Enzyme-Inspired Formulation of the Electrolyte for Stable and Efficient

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Enzyme-Inspired Formulation of the Electrolyte for Stable and Efficient Vanadium Redox Flow Batteries at High Temperatures Saleem Abbas,†,‡,∇ Jinyeon Hwang,†,∇ Heejin Kim,§,∇ Seen Ae Chae,∥ Ji Won Kim,∥ Sheeraz Mehboob,†,‡ Ahreum Ahn,⊥ Oc Hee Han,*,∥,#,¶ and Heung Yong Ha*,†,‡

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†

Center for Energy Storage Research, Korea Institute of Science and Technology (KIST), 14-gil 5, Hwarang-ro, Seongbuk-gu, Seoul 02792, Republic of Korea ‡ Division of Energy & Environmental Engineering, Korea University of Science & Technology (UST)-KIST School, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea § Electron Microscopy Center, Korea Basic Science Institute, Daejeon 34133, Republic of Korea ∥ Western Seoul Center, Korea Basic Science Institute (KBSI), Seoul 03759, Republic of Korea ⊥ Center for Computational Science and Engineering, Korea Institute of Science and Technology Information, Daejeon 34141, Republic of Korea # Graduate School of Analytical Science & Technology, Chungnam National University, Daejeon 34134, Republic of Korea ¶ Department of Chemistry & Nano Science, Ewha Womans University, Seoul 03760, Republic of Korea S Supporting Information *

ABSTRACT: Histidine, inspired by vanadium bromoperoxidase enzyme, has been applied as a homogeneous electrocatalyst to the positive electrolyte of vanadium redox flow battery (VRFB) to improve the performance and stability of VRFB at elevated temperatures. The histidine-containing electrolyte is found to significantly improve the performance of VRFB in terms of thermal stability estimated by the remaining amount of VO2+ in the electrolyte (61 vs 43% of a pristine one), energy efficiency at a high current density of 150 mA cm−2 (78.7 vs 71.2%), and capacity retention (73.2 vs 27.7%) at 60 °C. The mechanism of the catalytic functions of histidine with the chemical species in the electrolyte has been investigated for the first time by multinuclear NMR spectroscopy and first-principles calculations. The analyzed data reveal that histidine improves the kinetics of both charge and discharge reactions through different affinity toward the reactants and products as well as suppresses the precipitation of VO2+ by impeding the polymerization of vanadium ions. These findings are in good agreement with the improved chemical and electrochemical performance of the histidine-containing VRFB. Our results show a new type of chemical/electrochemical mechanism in the improved redox flow battery performance that may be essential in a new research arena for better performance of electrochemical systems. KEYWORDS: redox flow batteries, histidine, electrocatalyst, metal coordination, nuclear magnetic resonance, first-principles calculations, electrolyte stability

1. INTRODUCTION

large amounts of energy. Unlike many other RFBs, all vanadium redox flow batteries (VRFBs) are safe and have long lifespans by utilizing the redox-active materials solubilized

Redox flow batteries (RFBs) are rechargeable secondary batteries that utilize either metallic ions or organic molecules as electrolytes to convert electrical energy into chemical energy or vice versa.1−7 They are energy storage systems with energy capacities proportional to the amounts of redox-active materials in the electrolytes, and therefore capable of storing © XXXX American Chemical Society

Received: April 18, 2019 Accepted: July 3, 2019 Published: July 3, 2019 A

DOI: 10.1021/acsami.9b06790 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces in aqueous electrolytes.8−12 The system has storage capacities up to several hours which is an important requirement for integration with electrical power plants.13,14 However, current VRFBs suffer from relatively low energy density and energy efficiency (EE) when applied to practical situations, with their state-of-the-art maximum energy density and EE around 30 W h L−1 and 80%, respectively.15 Recently, the broad temperature adoptability of VERB has been comprehensively studied by evaluating the effect of a wide range of temperatures (−20 to 50 °C) on physicochemical and electrochemical properties of the vanadium electrolytes,16 single cell operation,17 cation and anion exchange membranes,18 and activities of electrocatalysts.19 The performance of VRFBs can be improved by enhancing solubility of vanadium ions in the electrolytes and stability at high temperatures. However, the VO2+ ions become increasingly unstable and prone to form irreversible V2O5 precipitates at high temperatures above 40 °C. The precipitates not only deteriorate the performance but also reduce the lifespan of VRFB.20−22 In order to tackle this problem, many attempts have been made to stabilize the vanadium electrolytes by introducing various types of additives directly into the electrolytes.23−27 Although some inorganic additives such as chloride and sulfate ions reportedly improved the performance to some degree,15,28−30 they might cause adverse effects including gas generation. On the other hand, organic additives could reduce side reactions and the irreversible precipitation by forming complex ions with vanadium ions even at high temperatures.31−35 Electrochemically stable organic additives may be able to improve the performance of VRFBs by facilitating the redox reactions, reducing the vanadium crossover, and preventing the precipitation of vanadium ions at high temperatures. Recently, stabilization of VRFB electrolytes has been reported with some amino acids having high solubility in water and strong coordination capability.35−39 However, the improvements in the real VRFB performance were reported only at or below room temperatures (RTs). Therefore, the net contribution of these amino acids toward the thermal stability of VRFB is yet to be verified at high temperatures. Among all amino acids, histidine (C6H9N3O2, Scheme 1a) exclusively has an imidazole ring which renders chemical endurance in a wide pH range where most amino

acids break down. Therefore, histidine has been used in various fields such as pH regulation, proton transfer, metal binding, phosphorylation, and catalysis of enzymes and proteins.40−42 Refer to Scheme 1b,c for reported metal ion−histidine structures.43,44 Scheme 1b is an example of histidine molecules chelating metal ions.43 An example of integration of vanadium and histidine is vanadium bromoperoxidase, an enzyme having a vanadium ion located in its catalytic center and coordinated with histidine (Scheme 1c, modified from Figure 6 in ref 44). The enzyme has a peculiar structure capable of withstanding harsh acidic environments and is a type of haloperoxidase found in seaweeds and some types of marine fungi. Vanadium bromoperoxidase is known to be thermodynamically stable even at high temperatures and high concentrations of substrates.45−48 In this work, inspired by vanadium bromoperoxidase, histidine has been applied as a homogeneous electrocatalyst to the positive electrolyte to improve the stability and performance of VRFBs at elevated temperatures. Electrochemical and spectroscopic analyses have been conducted to elucidate the kinetics, binding mechanism of histidine with vanadium ions, and molecular structures of the histidine− vanadium complex ions. In particular, the multinuclear NMR spectroscopy together with the first-principles calculations has been tried to get the information on the dynamic interactions of the chemical species such as histidine, water, sulfate ions, and vanadium ions in the electrolyte solutions. This approach is expected to reveal in more detail how histidine positively influences the performance of VRFBs. This study is the first ever attempt to analyze the influence of an enzyme-inspired homogeneous electrocatalyst, histidine, contained in the electrolyte solutions, on the VRFB performance, using NMR spectroscopy combined with the first-principles calculations.

2. EXPERIMENTAL SECTION 2.1. Electrolyte Preparation. VOSO4·nH2O salt (Newell Solution, 98.2%) was added to 3.0 M sulfuric acid (H2SO4, Daejung Chemicals) to obtain a 1.5 M VOSO4 + 3.0 M H2SO4 positive electrolyte (VO2+) solution. Histidine (Sigma-Aldrich, 99%) was mixed with the electrolyte and then stirred for 2 h. V3+ and VO2+ electrolyte solutions were prepared by electrochemical reduction and oxidation of the VO2+ electrolyte solution, respectively. Finally, 1.5 M VO2+, 1.5 M V3+, and 1.5 M VO2+ electrolyte solutions were confirmed by UV−visible spectroscopy (Cary 5000, Varian Inc.). 2.2. Electrochemical Analysis. Cyclic voltammograms (CVs) were taken by a PGSTAT 302 electrochemical workstation (Metrohm Autolab B.V.) using a three electrode cell. A 1.0 cm2 graphite plate, a Pt wire, and an Ag/AgCl electrode (BASi, MF-2052 RE-5B prepared with aqueous 3 M NaCl saturated with AgCl) were used as working, counter, and reference electrodes, respectively. To measure the electrochemical kinetics, a 1.5 M VOSO4 + 3 M H2SO4 (pristine electrolyte) solution was used. The voltage range was from 0 to 1.6 V with a scan rate of 20 mV s−1. For Randles−Sevcik plots, scan rates were varied from 1 to 100 mV s−1. The electrochemical impedance spectroscopy (EIS) was conducted with an IVIUMSTAT Electrochemical Interface (IVIUM Technologies) at open-circuit potential over the frequency range from 100 kHz to 0.1 Hz. 2.3. Thermal Stability Test. Thermal stability of the electrolytes was tested in an oven at 40 °C. After every 1 h interval, 0.2 mL was taken from the VO2+ electrolyte solution for the vanadium ion concentration analyses by UV−visible spectroscopy. All samples were diluted to the ratio 1:20 by using a 3.0 M H2SO4 solution for determining the concentrations and calibration curves of VO2+ and VO2+, which were obtained from the absorbance at 760 and 400 nm, respectively, based on Beer−Lambert’s law.

Scheme 1. (a) Histidine Molecule, (b) Zinc Ion−Histidine Molecule Complex Structure and (c) a Vanadium Ion Coordinated with Histidine of Vanadium Bromoperoxidase, an Enzyme

B

DOI: 10.1021/acsami.9b06790 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. CVs for (a) positive electrolytes with various concentrations of histidine at the scan rate of 20 mV s−1 (b) relationship between the anodic and the cathodic peak current densities versus the square root of applied scan rates for a pristine and a 0.05 M histidine-containing electrolyte, (c) a long term stability test with a 0.05 M histidine-containing positive electrolyte up to 2000 voltammetric cycles at the scan rate of 20 mV s−1 and (d−f) Nyquist plots of pristine and 0.05 M histidine-containing positive electrolyte measured at 25−60 °C. The corresponding equivalent circuit used for the fitting is given in the inset of (d).

Table 1. Electrochemical Parameters Obtained from CVs histidine concentration (M)

Vpaa (V)

Vpcb (V)

ΔV (mV)

jpac (mA cm−2)

jpcd (mA cm−2)

jpa/jpc

0.00 0.03 0.04 0.05 0.06 0.08 0.10

1.44 1.32 1.18 1.16 1.25 1.25 1.38

0.67 0.71 0.79 0.81 0.75 0.75 0.67

773.00 605.50 396.00 344.00 495.50 505.00 714.00

36.92 40.79 47.03 48.14 44.09 43.64 36.29

−17.91 −21.75 −26.82 −27.92 −23.97 −23.47 −18.67

2.06 1.88 1.75 1.72 1.84 1.86 1.94

a

Anodic peak potential. bCathodic peak potential. cAnodic peak current density. dCathodic peak current density. acid in D2O for 1H, tetramethylsilane for 13C, deionized water for 17O, and neat vanadium oxytrichloride for the 51V NMR experiments. 2.6. Calculation Details. Molecular geometries and their energy values were investigated using the M06-L functional with 6-31+g** basis set and Grimme’s D3 dispersion correction (zero damping), as implemented in the Q-Chem software package. Solvent effects were considered by applying the universal continuum solvation model (SM8).49−52

2.4. VRFB Cell Test. Performance of the electrolytes was tested through charge−discharge cycles by using a single cell having an active electrode area of 25 cm2. The single cell (ILDO F&C Co., Ltd.) consisted of a H2O2-treated Nafion 115 as a membrane, 5 mm-thick graphite felt (Nippon carbon, GF-5) electrodes and two graphite bipolar plates. Each bipolar plate was in contact with a brass current collector that was embedded in an end plate made of bakelite plastic. The graphite felt electrodes were compressed to 3 mm (40% compression) when assembling to the cell fixture. The total volumes of the electrolyte, anolyte, and catholyte solutions were 50 mL each and they were circulated through the cell at a flow rate of 50 mL min−1 by peristaltic pumps. The cutoff voltage window of charge− discharge cycles was set from 0.7 to 1.6 V, and the applied current density was varied from 50 to 150 mA cm−2. 2.5. NMR Analysis. All of the 1H, 13C, 17O, and 51V NMR experiments were carried out at RT using a Varian Unity INOVA or Bruker AVANCE III HD spectrometer at 11.7 T and 5 mm NMR tubes filled with 0.6 mL liquid samples. The 1H NMR spectra were taken with 5.0 μs pulse length (∼30° flip) and 5 s pulse repetition delay time. The 13C NMR spectra without proton decoupling were acquired with 4.5 μs pulse length (∼90° flip) and 5 s pulse repetition delay time. The 17O NMR spectra were taken with 14 μs pulse length (∼90° flip) and 1.5 s pulse repetition delay time. The 51V NMR spectra were acquired with 6.2 μs pulse length (∼60° flip) and 1 s pulse repetition delay time. The acquisition numbers were typically 32, 10000-30000, 400-6000, and 128-1088 for the 1H, 13C, 17O, and 51 V NMR experiments, respectively. Chemical shifts were externally calibrated with methyl group of 3-(trimethylsilyl)propane-1-sulfonic

3. RESULTS AND DISCUSSION 3.1. Electrochemical Analysis. 3.1.1. Cyclic Voltammetry. Cyclic voltammetry tests were performed to evaluate the electrochemical activities of the histidine-containing electrolytes. Figure 1a shows the CV of the positive electrolytes with various concentrations of histidine at the scan rate of 20 mV s−1. The oxidation and reduction peaks represent the redox reactions between VO2+ and VO2+. The anodic peak current density (jpa) and the cathodic peak current density (jpc) reflect the redox reaction rates in the electrolytes containing certain concentrations of histidine. On the other hand, the potential gap (ΔV) between the oxidation and the reduction peaks represents the kinetic barrier of the reactions.53,54 By comparing these two factors, the 0.05 M histidine-containing electrolyte is found to show the highest anodic and cathodic peak current densities (jpa = 48.14 mA cm−2, jpc = −27.92 mA cm−2), and the lowest overpotential (ΔV = 344 mV) which is less than a half of that for pristine electrolyte (773 mV). C

DOI: 10.1021/acsami.9b06790 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Additionally, the ratio of the peak current densities (jpa/jpc), a factor representing the reversibility of the vanadium redox reactions, indicates that the 0.05 M histidine-containing electrolyte has the highest reversibility with the value closest to 1 among the data as summarized in Table 1. The CV curves for pristine (1.5 M VOSO4 + 3.0 M H2SO4) and histidine-containing (1.5 M VOSO4 + 3.0 M H2SO4 + 0.05 M histidine) positive electrolytes at various scan rates (ν) from 1 to 100 mV s−1 were also obtained, given in Figure S1. The jpa and jpc values constantly increase with increasing the scan rates; however, the values seem more prominent at higher scan rates in the presence of histidine. This is a common trend generally observed in the electrolytes with additives as reported in the literature.35,38,39 Generally, various kinds of polarizations become more influential at high scan rates while the catalytic behavior of histidine overcomes these polarizations in a more effective way so the histidine-containing electrolyte shows better performance as compared to the pristine one. Figure 1b shows transformed Randles−Sevcik plots from Figure S1a,b. The correlation between jpa and ν1/2 is almost linear, implying the reaction is reversible regardless of the ν value. Likewise, jpc also linearly correlates with ν1/2. The slopes of the plots confirm that the histidine kinetically improves the performance of the positive electrolyte. These results indicate that the histidine, as a homogeneous electrocatalyst is able to improve the electrochemical kinetics and reduce the overpotentials of the vanadium redox reactions. Furthermore, the CV results for the 0.05 M histidine-containing electrolyte up to 2000 cycles, shown in Figure 1c, demonstrate that histidine is a very stable electrocatalyst as there is almost negligible performance fading over the 2000 cycles. In order to see how histidine affects the charge-transfer resistance process for the vanadium redox reactions, EIS was also conducted at various temperatures. The results show that histidine decreases the solution resistance (RS) as well as the charge-transfer resistance (RCT) at all temperatures. However, the decrement margin increases with increasing temperatures as shown in the Figure 1d−f and Table S1. It can be noticed that at 25 °C, the decrement in RCT of the histidine-containing electrolyte is almost 9% while it is about 37% at 60 °C as compared to that of the pristine one. This shows that histidine is even more influential at elevated temperatures where normally vanadium ions suffer with low stability. This unique effect of histidine is also further confirmed by the thermal stability test and charge discharge performance at elevated temperatures, which will be discussed in the following sections. 3.2. Thermal Stability. 3.2.1. UV−Vis Spectroscopy. It is well known that VO2+ ions become very unstable and form V2O5 precipitates at high temperatures.20,55 Figure 2 displays the concentration changes of VO2+ ions at 40 °C in the pristine and histidine-containing electrolyte solutions over time that were determined using UV−visible spectroscopy. The calibration curve for each ion was obtained using Beer’s law and the concentration was calculated using the measured absorbance.56 The VO2+ concentration in the pristine electrolyte solution drops to almost 43% from its initial value in 10 h. In contrast, the VO2+ concentration in the electrolyte solution containing 0.05 M of histidine drops only to 61% in the same duration, marking almost an 18% gain over the pristine one. This confirms that the histidine added to the electrolyte solutions is able to improve the thermal stability of the vanadium electrolyte at high temperatures by inhibiting the

Figure 2. Concentration change of VO2+ in pristine and histidinecontaining electrolyte solutions during the thermal stability test at 40 °C for 10 h.

formation of insoluble V2O5 precipitates through a stable interaction between histidine and vanadium ions. 3.2.2. Charge−Discharge Test. In order to examine the influence of histidine, performance of a VRFB single cell employing a positive electrolyte with an optimized histidine concentration of 0.05 M was investigated by charge−discharge experiments. The cell performance was evaluated in terms of Coulombic efficiency (CE), voltaic efficiency (VE), and EE. Figure 3a shows the changes in the EE of the cell at various temperatures from 25 to 60 °C. In Figure 3b, CE, VE, and EE values versus temperature are summarized. Compared to the negative effect of temperature on the pristine electrolyte, the histidine-containing electrolyte shows rather improved efficiencies at higher temperatures. This is because histidine prevents the irreversible precipitation of VO2+ into V2O5 and improves the reaction kinetics of the vanadium ions even at high temperatures as evidenced by thermal stability test and EIS results, respectively. Figure 3c compares the efficiencies of the cells at various current densities from 50 to 150 mA cm−2 at 60 °C. At all current densities tested, the histidinecontaining electrolyte maintains higher efficiencies than the pristine one. In Table S2, measured CE, VE, and EE of each electrolyte at different current densities are summarized. Figure 3d shows the capacity retentions of the cells at 60 °C. Even at the high temperatures, the capacity retention rate of the histidine-containing cell is highly maintained at 73.2% as compared to 27.7% of the pristine cell. Furthermore, this work has also been compared with some of the previous reports on the electrolytes containing amino acids as additives,35,37−39 as summarized in Table S3. All of these previous works have reported EE improvement investigated at current densities between 40 and 60 mA cm−2 while this work reports for higher current densities up to 150 mA cm−2. Similarly, the previous works have stated the capacity retention of VRFB measured at or below RT, whereas in this work it has been tested at 60 °C, that is, much higher than the well-known upper limit temperature (40 °C).20,55 The VRFB employing the histidine-containing electrolyte shows almost 7.5% gain of EE (Figure 3c) and almost 45.5% improvement in capacity retention (Figure 3d) at 60 °C as compared to the one with pristine electrolyte. This shows the advantages of histidine over previously reported amino acids as well as the broader temperature adoptability of VRFBs employing histidine. Moreover, a comparison of this work with some of the recent studies in other research fields of D

DOI: 10.1021/acsami.9b06790 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. Charge−discharge tests of a VRFB single cell with pristine or 0.05 M histidine-containing positive electrolyte: (a) energy efficiencies versus temperature at current density of 50 mA cm−2 for 30 cycles, (b) average efficiencies versus temperature at 50 mA cm−2, (c) efficiencies at various current densities at 60 °C and (d) charge−discharge capacity retentions over 40 cycles at 50 mA cm−2 and 60 °C.

VRFB like electrodes57−59 and membranes60 is also summarized in Table S4 showing that our results are comparable to other reports. After the charge−discharge operation at 60 °C, the graphite felt (GF) electrodes were analyzed by scanning electron microscopy (SEM) to observe the presence of any precipitate of V2O5 formed during the operation at high temperatures. The results (Figure S2) show that there is a significant amount of precipitates present on the surface of the GF used in the VRFB using the pristine positive electrolyte while the surface of the GF used in VRFB employing the histidine-containing electrolyte is almost clean, confirming that histidine significantly suppresses the precipitation of VO2+ into V2O4. 3.3. Interaction between Histidine and Vanadium Ions Analyzed by NMR Spectroscopy. To figure out the influence of histidine on the vanadium ions, their interaction geometry was investigated using NMR spectroscopy and the first-principles calculations. 3.3.1. 13C NMR Spectroscopy. A representative 13C NMR spectrum of a 0.1 M histidine solution is shown in Figure 4a and the peaks are assigned as marked on the spectrum by Jcoupling splitting patterns using the ChemBioDraw software (PerkinElmer, New York, U.S.A.). The chemical shift data are listed in Table S5. When an aqueous solution is strongly acidic as in the case of a 3.0 M H2SO4 + 0.1 M histidine solution, the peaks shift to the right side (to smaller ppms) as shown in Figure 4b. It is because most of the histidine molecules are protonated to a divalent state (C6H12N3O22+) at a pH of −0.48 in the 3.0 M H2SO4 solutions (Figure S3).61 This protonation of histidine will be further discussed in Section 3.3.2. In Figure 4c,e, the solutions containing 1.5 M VOSO4 exhibit larger chemical shifts than a 3.0 M H2SO4 + 0.1 M histidine solution because of bulk paramagnetic shifts which equally influence all of the histidine carbons through paramagnetic VO2+(IV) ions. In Figure 4c, the carboxyl (C′), Cα and Cβ signals are significantly broadened and even

disappear, while the signals from imidazole ring (Cγ, Cδ, and Cε peaks) are relatively less broadened. Because the dipolar paramagnetic contribution to relaxation is inversely proportion to r6 where r is a distance from the paramagnetic center to the observed nucleus, the abovementioned observation suggests that VO2+(IV) ions interact primarily with carboxyl and amino groups of histidine in the 1.5 M VOSO4 + 0.1 M histidine solution in the absence of H2SO4. In the H2SO4-containing solution of Figure 4d,e, the C′, Cα, and Cβ carbon signals clearly appear, indicating that the histidine molecules are apart from VO2+(IV) ions in the presence of H2SO4. The weaker binding affinity of VO2+(IV) to divalent histidine (−0.28 eV) than to sulfate or bisulfate ions (−0.74 and −0.41 eV for SO42− and HSO4−, respectively, calculated in this work) supports the separation of histidine from VO2+(IV) after adding H2SO4. In a statistical point of view, the number of sulfate ions at the solvation shell of VO2+(IV) shall be increased in the H2SO4 solution, leading to the decrease in population of histidine near VO2+(IV) ions. When the concentration of VOSO4 is lowered from 1.5 to 0.18 M (Figure 4f), the chemical shifts and peak broadening are almost identical to those of the solution without VOSO4 (Figure 4b) because there is a less chance that histidine can interact with VO2+. The latter result confirms that the chemical shifts observed in Figure 4c−e are strongly influenced by the paramagnetic VO2+(IV) center. In the 1.5 M VO2+(V) + 3.0 M H2SO4 + 0.05 or 0.1 M histidine solution (Figure 4g,h), a drastic variation of chemical shifts and/or peak widths observed with paramagnetic VO2+(IV) in Figure 4c,d is not observed because VO2+(V) ions are not paramagnetic. Instead, the peak broadening by the binding interaction between VO2+(V) and histidine is clearly seen: the peak C′ is significantly broadened and Cα is moderately broadened, compared to the spectrum in Figure 4b. This implies that the histidine also interacts with VO2+(V) ions primarily through the carboxylic acid group. When the molar ratio of VO2+(V) to histidine is reduced to 1 (Figure 4i), E

DOI: 10.1021/acsami.9b06790 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. 13C NMR spectra of various histidine-containing solutions: (a) 0.1 M histidine, (b) 3.0 M H2SO4 + 0.1 M histidine, (c) 1.5 M VOSO4 + 0.1 M histidine, (d) 1.5 M VOSO4 + 3.0 M H2SO4 + 0.05 M histidine, (e) 1.5 M VOSO4 + 3.0 M H2SO4 + 0.1 M histidine, (f) 0.18 M VOSO4 + 3.0 M H2SO4 + 0.18 M histidine, (g) 1.5 M VO2+ + 3.0 M H2SO4 + 0.05 M histidine, (h) 1.5 M VO2+ + 3.0 M H2SO4 + 0.1 M histidine, and (i) 0.18 M VO2+ + 3.0 M H2SO4 + 0.18 M histidine. The chemical forms of histidine in the figure may differ from the real ones in individual solutions because it depends on the pH values of the solutions. The spectra are not presented in the quantitative scale for their intensity.

amino (4H) and carboxylic acid (6H) protons are not detected because of fast proton exchange with water protons.62 On the other hand, 4H and 6H signals appear at 8 and 13 ppm, respectively, in the 3.0 M H2SO4 solutions (Figure 5b). This spectral change is due to the protonation of histidine at low pH, making the exchangeable protons stay longer at carboxylic acid and amino proton sites of histidine, which implies that the histidine exists in the divalent state in the 3.0 M H2SO4 solution. In the energy comparison between histidine molecules in different oxidation states (Figure S3), the divalent state is more stable than the monovalent state by 0.15 eV in the 3.0 M H2SO4 solution. The histidine proton signals are almost absent in the VOSO4 solution (Figure 5c) because of the paramagnetic effect, but a single proton signal is barely detected at 13.9 ppm which corresponds to the CH group of the imidazole ring. It is consistent with the 13C NMR results where the VO2+(IV) ions are away from the imidazole ring. This stronger paramagnetic effect for 1H spectra compared to 13 C spectra is due to the closer distance of proton than carbon to the paramagnetic VO2+(IV) center and the greater gyromagnetic ratio of 1H to 13C. In Figure 5d,e, the proton peaks screened by VO2+(IV) ions in Figure 5c are clearly visible because VO2+(IV) ions spend less time in the proximity

the signals become almost identical to those of Figure 4b, similar to the case of VOSO4 solutions. These similar behaviors of VO2+(IV) and VO2+(V) ions on the NMR spectra suggest that they interact with histidine almost in the same manner (through carboxylic acid and amino groups). It is also seen that only the peak width of tertiary carbon (Cγ) is kept narrow and minimally affected by vanadium ions, suggesting that the interactions between histidine and vanadium ions are mediated by protons through hydrogen bonding and/or proton exchange. 3.3.2. 1H NMR Spectroscopy. Figure 5 shows 1H NMR spectra for water proton (the largest peaks denoted with w) and histidine molecules (small peaks near the water peak) and the detailed values are presented in Tables S6 and S7, respectively. The chemical shift of water proton is additively affected by VOSO4 and H2SO4. For example, the presence of 3.0 M H2SO4 (Figure 5b) and 1.5 M VOSO4 (Figure 5c) induces the 1H chemical shift of water proton to 6 and 10 ppm, respectively, while the chemical shift in the mixed solution of 3.0 M H2SO4 + 1.5 M VOSO4 (Figure 5d,e) is 17−18 ppm which is close to the sum of those values observed for respective solutions. The proton spectrum of histidine in distilled water shows 4 groups of signals as assigned in the right side of Figure 5. For the 0.1 M histidine solution (Figure 5a), the signals from F

DOI: 10.1021/acsami.9b06790 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. 1H NMR spectra of various histidine-containing solutions: (a) 0.1 M histidine, (b) 3.0 M H2SO4 + 0.1 M histidine, (c) 1.5 M VOSO4 + 0.1 M histidine, (d) 1.5 M VOSO4 + 3.0 M H2SO4 + 0.05 M histidine, (e) 1.5 M VOSO4 + 3.0 M H2SO4 + 0.1 M histidine, (f) 0.18 M VOSO4 + 3.0 M H2SO4 + 0.18 M histidine, (g) 1.5 M VO2+ + 3.0 M H2SO4 + 0.05 M histidine, (h) 1.5 M VO2+ + 3.0 M H2SO4 + 0.1 M histidine, and (i) 0.18 M VO2+ + 3.0 M H2SO4 + 0.18 M histidine. For enlarged histidine signals, expanded spectra are shown at the right side. The chemical form of histidine in the figure may differ from the real ones in individual solutions because it depends on the pH values of the solutions. For the explanation of small peaks near 6.4 ppm in (c,d), refer to the note under Table S7 and Figure S4. Water signals are denoted with w. The spectra are not presented in the quantitative scale for their intensity.

confirms that even at 50 °C imidazole rings are not much involved in binding to VO2+(V) ions and the interaction of histidine with VO2+(V) ions is weaker at 50 °C. Furthermore, the possible crossover of histidine from positive to negative electrolyte during the charge−discharge operation is checked with 1H NMR spectroscopy, which has the best sensitivity among the NMR-sensitive nuclei to directly detect the presence of histidine. Small 1H signals of histidine are clearly shown in the negative electrolyte after the cell operation while no signals before the charge−discharge cycle, as shown in Figure S6. This confirms the crossover of histidine from the positive electrolyte to the negative electrolyte during the cell operation. 3.3.3. 17O NMR Spectroscopy. The 17O NMR spectra in Figure 6 show the signals from water (at 0−76 ppm) and sulfate/bisulfate ions (at 160−182 ppm). Table S8 summarizes the chemical shifts, peak widths, and peak area values. As in the 1 H NMR spectra, the 17O chemical shift of water in the mixed solutions (Figure 6d,e) is additively affected by each solution of H2SO4 (Figure 6b) and VOSO4 (Figure 6c). It is found that in the case of VOSO4 solutions, the signal from water molecules at the first solvation shell of VO2+(IV) is not visible because of the paramagnetic effect by VO2+(IV). As the 1.5 M VOSO4 solution (Figure 6c) has an equal molar number of SO42− and VO2+(IV) and the binding energy between

of histidine in the presence of H2SO4, as explained in Section 3.3.1. For the solutions having VO2+(V) ions (Figure 5g,h), the chemical shift and peak broadening are smaller than those of the VOSO4 solutions because the VO2+(V) is diamagnetic. Comparison of Figure 5g−i reveals that the peak width of the water proton peak is significantly broadened with increasing the concentration of VO2+(V). It suggests that the water molecules undergo a dynamic proton exchange in proportion to VO2+(V) concentrations; this is further supported by the 17 O NMR results discussed in the next section. When comparing Figure 5g,h to 5b, the amino proton signal (4H) at ∼8.2 ppm becomes sharper after adding VO2+(V) 15−30 times of histidine concentration, indicating the slower exchange rate of amino protons with water protons; this suggests that the VO2+(V) ions interact with the amino groups as well as with the carboxylic acid groups via hydrogen as explained in Section 3.3.1. Also, the NMR signals of histidine are less affected by VO2+(V) than those of water, implying that the histidine places at a higher solvation shell of the VO2+(V) ions than water. In view of the spectrum of a 1.5 M VO2+(V) + 3.0 M H2SO4 + 0.1 M histidine solution taken at 50 °C (Figure S5), the peaks only for 4H at ∼8.2 ppm and 6H at ∼13.1−13.4 ppm (amino and carboxyl acid protons, respectively) among histidine peaks are much broader than those at RT. This G

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Figure 6. 17O NMR spectra of various histidine-containing solutions: (a) 0.1 M histidine, (b) 3.0 M H2SO4 + 0.1 M histidine, (c) 1.5 M VOSO4 + 0.1 M histidine, (d) 1.5 M VOSO4 + 3.0 M H2SO4 + 0.05 M histidine, (e) 1.5 M VOSO4 + 3.0 M H2SO4 + 0.1 M histidine, (f) 0.18 M VOSO4 + 3.0 M H2SO4 + 0.18 M histidine, (g) 1.5 M VO2+ + 3.0 M H2SO4 + 0.05 M histidine, (h) 1.5 M VO2+ + 3.0 M H2SO4 + 0.1 M histidine, and (i) 0.18 M VO2+ + 3.0 M H2SO4 + 0.18 M histidine. The water signal is denoted with w and the spectra are not presented in the quantitative scale for their intensity.

VO2+(IV) and SO42− is quite strong (−0.74 eV), most of SO42− ions are bound to the paramagnetic VO2+(IV). Thus, the 17O NMR signal for sulfate/bisulfate ions is faintly observed at 181 ppm in Figure 6c. On the other hand, as the molar ratio of SO42− to VO2+ increases to 3 in the 1.5 M VOSO4 + 3.0 M H2SO4 solution (Figure 6d,e), the unbound sulfate/bisulfate ions that are detectable on NMR spectra become significant and, therefore, the signal for sulfate/ bisulfate ions is visible at 170 ppm. From the comparison of Figure 6g,h with 6b, the presence of VO2+(V) ions in a H2SO4 solution causes only a negligible change of the chemical shift of sulfate/bisulfate oxygen from 160 to 161 ppm, however, it results in a drastic change of the chemical shift of water oxygen from 5 to 74−76 ppm. The peak width broadening by VO2+(V) is also more significant for the water oxygen (from 60 to 1180−1310 Hz) than for the sulfate/bisulfate oxygen (from 98 to 440−470 Hz). This stronger influence of VO2+(V) on water than sulfate/bisulfate ions is ascribed to the fact that the first solvation shell of VO2+(V) is composed of water molecules. In contrast to the VO2+(IV) case, where the signals from the first solvation shell is not accessible, the interactions between ligand water and VO2+(V), such as bonding and dynamic exchange, are directly observable. Therefore, the broad single peak of water oxygen at 74 or 76 ppm in Figure 6g,h indicates that all water molecules are dynamically influenced by the VO2+(V) through ligand exchange. In the VO2+(IV) case, the NMR signals from bulk water which is observed at 13 ppm in Figure 6c and at 17−18 ppm in Figure 6d,e are presumably separated from the signals of coordinate water which is not detectable. Abovementioned

results suggest that the ligand exchange, that is (de)hydration, is quite feasible for VO2+(V) ions, while it is almost not allowed for the VO2+(IV) ion. The latter is verified using the data in Figure S7, from the first-principles calculations, where first and second dehydration energies of VO2+(V) are −0.09 and 0.13 eV, respectively, which is significantly smaller than those (0.22 and 0.94 eV, respectively) of VO2+(IV). 3.3.4. 51V NMR Spectroscopy. Though it was tried, 51V NMR signals of VO2+(IV) ions were not detected presumably because of paramagnetic peak broadening of VO2+(IV) ions, being consistent with the 1H, 13C, and 17O NMR data of the solutions containing VO2+(IV) ions described above, while those of VO2+(V) ions were observed as shown in Figure S8 and summarized in Table S9. The spectral parameters appeared to be influenced mainly by VO2+(V) but not by histidine. More detailed explanation of the data is in Supporting Information. 3.3.5. Vanadium−Histidine Binding. On the basis of NMR spectroscopy results, the most probable binding geometries between histidine and VO2+(V) ions are proposed as depicted in Figure 7. The formation of inner-sphere complexes between histidine and VO2+(V), having a V−O or V−N bond, is endothermic by 0.12−0.48 eV (Figure 7a). In contrast, the binding between them through the hydrogen bond, that is, outer-sphere complex, is exothermic by 0.52−0.61 eV (Figure 7b). Also, among different proton sites of histidine for binding VO2+(V), carboxyl, and amino groups are found to be much preferred to the imidazole ring. These results are consistent with the aforementioned 13C and 1H NMR results of histidine, where H

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consequently impeding the polymerization process to make V2O5 precipitates. Second, the existence of an optimal histidine concentration for the kinetics of vanadium redox reactions suggests that the Sabatier principle may work in this reaction system: that is, the interaction between histidine molecules and vanadium ions should neither be too strong nor too weak. Histidine seems to either improve or deteriorate the mass transfer of vanadium ions by electrostatic and steric manner, respectively, as explained in detail below. Divalent histidine molecules bind more easily to VO2+(V) ions than to VO2+(IV) ions because of the favorable binding energy by 0.26−0.35 eV (Figure 7b vs S10a). The resulting [VO2+(V)−histidine2+] complex has 3+ charge in total, and can respond to the electric field stronger than less-charged species including VO2+(IV) ions. During the discharge process (VO2+(V) → VO2+(IV)), an attractive electric field is applied to the positively charged species toward the electrode; this increases the concentration of reactant complex [VO2+(V)−histidine2+] near the electrode surface, improving the kinetics. Contrarily, during the charge process (VO2+(IV) → VO2+(V)), the electric field becomes repulsive for the positively charged species from the electrode, decreasing the concentration of product complex [VO2+(V)− histidine2+] near the electrode surface; this again enhances the reaction kinetics. In turn, the different affinity of histidine to reactant/product improves the kinetics of both charge and discharge reactions. On the other hand, the enlarged size and weight of the [VO2+(V)−histidine2+] complex compared to the bare VO2+(V) ion can deteriorate the overall kinetics by retarding the mass transport of VO2+(V) ions. As explained above, histidine can influence the vanadium redox reaction either positively or negatively; therefore, an optimal histidine concentration needs to be sought to improve the VRFB performance as observed in the CV tests in Figure 1. Another noteworthy thing is that, though the interaction between histidine and vanadium ions is not very strong, histidine dynamically influences all the vanadium ions through ligand exchange reactions in a homogeneous solution, which is in agreement with the NMR data explained above.

Figure 7. Representative (a) inner sphere and (b) outer sphere of VO2+(V)−histidine complexes. Numbers are binding energy in eV, where the positive (negative) value is endothermic (exothermic). Dashed lines indicate the hydrogen bonds. Each sphere matches green (V), red (O), blue (N), gray (C), and white (H), respectively.

NMR signals of carboxylic acid and amino groups are mostly affected while those of the imidazole ring are less influenced by vanadium ions. This is in contrast with the role of histidine as a metal-ion-binding site in a protein (vanadium bromoperoxidase) where the carboxylic acid and amino groups of histidine are engaged in peptide bonds and imidazole rings are involved in the binding to metal ions. 3.4. Effect of Histidine on VRFB Performance. The UV−visible and CV measurements have shown that the histidine, as a homogeneous electrocatalyst, improves the VRFB performance in terms of stability of electrolyte and kinetics of redox reactions. First, regarding the stability of the electrolyte, a significant reduction in the precipitate formation was observed in both chemical and electrochemical environments. According to the previous reports,55,63 the conversion of VO2+(V) ions into insoluble precipitates is a primary degradation mechanism of VRFB. This degradation process undergoes a hydrolysis-based polymerization process that begins with the proton loss from the coordinating water molecules. Therefore, the undesired precipitate formation may be suppressed by preventing the deprotonation and/or polymerization processes of vanadium ions. To examine the degree of deprotonation, energy penalties for detaching one proton from a coordinate water molecule, that is deprotonation energies, were evaluated with the first-principles calculations for VO2+(H2O)4 in various conditions (in a bare state, with a sulfate or bisulfate ion, and with a divalent histidine molecule) as shown in Figure S9. The deprotonation energies for the bare state, with HSO4−, and with SO42− ions were 0.52, 0.49, and 0.69 eV, respectively. These energy values, however, reduce to 0.39−0.46 eV when VO2+(V) ions are interacting with the divalent histidine molecules. The latter result implies that the histidine could not restrain the deprotonation of coordinate water molecules. Thus, the primary role of histidine for improving the stability of VO2+(V) ions is presumably related to the prevention of vanadium ion polymerization. In fact, because a histidine molecule is quite large in size and positively charged, once a VO2+(V) ion binds to a divalent histidine molecule they could repel another VO2+(V) ion electrostatically as well as sterically,

4. CONCLUSIONS In summary, this work discovers a new enzyme-inspired organic additive, histidine, for the positive electrolyte of VRFB as a homogeneous electrocatalyst. It improves the electrolyte stability at elevated temperatures by suppressing the conversion of VO2+ into insoluble V2O5 precipitates. At 40 °C, the concentration of VO2+ in the histidine-containing electrolyte remains 61% after a 10 h stability test while it reduces to 43% in a pristine electrolyte. It also improves the kinetics of redox reactions, resulting in a superior performance of the VRFB cell during charge and discharge processes: the VRFB cell with a histidine-containing electrolyte shows EE of 78.7% at 150 mA cm−2 and 60 °C compared to 71.2% with a pristine electrolyte, and the capacity retention of 73.2% as compared to only 27.7% with the pristine electrolyte. Multinuclear (1H, 13C, 17O, and 51V) NMR analyses and the first-principles calculations reveal that in contrast to the enzyme where imidazole rings bind to vanadium ions, the histidine additive dynamically interacts with vanadium ions through a carboxyl group and amino group forming an outersphere [VO2+(V)−histidine2+] complex. This dynamic complexation is found to prevent the aggregation of VO2+(V) ions and subsequent growth of V2O5 sterically and electrostatically, I

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Organic Approach to Persistent, Cyclable, Low-Potential Electrolytes for Flow Battery Applications. J. Am. Chem. Soc. 2017, 139, 2924− 2927. (6) Liu, J.; Zhang, J.-G.; Yang, Z.; Lemmon, J. P.; Imhoff, C.; Graff, G. L.; Li, L.; Hu, J.; Wang, C.; Xiao, J.; Xia, G.; Viswanathan, V. V.; Baskaran, S.; Sprenkle, V.; Li, X.; Shao, Y.; Birgit, S. Materials Science and Materials Chemistry for Large Scale Electrochemical Energy Storage: From Transportation to Electrical Grid. Adv. Funct. Mater. 2013, 23, 929−946. (7) Skyllas-Kazacos, M.; Chakrabarti, M. H.; Hajimolana, S. A.; Mjalli, F. S.; Saleem, M. Progress in Flow Battery Research and Development. J. Electrochem. Soc. 2011, 158, R55−R79. (8) Sun, B.; Skyllas-Kazacos, M. Modification of graphite electrode materials for vanadium redox flow battery application-I. Thermal treatment. Electrochim. Acta 1992, 37, 1253−1260. (9) Sun, B.; Skyllas-Kazacos, M. Chemical modification of graphite electrode materials for vanadium redox flow battery application-part II. Acid treatments. Electrochim. Acta 1992, 37, 2459−2465. (10) Rychcik, M.; Skyllas-Kazacos, M. Characteristics of a new allvanadium redox flow battery. J. Power Sources 1988, 22, 59−67. (11) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577−3613. (12) Mehboob, S.; Ali, G.; Shin, H.-J.; Hwang, J.; Abbas, S.; Chung, K. Y.; Ha, H. Y. Enhancing the performance of all-vanadium redox flow batteries by decorating carbon felt electrodes with SnO2 nanoparticles. Appl. Energy 2018, 229, 910−921. (13) Skyllas-Kazacos, M.; Menictas, C.; Lim, T. 12 - Redox flow batteries for medium- to large-scale energy storage A2 - Melhem, Ziad. Electricity Transmission, Distribution and Storage Systems; Woodhead Publishing, 2013; pp 398−441. (14) Chen, H.; Cong, T. N.; Yang, W.; Tan, C.; Li, Y.; Ding, Y. Progress in electrical energy storage system: A critical review. Prog. Nat. Sci. 2009, 19, 291−312. (15) Li, L.; Kim, S.; Wang, W.; Vijayakumar, M.; Nie, Z.; Chen, B.; Zhang, J.; Xia, G.; Hu, J.; Graff, G.; Liu, J.; Yang, Z. A Stable Vanadium Redox-Flow Battery with High Energy Density for LargeScale Energy Storage. Adv. Energy Mater. 2011, 1, 394−400. (16) Xiao, S.; Yu, L.; Wu, L.; Liu, L.; Qiu, X.; Xi, J. Broad temperature adaptability of vanadium redox flow battery-Part 1: Electrolyte research. Electrochim. Acta 2016, 187, 525−534. (17) Xi, J.; Xiao, S.; Yu, L.; Wu, L.; Liu, L.; Qiu, X. Broad temperature adaptability of vanadium redox flow battery-Part 2: Cell research. Electrochim. Acta 2016, 191, 695−704. (18) Xi, J.; Jiang, B.; Yu, L.; Liu, L. Membrane evaluation for vanadium flow batteries in a temperature range of −20-50 °C. J. Membr. Sci. 2017, 522, 45−55. (19) Liu, Y.; Liang, F.; Zhao, Y.; Yu, L.; Liu, L.; Xi, J. Broad temperature adaptability of vanadium redox flow battery-part 4: Unraveling wide temperature promotion mechanism of bismuth for V2+/V3+ couple. J. Energy Chem. 2018, 27, 1333−1340. (20) Skyllas-Kazacos, M.; Menictas, C.; Kazacos, M. Thermal Stability of Concentrated V(V) Electrolytes in the Vanadium Redox Cell. J. Electrochem. Soc. 1996, 143, L86−L88. (21) Kazacos, M.; Cheng, M.; Skyllas-Kazacos, M. Vanadium redox cell electrolyte optimization studies. J. Appl. Electrochem. 1990, 20, 463−467. (22) Rahman, F.; Skyllas-Kazacos, M. Vanadium redox battery: Positive half-cell electrolyte studies. J. Power Sources 2009, 189, 1212−1219. (23) Park, S.-K.; Shim, J.; Yang, J. H.; Jin, C.-S.; Lee, B. S.; Lee, Y.-S.; Shin, K.-H.; Jeon, J.-D. Effect of inorganic additive sodium pyrophosphate tetrabasic on positive electrolytes for a vanadium redox flow battery. Electrochim. Acta 2014, 121, 321−327. (24) Kausar, N.; Mousa, A.; Skyllas-Kazacos, M. The Effect of Additives on the High-Temperature Stability of the Vanadium Redox Flow Battery Positive Electrolytes. ChemElectroChem 2016, 3, 276− 282.

resulting in suppression of V2O5 precipitation. This work may open a new door to utilize biomolecules in redox flow batteries in the way the dynamic interactions of organic additives with electrolyte ions stabilize the active species as well as enhance the reaction kinetics, leading to much improved performance of RFBs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06790. CV of the electrolyte solutions at various scan rates, VFRB efficiencies at various current densities, physicochemical properties of electrolytes, SEM images of postcycling electrodes, additional NMR data, and the first-principles calculation results (relative energy of vanadium complex ions vs water coordination number, binding geometry and energy, and deprotonation enthalpy) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82-2-6908-6220 (O.H.H.). *E-mail: [email protected]. Phone: +82-2-958-5275 (H.Y.H.). ORCID

Saleem Abbas: 0000-0002-0438-2849 Heejin Kim: 0000-0003-3027-6983 Oc Hee Han: 0000-0003-1888-2323 Heung Yong Ha: 0000-0003-3114-2602 Author Contributions ∇

S.A., J.H., and H.K. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the KIST institutional program on the development of next generation batteries (Project code: 2E29641) and by the Korea CCS R&D Center (Korea CCS 2020 Project) grant funded by the Korea government (Ministry of Science and ICT) (KCRC2014M1A8A1049293). Asia BiBi, Deepika Gajanan Karmalkar, Cam Tu Le, Na Young Lee, Xiaoyan Lu, Jindou Yang, and Ji Soo Yoon are acknowledged for preliminary NMR investigation on pH dependence of histidine structures in aqueous solutions.

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REFERENCES

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DOI: 10.1021/acsami.9b06790 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.9b06790 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX