Simultaneous Regulation of Solvation Shell and Oriented Deposition toward a Highly Reversible Fe Anode for All-Iron Flow Batteries.

Developing low-cost all-iron hybrid redox flow batteries (RFBs) presents a practical alternative to the high-cost all-vanadium RFBs and is deemed vital for grid-scale energy storage applications. However, the intrinsically poor Fe anode reversibility associated with the deposition and dissolution of metallic iron greatly limits the cycling performance and long-term stability of all-iron hybrid RFBs. Herein, a highly reversible and dendrite-free Fe anode is reported for all-iron RFBs through regulation of polar solvent dimethyl sulfoxide (DMSO) on FeCl2 anolyte, which simultaneously reshapes Fe2+ solvation structure and induces controllable oriented Fe deposition. Combining both experimental and theoretical analyses, the polar DMSO additives prove effective in replacing H2 O molecule from the primary solvation shell of Fe2+ cation via the Fe2+ -O (DMSO) bond and meanwhile induces a fine-grained Fe nucleation on the preferred Fe (110) plane, which are responsible for the minimized hydrogen evolution and dendrite-free Fe deposition that significantly enhance Fe anode reversibility. The all-iron RFB based on the proposed FeCl2 -DMSO anolyte demonstrates an excellent combination of peak power density of 134 mW cm-2 , high energy efficiency of 75% at 30 mA cm-2 , and high capacity retention of 98.6% over 200 cycles, which presents the best performance of all-iron RFBs among previously reported research.

Coupling effect between the structure and surface characteristics of electrospun carbon nanofibres on the electrochemical activity towards the VO2(+)/VO(2+) redox couple.

In order to investigate the structure-function relationship of electrospun carbon nanofibres (ECNFs), polyacrylonitrile (PAN)-based electrospun carbon webs (ECWs) have been developed, consisting of ECNFs carbonized over the temperature range of 1000-1500 °C in a nitrogen atmosphere. The surface morphology, microstructure, composition, electrical conductivity and hydrophilicity of the ECNFs have been characterized. The electrochemical activity of the ECNFs towards the VO2(+)/VO(2+) redox reaction has been measured by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It is worth noting that the electrochemical performance of the ECNFs decreases firstly and then rises gradually with the increase in carbonization temperature, and a carbonization temperature of about 1300 °C is the turning point. This unusual phenomenon might be attributed to the coupling effect between the surface and structure characteristics of the ECNFs towards the VO2(+)/VO(2+) redox couple. The surface composition plays a leading role in the electrochemical activity of ECNFs carbonized over the temperature range of 1000-1300 °C; however, the edge planes of graphite crystallites which form during the high temperature range from 1300-1500 °C then become the dominant factor. Therefore, the electrochemical activity decreases with the reduction of functional groups on the surface from carbonization at 1000-1300 °C, and then increases with the addition of the edge planes of graphite crystallites from carbonization at 1300-1500 °C.

Unusual phenomena in the reduction process of vanadium(v) on a graphite electrode at high overpotentials.

An abrupt increase in the Tafel slope and the charge transfer resistance (Rct) of V(v) ions on a graphite electrode are both observed at a transition potential (EK). The possible vanadium complexes such as V2O3(3+) might result in the change of reduction mechanism and the unusual phenomena.

An Economical Composite Membrane with High Ion Selectivity for Vanadium Flow Batteries.

The ion exchange membrane of the Nafion series widely used in vanadium flow batteries (VFBs) is characterized by its high cost and high vanadium permeability, which limit the further commercialization of VFBs. Herein, a thin composite membrane enabled by a low-cost microporous polyethylene (PE) substrate and perfluorosulfonic acid (PFSA) resin is proposed to reduce the cost of the membrane. Meanwhile, the rigid PE substrate limits the swelling of the composite membrane, which effectively reduces the penetration of vanadium ions and improves the ion selectivity of the composite membrane. Benefiting from such a rational design, a VFB assembled with the PE/PFSA composite membrane exhibited a higher coulombic efficiency (CE ≈ 96.8%) compared with commercial Nafion212 at 200 mA cm-2. Significantly, the energy efficiency maintained stability within 200 cycles with a slow decay rate. In practical terms, the thin PE/PFSA composite membrane with low cost and high ion selectivity can make an ideal membrane candidate in VFBs.

[Spectrophotometry analysis of different valence state of vanadium in vanadium battery electrolyte].

In the present paper, oxalic acid was used to reduce V(V) ion to V(IV) ion, then its complex with V(IV) was formed. By this method, four valence states of vanadium ions had different characteristic absorption peaks in the UV-Visible range. Based on these characteristic absorption peaks, qualitative and quantitative spectrophotometric analysis methods for different valence states of vanadium in vanadium battery electrolyte were established. The results showed that the related coefficients of four standard curves of different valence states were greater than 0.999 0, linearity ranges were 0.326-2.445, 0.326 -2.445, 0.720-5.403, and 1.784-13.437 g x L(-1), respectively. The measurement of samples suggested that the spectrophotometric analysis method was suitable for analyzing the concentration of valence states of vanadium with the RSDs (n = 6) in the range of 0.594%-3.535%.

Oriented Proton-Conductive Nanochannels Boosting a Highly Conductive Proton-Exchange Membrane for a Vanadium Redox Flow Battery.

In this work, we propose a sulfonated poly (ether ether ketone) (SPEEK) composite proton-conductive membrane based on a 3-(1-hydro-imidazolium-3-yl)-propane-1-sulfonate (Him-pS) additive to break through the trade-off between conductivity and selectivity of a vanadium redox flow battery (VRFB). Specifically, Him-pS enables an oriented distribution of the SPEEK matrix to construct highly conductive proton nanochannels throughout the membrane arising from the noncovalent interaction. Moreover, the "acid-base pair" effect from an imidazolium group and a sulfonic group further facilitates the proton transport through the nanochannels. Meanwhile, the structure of the acid-base pair is further confirmed based on density functional theory calculations. Material and electrochemical characterizations indicate that the nanochannels with a size of 16.5 nm are vertically distributed across the membrane, which not only accelerate proton conductivity (31.54 mS cm-1) but also enhance the vanadium-ion selectivity (39.9 × 103 S min cm-3). Benefiting from such oriented proton-conductive nanochannels in the membrane, the cell delivers an excellent Coulombic efficiency (CE, ≈ 98.8%) and energy efficiency (EE, ≈ 78.5%) at 300 mA cm-2. More significantly, the cell maintains a stable energy efficiency over 600 charge-discharge cycles with only a 5.18% decay. Accordingly, this work provides a promising fabrication strategy for a high-performance membrane of VRFB.

A neutral polysulfide/ferricyanide redox flow battery.

Energy storage systems are crucial in the deployment of renewable energies. As one of the most promising solutions, redox flow batteries (RFBs) are still hindered for practical applications by low energy density, high cost, and environmental concerns. To breakthrough the fundamental solubility limit that restricts boosting energy density of the cell, we here demonstrate a new RFB system employing polysulfide and high concentrated ferricyanide (up to 1.6 M) species as reactants. The RFB cell exhibits high cell performances with capacity retention of 96.9% after 1,500 cycles and low reactant cost of $32.47/kWh. Moreover, neutral aqueous electrolytes are environmentally benign and cost-effective. A cell stack is assembled and exhibits low capacity fade rate of 0.021% per cycle over 642 charging-discharging steps (spans 60 days). This neutral polysulfide/ferricyanide RFB technology with high safety, long-duration, low cost, and feasibility of scale-up is an innovative design for storing massive energy.

Systematic Investigation of the Physical and Electrochemical Characteristics of the Vanadium (III) Acidic Electrolyte With Different Concentrations and Related Diffusion Kinetics.

Owing to the lack of systematic kinetic theory about the redox reaction of V(III)/V(II), the poor electrochemical performance of the negative process in vanadium flow batteries limits the overall battery performance to a great extent. As the key factors that influence electrode/electrolyte interfacial reactivity, the physicochemical properties of the V(III) acidic electrolyte play an important role in the redox reaction of V(III)/V(II), hence a systematic investigation of the physical and electrochemical characteristics of V(III) acidic electrolytes with different concentrations and related diffusion kinetics was conducted in this work. It was found that the surface tension and viscosity of the electrolyte increase with increasing V(III) concentration, while the corresponding conductivity shows an opposite trend. Both the surface tension and viscosity change slightly with increasing concentration of H2SO4, but the conductivity increases significantly, indicating that a lower V(III) concentration and a higher H2SO4 concentration are conducive to the ion transfer process. The electrochemical measurements further show that a higher V(III) concentration will facilitate the redox reaction of V(III)/V(II), while the increase in H2SO4 concentration only improves the ion transmission and has little effect on the electron transfer process. Furthermore, the diffusion kinetics of V(III) have been further studied with cyclic voltammetry and chronopotentiometry. The results show that an elevated temperature facilitates the V(III)/V(II) redox reaction and gives rise to an increased electrode reaction rate constant (k s) and diffusion coefficient [D V(III)]. On this basis, the diffusion activation energy (13.7 kJ·mol-1) and the diffusion equation of V(III) are provided to integrate kinetic theory in the redox reaction of V(III)/V(II).

A Novel Self-Binding Composite Separator Based on Poly(tetrafluoroethylene) Coating for Li-Ion Batteries.

In this study, a novel composite separator based on polytetrafluoroethylene (PTFE) coating layers and a commercial polyethylene (PE) separator is developed for high performance Li-ion batteries. This composite separator is prepared by immersing a PE separator directly into a commercial PTFE suspension to obtain a self-binding PTFE/PE/PTFE tri-layered structure. Then, the as-prepared composite separator is further treated with a H2O2/H2SO4 solution to enhance its electrolyte affinity. The results show that the coating layer, consisting of close-packed PTFE particles, possesses a highly ordered nano-porous structure and an excellent electrolyte wettability property, which significantly enhance the ionic conductivity of the composite separator. Due to the presence of the PTFE-based coating layer, the composite separator exhibits better thermal stability compared with the PE separator, reaching the thermal-resistant grade of commercial ceramic-coated separators. By using different separators, CR2032-type unit half-cells composed of a Li anode and a LiFePO4 cathode were assembled, and their C-rate and cycling performances were evaluated. The cell assembled with the composite separator was proven to have better C-rate capability and cycling capacity retention than the cell with the polyethylene separator. It is expected that the composite separator can be a potential candidate as a coating-type separator for high-performance rechargeable Li-ion batteries.

SPEEK Membrane of Ultrahigh Stability Enhanced by Functionalized Carbon Nanotubes for Vanadium Redox Flow Battery.

Proton exchange membrane is the key factor of vanadium redox flow battery (VRB) as their stability largely determine the lifetime of the VRB. In this study, a SPEEK/MWCNTs-OH composite membrane with ultrahigh stability is constructed by blending sulfonated poly(ether ether ketone) (SPEEK) with multi-walled carbon nanotubes toward VRB application. The carbon nanotubes disperse homogeneously in the SPEEK matrix with the assistance of hydroxyl group. The blended membrane exhibits 94.2 and 73.0% capacity retention after 100 and 500 cycles, respectively in a VRB single cell with coulombic efficiency of over 99.4% at 60 mA cm-2 indicating outstanding capability of reducing the permeability of vanadium ions and enhancing the transport of protons. The ultrahigh stability and low cost of the composite membrane make it a competent candidate for the next generation larger-scale vanadium redox flow battery.

[Research on the X-ray fluorescence spectrometry method to determine trace elements in kimberlite].

It is very important to detect trace elements for kilmberlite. Through improving the working conditions of X-ray fluorescence spectrometer and optimizing the analytical conditions, the determination method of trace elements, such as Sc, Cr, Ni, Y, Nb, La, in kimberlite was worked out. The method has been successfully applied to the determination of trace elements in over 2 thousand samples of kimberlite from Liaoning province. The detection limits of the method were relatively low (the detection limit of Sc droped from 9.54 to 2.83 micrograms.g-1 and the detection limit of La droped from 21.68 micrograms.g-1 to 9.18 micrograms.g-1), i.e. 2.83, 2.15, 2.20, 1.17, 1.05 and 9.18 micrograms.g-1 for Sc, Cr, Ni, Y, Nb and La, respectively. The precision of the method was very high with 2.10%-7.09% of RSD (n = 20). Compared with ICP spectrometry this method is satisfactory. The method has proven to be simple and rapid with low cost and high efficiency.