Novel functional polymer materials for electrochemical devices and gas separation applications

Abstract

Renewable and sustainable alternative energies have been the research areas of importance for both academia and industry amid the looming depletion of fossil fuels and the developing global warming crisis. Hydrogen has been regarded as a promising clean energy carrier that possesses high gravimetric energy density and zero greenhouse-gas emission property. The production and conversion of hydrogen fuel involve multiple electrochemical devices including fuel cells, water electrolyzers, and electrochemical hydrogen compressors, among which the electrolyte serves as an essential component. Although the polymer electrolyte membranes hold advantages over liquid electrolytes in terms of non-volatility, minor material corrosion, and negligible electrolyte leakage, there are still challenges that need to be addressed for better overall device performance. For example, fuel cells using anion exchange membrane (AEM) as electrolyte suffer from insufficient hydroxide conductivity and long-term alkaline stability; fuel cells using proton exchange membrane (PEM) as electrolyte (e.g., Nafion®) are expensive and have operating conditions limited to low temperatures and high relative humidities. This dissertation demonstrates the design, synthesis, and characterization of novel functional polymer membranes that show great potential for electrochemical devices and gas separation applications, which will ultimately benefit the adoption of alternative clean energy technologies and the emission reduction of greenhouse gases. The first part focuses on the design and preparation of polymer electrolyte membranes. In Chapter 2, a series of crosslinked AEMs based on SEBS block copolymer is prepared by a unique simultaneous polymer functionalization and crosslinking strategy. The product membranes offer improved hydroxide conductivity with excellent dimensional stability. A detailed investigation into the relationship between the degree of crosslinking/functionalization and membrane properties are discussed. In Chapter 3, phosphoric acid doped PEM materials are developed by employing the cation-anion ion-pair structures. The presence of ion-pair interactions improves the retention of doped phosphoric acids under non-anhydrous conditions. The resulting PEMs exhibit good proton conductivity at low-to-intermediate relative humidity range which is not achievable by commercial Nafion® or phosphoric acid-doped polybenzimidazole PEMs. As fossil fuel will not be completely replaced by renewable energy resources in the near future, the technology development of the emission reduction of greenhouse gases, especially CO2, is equally meaningful. The second part of this dissertation is focused on the material design and characterization of CO2/gas separation membranes. The polymer membrane-based CO2 separation technology offers benefits of energy efficiency, small footprint, low cost, and low maintenance compared to the traditional liquid amine absorption strategy. Chapter 4 presents SEBS-based polymer membranes that have favorable CO2 affinity by incorporating a short segment of the polar ether-bearing functional group. The permeability selectivity and solubility selectivity of CO2 are investigated as a function of the content of the grafted triethylene oxides. Chapter 5 discusses the effects of different amines on the CO2/gas separation properties. Tertiary amines are found to have minimal CO2 affinity under dry conditions, while quaternary ammoniums could enhance the membrane’s size-sieving ability.