BROADER CONTEXT

The controlled mixing of the water from a river and from the sea can
be used for producing electrical energy. In the two main existing
technologies, namely ``pressure retarded osmosis'' and ``reverse
electrodialysis'', the control of the mixing is obtained by
interposing membranes which can be crossed by either the water or the
ions. In the recently proposed ``capacitive mixing'' (CAPMIX)
technique, the freshwater and the saltwater flow through the cell in
alternating phases. The mixing is obtained by electrically storing the
ions inside two electrodes during the saltwater flow and releasing
them during freshwater flow. The electrodes can be either chemically
inert porous materials [D. Brogioli, Phys. Rev.  Lett. 103, 2009,
058501], or membrane-based ion-selective electrodes [B. B. Sales et
al., Env. Sci. Techn. 44, 2010, 5661], or chemically active materials,
like in batteries [F. La Mantia et al., Nano Lett. 11, 2011, 1810]. In
this paper, we present the analysis of activated carbon materials
produced with various methods and treatments and we show that their
behavior in CAPMIX experiments can display features coming both from
the electric double layer and from chemical reactions, including redox
reactions and chemical adsorption. By exploiting the new information,
we are able to improve the energy extraction, up to the value of 50 mW
per square meter of electrode.