Honeycombed-like nanosheet array composite NiCo2O4/rGO for efficient methanol electrooxidation and supercapacitors

Abstract

A series of novel honeycombed-like composites NiCo2O4/rGO-T (rGO: reduced graphene oxide, T: 200, 250 and 300 °C) were obtained by a pre-adjusted pH value aq. phase coprecipitation strategy assisted by citric acid, followed annealing in N2 flow. The NiCo2O4/rGO-250 shows the most uniformly highly-ordered honeycombed-like nanosheet array morphology consisting of ultrathin mesoporous nanosheets (~110 nm × 12 nm) interdigitated vertically grown on the rGO surface in both sides with the highest crystallinity, higher surface area (145 m2 g−1), bimodal pore size distribution (3.4 and 12.5 nm) along with the highly open macroporous networks. The NiCo2O4/rGO-250 electrode exhibits the highest current density for methanol oxidation reaction of 90 A g−1 at 0.6 V, as well as high cycling stability (94% after 500 cycles, returned to 98% with fresh electrolyte), which is the best one so far among reported catalysts with similar compositions. Simultaneously, NiCo2O4/rGO-250 electrode exhibits considerable specific capacitance of 1380 F g−1 at 1 A g−1, high rate performance (10 A g−1, 967 F g−1), and good cycling stability which remains 90% at 5 A g−1 after 1000 charge–discharge cycles for supercapacitor. The excellent electrochemical performance of the NiCo2O4/rGO-250 electrode can be attributed to the highly-ordered honeycombed-like mesoporous nanosheet array along with the greatly increased specific surface area exposing more active sites, the highly open macroporous networks formed by adjacent ultrathin NiCo2O4 nanosheets providing more ion/electron diffusion paths and effective contact area of electrolyte, and the strong NiCo2O4– rGO synergy endowing excellent conductivity and structural robustness, thus greatly facilitate the transfer of electrons and ions in electrode and at the electrolyte/electrode interface.