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Radical: A molecule or atom possessing
an unpaired electron.
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Rate: See C-rate.
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Rate constant: The rate of a chemical
reaction is proportional to the product of the concentrations of all the reactants
taking part in the reaction, with the "rate constant" the proportionality factor.
In other words, the rate constant is the rate of the reaction when all reactants
are present in unit concentration. The rate constant of a chemical reaction
is a function of temperature and pressure. For an electrode reaction the rate
constant is also a function of the electrode potential.
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Rated capacity: The total charge a battery
is able to deliver on discharge under some specified conditions. Usually expressed
in ampere-hours.
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Rate-determining step: The slowest elementary
reaction step in the series of steps comprising the overall reaction. The slowest
step will control the rate of the overall reaction. This is analogous to a traffic
situation, the time required to drive from the suburbs to downtown may completely
depend on the time spent in one traffic jam on the road. Abbreviated as "rds."
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Rational potential: The potential of
an electrode expressed against the potential of zero charge (pzc) of the same
electrode in the same solution. This provides a potential scale specific for
each electrode, with the origin (zero potential) always at the respective pzc.
Consequently, two electrodes having the same "rational potential" are typically
not at the same "potential" (as measured against any reference electrode). However,
the potential of these two electrodes is displaced by the same amount from their
respective pzc; therefore, the charge density in the double layer of the two
electrodes will be approximately equal. Consequently, the comparison of electrode
behavior at the same "rational potential" is more meaningful for some purposes
than a comparison at the same "potential on some arbitrary scale." The charge
densities of two electrodes at the same rational potential are only approximately
equal because the double-layer capacitance and its potential dependence varies
from electrode to electrode.
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RDE: Stands for rotating-disk electrode.
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rds: Stands for rate-determining step.
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Reactant: A chemical species that is
taking part in a chemical reaction by reacting (sometimes by itself, but usually
with other reactants) to form the products of the reaction. (electrode) reaction
mechanism: The totality of all the elementary reaction steps occurring in series
or parallel that fully defines the overall electrode reaction.
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Reaction overpotential (polarization):
The overpotential (alternatively called polarization) associated with a chemical
reaction (without charge transfer) step that is an elementary step in the overall
electrode reaction.
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Rechargeable battery: A battery in which
the chemical reaction system providing the electrical current is easily "chemically"
reversible. After discharging, it can be recharged by applying an electrical
current to its terminals. Some batteries can be recharged hundreds to thousands
times. See, e.g. the lead-acid battery. Also called "secondary" battery, and
"accumulator." Contrast with non-rechargeable battery. It operates as a galvanic
cell during discharge and as an electrolytic cell during charge. As a consequence,
the anode is the negative electrode during discharge, while it is the positive
electrode during charge; at the same time, the cathode is the positive electrode
during discharge, while it is the negative electrode during charge. This can
create a confusing situation, and it is preferable to refer to the electrodes
of a rechargeable battery as "positive" and "negative," because this designation
is independent of the operational mode. Unfortunately, this nomenclature is
not always followed. Often the "negative" electrode is designated as anode and
the "positive" electrode is designated as cathode. This naming convention is
a carry-over from the convention of the non-rechargeable battery.
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Recombining cell: A secondary
cell in which provision has been made for the products of overcharge reactions
to recombine so that no net change occurs to the composition of the cell system
as a result of overcharging.
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Rectifier: An electrical equipment that
converts alternating current into direct current.
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Redox battery: A rechargeable battery
with two redox electrodes contained in compartments typically separated by an
ion-exchange membrane E.g., a battery with iron and chromium redox couples in
the two compartments. During discharge, "ferric" (trivalent iron) cations are
reduced to "ferrous" (divalent iron) ions at one of the redox electrodes, while
"chromous" (divalent chromium) ions are oxidized to "chromic" (trivalent chromium)
ions at the other electrode. The reverse reactions occur during charging. An
advantage of this system is that the polarization losses are relatively small
because the redox reactions are typically fast and no solid phases are being
formed during the reaction. Also, the solutions containing the iron and chromium
salts can be stored in separate large tanks and circulated to a small "battery,"
permitting the storage of large amounts of energy.
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Redox couple: See redox reaction.
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Redox electrode: An inert electrode
(e.g., platinum, gold, carbon) the potential of which is controlled by a redox
reaction in solution. This is somewhat of a misnomer since all electrodes involve
oxidation/reduction. The distinction is that in the case of a redox electrode
both oxidized and reduced species are dissolved in the solution surrounding
the electrode.
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Redox potential: The potential of a
redox electrode.
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Redox reaction: A class of electrode
reactions involving oxidation/reduction of two dissolved species. E.g., iron
metal can exist in solution as a doubly positively charged ("ferrous") ion or
a triply positively charged ("ferric") ion. Such a system is often called a
"redox couple," such as the "ferrous/ferric" couple. The ferric ions can be
cathodically reduced to ferrous ions, or the ferrous ions can be anodically
oxidized to ferric ions. With these reactions, an inert electrode is used that
does not take part in any reactions under the conditions of the oxidation/reduction
of the ions. This electrode then acts only as a source or sink of electrons;
examples are: carbon, graphite, platinum, gold. Compare with a metal deposition/dissolution
reaction where one of the reacting species is a solid metal and the other species
is in solution.
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Reducing agent: A substance that is
affecting reduction by donating electrons to another substance. See oxidation/reduction.
Also called "reductant."
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Reductant: Alternative expression for
reducing agent.
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Reduction: The gain of electrons
by a chemical species. See oxidation/reduction
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Reference electrode:
An electrode that has a well known and stable equilibrium electrode potential
or a nonpolarizable electrode with a known and highly reproducible potential.
It is used as a reference point against which the potential of other electrodes
(typically that of the working electrode or measuring electrode) can be measured
in an electrochemical cell. In principle it can be any electrode fulfilling
the above requirements. In practice, there are a few commonly-used (and usually
commercially-available) electrode assemblies that have an electrode potential
independent of the electrolyte used in the cell. For some common reference electrodes
see e.g., the silver/silver-chloride electrode, calomel electrode, and hydrogen
electrode. Strictly speaking, there can be a small change in the potential of
these electrodes depending on the electrolyte because the presence of a liquid-junction
potential. This is very often (justifiably or not) ignored. The liquid-junction
potential is also minimized by the use of high concentration potassium chloride
as the filling solution of the reference electrodes, because the diffusion rate
of both ions is very closely the same in these solutions.
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Reserve battery: A non-rechargeable
battery that is stored in an "inactive" form until its intended, immediate use.
E.g., the battery is stored "dry," and it can be activated by injection of the
electrolyte. Or, a battery operating with molten salt electrolyte is stored
at a temperature below the electrolyte melting point, and it is activated by
the sudden application of heat to melt the electrolyte. The advantages of such
arrangements are an extremely long shelf life without practically any loss of
stored energy, and the possibility to produce a very powerful battery with very
reactive chemicals that would otherwise cause a very fast self discharge. Most
reserve batteries are made for military applications.
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Residual current (density): A small
faradaic current density flowing through an electrode under conditions when
zero faradaic current is expected (e.g., within the double-layer range). It
is caused by traces of impurities in the electrolyte. Also called "background
current (density)."
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Resistance: See resistivity.
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Resistance overpotential (polarization):
See solution ir drop.
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Resistivity (electrical): The measure
of a material's inability to carry electrical current. The measurement unit
of the resistivity (resistance) is the ohm. See also impedance. The reciprocal
of conductivity.
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Resistor: An electrical circuit element
with a fixed resistivity.
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Rest potential: See equilibrium potential.
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Reversal:
See cell reversal.
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Reversible electrode: An electrode with
a reversible electrode reaction.
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Reversible electrode reaction: A qualitative
term for a fast electrode reaction. There are, unfortunately, several meanings
attributed to the term "reversibility," resulting in possibly confusing situation.
An electrode reaction is considered reversible in the "electrochemical sense"
if the reaction is fast, that is, if the exchange current density of the electrode
reaction is large. In contrast, in the "chemical sense," reversibility indicates
that the reaction can proceed both in forward and backward (reverse) direction.
Also called nernstian reaction. Opposite: irreversible electrode reaction. See
also quasi-reversible electrode reaction. Both of the above described meanings
of reversibility are different from the meaning in the "thermodynamic sense."
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Reversible hydrogen electrode: A commonly
used reference electrode. A hydrogen electrode immersed directly into the electrolyte
of the electrochemical cell and usually (unless otherwise sated) operated with
one atmosphere pressure hydrogen gas. The equilibrium potential depends on the
hydrogen ion concentration (strictly speaking, activity) of the cell electrolyte.
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Rotating-disk electrode: A specialized
hydrodynamic electrode used in the study of the kinetics and mechanism of electrode
reactions and in electroanalysis for ensuring a known and controllable flow
of solution over the electrode. The flow control is achieved by using a flat
disc electrode that is rotated in the solution resulting in a defined hydrodynamic
boundary layer. Abbreviated as "RDE."
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Rotating-ring-disk electrode: A variant
of the rotating-disk electrode which includes a second electrode - a concentric
ring electrode - that is placed outside the disk and used to analyze the species
generated on the disk. The ring is electrically insulated from the disk so that
their potentials can be controlled independently. Abbreviated as "RRDE."
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Rotating-wire electrode: An electrode
made of metal wire (often platinum) rotated about its axis at a known and constant
velocity. It is used in the study of the kinetics and mechanism of electrode
reactions and in electroanalysis.
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Roughness factor: The ratio between
the true electrode area and the geometric electrode area.
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RRDE: Stands for rotating-ring-disk
electrode.
