A 2D Co-MOF nanosheet for boosting alkaline water splitting through electrocatalytic urea oxidation

Abstract

The alkaline hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) are gaining considerable interest for boosting the overall water splitting in the context of green hydrogen production with simultaneous urea removal from wastewater. In this work, we successfully synthesized a novel cobalt-based two-dimensional (2D) metal–organic framework (MOF), named Co-IDBA-MOF, by a solvothermal method using a mixed ligand system consisting of 2,2′-iminodibenzoic acid (IDBA) and 4,4′-bipyridine (Bpy). Single-crystal X-ray analysis of the Co-IDBA-MOF confirmed its layered 2D structure. The bulk specimen of the MOF was further characterized by powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric (TG) analysis, and UV-visible spectroscopic analysis. Field emission-scanning electron microscopic (FE-SEM), field emission gun-transmission electron microscopic (FEG-TEM) and atomic force microscopic (AFM) analyses uncovered the ultrathin 2D nanosheet-type morphology of the MOF, which facilitates the fabrication of 2D materials for the potential fabrication of real devices. This Co-IDBA-MOF exhibited good electrocatalytic performance in the alkaline HER at −0.241 V w. r. t. RHE at a current density of 10 mA cm⁻² (η₁₀) and a modest oxygen evolution reaction (OER) activity (1.66 V for 10 mA cm⁻² w. r. t. RHE) in an alkaline water medium. However, the anodic potential got drastically reduced to 1.55 V after the addition of 0.33 M urea due to the urea oxidation reaction (UOR). The lowering of the Tafel slope and the concomitant increase in double-layer capacitance for the alkaline hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) suggested improved kinetics for overall water splitting after urea addition. Further variations in the urea concentration and the concentration of electrode materials can tune the UOR activity. This work aims to design a novel Co-MOF-based electrode material for bifunctional activity and large-scale green hydrogen production via the UOR.