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.