In chemistry, an
electrochemical
Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outc ...
reaction mechanism is the step by step sequence of
elementary step A reaction step of a chemical reaction is defined as: ''"An elementary reaction, constituting one of the stages of a stepwise reaction in which a reaction intermediate (or, for the first step, the reactants) is converted into the next reaction inter ...
s, involving at least one outer sphere
electron transfer
Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons.
Electrochemical processes ar ...
, by which an overall chemical change occurs.
[
]
Overview
Elementary steps like
proton coupled electron transfer A Proton-coupled electron transfer (PCET) is a chemical reaction that involves the transfer of electrons and protons from one atom to another. The term was originally coined for single proton, single electron processes that are concerted, but the ...
and the movement of electrons between an electrode and substrate are special to electrochemical processes. Electrochemical mechanisms are important to all
redox chemistry
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
including
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
, redox active
photochemistry
Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible light (400–7 ...
including
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
, other biological systems often involving
electron transport chain
An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...
s and other forms of homogeneous and heterogeneous electron transfer. Such reactions are most often studied with standard three electrode techniques such as
cyclic voltammetry
Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment, the working electrode potential is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is r ...
(CV),
chronoamperometry
Chronoamperometry is an electrochemical technique in which the potential of the working electrode is stepped and the resulting current from faradaic processes occurring at the electrode (caused by the potential step) is monitored as a function of ...
, and bulk electrolysis as well as more complex experiments involving
rotating disk electrode A rotating disk electrode (RDE) is a working electrode used in three electrode systems for hydrodynamic voltammetry.Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications. New York: John Wiley & Sons, 2nd Edition, 2000. ...
s and
rotating ring-disk electrode A rotating ring-disk electrode (RRDE) is a double working electrode used in hydrodynamic voltammetry, very similar to a rotating disk electrode (RDE). The electrode rotates during experiments inducing a flux of analyte to the electrode. This sys ...
s. In the case of
photoinduced electron transfer the use of
time-resolved spectroscopy In physics and physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied after the illumination of a material occurs, ...
is common.
Formalism
When describing electrochemical reactions an "E" and "C" formalism is often employed. The E represents an
electron transfer
Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons.
Electrochemical processes ar ...
; sometimes E
O and E
R are used to represent oxidations and reductions respectively. The C represents a chemical reaction which can be any elementary reaction step and is often called a "following" reaction. In
coordination chemistry
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
common C steps which "follow" electron transfer are
ligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
loss and association. The ligand loss or gain is associated with a geometric change in the complexes
coordination sphere
In coordination chemistry, the first coordination sphere refers to the array of molecules and ions (the ligands) directly attached to the central metal atom. The second coordination sphere consists of molecules and ions that attached in various ...
.
:
n">Lnsup>2+ + e
− →
n">Lnsup>+ E
:
n">Lnsup>+ →
(n-1)">L(n-1)sup>+ + L C
The reaction above would be called an EC reaction.
Characterization
The production of
(n-1)">L(n-1)sup>+ in the reaction above by the "following" chemical reaction produces a species directly at the electrode that could display redox chemistry anywhere in a CV plot or none at all. The change in coordination from
n">Lnsup>+ to
(n-1)">L(n-1)sup>+ often prevents the observation of "reversible" behavior during electrochemical experiments like cyclic voltammetry. On the forward scan the expected diffusion wave is observed, in example above the reduction of
n">Lnsup>2+ to
n">Lnsup>1+. However, on the return scan the corresponding wave is not observed, in the example above this would be the wave corresponding to the oxidation of
n">Lnsup>1+ to
n">Lnsup>2+. In our example there is no
n">Lnsup>1+ to oxidize since it has been converted to
(n-1)">L(n-1)sup>+ through ligand loss. The return wave can sometimes be observed by increasing the scan rates so the following chemical reaction can be observed before the chemical reaction takes place. This often requires the use of
ultramicroelectrode An ultramicroelectrode (UME) is a working electrode used in a voltammetry. The small size of UME give them large diffusion layers and small overall currents. These features allow UME to achieve useful steady-state conditions and very high scan rat ...
s (UME) capable of very high scan rates of 0.5 to 5.0 V/s. Plots of forward and reverse peak ratios against modified forms of the scan rate often identify the rate of the chemical reaction. It has become a common practice to model such plots with electrochemical simulations. The results of such studies are of disputed practical relevance since simulation requires excellent experimental data, better than that routinely obtained and reported. Furthermore, the parameters of such studies are rarely reported and often include an unreasonably high variable to data ratio (ref?). A better practice is to look for a simple, well documented relationship between observed results and implied phenomena; or to investigate a specific physical phenomenon using an alternative technique such as
chronoamperometry
Chronoamperometry is an electrochemical technique in which the potential of the working electrode is stepped and the resulting current from faradaic processes occurring at the electrode (caused by the potential step) is monitored as a function of ...
or those involving a rotating electrode.
Electrocatalysis
Electrocatalysis is a
catalytic
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
process involving oxidation or reduction through the direct transfer of
electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
. The electrochemical mechanisms of electrocatalytic processes are a common research subject for various fields of chemistry and associated sciences. This is important to the development of
water oxidation
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
and
fuel cells catalysts. For example, half the water oxidation reaction is the reduction of protons to hydrogen, the subsequent
half reaction
A half reaction (or half-cell reaction) is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction. ...
.
:2H
+ + 2e
− → H
2
This reaction requires some form of catalyst to avoid a large
overpotential
In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly rela ...
in the delivery of electrons. A catalyst can accomplish this reaction through different reaction pathways, two examples are listed below for the homogeneous catalysts
n">Lnsup>2+.
:''Pathway 1''
:
n">Lnsup>2+ + e
− →
n">Lnsup>+ E
:
n">Lnsup>+ + H
+ →
n">MLnsup>2+ C
:
n">MLnsup>2+ + e
− →
n">MLnsup>+ E
:
n">MLnsup>+ + H
+ →
2MLn">2MLnsup>2+ C
:
2MLn">2MLnsup>2+ →
n">Lnsup>2+ + H
2 C
:''Pathway 2''
:
n">Lnsup>2+ + e
− →
n">Lnsup>+ E
:
n">Lnsup>+ + H
+ →
n">MLnsup>2+ C
:2
n">MLnsup>2+ → 2
n">Lnsup>2+ + H
2 C
Pathway 1 is described as an ECECC while pathway 2 would be described as an ECC. If the catalyst was being considered for solid support, pathway 1 which requires a single metal center to function would be a viable candidate. In contrast, a solid support system which separates the individual metal centers would render a catalysts that operates through pathway 2 useless, since it requires a step which is second order in metal center. Determining the reaction mechanism is much like other methods, with some techniques unique to electrochemistry. In most cases electron transfer can be assumed to be much faster than the chemical reactions. Unlike
stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
reactions where the steps between the starting materials and the
rate limiting step
In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate-determining step (RDS or RD-step or r/d step) or rate-limiting step. For a given reaction mechanism, the prediction of the ...
dominate, in
catalysis
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
the observed reaction order is usually dominated by the steps between the catalytic resting state and the rate limiting step.
"Following" physical transformations
During potential variant experiments, it is common to go through a redox couple in which the major species is transformed from a species that is soluble in the solution to one that is insoluble. This results in a nucleation process in which a new species plates out on the
working electrode
The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode sys ...
. If a species has been deposited on the electrode during a potential sweep then on the return sweep a stripping wave is usually observed.
:
n">Lnsup>+
(solvated) + e
− →
n">Lnsup>0
(solid) nucleation
:
n">Lnsup>0
(solid) → e
− +
n">Lnsup>+
(solvated) stripping
While the nucleation wave may be pronounced or difficult to detect, the stripping wave is usually very distinct. Often these phenomena can be avoided by reducing the concentration of the complex in solution. Neither of these physical state changes involve a chemical reaction mechanism but they are worth mentioning here since the resulting data is at times confused with some chemical reaction mechanisms.
References
{{Reflist
Electrochemical concepts