Cation-Assisted Water Oxidation with Crown Ether-Based Covalent Organic Frameworks

Covalent organic framework (COF)-based electrocatalysts for water oxidation are usually designed by inducing polarization into an organic framework. The most common methodology "heteroatom insertion" suffers from lower catalytic activity than metalated systems. Herein, we introduce a strategy of polarity induction into organic frameworks by complexing electrolyte's alkali metal using crown ether. We have synthesized a series of porous crystalline COFs with different sizes of crown ether units, having different alkali metal binding abilities. COF-catalysts having crown ethers in their backbone show oxygen and hydrogen evolution (OER and HER) performance in alkaline electrolytes. Electrocatalytic performances of various COF-catalysts have also been studied while changing the alkali metal of the electrolyte solution. The most efficient catalyst B18C achieved a current density of 20 and 100 mA cm^-2 at an overpotential (η₂₀ and η₁₀₀) of 287 ± 1 and 362 ± 8 mV (for the OER) in 1 M KOH (pH = 13.6), with good electrocatalytic stability (16 h chronopotentiometry study showed a 2 mV potential change at 25 mA cm^-2 current density). Notably, B18C COF also exhibited good OER performance in 1 M K₂SO₄ (pH = 6.3) with an overpotential (η₂₀) of 471 mV. Additionally, a promising alkaline HER activity of B18C (η₂₀ = 310 ± 5 mV) leads to an overall water-splitting performance in 1 M KOH (E_cell = 1.784 V @ 20 mA cm^-2).