Thermo-electrochemical cells have potential to generate thermoelectric voltage 1 order higher than that given by semiconductor materials. To overcome the current issues in thermoelectric energy ...conversion, it is of paramount importance to grow and fulfill the full potential of thermo-electrochemical cells. Here we report a rational supramolecular methodology that yielded the highest Seebeck coefficient of ca. 2.0 mV K–1 around ambient temperatures. This is based on the encapsulation of triiodide ions in α-cyclodextrin, whose equilibrium is shifted to the complexation at lower temperatures, whereas it is inverted at elevated temperatures. This temperature-dependent host–guest interaction provides a concentration gradient of redox ion pairs between two electrodes, leading to the eminent performance of the thermo-electrochemical cells. The figure of merit for this system, zT reached a high value of 5 × 10–3. The introduction of host–guest chemistry to thermoelectric cells thus provides a new perspective in thermoelectric energy conversion.
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The proton conductivities of the porous coordination polymers M(OH)(bdc−R) H2bdc = 1,4-benzenedicarboxylic acid; M = Al, Fe; R = H, NH2, OH, (COOH)2 were investigated under humid conditions. Good ...correlations among pK a, proton conductivity, and activation energy were observed. Fe(OH)(bdc−(COOH)2), having carboxy group and the lowest pK a, showed the highest proton conductivity and the lowest activation energy in this system. This is the first example in which proton conductivity has been widely controlled by substitution of ligand functional groups in an isostructural series.
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Proton conductivity through two-dimensional (2-D) hydrogen-bonding networks within a layered metal–organic framework (MOF) (NH4)2(H2adp)Zn2(ox)3·3H2O (H2adp = adipic acid; ox = oxalate) has been ...successfully controlled by cation substitution. We synthesized a cation-substituted MOF, K2(H2adp)Zn2(ox)3·3H2O, where the ammonium ions in a well-defined hydrogen-bonding network are substituted with non-hydrogen-bonding potassium ions, without any apparent change in the crystal structure. We successfully controlled the proton conductivity by cleavage of the hydrogen bonds in a proton-conducting pathway, showing that the 2-D hydrogen-bonding networks in the MOF truly contribute to the high proton conductivity. This is the first example of the control of proton conductivity by ion substitution in a well-defined hydrogen-bonding network within a MOF.
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A novel metal−organic framework (MOF), (NH4)2(adp)Zn2(ox)3·3H2O (1) was synthesized and its structure was determined. We propose three types of rational design to introduce proton carriers into MOFs. ...The simplest method is to introduce them directly as counterions such as NH4 +, H3O+, and HSO4 − into the pores of frameworks (type I). The second is to put acid groups on frameworks, the protons being provided from them (type II). The third is to incorporate acidic molecules into voids (type III). 1 demonstrated a combination of two of the concepts by introducing NH4 + ions using the anionic framework (type I) and putting carboxyl end groups of adipic acid in a honeycomb-shaped void (type III). 1 showed a superprotonic conductivity of 10−2 S cm−1 at ambient temperature, comparable to organic polymers such as Nafion, which is in practical use in fuel cells. This is the first example of an MOF to exhibit a superprotonic conductivity of 10−2 S cm−1 at ambient temperature.
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The electrical properties of a highly oriented crystalline MOF nanofilm were studied. This nanofilm has low activation energy and a proton conductivity that is among the highest value reported for ...MOF materials. The study uncovered the reasons for the excellent performance of this nanofilm and revealed a new pathway for proton transport in MOF materials; besides the channels inside a MOF, the surface of the MOF nanocrystal can also dominate proton transport.
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Metal–organic frameworks (MOFs) are desirable host materials to study and control the dynamics of molecules and ions such as lithium ions. We show the first study of a lithium ion-doped ionic liquid ...(IL) incorporated into a MOF and investigate its phase behavior and ionic conductivity. Moreover, for the first time, we have studied the dynamics of lithium ions in the micropores of the MOF in terms of the self-diffusion coefficient of the lithium ions. The IL was a mixture of EMI-TFSA (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide) with LiTFSA (lithium bis(trifluoromethylsulfonyl)amide), and the MOF was ZIF-8 (Zn(MeIM)2; H(MeIM) = 2-methylimidazole). The TFSA– anions showed a gradual decrease of mobility in the micropores at low temperatures, which indicates the absence of the apparent freezing transition. The mobility of the Li+ cations showed a slightly steeper decrease than that of the TFSA– anions at low temperature. The ionic conductivity of the (EMI0.8Li0.2)TFSA in the micropores was 2 orders of magnitude lower than that of the bulk (EMI0.8Li0.2)TFSA. However, the activation energy for the diffusion of lithium ions in the micropores of ZIF-8 was comparable with the bulk (EMI0.8Li0.2)TFSA. These results suggest that the Li+ cations diffuse through the micropores via the exchange of the solvating TFSA– anions, similar to the Grotthuss mechanism in proton conductivity.
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Structure-defined metal–organic frameworks (MOFs) are of interest because rational design and construction allow us to develop good proton conductors or possibly control the proton conductivity in ...solids. We prepared a highly proton-conductive MOF (NH4)2(adp)Zn2(ox)3·nH2O (abbreviated to 1· n H 2 O, adp: adipic acid, ox: oxalate, n = 0, 2, 3) having definite crystal structures and showing reversible structural transformations among the anhydrate (1), dihydrate (1·2H 2 O), and trihydrate (1·3H 2 O) phases. The crystal structures of all of these phases were determined by X-ray crystallography. Hydrogen-bonding networks consisting of ammonium ions, water molecules, and carboxylic acid groups of the adipic acids were formed inside the two-dimensional interlayer space in hydrated 1·2H 2 O and 1·3H 2 O. The crystal system of 1 or 1·2H 2 O (P21/c, No. 14) was changed into that of 1·3H 2 O (P1̅, No. 2), depending on water content because of rearrangement of guests and acidic molecules. Water molecules play a key role in proton conduction as conducting media and serve as triggers to change the proton conductivity through reforming hydrogen-bonding networks by water adsorption/desorption processes. Proton conductivity was consecutively controlled in the range from ∼10–12 S cm–1 (1) to ∼10–2 S cm–1 (1·3H 2 O) by the humidity. The relationships among the structures of conducting pathways, adsorption behavior, and proton conductivity were investigated. To the best of our knowledge, this is the first example of the control of a crystalline proton-conducting pathway by guest adsorption/desorption to control proton conductivity using MOFs.
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Complexation of copper(II) 2,3,9,10,16,17,23,24‐octahydroxy‐29H,31H‐phthalocyanine (CuPcOH) with copper(II) ions gives a two‐dimensional (2D) metal‐organic framework (MOF). This is the first report ...of a phthalocyanine‐based MOF. This 2D MOF was obtained as a black powder and showed an electrical conductivity of 1.6×10−6 S cm−1 at 80 °C. When this MOF is used as a cathode of lithium ion battery (LIB), large charge/discharge capacities of 151/128 mAh g−1 were obtained. In addition, it showed a good stability during 200 charge/discharge cycles. The obtained LIB performance mainly originates from the electrically conductive and redox‐active framework of the phthalocyanine‐based 2D MOF and its hierarchical microporous/mesoporous structure.
Complexation of the copper(II) phthalocyanine derivative, having four catechol moieties, with copper(II) ions gave a two‐dimensional MOF with hierarchical micro/mesoporous structure. The redox‐activity and conductivity of the framework rendered this Cu‐CuPc MOF a unique cathode material for the lithium‐ion battery system, showing large charge/discharge capacity with high cycle stability.
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The preparation of crystalline, ordered thin films of metal–organic frameworks (MOFs) will be a critical process for MOF-based nanodevices in the future. MOF thin films with perfect orientation and ...excellent crystallinity were formed with novel nanosheet-structured components, Cu–TCPP TCPP = 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin, by a new “modular assembly” strategy. The modular assembly process involves two steps: a “modularization” step is used to synthesize highly crystalline “modules” with a nanosized structure that can be conveniently assembled into a thin film in the following “assembly” step. With this method, MOF thin films can easily be set up on different substrates at very high speed with controllable thickness. This new approach also enabled us to prepare highly oriented crystalline thin films of MOFs that cannot be prepared in thin-film form by traditional techniques.
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