Few-layered reduced graphene oxide (RGO) is prepared from graphite by chemical exfoliation method. In half cell, RGO electrode delivers a specific capacitance value of 41 mF cm−2 at 0.5 mA cm−2 in 1.0 M KOH. An attempt is made to improve the capacitance properties of RGO by widening the operating voltage window and improving the charge storage capability through the use of metal oxide nanoparticles as electrolyte additives. The specific capacitance of RGO increases to 62 mF cm−2 and 87 mF cm−2 when ZnO and SiOx nanoparticles were uniformly dispersed in the electrolyte, respectively, at 0.5 mA cm−2. For a power density of 1.5 mW cm−2, the symmetric supercapacitor assembled using ZnO and SiOx nanofluid electrolyte delivers an energy density of 2.6 μWh cm−2 and 3.03 μWh cm−2, respectively, which is 2.7 and 3.1 times the value of energy density obtained for symmetric supercapacitor assembled using KOH electrolyte. The nanofluid electrolytes show high stability even after 60 d and the electrochemical performance of RGO is reproducible in the aged nanofluid electrolytes. The RGO electrode shows stable cycling for the tested number of 10000 cycles in all the electrolytes.
Understanding the charge storage mechanism of MnCO3 is essential to improve its capacitance performance. Herein, we report the charge storage mechanism of MnCO3 in aqueous Na2SO4 and Mg(ClO4)2 electrolytes studied systematically by using ex‐situ X‐ray diffraction and X‐ray photoelectron spectroscopy. Theoretical specific capacitance of MnCO3.H2O in the potential window of 0.1 to 1.0 V is 806 F g−1, however, it delivers a specific capacitance value of only 95 and 66 F g−1 in 0.1 M Mg(ClO4)2 and 0.1 M Na2SO4 electrolytes, respectively, which suggests that only a limited fraction of MnCO3 is participating in the charge storage. The ex‐situ X‐ray diffraction and X‐ray photoelectron spectroscopic studies reveal that the insertion and extraction of Mg2+‐ions into/from MnCO3 accompanied by redox reaction between Mn2+ and Mn1+ during charge/discharge are reversible and do not result in phase transformation in aqueous 0.1 M Mg(ClO4)2 electrolyte. In contrast, lattice expansion and contraction by insertion and extraction of Na+‐ions into/from MnCO3 result in a gradual transformation of MnCO3 into α‐MnO2 in aqueous 0.1 M Na2SO4 electrolyte.
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