The reliability of electric double-layer capacitors (EDLCs), long overlooked, is important for practical applications. For instance, issues like leakage current (leading to energy loss and heat generation), gassing (increasing internal pressure), and aging (causing capacity fading and cell resistance to rise) greatly affect the applicability of EDLCs. A hierarchical electrode design is developed in the present work to deal with these problems. The hierarchical carbon composites that consist of activated carbon (AC), zero-dimensional carbon nanospheres (CNSs) and one-dimensional CNTs are fabricated using a facile and cost-effective method, which can be easily scaled up for mass production. The charge-discharge properties, operation voltage, temperature profile during charging/discharging, leakage current, gas evolution, and self-discharge of the EDLCs are systematically investigated. The electronic conductivity of the electrodes is found to crucially influence the EDLC overall performance. A hierarchical carbon architecture with an appropriate AC/CNS/CNT constituent ratio can significantly improve charge-discharge capacitances, increase cell reliability, and decrease the aging-related degradation rate.