Molten Salt Synthesis and Electrochemical Evaluation of Na/Ag-Containing MnxOy Composites for Pseudocapacitor Applications

Different composites of manganese oxides (MnxOy) containing sodium (Na) and silver (Ag) were synthesized by the molten salt method with various MnSO4·H2O/NaNO3 (M/N) molar ratios (between 0.3 and 1), and different AgNO3 and NaOH amounts, obtaining two groups of materials: without the addition of AgNO3 (labeled as M/N) and with AgNO3 (labeled as M/N-A). As for the M/N group, the system with the lowest M/N ratio yielded the highest specific capacitance (160.5 F g−1), attributed to the formation of Mn3O4 and sodium birnessite. In the M/N-A group, the 1 M/N-0.5A system, produced with M/N ratio of 1 and addition of 0.5 g of AgNO3, exhibited the highest specific capacitance (229.1 F g−1), associated with the presence of Mn2O3, silver hollandite, and metallic Ag. This enhancement is attributed to the synergistic effects of Na+ and Ag+ ions, which improve charge transfer kinetics and electrochemical performance. It was demonstrated that decreasing the MnSO4·H2O/NaNO3 ratio in the M/N group and increasing AgNO3 content in the M/N-A group enhances the electrochemically active surface area. Galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques confirmed that the 1 M/N-0.5A system exhibited the best performance, characterized by high energy retention, stable cycling behavior, and low capacitance dispersion, indicating its strong potential as an active material for pseudocapacitor applications.