Molecular-Level Control over Ionic Conduction and Ionic Current Direction by Designing Macrocycle-Based Ionomers

Authors

Shyambo Chatterjee, University of Nebraska-LincolnFollow
Ehsan Zamani, University of Nebraska-LincolnFollow
Seefat Farzin, University of Nebraska-LincolnFollow
Iman Evazzade, University of Nebraska-LincolnFollow
Oghenetega Allen Obewhere, University of Nebraska-LincolnFollow
Tyler James Johnson, University of Nebraska-Lincoln
Vitaly Alexandrov, University of Nebraska-LincolnFollow
Shudipto Konika Dishari, University of Nebraska - LincolnFollow

ORCID IDs

Shudipto Konika Dishari https://orcid.org/0000-0003-1679-2332

Vitaly Alexandrov https://orcid.org/0000-0003-2063-6914

Date of this Version

5-11-2022

Document Type

Article

Citation

JACS Au 2022, 2, 1144−1159

doi:10.1021/jacsau.2c00143

Comments

© 2022 The Authors. Published by American Chemical Society

CC-BY-NC-SA

Abstract

Poor ionic conductivity of the catalyst-binding, submicrometer- thick ionomer layers in energy conversion and storage devices is a huge challenge. However, ionomers are rarely designed keeping in mind the specific issues associated with nanoconfinement. Here, we designed nature-inspired ionomers (calix-2) having hollow, macrocyclic, calix[4]arene-based repeat units with precise, sub-nanometer diameter. In ≤100 nm-thick films, the in-plane proton conductivity of calix-2 was up to 8 times higher than the current benchmark ionomer Nafion at 85% relative humidity (RH), while it was 1−2 orders of magnitude higher than Nafion at 20−25% RH. Confocal laser scanning microscopy and other synthetic techniques allowed us to demonstrate the role of macrocyclic cavities in boosting the proton conductivity. The systematic self-assembly of calix-2 chains into ellipsoids in thin films was evidenced from atomic force microscopy and grazing incidence small-angle X-ray scattering measurements. Moreover, the likelihood of alignment and stacking of macrocyclic units, the presence of one-dimensional water wires across this macrocycle stacks, and thus the formation of long-range proton conduction pathways were suggested by atomistic simulations. We not only did see an unprecedented improvement in thin-film proton conductivity but also saw an improvement in proton conductivity of bulk membranes when calix-2 was added to the Nafion matrices. Nafion−calix-2 composite membranes also took advantage of the asymmetric charge distribution across calix[4]arene repeat units collectively and exhibited voltage-gating behavior. The inclusion of molecular macrocyclic cavities into the ionomer chemical structure can thus emerge as a promising design concept for highly efficient ion-conducting and ion-permselective materials for sustainable energy applications.