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

Abstract

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.

Ion Exchange Conversion of Na-birnessite to Mg-buserite for Enhanced Cu2+ Removal via Membrane Capacitive Deionization (MCDI)

“…The cyclic voltammetry (CV) results of these LMOs/C (i.e., birnessite/C and/or buserite/C) electrodes obtained in a 1 M NaCl solution at various scanning rates (Figure S6) exhibit leaflike-shaped voltammograms at high scanning rates (i.e., 50, and 100 mV s −1 ), indicative of less ideal capacitive behaviors compared to carbon-based electrodes. 33,34 Note that these CV curves are in part distorted with redox peaks over the potential window of 0−0.8 V (Figure S6D), which are derived from the Faradaic reactions of redox-active MnO2, confirming the reversible redox activity of these composite electrodes. 46 In addition, the MgB/C electrode showed the largest integrated area under the CV curve, implying that it has the maximal specific capacitance, and probably the best electrosorption performance among these electrodes.…”

“…23,34,44 The O 1s spectra (Figure S2A, C, E) can be differentiated as the tetravalent oxide (Mn-O-Mn), the hydrated trivalent oxide (Mn-O-H), and the residual water molecular (H-O-H). 34 This finding is in good consistency with the FTIR spectra (Figure S3), which demonstrated the surface oxygen species are attributed to surface hydroxyl groups (~ 3415 cm −1 ), water…”

“…These results are further confirmed by the BJH pore size distribution (PSD) plots (Figure S5B), in which NaB and KB exhibited a narrow PSD centered at mean pore sizes of ~ S1, Supporting Information), which are in good assistance with the above particle size distribution data and also comparable to those in the previous reports. 4,28,34 Electrochemical Performance.…”

“…The electrochemical impedance spectroscopy (EIS) results (Figure S8B) (see the top left inset in Figure S8B), confirming that MgB/C has the highest interfacial ion migration rate among these electrodes, and thereby the best capacitive performance. 34,48 Capacitive Deionization Performance.…”

Decoupling Proton and Cation Contributions to Capacitive Charge Storage in Birnessite in Aqueous Electrolytes

“…The electrochemical charge storage behavior of birnessite is superior to other manganese oxide polymorphs [1–4] . In desalination systems, birnessite has shown higher reversible salt adsorption capacities as compared to tunneled or amorphous manganese oxides [5–7] . It is hypothesized that the large interlayer spacing of birnessite (∼7 Å) enables facile (de)intercalation of cations in neutral pH aqueous electrolytes [8,9] .…”