Designing Pores for Proton Conduction in Metal Phosphates, Coordination Polymers and Organic Polymers

Hydrogen fuel cells offer a promising route to clean, on-demand energy; however, efficiency and stability challenges have limited their widespread commercial adoption. One of the main issues lies in the sensitivity of the electrolyte of the hydrogen fuel cell which is the material responsible for transporting protons from the anode to the cathode. The current state of the art in electrolyte technology is Nafion®, a family of perfluorosulfonic acid polymers, which are highly conductive but suffer from strong humidity dependence. Since maintaining hydration requires complex water management systems, developing proton exchange membranes with reduced humidity dependence has become a major research focus. This thesis details investigations into several novel proton conductive materials, evaluating their stability, structure and humidity requirements. Chapter One provides an overview of hydrogen fuel cells, the design principles behind proton exchange membrane electrolytes and examples of materials that have achieved success in this field. Chapter Two outlines the specialized methods and characterization techniques employed. Chapter Three investigates the synthesis and conductivity of an amorphous proton conductive material derived from phytic acid, an inositol hexaphosphate ligand. A reproducible synthesis is detailed, a challenge owing to the amorphous nature, along with proton conductivity studies. Upon calcination, the material transforms into a mixture of AlP3O9 and AlPO4, which displays low conductivity. In Chapters Four and Five a 1-D lanthanum coordination polymer was synthesized from a 1-hydroxy-3-(4-imidazolyl)-propane-1,1-bisphosphonate ligand. This coordination polymer demonstrates solvent dependent flexing behaviour and moderate proton conductivity. When soaked in a 5M aqueous imidazole solution, the framework incorporates imidazole molecules in a highly stable, charge compensating manner. This enhances the material’s anhydrous proton conductivity relative to the unloaded form. Chapter Six explores synthetic pathways towards a metal ion templated material. Although the template did not incorporate into the polymer as intended, the reaction yielded a porous polymer upon template removal. Taken together, this thesis provides insights into strategies for designing stable, more humidity independent proton conductive materials for fuel cell applications.