Density Functional Theory analysis of clustering of ammonium, phosphonium, and phosphazenium cations with fluoride and hydroxide anions

Open Access
Author:
Labarge, Nicole Alyse
Area of Honors:
Chemical Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Michael John Janik, Thesis Supervisor
  • Themis Matsoukas, Honors Advisor
Keywords:
  • Anion Exchange Membrane
  • fuel cell
  • battery ammonium
  • phosphonium
  • phosphazenium
Abstract:
Both batteries and fuel cells commonly rely on the exchange of cations in order to facilitate electrical current, using Li+ in the case of batteries and H+ in fuel cells. Anion exchange batteries and fuel cells, while not widely used, have the potential to address some main issues with their cation exchange counterparts. The main challenge for fluoride batteries and HEMFCs is to increase anion conductivity of the membrane. The focus of my research is to computationally evaluate cations for interactions with F- and OH- that indicate good conductivity in an anion exchange membrane. In addition to simple pairing, cations and anions can also combine to form aggregates. The possible ion states in the polymer system can be approximated using a four state model: free ions, ion pairs, quadrupoles, and triple ions. Ion pairs consist of a cation and anion; quadrupoles consist of two cations and two anions, arranged so that all interact; and triple ions can exist as negative triple ions, comprised of two anions and one cation, or positive triple ions, comprised of two cations and one anion. The relative affinities for the ions for each state have conductivity implications: quadrupoles impede ion mobility, while charged species promote charge transfer. Fluoride and hydroxide were paired with ammonium, phosphonium, and phosphazenium ions with varied substituents. Density Functional Theory calculations were used to evaluate the stability of the various ion clusters, informing the selection of cations that may offer optimal properties to facilitate anion conduction.