Mixed electronic-ionic conductors based on host-guest architectures of metal-organic frameworks

The research of conductive metal organic frameworks (MOFs) has been focused on either electronic or ionic performances. However, electrode materials and oxidation catalysts often require mixed electronic-ionic conduction. Despite the demand, mixed electronic-ionic conductors derived from MOFs are still rare. By combining state-of-the-art conductivity mechanisms and host-guest chemistry, a special strategy for MOF-based host-guest architectures is proposed. The research of conductive metal organic frameworks (MOFs) has been focused on either electronic or ionic performances. However, electrode materials and oxidation catalysts often require mixed electronic-ionic conduction. Despite the demand, mixed electronic-ionic conductors derived from MOFs are still rare. By combining state-of-the-art conductivity mechanisms and host-guest chemistry, a special strategy for MOF-based host-guest architectures is proposed. The past two decades have witnessed the widespread development of metal-organic frameworks (MOFs) in various fields, ranging from catalysts, separation membranes, and adsorbents to energy and information materials and even life sciences.1Furukawa H. Cordova K.E. O’Keeffe M. Yaghi O.M. The chemistry and applications of metal-organic frameworks.Science. 2013; 341: 1230444Google Scholar,2Xie L.S. Skorupskii G. Dincă M. Electrically conductive metal−organic frameworks.Chem. Rev. 2020; 120: 8536-8580Google Scholar In particular, there has been an increasing research interest in their roles as electron-conducting or ion-conducting materials (Figure 1A).3Xia Z. Jia X. Ge X. Ren C. Yang Q. Hu J. Chen Z. Han J. Xie G. Chen S. Gao S. Tailoring electronic structure and size of ultrastable metalated metal–organic frameworks with enhanced electroconductivity for high-performance supercapacitors.Angew. Chem. Int. Ed. Engl. 2021; 60: 10228-10238Google Scholar,4Xu G. Otsubo K. Yamada T. Sakaida S. Kitagawa H. Superprotonic conductivity in a highly oriented crystalline metal-organic framework nanofilm.J. Am. Chem. Soc. 2013; 135: 7438-7441Google Scholar The former has promising applications in supercapacitors and information materials, while the latter has been intensively studied as ionic (or protonic) conductors for batteries and fuel cells. However, these two developments are in parallel. To the best of our knowledge, there have not been reports on their overlapping—mixed conductors based on MOFs—due to the currently employed design strategies and components. Therefore, in this Matter of Opinion, mixed conductors based on MOFs are proposed through the combination of both electronic and protonic/ionic conduction mechanisms. Charge (electrons and ions) transport modes in conductive MOFs can be described from both intrinsic and extrinsic perspectives (Figure 1B):(1)From an intrinsic principle, intrinsically electron-conductive MOFs can be sorted into categories of three types in terms of charge transfer mechanisms: (1) “through bond,” (2) "extended conjugation," and (3) “through space” (1a in Figure 1B). Type 1 and type 2 are usually achieved by employing variable-valence metal ions and highly conjugated ligands,5Sun L. Campbell M.G. Dincă M. Electrically conductive porous metal–organic frameworks.Angew. Chem. Int. Ed. Engl. 2016; 55: 3566-3579Google Scholar while type 3 can be enhanced through constructing two-dimensional MOFs with short layer distances. On the other hand, intrinsically ion-conductive MOFs (2a in Figure 1B) are regularly obtained by designing proton donor or proton acceptor sites (e.g. sulfonic acid groups and imidazolium groups) for the ligands or counter ions for the frameworks via Grotthuss or vehicle mechanisms.6Lim D.W. Kitagawa H. Proton transport in metal−organic frameworks.Chem. Rev. 2020; 120: 8416-8467Google Scholar(2)From an extrinsic principle, both types of conductive MOFs, in turn, can achieve electron (1b in Figure 1B) and ionic (2b in Figure 1B) conduction through constructing host-guest architectures. However, the design of mixed electronic-ionic conductors based on MOFs by a single-conduction mode (intrinsic-intrinsic mode, namely 1a+2a in Figure 1B, or extrinsic-extrinsic mode, namely 1b+2b in Figure 1B) is inefficient and would involve components that will interfere with each other. For instance, the sulfonic acid functional group will greatly change the electronic structure of the conjugated ligand and the layer distance of MOFs, thus reducing its capability of electron transport.7Mancuso J.L. Mroz A.M. Le K.N. Hendon C.H. Electronic structure modeling of metal−organic frameworks.Chem. Rev. 2020; 120: 8641-8715Google Scholar Furthermore, in the host-guest architecture, the nano-pore size of MOFs limits the inclusion of guests to single components such as small molecules, nanometallic particles, and so on. Therefore, the construction of individually functionalized host-guest architectures is an efficient strategy for the preparation of MOF-derived mixed conductors. Potentially viable strategies for mixed electronic-ionic conductors are listed in Figure 2A. Specifically, the host-guest architectures can be categorized into (1) intrinsically electron-conductive host MOFs with ion-conductive guest molecules (i.e., intrinsic-extrinsic mode, namely 1a+2b in Figure 1B), and (2) intrinsically ion-conductive host MOFs with electron-conductive guest molecules (i.e., intrinsic-extrinsic mode, namely 2a+1b in Figure 1B). Ion-conductive guests may include (1) Brønsted acids (including heteropoly acids), Brønsted bases, inorganic salts, organic salts, ionic liquids (or their mixtures), ionic plastic crystals, ionic liquid crystals, poly(ionic liquid)s, deep eutectic solvents, polyelectrolytes, ionic polymers (or their monomers), and so on; and (2) counterions (e.g., H3O+, NH4+, (CH3)2NH2+, and H2PO4–). On the other hand, electron-conductive guests can contain (1) metal nanoparticles (e.g., Ag, Au, and Cu); (2) electron-conductive molecules (e.g., C60, carbon nanotubes [CNTs], graphene, carbon black, and quantum dots); and (3) electrically conducting polymers (e.g., polyaniline [PANI], poly(3,4-ethylenedioxythiophene) [PEDOT], and polypyrrole [PPY]). Among all these potential guest candidates, ionic or electrically conducting organic polymers are noteworthy for the unique in situ polymerization of their precursors (monomers). During the formation of the [email protected] composites, the nano-channels of MOFs could serve as “nano-reactors” for the polymerization. Furthermore, due to the nano-confinement effect,8Tu W. Xu Y. Yin S. Xu R. Rational design of catalytic centers in crystalline frameworks.Adv. Mater. 2018; 30: e1707582Google Scholar the activation energy required for polymerization could be greatly reduced, which makes it easier to form interpenetrating architectures under moderate and controllable reaction conditions. Thus, a brand-new strategy is proposed and termed “interpenetrating networks” (Figure 2B). To highlight the concept of interpenetrating networks, an example based on coupling electron-conductive MOFs and proton-conductive hydrogen-bonded organic frameworks (HOFs) is given. Herein, {Cu(H2O)}(2,6-NDPA)0.5, wherein NDPA denotes naphthalenediphosphonic acid,9Peeples C.A. Kober D. Schmitt F.J. Tholen P. Siemensmeyer K. Halldorson Q. Çoşut B. Gurlo A. Yazaydin A.O. Hanna G. Yücesan G. A 3D Cu–naphthalene–phosphonate metal–organic framework with ultra–high electrical conductivity.Adv. Funct. Mater. 2021; 31: 2007294Google Scholar and 1,2,4-triazolium perfluorobutanesulfonate10Luo J. Jensen A.H. Brooks N.R. Sniekers J. Knipper M. Aili D. Li Q. Vanroy B. Wübbenhorst M. Yan F. et al.1,2,4-Triazolium perfluorobutanesulfonate as an archetypal pure protic organic ionic plastic crystal electrolyte for all-solid-state fuel cells.Energy Environ. Sci. 2015; 8: 1276-1291Google Scholar are chosen as the MOF and the HOF, respectively (Figure 2B). In this architecture, the HOF offers the proton source and proton transfer path, while the MOF provides the electron conduction path to ensure the proton and electron conductivity of the interpenetrating networks, respectively. In essence, this can be regarded as “HOF-in-MOF” or “MOF-in-HOF.” In conclusion, such materials will undoubtedly become a new research direction for conductive MOFs, host-guest chemistry, and mixed electronic-ionic conductors. J.L. acknowledges funding from National Natural Science Foundation of China (project no. 21776120), the starting grant (“One Hundred Talent Program”) from Sichuan University (project no. YJ202089), the Research Foundation - Flanders (FWO) (project no. G0B3218N), and the Innovative Teaching Reform Project for Postgraduate Education of Sichuan University (project no. GSALK2020009). The authors declare no competing interests.