Na2FeP2O7 as a Novel Material for Hybrid Capacitive Deionization

Abstract

Capacitive deionization (CDI) which removes ionic species from solution by applying electric energy to carbon based electrodes is one of excellent convergence technologies combined with energy storage technology and environmental systems. In recent decades, the technology for synthesizing new carbon materials has advanced, the electrical double layer capacitance in porous structures has become better understood, and novel deionization systems have been developed. Nevertheless, achieving a higher deionization performance is required for CDI to compete with reverse osmosis for deionization. The recently introduced Hybrid CDI (HCDI), which utilizes sodium manganese oxide (Na4Mn9O18) and carbon material, successfully demonstrated its superior deionization performance over conventional CDI systems. Despite the great promise of the HCDI system, the limited availability of aqueous based intercalation materials and the lack of information regarding the characteristics of the HCDI system operations are obstacles to the advancement of HCDI. Thus, we report a new HCDI system with sodium iron pyrophosphate (Na2FeP2O7), which is a promising material for sodium ion batteries due to its high capacity, low cost and environmentally benign nature. The major results of the HCDI system with Na2FeP2O7 showed a superior maximum deionization rate performance (0.081mgg−1s−1) with a comparable deionization capacity (30.2mgg−1) compared to the previous HCDI system with Na4Mn9O18. Furthermore, the analysis of the CDI Ragone plot revealed the hybrid behavior characteristics of this HCDI system and that the high deionization capacity originated from the high capacity of Na2FeP2O7 at a low current density, whereas the fast deionization rate originated from the supercapacitor system at a high current density. Consequently, this study on a new HCDI system with Na2FeP2O7 contributes to expanding the understanding of the kinetic properties of the HCDI system with respect to its diverse operations.