| Considering the low conductivity of pure NiCo2O4 powder materials and the required addition of conductive additive in the traditional slurry-coating technique electrochemical evaluation,we propose to prepare hierarchically structured NiCo2O4 nanosheets directly grown on conductive substrates,in order to increase electrochemically active surface area and facilitate diffusion of electrolyte in electrode.In the present thesis,novel nanosheets array-like NiCo2O4/nickel foam nanowall and scrobiculate NiCo2O4/rGO nanohybrids were obtained through a facile one-step solvothermal method and a one-step citric acid-assisted aqueous phase coprecipitation strategy,respectively,and their structure and morphology were investigated by comprehensive characterization methods.Moreover,methanol electrooxidation and supercapacitor performances were examined,and the essential relationship between structure and electrochemical performance was discussed.The main contents are as follows:(1)A serise of nanosheets array-like NiCo2O4/nickel foam nanowall(NCONW/NF-t,t refers to reaction time,t = 2?5?12?18?24 h)were obtained by a facile one-step solvothermal method.Systematic characterizations and time-dependent formation studies indicate that the roughness factor(Rf)of the nanowall structures is decreased with increasing reaction time because of the smaller sized NiCo2O4 crystals of constituent nanosheets obtained at longer reaction time due to serious ammonia coordination etching.The NCONW/NF-2 possesses the highest Rf of 1793 originated from its largest size of nanosheets(1388×29.8 nm)assembled orderly by interconnected and larger cubic NiCo2O4 crystals(ca.29.8 nm)forming uniform interparticle voids of(ca.22.1 nm).(2)Electrochemical test indicates that the series of NCONW/NF-t nanowall structures directly as electrodes show high methanol electrooxidation activity.Cyclic voltammetric(CV)results in 1 M KOH with 0.5 M CH3OH show that NCONW/NF-2 possesses the highest current density of 133.2 A g-1(0.6 V)and the lowest overpotential of 0.39V(vs.SCE),much higher than previously reported NiCo2O4 electrode materials.The current density maintained 87.5%(0.6 V)even after 500 cycles CV test,clearly demonstrating its high electrooxidation cycle stability.When used as electrodes for supercapacitors,the NCONW/NF-2 exhibits high specific capacitance of 1265 F·g-1 at 1 A·g-1 and rate capability(64.4%retention at 10 A·g-1).These results can be attributed to the highest Rf,numerous open spaces between neighboring ultrathin NiCo2O4 nanosheets with large interparticle voids,and strong interaction between the nanosheets and Ni foam scaffold,which can expose more electroactive sites and offer many facile diffusion paths for ion/electrolyte and greatly improve the electron/ion transfer within the electrode and at the electrode-electrolyte interfaces.(3)Honeycomb-like Nanosheets array NiCo2O4/rGO nanohybrids were obtained via a one-step citric acid-assisted aqueous-phase coprecipitation strategy,which is composed by ultrathin NiCo2O4 nanosheets of 130×12.6 nm vertical to the surface of rGO layers and cross-linked each other.Comparing with the pure NiCo2O4,NiCo2O4/rGO shows markedly higher methanol electro-oxidation performance(current density of 80 A/g at 0.6 V,onset potential 0.35 V(vs.SCE),the current density kept 87.5%(0.6 V)after 500 CV cycles)and supercapacitor performance(1330 F/g,1 A/g;72.7%,10 A/g).The better electrochemical performance of NiCo2O4/rGO can be attributed to its larger specific surface area and higher conductivity leading to more electroactive sites and many facile diffusion paths for ion/electrolyte within the electrode and at the electrode-electrolyte interfaces. |
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