On-chip micro-supercapacitors are promising miniaturized micro-power sources for wireless sensors, portable electronic devices, and implantable medical devices due to their advanced features such as high power densities, fast charge-discharge, and superior cycling lifetimes. Transition metal silicide nanowires can meet the desired requirements for electrode materials for on-chip micro-supercapacitors, as they provide advantages such as high conductivity, high surface area, effective electrolyte transport, and ease of fabrication and integration on silicon. In the present work, we introduce freestanding single-crystalline Co2Si nanowires directly synthesized on a Si substrate for application in a high-performance on-chip micro-supercapacitor. Compared with the previously reported supercapacitors comprising Si-based nanowires, the single-crystalline Co2Si nanowires-based supercapacitor exhibits good supercapacitor performance, namely, high areal capacitance (similar to 983 mu F cm(-2) at 2 mu A cm(-2)), high energy density (similar to 629 mu J cm(-2) at 2 mu A cm(-2)), and excellent cyclability (similar to 94% after 4000 cycles) in an ionic liquid electrolyte. To the best of our knowledge, this is the first report on the electrochemical performance of metal silicide nanowires directly grown on a Si substrate for supercapacitor application. Our results demonstrate the potential of metal silicide nanowires as electrode materials for on-chip micro-super-capacitor application.