12-09-2013, 04:00 PM
What Are Batteries, Fuel Cells, and Supercapacitors?
What Are Batteries.pdf (Size: 651.59 KB / Downloads: 22)
Introduction
Batteries versus Fuel Cells versus
Electrochemical Capacitors
Energy consumption/production that rely on the
combustion of fossil fuels is forecast to have a severe
future impact on world economics and ecology. Elec-
trochemical energy production is under serious con-
sideration as an alternative energy/power source, as
long as this energy consumption is designed to be
more sustainable and more environmentally friendly.
Systems for electrochemical energy storage and
conversion include batteries, fuel cells, and electro-
chemical capacitors (ECs). Although the energy stor-
age and conversion mechanisms are different, there
are “electrochemical similarities” of these three sys-
tems. Common features are that the energy-providing
processes take place at the phase boundary of the
electrode/electrolyte interface and that electron and
ion transport are separated. Figures 1 and 2 show
the basic operation mechanisms of the three systems.
Note that batteries, fuel cells, and supercapacitors
all consist of two electrodes in contact with an
electrolyte solution. The requirements on electron
and ion conduction in electrodes and the electrolyte
are given in Figure 1 and are valid for all three
systems.
Definitions
The following definitions are used during the
course of discussions on batteries, fuel cells, and
electrochemical capacitors.
A battery is one or more electrically connected
electrochemical cells having terminals/contacts to
supply electrical energy.
A primary battery is a cell, or group of cells, for
the generation of electrical energy intended to be
used until exhausted and then discarded. Primary
batteries are assembled in the charged state; dis-
charge is the primary process during operation.
A secondary battery is a cell or group of cells for
the generation of electrical energy in which the cell,
after being discharged, may be restored to its original
charged condition by an electric current flowing in
the direction opposite to the flow of current when the
cell was discharged.
Kinetics
Thermodynamics describe reactions at equilibrium
and the maximum energy release for a given reaction.
Compared to the equilibrium voltage () open ciruit
voltage, EOCV), the voltage drops off () “electrode
polarization” or “overvoltage”) when current is drawn
from the battery because of kinetic limitations of
reactions and of other processes must occur to
produce current flow during operation. Electrochemi-
cal reaction kinetics follow the same general consid-
erations as those for bulk chemical reactions. How-
ever, electrode kinetics differs from chemical kinetics
in two important aspects: (1) the influence of the
potential drop in the electrical double layer at an
electrode interface as it directly affects the activated
comples and (2) the fact that reactions at electrode
interfaces proceed in a two-dimensional, not three-
dimensional, manner. The detailed mechanism of
battery electrode reactions often involves a series of
physical, chemical, and electrochemical steps, includ-
ing charge-transfer and charge transport reactions.
Primary Batteries
Figure 15A shows the discharge reaction of a CuS
electrode in a Li-CuS cell. During the cell reaction,
Cu is displaced by Li and segregates into a distinct
solid phase in the cathode. The products of this
displacement type of reaction, Li2S and Cu, are
stable, and the reaction cannot be easily reversed.
Hence, the electrode reactions cannot be recharged
and the cell is considered to be a primary cell, as the
discharge reaction is not reversible. The Li electrode
in Figure 15B is discharged by oxidation. The formed
Li+ cation is going into solution. The reaction is
reversible by redeposition of the lithium. However,
like many other metals in batteries, the redeposition
of the Li is not smooth, but rough, mossy, and
dendritic, which may result in serious safety prob-
lems. This is in contrast to the situation with a lead
electrode in Figure 15C, which shows a similar
solution electrode. Here, the formed Pb2+ cation is
only slightly soluble in sulfuric acid solution.