Advances in Covalent Organic Frameworks Design for Proton Conduction

Superprotonic conductivity of ketoenamine covalent-organic frameworks grafted by imidazole-based units

To develop proton exchange membranes (PEMs) with robust structure stability and remarkable proton conductivity and explore their application in PEMs fuel cells have significant implications for realizing reduced carbon emission and environmental pollution. Covalent organic frameworks (COFs), as crystalline porous polymer material composed of organic monomers and connected by covalent bond, possess specific framework, inherent porosity, adjustable functional group and eminent thermal/chemical structure stability. Therefore, COFs display prominent superiorities in constructing rigid ordered proton transfer channels and improving fuel cell performance long‐term durability. In this review, the proton conduction properties of extrinsic proton‐conductive COFs (incorporating carriers into the pore), intrinsic proton‐conductive COFs (introducing conductive groups on the backbone) and combined extrinsic/intrinsic proton‐conductive COFs in the form of pressed pellets are discussed in detail. Meanwhile, proton‐conductive COFs related PEMs, including COFs‐related polymer‐based composite membranes, COFs‐based composite membranes and self‐supporting COFs membranes are also systematically summarized. In addition, the existing challenges are analyzed and future outlooks are addressed.

Advances in Emerging Crystalline Porous Materials.

Covalent organic frameworks (COFs) offer an irreplaceable platform for mass transport, as they provide aligned one-dimensional channels as pathways. Especially, proton conduction is of great scientific interest and technological importance. However, unlike proton conduction under humidity, anhydrous proton conduction remains a challenge, as it requires robust materials and proceeds under harsh conditions. Here, we report exceptional anhydrous proton conduction in stable crystalline porous COFs by integrating neat phosphoric acid into the channels to form extended hydrogen-bonding networks. The phosphoric acid networks in the pores are stabilized by hierarchical multipoint and multichain hydrogen-bonding interactions with the 3D channel walls. We synthesized five hexagonal COFs that possess different pore sizes, which are gradually tuned from micropores to mesopores. Remarkably, mesoporous COFs with a high pore volume exhibit an exceptional anhydrous proton conductivity of 0.31 S cm-1, which marks the highest conductivity among all examples reported for COFs. We observed that the proton conductivity is dependent on the pore volume, pore size, and content of phosphoric acid. Increasing the pore volume improves the proton conductivity in an exponential fashion. Remarkably, changing the pore volume from 0.41 to 1.60 cm3 g-1 increases the proton conductivity by 1150-fold. Interestingly, as the pore size increases, the activation energy barrier of proton conduction decreases in linear mode. The mesopores enable fast proton hopping across the channels, while the micropores follow sluggish vehicle conduction. Experiments on tuning phosphoric acid loading contents revealed that a well-developed hydrogen-bonding phosphoric acid network in the pores is critical for proton conduction.

Covalent organic frameworks (COFs) are widelyinvestigated for proton conduction due to their exceptional structural properties, with two primary strategies: designing intrinsically conductive COFs or loading guest molecules (e.g., imidazole). However, the loading strategy is plagued by critical drawbacks, including inconsistent loading quantities and leaching of guest molecules. Additionally, imine-linked COFs often exhibit structural instability in post-synthetic modification (PSM). To address these, we synthesized COF-316 featuring irreversible aromatic ether linkages and modified its nitrile group via PSM to, carboxyl (316-COOH) or amide (316-AM) groups. Modified COFs retained structural integrity, enhanced water vapor adsorption capacity, and improved proton conductivity. Remarkably, 316-AM demonstrated the highest proton conductivity of 2.55 × 10-2Scm-1 (100°C and 98% RH), among the highest intrinsic conductivities for COFs. Combined activation energy analysis and theoretical calculations clarified proton conduction mechanisms in all three COFs. This study provides valuable insights into functional group engineering for COF-based proton conductors.

Covalent-organic porous framework (COF) integrated hybrid membranes for energy and environmental applications: Current and future perspectives

Preparation of high temperature proton exchange membrane through covalent organic framework doped polyvinylidene fluoride nanofibers

Incorporating self-anchored phosphotungstic acid@triazole-functionalized covalent organic framework into sulfonated poly(ether ether ketone) for enhanced proton conductivity

Recent electrochemical energy conversion devices require more advanced proton conductors for their broad applications, especially, proton exchange membrane fuel cell (PEMFC) construction. Covalent organic frameworks (COFs) are an emerging class of organic porous crystalline materials that are composed of organic linkers and connected by strong covalent bonds. The unique characteristics including well-ordered and tailorable pore channels, permanent porosity, high degree of crystallinity, excellent chemical and thermal stability, enable COFs to be the potential proton conductors in fuel cell devices. Generally, proton conduction of COFs is dependent on the amount of water (extent of humidity). So, the constructed fuel cells accompanied complex water management system which requires large radiators and airflow for their operation at around 80 °C to avoid overheating and efficiency roll-off. To overcome such limitations, heavy-duty fuel cells require robust proton exchange membranes with stable proton conduction at elevated temperatures. Thus, proton conducting COFs under anhydrous conditions are in high demand. This review summarizes the recent progress in emerging COFs that exhibit proton conduction under anhydrous conditions, which may be prospective candidates for solid electrolytes in fuel cells.

As a new class of crystalline porous organic materials, covalent organic frameworks (COFs) have attracted considerable attention for proton conduction owing to their regular channels and tailored functionality. However, most COFs are insoluble and unprocessable, which makes membrane preparation for practical use a challenge. In this study, we used surface‐initiated condensation polymerization of a trialdehyde and a phenylenediamine for the synthesis of sulfonic COF (SCOF) coatings. The COF layer thickness could be finely tuned from 10 to 100 nm by controlling the polymerization time. Moreover, free‐standing COF membranes were obtained by sacrificing the bridging layer without any decomposition of the COF structure. Benefiting from the abundant sulfonic acid groups in the COF channels, the proton conductivity of the SCOF membrane reached 0.54 S cm−1 at 80 °C in pure water. To our knowledge, this is one of the highest values for a pristine COF membrane in the absence of additional additives.

High performance nanocomposite proton exchange membranes based on the nanohybrids formed by chemically bonding phosphotungstic acid with covalent organic frameworks

In recent years, covalent organic frameworks (COFs) have attracted enormous interest as a new generation of proton-exchange membranes, chemical sensors and electronic devices. However, to design high proton conductivity COFs, especially those with stimulus responsive performance remains a great challenge. Here, the first example of a light/heat switchable COF (COF-HNU9) has been synthesized by grafting a donor–acceptor Stenhouse adduct (DASA) within the channels of a β-ketoenamine-based COF. DASA groups in the nanopores of COF-HNU9 undergo a reversible open–closed photoisomerization upon visible light irradiation and are recovered by heating. Thus, COF-HNU9 exhibits not only a remarkably high proton conductivity, but also a highly effective switching performance. Under visible light irradiation at 98% RH, the proton conductivity of COF-HNU9 increases by three orders of magnitude at 25 °C and is up to 0.02 S cm−1 at 80 °C. Furthermore, the proton conductivity does not display any significant decrease even after 20 switching cycles. These results have been rationalized by a Grotthuss-type mechanism and verified by DFT calculations. The stimuli-responsive COF is conceptually confirmed by an optical control device with the light/heat switching proton conductive COF-HNU9 film, which is able to remote-control the illumination and switching off of an LED lamp without any current amplifier.

Proton conductivity is the paramount property of proton‐conducting materials that are playing significant roles in diverse electrochemical devices with applications in proton exchange membranes (PEMs) for fuel cells (PEMFCs). Considering the scarcity of fossil fuels, the development of clean and green renewable energy resources is in‐demand across the globe. Toward this direction, the development of solid‐state proton conductors is of significant interest. The higher structural tunability, lower density, good crystallinity, accessible well‐defined pores, excellent thermal and chemical stability of covalent‐organic frameworks (COFs) make them versatile platforms as proton conductors both under hydrous and anhydrous conditions. Taking advantage of such superior properties, the reports on proton‐conducting COFs have been increasing swiftly since 2014, which demands a summarization and comprehensive discussion for “at a glance” visualization and further development. In this review, the COFs are showcased as a newer class of solid‐state proton‐conducting material. A comprehensive discussion is presented by organizing the strategies taken to develop the proton‐conducting COFs. Establishment of structure–function relationships and the implementation of COFs as proton conductors are discussed as well. Moreover, challenges and future prospects are presented elaborately and are critically analyzed.

Covalent organic frameworks (COFs) hold promising potential as high-temperature proton conductors due to their highly ordered nanostructures and high specific surface areas. However, due to their limited functional groups and poor membrane-engineering properties, finding practical applications for COF-based proton-conducting materials still remains challenging. Herein, we proposed a universal strategy to fabricate proton-conducting composite membranes by the incorporation of sulfonic acid-bearing COFs and zwitterionic poly(ionic liquid)s (PILs) via in situ polymerization. Zwitterionic PILs with methanesulfonate counter ions can work as the intrinsic proton sources, and the sulfonic acid groups on the COF nanochannels can act as the extrinsic proton suppliers. Benefiting from the spatial nanoconfinement of long-range ordered nanochannels and the enhanced electrostatic interactions with PILs, the COFs with high densities of sulfonic acid groups can endow the as-prepared composite membrane (PIL@TpBD(SO3H)2) with a comparable anhydrous proton conductivity of 3.20 × 10−3 S cm−1 at 90 °C, which is much higher than that of conventional Nafion (~10−5 S cm−1 at 90 °C under anhydrous condition). 1H NMR DOSY spectra reveal that both the diffusion and dissociation of protons can be drastically facilitated upon nanoconfinement, demonstrating the promising efficiency of nanochannels in proton conduction. Moreover, the obtained composite membranes possess outstanding mechanical and thermal stability, which is crucial for their practical application. This study demonstrates proton conduction elevation in nanoconfined PILs and provides a promising insight into the engineering of stable COF-based proton-conducting materials.

Mimicking the interconvertible carbon allotropes of 2-dimensional (2D) graphene and 1-dimensional (1D) carbon nanotubes (CNTs), herein we report the in situ transformation of 2D π-conjugated covalent organic frameworks (COFs) sheet into 1D nanotubular structures via self-assembly the sheets at solvent interfaces. The facile "roll-sheets" self-assembly resulted in coaxial nanotubes with uniform cross-sectional diameter, which was realized for diazapyrene-based COFs but not for the corresponding pyrene COF, although both possess similar chemical structures. Upon replacing the carbon atoms at 2,7-positions of pyrene with nitrogen, contrasting optical and electronic properties were realized, reflecting the rolled structure of the conjugated 2D sheets. The nanotubes exhibited concerted electronic- and proton-conducting nature with stable conducting pathways at ambient conditions. The nitrogen centers act simultaneously as the site for charge carrier doping and proton acceptors, as evidenced by the high photo- and electrical conductivity, as well as the record proton conductivity (σ = 1.98 S cm-1) results. The present diazapyrene-based 1D nanotubular COF serves as a unique materials platform with electronic conduction in the wall and proton conduction in the core, respectively.