With the increasing world population and limited fossil energy resources, technologies that produce fresh water and clean energy in sustainable ways have become hot topics. Fuel cells, such as proton exchange membranes fuel cells (PEMFCs) using proton exchange membranes (PEMs) as solid electrolyte are regarded as promising clean energy devices for vehicle propulsion, stationary and portable power with high energy conversion, decent energy density and sustainability. Among the various methods for water purification, a membrane-based technology, i.e., reverse osmosis (RO), is the most widely used membrane-based technology for desalination applications due to their cost effectiveness and high energy efficiency. For both PEMFC and RO technologies, polyelectrolyte membranes or ion-selective membranes represent a core enabling component. Novel membranes are in high demand in both fields to address the challenges that existing membranes are facing, such as insufficient chemical stability (e.g., chlorine resistance for RO membranes) and mechanical stability (excessive swelling for PEMs). Sulfonated polysulfones have been recognized as attractive candidates for PEMs and RO due to their good chemical, thermal, mechanical stabilities and diverse structure varieties. However, the classic bisphenol A (Bis-A)- or biphenol (BP)-based sulfonated polysulfones showed inferior proton conductivities and oxidative stabilities for fuel cell application, and their water/salt selectivity still has a lot of room for improvement as RO membranes. As such, the development of new sulfonated polysulfones is of high interest to advance fuel cells and membrane desalination applications to a new level.To address these challenges, this dissertation reports the development of a novel series of high-free-volume ionic polymers containing bulky pentiptycene or phenolphthalein-based cardo units. Previous study in our group showed that incorporating triptycene units into sulfonated polysulfones could effectively enhance fuel cell and desalination performance due to the presence of unique molecular internal free volume intrinsic to the triptycene units. In this regard, we are motivated to extend the design of sulfonated polysulfone to include an even bulkier and more hierarchical iptycene unit-pentiptycene. Given more open clefts and free volume to facilitate small molecules transport, pentiptycene-based sulfonated polysulfones are highly promising to further enhance the performance of PEMFC and RO membranes.To expand the spectrum of membrane materials with diverse functionality, this dissertation also reported a novel platform of phenolphthalein-based polysulfones for high-performance PEMFC and water purifications. Phenolphthalein, well-known as a pH indicator, provides tremendous opportunities to achieve multifunctional polymeric materials via derivation or grafting reactions through the heterocyclic pendant lactone. Various strategies were explored to engineer the phenolphthalein-based polymers with a variety of functionality and morphologies such as cross-linking, multi-block copolymers, comb-shape polymers and their combinations, which, correspondingly, exhibited a wide range of tunable PEMFC and water purification performance to allow for establishing the fundamental structure-property relationships for these new membrane materials.