An extended S-heterocyclic organic compound with enhanced redox active sites for efficient and stable ammonium ion removal via capacitive deionization

Abstract

Innovations in electrode materials profoundly impact the performance breakthroughs of capacitive deionization (CDI) technology. Organic compounds with electrochemical activity constitute a significant family of electrode materials for CDI systems. However, many of them face challenges such as insufficient active sites and dissolution in the aqueous electrolyte during the electrosorption processes. The present study focuses on molecular engineering design of a novel sulfur heterocyclic organic compound, dibenzo[b,i]thianthrene-5,7,12,14-tetraone (DTT) through an in situ sulfidation reaction. The DTT molecule extends the π-conjugated plane of the previous 2,3-dichloro-1,4-naphthoquinone (2,3-d-1,4-NQ) through CS bonding, enhancing structural stability and significantly reducing the band gap (3.96 to 2.29 eV). Additionally, the number of redox-active groups (C=O) has doubled from 2 to 4, thereby resulting in the improved capability of pseudocapacitive interactions with NH4+. In situ Raman spectroscopy, combined with relevant ex situ characterization, has verified the redox mechanism of multiple CO in DTT. As electrode, DTT exhibits a high specific capacitance of 614.8 F g−1 (280 cycles at 1 A g−1) and maintains a capacitance retention of 91.8 % after 10,000 cycles at 10 A g−1. Additionally, DTT is applied in CDI for the NH4+ capturing, which exhibits a high NH4+ adsorption capacity of 72.1 mg g−1 and a rapid NH4+ removal rate of up to 9.2 mg g−1 min−1 (at 1.2 V) in CDI devices. The stable cyclic regeneration and favorable specific energy consumption promote the potential of DTT as a Faradaic electrode material for CDI applications.