Jingyu Xi

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.

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.

Ternary Platinum–Copper–Nickel Nanoparticles Anchored to Hierarchical Carbon Supports as Free-Standing Hydrogen Evolution Electrodes

Developing cost-effective and efficient hydrogen evolution reaction (HER) electrocatalysts for hydrogen production is of paramount importance to attain a sustainable energy future. Reported herein is a novel three-dimensional hierarchical architectured electrocatalyst, consisting of platinum-copper-nickel nanoparticles-decorated carbon nanofiber arrays, which are conformally assembled on carbon felt fabrics (PtCuNi/CNF@CF) by an ambient-pressure chemical vapor deposition coupled with a spontaneous galvanic replacement reaction. The free-standing PtCuNi/CNF@CF monolith exhibits high porosities, a well-defined geometry shape, outstanding electron conductivity, and a unique characteristic of localizing platinum-copper-nickel nanoparticles in the tips of carbon nanofibers. Such features render PtCuNi/CNF@CF as an ideal binder-free HER electrode for hydrogen production. Electrochemical measurements demonstrate that the PtCuNi/CNF@CF possesses superior intrinsic activity as well as mass-specific activity in comparison with the state-of-the-art Pt/C catalysts, both in acidic and alkaline solutions. With well-tuned composition of active nanoparticles, Pt42Cu57Ni1/CNF@CF showed excellent durability. The synthesis strategy reported in this work is likely to pave a new route for fabricating free-standing hierarchical electrodes for electrochemical devices.

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.

Facile approach to enhance the Pt utilization and CO-tolerance of Pt/C catalysts by physically mixing with transition-metal oxide nanoparticles

A very simple and promising method to design the anode catalyst architecture for direct alcohol fuel cells by physically mixing Pt/C catalyst with transition-metal oxide nanoparticles is presented and electrochemical measurements confirm that this unique catalyst structure has excellent activity toward alcohol and CO electro-oxidation.

A facile approach to fabricate free-standing hydrogen evolution electrodes: riveting tungsten carbide nanocrystals to graphite felt fabrics by carbon nanosheets

We report a novel three-dimensional architecture, consisting of tungsten carbide nanocrystals which are intimately riveted to graphite felt fabrics by carbon nanosheets (CNS@WC/GF). The as-prepared CNS@WC/GF monolith is utilized as a binder-free hydrogen evolution reaction electrode in both acidic and alkaline solutions. It demonstrates remarkable activity as well as durability.