- Email: [email protected]
AgCl reference electrodes: Fabrication and applications
Journal of Electroanalytical Chemistry 666 (2012) 25–31
Contents lists available at SciVerse ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Antifreezing Ag/AgCl reference electrodes: Fabrication and applications Jiao Yin a,b, Li Qi a, Hongyu Wang a,⇑ a b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China Graduate University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 6 August 2011 Received in revised form 15 November 2011 Accepted 18 November 2011 Available online 26 November 2011 Keywords: Antifreezing Ag/AgCl Reference electrodes Fabrication Applications
a b s t r a c t As a promising antifreeze, Ethylene glycol (EG) was employed for preparation of new types of antifreezing Ag/AgCl reference electrodes. Different proportions of EG were used as antifreezing additives in the inner electrolyte of Ag/AgCl reference electrode. The potential shifts of the resultant reference electrodes are less than 10 mV in the wide range of temperatures (25 40 °C), which extends their application at low temperatures. In addition, the proposed antifreezing electrodes maintain non-polarizable and reversible nature of the initial Ag/AgCl ones even in low-temperature environment. Finally, to ensure their stability at low temperatures, the antifreezing reference electrodes were applied in various low-temperature electrochemical systems. Ó 2011 Elsevier B.V. All rights reserved.
to the changes of temperature. As is well-known that for Ag/AgCl (KCl aq):
1. Introduction Reference electrodes, as indicator electrodes, are indispensable components of electrochemical systems for the reason that they can provide constant potential during the electrochemical measurement. There are mainly three kinds of common reference electrodes: normal hydrogen electrode (NHE), saturated calomel (Hg/Hg2Cl2) electrode (SCE) and silver/silver chloride (Ag/AgCl) electrode. In practice, the Ag/AgCl electrodes are preferably applied, because their potential is stable and well defined in relation to the potential of the NHE. Furthermore, their easy manipulation, green chemistry, and safety are convenient for the practical application. Such electrodes are commercially available in a variety of chemical systems such as sensors [1,2] and other related fields. Over the years, various Ag/AgCl reference electrodes based on traditional Ag/AgCl/aq.KCl have been proposed. For example, thick film Ag/AgCl reference electrodes have been fabricated by Cranny and Atkinson [3,4]. Matsumoto and Ohashi [5] have also developed micro-planar Ag/AgCl quasi-reference electrode for biosensor. In addition, the use of room temperature ionic liquids (RTILs) as electrolytes has promoted the development of Ag/AgCl (RTILs) reference electrodes [6–8]. Moreover, to meet the technical demands, solid state [9–12], micro [13] and micromachined [14] Ag/AgCl reference electrodes have also been constructed. All the Ag/AgCl reference electrodes above mentioned could be applied at room temperature. Nevertheless, the electrode potential is also sensitive ⇑ Corresponding author. Tel./fax: +86 431 85262287. E-mail Wang).
addresses:
[email protected],
[email protected]
1572-6657/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.11.021
(H.
ET ¼ ET0 RT=F ln aCl
ð1Þ
ET0 ¼ 0:23735 5:3783 104 t 2:3728 106 t2
ð2Þ
It does mean that the electrode potential has some definite relationship with temperatures and the ET0 can be calculated according to the temperature (t P 25 °C). However, the equation can only be applied at the temperature interval of t P 25 °C. Furthermore, the electrode potential of Ag/AgCl (aCl aq ¼ 1) vs. NHE at the temperature interval of 25 100 °C have been investigated and listed according to both experimental measurements and the theoretical calculation based on the above mentioned equations [15,16]. By contrast, higher temperature Ag/AgCl reference electrodes have also been fabricated to provide potential reference for hightemperature electrochemical system [17,18]. Similarly, to meet the industrial demand, Liu et al. [19] have developed antifreezing Cu/CuSO4 reference electrode suited to frozen soil determination. However, this type of antifreezing reference electrode is bulky and only suitable for engineering application. In addition, the antifreezing Cu/CuSO4 reference electrode has larger potential shift (more than 100 mV) in cold environment (2 °C) so that it cannot be applied in some low-temperature systems which require excellent stability or sensitive change of potential. Moreover, the electrochemical properties of antifreezing Cu/CuSO4 reference electrode were not fully investigated in Liu’s literature. Nevertheless, as a common reference electrode, little attention has been paid to studying the electrochemical properties of the traditional Ag/AgCl (KCl aq) or other new types of Ag/AgCl electrodes
26
J. Yin et al. / Journal of Electroanalytical Chemistry 666 (2012) 25–31
electrodes and working electrodes, respectively, below 10 °C. The conventional three-electrode system comprised a glassy carbon electrode (GCE, diameter, 3 mm, working electrode), a platinum flag counter electrode, and above mentioned antifreezing Ag/AgCl reference electrode, respectively. The antifreezing Ag/AgCl electrode (b) was taken as reference electrode for charge–discharge tests of low-temperature supercapacitor. The galvanostatic charge–discharge cycles of low-temperature supercapcitor were measured by a Land cell tester. The electrolyte of low-temperature supercapcitor was 5 M NaClO4 aqueous solution.
at low temperatures (t 6 0 °C). More importantly, it is sometimes necessary to use reference electrodes in low-temperature electrochemical experiments. Unfortunately, the internal electrolyte of Ag/AgCl (pure KCl aq) electrode will freeze at low temperatures (t 6 0 °C). Consequently, the reference electrodes employed for room temperature cannot be used in low-temperature environment. Furthermore, the traditional Ag/AgCl/aq.KCl cannot be employed for the low-temperature measurement due to significant potential fluctuation caused by freezing of internal electrolyte. Bearing the above situations in mind, we try to find suitable antifreeze which cannot only dissolve a certain amount of salt but also mix with the water well to lower the freezing point. It is well-known that polyhydric alcohol is often used as antifreeze in industry and miscible with water to get a homogeneous solution. Inspired by this, different proportions of Ethylene glycol (EG) were selected as antifreezing additives for internal electrolyte of Ag/AgCl electrode. The electrochemical properties of antifreezing Ag/AgCl electrodes both at room and low temperatures were investigated in detail, respectively. More importantly, these antifreezing Ag/ AgCl reference electrodes were applied in different low-temperature electrochemical systems.
3. Results and discussions 3.1. The fabrication process and structure of antifreezing Ag/AgCl reference electrode The fabrication process of Ag/AgCl electrodes. Fig. 1A shows a sketch of antifreezing Ag/AgCl electrode. The main fabrication process can be summarized as follows. The as-prepared Ag/AgCl electrode was inserted into glass tube which was filled with antifreezing electrolytes (containing EG, water and KCl). Pt wire at the bottom of glass tube was used as electrode-contact with testing solution. To keep the KCl saturated in the electrolyte, some solid KCl was placed at the bottom of glass tube. Finally, the electrode was sealed by parafilm and preserved in saturated KCl aqueous solution. The photograph of homemade Ag/AgCl reference electrode is shown in Fig. 1B. When the Ag/AgCl reference electrodes were put in low-temperature environment (20 °C), the electrolyte of common Ag/AgCl reference electrode was frozen in Fig. 1C. Nevertheless, the antifreezing Ag/AgCl reference electrode showed no obvious changes by appearances.
2. Experimental section 2.1. Materials and reagents Silver wires were obtained from National Chemical Reagent Company (Shanghai, China). Other chemicals (including Ethylene glycol, Potassium chloride, Potassium hexacyanoferrate and Sodium perchloride) were acquired from Beijing Chemical Factory (Beijing, China). Commercial Ag/AgCl electrode (saturated KCl aqueous solution) was indicated as common Ag/AgCl electrode in the following study. All the chemicals were of analytical grade and were used as received. The solutions in the present study were made using deionized water (Millipore Water System) with a resistance of 18.3 MX.
3.2. Evaluation of antifreezing Ag/AgCl reference electrodes Measurement of potential differences vs. common Ag/AgCl both at room and low temperatures. It is essential to evaluate the electrode potential with respect to those of existent reference electrodes for a new class of proposed reference electrode. The evaluation of those antifreezing Ag/AgCl electrodes at room temperature (25 °C) can be easily made through the results in Table S1 (see supporting information) using a value of E at a concentration of of 1 mol L1 KCl aq (test solution). The cell voltage of two electrodes, referred to by the following cell (3), E was measured with a CHI 660C electrochemical workstation. E values of a–e are 0.1, 2.8, 4.8, 5.2 and 6.7 mV vs. common Ag/ AgCl, respectively. Because the electrode potential of common Ag/
2.2. Fabrication of antifreezing Ag/AgCl reference electrodes Ag/AgCl electrodes were prepared by anodizing 0.5 mm diameter silver wires in diluted HCl aqueous solutions. The internal electrolytes of Ag/AgCl electrodes comprised EG, water and KCl. The solute of KCl dissolved in the following solvents (different proportions of EG and water) were the maximal concentrations of KCl solutions. The reference electrodes were named as a–e according to above different proportions of EG and water in internal electrolytes (shown in Table 1).
I
II
III
IV
V
VI
VII
Ag
AgCl
n1 wt%EG þ n2 wt%H2 O þ n3 MKCl
1MKCl
saturated KClaq
AgCl
Ag
fworkingelectrodeg
ftestsolutiong
2.3. Electrochemical measurments The electrochemical experiments were performed using a CHI 660 C electrochemical workstation (CH Instruments, Inc., Shanghai) and carried out using two-electrode and three-electrode systems, respectively. The two-electrode system consisted of a common Ag/AgCl reference electrode and the proposed Ag/AgCl working electrodes, respectively, above 10 °C. The proposed antifreezing Ag/AgCl electrodes were designated as reference
ð3Þ
freferenceelectrodeg
AgCl is 199 mV vs. NHE at 25 °C, the potential of those antifreezing Ag/AgCl electrodes with respect to NHE is estimated by adding 2.8, 4.8, 5.2 and 6.7 to 199 mV at 25 °C [7]. Then, the values of the electrode potential of antifreezing Ag/AgCl electrodes referred to as NHE are estimated to be 201.8, 203.8, 204.2 and 205.7 mV, respectively, at 25 °C. This is a rough estimation of electrode potential vs. NHE because the low liquid junction potential still exists. As a result, further investigations are required to accurately evaluate their electrode potential.
J. Yin et al. / Journal of Electroanalytical Chemistry 666 (2012) 25–31
27
Table 1 Internal electrolytes with different proportions of EG and water at 25 °C. Electrodes
x wt.% EG
y wt.% H2O
CKCl (maximal concentrations dissolved in mixtures of EG and water)
a b c d e
0 32.3 52.6 72.2 100
100 67.7 47.4 27.8 0
Saturated 3M 2M 1M 0.5 M
Furthermore, the content of EG has minor effect on the electrode potential of antifreezing Ag/AgCl electrode shown in Table S1-1 (see supporting information). The electrode potential between antifreezing Ag/AgCl electrode and common Ag/AgCl electrode is elevated with the increase of EG. This could be ascribed to the increase of phase-boundary potential between solid Ag/AgCl and liquid EG shown in cell (3) and Eq. (4). The phase-boundary potential between phase II and III in cell (3), /EG /AgCl could be formally expressed in terms of the transfer Gibbs energies of Ag+ and Cl ions shown in Eqs. (5) and (6) between the phases II and III, and:
DGEG!AgCl and DGEG!AgCl tr;Cl tr;Agþ /EG /AgCl ¼
i 1 h EG!AgCl DGtr;Agþ DGEG!AgCl tr;Cl 2F
AgClAgþ þ Cl
ð4Þ ð5Þ
AgClðsÞ þ e AgðsÞ þ ClðEGÞ
ð6Þ
However, the Ag/AgCl electrode is in contact with EG (containing Cl), there are common ions (Ag+, Cl) between phases II and III in cell (3). That is to say, the distribution of ionic species, primarily Cl, assures the thermodynamic equilibrium between the two phases (AgCl and EG), which leads to low phase-boundary potential even in the internal electrolyte of pure EG [12]. Anyway, the electrode potential of antifreezing Ag/AgCl electrodes still has slight deviation from the standard electrode potential of common Ag/AgCl at 25 °C due to the presence of phase-boundary potential. In the following study, the electrode potential at lower temperatures has been investigated. The electrode potential of antifreezing Ag/AgCl electrode vs. common Ag/AgCl at 5 °C has little shift with respect to that at 25 °C shown in Table S1-1 (see supporting information). When the temperature is lowered at 10 °C, the potential has bigger shift than that of temperature at 25 and 5 °C, respectively. With continuous decrease of tempera-
Fig. 2. Open-circuit-potential (OCP) of (A) common Ag/AgCl vs. common Ag/AgCl and (B) antifreezing Ag/AgCl vs. antifreezing Ag/AgCl (e vs. b) as a function of time in antifreezing electrolyte (52.6% EG + 47.4% H2O + 1 M KCl) at 20 °C, respectively.
ture, the internal electrolyte of common Ag/AgCl electrode froze at 20 °C shown in Fig. 1C, the electrode potential (antifreezing Ag/ AgCl electrodes vs. common Ag/AgCl) becomes rather unstable and has considerable shift. In addition, the pure KCl aqueous solution (1 M) cannot be used as test solution below 10 °C due to the formation of freezing coagulations. To measure the electrode potential below 10 °C, certain amounts of EG have been added into test solutions. To find out the reasons for potential shift, Fig. 2 shows open-circuit-potential (OCP) of two frozen common Ag/AgCl electrodes and two antifreezing Ag/AgCl electrodes (e vs. b) as a function of time in antifreezing test solution at 20 °C, respectively. During the measurement period, the potential shift of two antifreezing Ag/ AgCl reference electrodes was 1 mV. The frozen common Ag/AgCl electrodes have considerable potential fluctuation. This may be due to the fact that the internal electrolyte becomes solid and the Ag/AgCl cannot obtain normal response from Cl for the reason that ions cannot transfer smoothly in soild phase. Consequently, it cannot provide normal electrode potential. However, the antifreezing additives keep the internal electrolyte liquid so that AgCl still can achieve a reversible equilibrium with the solid silver and the chloride ions shown in Eq. (6). As a result, the Ag/AgCl has normal response from Cl at low temperatures. Equally, the other reference electrodes such as normal hydrogen electrode (NHE), saturated calomel electrode (SCE) have the same problems with common Ag/AgCl. The internal electrolytes of NHE and SCE will also become frozen at low temperature. Therefore, NHE and SCE
Fig. 1. (A) Diagrammatic sketch of proposed reference electrode, photograph of (B) antifreezing Ag/AgCl reference electrode, and (C) common Ag/AgCl electrode at 20 °C.
28
J. Yin et al. / Journal of Electroanalytical Chemistry 666 (2012) 25–31
Fig. 3. The sensitivity to changes of Cl concentration (0.1–3 M) for antifreezing electrodes (a–e) vs. common Ag/AgCl at 25 °C.
Fig. 5. Conductivity of internal electrolytes of various electrodes (a–e) saturated with maximal concentrations of KCl at 25 °C.
Fig. 4. Dependence of potential of the antifreezing reference electrodes on the polarization current (A) anodic and (C) cathodic at 25 °C, test solution: 1 M KCl aq, polarization time: 1 s, reference electrode: common Ag/AgCl electrode; polarization current (B) anodic and (D) cathodic at 40 °C, test solution: 52.6% EG + 47.4% H2O + 1 M KCl, polarization time: 1 s, reference electrode: electrode (e).
J. Yin et al. / Journal of Electroanalytical Chemistry 666 (2012) 25–31
cannot be taken as reference electrodes for electrochemical determination under lower temperatures. Faced up with this unsatisfactory situation, a favorable way to further investigate the stability of the antifreezing Ag/AgCl electrodes under such low temperature is to measure their potential versus themselves in the electrolyte of antifreezing test solutions. Thereafter, comparisons of potential differences can be made between room and low temperature and further comparisons also can be made between antifreezing electrodes and common Ag/AgCl electrode both at room and low temperatures, respectively. Tables S1-2 and S1-3 (see supporting information) list the potential differences of c, d and e vs. b, d
29
and e vs. c both at room and low temperatures, respectively. It exhibits that these antifreezing Ag/AgCl electrodes have little potential shifts (less than 11 mV). It is apparent that these antifreezing electrodes are also stable at low temperatures. 3.3. Constancy of the electrode potential of antifreezing Ag/AgCl reference electrodes against the change in KCl concentration When a Ag/AgCl electrode is placed in an aqueous solution, the silver chloride reaches a reversible equilibrium with the solid silver and the chloride ions shown in Eq. (5). At a steady state, the equi-
Fig. 6. (A) CV curves of a glassy carbon against proposed Ag/AgCl electrodes in the electrolyte of 1 M KCl + 0.01 M K3[Fe(CN)6] + 100% H2O at 25 °C. (B) CV curves of a glassy carbon against electrode b in the electrolyte of 1 M KCl + 0.01 M K3[Fe(CN)6] + 32.3% EG + 67.7% H2O, at 25 °C and 20 °C, respectively. (C) CV curves of a glassy carbon against electrode c in the electrolyte of 1 M KCl + 0.01 M K3[Fe(CN)6] + 52.6% EG + 47.4% H2O, at 25 °C and 40 °C, respectively. (D) CV curves of a glassy carbon against electrode d in the electrolyte of 1 M KCl + 0.01 M K3[Fe(CN)6] + 52.6% EG + 47.4% H2O, at 25 °C and 40 °C, respectively. (E) CV curves of a glassy carbon against electrode e in the electrolyte of 1 M KCl + 0.01 M K3[Fe(CN)6] + 52.6% EG + 47.4% H2O, at 25 °C and 40 °C, respectively.
30
J. Yin et al. / Journal of Electroanalytical Chemistry 666 (2012) 25–31
librium potential (E) related to this reversible reaction can be expressed by the Nernst equation [6,20,21]. Under the hypotheses of an ideal solution behavior and a definite temperature (e.g., 25 °C), the Nernst equation of the Ag/AgCl electrode can be simplified such that the electrode potential is associated with the chloride ion concentration via a slope of 59.16 mV/pCl. Consequently, the Ag/AgCl electrode will be theoretically predicted to have potential shifts in the test solutions containing various chloride ion concentrations. Fig. 3 shows potential of antifreezing Ag/AgCl electrodes (a–e) vs. common Ag/AgCl reference electrode at various concentrations of Cl. The potential shift of each electrode is less than 3 mV as the chloride ion concentration in the test solutions is stepwise increased from 0.1 to 3 M. The differences of potential shift between five electrodes are scrambled with increase of EG in internal electrolytes. Nevertheless, the potential differences are less than 10 mV among antifreezing Ag/AgCl electrodes, it does mean that the proportions of EG have minor effects on potential shifts. It is obvious that the electrode potential of proposed antifreezing reference electrode is not influenced by changes in the chloride ion concentration. This is likely because adding EG and solid KCl into internal electrolyte keeps the KCl solution saturated. Moreover, Pt wire used as electrode contact with test solution is sealed at the bottom of glass tube, which prevents serious leakage of KCl from tube into the test solution and the interference ions diffusing into the internal electrolyte. Hence, it can be as standard electrode under solutions of chloride ion concentration changing significantly. 3.4. Polarization An important characteristic of any reference electrode is a reversible and nonpolarizable property. Fig. 4A and C displays the potential changes of antifreezing reference electrodes system during and after a DC 1 s polarization with the current density of 0.002, 0.02 and 0.2 mA cm2, respectively, at room temperature. Table S2-1 (see supporting information) exhibits the time necessary for returning to open circuit potential ±3 mV after 1 s polarization with different current density at room temperature. It implies that the system returns to equilibrium within several seconds after polarization. Low polarization current density leads to smaller potential changes and fast returns to the open circuit potential, high polarization current density results in larger potential changes and slowly returns to the open circuit potential. The antifreezing reference electrodes need more time to return to the equilibrium than that of common Ag/AgCl reference electrode. This can be ascribed
to the fact that the solubility of KCl shown in Table 1 and the conductivity of internal electrolytes shown in Fig. 5 are gradually decreased with increase of EG. According to the literatures, the I–E curve of a cell with two ideal non-polarizable electrodes (working electrode and reference electrode) would like a pure resistance, which embodies a purely resistive load and exhibits their nonpolarizability [9,16,17]. Therefore, the polarization results reveal that the proposed reference electrodes are non-polarizable at room temperature. In addition, to investigate polarization and reversibility of antifreezing reference electrodes at low temperatures, Fig. 4B and D exhibits the potential changes of antifreezing reference electrode system during and after a DC 1 s polarization with the current density of 0.002, 0.02 and 0.2 mA cm2, respectively, at 40 °C. Table S2-2 (see supporting information) lists the time necessary for returning to open circuit potential ±10 mV after 1 s polarization with different current density. It reveals that it will take hundreds of seconds for system to return to the equilibrium owing to the increase of resistance in internal and external electrolytes. However, these electrodes are still non-polarizable. In other words, these types of electrodes have good reversibility and can be as feasible alternatives to develop commercial antifreezing reference electrodes both at room and low temperatures. 3.5. Applications as reference electrode both at room and low temperatures 3.5.1. Application of cyclic voltammetric measurement In order to validate the efficiency of these antifreezing electrodes, a series of typical cyclic voltammogram (CV) experiments were designed and conducted. KCl and K3[Fe(CN)6] were chosen as supporting electrolyte and redox probes, respectively. As shown in Fig. 6A, when the pure water was used as solvent and the antifreezing electrodes were used as reference electrodes, similar symmetric reversible curves and the same potential peaks are displayed at room temperature. These data confirm their availability indirectly. For the antifreezing Ag/AgCl reference electrodes, the shape of each CV curve shown in Fig. 6B–E was similar with one another both at room and low temperatures. Details of cathodic/ anode peak potentials and peak currents distribution of the CV curves are summarized in Table S3 (see supporting information). It denotes that the peak potentials have considerable shift and the peak currents reduce dramatically. The differences of voltammograms between room and low temperatures can be ascribed to the fact that the conductivities of internal electrolytes and test solutions are dramatically decreased at low temperatures.
Fig. 7. The constant-current charge/discharge cycling of low-temperature supercapacitor at 20 °C in the electrolyte of 5 M NaClO4 aqueous solution. Carbon as working electrode, a platinum coil as auxiliary electrode, and the antifreezing Ag/AgCl electrode (b) as reference electrode.
J. Yin et al. / Journal of Electroanalytical Chemistry 666 (2012) 25–31
However, the values of DEp show minor distinctions between room and low temperatures in the same electrolyte. Hence, the proposed electrodes can be used as reference electrode to characterize the electrochemical property of GCE even if at low temperatures. In addition, we can select different reference electrodes according to the temperature we choose. Electrode b and c are preferential to be employed in the range of 0 20 °C due to their approximation to common Ag/AgCl reference electrode. Below 20 °C, electrode d and e are primarily taken into consideration due to their precisions. 3.5.2. Application in low-temperature supercapacitor charge– discharge tests To further investigate the authenticity and stability of these antifreezing electrodes, the constant-current charge/discharge cycling of low-temperature supercapacitor was performed at 20 °C within the potential window 0–0.50 V vs. reference electrode b in the electrolyte of 5 M NaClO4 aqueous solution. As shown in Fig. 7, the linear variation of potential during both charging and discharging processes are observed for carbon electrode (electric double-layer capacitance), which reveals that the antifreezing electrode provides constant potential during the low-temperature electrochemical measurement. 4. Conclusions In summary, this study has successfully fabricated a series of antifreezing Ag/AgCl reference electrodes via adding different proportions of antifreeze additives (Ethylene glycol) into the inner electrolytes of Ag/AgCl electrodes. The electrochemical characterization and application results have shown that the proposed reference electrodes possess negligible potential shifts both at room and low temperatures, implying their common usage in different conditions. In addition, the property of insensitiveness to the concentration variation of Cl ions reveals their excellent stability, non-polarizable and reversible characters testify the possibility as reference electrodes. Furthermore, the practical utilization also proves their credibility as reference electrodes. Coupled with their inexpensiveness, low toxicity, easy manipulation and facile commercialization, promising applications in harsh conditions are envisioned.
31
Acknowledgements We thank Professors Er. Kang. Wang and Shao. Jun. Dong for helpful suggestions. This project was supported by National Basic Research Program of China (2011CB935702), the Foundation of Applied science and technology of Jilin Province (20090521), Scientific Research Foundation for the Returned Overseas Chinese Scholars and State Education Ministry (SRF for ROCS, SEM) and Hundred Talents Program of Chinese Academy of Sciences. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jelechem.2011.11.021. References [1] S. Ito, H. Hachiya, K. Baba, Y. Asano, H. Wada, Talanta 42 (1995) 1685–1690. [2] W.J. Parka, Y. Yi, J. Lee, B.C. Lee, O.K. Park, H.J. Lee, H. Lee, Talanta 81 (2010) 482–485. [3] A.W.J. Cranny, J.K. Atkinson, Meas. Sci. Technol. 9 (1998) 1557–1565. [4] I.Y. Huang, R.S. Huang, L.H. Lo, Sens. Actuators B. 94 (2003) 53–64. [5] T.A. Matsumoto, N. Ohashi, Anal. Chim. Acta. 462 (2002) 253–259. [6] T. Kakiuchi, T. Yoshimatsu, N. Nishi, Anal. Chem. 79 (2007) 7187–7191. [7] G.J. Janz, in: D.J.G. Ives, G.J. Janz (Eds.), Reference Electrodes, Academic Press, New York, 1961. Chapter 4. [8] G.A. Snook, A.S. Best, A.G. Pandolfo, A.F. Hollenkamp, Electrochem. Commun. 8 (2006) 1405–1411. [9] W.Y. Liao, T.C. Chou, Anal. Chem. 78 (2006) 4219–4223. [10] V.R. Gabriela, A.A.R. Giaan, A.G.V. Carlos, Sens. Actuators B. 110 (2005) 264– 270. [11] W. Vonaua, W. Oelbera, U. Guth, J. Henzeb, Sens. Actuators B. 144 (2010) 368– 373. [12] R. Maminska, A. Dybko, W. Wroblewski, Sens. Actuators B. 15 (2006) 552–557. [13] H. Suzuki, T. Hirakawa, S. Sasaki, I. Karube, Sens. Actuators B. 46 (1998) 146– 154. [14] B.J. Polk, A. Stelzenmuller, G. Mijares, W. MacCrehanb, M. Gaitan, Sens. Actuators B. 114 (2006) 239–247. [15] J.S. Lu, J.Y. Zhang, X.M. Yu, X.J. Zhang, Zhen. Jian. Inst. Tech. 2 (1991) 80–83. [16] C.L. Huang, J.J. Ren, D.F. Xu, Chin. J. Anal Chem. 24 (1996) 816–819. [17] Q. Zhan, P. Li, Z.Q. Bai, Appl. Sci. Tech. 32 (2005) 62–63. [18] P. Gao, X. Jin, D. Wang, X. Hu, G. Chen, J. Electroanal. Chem. 579 (2005) 321– 328. [19] L.L. Liu, C.J. Chen, Y.Q. Liu, H.Q. Li, OGST 21 (2002) 33–35. [20] A.J. Bard, L.R. Faulkner, Electrochemical Methods, 2nd ed., Wiley, New York, 2002. [21] R. Geely, J. Phys. Chem. 64 (1960) 652.
Contents lists available at SciVerse ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Antifreezing Ag/AgCl reference electrodes: Fabrication and applications Jiao Yin a,b, Li Qi a, Hongyu Wang a,⇑ a b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China Graduate University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 6 August 2011 Received in revised form 15 November 2011 Accepted 18 November 2011 Available online 26 November 2011 Keywords: Antifreezing Ag/AgCl Reference electrodes Fabrication Applications
a b s t r a c t As a promising antifreeze, Ethylene glycol (EG) was employed for preparation of new types of antifreezing Ag/AgCl reference electrodes. Different proportions of EG were used as antifreezing additives in the inner electrolyte of Ag/AgCl reference electrode. The potential shifts of the resultant reference electrodes are less than 10 mV in the wide range of temperatures (25 40 °C), which extends their application at low temperatures. In addition, the proposed antifreezing electrodes maintain non-polarizable and reversible nature of the initial Ag/AgCl ones even in low-temperature environment. Finally, to ensure their stability at low temperatures, the antifreezing reference electrodes were applied in various low-temperature electrochemical systems. Ó 2011 Elsevier B.V. All rights reserved.
to the changes of temperature. As is well-known that for Ag/AgCl (KCl aq):
1. Introduction Reference electrodes, as indicator electrodes, are indispensable components of electrochemical systems for the reason that they can provide constant potential during the electrochemical measurement. There are mainly three kinds of common reference electrodes: normal hydrogen electrode (NHE), saturated calomel (Hg/Hg2Cl2) electrode (SCE) and silver/silver chloride (Ag/AgCl) electrode. In practice, the Ag/AgCl electrodes are preferably applied, because their potential is stable and well defined in relation to the potential of the NHE. Furthermore, their easy manipulation, green chemistry, and safety are convenient for the practical application. Such electrodes are commercially available in a variety of chemical systems such as sensors [1,2] and other related fields. Over the years, various Ag/AgCl reference electrodes based on traditional Ag/AgCl/aq.KCl have been proposed. For example, thick film Ag/AgCl reference electrodes have been fabricated by Cranny and Atkinson [3,4]. Matsumoto and Ohashi [5] have also developed micro-planar Ag/AgCl quasi-reference electrode for biosensor. In addition, the use of room temperature ionic liquids (RTILs) as electrolytes has promoted the development of Ag/AgCl (RTILs) reference electrodes [6–8]. Moreover, to meet the technical demands, solid state [9–12], micro [13] and micromachined [14] Ag/AgCl reference electrodes have also been constructed. All the Ag/AgCl reference electrodes above mentioned could be applied at room temperature. Nevertheless, the electrode potential is also sensitive ⇑ Corresponding author. Tel./fax: +86 431 85262287. E-mail Wang).
addresses:
[email protected],
[email protected]
1572-6657/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.11.021
(H.
ET ¼ ET0 RT=F ln aCl
ð1Þ
ET0 ¼ 0:23735 5:3783 104 t 2:3728 106 t2
ð2Þ
It does mean that the electrode potential has some definite relationship with temperatures and the ET0 can be calculated according to the temperature (t P 25 °C). However, the equation can only be applied at the temperature interval of t P 25 °C. Furthermore, the electrode potential of Ag/AgCl (aCl aq ¼ 1) vs. NHE at the temperature interval of 25 100 °C have been investigated and listed according to both experimental measurements and the theoretical calculation based on the above mentioned equations [15,16]. By contrast, higher temperature Ag/AgCl reference electrodes have also been fabricated to provide potential reference for hightemperature electrochemical system [17,18]. Similarly, to meet the industrial demand, Liu et al. [19] have developed antifreezing Cu/CuSO4 reference electrode suited to frozen soil determination. However, this type of antifreezing reference electrode is bulky and only suitable for engineering application. In addition, the antifreezing Cu/CuSO4 reference electrode has larger potential shift (more than 100 mV) in cold environment (2 °C) so that it cannot be applied in some low-temperature systems which require excellent stability or sensitive change of potential. Moreover, the electrochemical properties of antifreezing Cu/CuSO4 reference electrode were not fully investigated in Liu’s literature. Nevertheless, as a common reference electrode, little attention has been paid to studying the electrochemical properties of the traditional Ag/AgCl (KCl aq) or other new types of Ag/AgCl electrodes
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electrodes and working electrodes, respectively, below 10 °C. The conventional three-electrode system comprised a glassy carbon electrode (GCE, diameter, 3 mm, working electrode), a platinum flag counter electrode, and above mentioned antifreezing Ag/AgCl reference electrode, respectively. The antifreezing Ag/AgCl electrode (b) was taken as reference electrode for charge–discharge tests of low-temperature supercapacitor. The galvanostatic charge–discharge cycles of low-temperature supercapcitor were measured by a Land cell tester. The electrolyte of low-temperature supercapcitor was 5 M NaClO4 aqueous solution.
at low temperatures (t 6 0 °C). More importantly, it is sometimes necessary to use reference electrodes in low-temperature electrochemical experiments. Unfortunately, the internal electrolyte of Ag/AgCl (pure KCl aq) electrode will freeze at low temperatures (t 6 0 °C). Consequently, the reference electrodes employed for room temperature cannot be used in low-temperature environment. Furthermore, the traditional Ag/AgCl/aq.KCl cannot be employed for the low-temperature measurement due to significant potential fluctuation caused by freezing of internal electrolyte. Bearing the above situations in mind, we try to find suitable antifreeze which cannot only dissolve a certain amount of salt but also mix with the water well to lower the freezing point. It is well-known that polyhydric alcohol is often used as antifreeze in industry and miscible with water to get a homogeneous solution. Inspired by this, different proportions of Ethylene glycol (EG) were selected as antifreezing additives for internal electrolyte of Ag/AgCl electrode. The electrochemical properties of antifreezing Ag/AgCl electrodes both at room and low temperatures were investigated in detail, respectively. More importantly, these antifreezing Ag/ AgCl reference electrodes were applied in different low-temperature electrochemical systems.
3. Results and discussions 3.1. The fabrication process and structure of antifreezing Ag/AgCl reference electrode The fabrication process of Ag/AgCl electrodes. Fig. 1A shows a sketch of antifreezing Ag/AgCl electrode. The main fabrication process can be summarized as follows. The as-prepared Ag/AgCl electrode was inserted into glass tube which was filled with antifreezing electrolytes (containing EG, water and KCl). Pt wire at the bottom of glass tube was used as electrode-contact with testing solution. To keep the KCl saturated in the electrolyte, some solid KCl was placed at the bottom of glass tube. Finally, the electrode was sealed by parafilm and preserved in saturated KCl aqueous solution. The photograph of homemade Ag/AgCl reference electrode is shown in Fig. 1B. When the Ag/AgCl reference electrodes were put in low-temperature environment (20 °C), the electrolyte of common Ag/AgCl reference electrode was frozen in Fig. 1C. Nevertheless, the antifreezing Ag/AgCl reference electrode showed no obvious changes by appearances.
2. Experimental section 2.1. Materials and reagents Silver wires were obtained from National Chemical Reagent Company (Shanghai, China). Other chemicals (including Ethylene glycol, Potassium chloride, Potassium hexacyanoferrate and Sodium perchloride) were acquired from Beijing Chemical Factory (Beijing, China). Commercial Ag/AgCl electrode (saturated KCl aqueous solution) was indicated as common Ag/AgCl electrode in the following study. All the chemicals were of analytical grade and were used as received. The solutions in the present study were made using deionized water (Millipore Water System) with a resistance of 18.3 MX.
3.2. Evaluation of antifreezing Ag/AgCl reference electrodes Measurement of potential differences vs. common Ag/AgCl both at room and low temperatures. It is essential to evaluate the electrode potential with respect to those of existent reference electrodes for a new class of proposed reference electrode. The evaluation of those antifreezing Ag/AgCl electrodes at room temperature (25 °C) can be easily made through the results in Table S1 (see supporting information) using a value of E at a concentration of of 1 mol L1 KCl aq (test solution). The cell voltage of two electrodes, referred to by the following cell (3), E was measured with a CHI 660C electrochemical workstation. E values of a–e are 0.1, 2.8, 4.8, 5.2 and 6.7 mV vs. common Ag/ AgCl, respectively. Because the electrode potential of common Ag/
2.2. Fabrication of antifreezing Ag/AgCl reference electrodes Ag/AgCl electrodes were prepared by anodizing 0.5 mm diameter silver wires in diluted HCl aqueous solutions. The internal electrolytes of Ag/AgCl electrodes comprised EG, water and KCl. The solute of KCl dissolved in the following solvents (different proportions of EG and water) were the maximal concentrations of KCl solutions. The reference electrodes were named as a–e according to above different proportions of EG and water in internal electrolytes (shown in Table 1).
I
II
III
IV
V
VI
VII
Ag
AgCl
n1 wt%EG þ n2 wt%H2 O þ n3 MKCl
1MKCl
saturated KClaq
AgCl
Ag
fworkingelectrodeg
ftestsolutiong
2.3. Electrochemical measurments The electrochemical experiments were performed using a CHI 660 C electrochemical workstation (CH Instruments, Inc., Shanghai) and carried out using two-electrode and three-electrode systems, respectively. The two-electrode system consisted of a common Ag/AgCl reference electrode and the proposed Ag/AgCl working electrodes, respectively, above 10 °C. The proposed antifreezing Ag/AgCl electrodes were designated as reference
ð3Þ
freferenceelectrodeg
AgCl is 199 mV vs. NHE at 25 °C, the potential of those antifreezing Ag/AgCl electrodes with respect to NHE is estimated by adding 2.8, 4.8, 5.2 and 6.7 to 199 mV at 25 °C [7]. Then, the values of the electrode potential of antifreezing Ag/AgCl electrodes referred to as NHE are estimated to be 201.8, 203.8, 204.2 and 205.7 mV, respectively, at 25 °C. This is a rough estimation of electrode potential vs. NHE because the low liquid junction potential still exists. As a result, further investigations are required to accurately evaluate their electrode potential.
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Table 1 Internal electrolytes with different proportions of EG and water at 25 °C. Electrodes
x wt.% EG
y wt.% H2O
CKCl (maximal concentrations dissolved in mixtures of EG and water)
a b c d e
0 32.3 52.6 72.2 100
100 67.7 47.4 27.8 0
Saturated 3M 2M 1M 0.5 M
Furthermore, the content of EG has minor effect on the electrode potential of antifreezing Ag/AgCl electrode shown in Table S1-1 (see supporting information). The electrode potential between antifreezing Ag/AgCl electrode and common Ag/AgCl electrode is elevated with the increase of EG. This could be ascribed to the increase of phase-boundary potential between solid Ag/AgCl and liquid EG shown in cell (3) and Eq. (4). The phase-boundary potential between phase II and III in cell (3), /EG /AgCl could be formally expressed in terms of the transfer Gibbs energies of Ag+ and Cl ions shown in Eqs. (5) and (6) between the phases II and III, and:
DGEG!AgCl and DGEG!AgCl tr;Cl tr;Agþ /EG /AgCl ¼
i 1 h EG!AgCl DGtr;Agþ DGEG!AgCl tr;Cl 2F
AgClAgþ þ Cl
ð4Þ ð5Þ
AgClðsÞ þ e AgðsÞ þ ClðEGÞ
ð6Þ
However, the Ag/AgCl electrode is in contact with EG (containing Cl), there are common ions (Ag+, Cl) between phases II and III in cell (3). That is to say, the distribution of ionic species, primarily Cl, assures the thermodynamic equilibrium between the two phases (AgCl and EG), which leads to low phase-boundary potential even in the internal electrolyte of pure EG [12]. Anyway, the electrode potential of antifreezing Ag/AgCl electrodes still has slight deviation from the standard electrode potential of common Ag/AgCl at 25 °C due to the presence of phase-boundary potential. In the following study, the electrode potential at lower temperatures has been investigated. The electrode potential of antifreezing Ag/AgCl electrode vs. common Ag/AgCl at 5 °C has little shift with respect to that at 25 °C shown in Table S1-1 (see supporting information). When the temperature is lowered at 10 °C, the potential has bigger shift than that of temperature at 25 and 5 °C, respectively. With continuous decrease of tempera-
Fig. 2. Open-circuit-potential (OCP) of (A) common Ag/AgCl vs. common Ag/AgCl and (B) antifreezing Ag/AgCl vs. antifreezing Ag/AgCl (e vs. b) as a function of time in antifreezing electrolyte (52.6% EG + 47.4% H2O + 1 M KCl) at 20 °C, respectively.
ture, the internal electrolyte of common Ag/AgCl electrode froze at 20 °C shown in Fig. 1C, the electrode potential (antifreezing Ag/ AgCl electrodes vs. common Ag/AgCl) becomes rather unstable and has considerable shift. In addition, the pure KCl aqueous solution (1 M) cannot be used as test solution below 10 °C due to the formation of freezing coagulations. To measure the electrode potential below 10 °C, certain amounts of EG have been added into test solutions. To find out the reasons for potential shift, Fig. 2 shows open-circuit-potential (OCP) of two frozen common Ag/AgCl electrodes and two antifreezing Ag/AgCl electrodes (e vs. b) as a function of time in antifreezing test solution at 20 °C, respectively. During the measurement period, the potential shift of two antifreezing Ag/ AgCl reference electrodes was 1 mV. The frozen common Ag/AgCl electrodes have considerable potential fluctuation. This may be due to the fact that the internal electrolyte becomes solid and the Ag/AgCl cannot obtain normal response from Cl for the reason that ions cannot transfer smoothly in soild phase. Consequently, it cannot provide normal electrode potential. However, the antifreezing additives keep the internal electrolyte liquid so that AgCl still can achieve a reversible equilibrium with the solid silver and the chloride ions shown in Eq. (6). As a result, the Ag/AgCl has normal response from Cl at low temperatures. Equally, the other reference electrodes such as normal hydrogen electrode (NHE), saturated calomel electrode (SCE) have the same problems with common Ag/AgCl. The internal electrolytes of NHE and SCE will also become frozen at low temperature. Therefore, NHE and SCE
Fig. 1. (A) Diagrammatic sketch of proposed reference electrode, photograph of (B) antifreezing Ag/AgCl reference electrode, and (C) common Ag/AgCl electrode at 20 °C.
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Fig. 3. The sensitivity to changes of Cl concentration (0.1–3 M) for antifreezing electrodes (a–e) vs. common Ag/AgCl at 25 °C.
Fig. 5. Conductivity of internal electrolytes of various electrodes (a–e) saturated with maximal concentrations of KCl at 25 °C.
Fig. 4. Dependence of potential of the antifreezing reference electrodes on the polarization current (A) anodic and (C) cathodic at 25 °C, test solution: 1 M KCl aq, polarization time: 1 s, reference electrode: common Ag/AgCl electrode; polarization current (B) anodic and (D) cathodic at 40 °C, test solution: 52.6% EG + 47.4% H2O + 1 M KCl, polarization time: 1 s, reference electrode: electrode (e).
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cannot be taken as reference electrodes for electrochemical determination under lower temperatures. Faced up with this unsatisfactory situation, a favorable way to further investigate the stability of the antifreezing Ag/AgCl electrodes under such low temperature is to measure their potential versus themselves in the electrolyte of antifreezing test solutions. Thereafter, comparisons of potential differences can be made between room and low temperature and further comparisons also can be made between antifreezing electrodes and common Ag/AgCl electrode both at room and low temperatures, respectively. Tables S1-2 and S1-3 (see supporting information) list the potential differences of c, d and e vs. b, d
29
and e vs. c both at room and low temperatures, respectively. It exhibits that these antifreezing Ag/AgCl electrodes have little potential shifts (less than 11 mV). It is apparent that these antifreezing electrodes are also stable at low temperatures. 3.3. Constancy of the electrode potential of antifreezing Ag/AgCl reference electrodes against the change in KCl concentration When a Ag/AgCl electrode is placed in an aqueous solution, the silver chloride reaches a reversible equilibrium with the solid silver and the chloride ions shown in Eq. (5). At a steady state, the equi-
Fig. 6. (A) CV curves of a glassy carbon against proposed Ag/AgCl electrodes in the electrolyte of 1 M KCl + 0.01 M K3[Fe(CN)6] + 100% H2O at 25 °C. (B) CV curves of a glassy carbon against electrode b in the electrolyte of 1 M KCl + 0.01 M K3[Fe(CN)6] + 32.3% EG + 67.7% H2O, at 25 °C and 20 °C, respectively. (C) CV curves of a glassy carbon against electrode c in the electrolyte of 1 M KCl + 0.01 M K3[Fe(CN)6] + 52.6% EG + 47.4% H2O, at 25 °C and 40 °C, respectively. (D) CV curves of a glassy carbon against electrode d in the electrolyte of 1 M KCl + 0.01 M K3[Fe(CN)6] + 52.6% EG + 47.4% H2O, at 25 °C and 40 °C, respectively. (E) CV curves of a glassy carbon against electrode e in the electrolyte of 1 M KCl + 0.01 M K3[Fe(CN)6] + 52.6% EG + 47.4% H2O, at 25 °C and 40 °C, respectively.
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librium potential (E) related to this reversible reaction can be expressed by the Nernst equation [6,20,21]. Under the hypotheses of an ideal solution behavior and a definite temperature (e.g., 25 °C), the Nernst equation of the Ag/AgCl electrode can be simplified such that the electrode potential is associated with the chloride ion concentration via a slope of 59.16 mV/pCl. Consequently, the Ag/AgCl electrode will be theoretically predicted to have potential shifts in the test solutions containing various chloride ion concentrations. Fig. 3 shows potential of antifreezing Ag/AgCl electrodes (a–e) vs. common Ag/AgCl reference electrode at various concentrations of Cl. The potential shift of each electrode is less than 3 mV as the chloride ion concentration in the test solutions is stepwise increased from 0.1 to 3 M. The differences of potential shift between five electrodes are scrambled with increase of EG in internal electrolytes. Nevertheless, the potential differences are less than 10 mV among antifreezing Ag/AgCl electrodes, it does mean that the proportions of EG have minor effects on potential shifts. It is obvious that the electrode potential of proposed antifreezing reference electrode is not influenced by changes in the chloride ion concentration. This is likely because adding EG and solid KCl into internal electrolyte keeps the KCl solution saturated. Moreover, Pt wire used as electrode contact with test solution is sealed at the bottom of glass tube, which prevents serious leakage of KCl from tube into the test solution and the interference ions diffusing into the internal electrolyte. Hence, it can be as standard electrode under solutions of chloride ion concentration changing significantly. 3.4. Polarization An important characteristic of any reference electrode is a reversible and nonpolarizable property. Fig. 4A and C displays the potential changes of antifreezing reference electrodes system during and after a DC 1 s polarization with the current density of 0.002, 0.02 and 0.2 mA cm2, respectively, at room temperature. Table S2-1 (see supporting information) exhibits the time necessary for returning to open circuit potential ±3 mV after 1 s polarization with different current density at room temperature. It implies that the system returns to equilibrium within several seconds after polarization. Low polarization current density leads to smaller potential changes and fast returns to the open circuit potential, high polarization current density results in larger potential changes and slowly returns to the open circuit potential. The antifreezing reference electrodes need more time to return to the equilibrium than that of common Ag/AgCl reference electrode. This can be ascribed
to the fact that the solubility of KCl shown in Table 1 and the conductivity of internal electrolytes shown in Fig. 5 are gradually decreased with increase of EG. According to the literatures, the I–E curve of a cell with two ideal non-polarizable electrodes (working electrode and reference electrode) would like a pure resistance, which embodies a purely resistive load and exhibits their nonpolarizability [9,16,17]. Therefore, the polarization results reveal that the proposed reference electrodes are non-polarizable at room temperature. In addition, to investigate polarization and reversibility of antifreezing reference electrodes at low temperatures, Fig. 4B and D exhibits the potential changes of antifreezing reference electrode system during and after a DC 1 s polarization with the current density of 0.002, 0.02 and 0.2 mA cm2, respectively, at 40 °C. Table S2-2 (see supporting information) lists the time necessary for returning to open circuit potential ±10 mV after 1 s polarization with different current density. It reveals that it will take hundreds of seconds for system to return to the equilibrium owing to the increase of resistance in internal and external electrolytes. However, these electrodes are still non-polarizable. In other words, these types of electrodes have good reversibility and can be as feasible alternatives to develop commercial antifreezing reference electrodes both at room and low temperatures. 3.5. Applications as reference electrode both at room and low temperatures 3.5.1. Application of cyclic voltammetric measurement In order to validate the efficiency of these antifreezing electrodes, a series of typical cyclic voltammogram (CV) experiments were designed and conducted. KCl and K3[Fe(CN)6] were chosen as supporting electrolyte and redox probes, respectively. As shown in Fig. 6A, when the pure water was used as solvent and the antifreezing electrodes were used as reference electrodes, similar symmetric reversible curves and the same potential peaks are displayed at room temperature. These data confirm their availability indirectly. For the antifreezing Ag/AgCl reference electrodes, the shape of each CV curve shown in Fig. 6B–E was similar with one another both at room and low temperatures. Details of cathodic/ anode peak potentials and peak currents distribution of the CV curves are summarized in Table S3 (see supporting information). It denotes that the peak potentials have considerable shift and the peak currents reduce dramatically. The differences of voltammograms between room and low temperatures can be ascribed to the fact that the conductivities of internal electrolytes and test solutions are dramatically decreased at low temperatures.
Fig. 7. The constant-current charge/discharge cycling of low-temperature supercapacitor at 20 °C in the electrolyte of 5 M NaClO4 aqueous solution. Carbon as working electrode, a platinum coil as auxiliary electrode, and the antifreezing Ag/AgCl electrode (b) as reference electrode.
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However, the values of DEp show minor distinctions between room and low temperatures in the same electrolyte. Hence, the proposed electrodes can be used as reference electrode to characterize the electrochemical property of GCE even if at low temperatures. In addition, we can select different reference electrodes according to the temperature we choose. Electrode b and c are preferential to be employed in the range of 0 20 °C due to their approximation to common Ag/AgCl reference electrode. Below 20 °C, electrode d and e are primarily taken into consideration due to their precisions. 3.5.2. Application in low-temperature supercapacitor charge– discharge tests To further investigate the authenticity and stability of these antifreezing electrodes, the constant-current charge/discharge cycling of low-temperature supercapacitor was performed at 20 °C within the potential window 0–0.50 V vs. reference electrode b in the electrolyte of 5 M NaClO4 aqueous solution. As shown in Fig. 7, the linear variation of potential during both charging and discharging processes are observed for carbon electrode (electric double-layer capacitance), which reveals that the antifreezing electrode provides constant potential during the low-temperature electrochemical measurement. 4. Conclusions In summary, this study has successfully fabricated a series of antifreezing Ag/AgCl reference electrodes via adding different proportions of antifreeze additives (Ethylene glycol) into the inner electrolytes of Ag/AgCl electrodes. The electrochemical characterization and application results have shown that the proposed reference electrodes possess negligible potential shifts both at room and low temperatures, implying their common usage in different conditions. In addition, the property of insensitiveness to the concentration variation of Cl ions reveals their excellent stability, non-polarizable and reversible characters testify the possibility as reference electrodes. Furthermore, the practical utilization also proves their credibility as reference electrodes. Coupled with their inexpensiveness, low toxicity, easy manipulation and facile commercialization, promising applications in harsh conditions are envisioned.
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Acknowledgements We thank Professors Er. Kang. Wang and Shao. Jun. Dong for helpful suggestions. This project was supported by National Basic Research Program of China (2011CB935702), the Foundation of Applied science and technology of Jilin Province (20090521), Scientific Research Foundation for the Returned Overseas Chinese Scholars and State Education Ministry (SRF for ROCS, SEM) and Hundred Talents Program of Chinese Academy of Sciences. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jelechem.2011.11.021. References [1] S. Ito, H. Hachiya, K. Baba, Y. Asano, H. Wada, Talanta 42 (1995) 1685–1690. [2] W.J. Parka, Y. Yi, J. Lee, B.C. Lee, O.K. Park, H.J. Lee, H. Lee, Talanta 81 (2010) 482–485. [3] A.W.J. Cranny, J.K. Atkinson, Meas. Sci. Technol. 9 (1998) 1557–1565. [4] I.Y. Huang, R.S. Huang, L.H. Lo, Sens. Actuators B. 94 (2003) 53–64. [5] T.A. Matsumoto, N. Ohashi, Anal. Chim. Acta. 462 (2002) 253–259. [6] T. Kakiuchi, T. Yoshimatsu, N. Nishi, Anal. Chem. 79 (2007) 7187–7191. [7] G.J. Janz, in: D.J.G. Ives, G.J. Janz (Eds.), Reference Electrodes, Academic Press, New York, 1961. Chapter 4. [8] G.A. Snook, A.S. Best, A.G. Pandolfo, A.F. Hollenkamp, Electrochem. Commun. 8 (2006) 1405–1411. [9] W.Y. Liao, T.C. Chou, Anal. Chem. 78 (2006) 4219–4223. [10] V.R. Gabriela, A.A.R. Giaan, A.G.V. Carlos, Sens. Actuators B. 110 (2005) 264– 270. [11] W. Vonaua, W. Oelbera, U. Guth, J. Henzeb, Sens. Actuators B. 144 (2010) 368– 373. [12] R. Maminska, A. Dybko, W. Wroblewski, Sens. Actuators B. 15 (2006) 552–557. [13] H. Suzuki, T. Hirakawa, S. Sasaki, I. Karube, Sens. Actuators B. 46 (1998) 146– 154. [14] B.J. Polk, A. Stelzenmuller, G. Mijares, W. MacCrehanb, M. Gaitan, Sens. Actuators B. 114 (2006) 239–247. [15] J.S. Lu, J.Y. Zhang, X.M. Yu, X.J. Zhang, Zhen. Jian. Inst. Tech. 2 (1991) 80–83. [16] C.L. Huang, J.J. Ren, D.F. Xu, Chin. J. Anal Chem. 24 (1996) 816–819. [17] Q. Zhan, P. Li, Z.Q. Bai, Appl. Sci. Tech. 32 (2005) 62–63. [18] P. Gao, X. Jin, D. Wang, X. Hu, G. Chen, J. Electroanal. Chem. 579 (2005) 321– 328. [19] L.L. Liu, C.J. Chen, Y.Q. Liu, H.Q. Li, OGST 21 (2002) 33–35. [20] A.J. Bard, L.R. Faulkner, Electrochemical Methods, 2nd ed., Wiley, New York, 2002. [21] R. Geely, J. Phys. Chem. 64 (1960) 652.