Maximizing the Electrochemical Performance of Supercapacitors by Using Seawater Electrolyte Instead of Acidic/Lithium-Based Electrolytes

Abstract

The electrochemical performance of supercapacitors (SCs) was evaluated by using different inorganic electrolytes: seawater, acidic-electrolyte (polyvinyl-alcohol (PVA)/H₃PO₄) and Li-based electrolyte (synthesized from expired LIB-electrodes and named as ERB-electrolyte). SCs made only with graphene electrodes and PVA/H₃PO₄ electrolyte exhibited a capacitance/energy-density of 421.4 F g⁻¹/58.5 W·h kg⁻¹. After adding the G/SiO₂/MgO (GSM) and G/SiO₂/MgO-MnO₂ (GSMM) nanocomposites to the SC electrodes, the capacitance increased by 36% and 69%, respectively. To develop an environmentally friendly SC, we substituted the acidic electrolyte with seawater or ERB electrolyte and compared their electrochemical performance. SCs made GSM and GSMM composites (seawater was the electrolyte) showed specific capacitances/energy-densities of 679.7 F g⁻¹/94.4 W·h kg⁻¹ and 852.3 F g⁻¹/118.5 W·h kg⁻¹, respectively, which were ∼20% higher compared with these for SCs made with acidic-electrolyte. SCs made with ERB-electrolyte and GSMM composite had a lower capacitance (683.3 F g⁻¹) in comparison with SCs made with GSMM/acidic electrolyte (710.4 F g⁻¹). Electrochemical-impedance- spectroscopy (EIS) analysis demonstrated that the lowest charge-transfer-resistance and series-resistance were obtained in SCs made with seawater-electrolyte, therefore, those SC had the most efficient ion storage/diffusion. Finally, UV-Vis/Raman/XPS studies revealed the presence of oxygen-vacancies, Mg²⁺/Mg⁰, Mn⁴⁺/Mn³⁺, and Si⁴⁺/Si³⁺/Si²⁺ species on the SC electrodes (active-redox-centers to store charge).