Enhancing materials for fuel cells and organic solar cells through molecular design

Abstract

In an effort to make alternative energy competitive to fossil fuels, research in improving efficiencies of solar cells and fuel cells have grown rapidly over the last few decades. One prominent strategy to improving the efficiencies in these devices focuses on engineering materials with customized molecular structure for enhancements in specific properties. Herein, new organic materials are designed and synthesized to enhance selected properties for applications in fuel cells and solar cells. In chapter 1, triptycene poly(aryl ethers) are synthesized and characterized for enhancing ion conductivity of ion exchange membranes in fuel cells. Triptycene motif is incorporated to increase charge density and fractional free volume in the membranes. In Chapter 1.2, sulfonated triptycene poly(ether ether ketone) (S-tripPEEK) is synthesized and studied for proton exchange membranes (PEM). Increasing fractional free volume in the membrane results in high water uptake at relative humidity (RH) from 10 %RH to 90 %RH and higher proton mobility in the membranes. S-tripPEEK membranes show proton conductivities of 334 mS/cm at 85 °C at 90 %RH and 0.37 mS/cm at 85 °C at 20 %RH as compared to 18.9 mS/cm and 0.0014 mS/cm observed in commercially available Nafion117[superscript TM] membranes. In Chapter 1.3, methylimidazolium triptycene poly(ether sulfone)s (MeIm-tripPES) are made for alkaline anion exchange membranes (AAEM) and are found to have ion conductivities of 104 mS/cm at 80 °C in water. Controlled nanophase separation with increased domain size contributed by the triptycene moiety lead to the high observed conductivities. However, the methylimidazolium functional groups on the membranes are not stable to alkaline conditions in the operation of a fuel cell. In Chapter 2, dithiolodithiole (C₄S₄) heterocycle was synthesized and studied as a new building block for organic photovoltaic materials. As an electron-rich fused-ring motif, C₄S₄ is expected to be a more effective electron donor. Comparison with analogous thiophene derivatives shows that C4S4 moiety raises the highest occupied molecular orbital (HOMO) by 0.7 - 1.0 eV, suggesting a stronger electron donating character than thiophene. Furthermore, crystal structures of C4S4 molecules show planarity in the molecule which further reduces the bandgap.