Electricity can be generated from the controlled mixing of salinity gradients (i.e., two waters with different salt concentrations). Capturing salinity gradient energy using existing technologies such as Pressure Retarded Osmosis and Reverse Electrodialysis are limited by membrane fouling and costs. Capacitive electrode technologies, like capacitive mixing, may be used to overcome the limitations of membrane-based technologies by taking advantage of capacitive double layer expansion (CDLE) at the electrode surface to generate power. However, due to intrinsically low charge storage capacities, researchers have begun to explore pseudo-capacitive electrode materials, such manganese oxides. The charge-storage capabilities of pseudo-capacitive materials differs from purely capacitive materials in that that they can store charge via faradaic, redox reactions between the cations and the oxide crystal structure (MnOO + Na⁺ + e^- = MnOONa). Presently, the exact charging mechanisms involved when manganese oxide electrodes harness salinity gradient energy are poorly understood. The goal of this study is to optimize the performance of a salinity gradient flow cell using MnO₂ electrodes by determining the charge-discharge mechanism of manganese oxide structures, which will facilitate rational material selection in the future. Material characterization tests, cyclic voltammetry analysis, and galvannostatic charge-discharge tests were used to characterize and compare a suite of manganese oxide powders that varied by crystal structure (amorphous, δ, γ, α, and β) and morphology (nanorod, sphere, urchin, and plate-like). Subsequently, flow cell experiments were performed with varying solution chemistries to obtain information on power production, stability, and cyclability. Preliminary flow cell experiments show that open circuit voltage window and power production for MnO₂ structures decreases in the order of amorphous, α, β, which follows the trend for BET surface area (amorphous > α > β). Additionally, electrochemical tests indicate the material is stable over multiple cycles and CV deconstruction shows that charge storage capabilities of MnO₂ may be largely attributed to surface reactions; suggesting that BET surface area may potentially be used as an indicator of performance for harnessing salinity gradient energy.