Phase- and Crystallinity-Tailorable MnO2 as an Electrode for Highly Efficient Hybrid Capacitive Deionization (HCDI)

Hybrid capacitive deionization (HCDI) is an emerging and promising technology for water desalination and has been extensively explored in recent years. Designing a structure-tailorable electrode material has been proved to be a valid strategy for achieving a higher salt adsorption capacity (SAC). In this study, MnO2 materials with tailorable phase compositions and crystallinities were prepared hydrothermally and then evaluated as electrodes for removal of ions from a NaCl solution in a membrane-free HCDI cell. MnO2 electrode materials tested in the HCDI system include poorly crystalline δ-MnO2 along with abundant amorphous phases (MnO2-1h); crystalline δ-MnO2 with plentiful amorphous MnO2 (MnO2-2h); MnO2 mixtures of α-, δ-, and amorphous MnO2 (MnO2-5h); and α-MnO2 nanowires with minor amorphous MnO2 (MnO2-12h). Our results revealed that the phase composition and the crystallinity of MnO2 materials govern their specific capacitances and thus the SAC values. When the cell voltage is 1.2 V, the lamellar-structured poorly crystalline MnO2-1h electrode with the lowest crystallinity demonstrates the highest SACs of 13.84 mg g–1 in 100 mg L–1 NaCl (1.71 mM) and 21.32 mg g–1 in 500 mg L–1 NaCl (8.56 mM) solutions, respectively. The desalination efficiencies of the MnO2−1h electrode are remarkable and much greater than other MnO2-based electrodes under similar conditions (e.g., NaCl concentrations, cell voltage, etc.). This study sheds light on the significance of understanding the fundamentals of both the phase composition and crystallinity in defining the desalination performance of MnO2 electrodes.