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

A series of novel honeycombed-like composites NiCo2O4/rGO-T (rGO: reduced graphene oxide, T: 200, 250 and 300 degrees C) were obtained by a pre-adjusted pH value aq. phase coprecipitation strategy assisted by citric acid, followed annealing in N-2 flow. The NiCo2O4/rGO-250 shows the most uniformly highly-ordered honeycombed-like nanosheet array morphology consisting of ultrathin mesoporous nanosheets (similar to 110 nm x 12 nm) interdigitated vertically grown on the rGO surface in both sides with the highest crystallinity, higher surface area (145 m(2) 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. (C) 2020 Elsevier Ltd. All rights reserved.