A systematic study on the key factors affecting the performance of electrochemical capacitor electrodes in solid electrolytes and their liquid electrolyte baselines was conducted. The study combined a test matrix of two types of activate carbons (AC) with different specific surface area, pore size and structures at various loadings in three solid and liquid electrolyte pairs. Working curves on loading vs. capacitance of these ACs in solid and corresponding liquid electrolytes revealed the correlation and interconnection of these material properties. A cross-sectional microscopic elemental analysis was used to identify and visualize the influence of these factors. When transition from liquid to solid polymer electrolytes, the infiltration of the electrolyte into the porous carbon plays a critical role in the performance of AC electrode especially at a high loading. When the precursor solution of polymer electrolyte was relatively less viscous, AC with an open structure and mesopores had similar capacitance as their liquid counterparts. A highly viscous precursor solution blocked some access of the polymer electrolytes into the bulk electrode, making infiltration less effective at high loading. This work shows an approach to project the performance of carbon electrodes in solid electrolytes and can provide directions for developing solid-state electrochemical capacitors. (c) The Author(s) 2019. Published by ECS.