Salinity gradients exist when two waters have different salt concentrations (e.g., when freshwater reaches seawater at coasts or when desalination brine is mixed with less saline water). Salinity gradients contain an immense amount of potential energy, which can theoretically provide 40% of the global electrical demand. One means to harvest salinity gradient energy is to use electrochemical techniques. In an electrochemical cell, a voltage can be created by using salinity gradients to alter electrode potentials in Capacitive Mixing or creating Donnan potentials across ion-exchange membranes in Reverse Electrodialysis. The main challenge with these techniques is that they yield power densities that are too low for commercialization (Capacitive Mixing: ~0.4 W/m², Reverse Electrodialysis: ~3 W/m²). Here, we combined these two types of potentials in a single electrochemical cell, greatly increasing power densities to values that exceed 12 W/m² when mixing synthetic freshwater and seawater. We examined how cell design and operating parameters as well as electrode and membrane materials influenced power production and energy efficiency. The results suggest that this type of cell may be used to effectively harvest salinity gradient energy at coasts and produce electricity from saline desalination brines.