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Abstract Ultrathin single crystalline nanoplates of Ni 12 P 5 are synthesized via a facile, template-free, wet chemical method. The thickness is about 6 nm with sizes tunable from 5 to 50 nm. The exposed plate surface is formed of { 2 ¯ 11 } lattice plane. Benefitting from the high electrical conductivity and the two dimensional (2D) nanostructures, the Ni 12 P 5 nanoplates exhibit excellent supercapacitor properties. The specific capacity is about 697 C/g at the current density of 1 A/g. Even at 20 A/g, it is measured as 446 C/g, with 64.0% retention indicating an outstanding rate capability. The charge storage mechanisms include a battery-type behavior related to the bulk redox processes and a double-layer capacitance arising from the 2D geometric morphology. In particular, the morphological effects accounts for about 63% of the total charge storage effects at the voltage sweep rate of 2 mV/s in the cyclic voltammetry measurement, and is even higher percentage-wise at a higher sweep rate. This is, by far, the best performance with facile synthesis techniques for single crystalline nickel phosphides. Graphical abstract Image 1 Highlights • Single-crystalline Ni 12 P 5 nanoplates by a facile, template-free wet chemical route. • Nanoplate surface is of { 2 ¯ 11 } crystal plane, and its thickness, about 7 nm. • The growth mechanism is by Kirkendall effect. • High specific capacity is 697 C/g at the current density of 1 A/g. • Surface term accounts for about 2/3 of the specific capacity, and the bulk, 1/3. [ABSTRACT FROM AUTHOR]