Phosphazenes for Energy Production and Storage: Applied and Exploratory Synthesis

Open Access
Author:
Hess, Andrew R
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 20, 2015
Committee Members:
  • Harry R Allcock, Dissertation Advisor
  • John V Badding, Committee Member
  • John B Asbury, Committee Member
  • Michael Anthony Hickner, Special Member
Keywords:
  • phosphazene
  • polyphosphazene
  • polymer electrolytes
  • DSSC
  • dye sensitized solar cell
  • lithium battery
  • lithium-ion battery
Abstract:
The work described in this dissertation involves progress toward phosphazene-based ion conducting materials with a focus on structure-property relationships and the use of the knowledge of those relationships to improve these materials. This dissertation also includes some more fundamental exploratory syntheses to probe the limits of phosphazene chemistry and discover structure-property relationships that may be useful in designing compounds to fulfill important technical requirements. Chapter 1 provides a brief introduction to polymers and polyphosphazenes as well as ion-conducting materials and the contribution of polyphosphazene chemistry to that field. Chapter 1 also provides a brief introduction to some of the analytical techniques employed in this dissertation. Chapter 2 begins with the use of organophosphates as stand-alone non-volatile and fire-retardant liquid electrolyte media for dye sensitized solar cells (DSSCs) as well as their use as plasticizer in polymer gel electrolytes intended for application in lithium batteries. These organophosphates are the smallest phosphorus containing model molecules investigated in this dissertation. A homologous series of oligoalkyleneoxy substituted phosphates was synthesized and the effect of the substituent chain length on viscosity and conductivity was investigated. Small, test-scale DSSCs were constructed and showed promising results with overall cell efficiencies of up to 3.6% under un-optimized conditions. Conductivity measurements were performed on polymer gel-electrolytes based on poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (MEEP) plasticized with the phosphate with the best combination of properties, using a system loaded with lithium trifluoromethanesulfonate as the charge carrier. These measurements yielded promising results with conductivities (at 30 °C) as high as 3.0x10-3 S·cm-1 at the highest plasticizer loading. In chapter 3 the effect of the cation of the charge carrier species on the anionic conductivity of DSSC type electrolytes is evaluated using hexakis(2-(2-methoxyethoxy)ethoxy)cyclotriphosphazene (MEE-trimer) as a small molecule model for MEEP. The iodides of lithium, sodium, and ammonium as well as the ionic liquid, 1-propyl-3-methylimidazolium iodide (PMII) were tested in various electrolyte concentrations. After identifying PMII as the most efficient salt additive for promoting anion conduction in the MEE-trimer systems, it was tested in MEEP-based DSSC polymer-gel electrolytes plasticized with propylene carbonate (PC). The MEEP/PMII type electrolytes were compared against and outperformed similar formulations based on poly(ethylene oxide). An overall cell efficiency of 1.88% was obtained. Chapter 4 extends the concept of poly[(oligoethyleneoxy)phosphazene] electrolytes by the synthesis of MEEP-analogues with various ammonium ions covalently bound to the polymer side chains in order to create a single-ion conducting polyelectrolyte. The motivation for this approach is that in this system the polymer serves as the conductive medium and provides the charge carriers. In this case iodide is the only ion that is free to move and carry charge since the cation is bound to the polymer. Two candidates from the series of polyelectrolytes were selected for extensive study, one in which the ammonium ion has a hexyl group, and another in which the ammonium ion has an MEE-group. The polymers were characterized by dielectric spectroscopy, X-ray scattering, and density functional theory calculations. In chapter 5 the concept of polyphosphazene based polyelectrolytes is extended further by quaternization of nitrogen atoms that are a part of the polymer backbone rather than a pendent group. The feasibility of the concept is first explored using cyclic-trimeric phosphazenes as small molecule models and is then extended to high polymeric systems. It was found that it is indeed possible to produce stable backbone-quaternized polyphosphazene electrolytes using strong methylation reagents such as methyl trifluoromethanesulfonate or trimethyloxonium fluoroborate. Unfortunately for their potential use in DSSCs, ion-exchange of the anions from the methylation reagents for iodide ion results in dealkylation or decomposition of the polyelectrolyte due to the high nucleophilicity of iodide. However, the conductivities of the unplasticized polyelectrolytes containing mobile trifluoromethanesulfonate ion are among the highest observed for unplasticized solid-state polymer electrolytes or polyelectrolytes. Chapter 6 begins a new section of exploratory synthesis meant to probe the limits of phosphazene chemistry and discover useful structure-property relationships as well as to improve on the synthetic procedures of the past. Thus, in chapter 6 a series of cyclotriphosphazenes is synthesized for evaluation as compounds with the ability to complex to metal surfaces or to form ceramic materials after pyrolysis. Several known compounds are resynthesized, many with improvements over previously published procedures either in yield or simplicity of reaction conditions. Also, two new cyclotriphosphazenes were synthesized in an attempt to produce a catechol-like side group that could mimic the metal complexation and adhesive abilities of mussel adhesive proteins which take advantage of similar catecholic moieties. Chapter 7 presents the synthesis of fluorine-bearing aryloxy phosphazenes in an attempt to overcome the deficiencies of some of the classical fluorinated organic polymers. Classical organic fluorinated polymers designed to be flexible often contain aliphatic C-H bonds which limits their thermal, chemical, and radiation stability. Poly(fluoroaryloxy)phosphazenes can offer many of the same advantages without the deficiencies of some of the more classical materials. The materials synthesized are excellent candidates for dielectric film materials in capacitors. In chapter 8 new ground is broken in phosphazene chemistry, wherein cycloalkanoxy groups, a type of side-group for phosphazenes that has been neglected in the past, are attached to the phosphazene skeleton. First, a single previous report of the synthesis of hexakis(cyclohexanoxy)cyclotriphosphazene was reproduced as a model system. Then a series of single substituent polyphosphazenes was synthesized in order to determine how large a saturated cyclic ring can be attached to the polymer backbone while still completing the replacement of the chlorine atoms attached to the backbone. Subsequently, a series of mixed substituent polymers containing both cycloalkanoxy groups (from cyclopentoxy to cyclooctanoxy) and the well-studied trifluoroethoxy group was synthesized and characterized in order to study larger saturated cyclic rings as polyphosphazene side-groups. These polymers are interesting not only from a fundamental scientific point of view, but may have applications in the field of membrane separations and are promising candidates for mechanical damping applications. Chapter 9 contains the author’s recommendations for the continuation of the research presented in this dissertation.