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 ( 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 band gap of promotes exciton and charge separation. The nitrogen atoms of triazine and azine units enable interactions with the sacrificial donor to the hole Moreover, the triazine unit the to promote electron to the reduction center of The azine linkage has been developed to synthesize the with a knot (Figure The knot and rhombic topology are to have extended π-conjugation over the In the presence of as a reduction center and as a sacrificial exhibits a hydrogen evolution rate of µmol h−1 The linkage yields COFs. A of COF, COF, and COF (Figures have been synthesized by condensing knot with and The COF, COF, and COF exhibit a band gap of and respectively. In the presence of and sacrificial the COF, COF, and COF generate hydrogen at a rate of and 5 µmol h−1 respectively. The high photocatalytic activity of the COF originates from the extended π-conjugation in the lattice owing to the presence of a linker. and have been designed to possess and blocks, and and as linker units, (Figures To the effect of and stacking on the hydrogen evolution these COFs have been synthesized under two distinct one is in a of in the presence of catalyst at for to yield and the other is conducted in a of in the presence of catalyst at for 3 to form a the the the highest hydrogen evolution rate of µmol h−1 in the presence of as a reduction center and sacrificial donor. This originates from the highest of light and charge carrier generation owing to the aligned donor–acceptor π-arrays, as by This that photochemical processes and photocatalytic activity are highly related to the ordered donor–acceptor interface evolution from water involves reactions in the the of COFs is The with has been by condensing knot with linker to achieve a and hydrophilic The a angle of exhibits a surface of and adsorbs water to achieve a of Noticeably, the has a band gap of at in the and exhibits a of is highly in hydrogen from water in the presence of sodium ascorbate sacrificial and reduction The system is over a continuous to show a hydrogen evolution rate of mmol h−1 which is to mmol h−1 by and mmol h−1 These results show that the of COFs is an to be in designing photocatalysts to achieve hydrogen evolution. A C=C bond linked (Figure has been synthesized via of knot and The band gap of is and its is Owing to the presence of knot and C=C bond is to be an polymer that would enable the transport of both electrons and The a with via with its units, electron from the skeleton to the The exhibits activity in hydrogen and oxygen the hydrogen evolution rate is µmol h−1 in a system of sacrificial donor and 3 while the oxygen evolution rate is µmol h−1 g−1 in a system as an electron acceptor and as a under visible light. a C=C bond linked (Figure with triazine knot and linker has been with a band gap of and at with a of at The a hydrogen evolution rate of µmol h−1 g−1 in an and 3 as the reduction by the condensing with