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A novel 3D electrochemically reduced graphene oxide (ERGO) porous gelatum material was self-assembled on graphite plate (GP) by the simple and controllable electrodeposition method to construct a new-type integrated electrode (ERGO-GP) for all vanadium redox flow batteries (VRFBs). Benefited from the ideal preparation method, a porous and gradient microstructure of ERGO was formed easily and facilitated the electrolyte, ions and electron transport simultaneously. Besides, the direct growth of ERGO on GP could reduce the contact resistance between the reaction layer and the conduction layer effectively. As a result, the electrochemistry and battery performance for the ERGO-GP as both negative and positive electrode of VRFBs were better than the common carbon felts (CF) and GP electrode system (CF-GP) due to the lower polarization. The obtained results also indicated the great promise of this material for application in other flow redox batteries and helped us to improve the design of carbon-based porous electrodes.

In order to improve the wettability and electrochemical activity of electrospun carbon nanofibers, the partially reduced graphene oxide doped electrospun carbon nanofibers have been successfully prepared by introducing graphene oxide into the electrospinning solution with a simple electrospinning and subsequent carbonization process. It is found that some of the partially reduced graphene oxide nanosheets with plenty of wrinkles are embedded between the nanofibers, which directly enhances the physical surface area. Despite the introduced graphene oxide is partially reduced during the carbonization process, the corresponding oxygen content is greatly increased, which results in the improved wettability and further a significant enhancement in the electrochemical surface area. It is worth noting that the electrocatalytic activity is almost unchanged, indicating the improved electrochemical response is attributed to the enhanced electrochemical surface area. Moreover, vanadium flow battery with such composites presents a remarkable improvement in the electrochemical performances such as energy efficiency and discharge capacity, further suggesting that such composites are promising high-efficient electrode materials for vanadium flow battery. Therefore, it is believed that the construction of carbon-based electrode with high electrochemical surface area through the anchoring effect of hydrophilic materials is an effective approach to obtain high-efficient electrode materials for vanadium flow battery.

We investigated the mismatched kinetics of electrode reactions in VRFB, RuO2, and Bi nanoparticles as the functional components electrodeposited on graphene porous gelatum-graphite bipolar plate (ERGO-GBP) integrated electrode and serving as anode and cathode, respectively. Benefitting from the unique structure and specific functionality, the novel integrated electrodes exhibit excellent electrochemical properties and functionality. The asymmetric VRFB with an RuO2-ERGO-GBP anode and a Bi-ERGO-GBP cathode shows better performance compared to the symmetric VRFB with ERGO-GBP as both anode and cathode, suggesting a feasible strategy to construct asymmetric VRFB with various functionalized ERGO-GBP integrated electrodes.

Carbon nanofibers with multi-scale pores have been easily constructed by synchronous water etching during the carbonization process of PAN nanofibers, reducing the additional consumption of energy and time. After etching by high-temperature water vapor, the fiber surface becomes more coarse, and large amounts of etched pits are formed, effectively increasing the electrode’s specific surface area and hydrophilicity. Oxygen content is also significantly increased, which may effectively increase the electrocatalytic active sites of the electrode. Electrochemical tests verified the improved electrocatalytic activity and increased effective surface area. As a result, the VRFB single cell with water vapor etched carbon nanofibers as its electrode shows higher battery efficiencies than that with pristine carbon nanofibers; the energy efficiency improves by nearly 9.4% at 200 mA·cm−2. After 100 charge/discharge cycles, the battery efficiency has no obvious attenuation, and the capacity attenuation rate of single cycle is nearly 0.26%，suggesting a satisfactory cycling stability. This green and simple method for constructing multi-scale porous carbon nanofibers electrode is expected to achieve large-scale production of high-performance electrode materials, and can be applied in various electrochemical energy storage systems.

Vanadium redox flow batteries (VRFBs) are an ideal choice for large-scale energy storage because they have the advantages of long cycle life, flexible design and high safety. However, the poor electrocatalytic activity of carbon-based materials results in a large polarization resistance and energy loss during charge/discharge that greatly limits their commercial viability. Here we report a hybrid electrode with a gradient bi-functional oxygen-containing groups for VRFBs. It consists of a hybrid material of graphene oxide (GO), reduced graphene oxide (rGO) and graphene foam (GF) that combines materials with a high electrocatalytic activity (GO) and a high electrical conductivity (GF). One side is enriched with functional groups and provides preferential redox reversibility for VO2+/VO2+ and V3+/V2+ redox couples because of the electrocatalytic nature of the many oxygen functional groups. The side with the low concentration of functional groups has a high electrical conductivity and facilitates electron transfer. As a result, this VRFB electrode achieves a low polarization, high discharge capacity, high energy density, and high energy efficiency and has great promise for use in VRFBs.

During the operation of vanadium redox flow battery, the cell stack can suffer from electrolyte leakage and material failure that significantly affect the overall performance of the battery, provided that the stack is improperly designed and assembled. In order to manufacture more reliable battery stacks without undergoing electrolyte leakage and mechanical failure, the stress distributions on all key components of the stack need to be known. In this study, three-dimensional mechanical models are developed to perform simulation analyses on stress distribution for the cell stacks. Stress distributions on key cell components under specified sealing gasket designs, assembling forces and number of cells in a stack are investigated for the single cell and multi-cell stacks, while potential material failure and damage for the stack components are also analyzed in accordance with maximum stress criterion and von Mises yield criterion depending on the material of the components. Simulations results successfully demonstrate the stress distribution and magnitude in specified stack design and assembly condition, and highlight the importance of mechanical analyses in developing flow battery stacks with superior sealing and mechanical performance for long-term use.

In this study, the electrochemical reactions along with the corresponding temperature change in the vanadium acid solutions have been investigated by the thermo-electro-chemistry method for the first time. It turned out that the electrochemical oxidation of V(II) and reduction of V(V) are both exothermic, so substantial heat could be created by the electrochemical reaction during the discharging process, which may adversely affect the stability and efficiency of the VRFB. The temperature change characters of different electrochemical redox reactions imply that lower temperature may be advantageous for the discharge process, while higher temperature is helpful to the charge process. Besides, oxygen evolution as a main side reaction along with the oxidation of V(IV) is seen to occur at a relatively lower potential and highly depends on the vanadium ion concentration in the electrolyte. Moreover, it is found that the V(III)/V(IV) solution is more likely to suffer from oxygen evolution in oxidation processes and hydrogen evolution in reduction processes than pure V(IV) and V(III) solutions. These results demonstrate that the thermo-electro-chemistry method shows great significance and application prospect in investigating the electrode kinetics and will be helpful to optimize the work conditions and related components in a VRFB.

Carbon nanofibers (ECNFs) electrode possess good electrochemcial reactivity towards the V3+/V2+ and VO2+/VO2+ redox couples in vanadium redox flow battery (VRFB), while its inferior mass transfer property limits the cell power density seriously due to the lower porosity and poorer permeability. In order to improve the mass transfer property, starch based carbon microsphere (SCMS) of a few micrometers is synthesized by a green hydrothermal method and introduced into the electrospun precursor solution. Subsequently, a novel spherical-filament binary carbon nanofibers electrode (ECNFs-SCMS) is constructed by the controllable electrospining technique along with the subsequent heat treatment processes. The novel binary carbon nanofibers exhibits more loose structure, improved hydrophilia, increased oxygen contents and more defects, which result in the simultaneous improvement of electrochemical reaction area, electrocatalytic activity and mass transfer kinetics. The single VRFB cell with ECNFs-SCMS as electrodes shows higher energy efficiency and maximun power density as compared to the one with pure ECNFs electrodes. The results in this work suggest that improving permeability is a feasible strategy to enhance the mass transfer property of carbon nanofibers electrode.

Aiming at the shortcoming of low specific surface area of the most commonly used carbon felt (CF) electrodes in vanadium flow battery (VFB), there are mainly two approaches to enhancing its specific surface area: anchoring effect and digging effect. Based on the digging effect, the CO2-etched CF has been fabricated by a simple heat treatment under the CO2 atmosphere. As expected, lots of etch pits are formed and evenly distribute on the fiber surface, indicating a greatly enhanced surface area. Meanwhile, the content of O in the CO2-etched CF increases obviously, meaning the formation of more oxygen-containing groups, which are conducive to the improvement of hydrophilicity and electrochemical surface area. Furthermore, the oxygen-containing groups newly generated during the CO2 etching process not only improve the hydrophilicity, but also enhance the electrocatalytic activity toward to the V3+/V2+ redox couples. Consequently, VFB with CO2-etched CF as its electrodes presents significantly improved energy conversion efficiencies, discharge capacity and cycling stability. This work supplies a convenient and efficient way to improve the VFB performance by enhancing the specific surface area of CF electrodes through digging effect.

The oxidation and reduction reaction mechanisms of the VO2 +/VO2+ redox couple have been studied to better understand the electrode reaction processes and further optimize the battery performance of vanadium flow battery (VFB) in our previous work. However, it is found that the negative electrode process of VFB plays a more important role in the improvement of the overall battery performance as research continues. Hence, the reduction reaction mechanism of V3+ in acidic solutions has been investigated systematacially in this work. Potentiodynamic polarization combined with rotating disk electrode have been applied to calculate the reaction orders (m) and the electron transfer number (n) of the reduction reaction of V3+. The results of this work demonstrated that the reduction reaction of V3+ is a single electron transfer step and H+ keeps out of the electrochemical reaction. Furthermore, the kinetic equation for the reduction reaction of V3+ has been established, and the theoretic polarization curve based on the established kinetic equation is well consistent with the experimental result, indicating an accurate prediction of the polarization behavior in V3+ acidic solutions.

Although electrospun carbon nanofibers (ECNFs) possess excellent conductivity, high surface area and good electrochemical activity toward vanadium redox couples, much surface area of ECNFs was still unutilized because of its poor hydrophilicity. CeO2 nanoparticles as an excellent hydrophilic agent are embedded in polyacrylonitrile (PAN) based carbon nanofibers by a simple electrospinning and subsequent carbonization process. The physicochemical characterizations show that the introduction of CeO2 nanoparticles slightly changes the structure and compositions of ECNFs, while the corresponding wettability is greatly improved. It is worth noting that the electrochemical surface area (ECSA) of CeO2/ECNFs is more than four times of that for ECNFs. According to the cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) results, the addition of CeO2 improves the electrocatalytic activity toward the negative reaction of vanadium flow battery (VFB) to some extent while has less effect on that toward the positive reaction, the significant improvement in the electrochemical performance of CeO2/ECNFs might be mostly ascribed to the remarkable enhancement in ECSA. In addition, the charge/discharge tests further verify that CeO2/ECNFs with high ECSA could significantly reduces the electrochemical polarization during the discharging process and results in an enhanced discharge capacity and energy efficiency. The accomplishment of this work provides a new concept that it might be more convenient and effective to improve the electrochemical performance of the electrode materials for VFB by increasing their ECSA.

In order to improve the utilization rate of the electrolyte and further reduce the energy storage cost, the physicochemical properties, electrochemical characteristics and charge/discharging behaviors of VFB with different concentration of VOSO4 and H2SO4 were investigated systematically. The physicochemical characterizations show that the viscosity increases with the increasing concentration of VOSO4 and H2SO4, and the conductivity increases with the increasing concentration of H2SO4 while decreases with the increasing concentration of VOSO4. Both CV and EIS results demonstrate that the electrolyte with 1.6 mol L−1 VOSO4 and 2.8 mol L−1 H2SO4 presents the best electrochemical performance because of the coupling effect of the viscosity, conductivity and electrochemical activity. Different with the half-cell electrochemical tests, the battery performance of VFB is not only dependent on the electrochemical activity of electrode/electrolyte interface, but also closely related to the conductivity of electrolyte and diffusion rates of the active particles between anolyte and catholyte. Taking the battery efficiencies and capacity into consideration, VFB with 1.6 mol L−1 VOSO4 and 2.8 mol L−1 H2SO4 exhibits the optimal electrochemical performance. The accomplishment of this work not only gives data support to the fundamental research of VFB, but also provides theoretical direction to the engineering application of VFB.

In order to investigate the effect of the conversion of carbon structure for electrospun carbon nanofibers (ECNFs) on their electrochemical activity toward vanadium redox couple, ECNFs with different graphitization degree are prepared through the catalytic graphitization process and their physico-chemical properties have been characterized systemically. The results show that both of the iron catalyst and catalytic graphitization temperature contributes to improve the graphitization degree of the ECNFs greatly. With the increase of iron (III) acetylacetonate (AAI) mass ratio and catalytic graphitization temperature, the graphitization degree of ECNFs increases continuously, while its electrochemical activity increases at first and then decreases. Despite the XPS results show that the O and N content may not have a significant effect on the electrochemical activity of ECNFs, the heteroatoms at the edge-plane definitely are beneficial to the vanadium redox reactions. Based on these, it can be concluded that plenty of edge planes of graphite crystallites are generated during the conversion of carbon structure and more heteroatoms at the edge-plane may act as the active sites for VO 2 + /VO 2+ redox reaction. It is to say, the carbon materials with an appropriate graphitization degree exhibit the best coupling effect of the conductivity and electrocatalytic activity, which have a big influence on the electrochemical activity of carbon materials.

The edge plane in carbon structure has good electrocatalytic activity toward vanadium redox reaction. To apply it in vanadium redox flow battery (VRFB) practically, the graphite nanopowders (GNPs) containing amounts of edge planes are used as electrocatalyst and embedded in the electrospun carbon nanofibers (ECNFs) by different mass ratios to make composite electrodes. The morphology and electrochemical activity of the GNPs and the composite electrodes containing them are characterized. Compared with the pristine ECNFs, the composite electrodes show much higher electrochemical activity. With the increase of GNPs content in composite electrodes, the electrochemical reversibility of the vanadium redox couples also increases. It proves the addition of GNPs can surely improve the electrochemical activity of ECNFs. Among the composite electrodes, the ECNFs containing 30 nm GNP by mass ratio of 1:50 show the best electrochemical activity, largest active surface area and excellent stability. Due to the high performance of GNP/ECNFs composite electrode and its relatively low cost preparation process, the GNPs are expected to be used as electrocatalyst in VRFB on a large scale to improve the cell performance.

Berberine (BBR), one of the major constituents of Chinese herb Rhizoma coptidis, has been reported to exert beneficial effects to various diseases, including Alzheimer's disease (AD). In the present work, we aimed to investigate the effects of BBR on neuronal cell death induced by homocysteic acid (HCA), which was considered as a risk of AD. BBR significantly reduced HCA-induced reactive oxygen species (ROS) generation, lactate dehydrogenase release and subsequent cell death. LY294002, the PI3K inhibitor, blocked the protection as well as the up-regulation of Akt phosphorylation of BBR. Taken together, our results indicate that BBR protects HCA-induced HT-22 cell death partly via modulating Akt pathway, suggesting BBR may be a promising therapeutic agent for the treatment of HCA-related diseases, including AD.