Exploring extended π-electron delocalization in organic electrodes for electrochemical energy storages Qi Chen 1 , Yoshiaki Shuku 1 , Rie Suizu 1 , Michio M. Matsushita 1 , Zhongyue Zhang 1,3 , Shunji Bandow 4 , Kunio Awaga 1,2 1 Department of Chemistry, Nagoya University, Japan, 2 Integrated Research Consortium on Chemical Sciences, Nagoya University, Japan, 3 International Research Organization for Advanced Science and Technology, Kumamoto University, Japan, 4 Department of Applied Chemistry, Meijo University, Japan Redox-active organic materials, known for their resource abundance, tunable functionalization, and adaptability for use in a variety of metal-ion batteries, have emerged as a potential substitution for conventional inorganic electrode materials in the next generation of energy storage systems. Although organic electrodes exhibit excellent performance in specific capacity, reaction kinetics, and cycling stability, their charge storage mechanisms are not well understood, especially with regard to multi-electron Faraday reactions. Herein, we employed electron spin resonance (ESR) spectroscopy and SQUID magnetometry to monitor the evaluation of species and concentration of paramagnetic intermediates step by step for two organic electrode materials. We have made the first observation that the π-electron state presents in the electrochemical intermediates for both materials: 2D fully conjugated Cu-THQ (THQ=Tetrahydroxy-1,4-benzoquinone) metal-organic framework and phenazine analogue of triptycene (Trip-Phz)/single-wall carbon nanotube (SWCNT) composites. Cu-THQ exhibits an ultra-high capacity of 390 mAh/g in Cu-THQ-Li battery, corresponding to 150% of the theoretical capacity based on two-electron reduction. This extra capacity is attributed to the graphite-like charge storage mechanism, where the π-electron band accepts/donates delocalized π electrons during redox. The delocalized π electrons in the fully conjugated MOF are identified by a temperature-independent ESR line and a temperature-independent paramagnetic term in magnetic susceptibility. In the case of Trip-Phz/SWCNT composites, specific capacitances of 876 and 480 F/g based on the mass of Trip-Phz were achieved for the Trip-Phz/SWCNT (1/5) and (5/1) composites in an aqueous supercapacitor, respectively, corresponding to one- electron and half-electron reduction per phenazine arm. After reduction, only the (5/1) sample exhibits a significant radical signal centered at a g-value of 2.0023 and with a narrow Dysonian line shape, indicating that the radicals are not located at the Trip-Phz molecule but at SWCNT, where the delocalized π electrons (also called free electrons) are located. This is supported by a large temperature-independent Pauli paramagnetism of 5.6×10 -3 emu/mol. The electrochemically induced charge transfer between the triangular Trip-Phz molecule and SWCNT is mainly due to their geometric matching and π-π interaction. It is noteworthy that the extended π-electron delocalization plays a crucial role in the fast kinetics of both composites, as they exhibit a pseudocapacitive behavior.
References 1. Q. Chen, O. Adeniran, Z. F. Liu, Z. Y. Zhang, and K. Awaga. Graphite-like Charge Storage Mechanism in a 2D π–d Conjugated Metal–Organic Framework Revealed by Stepwise Magnetic Monitoring. J. Am. Chem. Soc. 2023, 145, 2, 1062–1071. 2. R. Ushiroguchi, Y. Shuku, R. Suizu, and K. Awaga. Variable Host–Guest Charge-Transfer Interactions in 1D Channels Formed in a Molecule-Based Honeycomb Lattice of Phenazine Analogue of Triptycene. Cryst. Growth Des. 2020, 20, 12, 7593–7597.
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