Intrinsic Proton Conductivity Research Articles

Conjugated acid-base pairs in Keggin-type polyoxometalate-based metal-organic frameworks enhance proton conduction

To achieve high proton conductivities, it is pivotal to design and select appropriate functional organic ligands for constructing polyoxometalate-based metal–organic frameworks (POMOFs). In this work, the effects of functional groups upon the molecular electrostatic potentials and pKa of two organic ligands (4-abpt, theoretical pKa value of 25.13; Hdpt, theoretical pKa value of 9.24) are first investigated by theoretical calculations. And then, two new proton-conductive decorated POMOFs, {[Cu(4-abpt)2(H2O)][Cu(4-abpt)(H2O)][SiW12O40]}·6H2O (CUST-875) and {[Cu(Hdpt)(H2O)2] [Cu(Hdpt)][SiW12O40]}·2H2O (CUST-876), have been designed and obtained via hydrothermal method. Besides, their proton conduction performances and influencing factors also have been investigated in more details. It is found that more water molecules adsorption and the conjugated acid-base pairs played substantial roles in proton conduction. As a consequence, the three-dimensional framework CUST-875, composed of the higher theoretical pKa value organic ligand 4-abpt, exhibited a higher proton conductivity of 1.07 × 10−4 S cm−1 at 95 °C and 98 % relative humidity (RH), surpassing that of CUST-876 by three orders of magnitude. Additionally, CUST-875 possesses the ability to degrade H2O2, which readily generates during charging and discharging of battery and has adverse effects on the battery life. This work contributes new insights into the high efficiency preparation and structure–property relationships of new polyoxometalate (POM)-based crystalline proton conducting materials with relatively high intrinsic proton conductivities.

Two birds with one stone: A ferrocene-based zirconium(IV)-organic framework nanosheet denoting intrinsic high proton conductivity and acting as a fine dopant to chitosan-based composite membranes

Recently, it was demonstrated that employing metal-organic frameworks (MOFs) with prominent proton conductivity (σ) as fillers with organic substrates such as chitosan (CS) or Nafion is an effective approach for preparing composite membranes (CMs) with outstanding functionalities. Inspired by this, one extremely stable Zr(IV)-MOF (namely Zr-FDC) with a nanosheet structure produced by 1,1'-ferrocene dicarboxylic acid (H2FDA) was successfully manufactured in this research and subsequently used as a filler to synthesize a series of CS-based CMs via casting method. The alternating current (AC) impedance determinations manifested that both Zr-FDC and the related CS-based CMs (CS/MOF-x; x = 2, 4, 6, 8 being the mass percentage of Zr-MOF in the CM) showed ultrahigh σ values, indicating the structural advantages of the Zr-MOF. Further research verified that when the MOF doping quantity is 4%, the CM's (CS/MOF-4) σ is the greatest, being 2.22 × 10-2 S/cm, which is boosted by nearly tenfold at 100 °C and 98% relative humidity (RH) compared to the original MOF (3.2 × 10-3 S/cm). Furthermore, the CM exhibits superior thermal stability and tensile resistance. Finally, considering the structural features of the MOF and CS, activation energy data, and other determinations, we thoroughly theorized the proton conduction process within the MOF framework and CMs, referencing the subsequent proton exchange membrane design.

High intrinsic proton conduction in one three-dimensional indium(III)-organic framework built by amino terephthalic acid

It is imperative to carry out more research on the proton conduction in metal-organic frameworks (MOFs) made of main group metals. Inspired by this, we synthesized a 3D indium-based MOF, [In(BDC-NH3)(BDC-NH2)]∙1.5DMF∙1.5H2O (namely In-MOF 1), which is made up of [In(CO2)4] metal nodes and 2-amino terephthalic acid (H2BDC-NH2) organic linkers. Meanwhile, organic ligand functionalization has been proven to be one of the effective techniques to boost the proton conductivity of the corresponding metal-organic frameworks. In light of this, the functionalized MOF was successfully synthesized using In(III) ions and H2BDC-NH2, which not only has excellent water stability but also bear some hydrophilic groups such as –NH2 and carboxylate units, which may act as a pivotal role in improving the proton conductivity (σ). Consequently, the σ of In-MOF 1 was thoroughly examined. As expected, its optimal σ can reach 0.81 × 10−2 S cm−1 under 100 °C and 98 % relative humidity (RH), substantially higher than the In-MOF composed only of terephthalic acid. Finally, we estimated its activation energy (Ea) at 98 % and 68 % RHs using the Arrhenius equation, and the proton transport mechanism was further explored.

A novel comb-shaped polybenzimidazoles crosslinked sulfonic acid functionalized polyphosphazenes high temperature proton exchange membrane

Designing high temperature proton exchange membranes (HTPEMs) with high H+ conductivity and long durability has been challenging. Here, a comb-shaped polybenzimidazoles (CbPBI) and crosslinkable polyoxy (benzenesulfonic acid) phazenes (PBSP) proton conductors are prepared, then they are used to fabricate a series of PBSP-CbPBI membranes with good comprehensive performances. CbPBI provides high flexibility and large free volume, which facilitates the construction of proton transport channels. And PBSP is the bifunctional proton conductor and crosslinker that introduces abundant sulfonic acid groups while retaining the crosslinkable property. By strongly chemical binding with CbPBI, the basicity of the imidazole ring is enhanced, avoiding the leaching of PBSP, thus enabling the overall crosslinked membrane to exhibit a durable intrinsic proton conductivity. Benefiting from the construction of the dual crosslinked network (covalent and ionic crosslinking), the oxidative stability, dimensional stability, and fuel-blocking properties of the PBSP-CbPBI membranes perform well. The proton conductivity of PBSP(50)-CbPBI membrane reaches 0.163, 0.084, 0.057, and 0.043 S/cm, respectively, at 180 °C and different relative humidity (100 % RH, 50 % RH, 30 % RH, and 0 RH). Enduring hot-washing for 96 h, the less decay of membrane in conductivity and weight demonstrates good durability. The design strategy such as the construction of monolithic crosslinked macromolecular structures in PBSP-CbPBI membranes shows wide application prospects in HTPEMs.

Prominent Intrinsic Proton Conduction in Two Robust Zr/Hf Metal-Organic Frameworks Assembled by Bithiophene Dicarboxylate.

To date, developing crystalline proton-conductive metal-organic frameworks (MOFs) with an inherent excellent proton-conducting ability and structural stability has been a critical priority in addressing the technologies required for sustainable development and energy storage. Bearing this in mind, a multifunctional organic ligand, 3,4-dimethylthiophene[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), was employed to generate two exceptionally stable three-dimensional porous Zr/Hf MOFs, [Zr6O4(OH)4(DTD)6]·5DMF·H2O (Zr-DTD) and [Hf6O4(OH)4(DTD)6]·4DMF·H2O (Hf-DTD), using solvothermal means. The presence of Zr6 or Hf6 nodes, strong Zr/Hf-O bonds, the electrical influence of the methyl group, and the steric effect of the thiophene unit all contribute to their structural stability throughout a wide pH range as well as in water. Their proton conductivity was fully examined at various relative humidities (RHs) and temperatures. Creating intricate and rich H-bonded networks between the guest water molecules, coordination solvent molecules, thiophene-S, -COOH, and -OH units within the framework assisted proton transfer. As a result, both MOFs manifest the maximum proton conductivity of 0.67 × 10-2 and 4.85 × 10-3 S·cm-1 under 98% RH/100 °C, making them the top-performing proton-conductive Zr/Hf-MOFs. Finally, by combining structural characteristics and activation energies, potential proton conduction pathways for the two MOFs were identified.

Achieving High Proton Conductivity in MIL-91(Al) Aerogel.

The processability and sustainability of proton conductors are two important indicators of their application. Here, MIL-91(Al) with an intrinsic proton conduction framework originating from protonated phosphonate groups was cross-linked with poly(vinyl alcohol) (PVA) to obtain MIL-91(Al) aerogel through freeze-drying. This simple and inexpensive strategy not only facilitated the processing of MIL-91(Al) powder but also resulted in a molded MIL-91(Al) aerogel having a high proton conductivity of 1.02 × 10-2 S cm-1 at 70 °C and 100% relative humidity. Furthermore, MIL-91(Al) aerogel was recyclable and reusable, in line with the principles of environmental protection and sustainability. To the best of our knowledge, this is the first example of using a metal-organic framework aerogel as a proton conductor, which may develop a new model system in this field.

Proton pathways via free volumes: A positron annihilation lifetime spectroscopy (PALS) investigation of proton conductivity in SPEEK-PEG-TMOS composites

This study investigates the ionic conductivity and dielectric properties of SPEEK-PEG-TMOS solid-state polymer electrolytes across a broad frequency spectrum (0.1–1000 kHz). Proton transport mechanisms were explored over an extensive temperature range, and positron annihilation lifetime spectroscopy (PALS) was employed to elucidate the roles of free volumes in the transport mechanism. At elevated temperatures, sulfonic acid groups and free volume defects predominantly govern the conductivity in all polymer electrolytes. PEG incorporation significantly improved proton conductivity at low temperatures, while the presence of TMOS and higher PEG content have elevated conductivity at higher temperatures. Proton conductivity mechanisms at elevated temperatures are influenced by SPEEK's intrinsic proton conductivity, PEG's plasticizing effect, acid-base interactions, and the presence of free volumes. PALS analysis reveals temperature-dependent trends in the o-Ps lifetime, intensity, and free volume fraction, increasing linearly with temperature due to thermal expansion. PEG content modulated this trend, with lower PEG content samples displaying lower thermal expansion slopes, and higher PEG content samples exhibiting more substantial increases in hole-free volume. The o-Ps intensity displayed a temperature-dependent change attributed to PEG's thermal expansion limitations. These insights contributed to a comprehensive understanding of the intricate interplay between PEG content, thermal expansion, and free volume in SPEEK-PEG-TMOS composites. The study has implications for material science applications, particularly in the development of high-temperature, anhydrous proton-conductive systems.

Ligand-Functionalized MIL-68-type Indium(III) Metal-Organic Frameworks with Prominent Intrinsic Proton Conductivity.

Indium-based metal-organic frameworks (In-MOFs) have now become an attractive class of porous solids in materials science and electrochemistry due to their diverse structures and promising applications. In the field of proton conduction, to find more crystalline MOFs with splendid proton-conductive properties, herein, five three-dimensional isostructural In-MOFs, MIL-68-In or MIL-68-In-X (X = NH2, OH, Br, or NO2) using terephthalic acid (H2BDC) or functionalized terephthalic acids (H2BDC-X) as multifunctional linkages were efficiently fabricated. First, the outstanding structural stability of the five MOFs, including thermal and water stability, was verified by thermal analysis and powder X-ray diffraction. Subsequently, the H2O-mediated proton conductivities (σ) were fully assessed and compared. Notably, their σ evinced a significant positive correlation between the temperature or relative humidity (RH) and varied with the functional groups on the organic ligands. Impressively, their highest σ values are up to 10-3-10-4 S/cm (100 °C/98% RH) and change in this order: MIL-68-In-OH (1.72 × 10-3 S/cm) > MIL-68-In-NH2 (1.70 × 10-3 S/cm) > MIL-68-In-NO2 (4.47 × 10-4 S/cm) > MIL-68-In-Br (4.11 × 10-4 S/cm) > MIL-68-In (2.37 × 10-4 S/cm). Finally, the computed activation energy values under 98 or 68% RHs are assessed, and the related proton conduction mechanisms are speculated. Moreover, after electrochemical testing, these MOFs illustrate remarkable structural rigidity, laying a meritorious material foundation for future applications.

High intrinsic proton conduction in cerium(IV) metal-organic framework constructed by bromine-grafted terephthalic acid

Organic linkage functionalization has been demonstrated to be an efficient approach for improving the proton conductivity of metal-organic frameworks (MOFs). Herein, we successfully synthesized a functionalized MOF, UiO-66-Br2-Ce, employing cerium(IV) ion and 2, 5-dibromoterephthalic acid (H2BDC-Br2) ligand. Since high electronegativity and hydrophilicity bromine may play a major role in boosting the proton conduction efficiency of MOFs, we thoroughly investigated the proton conductivity of this three-dimensional porous MOF. As anticipated, its optimal proton conductivity can attain 0.61 × 10−3 S cm−1 (100 °C/98% relative humidity), indicating a noteworthy enhancement in comparison to UiO-66-Ce composed only of terephthalic acid. It is also worth noting that the MOF manifested great water resistance, acid and alkali resistance, and framework stiffness during electrochemical tests, denoting exceptional structural stability. Finally, a proton-conducting mechanism based on activation energy and structural analysis was proposed.

Achieving High Intrinsic Proton Conductivity via Functionalization of MOF-808 by Ionic Liquid

Achieving High Intrinsic Proton Conductivity via Functionalization of MOF-808 by Ionic Liquid

Proton Conduction Properties of Intrinsically Sulfonated Covalent Organic Framework Composites

The long–term stability of proton conductors is one of the most important factors in evaluating materials. Guest molecules can act as “bridges” for proton conduction channels and reside in the channels of covalent organic frameworks, but they are prone to leakage. Therefore, it is important to develop proton conductors with intrinsic proton conductivity. In this paper, we synthesized an intrinsically sulfonated covalent organic framework, TpPa–SO3H, which has a more stable proton conducting performance than that of TpPa@H2SO4 by loading guest molecules. Meanwhile, the proton conductivity of TpPa–SO3H was further improved by coating a superabsorbent polymer through an in situ reaction to obtain PANa@TpPa–SO3H (PANa: sodium polyacrylate). As a result, the modified composite exhibits an ultrahigh proton conductivity of 2.33 × 10−1 S cm−1 at 80 °C under 95% relative humidity (RH). The stability of PANa@TpPa–SO3H makes it an efficient proton transport platform with excellent proton conductivity and long–term durability.

Design, Preparation, and High Intrinsic Proton Conductivity of Two Highly Stable Hydrazone-Linked Covalent Organic Frameworks.

Assembling crystalline materials with high stability and high proton conductivity as a potential alternative to the Nafion membrane is a challenging topic in the field of energy materials. Herein, we concentrated on the creation and preparation of hydrazone-linked COFs with super-high stability to explore their proton conduction. Fortunately, two hydrazone-linked COFs, TpBth and TaBth, were solvothermally prepared by using benzene-1,3,5-tricarbohydrazide (Bth), 2,4,6-trihydroxy-benzene-1,3,5-tricarbaldehyde (Tp), and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (Ta) as monomers. Their structures were simulated by Material Studio 8.0 software and confirmed by the PXRD pattern, demonstrating a two-dimensional framework with AA packing. The presence of a large number of carbonyl groups as well as -NH-NH2- groups on the backbone is responsible for their super-high water stability as well as high water absorption capacity. AC impedance tests demonstrated a positive correlation between the water-assisted proton conductivity (σ) of the two COFs and the temperature and humidity. Under 100 °C/98% RH, the highest σ values of TpBth and TaBth can reach 2.11 × 10-4 and 0.62 × 10-5 S·cm-1, which are among the high σ values of the reported COFs. Their proton-conductive mechanisms were highlighted by structural analyses as well as N2 and H2O vapor adsorption data and activation energy values. Our systematic research affords ideas for the synthesis of proton-conducting COFs with high σ values.

Polydopamine-coated polyimide nanofiber to anchor HPW and construct the pocket-like composite membrane with excellent proton conductivity and stability

Solid heteropolyacids with high intrinsic proton conductivity, like phosphotungstic acid (HPW), were added to further enhance the proton conductivity of sulfonated poly ether ketone (SPEEK). The great solubility of solid acid within water, however, would produce a dramatic decline in stability and durability. In this study, we utilized polydopamine (PDA) coated polyimide (PI) nanofiber to form the acid-base pair between PDA@PI nanofiber and HPW/SPEEK to anchor the HPW. Additionally, a pocket was constructed in the sandwich-structure PDA@PI/SPEEK + HPW membrane to help better anchor the HPW. After 6 weeks of durability treatment during 60 °C and 100% RH, proton conductivity of PDA@PI/SPEEK + HPW-20 was 163.5 mS cm−1, which was only 23.0% lower than that of the initial state. By sharp contrast, it was 55.2% lower than the initial state for pristine SPEEK. And its swelling ratio is only 54% compared with 128% of pristine SPEEK under the same condition. We first revealed and investigated the acid-base pair created by PDA@PI and HPW/SPEEK, which was beneficial for proton conductivity and durability. This pocket-like PDA@PI/SPEEK + HPW membrane would inspire a wide application in PEM among excellent proton conductivity, better stability, with endurance.

The Role of Superadsorbent Polymers on Covalent Organic Frameworks‐Based Solid Electrolytes: Investigation of the Ionic Conductivity and Relaxation

AbstractThe scarcity of fossil fuels calls for immediate action toward the development of clean and renewable energy resources. In this context, proton exchange membrane fuel cells (PEMFCs) are gaining ever‐increasing attention as clean technology. Although covalent organic frameworks (COFs) do not usually exhibit high intrinsic proton conductivity (σ), they have been recently proposed as solid polymer electrolytes in PEMFCs, thanks to their high crystallinity and stability to acids and bases. Here, a simple strategy is presented to improve the performance of poor COF‐based proton conductors through addition of sodium polyacrylate (PANa) superadsorbent polymer. Electrochemical impedance spectroscopy investigations after activation at high temperature and relative humidity (RH) provide insights into the role of PANa, whose presence is key to preserve high σ at low RH. The humidity‐dependent X‐ray diffraction study reveals a strengthening of the stacking interaction along the COF (100) plane direction with increasing humidity, through the formation of H‐bonding, thus promoting proton hopping. The study of the dielectric properties as a function of PANa content enables to determine a Debye relaxation regime for the COF/PANa blend with a maximum relaxation frequency of 1513 and 6606 Hz for the pristine COF and the COF/PANa blend, respectively, at their maximum operating temperatures.

Sulfonic-Pendent Vinylene-Linked Covalent Organic Frameworks Enabling Benchmark Potential in Advanced Energy.

Both proton exchange membrane fuel cells and uranium-based nuclear techniques represent two green and advanced energies. However, both of them still face some intractable scientific and industrial problems. For the former, established proton-conduction materials always suffer one or another defect such as low proton conductivity, high activation energy, bad durability, or just small-scale product; while for the later, there still lacks available adsorbent to selectively recover of UO2 2+ from concentrated nitric acid (>1M) during the spent fuel reprocessing due to the deactivation of the adsorption site or the decomposition of adsorbent under such rigorous conditions. It is found that the above two issues can be well solved by the construction of sulfonic-pendent vinylene-linked covalent organic frameworks (COFs), since these COFs contain abundant sulfonic units for both intrinsic proton conduction and UO2 2+ capture through strong coordination fixation and vinylene linkage that enhances the stability up to 12M nitric acid (one of the best materials surviving in 12M HNO3 ).

High-performance sandwich-structure PI/SPEEK+HPW nanofiber composite membrane with balanced proton conductivity and stability

Due to its uncomplicated manufacturing procedure for large-scale industrialization and adaptable conductivity, the competitive proton exchange membrane (PEM) matrix employed is sulfonated poly (ether ether ketone). However, the conflict between the strong proton conductivity and high stability of SPEEK-based PEM constantly upsets the equilibrium; the primary chain of SPEEK will conduct more protons if additional sulfonic acid is introduced, but it also causes a high swelling ratio and lowers stability. To tackle this challenging issue, we coupled the high intrinsic proton conductivity of phosphotungstic acid (HPW) and the excellent stability of polyimide (PI) nanofibers in a single PEM. In particular, sandwich-structure hybrid membranes (PI/SPEEK + HPW) were created to reduce HPW loss under operational conditions, further improving stability. Consequently, the swelling ratio and conductivity of the premium sandwich-structure PI/SPEEK + HPW-20 were 29% and 223.4 mS cm−1 under 60 °C and 100% RH, their respective values were 8% larger than those of SPEEK. Its proton conductivity reached 170.9 mS cm−1 during 13 days of exposure under 60 °C and 100% RH, which would be 30% better than that of SPEEK over the same treatment. Protons in these membranes were transported through Grotthus and vehicle mechanism, and the establishment of an acid-rich layer at the surface among PI nanofiber with HPW/SPEEK, which was confirmed by FTIR, XPS, and TEM, is responsible for these superior performances. From our point of view, our findings might encourage more studies into the creation of high-performance PEM.

Pt‐Anchored‐Zirconium Phosphate Nanoplates as High‐Durable Carbon‐Free Oxygen Reduction Reaction Electrocatalyst for PEM Fuel Cell Applications

AbstractCommercially available platinum‐supported carbon (Pt/C) catalysts are the most widely used oxygen reduction reaction (ORR) electrocatalysts in polymer electrolyte membrane fuel cells (PEMFCs). However, inadequate active triple‐phase boundary formation and carbon oxidation in Pt/C during PEMFC operation shorten its lifetime and efficiency. In this direction, a new class of carbon‐free electrocatalysts for ORR is prepared by dispersing Pt nanoparticles on ZrP (Zirconium phosphates) nanoplates. In one case (ZrP@Pt), the Pt nanoparticles are found to be closely distributed and completely covering the ZrP nanoplates, whereas in the second case (Pt/ZrP), the Pt nanoparticles selectively restrict dispersion along the edges of the support. ZrP as the support displays an intrinsic proton conductivity of ≈0.5 × 10−4 S cm−1 at 70 °C, with an activation energy (Ea) of 0.19 eV. Pt/ZrP shows better durability after 3000 start‐stop cycles. The mass activity of Pt/ZrP is increased by 4.6 times compared to Pt/C, which exhibits a loss in mass activity by 1.37 times. The single‐cell level validation of ZrP@Pt, Pt/ZrP, and Pt/C as the electrocatalysts in PEMFC at an operating potential of 0.60 V shows the achievable current densities of 0.600, 0.890, and 0.890 A cm−2.

High proton conduction in two highly stable phenyl imidazole dicarboxylate-based Cd(II)-MOFs

In recent years, carboxylic acid-based MOFs have gathered a lot of attention because of their high stability and excellent proton conductivity, and are likely to be promising candidates as proton-exchange membranes. In this work, we synthesized two MOFs by reference method, namely [Cd4(μ3-PhIDC)2(μ4-PhIDC)2(H2O)]n (1) (H2PhIDC ​= ​2-phenyl-4,5-imidazole dicarboxylic acid), [Cd2(μ2-PhIDC)2(phen)2]n (2) (phen ​= ​1,10-phenanthroline), which have high water stability. Therefore, this work focuses on their intrinsic proton conductivity. The results showed that under the conditions of 98% RH/100 ​°C, the proton conductivities of 1 and 2 can achieve 2.71 ​× ​10−3 and 2.67 ​× ​10−3 ​S ​cm−1, respectively. Finally, based on the structural analysis and calculations of activation energies, the principle of proton transfer is inferred, which will afford new inspiration for the design of the proton conduction crystalline materials.

Covalent organic frameworks confining ultra-dense hydrated hydrogen-bond networks for efficient intrinsic proton conduction

Preinstalling proton donor groups in covalent organic frameworks to confine dense hydrated hydrogen-bond networks enables efficient intrinsic proton conduction.

Isotropic Anhydrous Superprotonic Conductivity Cooperated with Installed Imidazolium Molecular Motions in a 3D Hydrogen‐Bonded Phosphate Network

AbstractUtilizing molecular motion is essential for the use of anhydrous superprotonic molecular proton conductors (σ beyond 10−4 S cm−1) as electrolytes in hydrogen fuel cells. However, molecular motion contributing to the improvement of intrinsic proton conduction has been limited and little clarified in relation to the proton conduction mechanism, limiting the development of material design guidelines. Here, a salt with a three‐dimensional (3D) hydrogen‐bonded (H‐bonded) phosphate network with imidazolium cations installed inside was studied, whose components are known to exhibit molecular motions that contribute to proton conduction. Despite its anisotropic H‐bonded network, the salt exhibits isotropic anhydrous superprotonic conductivity exceeding 10−3 S cm−1 at ≈351 K, which is the first example for organic molecular crystal. Variable‐temperature X‐ray structural analysis and solid‐state 2H NMR measurements revealed significant 3D molecular motion of imidazolium cations, which accelerate proton conduction via the 3D H‐bonded phosphate network.