Nuclear reactors are fueled by 235 U, which is typically concentrated from mined uranium ore. However, estimates suggest that globally there are only ≈4.5 million tons of uranium ore on land, which represents a significant obstacle to the increased implementation of nuclear power. [3] However, uranium (present as uranyl ions) is abundant in seawater, and estimates suggest ≈4.5 billion tons are available in the oceans. This makes the extraction of uranium from seawater a priority if humans want to harness nuclear energy in the future. [4] Recently, much research effort has been focused on developing high capacity adsorbents that can recover uranium from seawater at good rates. [5] Porous materials such as metal oxides/sulfides, [6] porous organic polymers, [7] porous aromatic frameworks, [8] metal-organic frameworks, [9] biomass-based materials, [10] covalent organic frameworks, [11] and porous carbons [12] have all been explored as potential adsorbents for extracting uranium from seawater. It was found that the adsorption capacity and uranyl ion-binding affinity of sorbents can be significantly improved Uranium extraction from seawater provides an opportunity for sustainable fuel supply to nuclear power plants. Herein, an adsorption-electrocatalysis strategy is demonstrated for efficient uranium extraction from seawater using a functionalized iron-nitrogen-carbon (Fe-N x -C-R) catalyst, comprising N-doped carbon capsules supporting FeN x single-atom sites and surface chelating amidoxime groups (R). The amidoxime groups bring hydrophilicity to the adsorbent and offer surface-specific binding sites for UO 2 2+ capture. The site-isolated FeN x centres reduce adsorbed UO 2 2+ to UO 2 + . Subsequently, through electrochemical reduction of the FeN x sites, unstable U(V) ions are reoxidized to U(VI) in the presence of Na + resulting in the generation of solid Na 2 O(UO 3 •H 2 O) x , which can easily be collected. Fe-N x -C-R reduced the uranium concentration in seawater from ≈3.5 ppb to below 0.5 ppb with a calculated capacity of ≈1.2 mg g -1 within 24 h. To the best of the knowledge, the developed system is the first to use the adsorption of uranyl ions and electrodeposition of solid Na 2 O(UO 3 .H 2 O) x for the extraction of uranium from seawater. The important discoveries guide technology development for the efficient extraction of uranium from seawater.

Covalent organic frameworks (COFs) represent an emerging class of organic photocatalysts. However, their complicated structures lead to indeterminacy about photocatalytic active sites and reaction mechanisms. Herein, we use reticular chemistry to construct a family of isoreticular crystalline hydrazide-based COF photocatalysts, with the optoelectronic properties and local pore characteristics of the COFs modulated using different linkers. The excited state electronic distribution and transport pathways in the COFs are probed using a host of experimental methods and theoretical calculations at a molecular level. One of our developed COFs (denoted as COF-4) exhibits a remarkable excited state electron utilization efficiency and charge transfer properties, achieving a record-high photocatalytic uranium extraction performance of ~6.84 mg/g/day in natural seawater among all techniques reported so far. This study brings a new understanding about the operation of COF-based photocatalysts, guiding the design of improved COF photocatalysts for many applications.

Seawater contains uranium at a concentration of ≈3.3 ppb, thus representing a rich and sustainable nuclear fuel source. Herein, an adsorption–electrocatalytic platform is developed for uranium extraction from seawater, comprising atomically dispersed indium anchored on hollow nitrogen‐doped carbon capsules functionalized with flexible amidoxime moieties (In–N x –C–R, where R denotes amidoxime groups). In–N x –C–R exhibits excellent uranyl capture properties, enabling a uranium removal rate of 6.35 mg g −1 in 24 h, representing one of the best uranium extractants reported to date. Importantly, In–N x –C–R demonstrates exceptional selectivity for uranium extraction relative to vanadium in seawater (8.75 times more selective for the former). X‐ray absorption spectroscopy (XAS) reveals that the amidoxime groups serve as uranyl chelating sites, thus allowing selective adsorption over other ions. XAS and in situ Raman results directly indicate that the absorbed uranyl can be electrocatalytically reduced to an unstable U(V) intermediate, then re‐oxidizes to U(VI) in the form of insoluble Na 2 O(UO 3 ·H 2 O) x for collection, through reversible single electron transfer processes involving InN x sites. These results provide detailed mechanistic understanding of the uranium extraction process at a molecular level. This work provides a roadmap for the adsorption–electrocatalytic extraction of uranium from seawater, adding to the growing suite of technologies for harvesting valuable metals from the earth's oceans.

Covalent organic frameworks (COFs) can be designed to allow uranium extraction from seawater by incorporating photocatalytic linkers. However, often sacrificial reagents are required for separating photogenerated charges which limits their practical applications. Herein, we present a COF-based adsorption-photocatalysis strategy for selective removal of uranyl from seawater in the absence of sacrificial reagents. A series of ternary and quaternary COFs were synthesized containing the electron-rich linker 2,4,6-triformylphloroglucinol as the electron donor, the electron-deficient linker 4,4′-(thiazolo[5,4- d ]thiazole-2,5-diyl)dibenzaldehyde as the acceptor, and amidoxime nanotraps for selective uranyl capture (with the quaternary COFs incorporating [2,2′-bipyridine-5,5′-diamine-Ru(Bp) 2 ]Cl 2 as a secondary photosensitizer). The ordered porous structure of the quaternary COFs ensured efficient mass transfer during the adsorption-photocatalysis capture of uranium from seawater samples, with photocatalytically generated electrons resulting in the reduction of adsorbed U(VI) to U(IV) in the form of UO 2 . A quaternary COF, denoted as COF 2-Ru-AO, possessed a high uranium uptake capacity of 2.45 mg/g/day in natural seawater and good anti-biofouling abilities, surpassing most adsorbents thus far. This work shows that multivariate COF adsorption-photocatalysts can be rationally engineered to work efficiently and stably without sacrificial electron donors, thus opening the pathway for the economic and efficient extraction of uranium from the earth’s oceans.