Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 579 (2005) 321–328 www.elsevier.com/locate/jelechem
A quartz sealed Ag/AgCl reference electrode for CaCl2 based molten salts Pei Gao a, Xianbo Jin
a,*
, Dihua Wang a, Xiaohong Hu a, George Z. Chen
a,b,*
a
b
College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, PR China School of Chemical, Environmental and Mining Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK Received 10 November 2004; received in revised form 4 March 2005; accepted 5 March 2005 Available online 8 April 2005
Abstract A quartz sealed Ag/AgCl reference electrode was fabricated and studied in CaCl2 based molten salts. It performed satisfactorily in terms of reproducibility, reusability and stability in experiments that varied the temperature (700–950 C) and service time (from hours to days). The electric resistance of the reference electrode decreased from 105 to 103 X when increasing the molten salt temperature from 600 to 950 C, following well Arrhenius Law. The potential variation of the electrode upon changing the electrolyte composition (CaCl2, NaCl, KCl, and/or AgCl) suggested the selective conduction of Na+ ions and possibly Ca2+ ions through the thin-wall of the sealed quartz tube. Prolonged use (two to three days) of the reference electrode in the presence of both oxygen and molten chloride salt led to noticeable erosion of the quartz tube, particularly at the molten salt–quartz–gas triple phase boundary, which can be attributed to the formation of calcium and/or sodium silicates under the influence of oxygen present in the liquid and gas phases, respectively. 2005 Elsevier B.V. All rights reserved. Keywords: High temperature molten salts; Reference electrode; Quartz; Cyclic voltammetry
1. Introduction High temperature molten salts play important roles in modern electrochemical research and industrial practices. Recently, in molten CaCl2 at 900 C, electrochemical deoxygenation of metals, such as titanium and niobium, was investigated [1–3]. It was also reported that when a solid metal oxide, e.g., TiO2, was made into an cathode, regardless of the electronic conductivity, it could be electrochemically reduced in the same melt directly to the metal [4,5]. The possibility of using this new electro-deoxidation method has been acclaimed for the economical and environmentally * Corresponding authors. Tel.: +44 115 9514171; fax: +44 115 9514115 (G.Z. Chen). E-mail addresses:
[email protected] (X. Jin),
[email protected] (G.Z. Chen).
0022-0728/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.03.004
friendly production of many useful but currently expensive metals and semi-metals, typically Ti, Si, Nb and Cr, and their alloys [4–11]. However, in previous studies, except for some voltammetric observations from using a metal based pseudo-reference electrode coupled with a known electrode reaction as the internal reference [4,7,10], most results and analyses were linked with electrolysis in a two-electrode cell which only allowed the control of cell voltage or current. The cell voltages were then compared with thermodynamic data to gain further insight of the electrochemical reactions. Nonetheless, in electrochemical studies, a three-electrode cell incorporating a stable reference electrode is essential to avoid uncertainty of the electrode reactions. For example, in the electrolysis of solid metal oxides in molten CaCl2, the following reactions are all possible at a graphite anode:
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2O2 ! O2 ðgasÞ þ 4e 2þ
2:7 V ðvs: Ca=Ca ; 850 CÞ
ð1Þ
C þ O2 ! 2COðgasÞ þ 2e 2:2 V ðvs: Ca=Ca2þ ; 850 CÞ
ð2Þ
C þ 2O2 ! CO2 ðgasÞ 2:3 V ðvs: Ca=Ca2þ ; 850 CÞ
ð3Þ
2Cl ! Cl2 ðgasÞ þ 2e 3:3 V ðvs: Ca=Ca2þ ; 850 CÞ
ð4Þ
In pure molten CaCl2, oxygen is initially absent and only reaction (4) can take place. However, with oxygen ions entering the molten salt from the oxide cathode during electrolysis, reactions (1), (2) and/or (3) occur, leading to variation on the anodes potential. Under a fixed cell voltage, the cathode potential may consequently change greatly, bringing about unwanted reactions and a lower current or energy efficiency. For example, if the anode reaction is shifted from (4) to either (1) and (2) or (3), the cathode potential will shift negatively to invoke calcium deposition. A worse scenario is when the deposited calcium can react with and hence contaminate the metal product, e.g., chromium and silicon, on the cathode. Indeed, a number of research groups have observed that the electro-deoxidation of solid TiO2 in molten CaCl2 produced various products at different cell voltages or cathode potentials [12–15]. These observations argue strongly that a proper reference electrode is urgently needed for both fundamental research and industrial application of this novel electro-deoxidation method. However, a universal reference electrode has not so far been reported in the literature for high temperature (>700 C) molten salts. Instead, various metal pseudoreference electrodes immersed directly in the molten salt are often used but their potentials could be unstable and vary with many factors (e.g., the O2 concentration). Other types of reference electrodes such as Ca/Ca2+ in molten CaCl2 can give a specific potential but has the problem of quick dissolution of the calcium metal into the molten salt at high temperatures, leading to a short life time of the reference electrode and electronic conduction through the electrolyte. The Ag/AgCl electrode in its conventional form is a favourite reference electrode in many electrolytes with or without the Cl ion. However, in a chloride molten salt at temperatures higher than 700 C, it would be impossible to avoid the dissolution of AgCl (m.p. = 455 C, b.p. = 1550 C) and hence the contamination of the molten salt by the Ag+ ion. On the other hand, evaporation of the chloride melt at high temperatures (but below the boiling point) is commonplace, which can change the composition of the AgCl contain-
ing electrolyte, and hence the potential of the reference electrode. Therefore, a completely sealed structure is a preferred solution. At molten salt temperatures between 450 and 600 C, the common sodium glass exhibits a sufficiently high ionic conductivity and can be easily fabricated into a thin-wall tube to accommodate the reference electrode [16–18]. The sodium glass, however, cannot be used at temperatures higher than 700 C due to its low melt point. Thin-wall tubes with a sealed end of high temperature ceramic materials, such as Mullite (3Al2O3 Æ 2SiO2, m.p. = 1650 C), were also used to accommodate the Ag/AgCl reference electrode for molten salt studies [19,20]. In these ceramics, ionic conduction is usually achieved through the grain boundaries of the tubes thin-wall. Depending on the nature of the boundaries, ionic conduction can either involve both cations and anions of the electrolyte, similar to that in a porous diaphragm or a salt-bridge, or be selective to a specific ion. In selective ionic conduction, net ionic charge will form on each side of the membrane due to the specific ion accumulating on one side, and depleting on the other side of the membrane. This change will lead to a potential difference across the membrane and requires calibration. Nonetheless, these ceramics are very brittle when made very thin and their fabrication is difficult to achieve in a chemical laboratory, for example, using the glass-blowing technique. Quartz is commonly used in research at high temperatures up to 1200 C. It is also satisfactorily stable in molten chloride salts in the absence of, for instance, a strong reductant such as the calcium and sodium metals. Depending on purity, quartz becomes soft at a wide range of temperatures around 1400 C. Therefore, quartz fabrication by the glass-blowing technique is feasible using the hydrogen–oxygen flame in common laboratory practice with proper safety measures. Considering that silver has a melting temperature of about 962 C, the combination of quartz and Ag/AgCl could allow the upward rise of the working temperature to 950 C. This upper limit is sufficient for many high temperature processes in molten salts, particularly the electrochemical reduction of solid metal oxides [4– 11]. Like Mullite, thin-wall quartz tubes with a sealed end were also used to accommodate the Ag/AgCl electrode in previous research [21]. However, the working temperatures were not higher than 750 C, and the conduction mechanism was not investigated. More importantly, to our best knowledge, except for a recent communication [22], there has not been any reported molten salt work in which the reference electrode has been used continuously for 24 h or longer. Such a long service life for a reference electrode is very needed for many existing and future industrial electrolytic processes in molten salts where continuously monitoring and/or controlling the potentials of both anode and cathode are crucial.
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In this laboratory, a method for the preparation of sodium ion conducting quartz membrane was previously developed [22]. The membrane was then sealed at the end of a quartz tube into which the Ag/AgCl reference electrode and the relevant internal electrolyte were incorporated to produce a wholly enclosed quartz outer structure. The electrode was tested in molten chloride salts in the temperature range between 600 and 900 C and the results were satisfactory. However, the preparation of the membrane takes a very long time (>2 days) and it was found in later practice that the gas-flame sealed joining part between the membrane and the quartz tube occasionally failed without a clearly understandable cause. This paper reports the fabrication of a quartz sealed Ag/AgCl electrode without separate preparation of the ion conducting membrane, and various experimental tests of such made electrodes in molten chloride salts at temperatures between 700 and 950 C. The results demonstrate that, with a wall thickness of about 0.5 mm, the quartz sealed Ag/AgCl reference electrode can be satisfactorily applied for many times in electrochemical analysis experiments without compromising the quality of service, or can be continuously used for at least three days, as experimented so far, in constant potential electrolysis. It is anticipated that using the same fabrication technique and also the completely sealed quartz structure, many more reference electrodes can be made by employing different reversible high temperature redox couples such as metal/metal sulfide and metal/metal oxide. It should be pointed out that quartz and many other metal oxide based ceramics are unstable in molten fluoride salts and hence not suitable for the fabrication of a reference electrode to be used in these melts.
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longer than the above mentioned quartz tube was inserted rapidly into the heat-softened end of a silver rod (60 mm in length, 2.0 mm in diameter) at a depth of about 10 mm. The outer quartz tube prepared previously was filled with a mixture of AgCl, NaCl and KCl (molar ratio: 45% NaCl, 45% KCl and 10% AgCl) which had been dried at 400 C in air for two days. The tube was then heated to melt the contained salt mixture, and the inner Ag electrode was inserted into the molten salt. Several minutes later, the whole electrode was cooled slowly to room temperature. After cleaning and drying, the open end of the quartz tube was connected to a rotary vacuum pump. When the internal pressure of the quartz tube could not be further decreased, the hydrogen–oxygen flame was used to seal the construction at a position around the tungsten wire, as shown in Fig. 1, and the fabrication of the reference electrode was completed. 2.2. Performance tests Most performance tests of the new reference electrode, including reproducibility, reusability and stability, were carried out in a graphite crucible with a relatively small volume (23 mm in inner diameter, 150 mm in height, 3.5 mm in wall thickness). The reference electrode was also checked by performing constant potential electrolysis of pressed silica powder in a larger graphite cell (100 mm in inner diameter, 230 mm in height, 10 mm in wall thickness) [7]. In all experiments, the salts were of the AnalaR grade and were thermally dried in air before use. All electrochemical tests and measurements were operated under argon protection
2. Experimental 2.1. Preparation of the Ag/AgCl reference electrode The sealed quartz structure, which was to function as the ion conducting membrane of the Ag/AgCl electrode, was prepared by first sealing one end of a quartz tube (350 mm in length, 6.0 mm in external diameter and 1.5 mm in wall thickness) in the hydrogen–oxygen gas flame. While still in the flame, a positive air pressure was blown into the closed end which then expanded into a capsule whose size, shape and wall thickness were controlled by the blowing operation. After cooling, the quartz tube was ready to receive the Ag/AgCl electrode that consisted of a silver electrode and the internal electrolyte. The silver electrode was made as follows. After being manually polished briefly with sand paper for cleaning, a tungsten wire of 0.2 mm in diameter with a length
Fig. 1. A schematic diagram showing the quartz sealed Ag/AgCl reference electrode. 1. quartz tube with the sealed middle part; 2. silver rod; 3. mixture of AgCl, NaCl and KCl; 4. tungsten wire; 5. thinned and sealed lower end of the quartz tube as the ion conducting membrane.
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using the CHI660a electrochemical system (Shanghai Chenhua, China) that was controlled by a PC computer.
3. Results and discussion 3.1. Resistance The performance of the quartz sealed Ag/AgCl reference electrode in the molten salt depended strongly on its resistance that was mainly affected by the thickness of the ion conducting membrane, i.e., the sealed lower end of the quartz tube (see Fig. 1). The ionic resistance (R) of the reference electrode (Table 1) was measured in the mixed molten salts of LiCl, NaCl and KCl at different temperatures by the voltage-jump method as described below. When a small voltage jump (10–50 mV) was applied between the reference electrode and an inert counter electrode (graphite), by Ohms Law and treating the system as a resistor and capacitor connected in series, the ionic resistance can be determined by the applied voltage and the initial responding current within the first few microseconds. (This is comparable with the ac impedance measurement at high frequencies.) Because the ionic resistance of the bulk molten salt is relatively low, the measured resistance can be considered to be the ionic resistance of the reference electrode introduced by the silica tube wall. Each value in Table 1 is the average of at least five measured values. A plot of the voltage against the current was recorded and the resistance was read as the slope by linear fitting. The measurement was repeated at different temperatures. As it is revealed in Fig. 2, the linear relationship between the resistance and the reciprocal of temperature is characteristic of a thermally activated conduction in accordance with Arrhenius Law [23]. Considering that a modern potentiostat has commonly an input impedance of 105 X or higher, the resistance of the reference electrode is preferred to be smaller than 104 X. Therefore, the data in Table 1 demonstrate that the quartz sealed Ag/AgCl reference electrode can be used at temperatures higher than 700 C. Indeed, when using the reference electrode at temperatures lower than 700 C, current oscillation occurred. The resistance of the reference electrode was found to be almost independent of the composition of the molten salts used in this work, indicating an intrinsic property of the electrode. For example, the resistance of the electrode was about 104 X at 850 C in pure molten CaCl2,
Fig. 2. The linear correlation between the logarithm of the ratio of temperature and ionic conduction (T/R) of the reference electrode and the reciprocal of temperature (1000/T) of the mixed molten salts of LiCl + NaCl + KCl.
in agreement with that given in Table 1 measured in the mixed salts. 3.2. Potential The capability of providing a stable potential under service conditions is the most important specification of a reference electrode. Therefore, measuring the potential variation of a new reference electrode with changing the relevant experimental parameters is essential for assessment. In room temperature systems, many wellknown reference electrodes can be used as the reference to investigate a new reference electrode. However, this is not the case in high temperature molten salts. An alternative approach is to use one or more known redox active compounds as the internal references that exhibit characteristic responses when measured by an electrochemical means, such as cyclic voltammetry. Conventionally, the redox active compound is added into the electrolyte, which, however, has the drawbacks of contaminating the electrolyte and possibly interfering with the system to be investigated. In this laboratory, a quartz sealed tungsten disc electrode was previously studied in molten CaCl2 by cyclic voltammetry using a platinum wire pseudo-reference electrode [7]. The obtained cyclic voltammogram (CV) exhibited very re-producible and easily distinguishable characteristics related with the reduction of solid SiO2 and Ca2+, around and on the central W-disc, respectively. These electrochemical reactions are suitable inter-
Table 1 The resistance of the quartz sealed Ag/AgCl reference electrode in the melt of LiCl + NaCl + KCl at different temperatures Temperature (C) Resistance (X)
950
900 3
2 · 10
4 · 10
850 3
800 4
1 · 10
750 4
1.5 · 10
700 4
2.5 · 10
650 4
5 · 10
600 5
1 · 10
4 · 105
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nal references for investigating the new quartz sealed Ag/AgCl reference electrode. An additional advantage is that there is no need to add anything into the electrolyte except for the electrode itself. Therefore, in this work, the CVs of the W–SiO2 electrode, coupled with the new quartz sealed Ag/AgCl electrode, were recorded again in molten CaCl2, and two examples are shown in Fig. 3. It can be seen from these CVs (and others recorded at different scan rates) that the two current onset potentials indicated by ‘‘a’’ and ‘‘b’’ in Fig. 3, which correspond to the reduction of solid SiO2 to Si and of Ca2+ to Ca, respectively [7], are in good agreement with the calculated potential of the Ag/AgCl electrode using available thermodynamic data (‘‘a’’: 0.45 V vs. Si/SiO2; ‘‘b’’: 1.26 V vs. Ca/Ca2+). In the following discussion, the potentials are therefore reported with respect to either of these two current onset potentials. 3.3. Stability The stability of the new reference electrode was investigated against a number of experimental parameters, including working temperature, service time, and composition of the molten salts. In all these cases, CVs of the W–SiO2 electrode were recorded for obtaining the potential of the new reference electrode. Molten CaCl2 was used as the basic electrolyte and other salts were added when necessary. Once immersed in pure molten CaCl2, the quartz sealed Ag/AgCl reference electrode was capable of reaching a stable potential after a short initial stabilization time of about 15 min. Afterwards, the electrode exhibited considerably high stability and a very reproducible potential. In the experiments, the CV of the
Fig. 3. Cyclic voltammograms of the W–SiO2 electrode in molten CaCl2 (200mV/s, 850 C) at 15 min (solid line) and 25 h (dashed line) after the reference electrode was continuously immersed in the molten salt. (Diameter of W-disc: 300 lm.) Note that the current onset points, a and b, for the reduction of SiO2 and the deposition of calcium, respectively, are the same on the two CVs.
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W–SiO2 was recorded at intervals of around an hour, but the reference electrode stayed in the molten salt continuously. Fig. 3 shows two CVs recorded at very different times: one at 15 min and the other 25 h after the W– SiO2 working and the quartz sealed Ag/AgCl reference electrodes were inserted into the molten salt. Clearly, no obvious difference can be seen between the currents and potentials on the cathodic (reduction) branches of the two CVs. It should be mentioned that identical or very similar CVs were often recorded at different times of electrode immersion in the molten salt within the same potential window. However, in order to see the two superimposed CV curves in Fig. 3, different cathodic potential limits were purposely used, which resulted in the difference between the anodic (re-oxidation) branches of the CVs. Furthermore, in this work, CVs were record at a relatively fast potential scan rate, 200 mV/s. This was partly for comparison with previous work [7] and also because a faster scan causes less chemical change to the W–SiO2 electrode and is more suitable for repeated/prolonged tests. An alternative way of demonstrating the stability of the new reference electrode is to plot the characteristic potential of one of the recognized cathodic reactions, such as that indicated by ‘‘a’’ or ‘‘b’’ in Fig. 3, against the immersion time. Typical plots of the potential variation (DE) against the service time are displayed in Fig. 4 for measurements at two different temperatures, 850 and 950 C. It can be seen that within the tested experimental periods, the potential variation in both cases was smaller than ±5 mV which is within the typical experimental error range of cyclic voltammetry. In fact, in this and other research programmes of this laboratory, the same stability of the reference electrode had been observed continuously for more than three days. The influence of the electrolyte composition on the stability of the new reference electrode was also investigated by adding different chloride salts, i.e., KCl, NaCl
Fig. 4. The variation of the current onset potential of the SiO2 reduction (‘‘a’’ in Fig. 3) vs. the quartz sealed Ag/AgCl reference electrode, as described in the text, in molten CaCl2 at (d) 850 and (m) 950 C.
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and AgCl, to molten CaCl2. It was observed that the potential of the reference electrode was insignificantly affected by the addition of KCl and AgCl to a concentration of 10 wt% at 850 C. However, with increasing the concentration of NaCl in molten CaCl2, the recorded cyclic voltammograms shifted continuously in the negative direction, as shown in Fig. 5. Because the voltammogram shift happened to all reactions, including the reduction of solid SiO2 which is not expected to be affected by the addition of a small amount of NaCl, and there were not significant changes in the activities of other components in the molten salt and in other relevant parameters, e.g., temperature and electrode, it is clear evidence of the positive shift of the potential of the reference electrode. There are two well-known ion conduction mechanisms in solid materials. The first type of conduction is through relatively large physical defects in the solid membrane, such as micro-pores and grain boundaries. Because this type of conduction happens unselectively to both cations and anions, no potential difference should build up on the two sides of the membrane. The second type of ion conduction is selective to certain cations or anions but not to both. It can lead to unequal amounts/activities of the selected cation or anion on the two sides of the membrane, resulting in the membrane or Donnan potential which is proportional to the logarithm of the activity ratio of the selected ions on each side of the membrane. Previous work has demonstrated that quartz could become selectively conducting to Na+ at high temperatures by for example immersing the material in molten NaCl or its vapour in the presence of oxygen [22]. To further confirm the shift of the CV shown in Fig. 5 to be a result from the selective conduction of Na+ ion through the quartz membrane, the current onset potential of the SiO2 reduction is plotted against the logarithm of the concentration of NaCl in the molten salt.
Fig. 5. Cyclic voltammograms of the W–SiO2 electrode at 850 C and 200 mV/s in molten CaCl2 containing different amounts of NaCl, (a) 1, (b) 5 and (c) 10 wt%. The reference electrode was the quartz sealed Ag/AgCl electrode described in the text. (Diameter of W-disc: 100 lm.)
The results are displayed in Fig. 6, showing clearly a linear correlation. Nonetheless, it is noticed that the slope of the fitted straight line in Fig. 6 is 0.1522 V/decade. This value can be compared with the theoretical Donnan gradients at 850 C of 0.2228 V/decade for the conduction of a mono-cation and of 0.1114 V/decade for a di-cation. This discrepancy could have resulted from two origins. The first is the conduction of Na+ being accompanied by other ions, particularly the di-cation, Ca2+, which is plentiful in the molten salt but not in the internal solution of the reference electrode. If so, the influence of Ca2+ on the potential is expected because the variation of the mole fraction of NaCl is accompanied by that of CaCl2. The second is the use of the mole fraction of NaCl as the activity of the Na+ ion in the molten salt. The straight line shown in Fig. 6 suggests that the deviation might have been linear, which is in accordance with Henrys Law for a dilute solute (NaCl) or Raoults Law for the solvent (CaCl2). The implied conduction of Ca2+ through the quartz membrane is actually not surprising because of the similar ion radii between Na+ and Ca2+. Furthermore, as demonstrated in both Figs. 3 and 4, the quartz sealed Ag/AgCl electrode had performed very well in pure molten CaCl2, which could be an indication of Ca2+ conduction through the quartz membrane. It is worth mentioning that the capability of silica to accommodate and transport both Na+ and Ca2+ ions is well known to the glass industry that uses Na2O and CaO as the network modifiers to terminate the silica network and hence lower the softening temperature of silica based glass [24]. 3.4. Reproducibility and reusability A comparison was made between the quartz sealed Ag/AgCl electrodes prepared by different workers or at different times but following the same procedure as described in Section 2. In the experiment, 10 electrodes
Fig. 6. The linear correlation between the potential of the SiO2 reduction and the NaCl concentration (mole fraction) in molten CaCl2 at 850 C.
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4000 3000
Na
b
2000
a
1000 0
200
250
300
350
400
B.E. (eV) Fig. 7. XPS spectra of quartz glass before (a) and after (b) treatment in molten NaCl at 850 C in the presence of oxygen for two days.
3.5. Test in constant potential electrolysis In this laboratory, the quartz sealed Ag/AgCl electrode was and is still being used in constant potential electrolysis of, for example, solid silica (powder) in molten chloride salts [7,22], which usually lasted for a period typically from hours to days to prepare multiple samples. While results from these investigations are being collected and analysed, it is more relevant to report here the performance of the new reference electrode. In all these tests, the new reference electrode provided reliable continuous assistance to the electrolysis. However, after a very long service time (over two days), it was found that the surface of the outer quartz tube immersed in the molten salt, especially of the part at the boundary linking the molten salt and gas phase, showed signs of erosion. In more serious cases, the eroded quartz tube became brittle and prone to break, which prevented further use of the reference electrode. Because the erosion did not occur in experiments that, even though continued for days, involved only small quantities of cathodic and anodic products as in the cyclic voltammetric measurements, it might be related with reactions between quartz and the electrode reaction products. In the electrolysis of solid silica, O2 ions enter the molten salt from the oxide cathode and discharge at the anode to form oxygen containing gaseous products. To continue the process, a certain amount of O2 ions is maintained in the molten salt (but chloride ions have to be first removed anodically to maintain neutrality of the molten chloride salt). The O2 ions may attack quartz to form silicates via the following reaction: SiO2 þ xO2 þ xCa2þ ðor 2xNaþ Þ ! Cax SiO2þx ðor Na2x SiO2þx Þ
5000
CPS
were immersed in the same molten salt and the potential difference was measured between any pair of these electrodes. It was observed that the potential difference between these individual electrodes was usually smaller than 8 mV and never greater than 15 mV. The reference electrode was also successfully used for electrochemical analyses, such as cyclic voltammetry, in CaCl2 and its mixture with other salts. No visible damages to the electrodes outer tube were seen after such uses, even continuously for longer than two to three days, see Figs. 3 and 4. Once removed from the molten salt and cooled to room temperature in air, the electrode could be washed in water, dried and stored in open air. The same electrode could be re-used for many times (>10) without compromising the service quality.
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ð5Þ
Furthermore, the anodically formed oxygen gas can reach at the quartz–salt–gas triple phase boundary and invoke silicate formation via, for example, the following reaction:
SiO2 þ xCaCl2 ðor 2xNaClÞ þ x=2O2 ! Cax SiO2þx ðor Na2x SiO2þx Þ þ xCl2
ð6Þ
It was reported recently that, under a continuous flow of oxygen gas, silica powder could promote the dechlorination of CaCl2 at 900 C because of the formation of calcium silicate [25]. In this laboratory, visual observation and XRD analysis had confirmed that thin quartz slices could be enriched by sodium silicate at 850 C in molten NaCl under an oxygen containing atmosphere [22]. XPS analysis also confirmed the presence of sodium in such treated quartz glass but did not find the same in untreated samples, as shown in Fig. 7. These findings, together with the observation of more erosion at the triple phase boundary linking the molten salt, quartz tube and gas phase, suggest that reaction (6) is more problematic for the use of the quartz sealed Ag/AgCl reference electrode. However, because the problem involves oxygen in the gas phase, it may be prevented or slowed by using a more erosion resistant protection layer on the surface of the quartz tube. Various candidate materials are being tested for this purpose, including graphite.
4. Conclusion The fabrication and investigation of a stable and long life quartz sealed Ag/AgCl reference electrode for high temperature molten chloride salts are described in this paper. The method is based on the conventional glassblowing technique with the assistance of the hydrogen–oxygen flame. Such made reference electrodes could be continuously used in CaCl2 based molten salts at temperatures between 700 and 950 C. Due to the wholly sealed construction, the reference electrode is stable, temperature reversible, contamination free and
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re-usable in the same or different electrolyte systems. Erosion of the outer quartz tube was observed in prolonged electrolysis of solid silica powder in molten CaCl2, likely due to the formation of calcium or sodium silicates under the influence of electrochemically generated oxygen in both of the liquid and the gas phases.
Acknowledgements The Natural Science Foundation of China is gratefully acknowledged for financial support (Grant No. 20125308). G.Z.C. thanks the Ministry of Education of China for the award of the Cheung Kong Scholarship.
References [1] T.H. Okabe, T. Oishi, K. Ono, Metal. Trans. B 23 (1992) 583. [2] T.H. Okabe, M. Nakamura, T. Oishi, K. Ono, Metal. Trans. B 24 (1993) 449. [3] K. Hiroa, T.H. Okabe, F. Saito, Y. Waseda, K.T. Jacob, J. Alloys Compd. 282 (1999) 101. [4] G.Z. Chen, D.J. Fray, T.W. Farthing, Nature 407 (2000) 361. [5] G.Z. Chen, D.J. Fray, in: N.Y. Chen, Z.Y. Qiao (Eds.), Proceedings of 6th International Symposium on Molten Salt Chemistry and Technology, Shanghai University, Shanghai, China, 2001, p. 79. [6] A.J. Fenn, G. Cooley, D.J. Fray, L. Smith, Adv. Mater. Proc. 2 (2004) 51.
[7] X.B. Jin, P. Gao, D.H. Wang, X.H. Hu, G.Z. Chen, Angew. Chem. Int. Ed. 43 (2004) 733. [8] K. Ono, R.O. Suzuki, J. Organomet. Chem. 54 (2002) 59. [9] X.Y. Yan, D.J. Fray, Metal. Mater. Trans. B 33 (2002) 685. [10] T. Nohira, K. Yasuda, Y. Ito, Nat. Mat. 2 (2003) 397. [11] G.Z. Chen, E. Gordo, D.J. Fray, Metal. Mater. Trans. B 35 (2004) 223. [12] G.Z. Chen, D.J. Fray, J. Electrochem. Soc. 149 (2002) E455. [13] K. Dring, M. Jackson, R. Dashwood, H. Flower, D. Inman, in: F.H. Froes, M.A. Imam, H.M. Flower, D. Fray (Eds.), CostAffordable Titanium Symposium in Honor of Professor Harvey Flower, The Minerals, Metals & Materials Society, Warrendale, PA, 2004, p. 95. [14] K. Dring, R. Dashwood, D. Inman, in: Programme and Abstracts, MSDG Christmas Research Meeting, London, UK, December 2004. [15] Y. Katayama, B. Friedrich, in: 2004 Joint International Meeting – Fourteenth International Symposium on Molten Salts, Honolulu, Hawaii, October, 2004 (Abs# 2373). [16] J.OM. Bockris, G.J. Hills, D. Inman, L. Young, J. Sci. Instrum. 33 (1956) 438. [17] R.D. Caton Jr., C.R. Wolfe, Anal. Chem. 43 (1971) 660. [18] Y. Castrillejo, M.A. Garcia, A.M. Martinez, C. Abejon, P. Pasquer, G. Picard, J. Electroanal. Chem. 434 (1997) 43. [19] R.J. Labrie, V.A. Lamb, J. Electrochem. Soc. 106 (1959) 895. [20] A. Nishikata, J. Yamamuro, T. Tsuru, J. Electrochem. Soc. 161 (2004) B245. [21] A.M. Martinez, Y. Castrillejo, B. Borresen, M.R. Bermjo, M. Vega, J. Electroanal. Chem. 493 (2000) 1. [22] P. Gao, X.B. Jin, D.H. Wang, X.H. Hu, G.Z. Chen, Dian Huaxue (Electrochemistry) (2004), accepted (in Chinese). [23] M. Malki, P. Echegut, J. Non-Cryst. Solids 323 (2003) 131. [24] J. Sehgal, S. Ito, J. Am. Ceram. Soc. 81 (1998) 2485. [25] T. Kondo, T. Sukigara, K. Itoh, N. Azuma, A. Ueno, Y. Momose, Ind. Eng. Chem. Res. 42 (2003) 6040.
