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Mechanism and transfer behavior of ions in Nafion membranes under alkaline media
Author’s Accepted Manuscript Mechanism and transfer behavior of ions in Nafion membranes under alkaline media Jing Hu, Huamin Zhang, Wenbin Xu, Zhizhang Yuan, Xianfeng Li www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(18)31859-3 https://doi.org/10.1016/j.memsci.2018.08.057 MEMSCI16431
To appear in: Journal of Membrane Science Received date: 16 July 2018 Revised date: 24 August 2018 Accepted date: 26 August 2018 Cite this article as: Jing Hu, Huamin Zhang, Wenbin Xu, Zhizhang Yuan and Xianfeng Li, Mechanism and transfer behavior of ions in Nafion membranes under alkaline media, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.08.057 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.
Mechanism and transfer behavior of ions in Nafion membranes under alkaline media
Jing Hua, b, Huamin Zhanga, c, Wenbin Xua,c, Zhizhang Yuana,*, and Xianfeng Lia, c,* a
Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, 457 Zhongshan Road, Dalian 116023 (P. R. China). b
c
University of Chinese Academy of Sciences, Beijing 100049 (P. R. China).
Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023
(P. R. China).
Email: [email protected] ; [email protected].
*Corresponding author at: Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023 (P. R. China).
1
Abstract Nafion series cation exchange membranes are extensively investigated and applied in proton exchange membrane fuel cells and flow battery technologies because of their excellent stability and easy availability. However, a deep understanding of their ions transport mechanism and behavior under the alkaline based flow battery media is very limited. Here, the ion transport mechanism through Nafion membrane under alkaline medium is investigated by small-angle X-ray scattering and atomic force microscope techniques. The results indicate that a membrane showed a higher degree of phase separation and larger cluster radius in a NaOH solution than those in a KOH solution, which endows the membrane with a much higher ion conductivity in the NaOH solution. Density functional theory-based simulation also indicates that the adsorption and desorption between the Na+ and the –SO3- in Nafion is faster than those of K+. Inspired by these results, an alkaline zinc iron flow battery with a Nafion 212 membrane using NaOH as the supporting electrolyte exhibits a coulombic efficiency of ~99% and an energy efficiency of ~86% at 80 mA cm-2. This work offers insights into ways to obtain an improved battery performance for the existing as well as the emerging alkaline based flow battery technologies.
2
Graphical abstract
3
Keywords: Nafion membrane; Ion transfer mechanism; Density functional theory-based simulation; Zinc iron flow battery
4
1. Introduction In the past decade flow battery technologies received much attention due to their urgent need in wide application renewables like solar and wind power. Aqueous flow battery technologies, represented by vanadium flow battery and zinc bromine flow battery, are at the stage of commercial demonstration [1]. However, there remained some critical challenges, e.g. the relatively high cost of all-vanadium redox pairs, the relatively low electrochemical activity and corrosion of bromine couples. Hence, there appears some new redox couples and flow-type systems, e.g. alkaline quinone based and organic flow battery systems [2, 3], acidic Fe-V flow battery system [4] and neutral polymer based flow battery systems [5]. On the basis of design strategy of a flow battery, an ion conducting membrane is employed to separate the anode and cathode while still transfer charge-balancing ions to complete the internal circuit [6, 7]. The properties of an ion conducting membrane have great influence on the battery performance. Currently, numerous ion conducting membranes and membrane materials have been developed for battery applications. Among which, Nafion series cation exchange membranes are the most widely used membrane materials due to their high ion conductivity, excellent stability and easy availability [8]. It has been widely accepted that Nafion membranes transport protons through hopping mechanism and vehicle mechanism in proton exchange membrane fuel cell (PEMFC) and flow battery systems involving in acidic media [9]. And for those flow batteries using acidic supporting electrolyte such as vanadium flow battery and quinone bromine flow battery, a piece of Nafion membrane can normally endow them with high voltage efficiency because of the high proton conductivity of the membrane. 5
In addition to quinone bromine flow battery and Fe-V flow battery, of which acidic electrolyte is employed, most of recently reported aqueous flow battery systems are working under alkaline or neutral conditions, where the Nafion cation exchange membranes are still utilized prevailingly. However, critically different from acidic based flow batteries, alkaline based flow batteries using a Nafion membrane normally afforded a relatively low voltage efficiency (Table 1) since a different ion transport mechanism and behavior maybe involved in alkaline media. For instance, an all-soluble all-iron aqueous flow battery using a Nafion 212 membrane afforded a voltage efficiency of 78% at 40 mA cm-2 [10]. A polysulfide iodide flow battery using a Nafion 117 membrane showed a voltage efficiency of 72.15% at 15 mA cm -2 [11]. An alkaline zinc ferrocyanide flow battery with a Nafion 115 membrane delivered a voltage efficiency of 79% at 80 mA cm-2. Whereas the alkaline zinc ferrocyanide flow battery with a polybenzimidazole (PBI) anion exchange membrane demonstrated a voltage efficiency of 88% at the same condition [12]. Judging from the discussions above, Nafion cation exchange membranes tell a different story in alkaline media, where the fundamental aspects of Nafion membranes under alkaline media especially the ion transport mechanism through the membrane are unclear. This in turn results in lacking strategies to optimize as well as improve battery performance from membrane and electrolyte aspects. Therefore, it is of very importance to clarify the fundamental ion transport mechanism of Nafion membranes under alkaline condition, which would direct further improvement in the performance of the existing alkaline based flow battery systems and open up a new perspective on utilization of cation exchange membranes for the emerging alkaline based flow battery technologies.
6
Table 1 Battery performance of recently reported alkaline based flow battery systems using Nafion cation exchange membranes. Aqueous flow battery
Supporting
Membrane
Performance
(mA cm-2)
electrolyte
Fe(TEOA)OH/
Current density
CE(%)
VE(%)
NaOH
Nafion 212
40
93
78
KI/K2S2 [11]
KOH
Nafion 117+115
15
94
72
Zn/Fe [13]
NaOH
Nafion 212
80
99
76
DHAQ/K4Fe(CN)6 [14]
KOH
Nafion 212
100
99
84
DHBQ/K4Fe(CN)6 [15]
KOH
Nafion 115
100
99
66
ACA/K4Fe(CN)6 [16]
KOH
Nafion 212
100
99
63
FMN/K4Fe(CN)6 [17]
KOH
Nafion 212
80
99
55
This work
NaOH
Nafion 212
80
99
86
K4Fe(CN)6 [10]
In this study, aimed at gaining deep insight into the fundamental aspects of Nafion cation exchange membranes under alkaline based flow battery media, we present an experimental and computational strategy that links different physical and electrochemical behaviors of Nafion in different kinds of electrolytes. This strategy, utilized to an alkaline zinc iron flow battery as platform, demonstrates that a battery with Nafion membrane can afford high 7
performance in alkaline media as well. In terms of unraveling behaviors of Nafion under alkaline condition, the presented research is expected to provide insights into ways to afford high battery performance for the existing as well as the emerging alkaline based flow batteries. 2. Experimental Section 2.1 Materials Potassium ferrocyanide, potassium hydroxide, sodium chloride and potassium chloride were purchased from Tianjin Damao Chemical Reagent Factory. Sodium hydroxide was purchased from Tianli Chemical Reagent Co.,Ltd. Sodium ferrocyanide was bought from Sinopharm Chemical Regent Co.,Ltd. Zinc oxide was bought from Kermel Chemical Reagent Factory. These reagents were supplied with analytical grade. Other reagents were brought from Sigma Aldrich unless stated otherwise and used as received. 2.2 Battery Performance The alkaline zinc iron flow battery was assembled by sandwiching the Nafion membrane (Nafion 115, 211, 212, respectively) between two carbon felt electrodes, clamped by two graphite plates. The active area of the electrode is 48 cm2. All of these components were fixed between two stainless steel plates. The electrolyte was cyclically pumped through the corresponding electrodes in airtight pipelines. Charge-discharge cycling tests were conducted by ArbinBT 2000 at a constant current density of 80 mA cm-2. The charge process was controlled by the charge time to keep a constant charge capacity, while the discharge process was ended with the cut-off voltage of 0.1 V. 2.3 Small angle X-ray scattering (SAXS) 8
SAXS patterns of the Nafion membranes treated with ultrapure water, NaOH and KOH solution were recorded on a Rigaku Smart Lab between 0.6° and 5°. Before both tests, all the samples were soaked in ultrapure water (Nafion 212-H2O), 2 M NaOH (Nafion 212-NaOH), and 2 M KOH (Nafion 212-KOH), respectively for at least 48 h. The scattering vector was calculated according to equation: q 2 sin /
(1)
where 2θ is the scattering angle and λ is the scattering wave length, 0.1541 nm. The Bragg spacing was calculated as follows:
d
2 q
(2)
2.4 Atomic force microscope (AFM) The nanophase-separated morphology of membranes was probed by tapping mode AFM. Tapping mode AFM was performed using an Oxford instrument (CypherES). A silicone cantilever (55 µm long) with an end radius 7 ± 3 nm，resonance frequencies 850–2500 kHz and spring constants 38–184 Nm−1 was used to image the samples. 2.5 Swelling and electrolyte uptake To detect the electrolyte uptake and swelling of the membranes, the membranes were firstly soaked in ultrapure water (Nafion 212-H2O), 2 M NaOH (Nafion 212-NaOH) and 2 M KOH (Nafion 212-KOH), respectively for at least 48 h. The weight of the saturated membranes was then obtained after quickly wiping off the surface electrolyte using a tissue. The swelling is defined as the length ratio of the swollen membranes to the dry membranes, as shown in equation:
9
Swelling (%)
Ls Ld 100 Ld
(3)
where Ls and Ld are the lengths of the saturated and dry membranes, respectively. The water uptake is defined as the weight ratio of the absorbed electrolyte to the dry membrane, as shown in equation: Wateruptake(%)
Ws Wd 100 Wd
(4)
where Ws and Wd are the weights of the saturated and dry membranes, respectively. 2.6 OH- ion permeation and membrane ion conductivity tests The OH- ion permeation test was carried out using a diffusion cell shown in Fig. S4. The effective membrane area is 9 cm2. For the measurement, 80 mL alkaline solution (2 M KOH or 2 M NaOH) and 80 mL ultrapure water were added into the draw (left) side and feed (right) side, respectively. The pH or conductivity of right side solution was detected at a regular time interval by using Mettler Toledo pH meter model or conductivity model, respectively. The OH- concentration (COH-) was calculated from the pH data according to the following equation: pH = 14 + lg(COH-)
(5)
By monitoring the pH, the concentration of OH- thus can be obtained. All the experiments were conducted at room temperature. 2.7 Energy dispersive X-ray spectroscope (EDS) The surface element distribution of Nafion 212 membrane was recorded by scanning electron microscope (JSM-7800F) equipped with energy dispersive X-ray spectroscope (EDS). The EDS spectrum of the membrane surface confirmed that the Na-Nafion membrane treated with 10
KOH for 48 h was composed of the elements carbon, oxygen, fluorine, sulfur and potassium, but a little sodium, which suggested that Na-Nafion could be translated into K-Nafion in the KOH solution. Same thing, K-Nafion membrane treated with NaOH for 48 h was confirmed the elements of carbon, oxygen, fluorine, sulfur and sodium but a little potassium, which suggested that K-Nafion could be translated into Na-Nafion in the NaOH solution. 2.8 Density functional theory (DFT) calculations In the DFT calculation, structures of the Nafion membrane and the hydrated cations-(H2O)6 were optimized by B3LYP [18] and Def2-SVP [19], in which cations were located beside the Nafion structure at the most stable state. These calculations were carried out using the Gaussian 09 software package [20]. The binding energies were described by the Def2-TZVP basis sets. In our model, the ionic radius of hydrated potassium cation (K(H2O)6+) is approximately 2.70 Å from the mass center, and the ionic radius of hydrated sodium cation (Na(H2O)6+) is approximately 2.38 Å. Besides, the interactions between the cations and the Nafion 212 were provided during the calculation of the binding energies. The binding energies between the cations and the Nafion 212 membrane were estimated by the following equation: ENF-cation = ENF + E cation – E (NF+ cation)
(6)
where NF represented Nafion 212 membrane, cation represented K+ and Na+. where ENF and E
cation
are the total energy of Nafion 212 membrane and cation, E
(NF+ cation)
refers to the total energy of the hybrid system consisting of both Nafion 212 and cation. Thus, the calculated ENF-cation represents the binding energy between the cations and Nafion 212 membranes. 11
3. Results and discussion In our previous work [12], we reported an alkaline zinc iron using a PBI anion exchange membrane for stationary energy storage. As a reference, a piece of Nafion 115 cation membrane with a thickness of ~125 µm was employed. Unfortunately, the battery with a Nafion 115 membrane delivered a relative low voltage efficiency (VE) of 79% at 80 mA cm -2. It is expected that the utilization of thinner membranes like Nafion 212 (~50 µm) or Nafion 211 (~25 µm) can lower battery resistance and boost the battery performance. We thus firstly investigated the battery performance by using a Nafion 212 or a Nafion 211 membrane. Unexpectedly, the battery with different thickness of Nafion membranes showed similar performance (Fig. S1), which is critically different from that of vanadium flow battery, where a decreased coulombic efficiency (CE) and an increased VE is confirmed with reduced Nafion membrane thickness [21]. It is easy to understand the high CE (~100%) of the alkaline zinc iron flow battery even assembled the thinnest Nafion 211 membrane, which is resulted from the exclusion effect between the negatively charged sulfonic acid groups in Nafion and the redox couples (Fe(CN)63-/Fe(CN)64- and Zn(OH)42-). However, the alkaline zinc iron flow battery using Nafion membranes with different thickness showed a similar VE (~80%), suggesting that the thickness of a Nafion membrane has no significant impacts on an alkaline based battery but other factors may do. Hence, clarifying the underlying reasons behind this phenomenon will provide an important reference for the research and application of Nafion membranes in the existing and emerging alkaline based flow battery technologies. For this purpose, a sheet of Nafion 212 membrane was selected for further study. Normally, in alkaline media, where KOH or NaOH is employed, Nafion cation exchange 12
membranes are supposed to transfer K+ or Na+ via ion exchange mechanism, while reject OHbecause of exclusion effect between the negatively charged sulfonic acid groups and the OHanions [22, 23]. Actually, OH- can penetrate the membrane as well due to the swelling behavior of Nafion in aqueous media, which results from the hydrophobic Teflon backbone and hydrophilic sulfonic acid groups in the polymer backbone, further leading to the membrane with a phase separated structure. Thus to investigate the permeation rate of OHthrough the Nafion membrane, a diffusion experiment using a Nafion 212 membrane was carried out as shown in Fig. 1a. Remarkably, by using NaOH as the feeding solution, the OHpermeation rate, calculated from the slope of the line, is higher than does the membrane using KOH as the feeding solution. According to the principle of charge conservation, the permeation of OH- through Nafion membrane from the feeding side to the diffusion side would be inevitably associated with the permeation of K+ or Na+, which is mainly derived from the ion exchange mechanism (ion exchange between cations and the –SO3-) and the free diffusion of cations in the hydrophilic channels (formed by hydrophilic sulfonic acid groups in Nafion). The higher OH- permeation rate reflect higher cations permeation rate indirectly, that is, the transportation of Na+ through Nafion 212 membrane is faster than the K+.
13
Fig. 1. (a) The permeation rate of OH- by testing the PH in the diffusion side. (b) The permeation rates of Na+ and K+ by testing the conductivity of solution in the diffusion side. To further verify the permeation rates of Na+ and K+, another diffusion experiment using a NaCl and KCl as the feeding solution was conducted. The concentration of the cations in the diffusion side was then calculated according to the conductivity (Λm = (κ/c), where κ is the conductivity of solution in the diffusion side, and c is the concentration of the cations) [24]. The conductivity of NaCl or KCl with a certain concentration was first measured (Fig. S2). The molar conductivity (Λm) of NaCl in the diffusion side, obtained from the slope of the line, is lower than does the KCl, suggesting higher concentration of NaCl in the diffusion side (Fig. 1b), further confirming the higher permeation rate of Na+ through Nafion 212 membrane. Inspired by the above results, an alkaline zinc flow battery employing NaOH as the supporting electrolyte is expected to deliver a higher performance than does a battery using KOH as the supporting electrolyte. As expected, by using 2 M NaOH as the supporting electrolyte for both positive and negative side, the battery demonstrates a CE of ~100% and a VE of 86.46% at the current density of 80 mA cm-2. By contrast, when using 2 M KOH as the supporting electrolyte, a much lower VE of 79.44% is afforded at the same working current density (Fig. 2a). To further confirm the universal applicability, the performance of an alkaline 14
zinc iron flow battery with a Nafion 115 membrane is also tested. As expected, the battery with a Nafion 115 membrane demonstrates a much higher VE when using NaOH as the supporting electrolyte than does a battery using KOH as the supporting electrolyte at the current density of 80 mA cm-2 (Fig. S3). Fig. 2b shows that alkaline zinc iron flow battery using NaOH as supporting electrolyte demonstrated a lower charging voltage and higher discharging voltage than does a battery using the KOH as the supporting electrolyte, indicating that the Nafion 212 membrane in NaOH media has a lower resistance, which is consistent with permeation results. Furthermore, by using NaOH as the supporting electrolyte, an alkaline zinc iron flow battery afforded a stable performance over more than 100 cycles at the current density of 80 mA cm-2, maintaining an average CE of ~100% and an average EE of 82%, which is much higher than the previously reported alkaline zinc iron flow battery, where only a CE of 76% and an EE of 61.5% was demonstrated at the current density of 35 mA cm-2 [25]. In addition to the stable cycling performance, the battery showed a discharge capacity of ~7.5 Ah L-1 and a discharge energy of ~12 Wh L-1, respectively, yielding an electrolyte utilization of ~70% (theoretical capacity of 10.7 Ah L-1) at 80 mA cm-2 (Fig. 2c, d).
15
Fig. 2. (a) The battery performance of alkaline zinc iron flow battery with different kind of electrolyte (b) Charging-discharging curves of the alkaline zinc iron flow battery with different kind of electrolytes. (c) Long-term stability of the alkaline zinc iron flow battery. Insert, charge and discharge curves. (d) The corresponding discharge capacity and discharge energy values of 100 cycles. It is generally accepted that the ion conductivity of a Nafion membrane heavily depends on the microstructure. To uncover the underlying reasons for the differences in ion conductivity as well as the battery performance of Nafion membrane in different kinds of alkaline supporting electrolytes, the microstructures of Nafion 212 membranes, treated with ultrapure water, NaOH solution and KOH solution (denoted as Nafion 212-H2O, Nafion 212-NaOH, Nafion 212-KOH), respectively, were analyzed by the small-angle X-ray scattering (SAXS) 16
technique. The SAXS patterns in Fig. 3a shows a maximum scattering ionomer peak at around q = 1.49 nm-1 for the original Nafion 212 membrane, while the corresponding peaks of Nafion 212 membranes treated with NaOH and KOH solutions shifted to higher angle at around q = 1.71 nm-1 and q = 2.56 nm-1, respectively. This shift was mainly attributed to higher cationic electron density since the protons in the sulfonic acid groups were exchanged by Na+ or K+ after treating in NaOH and KOH solutions. The Bragg spacing d of Nafion 212-H2O, Nafion 212-NaOH, Nafion 212-KOH obtained from Bragg equation is 4.2 nm, 3.68 nm, and 2.45 nm, respectively (Table S1), indicating the sizes of hydrophilic clusters, which is in accordance with those reported in the previous literature [26]. The present results indicated that Nafion 212 membrane treated with NaOH solution has a higher degree of phase separation and larger cluster radius than does the one treated with KOH solution (Fig. 3b). Tapping mode atomic force microscope (AFM) was used to further identify the differences in phase-separated structure of Nafion 212 in different solutions. Fig. 3c shows the connective ion transfer channels of Nafion 212 membrane in ultrapure water. Nafion 212 membrane in NaOH solution exhibits a clear phase-separated structure as well (Fig. 3d). By contrast, the Nafion 212 membrane shows a relatively low degree of phase-separated structure in KOH solution (Fig. 3e), of which the dark regions in the phase image correspond to the soft regions of the highly hydrophilic ionic clusters while the light portions refer to the hard structure of hydrophobic polytetrafluoroethylene backbone chain along with regularly spaced shorter perfluorovinyl ether side-chains [27]. The higher degree of phase-separated structure of Nafion 212 membrane would present a higher membrane swelling behavior and absorb more electrolyte in the hydrophilic regions, thus affording the membrane with higher ion 17
conductivity. The large cluster radius together with high phase-separated structure of Nafion 212 membrane in NaOH solution contribute enormously to the enhanced VE in Fig. 2a and are expected to act as guidance for other alkaline based flow battery systems.
Fig. 3. (a) SAXS patterns of three kinds of different Nafion 212 membranes. (b) Scheme of ion cluster size of Nafion 212 membrane in NaOH and KOH solutions. AFM tapping mode phase images of the membrane: (c) Nafion 212-H2O, (d) Nafion 212-NaOH, (e) Nafion 212-KOH. The membrane properties like swelling, water uptake and ion permeability are closely related to the micro structure of the membrane. Therefore, we performed experimental validation on membranes with different degree of swelling and water uptake in macroscopic dimensions. The Nafion 212 membrane demonstrated the highest swelling (Fig. 4a) and electrolyte uptake 18
(Fig. 4b) in ultrapure water, while the lowest in KOH solution. A similar trend can be found in Nafion 115 as well (Fig. S5). This higher swelling and electrolyte uptake are in favor of faster transportation of ions in the membrane, which is determined by the microstructure of a membrane. The ion permeation tests were carried out using a diffusion cell as shown in Fig. S4 to identify the different ion transfer rate caused by different ion cluster size of Nafion 212 in NaOH and KOH solutions (Na-Nafion and K-Nafion). As expected, the Na-Nafion membrane demonstrated a much higher permeation rate in NaOH solution than does a membrane in KOH solution (Fig. 4c), which suggested that Na-Nafion could be translated into K-Nafion in the KOH solution. Energy dispersive X-ray spectroscope (EDS) measurement in Fig. S6 can also confirms this exchange. Barely phase-separated structure along with small cluster radius of K-Nafion, thus afforded the membrane with a much lower ion permeation rate in a KOH solution than does the membrane in a NaOH solution. Similar tendency was observed for K-Nafion in the NaOH and KOH solutions (Fig. 4d), where a higher permeation rate can be afforded in the NaOH solution. These results suggested again that a Nafion membrane exhibits a higher ion conductivity in a NaOH solution and an alkaline based flow battery employing Nafion as membrane is expected to achieve an improved battery performance by using NaOH as the supporting electrolyte.
19
Fig. 4. (a) Swelling and (b) electrolyte uptake of Nafion 212 membrane in ultrapure water, NaOH and KOH solution, respectively; Ion permeability of Na-Nafion membrane (c) and K-Nafion membrane (d) using NaOH and KOH as the feeding solution, respectively. Density Functional Theory (DFT) calculations were carried out to gain an in-depth understanding of the difference in ion cluster as well as the hydrophilic channel of the Nafion 212 membranes in different solutions. The structures of the hydrated K+ and Na+ are shown in Fig. S7. The ionic radius of hydrated potassium cation (K(H2O)6+) is 2.70 Å, which is relative larger than does a hydrated sodium cation (Na(H2O)6+ 2.38 Å), shown in Fig. S8. Hence, the K(H2O)6+ has a lager spatial resistance than that of (Na(H2O)6+ when entering into the ion cluster channel, which leads to a lower K+ conductivity. Considering the higher spatial 20
resistance of hydrated potassium cation, less water molecule can be brought into the hydrophilic region of the Nafion 212 membrane, which has been a contributing factor for the lower electrolyte uptake of the membrane in KOH solution. To further verify this, the hydration energy of hydrated K+ and Na+ were calculated as given in Fig. S8. The hydration energy of hydrated K+ cation that bonds with sulfonic acid group is 85.98 kcal mol-1, which is lower than that of the hydrated Na+ cation (108.50 kcal mol-1). This indicates that the ability of carrying water molecules of the hydrated K+ cation is weaker than that of the hydrated Na+ cation, thus further leading to a smaller cluster radius and a lower degree of phase-separated structure of the Nafion 212 membrane in KOH solution [28]. In addition to the membrane microstructure that affects the ion conductivity of a membrane to a great extent, the rapid adsorption and desorption rate between the cations and the ion exchange groups also has an enormous impact on membrane ion conductivity [29], which is expected to turn into an important contributor to enhance the ion conductivity of a membrane as well, and thus further boosting the battery performance. To verify the above speculation, the binding energy between the cations and sulfonic acid groups was then calculated according to DFT (Fig. 5). The binding energy of SO3- anion and Na+ cation (ENFSO3-Na) is defined as the energy difference between the single energies of SO3- anion (ENFSO3-) and cation (ENa+) and the total energy of hybrid system (E(NFSO3-+ Na+)). Thus, ENFSO3-Na = ENFSO3- + ENa+− E(NFSO3-+
+ Na ).
Here, E(NFSO3-+
+ Na )
refers to the total energy of the hybrid system
consisting of both NFSO3- anion and Na+. Similarly, ENFSO3-K = ENFSO3- + EK+ − E(NFSO3-+ K+). The simulation results were shown in Fig. 5 and Table 2, where the binding energy of Na+ (K+) and SO3- group is 120.61 kcal mol-1 (105.44 kcal mol-1). The larger binding energy of Na+ and 21
SO3- group indicated a faster adsorption and desorption rate between the Na+ and the SO3groups, which would promote the conductivity of Na+ in the Nafion 212 membrane. This served as another important factor that could account for the higher performance of an alkaline zinc iron flow battery when using NaOH as the supporting electrolyte than does a battery using KOH as the supporting electrolyte.
Fig. 5. The interaction of Nafion sulfonic membranes and cations. Here, the chain represents the partly segment structure of the Nafion sulfonic membrane. Table 2 The binding energy of Nafion and cations.
Binding energy (kcal mol-1)
ENFSO3-Na
ENFSO3-K
120.61
105.44
22
4. Conclusion In summary, we have investigated the mechanism and behavior of ions transfer in a Nafion 212 membrane under alkaline zinc iron flow battery media in very detail. The results indicated that a Nafion 212 membrane exhibits a higher swelling and electrolyte uptake in a NaOH solution than that of in a KOH solution, which was resulted from the higher degree of phase separated structure and larger cluster radius of the Nafion 212 in the NaOH solution. Density functional theory-based simulation verified that the adsorption and desorption between the Na+ cation and the –SO3- in Nafion is faster than those of K+ cation. The faster capacity of ion exchange for Na+ cation and higher degree of phase separation together with larger cluster radius of the Nafion 212 endow the membrane with a higher ion conductivity in a NaOH solution than that of in a KOH solution. As a consequence, an alkaline zinc iron flow battery with a Nafion 212 membrane using NaOH as the supporting electrolyte exhibited a much higher battery performance than does a battery using KOH as the supporting electrolyte. The results present here may provide effective ways to significantly improve the performance of the existing as well as the emerging alkaline based flow batteries. Acknowledgements The authors greatly acknowledge the financial support from China Natural Science Foundation
(Grant
Nos.
21206158),
Key
project
of
Frontier
Science,
CAS
(QYZDB-SSW-JSC032), CAS-DOE collaborative project and DICP funding (ZZBS201707). The authors also greatly thank Prof. Qiang Fu and Dr. Chuanhai Xiao at DICP for help with the atomic force microscope measurements. Appendix A. Supplementary material 23
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[9] S. Cukierman, Et tu, Grotthuss! and other unfinished stories, Biochim. Biophys. Acta. 1757 (2006) 876-885. [10] K. Gong, F. Xu, J.B. Grunewald, X. Ma, Y. Zhao, S. Gu, Y. Yan, All-Soluble All-Iron Aqueous Redox-Flow Battery, ACS Energy Lett. 1 (2016) 89-93. [11] Z. Li, G. Weng, Q. Zou, G. Cong, Y.-C. Lu, A high-energy and low-cost polysulfide/iodide redox flow battery, Nano Energy. 30 (2016) 283-292. [12] Z. Yuan, Y. Duan, T. Liu, H. Zhang, X. Li, Toward a Low-Cost Alkaline Zinc-Iron Flow Battery with a Polybenzimidazole Custom Membrane for Stationary Energy Storage, iScience. 3 (2018) 40-49. [13] K. Gong, X. Ma, K.M. Conforti, K.J. Kuttler, J.B. Grunewald, K.L. Yeager, M.Z. Bazant, S. Gu, Y. Yan, A zinc–iron redox-flow battery under $100 per kW h of system capital cost, Energy Environ. Sci. 8 (2015) 2941-2945. [14] K. Lin, Q. Chen, M.R. Gerhardt, L. Tong, S.B. Kim, L. Eisenach, A.W. Valle, D. Hardee, R.G. Gordon, M.J. Aziz, M.P. Marshak, Alkaline quinone flow battery, Science. 349 (2015) 1529-1532. [15] Z. Yang, L. Tong, D.P. Tabor, E.S. Beh, M.-A. Goulet, D. De Porcellinis, A. Aspuru-Guzik, R.G. Gordon, M.J. Aziz, Alkaline Benzoquinone Aqueous Flow Battery for Large-Scale Storage of Electrical Energy, Adv. Energy Mater. 8 (2018) 1702056. [16] K. Lin, R. Gómez-Bombarelli, E.S. Beh, L. Tong, Q. Chen, A. Valle, A. Aspuru-Guzik, M.J. Aziz, R.G. Gordon, A redox-flow battery with an alloxazine-based organic electrolyte, Nat. Energy. 1 (2016) 16102. [17] A. Orita, M.G. Verde, M. Sakai, Y.S. Meng, A biomimetic redox flow battery based on 25
flavin mononucleotide, Nat. Commun. 7 (2016) 13230. [18] C. Lee, W. Yang, R.G. Parr, Lee, C., Yang, W., Parr, R. G., Development of the Colle-Salvetti Correlation Energy Formula into a Functional of the Electron Density, Phys. Rev. B 37, 785 (1988), Phys. Rev. B: Condens. Matter. 37 (1988) 785-789. [19] F. Weigend, R. Ahlrichs, Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy, Phys. Chem. Chem. Phys. 7 (2005) 3297-3305. [20] T.G. Frisch MJ, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF, Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T, Montgomery JA Jr, Peralta JE, Ogliaro F, Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN, Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC, Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M, Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C, Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas Ö, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ Gaussian-09 revision D.01. Gaussian Inc, Wallingford CT, (2009). [21] Q. Zheng, F. Xing, X. Li, T. Liu, Q. Lai, G. Ning, H. Zhang, Investigation on the performance evaluation method of flow batteries, J. Power Sources. 266 (2014) 145-149. [22] Z.P. Li, B.H. Liu, K. Arai, S. Suda, A Fuel Cell Development for Using Borohydrides as the Fuel, J. Electrochem. Soc. 150 (2003) A868-A872. 26
[23] L. An, T.S. Zhao, An alkaline direct ethanol fuel cell with a cation exchange membrane, Energy Environ. Sci. 4 (2011) 2213-2217. [24] C.E. Ren, K.B. Hatzell, M. Alhabeb, Z. Ling, K.A. Mahmoud, Y. Gogotsi, Charge- and Size-Selective Ion Sieving Through Ti3C2Tx MXene Membranes, J. Phys. Chem. Lett. 6 (2015) 4026-4031. [25] J. Mcbreen, Rechargeable zinc batteries, J. Electroanal. Chem. Interfacial Electrochem. 168 (1984) 415-432. [26] H.S. Sodaye, P.K. Pujari, A. Goswami, S.B. Manohar, Probing the microstructure of Nafion‐117 using positron annihilation spectroscopy, J. Polym. Sci., Part B: Polym. Phys. 35 (2015) 771-776. [27] D. Guo, A.N. Lai, C.X. Lin, Q. Zhang, A. Zhu, Q. Liu, Imidazolium functionalized poly (arylene ether sulfone)s anion exchange membranes densely grafted with flexible side chains for fuel cells, ACS Appl. Mat. Interfaces. 8 (2016) 25279. [28] J. Ferrer, E. San-Fabián, Competition for water between protein (from Haloferax mediterranei) and cations Na+ and K+: a quantum approach to problem, Theor. Chem. Acc. 135 (2016) 228. [29] N.W. DeLuca, Y.A. Elabd, Polymer electrolyte membranes for the direct methanol fuel cell: A review, J. Polym. Sci., Part B: Polym.Phys. 44 (2006) 2201-2225.
27
Fig. 1. (a) The permeation rate of OH- by testing the PH in the diffusion side. (b) The permeation rates of Na+ and K+ by testing the conductivity of solution in the diffusion side. Fig. 2. (a) The battery performance of alkaline zinc iron flow battery with different kind of electrolyte (b) Charging-discharging curves of the alkaline zinc iron flow battery with different kind of electrolytes. (c) Long-term stability of the alkaline zinc iron flow battery. Insert, charge and discharge curves. (d) The corresponding discharge capacity and discharge energy values of 100 cycles. Fig. 3. (a) SAXS patterns of three kinds of different Nafion 212 membranes. (b) Scheme of ion cluster size of Nafion 212 membrane in NaOH and KOH solutions. AFM tapping mode phase images of the membrane: (c) Nafion 212-H2O, (d) Nafion 212-NaOH, (e) Nafion 212-KOH. Fig. 4. (a) Swelling and (b) electrolyte uptake of Nafion 212 membrane in ultrapure water, NaOH and KOH solution, respectively; Ion permeability of Na-Nafion membrane (c) and K-Nafion membrane (d) using NaOH and KOH as the feeding solution, respectively. Fig. 5. The interaction of Nafion sulfonic membranes and cations. Here, the chain represents the partly segment structure of the Nafion sulfonic membrane. Table 1 Battery performance of recently reported alkaline based flow battery systems using Nafion cation exchange membranes. Table 2 The binding energy of Nafion and cations.
28
Highlights •
Ions transfer behavior and mechanism in Nafion under alkaline media are clarified.
•
The ions transfer in Nafion is studied via experimental and computational method.
•
Nafion membrane shows higher ion conductivity in NaOH than those in KOH solution.
•
A battery using NaOH as supporting electrolyte shows an EE of ~86% at 80 mA cm-2.
29
PII: DOI: Reference:
S0376-7388(18)31859-3 https://doi.org/10.1016/j.memsci.2018.08.057 MEMSCI16431
To appear in: Journal of Membrane Science Received date: 16 July 2018 Revised date: 24 August 2018 Accepted date: 26 August 2018 Cite this article as: Jing Hu, Huamin Zhang, Wenbin Xu, Zhizhang Yuan and Xianfeng Li, Mechanism and transfer behavior of ions in Nafion membranes under alkaline media, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.08.057 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.
Mechanism and transfer behavior of ions in Nafion membranes under alkaline media
Jing Hua, b, Huamin Zhanga, c, Wenbin Xua,c, Zhizhang Yuana,*, and Xianfeng Lia, c,* a
Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, 457 Zhongshan Road, Dalian 116023 (P. R. China). b
c
University of Chinese Academy of Sciences, Beijing 100049 (P. R. China).
Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023
(P. R. China).
Email: [email protected] ; [email protected].
*Corresponding author at: Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023 (P. R. China).
1
Abstract Nafion series cation exchange membranes are extensively investigated and applied in proton exchange membrane fuel cells and flow battery technologies because of their excellent stability and easy availability. However, a deep understanding of their ions transport mechanism and behavior under the alkaline based flow battery media is very limited. Here, the ion transport mechanism through Nafion membrane under alkaline medium is investigated by small-angle X-ray scattering and atomic force microscope techniques. The results indicate that a membrane showed a higher degree of phase separation and larger cluster radius in a NaOH solution than those in a KOH solution, which endows the membrane with a much higher ion conductivity in the NaOH solution. Density functional theory-based simulation also indicates that the adsorption and desorption between the Na+ and the –SO3- in Nafion is faster than those of K+. Inspired by these results, an alkaline zinc iron flow battery with a Nafion 212 membrane using NaOH as the supporting electrolyte exhibits a coulombic efficiency of ~99% and an energy efficiency of ~86% at 80 mA cm-2. This work offers insights into ways to obtain an improved battery performance for the existing as well as the emerging alkaline based flow battery technologies.
2
Graphical abstract
3
Keywords: Nafion membrane; Ion transfer mechanism; Density functional theory-based simulation; Zinc iron flow battery
4
1. Introduction In the past decade flow battery technologies received much attention due to their urgent need in wide application renewables like solar and wind power. Aqueous flow battery technologies, represented by vanadium flow battery and zinc bromine flow battery, are at the stage of commercial demonstration [1]. However, there remained some critical challenges, e.g. the relatively high cost of all-vanadium redox pairs, the relatively low electrochemical activity and corrosion of bromine couples. Hence, there appears some new redox couples and flow-type systems, e.g. alkaline quinone based and organic flow battery systems [2, 3], acidic Fe-V flow battery system [4] and neutral polymer based flow battery systems [5]. On the basis of design strategy of a flow battery, an ion conducting membrane is employed to separate the anode and cathode while still transfer charge-balancing ions to complete the internal circuit [6, 7]. The properties of an ion conducting membrane have great influence on the battery performance. Currently, numerous ion conducting membranes and membrane materials have been developed for battery applications. Among which, Nafion series cation exchange membranes are the most widely used membrane materials due to their high ion conductivity, excellent stability and easy availability [8]. It has been widely accepted that Nafion membranes transport protons through hopping mechanism and vehicle mechanism in proton exchange membrane fuel cell (PEMFC) and flow battery systems involving in acidic media [9]. And for those flow batteries using acidic supporting electrolyte such as vanadium flow battery and quinone bromine flow battery, a piece of Nafion membrane can normally endow them with high voltage efficiency because of the high proton conductivity of the membrane. 5
In addition to quinone bromine flow battery and Fe-V flow battery, of which acidic electrolyte is employed, most of recently reported aqueous flow battery systems are working under alkaline or neutral conditions, where the Nafion cation exchange membranes are still utilized prevailingly. However, critically different from acidic based flow batteries, alkaline based flow batteries using a Nafion membrane normally afforded a relatively low voltage efficiency (Table 1) since a different ion transport mechanism and behavior maybe involved in alkaline media. For instance, an all-soluble all-iron aqueous flow battery using a Nafion 212 membrane afforded a voltage efficiency of 78% at 40 mA cm-2 [10]. A polysulfide iodide flow battery using a Nafion 117 membrane showed a voltage efficiency of 72.15% at 15 mA cm -2 [11]. An alkaline zinc ferrocyanide flow battery with a Nafion 115 membrane delivered a voltage efficiency of 79% at 80 mA cm-2. Whereas the alkaline zinc ferrocyanide flow battery with a polybenzimidazole (PBI) anion exchange membrane demonstrated a voltage efficiency of 88% at the same condition [12]. Judging from the discussions above, Nafion cation exchange membranes tell a different story in alkaline media, where the fundamental aspects of Nafion membranes under alkaline media especially the ion transport mechanism through the membrane are unclear. This in turn results in lacking strategies to optimize as well as improve battery performance from membrane and electrolyte aspects. Therefore, it is of very importance to clarify the fundamental ion transport mechanism of Nafion membranes under alkaline condition, which would direct further improvement in the performance of the existing alkaline based flow battery systems and open up a new perspective on utilization of cation exchange membranes for the emerging alkaline based flow battery technologies.
6
Table 1 Battery performance of recently reported alkaline based flow battery systems using Nafion cation exchange membranes. Aqueous flow battery
Supporting
Membrane
Performance
(mA cm-2)
electrolyte
Fe(TEOA)OH/
Current density
CE(%)
VE(%)
NaOH
Nafion 212
40
93
78
KI/K2S2 [11]
KOH
Nafion 117+115
15
94
72
Zn/Fe [13]
NaOH
Nafion 212
80
99
76
DHAQ/K4Fe(CN)6 [14]
KOH
Nafion 212
100
99
84
DHBQ/K4Fe(CN)6 [15]
KOH
Nafion 115
100
99
66
ACA/K4Fe(CN)6 [16]
KOH
Nafion 212
100
99
63
FMN/K4Fe(CN)6 [17]
KOH
Nafion 212
80
99
55
This work
NaOH
Nafion 212
80
99
86
K4Fe(CN)6 [10]
In this study, aimed at gaining deep insight into the fundamental aspects of Nafion cation exchange membranes under alkaline based flow battery media, we present an experimental and computational strategy that links different physical and electrochemical behaviors of Nafion in different kinds of electrolytes. This strategy, utilized to an alkaline zinc iron flow battery as platform, demonstrates that a battery with Nafion membrane can afford high 7
performance in alkaline media as well. In terms of unraveling behaviors of Nafion under alkaline condition, the presented research is expected to provide insights into ways to afford high battery performance for the existing as well as the emerging alkaline based flow batteries. 2. Experimental Section 2.1 Materials Potassium ferrocyanide, potassium hydroxide, sodium chloride and potassium chloride were purchased from Tianjin Damao Chemical Reagent Factory. Sodium hydroxide was purchased from Tianli Chemical Reagent Co.,Ltd. Sodium ferrocyanide was bought from Sinopharm Chemical Regent Co.,Ltd. Zinc oxide was bought from Kermel Chemical Reagent Factory. These reagents were supplied with analytical grade. Other reagents were brought from Sigma Aldrich unless stated otherwise and used as received. 2.2 Battery Performance The alkaline zinc iron flow battery was assembled by sandwiching the Nafion membrane (Nafion 115, 211, 212, respectively) between two carbon felt electrodes, clamped by two graphite plates. The active area of the electrode is 48 cm2. All of these components were fixed between two stainless steel plates. The electrolyte was cyclically pumped through the corresponding electrodes in airtight pipelines. Charge-discharge cycling tests were conducted by ArbinBT 2000 at a constant current density of 80 mA cm-2. The charge process was controlled by the charge time to keep a constant charge capacity, while the discharge process was ended with the cut-off voltage of 0.1 V. 2.3 Small angle X-ray scattering (SAXS) 8
SAXS patterns of the Nafion membranes treated with ultrapure water, NaOH and KOH solution were recorded on a Rigaku Smart Lab between 0.6° and 5°. Before both tests, all the samples were soaked in ultrapure water (Nafion 212-H2O), 2 M NaOH (Nafion 212-NaOH), and 2 M KOH (Nafion 212-KOH), respectively for at least 48 h. The scattering vector was calculated according to equation: q 2 sin /
(1)
where 2θ is the scattering angle and λ is the scattering wave length, 0.1541 nm. The Bragg spacing was calculated as follows:
d
2 q
(2)
2.4 Atomic force microscope (AFM) The nanophase-separated morphology of membranes was probed by tapping mode AFM. Tapping mode AFM was performed using an Oxford instrument (CypherES). A silicone cantilever (55 µm long) with an end radius 7 ± 3 nm，resonance frequencies 850–2500 kHz and spring constants 38–184 Nm−1 was used to image the samples. 2.5 Swelling and electrolyte uptake To detect the electrolyte uptake and swelling of the membranes, the membranes were firstly soaked in ultrapure water (Nafion 212-H2O), 2 M NaOH (Nafion 212-NaOH) and 2 M KOH (Nafion 212-KOH), respectively for at least 48 h. The weight of the saturated membranes was then obtained after quickly wiping off the surface electrolyte using a tissue. The swelling is defined as the length ratio of the swollen membranes to the dry membranes, as shown in equation:
9
Swelling (%)
Ls Ld 100 Ld
(3)
where Ls and Ld are the lengths of the saturated and dry membranes, respectively. The water uptake is defined as the weight ratio of the absorbed electrolyte to the dry membrane, as shown in equation: Wateruptake(%)
Ws Wd 100 Wd
(4)
where Ws and Wd are the weights of the saturated and dry membranes, respectively. 2.6 OH- ion permeation and membrane ion conductivity tests The OH- ion permeation test was carried out using a diffusion cell shown in Fig. S4. The effective membrane area is 9 cm2. For the measurement, 80 mL alkaline solution (2 M KOH or 2 M NaOH) and 80 mL ultrapure water were added into the draw (left) side and feed (right) side, respectively. The pH or conductivity of right side solution was detected at a regular time interval by using Mettler Toledo pH meter model or conductivity model, respectively. The OH- concentration (COH-) was calculated from the pH data according to the following equation: pH = 14 + lg(COH-)
(5)
By monitoring the pH, the concentration of OH- thus can be obtained. All the experiments were conducted at room temperature. 2.7 Energy dispersive X-ray spectroscope (EDS) The surface element distribution of Nafion 212 membrane was recorded by scanning electron microscope (JSM-7800F) equipped with energy dispersive X-ray spectroscope (EDS). The EDS spectrum of the membrane surface confirmed that the Na-Nafion membrane treated with 10
KOH for 48 h was composed of the elements carbon, oxygen, fluorine, sulfur and potassium, but a little sodium, which suggested that Na-Nafion could be translated into K-Nafion in the KOH solution. Same thing, K-Nafion membrane treated with NaOH for 48 h was confirmed the elements of carbon, oxygen, fluorine, sulfur and sodium but a little potassium, which suggested that K-Nafion could be translated into Na-Nafion in the NaOH solution. 2.8 Density functional theory (DFT) calculations In the DFT calculation, structures of the Nafion membrane and the hydrated cations-(H2O)6 were optimized by B3LYP [18] and Def2-SVP [19], in which cations were located beside the Nafion structure at the most stable state. These calculations were carried out using the Gaussian 09 software package [20]. The binding energies were described by the Def2-TZVP basis sets. In our model, the ionic radius of hydrated potassium cation (K(H2O)6+) is approximately 2.70 Å from the mass center, and the ionic radius of hydrated sodium cation (Na(H2O)6+) is approximately 2.38 Å. Besides, the interactions between the cations and the Nafion 212 were provided during the calculation of the binding energies. The binding energies between the cations and the Nafion 212 membrane were estimated by the following equation: ENF-cation = ENF + E cation – E (NF+ cation)
(6)
where NF represented Nafion 212 membrane, cation represented K+ and Na+. where ENF and E
cation
are the total energy of Nafion 212 membrane and cation, E
(NF+ cation)
refers to the total energy of the hybrid system consisting of both Nafion 212 and cation. Thus, the calculated ENF-cation represents the binding energy between the cations and Nafion 212 membranes. 11
3. Results and discussion In our previous work [12], we reported an alkaline zinc iron using a PBI anion exchange membrane for stationary energy storage. As a reference, a piece of Nafion 115 cation membrane with a thickness of ~125 µm was employed. Unfortunately, the battery with a Nafion 115 membrane delivered a relative low voltage efficiency (VE) of 79% at 80 mA cm -2. It is expected that the utilization of thinner membranes like Nafion 212 (~50 µm) or Nafion 211 (~25 µm) can lower battery resistance and boost the battery performance. We thus firstly investigated the battery performance by using a Nafion 212 or a Nafion 211 membrane. Unexpectedly, the battery with different thickness of Nafion membranes showed similar performance (Fig. S1), which is critically different from that of vanadium flow battery, where a decreased coulombic efficiency (CE) and an increased VE is confirmed with reduced Nafion membrane thickness [21]. It is easy to understand the high CE (~100%) of the alkaline zinc iron flow battery even assembled the thinnest Nafion 211 membrane, which is resulted from the exclusion effect between the negatively charged sulfonic acid groups in Nafion and the redox couples (Fe(CN)63-/Fe(CN)64- and Zn(OH)42-). However, the alkaline zinc iron flow battery using Nafion membranes with different thickness showed a similar VE (~80%), suggesting that the thickness of a Nafion membrane has no significant impacts on an alkaline based battery but other factors may do. Hence, clarifying the underlying reasons behind this phenomenon will provide an important reference for the research and application of Nafion membranes in the existing and emerging alkaline based flow battery technologies. For this purpose, a sheet of Nafion 212 membrane was selected for further study. Normally, in alkaline media, where KOH or NaOH is employed, Nafion cation exchange 12
membranes are supposed to transfer K+ or Na+ via ion exchange mechanism, while reject OHbecause of exclusion effect between the negatively charged sulfonic acid groups and the OHanions [22, 23]. Actually, OH- can penetrate the membrane as well due to the swelling behavior of Nafion in aqueous media, which results from the hydrophobic Teflon backbone and hydrophilic sulfonic acid groups in the polymer backbone, further leading to the membrane with a phase separated structure. Thus to investigate the permeation rate of OHthrough the Nafion membrane, a diffusion experiment using a Nafion 212 membrane was carried out as shown in Fig. 1a. Remarkably, by using NaOH as the feeding solution, the OHpermeation rate, calculated from the slope of the line, is higher than does the membrane using KOH as the feeding solution. According to the principle of charge conservation, the permeation of OH- through Nafion membrane from the feeding side to the diffusion side would be inevitably associated with the permeation of K+ or Na+, which is mainly derived from the ion exchange mechanism (ion exchange between cations and the –SO3-) and the free diffusion of cations in the hydrophilic channels (formed by hydrophilic sulfonic acid groups in Nafion). The higher OH- permeation rate reflect higher cations permeation rate indirectly, that is, the transportation of Na+ through Nafion 212 membrane is faster than the K+.
13
Fig. 1. (a) The permeation rate of OH- by testing the PH in the diffusion side. (b) The permeation rates of Na+ and K+ by testing the conductivity of solution in the diffusion side. To further verify the permeation rates of Na+ and K+, another diffusion experiment using a NaCl and KCl as the feeding solution was conducted. The concentration of the cations in the diffusion side was then calculated according to the conductivity (Λm = (κ/c), where κ is the conductivity of solution in the diffusion side, and c is the concentration of the cations) [24]. The conductivity of NaCl or KCl with a certain concentration was first measured (Fig. S2). The molar conductivity (Λm) of NaCl in the diffusion side, obtained from the slope of the line, is lower than does the KCl, suggesting higher concentration of NaCl in the diffusion side (Fig. 1b), further confirming the higher permeation rate of Na+ through Nafion 212 membrane. Inspired by the above results, an alkaline zinc flow battery employing NaOH as the supporting electrolyte is expected to deliver a higher performance than does a battery using KOH as the supporting electrolyte. As expected, by using 2 M NaOH as the supporting electrolyte for both positive and negative side, the battery demonstrates a CE of ~100% and a VE of 86.46% at the current density of 80 mA cm-2. By contrast, when using 2 M KOH as the supporting electrolyte, a much lower VE of 79.44% is afforded at the same working current density (Fig. 2a). To further confirm the universal applicability, the performance of an alkaline 14
zinc iron flow battery with a Nafion 115 membrane is also tested. As expected, the battery with a Nafion 115 membrane demonstrates a much higher VE when using NaOH as the supporting electrolyte than does a battery using KOH as the supporting electrolyte at the current density of 80 mA cm-2 (Fig. S3). Fig. 2b shows that alkaline zinc iron flow battery using NaOH as supporting electrolyte demonstrated a lower charging voltage and higher discharging voltage than does a battery using the KOH as the supporting electrolyte, indicating that the Nafion 212 membrane in NaOH media has a lower resistance, which is consistent with permeation results. Furthermore, by using NaOH as the supporting electrolyte, an alkaline zinc iron flow battery afforded a stable performance over more than 100 cycles at the current density of 80 mA cm-2, maintaining an average CE of ~100% and an average EE of 82%, which is much higher than the previously reported alkaline zinc iron flow battery, where only a CE of 76% and an EE of 61.5% was demonstrated at the current density of 35 mA cm-2 [25]. In addition to the stable cycling performance, the battery showed a discharge capacity of ~7.5 Ah L-1 and a discharge energy of ~12 Wh L-1, respectively, yielding an electrolyte utilization of ~70% (theoretical capacity of 10.7 Ah L-1) at 80 mA cm-2 (Fig. 2c, d).
15
Fig. 2. (a) The battery performance of alkaline zinc iron flow battery with different kind of electrolyte (b) Charging-discharging curves of the alkaline zinc iron flow battery with different kind of electrolytes. (c) Long-term stability of the alkaline zinc iron flow battery. Insert, charge and discharge curves. (d) The corresponding discharge capacity and discharge energy values of 100 cycles. It is generally accepted that the ion conductivity of a Nafion membrane heavily depends on the microstructure. To uncover the underlying reasons for the differences in ion conductivity as well as the battery performance of Nafion membrane in different kinds of alkaline supporting electrolytes, the microstructures of Nafion 212 membranes, treated with ultrapure water, NaOH solution and KOH solution (denoted as Nafion 212-H2O, Nafion 212-NaOH, Nafion 212-KOH), respectively, were analyzed by the small-angle X-ray scattering (SAXS) 16
technique. The SAXS patterns in Fig. 3a shows a maximum scattering ionomer peak at around q = 1.49 nm-1 for the original Nafion 212 membrane, while the corresponding peaks of Nafion 212 membranes treated with NaOH and KOH solutions shifted to higher angle at around q = 1.71 nm-1 and q = 2.56 nm-1, respectively. This shift was mainly attributed to higher cationic electron density since the protons in the sulfonic acid groups were exchanged by Na+ or K+ after treating in NaOH and KOH solutions. The Bragg spacing d of Nafion 212-H2O, Nafion 212-NaOH, Nafion 212-KOH obtained from Bragg equation is 4.2 nm, 3.68 nm, and 2.45 nm, respectively (Table S1), indicating the sizes of hydrophilic clusters, which is in accordance with those reported in the previous literature [26]. The present results indicated that Nafion 212 membrane treated with NaOH solution has a higher degree of phase separation and larger cluster radius than does the one treated with KOH solution (Fig. 3b). Tapping mode atomic force microscope (AFM) was used to further identify the differences in phase-separated structure of Nafion 212 in different solutions. Fig. 3c shows the connective ion transfer channels of Nafion 212 membrane in ultrapure water. Nafion 212 membrane in NaOH solution exhibits a clear phase-separated structure as well (Fig. 3d). By contrast, the Nafion 212 membrane shows a relatively low degree of phase-separated structure in KOH solution (Fig. 3e), of which the dark regions in the phase image correspond to the soft regions of the highly hydrophilic ionic clusters while the light portions refer to the hard structure of hydrophobic polytetrafluoroethylene backbone chain along with regularly spaced shorter perfluorovinyl ether side-chains [27]. The higher degree of phase-separated structure of Nafion 212 membrane would present a higher membrane swelling behavior and absorb more electrolyte in the hydrophilic regions, thus affording the membrane with higher ion 17
conductivity. The large cluster radius together with high phase-separated structure of Nafion 212 membrane in NaOH solution contribute enormously to the enhanced VE in Fig. 2a and are expected to act as guidance for other alkaline based flow battery systems.
Fig. 3. (a) SAXS patterns of three kinds of different Nafion 212 membranes. (b) Scheme of ion cluster size of Nafion 212 membrane in NaOH and KOH solutions. AFM tapping mode phase images of the membrane: (c) Nafion 212-H2O, (d) Nafion 212-NaOH, (e) Nafion 212-KOH. The membrane properties like swelling, water uptake and ion permeability are closely related to the micro structure of the membrane. Therefore, we performed experimental validation on membranes with different degree of swelling and water uptake in macroscopic dimensions. The Nafion 212 membrane demonstrated the highest swelling (Fig. 4a) and electrolyte uptake 18
(Fig. 4b) in ultrapure water, while the lowest in KOH solution. A similar trend can be found in Nafion 115 as well (Fig. S5). This higher swelling and electrolyte uptake are in favor of faster transportation of ions in the membrane, which is determined by the microstructure of a membrane. The ion permeation tests were carried out using a diffusion cell as shown in Fig. S4 to identify the different ion transfer rate caused by different ion cluster size of Nafion 212 in NaOH and KOH solutions (Na-Nafion and K-Nafion). As expected, the Na-Nafion membrane demonstrated a much higher permeation rate in NaOH solution than does a membrane in KOH solution (Fig. 4c), which suggested that Na-Nafion could be translated into K-Nafion in the KOH solution. Energy dispersive X-ray spectroscope (EDS) measurement in Fig. S6 can also confirms this exchange. Barely phase-separated structure along with small cluster radius of K-Nafion, thus afforded the membrane with a much lower ion permeation rate in a KOH solution than does the membrane in a NaOH solution. Similar tendency was observed for K-Nafion in the NaOH and KOH solutions (Fig. 4d), where a higher permeation rate can be afforded in the NaOH solution. These results suggested again that a Nafion membrane exhibits a higher ion conductivity in a NaOH solution and an alkaline based flow battery employing Nafion as membrane is expected to achieve an improved battery performance by using NaOH as the supporting electrolyte.
19
Fig. 4. (a) Swelling and (b) electrolyte uptake of Nafion 212 membrane in ultrapure water, NaOH and KOH solution, respectively; Ion permeability of Na-Nafion membrane (c) and K-Nafion membrane (d) using NaOH and KOH as the feeding solution, respectively. Density Functional Theory (DFT) calculations were carried out to gain an in-depth understanding of the difference in ion cluster as well as the hydrophilic channel of the Nafion 212 membranes in different solutions. The structures of the hydrated K+ and Na+ are shown in Fig. S7. The ionic radius of hydrated potassium cation (K(H2O)6+) is 2.70 Å, which is relative larger than does a hydrated sodium cation (Na(H2O)6+ 2.38 Å), shown in Fig. S8. Hence, the K(H2O)6+ has a lager spatial resistance than that of (Na(H2O)6+ when entering into the ion cluster channel, which leads to a lower K+ conductivity. Considering the higher spatial 20
resistance of hydrated potassium cation, less water molecule can be brought into the hydrophilic region of the Nafion 212 membrane, which has been a contributing factor for the lower electrolyte uptake of the membrane in KOH solution. To further verify this, the hydration energy of hydrated K+ and Na+ were calculated as given in Fig. S8. The hydration energy of hydrated K+ cation that bonds with sulfonic acid group is 85.98 kcal mol-1, which is lower than that of the hydrated Na+ cation (108.50 kcal mol-1). This indicates that the ability of carrying water molecules of the hydrated K+ cation is weaker than that of the hydrated Na+ cation, thus further leading to a smaller cluster radius and a lower degree of phase-separated structure of the Nafion 212 membrane in KOH solution [28]. In addition to the membrane microstructure that affects the ion conductivity of a membrane to a great extent, the rapid adsorption and desorption rate between the cations and the ion exchange groups also has an enormous impact on membrane ion conductivity [29], which is expected to turn into an important contributor to enhance the ion conductivity of a membrane as well, and thus further boosting the battery performance. To verify the above speculation, the binding energy between the cations and sulfonic acid groups was then calculated according to DFT (Fig. 5). The binding energy of SO3- anion and Na+ cation (ENFSO3-Na) is defined as the energy difference between the single energies of SO3- anion (ENFSO3-) and cation (ENa+) and the total energy of hybrid system (E(NFSO3-+ Na+)). Thus, ENFSO3-Na = ENFSO3- + ENa+− E(NFSO3-+
+ Na ).
Here, E(NFSO3-+
+ Na )
refers to the total energy of the hybrid system
consisting of both NFSO3- anion and Na+. Similarly, ENFSO3-K = ENFSO3- + EK+ − E(NFSO3-+ K+). The simulation results were shown in Fig. 5 and Table 2, where the binding energy of Na+ (K+) and SO3- group is 120.61 kcal mol-1 (105.44 kcal mol-1). The larger binding energy of Na+ and 21
SO3- group indicated a faster adsorption and desorption rate between the Na+ and the SO3groups, which would promote the conductivity of Na+ in the Nafion 212 membrane. This served as another important factor that could account for the higher performance of an alkaline zinc iron flow battery when using NaOH as the supporting electrolyte than does a battery using KOH as the supporting electrolyte.
Fig. 5. The interaction of Nafion sulfonic membranes and cations. Here, the chain represents the partly segment structure of the Nafion sulfonic membrane. Table 2 The binding energy of Nafion and cations.
Binding energy (kcal mol-1)
ENFSO3-Na
ENFSO3-K
120.61
105.44
22
4. Conclusion In summary, we have investigated the mechanism and behavior of ions transfer in a Nafion 212 membrane under alkaline zinc iron flow battery media in very detail. The results indicated that a Nafion 212 membrane exhibits a higher swelling and electrolyte uptake in a NaOH solution than that of in a KOH solution, which was resulted from the higher degree of phase separated structure and larger cluster radius of the Nafion 212 in the NaOH solution. Density functional theory-based simulation verified that the adsorption and desorption between the Na+ cation and the –SO3- in Nafion is faster than those of K+ cation. The faster capacity of ion exchange for Na+ cation and higher degree of phase separation together with larger cluster radius of the Nafion 212 endow the membrane with a higher ion conductivity in a NaOH solution than that of in a KOH solution. As a consequence, an alkaline zinc iron flow battery with a Nafion 212 membrane using NaOH as the supporting electrolyte exhibited a much higher battery performance than does a battery using KOH as the supporting electrolyte. The results present here may provide effective ways to significantly improve the performance of the existing as well as the emerging alkaline based flow batteries. Acknowledgements The authors greatly acknowledge the financial support from China Natural Science Foundation
(Grant
Nos.
21206158),
Key
project
of
Frontier
Science,
CAS
(QYZDB-SSW-JSC032), CAS-DOE collaborative project and DICP funding (ZZBS201707). The authors also greatly thank Prof. Qiang Fu and Dr. Chuanhai Xiao at DICP for help with the atomic force microscope measurements. Appendix A. Supplementary material 23
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Fig. 1. (a) The permeation rate of OH- by testing the PH in the diffusion side. (b) The permeation rates of Na+ and K+ by testing the conductivity of solution in the diffusion side. Fig. 2. (a) The battery performance of alkaline zinc iron flow battery with different kind of electrolyte (b) Charging-discharging curves of the alkaline zinc iron flow battery with different kind of electrolytes. (c) Long-term stability of the alkaline zinc iron flow battery. Insert, charge and discharge curves. (d) The corresponding discharge capacity and discharge energy values of 100 cycles. Fig. 3. (a) SAXS patterns of three kinds of different Nafion 212 membranes. (b) Scheme of ion cluster size of Nafion 212 membrane in NaOH and KOH solutions. AFM tapping mode phase images of the membrane: (c) Nafion 212-H2O, (d) Nafion 212-NaOH, (e) Nafion 212-KOH. Fig. 4. (a) Swelling and (b) electrolyte uptake of Nafion 212 membrane in ultrapure water, NaOH and KOH solution, respectively; Ion permeability of Na-Nafion membrane (c) and K-Nafion membrane (d) using NaOH and KOH as the feeding solution, respectively. Fig. 5. The interaction of Nafion sulfonic membranes and cations. Here, the chain represents the partly segment structure of the Nafion sulfonic membrane. Table 1 Battery performance of recently reported alkaline based flow battery systems using Nafion cation exchange membranes. Table 2 The binding energy of Nafion and cations.
28
Highlights •
Ions transfer behavior and mechanism in Nafion under alkaline media are clarified.
•
The ions transfer in Nafion is studied via experimental and computational method.
•
Nafion membrane shows higher ion conductivity in NaOH than those in KOH solution.
•
A battery using NaOH as supporting electrolyte shows an EE of ~86% at 80 mA cm-2.
29