ABSTRACT
The scarcity of fresh water, the depletion of fossil fuels, and the
ever-increasing demand for electric power are important issues
that receive increasing attention in a variety of branches of
science and technology. In all three cases, nanoporous carbon
electrodes, immersed in a liquid medium with charge carriers,
are being considered as device elements. For instance, in carbide-
derived carbon electrodes with nanometer-sized pores filled with
an ionic liquid, electric energy can be stored/released through
the adsorption/desorption of ionic charges on/from the surface of
the charging/discharging electrodes [Chmiola et al. (2006); Miller and Simon (2008); Merlet et al. (2012)]. Carbon electrodes are also being explored in capacitive devices to harvest sustainable
energy from mixing fresh river water with salty sea water [Brogioli
(2009); Brogioli et al. (2011); Rica et al. (2012); Sales et al. (2010)]. This salinity-gradient energy, or “blue” energy, is obtained from
charging up a pair of electrodes immersed in sea water (whereby
ions adsorb onto the electrodes at a low potential) and discharging
them again immersed in fresh water (whereby ions desorb from
the electrodes at a higher potential). This capacitive mixing process,
with brackish water as a waste product, intercepts the spontaneous
diffusion of ions from high to low salinity in much the same way
as heat engines intercept the heat flow from hot to cold heat
baths; for typical salt concentrations in river and sea water, these
“blue engines” can produce of the order of 2 kJ of energy per
liter of river water, in principle even completely reversibly [Boon
and van Roij (2011)]. The reverse process, which can be seen as
a “blue fridge,” is a desalination process in which two volumes of
initially brackish water are converted into a volume of fresh water
and a volume of brine by charging up the electrodes in one of
the volumes (which then desalinates due to ion adsorption onto
the electrodes, at a high potential) and discharging them in the
other volume (which then becomes more salty due to the release
of the ions from the electrodes, at a low potential) [Biesheuvel
(2009)]. Of course, the “blue fridge” requires a net energy input, and
ongoing research questions involve the efficiency and speed of such
processes.
