Novel Electrolyte Energy Storage Systems Investigators Allen J. Bard, Professor, Chemistry, The University of Texas at Austin Brent Bennett, Graduate Researcher, Materials Science and Engineering and Chemistry Netzahualcoyotl Arroyo-Currás, Graduate Researcher, Chemistry Jinho Chang, Graduate Researcher, Chemistry Robert Villwock, Associate Director, Center for Electrochemistry Jeremy P. Meyers, Assistant Professor, Mechanical Engineering Abstract

To enhance reliability of the electric grid while simultaneously incorporating renewable power sources into it, there is a pressing need for electrical energy storage, to increase the capability for dispatch and to accommodate the variable nature of those resources. There is, at present, very little energy storage on the grid, due in part to the high capital costs associated with electrochemical energy storage and the lack of flexibility in siting for other technologies.

In this project, we have approached enabling widespread deployment of grid-based storage by lowering the cost of such a system. We have reexamined the fundamentals of redox flow battery (RFB) technology and engaged in an effort in which new active redox couples were discovered and optimized, in pursuit of high efficiency and lower capital costs. We continue to seek transformative changes in the construction and composition of the RFB.

Major research accomplishments during this project include: a) Development of the first alkaline redox flow battery (a-RFB) with negligible crossover and capacity fade, including fundamental knowledge of how potentials of redox couples (e.g. of Fe, Co, and Mn) can be shifted with various novel ligands to increase voltage output, (b) Significant progress in our understanding, design, and testing of high energy-density RFB systems based on a nitrobenzene-bromine system, including understanding the Br-/Br2 reaction mechanism and identifying novel supporting electrolytes, and (c) Design and testing of a tin/bromine RFB and increased understanding of the Sn2+/Sn4+ reaction mechanism.

Introduction Efficient, cost-effective energy storage is vital to the effort to fully integrate

renewable power sources into the electric utility grid. While compressed-air and pumped-hydro storage plants hold the promise of large-scale economical storage, they both require special sites. To date, redox flow batteries (RFB) have shown promise, but are at present far too expensive to be widely deployed. The objective of this research has been to identify new electrolyte systems and cell designs that allow drastic cost reductions (removing this key barrier) while maintaining the high efficiency and ease of operation that are the hallmarks of RFB systems. Our approach brings together expert researchers with skills in chemistry, material science and characterization, electrochemical engineering, and mathematical modeling.

The requirements for large-scale electrical energy storage systems are quite different from existing battery systems. While batteries for portable and transportation applications place a premium on weight and volume, stationary energy storage systems have considerably less stringent requirements. Backup power systems support telecommunications and data centers, but are generally not expected to survive large numbers of charge/discharge cycles. For flow battery systems, one can specify independently the size of the electrochemical reactor (power capacity) and the size of the storage tanks of the free-flowing electrolyte streams (energy capacity). The ability to deliver the active material to the electrode surface by convection ensures that one can bypass mass-transport limitations that curtail the energy density of conventional batteries with solid-phase active materials. Moreover, since the charge and discharge cycles of most RFBs do not involve phase changes, the cycle life and stability can be much higher than conventional batteries.

Background Redox couples for flow-battery applications, represented by reaction in Equation 1,

O + ne- ⇌ R (1)

must satisfy a number of requirements: (i) both forms, O and R, must be highly soluble to minimize the storage volume and mass and to allow high mass transfer rates and current densities during charging and discharging; (ii) the formal potential, Eo’, of one couple must be highly positive, and Eo’ of the other highly negative to maximize the cell voltage and energy density; (iii) the heterogeneous reaction rate for the charging and discharging reactions at the inert electrodes should be rapid so that the electrode reactions occur at their mass transfer controlled rates; (iv) both O and R should be stable during generation and storage, and this stability pertains to reactions with solvent, electrolyte, atmosphere, and electrode materials, and, for metal complexes, stability with respect to ligand loss; (v) the materials should be safe, inexpensive, and abundant; (vi) the couple should not be corrosive and react with cell materials, or the storage vessel.

There are many redox flow battery chemistries, including iron-chromium, zinc-bromine, cerium-zinc, iron-vanadium, zinc air, vanadium air, and all-vanadium. The all-vanadium RFB (VRFB) uses vanadium (which can exist as +2, +3, +4, or +5 ions) in both electrolytes. This eliminates some problems with crossover from the positive to the negative side, because the negative half-cell uses the V(II)/(III) redox couple, and the positive half-cell uses the V(IV)/(V) redox couple. This system is now commercial, with and handful of companies now deploying VRFB technology, such as Prudent Energy (Bethesda, MD), REDT (London, England), H2 (Dajeon, S. Korea), and Schmid Energy (Freudenstadt, Germany). The DOE Global Energy Storage Database[1] lists eight VRFB installations in the U.S. that are planned or already operational.

Several new redox flow battery and hybrid flow battery technologies are moving toward commercialization, and the field is rapidly expanding, both in market size and in technology development. For example, EnerVault (Sunnyvale, CA) has completed a flagship iron-chromium RFB demonstration system at a scale of 250 kW and 1 MWh in Turlock, CA. The capital cost is said to be close to beating cost targets set by the DOE:

capital cost under $250/kW and levelized cost of energy under $0.20/kWh. Several companies are working to commercialize zinc-bromine hybrid flow batteries, including Premium Power (North Reading, MA), Primus Power (Hayward, CA), RedFlow Limited (Brisbane, Australia), and ZBB Energy Corporation (Menomonee Falls, WI). Eos Energy Storage (New York, NY) has announced a MW-scale system available for deliveries starting in 2016.[2] Their Aurora 1000|4000 product is a containerized 1 MW DC battery system providing four continuous hours of discharge, at a volume price of $160/kWh. Aquion Energy (Pittsburg, PA) is commercializing a hybrid sodium-water battery developed by Jay Whitacre at Carnegie Mellon University. That technology is now in volume production. Other companies include Cellstrom (EU, vanadium), EnStorage (Israel, hydrogen-bromine), and Deeya Energy (Fremont, CA, iron-chromium).

There remains an unmet need, and advances in the technology are still required to make grid-scale energy storage by flow batteries attractive to utility companies. LIPA (Long Island Power Authority) recently issued a request for proposals for grid-scale storage capable of delivering 12 hours of discharge at rated power, which exceeds the capability of conventional secondary batteries,[3] seeking as much as 1.63 GW in the form of new peaking or distributed generation, energy storage and demand response resources. However, LIPA later determined not to move forward on any of the proposals submitted.[4] In Texas, utility transmission company Oncor recently announced plans to seek state regulatory approval for up to 5 GW of grid-connected batteries, with deployment set to start in 2018 at an estimated cost of $5.2 billion,[5] based on a report Oncor commissioned from the Brattle Group.[6]

The major thrusts of our research have been directed at discovering the next generation of redox couples for flow batteries that are less expensive than current systems and work under different conditions, for example in alkaline solutions that show lower corrosion effects. As discussed in the following section, we have developed and demonstrated a novel Fe/Co alkaline system. We have also been interested in couples that show multielectron transfers (thereby increasing the energy efficiency) while still maintaining good electrochemical properties. We published studies of the Sn(IV)/(II)-bromine system, where the reaction mechanism has been elucidated with the detection of intermediates, while investigations of electrocatalysis with these couples continue. Finally, we have advanced understanding of systems that could be described as “electrochemical fuels,” by using liquids that themselves show redox properties. These are capable of much higher energy density storage than conventional RFBs, and we demonstrate these principles with the bromine/nitrobenzene system.

Results Redox Active Liquids for High Energy Density Flow Batteries: The Bromine/ Nitrobenzene Flow Battery

Interest in nonaqueous solvents for redox flow batteries (RFBs) has grown in recent years because they offer the possibility of RFBs with higher energy densities, new cell designs, and cheaper materials. However, because these solvents and the relevant supporting electrolytes are more expensive than aqueous solutions with common acids or bases—and have many other problems, including low conductivity and high toxicity—a nonaqueous flow battery likely needs to have 5–10 times the energy density of an aqueous flow battery to realize significant cost savings. With this goal in mind, the Bard group has begun investigating the use of redox active liquids to create a new class of nonaqueous flow batteries. The advantage of a redox active liquid is that it can be used as both the solvent and the redox active material. Its energy density is limited only by the concentration of the supporting electrolyte, which provides the counterions for the redox reaction. These supporting electrolyte concentrations may be able to exceed 5 M, compared to 2 M for the active salts in a typical aqueous flow battery. Combined with the increased voltage window available in nonaqueous solvents, we believe a five to tenfold increase in energy density is possible.

We have designed a high energy density flow battery using bromine (Br2) for the positive electrode reaction and nitrobenzene (NB) for the negative electrode reaction. We have found that the redox potential of the Br3

-/Br2 couple, which is more positive than Br-

/Br2 in NB, is ~ 0.8 V vs. N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD/TMPD+. couple), and that the NB/NB- couple is ~ 1.2 V vs. TMPD/TMPD+.. Therefore, a 2 V flow battery is possible with this system, compared to 1–1.5 V for aqueous systems. The theoretical energy density of this battery, assuming all NB in the negative electrode and a 60/40 mix of NB and Br2 in the positive electrode, is 377 Wh/L. The maximum solubility of our supporting electrolytes prevents reaching this value, but it is possible to approach 200 Wh/L with this all-liquid system, an energy density comparable to Li-ion batteries.

Our initial tests of this system identified three key problems. First, as shown in Figure 1, there is rapid capacity fade due to crossover of the Br2 product as the battery is charged. This problem is also evident the Zn-Br2 RFB system, and in that system it is common to use a quaternary ammonium salt to complex with the Br2 and prevent it from crossing over as rapidly. The second problem is the irreversibility of the Br-/Br2 reaction. Although this reaction is reversible on carbon electrodes in water, in nitrobenzene it is a complex two-step process through a stable tribromide ion intermediate, as shown in Figure 2. We can utilize the Br3

-/Br2 redox couple instead of the entire reaction, but even this couple shows a large peak splitting in cyclic voltammetry on glassy carbon (GC), which leads to a low voltage efficiency. Therefore, we have to use a catalyst (in this case Pt) to achieve a reasonable voltage efficiency (70%). Finally, the low conductivity of this solution prevents us from passing large currents without significant ohmic losses. Although NB has a relatively high dielectric constant (ε = 34.8), solutions of salts in solvents such as NB have conductivities that are two orders of magnitude below that of aqueous solutions (< 10 mS/cm compared to 0.5–1 S/cm). In this case, we are only applying a current density of 5 mA/cm2, which is far less than the typical current densities ( >100 mA/cm2) used in aqueous RFBs.

Figure 1: The first 10 charge-discharge cycling curves of the Br2/NB RFB. Catholyte - 20 mL 100 mM TBA-BR3 and 1 M TBA-BF4 in NB.; analyte - 1 M TBA-BF4 in NB. flow rate – 30 mL/min; 5 cm2 carbon paper (SGL) electrodes, with Pt electrodeposited on the positive electrode; 150 μm polypropylene separator (6 Celgard 2400 separators).

Figure 2: Cyclic voltammograms of the bromide/bromine (Br-/Br2) redox couple in nitrobenzene on platinum (red) and glassy carbon (black) electrodes. In this case, the initial solution is 6 mM Br2 and 0.1 M TEA-BF4 in NB such that the first wave (left to right) is the Br3

-/Br2 couple and the second wave is the Br-/Br3- couple.

Since the first of these three problems has been studied extensively in the Zn/Br2 system, we chose to focus on the latter two problems. Beginning with the irreversibility of the Br-/Br3

-/Br2 redox couples, we decided to study the reaction mechanisms in greater detail with an eye toward identifying why this reaction is less reversible on carbon than on Pt electrodes. This problem is of interest both from a fundamental standpoint – the oxidation of halides in nonaqueous solvents was first studied almost 60 years ago [7], but the individual reaction steps are not fully known - and from a practical standpoint – we want to increase the reversibility of these reactions without using an expensive catalyst.

Using scanning electrochemical microscopy (SECM), we were able to identify an

unstable intermediate during the oxidation of Br- to Br3-, which gave us confidence to

move ahead and study the full Br-/Br3-/Br2 reaction using cyclic voltammetry (CV) on Pt

electrodes.

The results of our studies suggest that the reaction follows an ECEC mechanism, as shown in Figure 3. By fitting this mechanism and the corresponding homogenous and heterogeneous reaction parameters to CVs at a wide range of scan rates (0.020–500 V/s) and reactant concentrations (2–20 mM), we can verify the accuracy of the mechanism and gain insight into the different features of this reaction. First, the equilibrium

Br- + Br2 ⇌ Br3- (2)

strongly favors Br3-, which pushes the Br2 reduction potential to more positive values and

makes the direct oxidation and reduction of Br3- possible. Both of these factors cause the

CV to be split into two waves, whereas in water, which has a much lower equilibrium constant for the Br- + Br2 ⇌ Br3

- reaction, there is only a single two-electron wave. The fact that Br- oxidation follows a different pathway (orange) than Br3

- reduction (blue),

Figure 3: Square scheme depicting the pathways for the Br-/Br3

-/Br2 redox reactions in NB. The Br-/Br3

- oxidation pathway is shown in orange, and the reduction pathway is shown in blue. The Br3

-/Br2 redox pathway is in purple.

both being ECEC reactions, explains the large irreversibility of this redox couple, which is analogous to the Sn2+/Sn4+ couple covered later in this report. On the other hand, the Br3

-/Br2 redox couple (purple) is a simpler EC reaction, which explains why this couple is more reversible.

Finally, by using this same mechanism to model the reaction on carbon electrodes, it was clear that the apparent heterogeneous kinetics of the electrochemical reactions, with the exception of reaction 3,

Br• + e ⇌ Br- (3)

are two orders of magnitude slower than on Pt. From this study, it is clear that order to improve the voltage efficiency of an RFB employing the Br3

-/Br2 redox couple, the kinetics of reactions 4 and 5 must be improved on carbon electrodes.

Br2 + e ⇌ Br2-• (4)

Br3• + e ⇌ Br3

- (5)

Another focus of our efforts has been the identification of supporting electrolytes that are both soluble and conducting in NB and to see if any there are any changes in the solution as we electrolyze it. In general, we find that salts that are soluble in NB tend to consist of large organic ions, such as tetrabutylammonium (TBA+). These significantly increase the viscosity of the solution and limit the maximum concentration to 2–2.5 M due to large volume changes. Fortunately, we were able to identify a smaller asymmetric cation (labeled ASY+ in the plot) that forms solutions in NB that are 3-4 times less viscous and far less resistive at high concentrations than their TBA+ counterparts, as shown in Figure 4.

Figure 4: Conductivity of quaternary ammonium salts, with both symmetric and asymmetric ions, as a function of electrolyte concentration in NB.

Ultimately, we were able to combine this asymmetric cation with trifluoromethane-sulfonate (triflate), a common asymmetric anion, and achieve a maximum solubility of over 3 M. At about 3.1 M, there are equal numbers of ASY+ and NB molecules, at which point it does not make sense from a practical standpoint to add any more salt.

We may be able to come closer to our goal of a 5 M supporting electrolyte by using asymmetric ions with lower molecular weights and less volume change upon mixing with NB. However, despite the significant improvements in conductivity achieved by using the smaller asymmetric salts, the conductivities of these solutions are still an order of magnitude less than similar acetonitrile solutions and two orders of magnitude less than strongly acidic and basic aqueous solutions. Furthermore, as we electrolyze these solutions and create large quantities of nitrobenzene anions, we see evidence of a 2–3 fold increase in viscosity and a corresponding decrease in conductivity.

Although we have not been able to explain this effect completely, it is clear that replacing the supporting electrolyte anion (BF4

-, triflate, etc.) with NB-. during electrolysis changes the intermolecular interactions between the anions, cations, and NB solvent. In order to utilize nitrobenzene as a solvent or a negative redox couple in an RFB, these problems must be addressed through cell design and operating parameters.

The Alkaline Fe/Co Redox Flow Battery We have developed the first redox flow battery (RFB) based on an alkaline

electrolyte. The laboratory prototype energy-storage device uses complexes of iron and cobalt in strong base to achieve energy efficiencies of 75% at current densities of 21 mA cm-2. The redox couples are negatively charged metal-ion complexes. This greatly reduces crossover through the cation-exchange membrane separator, and maintains crossover energy losses below 4–5% in 30 cycles. The voltage of a single cell is 0.93 V, and the RFB is designed with low-cost feedstock materials, lower corrosiveness than acidic electrolytes, and no evolution of gases during cycling.

Our motivation to develop such a device came from the idea that an alkaline electrolyte (such as aqueous NaOH) can achieve conductivities as high as 414 mS/cm,[8] a value that is comparable to the conductivity of 1 M H2SO4. Using an alkaline electrolyte offers the following advantages: a) the conductivity of the electrolyte can be adjusted to minimize undesired shunts within cell stacks; b) alkaline solutions are less corrosive with typical materials of construction than acids; c) ligand-modified redox couples can be used that are chemically stable and reversible, offering good cyclability and high efficiency during the electron transfer reactions.

Ligands can be used to improve solubility and to tune the formal equilibrium potential over wide potential ranges for electrolyte-phase redox couples. Increased solubility allows for higher electrolyte concentrations, increases the energy storage per unit volume, and reduces variability in the potential profile. By suitable choice of ligand the formal potential of a transition-metal ion couple can be shifted in the desired direction. Moreover, such complexes may show improved characteristics with respect to stability in comparison to the uncomplexed species.

The chemical principles related to the formation and properties of metal complexes are well developed and many potentially useful ligands have been reported and compiled.[9] This new work leverages our previous investigations of the electrochemical behavior of promising transition-metal ion complexes with a variety of ligands, including measurements of the formal potential, the electrode kinetics, reversability and the stability of reactants and products of the complexes in solution.[10] We have identified fifteen chemically stable, electrochemically reversible/quasi-reversible couples that are potential candidates for an alkaline flow battery. These chemistries have been extensively characterized and are compatible with carbon-felt electrodes.

The most successful of these new couples is based on coordination chemistries of Fe and Co with organic ligands in strong base, shown in Figure 5.

The electron transfer reactions involved are shown below:

Fe TEA OH ! + e ⇌ Fe TEA OH !!, E!! = −1.05  V  (vs. Ag/AgCl) (6)

Co mTEA H!O + e ⇌ Co mTEA H!O !,  E!" = −0.12  V (vs. Ag/AgCl) (7)

Figure 5: Cyclic voltammograms of candidate redox couples for an alkaline redox flow battery. The concentrations of both couples are 20 mM in Mn+ ions. CVs include: 2 mm diameter glassy carbon electrode (black), 20 mM [Fe(TEA)(OH)]- (blue), and 20 mM [Co(mTEA)(H2O)] (red), all in aqueous 5 M NaOH. TEA is triethanolamine and mTEA is 1-[bis(2-hydroxyethyl)amino]-2-propanol. The scan rate is v = 50 mV s-1. The complexes were synthesized using stoichiometric amounts of metal ions and ligands. ΔEpFe-TEA = 60 mV, ΔEpCo-mTEA = 73 mV. E0’

Fe = -1.05 V and E0’Co = -0.12 V.

The complexes used in the battery are synthesized in a one-step procedure with virtually 100% yield. This energy-storage device represents a successful attempt to extend cyclability of RFBs by chemical modification of the electrolytes. A patent and a peer-reviewed manuscript have been published.

The electrochemical performance of the alkaline battery proposed was evaluated in a high-efficiency flow cell (Figure 6) by electrochemical cycling, using concentrations of 0.5 M of each redox couple in solution. The design of the flow cell was based on a previous report.[11] An output voltage of 0.95 V and output currents on the order of 150 mA were obtained (using carbon felt electrodes, 4.7 cm x 4.7 cm x 0.5 mm, 11.05 cm3, ρ = 0.1 g/cm3).

As shown in Figure 7, cycling of the battery in this configuration achieved up to 40 cycles with 3.2 % crossover, and 76 % average energy efficiency when passing 21 mA cm-2. Coulombic efficiency was 99%. Concentrations up to 0.5 M of each redox couple have been tested with similar performance. The main source of internal resistance in the cell is the Nafion membrane, because the conductivity of Na+ ions (from the 5 M NaOH electrolyte) across the membrane is considerably lower than that of protons.

Adjusting the concentration ratio of ligand-to-metal has allowed us to reach maximum concentrations of 0.8 M Fe-TEA and 0.7 M Co-mTEA.

Figure 6: A schematic diagram of the flow cell for the alkaline redox flow battery. A commercial fuel cell from Fuel Cell Technologies (Albuquerque, NM) was modified to have an active area of 3.24 cm2. The RFB was assembled in a “zero-gap” configuration, meaning that the membrane, electrodes, and current collectors were in direct contact with each other. A Nafion® membrane (50 µm thick) from Alfa Aesar (Ward Hill, MA) served as the separator. The high surface area electrodes were 10AA carbon paper from SGL Technologies GmbH (Germany). The current collectors were Poco graphite plates with machined serpentine flow channels. Contact to the current collectors was made with nickel-plated copper plates.

Figure 7: Top. Charge-discharge cycling curves of the Co/Fe aRFB with the following conditions: catholyte - 20 mL of 50 mM Co-mTEA in 5 M NaOH; anolyte - 40 mL of 50 mM FeII-TEA and 25 mM FeIII-TEA in 5 M NaOH; 50 µm thick Nafion® membrane; flow rate: 40 mL min-1, current density: 21 mA cm-2. State of charge = 40%. Bottom. Detail of region between 3–5 hours.

Multielectron Redox Couples for RFB Applications — Tin/Bromine System Most of the research on redox couples for flow batteries involves one electron

transferred (i.e., n = 1 in Equation 1). However, by employing multielectron couples the storage density could be multiplied. A problem with such couples is that the mechanism of the half-reaction is more complex, and this may result in kinetic complications in the charge and discharge cycles. We have focused on tin-based systems for redox flow batteries. Sn(IV) bromide in an acidic bromide medium is a promising candidate because both redox couples, Sn(IV)/Sn(II) and Br2/Br- are dissolved in same solution, eliminating concerns of cross contamination. This work was encouraged by the design of a new tin-bromine RFB system with Sn(IV)/Sn(II) as an anode and Br2/Br- as a cathode.[12]

The fundamental understanding of the kinetics and mechanism of such electrode reactions (where n = 2) is very poor compared to the rather advanced state of n = 1 reactions. With a view toward discovering ways to improve poor electrochemical reversibility, considerable progress was made toward a complete understanding of the mechanism of the Sn(IV)/Sn(II) redox reaction. A key question has been whether the preferred reaction pathway is (a) two electrons simultaneously transferred interfacially through tunneling, or (b) two stepwise one-electron transfers occurring via a Sn(III) intermediate. We have sought to find and quantify a Sn(III) intermediate through electrochemical analysis methods such as fast-scan cyclic voltammetry and scanning electrochemical microscopy (SECM), and use these techniques to elucidate the mechanism and rate constants for multielectron reaction pathways.

We have reported the detection of a Sn(III) intermediate, Sn(III)Br4-, during

Sn(II)Br42-/Sn(IV)Br6

2- oxidation, building upon our previous work in which Sn(III)Br63-

was detected during Sn(IV)Br62-/Sn(II)Br4

2- reduction. Using these new data, we propose a mechanism for Sn(II)/Sn(IV) oxidation.

Fast scan cyclic voltammetry (FSCV) and scanning electrochemical microscopy (SECM) were used to detect Sn(III). FSCV has been successfully used with other half-reactions to observe two single-electron-transfer steps in overall two-electron-transfer reactions, allowing kinetic discrimination of the two electrochemical steps.[13] FSCV is also a powerful tool for detecting short-lived intermediates because the intermediates can be re-oxidized/reduced before undergoing homogeneous reactions to form more stabilized structures. This detection is possible when the characteristic time of cyclic voltammetry is shorter than homogenous reaction time of the intermediates.[14] SECM methods have been developed and used for the collection of intermediates.[15] In SECM, a 100% collection efficiency (CE) can be achieved in pure diffusion-controlled redox mediators at a reasonably close distance between a tip and substrate; this CE is much higher than that obtained from a rotating ring disk electrode (normally, 30–35%). Therefore, SECM can collect even traces of any generated intermediates.[16]

A one-electron-transfer step in the Sn(II)/Sn(III) redox reaction was observed by FSCV, using scan rates of ν ≥ 10 V/s. From tip generation/ substrate collection (TG/SC) mode in SECM, no further fast decomposition reactions of Sn(II)Br4

2- and Sn(IV)Br62-

were confirmed. In SECM studies, the intermediate Sn(III)Br4-, generated from the tip,

was collected by the reducing it to Sn(II)Br42- on the substrate at d~1 µm while holding

Esub = 0 V, where Sn(IV)Br62- was not reduced. The rate of the disproportionation and

bromide addition reactions of Sn(III)Br4- was estimated to be 1.0×109 M-1s-1 and

3.0×103 s-1, respectively, by a digital simulation based on SECM experiments. From the experimental evidence of the Sn(III) species in the Sn(IV)/Sn(II) redox reaction, different reaction pathways between Sn(IV)/Sn(II) reduction and oxidation, via the Sn(III) intermediates, were proposed, and the irreversibility of Sn(IV)/Sn(II) redox reaction was explained by the difference of the reduction potentials between Sn(IV)Br6

2-/Sn(III)Br63-

and Sn(III)Br4-/Sn(II)Br4

2-.

In the study of Sn(IV)/Sn(II) reduction previously reported, the main species of Sn(IV) and Sn(II) in 2 M HBr + 4 M NaBr solution were reported to be Sn(IV)Br6

2- and Sn(II)Br4

2-, respectively, based on Raman studies,[17,18] and the stability constants of Sn(II) with Br-.[19] Based on our cyclic voltammetry and SECM experiments, we propose the following Sn(II)/Sn(IV) oxidation reaction pathway:

Sn(III)Br4- + e- Sn(II)Br4

2- (8) 2 Sn(III)Br4

- Sn(II)Br42- + Sn(IV)Br6

2- (9)

Sn(III)Br4- + Br- Sn(III)Br5

2- (10) Sn(IV)Br5

- + e- Sn(III)Br52- (11)

2 Sn(III)Br52- Sn(II)Br4

2- + Sn(IV)Br62- (12)

Sn(IV)Br5- + Br- Sn(IV)Br6

2- (13)

The new anodic peak with one electron transfer and a small reversal peak in the CVs with fast scans and the collected current in SECM at Esub = 0 V are attributed to Sn(III)Br4

-/Sn(II)Br42- redox reaction (2). The Sn(III)Br4

- can either disproportionate to

Figure 8: A schematic description of the total reaction pathway in the Sn(IV)/Sn(II) redox reactions; black circles show main species in 2 M HBr / 4 M NaBr solution and blue circles show the Sn(III) intermediates detected by FSCV and SECM.

Sn(II)Br42- and Sn(IV)Br6

2- (3) or become Sn(III)Br52- with the addition of one

bromide (4). Sn(III)Br52- can be oxidized to Sn(IV)Br5

- with one electron transfer, which was not observed in our fast scan CVs, (5) or disproportionate to Sn(II)Br4

2- and Sn(IV)Br6

2- (6). Finally, Sn(IV)Br5- goes to Sn(IV)Br6

2- with the addition of Br- (7).

Figure 8 shows a schematic description of the reaction pathway for both Sn(IV)/Sn(II) reduction (red) and oxidation (green). The proposed mechanism explains the large irreversibility in Sn(IV)/Sn(II) redox reaction. In the reduction, Sn(IV)Br6

2- is reduced to Sn(II)Br4

2- through the Sn(III) intermediates, Sn(III)Br63- and Sn(III)Br5

2-. The reduced Sn(II)Br4

2- cannot be oxidized via the same route as that in the reduction because Sn(II)Br4

2- is thermodynamically the most stable form among the Sn(II)Brx2-x species in

2 M HBr + 4 M NaBr. It must first be oxidized to Sn(III)Br4-. Therefore, the

irreversibility is based on the difference in the reduction potentials between Sn(IV)Br62-

/Sn(III)Br63- and Sn(III)Br4

-/Sn(II)Br42-.

Conclusions The need for better and more options for grid-scale energy storage has been a barrier

to broader implementation of renewable energy sources. During this project we have made progress toward developing a basis for several flow-battery technologies that could improve grid-scale energy storage. Each of these represents a new, previously unexplored strategy for energy storage. The major research accomplishments include the following.

We have reported the development of the first alkaline redox flow battery (a-RFB), a novel Fe/Co alkaline system, which has negligible crossover and capacity fade. We gathered detailed fundamental knowledge of how potentials of redox couples (e.g. of Fe, Co, and Mn) can be shifted with various novel ligands to increase voltage output.

We have advanced understanding of systems that could be described as “electro-chemical fuels,” by using liquids that themselves show redox properties. These are capable of much higher energy density storage than conventional RFBs, where the redox molecules are solutes and are subject to solubility limits. We demonstrated these principles with the bromine/nitrobenzene system, understanding, designing, and testing this high energy-density RFB system, including understanding the Br-/Br2 reaction mechanism and identifying novel supporting electrolytes.

Finally, most flow-battery strategies to date have involved redox couples where only one electron is transferred. We explored multielectron couples with the aim of multiplying the possible energy density. In particular, we focused on tin-based systems, including Sn(IV) bromide in an acidic bromide medium. This is a promising candidate because both redox couples, Sn(IV)/Sn(II) and Br2/Br- are dissolved in same solution, eliminating concerns of cross contamination. An inherent problem with such multielectron couples is that the mechanism of the half-reaction is more complex, which may result in inefficiencies due to slow reaction kinetics in the charge and discharge cycles. We have elucidated the reaction mechanism of the Sn(IV)/(II)-bromine system, including the detection of Sn(III) intermediates. This increased understanding of the reaction mechanism holds promise for future development of RFBs that take advantage

of multielectron transfers (thereby increasing the energy efficiency) while still maintaining good electrochemical properties.

This research has advanced fundamental understanding of the controlling processes in novel flow batteries, enabling modifications in their chemistry so that they can deliver efficient energy storage. The new chemistry can inform related efforts and enable advances in the development of these devices and materials. Specifically, we have developed new understanding of how to tailor redox couples, and quantified competing factors that can tune the redox potentials. Our research considered the eventual large-scale application by working with both aqueous and nonaqueous systems, earth-abundant materials, and optimizing for manufacturability and cost. Moving forward, these insights can inform characterization and design efforts, and provide guidelines for scale-up.

The potential impact of this research on greenhouse gas emissions at a global scale is difficult to estimate, largely because the effect is indirect—the main motivation for grid-scale storage is to enable energy production options with more favorable carbon footprints. The fundamental leaps in chemistry, materials, and engineering knowledge provided by this work are a step toward the ability to build and operate commercial grid-scale storage for intermittent renewable energy sources. This type of large-scale storage is a natural complement to intermittent renewable generation methods, and to the degree we can enable those methods, we can directly reduce greenhouse gas emissions. These new RFB systems could provide new technology options for energy storage, enabling wider deployment of renewable energy sources on the electrical utility grid.

Publications, Patents and Presentations 1. Chang, J., Leonard, K. C., Cho, S. K., & Bard, A. J. Examining Ultramicroelectrodes for Scanning

Electrochemical Microscopy by White Light Vertical Scanning Interferometry and Filling Recessed Tips by Electrodeposition of Gold Anal. Chem. 84, 5159–5163 (2012).

2. Chang, J. Detection of Intermediate Sn(III) on Gold electrode through Fast Scan Cyclic Voltammetry and Scanning Electrochemical Microscopy, 2012 CEC Annual Workshop on Electrochemistry, Poster Session, February 11, 2012, Austin, TX.

3. Arroyo-Currás, N. Use of Scanning Electrochemical Microscopy (SECM) in the Evaluation of Redox Couples for Flow Battery Applications, 2013 CEC Annual Workshop on Electrochemistry, Poster Session, February 9, 2013, Austin, TX.

4. Chang, J. Detection of Intermediate Sn(III) in Sn(II)/Sn(IV) Oxidation on Gold electrode Through Fast Scan Cyclic Voltammetry (FSCV) and Scanning Electrochemical Microscopy (SECM), 2013 CEC Annual Workshop on Electrochemistry, Poster Session, February 9, 2013, Austin, TX.

5. Bennett, B. Electrochemical Study of the Bromide/Bromine Reaction in Nitrobenzene, 2013 CEC Annual Workshop on Electrochemistry, Poster Session, February 9, 2013, Austin, TX.

6. Bennett, B., Chang, J. & Bard, A. J. Redox Active Liquids for High Energy Density Flow Batteries: The Bromine/Nitrobenzene Flow Battery, 224th ECS Meeting, Poster Session, October 29, 2013, San Francisco, CA.

7. Bennett, B., Chang, J. & Bard, A. J. Redox Active Liquids for High Energy Density Flow Batteries: The Bromine/Nitrobenzene Flow Battery, 2014 CEC Annual Workshop on Electrochemistry, Poster Session, February 8, 2014, Austin, TX.

8. Arroyo-Currás, N., Hall, J. W., Dick, J. E., Bard, A. J. Co/Fe: The Alkaline Redox Flow Battery, 2014 CEC Annual Workshop on Electrochemistry, Poster Session, February 8, 2014, Austin, TX.

9. Chang, J. & Bard, A. J. Detection of the Sn(III) Intermediate and the Mechanism of the Sn(IV)/Sn(II) Electroreduction Reaction in Bromide Media by Cyclic Voltammetry and Scanning Electrochemical Microscopy J. Am. Chem. Soc., 136, 311−320 (2014).

10. Bennett, B. Redox Flow Batteries: How Cutting-Edge Chemistry Will Transform the Electric Grid, UT Energy Institute Student Research Showcase, April 24, 2014, Austin, TX.

11. Bennett, B., Chang, J. & Bard, A. J. Redox Active Liquids for High Energy Density Flow Batteries: The Bromine/Nitrobenzene Flow Battery, GCEP Research Symposium 2014, Poster Session, October 14, 2014, Palo Alto, CA.

12. Arroyo-Currás, N., Hall, J. W., Dick, J. W., Jones, R. A., & Bard, A. J. An Alkaline Flow Battery Based on the Coordination Chemistry of Iron and Cobalt J. Electrochem. Soc., 162 (3), A378-A383 (2015).

13. Bennett, B., Chang, J. & Bard, A. J. Redox Active Liquids for High Energy Density Flow Batteries: The Bromine/Nitrobenzene Flow Battery, 2015 CEC Annual Workshop on Electrochemistry, Poster Session, February 7, 2015, Austin, TX.

14. Bard, A. J. and Arroyo-Currás, N. A Redox Flow Battery that Uses Complexes of Cobalt and Iron with Amino-alcohol Ligands in Alkaline Electrolytes to Store Electrical Energy, U.S. Patent, PCT International Application WO 2015054260 A2, published April 16th, 2015.

15. Chang, J., Bennett, B. & Bard, A. J. Detection of an Unstable Intermediate and the Mechanism of Br-/Br3- Electro-oxidation on a Platinum Electrode in Nitrobenzene by Scanning Electrochemical Microscopy, in preparation.

16. Bennett, B., Chang, J. & Bard, A. J., Investigation of the Br-/Br3-/Br2 Reaction Mechanism on a

Platinum Electrode in Nitrobenzene by Cyclic Voltammetry, in preparation. 17. Bennett, B. & Bard, A. J. Application of Nitrobenzene as a Solvent and Negative Redox Couple in

a Redox Flow Battery: Conductivity and Viscosity of Novel Asymmetric Supporting Electrolytes as a Function of State of Charge, in preparation.

18. Arroyo-Currás, N., Hall, J. W., Jones, R. A., Bard, A. J. Structural, Electrochemical and Spectroscopic Characterizations of Fe(III/II)-TEA in Strong Base, in preparation.

References

1. http://www.energystorageexchange.org/ (accessed May 11, 2015). 2. Business Wire, Jan. 21, 2015, http://www.businesswire.com/news/home/20150121005210/en/Eos-

Energy-Storage-Introduces-Aurora-Battery-System (accessed May 11, 2015). 3. http://www.lipower.org/newscenter/pr/2013/101813-proposal.html (accessed April 27, 2014). 4. https://www.psegliny.com/page.cfm/AboutUs/PressReleases/120214-statement (accessed May 11,

2015). 5. https://www.greentechmedia.com/articles/read/Texas-Utility-Oncor-Faces-Opposition-on-Its-

5.2B-Bet-on-Distributed-Energy (accessed May 11, 2015). 6. http://www.brattle.com/system/news/pdfs/000/000/749/original/

The_Value_of_Distributed_Electricity_Storage_in_Texas.pdf (accessed May 11, 2015). 7. Popov, A. I.; Geske, D. H.; J. Am. Chem. Soc, 1958, 80, 5346–5349. 8. Foxboro, Technical Report: http://myweb.wit.edu/sandinic/Research/

conductivity%20v%20concentration.pdf (accessed April 3, 2013). 9. See, e.g., some excellent and comprehensive reviews in (a) Comprehensive Coordination

Chemistry: the Synthesis, Reactions, Properties and Application Wilkinson, G., Gillard, R. D., & McCleverty, J. A., Eds. Vols. 1–7, Oxford, England; New York: Pergmon Press, 1987; (b) Wilkins, R. G. Kinetics and Mechanism of Reactions of Transition Metal Complexes 2nd ed., VCH, New York, 1991. (c) Comprehensive Coordination Chemistry II: from Biology to Nanotechnology McCleverty, J. A. & Meyer, T. J., Eds. Vols. 1–10, Amsterdam; Boston; Elsevier Pergamon, 2004.

10. Arroyo-Currás, N. Use of Scanning Electrochemical Microscopy (SECM) in the Evaluation of Redox Couples for Flow Battery Applications, 5th Annual Center for Electrochemistry Workshop, Poster Session, February 9, 2013, Austin, TX.

11. Aaron, D.S.; Liu, Q.; Grim, G.M.; Papandrew, A.B.; Turhan, A.; Zawodzinski, T.A.; Mench, M.M.; J. Power Sources, 2012, 206, 450–453.

12. (a) Skyllas-Kazacos, M., Chakrabarti, M. H., Hajimolana, S. A., Mjalli, F. S., & Saleem, M. Progress in Flow Battery Research and Development J. Electrochem. Soc.158, R55–R79 (2011).

(b) Skyllas-Kazacos, M. Novel Vanadium Chloride/Polyhalide Redox Flow Battery J. Power Sources 124, 299–302 (2003).

13. (a) Pierce, D. T. & Geiger, W. E. Electrochemical Kinetic Discrimination of the Single-Electron-Transfer Events of a Two-Electron-Transfer Reaction: Cyclic Voltammetry of the Reduction of the bis(Hexamethylbenzene)ruthenium Dication J. Am. Chem. Soc..114, 6063–6073 (1992). (b) Stoll, M. E., Belanzoni, P., Calhorda, M. J., Drew, M. G. B., Félix, V., Geiger, W. E., Gamelas, C. A., Gonçalves, I. S., Romão, C. C., & Veiros, L. F. Stepwise Hapticity Changes in Sequential One-Electron Redox Reactions of Indenyl-Molybdenum Complexes:   Combined Electrochemical, ESR, X-ray, and Theoretical Studies J. Am. Chem. Soc. 123, 10595–10606 (2001). (c) Kaim, W., Reinhardt, R., Greulich, S., & Fiedler, J. Resolving the Two-Electron Process for the Couple [(C5Me5)M(N^N)Cl]+/[(C5Me5)M(N^N)] (M=Rh, Ir) into Two One-Electron Steps Using the 2,2’-Azobis(pyridine) N^N Ligand, Fast Scan Cyclovoltammetry, and Spectroelectrochemistry:   Detection of Radicals instead of MII Intermediates Organometallics 22, 2240–2244 (2003).

14. (a) Yang, H. & Bard, A. J. The Application of Rapid Scan Cyclic Voltammetry and Digital Simulation to the Study of the Mechanism of Diphenylamine Oxidation, Radical Cation Dimerization, and Polymerization in Acetonitrile J. Electroanal. Chem. Interfacial Electrochem. 306, 87–109 (1991). (b) Yang, H. & Bard, A. J. The Application of Fast Scan Cyclic Voltammetry. Mechanistic Study of the Initial Stage of Electropolymerization of Aniline in Aqueous Solutions J. Electroanal. Chem. 339, 423–449 (1992). (c) Yang, H., Wipf, D. O., & Bard, A. J. Application of Rapid Scan Cyclic Voltammetry to a Study of the Oxidation and Dimerization of N,N-dimethylaniline in Acetonitrile. J. Electroanal. Chem. 331, 913–924 (1992). (d) Baur, J. E., Wang, S., & Brandt, M. C. Fast-Scan Voltammetry of Cyclic Nitroxide Free Radicals Anal. Chem. 68, 3815–3821 (1996). (e) Andrieux, C. P., Hapiot, P., & Saveant, & J. M. Fast Chemical Steps Coupled with Outer-sphere Electron Transfers. Application of Fast Scan Voltammetry at Ultramicroelectrodes to the Cleavage of Aromatic Halide Anion Radicals in the Microsecond Lifetime Range J. Phys. Chem. 92, 5987–5992 (1988). (f) Andrieux, C. P., Audebert, P., Hapiot, P., & Saveant, J. M. Observation of the Cation Radicals of Pyrrole and of Some Substituted Pyrroles in Fast Scan Cyclic Voltammetry. Standard Potentials and Lifetimes. J. Am. Chem. Soc. 112, 2439–2440 (1990).

15. (a) Zhou, F. & Bard, A. J. Detection of the Electrohydrodimerization Intermediate Acrylonitrile Radical Anion by Scanning Electrochemical Microscopy J. Am. Chem. Soc. 116, 393–394 (1994). (b) Bi, S., Liu, B., Fan, F. R., & Bard, A. J. Electrochemical Studies of Guanosine in DMF and Detection of Its Radical Cation in a Scanning Electrochemical Microscopy Nanogap Experiment J. Am. Chem. Soc. 127, 3690–3691 (2005).

16. Unwin, P. R. Kinetics of Homogeneous Reactions Coupled to Heterogeneous Electron Transfer. In Scanning Electrochemical Microscopy; Mirkin, M. V. & Bard, A. J., Eds. Marcel Dekker, Inc.: New York, 2001; pp. 244–254.

17. Woodward, L. A. & Anderson, L. E. Raman Spectrum and Structure of the Hexabromostannate Ion in Aqueous Solution J. Chem. Soc. 1284–1286 (1957).

18. Taylor, M. J.; Coddington, J. M. The Constitution of Aqueous Tin(IV) Chloride and Bromide Solutions and Solvent Extracts Studied by 119Sn NMR and Vibrational Spectroscopy Polyhedron 11, 1531–1544 (1992).

19. Högfeldt. E. Stability constants of metal-ion complexes, Part A: Inorganic ligands, 1st ed.; IUPAC Chemical Data Series, No. 21; Pergamon Press: 1982; pp. 264.

Contacts Allen J. Bard: ajbard@mail.utexas.edu Brent Bennett: brent.bennett@utexas.edu Netzahualcoyotl Arroyo-Currás: narroyo@chem.ucsb.edu Jinho Chang: jcha@illinois.edu Robert Villwock: bvillwock@cm.utexas.edu Jeremy Meyers: jmeyers@molecularrebar.com