Electrocatalytic Water Splitting in Isoindigo-Based Covalent Organic Frameworks

Abstract

Developing a low-cost, robust, and high-performance electrocatalyst capable of efficiently performing both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) under both basic and acidic conditions is a major challenge. This area of research has attracted much attention in recent decades due to its importance in energy storage and conversion. Herein, we report the synthesis of two imine-linked isoindigo-based covalent organic networks I-TTA and I-TG (I=Isoindigo, TTA=4,4',4''-(1,3,5-triazine-2,4,6-triyl)-trianiline, TG=triamino-guanidinium hydrochloride salt). By introducing two amine core units with different planarity, such as triazine and ionic guanidinium units, we control the morphology, crystallinity, and corresponding electrocatalytic properties of the materials. The combination of isoindigo dialdehyde with a planar triazine core, leads to the formation of thin, highly crystalline, planar two dimensional (2D) nanosheets covalent organic framework (COF), I-TTA whereas its combination with ionic non-planar guanidinium core leads to an amorphous covalent organic polymer (COP), I-TG with a fibrous morphology. The sheet-like crystalline I-TTA COF shows better electrocatalytic activity compared to the amorphous fibrous I-TG COP. I-TTA exhibits a current density of 10 mA cm^-2 at an overpotential of ~134 mV for HER (in 0.5 M H₂SO₄) and ~283 mV for OER (in 1 M KOH). The electrocatalytic activity of the I-TTA COF in the OER exceeds that of other metal-free COFs. The catalytic activity is maintained even after 24 hours of chronoamperometry and 500 cycles of cyclic voltammetry (CV) at high scan rates.

Keywords: Covalent organic frameworks; Electrocatalysis; Isoindigo; Water splitting.