Ultracapacitors, due to their high specific power, long cycle life, and ability to bridge the power/energy gap between conventional capacitors and batteries/fuel cells, have attracted considerable attention for commercial applications such as plug-in hybrid electrical vehicles. Rational design of nanostructured composite electrode materials have demonstrated superior electrochemical properties in producing high-performance ultracapacitors. Incorporation of metal/metal oxide nanoparticles with appropriate size distributions into the interior regions of nitrogen-doped, activated carbon powders provides a feasible route to improve the energy density of ultracapacitor devices via the contribution of pseudocapacitance and improved electrolyte transport. In this project, we structurally tuned the porosity of commercially available, high-surface-area activated carbon powders to generate large amount of mesopores with pore widths in the range of 10-30 nm, which enables incorporation of niobium/niobium oxide nanoparticles. Nitrogen-doping of the activated carbon powders further enhances their capacitive performance. Such rationally designed high-surface-area nanocomposite carbon powders are fabricated into a monolithic hybrid capacitor electrode structure for significantly enhancing the energy density of ultracapacitors based on commercially available activated carbon powders.