Xinping Qiu

Self-assembled polyelectrolyte multilayer modified Nafion membrane with suppressed vanadium ion crossover for vanadium redox flow batteries

The crossover of vanadium ions through proton-exchange membranes such as those of Nafion is the chief reason that results in the low energy efficiency and high self-discharge rate of vanadium redox flow batteries (VRB). With respect to applicability, the ideal proton-exchange membrane used in VRB should possess simultaneously high proton conductivity and low vanadium ion permeability. Here, we report a novel approach using a polyelectrolyte layer-by-layer self-assembly technique to fabricate a barrier layer onto the surface of Nafion membrane by alternate adsorption of polycation poly(diallyldimethylammonium chloride) (PDDA) and polyanion poly(sodium styrene sulfonate) (PSS), which can suppress the crossover of vanadium ions. The Nafion–[PDDA-PSS]n membrane (n = the number of multilayers) obtained shows much lower vanadium ion permeability compared with plain Nafion membrane. Accordingly, the VRB with Nafion–[PDDA-PSS]n membrane exhibits a higher coulombic efficiency (CE) and energy efficiency (EE) together with a slower self-discharge rate than that of Nafion system. The highest CE of 97.6% and EE of 83.9% can be achieved at charge–discharge current density of 80 mA cm−2 and 20 mA cm−2, respectively.

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.

Synthesis and high rate properties of nanoparticled lithium cobalt oxides as the cathode material for lithium-ion battery

Nanoparticled lithium cobalt oxides were synthesized by coprecipitation in ethanol. Mechanical stirring was used to control the particle growth. The controlled grain size was below 100 nm with a narrow distribution. The structure has been examined using X-ray diffraction. On hand of an experimental cell it appears that the novel material has an exciting high charge and discharge rate: the capacity is 100 mA h/g for a 50C rate and 130 mA h/g for a 10C rate.

Mesocarbon microbeads supported Pt-Ru catalysts for electrochemical oxidation of methanol

Abstract The effects of mesocarbon microbeads support for platinum-ruthenium (Pt-Ru) catalysts on anode performance of the direct methanol fuel cell (DMFC) were investigated. The Pt-Ru catalyst with mesocarbon microbeads as support shows lower polarization characteristics than that with carbon black as support. The SEM of the cross-section of the catalyst layer with mesocarbon microbeads as support shows that there exist large pores and channels that are favorite for mass transfer in electrode reaction region. The effects of the cell operation temperature and the methanol concentration on the polarization curves of the anode electrode were also investigated.

ZrO2-Nanoparticle-Modified Graphite Felt: Bifunctional Effects on Vanadium Flow Batteries

To improve the electrochemical performance of graphite felt (GF) electrodes in vanadium flow batteries (VFBs), we synthesize a series of ZrO2-modified GF (ZrO2/GF) electrodes with varying ZrO2 contents via a facile immersion-precipitation approach. It is found that the uniform immobilization of ZrO2 nanoparticles on the GF not only significantly promotes the accessibility of vanadium electrolyte, but also provides more active sites for the redox reactions, thereby resulting in better electrochemical activity and reversibility toward the VO(2+)/VO2(+) and V(2+)/V(3+) redox reactions as compared with those of GF. In particular, The ZrO2/GF composite with 0.3 wt % ZrO2 displays the best electrochemical performance with voltage and energy efficiencies of 71.9% and 67.4%, respectively, which are much higher than those of 57.3% and 53.8% as obtained from the GF electrode at 200 mA cm(-2). The cycle life tests demonstrate that the ZrO2/GF electrodes exhibit outstanding stability. The ZrO2/GF-based VFB battery shows negligible activity decay after 200 cycles.

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.

The Microstructure and Character of the PVDF-g-PSSA Membrane Prepared by Solution Grafting

A new method based on a solution-grafting technique was used to prepare poly(vinylidene fluoride) grafted polystyrene sulfonated acid (PVDF-g-PSSA) proton exchange membrane. Polystyrene is easily grafted into PVDF, which has been treated in KOH solution. There is a linear relationship between the degree of grafting and the treatment time in KOH solution. Fourier transform infrared spectroscopy and differential scanning calorimetry were used to characterize the changes of the membranes microstructure after grafting and sulfonation. The conductivity of the solution-grafted PVDF-g-PSSA membranes was investigated by the four-probe dc technique and reached S cm−1 at 25°C. Gas chromatographic studies show that the methanol permeability of the PVDF-g-PSSA membrane is almost one order of magnitude lower than that of a Nafion-112 membrane.

Impedance studies on mesocarbon microbeads supported Pt-Ru catalytic anode

The impedance behaviors of direct methanol fuel cell anode using mesocarbon microbeads supported Pt-Ru catalysts were studied. A loop reflecting an inductive behavior appears at the low-frequency end only when the anode is at higher overpotential. The Nafion content in the catalyst, the MEA hot press condition and the effects of the cell temperature on the impedance behaviors of the anode electrode were also investigated.