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In Situ Techniques for Monitoring Electrochromism An Advanced Laboratory Experiment Hakan Saric-ayir and Musa Uce Chemistry Department, Ataturk Faculty of Education, Marmara University, 34722 Kadkoy, Istanbul, Turkey Atf Koca* Chemical Engineering Department, Engineering Faculty, Marmara University, 34722 Kadkoy, Istanbul, Turkey *
[email protected] The most common in situ technique used in science laboratories is in situ spectroelectrochemistry, which focuses primarily on the changes of transmission or absorption spectra during the electrochemical perturbations of a system. Published practical examples teaching the principles and applications of in situ spectroelectrochemistry are limited (1, 2). Heineman (1) developed several commonly used spectroelectrochemical methods with examples of typical applications. He also developed an electrochemical experiment describing analysis of an optically transparent thin-layer electrode with a spectroelectrochemistry technique (3). Similarly, there are few publications on the teaching of the application of spectroelectrochemistry for optical and electrochemical characterization of different electrochromic materials (4-6). Color changes in these studies were observed by the naked eye or proposed by spectral changes. It has been suggested that to describe a particular color, its scientific parameters must be identified because full-color characterization of an electrochromic system is essential for its application. For this purpose, an instrumental colorimetric measurement must be applied to record the chromaticity diagram and color parameters of a system. The combination of reaction-oriented electrochemistry with colorimetry in in situ electrocolorimetry allows for a more complete analysis of electrochromic materials. While the technique has been well developed during the last few decades, its application in various fields of chemistry has only recently become more widespread (7-9). There is no published practical example of teaching the principles and applications of in situ electrocolorimetry techniques. Thus, we developed an experiment that characterizes the electrochromism of methyl viologen, extensively investigated as a material for digital displays and smart windows applications (10, 11). In this experimental procedure, we describe the principles and applications of in situ electrocolorimetry combined with in situ spectroelectrochemistry. This experiment, accomplished in a 5 h laboratory period, is one of a series of graduating-project experiments assigned to fourth-year undergraduates in the chemical engineering department. Each fourth-year undergraduate of our department is required to prepare an experimental graduating project consisting of advanced research-like experiments to improve his or her literature skill, independent study ability, and understanding of
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basic scientific research prior to the graduation-thesis program. Depending on class size, available resources, and run-times, this experiment can be performed as a demonstration experiment or an experiment in the instrumental analysis or physical chemistry lab. Experimental Details Materials, Chemicals, and Instrumentation Indium tin oxide (ITO)-coated quartz slides were purchased from SPI Supplies and Structure Probe, Inc. (catalog no. 06445-CF). Methyl viologen dichloride hydrate (MV; 1,10 dimethyl-4,40 -bipyridinium dichloride), nafion perfluorinated solution (5 wt % in lower aliphatic alcohols and water, contains 15-20% water), NaCl, LiCl, acetonitrile, tetrabutylammonium perchlorate, propylene carbonate, LiClO4, and polymethyl methacrylate were purchased from Sigma-Aldrich, Inc. Electrochemical coating, in situ spectroelectrochemistry, and in situ electrocolorimetry measurements were performed with three-electrode cells using a computerized potentiostat-galvanostat from Gamry Inc. (model reference 600) combined with an OceanOptics QE65000 spectrophotometer. The working electrodes were MV-coated ITO, the auxiliary electrode was a platinum wire, and the reference electrode was an Ag/ AgCl. During the electrode-modification process, the ITO electrode was coated with the nafion perfluorinated solution and then MV was incorporated into Nafion electrochemically. An aqueous solution of 5 mM MV and 0.2 M LiCl was used as MVcoating solution. Two ITO slides (2.5-7.5 cm) were used for electrochromic display devices (ECD). One of them was coated with MV with the same procedure described above. The other one was coated with MV as the letters “J C E”. Hazards Methyl viologen dichloride hydrate (MV) is toxic by inhalation and ingestion and if absorbed through skin is a possible carcinogen. Nafion perfluorinated solution is flammable and harmful. Acetonitrile is a flammable liquid and causes eye, skin, and respiratory tract irritation. Tetrabutylammonium perchlorate is a strong oxidizer, causes irritation, and may be harmful
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 2 February 2010 10.1021/ed800048u Published on Web 01/12/2010
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Figure 2. In situ spectroelectrochemical responses of MV20 film recorded in MV-free 0.20 M LiCl aqueous solution. (A) Symmetric pulsed potential waveform with E1 = 1.0 V, τ1 = 50 s, E2 = -1.25 V, τ2 = 50 s and current responses for testing electrochromic stability of MV20 film. (B) Reflectance responses (at 391 nm vs time) of the MV20 film to the symmetric pulsed potential waveform.
Figure 1. In situ electrocolorimetric responses of MV20 film recorded in MV-free 0.20 M LiCl aqueous solution. (A) Pulsed potential waveform and current responses for testing electrochromic switching in the MV20 film. (B) Reflectance (% R) responses of the MV20 film to the pulsed potential waveform. (C) Chromaticity diagram representing color changes during applied pulsed potential waveform (a: bleaching coordinates and b: coloring coordinates).
if swallowed. Students should practice caution while performing this experiment, and wastes should be collected for disposal by a suitable authority. Results and Discussion This experiment is important to instructors who want to encourage students' interest in the science, technology, and engineering research fields as the lab creates a real-world instrumental application in analyzing material properties. Electrochromic films of MV were synthesized by electrochemical deposition on an ITO electrode modified with nafion polymer matrix by repeating cyclic voltammetry. To investigate the concentration effect of MV incorporated into the nafion, two different films were prepared. The film coated with 20 cycles is abbreviated as MV20 and the film coated with 50 cycles as MV50. (The results for the MV50 film are given in the supporting material.) 206
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Figure 3. Construction of the electrochromic display device (ECD) used for electrochemical writing of the letters “J C E”.
The spectroelectrochemical properties of the MV films were studied by applying potentials between -1.25 and þ1.0 V in MV-free 0.2 M NaCl aqueous solution. Reflectance (% R) changes during bleaching and during coloring were recorded. The high reflectance between 420 and 550 nm justified the blue color of the MV film. Three features of color;hue, saturation, and luminance;and the coordinate of the color (Yxy color space) of an electrochromic material were defined accurately by performing in situ electrocolorimetry measurements. The chromaticity diagram, reflectance changes, and current changes recorded during coloring the film are given in Figure 1. The electrochromic switching efficiency and stability of MV films were tested by performing repetitive potential step experiments. The current and reflectance (at 391 nm) responses of MV film versus time to a potential waveform with coloration potential -1.25 V and bleaching potential þ1.0 V are presented in Figure 2. Film stability and switching efficiency (coloring and bleaching times) were measured from the changes in current and reflectance recorded under -1.25 V and þ1.0 V pulses. Finally in this experimental study, a model ECD was designed to write “J C E” electrochemically. The ECD was built by arranging two ITO glasses coated with electrochromic MV films (one was coated as “J C E” letters and used as cathode)
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facing each other and separated by a gel electrolyte. The construction of the device is shown in Figure 3. When -2.5 V was applied, the letters “J C E” were written on the devise. The optical memory of an electrochromic material is defined as the time during which this material retains its color without applying potential. The ECD was polarized in the blue state by an applied -2.5 V potential for 5 s and then kept under open-circuit conditions for 60 s. The device was able to remember its color for 35 s. Conclusions This in situ spectroelectrochemical and in situ electrocolorimetric experiment with simultaneous recording of voltammetric, spectroscopic, and colorimetric characteristics of electrochromic MV films is an excellent experimental experience for students working in an instrumental analysis laboratory or physical chemistry laboratory. The experiment is organized to solve problems of many spectroscopic and electrochemical topics with perspective applications of these new in situ instrumental techniques. Students responded positively to this experiment because it integrates some organic, inorganic, and electroanalytical chemistry concepts with engineering research. The scope of this laboratory can be expanded into the study of (i) other common electrochromic materials (WO3, aniline, thiophene), (ii) other supporting electrolytes (HCl, NaCl, KCl, and so forth), (iii) the use of other electrode-modifying techniques, and (iv) other polymeric matrixes for the improvements in the modified-electrode preparation.
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Acknowledgment Financial support of this work is gratefully acknowledged : from the TUB ITAK (project no. MAG-106M286). Literature Cited 1. Heineman, W. R. J. Chem. Educ. 1983, 60, 305. 2. Plieth, W.; Wilson, G. S.; Gutierrez De La Fe, C. Pure Appl. Chem. 1998, 70, 1395. 3. DeAngelis, T. P.; Heineman, W. R. J. Chem. Educ. 1976, 53, 594. 4. Lawrence, D. J.; Stenger, J. G. Fabrication of Electrochromic Devices in an Undergraduate Laboratory. In Proceedings of the Fourteenth Biennial University/Government/Industry Microelectronics Symposium; Virginia Commonwealth University, Richmond, VA, Jun 17-20, 2001; DOI:10.1109/UGIM.2001.96030. 5. Hepel, M. J. Chem. Educ. 2008, 85, 125. 6. Forslund, B. J. Chem. Educ. 1997, 74, 962. 7. Pinheiro, C.; Parola, A. J.; Pina, F.; Fonseca, J.; Freire, C. Sol. Energ. Mater. Sol. Cell. 2008, 92, 980. 8. Bodrogi, P. Displays 2003, 24, 39. 9. Koca, A.; Bayar, S-.; Dinc-er, H. A.; Gonca, E. Electrochim. Acta 2009, 54, 2684-2692; DOI:10.1016/j.electacta.2008.11.028) 10. Mortimer, R. J.; Reynolds, J. R. Displays 2008, 29, 424. 11. Somani, P. R.; Radhakrishnan, S. Mater. Chem. Phys. 2002, 77, 117.
Supporting Information Available Instructor notes; experimental results; student handout. This material is available via the Internet at http://pubs.acs.org.
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