*Corresponding author. Tel.:+34 9 85119090; Fax: (+34) 985 297 662 E-mail address: clara@incar.csic.es Supercapacitor modified with Methylene Blue as redox active electrolyte Silvia Roldán, Marcos Granda, Rosa Menéndez, Ricardo Santamaría and Clara Blanco* Instituto Nacional del Carbón, CSIC, Aptdo. 73, 33080-Oviedo, Spain clara@incar.csic.es ABSTRACT. MWCNT-based supercapactors (SC) containing methylene blue (MB) as redox active electrolyte were studied. MWCNTs were employed as model of electrode active material due to their ideal double- layer behavior facilitates the investigation of the energy storage mechanisms involved. MB led to a cell capacitance enhancement equal to 4.5 times the original cell capacitance of MWCNTs in sulphuric acid with a capacitance reduction of only 12 % after 6000 charge-discharge cycles. The potential evolution of each electrode during galvanostatic cycling revealed that MB redox reaction develops in both electrodes simultaneously in the voltage range of 0-0.104 V and that this is the main cause of cell capacitance enhancement. Beyond this voltage range, the faradaic contribution from the MB redox reaction decreases because the anode behaves as a capacitative electrode with a rather reduced charge- capacity due to the small surface area of MWCNTs. By means of a modified assembly composed of a Nafion membrane and MB and sulfuric acid solutions located in the cathode and anode compartments, respectively, it was demonstrated the limiting role of the capacitative electrode in the cell charge- capacity in this type of hybrid devices.