Highly efficient capacitive deionization of brackish water with manganese vanadate nanorod decorated reduced graphene oxide electrode

Abstract

Capacitive deionization (CDI) utilizing metal oxide-decorated carbon-based materials has emerged as a promising process to convert saline water into freshwater due to its lower energy demand and robustness. In this study, manganese vanadate (MnV₂O₆) combined with 2-dimensional reduced graphene oxide (MVO@rGO) was synthesized via a hydrothermal method. Results show that the small-sized nanorods of MVO with the aspect ratio of 4–6 can be embedded onto the rGO surface at a hydrothermal temperature of 180 °C for 18 h in the presence of sodium dodecyl sulfate as the stabilizing agent. The as-synthesized MVO@rGO provides excellent electrochemical performance with specific capacitances of 208 and 201 F g⁻¹ at 5 mV s⁻¹ and 2 A g⁻¹, respectively. The MVO@rGO exhibits a high specific surface area of 374 m² g⁻¹ with a continuous pore size distribution ranging from 2 to 10 nm, which can provide sufficient active sites and channels for ion transport. The low impedance of MVO@rGO also accelerates the ion and electron transport inside the porous structure. Moreover, both electric double layer and faradaic capacitance contribute to the electrochemical performance of MVO@rGO, resulting in superior CDI performance. The symmetric MVO@rGO electrodes show an enhanced salt electrosorption capacity (SEC) of 49.3 mg g⁻¹ at 1.4 V in the 1000 mg L⁻¹ NaCl solution and excellent long-term stability toward salt removal for 50 cycles. The CDI Ragone plot also shows that the SEC is a function of initial NaCl concentration (100–1000 mg L⁻¹) and applied voltage (0.8–1.4 V). The superior electrochemical and CDI performance makes the MVO@rGO a promising electrode material for brackish water desalination and other electrochemical energy storage applications.