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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.

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It is found that triplet state lifetimes and intersystem crossing yields of individual molecules embedded in a polymer host at room temperature are not constant in time. The range over which the triplet lifetime of a single molecule varies during long observation times shows a strong similarity with the distribution of lifetime values obtained during short observation times of many individual molecules dispersed in space. The similarity is an elegant manifestation of the ergodic principle of statistical physics. 07.79.Fc, Single molecule microscopy and spectroscopy give new and unique insight in the complex behavior of individual emitters on the nanometer scale [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Measurements on individual molecules have two distinct advantages. First, phenomena that are usually hidden due to ensemble averaging can be directly observed, such as spectral and rotational jumps [1][2][3][4][5][6][7][8], photon (anti-)bunching [7,[9][10][11][12][13], and discrete photobleaching. Second, the monitoring of single molecule fluorescence forms a powerful way to probe the dynamics of the local environment on a nanometer scale. Therefore, single molecule detection allows the inhomogeneity of the ensemble to be directly related to the real time dynamics of the heterogeneity of the environment. In contrast, alternative techniques to study the composition of an inhomogeneously broadened ensemble like spectral hole-burning or pump-probe spectroscopies still average over a subset of the ensemble.For the purpose of this Letter, fluorescent molecules are considered as a three-level system. Besides the repetitive transitions between the singlet ground state ͑S 0 ͒ and the lowest singlet excited state ͑S 1 ͒ giving rise to fluorescence, the molecule has a small chance to undergo intersystem crossing (ISC) from S 1 to the lowest excited triplet state ͑T 1 ͒. As long as the T 1 state remains occupied, the S 0 2 S 1 transition does not occur and the fluorescence is interrupted temporarily. After decaying to S 0 the molecule starts fluorescing again. As a result, the fluorescence photons are emitted in bunches separated by dark periods when the molecule is in T 1 . This so-called photon bunching can be investigated in two ways. First, autocorrelation of the time intervals between detected photon counts yields the typical duration of the dark periods and thus the T 1 lifetime [7]. The disadvantage of this indirect method is that it yields only a mean value for the T 1 lifetime. Second, integration of the detected fluorescence photons over time intervals shorter than the duration of the dark periods can identify the time length of each excursion to T 1 in a direct way [11][12][13]. We have used the second method to obtain time-resolved T 1 state dynamics.The first measurements on the T 1 state of individual molecules were performed at cryogenic temperatures [1,[7][8][9]11]. It was found that T 1 lifetimes and intersystem crossing rates could vary among different molecules, which was attributed to loc...

Reverse electrodialysis (RED) is a method for directly extracting electrical energy from salinity gradients, especially from sea and river water. For the commercial implementation of RED, the electrode system is a key component. In this paper, novel electrode systems for RED were compared with existing systems on safety, health, environment, technical feasibility and economics. Systems with inert DSA-type electrodes and a NaCl-HCl supporting electrolyte with the reversible Fe 2? /Fe 3? redox couple or the [Fe(CN) 6 ] 4-/Fe(CN) 6 ] 3-couple achieved the highest ranking. Improvements of the electrode system are also discussed like the use of special stable metal electrodes, graphite electrodes, other reversible redox couples, capacitive electrodes and electrolytes with carbon particles.