Multiscale Tortuous Diffusion in Anion and Cation Exchange Membranes

Abstract

A fundamental understanding of water transport and morphology is critical for improving ionic conductivity in polymer membranes. In a series of random copolymer anion exchange and cation exchange membranes, we systematically investigate the influence of counterion type, side chain type, and degree of ionic functionalization on water transport using NMR diffusometry. Time-dependent water diffusion measurements reveal micrometer-scale heterogeneity of the hydrophilic network in these random copolymers. We propose a model in which the hydrophilic domain network in these membranes has micrometer-scale distributions of local nanometer-scale dead ends, leading to changes in tortuosity as a function of water content and membrane composition. We furthermore parse tortuosity into two length-scale regimes, one regime from nanometer (local) to bulk and another from micrometer to bulk, offering enhanced discrimination of the multiscale morphological structures that influence bulk transport. This study thus provides new insights into ionic polymer membrane morphology and diffusion behavior, with the ultimate goal of controlling polymeric materials for enhanced fuel cells and other separations applications.

Perfluorosulfonyl Imide versus Perfluorosulfonic Acid Ionomers in Proton‐Exchange Membrane Fuel Cells at Low Relative Humidity

“…[1,2] To achieve high conductivity, especially in the case of rigid chemical structuress uch as aromatic polymers, it is crucial to design an ionomer architecture that is able to self-organize into neat hydrophilic-hydrophobic phase-separated structures forming well-defined and intercon-nectedi onic nanochannelsw ith low tortuosity. [3][4][5][6] Much-improvedm orphologies were obtained in multiblock copolymers by alternating non-miscible highly sulfonated andn onsulfonated blocks within the copolymer chain, promoting the formation of membranes with well-definedh ydrophilic-hydrophobic microstructures, as extensively reported by McGrath and coworkers. [7][8][9][10] Recently,i ncreasing the acidity of the functional moiety has emerged as ap lausible approach to favor proton transport, and significantly higher proton conductivities were reported as compared to those of directly sulfonated aromatic ionomers.…”

“…The proton conductivity of ionomers generallyr ises with the hydration degree as ac onsequence of (i)the reduction of confinemente ffects at the nanoscale, [17,18,[43][44][45] (ii)the increase of connectivity, [5,46] and (iii)the development of structural diffusion. [18,22,[47][48][49][50][51] The proton conductivity as af unction of RH at 25 8Ci sp resented on Figure 3c for Si X/Y (red) and In X/Y (blue).…”

“…Aromatic ionomers are among the most studied alternative materials to benchmark perfluorosulfonic acid (PFSA) membranes (i.e., Nafion and its derivatives) for proton‐exchange membrane fuel cells (PEMFCs) . To achieve high conductivity, especially in the case of rigid chemical structures such as aromatic polymers, it is crucial to design an ionomer architecture that is able to self‐organize into neat hydrophilic–hydrophobic phase‐separated structures forming well‐defined and interconnected ionic nanochannels with low tortuosity . Much‐improved morphologies were obtained in multiblock copolymers by alternating non‐miscible highly sulfonated and nonsulfonated blocks within the copolymer chain, promoting the formation of membranes with well‐defined hydrophilic–hydrophobic microstructures, as extensively reported by McGrath and co‐workers …”

Selectivity of Transport Processes in Ion-Exchange Membranes: Relationship with the Structure and Methods for Its Improvement

“…This is the main reason for the increase in the flow of coions and a decrease in the selectivity of ion-exchange membranes with high water uptake. These regularities are typical for both cation-exchange and anion-exchange membranes [ 91 , 92 , 93 , 94 ].…”

Impact of Frictional Interactions on Conductivity, Diffusion, and Transference Number in Ether- and Perfluoroether-Based Electrolytes

“…23 Measurements of ion self-diffusion coefficients, D self,+ and D , self,-by pulsed-field gradient NMR (PFG-NMR) have provided valuable insight into the underpinnings of ion transport. [24][25][26][27][28][29][30][31][32] In ideal, dilute electrolytes, wherein the activity coefficients of the ions are unity, the relationships between self-diffusion coefficients and ion transport coefficients are relatively simple. In this limit, ionic conductivity is given by the Nernst-Einstein equation, which is often modified to give Eq.…”