5.6 Water in Electrochemical Energy Systems
209
are four main classes of the electrochemical systems: batteries, supercapacitors, fuel
cells, and electrolyzers, which differ by their characteristics and thus ranges of application.
All batteries are variations of the galvanic cell described above. The most popular
water-based battery is a lead–acid accumulator, which consists of a liquid electrolyte
(a mixture of sulfuric acid and water), lead electrodes, and a separator (rubber or
other porous material). The plates in the lead–acid battery contain an active material
that is continuously bathed in the electrolyte while oxygen and hydrogen gas are
released during charging. Due to the liquid convection and other effects described in
this book, the water-based battery cell tends to rapidly charge and discharge. Having
low power density (W/kg), but relatively high-energy density (W·h/kg), batteries are
used to start automobiles, and in large backup power supplies, grid energy storage,
off-grid household electric power systems, emergency lighting, and sometimes in
small vehicles.
A special case of batteries is flow batteries, which are close to fuel cells (see below)
by their operation principle. This type of electrochemical energy source stores electrical charge in scalable tanks of liquid electrolyte that is pumped near the electrodes
to extract electrons by means of a chemical reaction. The used electrolyte returns to
the tank and can be “recharged” by an external source of electricity. The two most
popular, the organic and the vanadium flow batteries, are water based. Flow batteries
are in demand as backup systems for stationary applications, such as wind power
stations and solar-power plants. They have very low-energy density and relatively
low charge and discharge rates, however, they can be easily scaled-up without a
reduction of their efficiency.
Supercapacitors cover the gap between electrolytic capacitors and batteries. They
store 100 times more energy per unit mass than electrolytic capacitors, accept and
deliver charge much faster than batteries, and have very long charge-discharge lifecycles. The construction feature of a supercapacitor is electrodes of high porosity,
and, thus, high surface area, so the electrolyte layer can be reduced to the thickness
of only one electric double layer, which provides very high capacitance. The aqueous
supercapacitor is very ecologically friendly as it contains only water and charcoal
electrodes. Nevertheless, the threshold of the electrolysis of water limits the maximal
voltage of such a supercapacitor to about 2 V. The properties of water presented in
this book pave the way for the further improvement of aqueous supercapacitors. The
main advantage of supercapacitors is that they have the highest power density among
electrochemical energy sources, which makes them indispensable in automobiles and
small planes, where they can effectively supplement fuel cells.
Fuel cells are open electrochemical systems which produce electric energy on
demand using different types of fuels (e.g., hydrogen and natural gas). The most
developed fuel cell type is an aqueous polymer-electrolyte fuel cell (PEMC).
16 This
electrochemical system transforms the chemical energy liberated during the electro16 Note that the solid-oxide fuel cell (SOFC) is one of the most promising types of the fuel cell for
portable applications, as it has the highest energy density. However, the high operation temperature
and the current level of material-production technology hinders its immediate application. The main
209
are four main classes of the electrochemical systems: batteries, supercapacitors, fuel
cells, and electrolyzers, which differ by their characteristics and thus ranges of application.
All batteries are variations of the galvanic cell described above. The most popular
water-based battery is a lead–acid accumulator, which consists of a liquid electrolyte
(a mixture of sulfuric acid and water), lead electrodes, and a separator (rubber or
other porous material). The plates in the lead–acid battery contain an active material
that is continuously bathed in the electrolyte while oxygen and hydrogen gas are
released during charging. Due to the liquid convection and other effects described in
this book, the water-based battery cell tends to rapidly charge and discharge. Having
low power density (W/kg), but relatively high-energy density (W·h/kg), batteries are
used to start automobiles, and in large backup power supplies, grid energy storage,
off-grid household electric power systems, emergency lighting, and sometimes in
small vehicles.
A special case of batteries is flow batteries, which are close to fuel cells (see below)
by their operation principle. This type of electrochemical energy source stores electrical charge in scalable tanks of liquid electrolyte that is pumped near the electrodes
to extract electrons by means of a chemical reaction. The used electrolyte returns to
the tank and can be “recharged” by an external source of electricity. The two most
popular, the organic and the vanadium flow batteries, are water based. Flow batteries
are in demand as backup systems for stationary applications, such as wind power
stations and solar-power plants. They have very low-energy density and relatively
low charge and discharge rates, however, they can be easily scaled-up without a
reduction of their efficiency.
Supercapacitors cover the gap between electrolytic capacitors and batteries. They
store 100 times more energy per unit mass than electrolytic capacitors, accept and
deliver charge much faster than batteries, and have very long charge-discharge lifecycles. The construction feature of a supercapacitor is electrodes of high porosity,
and, thus, high surface area, so the electrolyte layer can be reduced to the thickness
of only one electric double layer, which provides very high capacitance. The aqueous
supercapacitor is very ecologically friendly as it contains only water and charcoal
electrodes. Nevertheless, the threshold of the electrolysis of water limits the maximal
voltage of such a supercapacitor to about 2 V. The properties of water presented in
this book pave the way for the further improvement of aqueous supercapacitors. The
main advantage of supercapacitors is that they have the highest power density among
electrochemical energy sources, which makes them indispensable in automobiles and
small planes, where they can effectively supplement fuel cells.
Fuel cells are open electrochemical systems which produce electric energy on
demand using different types of fuels (e.g., hydrogen and natural gas). The most
developed fuel cell type is an aqueous polymer-electrolyte fuel cell (PEMC).
16 This
electrochemical system transforms the chemical energy liberated during the electro16 Note that the solid-oxide fuel cell (SOFC) is one of the most promising types of the fuel cell for
portable applications, as it has the highest energy density. However, the high operation temperature
and the current level of material-production technology hinders its immediate application. The main