Reactions between molecules can occur in two basic ways. The &st one is well known: molecules 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
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 outside a few specialist groups, is discussed. The relation of overpotential to the development of near-future technologies is indicated. A Stumbling Path Downwards
-
2 ~ 2 0 4Hz0 with geonoration of electrical energy. Hn a n d - ~ nmr; in'it 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 remained a t equilibrium, though in contact with the substrate 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 conversion, 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 reaction could (hypothetically, and for certain reactions) 352
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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 discussions of the electrical potentials of galvanic cells. Correspondingly, attempts to deduce structural inforrnation 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 considered 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 measurements of the variation of the reversible potential with temperature made possible determination of the reaction enthalpy, etc. Such macroscopic material was a center of classical ("traditional") physical chemistry. I n what way did this physical chemistry of thermodynamically 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 reversible condit,ion must be achieved for t,he emf measurement to he acceptable, a net react,ion rat,e (i.e., a current,) was an anathema. r\Tevert,heless, t,he trarlitional electrochemisls did have a t,heory of polarizat,ion, as departures from t,he reversible thermodynamic pot,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 exchanging 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,ionvalues. I t was assumed that the electron tmnsfcr occurring a t the interphase of metals with t,heir solut,ion occurred i?,a thermodynamically 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 displacement of the potential from t,he expected thermodynamic 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 it 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 concelltration 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 hypothetical 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 ~vorkt,o form a bubble depends on the gas-solutio~iiuterfacial 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 {.hepalrntisl was developed. Chemists said "At the interface 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 t o be related to the free energy changes for the reaclkrn occurring in the whole cell. But physicists claimed t h a t 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 potentials were small enough l o pass far eonce~rlmtionpolariantion.
The fact that large overvolt,ages were observed for hydrogen and oxygen evolution encouraged rntiondizntions of t,hese large overpote~tialsby 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 beginning 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. I n spite of the presence of the eminent physicist, hlott, and the theoretical electrochemist,, 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 concept 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 overpotential and the lack of knowledge of it. We may mention Tafel (1903), mho was still very much with thermodynamics. 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 surface 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,ialin 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 elect,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,eson 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
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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 solution. 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 thermodynamically 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, remarkably 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 commencement of the consideration of the fuel cell3 was entirely thermodynamic. The thermodynamically reversible galvanic cell was used in textbooks to show the relation of the cell potential, measured under thermody?ll"ainicallyreversible 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 understood. If one obtained the energy released in a chemical react,ion firstly as heat, and then converted this, via a heat engine, to work (and further, wit,hout intrinsic efficiency loss, to energy), there Tvas an efficiency 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 character of the electrochemical energy converter. It was more than a matter of the convenience of a di~ectconversion of chemical energy to elect,ricity without mov-
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ingparts. For this, other indirect met,hods (e.g., thermionic 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 ~toc ~ 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 neoessary 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 unrealized. 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 a t 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 electrodes of specific designs, the rate per real square centimeter (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 1 -. is ~ II. fnel . Attention -.~.p . . d- l ~ ~ - ~ is concentrated on the energyproduced 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,vof 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 PhD degree in the Electrochemistry Bacon, had obtained Group a t the Imperial College of Science and Technology (London), in 1951. ~~
~
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 selffunctioning 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 regarded 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 oocurrenoe 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 reaction) was due to Gurney, 1932. But i t was as first rejected by eledrochemists and only relt~ctantlytaken into alactrochemistry in the 1960'6. a) In pardel to these "nunors of overpotential," rn attempt to realize the free energy of chemical reactions directly as eleotrial 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 consequence of Nernst's folly. 4) Ideals of overpotentid developed without reference to self-driving electrochemiesl systems, and them stumhled forward in the 1960's (in the hands of m&m~) without an understanding of overpotential, and without comprehension being gained that the devices were macrc-acde-ups aimed at obtaining usefulenergy of a second general pathfw 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 ( H a + , 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 ourselvesto 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 hypothesis," very easy (and hence thermodynamically reversible) 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 overlaps with empty levels in the solution; otherwise, no significant electron transfer from metal to solution occurs. This is what the shifting of the electrode potential (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, at the thermodynamic reversible condition for platinum in a solution saturated with & (no net electron 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 1mA om-'. Then, we can get this acceleration 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 at thermodynamic revemibility (in its smundbg8, etc.) to provoke a specific emi.ra'~onr a v to a given acceptor in
+
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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
A, Graphical repnrent(~timof electron tunneling to a hydrded proton. The hydrogen mom .ofetmed s & s to the metal wrfacs formbg a metal-hydrogen IM-H) specie% 0, Energy level representotion of slesb u r s 3.
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 unfllkd levels b rolution, electmn tvnndr Lmvph the borrier. The shift of the Fsrmi level, or represented in the dlagrom, =sun os a result of pumping in of dedrom from an exlomcl source.
solution." It is a t once clear that the neoessary overpotentiil for a given ehemioal reaction will be a function 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)
Quantum Mechanics and Electric Reacfion Medunisms
- 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 eleotrone. Quite rightly, if II = 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~lamhkbits an overpotentiel, there can be no net reaction. The present physical explanation of the idea of overpotential has not yet been referred to a clear expesimental 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 semioonductor) in solution. One might assume, 8s an ex356 / Jourml o f Chemical Fdwation
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 chemiual 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 Electroohemiaal reactions, like the decay of nuclei, or the MBssbauer effect, are essentially quantum mechanical. 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 acjiicabii;ty in biological systems was kxpreased in 1969. in 1970, Wsguer sullgested critical experiments whereby electrical and chemical reaction path wuld bedilllwished.
2 ) The rlevtn,dt4e~-dcpo.itim of, e.g., rner~lifromsrtlutioui n IIIP pre.cl.rr:of wrt.tin r~rgklticrnatrr~.iI~ 3 TI,?f < m < . ~ t id ,!T ~c f r m T L ' l ~ s, t t d \lg i n ~ h Krdl c prwe.+ for extraction of Ti 4) The reaction of HSwith 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 reactions. For example, barrierless transition, i.e., t>ransitions 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.lenergy which is hypothetically obtainable becomes greater t,ha.nt,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, overpotemtial 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 potential necessary in order to make the reaction occur a t a finite mt,e. One cannot purchase-achievc-something 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 thermodynamically 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 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, metallurgists, 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 could
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~ns/~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~hermodynamicallyreversible. 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 countries, iu part,icular in those of t,he United Stat,es, by t,he same ~ n c i e u trit,es. Nirrety-nine percent, of biochemistry st,udent.s,for example, where a kno~dedgeof overpotential is pa.rticularly applicable, arid xhere the applicat,ion of tired, t~radit,ional,electrochemical principles is even now videspread, remain unam~reof 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 assumption 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,sof 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. Electroanalyticalchemists-oftengratuitously calledelectrochemistsare 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 overpotential (or, e.g., the exist,ence of adsorptive intermediates on elect,rodcs) are usually not ment,ioned in textbooks on electroanalytical chemistry. I n terms of numbers, there mould seem to be some ten electroanalytical chemists for each futidament,al electrochemist, and this fact, together \~-it,hthe confusion which most academics have (i.e., that analytical chemists 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 elem 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 overpotential by modern physical chemists. Physical chemistry tends to be increasingly divided into the so-called traditioual material (thermodynamic t,reat,ments of homogeneous and heterogeneous equilibria; solutions, including ionic solutions) and the material which is now rationalizable in atomic t e r m ~ t h esimple phenomena such as the exchange of energy between electron states of molecules in the gas phase, the effect of electromngnetic radiation on materials, et,c. When presented with material concertled vith electrochemical cells, all but a few scientist,^ recall the tmditional t,hermodynamic 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,entitled overpotential. The phenomenon is supposed t,o Volume 48, Number 6, June 1971
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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 reversibility, 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 presentday 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~ticlesunderwent diffraction. 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: "electrochemistry 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.
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use of electricity and electric modes of operating chemical 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 significant proportions by 2000 A.D. has recently been featured 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
O'M.. A N D R ~ o o u ,A. K. N., "Modern Electrooherniatry," vols. 1 and 2,Plenum Press. New York, 1970, Partioulerly Chapter I. B o ~ x m s I. , O'M.. AND Snmmv*8*~.S., "Fuel C e l l s T h e i r Eleotroohemis try," MeGraw-Hill. New York. 1970. BOOKRIS.J. O'M., " E l e c t r o c h e m i s t r ~ T h e Underdeveloped Science." J . Eleetioonal. Cham.. 9,408 (1965). CONWAY, B. E., A N D SALOMON, M.. "Electm~hemistry: Its Role I n Teaching Physioal Chemistry," I. C n w . Eoac.. 44,554 (1967). KITTEL,C.. "Intrmd~ctianto Solid State Physios:' John Wiley & Sona. Ino., New York. 1PW. ZIMAN,J. ,M.. Principles of the Theory of Solida," Cambridge University B o c ~ m a .I.
Press, 1965.
