A strategy in the rational design of the aligned carbon nanotube @ polyaniline (ACNT@PANI), arrayed on metal foils as supercapacitors, has been demonstrated towards the upper bound of the specific capacitance, namely a high surface area and nanometre thickness of PANI, good accessibility of ions and a low contact resistance. The structure of ACNT@PANI nanoelectrode arrays was investigated and correlated to the surface redox reaction, electronic and ionic transport and thus to the energy and power density of the supercapacitors. The thickness of the PANI film on each ACNT was found the most important parameter, which was optimized to achieve the specific capacitance (∼1100 F g⁻¹) of PANI close to the maximum theoretic value. A prototype supercapacitor was assembled, and a practical application was demonstrated. This device shows a higher specific capacitance, a higher specific power and a lower internal resistance but reduced cost and volume than conventional two-plate supercapacitors, which may provide opportunities for a smarter design of energy storage devices.