Batteries for Efficient Energy Extraction from a Water Salinity Difference

Abstract

The salinity difference between seawater and river water is a renewable source of enormous entropic energy, but extracting it efficiently as a form of useful energy remains a challenge. Here we demonstrate a device called "mixing entropy battery", which can extract and store it as useful electrochemical energy. The battery, containing a Na(2-x)Mn(5)O(10) nanorod electrode, was shown to extract energy from real seawater and river water and can be applied to a variety of salt waters. We demonstrated energy extraction efficiencies of up to 74%. Considering the flow rate of river water into oceans as the limiting factor, the renewable energy production could potentially reach 2 TW, or ∼13% of the current world energy consumption. The mixing entropy battery is simple to fabricate and could contribute significantly to renewable energy in the future.

Proof-of-Concept of a Zinc-Silver Battery for the Extraction of Energy from a Concentration Difference

“…The "capacitive mixing" (CAPMIX) technique [15] makes use of activated carbon electrodes, either directly dipped into the solution [16][17][18] or covered by perm-selective membranes [19][20][21][22][23]. Battery electrodes undergoing redox reactions are used in the "mixing entropy battery" [4,24] and in the concentration cell proposed in [25]. Since capacitors and batteries are collectively called accumulators, this family of techniques will be called "accumulator mixing" (AccMix).…”

Diffuse charge and Faradaic reactions in porous electrodes

“…When only a liquid or a gas phase contacts the electrode, porous electrodes are used to increase the surface area for charge transfer, thereby reducing the electrode overpotential (or interfacial resistance), as in fuel cell applications [5,6]. Porous electrodes are also used to increase the charge storage capacity of capacitive electrochemical cells, such as double-layer (DL) supercapacitors, which store electrons [7][8][9][10][11], capacitive deionization cells, which store ions for water desalination [2,[12][13][14][15][16][17][18][19][20][21][22][23][24], and capacitive energy-harvesting cells, which exploit the reverse process to extract energy by alternating contact of electrodes with water of low and high ionic strengths [25][26][27][28][29].…”

“…In desalination and energy-harvesting applications, parasitic Faradaic reactions can diminish the efficiency of the process and, thus, must be understood and must be quantified. In capacitive energy storage, Faradaic reactions can have a beneficial effect, boosting the energy density of the porous electrode by combining surface-based DL capacitance (storing electrostatic energy) with volumebased pseudocapacitance from Faradaic reaction products (storing chemical energy) [7,28,30,31]. These gains in energy density, however, come at the expense of losses in power density, and a general mathematical model would help to tailor this delicate balance for specific applications.…”

Modification of the surface of activated carbon electrodes for capacitive mixing energy extraction from salinity differences

“…Battery-like electrodes make use of materials that capture the ions in the solution by means of redox reactions [27,28,29,30]. The voltage of the electrodes with respect to the solution reflects the chemical potential of the ions in the solution, that changes according to the variation of their activity; this originates the voltage rise.…”