Performance Of Vanadium Redox Flow Battery Research Articles

Electrochemical Pretreatment of Carbon Fibre Electrodes and Its Effect on the Kinetics of Vanadium Redox Reactions

The practical importance of vanadium redox flow batteries (VRFBs) has made the kinetics of the redox reactions at carbon electrodes of great interest. The kinetics of the redox reactions at the positive and negative electrodes have been investigated in flow batteries and three-electrode cells for a range of carbon materials. The reports reveal that the kinetics of these reactions are not fully understood1,2 and discrepancies exist around which half-cell has slower kinetics1–7.Carbon surfaces are, however, very sensitive to changes in their environment which can then alter kinetics at their surfaces.8,9 A variety of investigations have shown either enhancement or inhibition of VII/VIII or VIV/VV kinetic rates by various pretreatments either electrochemical, chemical or thermal. There are, however, apparent disagreements amongst these reports. Pretreatment of electrodes by polarization at either positive or negative potentials has a very significant effect on the kinetic rates of both the VII/VIII or VIV/VV redox reactions. We have found3–5 that pretreatment of various carbon electrodes at positive potentials leads to inhibition of the rates of the VIV/VV reactions but enhancement of the rates of the VII/VIII reactions. Conversely, pretreatment at negative potentials leads to enhancement of the VIV/VV reactions but inhibition of the VII/VIII reactions. We will present results on the effect of electrochemical pretreatment of single carbon fibers extracted from carbon felt. The kinetic rates for both the VII/VIII and VIV/VV couples on these electrodes were measured and compared using linear sweep voltammetry, electrochemical impedance spectroscopy, and current measurements at constant potential. The surface of the carbon fibers after electrochemical treatments was characterized using x-ray photoelectron spectroscopy. These results are directly applicable to the performance of VRFBs which are typically operated using carbon felt electrodes. Acknowledgements The authors wish to acknowledge support from the Irish Research Council (IRC), the National Science Foundation, Sustainable Energy Pathways Program (NSF-1230236), and the IRC-Marie Curie International Mobility Fellowship in Science, Engineering and Technology (grant no. INSPIRE PCOFUND-GA-2008-229520).

Amino-silica modified Nafion membrane for vanadium redox flow battery

A hybrid membrane of Nafion/amino-silica (amino-SiO2) for vanadium redox flow battery (VRB) systems is prepared via the sol–gel method to improve the selectivity of the Nafion membrane, to reduce the crossover of vanadium ions, and to decrease water transfer across the membranes. The sulfonated pores of the pristine Nafion membrane are filled with amino-SiO2 nanoparticles localized by electrostatic interaction. The permeability of vanadium ions through the Nafion/amino-SiO2 hybrid membrane is determined by electrometric titration. The results indicate the crossover of vanadium ions through the hybrid membrane is 26.8% of the pristine Nafion membrane. The presence of amino-SiO2 in the hybrid membrane is verified by X-ray photoelectron spectroscopy (XPS). Nafion/amino-SiO2 hybrid membrane exhibits through plane conductivity about the same as the pristine Nafion membrane. The ion exchange capacity (IEC) of the hybrid membrane is 9.4% higher than that of the pristine Nafion membrane. In addition, Nafion/amino-SiO2 hybrid membrane exhibits a higher coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) over a range of current densities from 20 to 80 mA cm−2. The performance of VRB with Nafion/amino-SiO2 hybrid membrane varies little around a charge–discharge current density of 80 mA cm−2 for 150 cycles. Thus, the Nafion/amino-SiO2 hybrid membrane can suppress the vanadium ions crossover in VRB.

Recent Development of Nanocomposite Membranes for Vanadium Redox Flow Batteries

The vanadium redox flow battery (VRB) has received considerable attention due to its long cycle life, flexible design, fast response time, deep‐discharge capability, and low pollution emissions in large‐scale energy storage. The key component of VRB is an ion exchange membrane that prevents cross mixing of the positive and negative electrolytes by separating two electrolyte solutions, while allowing the conduction of ions. This review summarizes efforts in developing nanocomposite membranes with reduced vanadium ion permeability and improved proton conductivity in order to achieve high performance and long life of VRB systems. Moreover, functionalized nanocomposite membranes will be reviewed for the development of next‐generation materials to further improve the performance of VRB, focusing on their properties and performance of VRB.

Effect of mesocelluar carbon foam electrode material on performance of vanadium redox flow battery

Languid reaction rate of VO2+/VO2+ redox couple is a problem to solve for improving performance of vanadium redox flow battery (VRFB). To facilitate the slow reaction materials including large pore sized mesocellular carbon foam (MSU-F-C and Pt/MSU-F-C) are used as new catalyst. Their catalytic activity and reaction reversibility are estimated and compared with other catalysts, while cycle tests of charge–discharge and polarization curve tests are implemented to evaluate energy efficiency (EE) and maximum power density (MPD). Their crystal structure, specific surface area and catalyst morphology are measured by XRD, BET and TEM. The new catalysts indicate high peak current ratio, small peak potential difference and high electron transfer rate constant, proving that their catalytic activity and reaction reversibility are superior. Regarding the charge–discharge and polarization curve tests, the VRFB single cells including new catalysts show high EE as well as low overpotential and internal resistance and high MPD. Such excellent results are due to mostly unique characteristics of MSU-F-C having large interconnected mesopores, high surface area and large contents of hydroxyl groups that serve as active sites for VO2+/VO2+ redox reaction and platinums (Pts) supporting the MSU-F-C. Indeed, employment of the catalysts including MSU-F-C leads to enhancement in performance of VRFB by facilitating the slow VO2+/VO2+ redox reaction.

Influence of antimony ions in negative electrolyte on the electrochemical performance of vanadium redox flow batteries

Sb3+ ions are introduced into the negative electrolyte of vanadium redox flow batteries (VRFB), and their influence on the electrochemical performance of VRFB are investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The results show that the electrochemical activity and sluggish kinetics of V(II)/V(III) redox couple can be improved by the addition of Sb3+ ions, and the optimal concentration of Sb3+ ions is found to be 5mM. Meanwhile, Sb3+ ions can lead to an increase of the diffusion coefficient of V(III) species and a decrease of charge transfer resistance. Moreover, the VRFB cell using negative electrolyte with Sb3+ ions exhibits excellent cycling stability and high average energy efficiency, especially under high power operation. The energy efficiency (67.1%) of the VRFB employing electrolyte with 5mM Sb3+ ions is increased by 9.6% at a current density of 120 mA·cm−2, compared to the pristine one (57.5%). The improved electrochemical performance should be ascribed to the prominent catalytic effect of Sb particles, which are simultaneously electrodeposited onto the surface of graphite felts during operation of the flow cell and facilitate charge transfer process.

Influence of the basic membrane properties of the disulfonated poly(arylene ether sulfone) copolymer membranes on the vanadium redox flow battery performance

This article focuses on structure–property–performance relationship of directly copolymerized disulfonated poly(arylene ether sulfone) membranes (BPSH) utilized in vanadium redox flow batteries (VRFBs). Degree of disulfonation was systematically altered from 25 to 45M percent by introducing controlled amount of disulfonated monomer into the copolymer structure. For the first time with this study the relation has been established between VRFB cell performances and basic membrane properties such as ion exchange capacity (IEC), water uptake, proton conductivity, and vanadium (VO2+) permeability of BPSH membranes. BPSH with 25M percent disulfonation (BPSH 25) showed delay time during discharging process due to the fluctuation of the discharging voltage caused by poor proton conductivity, 23mScm−1. Therefore, columbic efficiencies became greater than 100 percent for BPSH 25 indicating higher discharging time than charging time. On the other hand, the best performance in the series was observed with BPSH 35. Since it had almost three times higher proton conductivity than BPSH 25 and about 100 times lower vanadium permeability (1.6×10−13m2s−1) than BPSH 45. Although BPSH 45 had the highest proton conductivity (140mScm−1), its performance was deteriorated due to higher vanadium permeability (1.6×10−11m2s−1). It was also demonstrated that water uptake values of the series scales with the vanadium permeabilities. As a result, several basic membrane parameters were controlled and presented as tunable properties, which were function of VRFB performance.

바나듐 레독스-흐름 전지에서 집전체의 전기화학적 특성

두 종류의 집전체(BP, bipolar plate)를 사용하여 바나듐 레독스-흐름 전지(V-RFB, vanadium redox-flow battery)의 성능을 평가하였다. V-RFB의 성능평가는 <TEX>$60mA/cm^2$</TEX>의 전류밀도에서 진행하였다. A 집전체를 사용한 V-RFB의 기전력(SOC 100%에서의 OVC)은 1.47V, B 집전체를 사용한 V-RFB의 기전력은 1.54V를 나타냈다. A 집전체를 사용한 V-RFB의 셀 저항은 충전시에 <TEX>$4.44{\sim}5.00{\Omega}{\cdot}cm^2$</TEX>을, 방전시에 <TEX>$3.28{\sim}3.75{\Omega}{\cdot}cm^2$</TEX>를 보였으며, B 집전체를 사용한 V-RFB의 셀 저항은 충전시에 <TEX>$4.19{\sim}4.42{\Omega}{\cdot}cm^2$</TEX>, 방전시에 <TEX>$4.71{\sim}5.49{\Omega}{\cdot}cm^2$</TEX>를 나타냈다. 각 집전체를 사용한 V-RFB의 성능은 5회 충방전 실험을 진행하여 평가하였다. A 집전체를 사용한 V-RFB는 평균 전류효율 93.1%, 평균 전압효율 76.8%, 평균 에너지효율 71.4%를 나타냈으며, B 집전체를 사용한 V-RFB는 평균 전류효율 96.4%, 평균 전압효율 73.6%, 평균 에너지효율 71.0%를 나타냈다. Two commercial carbon plates were evaluated as a current collector (bipolar plate) in the all vanadium redox-flow battery (V-RFB). The performance properties of V-RFB were test in the current density of <TEX>$60mA/cm^2$</TEX>. The electromotive forces (OCV at SOC 100%) of V-RFB using A and B current collector were 1.47 V and 1.54 V. The cell resistance of V-RFB using A current collector was <TEX>$4.44{\sim}5.00{\Omega}{\cdot}cm^2$</TEX> and <TEX>$3.28{\sim}3.75{\Omega}{\cdot}cm^2$</TEX> for charge and discharge, respectively. The cell resistance of V-RFB using B current collector was <TEX>$4.19{\sim}4.42{\Omega}{\cdot}cm^2$</TEX> and <TEX>$4.71{\sim}5.49{\Omega}{\cdot}cm^2$</TEX> for charge and discharge, respectively. The performance of V-RFB using each current collector was evaluated. The performance of V-RFB using A current collector was 93.1%, 76.8% and 71.4% for average current efficiency, average voltage efficiency and average energy efficiency, respectively. The performance of V-RFB using B current collector was 96.4%, 73.6% and 71.0% for average current efficiency, average voltage efficiency and average energy efficiency, respectively.

Study of Transient Behavior of Vanadium Redox Flow Battery at Varying Flow Rates and States of Charge

The Vanadium Redox Flow Battery (VRB) is one promising candidate for grid type storage because of its long life time, modularity, active thermal management, and good cycle life. There are various parameters that affect the performance of VRB for example: temperature, flow rate, state of charge etc. Flow rate is an important parameter, since the electrolyte flow rate influences the reaction rate of the vanadium ions. It is also important because it has a substantial influence on the system efficiencies.Ma et al. [1] investigated the effect of flow rate on the system capacity and the efficiency. Fetlawai [2] investigated the effect of flow rate on the charge-discharge characteristics and on the gas evolution. It was established that flow rate is one of the important parameters which should be controlled to obtain high efficiency of VRB. The effect of the flow rate on the transient behavior of the battery has not got much attention which motivated this study.We will present experimental results on the transient behavior of a VRB at various flow rates up to ca. 1 L/min. The battery was at 80% state of charge during experiment. It was observed that flow rate has a strong effect on the transient behavior of the battery (a typical examples is shown in Fig. 1). The load of the battery was varied in a regime typical for applications of VRBs. The results showed clear trends of the transient behaviour of the battery with changes of flow rate. The implications of these findings for the development of control systems in grid energy storage system design with VRBs will be discussed.[1] X. Ma, H. Zhang, C. Sun, Y. Zou, and T. Zhang, “An optimal strategy of electrolyte flow rate for vanadium redox flow battery,” J. Power Sources, vol. 203, pp. 153–158, Apr. 2012.[2] H. A.-Z. A.-Y. Al-Fetlawi, “Modelling and simulation of all-vanadium redox flow batteries.” 14-Apr-2011.[3]J. Chahwan, C. Abbey, and G. Joos, “VRB Modelling for the Study of Output Terminal Voltages, Internal Losses and Performance,” 2007 IEEE Canada Electr. Power Conf., pp. 387–392, Oct. 2007.

Ion exchange membranes for vanadium redox flow batteries

Abstract In recent years, much attention has been paid to vanadium redox flow batteries (VRBs) because of their excellent performance as a new and efficient energy storage system, especially for large-scale energy storage. As one core component of a VRB, ion exchange membrane prevents cross-over of positive and negative electrolytes, while it enables the transportation of charge-balancing ions such as H+, $${\text{SO}}_4^{2 - },$$ and $${\text{HSO}}_4^ - $$ to complete the current circuit. To a large extent, its structure and property affect the performance of VRBs. This review focuses on the latest work on the ion exchange membranes for VRBs such as perfluorinated, partially fluorinated, and nonfluorinated membranes. The prospective for future development on membranes for VRBs is also proposed.

Laser-perforated carbon paper electrodes for improved mass-transport in high power density vanadium redox flow batteries

In this study, we demonstrate up to 30% increase in power density of carbon paper electrodes for vanadium redox flow batteries (VRFB) by introducing perforations into the structure of electrodes. A CO2 laser was used to generate holes ranging from 171 to 421 μm diameter, and hole densities from 96.8 to 649.8 holes cm−2. Perforation of the carbon paper electrodes was observed to improve cell performance in the activation region due to thermal treatment of the area around the perforations. Results also demonstrate improved mass transport, resulting in enhanced peak power and limiting current density. However, excessive perforation of the electrode yielded a decrease in performance due to reduced available surface area. A 30% increase in peak power density (478 mW cm−2) was observed for the laser perforated electrode with 234 μm diameter holes and 352.8 holes cm−2 (1764 holes per 5 cm2 electrode), despite a 15% decrease in total surface area compared to the raw un-perforated electrode. Additionally, the effect of perforation on VRFB performance was studied at different flow rates (up to 120 mL min−1) for the optimized electrode architecture. A maximum power density of 543 mW cm−2 was achieved at 120 mL min−1.

Preparation and characterization of a novel layer-by-layer porous composite membrane for vanadium redox flow battery (VRB) applications

By the solution casting method, a novel porous membrane has been prepared for VRB by doping sulfonated poly(fluorenyl ether ketone) (SPFEK) with imidazole, and then imidazole was washed out by extraction with solution. The proton conductivity of porous membrane increased with increasing the content of imidazole, but proton/vanadium ion (H/V) selectivity decreased. Layer-by-layer (LbL) technique was used to improve the porous membrane with high selectivity. Moreover, the performance of VRB using SPFEK-20.7imidazole-(PDDA/PSS)8 membrane which is doped with 20.7 wt.% content of imidazole and then removed imidazole, and then deposited with eight LbL bilayers exhibits the highest columbic efficiency (CE) of 92.5% at 30 mA cm−2.

Sulfonated poly(ether ether ketone)/mesoporous silica hybrid membrane for high performance vanadium redox flow battery

Hybrid membranes of sulfonated poly(ether ether ketone) (SPEEK) and mesoporous silica SBA-15 are prepared with various mass ratios for vanadium redox flow battery (VRB) application and investigated in detail. The hybrid membranes are dense and homogeneous with no visible hole as the SEM and EDX images shown. With the increasing of SBA-15 mass ratio, the physicochemical property, VO2+ permeability, mechanical property and thermal stability of hybrid membranes exhibit good trends, which can be attributed to the interaction between SPEEK and SBA-15. The hybrid membrane with 20 wt.% SBA-15 (termed as S/SBA-15 20) shows the VRB single cell performance of CE 96.3% and EE 88.1% at 60 mA cm−2 due to its good balance of proton conductivity and VO2+ permeability, while Nafion 117 membrane shows the cell performance of CE 92.2% and EE 81.0%. Besides, the S/SBA-15 20 membrane shows stable cell performance of highly stable efficiency and slower discharge capacity decline during 120 cycles at 60 mA cm−2. Therefore, the SPEEK/SBA-15 hybrid membranes with optimized mass ratio and excellent VRB performance can be achieved, exhibiting good potential usage in VRB systems.

Analysis of Concentration Polarization Using UV-Visible Spectrophotometry in a Vanadium Redox Flow Battery

In this study, the concentration of vanadium ions in a carbon felt electrode was examined quantitatively using UV-visible spectrophotometry with respect to the current density and flow rate to examine the effect of the concentration overpotential on the performance of a vanadium redox flow battery (VRB). Experimental results revealed that the concentration of vanadium ions in the positive electrode is different from that observed in the positive electrolyte tank, and the deviation increases with increasing current density and decreasing flow rate. The concentration of V(IV) is higher in the positive electrode compared to the positive electrolyte tank after the discharging process, which leads to a premature cut-off in the discharge of the VRB. Therefore, this result suggests that it is important to improve electrolyte transport through the electrode for better performance of VRBs, and the measurement of the concentration of vanadium ions in the electrode could be useful for studying the concentration overpotential.

Performance enhancement in vanadium redox flow battery using platinum-based electrocatalyst synthesized by polyol process

Abstract Sluggish reaction rate of [VO]2+/[VO2]+ redox couple is an obstacle to be addressed in vanadium redox flow battery (VRFB). To improve the slow reaction rate, Pt/C catalyst synthesized by polyol method is suggested. Its catalytic activity, reaction reversibility and charge–discharge performance are evaluated by half cell and single cell tests, while its crystal structure, particle size and particle distribution are measured by XRD and TEM. The XRD and TEM measurements show the polyol Pt/C catalyst has larger electrochemically active surface (EAS) area and smaller particle size than commercial Pt/C catalyst. When catalytic activities of all the catalysts are estimated, the Pt-included catalysts demonstrate high peak current ratio, small peak potential difference and high electron transfer rate constant, confirming that their catalytic activity and reaction reversibility are excellent. In charge–discharge performance tests, the catalysts indicate high efficiencies as well as low overpotential and internal resistance. Excellent performances of the Pt-included catalysts are attributed to positively charged Pts that serve as active sites for activating [VO]2+/[VO2]+ reaction. Indeed, adoption of the Pt-included catalysts, especially, use of the polyol Pt/C consisting of uniform and small particles helps improve performance of VRFB.

Optimal Configuration Method of Hybrid Energy Storage System for Wind Farm

According to the demand of wind farm power fluctuations stabilize and the characteristics of hybrid energy storage system. Taking vanadium redox flow battery (VRB) and supercapacitor (SC) as research object, a hybrid energy storage system optimal configuration model is builted. Combined with expert systems and improved genetic algorithm proposed a hybrid energy storage system to optimize the allocation method. The method first established desire output curve of the wind farm based on grid energy saving efficiency and static voltage stability indicators, then introduce coordinate control strategy based on expert system to improved genetic algorithm to get the result of optimal configuration and verify the correctness of this model and method. Finally, analysis the performance of VRB and SC in a typical days wind farm output situation, verify the conclusions of experts coordinated control strategy can extend the vanadium battery life.

Layered zirconium phosphate sulfophenylphosphonates reinforced sulfonated poly (fluorenyl ether ketone) hybrid membranes with high proton conductivity and low vanadium ion permeability

Sulfonated poly(fluorenyl ether ketone) (SPFEK) membranes were modified with layered proton conductors zirconium phosphate sulfophenylenphosphonates (ZrPSPP) to improve the proton conductivity and mitigate vanadium ions crossover. The effects of ZrPSPP filler loading on the microstructure and chemical–physical property of SPFEK/ZrPSPP composite membranes as well as the vanadium redox flow battery performance are reported. The uniform dispersion of ZrPSPP sheets in the SPFEK matrix is investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). Since the sulfonic acid groups on the aromatic main-chains are less flexible, the proton channels in the pristine SPFEK membranes are relatively narrow and not well connected. The intensive sulfonic acid groups on ZrPSPP fillers increase the water uptake and the hydrophobic/hydrophilic microphase separation of the composite membranes, resulting in improved proton conductivity. The impermeable nanoplates demonstrate to be a potential physical barrier for vanadium ion diffusion. The VRB assembled with the SPFEK/5wt%ZrPSPP composite membrane shows longest discharge time and maximal coulombic efficiency among VRBs using the pristine SPFEK and other hybrid SPFEK/ZrPSPP and Nafion 117 membranes.

Optimizing membrane thickness for vanadium redox flow batteries

Two important intrinsic properties of proton exchange membranes for vanadium redox flow battery (VRFB) operation are proton conductivity and vanadium permeability. These characteristics are thickness-normalized quantities and depend on fundamental material parameters. However, the operational criteria of proton exchange membranes in these devices are the membrane resistance and vanadium crossover flux, both of which depend on membrane thickness. Herein, we explore the influence of the thickness of ion exchange capacity (IEC)-optimized sulfonated fluorinated poly(arylene ether) (SFPAE) membranes on their VRFB performance including charge/discharge behavior, charge depth, coulombic efficiency, voltage efficiency, energy efficiency and cell polarization. IEC-optimized SFPAE membranes with three different thicknesses (28μm, 45μm and 80μm) were prepared and tested in this study. It was found that the combined effects of the ohmic loss and electrolyte crossover loss in the VRFB, which were governed by membrane thickness, resulted in an optimal membrane thickness of 45μm for SFPAE under the conditions tested. Thicker membranes were observed to cause higher cell resistance while thinner membranes yielded larger vanadium crossover flux, both of which had negative impacts on the cell performance. The maximum power densities of the VRFBs assembled with 28μm, 45μm and 80μm SFPAE membranes were 267mWcm−2, 311mWcm−2 and 253mWcm−2 respectively, much higher than that of the VRFB assembled with N212 membrane, which was 204mWcm−2. These results supported our previous observation that SFPAE was superior to N212 with regard to VRFB performance. The data also indicated that there is an optimum membrane thickness for a given set of properties through which the cell performance can be significantly improved while keeping the membrane material constant.

Neural Network Predictive Control for Vanadium Redox Flow Battery

The vanadium redox flow battery (VRB) is a nonlinear system with unknown dynamics and disturbances. The flowrate of the electrolyte is an important control mechanism in the operation of a VRB system. Too low or too high flowrate is unfavorable for the safety and performance of VRB. This paper presents a neural network predictive control scheme to enhance the overall performance of the battery. A radial basis function (RBF) network is employed to approximate the dynamics of the VRB system. The genetic algorithm (GA) is used to obtain the optimum initial values of the RBF network parameters. The gradient descent algorithm is used to optimize the objective function of the predictive controller. Compared with the constant flowrate, the simulation results show that the flowrate optimized by neural network predictive controller can increase the power delivered by the battery during the discharge and decrease the power consumed during the charge.

Layer-by-layer self-assembly of PDDA/PSS-SPFEK composite membrane with low vanadium permeability for vanadium redox flow battery

Sulfonated poly(fluorenyl ether ketone) (SPFEK) membranes have been first modified by layer-by-layer (LbL) self-assembly of positively charged polyelectrolyte PDDA (poly(diallyldimethylammonium chloride)) and negatively charged PSS (poly(sodium styrene sulfonate)). The membranes were investigated as an ion exchange membrane for vanadium redox flow batteries (VRBs). The permeability of the vanadium ions in VRBs was effectively suppressed by depositing the LbL thin film on the SPFEK membrane. The permeability decreased with increasing the number of PDDA/PSS bilayers. For the membrane with two self-assembly bilayers of PDDA/PSS, 50% and 10% of vanadium ion permeability of a pristine SPFEK and Nafion 117 membranes can be afforded, respectively. Moreover, the oxidative stability of the PDDA/PSS-SPFEK composite membrane is improved remarkably compared with the pristine one. Consequently, the performance of VRBs using the PDDA/PSS-SPFEK composite membrane exhibits the highest coulombic efficiency (CE) of 82.1% at 30 mA cm−2 and the longest duration stability in the self-discharge test.

Nitrogen-Doped Carbon Nanotube/Graphite Felts as Advanced Electrode Materials for Vanadium Redox Flow Batteries.

Nitrogen-doped carbon nanotubes have been grown, for the first time, on graphite felt (N-CNT/GF) by a chemical vapor deposition approach and examined as an advanced electrode for vanadium redox flow batteries (VRFBs). The unique porous structure and nitrogen doping of N-CNT/GF with increased surface area enhances the battery performance significantly. The enriched porous structure of N-CNTs on graphite felt could potentially facilitate the diffusion of electrolyte, while the N-doping could significantly contribute to the enhanced electrode performance. Specifically, the N-doping (i) modifies the electronic properties of CNT and thereby alters the chemisorption characteristics of the vanadium ions, (ii) generates defect sites that are electrochemically more active, (iii) increases the oxygen species on CNT surface, which is a key factor influencing the VRFB performance, and (iv) makes the N-CNT electrochemically more accessible than the CNT.