Novel flow-electrode capacitive deionization with sodium-manganese oxide electrodes for enhancing desalination: Characterization, performance, and mechanism

Abstract

Flow-electrode capacitive deionization with cation intercalation deionization (FCDI-CID) enables continuous water desalination. Traditional carbon-based electrodes used in these systems often face limitations because of limited specific surface areas and insufficient electrical conductivities, reducing desalination efficiency. To address these issues, we incorporated sodium‑manganese oxide (NMO) electrodes, commonly used in sodium-ion batteries, into an FCDI-CID system (FCDI-CID-NMO). The rheological and electrochemical properties of the NMO electrodes were assessed and compared with those of conventional carbon-based electrodes (activated carbon [AC] and carbon black [CB]). The NMO electrodes exhibited superior dispersion, suspension stability, and a specific capacitance of 6.92 F·g⁻¹ than carbon-based electrodes. Additionally, it showed lower ohmic (3.53 Ω) and contact (50.47 Ω) resistances. Despite NMO's specific capacitance being inferior to that of AC (62.43 F g⁻¹), its Ohmic and contact resistances are significantly lower (3.84 Ω and 61.34 Ω, respectively). The desalination efficiency of CB and NMO improved by 10 %–35 % relative to AC, with NMO achieving a 10 %–20 % increase in energy-normalized removal of salt and charging efficiency, along with 20 %–30 % improvement in salt concentration efficiency, without compromising performance. The FCDI-CID-NMO system demonstrated stable desalination performance over a 48 h operational period. Mechanistic analysis revealed that CB possessed high electrical conductivity, beneficial for desalination, the NMO electrodes effectively captured Na⁺ through pseudocapacitive interactions, thereby exhibiting higher desalination efficiency than AC. Overall, this research addressed the issues of low desalination efficiency and clogging commonly associated with conventional carbon-based electrodes by introducing NMO electrodes as flow electrodes in an FCDI–CID system.