Charge storage mechanisms of electrospun Mn3O4 nanofibres for high-performance supercapacitors

Abstract

Mixed oxidation states of manganese oxides are widely used as the electrodes in supercapacitors due to their high theoretical pseudocapacitances. However, their charge storage mechanisms are not yet fully understood. In this work, the charge storage mechanism of Mn₃O₄ or Mn²⁺(Mn³⁺)₂O₄ nanofibres was investigated using a synchrotron-based X-ray absorption spectroscopy (XAS) technique and an in situ electrochemical quartz crystal microbalance (EQCM). The average oxidation state of the Mn in the as-synthesized Mn₃O₄ is +2.67. After the first charge, the average oxidation states of Mn at the positive and negative electrodes are +2.61 and +2.38, respectively. The significant change in the oxidation state of Mn at the negative electrode is due to phase transformation of Mn₃O₄ to Na_δMnO_x·nH₂O. Meanwhile, the charge storage mechanism at the positive electrode mainly involves the adsorption of counter ions or solvated SO₄²⁻. After the first discharge, the calculated Mn average oxidation numbers are +2.51 and +2.53 at the positive and negative electrodes, respectively. At the negative electrode, the solvated Na⁺ is desorbed from the electrode surface. At the same time, the solvated SO₄²⁻ is desorbed from the positive electrode. The mass change of solvated Na⁺ during charging/discharging is ca. 80 ng per cm² of the Mn₃O₄ electrode. A symmetric supercapacitor constructed from Mn₃O₄ nanofibres in 0.5 M Na₂SO₄ provides a working potential of 1.8 V, a specific energy of 37.4 W h kg⁻¹ and a maximum specific power of 11.1 kW kg⁻¹ with 98% capacity retention over 4500 cycles. The understanding of the charge storage mechanism of the mixed oxidation states of Mn²⁺(Mn³⁺)₂O₄ presented in this work could lead to further development of metal oxide-based pseudocapacitors.