Efficient power density amelioration strategy for capacitive salinity-gradient and pH-gradient energy harvesting systems*

Osmotic energy released from the mixing of  dilute and concentrated solutions is a new source of renewable energy. Our team developed previously an efficient capacitive concentration cell by using one single membrane and two inexpensive carbon-based capacitive electrodes. Such system converts efficiently the osmotic energy into electricity. It is safe, sustainable and environmental-friendly, but remains economically unviable today.

For power density amelioration,  we developed a promising strategy by using an additional booster system in the circuit. The booster system generates an alternative electric voltage in phase with the period of flux reversal. The net power density output is calculated as the difference between gross power density dissipated and the power density generated by the booster system. We  measured a maximal power density of 5.26 W.m^-2 (salinity difference of 0.17 mol.L^-1 and 5.13 mol.L^-1), corresponding to a 59.8% increase compared with the value 3.29 W.m^-2 without booster.

We extended this approach to processes involving solutions with pH gradients. These solutions are obtained by injecting CO₂ from plant stacks into electrolytes such as Na₂CO₃ and NaHCO₃. Under boosting strategy, we generated a higher net power density of 3.2 W.m^-2 (pH gradient of 2.4) compared with the result of 1.7 W.m^-2 reported in literature. A maximum net power density of 4 W.m^-2 is obtained for a higher pH difference. In both situations, the process approaches economical viability.