image: (a) Schematic illustration of the synthetic route for s-HATBAQ and a-HATBAQ, (b) FT-IR spectra of a-HATBAQ and s-HATBAQ, (c) Mass spectra of a-HATBAQ and s-HATBAQ, (d) 13C NMR spectra of a-HATBAQ and s-HATBAQ, (e) XRD patterns of a-HATBAQ and s-HATBAQ, (f) TGA curves of a-HATBAQ and s-HATBAQ
Credit: ©Science China Press
This study is led by Dr. Mingjun Hu (School of Materials Science and Engineering, Beihang University), Prof. Jun Luo (ShenSi Lab, Shenzhen Institute for Advanced Study, University of Electronic Science and Technology of China, Longhua, Shenzhen 518110, China) and Prof. Jun Yang (Beijing Institute of Nanoenergy & Nanosystems, Chinese Academy of Sciences, Beijing 101400, China).
Aqueous proton batteries have shown enormous potential in high-power and ultra-low temperature energy storage devices, but the scarcity of high-capacity and low-potential anodic materials still largely limits its energy density and thus the competitiveness in aqueous batteries. Recently organic electrode materials have shown enormous potential as high-capacity anode of proton battery due to its designable molecular structure and accessible abundant active sites. However, its capacity was still limited by low ultilization of active sites due to its low conductivity and complicated molecular structure effects. To further tapping the potential of organic electrode materials in energy storage, exploring the effect of precise molecular structure on the charge storage is essentially important for further improving electrode performance.
As is well known, material structures determine their properties and functions. For organic electrodes, their specific capacities were mainly affected by the density and ultilization of active sites. High specific capacity requires high active site density, but high-active site density usually incurs high steric hinderance and thus low ultilization of active sites. The contradiction poses an obvious challenge to the improvement of specific capacity of organic electrode. Therefore, there is a strong need to investigate the influence of steric hinderance of organic electrodes on their specific capacity. The electroactive stereoisomers with slight steric hinderance difference are the best model molecules to study the relationship between steric hinderance and specific capacity. It is interesting and meaningful to investigate and compare the synthesis and electrochemical properties of stereoisomers for guiding the molecular structure design and improving electrode performance. Up to now, there is no report that investigates the effect of precise molecular structure (steric hindrance) of stereoisomers on electrochemical properties.
In this work, the researchers reported the controllable synthesis of two isomers, symmetric hexaazatribenzanthraquinone (s-HATBAQ) and asymmetric one (a-HATBAQ), by adjusting the solvent, reaction temperature and catalysts, and observed the significantly different electrochemical properties of the two isomers. It is found that s-HATBAQ had 1.5 times higher specific capacity and better rate capability than a-HATBAQ in acid electrolytes, mainly ascribed to the weaker steric hindrance to the insertion of protons around active sites of s-HATBAQ, which has been confirmed by both experimental and DFT calculation results. To maximize the utilization of active sites, the researchers in-situ grow s-HATBAQ on reduced graphene oxide (rGO) for further improving the conductivity. As a result, s-HATBAQ-50% rGO composites delivered a record high specific capacity of 405 mAh g-1 at 0.1 A g-1 in 5 M H2SO4 electrolyte.
The researchers further developed an anti-freezing electrolyte (5 M H2SO4+0.5 M Mn(BF4)2), which showed an impressive ionic conductivity of 213.8 mS cm-1 at -80 ℃. When the electrolyte was applied to a swagelok cell with MnO2@CF-KOH as the cathode and s-HATBAQ-50% rGO as the anode, the proton full battery exhibited an excellent rate performance (210 mAh g-1 at 2 A g-1 and 111 mAh g-1 at 80 A g-1) and outstanding cycling stability with a capacity retention of 96 % after 26000 cycles at 5 A g-1. At -80 ℃, The battery showed an initial capacity of 91.9 mAh g-1 at 0.1 A g-1 and maintained over 1000 cycles with ~92% capacity retention at 0.5 A g-1. This work not only inspires the study of high-performance wide-temperature-range aqueous proton batteries but also highlights molecule-level precise structure regulation for improving the performance of organic electrode and advancing the development of organic energy storage.
Journal
National Science Review