Galvanic cell: An electrochemical cell
that converts chemical energy into electrical energy. A cell in which chemical
reactions occur spontaneously at the electrodes when they are connected through
an external circuit, producing an electrical current. E.g., in a fuel cell hydrogen
is oxidized at the anode by transferring electrons to the anode and the oxygen
is reduced at the cathode by accepting electrons from the cathode. During this
process the electrons are carried from the anode to the cathode through an outside
electrical circuit where the electrical current can drive a motor, light a light
bulb, etc. In contrast, in an electrolytic cell electrical power must be supplied
to force the non-spontaneous reverse reaction, the electrolysis of water. See
also electrochemical reaction, battery, and fuel cell. Also called "voltaic"
cell. The free energy change of the overall cell reaction is negative.
Galvanic series: See electromotive series.
Galvanizing: Process for coating iron
or steel with a thin layer of zinc for corrosion protection. It can be carried
out electrochemically by electroplating (called "electrogalvanizing") or by
"hot-dip" galvanizing consisting of immersing the substrate into molten zinc.
Galvanometer: A very sensitive ammeter
that can be used to measure currents in the range of one millionth to one billionth
of an ampere.
Galvanoplasty: A somewhat archaic, today
very seldom used, term for electroforming.
An electronic instrument that controls the current through an electrochemical
cell at a preset value, as long as the needed cell voltage and current do not
exceed the compliance limits of the galvanostat. Also called "amperostat."
Galvanostatic technique: An electrochemical
measuring technique for electrochemical analysis or for the determination of
the kinetics and mechanism of electrode reactions based on the control of the
current flowing through the system.
Gas electrode: Any electrode with one
of the reactants or products in the gaseous phase. The solution surrounding
the electrode is typically saturated with the gas. Essentially a redox electrode
with the "dissolved" gas as one of the potential determining species. E.g.,
the oxidized species for the "chlorine electrode" is the dissolved chlorine
gas, while the reduced species is the chloride ion in solution. Under equilibrium
conditions, the chemical potential of the gaseous chlorine is the same as that
of the dissolved chlorine. This may not be the case when current is flowing
through the electrode: during the electrolytic production of chlorine, the solution
may become supersaturated in chlorine if the nucleation of the gas bubbles requires
considerable activation energy.
Gassing: Gas evolution which
takes place towards the end of the charging of a battery.
Geometric(al) current density: Current
density calculated with the geometric electrode area.
Geometric(al) electrode area: The surface
area of an electrode calculated from its geometrical dimensions. Contrast with
true electrode area.
Glass electrode: A membrane electrode
with a thin glass membrane (usually in the form of a bulb at the end of a glass
tubing) sensing element. It is most often used as a pH electrode, but some glass
compositions can also be sensitive to the concentration of other cations (e.g.,
Grain boundary: The surface
separating two regions of a solid having different crystal orientations.
Gram-equivalent: An amount of a substance
equal in grams to its equivalent weight.
Gram-atom: A concept similar to gram-mole
except it relates to atoms rather than molecules.
Gram-mole: An amount of a compound equal
in grams to its molecular weight. E.g., the molecular weight of water is 18,
so 18 grams of water is called a gram-mole of water. This provides an atomistically
fundamental unit because one gram-mole of any material will contain the same
number of molecules (this is a very large number, called "Avogadro's" number).
One gram-mole of hydrogen gas contains the exactly same number of molecules
as one gram-mole of table salt (sodium chloride), even though the latter is
much heavier. The simplified expression of "mole" is often used in place of
"gram-mole" and also in place of gram-atom. It is usually obvious from the context
which meaning is appropriate. The measurement unit and symbol of the "gram-mole"
or "mole" is the "mol".
Gouy-Chapman model of the double layer:
A model of the electrical double layer. According to this model, the excess
ions are non-uniformly distributed in the vicinity of the electrode, their concentration
is the largest at the surface of the electrode, decreasing non-linearly till
they reach bulk concentration. The "thickness" of this so called "diffuse layer"
is variable, but it is typically around the order of magnitude of a millionth
of a centimeter.
Gouy-Chapman-Stern model of the double layer:
A model of the electrical double layer. The Gouy-Chapman model predicts an unrealistically
large surface concentration, because it assumes that the ions are infinitely
small and can get infinitely close to the surface of the electrode. The "Stern
modification" is essentially a combination of the Helmholtz and Gouy-Chapman
models. It assumes a "plane of closest approach" where a portion of the excess
ions reside and attaches to this a Gouy-Chapman type "diffuse layer."
The lead framework of a lead-acid battery plate which holds the active
material in place.
Grid-connected: An energy producing
system connected to the utility transmission grid. (Also called utility interactive.)
A set of electrodes within a cell which are connected in parallel.