Research background

Water based redox flow batteries (ARFBs) have the advantages of large scale, high device flexibility, long lifespan, and high safety, which can solve the problems of discontinuous, unstable, and uncontrollable sustainable energy generation. At present, the commonly used electrolytes for ARFB include metal electrolytes such as inorganic ions and metal complexes, as well as organic electrolytes such as quinone compounds. All vanadium flow battery (VRB) has been widely explored as a potential redox flow battery technology, and some MWh scale VRB demonstration systems have been installed. However, vanadium metal, as the redox material of ARFB, is easily affected by metal pollution and price fluctuations. In this context, organic compounds with electrochemical active groups such as phenolic hydroxyl, methoxy, sulfur, and nitrogen can achieve electron transfer and storage through redox reactions, and are expected to be used as energy storage electrolytes.

Lignin is a complex phenolic polymer that is inexpensive and abundant in nature, and can be easily obtained from the papermaking and cellulose biofuel industries. Inspired by the oxidation-reduction function of lignin in the energy conversion process, its potential as an oxidation-reduction active substance for energy storage has attracted widespread attention. Lignin has been used as an electrode for solid capacitors and has shown good performance. Studies have [1] assembled a lignin/polypyrrole hybrid electrode with good energy storage performance through the electrochemical polymerization of pyrrole in lignin sulfonate solution. Due to the presence of quinone groups in lignin, energy storage performance, including capacitance and lifespan, is improved.
However, there is limited research on the use of lignin as an active species in ARFB. As an electrolyte, lignosulfonate is limited by cycling durability, including loss of charging capacity and decrease in current efficiency during battery cycling. Research has shown that energy storage behavior depends on the content of redox active groups in lignin. Therefore, it can be attempted to optimize the lignin structure by introducing quinone groups, which can effectively improve energy storage performance. Anthraquinone (AQ) derivatives are often considered as electrolytes for ARFB, while 1,4-dihydroxyanthraquinone (1,4-DHAQ) is prone to cyclization reactions with unsaturated aldehydes such as coninal (G) and mustard aldehyde (S) in lignin structures. The authors of this study, Liang et al., addressed the issue of low electrochemical activity of lignin and poor chemical stability of 1,4-DHA, and prepared lignin anthraquinone derivatives (LAQDs) for ARFB.

Research has found that lignin anthraquinone derivatives have better chemical stability as active substances in ARFBs than anthraquinone. Moreover, cyclization with 1,4-DHAQ can enhance the electrochemical activity of lignin. The study found that the initial volume capacitance, capacity retention, and Coulombic efficiency of the derivatives of pine and cypress aldehyde anthraquinone [LAQD (G)] were determined to be 148.0 mAh L-1, 89.3%, and 99.0%, respectively. The initial volumetric capacitance, capacity retention, and Coulombic efficiency of the sine aldehyde anthraquinone derivative [LAQD (S)] were determined to be 132.1 mAh L-1, 81.2%, and 99.0%, respectively.


Figure 1 shows a possible reaction pathway diagram for the cyclization of different unsaturated aldehydes (pine aldehydes G and mustard aldehydes S) in 1,4-DHAQ and lignin. Unsaturated aldehydes in lignin, such as pine aldehydes and mustard aldehydes, provide positions for cyclization reactions. With the help of NaOH, Michael addition reactions occur to generate Michael adducts, which are then intramolecular cyclized by attacking the enol double bonds of unsaturated aldehydes to produce cyclization products.

The author subsequently investigated the effects of reactant molar ratio on the conversion rate of LAQDs, selectivity of cyclization products, yield of cyclization products, and degree of cyclization of lignin monomers, as shown in Figure 2. When the ratio of G to 1,4-DHAQ is 4:1, the yield of LAQD (G) is 71.7%, the selectivity is 72.3%, and the degree of cyclization of lignin monomers is 17.9%. When the ratio of S to 1,4-DHAQ is 4:1, the yield and selectivity of LAQD (S) are 69.8% and 69.0%, respectively, and the cyclization degree of S is 17.2%. When the amount of lignin monomer decreases, the yield of LAQDs significantly decreases and the recovery rate of AQ increases, indicating that excessive AQ does not promote cyclization. In the cyclization reaction, 1,4-DHAQ is an electrophilic reagent, with unsaturated double bonds on lignin monomers acting as nucleophilic reagents. 1,4-DHAQ provides four electrons to lignin, which also proves that the optimal molar ratio of reactants is 4:1.

Figure 2: Effect of reactant molar ratio on LAQDs conversion rate, cyclization product selectivity, cyclization product yield, and lignin monomer cyclization degree

The author also studied the effect of reaction temperature on the degree of cyclization, as shown in Figure 3. However, whether the temperature is increased or decreased, the degree of cyclization of lignin monomers does not decrease. At 0 ° C, the degree of cyclization is highest, and when the temperature is below 0 ° C, the conversion rate of AQ decreases. On the contrary, increasing the temperature above 0 ° C will accelerate the formation of by-products and reduce the yield of LAQD. And research has found that with the prolongation of reaction time, the degree of cyclization of lignin monomers increases, reaching the highest value at 50 minutes, indicating that the cyclization products form a stable structure.

图3：反应温度和反应时间对环化度的影响

作者还利用了循环伏安法对半电池体系进行测试，结果如图4所示，发现在正扫过程中，出现了多个氧化峰。这是由于1,4-DHAQ和G上醌结构的氧化电位不同造成的。在负扫描过程中，1,4-DHAQ上的醌结构更容易被还原，因此还原峰出现在- 0.67 V vs. SHE。与G用作电解质相比，LAQD(G)更容易被氧化和还原，这是因为引入了电化学活性更强的1,4-DHAQ，更有利于储能，同时，LAQD（S）在氧化还原电位方面具有相同的特性。

Figure 4: Cyclic voltammetry of 0.01M LAQD (G) and 0.01M LAQD (S) in 0.1 M KOH aqueous solution

In order to verify the energy storage performance of LAQDs, the author studied the use of LAQDs (G) for over 200 complete charging and discharging cycles of the battery (Figure 5a). When the current density is 40 mA cm-2, the open circuit voltage is 1.2V, and the initial volume capacity of 0.01M LAQD (G) reaches 148mAh L-1. After 200 cycles of charge and discharge, the volume capacity remains at 132.2mAh L-1, with a capacity retention rate of 89.3% and a Coulombic efficiency always maintained at about 99.0%. During a constant current cycle with a current density of 40 mA cm-2, the volumetric capacitance of 0.01M LAQD (G) reached 148mAh L-1. As the current density decreased, the capacitance also significantly decreased. Compared with 0.01M G (109mAh L-1) used as the negative electrode, the introduction of AQ can significantly increase the volume capacity, and the capacity retention rate of LAQD (G) is close to G (87.4%).

图5：LAQD(G)恒电位循环与恒电流循环

For batteries assembled with LAQD (S), when the current density is 1 mA cm-2, the open circuit voltage is 1.2V, and the initial volume capacity of 0.01M LAQD (S) reaches 132.1mAh L-1. After 200 cycles of charging and discharging, the volume capacity remained at 107.3 mAh L-1, with a capacity retention rate of 81.2% and a Coulombic efficiency consistently maintained at about 99.0%. During a constant current cycle with a current density of 1.0 mA cm-2, the volumetric capacitance of 0.01M LAQD (S) reached 132.1mAh L-1, and the capacitance significantly decreased with increasing current density. Research has found that LAQD (S) can achieve efficient energy storage at low current densities. As the current density increases, the voltage efficiency and energy efficiency of both LAQDs remain around 80%, indicating that both electrolytes have good charge discharge stability.

图6：LAQD(S)恒电位循环与恒电流循环

由于木质素是由三种醇单体形成的复杂酚类聚合物，作者在研究中将LAQD(G)和LAQD(S)按质量比3:7混合制备混合LAQDs电解液以模拟桉树木质素结构，并测试其电化学性能。作者以0.01 mol/L 混合 LAQDs 用作中间电解质制备半电池，进行循环伏安法。在正扫过程中，出现多个氧化峰，这是由于1,4-DHAQ上的醌结构和G上的醌结构的氧化电位不同所致，氧化电位分别为-0.150 V和0.185 V，这意味着两种LAQDs不会同时发生氧化，但氧化电位比较接近，当放电电位高于0.185 V 时，混合LAQD可以被氧化。

作者最后对采用混合电解质的全电池进行了测试。当电流密度为20 mA cm ^-2时，开路电压为1.4V，初始容量达到144.8mAh L ^-1。充放电200次循环后，体积容量为117.3mAh L ^-2，容量维持率为81.0%，库仑效率始终保持在约 99.0%。在电流密度为 20.0 mA cm ^-2的恒电流循环期间，0.01 M LAQD(S)的体积电容达到144.8 mAh L ^-1，这进一步表明G单元木质素具有良好的电化学活性、更高的电容量以及S单元木质素更强的化学稳定性。

Lignin anthraquinone derivatives have considerable energy storage potential as energy storage materials for aqueous redox flow batteries. This electrolyte is very suitable for the energy storage industry's demand for low cost, sustainability, and low toxicity. Compared with developed organic electrolytes, LAQDs have better energy storage performance. On the one hand, it can overcome the problem of poor chemical stability of anthraquinone, and on the other hand, it can improve the electrochemical activity of lignin based electrolyte materials. We have a hopeful attitude towards whether this non-metallic aqueous redox flow battery electrolyte material can truly be applied in the future, and achieve a significant cost reduction on the basis of further improving electrochemical performance.

[1] G. Milczarek, O. Inganäs, Renewable cathode materials from Biopolymer/Conjugated polymer interpenetrating networks, Science, 335 (6075) (2012), pp. 1468-1471, 10.1126/science.1215159.

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