Enrichment of γ-NiOOH in ultrathin metal-organic framework nanosheet arrays by linker engineering for urea-assisted natural seawater electrolysis.

Abstract

Metal-organic frameworks have been widely considered a potential alternative for noble metal catalysts for green hydrogen from seawater electrolysis, yet their performance is often limited by low activity and poor stability. Here, we propose a linker engineering strategy to optimize the phase composition of ultrathin Ni-MOF nanosheet arrays, aiming to enhance both activity and stability. We found that partial substitution of terephthalic acid (BDC) with electron-withdrawing tetrafluoroterephthalate (TFBDC) ligand alters the electronic structure and significantly promotes the formation of the catalytically active γ-NiOOH phase in Ni-TFBDC-2. This results in a 90 mV reduction in the overpotential for the oxygen evolution reaction at 50 mA cm^-2, surpassing the performance of a state-of-the-art RuO₂ catalyst, and is accompanied by an increased corrosion potential in seawater. Furthermore, the enrichment of the γ-NiOOH phase in Ni-TFBDC-2 effectively suppresses the passivation during urea oxidation reaction (UOR) in a seawater electrolyte, enabling the achievement of an industrially relevant current density of 0.8 A cm^-2. Operando characterizations reveal that Ni-TFBDC-2 undergoes an electrooxidation process to form Ni³⁺ species, which subsequently act as the active catalytic sites for the OER. Additionally, the urea-assisted natural seawater electrolyzer assembled with Ni-TFBDC-2 requires a low voltage of 1.76 V at 400 mA cm^-2 and demonstrates excellent durability over 170 h of continuous operation. This work offers a novel strategy to enrich the catalytically active phase in MOF-based electrocatalysts, aiming to achieve high activity and long-term stability during urea-assisted natural seawater electrolysis. It is noteworthy that different notable aspects, such as the durability of the materials after prolonged reaction, should be more thoroughly considered for practical applications on larger scales.