Short communicationA low potential solvent-miscible 3-methylbenzophenone anolyte material for high voltage and energy density all-organic flow battery
Graphical abstract
Introduction
Redox flow batteries (RFBs) offer an effective energy storage solution necessary to integrate massive amounts of intermittent renewable energy sources into the grid [[1], [2], [3]]. In RFBs, energy is stored by the electrochemical active materials that are dissolved in liquid electrolytes contained in external tanks. In discharge mode, the redox active species flow through an electrochemical cell or stack containing active electrodes allowing conversion from chemical to electrochemical energy to take place and release electrons to perform work. Compared to other batteries that store energy in solid electrodes, the unique structure of RFBs allows the power and energy to be decoupled and is highly scalable [[4], [5], [6]]. For traditional RFBs based on aqueous system, the energy density of most systems is less than 50 Wh L−1, limiting by the low electrochemical potential of water (<1.6 V) [7,8]. Options aimed at improving the energy density include maximizing the solubility of electroactive materials in electrolyte, increasing the open circuit voltage of the cell and the number of transferrable electrons of positive and negative redox reactions [9].
Redox-active organic molecules (ROMs) are considered to be alternative redox-active materials to conquer current plight due to their flexible structural design, potential high solubility in solvent and relatively low molecular weight [2,10,11]. Many ROMs as anolyte or catholyte active materials for RFBs have been reported. 2,2,6,6-Tetramethylpiperidinooxy (TEMPO) and its derivatives, as a typical nitroxide radical redox shuttle in Li-ion batteries previously, have been investigated as cathode materials for organic RFBs due to their electrochemical reversibility and stability [[12], [13], [14], [15]]. Wang et al. [16] reported a hybrid RFB using TEMPO as cathode with metal Li anode, which shows a theoretical open circle voltage of 3.5 V and a theoretical energy density of 126 W h L−1. A variety of 1,4-dialkoxybenzenes derivatives, such as 2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene and 2,5-di-tert-butyl-1-methoxy-4-[2ʹ-methoxyethoxy]benzene are another class of redox shuttle materials and also have been evaluated as positive active materials for nonaqueous RFBs [[17], [18], [19], [20]]. In addition, some heterocyclic redox molecules, such as quinoxaline and N-alkylated pyridinium derivatives [17,21], have been developed as anolyte materials for all-organic flow batteries because of their low potential and high solubility. Organic molecules containing carbonyl moieties are currently being considered for electrochemical energy storage [22]. Carbonyl-based electrochemical molecules have rapid reaction kinetics, extensively varied structures and cost-effective manufacturing, and have been employed in various electrochemical applications that include RFBs, supercapacitors and solid batteries [21,[23], [24], [25]]. 9-Fluorenone has been reported as anolyte for organic RFB, and showed a potential of −1.64 V vs. Ag/Ag+ and a solubility of 2 M in acetonitrile (MeCN) [26]. Benzophenone (BP) derivatives also have been used as anode active materials due to their good electrochemical and high soluble properties [27,28].
Herein, we report a new BP derivative 3-methylbenzophenone (3-MBP) as anolyte for organic RFBs. The liquid 3-MBP can be synthesized simply or purchased from commercial sources. The physical and chemistry properties of 3-MBP were investigated. By pairing 3-MBP with 1,4-di-tert-butyl-2-methoxy-5-(2-methoxyethoxy)benzene (DBMMB) as cathode, we constructed a high voltage and potentially high energy density organic RFB based on all liquid active materials, which can provide a prospective molecular model towards achieving high energy density RFBs.
Section snippets
Chemicals
3-MBP and MeCN were received from Alfa Aesar. 1,4-di-tert-butyl-2-methoxy-5-(2-methoxyethoxy)benzene and tetraethylammonium hexafluorophosphate (TEAPF6) were synthesized based on the reported methods [26,27], respectively. Solvents were distilled prior to use. All other chemicals were used as obtained without further purification.
Cyclic voltammetry
Cyclic voltammetry (CV) was carried out in a nitrogen-filled glovebox with a CHI 660D (Shanghai Chenhua Instruments Co., Ltd., China). A platinum strip electrode, a
Results and discussion
Fig. 1a presents the redox reaction of 3-MBP. The solution of 0.5 M TEAPF6/MeCN shows a wide electrochemical window of −2.6 to 1.2 V in Fig. S1 (in supporting information). The CVs of 0.1 M 3-MBP in 0.5 M TEAPF6/MeCN are shown in Fig. 1b. The oxidation and reduction peaks are distinctly identified at −2.14 V and −2.2 vs Ag/Ag+ at a scan rate of 0.1 V s−1, resulting in a half-wave potential of −2.18 V vs Ag/Ag+. The 5th, 100th and 200th CVs almost overlap with each other and only one pair of
Conclusions
In summary, we reported an easily available 3-MBP as liquid anolyte redox-active material for organic RFBs. It was demonstrated that the 3-MBP had good miscibility with organic solvents and a reversible redox process with a low potential. The electrochemical kinetics study showed that the redox reaction had a coefficient of 6.92 × 10−6 cm2 s−1 and was a diffused control process in TEAPF6/MeCN. A UV–vis study and BE tests demonstrated capacity retention over time with multiple cycles. The
Acknowledgements
This work was funded by a National Institute of Clean-and-Low-Carbon Energy Project (No. CF9300172123) and the National Natural Science Foundation of China (No. 21606004).
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