Ion Transport by Nanochannels in Ion-Containing Aromatic Copolymers Nanwen Li †,§ and Michael D. Guiver* ,†,‡ † National Research Council, Ottawa, Ontario K1A 0R6, Canada § School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Dr. NW, Atlanta, Georgia 30332, United States ‡ Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea ABSTRACT: The search for the next generation of highly ion-conducting polymer electrolyte membranes has been a subject of intense research because of their potential applications in energy storage and transformation devices, such as fuel cells, vanadium flow batteries, membrane-based artificial photosynthesis, water electrolysis, or water treatment processes such as electrodialysis desalination. Nanochannels that contain ionic groups, through which “hydrated” ions can pass, are believed to be of key importance for efficient ion transport in polymer electrolytes membranes. In this Perspective, we present an overview of the approaches to induce ion-conducting nanochannel formation by self- assembly, using polymer architecture such as block or comb- shaped copolymers. The transport properties of ion-containing aromatic copolymers are examined to obtain an insight into the fundamental behavior of these materials, which are targeted toward applications in fuel cells and other electrochemical devices. Challenges in obtaining well-defined nanochannel morphologies, and possible strategies to improve transport properties in aromatic copolymers having structures with the potential to withstand operation in electrochemical/chemical devices, are discussed. Opportunities for the application of ion-containing aromatic copolymer membranes in fuel cells, vanadium flow batteries, membrane-based artificial photosynthesis, electrolysis, and electrodialysis are also reviewed. Research needs for further improvements in ionic conductivity and durability, and their applications are identified. 1. INTRODUCTION The majority of world energy production is derived from the burning of fossil fuels (coal, oil, natural gas, etc.), which results in the emission of large amounts of the greenhouse gas carbon dioxide in addition to environmental pollution from sulfur and nitrogen oxides. Much effort has been expended on the development of membrane-based renewable energy sources and energy storage and transformation devices, such as polymer electrolyte membrane fuel cells, 1-4 redox flow batteries, 5 and hydrogen production for fuel cells (water electrolysis 6-9 and membrane-based artificial photosynthesis 10 ). These technolo- gies rely upon ion-containing polymer electrolyte membranes that separate and transport ions between the anode and cathode to balance the flow of electrons in an external circuit. 11 Therefore, they play a central role in determining the efficiency of the devices since ionic transport is a kinetic bottleneck compared with electrical conductivity. 12 These ion-containing polymeric membranes should meet several requirements: reasonable ionic conductivity, good chemical, hydrolytic and dimensional stability, durability in the actual device (electro- chemical/chemical) environment, mechanical toughness, ad- equate heat endurance, and low permeability to gas or liquid. 1,12 Typically, the membranes used in electrochemical devices consist of polymers with fluorinated or hydrocarbon backbones with ion exchange sites 13 (i.e., mostly sulfonic acid 1,12 or quaternary ammonium groups 3,4 ) that require water of solvation for effective ion transport to provide adequate device performance. Thus, ion-containing polymeric mem- branes are broadly divided into proton exchange membranes (PEMs) with fixed negatively charged functional groups such as sulfonic acid and anion exchange membranes (AEMs) with fixed positively charged functional groups such as quaternary ammonium, as shown in Figure 1. Perfluorosulfonic acid (PFSA) polymers have been commer- cialized for many years and were originally utilized in the chloralkali industry. 11 Among PSFAs, Nafion (DuPont) is well- known as one of the most promising state-of-the-art PEMs. 1,11 However, these materials have limitations in their utility and performance because of several shortcomings such as reduced proton conductivity at elevated temperatures (>80 °C) due to relatively easy dehydration, high methanol/gas diffusion, poor environmental recyclability and significant manufacturing costs. Received: October 31, 2013 Revised: February 10, 2014 Published: February 18, 2014 Perspective pubs.acs.org/Macromolecules Published 2014 by the American Chemical Society 2175 dx.doi.org/10.1021/ma402254h | Macromolecules 2014, 47, 2175-2198