-TES to A-TES and double H3PO4 loading increases from 4.89×10-2 mS/cm at 30 ℃ to 25.7 mS/cm at 140 ℃.The excellent proton transport ability under both hydrous and anhydrous conditions demonstrates a potential application in the polymer electrolyte membrane fuel cells.

High-performance polymer electrolyte membranes incorporated with 2D silica nanosheets in high-temperature proton exchange membrane fuel cells

Silica nanosheets(SN)derived from natural vermiculite(Verm)were successfully incorporated into polyethersulfone-polyvinylpyrrolidone(PES-PVP)polymer to fabricate high-temperature proton exchange membranes(HT-PEMs).The content of SN filler was varied(0.1-0.75 wt%)to study its influence on proton conductivity,power density and durability.Benefiting from the hydroxyl groups of SN that enable the formation of additional proton-transferring pathways,the inorganic-organic membrane displayed enhanced proton conductivity of 48.2 mS/cm and power density of 495 mW/cm^(2) at 150℃ without humidification when the content of SN is 0.25 wt%.Furthermore,exfoliated SN(E-SN)and sulfonated SN(S-SN),which were fabricated by a liquid-phase exfoliation method and silane condensation,respectively,were embedded in PES-PVP polymer matrix by a simple blending method.Due to the significant contribution from sulfonic groups in S-SN,the membrane with 0.25 wt%S-SN reached the highest proton conductivity of51.5 mS/cm and peak power density of 546 mW/cm^(2) at150℃,48%higher than the pristine PES-PVP membranes.Compared to unaltered PES-PVP membrane,SN added hybrid composite membrane demonstrated excellent durability for the fuel cell at 150℃.Using a facile method to prepare 2D SN from natural clay minerals,the strategy of exfoliation and functionalization of SN can be potentially used in the production of HT-PEMs.