Ion-controlled electronics use biocompatible electrolytes to realize multiple functions, such as ionic resistors, ionic diodes, and ionic transistors, endowing them with huge biologic application potentials, ranging from sensing to information processing. Biocompatible porous carbons with exceptionally high surface areas enable high capacitance for biologic ions electric-double-layer capacitive devices. However, we find an unexpectedly high content of water-soluble ionic impurities in commercial and custom designed carbons, which affects the capacitive behavior of carbons in the pure water electrolyte and thereby affects the precise control of ions transport induced by electric polarization of porous carbon electrodes. Complementary analytical techniques are used to analyze the commercial clean activated carbon ColorSorb (AC-CS) illuminating the influence of adsorbed ionic impurities for the electrosorption mechanism in a low concentrated bioactive choline chloride (ChCl) electrolyte. Moreover, the comparison of high purity activated carbon (AC-HP) with pure AC-CS suggests the roles of ionic impurities for the capacitive behavior in ChCl and pure water electrolytes. Our work encourages to reconsider the importance of adsorbed ionic impurities for the carbon's property and application in capacitive devices, and clarifies the influence of ionic impurity release from porous carbon for ion depletion and release processes.