University of Pretoria,
Keywords: mixed hydroxides, NiCo-MnO2, nitrates, electrochemical performance, supercapattery, energy density
Summary:Supercapacitors (SCs) are classified as emerging high power-delivery storage devices which are known to be able to hold much more electrical charge than standard parallel-plate capacitors . They are also easily able to discharge at a very fast rate in delivering the necessary high power as compared to batteries and thus can complement batteries in many domestic, commercial and industrial applications [1-2]. SCs also work in very low temperatures; an operating condition that prevents the adoption of many types of electrochemical batteries [3-4]. In this work, NiCo /NiCoMn-mixed hydroxides and ternary NiCo-MnO2 electrode materials were successfully synthesised by a force-driven hydrolysis of hydrated nickel, cobalt and manganese nitrate salts at 40 °C for 2 h with an additional annealing step adopted in producing the NiCo-MnO2 sample. The morphological, structural, compositional and textural characterization of the samples were obtained using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray powder diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and N2 physisorption respectively. The initial sample characterization confirmed bigger material agglomeration of the as-prepared mixed hydroxides compared to the NiCo-MnO2 sample which had small stacked sheet-like and porous morphology. Further sample analysis also confirmed a high degree of crystallinity in both the mixed hydroxides and NiCo-MnO2 samples with the elemental constituents existing in different oxidation states. One of the mixed hydroxides sample namely, NiCo(OH)2 exhibited a specific surface area (SSA) of approximately 3.40 m2 g-1 as compared to the ternary NiCo-MnO2 material which exhibited a higher SSA of 153.94 m2 g-1. The ternary NiCo-MnO2 electrode exhibited the highest specific capacity of 132.1 mAh g-1, compared to NiCo(OH)2 and NiCoMn-triple hydroxide (NiCoMn-TH) electrodes which exhibited a specific capacities of 110.3 and 64.36 mAh g-1 respectively at a current density of 0.5 A g-1. In addition, the ternary NiCo-MnO2 electrode exhibited a better cycling stability compared to NiCo(OH)2 electrode. Notably, an assembled hybrid asymmetric supercapattery with carbonized iron cations (C-FP) selected as negative electrode (NiCo-MnO2//C-FP), displayed a specific capacitance of 130.67 F g-1, high energy and power densities of 48.83 Wh kg-1 and 896.88 W kg-1 at 1 A g-1 respectively. An excellent cycling stability with a coulombic efficiency of 99.98% and capacitance retention of 96.78 % was recorded for up to 10,000 cycles within an operating voltage of 1.5 V, at a 3 A g-1 current density.