Pseudocapacitance of Mesoporous Spinel-Type MCo₂O₄ (M = Co, Zn, and Ni) Rods Fabricated by a Facile Solvothermal Route

Abstract

We present the structural properties and electrochemical capacitance of mesoporous MCo₂O₄ (M = Co, Zn, and Ni) rods synthesized by a facile solvothermal route without necessity to use templates. The Brunauer-Emmett-Teller specific surface areas of these mesoporous rods are found to be about 24, 54, and 62 m² g^-1 with major pore diameters of about 31, 15, and 9 nm for MCo₂O₄, M = Co, Zn, and Ni, respectively. X-ray photoelectron spectroscopy and X-ray diffraction studies reveal the phase purity of the samples with a predominant spinel-type crystal structure. The spinel crystal structure with lattice parameters of 8.118, 8.106, and 8.125 Å is obtained for MCo₂O₄, M = Co, Zn, and Ni, respectively. The transmission electron microscopy study reveals that the mesoporous rods are built by self-assembled aggregates of nanoparticles which are well-interconnected to form stable mesoporous rods. The electrochemical capacitor performance was investigated by means of cyclic voltammetry, galvanostatic charge/discharge cycling, and impedance spectroscopy in a three-electrode configuration. As a result, the spinel-type MCo₂O₄ rods exhibit high specific capacitances of 1846 F g^-1 (CoCo₂O₄), 1983 F g^-1 (ZnCo₂O₄), and 2118 F g^-1 (NiCo₂O₄) at a scan rate of 2 mV/s. Furthermore, the mesoporous spinel-type metal oxides show desirable stability in alkaline electrolyte during long-term cycling with excellent cycling efficiency.