Polymeric Membranes with Optimized Ion Selective Behaviors for Energy Generation and Storage

Abstract

In this thesis, we investigated and developed pressure retarded osmosis (PRO) and vanadium redox flow battery (VRFB) systems for energy generation and management. The PRO system utilizes osmotic pressure difference between two saline solutions to generate electricity with hydraulic turbines. The semipermeable membranes are the most important element in PRO system as the effective osmotic pressure difference is determined by the ion transport behavior of the membrane. Similarly, in VRFB using the redox reaction of vanadium ions to store the energy, the membranes play an important role because they prevent electrode shorting and fuel crossover. However, these ion selective membranes are difficult to fabricate due to the trade-off relationship between the permeability (PRO: water, VRFB: proton) and the ion selectivity. To overcome the challenges of these two systems, we optimized membrane fabrication by introducing novel polymers and post-treatments. This thesis consists of five chapters including an introduction, main text, and conclusions as follows.

In Chapter 1, PRO and VRFB technologies were overviewed. The basic principle and necessity of the applications were represented, and challenges to fabricate the membrane were investigated. As a promising method for fabricating membranes, the electrospinning using an electric field to spray the nanofiber was also explored. The stacked nanofibrous membrane (NFM) showed high porosity, large surface area, and huge pore size compared to conventional fabrication method such as phase separation, template synthesis, and self-assembly. To describe the parameters that affect membrane’s morphology, we classified the parameters into (i) intrinsic properties of the solution, (ii) operating conditions of electrospinning, and (iii) circumstance conditions in electrospinning chamber. As an electrospinning material, thermally rearranged (TR) poly(benzoxazole-co-imide) polymer and sulfonated poly(arylene ether sulfone) (SPAES, which is also referred to as BPSH in this thesis) polymer were used. The property of the polymers and their recent progress were discussed in this chapter. Although the electrospun membranes using BPSH and TR polymers (denoted as BPSH-NFM and TR-NFM, respectively) showed decent physical properties, they were challenged for mechanical strength and hydrophobicity, respectively. Therefore, post-treatments such as chemical cross-linking and direct fluorination were performed to enhance their properties.

In Chapter 2, for PRO application, thin film composite (TFC) membranes were fabricated via interfacial polymerization on BPSH-NFM and TR-NFM using 1, 3, 5-bezenetricarbonyl trichloride (TMC) and m-phenylene diamine (MPD) monomers. To optimize the intrinsic properties of the selective (or active) layer, chlorine modification with NaOCl solution was applied to TFC membranes (denoted as XBPSH-TFC-Cl and XTR-TFC-Cl, respectively). The physical and intrinsic properties of the fabricated membrane were carefully analyzed. The chlorine-modified TFC membranes showed higher water permeability than pristine TFC membranes, while maintaining good salt rejection values. As a result, the highly efficient XTR-TFC-Cl achieved a peak power density of 26.6 W·m−2 at 21 bar using 1 M NaClaq and deionized (D.I.) water as draw and feed solutions, respectively. According to modelling result, XTR-TFC-Cl was less affected by the adverse phenomena such as concentration polarization and reverse salt permeation, and showed lower performance reduction for various feed sources compared to XBPSH-TFC-Cl.

Although the research for XTR-TFC-Cl demonstrated the potential of the next generation TFC membrane, the PRO performance was measured using a 1M NaClaq draw solution, meaning that the efficiency of the PRO system was not maximized. Thus, in Chapter 3, the PRO performance was measured with 3M NaClaq as a draw solution to increase the osmotic pressure difference. However, since the internal concentration polarization (ICP) was severe in hyper saline solution, hydrophilization of support layer was required to reduce the ICP phenomenon. For the first time in osmotic-driven systems, we adopted novel one-step direct fluorination to increase hydrophilicity of the NFM. Direct fluorination increased the total surface energy of the NFM by boosting polar surface energy parameter, which eventually affected the formation of ‘ridge & valley’-like thin film composite membrane (TR-TFC-F5) through interfacial polymerization of the fluorinated NFM. TR-TFC-F5 showed unprecedented power density of 87.2 W·m-2 at 27 bar. For further studies, we assumed that the PRO system with TR-TFC-F5 is hybridized with membrane distillation (MD) for osmotic heat engine (OHE) system, which is closed-loop system converting waste heat into electrical energy. The OHE system with TR-TFC-F5 showed a power generation cost of only 203 $⋅MWh-1, which was less than half the cost observed using commercial membranes.

In Chapter 4, to develop new cost-effective and high performance ion exchange membranes (IEMs) for VRFB applications, BPSH membrane was coated with hydrophobic nano-cracks via atmospheric plasma treatment (P-BPSH). The effect of nano-cracks on the membranes’ ion transport properties was analyzed in detail, and plasma-coating number was optimized considering the VRFB performances. The ion-selective nano-crack surface significantly improved the proton selectivity, from 33.0 to 74.2, over vanadium (VO2+) ions. Consequently, the VRFB with P-BPSH showed higher energy and coulombic efficiencies compared to the VRFB using a pristine BPSH membrane. The energy efficiency of the P-BPSH (85.4%) is comparable to that of a Nafion® 117 membrane (85.1%). The improved battery performance demonstrated that the surface nano-crack layer effectively blocked the transport of vanadium ions without distinct reduction of the proton conductivity. This results suggest a promising strategy to enhance membrane proton selectivity over vanadium ions.

Finally in Chapter 5, the conclusion of this thesis and future research direction were represented in terms of membrane technology.