Tailorable Phase and Structural MnO2 as Electrode for Highly Efficient Hybrid Capacitive Desalination (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 regulatory microstructures were prepared hydrothermally and then evaluated as electrodes for removal of ions from NaCl solution in a membrane-free HCDI cell. MnO₂ electrode materials tested in HCDI system include poorly crystalline δ-MnO₂ with a lot of amorphous phases (MnO₂-1h), crystalline δ-MnO₂ with amorphous MnO₂ (MnO₂-2h), MnO₂ mixtures of α-, δ-, and amorphous MnO₂ (MnO₂-5h), and α-MnO₂ nanowire with minor amorphous MnO₂ (MnO₂-12h). It is notable that the phase composition along with the microstructures of MnO₂ materials rather than their surface areas determines the SAC values. When the cell voltage is 1.2 V, the lamellar structured MnO₂-1h electrode demonstrates the highest SACs of 13.84 mg g^-1 in 100 mg L^-1 NaCl, and 21.32 mg g^-1 in 500 mg L^-1 NaCl solution, respectively. The desalination efficiencies are remarkable and far 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 fundamental of both phase composition and microstructure in governing the desalination performance of MnO₂ electrodes.