Proton-conducting Materials Research Articles

Metal-Organic Frameworks and Other Crystalline Materials for Ultrahigh Superprotonic Conductivities of 10-2 S cm-1 or Higher.

Proton-conducting materials in the solid state have received immense attention for their role as electrolytes in proton-exchange membrane fuel cells. Recently, crystalline materials-metal-organic frameworks (MOFs), hydrogen-bonded organic frameworks (HOFs), covalent organic frameworks (COFs), polyoxometalates (POMs), and porous organic crystals-have become an exciting research topic in the field of proton-conducting materials. For a better electrolyte, a high proton conductivity on the order of 10-2 S cm-1 or higher is preferred as efficient proton transport between the electrodes is ultimately necessary. With an emphasis on design principles, this Concept will focus on MOFs and other crystalline solid-based proton-conducting platforms that exhibit "ultrahigh superprotonic" conductivities with values in excess of 10-2 S cm-1 . While only a handful of MOFs exhibit such an ultrahigh conductivity, this quality in other systems is even rarer. In addition to interpreting the structural-functional correlation by taking advantage of their crystalline nature, we address the challenges and promising directions for future research.

A novel synthesis approach to partially fluorinated sulfonimide based poly (arylene ether sulfone) s for proton exchange membrane

Innovation of novel low cost proton conductive materials with super acidity has been the ever-increasing thirst for PEMFCs. Sulfonimide groups have the strongest gas-phase super-acidity with excellent thermal and chemical stability. Therefore, a series of partially fluorinated sulfonimide functionalized poly(arylene ether sulfone)s (SIPAES-xx) were successfully synthesized by chemical modification of sulfonated polyarylethersulfone (SPAES). The SPAESs were synthesized first by the direct polymerization of 4,4′ -dichlorodiphenylsulfone (DCDPS), 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS), and bisphenol. Thereafter, all arylsulfonic acid groups were converted into more acidic sulfonimide acid groups using partial fluorinated fluorosulfonyl imide monomer. The effect of the conversion of arylsulfonic acid function into sulfonimide was evaluated through thermal and chemical analysis. 1H-NMR, FTIR, TGA, FE SEM, and AFM were used to illustrate the structure, thermal and chemical properties of (SIPAES-xx) membranes. The membranes showed IEC values of 0.78–1.41 mequiv./g with 6.7–40.6% water uptake for 20–40% ionic groups. The synthesized SIPAES-40 membranes showed comparable proton conductivity to Nafion® 117 at same conditions. Nevertheless, the aromatic sulfonimide remained stable up to 370 °C. Furthermore, the presence of fluorine within the sulfonimide polymer provided high dimensional stability and oxidative durability by protecting the polymer chain from oxidizing radical species. Therefore, the synthesized SIPAES-xx membranes have the potential features as a proton exchange membrane (PEM) materials in the fuel cell.

High Proton Conductivity Achieved by Encapsulation of Imidazole Molecules into Proton-Conducting MOF-808.

Metal-organic frameworks (MOFs), as newly emerging materials, show compelling intrinsic structural features, e.g., the highly crystalline nature and designable and tunable porosity, as well as tailorable functionality, rendering them suitable for proton-conducting materials. The proton conduction of MOF is significantly improved using the postsynthesis or encapsulation strategy. In this work, the MOF-based proton-conducting material Im@MOF-808 has been prepared by incorporating the imidazole molecules into the pores of proton-conducting MOF-808. Compared with MOF-808, Im@MOF-808 not only possesses higher proton conductivity of 3.45 × 10-2 S cm-1 at 338 K and 99% RH, superior to that of any imidazole-encapsulated proton-conducting materials reported to date, but also good durable and stable proton conduction. Moreover, the thermal stability of H-bond networks is much improved owing to the water molecules partially replaced by higher boiling point imidazole molecules. Additionally, it is further discussed for the possible mechanism of imidazole encapsulation into the pores of MOF-808 to enhance proton conduction.

Supramolecular hydrogen-bonded organic networks through acid–base pairs as efficient proton-conducting electrolytes

The research of developing new proton-conducting materials via a simple and cost-effective method is vital in fuel cell technology.

Fabrication of protonated g-C3N4 nanosheets as promising proton conductive materials

Protonated g-C3N4 nanosheets were prepared by refluxing a mixture containing g-C3N4 and 95% H2SO4 at 80 °C for 1 hour. The obtained g-C3N4 nanosheets showed a similar layer structure to that of g-C3N4 and high water adsorption capacities. They also depicted superior conductivities reaching up to 1.44 × 10-2 S cm-1 at 80 °C and 98% RH, with promising features for proton conductivity materials.

Протонпроводящие композиционные материалы на основе соединения Cs-=SUB=-6-=/SUB=-H(HSO-=SUB=-4-=/SUB=-)-=SUB=-3-=/SUB=-(H-=SUB=-2-=/SUB=-PO-=SUB=-4-=/SUB=-)-=SUB=-4-=/SUB=-

Composite proton-conducting materials xCs6H (HSO4) 3 (H2PO4) 4+ (1 - x) AlPO4 were obtained in the composition range (x = 0.9–0.7). The methods of x-ray phase analysis, impedance spectroscopy, electron scanning microscopy studied their physicochemical and transport properties.

Graphene oxide and sulfonated-derivative: Proton transport properties and electrochemical behavior of Nafion-based nanocomposites

An organo sulfonated-derivative (sGO) of the graphene oxide (GO) was synthesized using a basically new approach. NMR investigations (13C-MAS, PFG, relaxation) highlighted the structural and composition changes of the carbon surfaces subsequent to GO functionalization, as well as especially the different capacity of hydration and retention that the two GO and sGO materials manifest.Two self-diffusion coefficients for the water inside the interlayer spaces were detected, spaced by an order of magnitude. sGO evidences higher ability to incorporate and retain water up to 130 °C than GO, whereby much higher proton conductivity, approximately 3 mS cm−1 at 90%RH, confirming how a proper organo-modification of the graphene platelets with highly hydrophilic groups can favor the proton transport.Such proton-conductive materials GO-based can be exploit as additives in membranes for applications PEMFCs. Homogeneous and exfoliated nanocomposite membranes based were successfully prepared by using Nafion ionomer. The organo-derivative sGO produces a considerable gain in proton conductivity, also at very low RH conditions, due to the formation of extremely connected path for protons, which is, instead, absent in Naf-GO composite, where the chemical reaction between epoxy and sulfonic groups, of filler and polymer respectively, subtracts a fraction of acid functionalities for the proton exchange.

A robust zirconium amino acid metal-organic framework for proton conduction

Proton conductive materials are of significant importance and highly desired for clean energy-related applications. Discovery of practical metal-organic frameworks (MOFs) with high proton conduction remains a challenge due to the use of toxic chemicals, inconvenient ligand preparation and complication of production at scale for the state-of-the-art candidates. Herein, we report a zirconium-MOF, MIP-202(Zr), constructed from natural α-amino acid showing a high and steady proton conductivity of 0.011 S cm−1 at 363 K and under 95% relative humidity. This MOF features a cost-effective, green and scalable preparation with a very high space-time yield above 7000 kg m−3 day−1. It exhibits a good chemical stability under various conditions, including solutions of wide pH range and boiling water. Finally, a comprehensive molecular simulation was carried out to shed light on the proton conduction mechanism. All together these features make MIP-202(Zr) one of the most promising candidates to approach the commercial benchmark Nafion.

Enhancing Proton Conductivity of a 3D Metal-Organic Framework by Attaching Guest NH3 Molecules.

By reaction of a newly designed organic ligand, [3-(naphthalene-1-carbonyl)-thioureido] acetic acid (C10H7C(O)NHC(S)NHCH2COOH; H3L), with Cu(OAc)2, a metal-organic framework [(CuI4CuII4L4)·3H2O] n (1) containing unique mixed-valence [CuI4Cu4IIL4] subunits has been successfully synthesized and structurally characterized. MOF 1 displays a three-dimensional open framework bearing one-dimensional channels. Consequently, a new derivative MOF [CuI4CuII4L4] n-NH3 (2) is procured upon exposure of 1 to NH3 vapors from 28 wt % aqueous NH3 solution, which bears 2 NH3 and 4 H2O molecules in accordance with the elemental and thermal analyses. Both 1 and 2 exhibit relatively high water stability, whose proton conduction properties under water vapor have been researched. Notably, 2 shows an ultrahigh proton conductivity of 1.13 × 10-2 S cm-1, which is 2 orders of magnitude larger than that of MOF 1 (4.90 × 10-4 S cm-1) under 100 °C and 98% RH. On the basis of the structural data, Ea values, H2O and ammonia vapor absorptions, and PXRD measurements, the proton transfer mechanisms were suggested. This is an efficient and convenient way to obtain suitable and highly proton-conducting materials by attaching NH3 molecules.

Loading Acid–Base Pairs into Periodic Mesoporous Organosilica for High Anhydrous Proton Conductivity over a Wide Operating Temperature Window

The design and fabrication of high proton conductors over a wide working temperature range are significant for the development of proton exchange membrane (PEM) fuel cells, as fuel cell vehicle technology requires solid electrolytes that should have the ability of conductivity at both high and subzero temperatures. Here, we demonstrated that biphenylene-bridged periodic-mesoporous-organosilica (PMO) and its sulfonated derivatives (s-PMO) are good hosts to load proton carrier imidazole for such sophisticated proton-conducting materials. With the acid–base pairs, the resulting material Im@s-PMO shows efficient proton conductivity over the temperature range from −40 °C (1.12 × 10–6 S/cm) to 180 °C (7.11 × 10–3 S/cm) under anhydrous conditions; the maximum value is significantly higher than that of Im@PMO (2.30 × 10–4 S/cm, 180 °C). The interaction between imidazole and the sulfonate group obviously affects the conductivity, which was confirmed by thermogravimetric analysis, MALDI-TOF mass spectrum, Fourier t...

Synthesis of Hybrid Materials Based on MF-4SK Perfluorinated Sulfonated Cation-Exchange Membranes Modified with Polyantimonic Acid and Characterization of Their Proton Conductivity

Proton-conducting materials based on the MF-4SK membrane and polyantimonic acid have been synthesized; their dielectric properties and conductivity at varying temperature and relative air humidity have been studied. The introduction of polyantimonic acid particles leads to a significant increase in the proton conductivity of the membranes—from 3.2 × 10–5 to 2.5 × 10–4 S/cm—at low humidity. Studies of the proton conductivity as a function of temperature have shown that the doping of the MF-4SK membrane with polyantimonic acid leads to a decrease in the activation energy from 16.8 to 12.9 kJ/mol.

Metal Atom Clusters as Building Blocks for Multifunctional Proton-Conducting Materials: Theoretical and Experimental Characterization.

The search for new multifunctional materials displaying proton-conducting properties is of paramount necessity for the development of electrochromic devices and supercapacitors as well as for energy conversion and storage. In the present study, proton conductivity is reported for the first time in three molybdenum cluster-based materials: (H)4[Mo6Br6S2(OH)6]-12H2O and (H)2[Mo6X8(OH)6]-12H2O (X = Cl, Br). We show that the self-assembling of the luminescent [Mo6L8i(OH)6a]2-/4- cluster units leads to both luminescence and proton conductivity (σ = 1.4 × 10-4 S·cm-1 in (H)2[Mo6Cl8(OH)6]-12H2O under wet conditions) in the three materials. The latter property results from the strong hydrogen-bond network that develops between the clusters and the water molecules and is magnified by the presence of protons that are statistically shared by apical hydroxyl groups between adjacent clusters. Their role in the proton conduction is highlighted at the molecular scale by ab initio molecular dynamics simulations that demonstrate that concerted proton transfers through the hydrogen-bond network are possible. Furthermore, thermogravimetric analysis also shows the ability of the compounds to accommodate more or less water molecules, which highlights that vehicular (or diffusion) transport probably occurs within the materials. An infrared fingerprint of the mobile protons is finally proposed based on both theoretical and experimental proofs. The present study relies on a synergic computational/experimental approach that can be extended to other proton-conducting materials. It thus paves the way to the design and understanding of new multifunctional proton-conducting materials displaying original and exciting properties.

Transport properties of highly dense proton-conducting BaCe0.8–xZrxDy0.2O3–δ materials in low- and high-temperature ranges

Proton-conducting materials constitute a class of oxide compounds possessing the required properties for application as electrolytes for low- and intermediate temperature solid oxide cells. In the present investigation, new highly dense BaCe0.8−xZrxDy0.2O3−δ ceramic materials (x = 0.2 … 0.6, Δx = 0.1) are successfully prepared and their electrochemical properties are thoroughly characterised. The separation of total conductivity in bulk and grain boundary components along with ionic and electronic contributions is performed using 2-probe AC and 4-probe DC conductivity measurements. The obtained results reveal that the bulk region determines the transport properties of the materials, starting from ∼190 °C for x = 0.2 and 470 °C for x = 0.6, whereas at lower temperatures the total conductivity is controlled by the grain boundaries. According to high-temperature measurements performed in air and hydrogen atmospheres with a wide water vapour partial pressure variation, the Zr-enriched samples (in comparison with the Ce-enriched ones) exhibit a higher contribution of electronic conductivity in oxidising atmospheres and a lower contribution of proton conductivity in reducing atmospheres. The negative effects of Zr for Ce substitution on the transport properties are compensated by their higher chemical stability, motivating the optimal composition exploration required for the specified electrochemical devices.

(Invited) Proton Conductivity Enhancement in Polymer Thin Films By Molecular Orientation and Organized Structure

Highly proton-conductive polymers have long attracted the attention of researchers for use in energy conversion, sensors, catalysts, and other applications. From the viewpoint of scientific history of creating highly proton-conductive polymers, one fundamental approach is based on the strategy of phase-segregated structures with strong acid groups.[1-3] Protons are considered to be transported through the hydrophilic parts. In the recent polymer research fields, taking advantage of the interface extends the molecular design such as the oriented structure and organized structure for the highly proton conductive materials. Matsui et al. showed anisotropic proton conductivity in Langmuir–Blodgett thin films with well-defined lamellar structures [4]. Proton conductivity is enhanced by the formation of 2D hydrogen-bonding networks in multilayer nanosheets [5, 6]. In this presentation, the author demonstrates a new approach to enhance the proton conductivity of the polymer thin films using an interface that can modify the degrees of freedom for a polymer structure through interaction between the substrate surface and polymers.[7-12] The interface can modify the degrees of freedom for polymer structures through interaction for functional groups, wettability, and surface charge between the substrate surface and polymers. Some proton-conductive polymers exhibit an oriented structure determined by an IR p-polarized multiple angle incidence resolution spectrometry (pMAIRS) [13]. Their oriented thin films show unique proton transport properties compared to those of non-oriented samples.

Conjugated Microporous Polymers with Dense Sulfonic Acid Groups as Efficient Proton Conductors.

Proton-exchange membrane fuel cells, emerging as green and sustainable energy sources, have attracted extensive attention in recent decades. Porous organic polymers, which feature in high surface area values, tunable pore sizes, excellent thermal and chemical stabilities, and the flexibility to incorporate specific functional groups, have recently displayed their striking images as potential electrolytes for fuel cells. In this work, BO-CMP-1 and BO-CMP-2 that possess rich π-structure and permanent porosity and have high thermal and chemical stability were synthesized through Suzuki-Miyaura coupling reaction. Owing to their rigid structures and abundant electrophilic substitution positions, these two novel porous polymers were covalently decorated with dense sulfonic acid groups by postsulfonation, as denoted by SBO-CMP-1 and SBO-CMP-2. The proton conductivity of SBO-CMPs is systematically studied to evaluate their performance as proton-conductive materials. It was found that their performance is highly humidity- and temperature-dependent and they show relatively high proton conductivity. For SBO-CMP-1 and SBO-CMP-2, their proton conductivities are 1.29 × 10-2 and 5.21 × 10-3 S cm-1, respectively, at 70 °C and 100% relative humidity. Low activation energy values of 0.32 eV for SBO-CMP-1 and 0.40 eV for SBO-CMP-2 suggest the Grotthuss mechanism for proton conduction.

Ceramic and Transport Characteristics of Electrolytes Based on Mg-Doped LaYO3

Effect of magnesium on the sinterability, phase composition, microstructure, and transport properties of proton-conducting materials of composition LaY1–xMgxO3–δ (х = 0, 0.05, 0.1) was studied. Ceramic samples were obtained by using the citrate-nitrate synthesis method at various sintering temperatures (1250–1400°C). It was shown that, for the samples with x = 0.05 and 0.1, the relative density was no less than 95% at a sintering temperature of 1350°C, whereas undoped lanthanum nitrate has this density at 1450°C. An X-ray diffraction analysis and scanning electron microscopy demonstrated that introduction of a small amount of magnesium (x = 0.05) is sufficient for forming the single-phase and high-dense ceramics. Electrical conductivity data show that the LaY0.95Mg0.05O3–δ sample has high overall and ionic conductivities.

Unique Proton Transportation Pathway in a Robust Inorganic Coordination Polymer Leading to Intrinsically High and Sustainable Anhydrous Proton Conductivity.

Although comprehensive progress has been made in the area of coordination polymer (CP)/metal-organic framework (MOF)-based proton-conducting materials over the past decade, searching for a CP/MOF with stable, intrinsic, high anhydrous proton conductivity that can be directly used as a practical electrolyte in an intermediate-temperature proton-exchange membrane fuel cell assembly for durable power generation remains a substantial challenge. Here, we introduce a new proton-conducting CP, (NH4)3[Zr(H2/3PO4)3] (ZrP), which consists of one-dimensional zirconium phosphate anionic chains and fully ordered charge-balancing NH4+ cations. X-ray crystallography, neutron powder diffraction, and variable-temperature solid-state NMR spectroscopy suggest that protons are disordered within an inherent hydrogen-bonded infinite chain of acid-base pairs (N-H···O-P), leading to a stable anhydrous proton conductivity of 1.45 × 10-3 S·cm-1 at 180 °C, one of the highest values among reported intermediate-temperature proton-conducting materials. First-principles and quantum molecular dynamics simulations were used to directly visualize the unique proton transport pathway involving very efficient proton exchange between NH4+ and phosphate pairs, which is distinct from the common guest encapsulation/dehydration/superprotonic transition mechanisms. ZrP as the electrolyte was further assembled into a H2/O2 fuel cell, which showed a record-high electrical power density of 12 mW·cm-2 at 180 °C among reported cells assembled from crystalline solid electrolytes, as well as a direct methanol fuel cell for the first time to demonstrate real applications. These cells were tested for over 15 h without notable power loss.

Performance of Metal-Supported Proton-Conducting Solid Oxide Fuel Cells By Reactive Spray Deposition Technology

In order to make solid oxide fuel cells (SOFCs) more economically feasible, researchers are studying how to reduce the cost to produce these fuel cells. One method that researchers are using to reduce that production cost is to reduce the operating temperature of the fuel cell from ~1000°C to 600°C. Producing SOFCs with proton-conducting materials leads to many advantages that can make the overall device more economical. One major benefit of using these materials is the ability to operate the fuel cell at lower temperatures with no significant negative impact on conductivity as a result of the perovskite structure of these materials[1]. By using the lower operating temperature, it is possible to utilize more inexpensive materials for the internal components of the SOFC. Rather than using ceramic supports or high temperature alloy materials which can increase the cost of the SOFC, it is possible to use more inexpensive steels as a support for the cell. With the lower operating temperature, these materials would not degrade as fast as in traditional SOFCs and will allow for a longer lifespan and less degradation of the fuel cell. In order to further decrease the cost of the cell production, it is also possible to incorporate internal reforming which can allow the use of methane as a fuel rather than hydrogen at the lower operating temperatures. In this work, we will examine the use of Reactive Spray Deposition Technology (RSDT) as a method to produce these metal-supported proton-conducting solid oxide fuel cells. Use of the RSDT process in the production of low-temperature SOFCs has been well-documented, as previous studies show that the RSDT was able to deposit porous electrodes as well as dense electrolytes with satisfactory electrochemical performance [2]. The RSDT process allows for a direct deposition of the desired electrode and electrolyte materials onto a stainless steel or Crofer 22 APU® support and subsequent layers. In an effort to prevent oxidation of the metal supports, previous work has examined using diffusion blocking layers to allow for to use of Crofer 22 APU® in an RSDT application [3]. This work will use the proton conducting material Ba(Zr0.4Ce0.4Y0.1Yb0.1)O3 (BZCYYb4411) as the electrolyte material. Previous studies using BZCYYb have shown excellent ionic conductivity below 750°C [4]. Data from Yang et al. shows that the ionic conductivity of BZCYYb is significantly greater than the conductivity of yttria-stabilized zirconia, a traditional oxygen anion conducting SOFC material, at the same temperature [4]. That BZCYYb electrolyte is deposited on a Ni-BZCYYb cermet anode with an La0.6Sr0.4Co0.2Fe0.8O3 (LSCF6428) cathode. Following each deposition, the deposited layers are examined using scanning electron microscopy to check that each RSDT-deposited layer has the desired morphology, as shown in Figure 1. This work examines those cells and demonstrates the electrochemical performance given by the RSDT-produced metal-supported proton-conducting SOFCs.

Open-Framework Chalcogenide Showing Both Intrinsic Anhydrous and Water-Assisted High Proton Conductivity.

Proton-conducting materials have attracted increasing interest because of the promising technological applications as key components in various electrochemical devices. It is of great significance for technique application to seek superior proton-conducting materials, operating under both anhydrous and humidified conditions in a wide temperature range. Herein we demonstrate the proton conductance of an open-framework chalcogenide, (CH3NH3)2Ag4Sn3S8 (1), and the postsynthesis product 2 achieved by doping hydrochloric acid into 1. Hybrid 2 displays both intrinsic anhydrous and water-assisted high proton conductance, with σ = 1.87 × 10-4 S·cm-1 at 463 K under N2 atmosphere and 1.14 × 10-3 S·cm-1 at 340 K and 99% relative humidity, and these conductivities are comparable to that in the efficient metal-organic frameworks-based proton-conducting materials. Moreover, hybrid 2 shows excellent thermal stability and long-term stability of proton conduction.

Switching the Proton Conduction in Nanoporous, Crystalline Materials by Light.

Proton conducting nanoporous materials attract substantial attention with respect to applications in fuel cells, supercapacitors, chemical sensors, and information processing devices inspired by biological systems. Here, a crystalline, nanoporous material which offers dynamic remote-control over the proton conduction is presented. This is realized by using surface-mounted metal-organic frameworks (SURMOFs) with azobenzene side groups that can undergo light-induced reversible isomerization between the stable trans and cis states. The trans-cis photoisomerization results in the modulation of the interaction between MOF and guest molecules, 1,4-butanediol and 1,2,3-triazole; enabling the switching between the states with significantly increased (trans) and reduced (cis) conductivity. Quantum chemical calculations show that the trans-to-cis isomerization results in the formation of stronger hydrogen bridges of the guest molecules with the azo groups, causing stronger bonding of the guest molecules and, as a result, smaller proton conductivity. It is foreseen that photoswitchable proton-conducting materials may find its application in advanced, remote-controllable chemical sensors, and a variety of devices based on the conductivity of protons or other charged molecules, which can be interfaced with biological systems.