Hydroxide Ion Conduction through Viologen-Based Covalent Organic Frameworks (vCOFs): An Approach toward the Advancement.

Alkaline fuel cells rely on the movement of hydroxide anions (OH-) for their operation, yet these anions face challenges in efficient conduction due to their limited diffusion coefficient and substantial mass compared to proton (H+) transport. Within the covalent organic framework structure, ordered channels offer a promising solution for the OH- ion transport. Herein, we synthesized a cationic covalent organic framework (vTAPA) via the solvothermal-assisted Zincke reaction. vTAPA showcases excellent stability in harsh basic solution (12 M) and a wide range of pH. This framework facilitates OH- conduction through its one-dimensional network through the anion exchange process. We employed various tertiary ammonium salts (tetramethyl, tetraethyl, and tetrabutyl ammonium hydroxide) to exchange trapped anionic chloride ions inside the vTAPA structure with OH- ions. The density functional theory (DFT) study exhibited that the anion exchange process is very favorable, as the vTAPA framework offers preferable interaction sites for OH- ions. The impact of steric hindrance from these tertiary ammonium salts on the OH- conduction performance was extensively investigated. Butyl@vTAPA exhibited a high OH- ion conductivity of 1.05 × 10-4 S cm-1 at 90 °C under 98% relative humidity (RH). Our uniquely designed cationic covalent organic frameworks (COF) created a platform for a preferential transport network of hydroxide ions, and this is the first report of directly used COFs for hydroxide ion conduction without any vigorous postsynthetic modification.

Reticular synthesis of two-dimensional ionic covalent organic networks as metal-free bifunctional electrocatalysts for oxygen reduction and evolution reactions.

Bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are the heart of metal-air batteries, fuel cells, and other energy storage systems. Here, we report a series of a novel class of redox-active viologen-based ionic covalent organic networks (vCONs) which are directly used as metal-free bifunctional electrocatalysts towards ORR and OER applications. These vCONs (named vGC, vGAC, vMEL and vBPDP) were synthesized by the well-known Zincke reaction. The installation of redox-active viologen moieties among the extended covalent organic architectures played a crucial role for exceptional acid/base stability, as well as bifunctional ORR and OER activities, confirmed by the cyclic voltammetry (CV) curves. Among all of them, vBPDP showed high ORR efficiency with a half-wave potential of 0.72 V against a reversible hydrogen electrode (RHE) in 1 M KOH electrolyte. In contrast, vMEL demonstrated high OER activity with an overpotential of 320 mV at a current density of 10 mAcm-2 and a Tafel slope of 109.4 mV dec-1 in 1 M KOH electrolyte solution. This work is exceptional and unique in terms of directly used pristine ionic covalent organic networks that are used as bifunctional (ORR and OER) electrocatalysts without adding any metals or conductive materials.