Green energy storage chemistries based on neutral aqueous electrolytes

Widespread use of fossil fuels has resulted in many environmental problems. There is an urgent need to search for reliable green energy storage technologies. The electrical energy storage systems based on neutral aqueous solutions are attractive candidates compared to the present energy storage systems utilizing flammable and expensive organic electrolytes because of their improved safety, low cost and environmental friendliness. What is more, high-rate performance can be achieved due to the high ionic conductivity of the aqueous electrolytes. However, the lower electrochemical window of water limits the possible high output voltage and high energy density. Recently, many new concepts based on neutral aqueous energy storage chemistries including aqueous rechargeable batteries and supercapacitors have exhibited high power and energy densities, excellent cycling life and high Coulombic efficiency. The present paper reviews the latest advances in these new chemistries based on neutral aqueous electrolytes, and the challenges and outlooks in this field are briefly commented on and discussed.