Cyano-Triggered Strong Anion-π Interactions: Unlocking Anion-Cation Adsorption Bifunction

Due to the inherent differences in anion and cation adsorption mechanisms, designing a single Faradaic material that functions as an efficient symmetric capacitive deionization (CDI) electrode for desalination poses a significant challenge. Herein, by employing a simple n-type azabenzene compound, hexaazatrinaphthalene (HATN), as a template, we introduce strong electron-withdrawing cyano (C≡N) groups to drastically reduce its surface electron density, constructing an electron-deficient system with a positive surface potential and a highly positive permanent quadrupole moment (Q_zz), denoted as HCNAP. This unique electronic structure triggers anion-π interactions and consequently enables anion adsorption. Meanwhile, the C═N and C≡N groups on the HCNAP skeleton maintain the function of cation adsorption. As a proof of concept, the symmetric CDI device fabricated with HCNAP exhibits outstanding desalination performance in a 500 mg L^-1 NaCl solution, achieving a salt adsorption capacity of 53.88 mg g^-1 and a removal rate of 10.1 mg g^-1 min^-1. Theoretical calculations and experimental results clearly reveal the adsorption mechanism of Na⁺ and Cl^-. Besides, HCNAP presents favorable adsorption toward three additional cations and anions. This innovative strategy establishes a new paradigm for constructing bifunctional Faradaic electrodes for highly efficient desalination.