Electrocapacitive behavior of colloidal nanocrystal assemblies of manganese ferrite in multivalent ion electrolytes

Manganese based materials have been considered to be one type of important electrode materials in energy storage and conversion devices. Two types of colloidal nanocrystal assemblies (CNAs) of MnFe₂O₄ had been investigated as the electrode materials by using aqueous electrolytes containing bivalent and trivalent metal ions. Experimental results showed that CNAs assembled by 16 nm nanocrystals (MnFe-1Sf) displayed specific capacitance of about 32.0 F g⁻¹ and 43.2 F g⁻¹ at 0.1 A g⁻¹ in aqueous solutions containing Mg²⁺ or Al³⁺ ions, respectively. However, the capacitances of CNAs composed of large nanoparticles (MnFe-2 Ac) increased respectively up to around 58.2 F g⁻¹ and 45.1 F g⁻¹ at 0.1 A g⁻¹. MnFe-1Sf delivered the specific capacitance of 30.6 F g⁻¹ and 51.7 F g⁻¹ in electrolytes of Na₂SO₄ mixed MgSO₄ and MgSO₄ mixed ZnSO₄ at 0.1 A g⁻¹ while 36.2 F g⁻¹ and 44.4 F g⁻¹ for MnFe-2 Ac, respectively. MnFe-1Sf showed a better cycle stability than MnFe-2 Ac either in MgSO₄ or in Al₂(SO₄)₃ electrolytes. The wide potential window during cyclic voltammetry brings the electrochemical diversification of the CNAs based electrodes in various electrolytes. Based on the experimental data and crystalline features of the CNAs, the structure-property relationship has been discussed and analyzed.