Jarrod David Milshtein’s research while affiliated with Massachusetts Institute of Technology and other places

With commercial development of redox flow batteries (RFBs) underway and given the importance of capital cost, we analyze RFB supporting electrolyte charge carrier, membrane, open-circuit voltage (OCV), cell performance, and active species size as possible major cost drivers. Physics-based and techno-economic modeling includes a porous electrode model and accounts for chemical and material selection and amounts, stack and tank sizing, and flow-dependent pumping power. ¹ The 1D porous electrode model, validated in a prior study, ² couples mass transfer, charge transfer, and ohmic transport to estimate the electrode contribution to full cell area specific resistance (ASR). Primary findings for a 5-h discharge of an aqueous RFB are that present costs are in general $10 ~ 100/kWh lower with 1.4 V versus 1.0 V OCV, size selective separators (SSS) versus ion exchange membranes (IEM), as shown in Figure 1a. Large active species size alone would increase costs slightly due to higher viscosity and thus higher pumping cost, reduced mass transfer, and higher mass-based electrolyte cost versus lower molecular weight actives. However, large actives enable SSS use, and the slight incremental cost is more than offset by cost reductions from the high conductivity and inherent low cost of SSS membranes. In the future, the costs of different scenarios converge assuming significant cost reductions in the stack, membranes, and unit cost less materials. In Figure 1b, multiple options, led by H ⁺ /IEM with small active species and OCV > 1.3 V, can meet a Joint Center for Energy Storage Research target adjusted to$100/kWh for 5-h storage. Implications of the work here are: 1) recommendations on the path to enable low cost aqueous RFBs, and 2) a modeling tool and opportunity to quantify cost implications of additional factors held constant in the present study. One recommendation is the further exploration of IEMs using H ⁺ or Na ⁺ and SSS, as these membranes have relatively favorable likelihoods to enable system cost targets. We also suggest development work towards redox couples and conditions for high OCV, which enhances electrolyte energy content and stack power. Subsequently, engineering effort and adoption of RFBs is needed to enable significant reduction in cost through high volume manufacturing of stacks, membranes, and overall RFB systems. Lastly, one can use the present model to explore the effect of various additional parameters on cost. Such parameters include: the cell components (e.g., electrode properties, membrane thickness) affecting ASR, the operation (e.g., flow rate, V-I operating point) affecting ASR, and the application (e.g., discharge duration) affecting power-related cost (e.g., stack) versus energy-related cost (e.g., electrolyte volume, tank size). Acknowledgments This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. JDM acknowledges additional support from the NSF Graduate Research Fellowship. References: J. D. Milshtein, R. M. Darling, J. Drake, M. L. Perry, and F. R. Brushett, J. Electrochem. Soc. , 164 , A3883 (2017). J. D. Milshtein, K. M. Tenny, J. L. Barton, J. Drake, R. M. Darling, and F. R. Brushett, J. Electrochem. Soc. , 164 , E3265 (2017). Figure 1

Redox flow batteries (RFBs) are promising candidates for grid storage, but current systems have not met the stringent cost and/or safety requirements needed for widespread implementation. Replacing vanadium with organic compounds may lower materials cost, and utilizing non-aqueous (aprotic) electrolyte solvents, in place of water, could enable a 2- to 3-fold increase in operating voltage. Both features make non-aqueous RFBs candidates for large-scale stationary storage. A limited number of organic compounds have been reported as stable electron donors and acceptors, with even fewer materials being studied as small molecule two-electron donors and/or two-electron acceptors. Our recent efforts have focused on the development of highly soluble electron donors and acceptors with stable oxidized and reduced states. This presentation will focus on design strategies utilized to increase molecular stability in all relevant states of charge as well as solubility. In particular, we highlight the design, synthesis, and electrochemical analysis of organic redox couples. Results will be presented on the cycling of phenothiazine and naphthoquinone derivatives in flowing full cell battery prototypes.

Redox flow batteries (RFBs) are an attractive technology for grid-scale energy storage due to independent scaling of battery power and energy ratings. These devices utilize a liquid-phase electrolyte, with dissolved active species, which is pumped through a flow-through porous electrode, where the desired electrochemical reactions take place. Despite the critical role of mass transfer on flow cell performance, mass transfer rates within the porous electrodes of RFBs are rarely quantified. This modeling and experimental study quantifies and explains mass transfer rates in RFBs as a function of active species concentration, flow rate, and flow field design in a systematic fashion. Based on porous electrode theory, ¹ we develop a steady-state, one-dimensional model to describe electrode polarization, considering losses due to the electrolyte resistivity, charge transfer, and convective mass transfer. Combining the Butler-Volmer kinetic equation, linear mass transfer relationships, Ohm’s Law and several assumptions for a model redox cell, a second-order, dimensionless, and ordinary differential equation emerges for the potential as a function of position and two dimensionless parameters. Solved numerically, the model produces a series of dimensionless plots that illustrate how the RFB electrode behaves across a range of exchange and limiting current values. For each steady-state cell polarization (I-V characteristic), the model reveals an explanatory spatial variation in overpotential and current distribution across the electrode. The dimensionless nature and reduction to two parameters enable facile curve fitting of the model to experimental polarization curves, such as shown in Figure 1. In conjunction with the model, we implement a single electrolyte diagnostic flow cell technique, ² with an iron chloride electrolyte, to probe the polarization performance as a function of the aforementioned experimental parameters. Quantitative mass transfer coefficients ( k m ) are extracted by fitting the two model parameters to experimental polarization curves, and the only additional experimental data required is the electrolyte conductivity. In this work, power-law proportionalities between mass transfer coefficient and electrolyte velocity are revealed for 4 flow field types: flow through (FTFF), interdigitated (IDFF), parallel (PFF), and serpentine (SFF). Quantifying mass transfer rates for 4 common RFB flow fields offers mechanistic insight into transport phenomena and provides tangible parameters for future engineering optimization. In terms of mechanistic understanding, the FTFF measurements indicate that traditional mass transfer coefficient correlations for packed powder beds are shifted relative to porous carbon paper electrodes used here. The small k m values associated with the PFF and weak flow rate dependence confirms the findings of prior studies that the PFF does not promote forced convection in the porous electrode and is thus unsuitable for implementation in RFBs. ² Additionally, the surprisingly high mass transfer rates associated with the SFF and the intermediate velocity dependence of the IDFF raise interesting questions as to the role of mixed transport in flow field designs. The mass transfer coefficient data and correlations in this work can serve as a basis for more advanced computational studies, for optimizing electrolyte flow rate to balance electrochemical performance and pump work, and for more detailed system-level descriptions of technical performance and cost. Moreover, this combined modeling and experimental approach offers transparency and applicability to a range of porous electrode materials, electrolyte compositions, and flow field geometries for other flow batteries, or other flowable electrochemical systems, with porous electrodes. Acknowledgements J. D. M. and K. M. T. contributed equally to this work. This project was supported by the Joint Center for Energy Storage Research (JCESR), an Energy and Innovation Hub funded by the United States Department of Energy. J. D. M. acknowledges additional funding from the National Science Foundation Graduate Research Fellowship, and K. M. T. recognizes support from the MIT summer research program. References 1. J. Newman and K. E. Thomas-Alyea, Electrochemical Systems , 3rd ed., Chapter 22, John Wiley & Sons, Inc., Hoboken (2004). 2. R. M. Darling and M. L. Perry, J. Electrochem. Soc. , 161, A1381–A1387 (2014). Figure 1

Commercial redox flow batteries (RFBs) contain highly acidic and corrosive electrolytes, a cause for concern with widespread use to due concerns about safety and environmental contamination. Replacing these electrolytes with non-aqueous equivalents allows for a safer storage medium. Additionally, utilization of organic electroactive materials may lead to more scalable technologies that do not rely on mined materials such as vanadium and lithium. Furthermore, non-aqueous electrolytes could enable a 2- to 3-fold increase in operating voltage due to the wider operating voltage of non-aqueous systems. Despite the promise for a safer, scalable, higher energy system, the number of organic compounds reported in non-aqueous flow battery systems has been limited to a few classes of compounds, many of which suffer from instability and/or insolubility, especially in charged states. Our recent efforts have focused on the development of organic electron donors and acceptors with stable oxidized and reduced states. This presentation will focus on design strategies utilized to increase molecular stability and solubility in all relevant states of oxidation while keeping syntheses short, high yielding, and scalable. In particular, we highlight the design, synthesis, and electrochemical analysis of new phenothiazine and napthoquinone derivatives designed to serve as one- or two-electron donors. We show that tactical placement of substituents leads to improved stability and increased solubility. Additionally, a new approach to modification of redox potentials using strategic substituent placement will be presented.

Redox flow batteries (RFBs) are promising electrochemical devices for grid-scale energy storage due to their many favorable inherent attributes, such as decoupled power and energy ratings, as well as excellent performance and capacity retention. Recently, significant improvements in cell performance have made RFB systems more viable than ever. However, to meet the U.S. Department of Energy long range capital-cost target of $150 kWh ⁻¹ , additional cost reductions are needed. One promising route is new active species for RFBs designed for lower cost or improved performance. These next-generation RFB chemistries are likely to be engineered molecules or complexes, such as redox-active organic or organometallic compounds, which offer a multitude of possibilities. A key consideration in the selection of any new RFB chemistry is the supporting electrolyte, which has a significant influence on the membrane resistance, electrolyte conductivity, and electrolyte viscosity. All of these physical parameters have a major impact on RFB performance and cost. Therefore, it is useful to quantify changes in economic viability for various RFB chemistry options with different aqueous supporting electrolytes paired with different types of membranes, which is the focus of this work. A techno-economic model is used to estimate RFB-system costs for the different membrane and supporting electrolyte options considered herein. Variations in cell performance due to the working ion selection and electrolyte viscosity can yield battery cost differences in the$100’s kWh ⁻¹ , and this analysis allows for quantification of cost performance changes by selecting certain electrolyte characteristics. Beyond the conventional RFB design incorporating small active species and an ion-exchange membrane (IEM), this work also considers size-selective separators (SSS) as a cost-effective alternative to IEMs. The SSS concept utilizes nano-porous materials with no functionalization for ion selectivity, and active species that are too large to pass through the pores. Across the entire design space, SSS separators with low viscosity electrolytes offer the lowest RFB costs. Acknowledgements This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the United States Department of Energy. J.D.M. acknowledges additional financial support from the National Science Foundation Graduate Research Fellowship Program.

Due to their inherent scalability and long operational lifetimes, redox flow batteries (RFBs) have shown promise as grid-level energy storage systems that enable the integration of intermittent renewable energy sources and the improvement of non-renewable energy processes on the grid. ¹ However, additional cost reductions are required to reach proposed capital cost targets for widespread commercial implementation. ² While multiple efforts have focused on discovery and development of next-generation materials for RFBs, fewer studies have systematically investigated the impact of the physical properties of electrolytes on cell performance. This is particularly relevant as many new electrolyte formulations push towards higher concentrations and incorporate new redox structures (e.g., macromolecules), which improve capacity and reduce cost, but also result in higher viscosities and greater property variation as a function of state-of-charge. These factors are expected to strongly influence cell performance. Specifically, engineering an efficient, cost-effective reactor depends on mitigation of resistive losses, several of which depend on electrolyte viscosity, and this dependence has yet to be fully characterized within a RFB. Here, we use a single-electrolyte flow cell configuration ³ , coupled with a model iron-based electrolyte, to probe the impact of electrolyte viscosity on RFB losses. Through the use of glucose as a chemically and electrochemically inert solution thickener, we investigate polarization as a function of electrolyte viscosity, electrolyte flowrate, and flow field geometry. Experimental data is combined with an one dimensional porous electrode polarization model to extract ohmic, kinetic, and mass transfer contributions to cell resistance. Of particular interest are mass transfer rates in RFBs, which are rarely quantified, thus the observed trends in mass transfer coefficient are correlated in terms of dimensionless numbers (e.g., Reynolds, Schmidt) in a traditional power-law format. This study aims to link electrolyte properties and cell performance and to provide a scalable descriptions of mass transfer in RFBs. Acknowledgments The authors acknowledge the financial support of the Joint Center for Energy Storage Research, which was formed under the Office of Basic Energy Sciences within the Department of Energy. We thank A. Helal and G. H. McKinley for rheological guidance and aid in viscosity measurements. We also thank M. Z. Bazant for use of milling equipment to fabricate flow fields used in this work. References 1. Weber, A. Z. et al. Redox flow batteries : a review. J. Appl. Electrochem. 41, 1137–1164 (2011). 2. Dmello, R., Milshtein, J. D., Brushett, F. R. & Smith, K. C. Cost-driven materials selection criteria for redox flow battery electrolytes. J. Power Sources 330, 261–272 (2016). 3. Darling, R. M. & Perry, M. L. The Influence of Electrode and Channel Configurations on Flow Battery Performance. J. Electrochem. Soc. 161, A1381–A1387 (2014). Figure 1

The development of low-cost, large-scale energy storage technologies is required for increased reliance on non-polluting, intermittent renewable energy sources connected to our electrical grid. Our research focuses on the development of organic materials for charge storage in non-aqueous redox flow batteries (RFBs). At present, most commercial RFBs contain aqueous, vanadium-based electrolytes, which have been demonstrated on scales as large as 1.5 MW. With non-aqueous electrolytes, a wider electrochemical window allows for 2-3 times higher operating voltages, while avoiding the use of corrosive electrolytes. Most of the organic species reported as electron donors are only stable in the neutral and singly oxidized states; removal of a second electron results in an unstable dication. Here we sought to increase the stability of dication species without significantly raising molecular weight, thus preserving atom economy. Considering phenothiazine donors, we found that the addition of certain substituents led to improved stability of the dication form; specifically, electron-donating methoxy groups (Figure 1) are more effective in stabilization than non-conjugated substituents. This presentation will include the synthesis of these materials, as well as electrochemical and spectroelectrochemical analysis. Furthermore, the results of density functional theory calculations will be presented to evaluate the design of a stable two-electron donor. Figure 1. Structural representation of 3,7-dimethoxyphenothiazine derivatives (a) and cyclic voltammograms of N -ethyl-3,7-dimethoxyphenothiazine (DMeOEPT) showing reversible first and second oxidations (b). Figure 1

Redox flow batteries (RFBs) are promising candidates for grid storage, with a few large-scale systems currently in operation. However, current systems have not met the stringent cost and/or safety requirements needed for widespread implementation. Replacing vanadium with organic compounds may lower materials cost, and utilizing non-aqueous (aprotic) electrolyte solvents, in place of water, could enable a 2- to 3-fold increase in operating voltage. Both features make non-aqueous RFBs candidates for large-scale stationary storage. Currently a limited number of organic compounds have been reported as stable electron donors and acceptors, with even fewer materials being studied as small molecule two-electron donors and/or two-electron acceptors. Yet if the amount of charged stored within an individual molecule were raised without significantly increasing the molecular weight, then electrolyte capacity could be increased proportionally, assuming solubility of neutral and charged species is retained. Our recent efforts have focused on the development of highly soluble electron donors and acceptors with stable oxidized and reduced states. This presentation will focus on design strategies utilized to increase molecular stability in all relevant states of charge as well as solubility. In particular, we highlight the design, synthesis, and electrochemical analysis of phenothiazine and napthoquinone derivatives designed to serve as two-electron donors and two-electron acceptors, respectively. We show that tactical placement of substituents leads to improved stability of doubly oxidized and doubly reduced species, whilst retaining atom economy and that high solubility.

Non-aqueous redox flow batteries (NAqRFBs) have recently received considerable attention as promising high voltage, low cost grid-level energy storage technologies. Despite these attractive features, NAqRFBs are still at an early stage of development, and innovative design techniques are necessary to improve performance and decrease costs. In this work, we investigate multi-electron transfer, common ion exchange NAqRFBs. Common ion systems decrease the supporting electrolyte required for RFB operation, which subsequently improves active material solubility and decreases electrolyte cost. Voltammetric and electrolytic techniques are used to study the electrochemical performance and chemical compatibility of model redox active materials, Fe(bpy) 3 (BF 4 ) 2 and Fc1N112-BF 4 . These results help to disentangle complex cycling behavior observed in flow cell experiments. Further, a simple techno-economic model demonstrates the cost benefits of employing a common ion exchange NAqRFB, afforded by decreasing the salt and solvent contributions to the total chemical cost. This study offers the first analysis of the benefits of common ion exchange NAqRFBs and the demonstration of a 2e ⁻ flow cell. In addition, the compatibility analysis developed for asymmetric chemistries can apply to other promising active species, including organics, metal coordination complexes (MCCs) and mixed MCC/organic systems, enabling the design of low cost NAqRFBs.