Na 3 V 2 (PO 4 ) 3 has received a lot of attention as a prospective cathode for sodium-ion batteries (SIBs) owing to its strong 3D wide-open architecture for Na + transport. However, the practical application is still hampered by its poor electronic conductivity and weak Na + diffusion kinetics . In this paper, a novel Mg 2+ -doped Na 3 V 2 (PO 4 ) 3 flower-like cathode (Na 3+x V 2-x Mg x (PO 4 ) 3 /C, x = 0, 0.02, 0.04, 0.06) with abundant mesoporous has been synthesized by a convenient template-assisted sol-gel method. Mg 2+ doping successfully triggers V 4+ /V 5+ reversible redox with improved voltage plateau and reversible capacity. The hierarchical mesoporous structure combined with carbon layers is beneficial to the fast transport of Na + and e − , which guarantee improved kinetic properties and enhanced conductivity. Attributed to these competitive advantages, significantly improved electrochemical properties of Na 3.04 V 1.96 Mg 0.04 (PO 4 ) 3 /C (NVMP-4) are acquired, including extremely high specific capacity (123.8 mAh g −1 at 1C), excellent rate performance (73.8 mAh·g −1 at 100C) and exceptionally stable energy output with 89.1 % capacity retention after 3000 cycles at 20C. Additionally, when assembled with a hard carbon anode (HC), NVMP-4||HC provides an exceptional energy density of 216.9 Wh kg −1 at 0.2C. By enhancing nanostructure and rational surface engineering, this project offers fresh perspectives for creating superior electrodes, increasing the energy density of SIBs.
Dan Zhou,Mengmeng Liang,Yonglian Xia,Chao Li,Mingzheng Huang,Shuming Peng,Yi Huang
Extracellular polymeric substances (EPS) are high-molecular polymers secreted by microbes and play essential roles in metallic biogeochemical cycling. Previous studies demonstrated the reducing capacity of the functional groups on EPS for metal reduction. However, the roles of different EPS components in vanadium speciation and their responsible reducing substances for vanadium reduction are still unknown. In this study, the EPS of Bacillus sp. PFYN01 was fractionated via ultrafiltration into six components with different kDa (EPS >100 , EPS 100 – 50 , EPS 50 – 30 , EPS 30 – 10 , EPS 10 – 3 , and EPS <3 ). Batch reduction experiments of the intact cells, EPS-free cells, the pristine and fractionated EPS with V 5+ were conducted and characterized. The results demonstrated that the extracellular reduction of V 5+ into V 4+ by EPS was the major reduction process. Among the functional groups in EPS, C=O/C-N of amide in protein/polypeptide and C O of carboxyl in fulvic acid-like substances might act as the reductants for V 5+ , while C O in polysaccharide molecules and P O in phosphodiester played a key role in the adsorption process. The intracellular reduction was via translocating V 5+ into the cells and releasing V 4+ by the intracellular reductases. The reducing capacity of the fractionated EPS followed a sequence of EPS <3 > EPS 10 – 3 > EPS 50 – 30 > EPS 100 – 50 > EPS 30 – 10 > EPS >100 . The small molecules of fulvic acid-like substances and amino acids were responsible for the high reducing capacity of EPS <3 . EPS >100 had the lowest reducing capacity due to its macromolecular structure decreasing the exposure of the reactive sites. In addition to reduction, those intermediate EPS components may also have supporting functions, such as connecting protein skeletons and increasing the specific surface area of EPS. Therefore, the diverse effects of the EPS components cannot be neglected in vanadium biogeochemical cycling.
Tian Zhou,Zhangnan Xu,Mingyu Zhou,Zexin Zhou,Nianben Zheng,Zhiqiang Sun
Flow electrolysis is regarded as the most promising approach for producing high-quality and pure vanadium electrolytes. However, it faces challenges due to sluggish electrode reaction kinetics, resulting in high energy consumption and relatively low preparation efficiency. To address this issue, we propose a rapid electrode modification method that combines low-pressure plasma treatment with KMnO 4 etching. The catalyst MnO 2 , formed on carbon fiber, effectively enhances the electrode's electrocatalytic performance. The findings from the electrochemical analysis demonstrate that the modified electrode displays markedly enhanced electrocatalytic activity for the redox reaction of V 4+ /V 3+ . Flow electrolysis tests reveal that the electrolysis system with the modified electrode consumes 35.78 % less energy than the original graphite felt at a high current density of 300 mA/cm 2 , ten times higher than previously reported. Notably, the efficiency of the electrolyte prepared using the modified electrode in vanadium redox flow batteries (VRFB) achieves 98.13 % (coulombic efficiency), 86.88 % (volumetric efficiency), and 85.26 % (energy efficiency) at 100 mA/cm 2 , comparable to the electrolyte produced with the original electrode. This demonstrates the viability of employing the proposed graphite felt to efficiently fabricate V 3.5+ electrolytes with low energy consumption at high current densities.
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