Highly conjugated 2D COF/MOF composites for bifunctional electrocatalytic alkaline HER and OER with enhanced activity and stability

Abstract

Electrocatalytic water splitting, consisting of the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER), represents a promising renewable energy technology for producing ultra-high purity hydrogen through efficient energy conversion and storage. However, the practical implementation of this technology in an alkaline environment is hindered by the sluggish kinetics of both the HER and OER, which significantly limit the water splitting efficiency. Thus, the development of highly active and stable alkaline HER/OER electrocatalysts is urgently needed but remains challenging. In this work, we synthesized a novel two-dimensional (2D) highly conjugated COF/MOF composite (COF-C₄N/THQ-M) through a post-synthesis method. This method enables the controlled growth of a part of COF-C₄N at the edges of THQ-M MOF to prevent the structural disintegration of THQ-M, consequently enhancing the surface charge transfer efficiency and further improving the catalytic activity and stability of the composite. By regulating the metal sites, COF-C₄N/THQ-Co and COF-C₄N/THQ-Co₂Fe₁ are proposed to be the optimal alkaline HER electrocatalysts with an overpotential of 58 mV at −10 mA cm⁻² and the alkaline OER electrocatalysts with 314 mV at 10 mA cm⁻², respectively, which are superior to most of the reported non-precious metal electrocatalysts. The charge transfer characteristics and the alkaline HER and OER pathways were calculated based on DFT calculations to reveal the synergistic mechanism between COF-C₄N and THQ-M. This work provides a novel idea for developing high-performance bifunctional electrocatalysts for alkaline water splitting applications based on hybrid highly conjugated COF/MOF systems.

Keywords: Two-dimensional covalent organic framework; Metal–organic framework; COF/MOF composite; Electrocatalytic oxygen evolution reaction; Electrocatalytic hydrogen evolution reaction.