Covalent Organic Frameworks for Energy Conversions: Current Status, Challenges, and Perspectives

Open AccessCCS ChemistryMINI REVIEW1 Jan 2021Covalent Organic Frameworks for Energy Conversions: Current Status, Challenges, and Perspectives Shanshan Tao and Donglin Jiang Shanshan Tao Department of Chemistry, Faculty of Science, National University of Singapore, Singapore 117543 Google Scholar More articles by this author and Donglin Jiang *Corresponding author: E-mail Address: [email protected] Department of Chemistry, Faculty of Science, National University of Singapore, Singapore 117543 Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Fuzhou 350207 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000491 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Energy conversion into clean fuels is critical to society’s health benefits and sustainable future; thus, exploring materials to enable and facilitate energy conversions with reduced climate-related emissions is a central subject of science and technology. Covalent organic frameworks (COFs) are a class of polymers that enables predesign of both primary- and high-order structures and precise synthesis of long-range structures through one-pot polymerization. Progress over the past 15 years in chemistry has dramatically enhanced our capability of designing and synthesizing COFs and deepening our understanding to explore energy-converting functions that originate from their ordered skeletons and channels. In this minireview, we summarize general strategies for predesigning skeletons and channels and analyze the structural requirements for each type of energy conversion. We demonstrate synthetic approaches to develop energy conversion functions, that is, photocatalytic and electrocatalytic conversions. Further, we scrutinize energy conversion features by disclosing interplays of COFs with photons, holes, electrons, and molecules, highlighting the role of structural orderings in energy conversions. Finally, we have predicted the challenging issues in molecular design and synthesis, and thought of future directions toward advancement in this field, and show perspectives from aspects of chemistry, physics, and materials science, aimed at unveiling a full picture of energy conversions based on predesignable organic architectures. Download figure Download PowerPoint Introduction Energy conversion is driven by molecular systems that enable the input of photons and electrons/holes and the output of different energy forms.1 The most sustainable way is to use natural resources, including carbon dioxide, oxygen, and nitrogen in the atmosphere, as well as light and water, as the input to produce electricity, fuels, and value-added chemicals. Studies on photoenergy conversion to yield electricity have established a diversity of different photovoltaic systems; some have been widely used in power plants.2–4 In contrast, the utilization of light energy to produce chemical fuels and value-added compounds, represented by water splitting into hydrogen and oxygen, and carbon dioxide reduction into carbon monoxide and other derivatives, remains challenging goals.5–7 On the other hand, the usage of electricity to reduce carbon dioxide is remarkable with the production of value-added chemicals, while the reduction of oxygen on an electrode is a critical reaction in fuel cells relative to green energy generation. These transformations require a systematic design of catalytic systems that would indeed promote green energy transformations, encompassing efficiency, eco-friendly, and sustainability. The utilization of solar energy to split water into hydrogen and oxygen is an ideal plan for the production of green energy, as sunlight and water are both abundant natural resources, and hydrogen is a green chemical fuel.5 However, simple irradiation of water with sunlight could not split water into hydrogen and oxygen, as this reaction is accompanied by a large entropic thermodynamic penalty. Since the oil crisis in the 1970s, extensive and intensive studies have been conducted to develop catalysts that ease and promote water splitting driven by light energy.5 Nevertheless, a suitable catalyst that promotes this reaction efficiently is still inaccessible so far. Meanwhile, the photoreduction of carbon dioxide requires suitable catalysts so that the generation of electrons from excitons and the flow of electrons to the reduction center could be interlocked to promote the reaction. However, it is extremely difficult to merge photochemical events and catalytic cycles into one catalytic material. Distinct from photocatalysis, the electrocatalytic reduction of carbon dioxide and oxygen requires different structural criteria for designing catalysts, as the reaction occurs on an electrode and is driven by an electric field.6 In these cases, the injection of an electron from an electrode to the reduction center and the accumulation of carbon dioxide and oxygen on the electrode become key processes to control the reaction dynamics, and how the catalytic site promotes the reduction reaction determines the kinetics. In terms of resource availability, developing photocatalysts and electrocatalysts based on organic materials other than the traditional metal-based systems are essential for achieving sustainable transformations and energy conversions. Covalent organic frameworks (COFs) are a unique class of polymers as they integrate organic molecules into a predictable long-range-ordered structure.8–15 This pushes the boundaries of organic/polymer chemistry and materials science to achieve the unprecedented deterministic control of long-range structures and properties of synthetic macromolecules. COFs are formed via the polymerization of organic molecules (monomers) in which the chain propagation is controlled by a topology diagram so that each bond connection is spatially confined at either a two- or three-dimensional (2D or 3D) way or geometrically guided to produce extended, yet ordered 2D or 3D polymer architecture. The geometry and backbone of organic units predetermine the dimensionality, along with the shape and size of polygons in COFs (Figure 1). During polymerization, the 2D network crystallizes to form 2D layer frameworks driven by interlayer π–π stacking interactions; thus 2D COFs offer ordered columnar π-arrays, as well as aligned one-dimensional (1D) nanochannels (Figure 1). Therefore, the ordering structure of COFs is twofold: one is the periodic skeleton, and the other is the aligned channels. Owing to these two distinct features, COFs offer an irreplaceable platform for designing polymers to achieve a predesignable skeleton and pore. Undoubtedly, the above diverse structural features of COFs are inaccessible with traditional polymers and other porous materials, while the predesignablity of skeleton and channel opens a way to design not only structures but functions as well. Figure 1 | Formation of primary- and high-order structures of COFs via a one-pot reaction that enables polymerization and crystallization, illustrated with a tetragonal lattice. Key features of polymerization and fundamental factors at different structural levels are shown. COFs, covalent organic frameworks. Download figure Download PowerPoint By virtue of a broad diversity of building blocks, COFs have been explored using different design strategies to develop various energy conversion systems. In this minireview, we focus on exploring COFs for diversified energy conversions, including light-driven water splitting, carbon dioxide reduction, and the electrocatalytic reduction of oxygen and carbon dioxide. We summarize the design principles of COFs for each type of conversion and outline their synthetic approaches and fundamental properties to show the structural features of these catalytic systems. We scrutinize each energy conversion system by correlating structure with function to disclose the essence, mechanistic insights, and drawbacks, intended to examine the potential of each strategy and approach. Based on these analyses, we predict key fundamental issues to be challenged, envisioned future directions worthy of the studies, and clarify perspectives from chemistry, physics, and materials science to uncover a full picture of energy conversions. Design Strategies and Structural Features COFs explore topology diagram as a principle to design skeletons and pores,10 and the geometric combination of monomers, that is, knot and linker determines both the primary- and high-order structures. This predesignability is twofold (Figure 2): One is constructing the polymer backbone; its dimensionality and spatial orientation pattern are predetermined. The other is the presetting the pore size, shape, and wall interface. At the primary-order structural level, the backbone offers the 2D polymer, which is an atomic sheet with an alternately connected knot and linker over the 2D plane, while the topology pattern is predetermined by the geometry of monomers. The pores in the 2D sheet have discrete size and shape and consist of specific units extruded from the knot and linker, while these parameters are predesigned by monomers. Therefore, the primary-order structure is predetermined by a polymerization reaction in which knot and linker units are covalently connected along the x and y direction to form an ordered 2D polymer. At the high-order structure level, the 2D polymer sheet stacks to create layer frameworks, achieved via crystallization, and the high-order structure is predetermined by the noncovalent interactions between the 2D layers and controlled by the total free energy for crystallization. Figure 2 | (a) Backbone structural features. (b) Pore and channel structural features. Download figure Download PowerPoint The layer framework configuration constitutes π-columns at the knot and linker sites, with the π-columns linked topologically to form continuous π-arrays across the material (Figure 2a). By virtue of layer stacks, the framework creates extended 1D channels, which are independent of each other, and accessible from the top and bottom layers (Figure 2b). These channels possess discrete size and shape, while the pore walls are covered with the various C–H units extruded from the knot and linker units, B, O, or N atoms from linkages, and other substituents on the knot and linker. These atoms and units are sequenced continuously along the z-direction on the channel walls and form various wall interfaces that control channels function (Figure 2b). More interestingly, the channel walls could be predesigned to install multiple functional groups such as acid, base, catalytic site, radical, hydrophobic, and hydrophilic units via pore surface engineering.16,17 Organic units with C1, C2, C3, C4, and C6 symmetries that bear different reactive sites (Figure 3a) have been explored as monomers for preparing 2D COFs. Figures 3b–3d summarize the major geometry combinations of monomers for designing 2D COFs to achieve different backbones and channels. Except for self-condensation reactions, COFs have been designed with the [one knot + one linker] combination, with varying geometry combinations (Figure 3b). In these cases, the polymer architecture features isotropic tiling and regular polygonal pores across the framework. Figure 3 | (a) Monomer geometry. (b) Conventional strategy based on [one knot + one linker] for designing COFs. (c) Multicomponent strategy based on [one knot + two linkers] and [one knot + three linkers] for designing COFs. (d) Double-stage strategy based on [one knot + one linker] and [two knots + one linker] for designing COFs. COFs, covalent organic frameworks. Download figure Download PowerPoint We have explored a multicomponent strategy for designing COFs to achieve anisotropic lattice tiling and irregular polygonal shapes (Figure 3c).18 This strategy enables the [one knot + two linkers], [one knot + three linkers], or [two knots + one linker] combinations, and is applied to the hexagonal and tetragonal topologies. The multicomponent strategy offers 2D lattices with anisotropic tiling and unusual pore shapes, which are inaccessible to the [one knot + one linker] strategy. Interestingly, the aligned multicomponent in the lattice triggers unique electronic interactions to exhibit distinct properties and functions, while the special pore shape allows the creation of unique nanospace for confinement and molecular separation.18 The multicomponent strategy enhances the structural complexity considerably due to the anisotropic tiling and increases the diversity of COF members owing to the introduction of more patterns for varying combinations of components.18 The double-stage strategy develops the possibility of using two different linkages to synthesize COFs, based on one C1-symmetric unit that possesses two types of reactive sites (Figure 3d).19,20 We have established this strategy for designing COFs to show the [two C3 knots + one C1 linker] and [one C3 knot + one C1 linker] schemes used for hexagonal COFs, the [two C4 knots + one C1 linker] combination for tetragonal COFs, and the [two C2 knots + one C1 linker] diagram for the rhombic COFs.19 Noticeably, these geometrical combinations improve the structural complexity and diversity of COFs substantially. The topology diagram guides the growth of polymer chains into covalently linked 2D polymers, which crystallize to form crystalline porous frameworks via a one-pot reaction that enables both polymerization and crystallization (Figure 1). As monomers consist of rigid π-backbones, the above topology diagrams allow the design of different π-architectures, which are otherwise inaccessible with a supramolecular assembly of 1D polymers and single crystals of small organic compounds. The layered framework offers extended 2D topology in which the π-units stack face-to-face to maximize interactions, thereby casting a sharp contrast to single crystals, which tend to form herringbone alignment. COFs have been designed to achieve various structures with typical topologies, as shown in Figure 4a. To design a functional COF, a basic concept is to develop an interface based on the COF structure (Figure 4b). The key fundamental issue is to disclose and understand how COFs interplay with photons, excitons, phonons, electrons, holes, ions, and molecules, as these interactions determine the property and function of COFs. How to trigger and develop unique interplays with COFs are central subjects that pave the way to explore COFs as a class of novel materials, unique in structural and functional properties that could not be replaced with other materials. Figure 4 | (a) Typical topologies for 2D COFs with regular and irregular polygonal lattices. (b) Basic concepts and key fundamental issues involved in functional exploration. (c) Approaches to design functions based on skeletons, pores, and complimentary use of skeletons and pores. 2D COFs, two-dimensional covalent organic frameworks. Download figure Download PowerPoint Based on the structural features of COFs, we have developed the functions of COFs from three different approaches (Figure 4c). Based on ordered π-skeleton, COFs have been developed as semiconductors, light emitters, light-harvesting antennae, energy transfer, electron transfer, charge separation, photovoltaics, spin alignment, and topology insulators. By developing 1D channels with specific interfaces, COFs have been explored for adsorption, storage, confinement, recognition, and separation. In many cases, the function is triggered by the complementary effects of both skeleton and channel, such as catalysis, sensing, mass transport, energy storage, energy conversion, and biorelated functions. In this minireview, we focus on scrutinizing the COF systems for photocatalysis and electrocatalysis, related to energy conversion (Figure 4c, keywords in red). These catalytic properties originate from the structural feature of COFs; the π-frameworks consist of periodically aligned columnar π-arrays, while their spatial patterns are distinct from each other based on the topology. The π-arrays serve as light-harvesting antennae and offer pathways for charge carrier transport. With the diversity of building blocks, COFs have been developed into p-, n-, and ambipolar-type semiconductors.21–32 With the phenazine27,28 and C=C bond linkages,29–32 fully π-conjugated COFs have been synthesized so that carrier transport could be achieved over the 2D plane and along with the perpendicular π-column directions, leading to carrier transport across the material. On the other hand, the channels provide nanospace for accommodating reactants, so that they are proximate to the reaction centers while products could be timely released from the catalytic site. These structural features offer the chemical base for exploring photocatalytic and electrocatalytic systems. COFs for Photocatalysis Photocatalysts require the combination of a set of different properties, including the capability of light-harvesting, photoinduced electron transfer, charge separation, and charge transport into one material. COFs offer an irreplaceable way to design photocatalysts as they merge these properties in one material by developing different π-electronic interfaces to control and connect these processes. Indeed, by using different π-units, these photochemical events could be designed topologically and managed synthetically. These distinct features render the designed COFs photocatalytic systems capable of fulfilling the energy conversion. Light-driven hydrogen and oxygen evolutions Water splitting into hydrogen and oxygen offers a way to produce green energy as hydrogen contains high energy density and serves as a chemical input for fuel cells. The splitting process consists of two half-reactions: one from the hydrogen evolution and on the reduction side, and the other is oxygen generation, which is on the oxidation side. The hydrogen reduction process requires two electrons, while the oxygen generation involves four electrons; these reactions do not proceed under ambient conditions without catalysts. Light-driven water splitting needs semiconductors with suitable redox potential and band gap that meet the reduction and oxidation reactions. A simplified scenario for the catalyst is that it absorbs a photon and splits exciton into electron and hole, while the resulting electron is used to reduce water into hydrogen, while the hole triggers water oxidation to generate oxygen. Therefore, a catalyst must have a high enough lowest unoccupied molecular orbital (LUMO) level (>−4.02 eV) for the reduction of water and low enough highest occupied molecular orbital (HOMO) level (<−5.25 eV) for water oxidation (Figure 5a). On the other hand, the photochemical processes in these half-reactions involve light-harvesting, exciton splitting, and charge transport at the reaction centers. These processes must be merged with suitable interfaces to enable the continuous flow of electrons and holes to the catalytic sites (Figure 5b). It has remained challenging to merge these processes and combine two catalytic functions to fabricate one organic material. Figure 5 | (a) Redox potentials of water oxidation, water reduction, and carbon dioxide reduction as well as band structures required for COFs. (b) Photoinduced chemical processes involved in water oxidation, water reduction, and carbon dioxide reduction. Catalytic cycles are not shown. COFs, covalent organic frameworks. Download figure Download PowerPoint COFs are unique in that they predesign the π-electronic skeletons with knot and linker, as well as the linkage and hence, offer a well-defined molecular platform for designing HOMO and LUMO levels to tune semiconducting properties. Moreover, COFs have been developed into donor–acceptor structures,21–32 which offer super heterojunction for splitting excitons into electron and hole, while the donor and acceptor π-columns enable the transport of holes and electrons, respectively. These distinct features are inaccessible with other polymeric architectures and molecular frameworks. Thus, COFs are highly promising for developing photocatalysts to split water driven by light. A squaraine-linked copper porphyrin (CuP-SQ) COF (Figure 6a) is the first photocatalyst based on COFs that activate molecular oxygen to singlet oxygen under visible light.33 Owing to the π-conjugated squaraine linkage and ordered π-columns, CuP-SQ COF exhibits greatly enhanced photocatalytic activity, compared with monomeric copper porphyrin complex. A hexagonal TFPT-COF, based on 1,3,5-tris(4-formyl-phenyl)triazine (TFPT) and 2,5-diethoxy-terephthalohydrazide building blocks (Figure 6b) with triazole knot and hydrazone linkage adopts a planar conformation and possesses a band gap of 2.8 eV.34 TFPT-COF adsorbs visible light and triggers hydrogen evolution to achieve a rate of 230 µmol h−1 g−1 in the presence of sodium ascorbate sacrificial donor and a rate of 1.97 mmol h−1 g−1 for a system with triethanolamine (TEOA) sacrificial donor. These results demonstrate the possibility of COFs as a photocatalyst. Figure 6 | (a–t) COFs for photocatalytic oxygen activation and hydrogen evolution. COFs, covalent organic frameworks. Download figure Download PowerPoint Azine-linked COFs have been synthesized by condensing hydrazine with knot units bearing aldehyde groups.35 The azine linkage enables an extended π-conjugation in the network and offers dense nitrogen atoms on the channel walls as the N–N linker is short. Azine-linked Nx-COFs (Figures 6c–6f; x = 0−3) have been synthesized by integrating different Nx knots, that is, N0 phenyl, N1 pyridine, N2 pyrimidine, and N3 triazine units, respectively.36,37 The different number (x) of the nitrogen atom in the knot induces different planarity of the resulting frameworks, as the twisted angle between the knot and linker decreases in the order of N0 > N1 > N2 > N3. The triazine N3 knot yields a planar conformation of the 2D layer stacking to show a zero degrees (0°), demonstrating resistance to a twisted angle. Noticeably, the photocatalytic activity increases in the order of N0-COF < N1-COF < N2-COF < N3-COF. Indeed, the N0-COF, N1-COF, N2-COF, and N3-COF exhibit a hydrogen evolution rate of 0.023, 0.090, 0.438, 1.703 mmol h−1g−1, respectively. This performance of N3-COF originates from a multifold effect of the azine-linked triazine network. N3-COF with planar conformation and extended π-conjugation possesses an appropriate band gap of 2.6–2.7 eV. Its π-structure promotes exciton migration and charge separation. The nitrogen atoms of triazine and azine units enable hydrogen-bonding interactions with the sacrificial donor TEOA, contributing to the hole quenching. Moreover, the electron-deficient triazine unit stabilizes the negative charges to promote electron transfer to the reduction center of Pt nanoparticles.36 The azine linkage has been developed to synthesize the rhombic, A-TEBPY-COF, with a pyrene knot (Figure 6g). The pyrene knot and rhombic topology are expected to have extended π-conjugation over the framework.38 In the presence of platinum (Pt) nanoparticles as a reduction center and 10 vol % TEOA as a sacrificial donor, A-TEBPY-COF exhibits a hydrogen evolution rate of 98 µmol h−1 g−1. The β-ketoenamine linkage yields stable COFs. A series of β-ketoenamine-linked TP-BDDA COF, TP-EDDA COF, and TP-DTP COF (Figures 6h–6j) have been synthesized by condensing 1,3,5-triformylphloroglucinol (TFP) knot with 4,4'-(buta-1,3-diyne-1,4-diyl)dianiline, 4,4'-(ethyne-1,2-diyl)dianiline, and 4,4'-(ethyne-1,2-diyl)dianiline, respectively.39 The TP-BDDA COF, TP-EDDA COF, and TP-DTP COF exhibit a band gap of 2.31, 2.34, and 2.42 eV, respectively. In the presence of Pt cocatalyst and TEOA sacrificial donor, the TP-BDDA COF, TP-EDDA COF, and TP-DTP COF generate hydrogen at a rate of 324 ± 10, 30 ± 5, and 20 ± 5 µmol h−1 g−1, respectively. The relatively high photocatalytic activity of the TP-BDDA COF originates from the extended π-conjugation in the lattice owing to the presence of a diacetylene linker. Similarly, β-ketoenamine-linked TPCOF, AntCOF, TzCOF, and BtCOF have been designed to possess electron-donating TP and anthracene (Ant) blocks, and electron-accepting tetrazine (Tz) and benzothiadiazole (Bt) moieties as linker units, respectively (Figures 6k–6n).40 To compare the effect of crystallinity, porosity, and stacking mode on the hydrogen evolution rate, these COFs have been synthesized under two distinct conditions; one is carried out in a mixture of mesitylene/dioxane (4/1 vol) in the presence of acetic acid (6 M) catalyst at 120 °C for 7 days to yield COF120 series, and the other is conducted in a mixture of mesitylene/dioxane (1/2 vol) in the presence of acetic acid (6 M) catalyst at 150 °C for 3 days to form a COF150 group. Among the series, the BtCOF150 achieves the highest hydrogen evolution rate of 750 ± 25 µmol h−1 g−1, in the presence of Pt cocatalyst as a reduction center and TEOA sacrificial donor. This result originates from the highest capabilities of light absorption and charge carrier generation owing to the aligned donor–acceptor π-arrays, as evident by solid-state absorption spectroscopy. This result suggests that photochemical processes and photocatalytic activity are highly related to the ordered donor–acceptor interface structure. Hydrogen evolution from water involves reactions in the aqueous phase; the hydrophilicity of COFs is essential. The β-ketoenamine-linked FS-COF with fused-sulfone (FS) unit(Figure 6o) has been prepared by condensing TFP knot with dibenzo[b,d]thiophene sulfone (DBTS) linker to achieve a stable and hydrophilic framework.41 The FS-COF shows a contact angle of 23.6°, exhibits a Brunauer–Emmett–Teller (BET) surface area of 1288 m2 g−1, and adsorbs water to achieve a capacity of 67 wt%. Noticeably, the FS-COF has a band gap of 1.85 eV, emits at 670 nm in the solid-state, and exhibits a fluorescence lifetime of 5.56 ns. FS-COF is highly active in producing hydrogen from water in the presence of near-infrared absorbing dye WS5F cosensitizer, sodium ascorbate sacrificial donor, and Pt reduction center. The system is stable over a 50 h continuous run to show a constant hydrogen evolution rate of 16.3 mmol h−1 g−1, which is superior to Pd0/TpPa-1 (10.4 mmol h−1 g−1)42 cosensitized by Eosin Y and TpPa-COF-(CH3)2 (8.33 mmol h−1 g−1).43 These results show that the wettability of COFs is an important parameter to be considered in designing COFs-based photocatalysts to achieve hydrogen evolution. A C=C bond linked g-C40N3-COF (Figure 6p) has been synthesized via Knoevenagel polycondensation of 3,5-dicyano-2,4,6-trimethylpyridine (DCTMP) knot and 1,3,5-tris(4-formylphenyl)benzene) (TFPB) linker.44 The band gap of g-C40N3-COF is 2.36 eV, and its fluorescence lifetime is 3.31 ns. Owing to the presence of pyridine knot and C=C bond linkage, g-C40N3-COF is expected to be an ambipolar conducting polymer that would enable the transport of both electrons and holes. The g-C40N3-COF forms a complex with Pt ion via coordination with its pyridine units, promoting electron transfer from the skeleton to the Pt sites. The g-C40N3-COF exhibits activity in separated hydrogen and oxygen evolutions: the hydrogen evolution rate is 129.8 µmol h−1 in a system consisting of 10 vol % TEOA sacrificial donor and 3 wt % Pt, while the oxygen evolution rate is 2.5 µmol h−1 g−1 in a system containing AgNO3 (0.01 M) as an electron acceptor and La2O3 (0.2 g) as a buffer under visible light. Similarly, a shorter C-chain, C=C bond linked g-C18N3-COF (Figure 6q)45 with triazine knot and phenyl linker has been prepared with a band gap of 2.42 eV and emits at 510 nm with a fluorescence lifetime of 7.25 ns upon excitation at 365 nm. The g-C18N3-COF shows a higher hydrogen evolution rate of 292 µmol h−1 g−1 in an aqueous ascorbic acid solution (1 M) and 3 wt% Pt as the reduction catalyst. Inspired by the sp2c-COFs,14,32 condensing 4,4′,4″,4‴-(pyrene-1,3,6,8-tetrayl)tetrabenzaldehyde (Py-CHO) with 4,4′-(benzo[c][1,2,5]thiadiazole-4