Proton-conducting Materials Research Articles

Supramolecular assembly, structure, magnetic property, and proton conductivity of a nickel(II) hydrogen-bonded organosulfonate framework

A three-dimensional nickel(II) hydrogen-bonded organic framework (NiHOF), formulated as [Ni(H2O)2(EtOH)2(MeCN)2][nds] (NiHOF-HGNU-2, H2nds = 1,5-Naphthalenedisulfonic acid, HGNU = Huanggang Normal University), was synthesized and magnetically and electrically characterized. Single-crystal X-ray diffraction (SC-XRD) analysis revealed that extensive hydrogen bonding interactions formed between the [Ni(H2O)(EtOH)(MeCN)]2+cations and organodisulfonate anions nds2−, leading to a highly hydrogen-bonded network. Variable-temperature SC-XRD and thermogravimetric analysis both reveal the crystalline sample of NiHOF-HGNU-2 can be thermally stable up to 400 K. Magnetic investigations reveal the uniaxial magnetic anisotropy (D = -9.87 cm−1) of the distorted octahedral Ni2+ ions of NiHOF-HGNU-2 without slow magnetic relaxation. Alternating current impedance spectroscopy indicate NiHOF-HGNU-2 exhibits temperature- and humidity-dependent proton conduction behavior with the highest measured conductivity reaching 1.15 × 10−4 S cm−1 at 85 °C and 98 % relative humidity, suggesting a supramolecular proton conductor of the NiHOF-HGNU-2. The forgoing results not only provide a new Ni-based proton conductor but also an organodisulfonate strategy for building supramolecular proton-conducting materials.

Introducing sulfonic acid polymers into MOF nanochannels for ultra-high Ba2+ adsorption capacity and proton conductivity

Porous materials with a high concentration of sulfonic acid groups are highly desirable for applications such as radioactive Ba2+ adsorption, catalysis, and proton-conductive materials, but they are also challenging to synthesize. In this work, we introduce polymers containing sulfonic acid groups into the nanochannels of metal–organic frameworks (MOFs) through in-situ synthesis to enhance the sulfonic acid content. We use a alkene with sulfonic acid groups (2)-acrylamido-2-methylpropane sulfonic acid, AMPS) and a crosslinking agent N,N’-methylenebisacrylamide (MBAA) to synthesize two MOF composites, PAMPS@MIL-101 and PAMPS@MIL-101-SO3H, in the ultra-stable MIL-101 series through in-situ polymerization with free radical initiators. The two MOF composites have a high and uniform distribution of sulfonic acid groups in the MOF framework, which results in ultra-high barium ion adsorption capacity. The maximum adsorption amounts are 142.7 and 356.9 mg·g−1 for PAMPS@MIL-101 and PAMPS@MIL-101-SO3H, respectively, which are higher than those of most reported adsorbents. Importantly, the adsorption equilibrium can be achieved within 20 min. Moreover, both MOF composites, PAMPS@MIL-101 and PAMPS@MIL-101-SO3H, exhibit super proton conductivity, with values of 6.43 × 10−2 S·cm−1 and 1.43 × 10−1 S·cm−1 at 85 °C and 98 % RH, respectively. This method offers a new way of modifying MOF nanochannels with high sulfonic acid concentrations, and opens up new opportunities for developing radioactive Ba2+ adsorption materials and proton-conductive materials.

Superprotonic Conductivity in a Metalloporphyrin-Based SMOF (Supramolecular Metal-Organic Framework).

Metal-organic frameworks and supramolecular metal-organic frameworks (SMOFs) exhibit great potential for a broad range of applications taking advantage of the high surface area and pore sizes and tunable chemistry. In particular, metalloporphyrin-based MOFs and SMOFs are becoming of great importance in many fields due to the bioessential functions of these macrocycles that are being mimicked. On the other hand, during the last years, proton-conducting materials have aroused much interest, and those presenting high conductivity values are potential candidates to play a key role in some solid-state electrochemical devices such as batteries and fuel cells. In this way, using metalloporphyrins as building units we have obtained a new crystalline material with formula [H(bipy)]2[(MnTPPS)(H2O)2]·2bipy·14H2O, where bipy is 4,4'-bipyidine and TPPS4- is the meso-tetra(4-sulfonatephenyl) porphyrin. The crystal structure shows a zig-zag water chain along the [100] direction located between the sulfonate groups of the porphyrin. Taking into account those structural features, the compound was tested for proton conduction by complex electrochemical impedance spectroscopy (EIS). The as-obtained conductivity is 1 × 10-2 S·cm-1 at 40 °C and 98% relative humidity, which is a remarkably high value.

Comparative Analysis of Proton Conductivity in Two Zn-Based MOFs Featuring Sulfate and Sulfonate Functional Groups.

Metal-organic frameworks (MOFs) have shown promising potential as proton-conducting materials due to their tunable structures and high porosity. In this study, two novel MOFs had been successfully synthesized, one containing sulfate groups (MOF-1; [Zn4(TIPE)2(SO4)4(H2O)]·5H2O) and the other containing sulfonate groups (MOF-2; [Zn2(TIPE)(5-sip)(NO3)0.66]·0.34NO3·17.5H2O) (TIPE = 1,1,2,2-tetrakis(4-(1H-imidazole-1-yl)phenyl)ethene, H35-sip = 5-sulfoisophthalicacid), and the effect of the two groups on the proton conductivity of Zn-based MOFs had been investigated and compared for the first time. The proton conductivity of these MOFs was systematically measured at different temperatures and humidity conditions. Remarkably, the results revealed significant differences in proton conductivity between the two sets of MOFs. At 90 °C and 98% RH, MOF-1 and MOF-2 achieved optimal proton conductivity of 4.48 × 10-3 and 5.69 × 10-2 S·cm-1, respectively. This was due to the structural differences arising from the presence of different functional groups, which subsequently affected the porosity and hydrophilicity, thereby influencing the proton conductivity. Overall, this comparative study revealed the influence of sulfate and sulfonate groups on the proton conductivity of Zn-based MOFs. This research provided a feasible idea for the development of advanced MOF materials with enhanced proton conductivity and opened up new possibilities for their application in proton devices.

Controlled Assembly of a Dawson-like Antimoniotungstate-Supported Trefoil-type Trimer with Good Proton Conductivity Performance.

With the excellent properties of POM in the field of proton conductivity, the preparation of POM-based proton-conductive materials has burst into life. Herein, an unprecedented Sb-templated all-inorganic trimer Na8H18.64[(SbW14O52)3(Sb2W6.12Ru5.88O18)]·85H2O (1), which is based on tetravacant Dawson-like [SbW14O52]17- blocks and exhibits a trefoil type with D3 symmetry, has been successfully designed and synthesized by the assembly of simple materials with a one-pot hydrothermal method under acidic conditions. Also, compound 1 is systematically characterized by single-crystal X-ray diffraction, PXRD, ESI-MS, IR spectroscopy, UV-vis, elemental analysis, and TGA. Crystal structure data analysis demonstrates that compound 1 is constructed by a hexagonal prismatic heterometallic {Sb2W6.12Ru5.88O18} core and three equivalent {SbW14} units bridged through μ2-O atoms in periphery. Subsequently, further property experiments show that compound 1 exhibits high proton conductivity with a conductivity value (σ) of 3.07 × 10-2 S cm-1 at 75 °C and 80% relative humidity (RH). The activation energy of compound 1 evaluated by the Arrhenius plots is 0.22 eV, which indicates that the Grotthuss mechanism is dominant during the process of proton transfer.

Improving the proton conductivity of MOF materials by regulating the pore space

The construction of metal-organic frameworks (MOFs) with structural tunability, designability, and high stability is crucial in proton conduction studies. In this paper, MOF-801 and UiO-66 were used as precursors. A solvothermal synthesis method was employed to modulate the pore space of composite MOFs. The hydrogen-bonding network and structure within the pore cavities are adjusted by varying the molar ratios of fumaric acid and terephthalic acid to improve the proton transfer rate and proton conductivity of the materials. A total of 14 materials, MOF/MOG-801&UiO-66-X (X is the molar ratio of fumaric acid and terephthalic acid) were synthesised, the variation of proton conductivity with temperature of which was investigated at different tempertures and humidities. The results shows that MOG-801&UiO-66–1:1 has the best proton conductivity with 0.12 S cm−1 at 353 K and 100 % RH, which is 5 times higher than that of MOF-801&UiO-66–1:3 (2.54 × 10−2 S cm−1), 11 times higher than that of MOF-801 and 22 times higher than that of UiO-66. Based on the calculated activation energy and structural characterisation, the proton conduction mechanism of the composite was investigated. This work opens up novel research fields for the design of new proton-conducting MOF materials.

Water-chain mediated proton conductivity in mechanically flexible redox-active organic single crystals

Investigating the electrochemical features of proton-conducting flexible organic crystalline materials is crucial for the development of efficient energy storage and conversion devices.

Synthesis and proton-conductive behaviour of two MOFs with covalently bonded imidazoles in the channels.

Immobilization of imidazole molecules as proton carriers into MOFs to facilitate proton conduction is a general strategy for developing high proton conductive materials. Herein, we designed two imidazole substituted phthalic acid ligands and constructed two novel MOFs, {[Zr6(OH)16(H3L1)4]Cl8·20H2O}n [Zr-MOF; H3L1 = 2-(1H-imidazol-4-yl) methylaminoterephthalic acid] and {Gd(HCOO)(H2L2)2}n [Gd-MOF; H3L2 = 5-(1H-imidazol-4-yl)methylaminoisophthalic acid] and fully studied their porous nature, stability and water-assisted proton conduction. The resulting Zr-MOF exhibits a high proton conductivity of 1.82 × 10-2 S cm-1 at 98% RH and 80 °C, while Gd-MOF has a proton conductivity of 3.01 × 10-3 S cm-1 at 98% RH and 60 °C.

Superprotonic Conductivity of Guanidinium Organosulfonate Hydrogen-Bonded Organic Frameworks with Nanotube-Shaped Proton Transport Channels.

Grasping proton transport pathways and mechanisms is vital for the application of fuel cell technology. Herein, we screened four guanidinium organosulfonate charge-assisted hydrogen-bonded organic frameworks (HOFs), namely, GBBS, G 3 TSPHB, G 4 TSP, and G 6 HSPB, which possess high hydrogen-bonded density proton transport networks shaped like nanotubes. These materials were prepared by self-assembly through charge-assisted interactions between guanidinium cations and organosulfonate anions, as well as by host-guest regulation. At 80 °C and 93% RH, the proton conductivity of GBBS, G 3 TSPHB, G 4 TSP, and G 6 HSPB can reach 4.56 × 10-2, 2.55 × 10-2, 4.01 × 10-2, and 1.2 × 10-1 S cm-1, respectively, with superprotonic conductivity. Doping G 6 HSPB into the Nafion matrix prepared composite membranes for testing the performance of fuel cells. At 80 °C and 98% RH, the proton conductivity of 9%-G 6 HSPB@Nafion reached a maximum value of 1.14 × 10-1 S cm-1, which is 2.8 times higher than recast Nafion. The results showed that charge-assisted HOFs with high proton channel density have better proton transport properties, providing a reference for the design of highly proton-conducting materials.

A highly proton conductive perfluorinated covalent triazine framework via low-temperature synthesis

Proton-conducting materials are essential to the emerging hydrogen economy. Covalent triazine frameworks (CTFs) are promising proton-conducting materials at high temperatures but need more effective sites to strengthen interaction for proton carriers. However, their construction and design in a concise condition are still challenges. Herein, we show a low temperature approach to synthesize CTFs via a direct cyclotrimerization of aromatic aldehyde using ammonium iodide as facile nitrogen source. Among the CTFs, the perfluorinated CTF (CTF-TF) was successfully synthesized with much lower temperature ( ≤ 160 °C) and open-air atmosphere. Due to the additional hydrogen-bonding interaction between fluorine atoms and proton carriers (H3PO4), the CTF-TF achieves a proton conductivity of 1.82 × 10−1 S cm−1 at 150 °C after H3PO4 loading. Moreover, the CTF-TF can be readily integrated into mixed matrix membranes, displaying high proton conduction abilities and good mechanical strength. This work provides an alternative strategy for rational design of proton conducting media.

Synthesis of Phosphovanadate-Based Porous Inorganic Frameworks with High Proton Conductivity.

Materials with high proton conductivity have attracted significant attention for their wide-ranging applications in proton exchange membrane fuel cells. However, the design of new and efficient porous proton-conducting materials remains a challenging task. The structure-controllable and highly stable metal phosphates can be synthesized into layer or frame networks to provide proton transport capabilities. Herein, we have successfully synthesized three isomorphic metal phosphovanadates, namely, H2(C2H10N2)2[MII(H2O)2(VIVO)8(OH)4(PO4)4(HPO4)4] (C2H8N2 = 1,2-ethylenediamine; M = Co, Ni, and Cu), by the hydrothermal method employing ethylenediamine as a template. These pure inorganic open frameworks exhibit a cavity width ranging from 6.4 to 7.5 Å. Remarkably, the proton conductivity of compounds 1-3 can reach 1 × 10-2 S·cm-1 at 85 °C and 97% relative humidity (RH), and they can remain stable at high temperatures as well as long-term stability. This work provides a novel strategy for the development and design of porous proton-conducting materials.

Highly Stable Rare Earth Metal-Organic Frameworks for Fluorescence Recognition of Folic Acid, Proton Conduction, and Magnetic Refrigeration.

Four new isostructural rare earth metal-organic frameworks (RE-MOFs) were synthesized and full characterized, namely, {[(CH)2NH2]3[RE2(BTDBA)2(HCOO)]·5H2O·2DMF}n (H4BTDBA = (4',4'''-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis([1,1'-biphenyl]-3,5-dicarboxylic acid); RE = Eu (JXUST-34), Gd (JXUST-35), Tb (JXUST-36), and Dy (JXUST-37)). The single-crystal structures analysis shows that JXUST-34-37 are chain-based three-dimensional structures. Importantly, JXUST-34 exhibits excellent water, organic solvents, and acid-base stability, which can be used as a fluorescence sensor for folic acid and Al3+ with detection limits of 0.02 mM and 0.05 μM, respectively. The presence of free [(CH)2NH2]+ cations in the channels can engage the proton carrier during proton conduction. JXUST-34-37 display good proton conductivity, and the conductivities vary with relative humidity and temperatures, among which JXUST-37 has the highest conductivity of 9.66 × 10-3 S·cm-1 at 60 °C and 98% RH. The magnetic studies show that the -ΔSm of JXUST-35 reaches 16.13 J kg-1 K-1 at 2 K and ΔH = 7 T. JXUST-34-37 show multifunctional properties of fluorescence sensing, high proton conductivity, and magnetic refrigeration, which provides a new clue for the development of fluorescent-responsive, magnetic-refrigerant, and proton-conductive RE-MOF materials.

Polybenzimidazole-based polymers of intrinsic microporosity membrane for high-temperature proton conduction

Anhydrous proton conducting polymer membrane materials have raised much attention in the application of high-temperature proton-exchange membranes (HT-PEMs) fuel cells, which convert chemical energy into electrical energy. Polybenzimidazole-based polymers, as typical materials for HT-PEMs, their compact structures result in small fractional free volumes (FFV), which is not conducive to the adsorption and retention of proton carriers. Herein, a series of polybenzimidazole-based polymers of intrinsic microporosity (PIM-PBIs) were synthesized. PIM-PBIs possess the benzimidazole units and porous structures, and have good thermal stability and large specific surface area (100 ∼ 400 m2/g). PIM-PBIs are further embedded in polyvinylidene fluoride (PVDF) to form mixed matrix membranes for high-temperature proton conduction. The resulting membranes exhibit high proton conductivity up to 90.11 mS cm−1 at 140 ℃ and high phosphoric acid (PA) retention rate of 90.0 %. This work provides a facile method for the precise design and preparation of proton-conducting porous materials.

Hydrogen Exchange and Diffusion Kinetics at Elevated Temperatures in Proton-Conducting Solid Oxide Materials

Storage of purified hydrogen is one of the central challenges in addressing climate change and reducing our reliance on fossil fuels for energy conversion and storage, and therefore there is a global surge in research and development concerning hydrogen purification and storage. In this regard, we are studying proton conduction in solid oxide materials at elevated temperatures for applications in hydrogen separation and compression membranes. Hydrogen compression is the most recommended method to store hydrogen for automotive applications as it allows an increase in the hydrogen volumetric energy density.Traditionally the protonic conductivity in these materials is measured by indirect methods. For example, conductivity measurements in mixed gas atmospheres, comparing for example dry N2 with humidified N2, thereby allowing the contribution of protons to be evaluated. In this study, we for the first time report the evaluation of protonic conductivity in BaZr1-xCexY0.2O3−δ, BaZr0.1Ce0.7Y0.2–xYbxO3–δ and Ba7Nb4MoO20 by direct measurements afforded by the Isotope Exchange Depth Profiling technique with deuterium labelling. We also report the kinetics of H/D transport through the bulk materials and across metal-ceramic interfaces with a particular interest in the behaviour of the interface between the key Pd/Pd alloy catalyst component and the hydrogen-transporting oxide ceramic material. The transport and interface behaviour information will be of significance in designing hydrogen separation and compression membranes. Figure 1

Anhydrous superprotonic conductivity in a new zirconium acid phosphate: application of the BVSE method to proton-conducting materials

Anhydrous superprotonic conductivity in a new zirconium acid phosphate: application of the BVSE method to proton-conducting materials

Improved Solid-State Reaction Method for Scaled-Up Synthesis of Ceramic Proton-Conducting Electrolyte Materials

Improved Solid-State Reaction Method for Scaled-Up Synthesis of Ceramic Proton-Conducting Electrolyte Materials

Facile synthesis of a mixed matrix membrane based on PTA@MOF-199 nanoparticles with proton conductivity

Firstly, H3[P(W3O10)4]@MOF-199 nanoparticles (PTA@MOF-199) of uniform morphology, about 80-125 nm, were successfully synthesized by a simple one-step reaction under solvothermal conditions and then these as-prepared nanoparticles were incorporated into polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) with solution-cast technique to fabricate a series of proton-conducting mixed matrix membranes with different mass percentage of PTA@MOF-199. These membranes were characterized by XRD and FT-IR. Afterwards, the influence of the mass percentage of PTA@MOF-199 and temperature on the proton-conducting properties were evaluated. Our work confirmed that the polyoxometalate (POM) functionalized MOFs had great application potential as proton conductive materials and provided a basis for further studies on other POM@MOFs based proton-conducting membranes fabrication.

Novel proton-conducting materials based on a polyethylene terephthalate track-etched membrane modified with an N, P-containing ionic liquid

The development of novel membrane materials for hydrogen fuel cells, a promising environmentally friendly technology, represents a relevant research task. In this work, we propose an approach to creating proton-conducting membranes from an industrial polyethylene terephthalate (PET) dielectric track-etched film. An N, P-containing ionic liquid was used as a modifying agent, whose polymerization was carried out directly in the PET membrane pores. The ionic liquid was obtained using a novel approach to the directed synthesis of organophosphorus compounds from elemental phosphorus via the Trofimov-Gusarova reaction developed at the A.E. Favorsky Institute of Chemistry of the Siberian Branch of the Russian Academy of Sciences. The ionic liquid properties were characterized by NMR and IR spectroscopy. The application of the obtained N, P-containing ionic liquid onto a PET membrane was shown to yield a material exhibiting the required mechanical parameters for operation as proton-conducting membranes. The novel proton-conducting materials demonstrate a high proton conductivity of 77.76 mS·cm-1 at 353 K. The obtained proton-conducting membranes seem promising for application in hydrogen fuel cells, thus contributing to the development of effective alternative energy sources.

Proton conductive properties of one water-stable nickel(II) supramolecule bearing tri-pyridyl and tri-carboxylate ligand

The construction of highly stable complexes using azacyclic carboxylate ligands is an important way to obtain crystalline proton-conducting materials with high performance. In this paper, one three-dimensional supramolecule, [Ni(H2ttp)2]⋅3H2O (1) [H3ttp = 4-(2,4,6-tricarboxyphenyl)-2,2′,6,2′′-terpyridine] has been prepared according to literature approach with slight modification. Firstly, its molecular structure was determined by powder X-ray diffraction, infrared spectroscopy, and elemental analysis, followed by a thermogravimetric determination to confirm its thermal and water stability. The N2 and water H2O adsorption abilities were also measured. Electrochemical experiments have shown 1 to be a typical temperature and humidity-dependent proton conductor with an optimum proton conductivity of 4.00 × 10−4 S/cm (98% RH/100 °C). A large number of uncoordinated carboxylate units and crystalline water molecules, as well as the H-bonding networks inside the framework, provide effective channels for proton transportation. The proton-conductive mechanism was further postulated following the calculated value of activation energy.

Modulation of defects in metal organic gels to enhance anhydrous proton conduction from subzero to moderate temperature

Exploitation of solid-state proton-conducting materials with high anhydrous proton conductivity from subzero temperature (<273 K) to moderate temperature (>353 K) is a great challenge. Here, Brönsted acid-dopped zirconium-organic xerogels (Zr/BTC-xerogels) are prepared for anhydrous proton conduction from subzero to moderate temperature. Abundant acid sites and strong H-bonding interactions make the CF3SO3H (TMSA)-introduced xerogel gain high proton conductivity from 9.0 × 10-4 S cm−1 (253 K) to 1.40 × 10-2 S cm−1 (363 K) under anhydrous conditions, which are in the leading level. This provides a new possibility to develop wide-operating-temperature conductors.