Three-dimensional nano-nickel electrodes fabricated on silicon microchannel plates by electroless deposition are characterized by electrochemical impedance spectroscopy. Compared to the nickel sheet, adsorption of cations and anions is detected due to the larger surface area of the porous nickel structure and three-dimensional architecture of the silicon microchannel plates. Adsorption is sensitive to the frequency and increasing step of the external voltage and different desorption rates of 0.003 and 0.027 A/cm2 are observed from cations and anions, respectively. The experimental results indicate that adsorption is responsible for the anomalous increase of the impedance in the low frequency region.