To improve the transport of active species in the carbon nano-fibrous electrodes of a vanadium flow battery (VFB), a free-standing carbon nano-fibrous web with ultra large pores has been designed and fabricated through the horizontally-opposed blending electrospinning method in this study. The morphology, surface chemistry and electrochemical performances of the highly porous nano-fibrous web have been investigated and compared with the carbon nano-fibrous web prepared by traditional electrospinning. Benefiting from the much larger pore size and higher porosity of the carbon nano-fibrous web prepared by horizontally-opposed blending electrospinning, the concentration polarization of the vanadium flow battery is effectively reduced. As indicated by the single cell tests, the battery using horizontally-opposed blending electrospun carbon nano-fibrous web electrodes delivers much improved performance, especially at high current density. The voltage efficiency is 10.3% higher than that of the traditional electrospun carbon nano-fibrous web electrodes and the electrolyte utilization efficiency is twice as much as that of the traditional electrospun carbon nano-fibrous web electrodes at 60 mA cm−2. The results suggest that expanding the pore size could be one effective strategy to facilitate carbon nano-fibrous materials' applications for VRBs, and that the horizontally-opposed blending electrospun carbon nano-fibrous web is a promising electrode candidate for VFBs.

Ultrathin free-standing electrospun carbon nanofiber web (ECNFW) used for the electrodes of the vanadium flow battery (VFB) has been fabricated by the electrospinning technique followed by the carbonization process in this study to reduce the ohmic polarization of the VFB. The microstructure, surface chemistry and electrochemical performance of ECNFW carbonized at various temperatures from 800 to 1400 °C have been investigated. The results show that ECNFW carbonized at 1100 °C exhibits the highest electrocatalytic activity toward the V2+/V3+ redox reaction, and its electrocatalytic activity decreases along with the increase of carbonization temperature due to the drooping of the surface functional groups. While for the VO2+/VO2+ redox couple, the electrocatalytic activity of ECNFW carbonized above 1100 °C barely changes as the carbonization temperature rises. It indicates that the surface functional groups could function as the reaction sites for the V2+/V3+ redox couple, but have not any catalytic effect for the VO2+/VO2+ redox couple. And the single cell test result suggests that ECNFW carbonized at 1100 °C is a promising material as the VFB electrode and the VFB with ECNFW electrodes obtains a super low internal resistance of 250 mΩ cm2.The carbon nanofibers web has been made through the electrospinning method and used as vanadium flow battery (VFB) electrode independently. The results showed that it can significantly decrease ohmic polarization of single cell.Download high-res image (128KB)Download full-size image.

Free-standing activated carbon nanofibers (ACNF) were prepared through electrospinning combining with CO2 activation and then used for nonaqueous Li–O2 battery cathodes. As-prepared ACNF based cathode was loosely packed with carbon nanofibers complicatedly overlapped. Owing to some micrometer-sized pores between individual nanofibers, relatively high permeability of O2 across the cathode becomes feasible. Meanwhile, the mesopores introduced by CO2 activation act as additional nucleation sites for Li2O2 formation, leading to an increase in the density of Li2O2 particles along with a size decrease of the individual particles, and therefore, flake-like Li2O2 are preferentially formed. In addition, the free-standing structure of ACNF cathode eliminates the side reactions about PVDF. As a result, the Li–O2 batteries with ACNF cathodes showed increased discharge capacities, reduced overpotentials, and longer cycle life in the case of full discharge and charge operation. This provides a novel pathway for the design of cathodes for Li–O2 battery.Keywords: electrospinning; free-standing cathodes; hierarchically porous structure; Li−O2 battery; physical activation

Meso–macro hierarchical porous carbon (HPC) is prepared and used as a cathode material in Li–O2 batteries. The O2 diffusivity has been largely improved due to the unblocked macropores. As a result, a better pore utilization and extremely high discharge capacity is achieved.

Nitrogen enriched mesoporous carbon (N-MCS) with extremely high mesopore volume and nitrogen content is prepared through a one-step hard template method. The N-MCS cathode shows excellent discharge performance in lithium–oxygen batteries. The pore space is better utilized due to its optimized pore structure and uniformly incorporated N.