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Holey-engineered electrodes for advanced vanadium flow batteries
Author’s Accepted Manuscript Holey-engineered electrodes vanadium flow batteries
for
advanced
Yuchen Liu, Yi Shen, Lihong Yu, Le Liu, Feng Liang, Xinping Qiu, Jingyu Xi www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30691-2 https://doi.org/10.1016/j.nanoen.2017.11.012 NANOEN2313
To appear in: Nano Energy Received date: 21 July 2017 Revised date: 17 October 2017 Accepted date: 6 November 2017 Cite this article as: Yuchen Liu, Yi Shen, Lihong Yu, Le Liu, Feng Liang, Xinping Qiu and Jingyu Xi, Holey-engineered electrodes for advanced vanadium flow batteries, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.11.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Holey-engineered electrodes for advanced vanadium flow batteries
Yuchen Liua,b, Yi Shenc, Lihong Yud, Le Liua, Feng Liangb,** , Xinping Qiue,**, Jingyu Xia,*
a
Institute of Green Chemistry and Energy, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China b
The State Key Laboratory for Refractories and Metallurgy, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China c
School of Food Science and Engineering, South China University of Technology, Guangzhou 510640, China d
School of Applied Chemistry and Biological Technology, Shenzhen Polytechnic, Shenzhen 518055, China
e
Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China
* Corresponding author. ** Co-corresponding authors. E-mail address: [email protected] (J. Xi), [email protected] (X. Qiu), [email protected] (F. Liang)
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Abstract Vanadium flow battery (VFB) has received tremendous attention because of its advantages such as long lifespan, easy to scale and flexible operation. Fabricating novel electrodes with high power density and wide operating temperature is critical to promote the practical application of VFB for all-climate energy storage. In this work, we describe a well-controlled method to prepare holey-engineered porous graphite felt (PGF) electrodes, in which nanosized pores are evenly distributed on the microscale graphite fibers of the graphite felt. Owing to its excellent electrolyte wettability and greatly enhanced surface area, the as-prepared PGF electrode exhibits high electrochemical activity towards VO2+/VO2+ and V2+/V3+ redox couples. As a result, the VFB single cell assembled with PGF electrodes demonstrates outstanding rate performance under current density up to 300 mA cm-2. The resulting PGF electrode also exhibits superior long-term stability over 3000 charging-discharging cycles at a high current density of 150 mA cm-2, and wide temperature adaptability from -20 oC to 60 oC.
Keywords: vanadium flow battery; holey-engineered electrode; graphite felt; high-power-density; wide temperature
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1. Introduction Due to energy consumption and global environmental issues, it is very important to explore renewable energy [1]. However, environmentally sustainable sources of energy, such as wind and solar energy, are subjected to intermittence and fluctuation in electricity generation [2]. To address the issue of intermittency, large-scale electrical energy storage (EES) systems are necessary [3,4]. Among various large-scale EES systems, redox flow batteries (RFBs) with the attractive features of uncoupled power output and energy capacity, long cycle life, high safety, high efficiency and environmental friendliness are attractive for economically viable EES applications [5,6]. In particular, among RFBs, vanadium flow battery (VFB) capitalizes on the same metal element of four different oxidation states to operate positive and negative half-cells. Notably, this system can diminish cross-contamination of the active components [7,8]. Electrodes, which act as the place of the vanadium ions redox reaction, play a significant role in VFB. However, the catalytic activity of the electrodes such as graphite felts (GF) and carbon felts (CF) is far from desirable, to which the physical and chemical modification has been conducted. At present, the GF is a typical electrode applied in VFB due to its low cost, high stability, corrosion resistance and wide range of operating potential [9]. However, its poor catalytic activity and low specific surface area restrict the further application. Up to now, there are usually two categories to modify the GF. One is to deposit metals or metal oxides on the surface of the electrode material as catalysts, such as Ir, Au, Pt, Nb2O5, CeO2 and ZrO2, etc [10-16]. Nevertheless the noble metals are expensive and susceptible to hydrogen evolution, so that they are not practical. Additionally, low-cost WO3 and other metal oxides as catalysts have low conductivity. The other modification method is the introduction of carbon-based electrocatalysts. Recently, various carbon-based materials with good electronic conductivity and large specific surface areas have been reported such as graphene, carbon nanotubes, carbon dots and carbon nanowalls [17-22]. But the VFB single cell performance seems to be limited when they are operated at high charge and discharge current density. Moreover, some intrinsic treatments have been taken to improve the electrochemical activity of GF, for example thermal treatment [23], acid treatment [24] and electrochemical oxidation [25]. In general, the electrochemical activity of the pretreatment GF is still rather low, resulting in low energy efficiency and power density. Although some achievements have been made in boosting catalytic activity of GF-based electrodes, the specific surface area of GF is still insufficient. Besides, it is a great challenge to keep the nanomaterials decorated on GF stable during the long-term operation of VFB. Hence it is meaningful to modify the GF 3
electrode itself with excellent performance in VFB. To date, it has been reported that porous GF-based materials obtained by thermal etching or KOH etching are applied on lithium ion batteries [26], supercapacitors [27,28], and VFB [29-31], due to the improved specific surface area and durability of GF. Herein, we demonstrate a simple and cost-effective method to prepare holey-engineered GF (PGF) electrodes for high-power-density VFB application, as shown in Scheme 1. The size and depth of pores are tailored by varying the precursor solution concentration of the hydrothermal reaction and the time of the thermal reduction reaction. The optimized PGF has a specific surface area of 10.2 m2 g -1, two orders of magnitude higher than that of pristine GF (0.4 m2 g-1). The porous morphology of PGF has greatly enlarged surface area and improved wettability, so they can provide more active sites for electrochemical reaction of vanadium redox couples and reduce concentration polarization during mass transport. The PGF electrode shows highly enhanced electrochemical activity and reversibility in redox reactions and stability during the operation of VFB at high current densities on all-climate conditions.
Scheme 1. Schematic illustration of the PGF preparation procedure and the corresponding SEM images for each step.
2. Experimental Section Materials GF (5 mm of thickness) was purchased from Gansu Haoshi Carbon Fiber Co., Ltd. The GF was washed with ethanol, deionized water, and thermally treated at 420 oC for 10 h in air [16], then the thermal activated GF (TGF) was obtained. Other chemicals were of analytical grade and used without further
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purification. Preparation of PGF The preparation process of PGF is shown in Scheme 1. Briefly, FeOOH nanorods were firstly grown on the TGF by a previously reported hydrothermal method [32,33]. Then, the obtained TGF-FeOOH was converted to TGF-Fe3O4 by annealing in N2 gas at 900 oC with a heating rate of 5 oC min-1. Finally, the holey-engineered PGF was obtained by dissolving the Fe3O4 nanoparticles with concentrated HCl, and subsequently thermal activated in air at 420 oC for 10 h. Characterization Scanning electron microscopy (SEM) images were obtained by a field emission scanning electron microscope (ZEISS SUPRA®55) at 5 kV, and energy dispersive X-ray spectroscopy (EDX) was recorded at an acceleration voltage of 20 kV. Brunauer-Emmett-Teller (BET) surface area was measured by gas adsorption analyzer (BelSorp Max, ASAP 2020). Wide-angle XRD patterns were measured on a Bruker D8 Advance using Cu Kα radiation (40 kV, 40 mA, 10o min-1 from 10 to 70o). Raman spectra were recorded by a microconfocal Raman spectrometer (iHR320, Horiba) with a 532 nm laser excitation. The wetting property of the samples was analyzed by contact angle test (JC2000D1, Shanghai ZhongChen) which measured the contact angle by fitting a tangent to the three-phase point at room temperature. Electrochemical measurement Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted on a PARSTST 2273 electrochemical workstation using a three-electrode system [34]. A graphite plate and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. A solution of 0.1 M VO2+ in 2 M H2SO4 was used for positive tests and a solution of 0.1 M V3+ in 2 M H2SO4 was used for negative tests. EIS measurements were carried out at a polarization voltage of 0.75 V for the positive reaction and -0.4 V for the negative reaction, respectively, with an excitation signal of 5 mV over the frequency range from 100 kHz to 10 mHz. VFB single cell test The assembly of a VFB single cell was reported previously [35,36]. The size of electrode was 5 cm × 5 cm, and the compression ratio was controlled at 20%. The Nafion 115 membrane (7 cm × 7 cm) was boiled
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in 1 M H2SO4 solution for 1 h and then in deionized water for 1 h before being used as separator, and the thickness of the boiled Nafion 115 membrane is about 161 μm [37]. The electrolyte in both positive and negative half-cells was 50 mL of 1.5 M V3.5+ in 2 M free H2SO4 with a flow rate of 60 mL min-1. Rate performance of VFB was evaluated at current densities from 50 to 300 mA cm-2 under the voltage window of 0.8-1.65 V, while lifespan test was conducted at current density of 150 mA cm-2. Polarization curve was measured as described in Fig. S1. Wide temperature (-20–60 oC) test was performed at current density of 100 mA cm-2 via a thermostat (PU-80, Guangdong Hongzhan), as shown in Fig. S2. All the cell tests were carried out on a CT-3008 battery testing system (Neware Co., Ltd).
3. Results and Discussion 3.1 Characterization of PGFs Thermally activated GF (TGF) was used to prepare PGF via a three-step approach (Scheme 1). Firstly, through a simple hydrothermal process, vertical akaganeite nanorods (FeOOH NRs) uniformly grew on the surface of TGF (TGF-FeOOH NRs), as confirmed by X-ray diffraction analysis in Fig. S3 and the SEM image in Scheme 1b. Then, the as-prepared TGF-FeOOH NRs were annealed in a N2 atomosphere to obtain magnetite nanoparticles decorated TGF (TGF-Fe3O4 NPs). XRD analysis also confirms that akaganeite transforms into magnetite (Fig. S3). At this stage, the thermal reduction process etched the nanoparticles into the interior of the TGF, thereby enabling the formation of porous TGF surface. A SEM image of the TGF-Fe3O4 NPs shows that uniform nanoparticles are decorated in the inner and surface of the TGF (Scheme 1c). The reaction occurring in the annealing process was proposed as follows [26]: FeOOH Fe2O3 Fe2O3 + C Fe3O4+CO Finally, TGF-Fe3O4 NPs was immersed into concentrated HCl for removing of Fe3O4 at room temperature. The XRD pattern of the obtained PGF is the same as that of the TGF (Fig. S3), which demonstrates that Fe3O4 NPs have been totally removed. As shown in Scheme 1d, the surface of PGF is rough and pores are uniformly distributed with a diameter of 100-150 nm. To clarify the elemental composition of the samples, energy dispersive X-ray spectroscopy (EDX) elemental mappings were collected from the samples (Fig. S4a-d). In Fig. S4a, the surface of TGF is smooth and mainly contains two elements C and O. After hydrothermal process, the sample is mainly composed of three elements C, O and Fe with uniform distribution of vertical akaganeite in the carbon
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matrix (Fig. S4b), and the weight of the sample (50×50 mm) increases by 48 mg (Fig. S4e). Comparing the SEM images of the TGF-FeOOH NRs and TGF-Fe3O4 NPs, the mass load of the grown nanorods is reduced after the thermal reduction in the N2 atmosphere. The subsequent acid treatment made Fe3O4 NPs eliminate completely. Therefore the product is porous with high specific surface area (Fig. S4d) and the weight is reduced (Fig. S4e). The porosity and surface morphology of the PGF are adjusted by several crucial parameters, such as hydrothermal reaction time, thermal reduction reaction time and the concentration of the precursor. Firstly, hydrothermal reaction with 6 h is of benefit to obtain well-arranged holes on the surface of TGF (Fig. S5). Electrochemical measurements are carried out to ascertain the best reaction time. The sample with hydrothermal reaction time of 6 h shows the best activity (Fig. S6). Secondly, to study the effects of heating time in the pores, we change the thermal reduction reaction time. As shown in Fig. S7, under 3 h-thermal reduction reaction, pores on the surface of PGF are dense and uniform. And the PGF-3h sample exhibits the best electrochemical performance (Fig. S8). Hence, the 6 h-hydrothermal reaction and 3 h-thermal reduction reaction are the best preparation conditions. Since treatment in high temperatures reduces the oxygen functional groups on the surface of TGF [29], the samples were thermally activated at 420 oC for 10 h in air atmosphere. Comparing the morphology of noactivated porous GF (Fig. S9) with that of PGF (Fig. S7), it is believed that thermal activation will not change the morphology and structure of samples. The oxygen functional groups on the PGF surface may enhance its hydrophilicity. To investigate the electrolyte accessibility of samples, the PGF, TGF and GF were put into the positive and negative electrolytes to observe the wettability. As shown in Fig. S10a, PGF and TGF immediately sink to the bottom of the electrolyte, while GF floats on the surface of the electrolyte. For electrolyte dripping test, 10 μL of electrolyte droplet can be absorbed immediately by PGF and TGF, while the droplet cannot penetrate into the pristine GF (Fig. S10b). In the water contact angle test (Fig. S10c), water droplet is absorbed immediately by PGF and TGF, while the contact angle of GF is 149o. These results indicate that PGF has excellent wettability. Allowing for the effects of different concentrations of precursor on the pores, we chose three different concentrations of precursor to produce PGF. The SEM images show that reducing the concentration of precursor, the FeOOH NRs coated on the TGF become sparse and smaller (Fig. 1a). While increasing the concentration of precursor, the Fe3O4 NPs are partly embedded in the graphite fibers and the sizes of nanoparticles are bigger (Fig. 1b). It is worth mentioning that the obtained pores decorate uniformly on the 7
surface of PGF in all samples and the size of them become smaller as the concentration of the precursor solution decreases (Fig. 1c). This result corresponds to the size of the preceding Fe3O4 NPs, indicating that the size of pores can be well controlled. The three PGFs obtained from high (0.15 M FeCl3 + 1 M NaNO3), middle (0.05 M FeCl3 + 0.25 M NaNO3), and low (0.01 M FeCl3 + 0.05 M NaNO3) concentrations of precursor are denoted as H-PGF, M-PGF, and L-PGF, respectively. The BET surface areas of H-PGF, M-PGF and L-PGF are 10.2 m2 g-1, 29.5 m2 g-1 and 21.0 m2 g-1, respectively. Raman spectroscopy shows that there are no obvious differences in the D and G band of the three samples and the intensity ratios of the D and G band are almost the same (Fig. S11). Such results affirm that the graphite structure is not changed.
Fig. 1. SEM images of (a) TGF-FeOOH NRs, (b) TGF-Fe3O4NPs, (c) PGF obtained by different concentrations of precursor.
3.2 Electrochemical Evaluation of PGFs The electrochemical activity of the electrode is evaluated by a sample three-electrode cell [34]. Fig. 2a and b shows cyclic voltammetry (CV) curves of different PGF electrodes in positive and negative
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electrolyte. Typical CV curves of the VO2+/VO2+ redox couple are shown in Fig. 2a, the PGFs exhibit significantly improved redox onset potentials and current densities with comparison to the TGF. The peak current densities (Ip) of three PGFs are higher than that of TGF. More importantly, the peak potential separation (∆E) value in TGF (230 mV) has been decreased to 124 mV in H-PGF. Although the three PGFs show pores of various sizes (Fig. 1c), their CV curves have little difference. Ip, ∆E and redox onset potential can be used to estimate the catalytic activity of electrode materials [38]. Hence, the PGFs have better electrochemical activity than that of TGF, due to the increased electrochemical surface area which provides more active sites for VO2+/VO2+ redox reaction. The CV curves of the V3+/V2+ redox couple show a similar trend to those of the positive redox reactions (Fig. 2b). Obviously, the Ip of three PGFs are much higher than those of TGF, and the redox onset potentials and ∆E values of the PGFs are smaller than those of TGF, implying considerably improved catalytic activity of the PGFs. Among three PGFs, the H-PGF shows the best electrochemical activity. Additionally, we varied the scan rate of CV test from 1 to 10 mV s-1 to assess the mass transfer property of H-PGF and TGF (Fig. 2c and d). Redox current densities of two samples gradually increase when the scan rate is increased. It is found that higher current densities and smaller peak potential separation are produced by H-PGF at all scan rates. As shown in Fig. 2e, the Ip versus the square root of scan rate from the Randles-Sevcik equation was obtained [39,40]. The slopes of the H-PGF show higher values than those of the TGF in oxidation and reduction reactions, suggesting that a faster mass transfer process is achieved on the porous electrode. At the same time, the -Ipc/Ipa values are also gained at each scan rate in order to check the reversibility of the reaction (Fig. 2f). The -Ipc/Ipa values of TGF are about 0.6, which proves an unstable vanadium redox reaction with poor reversibility. However, the values of H-PGF are close to 1.0, which implies a reversible vanadium redox reaction occurring on the H-PGF. These results confirm that the surface with abundant pores provides more active sites for vanadium ions reaction and leads to high electrochemical activity.
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Fig. 2. CV curves of TGF and PGFs at a scan rate of 1 mV s-1 in (a) positive electrolyte and (b) negative electrolyte. CV curves of (c) H-PGF and (d) TGF at different scan rates in positive electrolyte. (e) Plots of the redox peak current density versus the square root of scan rate for PGF and TGF. (f) –Ipc/Ipa values. Nyquist plots of TGF and PGFs in (g) positive reaction and (h) negative reaction.
To further verify the enhanced electrochemical activity of the PGFs, electrochemical impedance spectroscopy (EIS) was conducted to measure the charge transfer resistance (Rct) during the redox reactions. All the EIS plots consist of a semicircle in the high-frequency region and a straight line in the low-frequency region (Fig. 2g and h). The high frequency intercept of the semicircle on the real axis is the bulk resistance (Rb), the diameter of the semicircle is the charge transfer resistance (Rct), and the slope of the straight line is the resistance of diffusion during the reaction process [41-43]. As for the positive reaction, the Rct value for the TGF is 24.1 Ω, which is much larger than those for PGFs (about 2.3 to 2.9 Ω, inset in Fig. 2g). As for the negative reaction, the Rct values for the PGFs are apparently smaller than that of the TGF (Fig. 2h), suggesting the redox reactions can be accelerated obviously on the PGFs since surface porous structure provides more active sites. The above results agrees well with the CV test as shown in Fig. 2a and b.
3.3 VFB Single Cell Performance The cycling performance of PGF electrodes was evaluated by using a VFB single cell. Nyquist plots of VFBs with PGF and TGF electrodes are shown in Fig. 3a and b, respectively. As can be seen from Fig. 3a, 10
the Rct values of the VFBs with PGF electrodes are from 0.8 Ω to 1.7 Ω. However, when using TGF as the electrode, the Rct is 10.5 Ω as showed in Fig. 3b, which is much larger than those of the PGFs. Additionally, the Rb values of VFBs with three PGF electrodes are around 32.4 mΩ，which are smaller than that of TGF electrode (38.2 mΩ), demonstrating outstanding wettability between vanadium electrolyte and PGF electrodes. The charge and discharge voltage profiles for the four electrodes are compared at a current density of 150 mA cm-2. For the three PGF electrodes, the voltages of charging branch are nearly the same, lower than that of TGF. The result observed for the discharging branch is quite contrary to the charging branch (Fig. 3c). It can be concluded the redox reactions on the PGF electrodes sustain much smaller electrochemical polarization during the cell operation [44-46]. Furthermore, within the same operation voltage window (0.8-1.65V), larger capacity is obtained from the VFBs with PGFs. The rate capability test of the VFBs with TGF and PGF electrodes was performed by gradually increasing current densities from 50 to 300 mA cm-2 (Fig. 3d-f). The Coulombic efficiency (CE) keeps the same value under the same current density among different VFBs due to the identical membranes and electrolytes. The value of CE increases with increasing current density, because the vanadium crossover time will be reduced by increasing current density [47]. The voltage efficiency (VE) varied greatly among electrodes, indicating that the modified electrode plays a key role in increasing the value of VE for VFB. At a current density of 300 mA cm-2, it is difficult to test the performance of the TGF electrode due to the significant increase in overpotentials. As shown in Fig. 3d, the VFB with H-PGF shows the highest VE value of 66.2% at 250 mA cm-2, which is 9.5% higher than that of TGF, and the VE of the VFB with M-PGF and L-PGF are 64.2% and 65.8%, respectively. This difference is caused by the different surface (pore) morphology of electrode with different size pores (Fig. 1), which leads to different surface areas appropriate for redox reaction. The energy efficiency (EE =VE×CE) value of VFBs with PGF electrode is much higher than that with TGF electrode at all tested current densities (Fig. 3e), and the EE value of VFB with H-PGF is 57.3% at the current density of 300 mA cm-2, which is higher than presently reported works (Fig. S12) [9,16]. On the other hand, the VFBs with PGFs deliver higher discharge capacities than that with TGF (Fig. 3f). As is well-known, the TGF contains a large number of oxygen-containing surface functional groups, which can improve the electrolyte wettability and the electrochemical activity of TGF simultaneously [48,49]. Increasing the specific surface area of the electrode can effectively increase the reaction active sites. Therefore, the holey-engineered PGFs demonstrate much better rate performance than that of TGF because of greatly enhanced surface area. According to previous report, the flow permeability though the porous 11
electrode with various pore sizes is different [50]. Although the specific surface area of the H-PGF is the smallest among the three PGFs, the H-PGF shows the best rate performance, which can be attributed to the tradeoff between the specific surface area and the porosity of the graphite felt electrode [51,52]. Considering that the H-PGF exhibits the best performance based on above CV, EIS, and single cell results, we choose this electrode to conduct comprehensive VFB evaluation for comparison with the TGF electrode.
Fig. 3. Nyquist plots of single cell: (a) PGF electrodes and (b) TGF electrode. (c) Charge-discharge curves of VFBs at current density of 150 mA cm-2. Rate performance of VFBs at different current densities: (d) CE, VE; (e) EE; (f) Discharge capacity.
Polarization curve is a powerful technology to evaluate the electrode activity in VFB [31]. The photograph of the VFB single cell used for polarization curve test is shown in Fig. S1, and the experiment is conducted at current density range from 0 to 1100 mA cm-2. As shown in Fig. 4, the VFB assembled with
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PGF electrode displays higher voltage than that with TGF electrode at all current densities, indicating smaller polarization of PGF electrode. Moreover, the PGF electrode demonstrates higher peak power density (712 mW cm-2 @ 925 mA cm-2) than that of TGF electrode (629 mW cm-2 @ 875 mA cm-2), which can lead to much lower material consumption for a VFB stack (i.e. reduce the total cost of the system).
Fig. 4. Polarization curves and power density curves of VFBs assembled with TGF and PGF electrodes.
The durability test of various electrodes was carried out at a high current density of 150 mA cm-2. As illustrated in Fig. 5a, it is clear that the CE (~95%) and EE (~75%) of VFB with PGF electrode are stable over 500 cycles, indicating the excellent stability of PGF electrode. On the contrary, the EE of VFB with TGF electrode decays gradually, especially after 350 cycles. After 500 cycles test, the positive and negative electrolytes of the VFB using PGF electrode were refreshed, and then another 500 cycles test was performed, which was repeated for five times. The super-long cycling performance of PGF based VFB, including six rounds of 500 cycles’ testing, is shown in Fig. 5b. The CE remains very stable at 95% during 3000 cycles (~1560 h), while the EE only shows a 3% attenuation. After each round of testing, EIS measurement was carried out to monitor the change of Rct and the Rb of the VFB (Fig. 5c). It is obvious that with the increase of the cycling round, the Rct increases, but the gap is getting smaller. The slightly increasing of Rct is probably due to the precipitation of V2O5 in the positive side [53]. However, the Rct is only 3.1 Ω after six rounds of testing, which is much lower than that of fresh TGF electrode (Fig. 3b). On the other hand, the Rb keeps relatively stable during 3000 cycles testing, revealing that the electron conductivity of PGF remains well. This further confirms the outstanding durability of the PGF electrode towards practical VFB application.
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Fig. 5. Long-term cycling performance of VFBs with PGF and TGF electrodes: (a) CE and EE at the current density of 150 mA cm-2, (b) super-long lifespan evaluation of PGF electrode, (c) Nyquist plots of VFB with PGF electrode after different rounds of testing.
Since the energy storage system is subjected to the constraints of climate and the environment, it is also a challenge to ensure the VFB operate normally at wide temperatures [43,54]. In order to further study the temperature impact on PGF electrode, the VFBs were evaluated at the temperatures range from -20 oC to 60 oC in a homemade all-climate research platform (Fig. S2) [55]. The EIS test was first performed for the VFBs at various temperatures, as shown in Fig. 6a-b. As the temperature rises, the electrolyte conductivity, electrochemical activity and diffusion property increase. Therefore, the Rct and Rb values of all VFBs decrease with increasing temperature because of the reduced electrochemical polarization [55]. Besides, the Rct and Rb of the PGF based VFB is much smaller than that of TGF based VFB at all temperatures. Then, all VFBs were ran for 10 cycles at each temperature point from -20 oC to 60 oC under current density of 100 mA cm-2. As shown in Fig. 6c, both VFBs show nearly the same CE at all temperatures due to the identical Nafion 115 membrane used. In addition, the CE decreases gradually with increasing temperature because of the accelerated vanadium ion crossover [47]. The EE of VFB with PGF electrode gradually increases with temperature, and reaches a maximum value of around 80% at 20-40 oC. The trend of EE with temperature can be explained by the combined effect of CE (decreasing with increasing temperature) and VE (rising with increasing temperature). Moreover, the EE of VFB with PGF electrode is much more stable and higher than that of TGF electrode at each temperature. After operating at 60 oC, the cell test
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comes back to 20 oC, and the EE of PGF based VFB returns to its original value, demonstrating its superior wide temperature adaptability. On the contrary, the EE of TGF based VFB reduces compared to its initial value, the poor electrochemical activity leads to more side reactions and consequently creates more deposition of vanadium pentoxide on the surface of the electrodes at high temperatures [55,56], as shown in Fig. S13. So the activity of the TGF electrode can’t be recovered and this indicates its poor stability during wide temperature operation. Consequently, the VFB with PGF electrode exhibits extremely superior cell performance during the whole testing temperature, indicating perfect stability and electrochemical activity of PGF at all-climate condition.
Fig. 6. All-climate performance of VFBs with PGF and TGF electrodes: (a) Nyquist plots of PGF based VFB, (b) Nyquist plots of TGF based VFB, (c) CE and EE at the current density of 100 mA cm-2.
4. Conclusions In summary, we have successfully prepared GF electrodes with a holey-engineered structure via a chemical etching approach. The sizes and depths of pores in carbon fibers can be readily tuned by varying reaction time and concentration of the precursor. This process is simple and does not involve any sophisticated instrument, which is beneficial to the scalable production. The generation of large number of 15
uniform pores significantly increases the specific surface areas of the electrode, which enhances its wettability of the electrolyte and provides more active sites for the reaction of vanadium ions, thereby resulting in better electrochemical activity and kinetic reversibility. The VFB assembled with the PGF electrodes exhibits increased discharge capacities and EE values under high current densities. The performance of VFB with PGF electrodes can retain for 3000 cycles without obvious decay under high current densities. The electrochemical activity of the PGF electrode is highly stable over a temperature range of -20 to 60 oC. The strategy disclosed in study could be generalized to other carbon-based electrodes. It could provide a new route to fabricate various electrodes which hold great promise for applications in energy storage and conversion.
Acknowledgements The authors appreciate financial supports from the National Natural Science Foundation of China (No. 21576154) and the Basic Research Project of Shenzhen City (Nos. JCYJ20170412170756603, JCYJ20170307152754218 and JCYJ20150630114140630).
Appendix A. Supplementary material Supplementary data associated with this article can be found in online version.
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Yuchen Liu received her B.S. degree from Wuhan University of Science and Technology. She is now pursuing the M.S. degree in Chemical Engineering and Technology at Wuhan University of Science and Technology under supervision of Prof. Feng Liang. She is also united training in the group of Prof. Jingyu Xi at Tsinghua University. Her research is focused on novel electrode materials of vanadium flow battery.
Yi Shen is currently an associate professor at the South China University of Technology (China). He obtained his Ph. D. degree from the Nanyang Technology University in 2012 and worked at the same institute as a Research Follow from 2012 to 2013. His research interests are electrocatalysts for fuel cells, hydrogen production and membrane separation.
Lihong Yu is an assistant professor in School of Applied Chemistry and Biological Technology, Shenzhen Polytechnic (China). She received her Ph.D in Physical Chemistry from The University of HongKong. She used to do research about the charge transfer dynamics under photoexcitation reacitions both in The University of HongKong and University of Rochester. Yu’s current research is focused on all-climate vanadium flow batteries for energy storage.
Le Liu is an assistant professor in Graduate School at Shenzhen, Tsinghua University (China). He received his B.S., M.S. and Ph.D. degrees from the department of physics, Tsinghua University (China), in 2003, 2006 and 2010. Then he worked as a postdoc at Tsinghua University from 2011 to 2012. His current research is focused on in situ detection of chemical reactions.
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Feng Liang earned his Ph.D. in Organic Chemistry at Wuhan University in 2003. He took his first faculty position at Wuhan University in 2004. He moved to Arizona State University (USA) in 2006, where he was an assistant research scientist at the BioDesign Institute. In 2012, he joined Wuhan University of Science and Technology, where he is currently a university professor and executive deputy dean of Institute of Advanced Materials & Nanotechnology (IAMN). His current research is focused on preparation and applications of gold and carbon nanomaterials. Xinping Qiu is a professor in Department of Chemistry, Tsinghua University (China). He is now the vice director of Clean Vehicle Consortium of US-China Clean Energy Research Center (CERC-CVC). He holds a B.S. and a Ph.D. from University of Science and Technology Beijing in 1988 and 1994, respectively. His research interests focused on the advanced power sources, such as lithium ion batteries, fuel cells and flow batteries. The main directions include new electrode materials for lithium ion battery, new catalysts for fuel cell, polymer electrolytes, electrode kinetics, the structure of porous electrode and new techniques for battery characterization. Jingyu Xi is an associate professor in Graduate School at Shenzhen, Tsinghua University (China). He received B.S. from Wuhan University (1998), M.S. from Lanzhou Institute of Chemical Physics (2011), Ph.D. from Shanghai Jiao Tong University (2004), and worked as a postdoc at Tsinghua University from 2005 to 2006. His current research is focused on electrochemical energy storage and conversion systems including flow batteries, fuel cells, and all solid-state lithium batteries.
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Graphical abstract
Highlights
Holey-engineered porous graphite felts (PGF) with tunable pore sizes are prepared.
The PGF electrodes exhibit improved wettability and enhanced activity towards vanadium electrolyte.
The PGF electrodes show excellent rate performance and outstanding durability in VFB.
The optimized PGF electrode demonstrates a maximum power density of 712 mW cm-2.
The PGF electrodes can operate in a wide temperature range of -20–60 oC.
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for
advanced
Yuchen Liu, Yi Shen, Lihong Yu, Le Liu, Feng Liang, Xinping Qiu, Jingyu Xi www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30691-2 https://doi.org/10.1016/j.nanoen.2017.11.012 NANOEN2313
To appear in: Nano Energy Received date: 21 July 2017 Revised date: 17 October 2017 Accepted date: 6 November 2017 Cite this article as: Yuchen Liu, Yi Shen, Lihong Yu, Le Liu, Feng Liang, Xinping Qiu and Jingyu Xi, Holey-engineered electrodes for advanced vanadium flow batteries, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.11.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Holey-engineered electrodes for advanced vanadium flow batteries
Yuchen Liua,b, Yi Shenc, Lihong Yud, Le Liua, Feng Liangb,** , Xinping Qiue,**, Jingyu Xia,*
a
Institute of Green Chemistry and Energy, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China b
The State Key Laboratory for Refractories and Metallurgy, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China c
School of Food Science and Engineering, South China University of Technology, Guangzhou 510640, China d
School of Applied Chemistry and Biological Technology, Shenzhen Polytechnic, Shenzhen 518055, China
e
Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China
* Corresponding author. ** Co-corresponding authors. E-mail address: [email protected] (J. Xi), [email protected] (X. Qiu), [email protected] (F. Liang)
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Abstract Vanadium flow battery (VFB) has received tremendous attention because of its advantages such as long lifespan, easy to scale and flexible operation. Fabricating novel electrodes with high power density and wide operating temperature is critical to promote the practical application of VFB for all-climate energy storage. In this work, we describe a well-controlled method to prepare holey-engineered porous graphite felt (PGF) electrodes, in which nanosized pores are evenly distributed on the microscale graphite fibers of the graphite felt. Owing to its excellent electrolyte wettability and greatly enhanced surface area, the as-prepared PGF electrode exhibits high electrochemical activity towards VO2+/VO2+ and V2+/V3+ redox couples. As a result, the VFB single cell assembled with PGF electrodes demonstrates outstanding rate performance under current density up to 300 mA cm-2. The resulting PGF electrode also exhibits superior long-term stability over 3000 charging-discharging cycles at a high current density of 150 mA cm-2, and wide temperature adaptability from -20 oC to 60 oC.
Keywords: vanadium flow battery; holey-engineered electrode; graphite felt; high-power-density; wide temperature
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1. Introduction Due to energy consumption and global environmental issues, it is very important to explore renewable energy [1]. However, environmentally sustainable sources of energy, such as wind and solar energy, are subjected to intermittence and fluctuation in electricity generation [2]. To address the issue of intermittency, large-scale electrical energy storage (EES) systems are necessary [3,4]. Among various large-scale EES systems, redox flow batteries (RFBs) with the attractive features of uncoupled power output and energy capacity, long cycle life, high safety, high efficiency and environmental friendliness are attractive for economically viable EES applications [5,6]. In particular, among RFBs, vanadium flow battery (VFB) capitalizes on the same metal element of four different oxidation states to operate positive and negative half-cells. Notably, this system can diminish cross-contamination of the active components [7,8]. Electrodes, which act as the place of the vanadium ions redox reaction, play a significant role in VFB. However, the catalytic activity of the electrodes such as graphite felts (GF) and carbon felts (CF) is far from desirable, to which the physical and chemical modification has been conducted. At present, the GF is a typical electrode applied in VFB due to its low cost, high stability, corrosion resistance and wide range of operating potential [9]. However, its poor catalytic activity and low specific surface area restrict the further application. Up to now, there are usually two categories to modify the GF. One is to deposit metals or metal oxides on the surface of the electrode material as catalysts, such as Ir, Au, Pt, Nb2O5, CeO2 and ZrO2, etc [10-16]. Nevertheless the noble metals are expensive and susceptible to hydrogen evolution, so that they are not practical. Additionally, low-cost WO3 and other metal oxides as catalysts have low conductivity. The other modification method is the introduction of carbon-based electrocatalysts. Recently, various carbon-based materials with good electronic conductivity and large specific surface areas have been reported such as graphene, carbon nanotubes, carbon dots and carbon nanowalls [17-22]. But the VFB single cell performance seems to be limited when they are operated at high charge and discharge current density. Moreover, some intrinsic treatments have been taken to improve the electrochemical activity of GF, for example thermal treatment [23], acid treatment [24] and electrochemical oxidation [25]. In general, the electrochemical activity of the pretreatment GF is still rather low, resulting in low energy efficiency and power density. Although some achievements have been made in boosting catalytic activity of GF-based electrodes, the specific surface area of GF is still insufficient. Besides, it is a great challenge to keep the nanomaterials decorated on GF stable during the long-term operation of VFB. Hence it is meaningful to modify the GF 3
electrode itself with excellent performance in VFB. To date, it has been reported that porous GF-based materials obtained by thermal etching or KOH etching are applied on lithium ion batteries [26], supercapacitors [27,28], and VFB [29-31], due to the improved specific surface area and durability of GF. Herein, we demonstrate a simple and cost-effective method to prepare holey-engineered GF (PGF) electrodes for high-power-density VFB application, as shown in Scheme 1. The size and depth of pores are tailored by varying the precursor solution concentration of the hydrothermal reaction and the time of the thermal reduction reaction. The optimized PGF has a specific surface area of 10.2 m2 g -1, two orders of magnitude higher than that of pristine GF (0.4 m2 g-1). The porous morphology of PGF has greatly enlarged surface area and improved wettability, so they can provide more active sites for electrochemical reaction of vanadium redox couples and reduce concentration polarization during mass transport. The PGF electrode shows highly enhanced electrochemical activity and reversibility in redox reactions and stability during the operation of VFB at high current densities on all-climate conditions.
Scheme 1. Schematic illustration of the PGF preparation procedure and the corresponding SEM images for each step.
2. Experimental Section Materials GF (5 mm of thickness) was purchased from Gansu Haoshi Carbon Fiber Co., Ltd. The GF was washed with ethanol, deionized water, and thermally treated at 420 oC for 10 h in air [16], then the thermal activated GF (TGF) was obtained. Other chemicals were of analytical grade and used without further
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purification. Preparation of PGF The preparation process of PGF is shown in Scheme 1. Briefly, FeOOH nanorods were firstly grown on the TGF by a previously reported hydrothermal method [32,33]. Then, the obtained TGF-FeOOH was converted to TGF-Fe3O4 by annealing in N2 gas at 900 oC with a heating rate of 5 oC min-1. Finally, the holey-engineered PGF was obtained by dissolving the Fe3O4 nanoparticles with concentrated HCl, and subsequently thermal activated in air at 420 oC for 10 h. Characterization Scanning electron microscopy (SEM) images were obtained by a field emission scanning electron microscope (ZEISS SUPRA®55) at 5 kV, and energy dispersive X-ray spectroscopy (EDX) was recorded at an acceleration voltage of 20 kV. Brunauer-Emmett-Teller (BET) surface area was measured by gas adsorption analyzer (BelSorp Max, ASAP 2020). Wide-angle XRD patterns were measured on a Bruker D8 Advance using Cu Kα radiation (40 kV, 40 mA, 10o min-1 from 10 to 70o). Raman spectra were recorded by a microconfocal Raman spectrometer (iHR320, Horiba) with a 532 nm laser excitation. The wetting property of the samples was analyzed by contact angle test (JC2000D1, Shanghai ZhongChen) which measured the contact angle by fitting a tangent to the three-phase point at room temperature. Electrochemical measurement Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted on a PARSTST 2273 electrochemical workstation using a three-electrode system [34]. A graphite plate and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. A solution of 0.1 M VO2+ in 2 M H2SO4 was used for positive tests and a solution of 0.1 M V3+ in 2 M H2SO4 was used for negative tests. EIS measurements were carried out at a polarization voltage of 0.75 V for the positive reaction and -0.4 V for the negative reaction, respectively, with an excitation signal of 5 mV over the frequency range from 100 kHz to 10 mHz. VFB single cell test The assembly of a VFB single cell was reported previously [35,36]. The size of electrode was 5 cm × 5 cm, and the compression ratio was controlled at 20%. The Nafion 115 membrane (7 cm × 7 cm) was boiled
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in 1 M H2SO4 solution for 1 h and then in deionized water for 1 h before being used as separator, and the thickness of the boiled Nafion 115 membrane is about 161 μm [37]. The electrolyte in both positive and negative half-cells was 50 mL of 1.5 M V3.5+ in 2 M free H2SO4 with a flow rate of 60 mL min-1. Rate performance of VFB was evaluated at current densities from 50 to 300 mA cm-2 under the voltage window of 0.8-1.65 V, while lifespan test was conducted at current density of 150 mA cm-2. Polarization curve was measured as described in Fig. S1. Wide temperature (-20–60 oC) test was performed at current density of 100 mA cm-2 via a thermostat (PU-80, Guangdong Hongzhan), as shown in Fig. S2. All the cell tests were carried out on a CT-3008 battery testing system (Neware Co., Ltd).
3. Results and Discussion 3.1 Characterization of PGFs Thermally activated GF (TGF) was used to prepare PGF via a three-step approach (Scheme 1). Firstly, through a simple hydrothermal process, vertical akaganeite nanorods (FeOOH NRs) uniformly grew on the surface of TGF (TGF-FeOOH NRs), as confirmed by X-ray diffraction analysis in Fig. S3 and the SEM image in Scheme 1b. Then, the as-prepared TGF-FeOOH NRs were annealed in a N2 atomosphere to obtain magnetite nanoparticles decorated TGF (TGF-Fe3O4 NPs). XRD analysis also confirms that akaganeite transforms into magnetite (Fig. S3). At this stage, the thermal reduction process etched the nanoparticles into the interior of the TGF, thereby enabling the formation of porous TGF surface. A SEM image of the TGF-Fe3O4 NPs shows that uniform nanoparticles are decorated in the inner and surface of the TGF (Scheme 1c). The reaction occurring in the annealing process was proposed as follows [26]: FeOOH Fe2O3 Fe2O3 + C Fe3O4+CO Finally, TGF-Fe3O4 NPs was immersed into concentrated HCl for removing of Fe3O4 at room temperature. The XRD pattern of the obtained PGF is the same as that of the TGF (Fig. S3), which demonstrates that Fe3O4 NPs have been totally removed. As shown in Scheme 1d, the surface of PGF is rough and pores are uniformly distributed with a diameter of 100-150 nm. To clarify the elemental composition of the samples, energy dispersive X-ray spectroscopy (EDX) elemental mappings were collected from the samples (Fig. S4a-d). In Fig. S4a, the surface of TGF is smooth and mainly contains two elements C and O. After hydrothermal process, the sample is mainly composed of three elements C, O and Fe with uniform distribution of vertical akaganeite in the carbon
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matrix (Fig. S4b), and the weight of the sample (50×50 mm) increases by 48 mg (Fig. S4e). Comparing the SEM images of the TGF-FeOOH NRs and TGF-Fe3O4 NPs, the mass load of the grown nanorods is reduced after the thermal reduction in the N2 atmosphere. The subsequent acid treatment made Fe3O4 NPs eliminate completely. Therefore the product is porous with high specific surface area (Fig. S4d) and the weight is reduced (Fig. S4e). The porosity and surface morphology of the PGF are adjusted by several crucial parameters, such as hydrothermal reaction time, thermal reduction reaction time and the concentration of the precursor. Firstly, hydrothermal reaction with 6 h is of benefit to obtain well-arranged holes on the surface of TGF (Fig. S5). Electrochemical measurements are carried out to ascertain the best reaction time. The sample with hydrothermal reaction time of 6 h shows the best activity (Fig. S6). Secondly, to study the effects of heating time in the pores, we change the thermal reduction reaction time. As shown in Fig. S7, under 3 h-thermal reduction reaction, pores on the surface of PGF are dense and uniform. And the PGF-3h sample exhibits the best electrochemical performance (Fig. S8). Hence, the 6 h-hydrothermal reaction and 3 h-thermal reduction reaction are the best preparation conditions. Since treatment in high temperatures reduces the oxygen functional groups on the surface of TGF [29], the samples were thermally activated at 420 oC for 10 h in air atmosphere. Comparing the morphology of noactivated porous GF (Fig. S9) with that of PGF (Fig. S7), it is believed that thermal activation will not change the morphology and structure of samples. The oxygen functional groups on the PGF surface may enhance its hydrophilicity. To investigate the electrolyte accessibility of samples, the PGF, TGF and GF were put into the positive and negative electrolytes to observe the wettability. As shown in Fig. S10a, PGF and TGF immediately sink to the bottom of the electrolyte, while GF floats on the surface of the electrolyte. For electrolyte dripping test, 10 μL of electrolyte droplet can be absorbed immediately by PGF and TGF, while the droplet cannot penetrate into the pristine GF (Fig. S10b). In the water contact angle test (Fig. S10c), water droplet is absorbed immediately by PGF and TGF, while the contact angle of GF is 149o. These results indicate that PGF has excellent wettability. Allowing for the effects of different concentrations of precursor on the pores, we chose three different concentrations of precursor to produce PGF. The SEM images show that reducing the concentration of precursor, the FeOOH NRs coated on the TGF become sparse and smaller (Fig. 1a). While increasing the concentration of precursor, the Fe3O4 NPs are partly embedded in the graphite fibers and the sizes of nanoparticles are bigger (Fig. 1b). It is worth mentioning that the obtained pores decorate uniformly on the 7
surface of PGF in all samples and the size of them become smaller as the concentration of the precursor solution decreases (Fig. 1c). This result corresponds to the size of the preceding Fe3O4 NPs, indicating that the size of pores can be well controlled. The three PGFs obtained from high (0.15 M FeCl3 + 1 M NaNO3), middle (0.05 M FeCl3 + 0.25 M NaNO3), and low (0.01 M FeCl3 + 0.05 M NaNO3) concentrations of precursor are denoted as H-PGF, M-PGF, and L-PGF, respectively. The BET surface areas of H-PGF, M-PGF and L-PGF are 10.2 m2 g-1, 29.5 m2 g-1 and 21.0 m2 g-1, respectively. Raman spectroscopy shows that there are no obvious differences in the D and G band of the three samples and the intensity ratios of the D and G band are almost the same (Fig. S11). Such results affirm that the graphite structure is not changed.
Fig. 1. SEM images of (a) TGF-FeOOH NRs, (b) TGF-Fe3O4NPs, (c) PGF obtained by different concentrations of precursor.
3.2 Electrochemical Evaluation of PGFs The electrochemical activity of the electrode is evaluated by a sample three-electrode cell [34]. Fig. 2a and b shows cyclic voltammetry (CV) curves of different PGF electrodes in positive and negative
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electrolyte. Typical CV curves of the VO2+/VO2+ redox couple are shown in Fig. 2a, the PGFs exhibit significantly improved redox onset potentials and current densities with comparison to the TGF. The peak current densities (Ip) of three PGFs are higher than that of TGF. More importantly, the peak potential separation (∆E) value in TGF (230 mV) has been decreased to 124 mV in H-PGF. Although the three PGFs show pores of various sizes (Fig. 1c), their CV curves have little difference. Ip, ∆E and redox onset potential can be used to estimate the catalytic activity of electrode materials [38]. Hence, the PGFs have better electrochemical activity than that of TGF, due to the increased electrochemical surface area which provides more active sites for VO2+/VO2+ redox reaction. The CV curves of the V3+/V2+ redox couple show a similar trend to those of the positive redox reactions (Fig. 2b). Obviously, the Ip of three PGFs are much higher than those of TGF, and the redox onset potentials and ∆E values of the PGFs are smaller than those of TGF, implying considerably improved catalytic activity of the PGFs. Among three PGFs, the H-PGF shows the best electrochemical activity. Additionally, we varied the scan rate of CV test from 1 to 10 mV s-1 to assess the mass transfer property of H-PGF and TGF (Fig. 2c and d). Redox current densities of two samples gradually increase when the scan rate is increased. It is found that higher current densities and smaller peak potential separation are produced by H-PGF at all scan rates. As shown in Fig. 2e, the Ip versus the square root of scan rate from the Randles-Sevcik equation was obtained [39,40]. The slopes of the H-PGF show higher values than those of the TGF in oxidation and reduction reactions, suggesting that a faster mass transfer process is achieved on the porous electrode. At the same time, the -Ipc/Ipa values are also gained at each scan rate in order to check the reversibility of the reaction (Fig. 2f). The -Ipc/Ipa values of TGF are about 0.6, which proves an unstable vanadium redox reaction with poor reversibility. However, the values of H-PGF are close to 1.0, which implies a reversible vanadium redox reaction occurring on the H-PGF. These results confirm that the surface with abundant pores provides more active sites for vanadium ions reaction and leads to high electrochemical activity.
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Fig. 2. CV curves of TGF and PGFs at a scan rate of 1 mV s-1 in (a) positive electrolyte and (b) negative electrolyte. CV curves of (c) H-PGF and (d) TGF at different scan rates in positive electrolyte. (e) Plots of the redox peak current density versus the square root of scan rate for PGF and TGF. (f) –Ipc/Ipa values. Nyquist plots of TGF and PGFs in (g) positive reaction and (h) negative reaction.
To further verify the enhanced electrochemical activity of the PGFs, electrochemical impedance spectroscopy (EIS) was conducted to measure the charge transfer resistance (Rct) during the redox reactions. All the EIS plots consist of a semicircle in the high-frequency region and a straight line in the low-frequency region (Fig. 2g and h). The high frequency intercept of the semicircle on the real axis is the bulk resistance (Rb), the diameter of the semicircle is the charge transfer resistance (Rct), and the slope of the straight line is the resistance of diffusion during the reaction process [41-43]. As for the positive reaction, the Rct value for the TGF is 24.1 Ω, which is much larger than those for PGFs (about 2.3 to 2.9 Ω, inset in Fig. 2g). As for the negative reaction, the Rct values for the PGFs are apparently smaller than that of the TGF (Fig. 2h), suggesting the redox reactions can be accelerated obviously on the PGFs since surface porous structure provides more active sites. The above results agrees well with the CV test as shown in Fig. 2a and b.
3.3 VFB Single Cell Performance The cycling performance of PGF electrodes was evaluated by using a VFB single cell. Nyquist plots of VFBs with PGF and TGF electrodes are shown in Fig. 3a and b, respectively. As can be seen from Fig. 3a, 10
the Rct values of the VFBs with PGF electrodes are from 0.8 Ω to 1.7 Ω. However, when using TGF as the electrode, the Rct is 10.5 Ω as showed in Fig. 3b, which is much larger than those of the PGFs. Additionally, the Rb values of VFBs with three PGF electrodes are around 32.4 mΩ，which are smaller than that of TGF electrode (38.2 mΩ), demonstrating outstanding wettability between vanadium electrolyte and PGF electrodes. The charge and discharge voltage profiles for the four electrodes are compared at a current density of 150 mA cm-2. For the three PGF electrodes, the voltages of charging branch are nearly the same, lower than that of TGF. The result observed for the discharging branch is quite contrary to the charging branch (Fig. 3c). It can be concluded the redox reactions on the PGF electrodes sustain much smaller electrochemical polarization during the cell operation [44-46]. Furthermore, within the same operation voltage window (0.8-1.65V), larger capacity is obtained from the VFBs with PGFs. The rate capability test of the VFBs with TGF and PGF electrodes was performed by gradually increasing current densities from 50 to 300 mA cm-2 (Fig. 3d-f). The Coulombic efficiency (CE) keeps the same value under the same current density among different VFBs due to the identical membranes and electrolytes. The value of CE increases with increasing current density, because the vanadium crossover time will be reduced by increasing current density [47]. The voltage efficiency (VE) varied greatly among electrodes, indicating that the modified electrode plays a key role in increasing the value of VE for VFB. At a current density of 300 mA cm-2, it is difficult to test the performance of the TGF electrode due to the significant increase in overpotentials. As shown in Fig. 3d, the VFB with H-PGF shows the highest VE value of 66.2% at 250 mA cm-2, which is 9.5% higher than that of TGF, and the VE of the VFB with M-PGF and L-PGF are 64.2% and 65.8%, respectively. This difference is caused by the different surface (pore) morphology of electrode with different size pores (Fig. 1), which leads to different surface areas appropriate for redox reaction. The energy efficiency (EE =VE×CE) value of VFBs with PGF electrode is much higher than that with TGF electrode at all tested current densities (Fig. 3e), and the EE value of VFB with H-PGF is 57.3% at the current density of 300 mA cm-2, which is higher than presently reported works (Fig. S12) [9,16]. On the other hand, the VFBs with PGFs deliver higher discharge capacities than that with TGF (Fig. 3f). As is well-known, the TGF contains a large number of oxygen-containing surface functional groups, which can improve the electrolyte wettability and the electrochemical activity of TGF simultaneously [48,49]. Increasing the specific surface area of the electrode can effectively increase the reaction active sites. Therefore, the holey-engineered PGFs demonstrate much better rate performance than that of TGF because of greatly enhanced surface area. According to previous report, the flow permeability though the porous 11
electrode with various pore sizes is different [50]. Although the specific surface area of the H-PGF is the smallest among the three PGFs, the H-PGF shows the best rate performance, which can be attributed to the tradeoff between the specific surface area and the porosity of the graphite felt electrode [51,52]. Considering that the H-PGF exhibits the best performance based on above CV, EIS, and single cell results, we choose this electrode to conduct comprehensive VFB evaluation for comparison with the TGF electrode.
Fig. 3. Nyquist plots of single cell: (a) PGF electrodes and (b) TGF electrode. (c) Charge-discharge curves of VFBs at current density of 150 mA cm-2. Rate performance of VFBs at different current densities: (d) CE, VE; (e) EE; (f) Discharge capacity.
Polarization curve is a powerful technology to evaluate the electrode activity in VFB [31]. The photograph of the VFB single cell used for polarization curve test is shown in Fig. S1, and the experiment is conducted at current density range from 0 to 1100 mA cm-2. As shown in Fig. 4, the VFB assembled with
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PGF electrode displays higher voltage than that with TGF electrode at all current densities, indicating smaller polarization of PGF electrode. Moreover, the PGF electrode demonstrates higher peak power density (712 mW cm-2 @ 925 mA cm-2) than that of TGF electrode (629 mW cm-2 @ 875 mA cm-2), which can lead to much lower material consumption for a VFB stack (i.e. reduce the total cost of the system).
Fig. 4. Polarization curves and power density curves of VFBs assembled with TGF and PGF electrodes.
The durability test of various electrodes was carried out at a high current density of 150 mA cm-2. As illustrated in Fig. 5a, it is clear that the CE (~95%) and EE (~75%) of VFB with PGF electrode are stable over 500 cycles, indicating the excellent stability of PGF electrode. On the contrary, the EE of VFB with TGF electrode decays gradually, especially after 350 cycles. After 500 cycles test, the positive and negative electrolytes of the VFB using PGF electrode were refreshed, and then another 500 cycles test was performed, which was repeated for five times. The super-long cycling performance of PGF based VFB, including six rounds of 500 cycles’ testing, is shown in Fig. 5b. The CE remains very stable at 95% during 3000 cycles (~1560 h), while the EE only shows a 3% attenuation. After each round of testing, EIS measurement was carried out to monitor the change of Rct and the Rb of the VFB (Fig. 5c). It is obvious that with the increase of the cycling round, the Rct increases, but the gap is getting smaller. The slightly increasing of Rct is probably due to the precipitation of V2O5 in the positive side [53]. However, the Rct is only 3.1 Ω after six rounds of testing, which is much lower than that of fresh TGF electrode (Fig. 3b). On the other hand, the Rb keeps relatively stable during 3000 cycles testing, revealing that the electron conductivity of PGF remains well. This further confirms the outstanding durability of the PGF electrode towards practical VFB application.
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Fig. 5. Long-term cycling performance of VFBs with PGF and TGF electrodes: (a) CE and EE at the current density of 150 mA cm-2, (b) super-long lifespan evaluation of PGF electrode, (c) Nyquist plots of VFB with PGF electrode after different rounds of testing.
Since the energy storage system is subjected to the constraints of climate and the environment, it is also a challenge to ensure the VFB operate normally at wide temperatures [43,54]. In order to further study the temperature impact on PGF electrode, the VFBs were evaluated at the temperatures range from -20 oC to 60 oC in a homemade all-climate research platform (Fig. S2) [55]. The EIS test was first performed for the VFBs at various temperatures, as shown in Fig. 6a-b. As the temperature rises, the electrolyte conductivity, electrochemical activity and diffusion property increase. Therefore, the Rct and Rb values of all VFBs decrease with increasing temperature because of the reduced electrochemical polarization [55]. Besides, the Rct and Rb of the PGF based VFB is much smaller than that of TGF based VFB at all temperatures. Then, all VFBs were ran for 10 cycles at each temperature point from -20 oC to 60 oC under current density of 100 mA cm-2. As shown in Fig. 6c, both VFBs show nearly the same CE at all temperatures due to the identical Nafion 115 membrane used. In addition, the CE decreases gradually with increasing temperature because of the accelerated vanadium ion crossover [47]. The EE of VFB with PGF electrode gradually increases with temperature, and reaches a maximum value of around 80% at 20-40 oC. The trend of EE with temperature can be explained by the combined effect of CE (decreasing with increasing temperature) and VE (rising with increasing temperature). Moreover, the EE of VFB with PGF electrode is much more stable and higher than that of TGF electrode at each temperature. After operating at 60 oC, the cell test
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comes back to 20 oC, and the EE of PGF based VFB returns to its original value, demonstrating its superior wide temperature adaptability. On the contrary, the EE of TGF based VFB reduces compared to its initial value, the poor electrochemical activity leads to more side reactions and consequently creates more deposition of vanadium pentoxide on the surface of the electrodes at high temperatures [55,56], as shown in Fig. S13. So the activity of the TGF electrode can’t be recovered and this indicates its poor stability during wide temperature operation. Consequently, the VFB with PGF electrode exhibits extremely superior cell performance during the whole testing temperature, indicating perfect stability and electrochemical activity of PGF at all-climate condition.
Fig. 6. All-climate performance of VFBs with PGF and TGF electrodes: (a) Nyquist plots of PGF based VFB, (b) Nyquist plots of TGF based VFB, (c) CE and EE at the current density of 100 mA cm-2.
4. Conclusions In summary, we have successfully prepared GF electrodes with a holey-engineered structure via a chemical etching approach. The sizes and depths of pores in carbon fibers can be readily tuned by varying reaction time and concentration of the precursor. This process is simple and does not involve any sophisticated instrument, which is beneficial to the scalable production. The generation of large number of 15
uniform pores significantly increases the specific surface areas of the electrode, which enhances its wettability of the electrolyte and provides more active sites for the reaction of vanadium ions, thereby resulting in better electrochemical activity and kinetic reversibility. The VFB assembled with the PGF electrodes exhibits increased discharge capacities and EE values under high current densities. The performance of VFB with PGF electrodes can retain for 3000 cycles without obvious decay under high current densities. The electrochemical activity of the PGF electrode is highly stable over a temperature range of -20 to 60 oC. The strategy disclosed in study could be generalized to other carbon-based electrodes. It could provide a new route to fabricate various electrodes which hold great promise for applications in energy storage and conversion.
Acknowledgements The authors appreciate financial supports from the National Natural Science Foundation of China (No. 21576154) and the Basic Research Project of Shenzhen City (Nos. JCYJ20170412170756603, JCYJ20170307152754218 and JCYJ20150630114140630).
Appendix A. Supplementary material Supplementary data associated with this article can be found in online version.
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Yuchen Liu received her B.S. degree from Wuhan University of Science and Technology. She is now pursuing the M.S. degree in Chemical Engineering and Technology at Wuhan University of Science and Technology under supervision of Prof. Feng Liang. She is also united training in the group of Prof. Jingyu Xi at Tsinghua University. Her research is focused on novel electrode materials of vanadium flow battery.
Yi Shen is currently an associate professor at the South China University of Technology (China). He obtained his Ph. D. degree from the Nanyang Technology University in 2012 and worked at the same institute as a Research Follow from 2012 to 2013. His research interests are electrocatalysts for fuel cells, hydrogen production and membrane separation.
Lihong Yu is an assistant professor in School of Applied Chemistry and Biological Technology, Shenzhen Polytechnic (China). She received her Ph.D in Physical Chemistry from The University of HongKong. She used to do research about the charge transfer dynamics under photoexcitation reacitions both in The University of HongKong and University of Rochester. Yu’s current research is focused on all-climate vanadium flow batteries for energy storage.
Le Liu is an assistant professor in Graduate School at Shenzhen, Tsinghua University (China). He received his B.S., M.S. and Ph.D. degrees from the department of physics, Tsinghua University (China), in 2003, 2006 and 2010. Then he worked as a postdoc at Tsinghua University from 2011 to 2012. His current research is focused on in situ detection of chemical reactions.
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Feng Liang earned his Ph.D. in Organic Chemistry at Wuhan University in 2003. He took his first faculty position at Wuhan University in 2004. He moved to Arizona State University (USA) in 2006, where he was an assistant research scientist at the BioDesign Institute. In 2012, he joined Wuhan University of Science and Technology, where he is currently a university professor and executive deputy dean of Institute of Advanced Materials & Nanotechnology (IAMN). His current research is focused on preparation and applications of gold and carbon nanomaterials. Xinping Qiu is a professor in Department of Chemistry, Tsinghua University (China). He is now the vice director of Clean Vehicle Consortium of US-China Clean Energy Research Center (CERC-CVC). He holds a B.S. and a Ph.D. from University of Science and Technology Beijing in 1988 and 1994, respectively. His research interests focused on the advanced power sources, such as lithium ion batteries, fuel cells and flow batteries. The main directions include new electrode materials for lithium ion battery, new catalysts for fuel cell, polymer electrolytes, electrode kinetics, the structure of porous electrode and new techniques for battery characterization. Jingyu Xi is an associate professor in Graduate School at Shenzhen, Tsinghua University (China). He received B.S. from Wuhan University (1998), M.S. from Lanzhou Institute of Chemical Physics (2011), Ph.D. from Shanghai Jiao Tong University (2004), and worked as a postdoc at Tsinghua University from 2005 to 2006. His current research is focused on electrochemical energy storage and conversion systems including flow batteries, fuel cells, and all solid-state lithium batteries.
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Graphical abstract
Highlights
Holey-engineered porous graphite felts (PGF) with tunable pore sizes are prepared.
The PGF electrodes exhibit improved wettability and enhanced activity towards vanadium electrolyte.
The PGF electrodes show excellent rate performance and outstanding durability in VFB.
The optimized PGF electrode demonstrates a maximum power density of 712 mW cm-2.
The PGF electrodes can operate in a wide temperature range of -20–60 oC.
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