e.q.03 understanding electrochemistry

7/29/2019 E.Q.03 Understanding Electrochemistry

1/3

Understanding Electrochemistry:Some Distinctive ConceptsLarry R. FaulknerUniversity of Illinois, Urbana, IL 61801

This article addresses a few basic ideas about electro-chemical systems tha t cause confusion among newcomers. Th eargumen ts are developed from the stu dent's general experi-ence with homogeneous chemistry. Th e chief goal is to syn-thesize a n understanding of the heterog eneity of an electro-chemical system and a chemical sense for the importantelectrochemical variables of pote ntia l, curr ent, an d charge.A fuller treat me nt begun along similar lines is available else-where (1).The points can he made in five simple state-ments.An Electrochemical System Is Not Homogeneous

Newcomers to electrochemistry have had much more ex-verience with homogeneous svstems tha n w ith heterogeneousones. Heteroge neities almo st always exist in electrochemicalsvstems. One cannot und erstand electrochemistrv without

interfa ce, the con centration of ferrocene will be drawn downat the surface, as shown in Figure l(h). Since the normalBrownian motion of molecules tend s to homogenize the so-lution, ther e is a ne t diffusion of new ferrocene molecules intothe depleted zone from more remote regions. Some moleculesdiffuse all the way to th e surface where they undergo reaction(3). Thus, there is a tendency for the system to transportreacta nts to the electrode from remote points, and the de-pleted zone w idens as the reaction proceeds. If there is stirring,

.fers a representation. INTERFACEElec trode reac tions, such as th e evolution of hydrogen, LAYERor the plating of copper, , ~ n i t i a l

Cu2++ 2e - u (21or the oxidation of ferrocene to ferricenium, 2 100 m s l o o m s

FeCpz- eCpz++ e(3! 5v7 han only take place at the interface between the electrode andthe solution of electrolyte. T he im porta nt point for the m o- 1msme nt is tha t the electrode can only affect or sense the par t of 0the solution in immediate contact with itself. No ferrocene can 0 1 0 2 0 0 1 0 2 0he oxidized if ther e is none a t the interface, even if it does exist D I S T A N C E / g msomewhere else in the cell. Figure 1. (a! Spatial structure of an electrochemicalsystem. (b) ConcentrationElectrode reactions, in fact, tend to make the composition profiles of ferroceneundergoing oxidation at the electrode. Distance is measuredin the nearby solution different from tha t further away. This from the electrode toward the bulk. Times are ela~sed eriodsrom start ofeffect is shown in Figure l (h ) for an electrode immersed in 1 electrolysis. c! concentrat ion profiles of ferricenium produced by oxidation ofm M ferrocene in acetonitrile. If reaction (3) akesplace at the ferrocene.

262 Journal of Ch em ical Education


2/3

then th e zone widens to a limit, because the stirring keeps thesolution homogenized beyond th at l imit . Ju st as the e lectro-reactant is deple ted near the interface , the product accumu-la tes nearhv. as shown in Fieure l (c ) .Th e nroduct eraduallv

th e comp osition is affected by an electrode reaction, is calledthe diffusion layer. It is large on a molecular scale, perhap slo4-lo7 A, bu t qu ite small on the usual dim ensions of cells,only 10-4-10-1 cm. Th e bulh is the part of the solution, farf rom the e lectrode (hu t s ti l l perhaps no m ore than 0.1 mmawav) where the comnosition is uniform.Of even smaller dimensions is the struct ure in the imm e-diate vicitity of the interface. It is intuitive that the p art of th esolution very near th e electrode, within a few molecular layers,m us t differ in struc ture from the characteristics of more re-mo te zones, simply because this p art of the system is forcedto interact with the electrode, which obviously has a verydifferent character from the bulk solvent. Likewise, the dis-tribu tion of mobile electrons in th e part of the electrode insides of th e interface have dim ensions of a few tens of ang-stroms, because th at is the scale on which intera tomic andintermolecular forces operate. Th e whole structu re is calledthe double layer .At th is noint . a verv imn ortant idea mu st be erasued. One, - .is tau ght ear ly to r e 6 on th e e lectroneutra l i ty of chemicalphases, i.e., on th e idea th at positive and negative charges existin equal numbers within a phase . This is not true of the sep-arat e phases which constitute the interface. Usually there aredifferences, so tha t phases retain ne t electrical charges. Thedifferences are small compared to t he total num ber of positivean d negative charges present, hence electroneutrality is a verygood approximation for most stoichiometric thinking. How-ever, th e excess charge has a big effect on the electrical ( andelectrochemical) properties of a phase.All conducting phases, such as metals an d electrolytes, tendto uush t he excess charges to their outer boundaries. T hus, the

is positively charged, then t he solution side has an equal excessof negative ions in th e interfacial region. One can show th atthe whole doub le layer is electrically neutral, even if both sidesare charged.Th e excess charge on the m etal can be changed a t will witha power supp ly, which is just a p um p for electrons. The sup plyforces electrons to en ter or leave the interface until the re-pulsion of charges remaining ther e will not allow the su pplyto remove or a dd m ore. The m etal can carrv nositive, zero, or

Many Things Can H appen at On ceAn electrode reaction can be ra ther complicated. Invariablyit takes d a c e in a mechanism of several s tens. Stud ents aref a mi l ia r k it h t h i s i de a, b u t i t is imp o r ta n t t o d e r s t a n d t h a t

the stem in electrode nrucesses are often very different inhind. T he re is at least'one heterogeneous electron-transferstep, l ike ( I ) , (Z), or (3) above. However, the re can also becoupled heterogeneo us processes of other types, such as (a )adsorntion or desorption of precursors, intermediate [e.g., Hatom; in (I)]or 6) igration of atoms across asurface during electrocrystallization of metals [e.g., Cu in (211(c) or surface-m ediated recomhination of atom s or radicals.Th ere can also be coupled homogeneous reactions in the so-lution . Acid-base reactions, metal-ligand chem istry, homo-geneous redox reactions, and radical-radical recomhinationorocesses are a uite comm on. These reactions can orecede orfollow the heterogen eous electron transfer.All of these chemical steps are kinetic events, and they all

proceed at rates described by rate constants. Th e overall rateof the electrode reaction is determi ned hv all of the individualrate s together, muc h as for a mechan ism of purely homoge-neous ste us. T he chief differences for electrode reactions a reth at the heterogeneous steps ( a) have ra te constan ts thatdep end o n th e stat e of charge of th e interface, i.e. , th e poten-t ia l of the e lectrode, and (b) have ra tes t ha t depend on con-centra t ion of reactants a t the surface.S ~ n c t .11, 1 ~ I e r t r ~ d 6ro,.c..s Q.. II I: t p l ~ 1 t l i ly :I 1hc el?,..1 r1&. .mj tn e r f a ch r i n rhe ~ ,vtra l le,iriitm r:jrt is 11ir ~ ~ I I V]Irrhi,h rextalil.. v3n IIV 11rou:hr to the cl c, tn & .r f n t rare 3rwli :~41 rt di~ci.. X I l h ~ i m r.ed .\lu,., !r(ln,> rl i - u s e ~ u l l sregarded as par t of th e overall mechanism, e ie n though i t isno t a chemical process. Mass trans po rt occurs hy diffusion(microscopic , random motion) , b y convection (hulk f low,stirring , movem ent of segments of solution), or by migration1ni~~t.11.111c I i m , ~ l u n i, I ] l ~ r t r i rie ld, ..\, cl> es ierc i n ,CII . I IC, w it A t a i n > ~ h c m 1 ~ . 1 1~ i ~ ~ . r m , i ~!r ,m a , . L m p l t e l ( c ~ r < ~ c t i e ~ i i i ~ . ~ i lit


3/3

r e f e r e n c ee l e c t r o d e

/ Iwork ing coun te re l e c t r o d e e l e c t r o d eFigure 2. Structure of an electrochemical ce ll The voltmeter has a high im-pedance, so current does not flow in the circuit between the working and ref-erence electrodes.

until its driving force is counterbalanced by the repulsionsbetween the excess charges. With a large excess of negativecharges, the potential is very negative and the energy ofelectrons on the electrode is hiph. Likewise, a large excess ofpositive charge implies a very positive potential and lowelectron energy. By changing the voltage on the power supply,the excess charge, the potential, and the electron energy canhe tuned continuously over a wide range.With this idea in mind , it is easy to unders tand oxidationand reduction a t the electrode. Consider a P t electrode incontact with a solution of ferrocene in acetonitrile. Ferrocene~ ~~has the property of being fairly easy to oxidize, is., it gives upan electron readily. Th e energy of th e highest electron in theferrocene molecule is, of course, determined by th e molecularstructure of ferrocene. and it has th e same value for all ferro-

possible. The st anda id potentia l for the ferrocenelferricen-ium couple is essentially th at critical energy. At more negativepotentials than E " , ferricenium would be reduced at theelectrode by the relatively high-energy electrons presentthere.Any electrode process is characterized by its own standardpot ent ial Eo. On the positive side ofE o, he oxidizedform ofthe couple is stable a t the electrode, and the reduced formtends to undergo oxidation if it reaches the electrode. Like-wise, the zone of potential more negative than E o s a regionwhere the reduced form is stable and the oxidized form tendsto be reduced. Actually, in a zone about 100mV wide centeredon En . tatistical eauilihrium of electrons on the electrode andon species in the shution permits mixtures of oxidized andreduced forms with appreciable amounts of both species.These ideas can tell us what m a y he possible a t a given en-erev, but the kinetics of the reactions determine whether anenergetically allowed reaction actually does occur. Some re-actions, such as (1)on Hg, are very sluggish and do not occura t appreciable rates unless a large excess potential beyondE ois applied. Others, like (31, are quite fast.Experimentally, one can measure only differences in po-tentials between two electrodes, so the word potential inelectrochemical usage means the voltage difference (Fig. 2)between a working electrode and an independent referenceelectrode. The reference electrode is usually arranged, so thatit does not pass current and is at equilibrium. It is constructed

nega t ive zoneFeCp; + e- eCp2transition zone

;".IFU positive zone

-1 w FeCp2-e-~ecp;W JW POSITIVE LIMIT

Figure 3. Illustration of po tentialas an expression of electron energy. At potential(a) ferricenium at the surface would be reduced to ferrocene.At potential (b)ferroc ene at the su rface is oxidized to ferricenium. The potential limits corre-spond to values for which the solvent. supporting electrolyte, or electrode ma-terial are oxidized or reduced.

so tha t i t contains both forms of a redox couule (e.a.. HflH ?.Hg/Hg2Clz, AgIAgCl) at fixed composition. ~l ec tr ok en er gi e~ iin the metallic uar t of the electrode are. therefore, fixed at avalue near thadcorresponding to E o for the coupl'e. This a r-rangement urovides an invariant reference uoint against whichworiing electrodes can he measured. -Suace is too limited here to discuss the manner in which an

cannot discuss the confusing matter of using potentials toextract thermodynamic information (e.g., free energies oreauilibrium constants) for chemical reactions. Careful dis-cissions are availahle in th e literature ( 3 , 4 ) .One Cannot C ontrol Both Current and PotentialSimultaneously

Once the energy available to a reaction has been determinedin anv chemical svstem, the reactlon uroceeds at a rate cor-respo"nding to tha t energy. In homogeneous kinetics, one candictate the temuerature and acceut the rate, or one can dictatethe rate and accept the corresponding temperature. Likewisein electrochemical systems one can set the potential of anelectrode and accept the corresponding current flow, or viceversa. Choosing one variable for control precludes any sepa-ra te influence over the other.The five major concepts discussed above give only a foun-dation for an understanding of electrochemistry. However,experience has shown that they do provide a sound hasis [orthe development of a contemporary working knowledge of thesubject.

Literature CitedIl l Fruikner, L. R.,n "Phvsieal M e t h d s in Mudern Chemical Analvsis? Vul. 4. Kuwana.

.".,~,,... p". ,,.14) Ba1d.A. J.. en d Faulkner. L. R.. "Electmchcmical Methods? Wiiey.New York. 1980,Chapt . 2.

264 Journal of Chemical Education

e.q.03 understanding electrochemistry

Documents