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

Abstract

Hydrothermal method is used to prepare layered Mn3(PO4)2·3H2O (MPO) powder with Na+ intercalation, which is written as Na-Mn3(PO4)2·3H2O (NMPO). Through material testing and characterization, it was proved that Na+ was successfully inserted into the interlayer. Through the testing of the electrochemical performance, it is found that the electrochemical performance of the Na–Mn3(PO4)2·3H2O that Na+ intercalated layer has been greatly improved compared with that of Mn3(PO4)2·3H2O. Two mixing electrochemical capacitors, MPO||AC and NMPO||AC, were constructed using MPO and NMPO electrodes and active carbon (AC) electrodes, respectively. The feasibility of their application in a salinity gradient energy extraction was successfully verified. When the electrolyte concentration is switched from 0.025 to 0.6 M for NaCl solution, the concentration response potential oscillation amplitude of the NMPO single electrode reaches 39 mV, the concentration response open circuit voltage of the MPO||AC mixing capacitor reaches 68 mV, and the peak short-circuit current reaches 6.3 A m–2. A four-step cycle assisted by constant voltage charging was conducted through an external power supply, The MPO||AC mixing capacitor obtained 1.5 J m–2 of electrical energy converted by water salinity gradient energy, the NMPO||AC mixing capacitor obtained 2.1 J m–2 of electrical energy.