We hypothesize that the key to obtaining higher efficiency and stability in PEC uranium extraction is the application of a proper external bias potential on the photocatalyst that can suppress the recombination of charge carriers and spatially separate the U deposition sites from the photocatalyst active sites. To examine this hypothesis, PEC experiments were conducted at varied bias potentials with synthetic U-containing water. Control experiments were also performed with illumination at open-circuit condition (i.e., PC U extraction), and in the dark with external bias (i.e., EC U extraction). Fast PEC U extraction kinetics were obtained at all tested bias potentials, and the total extraction efficiencies after 120 min operation reached 65.2%, 78.7%, and 63.8%, at applied bias potential of 0.0 V (all potentials reported in this work are relative to the standard calomel electrode (SCE) unless otherwise stated), 0.5 V, and 1.0 V respectively (Figure and). In the case of PC method, U extraction was much slower, where a total efficiency of 28.9% was achieved after 120 min operation. The results implied a significant role of the applied bias in promoting the U extraction efficiency of the PEC method. By comparing the U extraction efficiencies of the EC, PC, and PEC methods, obvious synergistic effects were observed in the PEC method: the synergy factor (calculated according to Supplementary Note 3 ) at 0.0 V, 0.5 V, and 1.0 V were 187%, 266% and 197%, respectively (Figure ). The bias potential might play two roles in PEC U extraction: (1) promoting the separation of charge carriers by triggering band bending within the photocatalyst 14 ; (2) driving direct EC U extraction on electrode surfaces . The first role was evidenced by the increased photocurrent with the increasing bias potential (Figure ). There was much less EC U extraction at the three bias potentials (≤7%) (Figure and Figure ), indicating an insignificant contribution of the second role. Furthermore, although the photocurrent increased with higher bias potential, an optimal value exists. This might be attributed to the production of oxidative species at a higher potential that can re-oxidize the precipitated U. of conduction band electrons from the photocatalyst to the cathode, the U deposition sites and the photocatalyst should be spatially separated, therefore higher stability and durability of the photocatalyst are expected. To test this expectation, we performed a reusability assay (Figure ). After each cycle, defined reaction solutions were directly refilled into the PEC reactor and the PC reactor without regeneration treatment of the TiO2 nanotube array (TNA) and the Ti electrode. The PEC method indeed showed high durability, as ~0.58 mg U could be extracted from 0.06 L synthetic U-containing water for each of the 8 tested 120-min operation cycles. On the contrary, in the first cycle of the PC method, only 0.28 mg U was extracted, and the extracted U decreased to 0.15 mg and 0.10 mg during the second and the third cycle, respectively. Hence, the PEC method had exceptional stability compared to the PC method. Ti electrode. The results show that, upon PEC treatment, the electrode was covered by a nanoparticles layer (Figure and), which contains U according to energy dispersive spectroscopy (EDS) analysis (Figure and Figure ). However, in the case of PC U extraction, no noticeable changes in the electrode morphology (Figure and) or U deposition (Figure and Figure ) was observed. We further analyzed the distribution of extracted U on both the TNAs and the Ti electrodes upon PC and PEC experiments (see Supplementary Note 5).