Organic Faradaic Desalination Cell Based on Polyimide Symmetric Electrodes

Abstract

Capacitive deionization (CDI) is an energy-efficient and environmentally friendly deionization technology. However, the maximum desalination capacity of conventional carbon-based CDI systems is in the range of 5–25 mg g-1, which compromises the use of this technology in practical applications [1]. Faradaic deionization (FDI) is a promising electrochemical technique, based on storing the ions within the particles/layers of the electrode materials either by intercalation and/or faradic reactions. By this mechanism, FDI has expanded the deionization values above 50 mg g-1. In the last ten years, research on FDI has primarily focused on battery electrode materials, especially in inorganic intercalation compounds, typically used for energy storage [2]. However, inorganic materials have several drawbacks, such as decomposition and dissolution issues, which affect long-term performance along with safety and sustainability aspects and consequently might limit their practical deployment.As an alternative, we introduce in this work for the first time an all-polymer deionization cell in combination with a rocking-chair desalination (RCD) mechanism with the aim of boosting the deionization performance [3]. A proof-of-concept next generation technology is accomplished by using a redox-active naphthalene-polyimide (poly[N,N′-(ethane-1,2-diyl)-1,4,5,8-naphthalenetetracarboxiimide], named as PNDIE), which is a well conductive and free-standing electrode material, as anode and cathode in a symmetric FDI system. Our approach relies on the synergistic combination of smart redox polymer selection i.e. PNDIE, and the advance configuration of this material into a buckypaper electrode architecture. A detailed electrochemical characterization under various operational conditions in three electrodes standard half-cell and in two electrode coin-cell configurations was performed. The results of those experiments provide critical information to build an all-polymer rocking chair system with practical PNDIE electrodes (92.2 mgPNDIE; 9.6 cm2). This novel FDI lab-cell demonstrated a remarkable desalination capacity (155.4 mg g-1 at 0.01 A g-1) together with a high salt-removal rate and productivity (3.42 mg g-1 min-1 at 0.01 A g-1 and 62 L h-1 m-2, respectively). In addition, long-term stability experiments provided salt adsorption capacity retention values over 95 % after 100 cycles (23 days). Compared to the state-of-the-art deionization technologies (CDI, Hybrid CDI and FDI) the reported technique exhibits superior electrochemical and deionization performance. These results support the idea of using organic materials as a viable alternative for robust and sustainable “water-energy” electrochemical applications.