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In this work, we propose and develop a novel electrode made of aligned microscale carbon fibers interweaved with highly porous carbon nanofibers for vanadium redox flow batteries. Such a unique combination not only endows the electrode with a high permeability due to the reduced tortuosity and large macropores, but also an ultra-large specific surface area for redox reactions. As a result, a vanadium redox flow battery equipped with the newly-developed electrode is able to achieve an energy efficiency of 79.3% at the current density of 400 mA cm−2, which is 9.9% higher than that with pure porous carbon nanofiber electrodes, and 14.1% higher than that with pure aligned fiber electrodes. More remarkably, the battery is capable of delivering a peak power density of as high as ∼1.9 W cm−2 and a limiting current density of ∼5000 mA cm−2. These results demonstrate that our strategy of interweaving porous carbon nanofibers between aligned microfibers effectively addresses the contradiction between permeability and surface area of porous electrodes, opening a new platform to fabricate high-performance electrodes for vanadium redox flow batteries.

Fabricating electrodes with large specific surface area (SSA) and high permeability has been the long-standing target in redox flow batteries (RFBs). In this work, we propose a novel ZIF-8-assisted etching approach to form holey fibers in the electrospinning process of aligned electrode structures. The etching approach allows the formation of holey fibers with small pores of ~50 nm, offering large active surface areas for redox reactions, while the aligned macrostructure with the holey fibers of 3–5 μm in diameter ensures a high permeability along the fiber direction. The application of the prepared electrodes to a vanadium redox flow battery (VRFB) enables an energy efficiency (EE) of 87.2% at the current density of 200 mA cm−2, which is 13.3% higher than that with conventional electrospun carbon electrodes. Even at high current densities of 300 and 400 mA cm−2, the battery still maintains energy efficiencies of 83.3% and 79.3%. More excitingly, the prepared electrode yields a high limiting current density of 4500 mA cm−2 and a peak power density of 1.6 W cm−2. It is anticipated that the present electrospinning method combining the ZIF-8-assisted etching approach with a way to form ordered fiber structures will allow even more high-performance electrodes for RFBs in the future.

Currently, lithium-sulfur (Li-S) batteries still suffer from fast capacity decay, poor coulombic efficiency (CE) and short cycling lifespan, which result from the severe shuttle effect issue caused by high solubility and rapid diffusion of lithium polysulfides (LiPSs) in organic electrolytes. Here, yolk-shell zinc oxide (YS-ZnO) spheres are synthesized and for the first time, applied as a host for Li-S batteries to tackle this challenge. The polar ZnO exhibits high chemical anchoring ability toward LiPSs while the unique yolk-shell structure not only provides an additional physical barrier to LiPSs but also enables much more uniform sulfur distribution, thus significantly suppressing LiPSs shuttling effect meanwhile promoting sulfur conversion reactions. As a result, the YS-ZnO enables the Li-S battery to display an initial specific capacity of 1355 mAh g−1 and an outstanding capacity retention capability (~89.44% retention rate) even after 500 cycles with the average CE of ~99.46% at the current of 0.5 C. By contrast, the capacity of conventional-ZnO-nanoparticles based battery severely decays to 472 mAh g−1 after cycling for 500 times. More impressively, the S/YS-ZnO based Li-S battery can maintain a low decay rate of 0.040% every cycle and high average CE of 98.82% over 1000 cycles at 3 C.

Developing electrodes with high catalytic activity and cycling stability is essential to make vanadium redox flow batteries (VRFBs) viable on a commercial level. In this work, hollow Ti3C2Tx spheres, in the form of 3D nanostructured MXene, are investigated as electrocatalysts for V3+/V2+ redox reactions. The first-principles study demonstrates that Ti3C2 exhibits a metallic behavior with a high electrical conductivity. The catalytic effect of the material is then verified experimentally by cyclic voltammetry and electrochemical impedance spectra test. The hollow Ti3C2Tx spheres decorated graphite felt electrodes are finally prepared and tested in the VRFB. It is shown that the prepared electrode enables the electrolyte utilization efficiency and energy efficiency to be as high as 80.1% and 81.3% at 200 mA cm−2, which are 41.7% and 15.7% higher than that of the battery with the pristine electrode. At the current density of 300 mA cm−2, the electrolyte utilization efficiency and energy efficiency can be still maintained at 62.9% and 75.0%, which are 44.5% and 12.8% higher than the battery with the XC-72 carbon nanoparticle decorated electrode. More impressively, the battery exhibits excellent stability with a high capacity retention rate and no energy efficiency decay over 500 charge-discharge cycles at a current density of 200 mA cm−2. The superior performance is ascribed to significant improvement in the electrochemical kinetics and enlarged active sites towards V3+/V2+ redox reactions by the decoration of hollow MXene spheres.

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.

The operating temperature of vanadium redox flow batteries (VRFBs) affects their performance and reliability. However, previous studies focused on evaluating the effects on the performance of lab-scale single cells, in which electrolyte flow rates and current densities are different from those in stack-scale VRFBs, leading to the lack of guidance for the design of stacks. In this work, we investigate thermal effects on the performance of stack-scale VRFBs. It is found that as the operating temperature increases from 25 to 50 °C, the discharge capacity increases by 42%, whereas the energy efficiency increases by 10%, implying that the temperature has greater effects on the discharge capacity than that on the energy efficiency. Additionally, the enhancement effect of temperature on the energy efficiency is gradually weakened with increasing flow rate, while that on the discharge capacity is almost unchanged. Furthermore, the enhancement effect of temperature on energy efficiency increases with the operating current density. Notably, an optimum operating condition of the stack-scale VRFBs is identified with a critical flow rate (2.88 ml min−1 cm−2) at 40 °C to achieve a high system efficiency. This work provides guidance for the design of stack-scale VRFBs with high performance and safety.

Polybenzimidazole (PBI)-based membranes are one of the most promising proton exchange membranes for vanadium redox flow batteries (VRFBs) due to their excellent ion selectivity. However, the relatively lower proton conductivity limits their application. Herein, a PBI membrane with both high proton conductivity and ion selectivity is prepared through a secondary phosphoric acid-doping method. The secondary-doped PBI membrane has a lower doping level in the surface layer while a higher doping level at the inner layer, forming a significant gradient-doped structure. In this structure, the former ensures an excellent ion selectivity while the latter enables a preferable proton conductivity. As a result, the VRFB with the secondary-doped PBI membrane exhibits an ultrahigh coulombic efficiency (CE) of 99.2% at the operating current density of 200 mA cm−2, which is significantly higher than that of the Nafion 212 membrane (97.7%), signifying an excellent ion selectivity. Meanwhile, the corresponding voltage efficiency (VE) is high up to 87.1%, which is also better than that of the Nafion 212 membrane (84.8%), indicating a high proton conductivity. Therefore, the secondary-doped PBI membrane might be a promising candidate for the highly efficient membrane for VRFB, and the secondary-doping method is simple and facile to realize engineering applications.

Catalytic denitrification, a promising technology for nitrate removal, is increasingly limited by the rising price of Pd. Replacing Pd with less-expensive Ru would significantly reduce the cost; however, Ru-based catalysts have been reported to perform inconsistently in denitrification applications, making their replacement prospects unclear. Herein, the surface oxidation of Ru catalysts was confirmed to be a key factor that inhibits activity. A series of Ru-Pd catalysts containing small amounts of Pd (0.5 wt%) was developed to eliminate the Ru surface-oxide layer through the spillover of hydrogen atoms activated on the Pd promoter. Ru-Pd/Fe3O4 exhibited superior catalytic activity to Ru-Pd/C and Ru-Pd/Al2O3 because the reducible carrier (Fe3O4) has a lower resistance to hydrogen spillover and diffusion, as determined experimentally and supported by density functional theory calculations. This study developed a method that eliminates ruthenium surface oxides in situ and restores its denitrification activity, further reducing the barrier to Ru replacing Pd in catalytic aqueous denitrification.

Fabricating fiber-based electrodes with a large specific surface area while maintaining high flow permeability is a challenging issue in developing high-performance redox flow batteries. Here, a sponge-like microfiber carbon electrode is reported with a specific surface area of as large as 853.6 m

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 propose a novel concept of energy storage that incorporates electrically rechargeable liquid fuels made of electroactive species, known as e-fuels, as the storage medium. This e-fuel energy storage system comprises an e-fuel charger and an e-fuel cell. The e-fuel charger electrically charges e-fuels, while the e-fuel cell subsequently generates electricity using charged e-fuels whenever and wherever on demand. The e-fuel energy storage system possesses all the advantages of conventional hydrogen storage systems, but unlike hydrogen, liquid e-fuels are as easy and safe to store and transport as gasoline. The potential e-fuel candidates have been identified to include inorganic electroactive materials, organic electroactive materials, and suspension of solid electroactive materials. In this work, we demonstrate an example e-fuel energy storage system for large-scale energy storage using inorganic e-fuels composed of V2+/V3+ and VO2+/VO2+ redox couples, and compare the performance of the e-fuel energy storage system with that of existing technologies. Results show that our e-fuel charger achieves a charge efficiency of as high as ∼94%, while the e-fuel cell is capable of delivering a peak power density of 3.4 W cm−2, which is 1.7 times higher than that of hydrogen fuel cells. More excitingly, the e-fuel energy storage system exhibits a round-trip efficiency of 80.0% and an electrolyte utilization of 83.0% at an ultra-high discharge current density of 1,000 mA cm−2, which are 19.9% and 67.3% higher than those of conventional vanadium redox flow batteries. This unprecedented performance allows a 27.0% reduction in the capital cost of the e-fuel energy storage system compared with that of vanadium redox flow batteries.

With the consideration of understanding the interplays between the electrolyte and electrode for the VO2+/VO2+ redox reactions at various electrodes and developing high-activity electrode materials for an all-vanadium redox flow battery, the reduction reaction kinetics of vanadium(V) ions on a platinum (Pt) electrode in comparison with that on carbon electrodes is investigated by steady-state potentiodynamic polarization and impedance spectroscopy measurements in sulfuric acid solutions with various pH and vanadium concentrations. No abnormal increase is observed neither in the cathodic Tafel slope nor the charge transfer resistance (Rct) at a transition potential (EK) on the Pt electrode. However, an abrupt change in the Tafel slope and Rct of vanadium(V) ions is observed on carbon electrodes. The above results indicate that the reduction reaction kinetics of vanadium(V) ions in acidic solutions on the Pt electrode show some unusual difference with that on carbon electrodes, and this is the first study to report such a new phenomenon. Additionally, possible mechanism pathways for the reduction reaction of vanadium(V) ions in acidic solutions are proposed for the above-mentioned carbon and Pt electrodes in this work.

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 work, we propose and fabricate a convection-enhanced flow field for aqueous redox flow batteries. Similar to the conventional single serpentine flow field, the new flow field here has an inlet and outlet, but the repatterned flow path induces higher pressure differences between adjacent flow channels over the entire electrode surface, thereby enhancing the under-rib convection between adjacent flow channels. The application of the new flow field to a vanadium redox flow battery enables substantial performance improvements. At 1.67 mL min−1 cm−2, which is a typical flow rate for a kilowatt-class VRFB stack, the energy efficiency, and electrolyte utilization reach 75.3% and 49.6% at a current density of 250 mA cm−2, increasing by 10.0% and 33.3% as opposed to the use of the conventional serpentine flow field, respectively. Three-dimensional numerical modelling shows that the improved performance is ascribed to the more uniform in-plane distribution of reactants and current with the new flow field. Another important finding of this work is that there is a critical flow rate above which energy efficiency tends to stabilize for a given current density.

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.

The redox flow battery is one of the most promising grid-scale energy storage technologies that has the potential to enable the widespread adoption of renewable energies such as wind and solar. To do so, the performance of redox flow batteries must be enhanced while the cost needs to be reduced. Electrodes are a key component where coupled electrochemical reactions and mass transport take place, and they play a critical role in determining the battery performance and system cost. This review summarizes recent developments in the design and fabrication of electrospun carbon fibers, which offers a bottom-up solution to the formation of electrodes with desired properties for high-performance flow batteries. The principles of electrospinning nanofibers and the key parameters that affect the morphologies of electrospun carbon fibers are discussed and summarized. The factors that influence electrode properties, including geometric structures, surface properties, electrical conductivity, mechanical strength, and wettability on the battery performances, are comprehensively elaborated with an emphasis on the electrode structure and surface properties that determine the mass transport and reaction kinetics. Finally, the scientific challenges and prospects of electrospun carbon fiber electrodes with maximized specific surface areas and hydraulic permeability are presented. This review offers insights into the design and development of advanced electrodes for next-generation flow batteries in the application of renewable energy storage.

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 use of polybenzimidazole (PBI)-based membranes in vanadium redox flow battery (VRFB) exhibits a high coulombic efficiency due to their excellent ion selectivity, but a low energy efficiency resulting from the low proton conductivity. In this work, we propose and develop a composite membrane, comprising a dense but thin PBI layer, and a porous but thick layer made of PBI electrospun nanofibers. The dense layer, 7.0 μm thick, not only can retain the high ion selectivity, but also facilities the proton conduction as a result of the reduced thickness. The porous layer, 23 μm in thickness, 2 μm in pore size, is tightly stuck to the dense layer to ensure the mechanical stability. A battery with the composite membrane demonstrates an energy efficiency of 82.0% at 150 mA cm−2, nearly 20% higher than that with a conventional PBI membrane. More impressively, this membrane enables the battery to be stably operated for 200 cycles with a decent capacity retention of 99.81% per cycle, far surpassing the commercialized Nafion 211 membrane with a similar thickness. Both the remarkable battery performance and the preferable stability suggest that the composite PBI membrane is a promising alternative for VRFB applications.

Iron–chromium flow battery (ICFB) is one of the most promising technologies for energy storage systems, while the parasitic hydrogen evolution reaction (HER) during the negative process remains a critical issue for the long-term operation. To solve this issue, In3+ is firstly used as the additive to improve the stability and performance of ICFB. The results suggest that In3+ not only effectively inhibits the HER but also accelerates the kinetics of Cr3+/Cr2+ and Fe3+/Fe2+ to some extent. More importantly, the ICFB using an optimal concentration of 0.01 M In3+ in the anolyte exhibits an energy efficiency of 77.0% at 200 mA cm−2, and a 36.3% higher capacity retention ratio than that of the pristine anolyte after 140 cycles at 160 mA cm−2. Furthermore, the capacity decay rate is low to 0.16% per cycle during the cycle test, which is much lower than that in the previous studies. Both the enhanced stability and electrochemical performance are attributed to the coupling effect of the inhibited HER and promoted kinetics of Cr3+/Cr2+ after In3+ is introduced. Therefore, In3+ as an anolyte additive can be a promising and cost-effective method to further improve the battery performance of ICFB and promote its engineering application.

In this work, the cycle life of vanadium redox flow batteries (VRFBs) is extended by resolving the inevitable loss of capacity and energy efficiency after long-term cycle operation. The electrolyte concentration, volume, and valence are rebalanced by mixing the electrolyte as well as adding a quantitative amount of a reducing agent. Without disassembling the battery, the energy efficiency decay induced by electrode degradations is almost completely restored by interchanging the positive and negative electrodes. By adopting this method, the capacity and energy efficiency after 500 cycles can be restored to 473 mAh and 90.8%, almost reaching the previous highest level at the beginning of the cycle test (478 mAh and 91.0%). In addition, the rate performance tests also demonstrate that even at a current density of as high as 300 mA cm−2, the energy efficiency of the recovered battery is only 0.8% lower than the new battery. All the results show that the recovery method reported here allows the cycle life of VRFBs to be greatly prolonged with no need to replace electrolytes and electrodes.

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.

The morphology, surface composition, wettability and the kinetic parameters of the electrochemically oxidized graphite electrodes obtained under different anodic polarization conditions have been examined by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle measurements, steady-state polarization and cyclic voltammetry (CV) tests, with an attempt to investigate the inherent correlation between the physicochemical properties and the kinetic characteristics for carbon electrodes used in an all-vanadium redox flow battery (VRFB). When the anodic polarization potential raises up to 1.8 V vs. SCE, the anodic corrosion of the graphite might happen and a large number of oxygen-containing functional groups generate. The VO2+/VO2+ redox reaction can be facilitated and the reaction reversibility tends to become better with the increasing anodic potential, possibly owing to the increased surface oxides and the resulting improved wettability of the electrode. Based on this, a real reaction kinetic equation for the oxidation of VO2+ has been obtained on the electrode polarized at 1.8 V vs. SCE and it can be also well used to predict the polarization behavior of the oxidized electrode in vanadium (IV) acidic solutions.

Compared with the basal-planes of highly oriented pyrolytic graphite (HOPG), the edge-planes of HOPG exhibit a significantly enhanced electrochemical performance, indicating of the better electrocatalytic activity of the edge-planes at graphite electrode toward vanadium redox reactions, especially for the positive reaction. In order to further investigate the electrocatalytic activity of the edge plane sites, glass carbon (GC) modified by graphite powders (GP) with different particle sizes, which possess different content of the edge-planes, are used as electrodes and their physicochemical properties such as the morphology, microstructure, surface composition, and specific surface area, in addition to the corresponding electrochemical characteristics have been characterized systematically. The total electrochemical activity toward vanadium redox reaction increases greatly with the decrease of the particle sizes for GP, which mainly attributes to the increasing surface area and exposed edge plane sites. It is worth note that a large surface area often means abundant edge plane sites, and it is mainly in favor of the enhancement of the current response, while the proportion of the active area such as the edge-planes in total surface area to a large extent determines the electrocatalytic activity.

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.

This work reports on a novel electrode prepared with electrospun nano- and micro-scale carbon fibers for aqueous redox flow batteries. Larger fibers, ~10 μm in diameter, form larger pores to provide pathways for electrolyte flow, while smaller fibers, ~1 μm in diameter, increase active surface area for redox reactions. Brunauer-Emmett-Teller and pressure drop tests show that the specific surface area of the prepared dual-diameter electrode is doubled as compared with the large-fiber electrode, while the estimated permeability is enhanced by 1.4 times as opposed to the small-fiber electrode. The application of the dual-diameter electrodes to a vanadium redox flow battery allows the battery to achieve an energy efficiency of 84.78% at the current density of 100 mA cm−2, which is 13.57% higher than that with small-fiber electrodes, and 3.91% higher than that with large-fiber electrodes. Even at a high current density of 200 mA cm−2, the battery with the prepared electrode can still maintain the energy efficiency of 74.18%, which is 5.5% higher than that with large-fiber electrodes, and the battery with small-fiber electrodes cannot be operated at such high current density. This dual-diameter fibrous structure provides inspirations for the future electrode design in aqueous redox flow batteries.

In order to improve the electrochemical performance of iron-chromium flow battery, a series of electrolytes with x M FeCl2 + x M CrCl3 + 3.0 M HCl (x = 0.5, 0.75, 1.0, 1.25) and 1.0 M FeCl2 + 1.0 M CrCl3 + y M HCl (y = 1.0, 2.0, 3.0, 4.0) are prepared, and the effect of electrolyte concentration on the electrochemical performance of iron-chromium flow battery are firstly investigated. The viscosity of electrolyte increases along with the increasing concentration of FeCl2, CrCl3 and HCl, however, the corresponding conductivity decreases with the increasing concentration of FeCl2 and CrCl3 but increases with the increasing HCl concentration. It is worth noting that the electrolyte with 1.0 M FeCl2, 1.0 M CrCl3 and 3.0 M HCl presents the best electrochemical performance due to the synergistic effect of viscosity, conductivity and electrochemical activity. Most importantly, iron-chromium flow battery with the optimized electrolyte presents excellent battery efficiency (coulombic efficiency: 97.4%; energy efficiency: 81.5%) when the operating current density is high up to 120 mA cm−2. This work can improve the battery performance of iron-chromium flow battery more efficiently, and further provide theoretical guidance and data support to its engineering application.

Enhancing the transport properties and enlarging the surface area of electrodes are critical for achieving a high-power-density aqueous redox flow battery. In this work, we design and fabricate a hierarchical and ordered carbon fibrous (CNF-AECF) electrode by in-situ growing a layer of carbon nanofibers on the surface of aligned electrospun carbon fibers. The aligned macroscopic structure provides electrolytes transport pathways, while the highly porous carbon nanofiber layer offers a large specific surface area of up to 108 m2 g−1, enabling abundant active sites for redox reactions. Cyclic voltammetry tests show that the positive side peak potential separation is reduced from 92.77 mV to as low as 55.01 mV, while the negative side peak potential separation is reduced from 92.77 mV to 88.65 mV at the scan rate of 10 mV s−1. The application of the as-prepared material to a vanadium redox flow battery as the positive electrode demonstrates an energy efficiency of 80.1% at the current density of 300 mA cm−2, and 75.0% at the current density of 400 mA cm−2, which is 5.0% and 6.6% higher than that with the pristine electrospun carbon fiber electrodes. All these results show that the custom-made carbon electrode with rationally designed geometric structures and surface properties offer the promise to achieve high power density for aqueous redox flow batteries.

To promote the universal application of renewable energy has become an important strategy for the energy security and sustainable economic development around the world. The proportion of renewable energy in the overall energy structure is greatly increasing in recent years, however, it is intermittent and unstable for the renewable energy generation, and the direct grid connection of the renewable energy generation will affect the stable operation of the power grid system. Supporting high-efficiency energy storage batteries to ensure the continuity and stability of power generation and power supply is an important way to achieve renewable energy development strategies. Thereinto, flow batteries (FBs) have a broad application prospect and huge market potential due to their safety, easy scale, flexible structure design and environmental friendliness. As the platform of the electrochemical reaction for FBs, the electrode is one of the key materials to determine its performance and cycle life. Currently the widely used electrode material of vanadium flow battery (VFB) is the polyacrylonitrile-based carbon felt (CF), and its low price and high stability make it the first choice of the electrode materials of FBs. However, the relatively lower electrochemical activity and specific surface area, in addition to higher electron resistance and transmission resistance of CF greatly limit the improvement of the battery performance and the reduction of the energy storage cost. For example, the power density of VFB is only 0.6 W cm-2, and its operating cost is about 650 $ kWh-1. In general, the basic requirements for the electrode materials of VFB as follows: i) high fluid permeability; ii) high specific surface area; iii) high electron conductivity; iv) low cost; v) high chemical and electrochemical stability. In order to achieve high electrolyte permeability, the conventional CF often possesses an average fiber diameter of up to 10 μm, resulting in a low electrochemical surface area. Therefore, only thicker electrodes can be used in VFB to ensure the electrochemical reaction interface, but this will result in a substantial increase in the ohmic resistance. In addition, the electrochemical activity of the electrode often depends on the oxygen-containing group on the surface of CF. However, the introduction of the oxygen-containing group also destroys the conductive network of the original graphite fiber simultaneously, thereby causing the loss of electronic conductivity of the electrode. The above several factors are coupled to each other, resulting in a very low power density of VFB. Fig. 1 Schematic diagram of the gradient carbon fiber electrode for FBs. Generally, the electron conductivity of the electrode is much larger than the ion conductivity of the electrolyte, and the ion migration path on the membrane side is short for FBs, so the electrochemical reaction tends to occur at the membrane side, which is often called the reaction zone; meanwhile the ion migration path at the plate side is long, which is not conducive to the reaction, so it is often caller the current collecting area. Thereinto, the former should has higher electrochemical activity, while the latter should possess higher conductivity. Therefore, the carbon fiber electrode for FBs should be constructed according to its specific structure characteristics. Based on this, a gradient carbon fiber electrode is proposed to resolve the contradiction among the conductivity, activity and transmission. Firstly, the fiber density exhibits a gradient distribution in the thickness direction of the electrode, the plate side with higher fiber density is used to reduce the contact resistance, while the membrane side with lower fiber density is used to reduce the mass transfer polarization. In addition, electrolyte will tend to flow through the membrane side due to its lower transmission resistance, which could ensure the supply of the active reactant. Secondly, the nano fibers are also gradiently distributed in the thickness direction of the electrode. The plate side of the electrode is mainly composed of graphite fibers with good conductivity, while the membrane side mainly consists of the composites of graphite fibers/nano fibers with good activity. And such gradient structure could result in an excellent electrochemical activity, a favorable conductivity and a preferable transmission performance simultaneously, successfully resolving the efficient coordination problem of charger transfer, electron conductivity and ion transport in FBs. Acknowledgement: This study was funded by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) and HKUST fund of Nanhai (Grant No.FSNH-18FYTRI01). Figure 1

Enhancing the mass transport from the flow field side to the membrane side without sacrificing the active surface area is a critical strategy for the design of electrodes in flow-field structured vanadium redox flow batteries (VRFBs). In this work, we design and prepare a novel porous electrode with a gradient distribution in pores, enabling a gradual decrease in permeability but an increase in active surface areas from the flow field side to the membrane side. This design not only increases the electrode utilization due to the enhanced mass transport near the flow field side, but also avoids the loss of active surface area near the membrane side. Numerical modeling results show that compared to the conventional electrode design, the gradient electrode design can promote the uniform distribution of local reaction current density and overpotential, leading to a lower charge voltage and higher discharge voltage. Experimental results show that at the current density of 240 mA cm−2, the battery with the gradient electrode design delivers a 69% higher discharge capacity than that with the conventional electrode design, demonstrating the superiority of the gradient electrode design strategy.

The pseudocapacitive properties of an electrochemically modified graphite electrode (MGE) in electrolytes with different pH are investigated, and the transformation of the corresponding reaction mechanism and contribution of pseudocapacitance to the overall capacitance are analyzed in detail by cyclic voltammetry, electrochemical impedance spectroscopy and self-discharge tests. At low pH, the pseudocapacitance is attributed to the redox reaction among hydroxyl, carbonyl and carboxyl groups, which takes up nearly 61% of the overall capacitance. As the pH increases from 0 to 2, the carboxyl groups begin to ionize, the proportion of the redox reaction between carbonyl and carboxyl groups is decreased, and the contribution of pseudocapacitance to the overall capacitance decreases too. With the pH further increasing, the carboxyl groups are almost completely ionized and the redox reaction occurs only between hydroxyl and carbonyl groups, the contribution of pseudocapacitance drops to 33% and the double layer capacitance begins to play a dominate role. Despite of a lower specific capacitance for MGE in neutral solution, a broader potential window is obtained and the energy density would be increased nearly 1.33 times as large as that in acidic solution. The results indicate that the electrochemical system of MGE with the neutral electrolyte may be a promising candidate for a high energy density supercapacitor.

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.

To improve the electrochemical activity of polyacrylonitrile (PAN)-based electrospun carbon nanofibers (ECNFs) toward vanadium redox couples, the multi-wall carbon nanotubes (CNTs) and Bi-based compound as electrocatalyst have been embedded in the ECNFs to make composite electrode, respectively. The morphology and electrochemical properties of pristine ECNFs, CNTs/ECNFs and Bi/ECNFs have been characterized. Among the three kinds of electrodes, the CNTs/ECNFs show best electrochemical activity toward VO 2+ / VO 2 + redox couple, while the Bi/ECNFs present the best electrochemical activity toward V 2+ /V 3+ redox couple. Furthermore, the high overpotential of hydrogen evolution on Bi/ECNFs makes the side-reaction suppressed. Because of the large property difference between the two composite electrodes, the CNTs/ECNFs and Bi/ECNFs are designed to act as positive and negative electrode for vanadium redox flow battery (VRFB), respectively. It not only does improve the kinetics of two electrode reactions at the same time, but also reduce the kinetics difference between them. Due to the application of asymmetric electrodes, performance of the cell is improved greatly.