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Behavior of sodium tungsten bronze electrode in alkaline solutions
Electroanalytical Chemistry and Interfacial Electrochemistry, 51 (1974) 99-106 © ElsevierSequoia S.A., Lausanne Printedin The Netherlands
99
BEHAVIOR O F S O D I U M T U N G S T E N B R O N Z E E L E C T R O D E IN A L K A L I N E S O L U T I O N S *
D. B. ~EPA, M. V. VOJNOVI(~, D. S. OVCIN and N. D. PAVLOVI(~ Institute for Chemistry, Technology and Metallurgy , and Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4/IV, 11000 Belgrade (Yugoslavia)
(Received 18th October 1973)
INTRODUCTION The first attempts to use sodium tungsten bronzes**, NaxWO3, in electrochemistry were made a long time ago 12. Recently, however, possible electrochemical applications of these electrodes in acid solutions have been extensively studied, e.9. oxygen electrode reaction 3 9, hydrogen electrode reaction 1°-15, redox reactions 1°'~6-19, carbon monoxide oxidation reaction 2°, and potentiometric measurements a9'21. The character of the surface of the bronze in acid solutions has been discussed also 22. Some evidence concerning attempts to apply the bronze electrodes in alkaline solutions exists in the literature (potentiometric titration L 21 and potential sweep measurements12). There are also some qualitative statements about the chemical stability of the bronzes in alkaline solutions in the presence of oxidizing species 2, 23--27. t Possible applications of the bronze electrodes in alkaline solutions (some are already proposed 19) can be hardly visualized if their electrochemical behavior in these media is not understood. This paper is aimed at the study of the behavior of the bronze electrodes in nitrogen saturated alkaline solutions under steady state conditions of measurement. EXPERIMENTAL All measurements were performed at the bronze electrode already described 15. Composition of the bronze was x = 0.80. An all-glass, three compartment cell was used. Sodium hydroxide solutions (prepared using p.a. chemical and conductivity water) of pH values varying between 11.6 and 14.0 were used. The sodium tungstate was of the p.a. grade too. Purified nitrogen was bubbled through the solution. Potential versus logarithm of the current density relationships were measured using conventional potentiostatic and galvanostatic steady state methods. Potentials were measured against the hydrogen electrode in the same solution. * Reported at the XVIIth Meeting of Serbian Chemical Society,Belgrade,January 1973. ** The term "bronze" will be used throughout for brevity.
100
D.B. ~EPA, M. V. VOJNOVIC, D. S. OVCIN, N. D. PAVLOVI(~
RESULTS
Figure 1 shows the potentiostatic steady state V l o g / relationship at the bronze electrode in nitrogen saturated alkaline solution. Between the oxygen and the hydrogen evolution reactions an anodic and a cathodic process occurs. It should be noted that the steady state at the bronze electrode in alkaline solutions is established slowly. Typical times needed to measure anodic and cathodic V log i relationships are given in Fig. 1.
i
i
i
i
i
ELECTRODE COMPLETELY
1.2 LU I
~ I
DISSOLVED
1.0
0.8
bd n 0.G
0.4 SLOPE: 120 mV 0.2
0.0
5~URS - 0,2
-0.4
I 6
5
-log (i/A
4
i
I
3
2
cm -2 )
Fig. 1. Typical potentiostatic V-log i relationship at Nao.8WO3 in 1 M N a O H ÷ 1 x 10 2 M Na2WO,.
The effect of the WO 2- anion concentration on the kinetics of anodic and cathodic processes is presented in Fig. 2. Galvanostatic V-log i relationships were measured in solutions of sodium tungstate of five concentrations (between 1 x 1'0-4 and 3 x 10 -2 mol 1-1) using 0.1 M N a O H as the supporting electrolyte. The full set of experiments was repeated in 0.01 and 1.0 M N a O H supporting electrolytes with a result equivalent to that presented in Fig. 2. In Fig. 3 the effect of pH on the anodic and the cathodic V-log i lines is presented. Potentials are refered at the pH-independent standard hydrogen electrode scale~ The rest potential, established at the bronze electrode in the alkaline solutions studied, moved about 75 mV to more negative potentials when the pH of the solution was increased by one unit. The current, determined from the intercept of the extrapolated anodic Tafel line to the rest potential, increases approximately linearly with pH of the solution, with a slope of about 0.4.
SODIUM TUNGSTEN BRONZE IN ALKALINE I
101
i
i
i
0.5
, . , , ~Iio~
D/x +
"6
W T
o
•
O
x
0
J
<_ W
0.3
,~
Q_
,~
o
SLOPE : 120 mV
o
~.......---~-~ '~
0.2
0~,~
0.1
°\
0.0
x
-0A
-0.2 -0.3
11 i 5
L
G -log (i/A
i 4
i 3
crn-2)
Fig. 2. Effect of the WO2 anion concentration on kinetics of cathodic and anodic processes at Nao.sWO3. The supporting electrolyte 0.1 M NaOH. Concentrations of Na2WO4 solutions: (©) 3x10 -2 ,(D) l x 1 0 - 2 ; ( x ) 3 x 1 0 3 ; ( A ) l x 1 0 - 3 ; ( O ) 3 x 1 0 `4 and(+) l x l 0 -4m011-1 . Figure 4 presents the pH dependence of the anodic process at the bronze electrode in solutions examined. A straight line with a slope close to - 1 2 0 mV per p H unit is obtained. In an independent set of experiments the effect of N a + ion concentration on the kinetics of the anodic and the cathodic processes was studied. A constant value of the pH in solutions of varying a m o u n t s of N a + ions was maintained by adding K O H . The overlap of both cathodic and anodic V-log i lines indicated zero effect of the Na + ion concentration on kinetics of both processes.
DISCUSSION (i) The anodic process It is assumed that the anodic process with the slope of the Tafel line close to 120 mV and pH-dependence of a b o u t - 1 2 0 mV per p H unit, is the bronze dissolution reaction with the W O 2 - anion as the product. This is in accordance with the qualitatively described behavior of the bronzes in alkaline media in the presence of oxidizing species 24'25'27 Hence, the overall reaction of the bronze oxidation to the WO~2- anion in alkaline solutions can be written as follows: NaxWO3+2OH-
= W O 2- + H z O + x N a
++xe
(1)
The fact that the sodium ion concentration does not affect the kinetics of
102
D.B. ~EPA, M. V. VOJNOVIC, D. S. OVCIN, N. D. PAVLOVIC
- 0~1
LLI "1-
-0,2
/
x12, 4
.J <_
+/+
Z ~ - 0.3 LU x ix"
13.3
x/ -0.4
-
0.5
-
0.6
14.0
°\
-0.7
-0.8
[
-log
I
I
6
5
0
|
I
I
4
3
(i//A ¢m-2)
Fig. 3. Effect of pH on kinetics of cathodic and anodic processes. The supporting electrolyte 1 x 10-2 M Na 2WO 4- Concentrations of NaO H solutions: (O) 0.01 ; ( [] ) 0.03 ; ( x ) 0.1 ; ( & ) 1.0 and ( + ) 5.0 mol 1- 1 I
I
t
141 -C
.-i > J
~--
0.3
hi
5n
-0.4
-0.5
i
i
i
12
13
14 pH
Fig. 4. pH dependence of the anodic process. Potentials taken at the constant current density of 1 x 1 0 - 4 A c m 2.
SODIUM TUNGSTEN
103
BRONZE IN ALKALINE
both cathodic and anodic processes indicates that the transfer of sodium from the bronze to the solution occurs independently and most probably before the cathodic and anodic processes start. Hence, prior to a discussion of a mechanism of the bronze dissolution reaction some facts concerning the character of the bronze surface in alkaline solutions must be pointed out. It is known that the surface of the bronze in contact with aqueous media spontaneously hydrates 2s. Uptake of water by the surface is the first step in a sequence of possible changes of the surface layer of the bronze. The hydration process follows definite stoichiometry and the following equation may represent a possible one: B - 1 NaxWO3+H=O = BX
1 NaxWO3_x(OH)2x
(2)
X
where B stands for the bulk phase of the bronze. It should be noted that, due to the non-stoichiometric character of the bronze, a domain of the surface, containing more than one basic structure unit (WO 6 octahedra29), is needed to satisfy the stoichiometry of the hydration process, i.e. formation of an integral number of O H groups. The further step in the course of spontaneous change of the surface of the bronze, which has been proved in acid solutions 8'9"22'3°, is the loss of sodium ions from the surface, due to an irreversible exchange with hydrogen ions 22' 30. The result is a hydrated tungsten oxide-like film with the same oxidation state of tungsten as of the initial bronze. If the stoichiometry proposed by eqn. (2) is followed, the loss of sodium from the surface in alkaline solutions can be represented by the equation: 1 1 B - - NaxWO3_x(OH)2 x = B - - W O 3 _ x ( O H ) ~ + N a + + O H X
X
(3)
Hence, the surface of the bronze at the rest potential in alkaline solutions is not bare, but covered with hydrated, sodium deficient, tungsten oxide-like species. The oxidation state of tungsten within such micro-domains of the surface, as presented by eqn. (3), is dependent on the prior composition of the bronze in this area. Obviously, at the surface there exist such domains with mean valency of tungsten and composition varying between five-valent and six-valent tungsten oxides, being statistically averaged at the valency of tungsten in the bulk phase of the bronze. When the steady-state is established, there exists a definite thickness of the surface film of this hydrated, tungsten oxide-like phase, being equilibrated by chemical dissolution on one side, and the occurrence of processes (2) and (3) in the bulk, at the other side. The bronze dissolution reaction will be discussed now as a process of oxidizing the surface oxide-like phase to a corresponding species analogous to the six-valent tungsten oxide and further dissolution of oxidized species. Therefore, the next sequence of steps is proposed to describe the mechanism of the anodic process: (1) oxidation of the W(V) in the surface layer to the W(VI): B - 1_WO3_x(OH)~ + O H - = S - 1 WO3_~(OH)2~+ e X
X
(4)
104
D. B. gEPA, M. V. VOJNOVIC, D. S. OVCIN, N. D. PAVLOVI(~
(2) chemical dissolution of the W(VI) surface species in alkaline media: B - ! WO3_x(OH)2 x + 2 O H - = B + x
x
E1
WO3_x(OH)2(a+x)
(5)
(3) depolymerization of polytungstate species formed in preceding step, since the only stable form of six-valent tungsten ionic species in alkaline solutions is the easily soluble WO 2- anion 31 34: WO3-x(OH)2o
+x)
WO 2-
+ --
x
H20
(6)
The total reaction of the anodic process, which occurs through the mechanism just proposed, can be written as follows: WO3_~(OH)~+(2+x)OH
=WO 2-+(x+l)H20+xe
(7)
Eqn. (7), together with eqns. (2) and (3) give the overall reaction of the bronze dissolution (1). Kinetic parameters calculated for the proposed mechanism, with the rate determining step given by eqn. (4), fit satisfactorily the experimental ones for the anodic process (cf. Table 1). TABLE 1 COMPARISON OF EXPERIMENTAL AND THEORETICAL KINETIC PARAMETERS FOR THE PROPOSED MECHANISM OF THE ANODIC PROCESS Parameter
Experimental
Theoretical
(dV/d log i)/mV
120 120 0
2.303 x 2 R T / F 2.303 x 2R T/F 0
- d V/dpH/mV
dV/dp[W042 ]
(ii) The cathodic process Experimental information concerning kinetics of the cathodic process presented in Figs. 2 and 3 is insufficient to define the mechanism. However, indirect information, a hidden reflection of the kinetics of the cathodic process, exists in the effects of pH and WO 2- anion concentration on the rest potential and the current obtained by extrapolation of the linear part of the anodic Tafel line to the rest potential. When the nature of the rest potential of the bronze electrode in alkaline solutions is considered, since it is not affected by the WO 2- anion concentration (cf. Fig. 2), it is clear that this is not the reversible potential of the dissolution of the surface oxide layer (eqn. 7). Hence, a discussion of the nature of the rest potential should be based on considerations of both the anodic process and a cathodic process. At least two reactions can be suggested as possible cathodic processes. (a) The cathodic process could be the reduction of the W(VI) within the sodium deficient, hydrated, tungsten oxide-like layer, which covers the surface of the bronze in the steady state, to the W(V) or even to the W(IV). In this case
SODIUM TUNGSTEN BRONZE IN ALKALINE
105
the rest potential of the bronze in alkaline solutions would be of the type of the oxide potential established at the metallic electrode covered with a non-stoichiometric layer having electronic conductivity 3s. (b) It has already been mentioned that the surface layer at the bronze is to some extent soluble in alkaline solutions. Chemical dissolution occurs most probably through polytungstate species. Polarographic studies of soluble polytungstates proved their stepwise reduction at the electrode to species of lower mean valencies of tungsten 4°'41. So, the cathodic process could be reduction of these polytungstates, if they were accumulated in solution in the vicinity of the electrode surface. However, from the experimental evidence available it is not possible to prove whether these species are accumulated in the neighbourhood of the electrode or not. It is reasonable to assume that their concentration in the vicinity of the electrode is determined by processes of diffusion to the solution (where their concentration is practically zero), of depolymerization 36-38 and disproportionation 39, since the only stable ionic species of tungsten in alkaline media is the WO 2 anion31 34. In this case the rest potential established at the bronze electrode in alkaline media would be the mixed potential. SUMMARY
Sodium tungsten bronze electrode of the composition x = 0.80 was studied, using steady state potentiostatic and galvanostatic methods, in nitrogen saturated alkaline solutions of different pH values and different WO~- concentrations. It is proposed that a sodium deficient, hydrated, tungsten oxide-like layer is spontaneously formed at the surface of the bronze in alkaline solutions. The anodic process is the oxidation of this surface layer with the WO 2- anion as the final product. Possible cathodic processes are discussed.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
I. Lifschitz and M. Reggiani, Gazz. Chim. ltal., 61 (1931) 915. E. O. Brimm, J. C. Brantley, J. H. Lorenz and M. H. Jellinek, J. Amer. Chem. Soc., 73 (1951) 5427. D. ~epa, A. Damjanovic and J. O'M. Bockris, Electrochim. Acta, 12 (1967) 746. A. Damjanovic, D. ~epa and J. O'M. Bockris, J. Res. Inst. Catal. Hokkaido Univ., 16 (1968) 1. J. H. Fishman, J. F. Henry and S. Tessore, Electrochim. Acta, 14 (1969) 1314. M. Voinov and H. Tannenberger in G. Sandstede (Ed.), Electrocatalysis Fuel Cells, Univ. Washington Press, Seattle, 1972, p. 101. J. McHardy and J. O'M. Bockris in G. Sandstede (Ed.), Electrocatalysis Fuel Cells, Univ. Washington Press, Seattle, 1972, p. 109. J. McHardy and J. O'M. Bockris, J. Electrochem. Soc., 120 (1973) 53. J. O'M. Bockris and J. McHardy, J. Electrochem. Sot., 120 (1973) 62. R. J. Mannan, Ph.D. Thesis, University of Pennsylvania, Philadelphia, 1967. J. O'M. Bockris, A. Damjanovic and R. J. Mannan, J. Electroanal. Chem., 18 (1968) 349. J. Vondrak and J. Balej, X X I CITCE Meeting, Prague, 1970, Extended Abstracts, p. 361. J. Vondrak and J. Balej, X X I I ISE Meeting, Dubrovnik, 1971, Extended Abstracts, p. 170. R. D. Armstrong, A. F. Douglas and D. E. Williams, Energy Convers., 11 (1971) 7. D. B. gepa, D. S. Ovcin and M. V. Vojnovi~, J. Electrochem. Soc., 119 (1972) 1285. M. Fukuda, K. Asai and T. Mitsumata, X V I I CITCE Meeting, Tokyo, 1966, Contribution No. A304. A. Weser and E. Pungor, Acta Chim. (Budapest), 59 (1969) 319. M. V. Vojnovid and D. B. Sepa, J. Chem. Phys., 51 (1969) 5344.
106
D.B. gEPA, M. V. VOJNOVI(~, D. S. OVCIN, N. D. PAVLOVIC
19 M. A. Wechter, H. R. Shanks, G. Carter, G. M. Ebert, R. Guglielmino and A. F. Voigt, Anal. Chem., 44 (1972) 850. 20 L. W. Niedrach and H. I. Zeliger, J. Electrochem. Soc., 116 (1969) 152. 21 A. G. Koksharov and V. F. Ust-Kachkintsev, Uch. Zap. Permsk. Gos. Univ., 111 (1964) 63. 22 M. V. Vojnovid, D. B. ~epa and D. S. Ovcin, Croat. Chim. Acta, 44 (1972) 89. 23 J. Philipp and P. Schwebel, Berichte, 12 (1879) 2234. 24 M. Straumanis, J. Amer. Chem. Soc., 71 (1949) 679. 25 L. E. Conroy and M. J. Sienko, J. Amer. Chem. Soc., 79 (1957) 4048. 26 R. A. Bernoff and L. E. Conroy, J. Amer. Chem. Soc., 82 (1960) 6261. 27 M. P. Morozova and G. P. Ovcharova, Zh. Fiz. Khim., 43 (1969) 1728. 28 V. Spitsyn and L Kaschtanoff, Z. Anal. Chem., 75 (1928) 440. 29 P. G. Dickens and M. S. Whittingham, Quart. Rev. Chem. Sot., 22 (1968) 30. 30 D. Sepa, Ph.D. Thesis, University of Belgrade, Belgrade, 1971. 31 J. F. Duncan and D. L. Kepert, J. Chem. Soc., (1961) 5317. 32 J. Chojnaka, Nukleonika, 12 (1967) 729. 33 L. A. Woodward and H. L. Robert, Trans. Faraday Soc., 52 (1956) 615. 34 J. S. Anderson and K. Saddington, J. Chem. Soc., Suppl. No. 2, (1949) 5381. 35 K. J. Vetter, J. Electrochem. Soe., 110 (1963) 597. 36 D. L. Kepert in F. A. Cotton (Ed.), Progress in Inorganic Chemistry, Vol. 4, Interscience, New York, 1962, p. 199. 37 O. Glernser, W. Holznagel and W. Hoeltje, Z. Anorg. Allgem. Chem., 342 (1966) 75. 38 V. Spitsyn and G. V. Kosmodemyanskaya, Zh. Neor 9. Khim., 11 (1966) 1397. 39 K. N. Mikhalevich and V. M. Litvinchuk, Zh. Neor 9. Khim., 3 (1958) 1846. 40 C. Tourne, Bull. Soc. Chim. Ft., 9 (1967) 3214. 41 P. Souchay, F. Chauvean and B. LeMeur, C.R. Acad. Sci. Paris, SOr. C, 270 (1970) 1401.
99
BEHAVIOR O F S O D I U M T U N G S T E N B R O N Z E E L E C T R O D E IN A L K A L I N E S O L U T I O N S *
D. B. ~EPA, M. V. VOJNOVI(~, D. S. OVCIN and N. D. PAVLOVI(~ Institute for Chemistry, Technology and Metallurgy , and Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4/IV, 11000 Belgrade (Yugoslavia)
(Received 18th October 1973)
INTRODUCTION The first attempts to use sodium tungsten bronzes**, NaxWO3, in electrochemistry were made a long time ago 12. Recently, however, possible electrochemical applications of these electrodes in acid solutions have been extensively studied, e.9. oxygen electrode reaction 3 9, hydrogen electrode reaction 1°-15, redox reactions 1°'~6-19, carbon monoxide oxidation reaction 2°, and potentiometric measurements a9'21. The character of the surface of the bronze in acid solutions has been discussed also 22. Some evidence concerning attempts to apply the bronze electrodes in alkaline solutions exists in the literature (potentiometric titration L 21 and potential sweep measurements12). There are also some qualitative statements about the chemical stability of the bronzes in alkaline solutions in the presence of oxidizing species 2, 23--27. t Possible applications of the bronze electrodes in alkaline solutions (some are already proposed 19) can be hardly visualized if their electrochemical behavior in these media is not understood. This paper is aimed at the study of the behavior of the bronze electrodes in nitrogen saturated alkaline solutions under steady state conditions of measurement. EXPERIMENTAL All measurements were performed at the bronze electrode already described 15. Composition of the bronze was x = 0.80. An all-glass, three compartment cell was used. Sodium hydroxide solutions (prepared using p.a. chemical and conductivity water) of pH values varying between 11.6 and 14.0 were used. The sodium tungstate was of the p.a. grade too. Purified nitrogen was bubbled through the solution. Potential versus logarithm of the current density relationships were measured using conventional potentiostatic and galvanostatic steady state methods. Potentials were measured against the hydrogen electrode in the same solution. * Reported at the XVIIth Meeting of Serbian Chemical Society,Belgrade,January 1973. ** The term "bronze" will be used throughout for brevity.
100
D.B. ~EPA, M. V. VOJNOVIC, D. S. OVCIN, N. D. PAVLOVI(~
RESULTS
Figure 1 shows the potentiostatic steady state V l o g / relationship at the bronze electrode in nitrogen saturated alkaline solution. Between the oxygen and the hydrogen evolution reactions an anodic and a cathodic process occurs. It should be noted that the steady state at the bronze electrode in alkaline solutions is established slowly. Typical times needed to measure anodic and cathodic V log i relationships are given in Fig. 1.
i
i
i
i
i
ELECTRODE COMPLETELY
1.2 LU I
~ I
DISSOLVED
1.0
0.8
bd n 0.G
0.4 SLOPE: 120 mV 0.2
0.0
5~URS - 0,2
-0.4
I 6
5
-log (i/A
4
i
I
3
2
cm -2 )
Fig. 1. Typical potentiostatic V-log i relationship at Nao.8WO3 in 1 M N a O H ÷ 1 x 10 2 M Na2WO,.
The effect of the WO 2- anion concentration on the kinetics of anodic and cathodic processes is presented in Fig. 2. Galvanostatic V-log i relationships were measured in solutions of sodium tungstate of five concentrations (between 1 x 1'0-4 and 3 x 10 -2 mol 1-1) using 0.1 M N a O H as the supporting electrolyte. The full set of experiments was repeated in 0.01 and 1.0 M N a O H supporting electrolytes with a result equivalent to that presented in Fig. 2. In Fig. 3 the effect of pH on the anodic and the cathodic V-log i lines is presented. Potentials are refered at the pH-independent standard hydrogen electrode scale~ The rest potential, established at the bronze electrode in the alkaline solutions studied, moved about 75 mV to more negative potentials when the pH of the solution was increased by one unit. The current, determined from the intercept of the extrapolated anodic Tafel line to the rest potential, increases approximately linearly with pH of the solution, with a slope of about 0.4.
SODIUM TUNGSTEN BRONZE IN ALKALINE I
101
i
i
i
0.5
, . , , ~Iio~
D/x +
"6
W T
o
•
O
x
0
J
<_ W
0.3
,~
Q_
,~
o
SLOPE : 120 mV
o
~.......---~-~ '~
0.2
0~,~
0.1
°\
0.0
x
-0A
-0.2 -0.3
11 i 5
L
G -log (i/A
i 4
i 3
crn-2)
Fig. 2. Effect of the WO2 anion concentration on kinetics of cathodic and anodic processes at Nao.sWO3. The supporting electrolyte 0.1 M NaOH. Concentrations of Na2WO4 solutions: (©) 3x10 -2 ,(D) l x 1 0 - 2 ; ( x ) 3 x 1 0 3 ; ( A ) l x 1 0 - 3 ; ( O ) 3 x 1 0 `4 and(+) l x l 0 -4m011-1 . Figure 4 presents the pH dependence of the anodic process at the bronze electrode in solutions examined. A straight line with a slope close to - 1 2 0 mV per p H unit is obtained. In an independent set of experiments the effect of N a + ion concentration on the kinetics of the anodic and the cathodic processes was studied. A constant value of the pH in solutions of varying a m o u n t s of N a + ions was maintained by adding K O H . The overlap of both cathodic and anodic V-log i lines indicated zero effect of the Na + ion concentration on kinetics of both processes.
DISCUSSION (i) The anodic process It is assumed that the anodic process with the slope of the Tafel line close to 120 mV and pH-dependence of a b o u t - 1 2 0 mV per p H unit, is the bronze dissolution reaction with the W O 2 - anion as the product. This is in accordance with the qualitatively described behavior of the bronzes in alkaline media in the presence of oxidizing species 24'25'27 Hence, the overall reaction of the bronze oxidation to the WO~2- anion in alkaline solutions can be written as follows: NaxWO3+2OH-
= W O 2- + H z O + x N a
++xe
(1)
The fact that the sodium ion concentration does not affect the kinetics of
102
D.B. ~EPA, M. V. VOJNOVIC, D. S. OVCIN, N. D. PAVLOVIC
- 0~1
LLI "1-
-0,2
/
x12, 4
.J <_
+/+
Z ~ - 0.3 LU x ix"
13.3
x/ -0.4
-
0.5
-
0.6
14.0
°\
-0.7
-0.8
[
-log
I
I
6
5
0
|
I
I
4
3
(i//A ¢m-2)
Fig. 3. Effect of pH on kinetics of cathodic and anodic processes. The supporting electrolyte 1 x 10-2 M Na 2WO 4- Concentrations of NaO H solutions: (O) 0.01 ; ( [] ) 0.03 ; ( x ) 0.1 ; ( & ) 1.0 and ( + ) 5.0 mol 1- 1 I
I
t
141 -C
.-i > J
~--
0.3
hi
5n
-0.4
-0.5
i
i
i
12
13
14 pH
Fig. 4. pH dependence of the anodic process. Potentials taken at the constant current density of 1 x 1 0 - 4 A c m 2.
SODIUM TUNGSTEN
103
BRONZE IN ALKALINE
both cathodic and anodic processes indicates that the transfer of sodium from the bronze to the solution occurs independently and most probably before the cathodic and anodic processes start. Hence, prior to a discussion of a mechanism of the bronze dissolution reaction some facts concerning the character of the bronze surface in alkaline solutions must be pointed out. It is known that the surface of the bronze in contact with aqueous media spontaneously hydrates 2s. Uptake of water by the surface is the first step in a sequence of possible changes of the surface layer of the bronze. The hydration process follows definite stoichiometry and the following equation may represent a possible one: B - 1 NaxWO3+H=O = BX
1 NaxWO3_x(OH)2x
(2)
X
where B stands for the bulk phase of the bronze. It should be noted that, due to the non-stoichiometric character of the bronze, a domain of the surface, containing more than one basic structure unit (WO 6 octahedra29), is needed to satisfy the stoichiometry of the hydration process, i.e. formation of an integral number of O H groups. The further step in the course of spontaneous change of the surface of the bronze, which has been proved in acid solutions 8'9"22'3°, is the loss of sodium ions from the surface, due to an irreversible exchange with hydrogen ions 22' 30. The result is a hydrated tungsten oxide-like film with the same oxidation state of tungsten as of the initial bronze. If the stoichiometry proposed by eqn. (2) is followed, the loss of sodium from the surface in alkaline solutions can be represented by the equation: 1 1 B - - NaxWO3_x(OH)2 x = B - - W O 3 _ x ( O H ) ~ + N a + + O H X
X
(3)
Hence, the surface of the bronze at the rest potential in alkaline solutions is not bare, but covered with hydrated, sodium deficient, tungsten oxide-like species. The oxidation state of tungsten within such micro-domains of the surface, as presented by eqn. (3), is dependent on the prior composition of the bronze in this area. Obviously, at the surface there exist such domains with mean valency of tungsten and composition varying between five-valent and six-valent tungsten oxides, being statistically averaged at the valency of tungsten in the bulk phase of the bronze. When the steady-state is established, there exists a definite thickness of the surface film of this hydrated, tungsten oxide-like phase, being equilibrated by chemical dissolution on one side, and the occurrence of processes (2) and (3) in the bulk, at the other side. The bronze dissolution reaction will be discussed now as a process of oxidizing the surface oxide-like phase to a corresponding species analogous to the six-valent tungsten oxide and further dissolution of oxidized species. Therefore, the next sequence of steps is proposed to describe the mechanism of the anodic process: (1) oxidation of the W(V) in the surface layer to the W(VI): B - 1_WO3_x(OH)~ + O H - = S - 1 WO3_~(OH)2~+ e X
X
(4)
104
D. B. gEPA, M. V. VOJNOVIC, D. S. OVCIN, N. D. PAVLOVI(~
(2) chemical dissolution of the W(VI) surface species in alkaline media: B - ! WO3_x(OH)2 x + 2 O H - = B + x
x
E1
WO3_x(OH)2(a+x)
(5)
(3) depolymerization of polytungstate species formed in preceding step, since the only stable form of six-valent tungsten ionic species in alkaline solutions is the easily soluble WO 2- anion 31 34: WO3-x(OH)2o
+x)
WO 2-
+ --
x
H20
(6)
The total reaction of the anodic process, which occurs through the mechanism just proposed, can be written as follows: WO3_~(OH)~+(2+x)OH
=WO 2-+(x+l)H20+xe
(7)
Eqn. (7), together with eqns. (2) and (3) give the overall reaction of the bronze dissolution (1). Kinetic parameters calculated for the proposed mechanism, with the rate determining step given by eqn. (4), fit satisfactorily the experimental ones for the anodic process (cf. Table 1). TABLE 1 COMPARISON OF EXPERIMENTAL AND THEORETICAL KINETIC PARAMETERS FOR THE PROPOSED MECHANISM OF THE ANODIC PROCESS Parameter
Experimental
Theoretical
(dV/d log i)/mV
120 120 0
2.303 x 2 R T / F 2.303 x 2R T/F 0
- d V/dpH/mV
dV/dp[W042 ]
(ii) The cathodic process Experimental information concerning kinetics of the cathodic process presented in Figs. 2 and 3 is insufficient to define the mechanism. However, indirect information, a hidden reflection of the kinetics of the cathodic process, exists in the effects of pH and WO 2- anion concentration on the rest potential and the current obtained by extrapolation of the linear part of the anodic Tafel line to the rest potential. When the nature of the rest potential of the bronze electrode in alkaline solutions is considered, since it is not affected by the WO 2- anion concentration (cf. Fig. 2), it is clear that this is not the reversible potential of the dissolution of the surface oxide layer (eqn. 7). Hence, a discussion of the nature of the rest potential should be based on considerations of both the anodic process and a cathodic process. At least two reactions can be suggested as possible cathodic processes. (a) The cathodic process could be the reduction of the W(VI) within the sodium deficient, hydrated, tungsten oxide-like layer, which covers the surface of the bronze in the steady state, to the W(V) or even to the W(IV). In this case
SODIUM TUNGSTEN BRONZE IN ALKALINE
105
the rest potential of the bronze in alkaline solutions would be of the type of the oxide potential established at the metallic electrode covered with a non-stoichiometric layer having electronic conductivity 3s. (b) It has already been mentioned that the surface layer at the bronze is to some extent soluble in alkaline solutions. Chemical dissolution occurs most probably through polytungstate species. Polarographic studies of soluble polytungstates proved their stepwise reduction at the electrode to species of lower mean valencies of tungsten 4°'41. So, the cathodic process could be reduction of these polytungstates, if they were accumulated in solution in the vicinity of the electrode surface. However, from the experimental evidence available it is not possible to prove whether these species are accumulated in the neighbourhood of the electrode or not. It is reasonable to assume that their concentration in the vicinity of the electrode is determined by processes of diffusion to the solution (where their concentration is practically zero), of depolymerization 36-38 and disproportionation 39, since the only stable ionic species of tungsten in alkaline media is the WO 2 anion31 34. In this case the rest potential established at the bronze electrode in alkaline media would be the mixed potential. SUMMARY
Sodium tungsten bronze electrode of the composition x = 0.80 was studied, using steady state potentiostatic and galvanostatic methods, in nitrogen saturated alkaline solutions of different pH values and different WO~- concentrations. It is proposed that a sodium deficient, hydrated, tungsten oxide-like layer is spontaneously formed at the surface of the bronze in alkaline solutions. The anodic process is the oxidation of this surface layer with the WO 2- anion as the final product. Possible cathodic processes are discussed.
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