High Proton Conductivity of One-Dimensional Ferrous Oxalate Dihydrate

Gas Sensors Based on Chemi-Resistive Hybrid Functional Nanomaterials

Chemi-resistive sensors based on hybrid functional materials are promising candidates for gas sensing with high responsivity, good selectivity, fast response/recovery, great stability/repeatability, room-working temperature, low cost, and easy-to-fabricate, for versatile applications. This progress report reviews the advantages and advances of these sensing structures compared with the single constituent, according to five main sensing forms: manipulating/constructing heterojunctions, catalytic reaction, charge transfer, charge carrier transport, molecular binding/sieving, and their combinations. Promises and challenges of the advances of each form are presented and discussed. Critical thinking and ideas regarding the orientation of the development of hybrid material-based gas sensor in the future are discussed.

Covalent Organic Frameworks: Design, Synthesis, and Functions

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

A proton transfer mechanism along the PO4 anion chain in the [Zn(HPO4)(H2PO4)]2- coordination polymer

In this study, we revisit the proton transfer mechanism in [Zn(HPO4)(H2PO4)]2-, a coordination polymer possessing high proton conductivity. In a previous report [N. Phattharasupakun, J. Wutthiprom, S. Kaenket, Th. Maihom, J. Limtrakul, M. Probst, S. S. Nagarkar, S. Horike and M. Sawangphruk, Chem. Commun., 2017, 53, 11786-11789], it was hypothesized that protons could move along the ImH+ chain involving phosphate anions within the polymer structure, with energy barriers >1.3 eV. Adopting M06-2X calculations to examine the reaction pathway, we observe that it is much more favorable for H+ to move along a one-dimensional channel formed by HPO42- and H2PO4- anions. Within a unit cell, the proton hopping process can be divided into three elementary steps. For the forward proton transfer direction, the maximum energy barrier is only 0.04 eV, while that of the backward direction is 0.27 eV. Even though the barriers of the backward direction seem to outreach the barriers of the forward direction, both are still low in comparison with those reported in the literature. Moreover, we also point out the involvement of PO4 rotation during the proton transfer process. Activation energies of 0.37 eV and 0.15 eV are required for single steps of rotation of the phosphate anion. Both H+ translation (hopping) and rotation steps of PO4 anions simultaneously participate in the course of proton transfer in the coordination polymer.

Functionality in metal-organic framework minerals: proton conductivity, stability and potential for polymorphism

Rare metal-organic framework (MOF) minerals stepanovite and zhemchuzhnikovite can exhibit properties comparable to known oxalate MOF proton conductors, including high proton conductivity over a range of relative humidities at 25 °C, and retention of the framework structure upon thermal dehydration. They also have high thermodynamic stability, with a pronounced stabilizing effect of substituting aluminium for iron, illustrating a simple design to access stable, highly proton-conductive MOFs without using complex organic ligands.

Modulating proton diffusion and conductivity in metal-organic frameworks by incorporation of accessible free carboxylic acid groups

Three multi-carboxylic acid functionalised ligands have been designed, synthesised and utilised to prepare the new barium-based MOFs, MFM-510, -511, and -512, which show excellent stability to water-vapour. MFM-510 and MFM-511 show moderate proton conductivities (2.1 × 10-5 and 5.1 × 10-5 S cm-1, respectively) at 99% RH and 298 K, attributed to the lack of free protons or hindered proton diffusion within the framework structures. In contrast, MFM-512, which incorporates a pendant carboxylic acid group directed into the pore of the framework, shows a two orders of magnitude enhancement in proton conductivity (2.9 × 10-3 S cm-1). Quasi-elastic neutron scattering (QENS) suggests that the proton dynamics of MFM-512 are mediated by "free diffusion inside a sphere" confirming that incorporation of free carboxylic acid groups within the pores of MOFs is an efficient albeit synthetically challenging strategy to improve proton conductivity.

A copper(ii)-coordination polymer based on a sulfonic–carboxylic ligand exhibits high water-facilitated proton conductivity

The coordination polymer {[Cu₂(sba)₂(bpg)₂(H₂O)₃]·5H₂O}_n encapsulates arrays of water molecules H-bonded to the framework displaying a high conductivity value of 0.94 × 10⁻² S cm⁻¹ with an activation energy of 0.64 eV.

2,5-Furandicarboxylic acid as a linker for lanthanide coordination polymers: the role of heteroaromatic π–π stacking and hydrogen bonding

Heteroaromatic carboxylate ligands are very intriguing components of coordination assemblies such as coordination polymers (CPs) due to their inherently multitopic structures.

Extra Water- and Acid-Stable MOF-801 with High Proton Conductivity and Its Composite Membrane for Proton-Exchange Membrane

Proton-exchange membranes (PEMs), characterized by selectively permitting the transfer of protons and acting as a separator in electrochemical devices, have attracted immense attention. The composite membrane, fabricated from organic polymer matrix and high proton-conducting metal-organic framework (MOF), integrates the excellent physical and chemical performances of the polymer and MOF, achieving collective properties for good-performance PEMs. In this study, we demonstrate that MOF-801 shows remarkable proton conductance with σ = 1.88 × 10^-3 S cm^-1 at 298 K and 98% relative humidity (RH), specifically, together with extra stability to hydrochloric acid or diluting sodium hydroxide aqueous solutions and boiling water. Furthermore, the composite membranes (denoted MOF-801@PP- X, where X represents the mass percentage of MOF-801 in the membrane) have been fabricated using the sub-micrometer-scale crystalline particles of MOF-801 and blending the poly(vinylidene fluoride)-poly(vinylpyrrolidone) matrix, and these PEMs display high proton conductivity, with σ = 1.84 × 10^-3 S cm^-1 at 325 K 98% RH. A composite membrane as PEM was assembled into H₂/O₂ fuel cell for tests, indicating that these membrane materials have vast potential for PEM application on electrochemical devices.

Proton conduction in a hydrogen-bonded complex of copper(ii)-bipyridine glycoluril nitrate

Bipyridine glycoluril (BPG), a urea-fused bipyridine tecton, forms a square-pyramidal secondary building unit with copper(ii) which further self-assembles to give a porous hydrogen-bonded complex. This complex displays a high proton conductivity of 4.45 × 10^-3 S cm^-1 at 90 °C and 95% relative humidity (RH). Chains consisting of coordinated water, solvent water and nitrate anions embedded in the complex are responsible for high proton conduction. The proton conduction pathway was corroborated by ab initio electronic structure calculations with molecular dynamics (MD) simulations using the Nudged Elastic Band (NEB) method. The theoretical activation energy estimated to be 0.18 eV is in close agreement with the experimental value of 0.15 eV which evidences a Grotthuss proton hopping mechanism. We thus demonstrate that the hydrogen-bonded complex encapsulating appropriate counter ions, coordinated water and solvent water molecules exhibts superprotonic conductivity.

Conductive Supramolecular Architecture Constructed from Polyoxovanadate Cluster and Heterocyclic Surfactant

Proton-conductive solid electrolytes are significant for fuel-cell battery technology. Especially for use in motor vehicles, proton conductors which work at intermediate temperatures (373–673 K) under an anhydrous atmosphere are desired to improve the fuel cell stability and efficiency. Inorganic–organic hybrid supramolecular architectures are a promising option for the realization of highly conductive proton conductors. Here, a hybrid layered crystal was synthesized for the first time by using an proton-containing decavanadate (V10) anion and a heterocyclic surfactant cation. A simple ion-exchange reaction led to the formation of an inorganic–organic hybrid of V10 by using dodecylpyridazinium (C12pda) as the heterocyclic surfactant. Single crystal X-ray analyses revealed that four C12pda cations were associated with one V10 anion, which was a diprotonated species forming a one-dimensional infinite chain structure through hydrogen bonds. Anhydrous proton conductivity was investigated by alternating current (AC) impedance spectroscopy in the range of 313–393 K, exhibiting a maximum value of 1.7 × 10−5 S cm−1 at 373 K.

Solvent-free synthesis and room temperature proton conductivity of new cobalt phosphite–oxalates

Two new cobalt phosphite–oxalates (1 and 2) have been synthesized under solvent-free conditions. Compound 2 possesses room temperature proton conductivity.

Efficient pure white light emission based on a three-component La:Eu,Tb-doped luminescent lanthanide metal–organic framework

A three-component Eu and Tb-doped La complex based on a semirigid tricarboxylate ligand is successfully constructed with high-efficiency white light emission (Φ = 11.22% at 345 nm).

Switching on the proton transport pathway of a lanthanide metal–organic framework by one-pot loading of tetraethylene glycol for high proton conduction

We enhance the proton conductivity of a lanthanide MOF compared to commercial Nafion by the one-pot loading of tetraethylene glycol.

Water stable oxalate-based coordination polymers with in situ generated cyclic dipeptides showing high proton conductivity

Presented here are two water stable oxalate-based coordination polymers with in situ generated cyclic dipeptides, which show high proton conductivities on the order of 10⁻³ S cm⁻¹ at 85 °C under 98% relative humidity.

Mesoporous Metal-Organic Frameworks with Exceptionally High Working Capacities for Adsorption Heat Transformation

Pore size is one of the most important parameters of adsorbents, and mesoporous materials have received intense attention for large guests. Here, a series of mesoporous coordination polymers underlying a new framework prototype for fast expansion of pore size is reported and the profound effect of pore size on adsorption heat transformation is demonstrated. Three isostructural honeycomb-like frameworks are designed and synthesized by combining ditopic linear metal oxalate chains and triangular tris-pyridine ligands. Changing the ligand bridging length from 5.5 to 8.6 and 9.9 Å gives rise to effective pore diameter from 20 to 33 and 37 Å, surface area from 2096 to 2630 and 2749 m² g^-1 , and pore volume from 1.19 to 1.93 and 2.36 cm³ g^-1 , respectively. By virtue of the unique and tunable isotherm shape of mesopores, exceptionally large working capacity up to 1.19 g g^-1 or 0.38 g cm^-3 for adsorption heat transformation can be achieved using R-134a (1,1,1,2-tetrafluroethane) as a working fluid.

Development of an All Solid State Battery Incorporating Graphene Oxide as Proton Conductor

Graphene oxide (GO) shows high proton conductivity (≈10-4 Scm-1), excellent mechanical stability, and electrical insulation property, which makes it an ideal candidate for use as a proton conducting solid state electrolyte. The prospects of using GO as single phase solid electrolyte in an all solid battery is presented herein. A battery with the cell configuration: Zn + ZnSO4•7H2O + graphite (anode) || GO (electrolyte) || MnO2 + graphite (cathode) is fabricated. Cyclic voltammetry confirms its rechargeable nature. The respective discharge capacity and power density of the cell are 360 μAh and 19.5 mW kg-1 at a constant current drain of 3 μA under the experimental conditions employed. GO based proton conductors are cleaner and cheaper than other solid electrolytes. The current study strongly suggests that GO can be used as a practical and beneficial component in solid state battery applications with low energy feedback.

Development of Ion-Conductive and Vapoluminescent Porous Coordination Polymers Composed of Ruthenium(II) Metalloligand

We synthesized two new porous coordination polymers (PCPs) {Ln₇(OH)₅[Ru(dcbpy)₃]₄·4nH₂O} (Ln₇-Ru₄; Ln = Ce, Nd) composed of the luminescent ruthenium(II) metalloligand [Ru(4,4'-dcbpy)₃]^4- ([4Ru]; 4,4'-dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) and lanthanide ions Ln³⁺ (Ln = Ce, Nd). These two PCPs Ln₇-Ru₄ are isomorphous with the previously reported PCP La₇-Ru₄, and the lattice constants (a, c, and unit cell volume V) changed systematically according to the lanthanide contraction. All three Ln₇-Ru₄ compounds have OH^- anion containing porous structures and a large number of hydrate water molecules within the pores, resulting in moderate ion conductivities (10^-6-10^-7 S cm^-1) at 90% relative humidity (RH) and 298 K. In contrast, the structural transformation of Ln₇-Ru₄ associated with water-vapor adsorption/desorption strongly depends on the lanthanide ion; the Ln₇-Ru₄ compounds with larger Ln³⁺ ions recover the original porous structure at lower relative humidities (RH). A similar trend was observed for the ion conduction activation energy, suggesting that the bridging Ln³⁺ ion plays an important role in the formation of the ion-conductive pathways. La₇-Ru₄ and Ce₇-Ru₄ exhibit vapochromic luminescence associated with water vapor adsorption/desorption, arising from the ³MLCT emission of [4Ru]. This vapochromic behavior is also affected by the replacement of the Ln³⁺ ion; the vapochromic shift of Ce₇-Ru₄ was observed at RH values (near 100% RH) higher than that of La₇-Ru₄. ³MLCT emissions of the [4Ru] metalloligand in Nd₇-Ru₄ were barely observable in the visible region, but sharp emission bands characteristic of 4f-4f transitions of the Nd³⁺ ion were observed in the near-infrared (NIR) region (arising from the ¹MLCT transition of [4Ru]), suggesting the transfer of energy from the [4Ru] ³MLCT excited state to the 4f-4f transition state of the Nd³⁺ ions.

A novel oxalate-based three-dimensional coordination polymer showing magnetic ordering and high proton conductivity

A novel example of an oxalate-based multifunctional material is reported here showing the coexistence of low-temperature magnetic ordering and high proton conductivity.

Anisotropic Li+ ion conductivity in a large single crystal of a Co(iii) coordination complex

Anisotropic Li⁺ ion conductivity has been observed in a large single crystal of a novel Co(iii) coordination complex attributable to the presence of directional ion channels with a suitable size.

Solution-based sequential modification of LiCoO2particle surfaces with iron(ii) oxalate nanolayers

Facile creation of hybrid metal oxide-core/MOF-shell structures is achieved by stepwise solution-based modification of the core surface with a framework compound.

An anchoring strategy leads to enhanced proton conductivity in a new metal–organic framework

An anchoring strategy affords a 900-fold enhancement in the proton conductivity of a new Zr-based metal–organic framework.

High Proton Conductivity of Zinc Oxalate Coordination Polymers Mediated by a Hydrogen Bond with Pyridinium

A novel metal-organic framework, (Hpy)2[Zn2(ox)3]·nH2O (n = 0, 1), having a pyridinium cation, was newly synthesized, and the crystal structures were determined. The hydrated compound shows a high proton conductivity of 2.2 × 10(-3) S cm(-1) at 298 K and 98% relative humidity. Single crystal XRD analysis revealed a rotational displacement factor for the hydrated pyridinium ring and elongated water site that is thought to cause the high proton conductivity.

Thermally Stable Super Ionic Conductor from Carbon Sphere Oxide

A highly stable proton conductor has been developed from carbon sphere oxide (CSO). Carbon sphere (CS) generated from sucrose was oxidized successfully to CSO using Hummers' graphite oxidation technique. At room temperature and 90 % relative humidity, the proton conductivity of thin layer CSO on microsized comb electrode was found to be 8.7×10(-3)  S cm(-1) , which is higher than that for a similar graphene oxide (GO) sample (3.4×10(-3)  S cm(-1) ). The activation energy (Ea ) of 0.258 eV suggests that the proton is conducted through the Grotthuss mechanism. The carboxyl functional groups on the CSO surface are primarily responsible for transporting protons. In contrast to conventional carbon-based proton conductors, in which the functional groups decompose around 80 °C, CSO has a stable morphology and functional groups with reproducible proton conductivity up to 400 °C. Even once annealed at different temperatures at high relative humidity, the proton conductivity of CSO remains almost unchanged, whereas significant change is seen with a similar GO sample. After annealing at 100 and 200 °C, the respective proton conductivity of CSO was almost the same, and was about ∼50 % of the proton conductivity at room temperature. Carbon-based solid electrolyte with such high thermal stability and reproducible proton conductivity is desired for practical applications. We expect that a CSO-based proton conductor would be applicable for fuel cells and sensing devices operating under high temperatures.