The demand for advanced energy storage devices such as supercapacitors and lithium-ion batteries has been increasing to meet the application requirements of hybrid vehicles and renewable energy systems. A major limitation of state-of-art supercapacitors lies in their relatively low energy density compared with lithium batteries although they have superior power density and cycle life. Here, we report an additive-free, nano-architectured nickel hydroxide/carbon nanotube (Ni(OH)2/CNT) electrode for high energy density supercapacitors prepared by a facile two-step fabrication method. This Ni(OH)2/CNT electrode consists of a thick layer of conformable Ni(OH)2 nano-flakes on CNT bundles directly grown on Ni foams (NFs) with a very high areal mass loading of 4.85 mg cm-2 for Ni(OH)2. Our Ni(OH)2/CNT/NF electrode demonstrates the highest specific capacitance of 3300 F g-1 and highest areal capacitance of 16 F cm-2, to the best of our knowledge. An asymmetric supercapacitor using the Ni(OH)2/CNT/NF electrode as the anode assembled with an activated carbon (AC) cathode can achieve a high cell voltage of 1.8 V and an energy density up to 50.6 Wh/kg, over 10 times higher than that of traditional electrochemical double-layer capacitors (EDLCs).