MECHANISM OF HYDROGEN EVOLUTION REACTION ON GOLD IN AQUEOUS SULFURIC ACID AND SODIUM HYDROXIDE*)

Transcription

1 Title MCHANISM OF HYDROGN VOLUTION RACTION ON GOLD IN SODIUM HYDROXID Authr(s)SASAKI, Takeshi; MATSUDA, Akiya CitatinJOURNAL OF TH RSARCH INSTITUT FOR CATALYSIS HOKK Issue Date Dc URL Type bulletin File Infrmatin 29(3)_P pdf Instructins fr use Hkkaid University Cllectin f Schlarly and Aca

2 J. Res. In st. Catalysis, Hkkaid Univ., Vl. 29, N.3, pp. 113 t 132 (1981) MCHANISM OF HYDROGN VOLUTION RACTION ON GOLD IN AQUOUS SULFURIC ACID AND SODIUM HYDROXID*) By Takeshi SASAKI**) and Akiya MATSUDA***) (Received Nvember 25, 1981) Abstract The hydrgen ver vltage n gld has been splitted int the cmpnents l71 and l72 which are caused respectively by the change f the free charge and the intermediate species n the electrde surface by the galvanstatic transient methd in aqueus sulfuric acid and sdium hydrxide. It has been fund that 7/1 is respnsible fr the prmtin f the rate f the electrn transfer step which is the discharge f hydrnium in in acid and that f Na + in in alkaline slutins, and that the recmbinatin f adsrbed hydrgen atms is rate-determining at lw current densities in acid. In alkaline slutins, hwever, there appears an vervltage cmpnent due t the specific adsrptin f sme species ther than the intermediate Na atm which may result in the irregular variatin f the plarizatin curves f the verall reactin. Intrductin The mechanism f the hydrgen evlutin reactin (h. e. r.) n gld has been discussed in acid slutins mainly based n the b-value f the Tafel slpe f the plarizatin curves in steady cnditins. I - 13 ) The Tafel slpe in acid sultins, hwever, are nted fr their wide variatins amng different authrs. In alkaline slutins very few investigatins have been reprted l8 ) because f difficulties in btaining reprducible results. On the ther hand, as pinted ut by Hriuti ct al,14l the theretically predicted b-value may vary frm 30 m V t the values higher than 120 m V depending upn the mutual interactin amng the intermediate species and activated cmplex f the reactin, even if the reactin prceeds thrugh a single mechanism. Therefre the b-value f the Tafel slpe cannt necessarily serve as a diagnstic criterin fr the mechanism f the h.e. r. *) This article is a part f Dctr thesis f T. S. in Hkkaid University (1974). Present address: **) Faculty f ngineering, **,,) Research Institute fr Catalysis, Hkkaid University, Sappr, 060 Japan. 118

3 114 T. SASAKI and A. MATSUDA It is desirable t divide experimentally the verall reactin int its cnstituent elementary steps and t cmpare directly their rates fr the elucidatin f the h. e. r. The present wrk is cncerned with the determinatin f the rate and vervltage f the cnstituent elementary steps f the h.e. r. n gld in acid and alkaline slutins by the galvanstatic transient methd. 15 ) Theretical basis f the galvanstatic transient methd The hydrgen vervltage r; may be caused by the change f free charge density and adsrbed quantity f intermediate species n the electrde surface. These tw cmpnents f vervltage may be separately determined by a galvanstatic transient methd by bserving the transient f the electrde ptential, r; - t curve, which is caused by adding a current pulse f cnstant height t the plarizing current. 15) If we bserve the vervltage transient at different time windws, then at the initial stage the change f r; may be caused by the change f free charge n the electrde surface, since charging up f the electric duble layer at the metal/slutin interface may be much faster than the frmatin f adsrbed intermediate species n the electrde surface. Therefre we can btain infrmatins n the differential capacity CD f the duble layer and the time cnstant '1 f the electrn leakage thrugh the duble layer frm the analysis f the initial r; - t curve by the equatin 15 ) where.di is the height f the current pulse and r; signifies the time derivative f r;, which can be identified with that f the electrstatic ptential W* f the electrde surface in cntact with the slutin when W* is changed by charging up f the duble layer. As pinted ut by Frumkin 16 ), W* can be expressed by the sum f several cmpnents ( 1 ) W* = W; + Xa + X SCM) + Xs + W S, (2 ) where W; is the cmpnent f the electrstatic ptential caused by the free charge n the electrde surface, Xa is the surface ptential due t the adsrbed *) The electrchemical ptential f the electrn Pe in the electrde metal can be expressed as Pe=-F(IJf*+W*)=-F(IJf+"V) in terms f the wrk functin W* f the electrde surface in cntact with the slutin r in terms f IJf and "V f the electrde surface in cntact with the gas phase.

4 115 Hydrgen l'/utin Reactin n Gld intermediate, XS(~!) is the surface ptential due t the specific rientatin f the slvent diple n the electrde surface, Xs and Iff s are respectively the surface and Vlta ptentials f the sultin. In the present treatment the last three terms f q. (2) are assumed t be cnstant. Therefre it fllws frm q. (2) that dw"; dx a -dt + -Cit ' (3 ) and 1; in q. (1) can be identified with 1p,,;. The vervltage cmpnent 7)1 caused by free charge n the electrde surface is given by the difference 7)1 = W"; -W"; (rev), (4 ) where W"; (rev) is W"; at the reversible state f the h. e. r. We assume here that the electrn leakage thrugh the duble layer ccurs nly thrugh the electrn transfer step f the h. e. r. Then the time cnstant 7"1 in q. (1) is given by where r l is the reactin resistance f the electrn transfer step which may be defined as and can be equalized t dlf f *. d/ by q. (3). If we knw the value f r l under a steady cnditin f plarizatin as a functin f the current density i, then it may be pssible t estimate the value f 7)1 at current density i by the integratin f q. (6) The vervltage cmpnent 7)2 caused by the adsrptin f the reactin intermediate n the electrde surface may be given by (5 ) ( 6 ) (7 ) 7)2 = Xa - Xa (rev) = 7) - 7)1 (8 ) under the presumptin that XS(M) =cnst at any electrde ptential. In this way it may be pssible t elucidate the mechanism f the h. e. r. frm the cmparisn f 7)1 and 7)2 in a steady state determined by the galvanstatic transient methd*). *) Recently ny17) has discussed the rle f the surface ptential due t the intermediate species in the kinetic law f the h. e. r. His treatment will be discussed separately elsewhere.

5 116 T. SASAKI and A. MATSUDA xperimental methd The electric circuit fr the bservatin f galvanstatic transients is shwn in Fig. 1. The plarizing current i was supplied t the test (T) and the cunter electrde (C) frm strage battery () thrugh high resistance (R 2 ). The hydrgen vervltage was determined by the ptential difference between (T) and the reference electrde (R) bserved n a memscpe scillscpe screen (Hughs Arcraft C., Mdel 105 A), the hmic part due t the slutin resistance being crrected n the scillscpe screen (M) by a differential pre-amplifer. A current pulse Lli was superimpsed n the plarizing current by a pulse generatr (YHP C. Mdel 214A) and the transient curves f vervltage were recrded n the scillscpe screen. A subsidiary circuit (K) was used fr the cmpensatin f the steady state vervltage in rder t bserve nly a transient part f vervltage n the screen. The arrangement f reactin apparatus is shwn in Fig. 2, where (A) and (B) are vessels respectively fr preparatin and pre-electrlysis f slutin R, T Test cell Fig. 1. lectric circuit fr galvanstatic transient measurement. Rh R 2, R3: Resistances fr current cntrl and measurement and fr cmpensatin f hmic part f vervltage; K: cmpensatin circuit fr statinary vervltage; PG, M: pulse generatr and mems cpe scillscpe.

6 117 Hydrgen ulutin Reactin n Gld ~-L~~~~~-J[----~~~ ~~2-----~~~ t Fig. 2. A B c D xperimental apparatus. Vessels fr preparatin f slutin (A), preelectrlysis (), test cell (e), slutin strage (D), ph-measurement () and purified S03 gas (F). (Pl)-(PS), (ql)-(q3) greaseless valves. and (C) the test cell, all f which were made f quartz glass, and (D) fr slutin strage and () fr ph measurement. All glass materials were preliminarily cleaned by steaming in hydrgen stream after treatments with ht mixture f cncentrated nitric and sulfuric acids. Sulfuric acid slutin was prepared frm S03 gas distilled in vacuum and triplly distilled water. Alkaline slutins were prepared frm sdium hydrxide and sdium sulfate recrystallyzed frm analytical reagent. The slutin in (A) was transferred t (B) by hydrgen pressure thrugh greaseless stp ccks (p) and (q). After pre-electrlysis f the slutin in (B) fr abut 50 hurs with 20 rna, the slutin was transferred int (C) by hydrgen pressure. After each run f kinetic measurements in (C) the slutin was transferred int (D) which was used fr the determinatin f ph in () r fr the determinatin f the cncentratin by the titratin methd. Gld fils f purity 99.99% (Jhnsn Matthey C.) used fr test and reference electrdes were purified by successive treatments with ht mixture f acids, ht distilled water, steaming in hydrgen stream and degassing under vacuum. Gld films prepared under vacuum by evaprating n t a grund glass surface were als used as the reference r test electrde.

7 118 T. SASAKI and A. MATSUDA xperimental results and discussin (1) Hydrgen evlutin reactin in aqueus sulfuric acids The effect f andic pre-treatment n the activity The activity f the gld hydrgen electrde in sulfuric acid was fund t increases by andic treatment and its activity was maintained fr abut an hur and then decreases gradually with time. 2.0 c Q) (; OU---~L-----~ L---L----ir~------~L--L t (5) Fig. 3. Decay curves f electrde ptential in 0.67 N H 2 S0 4 after andic treatment at 1.2 X 10-3 A/cm 2. Starting ptential (vs. r. h. e.): curve (1) V; curve (2)-1.4 V; curve (3)-1.64 V; curve (4)-1.8 V. In rder t get infrmatin n the surface state f the gld electrde after andic treatments the ptential decay in the andic regin was bserved. A typical example f the decay curves in 0.67 N H 2 S0 4 is shwn in Fig. 3. The xygen evlutin was bserved steadily at the ptential 1.8 V and the current density 1.2 X 10-3 A/cm 2 It is seen frm Fig. 3 that an arrest appears in the decay curve at V which may be attributed t an xidized state f the gld surface. The hydrgen adsrptin can nt practically be detected as seen frm curve (1). In rder t find the change f the surface area by andic treatments, the differential capacity Cn f the electrde was determined by the galvanstatic transient methd. Fig. 4 shws the relatin between lg (-ili/i;) vs. t btained frm the r; - t curves arund the equilibrium ptential after andic pretreatments. The value f Cn was btained frm the extraplatin f the linear part in this figure n the basis f eq. (1). The values f Cn

8 119 Hydrgen vlutin Reactin n Gld -4.5 '=: , -- 'fl ' t. (fl s) Fig. 4. ffect f andic treatment f gld fil f gemetrical area 0.6 cm 2 n the vervltage transients at reversible ptential in 0.3 N H 2 S0 4, expressed by lg (-Lli/lj) vs. t relatins. Line (a)-befre andic treatment; line (b)-after andic treatment fr 0.1 sec. at 1.2 V (vs. r. h. e.); line (c)-after andic treatment fr 10 sec. at 1.7 V (vs. r. h. e.). c.~ 01 Fig ~----~------~------~ J (mv) ffect f andic treatment n the steady state plarizatin curves btained at the same cnditin f the electrde as in Fig. 4. Current density is expressed in terms f the true surface area estimated frm the values CD.

9 120 T. SASAKI and A. MATSUDA f the gld fil f gemetrical area 0.6 cm 2 are respectively 13, 19 and 23 ftf fr curves (a), (b) and (c) respectively. It is seen frm this figure that 7'1 remains practically unchanged, but CD is increased by andic treatments. The surface area f the gld electrde f gemetrical area 0.6 cm 2 is estimated t be 0.72, 1.06 and 1.28 cm 2 frm these CD-values using 18 ftf/cm 2 fr the mercury electrde. 19 ) The increase f the surface area by andic treatment may be attributed t the rughening f the electrde surface as suggested frm the appearance f bending at the initial part f curve (c) in Fig. 4, as pinted ut by Kunimatsu. 18 ) The steady state plarizatin curves which crrespnd t curves (a), (b) and (c) in Fig. 4 are shwn in Fig. 5 expressed in terms f the current density using these surface areas. It is seen frm Fig. 5 that the increase f activity f the gld electrde by andic treatments cannt be explained by the increase f surface area f the electrde, cntrary t the arguments f Kuhn and Byrne. ) On the ther hand "I is practically nt affected by andic treatments, as seen frm Fig. 4. It can be cncluded frm these facts that the increase in activity f the gld hydrgen electrde by andic treatment may be attributed t the acceleratin f elementary steps which fllws the electrn transfer step (\j 8 0 "- <t '-') CI ~ ~----~------~----~------~ T) (mv). Fig. 6. Steady state plarizatin curves in aqueus H 2 S0 4 Cncentratin (N): (1)-1.3, (2)-0.67, (3)-0.30, (4)-0.12, (5)-0.077, (6)-0.037, (7)-0.012, (8) , (9)

10 121 Hydrgen vlutin Reactin n Gld The plarizatin curves fr steady cnditins Fig. 6 shws the plarizatin curves in varius cncentratin f H 2 S0 4 btained with an electrde f the highest activity after andic treatment. These curves were bserved at 25 C under strng stirring by frced hydrgen bubbles thrugh sintered quartz glass. As seen frm Fig. 6, the lg i vs. r; curves cincide with each ther at lw current densities, whereas a strng cncentratin dependence appears N ~ <t <t I 0 )( ~ 'T} (mv) Fig. 7. i vs. r; relatin in steady states in the neighburhd f the r. h. e. in aqueus H 2 S0 4. The cncentratins f slutins are the same as in Fig. 6.

11 122 T. SASAKI and A. MATSUDA at high current densities. Curve (1) which has Tafel slpe f 28 mv is nt affected by hydrgen bubbling, while curves (2)-(9) are strngly affected by stirnng. The strng effect f hydrgen bubbling n the plarizatin curves may be explained by the diffusin f hydrnium in frm the bulk f slutin t the electrde surface. The Tafel slpe f 28 mv in cncentrated slutins may be explained either by the diffusin f mlecular hydrgen frm the electrde surface t the bulk f slutin 20 } r by the recmbinatin f atmic hydrgen at lw cverage I4 15 } as rate-determining. As reprted by Schuldiner and Hare,!} hwever, the plarizatin curves f Pd-hydrgen electrde shw higher current density by abut ne rder f magnitude than curve (1) in Fig. 6. It may be cncluded frm these facts that the hydrgen evlutin n the gld electrde in sulfuric acid at lw vervltages is in favr f the recmbinatin mechanism. Furthermre the i -r; curves in steady states in the neighburhd f the reversible ptential in varius cncentratins f H 2 S0 4 frm 1.3 t N can well be expressed by a linear relatin independent f the cncentratin f the slutin as shwn in Fig. 7. We btain the exchange current density ir f the rate-determining step divided by its stichimetric number }..Ir as A/cm 2 by the equatin ir/}..ir= ~J /( ~i )~= using the value f (~n~ =0 calculated frm Fig. 7. Overvltage cmpnents and kinetic law f electrn transfer step Fig. 8 (1) shws a series f lg (- fli/i;) vs. t curves in 0.67 N H 2 S0 4 btained frm the r;-t curves bserved in varius strady state cnditins. As seen frm this figure, CD remains cnstant independent f the starting value f r; ranging frm 0 t -60 my, whereas 'I decreases with increasing vervltage ranging frm 14 f1s at r; = 0 t 7.8 f1s at - r; = 60 m V. The values f rl calculated frm '1 and CD in Fig. 8 (1) are pltted against the steady current density i in Fig. 8 (2). The vervltage cmpnent r;1 due t charging up f the electric duble layer at the metal/slutin interface can be calculated n the basis f eq. (7) by the graphical integratin f the rl - i curve in Fig. 8 (2). The lg i vs. r;1 plt btained frm Fig. 8 (2) is shwn in Fig. 9 by pen circles in the slid line, which shws the theretical relatin 21l F7J 1 F1l1 i = ilo(e-2rt-e2rt), ( 9) where ilo is the exchange current density f the electrn transfer step calculated by the equatin using the reactin resistance rio at r; = 0,. RT 1 tl=-f - rio (10)

12 123 Hydrgen vlutin Reactin n Gld.!='" ::::; -4.0 <l..!.. CI t (fls) Fig. 8, (1). lg (-Ji/~) vs. t relatin in 0.67 N H 2 S0 4 btained frm the transient curves at varius steady cnditins. Starting vervltage -1J (m V): (1)-0, (2)-9, (3)-19, (4)-48, (5) C\I 0 C: 0.4 c 0.2 ~ ~ ~ i. (ma/cm 2 ) (2). i-dependence f the reactin resistance rl estimated frm Fig. 9. The lg i vs. r;1 plt in 1.3 Nand 0.3 N H 2 S0 4 is als fund n the theretical line given by eq. (9) as shwn in Figs. 9 (2) and 9 (3). It is surprising that the rate f the electrn transfer step can be expressed in terms f the vervltage cmpent r;1 by a simple equatin (9) as in the case f the Hg-hydrgen electrde in acid slutin, althugh the range f r;1 in the present case is narrw. Fr cmparisn the plarizatin curve f the verall reactin is als shwn in Figs. 9 by brken lines.

13 124 T. SASAKI and A. MATSUDA Fig "7, r "7 (mv) (I) "7, r "7 (2) -150 (mv) -3.0 i, ""d'" p' P ri I _4.01!..' ~_--:-~--' "7, r "7 (mv) Plarizatin curves in steady states fr the electrn transfer step (slid line) and verall reactin (brken line) in 0.67 N H 2 S0 4 (1), in l.3 N H 2 S0 4 (2), in 0.30 N H 2 S0 4 (3). (3) (\I ~ <t ~ Fig ph ph-dependence f the exchange current density it f the electrn transfer step in aqueus H 2 S0 4.24) The ph-dependence f the exchange rate ilo f the electrn transfer step24) is shwn in Fig. 10. The gradient f lg ilo vs. ph equals abut It may be cncluded frm this result that the electrn transfer step f the hydrgen evlutin reactin n gld in sulfuric acid is the discharge f hydrnium in. The vervltage cmpnent 'fj2 due t the adsrbed intermediate species can be estimated as the difference 'fj-'fjl, frm Fig. 9. Fig. 11. shws the relatins between lg i vs. 'fj2 in 1.3, 0.67 and 0.3 N H 2 S0 4, As seen frm

14 125 Hydrgen vlutin Reactin n Gld I 2 3 N " <{... Cl ~--_-5..L.0---_-1...J _-15..J...0 7]1 r 7]2 (mv) Fig. 11. Steady states plarizatin curves f the elementary steps f the hydrgen evlutin reactin n gld in 0.3, 0.67 and 1.3 N H 2 S0 4 ; 1,2, 3-the discharge f hydrnium in; 1',2', 3'-the recmbinatin f adsrbed hydrgen atms. Fig. 11, lg i vs. "1J2 curves practically cincide with each ther irrespective f ph f the slutin with gradient abut 29 m V. These plarizatin curves can be explained by the recmbinatin f adsrbed hydrgen atms at lw surface cverage as predicted by Hriuti I4 ). Frm the cmparisn f the lg i vs. "1JI and lg i vs. "1J2 curves it can be seen that at lw current densities the recmbinatin f adsrbed hydrgen atms is rate-determining. In fact the rati f the exchange current densities i lo /i 20 falls in the range depending n the cncentratin f sulfuric acid, in cnfrmity with i lo /i 20 ;::;10 btained by Matsushima and ny22) by the istpic tracer methd, where i 20 is the exchange current density f the recmbinatin f adsrbed hydrgen atms. Hwever, we cannt neglect the pssibility that the discharge f hydrnium in in turn cntrls the rate at high current densities when the diffusin cntrl f hydrnium in is eliminated. (2) Hydrgen evlutin reactin in aqueus sdium hydrxide Plarizatin curves f the verall reactin The behavir f the plarizatin f the gld hydrgen electrde in sdium

15 126 T. SASAKI and A. MATSUDA hydrxide is quite different frm that in sulfuric acid. The actlvlty f the electrde in alkaline slutins changes spntaneusly with time, but it increases after andic treatments at 1.7 V r higher ptentials. Hwever, the activity is nt reprducible after andic treatment. Such circumstances make it difficult t elucidate the mechanism f the hydrgen evlutin reactin n the basis f the steady state plarizatin in alkaline slutins. The effect f andic pre-treatments n the steady state plarizatin in 8 X 10-3 and 0.56 N NaOH and in 0.43 N Na2S N NaOH is shwn in Figs. 12. It can be seen frm these figures that the activity f the gld hydrgen electrde in alkaline slutins is lwer than that in acid slutins by abut 3-4 in rder f magnitude. The distinctive feature f the plarizatin curves in alkaline slutins is that there appears a bending in the regin lwer than abut -r;=250 m V where the Tafel slpe is much higher than 120 m V. In the regin higher than 250 m V the plarizatin curves shw usual behavir with the Tafel slpe 120 m V. The bending f the plarizatin curve is mre cnspicuus with increasing ph f slutin. N ~ :: ~ ~ <[ > > TJ Fig. 12. (I) (mv) TJ (mv) TJ (mv) (2) (3) Plarizatin curves f the verall reactin in steady state f the hydrgen evlutin reactin n gld in alkaline slutins. (1) N NaOH, (2)--0.56N NaOH, (3) N Na2S N NaOH. (ahmmediately after intrductin f slutin int cell; (b)-after andic treatment fr several sec. at 1.65 V (vs. r. h. e.); (c)--after several hurs frm andic treatment. In rder t clarify the bending f the plarizatin curve, the pseudcapacity f the electrde was determined frm the decay curves f vervltage by the fllwing equatin 23l t C p = - ---;-. (11) 7J The r;-dependence f C p III varius slutins IS shwn III Fig. 13. As seen

16 127 Hydrgen vlutin Reactin n Gld Q, u Fig ] (mv) -600 "'-dependence f the pseud-capacity f the gld hydrgen electrde in alkaline slutins: (0) 0.56 N NaOH, ("') N NaOH, (0) N NaOH, (e) 0.43N Na Z S04 (ph=12.1), (&) 0.065N Na Z S04 (ph=1o.8). frm this figure, the high differential capacity is btained in the vervltage regin where the bending f the plarizatin curve appears and the value f C p is much higher in sdium hydrxide than in sdium sulfate slutin. In high vervltage regin f the Tafel slpe 120 m V, C p practically equals CD. The bending f the plarizatin curve may therefre be explained by the shift f the Vlta ptential f the electrde tward negative side due t a variety f specific adsrptin depending n the cmpsitin f the slutin. The reasn f the shift f the Vlta ptential which may result in the bending f the plarizatin curve will be discussed later after the specificatin f the reactin intermediate. Kinetic law f the electrn transfer step Fig. 14 shws a series f lg ( - din) vs. time curves in 4.3 X 10-2 N NaOH btained frm the r;-t curves started frm varius steady cnditins. As seen frm the figure, CD remains cnstant independent f r; in the range frm 0 t -720 m V. The increase f CD by andic treatment was nt bserved in alkaline slutins. The value f '1 remains cnstant at 460 fl sec in the range f - r; frm 0 t 400 m V, while in the range - r; > 400 m V it decreases with increasing vervltage, fr example '1 =89 fl sec at -r;=

17 128 T. SASAKI and A. MATSUDA.~ <J I CI t (,us) Fig. 14, (1). lg (-JiN) vs. t relatin btained frm the vervltage transients at different statinary cnditins in N NaOH. Starting vervltage - r; (m V): (1) 0-400, (2) 590, (3) 610, (4) 660, (5) 720. N 20 u i (ma/cm 2 ) (2). i-dependence f the reactin resistance 1"1 estimated frm Fig. 14, (1).

18 129 Hydrgen vlutin Reactin n Gld 716 m V. The values f rl calculated frm Fig. 14 (1) are pltted against the steady current density i in Fig. 14 (2). The value f r;l can be btained frm the graphical integratin f this curve. The lg i vs. r;l plt btained is shwn by pen circles in slid line in Fig. 15 tgether with the plarizatin curve f the verall reactin. The slid line in these figures shws the theretical line calculated by eq. (6) using the experimental value f i 10 ' It can be seen frm the cmparisn f lg i vs. r;l and lg i vs. r; curves that r;l is quite small as cmpared with r; in every slutin used. As nted '" c !:-----:::2-=00:::----::4-::-0::-0----:6:-:0:::0-_--:S:::: !:---_720=-O:O---_-:4-::-070-_-:6=-= ~--2:-:0~0, ,.;0~0,----6:-"0'-:0 7), r 7) (my) 7),.r 7) (my) 7], r 7) (my) Fig. 15. (I) (2) (3) Plarizatin curves in steady states fr the electrn transfer step (slid line) and verall reactin (brken line) in N NaOH (1), in N NaOH (2), in 0.43 N Na Z S04 (3). -/.O N ~ -3.0 <:(..., 0> Fig J 1 r 7J -400 (mv) -600 Plarizatin curve f the electrn transfer step in N NaOH n gld f different activities f the verall reactin. Brken lines are plarizatin curves f the verall reactin: (1) active state, (2) inactive state.

19 130 T. SASAKI and A. MATSUDA in the previus sectin, the plarizatin curves f the verall reactin in alkaline slutins shw wide variatin depending n the pre-treatments f the electrde. Hwever, the plarizatin curves lg i vs. Yjl are nt affected by pre-treatments f the electrde, as seen frm Fig. 16, which shws the lg i vs. Yjl curves btained under the different activities f the verall reactin. Fig. 17 shws the lg i vs. Yjl curves in varius cncentratin f sdium.~ Fig TJ, (mv) Plarizatin curves f the electrn transfer step f the h. e. r. in alkaline slutins; (1) 1.1 N NaOH, (2) 0.43 N Na 2 S04 (ph = 12.1), (3) N NaOH, (4) N NaOH, (5) N NaOH.

20 131 Hydrgen vlutin Reactin n Gld hydrxide r sdium sulfate. 20 ) The slid line in this figure shws the theretical ne given by q. (9). The lg i vs. 1)1 curve strngly depends n the cncentratin C Na + f Na+ in, as seen frm this figure, and lg ilo shws linear increase with increasing lg C Na + with gradient 1/2, as shwn in Fig. 18. It can be cncluded frm these facts that the electrn transfer step f the h. e. r. n gld in sdium hydrxide is the discharge f Na+ in, but nt the discharge f water mlecule, and its rate can be expressed by q. (9) as a functin f 1) ~ N ~ <l: -3.0.::; CI I Fig. 18. Dependence f exchange current density f the electrn transfer step n Na + in cncentratin25); (0) NaOH slutin; (6) 0.43 N Na Z S04 (ph 12.1); (0) N Na Z S04 (ph 10.8). On the ther hand, the gld h. e. in alkaline slutins indicated the bending f the plarizatin curves f the verall reactin and has high values f the pseud-capacity C p in the vervltage regin crrespnding t the bending f the plarizatin curves, as described in a previus sectin. The values f C p in this regin strngly depend n ph f the slutin rather than the cncentratin f sdium in, as seen frm Fig. 12. Therefre the Vlta ptential change f the electrde may be caused in this regin by the specific adsrptin f sme species in additin t the intermediate Na atm, prbably OH- in. This may be One f the reasns why the h. e. r. n gld in alkaline slutins is irreprducible and has high vervltage as cmpared with that in acid slutin.

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