Boosting Urea-Assisted Natural Seawater Electrolysis in 3D Leaf-Like Metal-Organic Framework Nanosheet Arrays Using Metal Node Engineering

Abstract

Metal node engineering, which can optimize the electronic structure and modulate the composition of poor electrically conductive metal-organic frameworks, is of great interest for electrochemical natural seawater splitting. However, the mechanism underlying the influence of mixed-metal nodes on electrocatalytic activities is still ambiguous. Herein, a strategic design is comprehensively demonstrated in which mixed Ni and Co metal redox-active centers are uniformly distributed within NH₂-Fe-MIL-101 to obtain a synergistic effect for the overall enhancement of electrocatalytic activities. Three-dimensional mixed metallic MOF nanosheet arrays, consisting of three different metal nodes, were in situ grown on Ni foam as a highly active and stable bifunctional catalyst for urea-assisted natural seawater splitting. A well-defined NH₂-NiCoFe-MIL-101 reaches 1.5 A cm^-2 at 360 mV for the oxygen evolution reaction (OER) and 0.6 A cm^-2 at 295 mV for the hydrogen evolution reaction (HER) in freshwater, substantially higher than its bimetallic and monometallic counterparts. Moreover, the bifunctional NH₂-NiCoFe-MIL-101 electrode exhibits eminent catalytic activity and stability in natural seawater-based electrolytes. Impressively, the two-electrode urea-assisted alkaline natural seawater electrolysis cell based on NH₂-NiCoFe-MIL-101 needs only 1.56 mV to yield 100 mA cm^-2, much lower than 1.78 V for alkaline natural seawater electrolysis cells and exhibits superior long-term stability at a current density of 80 mA cm^-2 for 80 h.

Keywords: MOF; mixed metal; natural seawater; urea electrolysis; water splitting.