“Unlocking the Potential of Polymorphs of MnO ₂ Nanomaterials for Intercalative Symmetrical Capacitive Deionization Devices”

The structural diversity of MnO 2 polymorphs plays a vital role in electrochemical performance, making them promising electrode materials for capacitive deionization (CDI), as they can undergo reversible ion intercalation in addition to surface adsorption of ions. Herein, insights into the performance of α, β, and γ polymorphs of MnO 2 for CDI applications in symmetrical geometry configuration, highlighting the impact of nanoscale structural and morphological attributes on desalination efficiencies, were investigated. The synthesized materials exhibit different morphologies, among others, and γ-MnO 2 with urchin-like morphology achieves a higher specific capacitance of 746.98 F/g at 0.5 A/g due to its crystalline and amorphous phase in nature with both edge- and corner-sharing [MnO 6 ] octahedra, outperforming α-MnO 2 (224 F/g) and β-MnO 2 (367.37 F/g). Further, γ-MnO 2 displays a superior salt adsorption capacity (SAC) of 16.94 mg/g compared to α-MnO 2 (9.50 mg/g) and β-MnO 2 (11.70 mg/g), with less energy consumption (0.5 J m/g), placing an advantage toward industrial-level desalination scale. The SAC performance is higher than the value reported for MnO 2 symmetrical-geometry configuration devices. As evidenced from XPS analysis, the higher SAC performance of γ-MnO 2 could be due to a disordered structure, with surface oxygen vacancies providing numerous active sites with both crystalline and amorphous phases, leading to improved electrochemical redox kinetics. These findings underscore the critical role of MnO 2 polymorphs in advancing sustainable and efficient intercalative CDI technologies.