Synthesis And Electrochemical Properties Of Binary Metal Oxides And Their Composites

NiCo2O4is a promising binary material which is widely used in Li-ion batteries,supercapacitors and electrocatalysts. This may be due to the intriguing advantagespossessed by this material such as low cost, relative abundance and environmentalfriendliness. More significantly, it has been reported that NiCo2O4possesses muchbetter electronic conductivity, at least two orders of magnitude higher than nickeloxide and cobalt oxide. The high electronic conductivity is of benefit to the fastelectron transfer in an electrode. Recently, there have been many reports on thesynthesis of NiCo2O4and its application in supercapacitors and fuel cells.(1) Nickel foam supported NiCo2O4nanosheet and nanocloth arrays are directlyprepared by a mild hydrothermal method and applied to methanol electro-oxidation.Several techniques including transmission electron microscopy (TEM), scanningelectron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectronspectra (XPS), Energy-dispersive spectroscopy (EDS) and nitrogen absorptionmeasurements have been employed to characterize the physicochemical properties ofas-prepared catalysts. The electrocatalytic performances are investigated by cyclicvoltammetry (CV), chronoamperometry (CA) measurements and electrochemicalimpedance spectroscopy (EIS). Impressively, NiCo2O4nanosheet and nanoclotharrays both deliver excellent electrocatalytic performance for methanolelectro-oxidation in alkaline medium, including very low onset potential (0.19V,0.18V), high current densities (111mA cm-2,134mA cm-2) and a desirableelectro-oxidation stability (90%,88%), respectively. The remarkable electrocatalytic activities of nickel foam supported NiCo2O4nanosheet and nanocloth arraysoriginate from their unique3D mesoporous architecture, which can provide adequateopen spaces and shorter ion diffusion paths to facilitate rapid ionic transportation.(2) Electrodeposition of nickel sulfide (Ni-S) on NiCo2O4(NiCo2O4@Ni-S)nanoplate arrays produces a new hierarchical core-shell functional-material. Weakcrystalline Ni-S nanosheets have been uniformly coated on the NiCo2O4nanoplatearray obtained by hydrothermal growth. When tested as an electrode forsupercapacitors, the NiCo2O4@Ni-S nanoplate arrays have been found to exhibit asignificantly improved areal capacitance of1.85F cm-2at a current density of8mAcm-2, good rate capability and cycling stability. In addition, the capacitance fading ofthe NiCo2O4@Ni-S arrays are attributed to the surface coarsening of the Ni-Snanosheets.