Sequential crystallization of sea urchin-like bimetallic (Ni, Co) carbonate hydroxide and its morphology conserved conversion to porous NiCo2O4 spinel for pseudocapacitors

Abstract

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.