Principles and Materials of Mixing Entropy Battery and Capacitor for Future Harvesting Salinity Gradient Energy

The salinity gradient energy, also known as “blue energy”, widely exists at the junction of river water and seawater, which is a promising renewable energy. When two solutions with different salt concentrations are mixed, they will spontaneously release energy. The blue energy is in the form of chemical energy. The available salinity gradient energy all over the world is huge, so the study of it is of great significance. In this review, various techniques and their principles for obtaining salinity gradient energy are reviewed, and the mixing entropy battery (MEB) technique is introduced in detail. The MEB can not only extract but also store electrochemical energy through a simple four-step cycle. We focus on the principles of MEB and the calculation of energy consumption during the process of mixing, as well as the anode materials, cathode materials, and natural electrolyte used in MEB. The electrolyte of the mixing entropy battery or capacitor is not limited to river and seawater; salt-lake brine, geothermal water, and urban wastewater can also be used, but further research is needed.