Laboratory Experiment pubs.acs.org/jchemeduc
An Inexpensive Device for Studying Electrochromism Jorge G. Ibanez,*,† Rodrigo Puente-Caballero,† Jonatan Torres-Perez,† Daniel Bustos,† Aranzazu Carmona-Orbezo,† and Fortunato B. Sevilla, III‡ †
Centro Mexicano de Quimica Verde y Microescala, Departamento de Ingenieria y Ciencias Quimicas, Universidad Iberoamericana, Prol. Reforma 880, 01219 Mexico, D.F. Mexico ‡ School of Sciences, University of Santo Tomas, Espana Boulevard, Manila 1015, Philippines S Supporting Information *
ABSTRACT: A novel procedure for the preparation of electrochromic WO3 films from readily available materials is presented. It is based on the electrochemical preparation of potassium tungstate from tungsten filaments of incandescent light bulbs in a potassium hydroxide solution. Tungstic acid is then produced by proton exchange using a superabsorbing polymer from a baby diaper. Simple air drying of the tungstic acid produces a tungsten(VI) oxide layer. On the surface of a copper foil or coupon or of a conducting glass, an electrochromic bleached (colorless)-to-colored (blue) transition of this oxide layer is easily observed by applying an external potential with a common power source in an electrochemical cell where the Cu (or the conducting glass) electrode is the cathode and a Pb electrode is the anode. This experiment can be performed in 2−3 h and is suitable for an applied inorganic chemistry or electrochemistry lab course. KEYWORDS: Upper-Division Undergraduate, Analytical Chemistry, Interdisciplinary/Multidisciplinary, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Electrochemistry, Electrolytic/Galvanic Cells/Potentials, Ion Exchange
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that have appeared in this Journal, the elegant preparation1,3 and characterization3 of electrochromic devices are especially illustrative. Unfortunately, most of the experimental procedures involve expensive equipment or reagents, which prevent their widespread use. The literature preparation of WO3 has been greatly simplified as follows: a. replacing the need for commercial potassium tungstate by producing it electrochemically from common incandescent light bulb filaments, b. replacing a commercial ion-exchange resin (required to produce tungstic acid from potassium tungstate) by a super absorbing polymer (SAP) available in baby diapers or feminine hygiene products, c. using a Cu foil or coupon to replace the more expensive (and unavailable in many countries) indium tin oxide (ITO) glass, and d. using an inexpensive Pb counter electrode to replace Pt in an electrochromic cell. This experiment involves a wide variety of chemical
olor changes derived from physical, chemical, and biological phenomena have fascinated humanity since early times. Among these, electrochromism is an eye-catching phenomenon characterized by a change in optical properties (e.g., color, reflectance) upon application of an external electrical field.1 Known since the mid-20th century, such changes are typically reversible and persistent2 and can be ascribed to one or more of the following: ion intercalation,3 formation of color centers, creation of band-to-band transitions, formation of defect complexes, and electrochemical oxidation or reduction reactions.2 A plethora of applications for electrochromic devices is in the near future or already in use, including digital displays,3 nonemissive large-scale displays,4 rear-view mirrors with variable reflectance,2 electrically controlled optical shutters for heat and light modulation,4 and smart windows powered by normal direct-current sources or by photovoltaic cells to provide dynamical control of incoming illumination.2,4 Some of the most popular electrochromic materials and their color changes from their neutral to their electrochemically induced states include: • Cu2O (initial, transparent; cathodic, black)2) • NiOx (initial, transparent; anodic, brown)4 • WO3 (initial, transparent; cathodic, blue)1,3,5−7 • Polyaniline, PANi (green, blue, colorless, or purple at different applied E)8 Thus, electrochromism is well suited for the teaching several principles and techniques. Of the several articles on this subject © 2012 American Chemical Society and Division of Chemical Education, Inc.
principles including electrochemical dissolution, acid−base reactions, ion exchange, acid dehydration, ion intercalation, and reversible electrochemical reduction−oxidation reactions. It can be performed in 2−3 h and is suitable for a general chemistry, inorganic chemistry, or electrochemistry lab course. Published: June 18, 2012 1205
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Laboratory Experiment
EXPERIMENTAL SECTION
Electrosynthesis of Potassium Tungstate
Potassium tungstate is prepared by the electrochemical dissolution of tungsten metal. For this, an electrochemical cell is prepared by adding 15 mL of 1 M KOH in a 20 mL beaker. A 5 cm long, 3 mm diameter graphite rod (of the type used for architectural sketches, e.g., Steadtler Mars HB) serves as the cathode and a homemade tungsten electrode, fabricated by carefully breaking 3 new 100 W incandescent light bulbs, removing their filaments, and intertwining them, serves as the anode. Alternatively, a tungsten rod (e.g., of the type found in welding supplies stores) also works well. The two electrodes are connected through alligator clips to any regulated dc power source and a cell voltage of 2−3 V is applied during ca. 10 min. This produces an aqueous solution of K2WO4. Longer times or higher voltages promote the undesired breakage of the filaments and the filaments may become red hot.
Figure 1. Setup of the electrochromic cell.
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HAZARDS The inner space of incandescent light bulbs (still available in most countries) is under vacuum to prevent oxidation of the tungsten filaments upon the natural heating produced by the passage of current (Joule effect), and so they implode upon breakage. The light bulbs should be broken individually with a hammer by placing each inside a thick plastic bag. Thick gloves, a lab coat, and appropriate eye protection should be worn. The fine pieces of glass should be adequately disposed. Although 1 M concentrations are significantly less dangerous than their concentrated counterparts, KOH, HCl, or H2SO4 solutions should not be in contact with your body as they may be an irritant or corrosive to human tissues. The material safety data sheets may be consulted if the instructor deems it necessary (see references in the Supporting Information). Electrochemical reactions at the voltages used in this experiment may be accompanied by evolution of hydrogen and oxygen. As mixtures of these gases can be explosive, the experiment should be performed in a well-ventilated area. If Pb metal is used as counter (auxiliary) electrode during the second electrolysis, avoid handling it with your bare hands. Because of the generation of Pb ions during this electrolysis, the resulting solution should be disposed of according to local regulations for hazardous waste.
Preparation of Tungstic Acid
Once the tungstate has been prepared, it can be converted into tungstic acid by acidification. To avoid the introduction of an additional (undesired) anion with the protons required for this step, ionic exchange is used. This can be achieved by humidifying a super absorbing polymer powder (SAP) obtained, for example, from a baby diaper or from a feminine hygiene product. For this, 1 g of the SAP is placed in a 20 mL beaker and 15 mL of 1 M HCl is added. The mixture is stirred and the SAP is squeezed against the walls of the beaker with a spatula and then the mixture is allowed to equilibrate for 1 h. Then, the excess HCl that forms the top liquid layer is removed. The remaining hydrated SAP is washed with 15 mL of deionized water and squeezed again. The slightly humid SAP is now in its protonated form and ready to convert the tungstate into tungstic acid. To the protonated SAP, 5 mL of the tungstate solution from the electrolysis step is added and the SAP is squeezed several times. The resulting liquid now contains enough tungstic acid, H2WO4, for the next step.
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Preparation of an Electrochromic Layer
With a Pasteur pipet, a small volume of the solution containing tungstic acid is drawn. Using the tip of the pipet as a “pen” and the dripping liquid as the “ink”, some letters are written on the surface of a Cu coupon or foil (e.g., 3 cm × 1 cm). Al, Zn, or Fe coupons can also be tested for this, although with less spectacular results. The resulting liquid layer is allowed to airdry for at least 30 min or until a whitish tone appears. The coupon contains enough colloidal WO3 as to produce an electrochromic effect in the next step. If transparent, conductive glass slides are available (e.g., ITO glass), the same procedure can be used to prepare the slide. In this case, the air-drying step may be substituted by drying the liquid layer with a hair dryer for 3 min (see pictures in the Supporting Information).
RESULTS AND DISCUSSION Potassium tungstate is electrochemically produced in the first cell: W(s) + 4H 2O(l) → WO4 2 − (aq) + 8H+(aq) + 6e−
(1)
WO4 2 − (aq) + 2KOH(aq) → K 2WO4 (aq) + 2OH−(aq) (2)
The SAP acts as an ion exchanger to convert potassium tungstate into tungstic acid: K 2WO4 (aq) + 2H+(aq) → H 2WO4 (aq) + 2K+(aq)
(3)
Upon water evaporation, tungstic acid dehydrates to form WO3:
Preparation of the Cell for Electrochromism
A small volume, 15 mL, of 1 M H2SO4 is placed in a 20 mL beaker. A lead rod (e.g., 5 cm long, 3 mm diameter) is used as the anode; any other material reasonably resistant to anodic electrochemical oxidation can also be used. The cathode is the Cu coupon with the electrochromic layer prepared earlier (Figure 1). The electrodes are connected through alligator clips to the power source and 2−3 V is applied. Changes on the WO3 layer are noted.
H 2WO4 (aq) → WO3 (s) + H 2O(g)
(4)
In the second electrochemical cell, the W(VI) (opaque white) is partially reduced at the cathode to W(V) (dark blue) with simultaneous H+ intercalation:1,3 WO3 (s) + x H+(aq) + x e− → HxWO3 (s)
(5)
The resulting effect is shown in Figure 2. 1206
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Laboratory Experiment
Figure 2. Electrochromism on a Cu coupon: (A) before and (B) upon application of potential. The anode is a Pb rod and the cathode is WO3-impregnated Cu coupon. The electrolyte is 1 M H2SO4.
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CONCLUSIONS An electrochromic device can be prepared from readily available materials. This preparation involves a wide variety of chemical principles, including electrochemical dissolution, acid−base reactions, ion exchange, acid dehydration, ion intercalation, and reversible electrochemical reduction−oxidation reactions.
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ASSOCIATED CONTENT
S Supporting Information *
Instructions for students (including a problem set); instructions for instructors (including answers to the student problem set). This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Experimental assistance by Elizabeth Garcia-Pintor and Claudia Camacho-Zuñiga is gratefully acknowledged. REFERENCES
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