BF3-Promoted Electrochemical Properties of Quinoxaline in Propylene Carbonate

Electrochemical and density functional studies demonstrate that coordination of electrolyte constituents to quinoxalines modulates their electrochemical properties. Quinoxalines are shown to be electrochemically inactive in most electrolytes in propylene carbonate, yet the predicted reduction potential is shown to match computational estimates in acetonitrile. We find that in the presence of LiBF4 and trace water, an adduct is formed between quinoxaline and the Lewis acid BF3, which then displays electrochemical activity at 1 – 1.5 V higher than prior observations of quinoxaline electrochemistry in non-aqueous media. Direct synthesis and testing of a bis-BF3 quinoxaline complex further validates the assignment of the electrochemically active species, presenting up to a ~26-fold improvement in charging capacity, demonstrating the advantages of this adduct over unmodified quinoxaline in LiBF4-based electrolyte. The use of Lewis acids to effectively “turn on” the electrochemical activity of organic molecules may lead to the development of new active material classes for energy storage applications. Page 1 of 29 RSC Advances R S C A dv an ce s A cc ep te d M an us cr ip t

Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. INTRODUCTION Stationary energy storage systems are needed to facilitate the widespread penetration of intermittent renewable electricity generators such as solar photovoltaic and wind turbines, and to improve energy efficiency of the electric grid 1 . Redox flow batteries (RFBs) may offer the best combination of cost, performance, and operational flexibility to meet these needs 2 . Unlike enclosed rechargeable batteries which house all components in a single cell, RFBs utilize the reduction and oxidation of electro-active species in flowable solutions or suspensions that are housed in external tanks and pumped to a power-converting electroreactor. To date, the vast majority of flow battery technologies are based on aqueous electrochemistry, with all-vanadium and zinc-bromine systems being the most successful 2d, 3 .
Redox-active organic molecules are particularly promising charge storage materials for RFBs because relevant properties like potential and solubility can be tuned through modifications of molecular structure. Aziz and co-workers recently described a bench-scale aqueous flow battery utilizing quinones as charge storage materials 4 . Furthermore, they employed quantum chemical computations to show a correlation between reduction potential and the degree of substitution with hydroxyl groups and demonstrated good agreement with experimental measurements in sulfuric acid media 4 . While the thermodynamic stability window of water is 1.23 V, sluggish hydrogen and oxygen evolution kinetics on electrode materials can enable higher cell voltages, for example lead-acid and zinc-bromine batteries. Deploying non-aqueous solvents offers an even wider window of electrochemical stability which enables non-aqueous RFBs to operate at cell potentials > 3.0 V, thereby leading to higher energy density and typically higher roundtrip efficiency, which together reduce the cost of energy. To take full advantage of this extended potential window, electrochemical couples must be developed with suitably different reduction potentials, high solubility, and good long term stability. To date, a selection of non-aqueous electrochemically active materials have been studied including transition metal centered coordination complexes 5 , transition metal centered ionic liquids 6 , and organic molecules 7 .
Quinoxalines are a promising family of redox-active materials due to their high intrinsic capacity (ca. 410 mAh/g for quinoxaline, assuming 2etransfer) and high solubility in carbonate To design and optimize quinoxalines for non-aqueous RFBs, their electronic properties as well as interactions with different electrolytes must be understood, controlled, and eventually manipulated. Moreover, given the sheer number of possible quinoxaline derivatives and electrolyte compositions, incorporating an understanding of electrolyte interactions into molecular design rules is critical to guiding future research directions. Here we describe the role of the Lewis acid BF 3 in the electrochemical behavior of quinoxaline in LiBF 4 -propylene carbonate electrolyte solutions. We used electrochemical studies in combination with explicit computational models to demonstrate the effect of various electrolyte constituents on the voltammetry and cycling behavior of quinoxaline. First, we characterized the effect of electrolyte composition and decomposition products on the electrochemical behavior of quinoxaline.
Second, we modeled the impact of electron-donating and electron-withdrawing substituent groups, as well as adducts formed from different types of electrolyte salts, on the thermodynamic and related electrochemical properties of quinoxalines. Third, the confluence of these experimental and computational efforts resulted in the design and synthesis of a novel substituted quinoxaline molecule exhibiting a 26-fold improvement in charging capacity compared to bare quinoxaline.

Computational Details
We employed the B3LYP/6-31+G(d) level of theory to compute the structure and energetics of all species using Gaussian 09 Software. The same level of theory was used to calculate zero point energies, free energy corrections (298 K, 1 atm pressure) and solvation energies. The SMD model was used to compute the solvation free energy by a single point energy calculation on the gas phase optimized geometry using water as the dielectric medium 12 . We find that this is an effective approximation for computing free energies of electrochemically active species in solution. We have optimized selected systems using the SMD solvent with a water dielectric medium model to include the solvation effects in determining the geometry and energy. For this study, changing the dielectric medium to acetone, dimethylsulfoxide, or methanol did not significantly affect the computed reduction potentials of quinoxaline derivatives (Table S1). The Gibbs free energy (298 K) in solution is computed as the sum of the free energy in the gas phase and the solvation energy. Upon computing the solution phase free energy change for reduction or oxidation process (∆G redox ), the reduction potential (E V redox ) is calculated via the following identity, E V redox = -∆G redox /nF, where n is the number of electrons involved in the reduction reaction and F is the Faraday constant. Thereafter, the computed reduction potential is referenced to a Li/Li + electrode, a typical standard used for non-aqueous Li-ion electrochemistry, via the following equation, E V redox (Li/Li + ) = E V redox -1.24 V, where 1.24 V represents the difference between the standard hydrogen electrode (SHE, -4.28 V 13 ) and Li/Li + reduction couple (-3.04 7 V). The addition of the constant '-1.24 V' is required to convert the free energy changes to reduction potential (Li/Li + reference electrode), a commonly used convention to compute the reduction potentials in solution 14 . The change in electron energy when going from vacuum to non-aqueous solution is treated as zero, similar to other reports 15 . Further details regarding the computation of reduction potential can be found elsewhere 15,16 .
It should be noted that the binding of the second electron to the mono-anion in the gas phase is thermodynamically uphill (negative electron affinity), while inclusion of solvation contributions favors the binding of the second electron. The negative electron affinities result in less accurate reduction potential, but agreement is reasonable in cases where experimental values are available. It was found that finite basis sets gives reasonable results in comparison to gas phase experimental results for gas phase temporary anions with negative electron affinities due to a cancellation of errors 17 . In general, quantum chemical calculations are able to compute the influence of different salt and solvent molecules on the reduction potential of a material of interest 16h,18 .

Synthesis of Quinoxaline (bis)trifluoroborane
Unless otherwise stated, all chemical reagents were obtained from commercial suppliers and used without purification. Solvents were purified in a solvent purification system with alumina columns. The synthesis procedure was modified from Martin et al 19

Electrochemical Analysis of Quinoxaline-Electrolyte Interactions
Quinoxaline is electrochemically active at 1.  To further study the promotional effect of LiBF 4 on the reduction current of quinoxalines, the concentrations of two quinoxaline species (2,3,6-TMQ and quinoxaline) and LiBF 4 were varied relative to each other. Figure 2 shows the change in peak current of the well-defined second reduction wave (E PC-ii ) which corresponds to the reduction wave at ~2.45 V vs. Li/Li + (Figure 1) of 2,3,6-TMQ and quinoxaline as a function of active species and LiBF 4 concentration. Figure   2a shows that increasing the 2,3,6-TMQ concentration from 5 to 50 mM, while holding the LiBF 4 concentration constant at 0.2 M, results in only slightly more than a two-fold increase in the current. In comparison, increasing the LiBF 4 concentration, while holding 2,3,6-TMQ concentration constant at 50 mM, leads to directly proportional increases in observed current.
Similar trends were observed for quinoxaline (Figure 2b). The voltammetry corresponding to these data points is presented in the Supporting Information ( Figure S4). These results indicate that the magnitude of the reduction current is due to an interaction between the active species and supporting electrolyte which, at these concentrations, has a stronger dependence on salt concentration. To determine if the presence of BF 3 is linked to the observed reduction current of quinoxalines, we spiked the electrolytes consisting of 2,3,6-TMQ or quinoxaline in Li triflate / PC with BF 3 •OEt 2 (Figure 3). Recall that 2,3,6-TMQ did not display any significant electrochemical behavior in the Li triflate / PC electrolyte (see Figure 1). Figure 3a shows  Quinoxaline is soluble and electrochemically active in select aqueous solutions 24 ; thus, it is imperative to consider the role of water contamination in the electrochemical behavior of quinoxalines in non-aqueous electrolytes. We note that the as-prepared (without subsequent drying) solutions contained significant amounts of water (> 100 ppm), even when the solvent and electrolyte were dried using activated molecular sieves. Therefore, we dried some of the solutions of quinoxaline after mixing it with electrolyte and then compared the voltammetry to as-prepared solutions. Following drying with activated molecular sieves, the peak reduction current of 0.05 M quinoxaline in 0.5 M LiBF 4 is only 0.5 µA (Black line, Figure S6). This is approximately an order of magnitude lower than as-prepared solutions in which the water content is 150 ppm (Red line, Figure S6). Although the voltammetry of the dried and asprepared quinoxaline display similar features (two reduction and oxidation waves), the magnitude of the reduction current corresponding to the dried quinoxaline is comparable to that of the baseline current seen in the presence of TBABF 4 (Supporting Information, Figure S2), indicating that quinoxaline is barely electrochemically active in the absence of water.
Importantly, the electrochemistry of quinoxaline in as-prepared Li triflate, which contained 200 ppm water, is comparable in magnitude to dried quinoxaline (Black line, Figure S7) and the background current as well (Supporting Information, Figure S2), indicating that water alone does not render quinoxaline electrochemically active.
To better understand the role of water in the non-aqueous electrochemistry of quinoxaline, we examined the effects of intentional water contamination on quinoxaline voltammetry. Adding water to the dried quinoxaline / LiBF 4 solution resulted in significant enhancement of the reduction current (Supporting Information, Figure S8a) and lead to a similar voltammetric fingerprint as described earlier by Brushett and co-workers 8 and is supported here by our computational results. Adding water to the quinoxaline / Li triflate solution ( Figure S8b) did not show a comparable effect on the quinoxaline voltammetry. Specifically, the magnitude of the reduction current and the shape of the voltammogram were not profoundly changed following water addition.
Our observation that trace water promotes the electrochemical properties of quinoxaline in some electrolyte but not others suggests that water itself does not directly electrochemically activate quinoxaline. Instead, the electrochemical behavior under study depends on a reaction between water and components of the LiBF 4 / PC electrolyte solution. Contamination by water is a well-known cause of degradation of LiBF 4 / PC electrolyte. 21 Furthermore, the aging of asprepared solutions of quinoxaline in LiBF 4 / PC, which contained 150 ppm of water, lead to increased reduction current as well, presumably due to the gradual degradation of the electrolyte following exposure to the water present in the quinoxaline stock ( Figure S9). These results justify our application of computational studies to predict the interaction of quinoxaline with various LiBF 4 degradation products, and the comparison of the calculated reduction potentials to the electrochemical data.

Computational Analysis of Quinoxaline-Electrolyte Interactions
Quantum chemical calculations are employed to better understand the quinoxaline-electrolyte interactions, particularly the role of BF 3 in the observed electrochemical behavior. Being a Lewis acid, BF 3 form adducts with basic quinoxalines which will, in turn, exhibit different electrochemical properties. A number of such scenarios are modeled for quinoxaline and 2,3,6-TMQ and the computed reduction potentials are compared to experimentally measured values.
Ion-pairing effects on the electrochemical mechanism and reduction potentials of carbonylcontaining molecules have been examined in non-aqueous media by others 25 . The formation of an ion-pair with the cation from the electrolyte stabilizes the electrochemically reduced anion, therefore shifting the electroreduction event towards positive potentials. We found that the calculated reduction potential of bare quinoxaline can be compared to previous results from Ames et al. in TBAPF 6 in acetonitrile. 9 In accordance with expectations from theory and the aforementioned previous studies, we observed that the electrochemical reduction potential of quinoxaline in LiBF 4 and LiPF 6 electrolyte was positively shifted compared to previous measurements. We found that quinoxaline did not appear electrochemically-active in TBABF 4 or NaBF 4 in propylene carbonate solution (Supporting Information, Figure S10).
In terms of the BF 3 binding with quinoxalines, from Table 1 Figure S11). The binding of two BF 3 to the same nitrogen atom (entry 5), a model which we note has unrealistic bonding, is energetically less favorable compared to two nitrogen atoms on opposite sides of the pyrazine hetereocycle in the quinoxaline. Based on the calculations, Lewis acids such as BF 3 bind more strongly with the quinoxaline than 2,3,6-TMQ due to the steric interaction from the methyl groups at positions 2 and 3 of the latter species. This is reflected in the complexation enthalpy of BF 3 with quinoxalines and in the N-B bond length in the quinoxaline-BF 3 complex. In their optimized geometries, shown in Figure 5, the N-B bond lengths are 1.72 Å and 1.75 Å for quinoxaline and 2,3,6-TMQ based complexes, respectively. This is also consistent with a relatively stronger coordination of quinoxaline with the BF 3 than the TMQ. Calculations presented in Table 1 suggest that the binding of BF 3 with quinoxaline and TMQ is different and the extent of binding may affect the redox properties of the molecule. To understand the ability of other quinoxaline derivatives to become active in the presence of BF 3 , we have computed enthalpies and free energies of complexation of BF 3 to seven selected quinoxaline derivatives, which are shown in molecules. The enthalpy of complexation of these molecules (entries 4 to 11) with BF 3 is not as strong as either quinoxaline or 2,3,6-TMQ indicating that the effect of salt or salt decomposition products in enhancing the electrochemical properties are minimal for these molecules.

Validation of Quinoxaline-2BF 3 Complex Activity
Having predicted the formation of an electro-active quinoxaline-BF 3 adduct through electrochemical experiments and quantum calculations, we sought to further verify the presence of this species by directly synthesizing and testing a quinoxaline (bis)trifluoroborane ((2BF 3 )Q) complex. The complex was synthesized as described above, and evaluated in Li triflate / PC electrolyte, in which quinoxalines were previously shown to be inactive (see Figure 1). Figure 6 shows the cyclic voltammograms for 5 mM (2BF 3 Figure 4). The presence of more than 2 reduction waves may indicate the presence of multiple electro-active species.  Figure 6 which also showed significant enhancement in the amount of electrochemically-active species. The charging and discharging capacities increased by about 10% of the maximal theoretical SOC over the course of cycling for both the quinoxaline and (2BF 3 )Q species. Interestingly, while the SOC for the quinoxaline-containing solution never exceeded 10% of the theoretical maximum SOC for an n = 2 ereduction, the SOC obtained for the (2BF 3 )Q-containing solution was approximately 50% of the theoretical maximum SOC for an n = 2 e -, or alternatively 100% SOC for an n = 1 e-reduction. Unexpectedly, coulombic efficiencies in excess of 100% were observed for both molecules and the discharge capacity also increased with time. Preliminary NMR analysis of the cycled quinoxaline solutions was inconclusive due to the presence of a large number of new peaks. This indicates that bulk electrolysis cycling generates electrochemically active side-products. In addition, we observed a gradual and irreversible change in solution color, from clear to dark blue for quinoxaline and clear to dark red for (2BF 3 )Q over the course of cycling, also pointing to structural evolution of quinoxaline. Therefore, further exploration of quinoxaline stability and the charge mechanisms are imperative to determining the fitness of quinoxalines as charge-storage materials in nonaqueous organic RFBs but are beyond the scope of the current paper. We aim to address these topics more specifically in subsequent studies.

Conclusions
We have investigated the impact of electrolyte composition on quinoxalines in PC solvent using electrochemical methods and quantum chemical simulations. Lewis acid salt decomposition products (i.e., BF 3 from LiBF 4 ) are found to bind strongly with the nitrogen

RSC Advances Accepted Manuscript
atoms of quinoxaline and increases electron affinity and thus raises the reduction potential. The observed activity in LiBF 4 (or LiPF 6 )-based electrolytes, between 2.4 -3.2 V vs. Li/Li + , is not due to quinoxaline alone but rather an electro-active quinoxaline-BF 3 complex. This is approximately 1 -1.5 V higher than previous observations of quinoxaline electrochemical behavior in non-aqueous media. The salt effect is further validated by synthesizing and testing a quinoxaline-2BF 3 complex, an optimal configuration according to quantum chemical calculations. As compared to quinoxaline in LiBF 4 -based electrolyte, synthesized (2BF 3 )Q demonstrates up to a 26-fold increase in charging capacity, using an electrolyte in which bare quinoxaline is inactive.
These results advance our understanding of the impact of electrolyte decomposition products on the electrochemical behavior of quinoxaline in propylene carbonate. The insight regarding electrochemical activation via BF 3 adduct-forming may lead to new classes of redox-active materials for non-aqueous flow battery applications. In continuing studies, we will employ more advanced electrochemical methods, including in situ spectroscopy, to focus on the structural evolution of quinoxaline during and after electrochemical reduction, as well as the precise role of the solvent in directing electrochemical properties.