Ind ian Journal of Clll.: mi stry VoL '-15 . May 2006, pp. I 144- 1 152

Electrochemical oxidation of ethanol on thin coating of platinum based material on nickel support

Prad ip Paul. Joyceta Bagchi & Swapan Kumar Bhatt acharya'''

Phys ical Chemi stry Secti on. Depa rtment of Chemi , rry. Jada vpur University. Kolkata 700 032 . lnd i,t

Emai 1: skbhatt7 @yahoo.co. in

Receit·ed 17 No t·ember 2005: rel'ised 16 Febr/1(1/T 2006

For the deve lopment of cost eflc:c tive direct a lcohol fuel ce ll s. the key point for anodic ox idat ion uf t: tha tlol is the search for a cheaper electrocata lys t whic h can effective ly increase the current density and decrease the anod ic overvol tage. In thi s respect. thin deposit s of plati num and co-ckpos it s of platin um and ru thenium on Ni- support frotn dillercnt baths. have been studied taking IM etha nol in IM NaOH so lution. Polarization and chronopotenti omctri c stud ies indicate the intrinsic superior ity of Ni/Pt-Ru e lectrodes over Ni/Pt and Ni e lectrodes. bu t the overa ll e lcc trocataly ti c ;tct ivity of the coated electrodes fo ll ows the order: i/Pt(PV /\ ) > Ni/Pt > Ni/Pt-Ru(PVA) > i/ Pt -Ru > Ni. (where PYA is polyv inyl alcohol). An increase in the roughness fac tor due to small er size ( 150nm ) of depos it of Pt than co-deposit o f Pt and Ru under the cx perimema l deposit ion condi tion may be the cause of the above mentioned orde r. Steady state polari zati on. cyc lic vo ltammetry. chronopotenti ometry and SEM imag..:s ha ve been used to correlate the cata lyti c a..: tivit y of such ,j t:c trock s wit h the charac teri stics of the depos its and surface morp hology.

IPC Code: Int. Cl. x C07B33/00; C25D: HOI MS/00

In recent years in fu el ce ll s, the di rect anodi c ox idati on of an alcohol, other than methanol, has been widely acl vocated t.2. Thi s is because of several problems assoc iated with a direct alcohol fu el ce ll (DAFC) wi th methanol as a fuel, e.g .. low ac ti vity of the state-o f- art electrocatalysts, anode poisoning by strongly adsorbed in te rmediates( mainly CO species) formed duri ng methanol dehydrogenati on in the process of methanol ox idati on, hi gh ex tent of methanol cross-over through Nafion- type membranes whi ch clepolari zes the air cathode, tox icity and hi ghl y in flammable nature of methanol with co mparati vely low boiling point (6SOC), etc. Therefore, other alcohols, particul arly those generated from huge bi omass resources, e.g., ethanol, which is obtained th rough the fermentati on of sugar containing raw materials, are considered as good alternati ves . Moreover, among va ri ous DAFCs. the cell with ethanol fuel prov ides a better theoretical energy density value (=8.0KWh/Kg) than that with methanol (=6. 1 KWh/ Kg). Both of these, however. provide less energy density than that of hydrocarbon or gasoline (= 10- 11 KWh/ Kg).

The complete electrochemical ox idation of ethanol occurs at the anode according to the overall reaction ,

. . . ( I )

which corresponds to a standard potent ial, £ " = 0.084 V vs. SHE, as ca lcul ated from the standard energy of formati on or the reactants and products 1• However. in alkaline medi a, C0 2 forms CO/ and scarcity of H10 + makes the reacti on favourab le whi ch is reflected by a hi gh negative va lue of the theoreti cal reversible cel l­potential and hence the actu al operating potent ial and prac ti cal efficienc/.4 fo r such oxida ti on in alka line medi a. Moreover, a wider possibilit y for se lect ion of electrode materials, better performance of oxygen cathode, littl e sensiti vity of organic fuels to the

t. s r, . d. I cl sur ·ace structure·· , etc., 1 n 1cate scvcra a vantages of using alkaline so lution in a DAFC. The recent works al so reveal that the best performing electrodes for DA FC are based on pl atinum metc:d ingrecli ents 1 2

.

The electrolytic act ivity of Pt-coatecl elect rodes is found to increase by introducing a second metal li ke Ru 7

·0

, Mo 10, Sn 11

·12

, Pcl 13, Rh 14

.16

• etc., or sometimes even a third metal 1• But, all these material s are qui te expensive. So, the development of a thin layer of such ex pensive binary or ternary metals on a cheaper el ectrode substrate like Ni foil for a cost effec ti ve DAFC, is the order of the clay. Moreover, any binary

PAUL e1 a/.: ELECTROOX IDATI ON OF ETHA OL OVER Pt -BASED DEPOSITS ON Ni SUPPORT 11 45

metal like Ni by a ll oy fo rmation w ith Pt may change the d-band character o f Pt and he nce change the e lectrocatal yti c property. Si gnificant improvement of e lectrocatalyti c activity has been observed by many workers on introducing a second metal like Mo, Rh , Sn , Pd, Ru with Pt. Of these, Ru is thou!!ht to be the bes t choice2

·17 But even with the different Pt-Ru

e lectrodes known so far, the efficiency of the DAFC is sti II insuffi c ient for prac tical app li cations 17

. Si nee shape, size and morpho logy of bimetals are likely to affect the sa id re lated ox idation 1R, effort s have been made to produce Pt and Pt- Ru depos its of different morpho logies, over 1i support and co mpare the electroche mical characteristics in context to ethano l oxidation.

Materials and Methods PreJ)aration of electrocatalysts

Ni-foi l (99.9+% gold leve ls, Aldrich) having thickness of 0.0 125cm, was taken as substrate for noble me tal deposits. The lower portio n of the foi l was poli shed, the middle portion covered with the Teno n tape (champion) and the upper portion used for e lec trical connection. The dri ed fo i I was weighed before and afte r deposition . Depos ited anode material s on cheaper Ni-support were obtained by applying a current density of SmAcm-2 for 5 min under galvanosratic condition from 2M HCI soluti ons containing respecti ve salt/salts. To prepare nickel supported platinum (Ni/Pt) and platinum-ruthenium composite (Ni/Pt-Ru) e lectrodes respectively, 2 wt% chloroplatinic acid (H2PtCI(, H20 ) and a 1: I (vlv) mixture of it with 2wt% ruthenium chloride (RuCb. 3 H20 ) (both from Arora Matthey Ltd) were used . Simil ar depos itio ns were made from a dilute suspensi o n of I o/o(wlv) polyvinyl alcohol (PYA) (Mol. wt= 125000, Lab Rasayan Co) co ll o id con­taining similar e lectrodes prepared fro m so luti o ns containing PY A. These wi II hereafte r be represented as Ni/Pt(PYA) and Ni/Pt- Ru (PYA ), respective ly . The reagen ts used were of AR grade and the water used fo r rinsing electrodes, prep<tration of so lutions, etc., was triply di still ed. The passage of current during me ta l depositi on, polarization and chronopotentio­metri c study was done by a constant current charger (DB-300. DB Electroni cs) at room temperature .

Electrochemical measurements

All e lectrochemical measurements were co nducted in a two-co mpartment g lass cel l using a conventional three-electrode assembly . The

re fe rence e lectrode used was Hg/HgO/OH -( l M) (MMO) which had an equilibrium e lec trode potenti a l - 0 . 1 Y with res pect to SHE. In all e lectroche mi cal measure ments , pote ntial data were recorded with respect to MMO. A large Pt-foil ( I em x I em) was used as a co unte r electrode. Steady s tate po lari zati o n data were recorded on ly after application of a constant current for a long time until a steady po tential was achi eved . A change in po tential o f lmY or less in a time period of 10 min was considered as the criterion of the steady state condition. The potential of all th e e lec trodes s tudi ed was noted by an EC digital multi mete r OM 610 48. Constant current chronopotentiometric study o n th e e lectrodes was pe rformed at a relatively hig h current density of 5mAcm-2 in the same solutions using the same charger, DB-300. Cyclic voltammetric s tudi es were carried out using a computer controlled pote ntiostat/galvanostat (AEW-2, Munists t, Sycope l. Scientific Ltd , UK) and cyc lic voltammogram o f each e lectrode was recorded at least for ten consecutive cyc les in alkaline medium with and without ethano l in a potential range between -800m Y and 500m Y at a sweep rate of 50m Y /sec. Nitrogen gas (XL grade) was bubbled occasionall y through th e soluti on for about 20 min before starting some of the experiments and it was observed that there was no significant difference vis-a-vis s imil ar exper iments where N2 was no t at all purged. To test the se lf corrosi on of Ni and thinly coated mod ifi ed Ni e lectrodes in 1M NaOH (AR , Merek) medium . galvanostatic po larization study was conducted both a long the anodic and cathodic directions in blan k soluti on which contained no ethanol bu t only J M

aOH. SEM study was carried o n a JEOL-JSM =-6360) scanning electron microscope at an <1Ccele­rating potential of 25 KY.

Results and Discussion Characterization of deposits

The amount of deposits on the Ni support to prepare different Pt-meta l coaterl Ni electrodes fo r depositi on time of 5 min from different baths with a constant galvanostatic current o f 5mAcm·2 , is a~ follow s (in kgm·2x I 00): Ni/Pt: 0.71: Ni/Pt (PYA ): 0.48; Ni/Pt-Ru: 0 .24 ; Ni/Pt-Ru (PY A): 0.32 . Thus, the amount of the Pt deposition is greater in the case o f Ni/Pt e lectrode than that in Ni/ Pt (PYA) elec trode. O n the other hand , the amount of composite Pt-Ru deposit is less in Ni/Pt-Ru than in Ni/Pt-Ru (PYA ) e lectrode. This apparent anomal y is however quite

1146 IND IAN J CHEM. SEC A, MAY 2006

expected as pol ymeric suspension of PV A opposes the diffu sion of large-s ized platinum co mpl ex fPtCi r,f to the electrode surface and thus dictates less depos iti on of Pt in Ni/Pt( PVA) than Ni/Pt electrode. Moreover, when PV A is added to the aqueous

. solution con taining both [PtCI6f and [Ru(H20 hCh], the latter is opposed more by viscous PV A suspension because [Ru(H20 hCI3] is larger in size than [PtCI6f. Th is causes relat ively more deposition of Pt and improves Pt:Ru molecular rati o in Pt-Ru co-depos it , res ulting in an increase in weight of deposit in Ni/Pt­Ru(PV A) electrode as compared to that in Ni/Pt-Ru electrode. Thus, one may conclude that PV A

....

suspensiOn, as expected, opposes the di ffusio n of large-sized complex molecules to the electrode surface, resulting in the characteri stic mass deposits.

Sud'acc characterization

SEM image of Ni/Pt catalyst as obtained by electro-depos ition for 5 min (Fig. I a) from a 2 N HCI so lution of chloroplatinic acid shows a uni form, ti ghtly bound and spheri cal crys tallites of Pt having an average radius of about 150 nm. In a few iso lated places, black portions peep through the wh ite dots of Pt. These are seemingly ex posed Ni, Ni/Pt all oy and Ni-ox ides as supported by EDX. SEM image of Ni/Pt-

-~-

1 -- ·,,.-···.,... 1 • .':"((',Y-.:; K·, ._,,: ..

h.

. !

.,-*\

,. ·'

... ~ ~· .Jt· ~. ( b) -~ ... e-_.·: :r--'·1 .. ,#

(d)

r ' .

, .'-2Sk•.• xte. eee a.:: eee? .JU-M&:T

Fig. 1- SEM photographs of (a): Ni/Pt e lect rode surface; (b): Ni/Pt-Ru electrode surface; (c): Ni/Pt-Ru e lectrode surface. ( I b) enl arged I 0,000 times; (d): Ni/Pt(PVA ) e lec trode surface; and, (c): Ni/Pt-Ru(PVA) electrode su rface

PAUL et a/.: ELECTROOXIDATION OF ET HANOL OVER Pt-BASED DEPOS ITS ON Ni SUPPORT 11 4 7

Ru catalyst (Fig. 1 b) shows a large extent of Ru­deposition as it can be recogni zed by severa l black spo ts which were detected as Ru by EDX. The white portion in Fig . I b when en larged total 10,000 times (Fi g. lc) , shows the bound spherical structure of Pt­Ru composite. But. these spheres are no t of equa l s ize and eq ui-spaccd like those observed in (Fig. I a) . The crysta lli tes observed in Fi g. Jc are of re latively greater and different sizes, and are poss ibl y because of different rates of deposition of binary metals , which vary with potentia l, sol ution composition , nature o f support. etc. , and constantly change w ith time during deposition . O vera ll , Ni/Pt- Ru electrode is not quite unifo rm in a ll of its parts , and has different compos iti ons in different porti ons . All these factors and the greater average size of the crysta ll ites as compared to those of Ni/Pt e lectrode, make it an e lec trode o f d ifferent val ue and nature. Figure ld dep icts the mi crotopography of Ni/Pt(PV A) e lectrode and reveals uniform ly packed round shaped (observed by further enlargement) structure of e lec trocatalytic partic le for Ni/Pt(PV A) electrode. But, unlike Ni/Pt e lectrode, it shows a multil ayered structure. For Ni/Pt­Ru (PV A) e lectrode (Fi g. I e), s imilar ti ghtl y packed, round shaped e lec trocatalytic particles were observed with an overall multil ayered puffy or spo ngy structure which ensures g reater corrosion and roughness factor of the elec trode.To get round shaped crystallites, 2N HC I was suffi c ient, but PV A was necessary to get multilayered structure for the further improve ment of the catalytic power.

Corrosion studies

Figure 2(a-c) depi cts the po tenti ostatic polari zation plo ts in bo th anodi c :md cathodic direct ions fo r bare and coated Ni e lec trodes immersed in I M NaO H solution . It was found from the evaluated corrosion parameters that the corros io n current density is of the o rder of 10-5 Acm-2 fo r all these three types of e lectrodes under study as expected 19

. T hi s may be due to the formation of oxide/hydroxide compound o n the ex posed N i and Ni-a lloy surface of the e lectrode. A s li ght difference in corrosion rate according to the sequence, Ni/Pt-Ru> Ni :::::: Ni/Pt, may be attribu ted to an increase of effective corros ive surface area of Ni/Pt-Ru e lectrode due to its non-uniform layered structure as confirmed by the SEM study. On the othe r hand, a comparatively tight framework of Ni/Pt electrode, as is evident from SEM study , is seemingly respons ible for its a lmost equal corros io n rate as that of bare Ni, desp ite its positio n be low Ni in the same

0 ~ ~ Cll z._ > ro :;::; c (j)

0 0..

1 - anode(NI) 0.9 2 - cathode(NI)

(a)

0.6

0.3 1

0.0

-0.3 2

-2.1 -1.8 -1.5 -1.2 -0.9 -0.6 -0.3

0.3 1 - anode(Ni/Pt) , / (b) 2 - cathode(Ni/Pt) / ~

0.0

-0.3

-0.6

1 /'

--~ ~,:;;: __ "~-~

\ -2.1 -1 .8 -1.5 -1 .2 -0.9 -0.6 -0.3

o.6 1 - anode(NI/Pt-Ru) 2- cathode(NI/Pt-Ru)

0.4

0 .2

0.0

-0.2

-2 .1 -1.8 -1 .5 -1.2 -0 .9 -0.6 -0 .3

log i(mA cm-2)

Fig. 2- Piot o f potentia l versus log i (mA cm-2) for I M NaO H of (a): bare Ni; (b): Ni/Pt ; (c) : Ni/Pt-Ru elec trodes. [Curve I: Anode: Curve 2: Cathode].

group in the periodic tab le. However, the nobl er behaviour of both the two e lectrodes than that of bare Ni is evident from the positi ve shift of the corros io n poten ti a l as observed by o thers 19

.

Ecorro( Ni ) < Ecorro(Ni/Pt) < Ecorro(Ni/Pt-Ru)

Thus, Ecorro reflects nobler behaviour of the coated e lectrodes as compared to pure Ni foil.

Cyclic voltammctrie studies

The cyclic vo ltam mog rams of the base N i-foi l and Ni/Pt-Ru e lectrode in the blank, viz., IM NaOH

1148 INDIAN J CHEM. SEC A. MAY 2006

< E

i ~ ::J u

3.0 ~~ -- NI/(NaOH) + EtOH)

I (a)

... .. .... NI/NaOH (blank)

J 1.5

/ I

0.0 I

;f' ·'. - .. j

1,5 /; 1/ J:

f 3.0 I

-1.5 -1.0 -0.5 0.0 0.5 1,0 1.5 2.0

3

0

-3

-6

V = -0.147V (c)

... Ni/Pt(1st cycle) -- NI/Pt(5th cycle)

-0.9 -0.6 -0.3 0.0

3

< E o i ~ ... ::J u -3

0.3 0.6

V =- 0.299V

2

0

-2

-4

3

0

-3

-- Ni/Pt-Ru/(NaOH + EtOH) (b) ····· ···· Ni/Pt-Ru/NaOH(blank)

-1.2 -0.8 -0.4 0 .0 0.4

V = 0.0447V (d) 1=1 .796mA

NI/Pl(PVA)(1st cycle) ···· ·· ··· NI/Pl(PVA)(3rd cycle) -- Ni/Pl(PVA)(5th cycle)

-0.9 -0.6 -0.3 0 .0 0.3 0.6

(e)

NI/Pt-Ru(PVA)(1st cycle) ··· ······ NI/Pt-Ru(PVA)(3rd cycle)

-6 -- Ni/Pt-Ru(PVA)(5th cycle)

-0.9 -0.6 -0.3 0.0 0.3 0.6

Potential, V(vs MMO)

Fig. 3---Z:yc li c voltammet ric profi les for (a) : N i-roil electrodes immersed in I M NaOH w ith (so lid line) and w itiH,ut (dasiH:d line) IM eth anol: (b): Ni/Pt-Ru electrode immersed in IM NaOH with (solid line) and without (dashed li ne) IM ethanol: (c ): Ni / l't electrode immersed in IM cthanolic so lution or I M NaO l-1 : (d): Ni/Pt(PVA) electrode immersed in IM cthanolic so lution or IM JOl-1. Jnd. (c): Ni/Pt-Ru(I'V A) electrode immersed in IM cthanolic solution of I M NaOH.

so lution, a rc shown in Fig. 3(a & b) respect ive ly . In both the vo ltammograms, hydrogen evoluti o n occurs at the starting pote ntial , v iz ., -1 .2V, whi ch is ev ident by the hig h cathod ic c urre nt and bubbl e formation. The greater va lue of the initial current for Ni / Pt-Ru over that for Ni-foil e lec trode may be att ri buted by the g reater ro ughness factor and better act ivity of Pt-Ru

. I I f I 'd . 1

Ill I over 1 e ectrocata ys t o r t 1e sa1 reactions-· . n bo th the vo ltammograms, sma ll anodic peak appears in the cathodic regio n. They are about -0 .9V for Ni/Pt-Ru and -0.7V for Ni . These peaks poss ib ly

correspo nd to the ox idati on of metal 2 1 -c~ according to

the reaction,

M + nOW ~ M (OH) n + ne .. . (2)

and re fl ect that on polari za tion such an ox idation is more favourabl e o n Ru-Pt rather th an o n Ni surface. ForNi, at around 0.48V , another small peak appears. Th is peak, as correc tl y me ntioned e lsewhcre

9·21

1s poss ibly an indication o f the tran s fonnati on c~ of

Ni(OHh to NiO(O H) .

PAUL era/. : ELECTROOXIDATION OF ETII A OL OVE R Pt-8/\SED DEPOSITS ON Ni SU PPORT 1149

Cyclic vol tammograms. (Fi g. 3) illustrate the ox idation of ethanol on bare Ni , Ni/Pt-Ru, Ni /Pt,

i/Pt(PV A) and Ni/Pt-Ru(PV A) electrodes respec­ti ve ly. The electroox iclati on of ethanol on bare Ni-foil is characteri zed by a persistent anodic peak within 5 cycles at about !250m V with respect to M MO electrode. This peak appears before 0 2 evolution. whi ch is indicated by a sharp increase of the current after 1500mV. This signifies delaying of 0 2 evolution in presence of ethanol clue to the occurrence o r more favourable ethanol ox idation on mainly NiO(O H) surface of Ni "~-26 . From the voltammograms for Ni/Pt and Ni/Pt(PV A) electrodes (Fig. 3c, 3d) respectively. it is quite ev iden t that in the course of multipl e scanning, there is a steady anodic shift of peak potential with a little incremen t of anodi c peak current. Thi s result may be attributed to an increased accumulati on of adsorbed intermediate oxidative species during potential cycling until an apparent steady conditi on is reached at about 5111 cyc le. The ox iclati ve products may i ncl ucle poisoning inter­mecliates like Pt-CO, Pt-CO H, etcc 7

.28 which can be

oxidi zed onl y at the higher potential "lJ . At the lower potentials, a small fraction of the surface is utili zed clue to the oxidation of the intermediate Pt-H only. whereas all different types of surface recluctants including the so called ' poison ' are ox idized at the hi gher potential s. in volvi ng the who le surface of the electrode. Larger va lue of peak current for

0.6

-0 ~ 0.3

II)

> -> 0.0

-c 2 -0.3 0 a.

-0.6

r:{'o

'j 2 717~ /; .- ~'

4/

I 1 N: /r~ 2 Ni/Pt ~ 3 Ni/Pt(PVA)

, v~~ 4 Ni/Pt-Ru

(a)

5 Ni/Pt-Ru(PVA)

0 30 60 90 120 150 180

Time, min

i/Pt(PV A) over that fo r Ni/Pt clectroclc is indicati ve of faster kinetics o r ethanol ox idation on the former electrode as compared to the latter. The reason behind thi s is possibl y the superimposable effect of greater di spersion of Pt-crysta iJites over that of lesser depos iti on (or load ing) of Pt particles fo r the Ni/Pt(PV A) electrode as compared to that of the Ni /Pt elec trode. Notabl y, for Ni/Pt electrode (Fig. 3c). a shoulder type of variation of current is observed at about 0.2V (w. r.t. MMO), bes ides an initi al anodic peak appearing at about -0. 15V. These two diffe rent regions possibly indicate ox idati on of adsorbed Pt- H and Pt-C bonds respect ively. These two regi ons however coalesce together for Ni/Pt (PV A) and Ni/Pt­Ru(PV A) electrodes, where Pt-crys tallites are well di spersed for whi ch there is a poss ibility of the resc uer M-OH intermediates . viz., (Ni-OH. Ru-OH etc.) being formed at the relatively lower anodic potential s.

Constant current chronopotcntiomctric studies

Variation of potential with time for each of the electrodes on app lication of a relatively large current density, viz., SmAcm·c, shows chronopotentiometric profiles (Fig. 4). These profiles reveal two potential regions for the overall reaction. The higher potenti al zone, 0.4-0.6V w. r.t. MMO. possibly signifies the zone for co mpl eti on of ox iclation due to the presence of M-OH bonds at these potent ial s as observed by

-0 .1 (b)

-0.2

-0.3

Ni/Pt-Ru

-0.4

-0.5

-4 -3 -2 -1 0 1

Fig. 4--(a): Plot of potential versus time for different electrodes studi ed in I M ethanolic solution of I M NaOH. and. (b): Plot of T t/2' _ 1tn

potential versus r1n 1,, J for di fferent electrodes studi ed in I M cthanol ic solution of I M NaOH.

I -

11 50 INDIAN J CHEM. SEC A, MAY 2006

others 17• Whereas the lower potenti al zone (-O.SV to

-0.2Y), al so observed by others1Y poss ibl y con·es­

poncls to the ox idation of MH bonds fo rmed by fas t dehydrogenation of alcohols. These profil es indicate that at any instance, fo r drawing the fi xed current from the system, the potenti al requirement of the anode vari es 111 the order: Ni/Pt-Ru(PVA) < Ni/Pt(PV A) < i/Pt < Ni, refl ecting an exac tl y reverse order of electrocatal ytic power. In other words, Ni /Pt­Ru(PV A) prov i.cles the max imum res istance towards electrode poisoning and Ni prov ides the minimum. The steady state potenti al achi eved at the hi gher potential zone, fo r the same current density draw n, follows, however, a diffe rent order: Ni/Pt(PV A) < ' i/Pt < Ni /Pt-Ru(PVA) < Ni/Pt-Ru < Ni. The reason

behind the required hi gher steady state values of potenti als fo r electrodes containing Ru , as compared to electrodes containing only Pt, seems possibly clue to the former's lesser ability towards dehydrogenation and greater ability towards ox ide fo rmati on IY.

20 which indirectly reduces the rate of dehydrogenation by covering many acti ve centers on the surfaces of the electrode. Moreover, cletai led anal ys is of chrono­potentiometric curves show that the transition time, r

increases in the order (given in minutes within parentheses): Ni (5) < Ni /Pt (3 1) < Ni/Pt(PV A) (33) <Ni/Pt-Ru (36) <Ni/Pt-Ru(PV A) (59), with an indica­ti on of the resistance offered by di fferent types of electrodes towards poisoning. Observed potential E vari es with time according to a straight line plot of E

T11 2 -1 112

versus [In !? ] , conforming to Eq. (3) :

0 :2

0.52

:2 0.48 rn > > 0.44 -· Ill - 0.40 c: Cll

0 c.. 0.36

f I -

-1 .0 -0.5 0.0 0.5

log(current density, mAcm .2

)

1.0

. RT T 112 - t 1n £= £ ---In II"

a.n F r -.. . (3)

indicating a di ffusion-acti va ti on co ntro lled qu asi reversibl e reacti on for the dehydrogenati on step of the ethanol ox idation. These plots are shmvn in Fig.4 (b). Lower slopes(i n vo lts) for electrodes containing Pt-Ru (N i/Pt-Ru :0.033; Ni/Pt-Ru(PV A):0.040; Ni/Pt :0.098: Ni/Pt(PV A):0.111 ) re fl ect greater o.n (=transmiss ion co-effi cient x no of electrons transferred) (N i/Pt­Ru :0.78; Ni/Pt-Ru(PV A): 0.64; Ni/Pt : 0.26: Ni/Pt(PV A):0.23) indicating bette r electrocatalyt ic acti vity than that fo r pure Pt elec trodes. On the other hand , hi gher negative value of the intercept, £ . (in volts) (N i/Pt-Ru :-0.5 12; Ni/Pt-Ru(PV A): -0.542: Ni/Pt: -0.343; Ni/Pt(PV A): -0.426), fo r Pt-Ru electrodes than that for Pt electrodes indi cates . . . II f 17"') I ~ . tntnnstca y power u · catalyti c na ture of Pt-Ru electrodes th an that of Pt electrodes.

Steady state polarization Figure Sa illustrates the polari zati on curves of bare

Ni and all the coated electrodes studied for anodic ox idati on of ethanol at roo m temperature. The curves reveal the linear Tafel region up to a potential of about O.SY . The order of the catalyti c ac ti vity of the electrodes is: Ni/Pt(PV A) > Ni/Pt > Ni/Pt-Ru(PV A) >N i/Pt-Ru > i, as it is ev ident from the decreased steady sta te potenti al value for the electrodes in the above order, at any fi xed current denslty within the region. The results show that there is no apparent improvement on introducing Ruin the catal ys t depos it of Pt.

0.441 (b)

0.42

0.40

0.38

0.36 0

0.34 Ni/Pt(PVA)

~~~~~~~~~~~ -1 .6 -1 .2 -0.8 -0.4 0 .0 0 .4 0 .8

log(current, mAmg .' )

Fig. 5--{a): Plot of steady state potential versus log (curn.:nt density. mA em- ~) for different electrodes studied immcrsec! in I M ethanolic solut ion of I M NaOH, and, (b): Plot of steady sta te potential versus log (current, mA cm.1

) for di f ferent electrodes studied. immersed in I M ethanoli c so lut ion of I M NaOH.

PAUL eTa/. : ELECTROOXIDATION OF ETHANOL OVER Pt-BASED DEPOSITS ON Ni SUPPORT 1151

Since the amount of deposit per unit area was different for the coated electrodes studied, it was throught that a plot of logarithm of current per unit weight of deposit versus potential might bring rewarding results . With this end in view, current per unit weight of deposit was calculated by dividing current per unit area by weight of deposit per unit area and the said plot was executed. The profile is depicted in Fig.Sb. From the profile it is evident that the overall electrocatalytic activity still follows the above mentioned order of activity, but the slopes of the profiles follow the order: Ni/Pt-Ru(PV A) < Ni/Pt-Ru < Ni/Pt(PV A) < Ni/Pt, reflecting a decreased overall transmission ability and hence catalytic activity of the e lectrodes in the order.

Conclusions From cyclic vo ltammetry, chronopotentiometry ,

polarization and SEM studies, we can conclude that among the different e lectrodes studied, Ni/Pt(PV A) gives the best performance. Overall electrocatalytic activity decreases in the order: Ni/Pt(PV A) > Ni/Pt > Ni/Pt-Ru(PVA) > Ni/Pt-Ru > Ni. The better perfor­mance of Ni/Pt(PV A) and Ni/Pt-Ru (PV A) than Ni/Pt and Ni/Pt-Ru respect ively may be due to , the spongy and multilayered structure of the deposit when it is generated from solutions containing PV A. The better e lectrocatalytic activity of Pt-deposits over the co­depos it of Pt and Rt' (Pt-Ru) may be due to (i) poor performance of Ni and Ru in the dehydrogenation step, particularly at the low overpotential, and, (ii ) blocking of surface by the rescuer intermediates M-OH (M = Ni, Pt, Ru) at the higher overpotential leading to a decrease in the availability of free surface for the initia l step of dehydrogenati on of C2H50H. Moreover, a greater roughness factor for Pt­e lectrodes than for Pt-Ru electrodes, :ts caused by a decrease in size of the former 's deposit-crystallites than that formed in case of co-deposition, seems poss ibly the reason behind the apparent greater e lectrocatalytic acttvtty of Pt-electrodcs at the working condition. However, the evaluation of the catalytic power per true unit area by e liminating the e ffect of roughness factor . may lead to a different order of intrinsic e lectrocatalytic activity of the e lectrodes studied. In thi s case, Ni/Pt-Ru(PV A) may appear to be the best with the following order of intrinsic e lectrocatalytic activity: Ni/Pt-Ru(PV A)> Ni/Pt-Ru> Ni/ Pt(PVA) > Ni/Pt > Ni , as reflected from chronopotentiometric and steady state polari zation studies.

Acknowledgement Financial assi stance from University with Potenti al

for Excellence Scheme of the UGC, New Delhi , is gratefully acknowledged (Rec/N/66/04 ).

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