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

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)2n·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.