Toward an Inexpensive Aqueous PolysulfideâPolyiodide Redox Flow

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Towards an inexpensive aqueous polysulfide-polyiodide redox flow battery Liang Su, Andres Badel, Changsu Cao, Jesse J Hinricher, and Fikile R. Brushett Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01476 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 8, 2017

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Industrial & Engineering Chemistry Research

Towards an inexpensive aqueous polysulfide-polyiodide redox flow battery Liang Sua,b, Andres F. Badelb, Changsu Caob,†, Jesse J. Hinricherb, Fikile R. Brushetta,b,* a b † *

Joint Center for Energy Storage Research, Argonne National Laboratory, Argonne, Illinois 60439, USA Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Current address: Department of Chemistry, Nankai University, Tianjin, China Corresponding author: [email protected]

Abstract Redox flow batteries (RFBs) hold promise for large-scale energy storage to facilitate the penetration of intermittent renewable resources and enhance the efficiency of non-renewable energy processes in the evolving electric power system. While all vanadium RFBs (VRFBs) represent the current state-of-the-art, their system price is near four-fold higher than the price targets outlined by the U.S. Department of Energy motivating research into cost reduction through increased energy density or reduced materials cost contributions. Motivated by the abundance and low cost of sulfur and iodine, herein, we explore the feasibility of an aqueous flow battery system using a polysulfide negative electrolyte and polyiodide positive electrolyte.

The

polysulfide-polyiodide flow battery (SIFB) has an open circuit voltage of 1 V and uses Na+ as the working ion to balance the charge on each electrode. Both positive and negative electrolytes display high coulombic efficiency close to 100%. Without significant system optimization, the SIFB gives a peak power of 65 mW/cm2 and stable cycling performance for more than 200 cycles (ca. 530 h) with a stabilized energy efficiency of ca. 50%. Techno-economic analysis show that the proposed SIFB has the potential to achieve lower prices than the VRFB and to meet the $100/kWh target but further research is needed with a particular focus on reducing the power cost contributions and developing ion-exchange membranes with improved conductivity and species selectivity.

Keywords: Polysulfide, polyiodide, redox flow battery, electrochemistry, techno-economic analysis

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1. Introduction Electrochemical energy storage is emerging as a critical technology to enable sustainable electricity generation by alleviating intermittency from renewable sources, reducing transmission congestion, enhancing grid resiliency, and decoupling generation from demand.1–3 As of 2010, pumped hydroelectric storage (PHS) accounts for ca. 99% of the overall worldwide storage capacity due in part to its high technology readiness level and cost competitiveness.4,5 However, geographic and environmental constraints limit the applicability of PHS to emerging needs and necessitate the development of alternative storage technologies. Redox flow batteries (RFBs) are rechargeable electrochemical devices that hold promise for storing and discharging megawatt hours (MWh) of electrical energy on the multi-hour (h) time scale and at the low system prices needed for economic viability. In a typical RFB, charge-storing active species are dissolved in liquid electrolytes which are stored in tanks. The electrolytes are pumped through an electrolytic stack, where the active species undergo a reduction or oxidation reaction on the porous electrode surfaces to charge or discharge the battery. Often referred to as the positive and negative electrolyte based on their respective redox potentials, the two electrolytes are separated within the stack by a selective membrane which enables ion transfer to balance charge but blocks transport of other components like the active species and solvents. Compared to enclosed batteries, flow batteries have several key advantages that are particularly relevant to grid storage including decoupled power and energy scaling, long operational lifetimes, easy maintenance, simplified manufacturing, high active-toinactive materials ratio (particularly at long storage durations), and improved safety characteristics.3,6,7 While numerous flow battery chemistries have been proposed, only a few technologies have been demonstrated at scale and none have experienced widespread commercial success due to technical and economic challenges.8 In recent years, the US Department of Energy (DOE) has established strict target prices for energy storage systems that reflect the low cost of electricity in the country. Notably, the Office of Electricity Delivery and Energy Reliability is targeting $150/kWh for a fully integrated 4 h discharge energy storage system, including installation and power conditioning equipment.9 The Advanced Research Projects Agency – Energy (ARPA-E) suggested a long-term energy cost of $100/kWh.10 However, current price estimates are significantly higher than these targets. For example, all-vanadium redox flow battery (VRFB) systems, arguably the most advanced 2 ACS Paragon Plus Environment

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flow battery technology, are on the order of $400-500/kWh.11–13 Despite these existing high prices, recent work has established that pathways exist for flow batteries to meet (and even surpass) this target either through increased energy density or reduced materials cost contributions.7,14 In the context of aqueous redox flow batteries, both organic15,16 and inorganic17,18 active compounds with highly abundant elements have shown their potential to meet the cost target. Note that this low system price must be achieved without compromising safety, scalability, or lifetime.

Pursuant to this goal, we aim to identify new or previously overlooked

inexpensive active materials with high abundance and low toxicity which, in turn, may enable energy storage at prices well below the trajectory of current leading technologies. Here, we evaluate the feasibility of a low cost aqueous flow battery based on a polysulfide negative electrolyte and polyiodide positive electrolyte. Widely available as an undesired by-product of natural gas and petroleum refining, sulfur holds promise as a charge storage material. High temperature molten sodium-sulfur batteries have long been considered for stationary applications19 and, more recently, nonaqueous lithium-sulfur batteries have been investigated as high energy density replacements for lithium-ion batteries in mobile applications.20 Sulfur also holds promise for flow batteries as polysulfides (Sn2-) can be dissolved in aqueous and nonaqueous at high concentrations. Indeed, polysulfide solubility is a confounding factor in nonaqueous lithium-sulfur batteries as the irreversible loss of active sulfur leads to capacity fade. Since their inception in the 1980s,21 polysulfide-based flow batteries have been sporadically investigated, typically in conjunction with a bromine/bromide couple as the positive electrolyte.22–24

Notably, research and development efforts on

polysulfide-bromine RFB culminated in the construction of the 12 MW / 100 MWh Little Barford Regenesys plant in the United Kingdom, that was ultimately never realized.25 While attractive from a performance and cost perspective, there are significant safety concerns associated with the bromine/bromide couple including high toxicity (0.1 ppm OSHA Permissible Exposure Limit26) and high vapor pressure (28.8 kPa at 25 °C

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).

The iodine/iodide couple presents a promising alternative, albeit at a lower redox potential, due to its improved safety characteristics, high aqueous solubility, and potential for low cost via high crustal abundance. Indeed, polyiodide-based electrolytes have been previously proposed in hybrid flow batteries coupled with zinc17 or


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protected Li negative electrodes.28 Together, these redox couples result in a polysulfide and polyiodide flow battery (SIFB) with an equilibrium cell voltage of ca. 1.0 V.29,30

Positive:

‫ܫ‬ଷି + 2݁ ି ↔ 3‫ି ܫ‬

E0 = 0.54 V vs. SHE

Negative:

ܵସଶି + 2݁ ି ↔ 2ܵଶଶି

E0 = -0.45 V vs. SHE

Overall:

3‫ ି ܫ‬+ ܵସଶି ↔ ‫ܫ‬ଷି + 2ܵଶଶି

Ecell = 0.99 V

In a recent report, Li et al. proposed a similar flow battery concept based on potassium polyiodide and potassium polysulfide and showed that the chemical costs of the polysulfide / polyiodide electrolytes are lower than that of vanadium electrolytes due to a combination of lower materials costs and higher active species solubility.31 However, the proof-of-concept cell performance is at least an order of magnitude lower than stateof-the-art all-vanadium cells.32 While continued improvements are expected through advances in component and cell design, acidic chemistries tend to have lower resistance than near neutral or alkaline chemistries due to the high mobility of protons through liquid electrolytes and solid Nafion®.29,33 Consequently, at a system-level, the chemical cost savings may be offset by higher reactor costs which, in turn, impacts the economic viability of a proposed technology. Thus, we investigate a similar flow battery chemistry, based on sodium polysulfide and sodium polyiodide, with a particular focus on connecting measured electrolyte and cell performance characteristics to system-level techno-economic assessment. In addition to quantifying the trade-offs between chemical and reactor costs, we highlight key challenges with this particular system that must be overcome to realize the promise of inexpensive energy storage.

2. Experimental 2.1. Chemicals Sodium sulfide (Na2S, anhydrous, > 95%), sulfur (S, 99.999%), sodium hydroxide (NaOH, 99.99%), sodium iodide (NaI, 99.9%), and iodine (I2, 99.8%) were purchased from Alfa Aesar. Sodium sulfate (Na2SO4, anhydrous, ≥ 99.0%) was purchased from Sigma-Aldrich. All aforementioned chemicals were used as received. 4 ACS Paragon Plus Environment

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Sodium triiodide (NaI3) electrolytes were prepared by mixing stoichiometric ratios of NaI and I2 at room temperature. Sodium polysulfide (Na2Sx) electrolytes were prepared by mixing stoichiometric ratios of Na2S and S at room temperature. All electrolytes were prepared using Millipore deionized water (18.2 MΩ·cm).

2.2. Flow cell set up The flow cell used in this study is similar to previous literature reports with interdigitated flow fields (IDFFs) and a geometric active area of 2.55 cm2.34,35 The backing plates were machined from polypropylene and the flow fields were machined from 3.18 mm thick impregnated graphite (G347B graphite, MWI, Inc.) Electrodes were cut (16.1 mm × 14.1 mm) from 280 ± 30 µm thick carbon paper (34AA, SGL Group). To improve the hydrophilicity of the as-received carbon paper, electrodes were first sonicated in isopropyl alcohol (SigmaAldrich) and then heated to ca. 45°C in a solution of concentrated sulfuric and nitric acid (H2SO4 : HNO3 = 3 : 1 by volume, Sigma-Aldrich). The electrodes were then soaked overnight in 1 M NaOH to neutralize the surface acidity. To increase electrochemically-active surface area, Vulcan XC72 carbon (VXC72, 250 m2/g surface area, Cabot Corporation) was added to both the positive and negative electrodes. Cobalt (IV) sulfide particles (CoS2, 200 mesh, 99.5%, Alfa Aesar) were used as an electrocatalyst on the negative electrode.

These

pretreatments did not change the electrode thickness. The modified carbon paper electrodes were layered two pieces thick and compressed by ca. 40% in the assembled flow cell, yielding a final total compressed electrode thickness of ca. 330 µm. Nafion® membrane (Ion Power Inc.) was used as a Na+-conductive membrane. To convert Nafion® membrane from a protonated form to a sodiated form, the as-received membrane was consecutively boiled in 3% hydrogen peroxide (Sigma-Aldrich), 0.25 M sulfuric acid, and 0.25 M NaOH for 1 h each. The treated membrane was dried at 120 °C under vacuum for 3 days and re-hydrated overnight before cell assembly.

These pretreatments did not change the membrane thickness.

Silicone rubber gaskets

(McMaster) sealed the separator and electrodes inside the cell. Capped glass scintillation vials (Wheaton) served as electrolyte reservoirs with holes drilled into the plastic cap to allow tubing in and out. A peristaltic pump (Masterflex L/S Series, Cole-Parmer) was used to drive electrolyte through the flow cell and reservoirs.


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Norprene tubing (1.6 mm inner diameter, Cole-Parmer) was used to circulate the electrolyte through the system. All flow cell materials were selected in part due to their compatibility with the electrolytes used in this study.

2.3. Flow cell testing All flow cell testing was performed using a Bio-Logic VSP300 potentiostat at room temperature (ca. 23 °C). The electrolyte flow rate was fixed at 10 mL/min throughout all experiments.

In the symmetric cell

configuration to study the stability of a single electrolyte across a range of SOCs, sodiated Nafion® 117 (NaN117, 178 µm nominal thickness) was used as the membrane. In the case of the iodine-iodine symmetric cell, the VXC72 loading of the iodine electrode on both sides of the cell was 0.3 mg/cm2. The working electrolyte was 5 mL of 1.5 M NaI + 0.5 M Na2SO4 with a nominal capacity of 134 mAh (at 0% SOC). The counter electrolyte was 15 mL of 0.75 M NaI + 0.25 M NaI3 + 0.5 M Na2SO4 with a nominal capacity of 402 mAh (at 50% SOC). The working electrolyte was first charged (NaI to NaI3) at 20 mA/cm2 to 75% SOC (capacity cutoff), and then discharged (NaI3 to NaI) at 20 mA/cm2 to -0.5 V (voltage cut-off). In the case of the sulfur-sulfur cell, the VXC72 and CoS2 loadings of the sulfur electrode on both sides of the cell were both 0.2 mg/cm2 (0.4 mg/cm2 total). The working electrolyte was 5 mL of 1 M Na2S2 + 1 M NaOH with a nominal capacity of 134 mAh (at 0% SOC). The counter electrolyte was 15 mL of 0.5 M Na2S2 + 0.25 M Na2S4 + 1 M NaOH with a nominal capacity of 402 mAh (at 50% SOC). The working electrolyte was first charged (Na2S2 to Na2S4) at 20 mA/cm2 to 75% SOC (capacity cut-off), and then discharged (Na2S4 to Na2S2) at 20 mA/cm2 to 0% SOC (capacity cut-off). The cell ohmic resistance was measured by impedance before galvanostatic cycling. In the full cell configuration to study the polarization behavior, sodiated Nafion® 212 (Na-N212, 51 µm nominal thickness) was used as the membrane. The VXC72 loading of the iodine electrode was 0.3 mg/cm2; the VXC72 and CoS2 loadings of the sulfur electrode were both 0.2 mg/cm2 (0.4 mg/cm2 total). The positive electrolyte was comprised of 5 mL 0.75 M NaI + 0.25 M NaI3 + 0.5 M Na2SO4; the negative electrolyte was comprised of 5 mL 0.5 M Na2S2 + 0.25 M Na2S4 + 1 M NaOH. These electrolytes represent a 1 M electron equivalent concentration with a total theoretical charge storage capacity of 268 mAh and a 50% state of charge (SOC). Galvanostatic polarization was performed to determine power performance and the polarization behavior for 6 ACS Paragon Plus Environment

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both charging and discharge processes of the cell. The cell voltage stabilized in ca. 10 seconds (s) and the discharge current was held for 30 s as to minimize any shift in cell SOC over the course of the experiment. In the full cell configuration to study the long-term cycling performance, Na-N117 was used as the membrane. The VXC72 loading of the iodine electrode was 0.7 mg/cm2; the VXC72 and CoS2 loadings of the sulfur electrode were both 0.5 mg/cm2 (1.0 mg/cm2 total). The positive electrolyte was comprised of 5 mL 1.5 M NaI + 0.5 M Na2SO4; the negative electrolyte was comprised of 5 mL 0.5 M Na2S4 + 1 M NaOH. These electrolytes represent a 0% SOC. The cell was pre-charged to 75% SOC (nominal) at 20 mA/cm2, which required 2.01 h to complete. The long-term galvanostatic cycling experiments were performed at a constant current density of 20 mA/cm2 for 1.34 h in the charge half cycle, which was equivalent to a 50% SOC swing and with a 0.4 V voltage cutoff in the discharge half cycle. The SOC range was selected to demonstrate cycling behavior without approaching performance limitations associated with extreme SOCs (e.g., mass transport limitations in the case of iodine, access of undesirable redox couples in the case of sulfur). The positive and negative electrolytes were refreshed every 50 cycles. Specifically, the cycled electrolytes were replaced with fresh electrolytes at 0% SOC followed by charging the “refreshed” cell to 75% at 20 mA/cm2 and the subsequent long-term cycling. The electrolyte pH before and after cycling was measured using a pH meter (Metrohm).

3. Results and discussion 3.1. Cell performance Cell performance was evaluated using a 2.55 cm2 flow cell that leverages the advanced cell architectures described in the state-of-the-art VRFB literature, namely a zero-gap interdigitated flow field (IDFF) and thin hydrophilic carbon-paper electrodes.36,37 The IDFF required that all electrolyte flows through a short path of porous electrode, enabling high current densities without developing an unacceptably large pressure drop. Thin carbon-paper electrodes offered a balance of high surface area, good mass transport, and low ohmic resistance. The surface area of both electrodes was increased through the addition of Vulcan carbon. Also, as polysulfide redox reactions are known to be slow on carbon surfaces, CoS2 powder was dispersed on the negative electrode to catalyze the Sn2- (n = 2 or 4) redox reactions.21 ACS Paragon Plus Environment

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To better understand the reversibility of the polyiodide and polysulfide redox processes in the flow cell, we first performed symmetric cell cycling using the same electrolyte on either side of the cell with Na-N117 membrane. This approach is similar to traditional bulk electrolysis techniques but allows for a more controlled electrolyte environment by removing the need for a counter electrode of dissimilar material and eliminating the possibility of working electrode contamination by side products generated from reactions at the counter electrode. Further, the flowing electrolyte improved mass transfer enabling higher concentration studies and the porous carbon electrodes were more relevant to flow battery applications than the typical reticulated vitreous carbon meshes.

In this study, it was necessary to ensure that the full working electrolyte capacity was

accessible without having to apply high cell voltages which can lead to data convolution through activation of additional redox events (e.g., higher order sulfides), irreversible side reactions (e.g., water splitting) or reactant electromigration. Thus, we employed a counter electrolyte volume (15 mL) that was three-fold greater than the working electrolyte volume (5 mL). Cells were cycled at 20 mA/cm2 with 75% capacity cut-off based on the working electrolyte volume.

Figure 1. Symmetric cell cycling at 20 mA/cm2 for (a) I-/I3- and (b) S22-/S42-. The theoretical capacity of the working electrolyte for both cases is 134 mAh. The oxidation processes are controlled by capacity cut-off (75%


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SOC).


The reduction processes are controlled by either capacity cut-off (0% SOC) or voltage cut-off,

whichever comes first.

Figure 1a shows a representative charge/discharge curves for the I-/I3- couple which show low overpotentials indicating facile redox kinetics17,28 and near symmetric profiles indicating a chemically reversible process. This reversibility could also be qualitatively observed by the change of electrolyte from colorless (NaI) to dark brown (NaI3) during charge (oxidation reaction), and vice versa during discharge (reduction reaction). At ca. 20 mAh during discharge (red curve), a minor stepwise decrease of the overpotential is observed, which we tentatively attribute to the dissolution of precipitated NaI3 during charge. Indeed, while NaI solubility as high as 7.8 M has been observed in DI water, the NaI3 solubility is significantly lower (< 0.2 M) in DI water, although, our flow cell appears to be able to operate at a slightly higher NaI3 concentration due to improved solubility in the presence of excessive NaI. The coulombic efficiency was 97% with the minor inefficiency attributed to high concentration overpotential at the end of discharge. This is further supported by the observation that, at the end of one cycle at 20 mA/cm2, the electrolyte can be further reduced by using a lower current density (2 mA/cm2, data not shown). Figure 1b shows a representative charge/discharge curves for the S22-/S42- couple which show higher overpotentials, indicating slower redox kinetics, and several plateaus which indicate the presence of multiple redox active species. Such complex electrochemical responses are welldescribed in the nonaqueous lithium-sulfur battery literature and have been linked to shifting solution phase chemical equilibria.38 Nevertheless, a coulombic efficiency of 100% was observed suggesting that despite these complexities, charge can be effectively stored and released in this electrolyte. To investigate performance at near practical concentrations, we selected positive and negative electrolyte compositions of 0.75 M NaI + 0.25 M NaI3 + 0.5 M Na2SO4 and 0.5 M Na2S2 + 0.25 M Na2S4 + 1 M NaOH, respectively. A Na-N212 was used as an ion-selective membrane to promote the Na+ transport and limit the crossover of the negatively charged redox active species (i.e., I-, I3-, S22-, and S42-). Figure 2a shows the discharge polarization and associated power density curves for the flow battery at 50% SOC. The measured cell open circuit voltage (OCV, ca. 1.00 V) was in good agreement with the predicted equilibrium cell voltage (0.99 9 ACS Paragon Plus Environment


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V). A peak power density of 65 mW/cm2 was measured at a current density of 140 mA/cm2 and a cell voltage of ca. 0.47 V. The polarization curve is dominated by kinetic losses in the lower current density region (ca. 0 – 20 mA/cm2) and ohmic losses in the higher current density region (ca. 20 – 140 mA/cm2). The area-specific resistance (ASR) of the flow cell during discharge was 4.0 Ω cm2, which is slightly lower than the prior report,31 likely due to the use of a thinner Nafion® membrane. However, these values are still about an order of magnitude lower than vanadium systems which have been reported32 due to a combination of lower cell open circuit voltage (1.0 V vs. 1.4 V) and reduced membrane and electrolyte conductivity. Doyle et al. reported a near five-fold difference in proton and sodium cation conductivity in hydrated Nafion® 117, 90.2 mS/cm and 18.7 mS/cm respectively.33 For reference, that same paper reported a potassium cation conductivity of 13.8 mS/cm in hydrated Nafion® 117. Figure 2b shows the polarization curves of both cell discharging and charging. The average charging ASR of 4.4 Ω cm2 was about 10% higher than observed on discharge, which we ascribe to asymmetry in the redox process between S22- and S42-, as evinced in Figure 1b.

Figure 2. (a) The discharge polarization of the polysulfide-polyiodide flow cell at 50% SOC. The black and red curves are the cell voltage and power density changes, respectively, with increasing current density. (b) The polarization curves of both charge (open square) and discharge (solid square) of the same flow cell at 50% SOC. The dotted horizontal line is the cell OCV.


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Figure 3 shows the galvanostatic cycling performance of the polyiodide-polysulfide flow battery at 20 mA/cm2 between the 25% and 75% SOC range. Selected charge/discharge voltage profiles of cycle 10, 25, and 40 after refreshing the electrolytes are plotted in Figures 3a-c. The coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) are summarized in Figure 3d. As the cycling proceeded (the black curves in Figures 3a-c), there was evolution of the plateaus, namely increased voltage during charge and decreased voltage during discharge, which, together lowering VE from ca. 55% in the 1st cycle to ca. 50% in the 50th. Alternately, the CE first decreased from ca. 100% to ca. 90%, and then increased to ca. 98% during the rest of the cycling. As a combined effect of VE and CE, the round-trip EE of this proof-of-concept cell stabilizes at ca. 50% after the 3rd cycle. It is worth mentioning that, for the given potential ranges (+0.54 ± 0.3 V vs NHE for iodine at near neutral pH and -0.45 ± 0.5 V vs NHE for sulfur at alkaline pH) and the electrode materials used (carbon, CoS2 on carbon), water splitting (oxygen evolution reaction on the positive electrode and hydrogen evolution reaction on the negative electrode) is not expected to be a concern. In addition, the pH values of positive and negative electrolytes are 6.2 and 13.5, respectively, before cycling and 8.8 and 13.2, respectively, after cycling, indicating a loss of ca. 0.15 M hydroxide ions in the negative electrolyte. The crossed over hydroxide ions might react with the triodide (I3-) to form iodate (IO3-).

The iodate is

electrochemically inert, resulting in the irreversible decay of the positive electrolyte capacity. In addition, the pH values of positive and negative electrolytes are 6.2 and 13.5, respectively, before cycling and 8.8 and 13.2, respectively, after cycling, indicating the minimal crossover of hydroxide ions. Accordingly, the evolution of VE and CE along opposite directions might be reconciled by considering active species crossover. With the membrane in contact with an electrolyte, the hydrated pore size is large enough to transport all species and Donnan exclusion effects were suppressed by high salt concentrations leading to higher crossover rates for the anionic active species.39 During cycling, both anionic species, Ix- and Sx2- species tended to migrate across the membrane from the negative side of the cell to the positive side of the cell based on the electric field. Of particular importance was the transport for Sx2- from the negative electrolyte to the positive electrolyte as this leads to the formation and precipitation of elemental sulfur (S0), via electro-oxidation, on the positive side of the 11 ACS Paragon Plus Environment


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cell, which resulted in the irreversible capacity decay of the cell. In fact, the color of polysulfide electrolyte changed from orange at the start of cycling to near colorless at the end of the cycling (Figure S1) and elemental sulfur was observed on the positive side of the cell (Figure S2), indicating the loss of active polysulfide species in the electrolyte. However, no redox feature of the crossed over sulfur species or iodine species can be identified on the post-cycling cyclic voltammograms. This is to be expected as, in the case of the positive electrolyte, any crossed over sulfur has precipitated within the flow cell and, in the case of the negative electrolyte, polyiodide species are not electrochemically active in alkaline electrolyte. Detailed mechanistic studies for the species crossover and capacity decay are under way. From a practical operation point of view, the irreversible species crossover that leads to electrolyte degradation necessitates periodic maintenance to refresh the system.40 This may be tolerable especially for a low cost active electrolyte(s) (such as the case in this study, which will be discussed in the next section) with a recoverable reactor performance. To this end, we further investigated the cell cycling behavior by refreshing both positive and negative electrolytes. As shown in Figures 3a-c, without any maintenance effort on the reactor (cell), the charge/discharge curves of refreshed electrolytes from selected cycles are all similar to the ones from the “new cell” (black curves). In fact, according to the “reproducible” efficiency numbers in Figure 3d, there is no sign of irreversible decay of the cell from the component level for over 200 cycles (530 h). Therefore, the proposed SIFB shows promise to achieve the long cycle life (> 5,000 cycles) and calendar life (> 10 years) required for economical grid storage.10 To fulfill this promise, future research effort should be focusing on preventing the decay and/or the crossover of active species, especially the polysulfide electrolytes, and on developing approaches to recycle or regenerate the spent electrolytes


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Figure 3. The galvanostatic cycling of the polysulfide-polyiodide flow cell at 20 mA/cm2 with the SOC range between 25% and 75%. (a), (b), and (c) are the charge/discharge curves of the 10th, 25th, and 40th cycle after each electrolyte refresh. The time span for each 50 cycles is ca. 132.5 h. The corresponding round-trip coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) for all cycles are summarized in (d).

3.2. Techno-economic analysis To estimate system price, we adopted the techno-economic framework developed by Darling et al.7 combined with selected cell performance metrics described in the previous section. The energy cost includes the chemical ACS Paragon Plus Environment

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cost (NaI, Na2SO4, Na2S4, NaOH), tank cost, discharge VE, round-trip CE, and SOC. The reactor cost includes the ASR, component cost factors, OCV, and discharge VE. In addition to the energy and power costs, the system price includes the balance-of-plant and additional costs. The balance-of-plant cost includes heating and cooling equipment, state-of-charge management, power electronics, and pumps, and the additional cost includes manufacturing cost, administration cost, R&D cost, and profit margin.7 Detailed calculations for the technoeconomic analysis in Figure 4 are provided in the supporting information. We note the model does not account for periodic electrolyte replacement as we assume the crossover issues can be mitigated. If, however, this is not the case, then electrolyte replenishment and regeneration will need to be considered as a cost contributor. The polyiodide redox couple represents both the largest chemical cost contributor (NaI) and the lowest solubility active compound (NaI3) in the proposed flow battery system. While the theoretical solubility limit of NaI is 7.8 M (in DI water), our experiments were conducted with 1.5 M NaI (in the presence of 0.5 M Na2SO4) with a SOC swing up to 50%. Thus, we first studied the impact of NaI concentration and SOC on the overall energy cost (including chemical cost and tank cost) of SIFB. As shown in Figure 4a, NaI has a higher impact on the energy cost of the system at lower concentration and lower SOC. For example, at 50% SOC, energy cost is only sensitive to NaI concentration below 1 M; at a given NaI concentration, the difference of energy cost between 25% and 50% SOC swing is much larger than that between 50% and 75% SOC swing. Low NaI concentration and low SOC result in a quickly decreased energy density, and consequently, sharply increased chemical cost (e.g., more electrolyte and larger tank). Increasing the sulfur concentration from 2 M, as demonstrated in the flow cell cycling, to 6 M, bounded by the solubility of Na2S2 at 25 °C, decreases the energy cost, the extent of which is also more dramatic at lower SOC. Increasing the NaI concentration and SOC of SIFB is achievable by, for example, increasing the operating temperature of the cell. Even at the demonstrated concentrations and SOC (yellow star in Figure 4a) along with their present cost, the overall energy cost of SIFB is lower than not only the present but also the future energy cost of VRFB. Figure 4b compares the system price (the summation of energy cost, power cost, balance-of-plant cost, and additional cost) of VRFB and SIFB. The upper and lower boundaries of both RFB systems reflect the present and future scenarios, respectively. At present, the high price of the SIFB results primarily from an ASR as high 14 ACS Paragon Plus Environment

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as 4 Ω cm2 (Figure 2a) in this proof-of-concept flow cell, which is significantly greater than that of state-of-theart VRFBs using Nafion® 212. At this point, charge transfer resistance on the negative electrode represents the main source of resistance in the cell (ca. 80% of total cell ASR). However, the contribution can be significantly reduced via more effective deployment of the catalyst(s) on the negative (sulfur) electrode as prior art indicates that resistances as low as 0.2 Ω cm2 are achievable.23 In addition, lower ohmic resistance (hardware + electrolyte + membrane) is possible through further cell-level optimization. Optimistically, the total cell ASR considering the ohmic resistance from membrane (Na-N212), charge transfer resistance from the redox reactions, and the mass transfer resistance may shrink to as low as 0.74 Ω cm2 (see details in supporting information), which provides a credible pathway to dramatically decrease the power cost to $80/kW. Along with its competitive energy cost, SIFB has the ability to outperform VRFB in terms of future cost at all energy / power (E/P) ranges, and shows the potential to meet the $100/kWh cost target with E/P between 6 and 10 h (purple band in Figure 4b), which is the duration of particular interest for a RFB system. Within this E/P range, Figure 4c and Figure 4d exhibit the sensitivity analysis of energy and power cost, respectively, on the overall system price. For all discharge durations, the decrease of energy cost or power cost leads to the decrease of the system price. Further, the impact of power cost on the system price reduction is more dramatic than that of energy cost, which signifies that future research effort should be focused on decreasing the power cost, primarily from better membranes and catalyst on sulfur side reactions.


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Figure 4. Techno-economic analysis of the polysulfide-polyiodide flow battery (SIFB). (a) The change of energy cost as a function of NaI concentration at different sulfur concentrations and cell SOC. The solid lines assume a total sulfur concentration of 2 M as demonstrated in this work. The dotted lines assume a total sulfur concentration of 6 M, which is consistent with the solubility of Na2S2 (3 M) at 25 °C. The NaI cost is fixed at $1/kg that is the best available present cost. The yellow star represents the experimentally determined condition as demonstrated in this work. For comparison, the present and future costs of vanadium, taken from Ref. 7 are labeled on the plot. (b) The system price of the SIFB (green) and the VRFB (orange) as a function of discharge duration (E/P). The upper and lower bounds in each case reflect the present and future states of the price. The purple band (6 – 10 h) is the typical discharge time for a redox flow battery. (c) Sensitivity analysis of energy ACS Paragon Plus Environment

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cost on the system price for different discharge durations (6 - 10 h). The right dotted line is the present energy cost and the left dotted line is the future energy cost. All other parameters for the calculation of the system price are assumed at the present level. (d) Sensitivity analysis of power cost on the system price for different discharge durations (6 - 10 h). The right dotted line is the present power cost and the left dotted line is the future power cost. All other parameters for the calculation of the system price are assumed at the present state.

4. Conclusions and perspective Herein, we explore the feasibility of a sodium polysulfide-polyiodide redox flow battery for achieving the low system prices necessary for grid energy storage. To this end, we performed cell level studies on a proof-ofconcept prototype to investigate individual electrolyte and full cell performance characteristics. The symmetric cell experiments informed good redox reversibility of both the sulfide- and iodide-based electrolytes at 75% SOC. The discharge peak power of polysulfide-polyiodide with nominal 1 M [e-] and at 50% SOC was determined to be 65 mW/cm2 with a total cell ASR of 4.0 Ω cm2 during discharge. The galvanostatic cycling of flow cell was demonstrated for 200 cycles at 20 mA/cm2 with a duration of ~ 530 h. The crossover of the polysulfide species was identified as the primary factor that results in both the inefficiency and the capacity decay. The results from these cell studies were used to inform a system-level techno-economic model, which seeks to predict the present and future state system prices for this conceptual battery and to compare the results to VRFBs and established targets. Techno-economic analysis revealed that energy cost is more sensitive to the actives concentration at low cell SOC and the system price of the SIFB is more sensitive to the power cost than to the energy cost. Furthermore, if reductions in power costs, via improved reactor performance, and energy costs, via increased actives concentration, can lead to a battery that meets the $100/kWh price target with a discharge duration between 6 and 10 h. Realizing the promise of this flow battery system will require overcoming a number of technical challenges. First, highly conductive and selective cation exchange membranes are necessary to achieve the low ASR (high power) and low active species crossover (high capacity retention) of the battery. Membrane selectivity remains a challenge for all asymmetric RFBs, which employ two different species for charge storage, which advantages 17 ACS Paragon Plus Environment


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single component systems, such as the VRFB, where species crossover reduces CE but does not lead to irreversible capacity fade. Second, despite the high solubility of NaI, the lower solubility of NaI3 limits the achievable energy density (and thus energy costs). Increases in NaI3 solubility may be achieved through increasing temperature or incorporating solubilized agents (e.g., ethanol 17). Third, this study only investigates the redox reactions between S22- and S42-, which accounts for only 25% of the theoretical capacity of 1675 mAh/g-S. Enabling deeper reductions to S2- at practical concentrations, without sacrificing electrochemical reversibility would significantly increase the SIFB energy density.

5. Acknowledgements This project was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. A.F.B gratefully acknowledges the SuperUROP program at MIT for financial support. We also thank Sam Pan and Prof. Yet-Ming Chiang for stimulating discussions. Finally, we appreciate the valuable feedback from members of the Brushett Group at the Massachusetts Institute of Technology throughout the project.

Supporting information includes the details of experimental description and the techno-economic analysis.

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Table of Content Figure (1.50 inch high, 3.33 inch wide)


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Toward an Inexpensive Aqueous PolysulfideâPolyiodide Redox Flow

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