0.
4-0.1
ution level
L。且-coa七 ed mag且et
Fig. 1, 4-necked ground glass joint flask cell
l 2 E
t(hxs)
4 5 6
Fig. 2. Potential recovery curve of the homogeneous cell Pt(p) IH3P04(O.980N)1 Pt (bright) at 300
C under non-stirring condition.
(254 ) ，在 不 問 金 屬 電 極 表 面 情 況
下
對. 氫 氫 離 子 之 電 化 學平
衡 研 究(255 )
H. H. Bauer, J. Electroanal. Chem., 16, 419 (1968). J. Brenet, Electrochim. Acta., 13,457 (1968).
M. Bonnem肘， G.BronoeI and M. Savy, Electrochim. Ac詞， 13， 659 (1968).
(8)
(9)
(10)
(11) Reddy
, “
Modern Electrochemistry" Vo1. 2,Plenum Press, N. Y. 1970. p. 862. O'M Bockris and A.K.N. J.
師大學報
Table 1
第二十三期
Results ofthe homogeneous ceIIsat 300
C under H2 gas (l atm)
çonstant potentiaI (volt) Homogeneous ceII CeII No. 0.526:t0.035 Pt(p) I H2 S04 (0.5 ION) I Pt (b) Pt(p) vs. C(g) 0.460:t0.035 0.371:t0.035 Pt(p) vs. Pt(b) 。.496 :t0.035 Pt(p) I H3P04 (0.980N) 1 Pt (b) 2 Pt(p) I KOH(0.328 N) I Pt (b) and C(g) 3 g: graphite b: bright; p: platinized; O
(.256 ) bright platinum. Thus the inequality (9) can be justified in this way.
It is interesting that the non-zero residual potential poses a serious question or paradox on the laws of thermodynamics. Let us consider there always exist a non-zero potential of the homogeneous cell, Pt (platinized) I H3P04 under H2 I Pt (bright). Upon being discharged, there is a current flowing. According to
拒 不 同
Fa叫叭 law，
a current must accompany an electrochemical reaction ateach 會
伽t仕ro叫od街e叭，s叩O 前圳tl昀la凶圳a前削t討i恤I
reaction must be H2
•
2H+ + 2 e一 ，表 面 情 況下
對since the platir叫 platinum is the negative electrode of 伽 cell. At the bright 于 platinum electrode, the only electrochemical reaction must be
2H+ + 2 e一→ H
2
氫 離于
之 電 化There is no other electrochemical reactions possible on either pl伽ized and 擎 bri
electrochemica1 reaction is a cydic mechanism which is not tolerated by the 究
laws of thermodynamics. This aspect has"been long overlooked. More work and thoughts should be concentrated on this very important aspect of electrochemistry as well as thermodynamics.
References
(1) D. MacGillavry, J. Chem. Phys., 19, 1499 (1951). (2) D. MacGillavry, ibid., 19,1195 (1951).
(3) G. J. Young and R.B. Rozelle, in
“
Fuel Cells", ed. by G. J. Young p. 23. Reinhold, N. Y. (1960).(4) T. P. Hoar. Proc. Roy. Soc. (London),A142, 628 (1933). (5) C. Wagner and W. Traud, Z. Electrochem., 44, 391" (1938). (6) T. F. L妞， J. Chinese Inst. Chem. Eng., 6, 71 (1974).
(7) W. M. Lat扭ler，“Oxidation Potentials弋 2nded., Prentice-Hall, N .Y. (1952).
=色'.
holds, then the inequality (4) is solely due to 'the inequality A、匙。If 主a
Ko ( 257 )
(5). There ani different concentrations of H2 and H+ on the surface of platinized and bright platinum. Thus the non-zerp residual potentials can be explained by the different adsorption effects of different electrode surface condi扯tions三 This
i旭s c∞ons邱ten叫t
with the idea of adsorption potential. On the other hand,if 包士
CH2
師大學報
一 c
已
C白2 holds, the inequality
(4)
issolely dueto the inequãlity (6). This means different rate constants on different electrode surface conditions. The present第二十三期
study c
,
m not determine which situation is correct.used in are activities rigorously, More mohlr concentiations. of Instead (7) (8) equation
(2)
and(3)
, we obtainRT. Kt> a 主 i ~=一一 ln ~~K 二丘 F Ko aH 2 RT Kt>'。三4 ð.<Þ= 一一 ln ~二三主斗 F Kú'à 主 2
Theremodynamically, if the homogeneous cell is in chemically equilibrium state, the activities of H+ and H2 are everywhere the sam~ either on platinized or bright platinum. Therefore the inequality ，!::，ç白手 !::'<Þ' , is solely due to the inequality.
KR' Ko'
In view of the platinized platinum being the negative electrode of the homo-geneous cell, it is easy to show the following eqality is true
弓在 KR Ko (9)
何一阿
豆豆< KòKinetically, the rate-determining step of electrochemical oxidation of H2 into H+
八
is the dissociation of hydrogen molecule into two hyd.rogen atóms. The electro-catalyst, platinum black, is known to be more effective in decomposing H2 into
two H atoms than bright platinum. Thus Kσ> K~. On the other hand, the reduc-tion rates of H+ into H2 can be considered about equal on the platinized and the
( 258 )
electrode. In the case of H2 -H+ electrochemical equilibrium, the Butler-Volmer equation for the platinized platinum electrode is
i=2FKoCHJ(1--F)的F/RT _ 2FK R C~+ e-ßð(þF/RT 、‘ .J -A JSE 、 在 wh叫o ar叭R a叫叫臼仙…C∞on叫叫州a羽叫呦伽t討ω枷io仙凹I叫dr間edO 血ω伽叫叫山叫叫ul比叫州ct州t“i肌帥郎叫叫叫t討伽伽叫i卸的ve
ð(þ i誌s the metal-solution potential. If the pl伽ized platinum electrode is 恥n團軍
極 表 面 ↑青 況
下
對 氫 氫 離 子 之 電 化 學 平衡 研 究circuited, there is no current flowing, Le_ i=O, so that it is obtained from equation
( 1) t he following equation KOCH 2eð(þF/RT = K R C~+ or RT _ KRC已+ ð(þ= 一一一ln 一二一 F KOCH 2
(2)
Simi1ar1y for the bright platinum, it can be obtained as
RT Kn'CJ+
Aφ= 一一-ln
F Ko' CÍI 2
(3)
It is experimentally found that the metal-solution potential of the platinized and bright platinum is different, Le. ð(þ手ð(þ'. Thus equations (2) and (3) lead to the following inequa1ity,
KR CiI++ K~ C;;+
Ko
C
H2
芋志可
7
The inequality (4) is due to the inequa1ity
(4) C~+ 干企 C
Ji*
CH2 • ç主 z (5) or 七 ， R 一， o k-K 4+MW -AU R • on k-Ka -P 、 d vd 4t qa u n-OLV .m hu O KU VA O (6)( 259 )
Our purpose of this study is to fmd out whether the potential of hydrogen-substrate metal can be influenced by the
of the platinum electrode and the electrode material such as graphite' under the equi1ibrium condition. Using the suitable electrolyte H3P04, a constant potential of 0.371 volt was found with hydrogen ion equi1ibrium at the inert
plat卸 ized) or (bright conditions surface 師大學報
bright platinum as the positive electrode. According to MacGi11avry's explanation, since hydrogen-dydrogen ion equi1ibrium can not be readi1y established at the
第二十三期
bright platinum, it is an irreversible electrode which might not obey Nernst equa國
tion. Therefore the adsorption potential (as used by MacGi11avry) between metal and solution of the irreversible electrode might be Øifferent from that of the reversible potential of platinized platinum. This is indeed confinned by the preseilt investigation. Although sulfuric acid is not so inert (might be reduced by H2, but after long discharge or standing
,
sulfate-sulfite equi1ibrium could a1so be reached),
large residua1 potential of 0.526 volt was obtained. In the case of usingKOH, the electrochemica1 reaction .at the two electrodes is different from other two cells as following:
H2+20H- 手 2 H20 + 2 e 一
The above electrochemica1 equi1ibration can be accelerated by platinized platinum,
but not by bright platinum or graphite. So there is potential difference between the platinized platinum and bright platinum or graphite.
It was also found that the residual potential between the platinized platinum and no-heat-treated bright platinum was less than that of the plat姐ized platinum and heat-treated bright platinum. This indicates that the electrode of heat-treated bright platinum is more irreversible than no~heat-treated platinum because of the smooth surface of heat-treated platinum. Therefore it is plausible that the active sites which catalyze the H2 -H+ electrochemical equi1ibrium is the edges or defect
。redicting the kinetic behavior of areas of pl~tinum metal. The smooth surface of bright platinum is most ineffective in catalyzing H2 -H+ electrochemical equilibrium.
Althoughthere is a controversy站 9 ， 10 about the validity of But1er-Volmer equation11, it is sti11 the best morlel for
(260 )
The results of the three cells using H2S04, H3P04 and KOH are shown in Table 1. For cell No. 1 using sulfuric acid, a constant potential of 0.526 volt was found which is the highest potential among the cells investigated. With pQ.osphoric acid which is a weaker acid, the constant potential of cell No. 2 was found to be 在 0.371 volt a前t 3忱 F叮叫伽 C臼ellNo. 3 us泌si扭ngpot個as鉛sh叫ly袖d伽r叫d臼阮ωe吼叭圳，tl仕伽t
were used, the results are 削 the constant potential of br:帆 platinum is 0 .496 爸 volt, that of gr吶iteis 0.460 volt rela伽川latinized platinum 伽trode.It must 麗 be noted that the potentials of these three c臼ells wer'昀'e hi蝴d叭伽咖ler盯ra剖t 伽 be峙e句gi扭nr口min
the discharge and then gradually 伽e叫 to the constant potentials as shown in 嘉
Table 1. The' pl枷i叫 platinum 你k~r耐 W叫le 叫枷e ele伽de i叫伽孟
伽e 岫宇 氫 離 Discussion 于之' 電 4化包
The 伽e閃吋伽elle倪ct仕r吋
r昀叫岫el呦l旭a前t忱l句Y， i削 b悅伽叫le忱lect的叫ct訂ω叫r叩叫O叫吻l卸枷yt惋e臼s. It is 'known that CN-, Cl一， Br- or 1- etc. can re叫 車
with the metals such as Cu, Hg, Ag and even with Pt. Therefore halide and cyanide 記
electrolytes were not used here. Phosphoric acid is the best electrolyte in the present investigation, since PO
.ï
could neither react with platinum, nor to be reduced by hydrogen. Although SO; can not react with platinum, but it could be reduced by hydrogen gas as the following reactionso;+H2+2H+
•
H2S03 + H20since the reduction potential of SO: in acid solution is EO
= 0.17 volt.7 As for hydroxide ion OH-, it could react with platinum, but could not dissolve platinum unlike hydrochloric acid wnich can dissolve platinum to become PtCI
6"
ion. If PtC16 is present in the solution, it can be deposited on the bright platinum because 五 the bright platinum is the positive electrode of the cell, thus the bright platinum becomes more or less platinized. So the residual potential of homogeneous cell using HCl solution is about 0.10 volt less than that using H3 P04 solution. The experimental from the homogeneous cell using HCl solution wi1l be reported later.( 261 )
potential to be meassured, the electric wires connecting the electrodes were removed and the potential was allowed to recover. Then under non-stirring condi-tion, open-circuit potential was measured with YEW Type 2722 DC Potentiometer. A typical potential recovery curve is shown by Fig. 2. After each experiment,
concentration of the solution in the flask was determined by acid-båse
titra-師大學報
the tion.
The chemicals H2 S04' H3 P04 and KOH used here are all of reagent grade.
第二十三期
Water was triply disti1led. High purity Marz grade Pt wire obtained from Material Research Corporation was used here. Two different surface treatments of platinum electrodes were made. The Marz grade platinum wire to be used as the bright platinum electrode was heated in a Bunsen flame to red hot and cooled in the air. The hea'C聖lreated platinum wire appears softer and brighter, apparently the surface of the platinum wire was more smooth arid less strained. The platinized platinum electrode was obtained by electrolyzing in 2% chloroplatinic acid solution as the negative pole. No lead acetate was used here. Graphite electrodes were spectral grade graphite rod with diameter O.6cm and length 3.7 cm. The graphite rod were obtained from Applied Research Laboratories Inc. which supplied the material for emission spectrographic analyzer.
Resu1ts
After completion of hydrogen bubbling, each cell was first discharged con-tinually for 10 days, only occasionally open-circuited for potential measurement. Then connecting wires were removed, the open-circuit potential would gradually recover as the typical potential recovery curve of Fig. 2. It usua11y took several hours to completely recover. In order to know whether the homogeneous cells for about 15 hours (overnight) and open-circuited to wait until the cell recovered to their constant potentials, then the potentia1 was measured. Usually the discharge opera-tions were repeated at least 5 times. If no decreasing tendency of the potential reached the equilibrium state or not, the ‘cell was discharged again
四
( 262 ) Experimental
The apparatus used was a 4-necked ground glass joint 250 ml flask fitted with different glass equipments as shown by Fig. 1. The central outlet and Qne of the side outlets were fitted with the glass stoppers with coarse platinum wire passing through them. The low end of the coarse platinum wire can be fastened with aid of a fme platinum wire or spot-welded with a spiral metal wire to be used as the electrode. The other two side outlets were fitted with gas-mcommg and g 品﹒
outgoing traps. The traps were fi11ed with the same solution as that in the flask.
甘le no-leakage structure of this apparatus was further assured by covering the four outlets with vaseline. In this experiment, 5-necked ground glass joint 250 ml flask was also used, the additiona1 out1et was for a third electrode. The present apparatus is the improved version of previous one reported6 befo
,re.
50 ml of solution to be investigated was placed 姐tothe 4-necked or 5-necked flask immersed in a thermostat controlled within O.IO
C. In order to minimize air dissolved in the solution, the disti11ed water used to prepare the solution was boiled or degassed under reduced ptessure at first. Hydrogen gas which was generated by electrolyzing 15% NaOH solu tion was passed through vanadous ch10ride solution to remove oxygen in the gas stream. The flask was first evacuated by an aspirator and hydrogen gas was bubbling into it. This was repeated several times. Finally, hydrogen gas was continually bubbling into the flask at least one hour to assure the complete absence of air in the flask. When the bubbling of hydrogen gas was completed, hot liquid vaseline was poured 扭tothe outlet cups of the flask to serve as sealing material when it solidified. Then this homogeneous cell was ready for discharge.
To discharge the celL, the two electrodes or three electrodes were connected with electric wires for a sufficient1y long time. In order to find out the difference in approaching the equi1ibrium state, some of the solutions were magnetically stirred; some of the solutions were not magnetically stirred. It was found that the equi1ibrium state can reach in the unstirred flask in a reasonable time (about 10 days), but in slower rate than stirred flask (about 5 days). When open-circuit
在 不 同 金 屬 電 極 表 面 情
事
對于
氫 離 子 之 電 化 學 平衡 研. 單是( 263 )
as adsorption potentials which do not obey the Nernst equation. Y oung and co-has measured the open-circuit potentials of hydrogen electrode with worker3
different substrate metal and correlated the potentials with their d-band structure. It was assum~d that open-circuit polarization, measured as the deviation from irreversible reversible potentials, is a measure of the energy loss
chemisorption at electrode. Mixed potential theory4,S has also been employed to caused by
師大學報
explain the potential deviation.
The conventional method of measuring the open-circuit potential of a single
ele~trode is done by dipping the sample electrode in a solution and its potential
第二十三期
is measuied relative to a reference electrode connecting the solution with a salt bridge. Most of the steady potentials of single electrode were done in this way. The conventional method is subjected to some difficulties. First, liquid junction potential of salt bridge is not completely negligible and also susceptible to the nature of solution used. Secondly, KCl in the salt bridge would slowly diffuse to the solution, therefore there is time limit before a reliable steady potential can be measured. In view of these difficulties, present experiment constructs a new type of electrochemical cell which -itself is a closed system and free of liquid junction. As an èxample, the cell Pt (platinized)
I
H3P04 under H2 gasl Pt (bright) consists of electrodes dipped into a homogeneous phosphoric acid solution which is twocovered by hydrogen gas. This kind of cell construction is called homogeneous èell. One electrode of the cell is platinized platinum. It behayes as a reversible hydrogen Other and . its potential can
electrode is bright platinum at which the hydrogen-hydrogen ion equilibrium can Nernst equation.
be predicted by the electrode
not be readily established. According to MacGillavry's explanation, it is a polariz-(irrevcrsible) electrode, which the Nernst equation might not hold. There-fore the metal-solution potential at the platinized platinum electrode might be able
different from the metal-solution potential at the bright platinum electrode, that is, the homogeneous cell could have a non-zero residual potdntial. The present the after study found that homogeneous cells have non-zero residual potentials,
(264 )
A Study of Hydrogen-Hydrogen lon Electrochemical
Equilibrium on the Different Surface Conditions of Substrate Metals
Tzu Fang Lin
Department of Chemistry National Taiwan Normal University
ABSTRACT 在 不 同 金 屬 造 極 表 面 情 況
-F
對 氫The homogeneous cells were constructed by dipping two electrodes
of 自
different surface conditions 詛to a homogeneous solution and covered by hydrogén gas. Through the exhaustive discharge, the equilibrium state can be reached inside the homogeneous cells and constant residual potential were measured. This study is concentrated on hydrogen-hydrogen ion electrochemical equilibrium..on different surface conditions of substrate metals. Large residual potentials were. found for the homogeneous cells consisting of reversible hydrogen electrode and irreversible hydrogen electrode. Although the nonzero residual potentials can be explained by adsorption or electric double layer effect, the question whether the non-zero residual potential can be justified in terms of the principles of thermodynamics remams open.In troduction
MacGi11avryl,2 has investigated the properties of the metal四s.olutionpotentials
of nickel in nickel-free hydroxide and phosphate solution. He found that the steady potentials of the metal-solution do not obey the Nernst equation. He con-cluded that the Nernst equation is only applicable to the ùnpolarizable (reversible) electrodes at which there. exist electrochemical equilibria due to mechanisms of reversible transfer of some ions between the electrode and solution. In the absence of any electrochemical equilibria, the metal-solution potentials are interpreted
子 之 電 化 學 平 衡 研 先
