Research Article | Open Access
Xuechun Lou, Hu Fu, Jian Xu, Yong Long, Su Yan, Haitao Zou, Bo Lu, Murong He, Mei Ding, Xiaobo Zhu, Chuankun Jia, "Cost-Effective Membrane and Advanced Electrode for Stable Polysulfide-Ferricyanide Flow Battery", Energy Material Advances, vol. 2022, Article ID 9865618, 11 pages, 2022. https://doi.org/10.34133/2022/9865618
Cost-Effective Membrane and Advanced Electrode for Stable Polysulfide-Ferricyanide Flow Battery
Based on inexpensive, safe, and environmentally friendly active redox species, neutral polysulfide-ferrocyanide redox flow batteries (PFRFBs) have attracted much attention for large-scale energy storage. However, the development of PFRFBs is undermined by the expensive commercial membrane materials as well as the sluggish polysulfide redox reactions. This work attempts to solve these critical problems by combining the economical membrane with the highly catalytic electrode. In specific, K+-exchanged sulfonated polyether ether ketone (SPEEK-K) membranes have been investigated in PFRFBs to replace the costly Nafion membrane. SPEEK-K with optimized degree of sulfonation enables the PFRFB high average coulombic efficiency of 99.80% and superior energy efficiency of 90.42% at a current density of 20 mA cm-2. Meanwhile, to overcome the kinetic limitations of polysulfide redox reactions, a CuS-modified carbon felt electrode is demonstrated with excellent catalytic performance, enabling the PFRFB higher and more stable energy efficiency over cycling. The combination of the cost-effective membrane with the catalytic electrode in one cell leads to a capacity retention of 99.54% after 1180 cycles and an outstanding power density (up to 223 mW cm-2). The significant enhancements of electrochemical performance at reduced capital cost will make the PFRFB more promising for large-scale energy storage systems.
The increasing severity of climate change is pushing a global energy transition from fossil fuels to renewable energy sources such as solar and wind. Due to the intermittent nature of these renewables, energy storage technologies play a critical role in integrating them into the grid. To facilitate the energy transition, the energy storage technologies are required to be efficient, safe, and affordable [1–3]. Owing to merits of design flexibility, deep-discharge capability, high efficiency, long cycle life, etc., redox flow batteries (RFBs) stand as attractive candidates for sizeable energy storage [4–6]. Currently, all-vanadium RFBs represent the most successful ones, which have been successfully demonstrated from the 1980s [7–9]. However, the high cost of the vanadium, low energy density (25 Wh/L), and strong acidity of the electrolytes hinder the widespread deployment of all-vanadium RFBs. Recently, organic RFBs based on organic redox-active species instead of metal elementals from mineral resources can potentially reduce the capital cost [10, 11]. However, the design of redox-active organic materials with desirable electrochemical performances remains challenging . To this end, many efforts have been directed to the investigations of new RFBs to achieve low cost [13–15], high energy density [16–19], low capacitance decay rate [20–22], and long lifetime [23–29].
Among emerging RFBs, neutral polysulfide-ferricyanide RFBs (PFRFBs) have attracted much attention because both the redox species of ferri/ferrocyanide and polysulfide are made from abundant, inexpensive, and environmentally benign raw materials . In fact, the ferri/ferrocyanide couple features well-defined redox behaviors and fast electron transfer kinetics  that has been extensively used in pairing with other electrochemically active reactants, such as quinone, anthraquinone, and benzoquinone-based anolytes [32–35]. On the other hand, the polysulfide that originated from cheap and abundant sulfur is also a promising redox-active material, which has been demonstrated for lithium-sulfur batteries [36–38] and polysulfide redox flow batteries [39–41]. In our previous work, a PFRFB was demonstrated using high concentrated ferricyanide and polysulfide as reactants, leading to a further improvement of the energy density. And the chemical cost of PFRFBs can achieve only 32.47 $/kWh when considering the expenses on redox and supporting materials . However, the polysulfide redox reactions, which are well known for the slow reaction kinetics, possess difficulties to match with the ferri/ferrocyanide cathodic side, resulting in unfavorable cell performance. Besides, although the redox materials in PFRFB system are inexpensive, the significant cost of around 800.00 $/m2  for commercial Nafion membrane is still hindering its market penetration .
Herein, we aim to address the two critical problems in PFRFBs by combining membrane and electrode optimizations. First, to replace the costly Nafion membrane, low-cost sulfonated polyether ether ketone (SPEEK) membranes (approximately 21.89 $/m2) [44–46] have been K+-exchanged for use in PFRFBs. By controlling the degree of sulfonation (DS), the K+-exchanged SPEEK (SPEEK-K) membrane shows balanced conductivity and ferricyanide permeability. Second, taking advantage of the catalytic effects of metal sulfides toward the polysulfide redox reactions [47–49], carbon felt (CF) deposited with CuS nanosheets (CuS-CF) has been investigated as the anodic electrode, which exhibits excellent charge transfer and catalytic properties for polysulfide redox reactions. The combination of the cost-effective membrane with the catalytic electrode in the PFRFB leads to excellent capacity retention of 99.54% as well as an outstanding power density. The scheme of the membrane and electrode designs is presented in Figure 1.
2. Materials and Methods
Polyether ether ketone (PEEK, 450PF) was purchased from Victrex. Sulfuric acid (H2SO4, 98 wt.%), dimethyl sulfoxide (DMSO), potassium ferricyanide (K3[Fe(CN)6]), potassium polysulfide (K2Sx, ≥40% K2S basis, was determined as 2), potassium chloride (KCl), copper sulfate pentahydrate (CuSO4·5H2O), and thiourea (CH4N2S) were ordered from Sinopharm Chemical Reagent Co. Ltd. All of them were used as received without any treatment. Nafion 212 membrane (N212) was supplied by Ion Power Inc., USA.
2.2. Preparation of SPEEK-K and N212-K Membranes
According to our previous work [44, 45], the DS of SPEEK was controlled to be about 47.0%, 57.0%, and 67.0%, respectively, which were measured by the titration method . For the preparation of SPEEK membranes, 1.60 g SPEEK was dissolved in 50.0 mL dimethyl sulfoxide (DMSO) under stirring at 80°C. The resulted solution was cast onto a homemade glass plate and dried in the oven at 100°C for overnight. After cooling down, the membrane was peeled off. The as-prepared SPEEK membrane and purchased N212 were pretreated before use. Prior to use, the membranes were heated in 1.0 M KOH solution at 80°C for 1 h followed by washing away residual KOH with deionized water at room temperature. The resulting K+-exchanged membrane was soaked in deionized water and ready to use. Based on the different DS of SPEEK, the membranes were named DS47-K, DS57-K, and DS67-K, respectively. The commercial N212 was treated following the same procedures and named N212-K as the reference.
2.3. Preparation of CuS-CF Electrode
The CuS-CF electrode was prepared according to a typical and one-step hydrothermal procedure. Prior to the deposition of CuS, CF was heated at 400°C for 12 h under air atmosphere to improve its wettability. Then, 0.61 g CuSO4·5H2O and 3.05 g CH4N2S were dissolved in 50.0 mL deionized water. After stirring for over 30 min, the resulted solution was transferred into a 100.0 mL Teflon-lined autoclave. Subsequently, a piece of heat-treated CF () was soaked in the solution. Then, the reactor was kept at 180°C for 4 h in an oven. Finally, the CuS-CF was collected, washed with deionized water, and dried at 80°C for 10 h before use.
2.4. Membrane Characterizations
A scanning electron microscope (SEM, Zeiss EVO 10) equipped with an energy-dispersive spectrometer (EDS) was used to detect the morphologies and element distributions of membranes. The surface morphologies of N212, SPEEK, N212-K, and SPEEK-K membranes were obtained by an atomic force microscope (AFM, Bruker Nano Inc. Dimension Icon). Other physicochemical properties were recorded according to reported methods, and details were presented as follows.
2.4.1. Ion Exchange Capacity (IEC)
IEC and DS were measured by typical chemical titration methods according to earlier studies [46, 51]. The membrane was soaked in a saturated NaCl solution for 48 h to ensure that all H+ of the membrane were liberated into the solution. After that, the solution was titrated with 0.01 M NaOH solution. IEC was calculated by the following:
and are the concentration and consumed volume of NaOH solution, respectively. represents the weight of dry membrane.
2.4.2. Swelling Ratio (SR)
By testing the length of wet and dry membrane, respectively, SR of membranes were calculated by Equation (2). Note that wet membranes refer to the membranes which were soaked in deionized water at room temperature for 48 h. All of them were dried in an oven at 80°C for overnight before measuring the length of dry membranes.
and are the length of wet membrane and dry membrane, respectively.
2.4.3. Area Resistance (AR) and Conductivity ()
The AR of membranes was measured by a resistance tester (DME-20, China) after the cell was circulated for 5 min in which the positive and negative reservoirs were both filled with 20.0 mL solution of 2.0 M KCl. The conductivity () of membranes was obtained according to
and refer to the thickness and working area (13.5 cm2) of the membrane, respectively. and are the cell’s resistance with and without the membrane, respectively.
2.4.4. Ion Permeability and Ion Selectivity
To verify the electrochemical active ion permeation through membranes, the H-shaped device separated by a piece of membrane (Figure S1) was utilized to measure the permeability of [Fe(CN)6]3-. The left compartment was filled with 0.1 M K3[Fe(CN)6] in 1.0 M KCl, and the right compartment was filled with 0.1 M CaCl2 in 1.0 M KCl. A UV-vis spectrometer (TU-1900) was used to monitor the concentration variations of [Fe(CN)6]3-. Samples were taken out from the CaCl2 compartment at an interval of 12 h. After each measurement, the sample was poured back into the same reservoir to keep the solution volume invariant. The ion selectivity is defined as the ratio of the K+ conductivity to the permeability of [Fe(CN)6]3- ions through membranes.
2.5. Electrode Characterizations
The surface morphologies of the electrode were observed by SEM (Zeiss EVO 10). The cyclic voltammetry (CV) tests were performed using a three-electrode cell configuration with carbon felt () as the working electrode, a platinum mesh as the counter electrode, and an Ag/AgCl (3.0 M KCl) reference electrode as the reference electrode at a VSP BioLogic workstation. The electrochemical impedance spectroscopy (EIS) tests were conducted with the same device with frequencies ranging from 10 MHz to 100 kHz at an amplitude of 15 mV.
2.6. Cell Tests
PFRFB cells were assembled by sandwiching the different membranes between CuS-CF anode and pristine CF cathode. Graphite plate and copper foil served as the current collector. The active area of the working electrode was 13.5 cm2. Potassium ferricyanide and potassium polysulfide were used as positive and negative active materials, respectively. Galvanostatic cycling performance was conducted on Neware battery testing systems at room temperature. The open-circuit voltage decay (OCV) and rate performance from 20 to 140 mA cm-2 at an interval of 20 mA cm-2 was measured by Arbin battery testing system (BT-I, Arbin, USA). Polarization curves were obtained by employing an optimized cell (electrode area was 4.0 cm2) at 100% state of charge (SOC) from the Arbin instrument (BT-I) with the current being gradually increased by 10 mA per second from 260 mA.
Catholyte with three different concentrations (0.1 M, 0.5 M, and 0.8 M K3[Fe(CN)6]) and anolyte with 2.0 M K2S were obtained by dissolving K3[Fe(CN)6] and K2S in 1.0 M KCl solution, respectively. The cell employed a 25.0 mL solution of K3[Fe(CN)6] in 1.0 M KCl as catholyte and a 15.0 mL solution of 2.0 M K2S in 1.0 M KCl as anolyte for cell performance tests, unless otherwise specialized. Both catholyte and anolyte were pumped between the storing tank and the corresponding compartment of the electrochemical cell by a peristaltic pump (Williamson Manufacturing Co., Ltd., UK).
3.1. SPEEK-K Membranes for PFRFBs
3.1.1. Morphologies of Membranes
After the K+ exchange, the cross-sectional morphology of SPEEK-K (DS57-K) becomes rougher with respect to that of SPEEK (Figure 2(a)). Besides, the surface morphologies of SPEEK-K membranes are rough, too (Figures 2(b)–2(d)). The same trend is observed for N212-K compared to N212 (Figures S2a and S2c). From DS47-K and DS57-K to DS67-K, more and more K+ ions are present which are in direct proportion to the amounts of H+ from the sulfonic acid groups. Moreover, the homogeneous distributions of K+ ions into the matrixes are verified by the EDS mapping results (Figures 2(b)–2(d)). To evaluate the influences of K+ on properties of membranes, AFM images have been recorded. Compared with N212 (Figure S2b) and SPEEK (Figure 2(e)) membranes, N212-K (Figure S2d) and SPEEK-K membranes (Figures 2(f)–2(h)) display more apparent dark regions. In particular, along with the increase of DS, from DS47-K and DS57-K to DS67-K, the dark regions from the AFM images gradually become bigger. These results suggest that K+ ions modify the water channels of the membranes and then increase the hydrophilicity.
3.1.2. Physiochemical Properties of Membranes
The thickness, IEC, SR, AR, and K+ conductivity values of the membranes have been measured and compared in Table 1. As anticipated, SPEEK-K membranes show higher IEC and SR values compared to N212-K due to the involvement of hydrophilic sulfonic acid groups. Moreover, the IEC and SR values of SPEEK-K membranes improve from 1.27 to 1.84 and from 3.81% to 4.85%, respectively, along with the increase of DS. Such results further confirm the role of sulfonic acid groups in boosting the absorption and the retention of water molecules within the membranes. Generally, high SR values benefit ion conductivity . As a result, the AR value is reduced from 1.59 Ω cm2 for N212-K to 0.80, 0.74, and 0.71 Ω cm2 for DS47-K, DS57-K, and DS67-K, respectively. Correspondingly, the K+ conductivity of the membranes increases from 16.84 mS cm-1 for N212-K to 17.16, 19.08, and 22.89 for DS47-K, DS57-K, and DS67-K, respectively.
Figure 3(a) shows the variations of [Fe(CN)6]3- concentration in the blank reservoirs, which reveal the permeability of membranes. The permeation rate of SPEEK-K membranes increases with the increase of DS, as high DS means more -SO3H groups in the polymer matrix, which effectively improves the K+ conductivity (Table 1) and accelerates the crossover of active species through the membranes. Nevertheless, all the SPEEK-K membranes exhibit lower permeability of active species compared with N212-K. According to Figure 3(b), the permeation rate of membranes is reduced from for the N212-K to for the highly sulfonated DS67-K, which is further decreased to and for DS57-K and DS47-K, respectively. Ion selectivity, which is defined as the ratio of the conductivity and the permeability, is normally used to reflect the overall performance of the membranes. As disclosed in Figure 3(b), all the SPEEK-K membranes demonstrate considerably higher ion selectivity values over N212-K. Among them, DS57-K shows the highest ion selectivity of , attributing to the suppressed ferricyanide permeation as well as the enhanced K+ conductivity.
3.1.3. Effects of Membranes on Cell Performance
To study the effects of the membranes on PFRFB cell performance and figure out the optimized membrane, N212-K and a series of SPEEK-K membranes with different DS have been continuously cycled in PFRFB cells at a current density of 20 mA cm-2. All these cells show the high coulombic efficiency (CE) (Figure S3a). It is noted that the cell with DS57-K membrane delivers an average energy efficiency (EE) of 90.42% and remains stable over 1051 cycles, while the EE of the cells with other membranes declines rapidly after 250 cycles due to the gradually increasing polarization of the cells (Figure S3b). Besides, compared with other membranes, the cell with DS57-K membrane demonstrates the highest discharge capacity retention (CR) after 1051 cycles (Figure S3c). In line with the cycling tests, PFRFBs using SPEEK-K membranes survive longer during the tests of open-circuit voltage decay with respect to that of the cell with N212-K (Figure S4). In particular, the cell with DS57-K shows the minimum self-discharge behavior, in good agreement with the highest ion selectivity of DS57-K.
Figure 4(a) shows the charge/discharge profiles of the cells with DS57-K and N212-K membrane, respectively. The cell using DS57-K exhibits a higher discharge capacity (60.8 mAh) than N212-K-based cell (53.8 mAh), which is in good agreement with the lower ion permeability and higher ion selectivity of DS57-K membrane. The cells which, respectively, employ N212-K and DS57-K membrane both exhibit high CE of around 99.80% over 1051 cycles. However, the EE of DS57-K-based cell is much higher and more stable than that of N212-based counterpart. The cell with DS57-K membrane enables a stable EE of 90.42% for 1051 cycles. When N212-K membrane is used, the EE is only 77.82% after 879 cycles (Figure 4(b)). Besides, the DS57-K-based cell shows the higher CR than N212-K-based cell (Figure 4(c)). In addition, as shown in Figure 4(d), the use of DS57-K membrane dramatically prolongs the self-discharge duration time of the PFRFB. The DS57-K-based PFRFB cell maintains about 717 hours, which is sixfold longer than that of N212-K-based cell (98 hours). All these results confirm that the DS57-K membrane efficiently retards the crossover of redox species owing to the SPEEK-K frameworks and the optimized DS.
3.2. CuS-CF Electrode for PFRFBs
SEM images reveal the significant differences between pristine CF (Figures 5(a) and 5(b)) and CuS-CF (Figures 5(c) and 5(d)) on the morphologies, indicating the successful growth of CuS on the surfaces of the CF. In specific, the surfaces of pristine CF are clean and smooth, while CuS-CF presents a flower-like bump structure, and these bumps are composed of numerous interconnected nanosheets. This flower-like structure can provide more active sites for redox reactions to occur. Figure S5 shows the EDS mapping results of the CuS-CF, which confirms the homogeneous distribution of S and Cu elements on CF.
As shown in Figure 6(a), multiple pairs of redox peaks can be observed on CuS-CF, while only one major pair of redox peaks can be clearly identified on the pristine CF electrode. The emergence of multiple polysulfide redox peaks indicates the excellent catalytic performance of the CuS-CF electrode. As illustrated in Figure 6(b), the EIS tests have been employed to understand the electrochemical properties of pristine CF and CuS-CF. The charge transfer resistance of CuS-CF is 2.14 Ω, which is lower than that of pristine CF (3.34 Ω). This result suggests that the CuS-CF electrode exhibits a faster electron transfer speed between the electrode interface and the electrolyte. When the scanning rate varied from 1 to 50 mV s−1, the main redox peaks display higher current densities and smaller separation of peak potentials at all scan rates on CuS-CF than those on pristine CF (Figures S6a and S6b). These results indicate that the surface coverage of flower-like CuS offers more active sites for polysulfide redox reactions and leads to higher electrochemical activities. Similar phenomena have also been observed in earlier reports [52, 53].
To evaluate the influences of electrodes on the cell performance, PFRFB cells have been assembled using pristine CF and CuS-CF as electrodes, respectively. As disclosed by the charge/discharge curves (Figure 6(c)), the CuS-CF-based cell exhibits a smaller polarization in the first cycle compared with the pristine CF-based cell. Such difference is further enlarged during the cycling process. The pristine CF-based cell shows evident IR drop, which is negligible when CuS-CF electrode is utilized. Correspondingly, the CuS-CF electrode enables more stable EE of the PFRFB cell over cycling, which remains at 75.80% after 879 cycles, compared to that of 66.54% after 751 cycles when CF electrode is used (Figure 6(d)). These significant improvements of cell performance that originated from electrodes can be explained by the more catalytic sites and active sites in CuS-CF .
3.3. Comprehensive PFRFB Performance
To explore the synergistic effects of the two key materials, CuS-CF is used as the anodic electrode in combination with the optimum DS57-K as the ion-exchange membrane in one PFRFB cell. As shown in Figure 7(a), at the beginning, the discharge capacity (DC) can achieve as high as 99.54% of the theoretical value. The CE keeps very stable over the long-term cycling test (99.95% after 1180 cycles), and the average EE of the PFRFB is 75.41% even at a high current density of 50 mA cm-2. In this study, the capacity of the cell is determined by the catholyte. When the concentration of redox-active material (K3[Fe(CN)6]) in the catholyte is increased to be 0.5 M and 0.8 M, the DC values display apparent enhancements (Figures 7(b) and 7(c)). Besides, when 0.5 M K3[Fe(CN)6]+2.0 M KCl is employed as the catholyte, the cell delivers outstanding performance at a current density of 20 mA cm-2, including the CE of 99.28%, the VE of 80.46%, and the EE of 79.27%. These key parameters remain steady for more than 230 cycles (Figure 7(b)). When the catholyte contains 0.8 M K3[Fe(CN)6]+2.0 M KCl (Figure 7(c)), the resulting cell exhibits minor capacity decay during 60 cycles (180 testing hours) at a current density of 20 mA cm-2, and the capacity decay rate is 0.0709% per hour. The average CE, EE, and VE of this more concentrated PFRFB cell achieve 97.21%, 83.41%, and 85.23%, respectively.
Rate performance of the PFRFB cell was collected from 20 to 140 mA cm-2 with an interval of 20 mA cm-2. The CE maintains at nearly 100% under all applied current densities, suggesting the high stability of both the DS57-K membrane and the CuS-CF electrode. The VE and the EE decrease strikingly with the increment of current density from 100 mA cm-2 to 140 mA cm-2, which may be related to the increase of electrochemical polarization and ohmic polarization of the cell. Despite that, when the current density returns to 20 mA cm-2, both the VE and the EE regain the initial high values. In addition, the polarization curves confirm that the power density of the cell becomes higher along with the concentration of redox-active materials in the catholyte increasing (Figure S7). For comparison, a PFRFB cell with pristine CF as electrodes and N212-K as the ion-exchange membrane (the reference cell) was assembled and tested. From the polarization curves, the peak power density of the cell with optimized electrodes and membrane reaches 223 mW cm-2, which is higher than 199 mW cm-2 of the reference cell (Figure 7(e)). Therefore, by the optimizations of membrane and electrode materials, the cost of the PFRFB system is reduced and the redox reaction kinetics of polysulfide species is improved.
In summary, we report a neutral PFRFB system by using an affordable SPEEK-K membrane and a highly catalytic CuS-CF electrode. By the optimization of DS, the DS57-K membrane enables the PFRFB cell high average CE of 99.80% and superior EE of 90.42% over 1051 cycles at a current density of 20 mA cm-2, surpassing the commercial N212 membrane in terms of both performance and affordability. Meanwhile, to match with the fast kinetics of the catholyte redox reactions, CuS-CF electrode is demonstrated with excellent catalytic effects on the polysulfide redox reactions, leading to the reduced polarization of the cell over long-term cycling. By combining the DS57-K membrane with the CuS-CF electrode, the PFRFB cell exhibits high efficiencies even at a high current density of 50 mA cm-2. Furthermore, with such strategy, a high concentrated catholyte with 0.8 M K3[Fe(CN)6] is employed in the proposed PFRFB cell, leading to an outstanding power density of 223 mW cm-2. Therefore, advances achieved in this study promote the development of neutral PFRFBs for high-performance and low-cost energy storage systems.
The authors declare that the main data supporting the findings in this study are available within the article and its supplementary information. Additional data are available from the corresponding authors upon reasonable request.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this article.
M.D., X.Z., and C.J. proposed the project, designed the experiment, and revised the manuscript. X.L. performed the most experiments and prepared the initial draft. H.F., J.X., and Y.L. characterized the electrodes, analyzed the data, and prepared the figures. S.Y. and H.Z. prepared and characterized the membranes. B.L. and M.H. performed optical measurements.
We acknowledge financial support from the 100 Talented Team of Hunan Province (XiangZu  91), the “Huxiang High-Level Talents” program (2019RS1046 and 2018RS3077), the Open Fund of National Engineering Laboratory of Highway Maintenance Technology (Changsha University of Science and Technology) (kfj170105), the Natural Science Foundation of Hunan Province (2020JJ5566), and the Outstanding Young Talent Project of Education Department of Hunan Province (19B029).
Figure S1: a diffusion cell for permeability measurements. Figure S2: characterizations of the N212 membrane. Figure S3: comparisons of cell performance with different membranes. Figure S4: self-discharge behaviors of the PFRFB cells with different membranes. Figure S5: the SEM and EDS mapping images of CuS-CF. Figure S6: the CV curves on pristine CF and CuS-CF electrode at different scanning rates, respectively. Figure S7: the polarization curves of cells with different concentrations of K3[Fe(CN)6] in the catholyte. Methods S1: cost calculations. Table S1: cost of membranes. (Supplementary Materials)
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