The additive effect of zwitterion and nano-particles on ion dissociation in polyelectrolytes

To realise the battery potential of gel polyelectrolytes greater ion dissociation, ultimately leading to higher conductivities, must be achieved. Higher conductivities will result through increasing the ion-dissociating properties of the gel polyelectrolyte. The poor degree of ion dissociation arises as the active ion tends to remain in close proximity to the backbone charge. Nano-particle inorganic oxides, and zwitterionic compounds have been shown to act as dissociation enhancers in certain polyelectrolyte systems. In an attempt to further increase ion dissociation the addition of both TiO₂ nano-particles and a zwitterionic compound based on 1-butylimidazolium-3-N-(butanesulphonate) were added to the gel polyelectrolyte system poly (Li-2-acrylamido-2-methyl-1-propane sulphonate-co-N,N′-dimethylacrylamide), poly(Li-AMPS-co-DMAA) to determine if a synergistic effect occurs. Two different solvents were used to determine the breadth of applicability of the additive effect. The use of both dissociators resulted in the maximum ionic conductivity being achieved at lower nano-particle concentrations when compared to an identical system without zwitterion.

Plasticized single conductung polyelectrolytes based on poly (AMPS)

Poly(2-acrylamido-2-methyl-propane-1-sulphonic acid), poly(AMPS), has been ion exchanged with lithium and sodium to form alkali metal ion conducting polyelectrolytes. In the pure form these materials are rigid and would thus show limited conductivity. However addition of water or dimethylsulphoxide, as plasticizers, increases the conductivity by several orders or magnitude. The thermal analysis and NMR relaxation studies of these systems suggest that the increase in conductivity is as a direct result of increased ion mobility although the FTIR evidence still suggests significant ion association consistent with weak electrolytes. Although the Tg's of the sodium form of the polymer were higher, this system displayed higher conductivities than lithium which can be explained by a greater degree of ion dissociation and hence a larger number of charge carriers in the case of sodium poly(AMPS).

Ionic conductivity of polymer gels deriving from alkali metal ionic liquids and negatively charged polyelectrolytes

We have prepared polymer gel electrolytes with alkali metal ionic liquids (AMILs) that inherently contain alkali metal ions. The AMIL consisted of sulfate anion, imidazolium cation, and alkali metal cation. AMILs were mixed directly with poly(3-sulfopropyl acrylate) lithium salt or poly(2-acrylamido-2-methylpropanesulfonic acid) lithium salt to form polymer gels. The ionic conductivity of these gels decreased with increasing polymer fraction, as in general ionic liquid/polymer mixed systems. At low polymer concentrations, these gels displayed excellent ionic conductivity of 10⁻⁴ to 10⁻³ S cm⁻¹ at room temperature. Gelation was found to cause little change in the ⁷Li diffusion coefficient of the ionic liquid, as measured by pulse-field-gradient NMR. These data strongly suggest that the lithium cation migrates in successive pathways provided by the ionic liquids.

Boroxine ring compounds as dissociation enhancers in gel polyelectrolytes

In order to combine the advantages of both traditional gel electrolytes and polyelectrolytes, a novel polyelectrolyte which incorporates a boroxine ring-containing anion-trapping agent has been explored. Poly(lithium 2-acrylamido-2-methyl-1-propanesulfonate) (PAMPSLi), ethylene carbonate (EC) and tri(methoxyethoxyethoxyethoxy)boroxine (TME₃Bx) were combined to prepare various gel systems. The thermal properties and conductivities of these gels have been investigated. A conductivity of 10^−3.6 S cm⁻¹ at 20 °C has been achieved in a gel polyelectrolyte system with a molar ratio of [EC]:[TME3Bx]:[Li⁺]=24:1.7:1. Temperature-dependent NMR measurements indicated that a significant interaction exists between the boroxine ring and the polyelectrolyte.

Enhancement of ‘dry’ proton conductivity by self-assembled nanochannels in all-solid polyelectrolytes

Proton transport has been recognized as an essential process in many biological systems, as well as electrochemical devices including fuel cells and redox flow batteries. In the present study, we address the pressing need for solvent-free proton conducting polymer electrolytes for high-temperature PEM fuel cell applications by developing a novel all-solid polyelectrolyte membrane with a self-assembled proton-channel structure. We show that this self-assembled nanostructure endows the material with exciting ‘dry’ proton conductivity at elevated temperatures, as high as 0.3 mS cm−1 at 120 °C, making it an attractive candidate for high-temperature PEM fuel cell applications. Based on the combined investigation of solid-state NMR, FTIR and conductivity measurements, we propose that both molecular design and nano-scale structures are essential for obtaining highly conductive anhydrous proton conductors.

Superhydrophobic polyimide films with a hierarchical topography : combined replica molding and layer-by-layer assembly

Artificial superhydrophobic surfaces with a hierarchical topography were fabricated by using layer-by-layer assembly of polyelectrolytes and silica nanoparticles on microsphere-patterned polyimide precursor substrates followed with thermal and fluoroalkylsilane treatment. In this special hierarchical topography, micrometer-scale structures were provided by replica molding of polyamic acid using two-dimensional arrays of polystyrene latex spheres as templates, and nanosized silica particles were then assembled on these microspheres to construct finer structures at the nanoscale. Heat treatment was conducted to induce chemical cross-linking between polyelectrolytes and simultaneously convert polyamic acid to polyimide. After surface modification with fluoroalkylsilane, the as-prepared highly hydrophilic surface was endowed with superhydrophobicity due to the bioinspired combination of low surface energy materials and hierarchical surface structures. A superhydrophobic surface with a static water contact angle of 160 degrees and sliding angle of less than 10 degrees was obtained. Notably, the polyimide microspheres were integrated with the substrate and were mechanically stable. In addition, the chemical and mechanical stability of the polyelectrolyte/silica nanoparticle multilayers could be increased by heat-induced cross-linking between polyelectrolytes to form nylon-like films, as well as the formation of interfacial chemical bonds.

Compositional effects on structure and transport properties in a polyelectrolyte gel

The copolymerization of lithium 2-acrylamido-2-methyl-1-propane sulfonate (LiAMPS) with N,N ′-dimethylacrylamide has yielded polyelectrolyte systems which can be gelled with an ethylene carbonate/N ′,N ′-dimethylacetamide solvent mixture and show high ionic conductivities. 7Li linewidth and relaxation times as well as 1H NMR diffusion coefficients have been used to investigate the effect of copolymer composition as well as copolymer concentration in the gel electrolyte with respect to ionic transport and polyelectrolyte structure. It appears that ion association is likely even in the case of low lithium salt concentration; however a rapid exchange exists between the associated and non-associated lithium species. Beyond 0.2 M of LiAMPS, both the conductivity and solvent diffusion reach a plateau, whilst lithium ion linewidth and spin-spin relaxation are suggestive, on average, of a less mobile species. The thermal analysis data is also supportive of this association effectively leading to a form of phase separation on the nanoscale, which gives a lower overall activity of lithium ions in the solvent rich regions beyond about 0.2 M of LiAMPS, thereby leading to an increase in the final liquidus temperature of the binary liquid solvent from –9 to +5°C.

Enhancement of ion dissociation in polyelectrolyte gels

High conductivity in single ion conducting polymer electrolytes is still the ultimate aim for many electrochemical devices such as secondary lithium batteries. Achieving effective ion dissociation in these cases remains a challenge since the active ion tends to remain in close proximity to the backbone charge as a result of a low degree of ion dissociation. A unique aspect of this dissociation problem in polyelectrolytes is the repulsion between the backbone charges created by dissociation. One way of enhancing ion dissociation in polyelectrolyte systems is to use copolymers in which only a fraction (<20%) of the mer units are charged and where the comonomer is itself chosen to be polar and preferably to be compatible with potential solvents. We have also found that certain dissociation enhancers based on ionic liquids or boroxine ring compounds can lead to high ionic conductivity. In the cases where an ionic liquid is used as the solvent in a polyelectrolyte gel, the viscosity of the ionic liquid and its hydrophilicity are critical to achieving high conductivity. Compounds based on the dicyanamide anion appear to be very effective ionic solvents; polyelectrolyte gels incorporating such ionic liquids exhibit conductivities as high as 10⁻² S/cm at room temperature. In the case of boroxine ring dissociation enhancers, gels based on poly(lithium-2-acrylamido-2-methyl-1-propanesulfonate) and ethylene carbonate produce conductivities approaching 10⁻³ S/cm. This paper will discuss these approaches for achieving higher conductivity in polyelectrolyte materials and suggest future directions to ensure single ion transport.

Zwitterionic additives for electrochemical devices

Zwitterionic electrolytes such as N-methyl-N-(n-butanesulfonate) pyrrolidinium are added to electrolyte compositions such as polyelectrolytes, ionic liquid electrolytes and molecular solvent electrolytes (for example, lithium hexafluorophosphate) to improve conductivity of the ion species, such as lithium, in the electrolyte. This has application to lithium based energy storage devices such as batteries and supercapacitors.

Zwitterion effect in polyelectrolyte gels based on lithium methacrylate-N,N-dimethyl acrylamide copolymer

Zwitterionic compounds such as those based on 1-butylimidazolium-3-(n-butanesulfonate) have previously been shown to have positive effects on the transport properties of polyelectrolytes. The addition of the zwitterion has been found to, in some cases, increase the dissociation of the lithium ion and enhance the conductivity by almost an order of magnitude. In this work, we report the effects of adding the above-mentioned zwitterion into the polyelectrolyte gel system poly(lithium methacrylate-co-N,N-dimethyl acrylamide); the anionic group being a stronger base leads to different behaviour for this copolymer compared to previous work. Polyelectrolyte gels based on dimethyl sulfoxide and polyether solvents were investigated to determine the breadth of applicability of the zwitterion in improving lithium ion transport. Impedance spectroscopy and pulse field gradient-NMR diffusion indicate an increase in the number of available charge carriers with zwitterion addition in some gel systems, however, the effect is not universal.

Polymer-in-ionic-liquid electrolytes

In order to achieve high conductivity in a polymer electrolyte, polymer-in-ionic-liquid electrolytes have been explored. It is found in this study that poly[vinylpyrrolidone-co-(vinyl acetate)] (P(VP-c-VA)) in 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl) amide (EtMeIm+Tf2N−) and poly(N,N-dimethyl acrylamide) (PDMAA) in trimethyl butyl ammonium bis(trifluoromethane sulfonyl) amide (N1114+Tf2N−) produce ion-conducting liquids and gels. The P(VP-c-VA)/ EtMeIm+Tf2N− mixture has a conductivity around 10−3 S · cm−1 at 22 °C, for copolymer concentrations up to 30 wt.-%. Thermal analysis shows that the Tg of the P(VP-c-VA)/ EtMeIm+Tf2N− system is well described by the Fox equation as a function of polymer content. Poly(methyl methacrylate) (PMMA)/ EtMeIm+Tf2N− gel electrolytes were prepared by in-situ polymerisation of the monomer in the ionic liquid. In the presence of 0.5–2.0 wt.-% of a crosslinking agent, these PMMA-based electrolytes displayed elastomeric properties and high conductivity (ca. 10−3 S · cm−1) at room temperature.

Polyelectrolyte-in-ionic-liquid electrolytes

Novel polymer electrolyte materials based on a polyelectrolyte-in-ionic-liquid principle are described. A combination of a lithium 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSLi) and N,N′-dimethylacrylamide (DMMA) are miscible with the ionic liquid, 1-ethyl-3-methylimidazolium dicyanamide (EMIDCA). EMIDCA has remarkably high conductivity (≥ 2 · 10−2 S · cm−1) at room temperature and acts as a good solvating medium for the polyelectrolyte. At compositions of AMPSLi less than or equal to 75 mol-% in the copolymer (P(AMPSLi-co-DMAA)), the polyelectrolytes in EMIDCA are homogeneous, flexible elastomeric gel materials at 10 − 15 wt.-% of total polyelectrolyte. Conductivities higher than 8 · 10−3 S · cm−1 at 30 °C have been achieved. The effects of the monomer composition, polyelectrolyte concentration, temperature and lithium concentration on the ionic conductivity have been studied using thermal and conductivity analysis, and pulsed field gradient nuclear magnetic resonance techniques.

ZWITTERIONIC ADDITIVES FOR ELECTROCHEMICAL DEVICES

Zwitterionic electrolytes such as N-methyl-N-(n-butanesulfonate) pyrrolidinium are added to electrolyte compositions such as polyelectrolytes, ionic liquid electrolytes and molecular solvent electrolytes (for example, lithium hexafluorophosphate) to improve conductivity of the ion species, such as lithium, in the electrolyte. This has application to lithium based energy storage devices such as batteries and supercapacitors.

Electrochemical behavior of CrN coating for polymer electrolyte membrane fuel cell

CrN films on a bipolar plate in polymer electrolyte membrane fuel cells have several advantages owing to their excellent corrosion resistance and mechanical properties. Three CrN samples deposited at various radio frequency (RF) powers by RF magnetron sputtering were evaluated under potentiodynamic, potentiostatic and electrochemical impedance spectroscopy conditions. The electrochemical impedance spectroscopy data were monitored for 168 h in a corrosive environment at 70 °C to determine the coating performance at +600 mV_SCE under simulated cathodic conditions in a polymer electrolyte membrane fuel cell. The electrochemical behavior of the coatings increased with decreasing RF power. CrN films on the AISI 316 stainless steel substrate showed high protective efficiency and charge transfer resistance, i.e. increasing corrosion resistance with decreasing RF power. X-ray diffraction confirmed the formation of a CrN(200) preferred orientation at low RF power.