Broadband Electrical Spectroscopy Research Articles

Three-dimensional Catenated 1-ethyl-3-methylimidazolium Halotitanate Ionic Liquid Electrolytes for Electrochemical Applications

A new family of ionic liquid based electrolytes ([EMImCl/(TiCl4)1.4]/(δ-MgCl2)x) for electrochemical applications is proposed. These materials are obtained by direct reaction of 1-ethyl-3-methylimidazolium chloride (EMImCl) with titanium(IV) chloride (TiCl4), and doped with increasing amounts of δ-MgCl2. Modulated differential scanning calorimetry (MDSC) measurements show that in these electrolytes the glass transition temperature, Tg, occurs at −36°C, and that the crystallization and melting transitions are inhibited by the presence of large and highly amorphous charge delocalized anionic domains. Vibrational studies indicate that in the electrolyte the anionic domains consist of large monomeric and dimeric 3D-catenated halotitanate clusters, in which magnesium chloride units are coordinated to Ti species, and whose equilibrium concentration is modulated by the concentration of δ-MgCl2. Density functional theory (DFT) based electronic structure calculations were undertaken to understand the structural features of these materials and to confirm the vibrational frequency assignments. Broadband electrical spectroscopy (BES) studies reveal that the electric response of these materials is modulated by polarization and dielectric relaxation events. These measurements revealed the correlations existent between conductivity mechanisms and the dielectric relaxations of the host matrix of the electrolytes. At 40°C, the conductivity is found to depend on the δ-MgCl2 concentration and approximately on the order of 10−4Scm−1. Preliminary electrochemical measurements reveal a reversible deposition and stripping process of a Mg-Ti alloy, with an average columbic efficiency of 99.3% and a deposition overvoltage of only 10mV.

(Keynote) Mechanisms in Ion Conducting Polymer Materials By Broadband Electrical Spectroscopy (BES)

The charge transfer mechanisms of ion conducting polymer materials (ICPMs) is of crucial importance both for fundamental research and for a host of practical applications, including primary and secondary batteries, fuel cells, dye-sensitized solar cells, supercapacitors and sensors. A wide variety of ICPMs has been proposed, based on: (a) different families of polymer electrolytes; (b) ionic liquids (ILs); and (c) classical ion-conducting ceramics. In these materials, the long-range charge transfer events take place owing to complex processes, which involve several possible relaxation phenomena, such as: (a) ion hopping events between ion coordination sites; (b) relaxation modes of the host matrix; and (c) polarization effects occurring at the interfaces between the different domains characterizing the materials. Broadband electrical spectroscopy (BES) is a powerful tool for the accurate investigation of the roles played by electrical relaxation events in the charge transfer processes. Indeed, BES allows to carefully detect the fundamental relaxations governing the long-range charge transfer mechanisms and to correlate them to the morphology of ion-conducting materials. This presentation overviews results of the application of BES in the study of the charge transfer mechanisms of a variety of ICMs, including: (a) polymer electrolytes based on alkaline and alkaline-earth ions; (b) pristine and hybrid inorganic-organic proton-conducting and anion-conducting membranes. The general phenomena and the fundamental theory underlying the interpretation of the events characterizing the electric response of the materials is also described. Finally, the models adopted for the interpretation of conductivity mechanisms are described and a unified conductivity mechanism is proposed.

A Polyketone-based Anion Exchange Membrane for Electrochemical Applications: Synthesis and Characterization

An anion exchange membrane (AEM) was made with a modified polyketone (PK). AEMs of polyamines were prepared in a three-step procedure: (I) PK synthesis using ethylene and carbon monoxide, supported by a Pd catalyst, followed by the introduction of 1,2-diaminopropane to yield the polymeric amines; (II) solvent casting of the modified PK with a low degree of amination; (III) iodomethylation to form the AEM (PK-PDAPm(I)), followed by ion exchange with KOH (PK-PDAPm(OH)).The structure of the modified polyketone was characterized using FT-IR, and UV–vis spectroscopy, demonstrating the successful introduction of amine in the PK. The conductivity of the AEM was studied using broadband electric spectroscopy (BES) in the temperature range from −100 to 120°C: the highest value of 9×10−4 S·cm−1 was reached at 120°C for the ionic conductivity of PK-PDAPm(I), followed by PK-PDAPm(OH) with values of the same order of magnitude (10−4 S·cm−1). Thermogravimetry showed that the material is thermally stable up to 200°C.

Electric Response and Conductivity Mechanism in H3PO4‑Doped Polybenzimidazole-4N−HfO2 Nanocomposite Membranes for High Temperature Fuel Cells

Relaxation and polarization phenomena of phosphoric acid-doped [PBI4N(HfO2)x](H3PO4)y nanocomposite membranes for high-temperature proton-exchange membrane fuel cells are studied using Dynamic Mechanical Analysis (DMA) and Broadband Electrical Spectroscopy (BES). The membranes are obtained by casting combinations of a polybenzimidazole polymer (PBI4N) with increasing amounts of hafnium oxide nanofiller, resulting in [PBI4N(HfO2)x] hybrid systems with 0≤x≤0.32. Phosphoric acid at varying content levels (0÷18wt%) is used as a doping agent, giving rise to [PBI4N(HfO2)x](H3PO4)y membranes. DMA and BES studies lead us to determine that the electric response of the membranes is modulated by polarization phenomena and by α and β dielectric relaxation events of the polymer matrix. Additionally, the experimental results suggest that in [PBI4N(HfO2)x](H3PO4)y membranes the conductivity occurs owing to three conductivity pathways: two mechanisms involving inter-domain proton migration phenomena by “hopping” events; and one mechanism in which proton exchange occurs between delocalization bodies. These results highlight the significant effect of the hafnium oxide nanofiller content on the conductivity of [PBI4N(HfO2)x](H3PO4)y where, at x≥0.04, demonstrates conductivity higher (9.0×10−2S/cm) than that of pristine H3PO4-doped PBI4N (4.8×10−2S/cm) at T≥155°C.

Interplay Between Structure and Conductivity in 1-Ethyl-3-methylimidazolium tetrafluoroborate/(δ-MgCl2)f Electrolytes for Magnesium Batteries

The synthesis, physicochemical properties and conductivity mechanism of a family of ionic liquid-based electrolytes for use in secondary Mg batteries are reported. The electrolytes are obtained by dissolving controlled amounts of δ-MgCl2 salt into the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) which acts as a solvent. δ-MgCl2 consists of an inorganic ribbon of Mg atoms covalently bonded together through bridging chlorine atoms. Due to this peculiar structural motif, with respect to the electrolytes based on conventional Mg salts, it is possible to achieve electrolytes of higher Mg concentration. Thus, concatenated anionic complexes bridged via halogen atoms are formed, improving the electrochemical performance of these materials. Electrolytes with a general formula EMImBF4/(δ-MgCl2)f with f ranging from 0 to 0.117 are obtained. The composition of the obtained materials is determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The properties of these systems are investigated by means of Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and vibrational spectroscopy in both medium (MIR) and far infrared (FIR). Finally, Broadband Electrical Spectroscopy (BES) is carried out with the aim to elucidate the electrical response of the electrolytes in terms of their polarization and relaxation phenomena and to propose a conductivity mechanism. At 20°C the highest conductivity (0.007S/cm) is observed for the electrolyte with cMg=0.00454molMg/kgIL.

N-Functionalised Polyketone Ion Exchange Membranes for Aemfcs

Anion-exchange membrane fuel cells (AEMFCs) provide significant advantages over their proton-exchange membrane counterparts. In the alkaline environment, the oxygen reduction reaction (ORR) is more facile, there is diminished fuel crossover, and a greater flexibility regarding fuel and catalyst choice. The membrane at the heart of AEMFCs not only facilitates the ion exchange but also separates the fuel feedstocks and acts as a support for the membrane-electrode assembly (MEA). However, to date there are still no membrane materials that satisfy all the needs (long-term stability in alkaline environment, high ionic conductivity, low swelling and good structural integrity) for use in AEMFCs and this remains one of the larger obstacles for further AEMFC development. The amination and subsequent quarternisation of polyketone leads to a new family of ionomers containing N-substituted pyrrole moieties. The degree of amination can be controlled by manipulating reaction conditions, allowing the composition and resulting structural properties of the polymer to be tuned. Membrane fabrication results in thermally stable (TD > 250 oC), structurally robust polymer electrolytes that exhibit ionic conductivity ( > 10-3 S cm-1). These new solid-state ion conducting materials have the potential to be used in a variety of applications including AEMFCs. Here we present an in-depth study focusing on the structure-property relationships of this new polypyrrole/polyketone polymer. A variety of analytical techniques are used to probe the thermal and structural properties of the polymers, these include high-resolution thermogravimetric analysis, modulated differential scanning calorimetry, dynamic mechanical analysis, vibrational, NMR and UV-Vis spectroscopies. In addition, broadband electrical spectroscopy is used to gauge the interplay between the structural properties and electrical response. Figure 1

(Invited) 3D-Catenated EMImCl/(TiCl4)1.4/(δ-MgCl2)x Ionic Liquid Electrolyte for Mg Secondary Batteries

The rapid advance in the fields of portable electronics, load leveling and peak shaving for the power grid and zero-emission automotive applications require the development of new and improved electrical energy storage systems [1,2]. Since the 90’s major improvements have been achieved in magnesium battery technology [3-6]. In comparison to Li, Mg offers the following advantages: (i) a higher volumetric capacity (3832 vs. 2062 mAh·cm-3); (ii) far greater abundance in the Earth’s crust, lowering the costs; (iii) a safer operation and a better compatibility with the environment; and (iv) an acceptable standard reduction potential (-2.36 vs. -3.04 V) [7-9]. The main roadblock for these devices is the development of an efficient and stable electrolyte that is able to reversibly deposit and strip magnesium. Although Grignard and other organo-magnesium compounds exhibit good electrochemical performances [9], they do not exhibit an optimal stability due to their high vapor pressure and flammability. Ionic liquids dissolving a Mg salt with a high crystalline disorder were proposed as promising alternative electrolytes to organo-Mg systems owing to their good electrochemical performance and lack of flammability and thermal stability issues [10]. In the present work a new family of electrolytes is proposed, based on 1-ethyl-3-methylimidazolium chloride (EMImCl), titanium(IV) chloride (TiCl4) and increasing amounts of δ-MgCl2. Specifically, four EMImCl/(TiCl4)1.4/(δ-MgCl2)xelectrolytes, with 0.00 ≤ x ≤ 0.23 are prepared and extensively characterized. The chemical composition was determined by Inductively-Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The thermal stability was gauged using High-Resolution Thermo Gravimetric Analysis (HR-TGA) and the phase transitions are highlighted with Modulated Differential Scanning Calorimetry (MDSC). Chemical interactions were studied through Fourier-Transform spectroscopy in the medium and far infrared (FT-MIR and FT-FIR) regions and confocal micro-Raman spectroscopy. The electrochemical performance was studied with: (i) Cyclic Voltammetry (CV), to probe Mg deposition and stripping (Fig. 1); (ii) Linear Sweep Voltammetry (LSV), to evaluate the electrochemical stability window; (iii) Chronopotentiometry (CP) experiments coupled with ICP-AES, to confirm and quantify the Mg deposition; and (iv) Broadband Electrical Spectroscopy (BES), to elucidate the long-range charge migration mechanisms of the electrolytes. High level density functional theory (DFT) based electronic structure calculations were undertaken to elucidate structures and vibrational frequency assignments. References 1 M. Armand and J. M. Tarascon Nature 451, 652-657, (2008). 2 B. Dunn, H. Kamath and J. M. Tarascon Science 334, 928-935, (2011). 3 V. Di Noto and M. Fauri, Batterie Primarie (Non Ricaricabili) e Secondarie (Ricaricabili) a Base di Elettroliti Polimerici Basati su Ioni Magnesio, PD99A000179, (1999). 4 V. Di Noto and M. Fauri, Magnesium-based Primary (Non Rechargeable) and Secondary (Rechargeable) Batteries, PCT/EP00/07221, (2000). 5 V. Di Noto, M. Fauri, G. De Luca and M. Vidali, (1998). 6 V. Di Noto, S. Lavina, D. Longo and M. Vidali Electrochim. Acta 43, 1225-1237, (1998). 7 D. Aurbach, G. S. Suresh, E. Levi, A. Mitelman, O. Mizrahi, O. Chusid and M. Brunelli Adv. Mater. 19, 4260-4267, (2007). 8 T. D. Gregory, R. J. Hoffman and R. C. Winterton J. Electrochem. Soc. 137, 775-780, (1990). 9 J. Muldoon, C. B. Bucur, A. G. Oliver, T. Sugimoto, M. Matsui, H. S. Kim, G. D. Allred, J. Zajicek and Y. Kotani Energy and Environmental Science 5, 5941-5950, (2012). 10 F. Bertasi, C. Hettige, F. Sepehr, X. Bogle, G. Pagot, K. Vezzù, E. Negro, S. J. Paddison, S. G. Greenbaum and M. Vittadello ChemSusChem 8, 3069-3076, (2015). Figure 1

(Invited) The Implications of Cation Clustering in Anion Exchange Membranes on Conductivity and Mechanical Properties

The potential of anion exchange membrane (AEM) fuel cells to provide inexpensive compact power from a wider variety of fuels than is possible with a proton exchange membrane (PEM) fuel cell, has continued to drive the research interest in this area. Alkaline catalysis in fuel cells has been demonstrated with non-precious metal catalysts, and with a variety of fuels beyond H2 and methanol. Alkaline fuel cells (AFCs), based on aqueous solutions of KOH, have serious drawbacks associated with system complexity and carbonate formation. Anion exchange membrane (AEMs) fuel cells have a number of advantages over both PEM fuel cells and traditional AFCs; however, although anionic conductivity in AEMs can be comparable to PEMs the chemical stability of membrane attached cations in hydroxide is still not always sufficient for practical applications. Recently, it has been recognized that a number of advanced cations, may give AEMs the needed chemical stability. In some circumstances simple trimethyl benzyl ammonium cations are stable up to 60oC allowing us to being to study hydroxide and water transport in these systems. We use in-plane conductivity and multi-nuclear PFGSE to measure self-diffusion coefficients of the water and where possible, the ion, i.e. F-, carbonate and bicarbonate. Together with temperature and RH dependent SAXS we couple this information with the morphological changes in the materails. By their nature these organic cations form dipoles, which have a tendency to interact, see Figure below for a trimethylbenzyl cation with bromide for illustrative purposes. For maximum ion transport the cation should be distributed along the polymer chain, however, cation clustering is a common phenomena. It occurs in phase separated diblock polymers when an attempt is made to raise the IEC, no net increase is observed in ionic conductivity above a certain IEC. In many other less structured polymers it occurs in the fully humidified state when the distributed disordered cations order in agglomerates at a disorder/order Tδ. This Tδ is often observed in the operating temperature range expected for a device, in both DMA, DSC, and broadband electric spectroscopy. Interestingly, even though the SAXS clearly shows an agglomeration on the nano-scale, little difference is seen in the bulk conductivity with temperature. However, steric hindernce effects are expected to discourage aggregation, and may allow the cation to tune all properties of the AEM. As these phenomena are so prevalent in AEM materials the implications and science behind them are being investigated by us and will be discussed in this presentation. The implications of more complex cations proposed for stability in AEMs will be described. Figure 1

Property-Relaxation Correlations in 3D-Siloxane/Polyether Hybrid Polymer Electrolytes

The thermomechanical and transport properties of a series of hybrid polymer electrolytes are examined by means of differential scanning calorimetry, rheological analysis and broadband electric spectroscopy. The electrolytes are composed of 3D-oligosiloxane defect clusters grafted with polyether chains and doped with LiClO4, with concentration ranging from 0 to 1.4 mol·kg–1. The thermomechanical properties are mainly modulated by the balance of interactions taking place within the polyether domains. The materials show low Tg and no crystallization in a wide salt concentration range, while the mechanical modulus, between 104 and 105 Pa, is stable up to at least 100 °C. A detailed electric characterization, combined with the results from vibrational spectroscopy analysis, elucidates the factors influencing the transport properties. The conductivity reaches 8 × 10–5 S·cm–1 at 30 °C for intermediate salt concentrations. The sluggishness of the host matrix appears to be the limiting factor depressing the conduc...

(Invited) The Use of Broadband Electric Spectroscopy to Probe the Relaxations and Polarizations in Phase Seperated Polymer Electrolytes

There has been a recent surge in interest in the use of anion exchange membranes (AEMs) as the separator in many electrochemical conversion devices such as fuel cells, electrolyzers, redox flow cells, and for uses in electrochemical water purification processes. This is because electrocatalysis is generally more facile in alkaline conditions, and so these devices could potentially operate with non-precious metal catalysts and with catalysts capable of oxidizing or generating fuels beyond hydrogen. An AEM with high anionic conductivity would overcome many of the disadvantages of proton exchange membrane devices that need precious metal catalyst and work best with hydrogen. Improved AEMs are also required for water purification applications. Realization of AEM devices, therefore, would revolutionize electrochemical energy conversion, as inexpensive versatile devices could be made available. While much attention has been focused on generating new AEMs with stable cations and backbones, very little attention has been paid to the design of AEMs to maximize ionic conductivity and water transport. The work presented here represents the first detailed study of the thermomechanical, polarization, and dielectric phenomena related to morphology in a series of AEMs. In detail in this presentation, the micro-structure is studied by Small Angle X-ray Scattering (SAXS), the thermal stability and transitions are detected by High-Resolution Thermogravimetry (HR-TG) and by Modulated Differential Scanning Calorimetry (MDSC), respectively. The mechanical properties and transitions are investigated by Dynamic Mechanical Analysis (DMA). Broadband Electrical Spectroscopy (BES) is used to study the electric response and the conductivity mechanisms of the membranes in the frequency and temperature range of 0. 01—107 Hz and -105 — 100 °C, respectively. BES is a powerful technique, which allows us to fully understand the overall electric response and dielectric relaxations of the membranes, both in a completely dry state and after full hydration, thus allowing clarification of the role played by both ion solvation phenomena and water cluster structure on performance of these materials. The integration of the information acquired by the above techniques allows us to propose a reasonable mechanism for the long-range charge migration processes in polyvinylbenzyltrimethylamne bromide-block-polymethylbutylene [PVBTMA][Br]-b-PMB, it hydoxide analogue and the random polymers of the same chemistry crosslinked by UV and thermal methods. We have also extended this study to polyphenylene-b-[PVBTMA][OH] and the pentablock quaternary ammonium functioalized analogue of the sulfonated polymer, Nexar. In amny of these polymers we observe an order-disorder transition, Tδ, where dispersed cations aglomorate due to the interactions of their dipoles. As Tδ often occurs above 0°C this phenomenom is of nterset to the development of practical devices. This study has important consequences for the design of next generation revolutionary AEMs.

(Invited) Conductivity and Relaxation Phenomena in Ion Conducting Materials By Broadband Electric Spectroscopy

Ionically conducting materials (ICM) are of great importance for the fabrication of portable batteries for electronic devices such as computers, tools, video and still cameras, and for the development of fuel cell and battery-powered electric vehicles, dye-sensitized solar cells, supercapacitors and sensors.1 It has been suggested that conductivity in ICMs occurs via a number of different processes. The predominant conductivity processes are attributed to: a) the charge migration of ions between coordination sites in the host materials;2-5 and b) the increase of conductivity due to relaxation phenomena involving the dynamics of the host materials.2-5 Ions “hopping” to new chemical environments can lead to successful charge migration only if ion-occupying domains relax via reorganizational processes,2-5 which generally are coupled with relaxation events associated with the host matrix. Here, it will be described in a concise fashion, the instruments used to comprehensively study the electric response of ionic conductors. To provide the reader with the basic tools necessary for understanding broadband electric spectroscopy,6-8 the first part will review the general phenomena and basic theory behind each type of electric response that materials may exhibit when they are subjected to static or dynamic electric fields. This will be achieved by focusing on the practical use of equations, while referring to specialized texts for detailed explanations of the equations. The second part of this presentation will describe in detail the strategies of data analysis, which is accomplished using specific empirical or theoretical models (Fig. 1). The third part will detail the methodologies for accurate data collection. Acknowledgements The authors thank the Strategic Project “From materials for Membrane electrode Assemblies to electric Energy conversion and SToRAge devices” (MAESTRA) of the University of Padova for funding this activity. References Polymer Electrolytes: Fundamentals and Applications; Sequeira, C.; Santos, D., Eds.; Woodhead Publishing Limited: Oxford, 2010.Di Noto, V. J. Phys. Chem. B 2000, 104, 10116.Di Noto, V.; Vittadello, M.; Lavina, S.; Fauri, M.; Biscazzo, S. J. Phys. Chem. B 2001, 105, 4584.Di Noto, V. J. Phys. Chem. B 2002, 106, 11139.Di Noto, V.; Vittadello, M.; Greenbaum, S. G.; Suarez, S.; Kano, K.; Furukawa, T. J. Phys. Chem. B 2004, 108, 18832.Di Noto, V.; Giffin, G. A.; Vezzù, K.; Piga, M.; Lavina S. Broadband Dielectric Spectroscopy: A Powerful Tool for the Determination of Charge Transfer Mechanisms in Ion Conductors, in Solid State Proton Conductors: Properties and Applications in Fuel Cells, P. Knauth, M. L. Di Vona, Eds., John Wiley & Sons Weinheim, Germany, 2012.Runt, J. P.; Fitzgerald, J. J. Dielectric Spectroscopy of Polymeric Materials: Fundamentals and Applications; American Chemical Society: Washington, D.C., 1997.Schoenhals, A.; Kremer, F. Broadband Dielectric Spectroscopy; Springer-Verlag: Berlin, 2003.Di Noto, V.; Guinevere, G.A.; Vezzù, K.; Nawn, G.; Bertasi, F.; Tsai, T.-H.; Maes, A.; Seifert, S. ; Coughlin, B.; Herring, A. Interplay between solid state transitions, conductivity mechanism, and electrical relaxations in a [PVBTMA][Br]-b-PMB diblock copolymer membrane for electrochemical applications. Phys Chem Chem Phys 2015, 17, 31125-311139. Figure 1

Dielectric relaxations of polyether-based polyurethanes containing ionic liquids as antistatic agents.

Dielectric properties of polyurethanes containing poly(propylene oxide) (PO) and poly(ethylene oxide) (EO) units are discussed, along with the results of direct current (DC) measurements and broadband electrical spectroscopy (BES) studies. The dielectric properties of polyether-containing polyurethanes (PUs) are compared to those of PUs containing 1000 ppm of ionic liquids (ILs) as antistatic agents. The effects of the chemical environment of these ILs, including anion-fixed polymers (PU-AF), cation-fixed polymers (PU-CF), and a simple mixture of IL with the PUs (PU-IL), are compared and discussed on the basis of ion mobility. DC measurements suggest that the charge current is attributed not only to the electrode polarization but also to continuous dielectric relaxation. BES studies elucidate that both fast and slow relaxations are taking place in EO-rich domains in pristine PU and PU-AF. The activation energies of the slow relaxation and of the ionic conductivity are similar, suggesting that the ionic conductivity of these materials is attributed to the ion exchange reaction in EO/ion complexes. In contrast, only fast relaxations are observed in the domains mostly comprised of ion-depleted EO in the PUs containing "free" anions, i.e., PU-CF and PU-IL. This suggests that [Tf2N](-) ligands are weakly interacting with the EO chains and contribute effectively to the ion conduction. Thus, enhanced ionic conductivity is observed in PU-CF and PU-IL, yielding sufficient antistatic effects. Taking into account its long shelf life, arising from the lack of IL bleed-out, PU-CF is concluded to be the most promising candidate.

Thin, Robust, Stable, High Performance Anion Exchnage Membranes for Elecrochemical Energy Conversion Applications

The potential of anion exchange membrane (AEM) fuel cells to provide inexpensive compact power from a wider variety of fuels than is possible with a proton exchange membrane (PEM) fuel cell, has continued to drive the research interest in this area. Alkaline catalysis in fuel cells has been demonstrated with non-precious metal catalysts, and with a variety of fuels beyond H2 and methanol. Alkaline fuel cells (AFCs), based on aqueous solutions of KOH, have serious drawbacks associated with system complexity and carbonate formation. Anion exchange membrane (AEMs) fuel cells have a number of advantages over both PEM fuel cells and traditional AFCs; however, although anionic conductivity in AEMs can be comparable to PEMs the chemical stability of membrane attached cations in hydroxide is still not always sufficient for practical applications. The real issue is water transport; water is both a product and a reactant in these systems and wet cations are much more stable than dry. Wetted cations are alos much more stable than dry cations. So an understanding of water in these membranes is essential. Here we discuss water and anion transport in a series of thin mechanically robust state of the rt experimental anion exchange membranes. The membranes are generally constructed from an isoprene block and a vinyl benzyl bromide block, either randomly or in di-, tri- or penta- block configurations. Post quaterniaztion leads to functionalized AEMs. We use electrochemical impedance spectroscopy to measure anion conductivity, multi-nuclear pulse field gradient spin echo NMR to measure self-diffusion, and broadband electric spectroscopy to measure the relaxation processes in these polymers. This information is coupled with microscopy and SAXS to explore the polymer morphology. Putting transport and morphology together allows us to describe a complete picture of water and anion transport in these systems. We have begun to build direct fueled AEM fuel cells and electrolyzers for hydrogen and ammonia production from these membranes and their related ionomers. We will alos show a few examples of the AEMs practical uses in single cell devices in this talk.

Membranes from Blended Ionomer/PVDF Nanofibers: II. Interplay Between Properties and Electric Response

Fact of Fiction: Do ion-exchange membranes really require complete water saturation in order to possess desirable conductivity? The fabrication of new proton-conducting membranes with a high conductivity at high temperature and low relative humidity conditions is a critically important research challenge for fuel cell developers [1]. Similarly, high conductivity and low crossover membranes are needed in redox flow batteries. Recently, membranes were made by nanofiber electrospinning solutions of perfluoroionomer + PVDF, followed by hot-pressing the resulting blended fiber mat into a dense membrane. These films have unusual and highly desirable properties for use in hydrogen/air fuel cells and/or aqueous flow batteries. In particular, some blended fiber membranes exhibit unusually high proton conductivity at very low water contents. The effect of processing on the complex interplay between the chemical composition, structure and conductivity mechanism of these innovative membranes is still poorly understood. Here we present a complete analysis of the physical-chemical properties as well as the conductivity mechanism of Nafion-PVDF (90wt%/10wt%) membranes obtained from a single fiber mat. These results are contrasted with data collected using a Nafion/PVDF blended membrane prepared by conventional solvent casting. The membranes are extensively characterized by thermoanalytical techniques (water uptake determination, HR-TGA, MDSC, DMA), vibrational spectroscopy (FT-IR ATR) [3], and broadband electric spectroscopy (BES). A Nafion-PVDF (90wt%/10wt%) membrane obtained from an electrospun single fiber mat shows a very low level of water (3 w%) and exhibits high conductivity. The two polymers are found to be well mixed with chains of Nafion surrounding those of PVDF. In addition, there is no evidence of bulk water domains but rather the water molecules are aligned along the polymer chains facilitating a fast proton transfer mechanism. The added benefit of the low water uptake is that there is negligible swelling of the membrane, rendering membranes of this type suitable for a variety of applications. BES measurements are carried out from -100 to 120°C and from 0.03 to 107Hz and are able to elucidate different kind of conductivity mechanisms. The electric response of the Nafion-PVDF (90/10) mat (Figure 1a) and the blend fiber membrane (Figure 1b) are very different from those of Nafion [4], while that obtained from solvent casting (Figure 1c) is similar to Nafion for T>0°C and to mat membrane at T<0°C. Acknowledgements This research was funded by the Strategic Project of the University of Padua “Materials for Membrane-Electrode Assemblies to Electric Energy Conversion and Storage Device (MAESTRA)” and by Veneto Nanotech S.C.p.a. (Venice). REFERENCES [1] V. Di Noto, T.A. Zawodzinski, A.M. Herring, G.A. Giffin, E. Negro, S. Lavina, Int. J. Hydrogen Energy 37, 6120 (2012). [2] J.B. Ballengee, P.N. Pintauro, J. Membr. Sci. 442, 187 (2013). [3]. E. Negro, M. Vittadello, K. Vezzù, S. J. Paddison, V. Di Noto, Solid State Ionics 252, 84 (2013). [4] V. Di Noto, M. Piga, G. Pace, E. Negro, S. Lavina, ECS Trans. 16, 1183 (2008). Figure 1

Hydroxide Transport in Next Generation Anion Exchange Membranes

The potential of anion exchange membrane (AEM) fuel cells/electrolyzers and other devices to provide inexpensive compact power from a wider variety of fuels than is possible with a proton exchange membrane (PEM) fuel cell, has continued to drive the research interest in this area. Alkaline catalysis in fuel cells has been demonstrated with non-precious metal catalysts, and with a variety of fuels beyond H2and methanol. Alkaline fuel cells (AFCs), based on aqueous solutions of KOH, have serious drawbacks associated with system complexity and carbonate formation. AEM fuel cells have a number of advantages over both PEM fuel cells and traditional AFCs; however, although anionic conductivity in AEMs can be comparable to PEMs the chemical stability of membrane attached cations in hydroxide is still not always sufficient for practical applications. The real issue though, is water transport; water is both a product and a reactant in these systems, and wet cations are much more stable than dry. So an understanding of water in these membranes is essential. Recenetly we have fabricated a series of membrnes with sufficent stability that we are able to measure the hydroxide conductivity in them at elevted temperatures while being humidified. Here we discuss water and hydroxide transport in a series of thin mechanically robust state of the art experimental AEMs. The membranes are generally constructed from an isoprene or ethylene block and a vinyl benzyl bromide block, either randomly or in di-, tri- or penta- block configurations. Post quaternization leads to functionalized AEMs. We use electrochemical impedance spectroscopy to measure anion conductivity, multi-nuclear pulse field gradient spin echo NMR to measure self-diffusion, and broadband electric spectroscopy to measure the relaxation processes in these polymers. This information is coupled with microscopy and SAXS to explore the polymer morphology. Putting transport and morphology together allows us to describe a complete picture of water and anion transport in these systems.

Understanding Polymer Ion Clustering and Its Implications for Fast Ion Transport in Polymer Electrolyte Membranes

Advanced polymer electrolyte membranes have the potential to enable new electrochemical devices, as acid functionalized membranes are aimed at higher and drier operating conditions and anion exchange membranes (AEMs) allow catalysis in alkaline media. However, little is known about transport in these newer materials. Alkaline fuel cells (AFCs) have been widely investigated since the 1960s. The perceived advantages of alkaline electrolytes (e.g., KOH solution) used in AFCs include the applications of non precious metal catalysts and increased fuel flexibility. However, the use of liquid alkaline electrolytes have disadvantages such as maintaining containment of the electrolyte, component corrosion, and reaction with CO2 to form insoluble carbonates that can block the electrodes, which must be scrubbed, necessitating the use of pumps and dramatically lowering system power density. There is currently great interest in using AEMs in electrochemical devices, as the carbonate anion is mobile in these materials, the elimination of the liquid electrolyte increases system simplicity, and dramatically higher power densities can be achieved. Two limitations of AEMs must be overcome to enable practical applications: their inherent low conductivity (compared to proton exchange membranes) and the chemical stability of the organic cations that are susceptible to nucleophile attack by hydroxide in the operating device. A large number of new chemistries have been proposed to overcome the two issues of needing high ionic conductivity and chemical stability. However, in order to design next generation AEMs we must correlate the cation chemistry with other membrane properties, e.g. anion and water transport, morphology, water absorption, in order to better understand the performance of the AEM. Simple quaternary ammonium cations have been studied extensively as they provide good model systems and in certain membranes have been shown to have adequate stability, but they do not provide a route to the thousands of hours of transient operation required in many real devices. Various 2nd generation cations have been proposed for enhanced chemical stability including phosphonium, pydridinium, sulfonium, imidazolium, guanidinium, and complex metal cations, e.g. (bis-terpyridine Ru). In all cases it is rare, especially when the membranes are not water saturated, to observe simple Arrhenius behavior for ionic conductivity in these systems. Generally the ion pairs in the material are not fully dissociated and the ionic movement is coupled either to the relaxations of the side group, VTF behavior, or the polymer backbone. In addition we are increasingly observing that the ions on the polymer backbone relax from a distributed configuration to a thermodynamically proffered clustered state in which ion transport becomes hindered. Very Often this transition as observed by broadband electric spectroscopy, electrochemical impedance spectroscopy, or Pulsed Field Gradient Spin Echo NMR spectroscopy are below or at the temperature of operation of the devices in which the materials are designed to function. Obviously, understanding these processes and designing materials that do not undergo such limitations is crucial to the design and implementation of next generation electrochemical energy conversion devices. In this talk we will show examples of how ionic conductivity is affected by the properties of the polymer matrix.

Interplay Between Vibrational Modes and Relaxations in Electrolytes for Secondary Magnesium Batteries Based on Haloaluminate Ionic Liquids

The search for electrolytes with enhanced performance is a crucial point for the development of efficient secondary Mg batteries.1 Early work on Mg deposition and stripping from Grignard-based non-aqueous solution were proposed in 1990 by Gregory et al.,2 while the first example of polymer electrolytes comprising PEG400 and δ-MgCl2 was proposed in 1998.3 In the following years, electrolytes based on EthylMgBr and PEO,4 Grignard,5,6 and other organo-Mg7compounds were explored. Among all proposed materials, Grignard and organo-Mg compounds, suffer from several drawbacks associated with their chemical stability in ethereal-based solvents, which are characterized by a high vapor pressure and flammability. On the contrary, Ionic Liquids (ILs) seem a very appealing class of materials for applications in Mg secondary batteries due to their: a) very low volatility, high thermal stability and non-flammability; b) high ion density and high conductivity; c) wide electrochemical stability window; and d) easiness of synthesis by evaluating the best cation and the anion for each application.8 There are still several aspects that are not fully understood regarding the use of ILs in electrochemical devices. Indeed, the conductivity mechanisms, the formation of the solid-electrolyte interface (SEI) and long-term performance of these systems are still open questions. Nevertheless, from a fundamental point of view, understanding the interplay between the ILs relaxations and the charge carrier migration is crucial in order to clarify the effect of the ILs matrix on the conductivity mechanism of Mg2+ions. Here we present a family of electrolytes based on 1-ethyl-3-methylimidazolium iodide, aluminium iodide, and δ-MgI2 8 with general formula [EMImI/(AlI3)m]/(δ-MgI2)n. The short-range structural features and the interactions in the electrolytes are elucidated by coupling Raman and Infrared (both in the medium and in the far infrared) spectroscopy with DFT calculations. The detailed electrical response of the [EMImI/(AlI3)m]/(δ-MgI2)n materials in terms of polarization and relaxation events at temperatures higher and lower than the melting of EMImI/(AlI3)m are investigated by using Broadband Electrical Spectroscopy (BES). The results allow us to correlate the dielectric relaxation of the imidazolium cations with the overall long-range charge migrations, thus elucidating the interplay existing between conductivity and nanostructure of this new class of IL. References (1) Muldoon, J.; Bucur, C. B.; Oliver, A. G.; Sugimoto, T.; Matsui, M.; Kim, H. S.; Allred, G. D.; Zajicek, J.; Kotani, Y. Electrolyte Roadblocks to a Magnesium Rechargeable Battery. Energy Environ. Sci. 2012, 5, 5941–5950. (2) Gregory, T. D.; Hoffman, R. J.; Winterton, R. C. Nonaqueous Electrochemistry of Magnesium. J. Electrochem. Soc. 1990, 137, 775–780. (3) Di Noto, V.; Lavina, S.; Longo, D.; Vidali, M. A Novel Electrolytic Complex Based on δ-MgCl2 and Poly(ethylene Glycol) 400. Electrochim. Acta 1998, 43, 1225–1237. (4) Liebenow, C. A Novel Type of Magnesium Ion Conducting Polymer Electrolyte. Electrochim. Acta 1998, 43, 1253–1256. (5) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Prototype Systems for Rechargeable Magnesium Batteries. Nature 2000, 407, 724–727. (6) Lu, Z.; Schechter, A.; Moshkovich, M.; Aurbach, D. On the Electrochemical Behavior of Magnesium Electrodes in Polar Aprotic Electrolyte Solutions. J. Electroanal. Chem. 1999, 466, 203–217. (7) Liebenow, C.; Yang, Z.; Lobitz, P. The Electrodeposition of Magnesium Using Solutions of Organomagnesium Halides, Amidomagnesium Halides and Magnesium Organoborates. Electrochem. Commun. 2000, 2, 641–645. (8) Electrochemical Aspects of Ionic Liquids; Ohno, H., Ed.; 2nd ed.; John Wiley & Sons: Hoboken, NJ, USA, 2005; Vol. 127.

Hybrid Polymer Electrolytes Based on Linear Siloxane Networks and Crosslinked Polyether Domains: Interplay Between Composition and Properties

In this study, a new class of hybrid polymer electrolytes was prepared and characterized. The materials consisted of a matrix based on linear oligosiloxane networks and partially crosslinked polyether domains, doped with Lithium bis(trifluorosulfonyl)imide (LiTFSI). These materials are characterized by a maximum conductivity of 8∙10-5 S/cm at 30 °C, and high thermo-mechanical stability, with decomposition temperature of 250 °C and a storage shear modulus of almost 105 Pa at 100 °C. Compositional parameters such as salt, cross-linker concentration, and polyether chain length were varied, and the materials were characterized by several analytical techniques. Vibrational spectroscopy provided details of the hybrid structure and the effect of salt doping. The thermo-mechanical properties were studied by thermogravimetric analysis, differential scanning calorimetry and rheological analysis. Finally, the transport properties were analyzed by means of broadband electric spectroscopy. The study made it possible to describe the relationships between composition, morphology, thermo-mechanical and transport properties of the synthesized hybrid electrolytes, with particular emphasis on the interplay between polymer dynamics and conduction mechanism. Figure 1

Single-Ion-Conducting Nanocomposite Polymer Electrolytes for Lithium Batteries Based on Lithiated-Fluorinated-Iron Oxide and Poly(ethylene glycol) 400

A poly(ethylene glycol) 400 (PEG400) matrix doped with different amounts of a fluorinated Fe2O3-based nanofiller (LiFI) featuring a Li+-functionalised surface gives rise to nanocomposite polymer electrolytes (nCPEs) that demonstrate single-ion conduction. A family of nCPEs with general formula [PEG400/(LiFI)y] and y=nFe/nPEG400 ranging from 0 to 8.15 are prepared; they are characterized by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), High-Resolution Thermogravimetric Analysis (HR-TGA), Differential Scanning Calorimetry (DSC), and Fourier-transform vibrational spectroscopy in both the medium (MIR) and far (FIR) infrared. The Li+ transference number, tLi+,is determined and Broadband Electrical Spectroscopy (BES) is used to elucidate the electrical response of the materials in terms of polarization and relaxation events. The combination of the information obtained by all the aforementioned techniques enables us to present a possible conduction mechanism for these nCPEs single-ion conducting systems.

Nanocomposite membranes based on polybenzimidazole and ZrO2 for high-temperature proton exchange membrane fuel cells.

Owing to the numerous benefits obtained when operating proton exchange membrane fuel cells at elevated temperature (>100 °C), the development of thermally stable proton exchange membranes that demonstrate conductivity under anhydrous conditions remains a significant goal for fuel cell technology. This paper presents composite membranes consisting of poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (PBI4N) impregnated with a ZrO2 nanofiller of varying content (ranging from 0 to 22 wt %). The structure-property relationships of the acid-doped and undoped composite membranes have been studied using thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis, wide-angle X-ray scattering, infrared spectroscopy, and broadband electrical spectroscopy. Results indicate that the level of nanofiller has a significant effect on the membrane properties. From 0 to 8 wt %, the acid uptake as well as the thermal and mechanical properties of the membrane increase. As the nanofiller level is increased from 8 to 22 wt % the opposite effect is observed. At 185 °C, the ionic conductivity of [PBI4N(ZrO2 )0.231 ](H3 PO4 )13 is found to be 1.04×10(-1) S cm(-1) . This renders membranes of this type promising candidates for use in high-temperature proton exchange membrane fuel cells.