(463b) Realizing the Potential of Micro-Phase Separated Block Copolymer Electrolytes: Ion Domain Connectivity Plays a Prominent Role in Ion Conduction

Single ion conducting polymer electrolyte membranes (PEMs) are found at the heart of many electrochemical devices (e.g., fuel cells, flow batteries, electrolyzers, electrodialysis etc.).(1) The key requirements for such materials include high ionic conductivity, electron insulation, mechanical resilience, selectivity (i.e., high transference #), and chemical, thermal, and mechanical stability.(2, 3) Ionic conductivity is particularly important because it strongly influences the ohmic overpotential in the said electrochemical devices affecting overall device efficiency.(1, 4)

Block copolymer electrolytes represent an attractive set of PEM materials because their micro-phase architecture yields greater ion conduction over their random copolymer counterparts.(5-7) Furthermore, facile and controllable polymerization reactions allow researchers to access a variety of morphological architectures, while engineering of the block copolymerâ??s self-assembly influences domain alignment.(8-12) However, there is lack of systematic studies correlating molecular level structural design to bulk material properties like ion transport.

In this work, a model lamellae-forming diblock copolymer electrolyte system was manipulated to examine the extent of ion domain connectivity on ion conduction.(13, 14) Careful control of the model systemâ??s volume fraction gave diblock copolymer structures with slightly rich electrolyte domains or slightly rich hydrophobic domains. The slightly rich electrolyte domains had greater contiguous area fraction while simultaneously demonstrating substantially lower terminal defects. Having a contiguous area fraction from 0.95 to 1.0 resulted in a 2x improvement in ionic conductivity over a non-micro-phase separated block copolymer electrolyte or a micro-phase separated block copolymer electrolyte with very poor connectivity (i.e., a system with larger volume fraction of hydrophobic blocks). Incremental adjustment in the extent of connectivity revealed an exponential growth curve for ion conductivity as a function of contiguous area fraction. Furthermore, the benefits that a micro-phase separated block copolymer electrolyte affords in terms of ion conductivity were not realized when the block copolymer electrolyte had poor connectivity. The results of this work have far reaching implication into the rationale design of PEM materials based upon block copolymer designs. This talk will emphasize the importance of maximizing ion domain connectivity while taking great strides to minimize terminal defects to boost ion transport in PEM materials. This talk will close with future directions on how molecular level engineering of block copolymer electrolytes offer the potential to shed light on how other structural features (e.g., tortuosity(15), grain boundaries(5), and counterion condensation) alter ion transport in PEM materials(16, 17).

References

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7. Y. Schneider et al., Ionic Conduction in Nanostructured Membranes Based on Polymerized Protic Ionic Liquids. Macromolecules 46, 1543-1548 (2013).

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17. K. M. Beers, N. P. Balsara, Design of Cluster-free Polymer Electrolyte Membranes and Implications on Proton Conductivity. ACS Macro Letters 1, 1155-1160 (2012).