Exploring High Power Nonaqueous Flow Cell Design

The redox flow battery has received significant research interest as a large scale energy storage system for renewable-energy sources. Although the all-vanadium aqueous redox flow battery has been most developed, its energy density is not sufficient mainly because of narrow electrochemical stability window of water. Therefore, recent efforts have shifted to developing nonaqueous redox flow batteries because they can achieve higher voltage due to wider electrochemical stability window of nonaqueous electrolytes. However, nonaqueous redox flow batteries generally suffer from insufficient energy densities because of low solubility of organic active materials in nonaqueous electrolytes. In this study, we designed and synthesized two highly soluble organic active materials, benzoquinone 1 and naphthoquinone 2 having diethylene glycol mono methyl ether groups. Both 1 and 2 were highly soluble in propylene carbonate (PC). Charge-discharge properties of the batteries using 1 and 2 as organic active materials were measured in LiBF4/PC using a cell equipped with a carbon fiber anode, a lithium-metal anode, and a glass filter separator. The charge-discharge measurements showed that 1 and 2 exhibited high voltages of ca. 3 V and good charge-discharge properties. Notably, 2 showed a higher energy density than conventional all-vanadium aqueous redox flow batteries.

Redox flow battery has been widely considered as a promising large-scale stationary energy storage technology to stabilize the power grid and promote integration of renewables. The unique cell structure enables separation of energy and power thus offering tremendous design flexibility for different E/P applications. The limited voltage window in traditional aqueous flow batteries narrows down the intrinsic energy capacity. Therefore, nonaqueous redox flow batteries are being pursued because of the wider voltage window of nonaqueous electrolytes. The current nonaqueous flow battery research is focused on the flow chemistry study and redox materials development.1 Among them, all-organic flow batteries is an important type and use organic electroactive materials at both positive and negative sides, taking advantage of the structural diversity and tunability of organic materials.2However, the current performance is limited by low redox material concentration, cell efficiency, cycling stability, and current density. Especially, development of suitable negative side redox materials is considered more challenging. Here we report several high performance all-organic redox flow chemistries with cell voltages higher than 2V. These flow cell tests demonstrated remarkable cell efficiencies and high operational current densities that exceed those of most reported non-aqueous flow chemistries. A mechanistic study was carried out to gain fundamental understanding of performance degradation in these flow batteries, indicating that the chemical stability of the charged species in various supporting solvents and salts plays a critically important role in achieving decent flow cell cycling stability. This finding can serve as an important guidance for the selection and design of stable redox electrolyte systems. Figure 1. (a) Cyclic voltammetry (CV) and (b) cycling capacity of an all-organic flow chemistry based on 9-fluorenone (negative) and 2,5-di-tert-butyl-1-methoxy-4-[2’-methoxyethoxy]benzene (positive).3 Figure 1

Recent progress in organic redox flow batteries: Active materials, electrolytes and membranes

Redox flow batteries (RFBs) are promising candidates for grid-scale energy storage, but further cost reductions are needed for widespread use.1 As such, redox-active organic molecules and metal-coordination complexes are emerging as charge storage materials considering their redox potential, solubility, and stability can be tailored through molecular functionalization.2,3 Moreover, a number of these compounds can support multiple electron transfers, affording dramatic increases in energy storage capacity.4,5 However, the nature of the electron transfer events, concerted or sequential, plays a key role in determining whether the added capacity can be accessed efficiently. Specifically, depending on molecular features and cell operation, voltaic inefficiencies may be incurred which, in turn, offset any benefits of increased capacity. Therefore, we sought to quantify these losses and their relationship to molecular- and cell-level parameters to better understand the feasibility of multi-electron transfer in RFBs.In this presentation, we use a zero-dimensional electrochemical model, accounting for thermodynamic, kinetic, and mass transfer, to simulate the performance of two-electron compounds in redox flow cells. First, using a single half-cell, we show that voltaic efficiency is significantly influenced by the average redox potential and potential difference between the redox events. We also discuss the effects of different mass transfer coefficients and comproportionation reaction rates. Second, using a full cell, we consider the effects of utilizing two-electron compounds in both half-cells and subsequent effects of charge imbalance, both resulting in further inefficiencies. By understanding the operational tradeoffs of multi-electron compounds, we can develop design criterion to guide molecular engineering efforts towards more efficient high energy density electrolytes for RFBs.AcknowledgementsThis work was funded by the National Science Foundation (NSF) under Award Number 1805566. B.J.N gratefully acknowledges the NSF Graduate Research Fellowship Program under Grant Number 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.References(1) Chalamala, B. R.; Soundappan, T.; Fisher, G. R.; Anstey, M. R.; Viswanathan, V. V.; Perry, M. L. Redox Flow Batteries: An Engineering Perspective. Proceedings of the IEEE 2014, 102 (6), 976–999.(2) Hogue, R. W.; Toghill, K. E. Metal Coordination Complexes in Nonaqueous Redox Flow Batteries. Current Opinion in Electrochemistry 2019.(3) Ding, Y.; Zhang, C.; Zhang, L.; Zhou, Y.; Yu, G. Molecular Engineering of Organic Electroactive Materials for Redox Flow Batteries. Chem. Soc. Rev. 2018, 47 (1), 69–103.(4) Kowalski, J. A.; Casselman, M. D.; Kaur, A. P.; Milshtein, J. D.; Elliott, C. F.; Modekrutti, S.; Attanayake, N. H.; Zhang, N.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A. A Stable Two-Electron-Donating Phenothiazine for Application in Nonaqueous Redox Flow Batteries. J. Mater. Chem. A 2017, 5 (46), 24371–24379.(5) Sevov, C. S.; Fisher, S. L.; Thompson, L. T.; Sanford, M. S. Mechanism-Based Development of a Low-Potential, Soluble, and Cyclable Multielectron Anolyte for Nonaqueous Redox Flow Batteries. J. Am. Chem. Soc. 2016, 138 (47), 15378–15384.

Renewable energy sources such as wind and solar are replacing fossil fuels for electricity generation. However, intermittency of wind and solar limits their wide-spread adoptions. The energy fed into the power grid must be matched with the consumer energy demand to prevent blackouts and destabilization of the grid [1, 2]. Recently, redox flow batteries (RFBs) have gained practical interest among the other energy storage technologies in light of their long lifetime, independent sizing of power and energy, high round-trip efficiency, scalability and design flexibility, fast response, and low environmental impact [3-5]. Organic redox-active materials have recently received attention as they provide competitive electrochemical characteristics, flexible design, and they are abundant in nature [6]. Aqueous designs face commercial difficulty because RFBs have low energy and power density due to the limited cell voltage of 1.23 V. The limited voltage is due to the evolutions of hydrogen and oxygen in the water electrolysis [7]. Solvent substitution is one solution to enable higher energy densities in RFBs, using non-aqueous solution also provides a large design space for enhancement of material solubility, cell potential and the number of electrons stored in the redox species [7-8].In this study, a new organic redox molecule, tetra amino anthraquinone (Disperse Blue 1: DB-1), is evaluated and compared with other organic systems reported in the literature [5, 7] such as benzoquinone (BQ), naphthoquinone (NQ), anthraquinone (AQ), tetramethyl piperidinyloxyl (Tempo), and phenylenediamine (PD) in non-aqueous solvent by means of cyclic voltammetry. A three-electrode system was utilized to conduct cyclic voltammetry (CV) experiments using glassy carbon working electrodes. The battery performance was evaluated using a flow cell with an electrode area of 2.5 cm2. The electrolytic solution, containing 40 mM DB-1 solution in dimethyl sulfoxide solvent (DMSO) and 1 M Bis (trifluoromethane) sulfonimide lithium salt, was circulated through the cell at a flow rate of 10 cm3 min-1. Graphite felt and Nafion 115 were used as the electrode and membrane, respectively.In addition, density functional theory (DFT) calculations were used to better understand the electrochemical behavior of the active quinone molecules at different oxidation states. Figure 1 shows the molecular orbital energy levels (HOMO and LUMO) of the DB-1 organic dye and other similar organic molecules obtained by DFT calculations in DMSO. A relatively small HOMO-LUMO gap means a lower overpotential required for the oxidation and reduction processes [8]. The DB-1 had narrower bandgaps (<3 eV) than other quinone molecules (> 3.9 eV), suggesting that the selected molecule has better kinetics than other organic molecules. The result describes a systematic evaluation of DB-1 as a redox active material for energy storage applications by means of electronic structures, electrochemical properties, and flow cell studies. Voltammetric studies and DFT calculations (in DMSO solvent) demonstrated that this was capable of forming both anions and cations at electrode potentials ranging from 1.7 to 4.4 V vs. Li, involving up to 6 electron-transfers and exhibiting one of the highest electrode potentials (up to 4.4 V vs. Li) in the literature. Since these quinone derivatives can be extracted from biomass materials, the proposed organic RFB system provides a greener and more sustainable option for grid-scale energy storage applications than metal based redox systems. The proposed chemistry in this work showed high energy efficiency (ca. 68 %) at a current density (20 mA cm-2) with close to 100% capacity retention. These results reveal that the molecular tuning of quinone structures is promising for RFBs applications. Figure 1

Nonaqueous redox flow batteries (NRFBs) are attractive devices for energy storage due to an increased potential window and the manifold possibilities they offer in terms of redox-active architectures and chemistries available for increasing energy density and battery performance. Our collaborations in the context of the Joint Center for Energy Storage Research (JCESR) have shown that pairing commercial nanoporous separators with redoxmer architectures[1] such as Redox-Active Polymers (RAPs),[2] Redox-Active Colloids (RACs),[3] and Redox-Active Oligomers (RAOs),[4] effectively prevent crossover of the active component within the flow system. The choice of redoxmer size and chemical makeup impact profoundly the electrochemical dynamics of the NRFB, and it is clear that optimal performance is only attained with a balanced pairing between redoxmer and separator membrane. In this presentation, I will discuss the design principles underlying size-exclusion NRFBs and some current directions on how nanoporous separators enable new strategies for long-term performance in the flow battery. I will highlight how the progression from high polymers to oligomers modifies the charge transfer characteristics of NRFB electrolytes and the implications of the dynamics of these redoxmer solutions on the size-exclusion approach.[5] Unlike small molecules, the electrochemical reactivity of solution-phase redoxmers is strongly modulated by the properties of both surface confined and freely diffusing species. Building from this knowledge, the recent introduction of Polymers of Intrinsic Microporosity (PIM) separators[4] into the NRFB scene area allows new opportunities in the design of redoxmers with improved charge transfer characteristics and diffusivity without sacrificing their size-exclusion performance. Finally, the use of a size-exclusion approach enables advanced concepts for long-term maintenance of a battery system. I will discuss recent results where controlled de-polymerization reactions are used for maintaining the state-of-health of NRFB components, highlighting the role of component separation using our strategy. Altogether, the combination of novel structural motifs and the use of new nanoporous materials, results in new directions to make better NRFBs that exploit the versatility of a new type of reacting redoxmer fluids for energy storage.

Electrochemical techniques have been recognized as an environmentally friendly and sustainable synthetic way to form organodisulfides. However, searching for optimum conditions which suffers from time/material‐consuming caused by the uncertainty of reactant consumption has hindered its rapid and large‐scale development. Inspired by advanced nonaqueous redox flow batteries (NARFBs) technology, it is proposed a smart flow electrosynthesis (SFE) method of organodisulfides that the voltage curve of NARFBs can be utilized as a precise indicator to reflect the desired information about reactants and distinguish the end point of reaction automatically. This electrochemical method also exhibits certain universality and scalability. Additionally, organodisulfides generated in electrolytes can be used as active species for NARFBs without further purification, and their electrochemical properties are easily adjusted by changing raw materials, which effectively alleviate the waste in complex synthesis steps for optimizing and designing active materials separately. An organodisulfide dervied from isopropyl alcohol and carbon disulfide shows excellent cycling life (1000 cycles) with low capacity fade rate (0.024% per cycle). Taking advantages of the inherent NARFBs, this work not only proves a SFE strategy, but also supplies a green and low‐cost molecular engineering scheme for designing electroactive materials for energy storage.

Redox flow batteries (RFBs) with high energy densities are essential for efficient and sustainable long‐term energy storage on a grid scale. To advance the development of nonaqueous RFBs with high energy densities, a new organic RFB system employing a molecularly engineered tetrathiafulvalene derivative ((PEG3/PerF)‐TTF) as a high energy density catholyte was developed. A synergistic approach to the molecular design of tetrathiafulvalene (TTF) was applied, with the incorporation of polyethylene glycol (PEG) chains, which enhance its solubility in organic carbonate electrolytes, and a perfluoro (PerF) group to increase its redox potential. When paired with a lithium metal anode, the two‐electron‐active (PEG3/PerF)‐TTF catholyte produced a cell voltage of 3.56 V for the first redox process and 3.92 V for the second redox process. In cyclic voltammetry and flow cell tests, the redox chemistry exhibited excellent cycling stability. The Li|(PEG3/PerF)‐TTF batteries, with concentrations of 0.1 M and 0.5 M, demonstrated capacity retention rates of ~94 % (99.87 % per cycle, 97.52 % per day) and 90 % (99.93 % per cycle, 99.16 % per day), and the average Coulombic efficiencies of 99.38 % and 98.35 %, respectively. The flow cell achieved a high power density of 129 mW/cm2. Furthermore, owing to the high redox potential and solubility of (PEG3/PerF)‐TTF, the flow cell attained a high operational energy density of 72 Wh/L (100 Wh/L theoretical). A 0.75 M flow cell exhibited an even higher operational energy density of 96 Wh/L (150 Wh/L theoretical).

The development of redox flow batteries (RFBs) has been one of the primary responses to the need to store harvested renewable energy for improving grid reliability and utilization.1–4 In this study, we synthesized and characterized seven types of Phenyl acrylate (PA) backbone-based anion-exchange membranes (AEM) and explored the performance of AEMs in non-aqueous redox flow batteries under ideal conditions. IR data (Figure 1 ) confirms the incorporation of ion carriers in the PA backbone. Methacroylcholine chloride (MACC) was used as an anion carrier, N, N-Methylenebis acrylamide (MBAA) was used as a cross-linker, and 1-Hydroxycyclohexyl phenyl ketone (HCPK) was used as a photoinitiator. Performance was compared based on high stability in a non-aqueous solvent, high permeability to the charge-carrying ion, low electric cell resistance, low cross-over of the redox-active molecules, high mechanical property, and low cost. The best membraned will be found which will show no cross-over of redox-active electrolytes after 1000 battery cycle experiments and exhibit low resistance and high structural/chemical stability in MeCN solvent. References (1) Mazumder, Md. M. R.; Burton, A.; Richburg, C. S.; Saha, S.; Cronin, B.; Duin, E.; Farnum, B. H. Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate. Inorg. Chem. 2021, 60 (17), 13388–13399. https://doi.org/10.1021/acs.inorgchem.1c01699.(2) Mazumder, M. M. R.; Farnum, B. H. Two One Electron vs One Two-Electron Redox System of Nickel Diethyldithiocarbamate for Redox Flow Battery; ACS, 2021. https://doi.org/10.1021/scimeetings.1c00951.(3) Mazumder, M. M. R. Ni(IV/II) Redox Couple 2e- Reversibility Improvement for Redox Flow Battery. ECS Meet. Abstr. 2021, MA2021-02 (5), 1855–1855. https://doi.org/10.1149/ma2021-0251855mtgabs.(4) Islam, R.; Mazumder, M. M. R.; Farnum, B. H. Solvent Dependent Spectroscopic and Electrochemical Studies of Nickel (II) Diethyldithiocarbamate for Energy Storage. ECS Meet. Abstr. 2021, MA2021-01 (1), 54–54. https://doi.org/10.1149/ma2021-01154mtgabs. Figure 1

Redox flow batteries (RFBs) are a burgeoning electrochemical platform for long-duration energy storage, but present embodiments are too expensive for broad adoption. Nonaqueous redox flow batteries (NAqRFBs) seek to reduce system costs by leveraging the large electrochemical stability window of organic solvents (>3 V) to operate at high cell voltages and to facilitate the use of redox couples that are incompatible with aqueous electrolytes. However, a key challenge for emerging nonaqueous chemistries is the lack of membranes/separators with suitable combinations of selectivity, conductivity, and stability. Single-ion conducting ceramics, integrated into a flexible polymer matrix, may offer a pathway to attain performance attributes needed for enabling competitive nonaqueous systems. Here, we explore composite polymer-inorganic binder-filler membranes for lithium-based NAqRFBs, investigating two different ceramic compounds with NASICON-type (NASICON: sodium (Na) superionic conductor) crystal structure, Li1.3Al0.3Ti1.7(PO4)3 (LATP) and Li1.4Al0.4Ge0.2Ti1.4(PO4)3 (LAGTP), each blended with a polyvinylidene fluoride (PVDF) polymeric matrix. We characterize the physicochemical and electrochemical properties of the synthesized membranes as a function of processing conditions and formulation using a range of microscopic and electrochemical techniques. Importantly, the electrochemical stability window of the as-prepared membranes lies between 2.2-4.5 V vs Li/Li+. We then integrate select composite membranes into a single electrolyte flow cell configuration and perform polarization measurements with different redox electrolyte compositions. We find that mechanically robust, chemically stable LATP/PVDF composites can support >40 mA cm-2 at 400 mV cell overpotential, but further improvements are needed in selectivity. Overall, the insights gained through this work begin to establish the foundational knowledge needed to advance composite polymer-inorganic membranes/separators for NAqRFBs.

To increase renewable energy generation to the electrical grid, intermittent energy sources, such as wind and solar energy, need a long-term, large scale energy storage method. Redox flow batteries (RFB) are an attractive solution as they allow for long term storage in various redox states of an organic or inorganic compound. The ideal redox-active compound for a RFB should have a high energy density, which can be achieved through a large potential difference in redox events, high concentration of charge carrier, or by increasing the number of electrons transferred during a given redox event. In particular, nonaqueous RFB (NRFB) are promising for building a system with a large potential difference for redox events due the large electrochemical solvent window (~5 V for acetonitrile) as compared to water (~2 V). While metal complexes featuring redox active ligands have been thoroughly investigated as carriers in NRFBs, they are often too expensive for widespread use. This has led to increasing interest in organic carriers. Benzophenone is a promising candidate for an organic negolyte due to its high solubility in acetonitrile and the presence of a reversible reduction at -2.1 V vs. Ag/Ag+. Unfortunately, the durability of benzophenone during cycling is low. Here, we discuss the degradation products of benzophenone during cycling and show how the incorporation of sterically bulky groups ortho to the benzophenone carbonyl improves durability. We then evaluate the utility of these benzophenone derivatives as negolytes in a NRFB flow cell.

Quinoxaline (Q) is an excellent candidate for anolyte active materials in nonaqueous redox flow batteries (NRFBs) because of its high solubility, low‐reaction potential, and two transfer electrons in organic solvents. However, systematic and in‐depth studies on the electrochemical performance of Q under nonaqueous conditions are still needed. Herein, a systematical study is conducted on the effect of ion species on Q reaction in nonaqueous solvent acetonitrile through a combination of electrochemical measurements and theoretical analysis. In accordance with the rotating disk electrode analysis, the diffusion coefficients of Q under Na+ and tetrabutylammonium (TBA)+ conditions reach 5.031 × 10−6 and 8.563 × 10−6 cm2 s−1, respectively, and the kinetic rate constants are 7.81 × 10−3 and 4.76 × 10−3 cm s−1, respectively. According to in situ UV–vis analysis, Q presents the best electrochemical reversibility under tetrabutylammonium hexafluorophosphate (TBAPF6) and a low‐reaction potential of −1.98 V versus Ag/Ag+, making it a very promising anolyte active material. By coupling with the 1,4‐di‐tert‐butyl‐2,5‐bis (2‐methoxyethoxy)benzene (DBBMEB) catholyte, the DBBMEB‐Q NRFB, which the battery voltage reaches above 2.5 V under both Na+ and TBA+ conditions, is demonstrated. The methodology adopted in this work provides a design method for the high‐voltage and high‐energy‐density redox flow batteries.

Polyoxometalates (POMs) are a class of metal-oxygen cluster molecular materials formed by covalently linking transition metal atoms and oxygen atoms. Those molecular cluster materials also shows multiple electron reversible redox capabilities and high solubility in solution. Thus, POMs also recently present interesting applications as redox active electrolyte in redox flow batteries (RFBs). In this review, we mainly focus on the structural design of POMs and analysis factors affecting the electrochemical properties of POMs. Spectroscopic characterizations are introduced to deeply explore the structural changes of POMs during the redox process. Furthermore, we summarize the cases of POMs used as liquid-phase redox couple in aqueous or non-aqueous redox flow batteries and discuss the advantages and deficiency of POMs with various configurations.

Recent advances in redox flow batteries (RFB) have made them a viable option for grid-scale energy storage. Vanadium redox flow batteries (VRFB) are the most prominent and commercially viable, however, when compared to lithium-ion batteries they have low energy density due to low cell potentials (U = 1.5 V in VRFB). To boost cell potentials, researchers have begun exploring organic platforms. These can be engineered to exhibit extreme potentials while offering high tunability, earth abundance, and cost-effectiveness. Here, we investigated the viability of phosphorus containing organic molecules as anolytes in RFB. Through molecular engineering, we developed a series of novel, sustainably derived, organic anolytes with highly negative redox potentials (–2.0 to –2.5 V vs. Fc/Fc+) that operate in acetonitrile and dimethylformamide, allowing for a wide cell potential. Through electrochemical studies using cyclic voltammetry, charge/discharge cycling, and various mechanistic studies (decomposition studies, EPR analysis, X-ray crystallography), we found an excellent candidate. Guided by this discovery, we tuned our system to create a long-lived anolyte which exhibited no capacity fade over 350 cycles with an average coulombic efficiency of 99.3%. When paired with an appropriate catholyte (e.g. E1/2 ≥ 0.5 V), an organophosphorus-based RFB could double the cell potential (U ~ 3V) and energy density found in VRFB. This performance establishes our anolyte as one of the most negative and stable reported in non-aqueous RFB. Figure 1

This dissertation examines the process of energy conversion on fuel cells and the storage of energy in the form of redox flow batteries. In Chapter 1, I will discuss the fundamentals of the polymer electrolyte membrane fuel cells for the conversion of chemical energy, stored in the form of hydrogen and oxygen, into electrical energy and the current technologies and limitations of these devices. I will also discuss the operating principles of the redox flow battery for applications in grid energy storage with a special focus on the non-aqueous vanadium redox flow battery and the space that exists for improvement.In chapter 2, I will discuss the work that was done to investigate the synthesis pathway of single-atom MnN4 catalyst sites. Using in-temperature X-ray absorption spectroscopy (XAS) we were able to unravel the mechanism of site evolution during pyrolysis. We show that the formation of manganese oxides (MnOx) is the initial state of the precursor at room temperature, and the emergence of the manganese-nitrogen coordination (MnN4) site begins at 750 °C and continues until reaching the upper limit of 1000 °C. The competition between the MnOx and MnN4 is identified as the primary factor governing the formation of MnN4 sites during pyrolysis. Chapter 3 investigates the process of MnNC catalyst development based on our understanding of the MnN4 site evolution pathway developed in chapter 2. This knowledge led us to utilize a chemical vapor deposition (CVD) method to produce MnN4 sites to bypass the evolution route involving the MnOx intermediates. The Mn-N-C catalyst synthesized via CVD shows improved ORR activity over the Mn-N-C synthesized via traditional synthesis by pyrolysis of a mixture of Mn, N, and C precursors. I will discuss our work in the field of redox flow batteries in chapter 4. Our investigation shows how solvent selection can change the electrochemical parameters of vanadium acetylacetonate (V(acac)3). We demonstrated that strong correlations of solvent properties such as polarity, viscosity, and longitudinal relaxation time can be used to predict the electrochemical behavior of V(acac)3. We propose this in-depth analysis of solvent effects on the electrochemical properties can be extended to other systems and give a principle-based design methodology for examining solvent and supporting electrolyte. This provides an alternative to the trial-and-error approach oft employed in solvent selection for non-aqueous redox flow batteries. In chapter 5 I will discuss the degradation process of the metal organic complexes with non-innocent ligands in redox flow battery conditions. Here we demonstrate the deposition onto the working electrode after cycling at low potentials. We examine the degradation mechanism electrochemically and through spectroscopy and propose a pathway the initiates from an intra molecular diradical combination mechanism. Lastly, in chapter 6, I will discuss the conclusions drawn from these research topics and propose avenues for future work and give a scope of how this work fits into the current understanding of energy conversion and storage. --Author's abstract