-
Reactions between molecules can occur in two basic ways. The
&st one is well known: mole- cules in excited states collide.
The other is novel: the reactant entities do not collide but meet
with separated sites of electronic conductors, in contact.
Electrons are given by reactant A to the conductor, and reactant B
at another site accepts them (Fig. 1). I n
2 ~ 2 0 - 4Hz0 with geonoration of electrical energy. Hn a n d -
~ n mr; in'i- t i d y adsorbed from solution, on electrodes in the
form of atoms. Tho porous portition in the middle d the sell
preventsthe mixing of Hz and 0%.
the functioning of this second basic path in chemical
reactivity, a central concept is the "overpotential" controlling
the partial reactions. It is the shift of the Fermi level in the
electronically conducting phase from that which it would have had,
had the reaction re- mained a t equilibrium, though in contact with
the sub- strate ensemble. This article portrays the evolution of
the concept of overpotential, its significance to pure chemistry
and its relevance to applied chemistry in the context of the
necessity to avoid the continued injection of COz and pollutants
into the atmosphere.
Overpotential is the controlling factor in the rate of important
practical processes, for example, the rate of decay of metals, an
important form of energy conver- sion, certain syntheses, the power
produced from energy storers, some biological processes. Phenomena
which have no analog in the collisional path for reactions occur:
the barrier controlling the rate of a reaction could be removed;
more than 100% of the heat of re- action could (hypothetically, and
for certain reactions) 352 / Journal of Chemical Education
be realized as electrical energy. Explanatory analogs of
overpotential are suggested.
The fact that a concept with such a wide applicability in
chemistry, metallurgy, and biology is not known out- side a few
specialist groups, is discussed. The relation of overpotential to
the development of near-future tech- nologies is indicated. A
Stumbling Path Downwards
I n order to understand the slowness of the evolution of the
concept of overpotential, and the electrical path for chemical
reactions, it is necessary to recall that the teaching of
(particularly physical) chemistry in the first half of the century
had a different flavor from that of physicochemical texts after
about 1950. The main difference was the bastion-like position
formerly given to classical thermodynamics. Theoretical chemistry
in the first fifty years of this century meant, effectively,
classical thermodynamics.
Looking back, now, it is easy to see that, a t high tide,
thermodynamics was sometimes applied to situations the equilibrium
state of which was largely in the eye of the researcher. A good
example is in pre-1950 discus- sions of the electrical potentials
of galvanic cells. Cor- respondingly, attempts to deduce structural
inforrna- tion from the activity coefficient of entities in liquid
mixtures, interpreted in terms of specific complexes, often used to
be published. This kind of chemistry was still being published in
the '50's.
In the '30's and '401s, the quantum theory, as taught in
chemistry, was a special-case affair, something which had to be
activated to deal with quantum effects in nuclear decay, or
specific heats at lotu temperatures. The prevading of much of
chemical thought by the quantum theory was not evident even in
Schrodinger's book, "What is Life?," written in 1943.
The change of attitude from a classical approach is greatest in
fundamental treatments of interfaces under charge transfer control.
Until the 1940's the treatment of this (largely in cells) was
entirely thermodynamic. The reaction occurring a t the interface
was not con- sidered in any mechanistic way, but in terms of cell
potentials and free energy changes. A leading section in
physicochemical texts of the pre-1950 era was devoted to the
"reversible galvanic cells," whereby measure- ments of the
variation of the reversible potential with temperature made
possible determination of the reac- tion enthalpy, etc. Such
macroscopic material was a center of classical ("traditional")
physical chemistry.
I n what way did this physical chemistry of thermo- dynamically
reversible changes deal with the disturbing fact that currents
could be drawn from galvanic cells,
-
gases evolved, even new organic compounds produced? As the
warnings of the t,exts were always t,hat t,he re- versible
condit,ion must be achieved for t,he emf mea- surement to he
acceptable, a net react,ion rat,e (i.e., a current,) was an
anathema. r\Tevert,heless, t,he trarli- tional electrochemisls did
have a t,heory of polarizat,ion, as departures from t,he reversible
thermodynamic po- t,entials upon passage of current were called.
As, according t,o the classical reversible thermodynamic theory,
the potential developed' is proportional t,o log (concent,ration of
ion i u confact with electrode exchang- ing elect,rons with it,),
t,hen the passage of a net current served (only) to create a
difference bct,wecn the bulk and interfacial concent~rat,ion
values. I t was assumed that the electron tmnsfcr occurring a t the
interphase of metals with t,heir solut,ion occurred i?, a thermody-
namically reversible wau (in spite of the relatively high net,
react,iou mte). This, t,he t,raditional electrochemist
hypothesized, was t,he origin of t,he polarization of dis-
placement of the potential from t,he expected thermo- dynamic value
which began to be called "overvoltage" in the 1930's. The t,acit
postulate that t,he electron exchange remained suficient,ly rapid
for the met:& solutiou interface to remain at equilibrium \ixs
seldom discussed; aud vhen Smits in 1924 suggested t,h:lt i t
might. not be true, he was met wit,h a surly silence. The chemists
and physicists of the time (1910-50, say) did admit that there
\!-ere cases in which the concell- tration overvoltage suggested
(see above) could not work. A reserve model was developed for
these, and in it the displacement of potential observed on passage
of a current \yes needed to overcome the work necessary to form a
gas bubble from, e.g., the H, or O2 evolving. Values of "bubble
overvoltages" were t,abulated for, say, hydrogen evolution on :I
number of metals. As the departure of the electric potential of a
metal varies with t,he velocit,y of electron flow into or out of
it,, such misapprehended tabulations mere analogous to hypo-
thetical tables made of, say, "wind resistance" for bodies of
various shapes, with no attempt to standardize the speed. The
neglect of the necessity to rationalize the large differences
between overpotentials observed on different metals for t,he same
reaction (the ~vork t,o form a bubble depends on the gas-solutio~i
iuterfacial tension) is difficult to understand.
Unt,il about 1950, the at,titude toward overvolt,age in
physicochemical texts (if it mas mentioned at, all) seemed couched
in terms which do now seem very curious. Because many cells "worked
with t,he Nernst approach, those which did not were regarded as
dissident-thermodynamically reversible basically but befouled by
some regrettable artifact of doubtful origin.
' It was never clear in the traditional electrachemislry of
colls where {.he palrntisl was developed. Chemists said "At the
inter- face of metal u,ilh solution," becsose the rearlions clearly
took place there, and the sum of t,he two pulenlinls st. the
electrodes of a cell had to be related to the free energy changes
for the re- aclkrn occurring in the whole cell. But physicists
claimed that the polentirrls of electric cells u w e of the same
order as that of the metal-metal potential diflerencc, formed st,
t.he jonclian (hcnco outside the soliltion) of tho two m e l d
electrodes.
a "Working well" meant that when the cell passed cmrent the
deviations from the thermodynamic values of the electrode po-
tentials were small enough lo pass far eonce~rlmtion polarian-
tion.
The fact that large overvolt,ages were observed for hy- drogen
and oxygen evolution encouraged rntiondizn- tions of t,hese large
overpote~tials by theories to do with gases. It was suspected t,hat
the gas evolved on the passage of current formed insulat,ing
bubbles on the surface. Passage of current through this resist,nnce
was thought t,o lead to a high overpotential in an ohmic way. This
writer's research supervisor (now deceased), a dynamic and
mustachio'd man of military mien and at,tit,udes, with a
super-thermodynamic approach, was frankly i m p d e n t with (and
sometimes contemptuous of) electrodes which exhibited
overpotential. "Pitch int,o t,hem," he briskly advised the author a
t the begin- ning of his graduate studies in London (1943). "See if
you can spot anything on 'em. Damned gas films again-or probably
merely bubbles."
I n 1947, o~erpot~ential was considered a t a Faraday S0ciet.y
meet,ing. In spite of the presence of the emi- nent physicist,
hlott, and the theoretical electrochem- ist,, Audubert, no hint of
the ubiquitous character of overpotential in int,erphasial
reactions in general emerged; nothing was suggested in respect to
the part it, might play in chemical reactions; and no mention was
made of applications of the concept in met,allurgy, engineering,
biophysics, etc. Upwards
In parallel wit,h these stumblings under the yoke of the attempt
to discuss thermodynamically a situation far from equilibrium, some
progress toward t,he con- cept of overpotential did occur before
1950. An account of them would take us far from the goal of
contrasting t,he breadth of phenomena associated wit,h overpoten-
tial and the lack of knowledge of it. We may mention Tafel (1903),
mho was still very much with thermo- dynamics. loor t,he hydrogen
evolution reaction, 2H+ + 2 e -. H,, he t,hought that hydrogen
atoms arrived on t,he surface so easily that they could indeed be
said to affect the elect,ric:~l potent,ial of the interface in a
thermodynamically reversible way. Thereafter, the chemical
recombination to molecules was relatively slow. I'or a given
current densit,y in the outer circuit, the hydrogen surface
concentrations were hence greater than those of equilibrium. This
displacement of sur- face pressure could be related
t,hermodynamically to a shift in pot,ential from that at. zero
current,, i.e., zero net reaction velocity, and t,hus provide an
overpotential ( T ) via the Nernst equation
Erdey-Gruz and Volmer (1930) were the first to re1at.e reaction
rate to overpot,ent,ial in the right way, i.e.
-
(where nF/RT = a constant at a given temperature) although
somewhat for the wrong reasons. Yor them, overpotential was t,he
change of the pot,ential difference across the double layer
necessary t,o do sufficient elec- t,rostat.ic work on ions t,o
cause them to jump from sites on the solution side of t,he double
layer over an energy barrier t,o sit,es on the clect,rode
surface.
Gurney had a correct formulat,ion in 1932: it is the origin of
modern ideas. He bad just formulated, wit,h Gamow, the theory of
radioactive decay-electron
Volume 48, Number 6, June 1977 / 353
-
tunneling out of the nucleus- and he saw in the brand new ideas
of quantum mechanics the basis of a new electrochemistry. For him
overpotential was the shift in the Fermi level necessary to allow
the electron in t,he metal to have energies overlapping with vacant
acceptor levels in molecules adjacent to the electrode in the solu-
tion.
The details of these beginnings are less important than to note
that the overpotential referred to was that a t a single phase
boundary. The orientation of the papers was still
potential-centric-how does one find a reason for this "deviation"
from the thermodynam- ically reversible potential upon passage of a
current across the interphase? There was no suggestion that. the
reactivity of chemical systems might be in terms of equal and
opposite electronic transfer velocities between absorbed reactants
and an electronically conducting substrate, each partial reaction
controlled by its own ~otential. Reactions had not yet been coupled
with overpotential, a happening to occur thirty years later.
Sideways
During the developnlent of the idea of overpotential, a
different development mas also talcing place, remark- ably
independently of the concept on which it was, later on, to depend.
The period of this development was longer, from about 1842 to about
1960. The subject concerned the conversion of the energy change in
a chemical reaction directly to available electricity.
As with the discussion of overpotential, the com- mencement of
the consideration of the fuel cell3 was entirely thermodynamic. The
thermodynamically re- versible galvanic cell was used in textbooks
to show the relation of the cell potential, measured under thermo-
dy?ll"ainically reversible conclitions, and the free energy change
of the overall reaction in the cell. A deviat,ion from t,he
thermodynamically reversible condition causes a current to pass
(e.g., forming or decomposing water according to lcig. I) , and, if
the deviation of the potential fronz the reversible value i s
infinitesimally small, then the current flowing, multiplied by the
(thermodynamically reversible) cell potential, provides watts,
which if the flow is continued until onc mole of Hz and a half mole
of Oz are consumed, will give energy in electrical units equal to
the free energy change AG for Hz + 1/202 + HzO.
It was Ostwald who in 1894 gave voice to the wider significance
of this mode of conversion of chemical to electrical energy, to
advocate an enti7"ely different path in the developnzent of
techuology from that which was at that time in an early stage and
which involved the Otto engine. Carnot's theorem was already well
under- stood. If one obtained the energy released in a chemi- cal
react,ion firstly as heat, and then converted this, via a heat
engine, to work (and further, wit,hout in- trinsic efficiency loss,
to energy), there Tvas an effi- ciency of perhaps 204070. If there
could be engineered a device which avoided the couversion of heat
to work, working at a finite mt,e, it would seem possible to have
efficiencies (referred to the fract,ion of energy realizable
compared with AH for the reaction) of perhaps 95% (i.e., of AGIAH).
Ostwald st,ressed the unique char- acter of the electrochemical
energy converter. It was more than a matter of the convenience of a
di~ect con- version of chemical energy to elect,ricity without
mov-
ingparts. For this, other indirect met,hods (e.g., thermi- onic
converters) were available. But these, too, were heat engines and
suffered the same Carnot limitation on the fract,ion of the energy
which they could convert as did more conventional heat engines.
The principle of the fuel cell had been demonstrated by Groves
in 1842. Work, part,icularly in Germany (Bauer) and France
(Jacques), and t,hen in Russia (Davt,yau), was commenced in pursuit
of Ostwald's objective. However, Ostwald had said not,hing about
~v&~otent,ial.
Here, Ost,wald and t,he early fuel cell workers suffered nnder
t,he cent,ral error of the Nernstian ~ D D I ' o ~ c ~ to ~~~~~~ ~~
~~
electrode potentials out of equilibrium. hey had assumed that
the charge transfel step took place in equilibrium, that any
departures from equilibrium actually observed mere due to
difficulties in t,ransport to and from the surface, or the singular
maladies of legendary gas films, etc. But these were matters of
engineering. A basic misunderstanding existed that electron
exchange processes between electronic and ionic conductors were at
equilibrium and that no neoes- sary shift of potential would occur
from t,he reversible value, i.e., overpotential as a7a intrinsic
phenomenon of a n electric couple in which a reaction i s
occurring, was un- realized. It seems reasonable to refer to this
concept as "Nernst's Hypothesis,"4 i.e., the assumption that
A + c e B
was maintained a t a pseudo-equilibrium at all practical
velocities. In fact, the feeling that this was correct persisted
among most physical chemists well into the '40's. They pointed out
that electron transfer, e.g., between metals, or semiconductors,
seemed to occur without kinetic hindran~e.~
Because of the legendary reputation of Nernst and the persistent
"querulous" occurrence of overpotential for finite reaction
velocities a t interfaces, attempts to realize Ostwald's
suggestions were shackled. Some success was obtained empirically in
the following way. (1) By using systems a t high temperature,
Nernst's hypothesis was made to be less inapplicable (i.e., kinetic
hindrances were reduced). (2 ) By using porous elec- trodes of
specific designs, the rate per real square centi- meter (area)
could be retained small (and hence the overpotential reduced)'
while the rate or current per external square centimeter remained
practical. Hence, already in 1955, Bacon, with little advertised7
attention to the intrinsic, quantum mechanical and solid-state
physics aspects of overpotential, demonstrated the first practical
fuel cell, an energy converter of 5 kw and an efficiency of some
70%.
a r i m ~ p 1 i s II. fnel d l . Attention is concentrated on
the en- ..--.. . -. ~ . - ~ ~ - ~ ~~ ~ ergyproduced per mole of
fuel consumed.
-' And to its acceptance and perpetuation as "Nernst'sfolly." 6
The late Nobel laureate electrochemist, Heyrovsky, still
insisted on Nernst's hypothesis in the 1050's, and indeed was so
anxious about the ~ossibilit,v of its breakdown that he is reported
to have threatened to have one of his coworkers, who wrote against
it, removed to a low-ranked appointment at an institute in a town
on his country's border.
8 Overpotential increases with log rate. 7 R. G. H. Watson, who
was one of a team of two working with
Bacon, had obtained PhD degree in the Electrochemistry Group a t
the Imperial College of Science and Technology (Lon- don), in
1951.
354 / Journal o f Chemical Education
-
Thus, just as there was a developwent of ideas on overpotential
which were not a t the time related to the functioning of entire
electric cells (ssd the electric path for chemical reactions), so
the maw-realization of that mode (actual fuel cells) stumbled
forward wi6hout being connected to the overpotential which in fact
determines what fraction of the energy of the self- functioning
cell bas to be wasted in making the cell work and deliver energy at
a certain power.
It is helpful to summarize the stumblings which led so
hesitantly to overpotential as a broad concept.
1) In the development of chemistry from the thermodynamic to the
quantum mechanicd, electrochemical systems were re- garded as "the
most thermodynamic of all" because they were r m d as examples of
near-ideal thermodynamically reversible systems. When a net current
was pawed through a cell, the deviation of the potentid from a
reversible value which oocurred (the overpotential) was not
recognized as inbn'rzs*! b the oo- currenoe of a reaction with
interphasial c h g e transfer (see below). I t was given
special-weinte'pretation.
2) The first formulation of overpotentid as an intrinsic prop
erty of a ehhxge-transfer syatem (no overpotential, no net re-
action) was due to Gurney, 1932. But i t was as first rejected by
eledrochemists and only relt~ctantly taken into alactrochem- istry
in the 1960'6.
a) In pardel to these "nunors of overpotential," rn attempt to
realize the free energy of chemical reactions directly as eleo-
trial energy was going on. I t offered a remarkable avoidance of
the consequences of Cawot's theorem. But it was befogged by the
incorrectness of "Nernst's hypothesis" (thermodynamic reversibility
of electron transfer reactions at interlam), the con- sequence of
Nernst's folly.
4) Ideals of overpotentid developed without reference to
self-driving electrochemiesl systems, and them stumhled for- ward
in the 1960's (in the hands of m&m~) without an under- standing
of overpotential, and without comprehension being gained that the
devices were macrc-acde-ups aimed at obtaining usefulenergy of a
second general path fw chaicol rsadiudt~.
Overpotential The Mochinefy of Overpotenfiol
The formal definition of overpotential is "the change of
potential of the electron-conducting phase when reaction rate
across its interlace with the ion-conducting phase with which it is
in contact, is changed from zero to a certain velocity." It is easy
to explain the physical significance of overpotential, for an
amhetypal reaction is the transfer of an electron from the electron
conductor to an entity (Ha+, say) a few angstroms from the solid
phase, and in solution. Two points are relevant:
1) The energy situation for the electron is analogous to that of
a particle in a nucleus. It does not have, at room temperature, d c
i e n t energy to jump out of its position, classically. The only
way it can exit to some suitable state in the solution is by
utiliaing the quantum mechanical property of transfer through a
barrier.
2) The electron cannot pass from a state in the metal to a state
in the solution without being "received" in some state in which the
electron energy will be the same as that in the metal. Hence, a
primary condition for a given reaction velocity will be that there
are suitable electron levels in the metal in whieh the electron has
an energy equal to the energy of empty states in some particles in
the solution
For the purpose of this explanation of the physical meaning of
overpotential the states in the solution (for example, their
distribution law) will not be considered further. But consider the
electronic states in the
metal. We shall neglect the subtlety of s u & ~ states and
confine ourselves to the Fermi distribution as shown in Figure 2.
For electrons which have an energy IEF + kT1, there are a
negligible number of electrons in them. We can wmider, therefore,
that the reaebant electrons are only those at the Fermi level. At
room temperature, the rate of escape of electrons from the metal
clsssically is negligible. Electrons have therefore to pass quantum
mechanically through the barrier a t the electrode, as shown in
Figure 3, to reach the acceptor states (e. g., in H of HaO+) in
solution.
It may be that the situation which these electrons face is then
suitable, i.e., as portrayed in Figure 3. I n such a case,
tunneling occurs easily. This would represent what has been called
here "Nernst's hypothe- sis," very easy (and hence
thermodynamically reversi- ble) electron passage from metal to
solution. More usually, howwer, there has to be a significant shift
of the electronic levels so that the new electron level over- laps
with empty levels in the solution; otherwise, no significant
electron transfer from metal to solution occurs. This is what the
shifting of the electrode po- tential (i.e., ovelpotmtial) d ~ : it
alkm the energy of the available electrons, until they are in a
statein which they overlap with a suitable number of empty acceptor
statw in the solution.
Thus, a t the thermodynamic reversible condition for platinum in
a solution saturated with & (no net elec- tron transfer rate,
electrode to solution), the rate of passage in the direction
electrade to HaO+ is 10-l1 mole of electrons om-* sec-1. We wish to
have a net reaction rate from metal to mlution of, say, 10- mole
electrons ~ m - ~ mc-', a rate equivalent to a e m n t density of 1
mA om-'. Then, we can get this accelera- tion by change of the
electron level in the metal by about 0.36 V. This change is the
overpotential. Thus a physical definition of overpotential would
be: "the energy in electron volts by which an electron in a metal
has to be excited from the energy it has in the metal a t
thermodynamic revemibility (in its smundbg8, etc.) to provoke a
specific emi.ra'~on r a v to a given acceptor in
Volume 48. Number 6, lune 1971 / 355
-
b u r s 3. A, Graphical repnrent(~tim of electron tunneling to a
hydrded proton. The hydrogen mom .o fetmed s & s to the metal
wrfacs formbg a metal-hydrogen IM-H) specie% 0, Energy level
representotion of sles- ttan tunneling through a barrier to a
hydrwbd proton. The bmken lines reprerefit lhe shift of the Formi
level and the energy barrier by #r rrp plierrtion of a cathodic
potential when Ea (d becomes equal to the un- fllkd levels b
rolution, electmn tvnndr Lmvph the borrier. The shift of the Fsrmi
level, or represented in the dlagrom, =sun os a result of pump- ing
in of dedrom from an exlomcl source.
solution." It is a t once clear that the neoessary over-
potentiil for a given ehemioal reaction will be a func- tion of the
metal and the acceptor in solution (i.e., an electrocatalysis will
exist). It is easy to show that rate of electron emission = k
(e-*hFfaT) - e('-*)(@'/RT)f
(4) where k is a rate constant characteristic of the electron
conductor from which the emission is occurring; or is a constant,
often I/*; 8' is the charge in a mob of eleo- trone. Quite rightly,
if I I = 0, the rate is zero, whereas if the hypothesis we have
attributed to Nernst were applicable, the rate could have been
finite. Unless a syste~la mhkbits an overpotentiel, there can be no
net reac- tion.
The present physical explanation of the idea of over- potential
has not yet been referred to a clear expesi- mental situation, it
having been said that emission of eleetrons from a metal to the
acceptor H,O+ was considered. We should need some eledrochemioal
circuit, a counter electrode, and driving power source. Let us
consider, however, what would occur if one did not have any of
that, but just a piece of metal (or semi- oonductor) in solution.
One might assume, 8s an ex-
ample, that there are CUM ions in solution, and that there is
also formaldehyde. The latter can undergo the de-electmnation
reaction, as
HCHO f KO - HCOOH + 2H+ + 26- (5) in whkh electrons are given to
the conductor a t a rate which we know is a function of
overpotential, as given by eqn. (4). Simultamusly, the Cuy+ ions
nndergo electronation reaction at Werent sites on the wnductor,
as
GUS+ + 2*- -+ cu (6) These reactions occurring on the same
eondnctor are shown in Fjgure 4. There will be an overpotential
at
which the two partial reaction rates become equal, one reaction
donating electrons to the substrate at some sites and one
attracting electrons at others. A chemi- ual reaction is occurrim.
but electmebemicallv. and the main controllii quanTity about the
net r& of this reaction is the merpotmtial concerned for each
partial reaction.8 Quantum Mechanics and Electric Reacfion
Medunisms
Electroohemiaal reactions, like the decay of nuclei, or the
MBssbauer effect, are essentially quantum mechani- cal. There wuld
be no electrochemical reaction path in classical mechanics,
because, according to it, the rate of electron emission to a
solution at room temperature is negligible. Systems in Which an
ElecfrochemimI Readion Path Has Been Edablished
A number of situations am 1) The ourrentleas d e w of metals and
mmimnductors in solu-
tion C'corrosion")
' A qualitative concept of this type was first stated by Hoar
and Evans for the currentleas dirsolution of metalsand developed bv
Wuener and Traud for the dissolution of Zn in HCI. An ac-
jiicabii;ty in biological systems was kxpreased in 1969. in 1970,
Wsguer sullgested critical experiments whereby electrical and
chemical reaction path wuld bedilllwished.
356 / Jourml o f Chemical Fdwation
-
2) The rlevtn,dt4e~- dcpo.itim of, e.g., rner~lifromsrtlutiou i
n I I I P pre.cl.rr:of wrt.tin r~rgklt ic rnatrr~.iI~
3 TI,? f < m < . ~ t i , ! ~ d Tc f r m T L ' l ~ , s t t
d \lg i n ~ h c Krdl prwe.+ for extraction of Ti
4) The reaction of HS with nitrobenzene in the presence of Pt
Novel Effects
A number of effects arise in electrochemical react,ions which do
not have an analog in classical chemical reac- tions. For example,
barrierless transition, i.e., t>ransi- tions in which t,he
thermal barrier for t,he bond breaking molecular (Maxwellian) part
of t,he reaction is negated by a sufficiently large overpotential.
Exceedingly fast reaction rates would be expected.
Another example is energy conversion to elect,ricit,y for
reactions in which t,here is a positive entropy change as in CO +
'/SO, - Con. In such a sit,uation, AG is numerically greater than
AH; and if heat,ing effects (e.g., 12R) are low, a net coolbig will
occur, so that heat is absorbed from the surroundings and the
maximum elect,rica.l energy which is hypothetically obtainable be-
comes greater t,ha.n t,he chemical energy of t,he reaction
occurring in the cell.
Some Analogues of Overpotential
A mention of some analogs of overpotent,ial may be useful to
help a wider understanding of t,he concept,. Thus, when the
reaction rate is small, overpotem- tial is also small; but
~verpotent~ial increases wit,h increase of rate. There is, thus,
some qualitative analogy to resistance in flow of fluids over
solids. lpor self-acting cells (t,he micro fuel cells envisaged
here), there is some resemblance to a graduated purchase tax: if
one spends available energy to do something (buy something), one
has to give up a part of it (tax). Thus, the overpotential is a
loss on the thermodynamic po- tential necessary in order to make
the reaction occur a t a finite mt,e. One cannot
purchase-achievc-some- thing without "wastirig" part of the
purchase price or1 the tax.
Another aspect of overpotential for self-operating cells is that
it is a kind of efficiency factor: the more the electron levels
shift in the metals, t,he less the ther- modynamically available
energy in the process can be used in production of the material
concerned.
Why Is Spread of a Knowledge of the Concept of Overpotential
among Scientists So Slow?
The phenomenological definition of overpotential could be found
in papers in the literature before 1940. A quantum mechanical
interpretation of overpot,ential was first given, though not
accepted, in 1932. I t is, however, an astounding fact that
chemists, metallur- gists, and biologists in 1970 are generally
unaware of the definit,ion, or concept, of overpot,ential, even at
a phenomenological leveLg Even more remarkable, most scient,ists
(particularly biologists, and some material's
V h u s , effectively all scientists know that thermal reaction
rate = ~onst.e-"/"~
hut effeclively no scientisls know thal
elect,ricsl reaction ra1.e = c ~ n s / ~ e - " ( ~ ~ / ~ ~ ) I t
seems probable that many more nnnl.urally occurring reactions take
place by mechanisms in which the lat,ler is a more spplicable
relatiun t,han the former.
science workers) continue deep in Nernst's folly, i.e.,
implicitly assume that electron t,ransfer react,ions a t interfaces
are t~hermodynamically reversible. k'urt,her, the heatment of
ancient chemical t,hemes, e.g., the thermodynamics of reversible
cells, contiuues to be imparted to st,uderlts in the universities
of some coun- tries, iu part,icular in those of t,he United
Stat,es, by t,he same ~ncieut rit,es. Nirrety-nine percent, of
biochem- istry st,udent.s, for example, where a kno~dedge of over-
potential is pa.rticularly applicable, arid xhere the appli-
cat,ion of tired, t~radit,ional, electrochemical principles is even
now videspread, remain unam~re of overpotential, the breakdown of
reversibiMy, the possibilit,y of an electrical path for chemical
reactions, etc. Here, also, books on thermodynamics of biological
reactions are written, but all with the exceedingly improbable
assump- tion that the reactions occur without loss of energy due to
an overpotential; and the questionable assumpt,ions that, they a11
occur by a. classical, collisional mechanism.
One cause of thc curious hiatus in knodedge among scient,ist,s
of overpotential lies in a lack of knodedge of t,he precise (in
particular the theoretical) significance of the term among all but
a few hundred fuudament?l electrochemists. The rationalization of
overpotential in quantum mechanical terms, and its relation to
solid state physics, electrocntalysis, etc., have not as yet
emerged from the specialist literature. Electroanalyti-
calchemists-oftengratuitously calledelectrochemists- are not axare
of t,he connections of overpotential t,o metal properties. Their
subject depends mainly on transport in solutions and the
applicat,ion of the classical Nernstian concepts; :md modern
dheories of overpo- tential (or, e.g., the exist,ence of adsorptive
interme- diates on elect,rodcs) are usually not ment,ioned in text-
books on electroanalytical chemistry. In terms of numbers, there
mould seem to be some ten electro- analytical chemists for each
futidament,al electro- chemist, and this fact, together \~-it,h the
confusion which most academics have (i.e., that analytical chem-
ists who utilize electrochemical methods are in fact
electrochemists) very badly confuses scientists from other levels
who inquire about the present, position in the field.
Correspondingly, t,he absence until 1970 of an ele- m e n t ~
modern t,extbook on electrochemistry in English has been a
hindrance.
However, it seems justified to infer that a uegat,ive
psychological factor may be present, particularly in respect t,o
the absence of comprehension of overpoten- tial by modern physical
chemists. Physical chemistry tends to be increasingly divided into
the so-called tradi- tioual material (thermodynamic t,reat,ments of
homo- geneous and heterogeneous equilibria; solutions, in- cluding
ionic solutions) and the material which is now rationalizable in
atomic t e r m ~ t h e simple phenomena such as the exchange of
energy between electron states of molecules in the gas phase, the
effect of electromng- netic radiation on materials, et,c. When
presented with material concertled vi th electrochemical cells, all
but a few scientist,^ recall the tmditional t,hermody- namic
t,reatment to which he has been exposed. He can hardly t,hink of
auyt,hing more clearly classical and traditional than
electrochemical cells. He is then asked to consider a concept which
to him is ne~v, en- titled overpotential. The phenomenon is
supposed t,o
Volume 48, Number 6, June 1971 / 357
-
be entirely "quantal" (in galvanic cells?!) and to be
rationalizable a t an atomic level. Being now in a state of
strained incredulity (no Nernst's equation, no re- versibility, all
quantal, solid-state physics involved!), he then learns that the
phenomenon concerned is the controlling factor in some surface
reactions which go on around him, and may also play a part in
controlling, e.g., his rate of digestion. Lastly, upon checking
into the matter, he learns that there is no account of the
phenomenon in any pre-1970 textbook, and that many (perhaps most?!)
of those who are called electrochemists don't recognize a
definition of overpotential in terms of shift of Fermi level, and
are pretty vague about whether it has any connection to the
solid-state physics of the substrate. I t is too much. The relation
of a present- day physical chemist to overpotential resembles that
of a physicist toward the beginning of the century who was faced
with some tale about mass changing with velocity, or when told
t,hat pa~ticles underwent diffrac- tion. It is too revolutionary,
seems to claim too much, and totally upsets stable ideas which were
at the center of the modern chemist's Weltanschanug: "electrochem-
istry is part of traditional chemistry!" Some Unfortunate
Sociological Consequences
Delay in the spread of knowledge of modern aspects of
electrochemistry is unfortunate because we are in a time in which
technology must turn exclusively to the
--
LO Calculstions of the greenhouse effect in causing a l m rise
in world sea levels varv from 20 to 50 vr. Thev aresubject to wide
errors because they-do not yet acc&nt for tce negative feedback
effect of increased cloud formation. Same unaccounted factors,
however, act to make the predicted time of rapid (1 ft/yr) rise of
the seas too conservative. Thus, the polar ice itself is now being
reduced in brightness because of deposition on the surface of solid
matter. Hence, the ice now absorbs heat incrensingly.
use of electricity and electric modes of operating chemi- cal
reactions.
1) It is clear (if one takes the viewpoint of the citizen and
not that of the owner of the oil wells) that the wrong path in
energy conversion was taken in 1894. Had Ostwald's advice been
followed, the dirt of the cities (oil and coal fumes), their noise
(internal combustion engines), and the pollution of the air and
some of that of the water would have been simply absent.
2) The danger of the rise of sea level reaching signifi- cant
proportions by 2000 A.D. has recently been fea- tured in a speech
by Mr. Daniel P. Moynihan, President Nixon's former representative
for urban affairs. Only an electrical and electrochemically based
technology can avoid it. Much of the injection of COz into the
atmosphere must cease during the next few decades.'0
3) The principal energy sources of the future will be solar,
atomic, and geophysical, and hence energy will he available
exclusively in the form of electricity. It will also be very much
cheaper than at present. I t would be disastrous not to be prepared
to use it. But its intelligent use-and our survival-at least in
chemical processes, energy conversion, metallurgy, engineering,
etc., depends on a widespread comprehension of the electrochemical
concept of overpotential. Suggestions for Further Reading Boc~ma.
I. O'M.. AND R ~ o o u , A. K. N., "Modern Electrooherniatry,"
vols. 1 and 2, Plenum Press. New York, 1970, Partioulerly
Chapter I. Bo~xms, I. O'M.. AND Snmmv*8*~. S., "Fuel Ce l l sThe i
r Eleotroohemis
try," MeGraw-Hill. New York. 1970. BOOKRIS. J. O'M., "Elec t
rochemis t r~The Underdeveloped Science."
J . Eleetioonal. Cham.. 9,408 (1965). CONWAY, B. E., AND
SALOMON, M.. "Electm~hemistry: Its Role In Teaching
Physioal Chemistry," I. Cnw. Eoac.. 44,554 (1967). KITTEL, C..
"Intrmd~ctian to Solid State Physios:' John Wiley & Sona.
Ino.,
New York. 1PW. ZIMAN, J. ,M.. Principles of the Theory of
Solida," Cambridge University
Press, 1965.
358 / Journal of Chemical Education
