Clean and renewable salinity gradient energy can be harvested using reverse electrodialysis (RED). The electrode system is an essential part to convert ionic current into electrical current. In this study, a typical 0.10 × 0.10 m2 RED stack with a cross-flow configuration was used to test carbon-based slurry electrodes (CSEs) to replace the usual redox solutions, like hexacyanoferrate, to enhance the RED process’ sustainability, stability, and economic value. Six different slurry compositions comprising activated carbon, carbon black, and graphite powder were tested. The CSE characteristics were systematically studied by measuring viscosity, electrode compartment pressure drop, maximum current density, stability, and performance of power density and energy efficiency. Using a single membrane configuration, the CSE ran continuously for 17 days with a stable output. The application of CSEs for RED, with artificial seawater and river water, using mixing activated carbon and carbon black at a total concentration of 20 wt %, resulted in the best performance with a net power density of 0.7 W·m-2. Moreover, higher current densities up to 350 A·m-2 were tested for ED and shown to be feasible until 150 A·m-2. CSEs show promising versatility for different application modes.