Large surface areas while maintaining a low mass transport resistance is a critical criterion for the optimal design of electrode structures for aqueous redox flow batteries. However, for conventional micro-scale electrode structures, increasing surface areas will lead to an increase in the mass transfer resistance. In this work, a composite electrode with a gradient porosity distribution is fabricated through combining two different carbon felt layers of different porosities. The smaller-porosity layer, offering a larger surface area, is placed adjacent to the membrane, while the larger-porosity layer, providing a smaller mass transfer resistance is placed on the flow field side. The thickness ratio of the two layers is adjusted in terms of the battery performance while applied in the vanadium redox flow battery. It is demonstrated that the battery with the structure-optimized composite electrode achieves a high energy efficiency of 82.7 % at 200 mA.cm(-2) at an electrolyte flow rate of 30 mL.min(-1), and delivers a discharge capacity of 240 mAh at 400 mA.cm(-2), which is 2.18 times that of the conventional graphite felt electrode. This work offers an idea for the structural design of high-performance aqueous redox flow batteries.

Employing chloride ions as additive in electrolytes is proposed for all vanadium redox flow batteries. The influences of different concentrations of chloride ions in the electrolyte on the battery performance are investigated. It is found that the chloride ions improve the reaction activity of VO2+/VO2+ redox couples. At a current density of 200 mA cm(-2), the battery with an optimum chloride ion concentration of 0.04 M attains an energy efficiency and vanadium utilization ratio of 82.5% and 86.3%, which are 2% and 4.1% higher than those without the additives. Further, at 400 mA cm(-2), the battery with 0.04 M chloride ions shows a high rate capability by delivering the specific discharge capacity of 5.95 Ah L-1. Moreover, the battery with 0.04 M chloride ions exhibits the capacity retention rate of 83.1% after one hundred cycle test, which is nearly 12% higher than that of pristine battery. The results suggest that chloride ions, as low-cost additives in electrolytes, offer great promise for high-performance vanadium redox flow battery application.

Electrospun carbon nanofibers (CNFs) are regarded as potential electrode materials for vanadium redox flow batteries (VRFBs) due to the advantages of a large surface area and good electrochemical activity. However, woven CNFs with randomly distributed fibers are limited by poor permeability of the electrolyte, thus leading to a large mass‐transport polarization during battery operation. To address this issue, aligned carbon nanofiber (ACNF) webs are fabricated and applied in the battery with fibers parallel to the flow channel. Cyclic voltammogram results show that the as‐prepared aligned electrode exhibits better electrochemical activity for both the VO2+/VO2+ and V2+/V3+ redox reactions than that of commercial carbon paper (CP) and random electrodes, attributing to its higher surface area and favorable surface activity. The battery with the ACNF electrode achieves higher voltage efficiency (87%, 60 mA cm−2) than that of CP and random electrodes. Moreover, the ordered structure of the aligned electrode further enhances mass transport effectively, thus lowering the concentration loss. The polarization curve measurements also show that the limiting current density of the aligned electrode is 25% higher than that of the random electrode. All these results demonstrate that the aligned electrode is promising in VRFBs.

An accurate prediction of ion selective adsorption in ion exchange membranes is essential to reflect the role of fixed charges on ion transport through the membrane in vanadium redox flow batteries (VRFB). Unlike those empirical models reported in the literature, this work reports on a new ion selective adsorption model with the Donnan effect considered for movable ions distributed in the membrane pores only. This model, no longer relying on empirical coefficients, is then applied to the calculation of ion transport through membranes in VRFBs. The model shows a more accurate prediction of vanadium crossover and membrane conductivity, and enables to capture the effect of key membrane properties on battery performance. It is found that (i) an increase in H2SO4 concentration reduces the electrolyte imbalance and improves the coulombic efficiency; (ii) an increase in membrane porosity significantly improves the membrane effective conductivity; (iii) the change of fixed charges should be careful to balance all performances. Therefore, membrane properties and operating conditions need proper adjustment to improve the battery performance, and our VRFB model is a good tool to help membrane optimization.

In this work, we conceived and fabricated a three-electrode electrochemical cell and transparent vanadium redox flow battery to in-situ investigate the hydrogen evolution reaction during battery operation. Experimental results show that operating temperature has a strong influence on the HER rate. In particular, compared with V3+ reduction reaction, HER is more sensitive to temperature variation. It is also found that, contrary to the conventional wisdom that side reactions occur at the late stage of the charge process, H-2 evolves at a relatively low SOC. About 0.26 and 1.94 mL H-2 were collected at an early (SOC lower than 20%) and end of the charge process, respectively, suggesting that attention to the hydrogen formation at the negative electrode in the early charge process should also be paid to during longterm battery operations. Moreover, the produced hydrogen gas at the negative side prefers to form macroscopically observable bubbles onto the electrode surface, covering the active sites for vanadium redox reactions, while oxygen evolution (including CO2 production) at the positive side corrodes electrode surface and introduces certain oxygen-containing functional groups. (C) 2017 Elsevier Ltd. All rights reserved.

In this work, we report an environmentally friendly and low-cost approach to synthesize an all biomass-derived electrode from cotton through a pyrolysis process and apply this novel electrode to a VRFB for the first time. It is demonstrated that this carbonized cotton (CC) electrode presents a higher BET surface area, a larger number of oxygen-containing functional groups, a better wettability and higher catalytic activity for vanadium reactions than commercial carbon papers (CPs) do. As a result, the CC electrode improves the reversibility toward VO2+/VO2+ by 100 mV lower peak separation as compared with commercial CPs. Moreover, the VRFB assembled with the prepared electrodes delivers a high voltage efficiency of 75.4% at a current density of 100 mA cm (2), outperforming the oxidized CPs (68.3%), and a better rate performance during the charge-discharge test at various current densities. These results suggest that the carbonized cotton electrode offers a great promise for the large-scale application in VRFBs. (C) 2017 Elsevier Ltd. All rights reserved.

It has recently been demonstrated that the use of anion exchange membranes (AEMs) in vanadium redox flow batteries (VRFBs) can reduce the migration of vanadium ions through the membrane due to the Donnan exclusion effect among the positively charged functional groups and vanadium ions. However, AEMs are plagued by low chemical stability in harsh chemical environments. Here we propose and fabricate a pyridinium-functionalized cross-linked AEM for VRFBs. The pyridinium-functionalized bromomethylated poly (2,6-dimethyl-1,4-phenylene oxide) exhibits a superior chemical stability as a result of the strengthened internal cross-linking networks and the chemical inertness of the polymer backbone. Therefore, the membrane exhibits littler decay in a harsh environment for 20 days during the course of an ex situ immersion test. A cycling test also demonstrates that the VRFB assembled with the membrane enable to retain 80% of the initial discharge capacity over 537 cycles with a capacity decay rate of 0.037% cycle−1. Meanwhile, the membrane also shows a low vanadium permeability and a reasonably high conductivity in supporting electrolytes. Hence, all the measurements and performance tests reported in this work suggest that the membrane is a promising AEM for redox flow batteries to achieve excellent cycling stability and superior cell performance.

MnO2 has been demonstrated to be an effective catalyst for Zn-air batteries, but suffers from cripplingly low cell performance due to its limited electrical conductivity. In this work, we report a facile process for preparing the MnO2/C air cathode by directly anchoring the MnO2 onto Ketjen Black (KB) via an in-situ redox reaction. It is demonstrated that a Zn-air battery installed with the proposed MnO2/KB air cathode outperforms that installed with a commercial Pt/C cathode. Specifically, the MnO2/KB cathode presents a more positive ORR onset potential and a larger current density compared with that of the Pt/C cathode. Under ambient air, the prepared MnO2/KB air cathode allows the battery to reach a peak power density of 133.17 mW cm(-2) when operated at a current density of 188.51 mA cm(-2), which is among the highest values in the literature. More impressively, the battery installed with the proposed cathode can be operated at a high current density of up to 100 mA cm(-2) with a voltage discharge plateau larger than 1.0V. These results indicate that the MnO2/KB electrode offers a promising option for both alkaline fuel cells and metal-air batteries. (C) 2016 Elsevier Ltd. All rights reserved.

Vanadium redox flow batteries (VRFBs) with their high flexibility in configuration and operation, as well as long cycle life are competent for the requirement of future energy storage systems. Nevertheless, due to the application of perfluorinated membranes, VRFBs are plagued by not only the severe migration issue of vanadium ions, but also their high cost. Herein, we fabricate semi-interpenetrating polymer networks (SIPNs), consisting of cross-linked polyvinylpyrrolidone (PVP) and polysulfone (PSF), as alternative membranes for VRFBs. It is demonstrated that the PVP-based SIPNs exhibit extremely low vanadium permeabilities, which contribute to the well-established hydrophilic/hydrophobic microstructures and the Donnan exclusion effect. As a result, the coulombic efficiencies of VRFBs with PVP-based SIPNs reach almost 100% at 40 mA cm(-2) to 100 mA cm(-2); the energy efficiencies are more than 3% higher than those of VRFBs with Nafion 212. More importantly, the PVP-based SIPNs exhibit a superior chemical stability, as demonstrated both by an ex situ immersion test and continuously cycling test. Hence, all the characterizations and performance tests reported here suggest that PVP-based SIPNs are a promising alternative membrane for redox flow batteries to achieve superior cell performance and excellent cycling stability at the fraction of the cost of perfluorinated membranes. (C) 2016 Elsevier B.V. All rights reserved.