A versatile protocol for the ionothermal synthesis of nanostructured nickel compounds as energy storage materials from a choline chloride-based ionic liquid

Abstract

A versatile ionothermal strategy is proposed to synthesize nanostructured nickel compounds as energy storage materials, such as Ni(NH₃)₆Cl₂ crystals, nanosheet-like NiCl₂, nanoflower-like (NF) α-Ni(OH)₂, and mesoporous NF-NiO. The primal solution is a choline chloride (ChCl)–urea mixture-based ionic liquid with nickel chloride hexahydrate (NiCl₂·6H₂O) dissolved. The proposed ionothermal protocol is attractive and environmental friendly because ChCl and urea are naturally biocompatible compounds. It has been reported that Ni(NH₃)₆Cl₂ is an indirect and reversible hydrogen storage material for fuel cells. Herein, we demonstrate that Ni(NH₃)₆Cl₂ crystals with octahedral geometry and an open structure can be obtained by maintaining the primal solution in a sealed vessel at 150 °C for 4 hours. A kinetic-to-dynamic growth mechanism is proposed to elucidate the formation of the interesting Ni(NH₃)₆Cl₂ structure. Further annealing of the Ni(NH₃)₆Cl₂ crystals at 300 °C produces sheet-like NiCl₂, which can be utilized as an active material in sodium-ion batteries. Interestingly, with the addition of a small amount of water into the primal solution which has been kept at 150 °C and open to the air for 40 minutes, NF α-Ni(OH)₂ can be obtained. Further annealing of the NF α-Ni(OH)₂ at 300 °C leads to the formation of NF-NiO with a mesoporous structure, which is demonstrated to be a promising pseudo-capacitive electrode material in a KOH aqueous solution. The rapid activation process, good cycling performance, and high specific capacitance of the NF-NiO electrode are attributed to its high surface area (72.9 m² g⁻¹) and the mesoporous structure. We expect that the ionothermal method will have implications for the use of ChCl–urea mixture-based ionic liquids in the large-scale synthesis of advanced functional materials.