All Liquid Electroactive Materials for High Energy Density Organic

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All Liquid Electroactive Materials for High Energy Density Organic Flow Battery Xueqi Xing, Qinghua Liu, Wenqiang Xu, Wenbin Liang, Junqing Liu, Baoguo Wang, and John P. Lemmon ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01874 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Applied Energy Materials

All Liquid Electroactive Materials for High Energy Density Organic Flow Battery Xueqi Xing, † Qinghua Liu, † Wenqiang Xu, † Wenbin Liang, † Junqing Liu, † Baoguo Wang ‡ and John P. Lemmon *†

†National

Institute of Clean-and-Low-Carbon Energy, Beijing 102211, China

‡Department

of Chemical Engineering, Tsinghua University, Beijing 100084, China

KEYWORDS: redox flow battery, active material, organic electroactive material, energy density, energy storage

ABSTRACT: Non-aqueous redox flow batteries (RFBs) are a promising energy storage technology that enables increased cell voltage and high energy capacity compared to aqueous RFBs. Herein, we firstly report a novel approach to substantially increase the energy density based on the miscible liquid redox materials 2,5-di-tert-butyl-1-methoxy-

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4-[2'-methoxyethoxy]benzene catholyte and 2-methylbenzophenone anolyte. This system has a high theoretical cell voltage of 2.97 V and a calculated energy density of 223 Wh L−1 that is much higher than previously reported non-aqueous organic RFBs. Our reported redox flow chemistry displays excellent electrochemical performance and stability under cyclic voltammetry, bulk electrolysis and flow cell conditions. A proof-ofprinciple RFB delivers a columbic efficiency of 95% and energy efficiency of 70% and represents significant progress towards high energy density RFBs.

Redox flow batteries (RFBs) are a promising candidate towards achieving scalable electrical energy storage technology which can solve the imbalance between electricity generation and consumption by alleviating the intermittency of renewable energy sources (e.g., wind and solar).1-8 The electrolytes containing redox-active material are circulated through the positive and negative electrode chambers, completing the conversion between electrical and chemical energy.3,9,10 Upon this unique feature, the power and energy density decouple, thereby modulating the power and energy


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independently according to storage demand.6,11,12 Conventional RFBs utilizing water as the solvent generally have a low energy density of ~50 Wh L-1 for most RFB systems which is limited by the narrow electrochemical window of water (< 1.6 V).13-15 This technical hurdle along with other issues has limited the widespread market penetration of RFB technologies.16 Moreover, the current cost of mature all-vanadium RFB technology is substantially higher than the U.S. Department of Energy (DOE)’s cost target of $100 kW h−1 which is mainly due to the high cost of redox-active materials and low energy densities.17,18 Increasing the energy density of RFBs is a viable path to reducing cost while expanding energy storage applications to meet the demands of an increasingly growing storage market.

Recent studies in non-aqueous RFBs have focused on exploring new redoxactive materials that increase both the voltage and energy density. For example, non-aqueous RFBs employing metal coordination (such as vanadium acetylacetonate, iron(II) and cobalt(II) tris(1,10-phenanthroline) and cobalt(II) and vanadium(III) trimetaphosphate),19-22 or organic electrochemical compounds23-25


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as the active species have demonstrated cell voltages of more than 2.0 V, which is almost two times that of the conventional RFBs. Moreover, the energy density of RFBs based on organic active molecules has also been increased through selective functionalization strategies which led to increased solubility in the electrolyte.10,26,27 However, the solubility of the active material can be significantly limited when dissolved in a high molarity supporting electrolyte which increases the risk of precipitation leading to lost energy capacity.5 Increasing the number of charge transfer electrons through multi-electron redox reactions in either the anolyte or catholyte has led to more than double the energy density when compared to conventional single-electron transfer chemistries.28,31 However, based on theoretical voltages and other factors, there is plenty of room for further increases which can result in large cost reductions for RFB energy storage systems. Organic redox couples can be tailored to increase chemical potential, solubility and other important properties that are highly advantageous in achieving breakthrough energy densities for RFB energy storage systems.


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Organic electroactive molecules 2,5-di-tert-butyl-1-methoxy-4-[2ʹmethoxyethoxy]benzene (DBMMB) and benzophenone have been used as redox species for non-aqueous RFBs.25,32,33 The liquid DBMMB at room temperature is miscible with organic solvent, such as acetonitrile (MeCN), and 1,2dimethoxyethane (DME). Benzophenone has been reported to be highly soluble in MeCN and possess a low half-wave potential necessary for an anolyte.24 Using molecular engineering we screened and identified a liquid organic anolyte 2methylbenzophenone (2-MBP) that significantly increased the miscibility in common electrolyte solvents such as MeCN, DME and dimethyl carbonate while maintaining the redox properties.


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Figure 1. (a) Cyclic voltammetry (CV) curves of 0.1 M 2-MBP/0.1 M DBMMB in 0.5 M tetraethylammonium hexafluorophosphate (TEAPF6)/MeCN scanned at 0.1 V s−1 for 5th, 50th, 100th, 200th, 300th cycle; arrows indicate the scan direction. (b) Scheme of


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positive and negative side redox reactions of the cell. (c) Linear sweep voltammetry curves of 0.005 M 2-MBP/0.005 M DBMMB in 0.5 M TEAPF6/MeCN at a voltage scan rates of 5 mV s−1. (d) Linear Levich plots of limiting current density (iL) versus square root of rotation rates (ω1/2).

The redox potential of the active species was determined by cyclic voltammetry (CV). Figure 1a shows the CV curves of an electrolyte solution containing mixed redox-active compound 2-MBP and DBMMB with an equal molarity (0.1 M) in 0.5 M tetraethylammonium hexafluorophosphate (TEAPF6)/MeCN. There are two well-defined oxidation and reduction peaks clearly observed at −2.26 and 0.71 V versus Ag/Ag+ in potential scan range of −2.6 to 1.1 V at 0.1 V s−1, corresponding to the half-cell reaction of 2-MBP/2-MBP·− and DBMMB/DBMMB·+ (Figure 1b). Under these conditions, 2-MBP displays one of the lowest potential values amongst reported anolytes for non-aqueous RFBs.16 The system of 2MBP/DBMMB reaches a theoretical cell potential of 2.97 V, which is one of the highest among reported single-electron non-aqueous organic RFBs.23,24,25,33


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Combined with the miscibility of both redox materials, the energy density of this cell has the potential to reach the calculated value of 223 Wh L−1 (based on our extrapolated solubility data of 5.6 M for 2-MBP). The electrolyte solution was cycled continuously 300 times and the CV curves of 5th, 50th, 100th, 200th, 300th cycle overlap with each other almost entirely, indicating high electrochemical stability for both redox species under CV scanning conditions.

To further characterize the electrochemical kinetics of both reactions, linear sweep voltammetry (LSV) was performed with a glassy carbon rotating disk electrode. LSV curves of DBMMB and 2-MBP with a rotating ranging from 50 to 900 rpm are shown in Figure 1c. Two well-defined plateau shapes are observed for DBMMB and 2-MBP by sweeping positively from 0.2 to 1.1 V versus Ag/Ag+ for DBMMB and sweeping negatively from −1.9 to −2.6 V versus Ag/Ag+ for 2MBP, indicating mass-transport-controlled limiting current density (iL). Levich plots (Figure 1d) were constructed using the iL and the square root of rotation speeds (ω1/2). Using the slopes obtained from the fitted linear Levich plots, the


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diffusion coefficients D are estimated to be 4.96 × 10−6 cm2 s−1 for the DBMMB and 7.18 × 10−6 cm2 s−1 for the 2-MBP, respectively.


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Figure 2. (a) CV curves of 0.005 M 2-MBP/0.005 M DBMMB in 0.5 M TEAPF6/MeCN at different scan rates from 0.01 to 0.4 V s−1. (b, c) Plots of Ψ versus peak separation (ΔEp) of the reduction peak and oxidation peak of DBMMB and 2-MBP. (d, e) Linear relationship between Ψ and square root of scan rates (ν-1/2) of DBMMB and 2-MBP.

Figure 2a shows the voltamogram of anolyte and catholyte sweeping at different scan rates from 0.01 to 0.4 V s−1. The peak current densities of the redox couples of 2-MBP/2-MBP·− and DBMMB/DBMMB·+ display linear relationship with the square root of scan rate (ν1/2, Figure S1), suggesting both the reactions are reversible and diffusion controlled. The electron transfer rate constants (k0) are calculated using the Nicholson’s method based on the CV data. The Ψ values (Figure 2b and 2c) are calculated by Equation S2 (in supporting information). Then k0 is estimated by the slope of the linear plots of Ψ value and the negative square root of scan rate (ν−1/2, Figure 2d and 2e). The k0 for DBMMB and 2-MBP are calculated to be 0.9 × 10−2 cm s−1 and 0.8 × 10−2 cm s−1 respectively.


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Figure 3. (a) Bulk electrolysis test of 0.005 M 2-MBP in 0.5 M TEAPF6/MeCN at a charge rate of 1C; inner picture is the first five cycles. (b) The 6-30 cycles of 0.1 M 2MBP/0.1 M DBMMB in 0.5 M TEAPF6/MeCN at a charge/discharge current density of 7.5 mA cm−2. (c) Capacity, columbic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) versus cycle numbers.


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The electrochemical stability of 2-MBP as anolyte for organic RFBs was evaluated in a bulk electrolysis (BE) cell (Figure S2). Figure 3a shows the cycles of 0.005 M 2-MBP in 0.5 M TEAPF6/MeCN at a charge rate of 1C. The galvanostatic cycling of cutoff is set at 50% of the theoretical capacity value or the voltage of −2.4 V, whichever reached first. When the neutral electrolyte solution is charged, the colorless 2-MBP is reduced to the teal 2-MBP·− radical anion (Figure S3). The charge and discharge plateaus are clearly observed in the voltage curves (Figure 3a insets). After 10 cycles, the columbic efficiency (CE) is stable at an average value of ~ 95%.

The redox couple performance was further characterized using a flow battery with a porous Daramic 250 membrane as separator and two stacked 3 pieces of Toray carbon paper (treated under N2 at 600 oC for 24 h before to use) as electrodes. A mixed-reactant electrolyte of 0.1 M 2-MBP/0.1 M DBMMB in 0.5 M TEAPF6/MeCN used as catholyte and anolyte was cycled at a constant current density of 7.5 mA cm−2, which gives a theoretical energy density of 4 Wh L−1. The


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cutoff voltages are set to 3.7 V for charge process and to 1.9 V for discharge process. Figure 3b shows the cycling performance of the 6-30 cycles. The charge and discharge voltage plateaus are clearly observed in the cell voltage profiles, locating at 3.4-3.6 V and 2.6-2.8 V respectively. The cycling capacity and efficiency over 50 cycles are presented in Figure 3c. As shown, the capacity gradually decreases with increasing cycle number. In the first ten cycles, a columbic efficiency (CE) near 99% is achieved. Further cycling shows the CE is stabilized with an average value of about 95% while the voltage efficiency (VE) and energy efficiency (EE) stabilize at average values of 73% and 70%, respectively.

In summary, we firstly report a non-aqueous RFB using all-liquid redox-active materials that has the potential to achieve high energy density. The RFB chemistry is based on a promising anolyte candidate material 2-MBP that is miscible with organic solvents and has excellent electrochemical properties. With a low redox potential value of −2.26 V versus Ag/Ag+, high cell voltages can be possibly achieved with common organic


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catholytes. The reported 2-MBP/DBMMB system here showed a theoretical cell voltage of 2.97 V based on CV data, which is higher than that of the 2,1,3benzothiadiazole/DBMMB system (2.36 V). The estimated theoretical energy density can reach as high as 223 Wh L−1, which represents vast improvement when compared to reported non-aqueous organic systems. Using a proof-of-concept RFB cell with 0.1 M concentration of redox active species, the 2-MBP/DBMMB system showed stable CE of ~95% and EE of ~70% over 50 cycles. Although our reported energy density for the experimental cell is much less than the estimated theoretical energy (4 Wh L-1 versus 223 Wh L-1 respectively), significant increases in performance and cycle life can be gained through the improvement of critical cell parameters such as separator, electrode materials and flow design. Our novel approach of molecular engineering miscible, high voltage, organic-based redox couples combined with future work on the optimization of cell materials can lead towards improved performance with significant cost reductions that open new energy storage applications for RFBs.


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ASSOCIATED CONTENT

Supporting Information The following files are available free of charge.

General experimental methods for materials, cyclic voltammetry, linear sweep voltammetry measurements, solubility tests, bulk electrolysis tests, flow cell tests, and supplementary figures (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the Low-cost Redox Flow Battery for Large-scale Energy Storage Program of National Institute of Clean-and-LowCarbon Energy (Grant CF9300172123) and the National Natural Science Foundation of China (Grant 21606004).

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All Liquid Electroactive Materials for High Energy Density Organic

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