Mechanism and Stability of a Redox Supercapacitor Based on Methylene Blue: Effects of Degradation of the Redox Shuttle

The mechanism of operation of a supercapacitor based on methylene blue (MB) as redox shuttle in cells containing activated carbon as electrodes and aqueous electrolyte has been studied. Attention is focused on the instability of these molecules which can undergo secondary reactions that impact negatively on the cell, generating fluctuations in its performance and reducing its cycling life. It is demonstrated that MB interacts strongly with activated carbons and modifies their structure, decreasing the resistance of the electrodes and increasing the energy density of the cell by more than 40%. It is also shown that MB is not stable in acidic electrolyte at the high potentials attained by the electrodes of the supercapacitor. Demethylation byproducts and oxidized derivates of the MB were detected after cycling by means of electrospray ionization mass spectrometry. These products were observed to cause a constant change in the equilibrium potential of the cell, substantially modifying the storage mechanism and the ranges of operation potential of the electrodes.