Review—Technologies and Materials for Water Salinity Gradient Energy Harvesting

Abstract

Obtaining energy from renewable resources is a powerful measure in eliminating the impact of greenhouse gas and environmental pollution. At present, only 20% of energy comes from renewable energy sources, such as wind, solar, ocean, biomass, and geothermal energy. Sea water is the most abundant natural resource, accounting for more than 97% of total water resources. From a practical standpoint, it is very desirable to use seawater for conversion and energy production. Salinity gradient energy is the chemical potential energy difference between two kinds of water with different salt concentrations. It has a large energy density and is considered to be the most promising ocean energy. Here, we discuss the important features of energy production technologies with salinity gradient energy. The principles and related materials and important finding of the available technologies, such as pressure-retarded osmosis, reverse electro-dialysis, capacitive mixing, concentration capacitor, and mixing entropy battery, have been described in this review. We also found that the current research mainly uses seawater to harvest salinity gradient energy. Because of its high concentration, the salinity gradient energy of salt lake brine is larger, it can be researched by further experiments.

Salinity Gradient Energy Harvesting via Na-Mn3(PO4)2-Based Electrochemical System

“…The cycle curve is drawn by using the changes of capacitor voltage and electric quantity in the four step cycle process as a measure of whether the system generates net energy. ,, The cycle curve is similar to the Carnot cycle diagram of a heat engine, as Figure d shows. The area enclosed by the voltage changes with the electric quantity is approximate to a parallelogram, E = ∮ V ·d Q , and if the discharge voltage is higher than the charging voltage without changing the electricity quantity, it will be a counterclockwise cycle, so the cycle is a productivity process.…”

“…In our study, we did not use inert porous electrodes, but instead used pseudocapacitive electrodes with a high specific capacity and a low self-discharge. This technology does not require any expensive ion exchange membranes and can also achieve good salinity gradient energy extraction results. , A hydrothermal method is used to prepare pure layered manganese phosphate hydrate powder with sodium ion intercalation, which is called Na-Mn 3 (PO 4 ) 2 ·3H 2 O. Through material testing and characterization it is found that the electrochemical performance of the Na-Mn 3 (PO 4 ) 2 ·3H 2 O that the Na + preintercalated layer has been greatly improved compared with that of Mn 3 (PO 4 ) 2 ·3H 2 O.…”

“…11,12 A hydrothermal method is used to prepare pure layered manganese phosphate hydrate powder with sodium ion intercalation, which is called Na-Mn 3 (PO 4 ) 2 •3H 2 O. Through material testing and characterization it is found that the electrochemical performance of the Na-Mn 3 (PO 4 ) 2 •3H 2 O that the Na + preintercalated layer has been greatly improved compared with that of Mn 3 (PO 4 ) 2 • 3H 2 O.…”

Current Source Strategy for Energy Injection from a CapMix Cell

“…However, none of these applications involve voltage levels comparable to those present in CapMix cells. As already indicated at the end of Section 1, the literature is mainly focused on the charging process related to mobility applications [32,33] when the storage system is based on both supercapacitor and battery systems, but nothing has been found related to CapMix cells, which typically are loaded with a resistor [34], as primary sources with polarity inversion and very low voltage for energy transfer to the grid.…”

Electrochemical Conversion of Salinity Gradient Energy via Molybdenum Disulfide Electrode

“…16 The anions and anions in the high concentration solution enter the low concentration solution under the action of osmotic pressure to form an ion flow. [17][18][19] The electrodes on both sides form a circuit by external resistance, and finally form a current, [20][21][22] but it has the same difficulties as the PRO of high cost and easy pollution. 23 The CapMix extracts salinity gradient energy through electrode potential difference, 6,7,24 which includes capacitive double-layer expansion (CDE), capacitive Donnan potential (CDP) and mixing entropy battery (MEB).…”