A high power density and long cycle life Vanadium Redox Flow Battery

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A high power density and long cycle life vanadium redox flow battery H.R. Jiang, J. Sun, L. Wei, M.C. Wu, W. Shyy, T.S. Zhao * Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Vanadium redox flow battery Large-scale energy storage Mass/ion transport Charge-discharge performance Cycling performance

Increasing the power density and prolonging the cycle life are effective to reduce the capital cost of the vanadium redox flow battery (VRFB), and thus is crucial to enable its widespread adoption for large-scale energy storage. In this work, we analyze the source of voltage losses and tailor the design of the battery to simultaneously minimize the ohmic resistance, maximize the transport of electrolytes, and boost the surface area and activity of electrodes.

These strategies collectively result in an unprecedented improvement in the performance of VRFBs. At the current densities of 200, 400 and 600 mA cm 2, the battery achieves the energy efficiencies of 91.98%, 86.45% and 80.83%, as well as the electrolyte utilizations of 87.97%, 85.21% and 76.98%, respectively. Even at an ultra-high current density of 1000 mA cm 2, the battery is still able to maintain an energy efficiency of as high as 70.40%. It is also demonstrated that the battery can deliver a high peak power density of 2.78 W cm 2 and a high limiting current density of ~7 A cm 2 at room temperature. Moreover, the battery is stably cycled for more than 20,000 cycles at a high current density of 600 mA cm 2. The data reported in this work represent the best chargedischarge performance, the highest peak power density, and the longest cycle life of flow batteries reported in the literature.

1. Introduction With the rapid development of renewable energies such as wind and solar powers which are intermittent in nature, the large-scale energy storage systems have attracted increasing attention from both academic and industrial fields, primarily due to the fact that the direct usage of the electricity generated from these renewable energies would destabilize the power grid [1–4]. Although lithium-ion batteries have been widely used ranging from electronic devices to electric vehicles, their applications in large-scale energy storage are hindered by the poor scalability, poor design flexibility, short cycle life, and safety concerns [5–7].

Fortunately, the redox flow battery that possesses the advantages including decoupled energy and power, high efficiency, good reliability, high design flexibility, fast response, and long cycle life, is regarded as a more practical candidate for large-scale energy storage [8–11]. Among the state-of-the-art redox flow batteries, the vanadium redox flow batteries (VRFBs) show the most promise for widespread commercial application, because the same element of vanadium is adopted as both the negative and positive electroactive materials, and therefore the severe cross-contamination issue in flow batteries is eliminated [12,13].

The typical schematic representation of VRFBs is shown in Fig. S1. It is seen that the system is composed of two tanks with electrolytes and a stack with two electrodes separated by a membrane. The electrolytes

containing electroactive materials are pumped through the porous electrodes, at which the redox reactions happen. Theoretically, the VRFBs can output a standard voltage of 1.26 V through the following electrochemical reactions.

Positive side: þ

VOþ 2 þ 2H þ e

discharge

⇄ VO2þ þ H2 O

charge

E0 ¼ 1:01 V vs: SHE

Negative side: V2þ

discharge

⇄ V3þ þ e

E0 ¼ 0:25 V vs: SHE

charge

Overall: 2þ VOþ þ 2Hþ 2 þV

discharge

⇄ VO2þ þ V3þ þ H2 O

charge

E0 ¼ 1:26 V

In the charge process, V3þ is reduced to V2þ at the negative side, and VO2þ is oxidized to VOþ 2 at the positive side. In the discharge process, the reversible reactions occur.

* Corresponding author.

E-mail address: metzhao@ust.hk (T.S. Zhao). https://doi.org/10.1016/j.ensm.2019.07.005 Received 17 June 2019; Accepted 4 July 2019 Available online xxxx 2405-8297/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: H.R. Jiang et al., A high power density and long cycle life vanadium redox flow battery, Energy Storage Materials, https:// doi.org/10.1016/j.ensm.2019.07.005

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2. Methods

membrane (anion exchange membrane (AEM), Fumasep, 50 μm), polybenzimidazole (PBI) membrane (porous membrane, PBI Performance Products, ~50 μm), Nafion 211 membrane (cation exchange membrane (CEM), Dupont, 25 μm) and Nafion 212 membrane (CEM, Dupont, 50 μm). The PBI membrane was pretreated by immersing it in 3 M H2SO4 for a week before use [24], while others were used as received. The negative electrolyte was 20 mL solution containing 1.1 M V3þ þ 3 M H2SO4 and the positive electrolyte was 20 mL solution containing 1.1 M VO2þ þ 3 M H2SO4. 0.5 mmol Bi was electrodeposited on the negative electrode before the charge process. The electrolytes were circulated at a constant flow rate by a 2-channel peristaltic pump (Longer pump, BT600-2J). N2 gas was bubbled to exhaust the air in the cell and tanks before all the battery tests.

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were conducted on the Autolab (PGSTAT30) workstation. A typical three-electrode electrochemical cell was used, and the graphite felt, saturated calomel electrode (SCE) and platinum mesh acted as the working, reference and counter electrodes. The CV and EIS measurements were conducted in a solution containing 0.1 M V4þ þ 3 M H2SO4. In the CV tests, the negative and positive reactions were characterized by adopting the voltage windows of 0.7 to 0.3 V (vs. SCE) and 0.6–1.1 V (vs. SCE), respectively. In the EIS tests, fixed voltages of 0.5 V (vs. SCE) and 0.9 V (vs. SCE) were set for V2þ/V3þ and VO2þ/VOþ 2 redox reactions. The polarization curve was tested at 100% SOC with a voltage reduction rate of 50 mV s 1. The scanning electron microscope (JEOL-6700 SEM) was used to observe the surface morphologies, and the X-ray photoelectron spectroscopy (XPS) characterization with a Physical Electronics PHI 5600 multi-technique system was adopted to analyze the surface properties. N2 adsorption/desorption measurement was conducted with a gas analyzer (ASAP2420, Micromeritics) after degassing, and the Brunauer-Emmett-Teller (BET) method was adopted to calculate the specific surface areas. The X-ray diffraction (XRD) characterization was carried out by an XRD system (model PW 1825) with the angle ranging from 10 to 90 at the scanning rate of 5 min 1. The surface properties of electrodes were evaluated by a Micro-Raman spectrophotometer (Renishaw RM 3000) at 514 nm existing wavelength. Mercury Intrusion Porosimetry tests were conducted by AutoPore IV 9500 V1.09 (Micromeritics Instrument Corporation, USA) with a maximum injection pressure of 400 MPa. The crossover of VO2þ through membranes was determined by a homemade dialysis cell containing two chambers with the same volume (50 mL). Chamber A was filled with 1.1 M MgSO4 þ 3.0 M H2SO4, and Chamber B was filled with 1.1 M VOSO4 þ 3.0 M H2SO4.

1 mL solution samples were collected from Chamber B at regular time intervals, and then 1 mL solution with 1.1 M MgSO4 þ 3.0 M H2SO4 was immediately added to Chamber B to maintain a constant solution volume. The concentrations of vanadium ions were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES).

2.1. Experimental methods

2.2. Computational methods

The battery setups with serpentine and interdigitated flow-field structures were both homemade. The channels with the depth of 1.5 mm and width of 1.0 mm were machined on the graphite plate. The battery tests were all performed on the Arbin BT2000 (Arbin Instrument, Inc.). The charge cutoff voltage was 1.65 V and the discharge cutoff voltage was 1.0 V. Both the negative and positive electrodes were one piece of separated graphite felt (Sigracell SGL carbon, GFA6 EA) with an active area of 2.0 cm 2.0 cm. The original graphite felt was named as GF. The traditional thermally treated graphite felt was prepared by annealing it in a muffle furnace under the ambient air at 400 C for 8 h, and named as treated GF. The multiscale graphite felt was fabricated by a simple yet effective method, which annealed GF in a muffle furnace under the ambient air at 500 C for 8 h, and named as multiscale GF. When further increasing the electrode fabrication temperature to 600 C, the GFs were burnt out due to the intensive oxidation reaction under this high temperature. The membranes adopted here were FAP-450

A 3-D numerical simulation was performed to investigate the effect of flow field design and flow rate on the velocity profiles of electrolyte in the porous electrodes. The computational domain included the flow field (serpentine or interdigitated) and a porous electrode with the projected area of 2 2 cm2. The continuity equation of the electrolyte was applied to the entire computational domain with the assumption of incompressible flow and is given as:

Although VRFBs possess attractive features, their widespread commercial adoption is still greatly hindered by the high capital cost, primarily due to the poor battery performance and the high cost of the electroactive materials [14]. One straightforward yet effective method to decrease the capital cost is to maximize the operating current density, whilst maintaining a high energy efficiency to reduce the size of the stack and a high electrolyte utilization to enable the effective usage of electroactive materials. Unfortunately, enhancing the operating current density would inevitably increase the cell polarizations, leading to the decreased energy efficiency and electrolyte utilization. Therefore, developing high-performance VRFBs needs to reduce the voltage losses including activation loss, ohmic loss and concentration loss to boost the voltage efficiency and electrolyte utilization, as well as minimize the vanadium crossover to improve the coulombic efficiency.

To this end, various approaches focusing on the key components [15–20] and operating conditions [21,22] have been applied to enhance the battery performance in the past decades. With these efforts, the charge-discharge performances of VRFBs have been greatly improved, and finally, Zhao et al. achieved an energy efficiency of 81.7% at a current density of 400 mA cm 2 [23]. In spite of the progress, the state-of-the-art VRFBs still suffer from several critical issues, which greatly hinder their widespread applications. Firstly, previous methods to enhance the battery performance are generally very complicated, and the electrode needs to be activated at a high temperature under protective gases (e.g. N2 and Ar) to increase the specific surface area, obstructing their mass production. Secondly, unlike the achievements in enhancing the operating current density and energy efficiency, few attentions have been paid to increase the battery’s electrolyte utilization and cycling stability, which are equally crucial to the development of flow batteries.

More importantly, the source of voltage losses in flow batteries is still poorly systematically analyzed, and a simple yet effective design strategy for high-performance VRFBs is still missing. Finally, it has been a longstanding goal for both academic and industrial fields to further enhance the VRFB performance to an unprecedented level, the success of which will undoubtedly promote the application of flow batteries in large-scale energy storage.

Therefore, in this work, we systematically analyze the source of voltage losses and judiciously tailor the design of the battery to simultaneously minimize the ohmic resistance, maximize the transport of electrolytes, and boost the surface area and activity of electrodes. With these efforts, it is exciting to find that the rationally designed VRFB is able to be operated at a high current density of 600 mA cm 2 with an energy efficiency of 80.83% as well as an electrolyte utilization of 76.98%, and be stably cycled for more than 20,000 cycles, representing the best performance for flow batteries in the open literature.

r ! u ¼0

(1)

The momentum equations of electrolyte were Navier-Stokes equations in the flow channels and Brinkman equations in the porous media:

u rÞ! u ¼ rp þ r μ r! ρð! u þ ðr! u ÞT

2

! þ f

(2)

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hμ

! i μ ρ ! u ! þ βF j! ð u rÞ ¼ rp þ r uj ! u þ f r! u þ ðr! u ÞT ε ε ε k

focuses on identifying the suitable membrane for high-performance VRFBs with high operating current density and high power density [27]. Herein, the properties of typical CEMs (Nafion membranes) and porous membranes (PBI membrane) are firstly evaluated and the corresponding battery performances are compared, as shown in Fig. 1. Fig. 1a depicts the concentration change of VO2þ ions in the MgSO4 chamber across different membranes. It is shown that the concentration of VO2þ permeating the Nafion 212 membrane is lower than the Nafion 211 membrane, but several tens of times higher than the PBI membrane, indicating the porous membrane can achieve an extremely low vanadium permeability through the pore size exclusion. The self-discharge curves of VRFBs with different membranes in Fig. 1b reveal the open circuit voltage of the batteries gradually decreases as time goes by, and then undergoes a sharp voltage drop to 0.8 V, which is due to the disappearance of VOþ 2 ion in the positive electrolyte. The self-discharge time to reach 0.8 V is 102 h and 180 h for VRFBs with Nafion 211 and Nafion 212 membranes, but the voltage can still maintain at 1.46 V for that with PBI membrane after 280 h. The charge-discharge curves of VRFBs with different membranes at the current density of 200 mA cm 2 are presented in Fig. 1c. Results show that although PBI and Nafion 212 membranes has low vanadium permeability than the Nafion 211 membrane, the batteries with them exhibit higher charge/discharge overpotentials and lower charge/discharge capacities than that with Nafion 211 membrane. Fig. 1d–f summarize the coulombic efficiency, voltage efficiency, energy efficiency and electrolyte utilization at varying current densities.

At the current density of 200 mA cm 2, attributed to the low vanadium permeability, the battery with PBI membrane shows a much higher coulombic efficiency (99.74%) than that with Nafion 211 (96.98%) and Nafion 212 (97.23%) membranes. However, the batteries’ voltage efficiency decreases in the sequence of Nafion 211 membrane (81.09%) > Nafion 212 membrane (78.01%) > PBI membrane (68.35%), which is opposite to the tendency of coulombic efficiency. This phenomenon is due to fact that although the membrane with low vanadium permeability can reduce the crossover of vanadium ions to enhance the coulombic efficiency, it also hinders the transport of protons, which leads to a higher cell resistance and thus lower voltage efficiency. Therefore, a tradeoff between the voltage efficiency and coulombic efficiency exists

(3) where ε is the porosity of the porous electrode, p is the pressure, μ represents the dynamic viscosity of the fluid and k denotes the permeability of the porous electrode. βF is the Forchheimer drag coefficient which is ! neglected in the simulation. And f is the body force acting on the flow, which is zero here as the gravity is not taken into consideration.

Continuous velocity and pressure were applied at the interface of the flow channel and the porous media. The porosity and permeability of porous electrodes were set to 0.9 and 1.75 10 11 m2, respectively.

3. Results and discussion The performance of VRFB is closely related to the properties of membrane, as it not only provides transport pathways for protons, but also blocks the crossover of vanadium ions [25]. Ideally, the membrane of VRFB should have high ionic conductivity and low vanadium permeability, not to mention other requirements such as good chemical and thermal stability, high mechanical strength, and low cost. Typically, three kinds of membranes, i.e., AEMs, CEMs and porous membranes, are adopted for VRFBs. The AEMs contain positively charged functional groups to transport negative ions and repel the vanadium ions through the electrostatic repulsion, which reduces the crossover issue for vanadium ion. However, the AEMs generally suffer from the poor chemical stability at highly oxidative media, hindering its long-term operation in VRFBs [26]. In addition, as shown in Fig. S2, the battery with FAP 450 membrane shows large overpotential and high coulombic efficiency of >99%. The CEMs adopt negatively charged functional groups to transport protons, and achieves the high ionic conductivity with good chemical stability, making them the most commonly used membrane for state-of-the-art VRFBs. In addition, the porous membranes have attracted increasing attention as it can reduce the crossover issue by the pore size exclusion, and ensure a high coulombic efficiency with good cycling stability of the battery. Although some works have been taken to investigate the properties of various membranes for VRFBs, seldom of them

Fig. 1. (a) Concentration change of VO2þ ions in the MgSO4 chamber across different membranes. (b) Self-discharge curves of fully charged VRFBs with different membranes. The charge-discharge curves of VRFBs with different membranes at the current density of (c) 200 mA cm 2. The (d) coulombic and voltage efficiencies, (e) energy efficiency and (f) electrolyte utilization of VRFBs with different membranes.

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area of carbon fibers without reducing the size of large pores between them. Following this strategy, several works have been taken to create secondary pores on carbon fibers [23,30], but the fabrication of these electrodes requires complicated procedures and the achieved specific surface area is still relatively low. Therefore, to boost the specific surface area and activity of electrodes, the novel multiscale porous electrodes which simultaneously contain primary, secondary and tertiary pores, are designed and fabricated. More importantly, the fabrication process of this multiscale electrode is simple and thus suitable for mass production, which can promote the widespread application of this kind of high-performance electrodes for VRFBs. Fig. 2a–c shows the SEM images of GF at different magnifications. It is seen that the electrode is formed by the interconnected carbon fibers, whose surface is smooth with some impurities on it. After traditional thermal treatment, as shown in Fig. 2d, some impurities are burnt out and the carbon fiber surface becomes rougher than that of the original GF. Remarkably, the carbon fibers of multiscale GF possess densely packed large pores with the diameter of ~400 nm, and each large pore contains abundant micro-pores on the surface, as shown in Fig. 2e and f. Nitrogen sorption isotherms in Fig. 2g confirm the existence of large amount of micro-pores on multiscale GF, which are not found in pristine and treated GFs. As a result, the specific surface area increases dramatically from 1.74 m2 g 1 of GF and 2.30 m2 g 1 of treated GF to 167.44 m2 g 1 of multiscale GF, suggesting a two-order increase compared to the traditional ones. Moreover, the pore

for VRFBs with various membranes. Another interesting thing is that, unlike previous finding at low operating current densities that the VRFB should balance the voltage efficiency and coulombic efficiency to obtain a high energy efficiency [28], we demonstrate that a membrane with high conductivity is more promising for high-performance VRFBs even though it will slightly sacrifice the coulombic efficiency. This is because the voltage losses dominate the battery performance and the coulombic efficiency is generally higher than 95% at such high current densities. In addition, the use of thin membrane (e.g. Nafion 211, 25 μm) can also decrease the material consumption, thus reducing the capital cost of the whole system [29]. Adopting the tailored membrane, it is noted that the battery can achieve energy efficiencies of 78.64% and 70.67% at the current densities of 200 and 300 mA cm 2, respectively.

In addition to the membrane, the electrode is another key component that determines the performance of VRFBs, because it provides not only the active sites for redox reactions, but also the pathways for mass/ion transport. However, owing to the limited surface area of traditional graphite felts, VRFBs assembled with such electrodes present poor performance [30]. One approach to increase the specific surface area is to increase the proportion of carbon fibers by decreasing the diameters, but this approach inevitably reduces the hydraulic permeability of the electrode, and thus increases the concentration loss of the battery. To circumvent the conflict between specific surface area and hydraulic permeability, another more promising approach is to improve the surface

Fig. 2. The SEM images of (a–c) GF at different magnifications, (d) treated GF, and (e, f) multiscale GF at different magnifications. The (g) nitrogen sorption isotherm plot, (h) pore size distribution below 100 nm (measured by nitrogen adsorption-desorption experiment), and (i) pore size distribution of macropores (measured by mercury intrusion porosimetry) of different electrodes.

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Fig. 3. The (a) Raman spectroscopy and (b) XRD patterns of different electrodes. The O1s core-level spectra of (c) GF, (d) treated GF and (e) multiscale GF. (f) Distribution of types of oxygen-functional groups for different electrodes.

size distributions in Fig. 2h and i reveal that the developed multiscale GF has the primary pore of ~40 μm formed by the voids between interconnected carbon fibers, the secondary pore of ~400 nm formed on the surface of carbon fibers, and the tertiary pore of ~1.5 nm formed inside the secondary pores. It is thus anticipated that this novel electrode design can not only ensure sufficient macroscopic pathways for electrolyte flow, but also boost the surface area for redox reactions.

The surface properties of prepared electrodes are then analyzed in details. The Raman spectroscopy of GF, treated GF and multiscale GF are shown in Fig. 3a. Two peaks located at around 1340 and 1580 cm 1 are the D band and G band, representing the disordered and graphitic carbon, respectively [31]. It is found that the ID/IG of GF is 0.99, and it increases to 1.02 and 1.12 for treated GF and multiscale GF, indicating the enriched defects on carbon fibers. Fig. 3b presents the XRD patterns, and the two significant diffraction peaks located at 26.4 and 43.6 correspond to the (0 0 2) and (1 0 0) planes of graphitic carbon [31]. Moreover, the peak intensity decreases in the order of GF > treated GF > multiscale GF, demonstrating more disordered structures are formed on the multiscale GF. The O1s core-level spectra of GF, treated GF and multiscale GF are shown in Fig. 3c–e, and the distribution of types of oxygen-functional groups are summarized in Fig. 3f and Table 1. Here, the O 1s core-level spectra can be deconvoluted into four peaks at binding energies of 534.7, 533.8, 532.5 and 531.5 eV, corresponding to H–OH, – O bonds, respectively [32,33]. The contents of COOH, C–OH and C– oxygen-functional groups increase in the sequence of GF (3.02%) < treated GF (4.95%) < multiscale GF (7.03%), representing

more oxygen-functional groups are generated on the multiscale GF. In – O group dominates the oxygen functional groups for all addition, the C– the samples, and its contents are1.12%, 2.87% and 3.27% in GF, treated GF and multiscale GF, respectively. All these results indicate that apart from the enhanced specific surface area, the multiscale GF can also simultaneously increase the contents of defects and oxygen-functional groups, which are beneficial to its electrochemical performances.

To verify the electrochemical performances of the prepared electrodes, CV and EIS tests are conducted, as shown in Fig. 4. It is revealed that the GF exhibits the poor electrochemical performance with a large peak potential separation, low peak current density and high charge transfer resistance, resulting from its low specific surface area and low content of oxygen-functional groups. On the contrary, the electrochemical performances of the treated GF and multiscale GF are dramatically improved, and the multiscale GF stands out among samples studied. The peak potential separations of the treated GF are 166 and 185 mV for the negative and positive redox reactions, respectively, but they decrease to 150 and 137 mV for the multiscale GF. The CV curves of the multiscale GF at various scan rates for V2þ/V3þ and VO2þ/VOþ 2 redox reactions are shown in Fig. 4c and d, respectively, and the obvious oxidation and reduction peaks are observed at all scan rates. In addition, the peak currents are found to be linearly proportional to the square root of the scan rate, indicating that the reactions are controlled by transport in the scan rate range [34]. Fig. 4e and f exhibit the EIS results of various electrodes at 0.5 and 0.9 V vs. SCE, which demonstrates the multiscale GF has the lowest charge transfer resistance and the best electrochemical performance, consistent with the CV results.

Then, the real performance of the multiscale electrode is evaluated in a single cell. The charge-discharge curves of VRFBs with GF, treated GF and multiscale GF at the current densities of 200 and 300 mA cm 2 are shown in Fig. 5a–b. It is found that the battery with multiscale GF electrodes exhibits the highest charge-discharge performance, which is attributed to its high specific surface area and high content of oxygenfunctional groups, in coordinate with the CV and EIS results. The coulombic efficiency, voltage efficiency, average voltage, energy efficiency and electrolyte utilization of VRFBs with different electrodes are shown in Fig. 5c–f. As the same membrane is used, the coulombic

Table 1 Elemental composition and distribution of types of oxygen-functional groups on original GF, treated GF and multiscale GF. Samples

C 1s (%)

O 1s (%)

C¼O (%)

C–OH (%)

COOH (%)

H–OH (%)

GF Treated GF Multiscale GF

96.98 95.05 92.97

3.02 4.95 7.03

1.12 2.87 3.29

1.56 1.33 1.23

0.23 0.47 0.92

0.11 0.28 1.59

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Fig. 4. CV curves of GF, treated GF and multiscale GF with the potential windows of (a) 0.7 to 0.3 V vs. SCE and (b) 0.6–1.1 V vs. SCE at the scan rate of 10 mV s 1.

CV curves of the multiscale GF for (c) V2þ/V3þ and (d) VO2þ/VOþ 2 redox reactions at various scan rates. Nyquist plots of GF, treated GF and multiscale GF at the voltage of (e) 0.5 V vs. SCE and (f) 0.9 V vs. SCE.

Fig. 5. The charge-discharge curves of VRFBs with different electrodes at the current densities of (a) 200 and (b) 300 mA cm 2. The (c) coulombic and voltage efficiencies, (d) average voltage, (e) energy efficiency changes as cycle numbers, and (f) electrolyte utilization of VRFBs with different electrodes. Nafion 211 membrane is used in the tests.

efficiencies are almost the same for all cases. At all investigated current densities, the battery with multiscale GF electrodes shows the lowest average overpotentials, and the highest voltage efficiency, energy efficiency as well as electrolyte utilization. At the current density of 400 mA cm 2, the battery achieves an energy efficiency of 80.95% and an electrolyte utilization of 72.17%. Even at a high current density of 800 mA cm 2, the battery can still maintain an energy efficiency of

66.73%.

To further boost the activity of electrodes, there are generally two methods: one is creating deeper and smaller pores on the carbon fibers, and the other is depositing nanoparticles on the primary pores [35,36].

Although the first method is effective to increase the specific surface area, its real application in further enhancing the battery performance is limited because of the following factors: i) excessive etching would

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decrease the size of carbon fibers, reducing the absolute active surface area for redox reactions; ii) excessive etching would deteriorate the electrical conductivity of carbon fiber surface, thus increasing the ohmic resistance; iii) too deep or small pores are difficult to be fully utilized due to the insufficient mass transport; iv) the catalytic effect of oxygen-functional groups and edged carbon atoms is limited compared to some effective catalysts. Therefore, instead of further creating pores on carbon fibers, in this work, the specific surface area is enhanced by in-situ electrodepositing bismuth nanoparticles (BiNPs) on the surface of carbon fibers in the negative multiscale GF, as the negative electrode is responsible for ~80% voltage losses and is the limiting step for high-performance VRFB [37]. The SEM image of the electrode with BiNPs (Fig. 6a) shows that the BiNPs with a small particle size of ~50 nm are well distributed on the carbon surface, which can effectively increase the specific surface area of electrodes and thus benefit the battery performance. The XRD pattern of the prepared multiscale electrode with BiNPs is exhibited in Fig. 6b, where the well distinguished peaks confirm the existence of Bi. Meanwhile, the deposited BiNPs can also act as the active sites to promote the vanadium redox reactions by reducing the reaction energy barrier, as evidenced in Fig. 6c with the increased peak currents and reduced onset potentials. The charge-discharge curves of VRFBs with and without BiNPs at the current densities of 400 and 500 mA cm 2 are shown in Fig. 6d and e, respectively. It is clearly seen that the battery with BiNPs exhibits lower charge and discharge

overpotentials, leading to the higher charge and discharge capacities.

The corresponding average voltage and efficiencies at different current densities are displayed in Fig. 6f–i. It is shown that the battery with the multiscale GF containing BiNPs exhibits lower average charge voltage, as well as higher discharge voltage, voltage efficiency and energy efficiency.

Furthermore, the battery with BiNPs exhibit a higher coulombic efficiency than that without BiNPs, which is due to the existence of BiNPs can suppress the hydrogen evolution reaction [38]. Additionally, it is interesting to find that at the current density of 200 mA cm 2, the VRFB with BiNPs shows a lower electrolyte utilization (86.26%) than that without BiNPs (87.57%), but it surpasses its counterpart at the current densities higher than 200 mA cm 2. This phenomenon can be explained as follows. The electrodeposition process consumes part of VO2þ in the positive electrolyte, thus reducing the capacities. When the current density is low, the polarization is not that large, and the enhanced performance cannot offset this part of capacity loss. On the contrary, the polarization greatly increases as the current density increases. As a result, the enhanced capacity resulting from the reduced polarization is able to exceed the consumed capacity by electrodepositing Bi. Notably, after adopting BiNPs on multiscale GF, the assembled VRFB can achieve an energy efficiency of 83.46% and an electrolyte utilization of 75.42% at the current density of 400 mA cm 2, as well as an energy efficiency of 80.31% and an electrolyte utilization of 67.78% at the current density of 500 mA cm 2.

Fig. 6. (a) SEM image and (b) XRD pattern of multiscale GF þ BiNPs. (c) CV curves of multiscale GF in solutions with and without Bi3þ with the potential windows of 0.7 to 0.1 V vs. SCE at the scan rate of 10 mV s 1. The charge-discharge curves of VRFBs with multiscale GF and multiscale GF þ BiNPs at the current densities of (d) 400 and (e) 500 mA cm 2. The (f) coulombic and voltage efficiencies, (g) average voltage, (h) energy efficiency changes as cycle numbers, and (i) electrolyte utilization of VRFBs with multiscale GF and multiscale GF þ BiNPs. 7

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flow fields, and Fig. 7c summarizes the velocity profiles at y ¼ 10 mm. It is revealed that the interdigitated flow field enables a much higher velocity at both the under-rib and under-channel regions compared to the serpentine flow field, which is able to enhance the transport of reactants, lower the concentration loss and benefit the electrolyte utilization.

Additionally, the enhanced mass transport of Bi3þ can also promote a more uniform distribution of electrodeposited BiNPs in the porous electrodes, which effectively increases the active sites for redox reactions.

The charge-discharge curves of VRFBs with serpentine and interdigitated flow fields at the current density of 400 and 600 mA cm 2 are shown in Fig. 7d and e. It is seen that the battery with the interdigitated flow field exhibits lower charge/discharge overpotentials and higher charge/ discharge capacities. Note that the enhanced capacities are not only attributed to the reduced activation and ohmic losses, but also due to the reduced concentration loss caused by the enhanced transport properties, as evidenced by the prolonged discharge curves at the final state with the low state of charge. The coulombic and voltage efficiencies, average charge/discharge voltages and energy efficiency are shown in Fig. 7f–h.

Due to the reduced polarizations, the VRFB with the interdigitated flow field presents higher voltage efficiency, lower voltage losses and higher energy efficiency compared to that with the serpentine flow field. At the current densities of 400, 500 and 600 mA cm 2, the designed battery achieves high energy efficiencies of 85.72%, 83.12% and 80.46%,

Based on the above-mentioned designs focusing on reducing the activation and ohmic losses, we have successfully achieved an excellent charge-discharge performance in VRFBs. However, the electrolyte utilization is still relatively low, indicating that a large percentage of precious active materials are wasted. To further enhance the electrolyte utilization, the other polarization loss, i.e., concentration loss, needs to be sufficiently reduced to prolong the charge and discharge curves, especially at the final states of charge and discharge processes. The concentration loss is determined by the ion and mass transports inside the porous electrodes, which is closely related to the design of the flow field as it serves to distribute electroactive species into the electrodes.

Although serpentine flow field is used in the above designs, an issue associated with the serpentine flow field is that the under-rib convection is only forced by the small pressure drop between neighboring continuous channels, and thus the fraction of flow into the electrode is limited.

In contrast, the interdigitated flow field possesses discontinuous flow channels by placing lands between inlet and outlet channels, and all electrolytes are forced through the porous media as it is the only hydraulic connection, indicating it has great promise to further enhance the mass transport inside the porous electrode. Hence, the interdigitated flow field is adopted to further improve the battery performance, especially the electrolyte utilization. Fig. 7a and b shows the simulated velocity field in the porous electrode of VRFBs with serpentine and interdigitated

Fig. 7. Simulated velocity field in the porous electrode of VRFBs with (a) serpentine and (b) interdigitated flow fields at z ¼ 200 μm. (c) Velocity profile in the porous electrode at y ¼ 10 mm. The charge-discharge curves of VRFBs with serpentine flow field and interdigitated flow field at the current densities of (d) 400 and (e) 600 mA cm 2. The (f) coulombic and voltage efficiencies, (g) average voltage, (h) energy efficiency changes as cycle numbers and (i) electrolyte utilization of VRFBs with serpentine flow field and interdigitated flow field.

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Fig. 8. Simulated velocity field in the porous electrode of VRFBs at the flow rates of (a) 20, (b) 50 and (c) 80 mL min 1. The charge-discharge curves of VRFBs at different flow rates at the current densities of (d) 400 and (e) 600 mA cm 2. The (f) coulombic and voltage efficiencies, (g) average voltage, (h) energy efficiency changes as cycle numbers and (i) electrolyte utilization of VRFBs at different flow rates.

70.07% and 63.21%, which are 8.45%, 12.47%, 16.51%, 21.28% and 26.20% higher than that with the serpentine flow field. This high electrolyte utilization can greatly decrease the usage of precious vanadium ions, reduce the high capital cost of the system, and promote the

respectively. More remarkably, the designed battery also exhibits superior electrolyte utilizations, as shown in Fig. 7i. At the current densities of 400, 500, 600, 700 and 800 mA cm 2, the VRFB with the interdigitated flow field delivers electrolyte utilizations of 83.87%, 80.25%, 75.64%,

Fig. 9. (a) The charge-discharge curves of the designed high-performance VRFB at the current densities ranging from 200 to 1000 mA cm 2. (b) The coulombic, voltage and energy efficiencies of the designed high-performance VRFB. 9

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efficiency, average charge/discharge voltages, voltage efficiency and energy efficiency are almost the same. This unique phenomenon clearly illustrates the limitation of merely using the efficiencies to evaluate the battery performance. To be specific, the efficiencies are just the ratios between the discharge capacity/voltage/energy and charge capacity/ voltage/energy, and thus some crucial information, such as real discharge/charge capacities, are covered up. On the contrary, the electrolyte utilization in Fig. 8i reveals that the batteries operated at 50 and 80 mL min 1 have similar electrolyte utilizations, and are much larger than that operated at 20 mL min 1, which is well consistent with the charge-discharge curves. In this regard, to comprehensively evaluate the performances of VRFBs, not only the efficiencies, but also the chargedischarge curves and electrolyte utilizations, should be considered.

After the above systematical design, the charge-discharge performance of the high-performance VRFB is summarized in Fig. 9 and Table S1. Fig. 9a shows the charge-discharge curves of the designed highperformance VRFB at the current density ranging from 200 to 1000 mA cm 2, showing the battery has low polarizations and high charge-discharge capacities. Fig. 9b exhibits the corresponding coulombic, voltage and energy efficiencies of the designed VRFB. It is found that the battery achieves energy efficiencies of 91.98%, 86.45% and 80.83% at the current density of 200, 400 and 600 mA cm 2, which represents the best charge-discharge performance for VRFBs in the open literature. More impressively, the battery can also be operated at an ultrahigh current density of 1000 mA cm 2 with an energy efficiency of 70.40%. To the best of our knowledge, this is the first time for a flow battery to be efficiently operated at such a high current density, demonstrating a dramatic performance enhancement for VRFBs. Another exciting finding is that the battery can maintain high electrolyte utilizations when operated at high current densities. Specifically, at the current densities of 200, 300, 400, 500, 600, 700, 800 and 1000 mA cm 2, the battery displays high electrolyte utilizations of 87.97%, 87.55%, 85.21%, 81.60%, 76.98%, 71.13%, 64.06% and 46.08%, respectively, which can greatly reduce the high capital cost of VRFBs. Moreover, we also test the VRFB performance at a higher vanadium concentration of 1.5 M, as shown in Fig. S3. It is seen that the designed VRFB can still achieve a voltage efficiency of 81.25% and an electrolyte utilization of 77.75% at a high current density of 600 mA cm 2, which is comparable with those at 1.1 M vanadium concentration, demonstrating the concentration of electrolyte is not the origin for the high performance achieved in this work. The polarization curve and power density of the designed high-performance VRFB are then tested at room temperature (25 C) and shown in Fig. 10. Strikingly, the battery is capable of delivering a high limiting current density of ~7 A cm 2, and a high peak power density of 2.78 W cm 2, representing the highest peak power density for flow batteries in the open literature, which is even higher than that of commercialized fuel cells. Another

Fig. 10. The polarization curves and power densities of the designed highperformance VRFB at room temperature.

widespread commercialization of VRFBs. In addition to the flow field design, the mass and ion transports are also decided by the flow rate of electrolyte. The high flow rate can increase the pressure drop between neighboring channels, and therefore enhance the under-rib convection of electrolyte. Moreover, it can also maintain the high concentration gradient between channels and porous electrodes, thus strengthening the diffusion of reactants. Therefore, the effect of flow rate on the battery performance is further investigated, as shown in Fig. 8. Fig. 8a–c exhibit the simulated velocity field in the porous electrodes at various flow rates, which clearly shows that with the increased flow rate, the electrolyte achieves enhanced velocity in the porous electrodes, leading to more sufficient reactants for redox reactions. Fig. 8d and e present the charge-discharge curves of VRFBs under the flow rate of 20, 50 and 80 mL min 1 at the current densities of 400 and 600 mA cm 2. It is found that when increasing the flow rate from 20 to 50 mL min 1, the discharge and charge overpotentials are decreased while the capacities are increased. This is because increasing the flow rate not only enhances the mass and ion transports to reduce the concentration loss, but also decreases the activation loss by increasing the local concentration of reactants, as evidenced by the Butler-Volmer equation. However, when further increasing the flow rate from 50 to 80 mL min 1, almost no change is observed in the charge-discharge curves, indicating the mass and ion transports are no longer the limiting step when the flow rate is higher than 50 mL min 1. Fig. 8f–h shows the coulombic, voltage and energy efficiencies as well as the average voltages of VRFBs at different flow rates. It is interesting to find that although the batteries operated at the flow rates of 50 and 80 mL min 1 show obvious better performances than that operated at 20 mL min 1 in the charge-discharge curves, the calculated coulombic

Fig. 11. The long-term cycling performance of the designed high-performance VRFB at the current density of 600 mA cm 2.

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Fig. 12. The comparison of (a) charge-discharge performance and (b) cycling performance of the designed high-performance VRFB with previous works.

important finding is that at the present stage, the activation polarization has been minimized to a low value and the ohmic polarization dominates the voltage loss. Therefore, future efforts should be taken not only to reduce the contact resistance between neighboring components, but also to judiciously tailor the ion and mass transports in the porous electrode to furtuer reduce the ohmic polarization and enhance the battery performance. In addition to those, the performance of VRFBs is also decided by the cycling stability, especially considering they are developed for longterm and large-scale energy storage. Therefore, a long-term cycling test is further conducted for the rationally designed high-performance VRFBs, as exhibited in Fig. 11 and S4. Excitingly, the battery is able to be stably operated for more than 20,000 cycles (about 8 months) at a high current density of 600 mA cm 2, demonstrating the excellent long-term cycling stability. Fig. 12a and b compare the charge-discharge performance and cycling performance reported here with those in the open literature [16, 23,36,39–48], and both of them demonstrate the unprecedented performance achieved in this work.

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4. Conclusion To conclude, the strategies to boost the performance of VRFBs and methods to evaluate the battery performance are discussed in details in this work. Our results show that the VRFB can achieve an energy efficiency of 80.83% and an electrolyte utilization of 76.98% at a high current density of 600 mA cm 2, as well as deliver a high peak power density of 2.78 W cm 2 and a limiting current density of ~7 A cm 2 at room temperature. Additionally, the battery can be stably cycled for more than 20,000 cycles without obvious decay at 600 mA cm 2, demonstrating the excellent cycling stability. To the best our knowledge, the performances reported here stand for the best charge-discharge performance, the highest power density and the longest cycle life for flow batteries. We believe our results can light the ways to fabricate highperformance VRFBs, which would benefit both the academic and industrial fields.

Acknowledgements The work described in this paper was fully supported by a grant from the Research Grant Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.ensm.2019.07.005.

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