Complexes Containing Redox Noninnocent Ligands for Symmetric

Article pubs.acs.org/JPCC

Complexes Containing Redox Noninnocent Ligands for Symmetric, Multielectron Transfer Nonaqueous Redox Flow Batteries Pablo J. Cabrera,† Xingyi Yang,‡,§ James A. Suttil,† Krista L. Hawthorne,‡ Rachel E. M. Brooner,† Melanie S. Sanford,*,† and Levi T. Thompson*,‡ †

Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States Department of Chemical Engineering, University of Michigan, 2300 Hayward Street, Ann Arbor, Michigan 48109, United States

‡

S Supporting Information *

ABSTRACT: Redox flow batteries (RFBs) hold promise for use in large-scale energy storage applications, but new electrolyte chemistries are needed to significantly enhance their energy densities and lower their cost. The energy density is governed by the cell voltage, active species concentration, and number of electrons transferred at each electrode. Nonaqueous solvents offer wider voltage windows than water; however, most if not all of the previously reported active species have low solubilities and/or are limited to single electron transfer at each electrode. In this paper we describe the design, synthesis, and characterization of metal coordination complexes containing noninnocent ligands that demonstrate enhanced solubilities at different oxidation states along with multiple electron transfers. In particular, a series of ester-functionalized chromium bipyridine complexes are demonstrated that afford six reversible redox couples over ∼2 V and solubilities approaching 1 M. These characteristics allow the same complex to be used at the negative and positive electrodes. Using an electrolyte consisting of the tris(4,4′-(bis(2-(2-methoxyethoxy)ethyl)ester)-2,2′-bipyridine)chromium complex ([Cr(L3)3]) in acetonitrile, we demonstrate two reversible electron transfers at each electrode in an unoptimized, symmetric H-cell with efficiencies of ∼70%. With further enhancements in the electrolyte chemistry and cell design, this approach could lead to the demonstration of highly energy dense RFB chemistries for grid-scale storage applications.

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∼1.5 V. As such, with single electron transfer chemistries and solubilities of ∼2 M, the maximum energy densities6,8 available from commercial aqueous RFBs are ∼25 W h L−1, although progress is being made toward increasing energy densities in these systems.9 Another promising approach for improving the energy densities involves the use of electrolytic solutions based on organic solvents that are stable over wide voltage windows, in combination with active species that undergo multiple redox events within these windows.10 There are several reports describing metal coordination complexes (MCCs) that possess two or more reversible couples in organic solvents.11−17 Active species with two redox couples can be used on both sides of a nonaqueous RFB cell, thereby minimizing inefficiencies associated with crossover.3,11,18 In the current study, we sought MCCs with four or more redox couples within the organic solvent stability window, so as to enable multiple electron transfers from the same species at the negative and positive electrodes of the cell.

INTRODUCTION With increasing efforts to incorporate renewable energy sources such as wind and solar into the electrical grid, the need for reliable and inexpensive energy storage has increased.1−4 Redox flow batteries (RFBs) offer great promise to meet the demands of grid-scale storage, as their power and energy outputs can be independently scaled.4 In addition, inefficiencies and instabilities due to battery self-discharge are minimized in these systems because the active electrolyte solutions are stored in reservoirs that are physically separated from the electrochemical conversion cell.1−5 These attractive features have led to the development of RFBs primarily based on aqueous electrolyte solutions.6,7 However, aqueous RFBs are fundamentally limited by the narrow electrochemical stability window of water.1−3,5,6 The energy density (Ê ) of an RFB is determined by the number of electrons transferred at the electrodes (n), cell potential (Vcell), and concentration of the redox active species (Cactive).3 Equation 1 provides an approximation of the theoretical energy E ̂ = 0.5nVcellCactiveF

(1) Received: April 14, 2015 Revised: May 27, 2015 Published: May 28, 2015

density for an RFB (F is Faraday’s constant). For aqueous RFBs, the thermodynamic stability of water limits the maximum Vcell to © 2015 American Chemical Society

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Scheme 1. Identifying Multielectron Active Species for Nonaqueous RFBs via Metal Complexes Bearing Redox Noninnocent Ligands

commercial RFBs, an active species capable of transferring 2 electrons at each electrode over a 2 V window would need to have a solubility approaching 0.7 M. Our initial efforts toward this goal have focused on chromium complexes containing ester-substituted bipyridine ligands that can be tuned to significantly enhance their solubility in acetonitrile. Our interest in bipyridine complexes was inspired by prior work using iron,25,26 nickel,23 and ruthenium19−21 bipyridine complexes as active species for nonaqueous RFBs,27−34 and the ease with which the bipyridine scaffold can be derivatized. The complexes developed herein have been applied to the first symmetrical nonaqueous RFB cell with multiple electron transfers at both the positive and negative electrodes. Theoretical energy densities for these materials approach those of commercial aqueous RFBs. With further optimization, this strategy holds great promise for developing RFBs with energy densities that significantly exceed those of aqueous systems.

To this end, MCCs incorporating redox-active (noninnocent) ligands are attractive targets (Scheme 1). The ligands not only contribute to solubilizing the metal complexes, but can enable the complex to undergo synergistic, multielectron redox chemistry at the metal center and ligand. As described below, there have been a few prior reports of the use of MCCs with noninnocent ligands in RFBs.19−23 However, multielectron transfer at both electrodes was not accomplished due to limitations of the specific complexes. Furthermore, these systems demonstrated very low cell energy densities. In one example, Matsuda reported the use of electrolytes based on a ruthenium bipyridine complex in acetonitrile.19−21 This complex exhibits four reversible redox couples over a nearly 3 V window; however, in the charge/discharge experiments, only one couple was accessed at the positive and negative electrodes, respectively. The couple at the negative electrode was relatively irreversible, a likely reason for limiting the operating voltage window to two couples. Anstey et al.22 recently described the use of a vanadium complex containing noninnocent dithiolate ligands. This complex exhibits three redox couples over a 2.3 V window in acetonitrile; however, the test cells were only cycled over a 1.1 V window. As such, only one couple was accessed at each electrode. The authors suggest that irreversible side reactions occur beyond operating voltages of 1.1 V. Doo et al.23 reported results for electrolyte solutions containing mixtures of iron and nickel bipyridine complexes in propylene carbonate. The combined electrochemistries of these complexes enabled two electron transfer events at the negative electrode based on the redox chemistry of the Ni complex, as well as one electron transfer for the positive electrolyte based on the redox chemistry of the Fe complex, with an operating voltage of 2.2 V. However, the use of electrolytes with mixtures of active species could significantly limit the overall energy density, because solvents typically have a limited capacity for the total solute,24 resulting in a trade-off between solubilities of the species (one is active at the negative electrode and the other at the positive electrode). Furthermore, the currently available membrane materials are susceptible to permeability of the active species, leading to crossover losses. The goal of the current research was to develop MCCs with noninnocent ligands that afford multielectron transfers and increased solubilities for use in symmetric cells (i.e., the same active species in the positive and negative electrolytes) with multiple electron transfers at both electrodes. Importantly, to achieve energy densities that are comparable to those for

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EXPERIMENTAL METHODS Synthesis Materials and Methods. All syntheses were conducted under an oxygen-free atmosphere either in a nitrogenfilled glovebox or using standard Schlenk line techniques unless stated otherwise. Dichloromethane and diethyl ether were purified using an Innovative Technologies solvent purification system consisting of a copper catalyst, activated alumina, and molecular sieves. Triethylamine was purified by distillation from CaH2. Dimethyl [2,2′-bipyridine]-4,4′dicarboxylate (L1),35 tetrakis(acetonitrile)chromium tetrafluoroborate (S2),36 [2,2′-bipyridine]-4,4′-dicarboxylic acid (S1),37 and [Cr(L1)3]3+ 38 were prepared according to published procedures. All remaining reagents were purchased from commercial sources and used as received. NMR spectra were obtained on a Varian VNMRS 700, Varian VNMRS 500, Varian Inova 500, or Varian MR400 spectrometer. 1H and 13C chemical shifts are reported in parts per million relative to tetramethylsilane (TMS), with the residual solvent peak used as an internal reference. NMR multiplicities are reported as follows: singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), broad (br). Coupling constants (J) are reported in hertz. Infrared (IR) spectroscopy for the Cr3+ complexes was performed on a PerkinElmer Spectrum BX Ft-IR spectrometer using an attenuated total reflection (ATR) attachment. IR spectroscopy for the Cr0 complexes was performed on a Thermo Scientific Nicolet iS-10 spectrometer using KBr pellets. Melting points 15883

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shown to have reasonable performance in nonaqueous RFB applications.39,40 Our initial studies focused on the methyl ester-substituted bipyridine ligand L1. The CrIII complex of L1 ([Cr(L1)3]3+) was reported to exhibit six reversible electron transfer events over a 2 V window in CH3CN.38 However, this complex has low solubility in CH3CN (the saturated solution is 0.13 M). Prior work from our groups demonstrated that the incorporation of flexible and polar substituents onto ligands can significantly enhance the solubility of MCCs in acetonitrile.41 This enhancement is likely due to minimization of solid-state packing interactions. On this basis, we hypothesized that the introduction of ethylene glycol ether chains on the ester functionality of the bipyridine ligands would enhance the solubility of their Cr complexes relative to the methyl-substituted analogue. Ligands L1, L2, and L3 were prepared by Jones oxidation of 4,4′-dimethylbipyridine37 followed by esterification with oxalyl chloride and the appropriate alcohol (Scheme 2). The CrIII

were determined with a Mel-Temp 3.0, Laboratory Devices Inc., United States, instrument and are uncorrected. Mass spectral data were obtained on a Micromass magnetic sector mass spectrometer in electrospray ionization mode. Elemental analyses were carried out at Atlantic Microlab in Norcross, GA. UV−vis data for CrIII complexes were collected on a Shimadzu UV-1601 UV−vis spectrometer. UV−vis data for Cr0 complexes were collected on an Implen NanoPhotometer P 300 UV−vis spectrophotometer. Powder X-ray diffraction (PXRD) data were obtained using a Bruker D8 Advance powder X-ray diffractometer with a sealed tube X-ray source (Cu) operating at 1.6 kW. Centrifugation was performed on a Sorval ST 16 centrifuge from ThermoScientific. Detailed synthesis procedures are described in the Supporting Information. Electrochemical Analysis. All electrochemical analyses were carried out in an argon-filled glovebox (MBraun). The supporting electrolyte was electrochemical-grade tetrabutylammonium tetrafluoroborate (Sigma). The solvent was acetonitrile (Sigma, anhydrous, 99.8%). Cyclic voltammetry was performed with an Autolab PGSTAT302N potentiostat/galvanostat (Ecochemie, The Netherlands) and a 600D potentiostat (CH Instruments, United States). Cyclic voltammetry was carried out in a three-electrode electrochemical cell, consisting of a glassy carbon disk working electrode (0.07 cm2, BASi), a Ag/Ag+ quasi-reference electrode (BASi) with 0.01 M AgBF4 (Sigma) in acetonitrile, and a platinum wire counter electrode (23 cm, ALS) separated by a glass frit. The glassy carbon disk electrode was polished using micrometer aluminum oxide polishing paper (9 and 0.3 μm, Fiber Instrument) and then sonicated in deionized water. All experiments were run at a scan rate of 100 mV s−1 in an acetonitrile electrolyte containing 1 mM chromium complex and 0.1 M (TBA)BF4 (TBA = tetrabutylammonium), unless otherwise noted. Charge/discharge measurements were carried out with a Maccor 4000 series battery tester in an H-cell. The H-cell was purchased from Adam’s & Chittenden. A Celgard 2325 separator was soaked in 0.5 M (TBA)BF4 in acetonitrile for 24 h prior to cell assembly. The electrolyte contained 0.01 M chromium complex and 0.5 M (TBA)BF4. Cells were charged at a current of 0.64 mA, with voltage cutoffs set on both the anode and cathode, referenced to a Ag/Ag+ reference electrode placed in each side of the cell. The voltage cutoff during charging for the cathode was 0 V, and the voltage cutoff for the anode was −1.35 V. A charge limit of 0.001 A h (4.0 C) was placed on the cell to protect the complex. The discharge conditions were similar, with a discharge current of 0.64 mA and voltage cutoffs of −1.2 and −1.35 V for the cathode and anode, respectively.

Scheme 2. (a) Synthesis of 4,4′-Ester-Substituted Bipyridine Ligands L1, L2, and L3 and (b) Synthesis of Functionalized [Cr(L)3]3+ Complexes

complexes of L1−L3 were synthesized via reaction of the ligand with [Cr(CH3CN)4](BF4)2 followed by 1e− oxidation with AgBF4 (Scheme 2).38 [Cr(L1)3]3+, [Cr(L2)3]3+, and [Cr(L3)3]3+ were characterized via high-resolution mass spectrometry, elemental analysis, and infrared spectroscopy. Solubility. The solubilities of the CrIII complexes in CH3CN were determined using UV−vis spectroscopy. Acetonitrile was selected as the solvent on the basis of its wide electrochemical window (∼5 V) and low viscosity. It is also a common solvent for electrochemical characterization.42 Consistent with our hypothesis, both complexes containing the ethylene glycol ether-derived ligands, [Cr(L2)3]3+ and [Cr(L3)3]3+, showed more than a 4-fold enhanced solubility relative to [Cr(L1)3]3+ (maximum solubilities of 0.63, 0.54, and 0.13 M, respectively). The [Cr(L)3]3+ complex represents the fully charged species in the RFB catholyte. We note that solubility is likely to change as the overall charge of the complex changes. In the worst case, a saturated solution of [Cr(L)3]3+ could, when discharged, access species with significantly lower solubilities than the starting complex and result in precipitation of the active material. Surprisingly, while the impact of the oxidation state on the solubility in aqueous vanadium RFBs has been studied,43 to our

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RESULTS AND DISCUSSION Synthesis of MCCs. Several design criteria were used to select redox-active ligand/metal combinations for this study. First, we targeted redox-active organic ligands that are readily accessible by simple synthetic routes. It is also desirable that the ligands be modular, such that solubilities can be systematically tuned. Additionally, we focused on earth-abundant first-row metals. Finally, we carefully selected the metal−ligand combinations, such that there is redox synergy between the metal center and organic ligand (i.e., redox noninnocence is observed). Chromium bipyridine complexes meet all of the above criteria. Synthetic approaches to access substituted bipyridine ligands are well-documented and can be conducted on a large scale.37 These ligands can be decorated with diverse substituents that can be used to modify solubilities. Additionally, Cr is an earth-abundant metal, and other Cr-based redox-active materials have been 15884

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voltammetry experiments was confined to between −2.5 and +1.5 V. Table 1 lists the redox potentials (E1/2) and peak current ratios for the [Cr(L1)3]0, [Cr(L1)3]3+, [Cr(L3)3]0, and [Cr(L3)3]3+ complexes. The couple at the most positive potential is designated as the first redox couple. The redox potentials (E1/2) were determined as the average of the cathodic and anodic potentials for each couple, and the peak current ratios (ipox/ipred) were the ratios of the reduction peak current to the oxidation peak current after deconvolution of the voltammogram. A peak current ratio of unity represents a perfectly reversible redox couple. As demonstrated by Shores,38 the [Cr(L1)3]3+ complex exhibits six reversible redox couples (Figure 1A). The voltammetry of this

knowledge there have been no systematic investigations of the corresponding effects in nonaqueous systems. We anticipated that the largest difference in solubility would be between the fully discharged neutral [Cr(L)3]0 complex and the charged [Cr(L)3]3+ complex. This proposal is supported by a recent report showing that N-alkylation of ((dimethylamino)methyl)ferrocene affords a quaternary ammonium salt with a 20-fold enhanced solubility relative to the neutral species.8 Thus, we compared the solubilities of the [Cr(L)3]0 and [Cr(L)3]3+ complexes in CH3CN. The neutral complexes [Cr(L1)3]0, [Cr(L2)3]0, and [Cr(L3)3]0 were prepared via the reaction of the ligand with [Cr(CO)6].44 As anticipated, the neutral complexes were significantly less soluble than those bearing a +3 charge (Scheme 3). In the neutral species, the complexes bearing the Scheme 3. Solubility of [Cr(L)3]3+ and [Cr(L)3]0 in Acetonitrile

Figure 1. Cyclic voltammograms for (A) 1 mM [Cr(L1)3]3+ and (B) 0.48 mM [Cr(L1)3]0 (saturated solution) in acetonitrile. The labels (1st, 2nd, etc.) correspond to the labels used in Table 1. Conditions: scan rate of 100 mV s−1, 0.1 M (TBA)BF4 supporting electrolyte, glassy carbon working electrode, fritted platinum wire counter electrode, and Ag/Ag+ reference electrode.

methyl-substituted ester and the methoxyethyl-substituted ester showed dramatically reduced solubilities (0.3% and 1.6% of the charged species solubilities, respectively). However, the more polar and flexible 2-(2-methoxyethoxy)ethyl-substituted ester complex maintained approximately 40% of the solubility observed in the +3 oxidation state. Cyclic Voltammetry. (TBA)BF4 (0.1 M in acetonitrile) was used as the supporting electrolyte for all of the electrochemical characterization, as its stable potential window is greater than that of the Cr complexes. Evidence for decomposition of the Cr complexes was observed at potentials more negative than −2.5 V vs Ag/Ag+; therefore, the potential window for all cyclic

complex indicates high reversibility on the basis of the peak height ratios listed in Table 1. Redox couples 4, 5, and 6 are all close in potential, causing the peaks to merge together slightly. The cyclic voltammogram for the [Cr(L1)3]0 complex shows six redox couples at potentials similar to those observed for the [Cr(L1)3]3+ complex (Figure 1B). However, the reversibility of the redox couples is significantly decreased. In particular, the peak height ratios for redox couples 1 and 5 are 1.56 and 0.34, respectively, which are far from the ideal ratio of 1.0 (Table 1). Also, the low solubility of the [Cr(L1)3]0 complex (0.48 mM) limits the

Table 1. Summary of Cyclic Voltammetry Data for the L1 and L3 Complexes in the Charged and Neutral Statesa compound [Cr(L1)3]3+ [Cr(L1)3]0 [Cr(L3)3]3+ [Cr(L3)3]0

E1/2 ipred/ipox E1/2 ipred/ipox E1/2 ipred/ipox E1/2 ipred/ipox

1st

2nd

3rd

4th

5th

6th

−0.20 1.02 −0.18 1.56 −0.17 1.06 −0.18 1.3

−0.61 1.04 −0.59 0.95 −0.58 0.9 −0.58 1.22

−1.14 1.04 −1.12 1.02 −1.12 0.98 −1.14 1.13

−1.67 1.03 −1.65 1.12 −1.63 0.98 −1.66 1.06

−1.86 0.94 −1.83 0.34 −1.82 1.02 −1.86 0.98

−2.03 0.95 −2.01 1.20 −2.0 1.06 −2.01 1.32

a

E1/2 (V) was calculated by taking the average of the cathodic and anodic potentials for each redox couple. The ipred/ipox ratio was obtained from the deconvoluted voltammograms. 15885

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Figure 3. Schematic of an electrochemical H-cell used for testing charge/discharge performance of flow battery active species.

provides the opportunity to quantitatively interpret the H-cell results. The cell is charged and discharged by applying a constant current across the graphite electrodes, and the overall cell potential is recorded over time. The potentials observed on each electrode should correspond to the potentials of the active redox couples observed in the cyclic voltammogram. Figure 4 shows results from constant current charge/discharge experiments with [Cr(L3)3]0. The cycling protocol was designed

Figure 2. Cyclic voltammograms for 1 mM (A) [Cr(L3)3]3+ and (B) [Cr(L3)3]0 in acetonitrile. The labels (1st, 2nd, etc.) correspond to the labels used in Table 1. Conditions: scan rate of 100 mV s−1, 0.1 M (TBA)BF4 supporting electrolyte, glassy carbon working electrode, fritted platinum wire counter electrode, and Ag/Ag+ reference electrode.

1e− couples are present at potentials very similar to those of the L1 complexes, with redox couples 4, 5, and 6 being closer in potential (Table 1). While potentials (E1/2) for the redox couples are similar, the electrochemistry for [Cr(L3)3]0 appears to be less reversible than that for [Cr(L3)3]3+ on the basis of the peak current ratios (see Table 1). Nevertheless, the reversibility of the redox couples for [Cr(L3)3]0 is greatly improved relative to that for [Cr(L1)3]0. The similar potentials between the charged and neutral L3 complexes indicate consistent electrochemical behavior independent of the oxidation state of the complex. Overall, the [Cr(L3)3]3+ complex shows reversible electrochemistry, with redox potentials that are slightly more positive than those for [Cr(L1)3]3+. The more positive potentials help to avoid extreme overpotentials across the battery, which could cause deterioration of the electrolyte. The separation between the first and sixth peaks indicates a maximum cell voltage of ∼1.8 V. While this voltage window is comparable to that of other nonaqueous redox couples, the number of redox couples that can be accessed with the noninnocent Cr complexes should increase the energy storage density at a given concentration relative to that of other nonaqueous chemistries.11−17,19−23,25−34,45−53 Charge/Discharge Measurements for [Cr(L3)3]0. On the basis of its attractive solubility profile and the six accessible redox couples, [Cr(L3)3]0 was selected for charge/discharge measurements in an H-cell (a schematic is shown in Figure 3). The H-cell configuration approximates the conditions of a flow system but allows for characterization with small volumes of electrolyte. Graphite electrodes similar to those used in flow cells and a microporous plastic separator (Celgard) were employed. A stir bar was placed in each reservoir to provide adequate mixing during charge/discharge experiments. Additionally, a Ag/Ag+ reference electrode was placed in each chamber to record the individual potential profiles at the positive and negative electrodes during cycling. The use of reference electrodes allows determination of the absolute potentials in situ during cell operation, and thus

Figure 4. Cycles 2−5 from charge/discharge measurements at ±0.64 mA for the [Cr(L3)3]0 complex in H-cell testing for (A) the total cell voltage, (B) the positive electrode potential as referenced to a Ag/Ag+ electrode, and (C) the negative electrode potential as referenced to a Ag/Ag+ electrode. The working electrode (graphite plate) was pretreated at 500 °C for 5 h. Separator: Celgard 2325. Electrolyte: 0.01 M [Cr(L3)3]0 and 0.5 M (TBA)BF4 in acetonitrile. Voltage cutoffs were set on both the negative and positive electrodes to avoid unwanted side reactions.

to accomplish multielectron transfer at each of the electrodes. In particular, the objective was to access the first, second, and third couples at the negative electrode and the fourth, fifth, and sixth couples at the positive electrode. Cutoff voltages were set to allow access to all redox couples, while avoiding irreversible side reactions at high overpotentials. During charge, we used a cathodic voltage cutoff of 0 V and an anodic voltage cutoff of −2.3 V (vs a Ag/Ag+ reference). During discharge, we used a cathodic voltage cutoff of −1.2 V and an anodic voltage cutoff of −1.35 V. However, during a preliminary experiment, a cell that was allowed to reach the charging cutoff voltages showed highly 15886

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A symmetric, multielectron RFB offers opportunities to enhance the energy density of an RFB and to mitigate performance losses associated with crossover of the active species. However, a number of challenges must be addressed to translate the research findings to commercial applications. For example, the cost of the MCCs reported herein is estimated to be 5 times that required for commercial viability (full details of this estimate are provided in the Supporting Information). This estimate is based on a recent analysis proposing that materials costs for RFBs are proportional to the equivalent weight of the active species (equivalent weight = molecular weight per electron transferred).54 To enhance the commercial viability, it will be essential to lower the equivalent weight by (1) decreasing the molecular weight of the MCC and/or (2) increasing the number of electrons transferred. In addition, significant advancements are still needed in solubility, targeting >1 M at all states of charge. Finally, it will be crucial to conduct detailed studies of materials stability during battery cycling to assess (1) how the MCC structure impacts the cycling stability and (2) what chemical decomposition pathways contribute to degradation. Fundamental research aimed at addressing all of these issues is ongoing in our laboratories and will be reported in due course.

irreversible behavior and could not be cycled. Therefore, the charge was limited to 0.0011 A h (4.0 C) to protect the reversibility of the complex. The results indicate that this limit allowed the cell to access four of the six redox couples, with two couples at each electrode. The overall cell potential is illustrated in Figure 4A. For the charging segment, two plateaus (1.1 and 1.7 V) are observed. These plateaus represent two single electron transfer processes (multielectron behavior). During discharge, the potential smoothly decreases, without obvious plateaus. The energy density after charging for the cell is 0.15 W h L−1 at the ∼27% state of charge (SOC). For the maximum [Cr(L3) 3 ] 0 concentration (0.21 M), the total energy density would be 10.2 W h L−1 at the full SOC. The charge/discharge behavior can be further understood from the negative and positive electrode profiles. Parts A and B of Figure 4 illustrate the electrode potentials referenced to a Ag/Ag+ electrode. During the charging segment, two plateaus (ca. −0.6 and −0.2 V) are present for the positive electrode (Figure 4B). These potentials are consistent with the first and second redox couples in the cyclic voltammogram (−0.2 and −0.6 V, respectively, Figure 2B). Notably, two distinct plateaus are observed, although the cell only reached the 27% SOC. It is likely that the low active species concentration combined with suboptimal transport in the H-cell contributes to a low local limiting current for the first oxidation (second redox couple), at which point the first couple would be accessed. Nevertheless, the two distinct plateaus demonstrate multiple electron transfer events from a single active species. The potential profile for the negative electrode does not contain clearly discernible plateaus. This is likely due to the close spacing of the fourth, fifth, and sixth redox potentials and the low voltage (about −1.9 V) achieved during charging. As a consequence, the negative electrode only appears to have accessed the fourth and fifth couples. This corresponds with the observation that only two couples were accessed at the positive electrode. The average Coulombic efficiency for the cell was 68%. While crossover of the active species through the microporous separator is a likely source of the inefficiency, permeability measurements suggest that it is not the sole source. In addition to crossover, it is possible that irreversible chemical interactions occur between the active species and the electrodes or separator. Other possible sources include decomposition of the active species and self-degradation of the electrolytic solutions (e.g., interaction of the active species and supporting electrolyte). Electrochemical decomposition of the active species can be ruled out due to the potential cutoffs set on both the negative and positive electrodes, as well as the Coulombic limit placed on the charge. The charge−discharge profiles are easily explained by the cyclic voltammograms, suggesting that the inefficiencies are primarily due to chemical, as opposed to electrochemical, phenomena. The specific causes for the inefficiencies are being further investigated.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis, purification, and characterization of ligands and Cr3+ and Cr0 complexes, solubility procedures, powder X-ray diffraction data of MCCs before and after solubility studies, cyclic voltammograms for all MCCs, and estimated cost of [Cr(L3)3]3+ for grid-scale applications. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03582.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address §

X.Y.: Optimal CAE Inc., 47802 W. Anchor Ct., Plymouth, MI 48170, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described in this paper was supported by the Joint Center for Energy Storage Research (JCESR), a U.S. Department of Energy Energy Innovation Hub. J.A.S. also thanks the University of Michigan Phoenix Memorial Energy Institute for a Partnerships for Innovation in Sustainable Energy Technologies (PISET) fellowship. We gratefully acknowledge Prof. Paul Rasmussen for helpful discussions and Bryant R. James for the PXRD characterization.

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CONCLUSIONS In summary, in this paper we describe the use of chromium bipyridine complexes in a symmetrical RFB capable of multielectron transfer at each electrode. The solubilities of these complexes can be significantly enhanced via ligand engineering, with minimal perturbation of the electrochemical properties. Charge/discharge experiments are in agreement with the cyclic voltammetry, and demonstrate the accessibility of two redox couples at both the negative and positive electrodes in an H-cell.

REFERENCES

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.5b03582 J. Phys. Chem. C 2015, 119, 15882−15889

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DOI: 10.1021/acs.jpcc.5b03582 J. Phys. Chem. C 2015, 119, 15882−15889