Electrodeposited MnO2 For Pseudocapacitive Deionization: Relating Deposition Condition and Electrode Structure to Performance

Nano-manganese oxide and reduced graphene oxide-incorporated polyacrylonitrile fiber mats as an electrode material for capacitive deionization (CDI) technology

Capacitive deionization (CDI) is a trending water desalination method during which the impurity ions in water can be removed by electrosorption. In this study, nano-manganese dioxide (MnO2) and reduced graphene oxide (rGO)-doped polyacrylonitrile (PAN) composite fibers are fabricated using an electrospinning technique. The incorporation of both MnO2 and rGO nanomaterials in the synthesized fibers was confirmed by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The electrochemical characteristics of electrode materials were examined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and constant current charge-discharge cycles (CCCDs). The specific capacitance of the PAN electrode increased with increasing MnO2 and rGO contents as well as when thermally treated at 280 °C. Thermally treated composite fibers with 17% (w/w) MnO2 and 1% (w/w) rGO (C-rGOMnPAN) were observed to have the best electrochemical performance, with a specific capacitance of 244 F g-1 at a 10 mV s-1 scan rate. The electrode system was used to study the removal of sodium chloride (NaCl), cadmium (Cd2+) and lead (Pb2+) ions. Results indicated that NaCl showed the highest electrosorption (20 472 C g-1) compared to two heavy metal salts (14 260 C g-1 for Pb2+ and 6265 C g-1 for Cd2+), which is most likely to be due to the ease of mass transfer of lighter Na+ and Cl- ions; When compared, Pb2+ ions tend to show more electrosorption on these fibers than Cd2+ ions. Also, the C-rGOMnPAN electrode system is shown to work with 95% regeneration efficiency when 100 ppm NaCl is used as the electrolyte. Hence, it is clear that the novel binder-free, electrospun C-rGOMnPAN electrodes have the potential to be used in salt removal and also for the heavy metal removal applications of water purification.

Bench-scale demonstration of CO2 capture with an electrochemically driven proton concentration process

A bench-scale demonstration of CO₂ capture from industrial flue gas with an electrochemically driven proton concentration process was demonstrated.

A Study of MnO2 with Different Crystalline Forms for Pseudocapacitive Desalination

Recent research on materials for capacitive deionization (CDI) has shown that intercalation materials have salt removal capacities (>40 mg g^-1) much higher than those of carbon materials (∼15 mg g^-1). However, little work has been done to elucidate the relationship between the microstructure of an intercalation material and its desalination performance. Herein, we report the desalination performances of various crystalline forms of MnO₂ in a hybrid CDI setup without the use of ion-exchange membranes. MnO₂ materials used in our experiments were either poorly crystalline or crystalline forms of 1D hollandite α-MnO₂, 2D birnessite δ-MnO₂, and 3D spinel λ-MnO₂. X-ray photoelectron spectroscopy performed on electrochemically cycled MnO₂ showed redox changes associated with intercalation processes in crystalline MnO₂, whereas poorly crystalline MnO₂ showed no such changes. It was further shown that surface adsorption is dominant in poorly crystalline MnO₂ and that poorly crystalline forms of α-MnO₂ and δ-MnO₂ exhibited the highest salt removal capacities of 0.17 and 0.16 mmol g^-1 (9.93 and 9.35 mg g^-1), respectively. These performances are comparable to state-of-the-art carbon materials and are remarkable considering the low surface areas (<400 m² g^-1) of MnO₂ materials.

Characterizing the Impacts of Deposition Techniques on the Performance of MnO2 Cathodes for Sodium Electrosorption in Hybrid Capacitive Deionization

Capacitive deionization (CDI) is currently limited by poor ion-selectivity and low salt adsorption capacity of porous carbon electrodes. To enhance selectivity and capacity via sodium insertion reactions, carbon aerogel electrodes were modified by depositing amorphous manganese dioxide layers via cyclic voltammetry (CV) and electroless deposition (ED). MnO₂-coated electrodes were evaluated in a hybrid capacitive deionization system to understand the relationship between oxide coating morphology, electrode capacitance, and sodium removal efficacy. Both deposition techniques increased electrode capacitance, but only ED electrodes improved desalination performance over bare aerogels. SEM imaging revealed ED deposition distributed MnO₂ throughout the aerogel, while CV deposition created a discrete crust, indicating that CV electrodes were limited by diffusion. Sodium adsorption capacity of ED electrodes increased with MnO₂ mass deposition, reaching a maximum of 0.77 mmol-Na⁺ per gram of cathode (2.29 mmol-Na⁺ g-MnO₂^-1), and peak charge efficiency of 0.95. The presence of MnO₂ also positively shifted the electrode potential window of sodium removal, reducing parasitic oxygen reduction and inverting the desalination cycle so that energy discharge coincides with salt removal (1.96 kg-NaCl kWh^-1). These results highlight the importance of deposition technique in improving desalination with MnO₂-coated electrodes.