Le Liu

Properties investigation of sulfonated poly(ether ether ketone)/polyacrylonitrile acid-base blend membrane for vanadium redox flow battery application.

Acid-base blend membrane prepared from sulfonated poly(ether ether ketone) (SPEEK) and polyacrylonitrile (PAN) was detailedly evaluated for vanadium redox flow battery (VRFB) application. SPEEK/PAN blend membrane exhibited dense and homogeneous cross-section morphology as scanning electron microscopy and energy-dispersive X-ray spectroscopy images show. The acid-base interaction of ionic cross-linking and hydrogen bonding between SPEEK and PAN could effectively reduce water uptake, swelling ratio, and vanadium ion permeability, and improve the performance and stability of blend membrane. Because of the good balance of proton conductivity and vanadium ion permeability, blend membrane with 20 wt % PAN (S/PAN-20%) showed higher Coulombic efficiency (96.2% vs 91.1%) and energy efficiency (83.5% vs 78.4%) than Nafion 117 membrane at current density of 80 mA cm(-2) when they were used in VRFB single cell. Besides, S/PAN-20% membrane kept a stable performance during 150 cycles at current density of 80 mA cm(-2) in the cycle life test. Hence the SPEEK/PAN acid-base blend membrane could be used as promising candidate for VRFB application.

CeO2 decorated graphite felt as a high-performance electrode for vanadium redox flow batteries

In this work, CeO2 nanoparticle decorated graphite felts (CeO2/GFs) were prepared by a facile precipitation method. The corresponding CeO2/GF composites containing different contents of CeO2, i.e. 0.1, 0.2, 0.3, 0.5 wt% were synthesized individually as electrodes for vanadium redox flow battery (VRFB) application. Scanning electron microscopy and X-ray diffraction analysis indicated the homogeneous dispersion of CeO2 nanoparticles on GF. The cyclic voltammetry results revealed that the CeO2/GFs exhibited higher activity and better reversibility towards the VO2+/VO2+ redox reaction compared with the pristine GF. Among all the electrodes, 0.2 wt% CeO2/GF demonstrated the best electrochemical properties, thus nominating CeO2 content of 0.2 wt% as an optimum content. The VRFB single cell tests indicated that 0.2 wt% CeO2/GF showed the highest energy efficiency of 64.7% at the current density of 200 mA cm−2, which was significantly higher than that of the pristine GF (53.9%). Furthermore, the cycle life test of a VRFB single cell demonstrated the outstanding stability of the CeO2/GFs electrode.

Insights into the Impact of the Nafion Membrane Pretreatment Process on Vanadium Flow Battery Performance

Nafion membranes are now the most widely used membranes for long-life vanadium flow batteries (VFBs) because of their extremely high chemical stability. Today, the type of Nafion membrane that should be selected and how to pretreat these Nafion membranes have become critical issues, which directly affects the performance and cost of VFBs. In this work, we chose the Nafion 115 membrane to investigate the effect of the pretreatment process (as received, wet, boiled, and boiled and dried) on the performance of VFBs. The relationship between the nanostructure and transport properties of Nafion 115 membranes is elucidated by wide-angle X-ray diffraction and small-angle X-ray scattering techniques. The self-discharge process, battery efficiencies, electrolyte utilization, and long-term cycling stability of VFBs with differently pretreated Nafion membranes are presented comprehensively. An online monitoring system is used to monitor the electrolyte volume that varies during the long-term charge-discharge test of VFBs. The capacity fading mechanism and electrolyte imbalance of VFBs with these Nafion 115 membranes are also discussed in detail. The optimal pretreatment processes for the benchmark membrane and practical application are synthetically selected.

State of charge monitoring for vanadium redox flow batteries by the transmission spectra of V(IV)/V(V) electrolytes

A method is presented for monitoring the state of charge (SOC) of vanadium redox flow batteries (VRBs) by the transmission spectrum of the positive electrolyte [V(IV)/V(V)]. We use the transmission spectrum rather than the absorption spectrum for better signal noise ratio. In order to solve the complicated relations between the spectrum and the SOC of the VRB, the entire shape of the spectrum is utilized instead of the data at one wavelength. We experimentally demonstrate that SOCs from 0 to 100xa0% can be monitored by comparing the transmission spectra of the positive electrolytes [V(IV)/V(V)] with the database, which is previously built by detecting standard samples. The method has potential ability to monitor the level of imbalances of the VRB, which is important for rebalancing the electrolyte and restoring the capacity loss of a VRB.

An on-line spectroscopic monitoring system for the electrolytes in vanadium redox flow batteries

In this work, an on-line electrolyte spectroscopic monitoring system (OESM) is developed for long term monitoring of the electrolytes in a charge–discharge cycling VRB. We demonstrated experimentally that the transmittance spectra of the positive/negative electrolyte in a cycling VRB can be on-line monitored. With appropriate calibration, parameters such as the state of charge (SOC) of the electrolytes can be calculated. The system offers a tool for further research on the complex relationships between the spectra and the composition of the electrolyte in a VRB, and provides a dynamic method to study the kinetics of the electrolyte imbalance in VRBs.

Rational use and reuse of Nafion 212 membrane in vanadium flow batteries

Nafion series membranes are widely applied in vanadium flow batteries (VFB) as benchmark separators because of their extremely high chemical/mechanical stability. However, the serious vanadium ions crossover and comparably high price still hinder the large-scale application of Nafion membranes in VFB. Rational use and reuse of Nafion membranes is expected to overcome these two critical issues, which would greatly enhance the cycling performances and reduce the cost of VFB. In this study, we chose the relatively thin (50 μm) and cheap Nafion 212 membrane to investigate the rational use (for fresh membranes) and reuse (for used membranes) protocols in VFB. The structure-property relationship of various pretreated (as-received, water wetted, and acid boiled) Nafion 212 membranes is studied comprehensively. The results demonstrate that the wet Nafion 212 membrane can achieve superior VFB performances including 96% of coulombic efficiency, 77% of energy efficiency, and 0.11% per cycle of capacity fading at a higher current density of 120 mA cm−2. Our attempt also reveals that the characters of the reused Nafion 212 membranes, such as micro and macro morphologies, mechanical properties, rate and cycling performances, have been well maintained, even after the strict testing procedures which including frequent assembly/disassembly (12 times) of battery and super long period of operation (1500 h), demonstrating that Nafion 212 membrane can be used repeatedly in VFB.

Online Spectroscopic Study on the Positive and the Negative Electrolytes in Vanadium Redox Flow Batteries

Traditional spectroscopic analysis based on the Beer-Lambert law cannot analyze the analyte with high concentration and interference between different compositions, such as the electrolyte in vanadium redox flow batteries (VRBs). Here we propose a new method for online detection of such analytes. We demonstrate experimentally that, by comparing the transmittance spectrum of the analyte with the spectra in a preprepared database using our intensity-corrected correlation coefficient (ICCC) algorithm, parameters such as the state of charge (SOC) of both the positive and the negative electrolytes in the VRB can be online monitored. This method could monitor the level of the electrolytes imbalance in the VRB, which is useful for further rebalancing the electrolyte and restoring the capacity loss of the VRB. The method also has the potential to be used in the online detection of other chemical reactions, in which the chemical reagents have high concentration and interferences between different compositions.

A low-cost average valence detector for mixed electrolytes in vanadium flow batteries

The decay in capacity has hindered the applications of vanadium redox flow batteries (VFBs), which are promising energy storage devices with many benefits. Mixing positive and negative electrolytes can recover some of the lost capacity but is ineffective towards the increasing average valence of the mixed electrolyte caused by the side reactions. In this study, a low-cost optical average valence detector has been developed to monitor the average valence of the mixed electrolyte in VFBs. We demonstrate experimentally that with the aid of the average valence detector, the capacity of VFBs can be regularly recovered via electrolyte mixing and online electrolysis. This low-cost average valence detector has great potential for recovering the decayed capacities of VFBs in large-scale energy storage stations, which consist of thousands of VFBs.