Tuning the Perfluorosulfonic Acid Membrane Morphology for Vanadium Redox-Flow Batteries.

The microstructure of perfluorinated sulfonic acid proton-exchange membranes such as Nafion significantly affects their transport properties and performance in a vanadium redox-flow battery (VRB). In this work, Nafion membranes with various equivalent weights ranging from 1000 to 1500 are prepared and the morphology-property-performance relationship is investigated. NMR and small-angle X-ray scattering studies revealed their composition and morphology variances, which lead to major differences in key transport properties related to proton conduction and vanadium-ion permeation. Their performances are further characterized as VRB membranes. On the basis of this understanding, a new perfluorosulfonic acid membrane is designed with optimal pore geometry and thickness, leading to higher ion selectivity and lower cost compared with the widely used Nafion 115. Excellent VRB single-cell performance (89.3% energy efficiency at 50 mA·cm-2) was achieved along with a stable cyclical capacity over prolonged cycling.

Capacity decay mechanism of microporous separator-based all-vanadium redox flow batteries and its recovery.

The results of the investigation of the capacity decay mechanism of vanadium redox flow batteries with microporous separators as membranes are reported. The investigation focuses on the relationship between the electrochemical performance and electrolyte compositions at both the positive and negative half-cells. Although the concentration of total vanadium ions remains nearly constant at both sides over cycling, the net transfer of solution from one side to the other and thus the asymmetrical valance of vanadium ions caused by the subsequent disproportionate self-discharge reactions at both sides lead to capacity fading. Through in situ monitoring of the hydraulic pressure of the electrolyte during cycling at both sides, the convection was found to arise from differential hydraulic pressures at both sides of the separators and plays a dominant role in capacity decay. A capacity-stabilizing method is developed and was successfully demonstrated through the regulation of gas pressures in both electrolyte tanks.

Bismuth nanoparticle decorating graphite felt as a high-performance electrode for an all-vanadium redox flow battery.

Employing electrolytes containing Bi(3+), bismuth nanoparticles are synchronously electrodeposited onto the surface of a graphite felt electrode during operation of an all-vanadium redox flow battery (VRFB). The influence of the Bi nanoparticles on the electrochemical performance of the VRFB is thoroughly investigated. It is confirmed that Bi is only present at the negative electrode and facilitates the redox reaction between V(II) and V(III). However, the Bi nanoparticles significantly improve the electrochemical performance of VRFB cells by enhancing the kinetics of the sluggish V(II)/V(III) redox reaction, especially under high power operation. The energy efficiency is increased by 11% at high current density (150 mA·cm(-2)) owing to faster charge transfer as compared with one without Bi. The results suggest that using Bi nanoparticles in place of noble metals offers great promise as high-performance electrodes for VRFB application.

Capacity decay and remediation of nafion-based all-vanadium redox flow batteries.

The relationship between electrochemical performance of vanadium redox flow batteries (VRBs) and electrolyte composition is investigated, and the reasons for capacity decay over charge-discharge cycling are analyzed and discussed herein. The results show that the reasons for capacity fading over real charge-discharge cycling include not only the imbalanced vanadium active species, but also the asymmetrical valence of vanadium ions in positive and negative electrolytes. The asymmetrical valence of vanadium ions leads to a state-of-charge (SOC)-range decrease in positive electrolytes and a SOC-range increase in negative electrolytes. As a result, the higher SOC range in negative half-cells further aggravates capacity fading by creating a higher overpotential and possible hydrogen evolution. Based on this finding, we developed two methods for restoring lost capacity, thereby enabling long-term operation of VRBs to be achieved without the substantial loss of energy resulting from periodic total remixing of electrolytes.