Pendent carboxylic acid-fuelled high-performance uranium extraction in a hydrogen-bonded framework and prolifically improved water oxidation via post-metalation-actuated composite fabrication

Abstract

Pore functionality engineering in metal–organic frameworks (MOFs) benefits monitoring of uranium contamination via fluoro-sensing, and further promises its effective extraction. Alternatively, mixed-metal-based MOF-composites aid high-performance electrochemical oxygen evolution reaction (OER). Herein, we strategically built a fish-bone-shaped Co(II)-framework that contains pendent carboxylic acid moieties and displays high hydro-chemical stability. The activated MOF demonstrates selective and ultra-fast turn-off fluoro-detection of UO₂²⁺ in uranium-spiked water and simulated seawater with high recyclability. The detection limit (0.13 μM) outperforms that of the majority of sensory MOFs. The layer-stacked framework shows regenerable extraction of U(VI) ion with fast kinetics. Besides notable saturated uptake (129.8 mg g⁻¹), uranium adsorption capacity reveals minor alteration over multiple cycles or in the presence of interfering cations. Apart from in-depth experimental support, the sensing and scavenging mechanisms are explicitly validated through a unique molecular scissoring approach by employing a pendent-functionality-truncated isostructural framework. The results mutually confirm the free –COOH group as a task-specific site for uranium detection and extraction. Redox-active Co(II) nodes further assist this MOF in alkaline medium electrochemical OER via a quasi-reversible Co²⁺/Co³⁺ couple with 331 mV overpotential and 62 mV dec⁻¹ Tafel slope. Suitably oriented carboxylic acids benefit anchoring of Ni(II) to yield a hetero-bimetallic composite with increased active sites. Interestingly, the overpotential of this post-metalated material (308 mV) is far less than that of the pristine MOF. Significantly, the Tafel slope is 1.6 times reduced (38 mV dec⁻¹) and ranks among top-tier OER catalysts, including benchmark (RuO₂ and IrO₂)/commercial (Co₃O₄ and NiO) ones. Both the pristine and post-modified MOFs divulged >97% faradaic efficiencies with 30 h chronopotentiometric stability and reusability up to 1500 cycles. Remarkably, the turnover frequency displays a 4-fold increment, subsequently boosting charge-transfer resistance by 3.5 times due to hetero-bimetallic synergy. The results set an example to drastically improve the OER performance in hydrogen-bonded MOFs via post-metalation of free carboxylic acids.