CNT/copper hexacyanoferrate: A superior Faradic electrode for ammonium ion removal with stable performance and high capacity

The discharge of ammonium ion (NH4+) not only seriously threatens the ecological safety of the environment, but also causes the waste of valuable resources. Capacitive deionization (CDI) is an environmentally friendly and efficient NH4+ ion treatment and recovery technology. Herein, the NH4+-rich copper hexacyanoferrate (N-CuHCF) and carbon nanotube (CNT) complex (CNT/N-CuHCF) was formed by a simple and fast co-precipitation-ion exchange method, and used as the Faradic electrode to store NH4+ in the hybrid capacitive deionization (HCDI) system. Intriguingly, the etching efficiency of the ion exchange process produces the interconnected nanonetwork structure, endows the prepared material with larger surface area and abundant redox active sites, guarantees high pseudocapacitance contribution and reversible NH4+ intercalation/deintercalation, thus enabling an excellent ammonium ion removal performance. The CNT/N-CuHCF demonstrates a superior specific adsorption capacity (SAC) of 120.2 mg g−1, a low specific energy consumption (126.1 kJ molNH4Cl−1), and remarkable cycling stability. In addition, compared with Na+ and Mg2+, CNT/N-CuHCF has better affinity and selectivity for NH4+. The NH4+ intercalation mechanism and structure stabilization mechanism of N-CuHCF were revealed through a series of ex situ characterizations. This work provides some new illuminating insights for the design of high-performance ammonium ion removal CDI electrodes, and lays a foundation for the recovery of ammonium resources.