Ion-Specific Effects on Ion and Polyelectrolyte Solvation

Ion-specific effects on aqueous solvation of monovalent counter ions, Na+ ${^+ }$ , K+ ${^+ }$ , Cl- ${^- }$ , and Br- ${^- }$ , and two model polyelectrolytes (PEs), poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA) were here studied with ab initio molecular dynamics (AIMD) and classical molecular dynamics (MD) simulations based on the OPLS-aa force-field which is an empirical fixed point-charge force-field. Ion-specific binding to the PE charge groups was also characterized. Both computational methods predict similar response for the solvation of the PEs but differ notably in description of ion solvation. Notably, AIMD captures the experimentally observed differences in Cl- ${^- }$ and Br- ${^- }$ anion solvation and binding with the PEs, while the classical MD simulations fail to differentiate the ion species response. Furthermore, the findings show that combining AIMD with the computationally less costly classical MD simulations allows benefiting from both the increased accuracy and statistics reach.

Ultrafast Proton Conduction in an Aqueous Electrolyte Confined in Adamantane-like Micropores of a Sulfonated, Aromatic Framework

Sulfonated, cross-linked porous polymers are promising frameworks for aqueous high-performance electrolyte-host systems for electrochemical energy storage and conversion. The systems offer high proton conductivities, excellent chemical and mechanical stabilities, and straightforward water management. However, little is known about mass transport mechanisms in such nanostructured hosts. We report on the synthesis and postsynthetic sulfonation of an aromatic framework (SPAF-2) with a 3D-interconnected nanoporosity and varying sulfonation degrees. Water adsorption produces the system SPAF-2H20. It features proton exchange capacities up to 6 mequiv g-1 and exceptional proton conductivities of about 1 S cm-1. Two contributions are essential for the highly efficient transport. First, the nanometer-sized pores link the charge transport to the diffusion of adsorbed water molecules, which is almost as fast as bulk water. Second, continuous exchange between interface-bound and mobile species enhances the conductivities at elevated temperatures. SPAF-2H20 showcases how to tailor nanostructured electrolyte-host systems with liquid-like conductivities.

A Pair of Multifunctional Cu(II)-Dy(III) Enantiomers with Zero-Field Single-Molecule Magnet Behaviors, Proton Conduction Properties and Magneto-Optical Faraday Effects

Multifunctional materials with a coexistence of proton conduction properties, single-molecule magnet (SMM) behaviors and magneto-optical Faraday effects have rarely been reported. Herein, a new pair of Cu(II)-Dy(III) enantiomers, [DyCu2(RR/SS-H2L)2(H2O)4(NO3)2]·(NO3)·(H2O) (R-1 and S-1) (H4L = [RR/SS] -N,N'-bis [3-hydroxysalicylidene] -1,2-cyclohexanediamine), has been designed and prepared using homochiral Schiff-base ligands. R-1 and S-1 contain linear Cu(II)-Dy(III)-Cu(II) trinuclear units and possess 1D stacking channels within their supramolecular networks. R-1 and S-1 display chiral optical activity and strong magneto-optical Faraday effects. Moreover, R-1 shows a zero-field SMM behavior. In addition, R-1 demonstrates humidity- and temperature-dependent proton conductivity with optimal values of 1.34 × 10-4 S·cm-1 under 50 °C and 98% relative humidity (RH), which is related to a 1D extended H-bonded chain constructed by water molecules, nitrate and phenol groups of the RR-H2L ligand.

Proton conductivity dependence on the surface polymer thickness of core-shell type nanoparticles in a proton exchange membrane

The proton exchange membrane (PEM) is the main component that determines the performance of polymer electrolyte fuel cells. The construction of proton-conduction channels capable of fast proton conduction is an important topic in PEM research. In this study, we have developed poly(vinylphosphonic acid)-block-polystyrene (PVPA-b-PS)-coated core-shell type silica nanoparticles prepared by in situ polymerization and a core-shell type nanoparticle-filled PEM. In this system, two-dimensional (2D) proton-conduction channels have been constructed between PVPA and the surface of silica nanoparticles, and three-dimensional proton-conduction channels were constructed by connecting these 2D channels by filling with the core-shell type nanoparticles. The proton conductivities and activation energies of pelletized PVPA-coated core-shell type nanoparticles increased depending on the coated PVPA thickness. Additionally, pelletized PVPA-b-PS-coated silica nanoparticles showed a good proton conductivity of 1.3 × 10^-2 S cm^-1 at 80 °C and 95% RH. Also, the membrane state achieved 1.8 × 10^-4 S cm^-1 in a similar temperature and humidity environment. Although these proton conductivities were lower than those of PVPA, they have advantages such as low activation energy for proton conduction, suppression of swelling due to water absorption, and the ability to handle samples in powder form. Moreover, by using PS simultaneously, we succeeded in improving the stability of proton conductivity against changes in the temperature and humidity environment. Therefore, we have demonstrated a highly durable, tough but still enough high proton conductive material by polymer coating onto the surface of nanoparticles and also succeeded in constructing proton-conduction channels through the easy integration of core-shell type nanoparticles.

Acid-in-Clay Electrolyte for Wide-Temperature-Range and Long-Cycle Proton Batteries

Proton conduction underlies many important electrochemical technologies. A family of new proton electrolytes is reported: acid-in-clay electrolyte (AiCE) prepared by integrating fast proton carriers in a natural phyllosilicate clay network, which can be made into thin-film (tens of micrometers) fluid-impervious membranes. The chosen example systems (sepiolite-phosphoric acid) rank top among the solid proton conductors in terms of proton conductivities (15 mS cm^-1 at 25 °C, 0.023 mS cm^-1 at -82 °C), electrochemical stability window (3.35 V), and reduced chemical reactivity. A proton battery is assembled using AiCE as the solid electrolyte membrane. Benefitting from the wider electrochemical stability window, reduced corrosivity, and excellent ionic selectivity of AiCE, the two main problems (gassing and cyclability) of proton batteries are successfully solved. This work draws attention to the element cross-over problem in proton batteries and the generic "acid-in-clay" solid electrolyte approach with superfast proton transport, outstanding selectivity, and improved stability for room- to cryogenic-temperature protonic applications.

Stable Lanthanide Metal-Organic Frameworks with Ratiometric Fluorescence Sensing for Amino Acids and Tunable Proton Conduction and Magnetic Properties

Four new isostructural lanthanide metal-organic frameworks (MOFs), namely {[Ln(DMTP-DC)_1.5(H₂O)₃]·DMF}_n [H₂DMTP-DC = 2',5'-dimethoxytriphenyl-4,4″-dicarboxylic acid; Ln^III = Eu^III (1), Gd^III (2), Tb^III (3), and Dy^III (4)], have been synthesized and characterized. Single-crystal structure analysis reveals that 1-4 are three-dimensional Ln-MOFs with rich H-bonding of coordinated H₂O molecules in the network channels. The X-ray diffraction patterns indicate that Ln-MOF 1 displays good stabilities in organic solvents and aqueous solutions with distinct pH values. Both 1 and 3 show characteristic emission of Ln^III ions. Ln-MOF 1 can be used as a ratiometric fluorescence sensor for arginine and lysine in aqueous solution, and the detection limits are 24.38 μM for arginine and 9.31 μM for lysine. All 1-4 show proton conductivity related to relative humidity (RH) and temperature, and the maximum conductivity values of 1-4 at 55 °C and 100% RH are 9.94 × 10^-5, 1.62 × 10^-4, 1.71 × 10^-4, and 2.67 × 10^-4 S·cm^-1, respectively. The value of σ increases with the decrease in ionic radius, indicating that the radius of the Ln^III ions can regulate the proton conductivity of these MOFs. Additionally, 2 exhibits a significant magnetocaloric effect (MCE) with a magnetic entropy change (-ΔS_m) of 18.86 J kg^-1 K^-1 for ΔH = 7 T at 2 K, and 4 shows weak field-induced slow relaxation of magnetization. The coexistence of good fluorescence sensing capability, attractive proton conductivity, and relatively large MCE in Ln-MOFs is rare, and thus, 1-4 are potentially multifunctional MOF materials.

Suitable acid groups and density in electrolytes to facilitate proton conduction

Proton conducting materials suffer from low proton conductivity under low-relative humidity (RH) conditions. Previously, it was reported that acid-acid interactions, where acids interact with each other at close distances, can facilitate proton conduction without water movement and are promising for overcoming this drawback [T. Ogawa, H. Ohashi, T. Tamaki and T. Yamaguchi, Chem. Phys. Lett., 2019, 731, 136627]. However, acid groups have not been compared to find a suitable acid group and density for the interaction, which is important to experimentally synthesize the material. Here, we performed ab initio calculations to identify acid groups and acid densities as a polymer design that effectively causes acid-acid interactions. The evaluation method employed parameters based on several different optimized coordination interactions of acids and water molecules. The results show that the order of the abilities of polymer electrolytes to readily induce acid-acid interactions is hydrocarbon-based phosphonated polymers > phosphonated aromatic hydrocarbon polymers > perfluorosulfonic acid polymers ≈ perfluorophosphonic acid polymers > sulfonated aromatic hydrocarbon polymers. The acid-acid interaction becomes stronger as the distance between acids decreases. The preferable distance between phosphonate moieties is within 13 Å.

Ion-Exchanged UPG-1 as Potential Electrolyte for Fuel Cells

Proton-exchange membrane fuel cells are an attractive green technology for energy production. However, one of their major drawbacks is instability of the electrolytes under working conditions (i.e., temperature and humidity). Some metal-organic frameworks (MOFs) have recently emerged as promising alternative electrolyte materials because of their higher stability (compared with the organic polymers currently used as electrolytes), proton conductivity, and outstanding porosity and versatility. Here, we present ionic exchange in a microporous zirconium phosphonate, UPG-1, as an efficient strategy to enhance its conductivity and cyclability. Thus, labile protons of the hybrid structure were successfully replaced by different alkali cations (Li⁺, Na⁺, and K⁺), leading to 2 orders of magnitude higher proton conductivity than the pristine UPG-1 (up to 2.3 × 10^-2 S·cm^-1, which is comparable with those of the commercial electrolytes). Further, the proton conductivity was strongly influenced by the MOF hydrophilicity and the polarization strength of the cation, as suggested by molecular simulation. Finally, a mixed-matrix membrane containing the best-performing material (the potassium-exchanged one) was successfully prepared, showing moderate proton conductivity (up to 8.51 × 10^-3 S·cm^-1).

Gyroid-Nanostructured All-Solid Polymer Films Combining High H+ Conductivity with Low H2 Permeability

Gyroid-nanostructured all-solid polymer films with exceedingly high proton conductivity and low H₂ gas permeability have been created via crosslinking polymerization of mixtures of a zwitterionic amphiphilic monomer and a polymerizable imide-type acid that co-organize into bicontinuous cubic liquid-crystalline phases. The gyroid nanostructures are visualized by reconstructing a 3D electron map from the synchrotron X-ray diffraction patterns. These films exhibit high proton conductivity of the order of 10^-1 S cm^-1 and extremely low H₂ gas permeability of the order of 10^-15 mol m m^-2 s^-1 Pa^-1 . These properties can be ascribed to the presence of the ionic liquid-like layer along the gyroid minimal surface. Since these two characteristics are required for improving the performance of proton-exchange membrane fuel cells, the present membrane design represents a promising strategy for the development of advanced devices, pertinent to establishing sustainable energy sources.

Precise Molecular-Level Modification of Nafion with Bismuth Oxide Clusters for High-performance Proton-Exchange Membranes

Fabricating proton exchange membranes (PEMs) with high ionic conductivity and ideal mechanical robustness through regulation of the membrane microstructures achieved by molecular-level hybridization remains essential but challenging for the further development of high-performance PEM fuel cells. In this work, by precisely hybridizing nano-scaled bismuth oxide clusters into Nafion, we have fabricated the high-performance hybrid membrane, Nafion-Bi₁₂ -3 %, which showed a proton conductivity of 386 mS cm^-1 at 80 °C in aqueous solution with low methanol permeability, and conserved the ideal mechanical and chemical stabilities as PEMs. Moreover, molecular dynamics (MD) simulation was employed to clarify the structural properties and the assembly mechanisms of the hybrid membrane on the molecular level. The maximum current density and power density of Nafion-Bi₁₂ -3 % for direct methanol fuel cells reached to 432.7 mA cm^-2 and 110.2 mW cm^-2 , respectively. This work provides new insights into the design of versatile functional polymer electrolyte membranes through polyoxometalate hybridization.

Proton Conductive Zr-Phosphonate UPG-1-Aminoacid Insertion as Proton Carrier Stabilizer

Proton exchange membrane fuel cells (PEMFCs) are an attractive green technology for energy generation. The poor stability and performances under working conditions of the current electrolytes are their major drawbacks. Metal-Organic Frameworks (MOFs) have recently emerged as an alternative to overcome these issues. Here, we propose a robust Zr-phosphonate MOF (UPG-1) bearing labile protons able to act a priori as an efficient electrolyte in PEMFCs. Further, in an attempt to further enhance the stability and conductivity of UPG-1, a proton carrier (the amino acid Lysine, Lys) was successfully encapsulated within its porosity. The behaviors of both solids as an electrolyte were investigated by a complete experimental (impedance spectroscopy, water sorption) and computational approach (MonteCarlo, water sorption). Compared with the pristine UPG-1, the newly prepared Lys@UPG-1 composite showed similar proton conductivity but a higher stability, which allows a better cyclability. This improved cyclability is mainly related to the different hydrophobic-hydrophilic balance of the Lys@UPG-1 and UPG-1 and the steric protection of the reactive sites of the MOF by the Lys.

Non-humidified fuel cells using a deep eutectic solvent (DES) as the electrolyte within a polymer electrolyte membrane (PEM): the effect of water and counterions

In this research, deep eutectic solvents (DESs) were prepared and employed as the electrolyte in Nafion membranes. Different factors, such as the water content and Nafion counterions (H⁺, Li⁺, Na⁺ and K⁺), which could influence the PEM performance, were evaluated. The obtained results showed that the presence of water may have a constructive or destructive effect on the DES and Nafion/DES properties, which should be considered for their final applications. Also, the electronegativity of the counterion can significantly influence the Nafion/DES proton conductivity. The prepared Nafion/DES composite membranes showed superconducting properties as a result of a Grotthuss-like mechanism for proton conduction. The conductivities of the prepared membranes were compared to those of other membranes based on an upper bound concept, which showed the potential use of DESs as a promising alternative to conventional ionic liquids. Finally, the fuel cell performances of the prepared membranes at different temperatures were evaluated.

Synergistic adsorptions of Na2CO3 and Na2SiO3 on calcium minerals revealed by spectroscopic and ab initio molecular dynamics studies

FTIR, XPS, and ab initio molecular dynamics studies demonstrated that sodium silicate (Na₂SiO₃) adsorbs on fluorite with a higher affinity when they are treated beforehand by sodium carbonate (Na₂CO₃) due to proton exchange(s).

The synergistic effects between sodium silicate (Na₂SiO₃) and sodium carbonate (Na₂CO₃) adsorbed on mineral surfaces are not yet understood, making it impossible to finely tune their respective amounts in various industrial processes. In order to unravel this phenomenon, diffuse reflectance infrared Fourier transform and X-ray photoelectron spectroscopies were combined with ab initio molecular dynamics to investigate the adsorption of Na₂SiO₃ onto bare and carbonated fluorite (CaF₂), an archetypal calcium mineral. Both experimental and theoretical results proved that Na₂CO₃ adsorbs onto CaF₂ with a high affinity and forms a layer of Na₂CO₃ on the surface. Besides, at low Na₂SiO₃ concentration, silica mainly physisorbs in a monomeric protonated form, Si(OH)₄, while at larger concentration, significant amounts of polymerised and deprotonated forms are identified. Prior surface carbonation induces an acid–base reaction on the surface, which results in the formation of the basic forms of the monomers and the dimers, i.e. SiO(OH)₃^– and Si₂O₃(OH)₄^2–, even at low coverage. Their adsorption is highly favoured compared to the acid forms, which explains the synergistic effects observed when Na₂SiO₃ is used after Na₂CO₃. The formation of the basic form on the bare surface is observed only by increasing the surface coverage to 100%. Hence, when Na₂CO₃ is used during a separation process, lower Na₂SiO₃ concentrations are needed to obtain the same effect as with lone Na₂SiO₃ in the separation process.

Proton-Conducting Poly-γ-glutamic Acid Nanofiber Embedded Sulfonated Poly(ether sulfone) for Proton Exchange Membranes

Development and fabrication of novel proton exchange membranes (PEMs) with excellent performance have a great significance to the commercial application of PEM fuel cell. Inspired from the proton-conducting mechanism, γ-poly(glutamic acid) (γ-PGA) nanofibers (NFs) are first fabricated by solution blowing with the help of polylactic acid (PLA) and designed to form amino acid arrays as efficient proton channels for PEMs. The NFs with 50% γ-PGA exhibit a high proton conductivity of 0.572 S cm^-1 at 80 °C/50% relative humidity (RH), and 1.28 S cm^-1 at 40 °C/90% RH. Density functional theory is carried out to explain the mechanisms of proton hopping in γ-PGA, and the activation energy barriers from NH to COO^- for trans and cis conformations under anhydrous conditions are only 0.64 and 0.62 eV, respectively. Then the γ-PGA/PLA NFs are incorporated into sulfonated poly(ether sulfone) to prepare PEMs, which show remarkable performance compared with the Nafion membrane. The composite membrane with 30 wt % NFs exhibits the highest proton conductivity (0.261 S cm^-1 at 80 °C/100% RH). The direct methanol fuel cells with this membrane show a maximum power density (202.3 mW cm^-2) among all of the PEMs, showing great application potential in the field of PEMs.

Biomimetic Polyelectrolytes Based on Polymer Nanosheet Films and Their Proton Conduction Mechanism

We report a biomimetic polyelectrolyte based on amphiphilic polymer nanosheet multilayer films. Copolymers of poly( N-dodecylacrylamide- co-vinylphosphonic acid) [p(DDA/VPA)] form a uniform monolayer at the air-water interface. By depositing such monolayers onto solid substrates using the Langmuir-Blodgett (LB) method, multilayer lamellae films with a structure similar to a bilayer membrane were fabricated. The proton conductivity at the hydrophilic interlayer of the lamellar multilayer films was studied by impedance spectroscopy under temperature- and humidity-controlled conditions. At 60 °C and 98% relative humidity (RH), the conductivity increased with increasing mole fraction of VPA ( n) up to 3.2 × 10^-2 S cm^-1 for n = 0.41. For a film with n = 0.45, the conductivity decreased to 2.2 × 10^-2 S cm^-1 despite the increase of proton sources. The reason for this decrease was evaluated by studying the effect of the distance between the VPAs ( l_VPA) on the proton conductivity as well as their activation energy. We propose that for n = 0.41, l_VPA is the optimal distance not only to form an efficient two-dimensional (2D) hydrogen bonding network but also to reorient water and VPA. For n = 0.45, on the other hand, the l_VPA was too close for a reorientation. Therefore, we concluded that there should be an optimal distance to obtain high proton conductivity at the hydrophilic interlayer of such multilayer films.

Coordination polymers from alkaline-earth nodes and pyrazine carboxylate linkers

A new series of alkaline-earth-metal based coordination polymers were synthesized by using a pyrazine-2,5-dicarboxylic acid (2,5-H2pzdc) ligand under hydrothermal conditions. These compounds show a variety of structural topologies, reflecting the variable coordination geometries of the alkaline-earth ions as well as the key role of the metal precursor salts. Ca, Sr, and Ba give porous three-dimensional compounds, namely [Ca(2,5-pzdc)(H2O)2]·H2O (1), [Sr(2,5-pzdc)(H2O)4]·H2O (3), [Ba(2,5-pzdc)(H2O)4]·2H2O (4) and [Ba(2,5pzdc)(H2O)2] (5), that feature one-dimensional hydrophilic channels which are filled with water molecules. The Sr compound retains its structure when the lattice water molecules are removed, while the other compounds undergo a structural rearrangement. The hydrophilicity of the Sr compound combined with its high stability even in the absence of guest molecules are the key characteristics that determine its good water adsorption and proton conductivity properties.

Development of novel polymer electrolyte membranes based on a benzothiadiazole unit

We developed novel aromatic polymer electrolyte membranes based on the benzothiadiazole unit with high swelling resistance and proton conductivity.

Nanostructured Ion-Exchange Membranes for Fuel Cells: Recent Advances and Perspectives

Polymer-based materials with tunable nanoscale structures and associated microenvironments hold great promise as next-generation ion-exchange membranes (IEMs) for acid or alkaline fuel cells. Understanding the relationships between nanostructure, physical and chemical microenvironment, and ion-transport properties are critical to the rational design and development of IEMs. These matters are addressed here by discussing representative and important advances since 2011, with particular emphasis on aromatic-polymer-based nanostructured IEMs, which are broadly divided into nanostructured polymer membranes and nanostructured polymer-filler composite membranes. For each category of membrane, the core factors that influence the physical and chemical microenvironments of the ion nanochannels are summarized. In addition, a brief perspective on the possible future directions of nanostructured IEMs is presented.

A durable anion conducting membrane with packed anion-exchange sites and an aromatic backbone for solid-state alkaline fuel cells

A durable polyelectrolyte membrane with highly cross-linked aromatic packed anion-exchange sites that enable high OH⁻ ion transport, simultaneously restricting methanol cross-over for solid-state alkaline fuel cells.