The charge storage mechanism of manganese oxide and activated carbon has been studied in aqueous medium in order to optimise an asymmetric (or hybrid) supercapacitor based on these two materials as positive and negative electrode, respectively. Amorphous manganese oxide can be polarised up to potentials of 1.2 V in neutral medium. Under negative polarisation, a pseudocapacitive behaviour of activated carbon has been demonstrated, that is related with reversible hydrogen adsorption in the pores. It allows carbon to be polarised at potential values far from the thermodynamic decomposition of the electrolyte. Balancing the mass of these two materials with pseudocapacitive properties results in a practical cell voltage of 2 V in aqueous medium, with energy densities close to the values obtained with electric double layer capacitors working in organic electrolytes, while avoiding their disadvantages. (c) 2005 Elsevier B.V. All rights reserved.