Yang Yang,Yan Xiang,Yuewen Yang,Xian Xie,Faheem Mushtaq,Ruiqin Zhang,Walid A. Daoud
Among multivalent redox flow batteries, the Zn-based redox flow battery (RFB) has the advantages of high energy density, nontoxicity, and low cost. However, the severe dendrite issue hinders its deployment. Therefore, the Sn-based RFB is reported as a prospective alternative, particularly in alkaline systems, due to its lower redox potential and four electrons transfer in redox reactions. However, the continuous growth of Sn during charge forms large Sn bulk, which increases the risk of “dead Sn” formation. By combining experiments and theoretical calculations, urea is proposed as an electrolyte additive to regulate Sn deposition on graphite felt. This is achieved through the enhanced adsorption and robust reaction of Sn(OH) 6 2− with graphite felt. Consequently, a more uniform morphology ensues, reducing “dead Sn” formation and thus improving the average discharge voltage and areal capacity. Moreover, the Columbic efficiency becomes more stable, extending the cycle life to over 420 h (>200 cycles) without deep discharge during cycling. This study demonstrates the role of urea in achieving high-performance Sn-I flow battery with long cycle life, paving the way for further development of metal-based hybrid RFBs.
Yiyang Cui,Kai Zheng,Ruoqing Sun,Zhiwei Yuan,Dengfeng Guo,Juan Xu,Xiaofei Yu,Jian Zang,Jianyu Cao
Aqueous phenazine redox flow batteries hold great potential in large-scale energy storage. The chemical/electrochemical properties of phenazine compounds are greatly influenced by their functional groups and electrode catalytic materials. In this study, we present a facile synthesis of the multi-substituted phenazine derivative containing hydroxyl, methyl and carboxyl groups, namely, 7(8)-hydroxy-8,9(6,7)-dimethylphenazine-2-carboxylic acid (HDMPC), which serves as an anolyte material and exhibit sufficient negative redox potential, quasi-reversible redox kinetics, and high aqueous solubility in LiOH aqueous electrolyte with moderate alkalinity. Pairing with lithium ferrocyanide catholyte in 1 M LiOH (pH∼12.3), with the promotion of a carbon-supported CuO electrocatalyst, the HDMPC anolyte delivers a high open circuit voltage of ∼1.16 V, average energy efficiency of 80.23 % and a low temporal fade rate of ∼0.0086 % per cycle (equivalent to ∼0.38 % per day) for over 200 cycles at 20 mA cm −2 . The post-analysis further reveals that tautomerization of the charged HDMPC rather than crossover, is responsible for most of the fade of cycling capacity during electrochemical cycling.
Mengna Qin,Guibao Wu,Kai Zheng,Xiaofei Yu,Juan Xu,Jianyu Cao
The pH-neutral aqueous redox flow battery (ARFB) is one of the most attractive flow batteries due to its non-corrosiveness, low-cost, and wider electrochemical stability window compared to those with acidic or alkaline electrolytes. However, there are few families of organic redox-active molecules available as catholyte materials for pH-neutral ARFBs. Herein, through incorporation of quaternary ammonium moiety into the redox-active phenoxazine nucleus, an organic catholyte material, N , N , N -trimethyl-2-(10H-phenoxazin-10-yl) ethan-1-aminium chloride (NEt-POZ) is achieved for pH-neutral ARFBs. The NEt-POZ has a high redox potential of 0.79 V versus SHE and rapid redox kinetics (0.23 cm s −1 ) as well as high water-solubility (∼2.6 M in H 2 O). Paired with a methyl viologen (MV) anolyte in the pH-neutral aqueous electrolyte, a viologen-phenoxazine ARFB full cell is assembled, which shows an open circuit voltage of ∼1.23 V. However, the results of long-term cycling experiments indicate low Coulomb efficiency and rapid declining capacity of the NEt-POZ catholyte. The mechanism analysis unveils that the unstable doubly oxidized tri-cations (NEt-POZ 3+ ) formed via the disproportionation of radical di-cations (NEt-POZ 2+ ) in solution readily react with chloride anions to form poly-chlorinated derivatives, which precipitate from the catholyte due to their poor water solubility, leading to a rapid decrease in capacity. When there is no chloride present in the electrolyte solution, highly reactive NEt-POZ 3+ cations can interact with other counter-anions (such as sulfate and carbonate ions) and even water to form complex adducts.
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