Ultra-small, size-controlled Ni(OH)2 nanoparticles: elucidating the relationship between particle size and electrochemical performance for advanced energy storage devices

Nanosizing is the fashionable method to obtain a desirable electrode material for energy storage applications, and thus, a question arises: do smaller electrode materials exhibit better electrochemical performance? In this context, theoretical analyses on the particle size, band gap and conductivity of nano-electrode materials were performed; it was determined that a critical size exist between particle size and electrochemical performance. To verify this determination, for the first time, a scalable formation and disassociation of nickel-citrate complex approach was performed to synthesize ultra-small Ni(OH)2 nanoparticles with different average sizes (3.3, 3.7, 4.4, 6.0, 6.3, 7.9, 9.4, 10.0 and 12.2 nm). The best electrochemical performance was observed with a specific capacity of 406 C g−1, an excellent rate capability was achieved at a critical size of 7.9 nm and a rapid decrease in the specific capacity was observed when the particle size was <7.9 nm. This result is because of the quantum confinement effect, which decreased the electrical conductivity and the sluggish charge and proton transfer. The results presented here provide a new insight into the nanosize effect on the electrochemical performance to help design advanced energy storage devices. There is acritical size for nanoparticles that maximizes their electrochemical performance in energy storage devices, show a team in China. This finding goes against the conventional wisdom that smaller is better for the electrode material of such devices. Electrochemical energy storage devices such as batteries and supercapacitors are attractive power sources. One way to boost their performance has been to reduce the size of their electrode materials. Xingbin Yan and co-workers at the Lanzhou Institute of Chemical Physics prepared nine sets of Ni(OH)2 nanoparticles having various average sizes in the range 3.3 to 12.2 nanometers. They found that the best electrochemical performance was obtained at a size of 7.9 nanometers. Below that size, the specific capacity dropped off rapidly due to the quantum confinement effect and slower charge and proton transfer. Ultra-small Ni(OH)2 nanoparticles with different average sizes are prepared in large scale, and the best electrochemical performance is obtained at the critical size rather than the smallest size, which provides a new insight on nanosize effect on electrode materials in energy storage.