Polymer Electrolyte Membranes for Facilitated Molecular Transport and Separation

Abstract

Electrolyte membranes capable of fast and selective transport of a target penetrant such as ion ad gas are promising for various electrochemical energy conversion and storage devices. Accordingly, understanding the specific interactions between penetrant and transport medium in a molecular-level is essential to development of advanced polymer electrolyte membranes because of their great influences on the transport phenomena. This dissertation comprises 5 chapters. Chapter 1 is Introduction, where different fields of applications are exploited here: bipolar membrane, vanadium redox flow battery, and carbon capture and storage are described briefly. Chapter 2 is on the effects of water structure on the water dissociation and the proton transport occurring in the bipolar membrane junction. In particular, Chapter 2 2 describes the formation of a unique ice-like cluster triggered by partially dissociative bound waters on the α-Fe2O3 catalyst surface. The electrochemical analyses prove that both the reaction rate of water dissociation and the transport of produced ions such as H+ and OH- within BPM are significantly increased by changing the strength and size of hydrogen bonding network at the interface between α-Fe2O3 catalyst and water. As a result, the overpotential of water dissociation is reduced, competitive with or even better than that of the commercial BPM (BP-1E, Astom), indicating its suitability for efficient water electrolysis with high durability. In Chapter 3, it was demonstrated that molecular-sieving nanochannels (~0.84 nm) generated inside a graphene oxide (GO) laminate efficiently restrict the transport of large-sized vanadium ions, while allowing the transport of small-sized H+. In particular, a sub-5 nm and highly selective GO nanofilm is successfully prepared to improve the H+ flux and also the H+/V selectivity for the vanadium redox flow battery (VRFB). The GO-coated thin-film composite membrane shows much higher H+ flux with an exceptionally high H+/V selectivity (H+ permeation rate / VO2+ permeation rate, up to 850), leading to a much more enhanced VRFB performance in terms of the energy efficiency (EE: 84.7%), compared to the benchmark Nafion membrane (EE: 69.2%), at 20 mA cm-2. 3 In Chapter 4, formation of a reversible complex between CO2 and bound waters coordinating an alkaline metal cation (Lewis-acidic water) is demonstrated for the first time. Such a unique property of water can facilitate CO2 permeation by offering an additional carrier-mediated pathway, which is distinct from the commonly recognized bicarbonate or carbonate ion-mediated CO2 transport. As a results, an exceptionally high CO2 permeance of 4,650 gas-permeation units (GPU) with an ideal separation factor of 1,500 was achieved for CO2/N2, which lies far above the upper bound of the selectivity-permeability trade-off curve, without any significant performance degradation over 6 months. In conclusion, this dissertation emphasizes the understanding of the effects of electrochemical interactions on the transport behaviors of ions and gases. Based on these findings, furthermore, advanced polymer electrolyte membranes have been developed, realizing energy-efficient, environmental-friendly, and sustainable membrane applications. For future prospects, in Chapter 6, preliminary studies for catalyst roles that allow water molecules to become ice-like structure are briefly introduced. It is demonstrated that pKa of the bound waters of the catalyst surface plays important roles in changing the hydrogen bond strength of surrounding water molecules. Interestingly, the weak acidic bound waters (~ pKa: 5.0) were found to be optimal for promoting the ice-like structure. Notably, a volcano relation between the pKa 4 value and a fraction of ice-like waters was observed, like a Sabatier plot, responsible for the water dissociation performance of the bipolar membrane. A more rigorous future study will be able to provide a paramount insight into the basic principles of the catalytic roles affecting the water dissociation mechanism.