Communication Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Using pH Indicator To Demonstrate Supercapacitor Reactions Emily Traver, Jamie E. Stark, Gianna Aleman Milán,* and Heather A. Andreas* Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada
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S Supporting Information *
ABSTRACT: In this supplement to our second-year supercapacitor lab (Bringing Real-World Energy-Storage Research into a Second-Year Physical-Chemistry Lab Using a MnO2Based Supercapacitor. J. Chem. Educ. 2018, 95 (11), 2028− 2033), we incorporate a color indicator as indirect evidence for the manganese oxide’s pseudocapacitive reaction (MnO2 + H+ + e− ⇌ MnOOH). Using the evidence of pH changes (the methyl red goes from light yellow to red during an oxidative potential hold), students can make the link between oxidation state changes and the applied potential of the electrochemical experiment. Herein, we optimize the indicator concentration and electrochemical parameters to ensure the color changes are evident. Optimal responses arise when 15 μM methyl red is coupled with a 2 min, 0.8 V potentiostatic hold. KEYWORDS: Second Year Undergraduate, Laboratory Instruction, Hands-On Learning/Manipulatives, Physical Chemistry, Inorganic Chemistry, Applications of Chemistry, Electrochemistry, Materials Science, Oxidation/Reduction
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base titrations that students often see in first year chemistry. We find that giving students an extra chromatic effect also further engages their interest while giving us a pathway in the lab to link the electrochemical oxidation/reduction of the pseudocapacitive reaction with the chemical changes the students can now visualize in the cell.
INTRODUCTION In our previous lab, entitled “Bringing Real-World Energy Storage Research into a Second-Year Physical-Chemistry Lab Using a MnO2-Based Supercapacitor”,1 we introduced students to the important fields of energy storage, supercapacitors, and pseudocapacitance. Briefly, supercapacitors store energy through very rapid redox reactions, called “pseudocapacitive reactions”. In that lab, we show students some of the real-world methods used to evaluate energy storage systems. We demonstrate this using manganese oxide supercapacitorelectrode material. This material undergoes a pseudocapacitive reaction: MnO2 + M+ + e− F MnOOM
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EXPERIMENTAL SECTION The preparation of the solutions and electrodes for the lab has been described previously.1 Herein, we describe the changes to that procedure and the optimization of the pH indication. A stock solution of the methyl red indicator was made from 0.5 g of methyl red (Sigma-Aldrich) dissolved in 300 mL of methanol, then diluted with 200 mL of 18 MΩ cm water, resulting in a final methyl red concentration of ca. 3.5 mM. The electrochemical testing was conducted in a 150 mL beaker, using a manganese oxide working electrode (described previously1), a Ag/AgCl reference electrode, and a stainlesssteel scoopula counter electrode. The electrolyte was 140 mL of 0.5 M Na2SO4, to which was added stock methyl red to produce concentrations ranging between 0 and 30 μM, in 5 μM increments. At each indicator concentration, a 0.8 V potentiostatic hold was applied for 5 min to oxidize the manganese oxide, release protons, and effect the pH change. Pictures taken every 30 s captured any color change in the cell. Then, two cyclic voltammograms (CVs) were recorded between
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where M+ is an electrolyte cation or proton. Students use cyclic voltammetry to oxidize the manganese oxide to MnO2 and reduce it to MnOOM. However, there is no direct evidence for the student that reaction 1 proceeds during the electrochemistry. While it is not currently possible to track the manganese oxidation state or oxide type during the electrochemical measurements, we can provide indirect evidence for this reaction by showing the pH changes that result from the reaction (when M+ is a proton) using a methyl red color indicator. This communication provides a method for incorporating the color pH indicator into the established lab experiment, with an additional electrochemical step to cause sufficiently large pH changes to be visually obvious. Using a color indicator gives students visual evidence of the pseudocapacitive reaction and highlights uses of pH and indicators beyond the typical acid− © XXXX American Chemical Society and Division of Chemical Education, Inc.
Received: March 28, 2019 Revised: May 24, 2019
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DOI: 10.1021/acs.jchemed.9b00271 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
First, comparing the left-most column (before the application of the 0.8 V hold), at low methyl red concentrations, the solution is slightly more orange than expected since the acidity of the methyl red drops the solution pH somewhat. At higher methyl red concentrations, the solution is darker and the extra acidity due to the methyl red results in more deeply orange solutions. While we tested higher concentrations, above 30 μM the solution becomes too dark for pH changes to be easily seen. The manganese oxyhydroxide undergoes oxidation during the positive sweep of the CV cycling, but the amount of oxidation is small, resulting in very small local pH changes; no color change is seen with the naked eye. To effect a more significant release of protons, a potentiostatic hold at an oxidizing 0.8 V potential was incorporated into the experimental procedure (a modified lab manual procedure is provided in the Supporting Information). The 0.8 V potential is sufficient to oxidize the manganese oxide without being too positive to cause film degradation. The color changes associated with increasing hold times are seen along each row in Figure 1. Clearly, the applied potential causes distinct pH changes around the working electrode. A 2 min hold results in a clear red hue. There is no benefit to longer hold times since there is no significant color increase, and at long times (e.g., 5 min) the color decreases as protons diffuse away from the electrode. Figure 1 also demonstrates that, for low concentrations (≤10 μM), the color change is faint (though it is slightly easier to see with the naked eye than is demonstrated with the pictures in Figure 1). Conversely, when 25 μM is exceeded, the bulk solution is too dark to see the color change effectively. Thus, the optimal methyl red concentration range for an obvious red color is between 15 and 25 μM, translating to between 0.6 and 1 mL of 3.5 mM methyl red stock solution in the 140 mL electrolyte. We note that, even with the potentiostatic hold, the local pH changes are still small; thus, to optimally view these color changes, students should place a white piece of paper behind the electrochemical cell and look along the plane of the electrode (as shown in Figure 1). We encourage our students to take pictures or video during the potential hold. Conceptual links can be made to spectroscopy (particularly UV−vis spectroscopy) at this point, since viewing the electrode face-on shows essentially no color change, but a distinct color is seen when viewing along the electrode plane; the difference arises from the variation in the Beer’s law path length. When viewed face-on, the path length is only 1−2 mm (i.e., the depth of the red solution on each side of the electrode), whereas when viewed along the electrode plane, the path length (region of pH changes) extends the whole length of the 1 cm electrode face. In the original lab experiment,1 students were asked to calculate several real-world parameters for evaluating energy storage systems: film thickness, theoretical capacity, film usage, Coulombic efficiency (CE), and energy efficiency (EE). Film thickness and theoretical capacity are calculated from film deposition data and are unaffected by the changes proposed herein. Figure 2 shows the other parameters calculated from the CVs collected at different indicator concentrations (see Supporting Information for sample calculations). We see from Figure 2a that there is no significant difference in the CVs recorded in the different solutions; the methyl red is not electrochemically active, nor does it influence the manganese oxide electrochemical response appreciably. Additionally, Figure 2b−d shows that there is no change in CE (Figure 2b), EE (Figure 2c), or film usage (Figure 2d) at the different indicator concentrations. Clearly, adding methyl red in the range of
0.4 and 0.8 V with a 10 mV/s sweep rate, to test if the potential hold or indicator concentration impacted the manganese oxide pseudocapacitive response. Since a different manganese oxide electrode was used for each test, and each electrode has a slightly different amount of active film (due to variations in electrodeposition), the CV currents were normalized to the deposition charge (Qdep) to eliminate film variation effects.
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HAZARDS Methyl red has a D2B (toxic material: mutagen) WHMIS classification (Canada). It may be harmful if inhaled or swallowed and may cause skin or eye irritation. Safety glasses, lab coats, and gloves should be worn.
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RESULTS AND DISCUSSION The Na2SO4 electrolyte has a pH of ∼6.5, and we expect only very small changes in pH during the electrochemical experiment; thus, methyl red is the ideal indicator since it exhibits a yellow color at high pH values (>6.2), is orange between 6.2 and 4.4, and is red at low pH (
