The major components of the global water cycle involve distillation of water from oceans by evaporation, precipitation, and collection of the resulting fresh water in rivers, lakes and aquifers, with mixing of fresh water and salt water in estuaries. Solar energy drives this cycle, creating a significant salinity difference between seawater and fresh water. The entropic energy created by the difference in water salinities is normally dissipated when river water flows into the sea. This reduction in free energy due to the mixing is estimated at about 2.2 kJ of free energy per liter of fresh water that flows into the sea (based on the osmotic pressure difference between fresh water and sea water). To date this significant and renewable energy source has not been harnessed, although several types of technologies have been proposed in order to take advantage of this renewable energy source.
Past suggestions for capturing energy from the mixing of seawater and fresh water include: pressure-retarded osmosis, based on semipermeable membranes; reverse electrodialysis, based on ion selective membranes; concentration electrochemical cells; and devices exploiting differences in vapor pressures. Low energy efficiencies, high costs, and short lifetimes (e.g., fouling) of membranes have prevented a large-scale utilization of membrane-based techniques for energy extraction. Implementation of the vapor pressure method has been difficult due to its reliance on a relatively small pressure difference, resulting in unstable power output. Concentration electrochemical cells produce energy from the concentration difference of chloride ions in two separate half cells, and thus they generally fail to capture at least half of the available energy (since free energy is stored nearly equally by both anions and cations). Moreover, the use of membranes to separate water into two compartments produces high internal resistances, and high energy losses, resulting in a theoretical energy conversion efficiency of less than about 42%.
More recently, a method for extracting energy from both cations and anions from the mixing entropy of seawater and river water was proposed using an electrochemical double layer capacitor technology, with activated carbon electrodes. Despite the promise of this technology, the use of supercapacitor electrodes has several technical challenges, resulting in an energy conversion efficiency of less than about 24%. These challenges include high sensitivity to impurities and dissolved oxygen, causing self-discharge, and the use of electrode material interface for energy storage, limiting the amount of charge that can be stored to the surfaces of the electrodes. In addition, a pre-charge voltage had to be applied to the electrodes to adsorb anions and cations at the surfaces of the electrodes.
It is against this background that a need arose to develop the batteries and related systems and methods described herein.