Nanostructured block copolymer electrolytes
Image credit: Peter Allen.
Researchers within Professor Paul F. Nealey’s group at the University of Chicago’s Institute for Molecular Engineering and the Materials Science Division at Argonne National Laboratory have developed a new scheme to construct single ion conducting, nanostructured block copolymer electrolyte (BCE) thin films. The research was published in the journal Chemistry of Materials and the newly reported BCEs demonstrate remarkably high anion conductivity. The work utilized both the Center for Nanoscale Materials and the Advanced Photon Source user facilities at Argonne National Laboratory.
Polymer electrolytes are ubiquitous materials for electrochemical cells, such as fuel cells, flow batteries, electrolyzers, redox-based resistive switching, etc. One variant of polymer electrolytes are block copolymer electrolytes containing micro-phase separated ionic and non-ionic domains. The micro-phase separation enables lower concentrations of ionic carriers to achieve high ionic conductivities while being mechanically resilient. However, engineering precise nanostructured BCE films is a difficult proposition. The work by Arges et al. first prepared a perpendicular oriented, self-assembled block copolymer of poly(styrene-b-2-vinyl pyridine). This block copolymer framework contained tertiary amine moieties that were converted to n-methylpyridinium cations, fixed charge carriers, using a novel chemical vapor infiltration reaction – a Menshutkin reaction between methyliodide vapor and the tertiary amine. The authors envision the platform as a valuable tool to elucidate how the salient structural features of BCEs influence ion conductivity.
Connecting structure and morphology to bulk transport properties, such as ionic conductivity, in nanostructured polymer electrolyte materials is a difficult proposition because of the challenge to precisely and accurately control order and the orientation of the ionic domains in such polymeric films. In this work, poly(styrene-block-2-vinylpyridine) (PSbP2VP) block copolymers were assembled perpendicularly to a substrate surface over large areas through chemical surface modification at the substrate and utilizing a versatile solvent vapor annealing (SVA) technique. After block copolymer assembly, a novel chemical vapor infiltration reaction (CVIR) technique selectively converted the 2-vinylpyridine block to 2-vinyl n-methylpyridinium (NMP+ X–) groups, which are anion charge carriers. The prepared block copolymer electrolytes maintained their orientation and ordered nanostructure upon the selective introduction of ion moieties into the P2VP block and post ion-exchange to other counterion forms (X– = chloride, hydroxide, etc.).
The prepared block copolymer electrolyte films demonstrated high chloride ion conductivities, 45 mS cm–1 at 20 °C in deionized water, the highest chloride ion conductivity for anion conducting polymer electrolyte films. Additionally, straight-line lamellae of block copolymer electrolytes were realized using chemoepitaxy and density multiplication. The devised scheme allowed for precise and accurate control of orientation of ionic domains in nanostructured polymer electrolyte films and enables a platform for future studies that examines the relationship between polymer electrolyte structure and ion transport.
Christopher G. Arges, Yu Kambe, Hyo Seon Suh, Leonidas E. Ocola, and Paul F. Nealey, “Perpendicularly Aligned, Anion Conducting Nanochannels in Block Copolymer Electrolyte Films,” Chemistry of Materials, 2016, DOI: 10.1021/acs.chemmater.5b04452, Published Online December 18, 2015.