electrocatalysis: a futuristic view

htt. J. Hrdrogen Energy, Vol. 17, No. 6, pp. 423-444, 1992. Printed in Great Britain.

0360-3199/92 $5.00 + 0.00 Pergamon Press Ltd.

© 1992 International Association for Hydrogen Energy.

ELECTROCATALYSIS: A FUTURISTIC VIEW

J. O'M. BOCKRIS and Z. S. MINEVSKI

Department of Chemistry, Texas A&M University, College Station, TX 77843, U.S.A.

(Received for publication 27 January 1992)

Abstract--Bowden and Rideal [Proc R. Soc. A. 120, 59 (1928)] were the first to carry out a systematic series of measurements of the same electrode reaction, hydrogen evolution, on a number of metals. However, they did not have an electrocatalytic attitude and consequently did not directly compare the rate constants from such measurements. The word -electrocatalysis'" was first used by Grubb [Low Temperature Hydrocarbons, 17th Annual Power Sources Con- ference, Atlantic City (1963)] in 1963 in connection with the investigations of fuel cells. However, the first interpretation of electrocatalysis came before that in a famous paper by Horiuti and Polanyi [Acta Physicochini., U.S.S.R. 2, 505 (1935)] and the basic diagram of this paper is given. Here the rate of the reaction is controlled by the heat of activation and there is a relationship between this quantity and heat of adsorption

AAH "~ = 3 ZXAHAj~.

This seminal theory of 1935 has played a part in various guises in the development of electrocatalysis till today. In this document, examples of application of this theory will be given in the form of recent progress made and then the discussion will change to a presentation of near future tasks, the presentation ending by an attempt to predict some possible electrocatalytic situations beyond 2050.

APPLICATIONS OF THE HORIUTI AND POLANYI THEORY TO MODERN EXAMPLES OF ELECTRO-

CATALYSIS [ 1 - 3 ]

1. The evolution o f oxygen on perovskites

The evolution of oxygen is of importance in photosynthesis but also in the functioning of electrolysers which may be the basis of the production of hydrogen as a clean fuel. The application of the Horiuti and Polanyi theory [3] (Fig. 1) to oxides such as perovskites is not easy to comprehend because one is confronted with an apparent O - O bonding. However, the difficulty is seen to vanish when one looks at the schematic model for the surface of a perovskite and this is shown in Fig. 2 [4] .

Thus, the transition metal appears on the surface. It is indeed possible for there to be a bonding between, e.g. OH and the electrode surface.

A likely electronic structure for OH is ls22s2(2p, + l,)22p~2pl. The bonding state in oxygen is 2pt orbital, sigma bonded to a hydrogen ls state. The oxygen 2p, and 2p~ orbitals remain as anti-bonding. The d-electron configuration of a transition metal ion at the surface of perovskites can be viewed as given in the diagram in Fig. 3.

Possible orbital interactions between the transition metal ion at the surface of the perovskite and the OH species are as follows. The d: 2 of a transition metal ion will overlap with the 2p: + ls orbital of a OH forming a sigma-type orbital, while the d,~(d,:) orbital will interact with 2p,(2p,) of the OH giving a r - type orbital. Neither the

d, -~ ,-' nor the d~, orbital will mix with the OH orbital because of symmetry conservation.

Since the bonding orbital of the M : - O H bonding orbital is readily occupied by electrons of the OH species, electrons from the d orbitals of the surface transition metals and perovskites will occupy the anti-bonding orbital of M: - OH.

Correspondingly, the bond strength of M : - O H will decrease as the number of the d-electrons in the transition metal increases.*

The uniformity of the behaviour among the six perovskites examined is shown in Fig. 4, where log i is plotted at r/ = - 0 . 3 against the M - O H bond strength in accord with the theory of Horiuti and Polanyi [3] .

Thus the change in MOH bond strength affects in a linear fashion the logarithm of the rate constant. The mechanism which was worked out for the evolution on these materials is the easy electron transfer between M :+ and OH as the first reaction. The rate determining step then will be the removal of the OH adsorbed to the surface by the deposition of a further O H - , a step known as "electrochemical

*Band theory by Matsumoto and its introduction involves the counter difficulty that there are no bond concepts, which seem fruitful in the catalytic discussions. In Matsumoto the rate determining step changes considerably with the metals within the perovskites whereas the uniformity which has been shown in the present work over six different perovskites speaks for a constant bond.

423

424 J. O'M. BOCKRIS and Z. S, MINEVSKI

p E ~

~ _ _ _ _ _ _ _ _ C

AAH °x: ~A AHI~Is >

X~B Fig. I. The effect of the variation of the AAH on the potential

energy-distance diagram.

resorption" in the analogous reaction for hydrogen evolution.

Subsequent processes involve the decomposition of physically absorbed hydrogen peroxide, yielding O2 in steps c and d (see mechanism, Fig. 5).

In accord with the statements made earlier in respect to the effect of d-electrons, the corresponding graph is shown in Fig. 6 in which there is a plot of log i at 0.3 overpotential against the number of d-electrons. A steady increase is seen.

The basic diagrams for potential energy in the Horiuti and Polanyi style are shown in Fig. 7, while in Fig. 8 the corresponding volcano plot is plotted from where it can be seen that nickel may show the maximum rate (though see other possibilities below).

O H - O H -

O H - O H -

s o l u t i o n

H H H 0 H 0

i I 0 , 0 I

s u r f a

® A ion ( lan than ide) • B ion ( t rans i t ion metal , M =) O Lat t ice ox ide ion (O~-) O P ro tona ted ox ide ion (OLH-)

Fig. 2. Schematic model for the active surface of the perovskite, in which transition metal B ion is electrochemically active [4].

Finally, the number of electrons occupying the anti- bonding orbital in M: + is shown to be proportional to the log i0 of the catalytic evolution of oxygen (Fig. 9)

There is a possibility of predicting future electrocatalysis by utilizing the last diagram, with the anti-bonding for the orbital and on this basis one might predict that there could

g

LU

UJ

B ulk

as

t2g

Surface

; \

t ___ da

"' d,z, dyz

MO6 MOs

d 2 d 3 d 4 d 5 d e d ~

I ' I

V 3+ C r 3. Mn 3" Fe z* C o a* Ni 3+

(h.s.) Mn3. Ni 3"

G. a "

' , , / ~ . - y , ~ . . ~, 2~.. ~p, ,. , . ,,J

" ~ fi~'- ~ ~4 - -2p , + ls '~ ,~rl--2p, + ls t

~ z ~ OH

Fig. 3. d-Electron configuration of transition metal ions at the surface of perovskites (above); MO diagrams for the Mz-OH bonding at the surface of perovskites; manganites and nickelates (below) [4].

ELECTROCATALYSIS: A FUTURISTIC VIEW 425

-4

=> co -5

? E

-7

-8

Ni

~ ~Cr

r i r i /

120 140 160 180

M-OH Bond S t reng th (kcal mot -~)

Fig. 4. Current density (based on real surface area) for oxygen evolution on perovskites at an over potential of 0.3 V vs M - O H bond strength. The transition metal ions (M) in perovskites are

indicated with different symbols [4].

be a possible better catalyst in lanthanum cupprate, LaCuO3, because its d 8 electrons would contain a greater number of anti-bonding positions. There is a difficulty with the lack of stability of the three valent cupric ion in solution and it might be possible to stabilize it by replacing some of the Cu 3÷ ions by Cu 2÷ in a compound such as La] _~ Ce~ CuO3.

The Otagawa work on the perovskites illustrates a modern application of the Horiuti and Polanyi [3] theory in a very successful case.

2. T h e r e d u c t i o n o f o x y g e n o n m a c r o - c y c l e s i n c l u d i n g

p o r p h y r i n s

The macro-cycle s are highly conjugated structures and are relatively conducting. They are used as adsorbed layers on graphite [5 ] . Much is due to the M6ssbauer studies of Scherson who has shown that in the iron based phtalo- cyanites, the iron forms an octahedral complex with O H - in an axial position.

Reduction occurs on the metal as with the porphyrins. Without the metal the macro-cycle has little activity.

Langmuir

a V i a In i

rate-determining step j,b 0 "+ 0 0 "+ 1

(a In i l

a In Temkin COH')V,I -¢

a V ia In i 8---~ 0---~

"0 1 N A a A e N A a A e eonditior

1. M + O H - "-* M O H + e- 2. 2 M O H - ~ M O + M + HaO

3. 2 M O - ~ 2 M + 0 2

1. M + O H ' ' ~ M O H + e" 2. M O H + OH" -~ M O + H 2 0 + e-

3. 2 M O ~ 2 M + 0 2

I . M + O H - ~ M O H + e" 2. MOH + OH- --> MO + H20

3. Mo- "-+MO + e-

4. 2 M O ~ 2 M + O2

I. M z + OH- "+ MzOH + e" 2. MzOH --+ Mz+'OH + e"

3. 2Mz+tOH + 2OH- ~ 2M z + H20 + 02

I . M + OH- ~ M O H + e" 2. M O H + O H ' ~ M O + H 2 0 + e"

3. MO + OH" ~ MO,H

4. MO2H" + OH- ~ MO 2" + H,O + e"

5. MO_ t- "~M + O: + e-

(I) Bockris's Oxide Path 4 2 R T / F 1 2 R T I 2 F - 2

1 R T / 4 F - 4

0 2 R T / F 0.5 roll ~ r o R T / F 1 ro l l >> r

0 R T / 2 F R T / F 2 1 K2 ~ 1 R T / 4 F R T / 3 F 2 1 K2 < < 1

(II) Bockris's Electrochemical Path 2 2 R T I F 1 2 2 R T / 3 F 2 R T / F 2 1

1 R T / 4 F - 4 0

2 R T / F 1 ro l l ~ r o R T / F 1.5 roH >> r

R T I 2 F R T / F 2 1 K2 ~ 1 R T ] 4 F R T / 3 F 4 3 K , << 1

(III) Krasil 'shchikov's Path 2 2 R T / F 1 2 R T / F - 2 1 ~ 1 roll ~ r o

2 R T / F 1.5 ro t t >> r 2 2 R T / 3 F 2 R T / F 2 0 2 R T / F 0 K2 ~ 1

2 R T / F 1 K2 << 1 1 R T ] 4 F ~ 4 0 R T / 2 F R T / F 2 1 K3 ~ 1

R T i 4 F R T ] 3 F 2 1 K3 << 1

(IV) O'Grady's Path 2 2 R T / F 1 2 2 R T / 3 F 2 R T / F 1

1 R T I 4 F ~ 4

0 2 R T / F 0 r , ~ r 3 e R T / F 0.5 r, >> r~

2 R T / 2 F R T / F 4 3 Ka ~ 1 R T / 4 F R T ] 3 F 4 3 K2 << 1

(V) Kobussen's Path 1 2 R T / F 1 1 2 R T / 3 F 2 R T / 3 F 2 1 2 R T / F 1 roll - r o

R T / F 1.5 roll >> r 1 R T I 2 F ~ 3 I ~ 1 K2 ~ 1

R T / F 2 Ka < < 1 1 2 R T / 5 F 2 R T / F 4 1 2 R T / F 1 K~ ~ 1

2 R T / F 2 K3 << 1 1 2 R T / 7 F ~ 4 0 R T / F 2 R T / F 1 0.5 K4 ~ 1

R T / 2 F 2 R T / 3 F 1 1.5 K, << 1

Fig. 5. Diagnostic criteria of proposed paths for oxygen evolution reaction [4].

426

-4

J. O'M. BOCKRIS and Z. S. MINFVSKI

i - 4

> -5 co o II

-6 f-- E o

"- -7 o

-8

C o / N i

V ~ / C r

d'2 I i L t I

d 3 d a d5 d 6 c~7

N u m b e r of d-e lec t rons

Fig. 6. Current density for oxygen evolution on perovskites at an overpotential of 0.3 V vs number of d electrons of transition metal ions in perovskites. The transition metal ions in perovskites are

indicated with different symbols [4].

Ni -5 Hypothetical Relation i " ~ Real Relahon

I / \~ I Mo? *' \ C o / . \ / o\ /

- 7 i / ~M n

Rh?/ k / B ~ * C r

-8 I 40 810 120 160 200

M-OH Bond Strength (kcal mol -~)

Fig. 8. Hypothetical volcano plot for oxygen evolution on perovskites. Transition metal B ions in perovskites are indicated with

different symbols [4].

i 2~

E LU % E

g_

\ \ / "

\ \ \ / A / M:..H,O,

X t' \ Proton Transfer \ \ / ~

\~.~Nic k elate. / / / I

M z - O H + e - / N ~ j ' / ~ ' " M I"- O H + O H , ~ O=

+ e "

Reaction Coord ina te

Fig. 7. Potential energy diagrams for a rate-determining OH resorption mechanism [4].


0 4

0 .6

0.8

1.0

£7 1.2

_1 't.4

1.8

2.01

C o A / N I

Number of Electrons Occupy ing the An t i bond ing Orbita/s of M L O H

Fig. 9. Tafel line intercept (Tafel parameter a, based on real surface area) of oxygen evolution on perovskites vs number of electrons occupying the antibonding orbitals of Mz-OH. The averaged values for high spin and low spin cases for cobaltates. The transition metal ions in perovskites are indicated with different

symbols [4].

Indeed, there is a relation somewhat similar to the perovskites in the fact that there is a sequence of activities (c.f. Savy et al. [6 ] , such that Fe > Co > M n N i . These mechanism investigations have been carried out, particularly by Appleby [7] .

Now, the remarkable fact is that although the porphyrins are heated in helium at 800 K there is an increased catalytic activity after this heating (which must destroy the structure).

It seems that the mechanism of macro-cycle effectiveness is simply one of isolating the central atom. Thus, in most catalysis by means of adsorption for materials on carbon the difficulty is a tendency of the atoms to coagulate and finally to hide much of their catalytic power within larger crystals.

The ultimate use of metal atoms is to make them as single atoms and a reasonable theory is that of what the macro- cycles do. This is, of course, negative to the concepts of Coleman and Anderson [8] who have made much of the face-to-face porphyrins in subtle structures of the porphyrins themselves. These structures are decomposed by heating in helium and cannot be an explanation of the fine catalytic activity of these materials. This is best explained by the bonding (in the Horiuti and Polanyi sense) to the individual atoms. The catalysis would then occur depending on the rate determining step with increasing bond strength (the RDS is the first step) or with decreasing bond strength if it is the second (desorption) step (Fig. 10).

3. The photoelectrocatalytic reduction of carbon dioxide

The photoelectrocatalytic reduction of carbon dioxide is one of the principal reactions in nature and indeed it is the basis to all living activity. Thus, the conversion of solar

/

Fig. 10. Syn diastereomer of the ~-linked face-to-face porph),rin with four-atom bridge. The metal-metal distance is - 4 A.

light to green plants is carried out by means of the photosynthesis reaction

CO2 + H20 ~ C H 2 0 + O :

and these plants are eaten by carnivores which are eaten by man, etc. Were it possible photo catalytically to reduce the CO2 from the atmosphere where it is present at about 0.4%, it would be an effectively infinite supply of car- bonaceous materials in a form which could give rise to food and textiles.

Tanaguchi working at Texas A&M University from 1982 to 1983 made some interesting advances in this field. He showed [9] that there was a photocatalytic effect of ammonium ions in this situation (Fig. 11), and then thereafter, Tanaguchi with Aurian Blajeni established certain important reaction mechanisms in respect to the photocatalytic reduction involving the previous reduction of ammonium or ammonium analogues such as NR4 + [10] .

From the analysis of the structure of cadmium telluride it is seen that there is evidence for a heterogeneous reaction

-0.4

-0.7.

< E -05

-0 1 !°51Acre I _01.5 I

-1.0

#

H I/

J/ ......... ,,?." ................. .o.:,k.

f i i -1.5 -2.0 -2.5

E/V vs SCE -3.0

Fig. 11. Current-potential curves at a p-CdTe electrode in DMF solution containing 5% water under irradiation with monochromatic light of 600 nm. 0.1 M NH4CIO4 for A (under Ar) and B (under CO2) were used as the supporting electrolyte

[10].

428 J. O'M. BOCKRIS and Z. S. MINEVSKI

I E u

P CdTeC02 70 mV /

POTENTIAL/V ( N H E )

Fig. 12. Current-potential curves at a p-CdTe electrode in DMF-0.1 M TEAP/5% H20 [11].

on the surface, i.e. that some tellurium is coming out from the surface of the catalyst. Tanaguchi established that CdTe was the most effective photoelectrode for the reduction of CO~ (Fig. 12). Bockris and Wass [11] then established that the crown ethers were the best catalysts to use with the cadmium telluride in order to increase further the artificial photo reduction of CO2. The question is, what is the mechanism of this catalysis? In Fig. 13. one sees the adsorption of the 15-crown-5-ether on p-silicon in acetonitrile, 0.1 M lithium fluoride with 1% water. There is a potential variation and this gave rise then to the sugges- tion of Bockris and Wass that the mechanism would be as follows

NR4 + + e- ~ NR'4,o, (A sites) (a)

NR'4ad~ + CO2~,d, -~ NR'4 + CO2ads (b)

C O 2 , a , + H + + e - - C O , d ~ + O H RDS (c)

OH + H + ~ H20. (d)

The clue to the mechanism was given by the fittingness of certain radicals (Fig. 14). The reaction rate passed through a maximum at the 15-crown-6 and then declined afterwards. Finally, part of the mechanism with the addition of the crown ethers can be seen in Fig. 15. Thus, in the crown ether case the velocity of the step (a) in the sequence ( a - d ) becomes coupled to the following photoreaction

CO2~d~+H* + e ~ O H +CO~a~ (B sites). (e)

The velocity of NR~ radical production increases due to the change in the locale of the initial site NR;. Displace- ment is shown in Fig. 16 (a) and (b). In this way the rate of A sites increases, and feeds back onto the rate of supply of C O ; to the neighboring B sites. The degree of effect is coupled to the fitting of the NR~ to the crown ether and

Z

m ~ .xm

1200 1150 I100 1050 1000 950 WA~NUICBERS

Fig. 13. Absorption spectra of 15-crown-5-ether on p-Si in acetonitrile 0.1 M LiF/1% H20 as a function of potential [ 11 ].

this in turn helps to ease electron transfer to the N centre of the adsorbed cations and increases the CO2 production and hence the rate of the photoreduction reaction.

Finally, it may be said that in order to make a self- activating system for production of carbon dioxide we need several orders more of electrocatalysis, but this does not seem impossible after improvements made in the work done in Texas by Tanaguchi and by Wass [ 9 - 1 1 ].

EVOLUTIONARY CONCEPTS WHICH ARE AT THE FRONTIER OF PRESENT ELECTROCATALYSIS,

1990 - 2000

The gradual and halting advance of the understanding of the mechanism of electrocatalysis allows us to begin to see how we could design surfaces which would optimize catalysis.

1. Designer surfaces It was found by Bockris and McHardy [ 12] in 1973 that

the rate of oxygen reduction at a given potential on the sodium tungsten bronze crystal increased by up to four orders of magnitude as platinum was introduced into the


Fig. 14. Molecular drawing depicting the size of 18-crown-6-ether with the tetraethyl ammonium cation fitting into the central cavity

[11].

crystal, and at the maximum platinum concentration, about 400 dpm, the rate approached that on metallic platinum. It turns out, on further calculation that the rate per square cen- timeter of the oxygen activity on pure platinum is exceeded by about 1000 times.

The model proposed for this was that by Boudart [ 13] and he thought of a spillover happening: surface diffusion of reaction intermediates from the catalyst to its support.

Applying these ideas to the case of sodium tungsten bronze it is possible to utilize the work of Wroblowa et al .

[141.

They showed that there was a variation of the adsorption of heat for oxygen onto platinum which obeyed the law

AHAa, = 12(1 -- 0) kcal m o l - '

The spillover reduces 0 on the surface and an emptying of the Pt surface of O would reduce the ,SH,,~ by about 6 kcal mol ~. Thus, the reaction rate would increase by about 1000 times.

It seems reasonable to assume that the only active platinum (platinum which spills its oxygen onto the sur- rounding tungsten bronze) is an angular zone around the edge of the inclusion of the platinum particle in the surface of the tungsten bronze. If the radius of this inclusion is r and the average width of the active zone is z, the fractional area of the platinum particles involved in the spillover will be about 27rrz/Trr 2 or 2z/r . The area of active platinum, A, per unit area of crystal will thus be given by

A = 2 z / r

where A is the total fraction of the surface occupied by platinum (Fig. 17).

This explanation serves to illustrate the concept of designer surfaces, i.e. surfaces which utilize knowledge of the mechanism of the reaction to attain subtle effects.

Another example of this kind refers to the FTIR work of Bewick [ 15] for adsorption of hydrogen on platinum and rhodium.

Bewick found that the surface of platinum was covered with hydrogen under cathodic conditions but that the reactivity which was the expression of the catalysis of the hydrogen evolution reaction on platinum took place on certain peaks of the platinum structure, on certain sites, and was low in average coverage.

Such a result of Bewick's work does rationalize the 1952 result of Bockris and Potter [ 16]. They ascertained that on nickel during the hydrogen evolution from alkaline solution, there was a full coverage of hydrogen on the surface, but contradictorily they found that the stoichiometric number was 2 for the reaction. In this case the RDS is the proton discharge reaction from water and under these cir- cumstances it is easy to show that 0act~,e must be ,~ 1.

SITE A

e - C-W NR 4 CO~.

"---.-~ N R; - - . J -'--- C 0 2

-...%

S I T E B

C O~ + H - - > C O + O H -

PLANE OF ELECTRODE Fig. 15. Diagram of the nonphotoactive sites (A) and the photoactive sites (B) on p-CdTe semiconducting electrode [11 ].

430 J. O'M. BOCKRIS and Z. S. MINEVSK1

DOUBLE LAYER STRUCTURE OF ADSORBED CROIAN ETHERS

( a )

aA / /

( b ) 3 A , / /

/ /

/ /

I

I I I I

I I i t

I I t I I

HELMHOLTZ PLANE

OUTER INNER HELMHOLTZ

PLANE

Fig. 16. ModeLs of the electrode surface (a) in the presence of ammonium ions only and (b) with the addition of crown ethers [ 11]

These thoughts lead to that of the distribution of the energy of sites on surfaces and this is a subject which has received attention from Nikitas [ 17].

One would, of course, expect the distribution of sites to be exponential with respect to energy, but it turns out that in order to explain the Temkin adsorption isotherm which is seen on so many materials, it is necessary to assume the distribution shown in Fig. 18.

In Fig. 18(a) one can see the exponential character of the distribution of sites, and then in Fig. 18(b) is the probability of adsorption which clearly increases greatly with the increase of energy of the sites.

ik"rI~ ~ _ Z Z A

Fig. 17. Schematic diagram of an inclusion particle [ 12].

The product of the Nu and Pu is shown in Fig. 18(c) which clearly must pass through a maximum.

What this means, therefore, is that for a given crystal there is a certain energy band which will be of the order of 3 kcal in width and this will be the band in which most of the adsorption occurs.

Now, if one then looks at Fig. 18(d) this shows the NuPu of a number of catalysts for the same reaction. It is clear that for each catalyst (same reaction) there is a zone in which NoPu will be maximal.

Now, let us consider the corresponding behaviour of the rate constant as a function of the adsorption energies on the catalyst. There will be two general types of behaviour of rate constant with the catalytic sites. On the one (corresponding to the original Horiuti and Polanyi model) in which the rate determining step is discharge of a radical upon a site (analogous to proton discharge) then it is clear that the value of the rate constant will be exponential with the heat of adsorption as shown in Fig. 18(e).

On the other hand, if the electrochemical desorption mechanism is rate determining, i.e. the radical has to displace another radical before it is adsorbed as with the oxygen evolution of perovskites, then the reverse will be obtained and Fig. 18(f) will be true.

One can begin to appreciate the specificity of catalysts. In Fig. 18(c), one sees that a certain catalyst has a specific zone of activity in a certain energy range and the question is whether or not this overlaps with a region of a high value

ELECTROCATALYS1S: A FUTURISTIC VIEW 431

(a)

Nu

U ~

(b)

~,,u

U~

(c)

Nul"~,u

(e)

I I

U

U

Fig. 18. Distribution of sites, probability of adsorption and

(d)

I I

U

(0

) !D

11

rate constants as a function of adsorption energy of a catalyst.

of the rate constant. If it does, this is an excellent catalyst. For example, if one took catalyst A in Fig. 18(e), it would do poorly with the situation shown in Fig. 18(e), whilst doing well with the situation in Fig. 18(f), and, of course, the catalyst C would do well with the situation in Fig. 18(e) but not in the situation given in Fig. 18(t).

Now, these things are clear for simple reactions such as the proton discharge reaction, and even for much more complex mechanisms such as the discharge of O H - on perovskites.

The situation is more difficult with the oxidation of hydrocarbons. It was found by Bockris and Stoner [18] that the most likely steps which would be rate determining are the chemical surface reaction of breaking either C - C or C - H bonds, since both of them are favoured.

Now, the point is that in complex mechanisms of marly steps like this it may be necessary to involve more than one type of surface. Here the spillover ideas would occur because one could designate a complex surface and if one found the optimal situation for each of, say, six steps, and found out that the optimal energy range for each step is AH,, AH2 . . . . one then would strive to find surfaces with

patches which corresponded to each of these optimal steps. Now, of course, this would presuppose a good deal of

mechanism knowledge but mechanism work can be made easier in the future by the type of computerization which has been suggested by Harrison.*

Thus, Fig. 19 shows the type of surface which is meant here. Of course, it would be important that the size of the patches is small because we have to rely upon spillover between the patch where the reaction is step A carried out quickly at its critically optimal heat of adsorption range and then diffusing over to patch B where there is an optimal situation for the next consecutive reaction and so forth. Now, how big would such patches be? One has to relate the

*Thus, Harrison suggested that it would be possible to program computers to carry out the entire experiment, the entire mechanism determination, in a few hours, the computer working to carry out half a dozen or so standard experiments and obtaining a pattern for the reaction. It would then recognize this pattern and thus deter- mine the reaction. Such methods have been put into practice by the Japanese researcher Motto.


Fig. 19. Illustrative concept of designer surfaces.

rate of the reaction to the rate of surface diffusion. Thus, if the lifetime of the radical on the surface is r then one has to have a situation where A2/2D < r, i.e. the time of passage from one site to another is less than the residence time of the radical upon the surface where D is the surface diffusion coefficient of the radicals concerned.

How could one approach the attainment in practice of such relatively complex ideas as these? It might be possible to do it with alloys, with cermets,* or with metal organic compounds. But probably the eventual home of such ideas may be with electronically conducting polymers which could be united then with metal additives and could give a really large variety of possible sites (or patches) on the metal surface.

Clearly here we are talking 10 years in advance (cf. the title of this study) but at any rate, it gives an idea whereby very fast catalysis may one day be obtainable.

2. Designer solutions

The first ever publications on this were due to Appleby [19] , and then Appleby and Baker [20] . t

The usual acid used for oxygen reduction in fuel cells had been phosphoric acid and the origin of this which came with the General Electric work around 1963 is worth stating. It was realized by the early 1960s that the principal difficulty of commercializing fuel ceils would be the oxygen reduction reaction, and that the way to solve this problem was electrocatalysis. However, clever electrocatalysis, designer electrodes, etc., were not then available and for this reason an engineering solution was proposed. The idea was that

*It is possible that the excellent catalytic properties of some amorphous materials could be due to the fact that such materials would have large variety of states upon the surface of various adsorption values and thus perhaps offer something like the designer surfaces given above.

Note that these ideas are somewhat confirmed by the fact that the promotion of single crystallinity gives very little improvement in the reactivity on surfaces because, of course, the single crystallinity decreases the number of possible available sites of different adsorption strengths.

tAppleby admits having discussed the matter with others earlier.

the constant want of engineers, of course, when they want something to go faster, is to " ra ise the temperature" . However, this brings with it an engineering difficulty and that is that the pressure rises, too, so that one has to buy high pressure equipment, which is expensive. For this reason, then, the concept was that if one could utilize a concentrated solution so that the aqueous part of the vapor pressure would be very small, high temperatures would be possible at room pressures. This was indeed achieved with phosphoric acid solutions of 98% and there it is possible to raise the temperature to about 190°C at room temperature without boiling off the electrolyte.

The engineers had a poor realization of the difficulties they were incurring by this solution, including low conductivity because phosphoric acid is basically a molecular compound and does not dissociate well as do some acids, even at high temperatures; and the fact that at lower temperatures, but well above room temperatures, the mix- tures would freeze out and cold start was out of the question.

Appleby's solution was to use trifluoromethane sulfonic acid. Why this peculiar structure, it is not clear but there are empirical reasons, perhaps, for seeking something with fluorine because fluorine is well known to be filic to oxygen, so that the presence of a compound which con- tained fluorine would be likely to increase the amount of dissolution which could be accepted.

Corresponding to this, sulphonic acids are known to be highly dissociated so that the possibility of getting enhanced conductivity, decreased IR drop, and the amount of solubility was at once there.

Low temperature experiments carried out by Appleby and Baker and by Srinivasan et al. [21 ] gave rise to start- ling increases in i0 for oxygen reduction (around 104 times better for the sulfonic acid) but these extreme thoughts were put away when the temperatures were increased to those which corresponded to actual fuel cell conditions. Thus, when the oxygen dissolution reaction in phosphoric acid at 190 K was compared with that around 363 K TFMSA, the ratio was decreased though still in favour of TFMSA by between 10 and 1130 times.

The challenge then came to the physical electrochemists: why? The solution to this problem was largely worked out at Texas A&M University by a series of measurements in which Habib, Scharifker and Zelenay took part [22, 23] , in that order.

The solubility at 423 K of oxygen in TFMSA is about 10 7 M cm 3 and the solubility of oxygen in 98% H3PO4 at 423 K is about 10 7, the corresponding value in TFMSA 9.5 M at only 358 K is about 3 x 10 -7 or some 30 times bigger. This would account then lor more than an order of magnitude increase in the limiting current and the corresponding increase in power per unit weight of the fuel cell.

Now, correspondingly, the diffusion coefficients for 9.5 M TFMSA at 358 K is 23 x 10 6 and the phosphoric acid does not catch up to this until 423 K so that the diffusion coefficient at 363 K is clearly larger than that for phosphoric acid at the highest temperature measured.


In general, one can say the diffusional properties are more than an order of magnitude better in the TFMSA.

It is relatively easy to interpret this. First of all, in respect to the solubility the fluorine acts in respect to its interaction with the oxygen in a positive way (artificial blood is made of fluorides) but the most marked interpretation on the side of structure can be seen in Fig. 20 where an attempt is made to show the results of some entropy calculations which have arisen from the dependence of the solubility on temperature. One can see from this the difficulty that 02 has in getting through the netted-up structure of the phosphoric acid whereas in the case of the TFMSA nothing like this exists and it is a much easier passage for the oxygen - - largely through water.

These aspects explain the superior properties of transport in the TFMS but they do not explain why the overpotential is lower for this is a matter of the rate constant. The investigations here are not complete but some insight has been obtained into them by a paper by Bhardwaj et al. [24] . These workers tackled the measurement of the individual activity coefficients in phosphoric acid and TFMSA and they measured individual proton activities between 150 and 90 K. Their basiciidea was to set up a transfer cell in which one of the electrodes was in the D e b y e - H u c k e l range and therefore its activity coefficient would be calculated. Thus, when the EMF of the overall cell was expressed, it had only one unknown, the activity coefficient in the concentrated solution.

One of the things that had to be found out was the transport number of the proton. This was done by radioac- tive radio tracer techniques.

Remarkable results were obtained in this way and some of them are shown in Fig. 21 (a) of the Bhardwaj paper and can be compared with Fig. 21(b). As the concentration increases there is an enormous increase in the activity coefficient of the proton in TFMSA at high concentrations. It is true that this somewhat declines with temperature, but it can be seen in Fig. 22(a) that the absolute values of the proton activities in TFMSA are one order of magnitude larger than those in phosphoric acid and as the concentration increases past 10 M this ratio increases to about 102. Both activity coefficients are enormous (in the region of 103- 104).*

The effect of this designer acid, TFMSA, upon the entire question of the reduction of oxygen ends up as being a structural matter. It can be understood why the diffusion coefficient and the solubilities are both greater and to some extent understood why the rate constant is greater (for this

(a)

% x 4-

g

O-

10

9

8

7

6

5

H 3 P 0 4

. / " : :,Oo M * 14 .7M

I I I I I I I I 20 4 0 6 0 80 100 120 140 160

(b) 8.5

7 5

6 5

+ = 5 5

4 5 .J

35

2 5

1 2O

* ~ , . CF3SO3H

+ +

÷ ÷

• TOM = ~ ÷ 9 .0M

, , , , - _ L ' / T M ,

, o ,oo T e m p e r o t u r e °C

Fig. 21. Plot of proton activity vs temperature °C for (a) I"I3PO4 and (b) CF3SO3H [24]

Fig. 20. Molecular motions of oxygen in phosphoric acid: (1) vibration, (2) hindered translation and (3) diffusion. Intra- molecular bonds are indicated in black, intermolecular (hydrogen) bonds are in white. The dashed lines show the hydrogen bonds involved in the different motions of 02 in the H3PO4 matrix [23].

*This extraordinary value is not to be wondered at for 98% solutions in which most of the water has been withdrawn into the acid to solvate the ion so that the addition of further concentrations are like adding a solute to almost no solvent and thus obtaining tremen- dous real concentrations (or activities).


(a) o

- 0 . 2

- 0 . 4

- 0 . 6

- 0 . 8

- 1 . 0

- 1 . 2

- 1 . 4

- 1 . 6 + 0

_J 8

(b) 6

H3 P04 ' * * ~

I I I I I I I I 0.5 1.0 1.5 2 .0 2.5 :5.0 :5.5 4 0

• 2 5 *C

+ 60=C * 8 0 *C . / .

CF3 SO3H /~..~ *

-~ L I ~' I I I h 0.10 031 1.00 1.73 2.25 2 6 4 3 0 0 3 .08

vr~ ME

Fig. 22. Plot of log "}In ~ VS %~C at different temperatures for (a) H3PO4 and (b) CF3SO3H [24]

is directly proportional to the proton activity). The analysis of the rate constant and its dependence on the surroundings is not yet fully solved, but at any rate the present situation with the comparison between these two acids is that, looked at in the long term, there is very much to be said in favour of designing acids in a rational way.

In fact, this is an example which can be used to point out that the science of electrocatalysis is by no means limited to the considerations of the surface. Here is a difference from gas catalysis because there the surface dominates everything whereas in the electrochemical case it is the transition state and the energy during the reaction, which are seen to be half dependent upon the surface and half dependent upon the solution. This may be changed as has now been shown, and looking towards the future it obviously will be a matter of understanding the transition state, calculating it, and then designing the solvent to suit it and maximise the rate.*

*One may ask why it is that the TFMSA is now not used in practical fuel cells and the answer is that it is not as stable as one might want for a year long use which is a typical example of "compen- sating factors". Looked at, however, as an exercise for the future, the work on the designer acid has obviously been well worthwhile and one looks forward to an outbreak of a great deal more design of solutions for electrochemical reactions.

Q U A N T U M MECHANICAL APPROACH

To understand the possibilities of new electrocatalysis from quantum mechanical considerations the concepts of adiabatic and non-adiabatic reactions must be explained. Consider the curves in Fig. 23, thus, curve 1 which has the normal shape of a potential energy curve represents the potential ene rgy-d i s t ance relationship under a situation in which the represented point remains in the same ground electronic state. This is the curve normally plotted. The behaviour there is called "ad iaba t ic" .

However, as was pointed out many years ago [25, 26] there is a loss mechanism in the fact that when the representative point approaches the saddle point of the potential energy surface, instead of "going ove r " and reaching its goal on curve 1 it may transfer to curve 2.

Curve 2 represents an activated state above that of the activated state of the lower curve. The result of the transfer of the representative point from curve E1 to curve E2 is that (suppose that the representative points are going from left to right) there will be a slide back down this curve to the starting point.

The equation deduced (under many approximations) by Landau and by Zener (separately) is that the probability of non-adiabatic behaviour would be given by:

P ..... d = e x p [ -27rV]2

It can be seen, therefore, from the equation that one of the principal aspects of the degree of non-adiabaticity is I/t2, the potential energy operator. If this is sufficiently small then there would be a tendency for the reactant to slip onto the upper curve.

Now, the general concept in the literature is that reactions are adiabatic. This is stressed by Marcus and his colleagues

LLI

E"

'5

== LLI

~o c~ b

~ : ~ b y 1 2 ( R o )

. V - o , E 1 ~ ~ 1

R o

Reaction coordinate, R

Fig. 23. The plot of energy of the reacting system as a function of reaction coordinate. Arrow a represents adiabatic and b represents non-adiabatic motion in the region of closest approach

of two potential curves.


[27] in their continuum dielectric theories of electrode reactions.

The assumption of adiabaticity is not well based and in 1977, Khan, Wright and Bockris [28] showed by a F e r m i - G o l d e n Rule calculation that the kappa value for redox reactions could be as low as 10 -4.

Such work was confirmed later by Newton and others [29] . They certainly found somewhat larger values, for example, 10 :, but the qualitative results were the same, simple redox reactions do not behave in an adiabatic way. One has to take into account a K value which is less than o n e .

It can be assumed that for reactions something of the order of 1 0 - 100 times is " in hand" in the value of K. In principle, therefore, one could increase the reaction rate if one could control V~_~ or the other factors which control K.

Thus, for reactions which involve several groups, it may be possible to find ways of relating V~2 to the structure of the groups and adjusting these so that the non-adiabatic aspects of the reaction become less important.

Recent work by Weaver [30] has stressed the non- adiabatic properties of complex redox reactions but the kappa factors of less than one are probably endemic throughout chemistry.

PRACTICAL ASPECTS OF ELECTROCATALYSIS

I. Rate o f electrode reaction

Practical ways to increase the rate of electrode reactions divide themselves into two pans. The first part concerns bringing about conditions to increase the reaction rate and the second, parallel increases in transport velocity to the electrode which would have to be achieved to match the increases in these transport rates.

The use of heat pulses. There are three ways in which heating of the electrode could be used. The electrode consists of a thin sheet of metal on a non-conductor, such as glass.

In the first a lateral current is passed across the conducting layer, from top to bottom as it were, and this current can be a d.c. current from an external source and is meant to heat the electrode above the boiling point of the solvent.

The result of this will be that the solvent will boil in the vicinity of the electrode causing streams of bubbles and great agitation which is excellent from the point of view of increasing the transport properties in addition to which, of course, it increases the rate of the reaction.

There is another concept which has not yet been applied but there could be a possibility of heating the electrode to a high temperature. Calculations have been made in this direction and they have shown that if one heats the electrode to 1000 K for 10 _3 S, one can avoid boiling in the diffusion layer. The heat pulses can be repeated every 10-~ s and give rise to an overall increase in the reaction heat by 100 times. The thickness of the electrode must be less than 0.1 #m.

A parallel con.cept to this which could be a general one in electrode kinetics is to assume that the local current den-

sity in practical electrode kinetic reactions is ca 10 -~ A cm-2 and then calculate the thickness of the electrode and the glass which would give heating itself as a result of the FR loss due to this current. In this way, these reactions would heat the electrode and increase the reaction rate.

The use ofelectricalpulses. A start on this has been made by Ghorogchian and Bockris [31] who created an apparatus in 1987 which gave rise to increases of about two times in the steady state velocity of the hydrogen evolution reaction (Fig. 24).

However, the electrical pulse approach could be exploited further.

On the one hand there is the aspect of surface recrystallization. Such experiments were earlier carried out by Bockris and Kita [32] . What they did was to take a small sphere of metal and carry pulses out across it. They made the life of the new surface vary from 1 #s to 1 s and found that the rate constant was high below 10 ~ s but then went down (Fig. 25).

Aria [33 ] has shown in more recent times that the crystal face of an electrode can be " a r r a n g e d " by certain pulses and indeed one type of crystal face can be stressed more than another type so that one can, in a sense, bring out a certain crystal plain by a certain regime of pulsing.

Such pulsing can be supplemented by pulsing forms, for example, by a.c. pulsing, triangular wave pulsing, etc. The question of how much electrode reactions can be affected by pulsing seems an under-researched topic in electrocatalysis.

W m C

'c ri POLE ~ ' . :

ii llUt -I

MAGNETIC NORTH

POLE

C PLEXIGLASS CONTAINER D STAINLESS STEEL DISK S RUBBER SEAL B SELFLUBRICATED BEARING P PULLEY W ALUMINUM SUPPORT WALL

Fig. 24. Schematic diagram of an in-situ homopolar generator and electrolyser [31 ].


10

E

5

0

-2 -I 0 I 2

Log I Is)

2. Destruction of wastes

Guruswamy and Bockris [34] suggested that it would be possible to utilize the Gratzel [ 35 ] approach by making on a mass produced basis colloidal particles which contain catalytic elements in small patches on the surface.

It would be possible perhaps to use Fe203 because the energy gap is much less. The major difficulty of using Fe203, that the conductivity is poor, will be overcome with the small particles because here there is little or no control by diffusion. Therefore, the low diffusion coefficient is not important.

The major difficulty in the method used for wastes destruction would be how to suspend the wastes in such a way that there was plenty of sun insulating the container without it being obscured by the wastes themselves, so the wastes would have to be diluted and this may reduce the possibility of use of catalysts in oil spills although the particles were to be made floatable (e.g. micro glass spheres coated with the Fe203, etc.) then there might be a possibility of the catalyst particles floating on the surface (Fig. 26).

Fig. 25. Change of over potential at i = 8.7 × 10-' A cm 2 with time of contact with solution after anodic dissolution. Open circle, dot: He-quenched electrode; square box, triangle, × : elec- trodeposited electrode; dark circle: oxide film electrode [32].

3. Destruction of wastes in general

Wastes can probably be electrocatalytically destroyed in a very general way by utilizing the following procedures, for example.

Instead of the methods recently developed by Kaba et al. [36] and Bhardwaj and Tennakoon it might be better to

• o

I~+H +

Fig. 26. Use of colloidal semiconductors for waste disposal.


WASTE ELECTROCHEMICAL PROCESSOR

WASTEIN

TOC-8.76g

WASTE

MULCHING

URINE+FECES

PRETREATMENT

CO2 H2

ELECTROCHEMICAL CELL

Fig. 27. Waste electrochemical processor.

TOC=I.7g

CLEAN

WATER

utilize low temperature molten salts, for example, KOH at 250 K and then inject oxygen around the electrode whereupon the cathodic reaction would be:

02 + 2e -- 0 -2

and then in the other electrode to introduce the wastes in concentrated mashed form at the very minimum of colloidal size and not more than 100 A in concentrated suspension whereupon reactions of the type

R - C O O - -- - C O 2 + 2R + 2e

may well take place, i.e the expected evolution would be a hydrocarbon and CO2 rather as in a Kolbe reaction.

Of course, this needs research, but it has already been proved that the fecal wastes with urine can be removed to > 9 0 % in aqueous solution method [37] .

In addition to this, ozone can be produced in an electrochemical way and then used to deal with the particles from highly toxic materials such as PCBs if the particle size is sufficiently small [38] .

We are on the edge here, on the frontier, that is what this review is about. The destruction of wastes seems to be one of the most useful areas of electrochemical investigation at this time (Fig. 27). The best idea, of course, would be to make a fuel cell which worked on wastes and destroyed them or converted them to something innocuous at the same time removing them and producing electrical power.

One of the products of this waste destruction is bound to be CO,, and this can be utilized further without being let into the atmosphere. The CO2 generated from the oxidation of carbonous waste can be used to enhance crude oil

extraction in the oil fields. The factors which contribute to the increase of oil recovery are crude oil viscosity reduction and swelling of crude oil.

Carbon dioxide used in the oil recovery can be later recycled along with the hydrocarbon vapours and thus the CO2 which is produced in the oxidation of waste eventually may not be released in the atmosphere.

4. Use of high temperature superconductors in electrocatalysis

Bockris and Wass [39] were the first to show that high temperature superconductors could be utilized in electrochemical reactions at low temperatures. They worked with frozen solutions containing HC104 and evolved hydrogen on YBa,.Cu~O7 x where 0 < x < 0.5.

The interesting result which may have electrocatalytic overtones is given by Fig. 28.

Here one sees at the temperature at which the super- conducting state tunes in, that the current does tend to rise. The idea of the application to catalysis is that as the basic reason for hot superconductivity is the existence of copper pairs of non-interacting electrons, then these might go on through the double layer and, correspondingly, not interacting, transfer without overpotential.

Something of this is seen here (increase of current as superconductivity switches on), but why there is a decrease at still lower temperatures is not yet known. There is an opening for exerting new kinds of electrocatalysis.

5. Electrocatalysis on electronically conducting polymers

The essence of the science of electrocatalysis is the ability to adjust the properties of surfaces to fit the requirements


- 4

04 - 6 - i

E O

,a~ - 8 -

.,,o - 1 0 -

¢3 0 ._/

- 1 2 -

- 1 4 I I t 2 3 4 14 15 16

0.5

\

t l I \ I I t I I I 5 6 7 8 9 10 11 12 13

T -1 x l O 0 0 / K -1

b)

0.4

E o

< 0.3 Q.

"T"

0.2

0.1 9 10 11 12 13 14 15

T -1 x l O 0 0 / K "1

Fig. 28. Hydrogen evolution reaction on YBa~(Cu0 ~Pd0 ~)/O7-, superconductor in HCIO4 ' 5.5H20 [39].

of a reaction and its kinetics. Until the 1960s it was thought that electrochemistry had available only some dozen or so metals. Thereafter the field was expanded by the aggrega- tion to it of many semiconducting oxides and sulfides.

The electronically conducting polymers present poten- tially another wide panorama of possible surfaces. At the moment the relationship between the degree of conductivity and the precise structure of the organic material is not yet known. However, this gap will be overcome in time.

Electronically conducting polymers possess a high degree of surface states [40]. In this respect the surface of electronically conducting polymer is more metallic than that of the classical semiconductors such as germanium or silicon.

It is a rule that the adsorption of organic compounds takes place in the vicinity of the potential of zero charge, pzc. It is therefore necessary to adjust the surface of the semiconductor to have a pzc which is near to the reversible reaction. Thus, for example, if the surface is to be used in the oxidation of hydrocarbons, the pzc would be somewhere in the region of 0 . 2 - 0 . 4 more positive than the reversible potential because the working potential for the reaction will be a few tenths of a volt more positive than the reversible potential so that it is important to get a high degree of adsorption at this point.

After that it is a matter of finding the rate determining step and promoting bonds and groups to serve it as the semiconductor which would correspond to maximization of the rate of that step.

Eventually the adsorption on electronically conducting organic substances may be advantageous. It is possible that there already exists a number of structures which do have bonds appropriate to certain reactions. What has to be learned here is how to introduce the enzyme on to the surface of the organic semiconductor without damaging it, i.e. whilst it retains its catalytic properties. It has been shown, for example, by means of ellipsometry [41] that some enzymes adsorb on surfaces only to undergo a toppling over and finally decrepitation upon the surface (Fig. 29).

ELECTRICAL POWER PROM THE USE OF BACTERIA

Bioelectrochemical fuel cells can be used in detection and rapid enumeration of bacteria, in the field of biosensors and also may be used as energy transfer or energy storage units [ 421 .

A bioelectrochemical fuel cell generates electrical energy utilizing biological redox reactions as the driving force. Here, biological components can be whole organisms or even isolated enzymes. Bacteria containing biofuel cells have already been commercialized, and a biocell fuelled by powdered rice husks has produced 40 mA at 6 V.

These biofuel cells consist of anodic and cathodic com- partments separated by an ion exchange membrane which allows the charge transport. In the anodic compartment electrons are transferred from the biological system to the anode under anaerobic conditions. This electron transfer can be carried out by redox-mediators, i.e. electron transport vehicles between biological systems and the anode. The electrons then enter the external circuit which is completed by suitable cathode. A widespread application in fuel cells has an oxygen diffusion cathode, while as a biocatalyst, Escherichia coli and as a redox-mediator, 2- hydroxy-l,4-naphthoquinone (HNQ) can be used. In order to increase the charge transfer rate a three-dimensional packed bed graphite electrode is preferred to the use of the glassy carbon disc as anode. In addition, the polarization effects (slow mass transport of substrates or products) are decreased due to the smaller distance between the anode and biocatalyst.

By using a packed bed anode the power output of the biofuel cell can be increased. An improved output of such a biofuel cell using a bed packed anode is shown in Fig. 30. By applying this kind of anode, higher values for current and voltage are achieved. At the same time, a rapid voltage drop due to polarization effects in the anodic compartment is shifted towards higher current values. The other advan- tage of this biofuel cell is in the use of the oxygen gas diffusion cathode. Since oxygen is taken from air via the pore system of the cathode, there is no need to purge the anodic compartment with nitrogen in order to avoid oxygen diffusion from the cathodic compartment. The oxygen gas diffusion electrode consumes protons and so the pH change of


4 / I

i ~

I f l l l l l l l l l l l l l l l l l l / t / / I / ,

a

7

I " 1

I 4 1 I / I ~, I 7

I I I I l l / f l l l l I I / l l l [ l l l f i l l ! I ' I I I I I l [ l l I I I ] II l i t I l l l l l l l l l

b c

FAD--

e~ 14o,&

G

50~

Fig. 29. Schematic representationoftheadsorptionprocessesofglucoseoxidase. (a) Ifthe potential is much more positive than the potential of the zero charge. (b) If the potential is close to the pzc. (c) Final state when the enzyme unfolded.

400 >

300

2OO

IO0

I - 4 -3 -2

Log [ / A

Fig. 30. Polarization characteristics of the biofuel cell: -- plate anode; O, packed bed anode [41].

the anolyte is decreased during the long fuel cell runs. This means that the only acidity of the anolyte can come due to the microbial secretion of acidic metabolites under the oxygen-limited conditions.

ELECTROCHEMISTRY BEYOND 2050

One of the authors began electrochemistry research in 1943 and has completed almost 50 years of electrochemical research and has seen in this time enormous changes. In 1943 there were no potentiostats, no measurements of spectra on surfaces and no electrode kinetic reasoning, The typical treatment was thermodynamic in type and diffusion control in solution was the only one known.

What about the next 5 0 - 6 0 years? There will be, of course, enormous changes once more, but less than those

of the explosive outgrowths of electrochemistry of the last 50 years, for we have seen it change from an ancient and thermodynamic frozen realm to one of the great frontiers of science.*

In 1965, the present author wrote an article called "Elec- trochemistry: The Underdeveloped Science" [43] and if one looks back to the predictions made there, only about half of them came true in the next 10 years and about one- quarter in the next 20. Hence, one must be careful with one's prognostications - - for the technological implications or developments sometimes take place more slowly, because they are hampered by economic and political restraints, for example, the wish of powerful corporations not to disturb a known technology in which investments have been made.

With these warnings, therefore, the following remarks attempt to speculate towards the horizon of 2050.

1. Exper imen ta l

Szklarczyk and Bockris were able to see atoms in 1990 on an electrode in solution [44] for the first time (Fig. 31), It seems likely that, by 2020, or so, it will be a routine matter to watch atoms on the electrode surface and to see reactions occurring. One thinks of X-rays and neutron dif- fraction but Szklarczyk and Bockris used STM.

*In 1981, according to Vijh, more than 27 % of the most quoted papers in all science were in electrochemical topics.

440

STM: M

J. O'M. BOCKRIS and Z. S. MINEVSKI

Sl [u A t o m i c R e s o l u t i o n

Lead E l e c [ r o d e p o s i [ e d

Before E l e c t r o d e p o s i [ i o n Af te r

~-,-,,..-, ,.. ,[..-, ,.,._,..~.-,:,...-....:. I,,7,* : . ; _ % ' ; , 4 9 g * 7 . , _ ~ . ' . ~ . , . ' ~ . ' . , , ' . , , , . , , , * ' o . " . ' * ' . . ' . • . |2.1~-t'~,~-iI .. 'e;s~,e.,1-*,,-B~ea-.e~..*v-, *~ ' ,% . ",

" ' , " , ' .<' . 'd ' . '=" - : " . ,P" ' ; ' ' . . - , . ' * , . % , ; ' * * . ' r .',:'* " . ' , , * , , ' * . ' . . % * , - . ' <'.'<'. ,d' . '<' . '~:. ' ,; ' . '< , ' 4 4 ,'. ;:': U::?V?::>'::~:':!:Si':;:~?i::"- ti_L_/__~ ~ - - ~ . ~ Zt.223

on HOPG*

E l e c t r o d e p o s i t i o n

C-C d i s t a n c e : 2.4 ff P b - P b d i s t a n c e : 3.6 /~ X-Y= 9,~x9/~ per grid; Vb~s = - 3 0 mY; i t = 1.5 nA

Fig. 31. The image of graphiteoSUrface (left) and the image of lead deposited on graphite surface (right). Grid dimension X x Y = 9 x 9 A. V~i,~ = - 30 mV, i, = 1.5 nA [44] .* Highly oriented pyrolytic graphite.

By 2020 it should be possible to adjust double layers mechanically within the 5 A region. EXAFS should allow us to see many layers in the diffuse layer out into the solution.

This ability actually to watch what happens on the atomic level would be one of the great quantum leaps of science and would make obsolete very many indirect techniques.

2. The structure of the interfacial region and a descriptive theor 3'

It seems likely that the theory of the interfacial region will be transformed in the time period to a largely quantum mechanical approach. Thus, since Gary, the approaches have been electrostatic.

One of the earliest and neglected achievements of quantum theory is the surface state theory of Tamm (1928) and this showed that stationary states exist just outside metal surfaces in a vacuum. It may be possible to apply such types of reasoning to solve a Schrodinger equation for the stationary state of electrons in the double layer and then extend to a consideration of ions (a Block function for ions) in the Helmholtz double layer and beyond.

3. Electrode kinetics

There are four pointers to the future which are worth mentioning here,

(I) The electrocatalytic barriers which have been the focus of attention since perhaps 1950 at a molecular level will come down. The use of designer electrodes and designer solutions will increase i0 values to 1 A cm -2 at a planar electrode for 100°C.

(2) Enzyme species will be stabilized in their landing on electrode surfaces and be able to be of direct use to increase the velocity of reactions. One must make the tunneling distance between the Heme group and the electrode less than 20 ,~ by the addition of "relay stat ions", i.e. redox materials which have been incorporated in the enzyme so that the distance between each one is not more than 20 ,~. The other aspect is to design the surface of the electrode by adsorbing upon it suitable organic layers so that the enzyme when it comes in to land remains intact.

If this can be done widely, then the selectivity of enzymes in solutions can be replicated on the electrode and it may be possible to have a fantastic range of biosensors, specific to individual molecules. If a given disease produces some characteristic or perhaps a specifically molecular signature, it may be possible to indicate the presence of trial disease c at an early state.

(3) The self heating of surfaces. If electrodes consist of evaporated films upon glass non-conductors then it would be possible to make them sufficiently thin (0.1 /zm, say) then currents of minimal amounts, in the milliamp range, will heat the electrodes to 100°C and the advantages of this


self-heating can be seen in the increase in velocity of the reactions. This must, of course, be balanced against the energy loss of the IR drop causing the heating. Rough calculations show that a suitable balance can be met.

(4) With the lowering of barriers expected during the next 50 years the major aspect of electrode kinetics will become increasingly the transport limits. How much should the limiting current be? This has been and indeed is, an Achilles heel of the electrochemical approach to reactivity because chemical reactions are three-dimensional but if one makes an electrochemical reaction it is a two-dimensional one. Let us see how much we can increase the limiting current from that at a plainer electrode.

(a) Rotating disk. This is a conventional method but it may be upped during the next 10 years to have very high velocities of rotation, perhaps as much as 50,000 rpm or perhaps 100,000 rpm. Then one may get to about 30 times increase in the l imit ingcurrent , really not very much if we want to take 10 A cm - as a not unreasonable current density (with solution flow to dissipate the heat).

(b) Micro-arrays. The possibility of increasing the current density at individual micro-electrodes is, of course, enormous and even a micro-electrode with a 0.1 #m tip will indicate an increase of around 10 4 times in current density. However, if the current is small it is not obvious how micro-electrodes can be used. On the other hand, micro- electrodes can be used to increase the current density. The conditions that must be met are the following: (1) the rate of reaction of the electroactive compound of interest at the electrode should be under diffusion control, and (2) the distance between active regions in the array should be smaller than the diffusion layer thickness.

Suppose that we have a 0.01 M solution of an organic compound. Under usual conditions, the diffusion-limited current density would be

il = nFDc/6 ~ 1 mA cm-Z

If instead of having a totally active electrode we have an array of 105 platinum microdiscs, each of 5/~m radius, embedded in epoxy and with a total surface area for the array of 1 cm 2. The distance between neighbouring microdiscs would be of the order of 1/x/nn = 1/(105) ~/2 ..~ 3 × 10 -3 cm, i.e. less than the diffusion layer thickness. Thus the limiting current to the entire assembly would still be the same as above (ca 1 mA cm z), but the average real current density would be 1 mA/0.08 cm2), i.e. more than 10 times larger. For smaller active patches of surface the enhancements are even greater. If the 105 microdiscs have 2 /zm radius, than the active area would be only 0.0125 cm 2 and the enhancement factor for the real current density would be about 80.

The figures above refer to the average current density at the microdiscs. At each microdisc, however, the current density will not be uniform: it will be largest at the edge and smaller toward the center, so that the enhancements in real current densities will be probably even larger than those estimated above.

(c) Packed beds. This device originated by Goodridge and Fleischmann [45] has some possibilities and the use of

these three-dimensional packed bed electrodes increases the current of the cell. The basic equation for metal deposition in a packed bed under diffusion controlled conditions follows the equation

C~ = Coe Nx

where C~ and Co are the outlet and inlet concentrations respectively, and they are passing through a packed bed of height L, while the parameter X is defined as

t~U X =

DAe

where all symbols have their usual meanings and e is the fractional free volume in the matrix (Fig. 32). Since the area per unit volume for a packed bed containing spherical particles is

(1 - e ) 3

then the total current is given by

1 = nFAuCo [ 1 - e x p ( - l/X) ].

Taking the radius of a spherical particle as 0.1 /~m, e = 0.4 and considering a bed thickness of 1 mm then L = 0.1 cm, and this all leads to the output current, in the flow cell, of 8 × 104A.

Thus, one of the unsolved areas of electrochemistry which certainly would need research in these next 50 years is how to impel material on electrodes, e.g. by means of the use of jets so that the material reaches the electrode sufficiently quickly to be utilized well. Here one thinks, perhaps of the use of multiple dendritic arrays in which the large number of dendrite spikes gives in effect a large number of micro-electrodes in a certain volume.

4. Bioelectrochemis try

Bioelectrochemistry is the ultimate frontier of electrochemistry. There is an immense prospect here and only three areas will be mentioned.

f PACKED ~ I D O 0 • • O O 0 I r CAr~OE~ / ooooooooI I

[OOOOOOO0 I[ q-v-::"l

ANOD O O O 0 0 O O o OOO • OOO~O

I O O O m q l ' • O I I LOOO••OOO J

'°"2>

. . . . . . °

Fig. 32. Schematic diagram of packed bed electrochemical cell.


(1) The use of artificial prosthetics in the body will increase enormously. Here the work of Sawyer and Srinivasan [46] must be mentioned in establishing electrochemical conditions under which blood does not clot, i.e. the surface must be more negative than 0.6 on the hydrogen scale.

(2) The widespread use of a fuel cell type concept in biological energy conversion can be extended to other parts of the body. Does the body work like an electrochemical machine due to electrical power coming from the fuel cell elements in mitochondria giving rise to ATP which takes the energy to locations where it is needed? There, the reverse reaction of the electrosynthesis of ATP from ADP may occur so that electrons and protons are delivered to the appropriate areas from the energy stored in ATP.

It is likely that this is the direction for bioelectrochemical energy conversion to develop. Protein structures will be seen as electrodes and wires within the membrane, in a semiconductor sense [47] .

(3) Lastly, we may mention one of the great unopened doors of Science, the concept of consciousness and its loca- tion in an electro-biological model. The model of how one raises one 's arm is incomplete: it goes well until one gets back to the brain and then the question of who decides to decide comes in, and thus concepts of intention are fuzzy. They can be resolved if we understand something of the bioelectrochemistry of the brain. This is indeed the highest frontier we shall mention in this study and whether it will be climbed by 2050 we do not know.

5. Material science and corrosion

In solutions in which the pH is neutral or alkaline it is the oxygen reduction reaction which is the partner reaction of corrosion. It is not so difficult to dispel oxygen from solution by competing cheap gases etc., so that the wide scale use of deoxygenation of solutions is a simple concept which needs implementation. A similar idea is to eliminate water. If the solution is concentrated enough (10 M and upwards) the hydration of the ions removes the water so that it itself reduces the corrosion rate [48] . Thirdly, the idea of " l iv ing polymers" may be developable. This comes out of a recent study by Bockris and Yang [49] on the activity of the acetylenic alcohol octynol and its inhibition of the corrosion of stainless steel by hot HC1 (Fig. 33).

Thus, Yang showed that when the temperature approached 100 K the refractive index increased with time in such a manner that it indicated that layer after layer of the octynol was forming and this ability of certain inhibitors then to form non-conducting layers on all parts of the surfaces with which they are in contact, even in crevices, must be developed from the point of view of molecular structure (quantum mechanical design calculations are feasible).

Last of all, we may mention erosion as a frontier area here which will be penetrated and climbed by 2010, perhaps 2020. The relationship between erosion and corrosion is poorly researched. We do not know well how the parameters of corrosion and corroding surfaces change with impact and what the local impacts are in typical cases. We do not know, for example, what is the ductility of the

F e / O . O 2 M H C I , - 0 . 4 2 V / N H E , T = 7 3 *C, 1- o c t y n - 3 - o l

Delta(sub) - Delto(ads)/degree 5 5

1-octyn-3-ol 3 0 • O.O01M + O.O03M +++ +++

+*÷ 2 5 +

++~++ 2 .O ++

~it+++¢+ + + + +++ + 4 ~

o ~ "

0 5

I I I I I 1 0 10 20 30 40 50 60

T i m e / r a i n

Fig. 33. ~,ub-2~,a, vs time relations for the adsorption of various concentrations of l-octyn-3-ol on Fe at -0.42 V (NHE) at 73°C

[491.

passive layer. Most practical corrosion is erosion related and it is a very important area to research.

6. Energy conversion and storage

Energy conversion may in the future not be hosted by electrochemical fuel cells (though monolithic fuel cells may give 10 kW lg -~) [50] . But we may be able to utilize the fugacity of deuterium in voids to cause internal fusion reactions. We are, of course, here in an extremely controversial area, but the book by Lifchitz and Pitewsky [51] has a chapter in which there is a discussion of the properties of materials at extremely high pressures and there it is stated that when the pressure reaches 107 atm the ions at this pressure will start capturing electrons and emitting neutrons. Now the whole difficulty of the imagining of a fusion reaction in a solid at low temperatures is the coulomb barrier, the penetration of which seems impossible to overcome because of the repulsion between D + and electrons. If fugacity in voids reaches 1017 atm, then controlled fusion may be obtained (Fig. 34) [52] .

One thinks, for example, of the possibility of artificially inducing many cavities in metals - - porous metals - - and then inducing the deuterons into these voids to cause fusion to take place in a controlled way.

Bockris and Subramanian [53] showed that the fugacity produced in the internal cracks in metals is highly mechanism dependent, i.e. it depends on the surface reaction of the metal and one can show that:

FD2 = e -*nF/RT

where 1/2 < x < 2 and from this reasoning it turns out that to get to 1017 for the fugacity in these voids the over potential must be between 0.6 and 2.2 V, depending upon the mechanism [54] .

7. Environmental

The use of bacteria to provoke conversion of waste material to give power to fuel cells is an idea which should

ELECTROCATALYSIS: A FUTURISTIC VIEW

OVERPOTENTIAL < 1 VOLT

fD > 1017bar

D PLASMA ....

VOID IN METAL

t

443

A z + e - = Az_ 1 + v

D + / / D"

17 f < l O

PROBABILITY OF TUNNELING < 10 -60

D

17 f > l O EASY PENETRATION

Fig. 34. Loss of charge by nucleus under high fugacity [52].

be accomplished technologically by 2000 and it would mean that environmentally clean waste removal can in future not cost energy but be the source of electrical power. In this respect the making of very small particles is essential because the reaction at the electrode is clearly with molecules which dissolve from these particles and the dissolution rate will increase proportionally to the area available. If one is thinking then of, e.g. domestic rubbish, as a source of energy, then the colloidization of this rubbish is a necessary preliminary to its electrochemical consump- tion.

Last of all, within the environmental area one may mention the monolithic fuel cell [50]. Such an energy conver- tor should easily make electrochemical energy conversion, e.g. as photo derived hydrogen as the general source of power for brushless and lightweight electric motors. This concept is not well known as yet, but basically it consists of a series of tubes which are parallel. The fuel circulates through one tube and the depolarizer through the other. The reaction occurs across the tube membrane and because the distance between cathode and anode is small, there is little IR drop. In fact, the electrode can be limitlessly big or as big as the fuel supply will allow. One can regard this as a final overcoming of the accusation that the electrode reactions are two-dimensional. An attempt to sketch such a concept is given in the diagram of Fig. 35.

Acknowledgements--The authors, particularly ZM, are grateful to the Welch Foundation for the support of their work in the form of fellowships and grants for our equipment.

T Direction of flow of O=ions and electrons

ltir electrode ~ Interconnection

Fuel electrode ~ Electrolyte

Fig. 35. Monolithic electrode [50].

REFERENCES

1. F. P. Bowden and E. Rideal, Proc. R. Soc. AI20, 59 (1928). 2. W. T. Grubb, Low Temperature Hydrocarbons, 17th Annual

Power Sources Conf., Atlantic City (1963). 3. J. Horiuti and M. Polanyi, Acta Physicochim., U.S.S.R. 2,

505 (1935). 4. J. O'M. Bockris and Takaaki Otagawa, J. Electrochem. Soc.

131, 290 (1984). 5. E. Yeager, D. Scherson and C. A. Fiero, ACS Symp. Series

44, 535 (1985). 6. M. Savy, P. Andro and C. Bernard, Electrochim. Acta 19,

403 (1974). 7. A. J. Appleby, J. Fleisch and M. Savy, J. Catal. 44, 281

(1976). 8. J. P. Collman, P. Denisevich, Y. Konai, M. Marrocco, C.

Koval and F. C. Anson, J. Am. Chem. Soc. 102, 6027 (1980). 9. 1. Taniguchi, B. Aurian-Blajeni and J. O'M. Bockris, J. Elec-

troanal. Chem. 157, 179 (1983). 10. I. Taniguchi, B. Aurian-Blajeni and J. O'M. Bockris, Elec-

trochim. Acta. 29, 923 (1984). 1 I. J. O'M. Bockris and J. C. Wass, J. Electrochem. Soc. 136,

2521 (1989). 12. J. McHardy and J. O'M. Bockris, J. Electrochem. Soc. 120,

53, 61 (1973). 13. M. Boudart, M. A. Vannice and J. E. Benson, Z. Physikal.

Chem. 171, 64 (1969). 14. H. Wroblowa, M. L. B. Rao, A. Damjanovic and J. O'M.

Bockris, J. Electroanal. Chem. 15, 139 (1967). 15. R. J. Nichols and A. Bewick, J. Electroanal. Chem. 243,445

(1988). 16. J. O'M. Bockris and E. C. Potter, J. Chem. Phys. 20, 614

(1952).


17. P. Nikitas, Electrochim. Acta. 33, 647 (1988). 18. J. O'M. Bockris, E. Gileadi and G. Stoner, J. Chem. Phys.

73, 427 (1969). 19. A. J. Appleby, J. Electrochem. Soc. 117, 328 (1970). 20. A. J. Appleby and B. Baker, J. Electrochem. Soc. 125, 404

(1970). 21. S. Srinivasan, A. J. App!eby and B. Baker. 22. M. A. Habib and J. O'M. Bockris, J. Electrochem. Soc. 132,

108 (1984). 23. P. Zelenay,B. R. Scharifker, J. O 'M. Bockris and D. Ger-

vasio, J. Electrochem. Soc. 134, 2714 (1987). 24. R. C. Bhardwaj, M. A. Enayetullah and J. O'M. Bockris, J.

Electrochem. Soc. 137, 2070 (1990). 25. L. Landau, Z. Phys. Sowjet. 2, 46 (1932). 26. C. Zener, Proc. R. Soc. (London) A137, 696 (1960). 27. R. A. Marcus, J. Chem. Phys. 43, 679 (1965); Electrochim.

Acta. 13, 995 (1968). 28. S. U. M. Khan, P. Wright and J. O'M. Bockris, Electrokhim.

13, 914 (1977). 30. G. E. McManis, A. K. Mishra and M. J. Weaver, J. Phys.

Chem. 86, 5550 (1987). 31. J. Ghoroghchian and J. O 'M. Bockris, Int. J. Hydrogen

Energy 10, 101 (1984). 32. J. O 'M. Bockris and H. Kita, J. Electrochem. Soc. 109, 928

(1962). 33. A. Visintin, J. C. Canullo, W. E. Triaca and A. J. Arvia, J.

Electroanal. Chem. 239, 67 (1988). 34. V. Guruswamy and J. O'M. Bockris, Energy Res. 91 (1984). 35. M. Gratzel, in R. E. White, J. O'M. Bockris and B. E. Con-

way (eds), Modern Aspects of Electrochemistry, Vol. 15, p. 83 (1983).

36. L. Kaba, G. D. Hitchens and J. O'M. Bockris, J. Elec- trochem. Soc. 137, 1341 (1990).

37. R. C. Bhardwaj and C. Tanakoon, Report for NASA Research Grant NAGW 1779 (August 1991).

38. G. Sartori, Private communication.

39. J. O'M. Bockris and J. Wass, J. Electroanal. Chem. 267, 329 (1989).

40. J. O'M. Bockris and D. Miller, Conducting Polymers, Spec. Appl., Proc. Workshop, Sintra, Portugal, July 1986, pp. 1 - 3 6 .

41. A. Szucs, G. D. Hitchens and J. O'M. Bockris, J. Elec- trochem. Soc. 136, 3745 (1989).

42. D. Sell, P. Kramar and G. Kreysa, Appl. Microbiol. Bio- technol. 31, 211 (1989).

43. J. O'M. Bockris, J. Electroanal. Chem. 9, 408 (1965). 44. M. Szklarczyk and J. O'M. Bockris, J. Electrochem. Soc.

137, 452 (1990). 45. F. R. Backurst, F. Goodridge, R. E. Plimley and M. Fleisch-

mann, Nature 211, 55 (1969). 46. P. S. Chopra, S. Srinivasan, T. Lucas and P. N. Sawyer,

Nature 215, 1494 (1967).

47. J. O'M. Bockris, F. Gutmann and M. A. Habib, J. Biol. Phys. 10, 435 (1985).

48. N. G. Smart, M. Gamboa-Aldeco and J. O'M. Bockris, Corros. Sci., submitted.

49. J. O'M. Bockris and B. Yang, Electrochim. Acta 36, 1333 (1991).

50. D. C. Fee, R. K. Steunenberg, T. D. Claar, R. B. Peoppel and J. P. Ackerman, National Fuel Cell Seminar Abstr, p. 74 (1983).

51. E. M. Lifshitz and L. P. Pitewsky, Properties of Matter at Very High Density in Statistical Physics, Part I, Vol. 5, p. 317 (1963).

52. J. O'M. Bockris, Chin-Chung Chien, D. Hodko and Z. Minevski, Int. J. Hydrogen Energy 17, 383 (1992).

53. J. O'M. Bockris and P. K. Subramanan, Electrochim. Acta 16, 2169 (1971).

54. J. O'M. Bockris, D. Hodko and Z. Minevski, Proc. 180th Meeting of the Electrochemical Soc. (1991); Hydrogen Storage Materials, Batteries and Electrochemistry, in press.

electrocatalysis: a futuristic view

Documents