Review—Technologies and Materials for Water Salinity Gradient Energy Harvesting

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.