Energy-Saving Hydrogen Production from Electrocatalytic Oxidation of Urea in Seawater over Mixed-Linker Mn1-xCox Metal-Organic Frameworks with Open Metal Sites.

Abstract

Seawater electrolysis offers a sustainable and cost-effective solution for hydrogen production, addressing both the energy crisis and global warming. Herein, a new Co(II)-based metal-organic framework [Co3(μ4-oba)2(μ3-pca)2(DMF)2]n·nDMF (Co-MUM-5) with the sql topology was assembled from two types of linkers, namely, 4,4'-oxybis(benzoic acid) (H2oba) and 4-pyridinecarboxylic acid (Hpca). A series of bimetallic derivatives of Co-MUM-5 (Mn1-xCox-MUM-5) was synthesized, aiming at the development of new electrocatalytic materials for the urea oxidation reaction (UOR) and the oxygen evolution reaction (OER). Exceptionally low potentials of 1.62 V (391 mV overpotential) and 1.87 V (640 mV overpotential) were shown by Mn0.15Co0.85-MUM-5 at 10 and 100 mA cm-2 current densities in the OER. Following the addition of 1 M KOH + 0.5 M NaCl + 0.33 M urea, Mn0.15Co0.85-MUM-5 also revealed low UOR potentials of 1.40 and 1.51 V at 10 and 100 mA cm-2, respectively; these values are considerably reduced (by 220 mV) compared to the OER. Evaluation of hydrogen production using water containing urea showed that only a 1.51 V cell voltage is required for the Mn0.15Co0.85-MUM-5 electrode to yield the current density of 10 mA cm-2. This study highlights the application of bimetallic MOFs with open metal sites as promising electrocatalysts in UOR, and their use in energy conversion systems.