Lihong Yu

SPEEK/Graphene oxide nanocomposite membranes with superior cyclability for highly efficient vanadium redox flow battery

A series of novel composite membranes, based on sulfonated poly(ether ether ketone) (SPEEK) with various graphene oxide (GO) loadings, were employed and investigated in vanadium redox flow battery (VRFB) for the first time. The scanning electron microscopy images of the composite membranes revealed the uniform dispersion of GO nanosheets in the polymer matrix due to the interaction between GO and SPEEK, as confirmed by Fourier transform infrared spectra. The mechanical and thermal parameters of the composite membranes increased, while the VO2+ permeability decreased with increasing GO content. Random embedding of GO nanosheets in the membranes can serve as effective barriers to block the transport of vanadium ion, resulting in a significant decrease of vanadium ion permeability. The VRFB assembled with the composite membrane exhibited highly improved cell parameters and strikingly long cycling stability compared with commercial Nafion 117 membrane. With the protection of porous PTFE substrate, the pore-filling SPEEK/GO composite membrane based on VRFB ran for 1200 cycles with relatively low capacity decline.

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.

A recast Nafion/graphene oxide composite membrane for advanced vanadium redox flow batteries

A graphene oxide (GO) nanosheet incorporated recast Nafion membrane (rNafion/GO) is applied to a vanadium redox flow battery (VRFB) as a promising ion-exchange membrane to reduce the vanadium permeation. Randomly embedded GO nanosheets in the composite membrane can serve as effective barriers to block the transport of vanadium ions, resulting in significant decrease of vanadium ion permeability. The rNafion/GO composite membrane exhibits a dense and homogeneous cross-section morphology as shown by SEM images. The hydrogen-bonding interaction between GO nanosheet filler and Nafion matrix is beneficial for the improvement of mechanical stability. Owing to the good balance of proton conductivity and vanadium ion permeability, the VRFB single cell performance of the rNafion/GO membrane shows higher coulombic efficiency (96% vs. 91%) and energy efficiency (85% vs. 80%) than the pure rNafion membrane at a current density of 80 mA cm−2. A long cycling test confirms that the rNafion/GO membrane has superior chemical and mechanical stability and can be repeatedly used in a VRFB without any damage, revealing the advancement for practical VRFB application.

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.

Polydopamine coated SPEEK membrane for a vanadium redox flow battery

In this work, sulfonated poly(ether ether ketone) (SPEEK) membranes have been first modified by a polydopamine (PDA) coating and then investigated as an ion exchange membrane for a vanadium redox flow battery (VRFB). The SEM results showed that the PDA film was successfully coated on the surface of the SPEEK membrane. The PDA film can serve as a blocking layer to protect the substrate SPEEK membrane from being corroded by the highly oxidative vanadium electrolyte. The obtained PDA coated SPEEK membranes showed much lower vanadium ion permeability, accompanied by higher mechanical strength and thermal stability than the pristine SPEEK membrane. Consequently, the VRFB with the optimal PDA/SPEEK membrane exhibited much higher coulombic efficiency and better discharge capacity retention than the pristine SPEEK membrane. This indicates that the PDA coated SPEEK membranes are of significant interest for VRFB applications.

Structure–property relationship study of Nafion XL membrane for high-rate, long-lifespan, and all-climate vanadium flow batteries

Vanadium flow batteries (VFB) have become one of the most promising large-scale energy storage technologies owing to their overwhelming advantage in lifespan and power. Development of highly efficient, durable and low-cost ion exchange membranes is essential to the practical application of VFB. In this work, we report an ultra-thin sandwich structure membrane (Nafion XL), as a promising alternative to the widely used Nafion 212 and 115 membranes. By combination of the mechanical reinforcement though the microporous PTFE middle layer (∼10 μm) and fast proton transport through the two dense Nafion outer layers (∼10 μm), the Nafion XL membrane shows lower area resistance and slower vanadium ion crossover than the Nafion 212 and 115 membranes. As a result, the Nafion XL membrane-based VFB single cell demonstrates excellent rate capability (40–400 mA cm−2), superior long-term cycling stability (2200 cycles@120/200 mA cm−2) and wide temperature adaptability (−20 to 50 °C). The results reported herein are beneficial for the development of high-rate, long-lifespan, and all-climate VFB for sustainable energy storage.

Electrospun polyacrylonitrile nanofiber mat protected membranes for vanadium flow batteries

A cheap, efficient and durable proton exchange membrane is essential for vanadium flow batteries (VFB). The sulfonate poly(ether ether ketone) (SPEEK) membrane is a leading candidate to replace the widely used Nafion series membranes because of high proton to vanadium ion selectivity and low-cost. However, poor chemical/mechanical stability still hinders the practical application of the SPEEK membrane in VFB. Herein, we report a simple and robust strategy by using an electrospun polyacrylonitrile (PAN) nanofiber mat as a protection layer to improve the stability of the SPEEK membrane. The PAN nanofiber mat can prevent the damage to the SPEEK membrane from the vanadium electrolytes, graphite (carbon) felt electrodes and sealing gaskets, without affecting the VFB performance. Therefore, the PAN nanofiber mat–SPEEK–PAN nanofiber mat (P–S–P) sandwich membrane demonstrates excellent rate performance and superior cycling stability over 1000 charge/discharge cycles at a current density of 120 mA cm−2.