A study on proton conductivity in organic ionic plastic crystal electrolytes

Although, ionic liquids (ILs) and organic ionic plastic crystals (OIPCs) have been known for more then half a century, their unique features as solvent free non- aqueous electrolyte systems for various electrochemical applications have received a significant amount of attention in the present decade. The proton-conducting versions of ILs are known as protic ionic liquids (PILs), which display high proton conductivity and high thermal stability, which makes them a suitable candidate for proton conducting electrolyte membrane fuel cell (PEMFC) technology. On the other hand, OIPCs can be regarded as the solid versions of these ILs with several additional advantages over ILs. Many OIPCs display liquid like ionic conductivities in their high temperature plastic crystal phase, while being solid-state conductors they can easily be fabricated into a stand-alone separator in the PEMFC configuration. However, most of the recent developments in PEMFCs using these organic ion-conducting electrolytes remained focused on ILs and PILs and apparently no attention have been paid to OIPCs as potential proton conductors. The thesis mainly concentrates on the investigation of proton conductivity in OIPCs, with the aim to develop a new class of solid-state proton conducting electrolyte for PEMFC technology. The current study is based on the hypothesis that the target ion (H+) diffusivity can be achieved by doping suitable acids into the OIPC matrix. In such a case, the basic purpose of the plastic crystal is to act as a solid solvent for fast H+ transport. The studies initially started with an investigation of proton conductivity in choline dihydrogen phosphate [Choline][DHP]. However, the preliminary investigations by electrochemical characterization [cyclic voltammetry (CV)] did not show any sign of proton conduction in the neat plastic crystal material even at elevated temperatures ∼ 100 – 120 °C. We proceeded to intentionally introduce proton conduction in this material by acid doping, we carried out a detailed NMR studies using a combination of various 1D and 2D techniques to explore the H+ dynamics and H+ transport mechanism in the [Choline][DHP]. The studies revealed that the exchange of protons between the hydroxyl (OH) group of [Choline]+ cation and the OH groups of [DHP]− anions provides a mechanism of proton transport by structural diffusion through the hydrogen-bonding network. Based on the possibility of H+ conduction in this material, mixtures of [Choline][DHP] with varying concentrations of H3PO4 were prepared and characterized. The choice of H3PO4 as dopant was based on the concept that [DHP]− anions formed by the deprotonation of acid is in common with the plastic crystal matrix. In this work, we found that the H+ conduction was quite high and measurable in the higher acid concentration mixture compared to the neat material, however, the amount of acid required for sufficient proton conduction led to a mixed phase material. XRD analysis of higher H3PO4 containing [Choline][DHP] sample showed that the second phase might be an amorphous solid or more likely a liquid phase. Since, the multiphase proton conducting materials are less desirable than fully solid-state materials, our next attempt was to explore the possibility of using highly dissociating strong acids, triflic acid (TfOH) and bis(trifloromethanesulfonyl)amide [HN(Tf)2] as potential dopants. It was found that even low concentrations of these super acids ∼ 4 mol % is sufficient to enhance the conductivity of neat plastic crystal by several orders of magnitude without disrupting the crystalline phases. The 4 mol% [HN(Tf)2] containing sample also display significantly high proton diffusivity and high proton reduction currents, and therefore a solid state conductor with facile H+ dynamics has been achieved. Freestanding OIPC membranes have been prepared by impregnating acid doped [Choline][DHP] materials into porous cellulose acetate (CA) membranes. These membranes were characterized for thermal stability, phase behaviour, ionic conductivities and H+ reduction currents. The study reveals that the impregnated membranes display high thermal stability, high ionic conductivities and facile H+ reduction currents especially in the temperature range of interests ~ 110 °C. Based on the high proton conductivities, two of these membranes 18 mol% H3PO4 and 4 mol% HN(Tf)2 were tested in fuel cell experiments. Though, the membranes showed promising properties as solid-state proton conductors, the fuel cell performance of these membranes was poor. This was attributed to [DHP]− anion adsorption and inappropriate membrane electrode assembly (MEA) design to test the materials of this kind. The problem of the possible deactivation of Pt surface due to the adsorption of [DHP]− anions led us to synthesize a plastic crystal based on non-hydrated anion such as Tf^-. As per our hypothesis, the new plastic crystal [Choline][Tf] exhibited significantly high ionic conductivity, high thermal stability and plastic crystal behaviour over a broad range of temperature. The acid doped materials showed H+ diffusion twice faster than the matrix ions in the high temperature phases, which makes the acid doped material a potential proton conductor for fuel cell application. A detailed study was then carried out in order to understand the mechanism of ionic conduction in these solid-state materials. The study revealed that all samples display an Arrhenius-like thermally activated conduction mechanism of ionic transport with remarkably low activation energy (Ea) values in the high temperature phases, suggesting liquid like ionic conductivities in these samples. The variable temperature XRD studies of neat Choline Tf and 4 mol% HTf samples showed that both adopt monoclinic structure in phase II and a highly symmetric cubic crystal structure in phase I. The XRD studies also show the thermal expansion of lattice in these materials with increasing order of temperatures. As the mechanism of ionic conductivity in crystalline systems is associated with the generation and expansion of defects, positron annihilation lifetime spectroscopy (PALS) experiments were also carried out. A linear variation of temperature dependence of ionic conductivities as a function of defect size suggests a defect assisted conductivity mechanism in these materials. Moreover, a slight reduction in the defect size and concentration was observed with increasing concentration of HTf, suggesting the rigidifying effects of the added acid. The static and diffusion NMR studies suggest the possibility of Frenkel and Schottky type defects in the neat Choline Tf, while in the case of acid containing Choline Tf samples, the Frenkel, Schottky and divalent cation defects may occur. The studies show that the H+ migration in the acid doped samples is associated with the cation dynamics, where the OH groups of Choline+ cations act as potential pathways for fast proton transport. A mixed Grotthuss and vehicular mechanism is suggested for fast H+ migration in the acid containing samples. Perusing the main aim of developing new proton conducting OIPCs, a novel class of phosphonium cation based organic ionic salts were synthesized and characterized, where the study involves a comparison of these phosphonium salts with their ammonium analogues. Of the four phosphonium salts (P4,4,4,H NO3, P4,4,4,H N(Tf)2, P4,4,4,H CH3SO3, P4,4,4,H Tf), which were synthesized, only P4,4,4,H Tf was found to be high temperature protic OIPC. The rest of them were categorized as protic ionic liquids (PILs). In this study, we found that the phosphonium based PILs show high thermal stability and high ionic conductivity compared to their corresponding ammonium analogues. The trends in the increasing thermal stability of these protic salts as a function of increasing acid strength were found to be consistent with the proton energy level differences between the component acid and base species forming the salt. Moreover, the electrochemical studies revealed that the phosphonium cation is more acidic (~ +150 mV for H+ reduction potential) than its ammonium analogue, a property that can be helpful for facile oxygen reduction reaction (ORR) at the cathodic side of PEM fuel cell.