Figure S1 shows the configuration of the RED stack. The resistance of the electrodes, its corresponding compartments and the last shielding membrane was measured and used a correction for all measurements. Figure S1: Configuration of the RED stack.

a b s t r a c tThe purpose of reverse electrodialysis (RED) is to produce electricity upon the mixing of two solutions. We studied the power density (W/m 2 ) and the energy efficiency (the amount of energy produced from specified volumes of river and sea water in relation to the thermodynamic maximum). With a stack of 50 cells (of 10 cm × 10 cm), a power density of 0.93 W/m 2 was obtained with artificial river water (1 g NaCl/L) and artificial sea water (30 g NaCl/L), which is the highest practical value reported for RED. This value is achieved due to an optimized cell design using a systematic measurement protocol.The main factor in the power density is the cell resistance. With the used membranes (Fumasep FAD and FKD) and a spacer thickness of 200 m, a cell resistance of 0.345 is measured under RED conditions. This is about one and a half times the value as expected from the contribution of the individual components. This high value is probably caused by the shielding effect of the spacers. The largest contribution to this resistance (about 45%) is from the river water compartment.The hydrodynamic loss resulted in a maximal net power density of about 0.8 W/m 2 at a flow rate of 400 mL/min. At this optimum the consumed power for pumping is 25% of the total generated energy. The majority of the pump power is lost in the manifolds.Multistage experiments were performed at maximal power conditions (a current density of about −30 A/m 2 and at a flow rate of 300 mL/min). At these conditions the theoretical energy efficiency is maximal 50%. In practice however, the energy efficiency of a single stack is 9%. The effluent concentrations of the so operated stack are used for a second experiment and so on, simulating a multistage operation. With 3 stages a cumulative energy efficiency of 18% is achieved. A fourth stage did not increase this value. The power density of the 3 stages together was 50% of the power density of the first stage, indicating that energy efficiency and power density are counteracting.Further increase of power density and energy efficiency can be obtained with a better spacer and manifold design. A more open spacer is beneficial for RED in two ways: less shielding and lower pressure drop. Less shielding decreases the electrical resistance of the cell. A lower pressure drop permits the use of thinner water compartments, resulting again in a decreased electrical resistance of the cell and an improvement of the power density.

The entropy increase of mixing two solutions of different salt concentrations can be harnessed to generate electrical energy. Worldwide, the potential of this resource, the controlled mixing of river and seawater, is enormous, but existing conversion technologies are still complex and expensive. Here we present a small-scale device that directly generates electrical power from the sequential flow of fresh and saline water, without the need for auxiliary processes or converters. The device consists of a sandwich of porous "supercapacitor" electrodes, ion-exchange membranes, and a spacer and can be further miniaturized or scaled-out. Our results demonstrate that alternating the flow of saline and fresh water through a capacitive cell allows direct autogeneration of voltage and current and consequently leads to power generation. Theoretical calculations aid in providing directions for further optimization of the properties of membranes and electrodes.

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