Effect of various porous nanotextures on the reversible electrochemical sorption of hydrogen in activated carbons

The reversible hydrogen storage capacity of three series of activated carbons (ACs) prepared from different precursors by KOH, CO2 and steam activation is determined by electrodecomposition of an alkaline water solution and is correlated with the nanotextural parameters of ACs. Galvanostatic charge/discharge appears as a precise quantitative method for estimating the hydrogen sorption capacity, whereas, cyclic voltammetry supplies a very useful information on the electrosorption mechanism. For the ACs studied, the hydrogen sorption capacity is not linearly related with any of the porosity parameters commonly used in other publications, such as the Dubinin-Radushkevich micropore volumes determined by nitrogen or carbon dioxide adsorption, V-DRN2 and V-DRCO2. In particular, an important discrepancy is observed for the KOH activated materials, suggesting that this treatment may provoke changes of pore shape. A better correlation is found considering the nanopore size distribution obtained from CO2 adsorption by the DFT method. The amount of hydrogen reversibly adsorbed demonstrates a proportional trend with the volume of micropores smaller than 0.6-0.7 nm. However, in all cases, a part of the micropore volume estimated by CO adsorption is ineffective, suggesting that some ultramicropores are involved in irreversible trapping of hydrogen. (c) 2005 Elsevier Ltd. All rights reserved.