Team:Hong Kong-CUHK/Project/Mixing Entropy Battery

Mixing-entropy battery

1.     Introduction of mechanism

In the nature, the water cycle is driven by solar energy. One part of the water cycle is that solar energy evaporates seawater to become freshwater through inland precipitation. From another point of view, during evaporation, the entropy of different ions in the sea, mainly sodium, chloride and potassium, decreases as ion concentration is elevating. It is a common phenomenon that when a concentrated solution of a certain solute, such as sodium chloride, is diluted, the entropy of the solvent increases. Thus the ocean is actually a gigantic energy reservoir. Its energy is transformed from solar energy and stored as salinity potential. When seawater mixes with freshwater from river, massive amount of energy is released.

Fig.1  Cycles of mixing-entropy battery.

Recently, some institutes are seeking efficient methods for extracting energy when mixing seawater and freshwater. Mixing-entropy battery is thus designed to convert salinity potential into electricity¹(Fig. 1). One pair of electrodes can specifically bind sodium ions or chloride ions, thus separating the charge when they are immersed in high salinity solution, while decreasing the sodium chloride concentration in the solution. During this process, electrons in the cathode flow across electrical wires and reach the anode when there is a complete electric circuit, since the immobile negative charges (chloride) accumulating in the cathode repels electrons, while immobile positive charges (sodium) accumulating in the anode attract electrons. When the electrodes achieve equilibrium with the solution, there is no electric current anymore. The next step is to immerse full-loaded electrodes in freshwater. Due to the salinity difference, sodium ions and chloride ions are released from the electrodes and those exceeded electrons in anode flow back to cathode to resume the original state. When the equilibrium is achieved, the electrodes are re-immersed to high salinity water to start another cycle¹.

However this method only has high efficiency near estuaries. To overcome this obstacle, we redesigned this method using halorhodopsin-transformed E. coli. In our project, we fabricated the pair of electrodes according to L. Mantia’s method¹ and withdrew currents from the battery. This is the first attempt to generate electricity from light energy by microorganism system in iGEM competition.

References

1.        La Mantia, F. et al.Batteries for Efficient Energy Extraction from a Water Salinity Difference. Nanoletters 0-3(2011).at <http://pubs.acs.org/doi/abs/10.1021/nl200500s>

2.        Guo, W. et al. Energy Harvesting with Single-Ion-Selective Nanopores: A Concentration-Gradient-Driven Nanofluidic Power Source. Advanced Functional Materials 20, 1339-1344(2010).