Self-Standing Porous Aromatic Framework Electrodes for Efficient Electrochemical Uranium Extraction

Electrochemical uranium extraction from seawater provides a new opportunity for a sustainable supply of nuclear fuel. However, there is still room for studying flexible electrode materials in this field. Herein, we construct amidoxime group modified porous aromatic frameworks (PAF-144-AO) on flexible carbon cloths in situ using an easy to scale-up electropolymerization method followed by postdecoration to fabricate the self-standing, binder-free, metal-free electrodes (PAF-E). Based on the architectural design, adsorption sites (amidoxime groups) and catalytic sites (carbazole groups) are integrated into PAF-144-AO. Under the action of an alternating electric field, uranyl ions are selectively captured by PAN-E and subsequently transformed into Na2O(UO3·H2O)x precipitates in the presence of Na+ via reversible electron transfer, with an extraction capacity of 12.6 mg g–1 over 24 days from natural seawater. This adsorption–electrocatalysis mechanism is also demonstrated at the molecular level by ex situ spectroscopy. Our work offers an effective approach to designing flexible porous organic polymer electrodes, which hold great potential in the field of electrochemical uranium extraction from seawater.


Physicochemical Adsorption Experiments
To investigate the physicochemical adsorption performance, PAF based electrodes was immersed into uranyl−spiked seawater solution with stirring.At specific time intervals, the uranium concentration in the solution was analyzed.The adsorption kinetics were performed at an initial concentration of 32 ppm.The isotherm experiments were conducted under certain initial concentrations of 5~120 ppm.

Data Analysis
The removal rate (R%) and the adsorption capacity (q, mg g −1 ) were calculated through the following equations: where C 0 and C e (mg L −1 ) are the initial and the equilibrium concentration of uranium in the aqueous solution, respectively.V (L) is the volume of the solution, and m (g) is the mass of the PAF material in the electrodes.

Electrochemical Uranium Removal
All electrochemical uranium adsorption experiments were conducted in a standard two−electrode system using a graphite rod as the anode and self-standing PAF based electrode as the cathode.Experiments were conducted over the voltage range from 0 V to −5 V using a frequency of 400 Hz during the tests.The pH of the uranyl−spiked seawater was adjusted to 5.0.The concentrations of the tested uranyl−spiked seawater solution were determined at specific time intervals.
For the selectivity experiments, the electrochemical uranium removal was performed in 10 ppm U-spiked real seawater solutions containing different interfering ions.The interfering ions included 10 ppm VO 3 − , Cu 2+ , Sr 2+ , Zn 2+ , Co 2+ , Ba 2+ and Ni 2+ , which were much higher than their actual concentrations.After the electrochemical removal, the concentrations of different metal ions were recorded.
For the reusability assay, fresh 10 ppm uranium spiked real seawater was used to run the electrochemical uranium removal.After one removal process, the U-loaded electrode was treated by eluting the bound uranium with 0.1 M HNO 3 and the blending solution of 1.0 M Na 2 CO 3 and 0.1 M H 2 O 2 .After this treatment, the electrode was reused for the new electrochemical uranium removal cycle.Ten consecutive cycles were performed under similar conditions.

Uranium Extraction from Natural Seawater
For purpose of examining the uranium extraction capacity of the PAF based electrode in natural seawater, the self-standing electrode was assembled in a flow device.The natural seawater without adjusting the pH value was forced through the electrode at a water flow of 5L S6 h −1 .Meanwhile, the voltage ranges from 0 V to −5 V using a frequency of 400 Hz were also applied to the electrodes.On specific days, the concentrations of residual uranium in the natural seawater were determined.The effect of the applied voltage on the uranium removal was investigated at different negative potentials from −1 V to −6 V.As the potential increased, the electrochemical uranium removal improved gradually and tended to the equilibrium after the potential of −5 V. High voltage could lead to the strong interaction between uranyl and PAF-E.Thus, the alternating voltage between −5 V and 0 V was applied in this work.

Scheme 2 .Scheme 3 .
Scheme 2. Synthesis routes and the relevant chemical structures of ETCB.

Figure S1 .
Figure S1.CV curves of the electropolymerization of only TCB (a), only NCP (b) and the

Figure S2 .
Figure S2.Mechanism of monomer oxidation, crosslinking, and reduction during the

Figure S3 .
Figure S3.Optical image of the self-standing porous aromatic framework electrodes.

Figure S4 .Figure S7 .
Figure S4.SEM images with low magnification for pure carbon cloth (a) and PAF-E (b).

Figure S17 .Figure S18 .
Figure S17.The formation process of yellow flocs on the PAF electrode during electrochemical

Figure S19 .Figure S20 .Figure S21 .
Figure S19.SEM image of the PAF-E after the electrochemical uranium extraction and its