Catalytic activity is primarily a surface phenomenon, however, little is known about Co₃O₄ nanocrystals in terms of the relationship between the oxygen reduction reaction (ORR) catalytic activity and surface structure, especially when dispersed on a highly conducting support to improve the electrical conductivity and so to enhance the catalytic activity. Herein, we report a controllable synthesis of Co₃O₄ nanorods (NR), nanocubes (NC) and nano-octahedrons (OC) with the different exposed nanocrystalline surfaces ({110}, {100} and {111}), uniformly anchored on graphene sheets, which has allowed us to investigate the effects of the surface structure on the ORR activity. Results show that the catalytically active sites for ORR should be the surface Co²⁺ ions, whereas the surface Co³⁺ ions catalyze CO oxidation and the catalytic ability is closely related to the density of the catalytically active sites. These results underscore the importance of morphological control in the design of highly efficient ORR catalysts.

We report kinetic control over and mechanistic studies on the formation of sea urchin-like, bimetallic (Ni, Co) carbonate hydroxidevia a sequential crystallization process, which was facilely converted to porous NiCo₂O₄ spinel with a conserved morphology, an excellent candidate material for pseudocapacitors. The formation of bimetallic carbonate hydroxide was found to start with the nucleation of monometallic nickel carbonate hydroxide evolving into flower-like microspheres. This was followed by the nucleation and growth of the bimetallic carbonate hydroxide nanorods from and on the nanoplates in the flower-like microspheres by localized dissolution-recrystallization, leading finally to the sea urchin structure. After calcination, a morphology conserved NiCo₂O₄ spinel nanostructure was formed, which uniquely comprises hierarchical, interconnected pores with high specific surface areas suitable for fast electron and electrolyte transport. This, in tandem with the rich redox reactions of nickel cobaltite spinel and their at least two orders of magnitude higher electric conductivity than those of nickel oxides and cobalt oxides alone, renders the novel nanostructures ideal candidates for pseudocapacitors. Indeed, the porous NiCo₂O₄ nanostructure with a specific surface area of up to 198.9 m² g⁻¹ has exhibited higher specific capacitances (658 F g⁻¹ at 1 A g⁻¹) than the monometallic cobalt oxides (60 F/g at 1 A g⁻¹) and nickel oxides (194 F g⁻¹ at 1 A g⁻) with similar porous nanostructures. Significantly, even at a high current density of 10 A g⁻¹, the pseudocapacitor made of NiCo₂O₄ porous materials retained high specific capacitances of 530 F g⁻¹ with excellent cycling stability. In all, the simple, scalable syntheses and the excellent supercapacitor performance reported here portend large scale applications of these novel materials in energy storage.

We report a facile and green method to synthesize a new type of catalyst by coating Pd nanoparticles (NPs) on reduced graphene oxide (rGO)-carbon nanotube (CNT) nanocomposite. An rGO–CNT nanocomposite with three-dimensional microstructures was obtained by hydrothermal treatment of an aqueous dispersion of graphene oxide (GO) and CNTs. After the rGO–CNT composites have been dipped in K₂PdCl₄ solution, the spontaneous redox reaction between the GO–CNT and PdCl₄²⁻ led to the formation of nanohybrid materials consisting rGO–CNT decorated with 4 nm Pd NPs, which exhibited excellent and stable catalytic activity: the reduction of 4-nitrophenol to 4-aminophenol using NaBH₄ as a catalyst was completed in only 20 s at room temperature, even when the Pd content of the catalyst was 1.12 wt%. This method does not require rigorous conditions or toxic agents and thus is a rapid, efficient, and green approach to the fabrication of highly active catalysts.