Issue 2, 2023

Hydroxide ion-conducting viologen–bakelite organic frameworks for flexible solid-state zinc–air battery applications

Abstract

Adaptable polymer-based solid-state electrolytes can be a game-changer toward safe, lightweight flexible batteries. We present a robust Bakelite-type organic polymer covalently decked with viologen, triazine, and phenolic moieties. Its flexible structure with cationic viologen centers incorporates counter-balancing free hydroxide ions into the polymeric framework. By design, the aromatic groups and heteroatoms in the framework can be activated under an applied potential to prompt a push–pull drive, setting off the towing of hydroxide ions via weak electrostatic, van der Waals, and hydrogen-bond interactions. The frontier orbitals from a DFT-modeled structure certify this. The hydroxyl-polymer requires minimal KOH wetting to maintain a humid environment for Grotthuss-type transport. The hydroxide ion conductivity reaches a value of 1.4 × 10−2 S cm−1 at 80 °C and 95% RH, which is retained for over 15 h. We enhanced its practical utility by coating it as a thin solid-state separator-cum-electrolyte on readily available filter paper. The composite exhibits a conductivity of 4.5 × 10−3 S cm−1 at 80 °C and 95% RH. A zinc–air battery (ZAB) constructed using this polymer-coated paper as electrolyte yields a maximum power density of 115 mW cm−2 and high specific capacitance of 435 mA h g−1. The power density recorded for our ZAB is among the best reported for polymer electrolyte–based batteries. Subsequently, the flexible battery fabricated with IISERP-POF11_OH@FilterPaper exhibits an OCV of 1.44 V, and three batteries in series power a demo traffic signal. To underscore the efficiency of hydroxide ion transport through the complex multifunctional backbone of the polymer, we calculated the diffusion coefficient for OH (Exp: 2.9 × 10−5 cm2 s−1; Comp. 5.2 × 10−6 cm2 s−1) using electrochemical methods and MD simulations. Climbing-edge NEB calculations reveal a large energy barrier of 2.11 eV for Zn2+ to penetrate the polymer and identify hydroxide ions within the polymer, suggesting no undesirable Zn2+ crossover. Our findings assert the readily accessible C–C-linked cationic polymer's capacity as a solid-state electrolyte for ZABs and any anion-conducting membrane.

Graphical abstract: Hydroxide ion-conducting viologen–bakelite organic frameworks for flexible solid-state zinc–air battery applications

Supplementary files

Article information

Article type
Communication
Submitted
29 Sept. 2022
Accepted
24 Nov. 2022
First published
25 Nov. 2022

Nanoscale Horiz., 2023,8, 224-234

Author version available

Hydroxide ion-conducting viologen–bakelite organic frameworks for flexible solid-state zinc–air battery applications

D. Rase, R. Illathvalappil, H. D. Singh, P. Shekhar, L. S. Leo, D. Chakraborty, S. Haldar, A. Shelke, T. G. Ajithkumar and R. Vaidhyanathan, Nanoscale Horiz., 2023, 8, 224 DOI: 10.1039/D2NH00455K

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