Deep Eutectic Solvent for Facile Synthesis of Mn 3 O 4 @N-Doped Carbon for Aqueous Multivalent-Based Supercapacitors: New Concept for Increasing Capacitance and Operating Voltage.

The capacitance and operating voltage of supercapacitors as well as their energy density have been increased by development of different materials and electrolytes. In this paper, two strategies, for the first time, were used to improve energy density: Mn 3 O 4 - and N-dual doped carbon electrode and aqueous mixture of multivalent ions as electrolyte. Mn 3 O 4 - and N-dual doped carbon was prepared by a novel and cost-effective procedure using deep eutectic solvent. XRD, XPS, and FTIR confirmed presence of Mn 3 O 4 and nitrogen, while SEM and EDS elemental mapping showed micrometer-sized nanosheets with uniform distribution of C, O, N, and Mn atoms. Charge storage behavior of carbon was tested in aqueous multivalent-based electrolytes and their mixture (Ca 2+ -Al 3+ ). Regarding both specific capacitance and workable voltage, the Ca 2+ -Al 3+ mixed electrolyte was found as the best optimal solution. The calcium addition to the Al-electrolyte allows the higher operating voltage than in the case of individual Al(NO 3 ) 3 electrolyte while the addition of Al 3+ ion in the Ca(NO 3 ) 2 electrolyte improves the multivalent-ion charge storage ability of carbon. As a result, the specific energy density of two-electrode Mn 3 O 4 @N-doped carbon//Al(NO 3 ) 2 +Ca(NO 3 ) 2 //Mn 3 O 4 @N-doped carbon supercapacitor (34 Wh kg -1 at 0.1 A g -1 ) overpasses the reported values obtained for Mn-based carbon supercapacitors using conventional aqueous electrolytes.