Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Bringing Real-World Energy-Storage Research into a Second-Year Physical-Chemistry Lab Using a MnO2‑Based Supercapacitor Felicia Licht, Gianna Aleman Milań ,* and Heather A. Andreas* Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada
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S Supporting Information *
ABSTRACT: As the need for alternative energy becomes increasingly important, energy research and related industries are rapidly expanding. This lab incorporates current energy-storage research into a second-year lab that instills real-world, industry-relevant knowledge and skills while teaching and reinforcing physical-chemistry concepts. A manganese oxide electrode, aqueous-Na2SO4-electrolyte supercapacitor system is used because it has no air or water sensitivity, unlike most battery technologies, so it is easy to implement in an undergraduate-lab setting. Manganese oxide is an increasingly popular supercapacitor material, and this lab introduces the concept of pseudocapacitance, in which current flows while still being governed by the Nernst equation (i.e., at equilibrium). Students conduct realistic and industrially relevant electrochemical experiments; they electrodeposit manganese oxide films and test them using cyclic voltammetry. Students compare the manganese oxide results to those from a nonpseudocapacitive system (i.e., a poor supercapacitor). In doing so, they learn the concepts of charge storage and energy and power (and their important differences), while reinforcing the physical-chemistry topics of thermodynamics and kinetics, all within a frame of familiar electrochemical knowledge (i.e., the Nernst equation). This lab can be completed in one 4 h laboratory period or in a 3 h period if the solutions are provided to the students or they prepare them a week in advance. Student interest and engagement is heightened by their being able to see the real-world applications and skills. KEYWORDS: Second-Year Undergraduate, Laboratory Instruction, Hands-On Learning/Manipulatives, Physical Chemistry, Inorganic Chemistry, Applications of Chemistry, Electrochemistry, Materials Science, Oxidation/Reduction, Thermodynamics
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INTRODUCTION Today’s university students were raised with a growing acknowledgment of global warming; the impending oil crisis; and a recognition of the need to switch to more sustainable energy sources, such as solar, wind, and tidal sources. This lab introduces students to the important field of energy storage, where energy produced (e.g., during daylight hours) is stored in order to deliver that energy later (e.g., at night). We build on students’ familiarity with batteries, currently the most common energy-storage system, and extend that knowledge to supercapacitors (also called electrochemical capacitors), a relatively new energy-storage technology that is gaining prominence.1−7 Students use energy storage to power their electronic devices and are increasingly likely to find jobs in energy-related fields; however, there are very few undergraduate chemistry laboratories that introduce students to the evaluation of energy-storage materials and provide the industrially relevant skills students will need. There are currently no laboratories focusing on supercapacitors and only few on batteries8−10 and fuel cells.11,12 Of these laboratories, only Blake’s Li ion− graphene lab8 describes the electrochemical parameters used in industry to evaluate energy-storage materials (e.g., capacity, © XXXX American Chemical Society and Division of Chemical Education, Inc.
etc.), although it does not evaluate the energy-related parameters covered herein. Additionally, that lab requires glovebox access because of the battery material’s air and water sensitivity; the lab herein has no such sensitivity. Supercapacitors use much safer materials than most batteries and are therefore easy to study in a second- or third-year undergraduate setting. Moreover, students completing this lab use the same materials,7,13−26 synthesis procedures,23−27 and system evaluation28,29 that we do in research laboratories and that is used in industrial supercapacitor and battery evaluations; this means that the skills, analysis experience, and knowledge are transferable to a wider industrial scope, which increases student excitement and engagement in the lab. This lab reinforces first- and second-year physical-chemistry concepts such as thermodynamics, kinetics and equilibria using realistic electrochemical measurements. Although the lab is geared toward physical chemistry, the materials synthesis and energy evaluations are also appropriate for an inorganic lab. Often, second-year students have only learned the Nernst Received: June 15, 2018 Revised: September 4, 2018
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DOI: 10.1021/acs.jchemed.8b00454 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
≥99.99% trace metal basis, polished with sandpaper until shiny) using a 20 s hold at 10 mA in 2 M KCl. The AgCl coating is slightly pink-brown. This deposition is conducted in a two-electrode cell with a lab scoopula as the other electrode. The AgCl-coated wire is placed inside a glass frit (ALS Company, sample holder, 60 × 68 mm) filled with either a 1 M or saturated KCl, and the reference electrode is stored in KCl when not in use. The potentials provided in this paper are versus Ag/AgCl with a 1 M filling solution; if a saturated filling solution is to be used, the potentials can all be shifted by +38 mV (i.e., a 0.4 V here would shift to 0.438 V vs Ag/AgCl (saturated KCl)). During AgCl formation, students will observe bubbles forming on both electrodes; with guidance, students deduce that the water is breaking down (forming O2 on the silver and H2 on the scoopula). The pseudocapacitive manganese oxide films were tested using cyclic voltammetry (CV) in 0.5 M Na2SO4, with 15 cycles in a 0.4−0.8 V potential window at a 10 mV s−1 sweep rate. To test a nonpseudocapacitive system, a bare stainlesssteel electrode (without manganese oxide film) was examined using five CV cycles in 2 mM MnSO4, with a 0.0−1.5 V potential window and a 10 mV s−1 sweep rate. The reference electrode was the Ag/AgCl and the counter electrode was the scoopula. Each electrode was thoroughly rinsed with deionized water before and after the experiment.
equation; however, the Nernst equation only applies to systems at equilibrium (i.e., typically where there is no current flow), often limiting us to open-circuit voltage measurements versus temperature or concentration. Conveniently, however, supercapacitors use pseudocapacitive materials, a class of materials for which the kinetics are so fast that they are under Nernstian control1−3 even though current flows (more detail is provided in the lab manual in the Supporting Information). The term “pseudocapacitive” indicates that the electrochemical response for these materials mirrors that of a conventional capacitor, opening discussions of charge storage through redox reactions versus separation of electrical charge. Additionally, supercapacitors are high power, whereas batteries are high energy, inviting comparisons between them and their commercial applications and providing a platform to explain the difference between power and energy. Thus, supercapacitors are ideal for introducing real-world energy-storage knowledge in a second- or third-year chemistry lab using familiar concepts.
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EXPERIMENTAL SECTION This lab requires 4 h: 1 h of solution preparation that could be done the week before and a 3 h lab. Students work in groups of two or three. This lab is appropriate for second- or third-year students, and the level of discussion can be modified to fit the group or individual student level. Manganese oxide films were formed on stainless steel (SS 304, McMaster-Carr) that had been cleaned by sequential washing with deionized (18 MΩ·cm) water, methanol, and deionized water and dried with a Kimwipe between each step. Three 10 × 1 cm strips were cut using scissors and wrapped with Parafilm, leaving a 2 × 1 cm deposition area on one end and some bare stainless steel at the other for electrical connection to the instrument (Figure 1a).
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HAZARDS The materials have no water or air sensitivity. MnSO4 has a D2B (chronic toxicity) WHMIS (Canada) classification with specific-target-organ toxicity (lungs, liver, blood) upon repeated exposure and acute aquatic toxicity. Na2SO4 is not a DOT-controlled substance (United States), nor is it controlled under WHMIS. For both MnSO4 and Na2SO4, irritation may occur upon contact with skin or eyes, and ingestion and inhalation should be avoided. Safety glasses, lab coats, and gloves should be worn. There is a slight risk of electrical shock. Ensure that there is no metal-to-metal contact between the electrodes or with the clamps or retort stand, and students should not touch the electrodes when the electrochemical cell is running. Also, care should be taken when handling the stainless steel as cutting can produce extremely sharp edges.
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RESULTS AND DISCUSSION
The manganese oxide electrodeposition results in a thin brown film (Figure 2a), sometimes with a thin-film-interference rainbow, which can be discussed if appropriate. Achieving a uniform film is relatively facile; although inadequate stainless-
Figure 1. Electrodes (a) and electrochemical cell (b) for manganese oxide electrodeposition.
Manganese oxide was electrodeposited in a stirred, room temperature 0.2 M MnSO4 aqueous solution (made from powder, purity ≥99%, Sigma-Aldrich, in deionized water) using a 1 min potential hold at 1.0 V versus the Ag/AgCl reference electrode (commercial or homemade, described below) and a lab-scoopula counter electrode (Figure 1a,b). All electrodes were rinsed with deionized water before and after deposition; the manganese oxide films were stored in 0.5 M Na2SO4 until use to prevent drying and cracking. Students make their own Ag/AgCl reference electrodes because it is simple and because they are familiar with the oxidation reaction from their first year. The AgCl is galvanostatically deposited on clean Ag wire (0.5 mm, Aldrich,
Figure 2. Manganese oxide films on stainless steel, illustrating (a) different deposition times (left to right: 30 s, 1 min, 2 min, and 4 min) and (b) poor versus uniform films. B
DOI: 10.1021/acs.jchemed.8b00454 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
steel cleaning results in spots or fingerprints (see Figure 2b), allowing students to self-evaluate their lab cleanliness. By integrating the electrodeposition current over time (Figure 3), students calculate the deposition charge (Qdep).
Q theor = zFn MnO2
Calculations of theoretical capacity are common in battery and supercapacitor research, and the theoretical capacity is compared with the real measurement of the charge stored (see below). Students also use Qdep to calculate the film thickness: thickness =
They then estimate the number of moles of MnO2 electrodeposited (nMnO2) by assuming 100% of the Qdep forms MnO2 and using eq 1: Q dep (1)
zF
Q depM MnO2 FzAρ
(3)
where MMnO2 and ρ are the molar mass and density for MnO2, and A is the deposition area. We ask students to predict whether the theoretical capacity would double if the film thickness is doubled; it should because the theoretical capacity assumes that all the MnO2 (i.e., throughout the whole film thickness) reacts. When the manganese oxide film is tested electrochemically, a typical manganese oxide CV is seen (Figure 4a). This CV has a similar shape to that of a conventional capacitor (Figure 4b), but its charge-storage mechanism is based on redox reactions instead of separation of electric charge; this is the origin of the term “pseudocapacitance”. Students use the similarity in CV shape as evidence that manganese oxide is pseudocapacitive. Indeed, the rectangular shape and the fact that the CV is a mirror image around the zero-current axis, are diagnostics used in supercapacitor literature to define a pseudocapacitive system.1−3 The mirror-image response shows that within any potential interval, approximately the same anodic charge and cathodic charge is passed (i.e., the stored charge is approximately equal to the charge delivered). In the lab period, it is useful to remind students that in ideal energy-storage systems, 100% of the stored charge is delivered to the device; any charge that goes to parasitic processes is wasted. For instance, in the initial manganese oxide CV, there is an oxidation wave above 0.7 V (Figure 4a) that has no corresponding reduction, so the charge that goes into this oxidation is lost or wasted. The oxidation-wave reaction is currently unknown (it is being studied in the Andreas lab); we use this as an opportunity to talk about “real” research. We expose students to the idea that not all answers are currently known, and we encourage them to make educated guesses regarding the identity of this reaction but without increasing student anxiety (e.g., no significant impact on student grades). Possible explanations include: oxidation of Mn2+ trapped during electrodeposition; oxidation of H2O to O2, and irreversible
Figure 3. Current−time profile for manganese oxide electrodeposition.
n MnO2 =
(2)
−
where F is Faraday’s constant (96,485 C/mol e ), and z is the number of electrons in the deposition reaction (Mn2+ + 2H2O → MnO2 + 4H+ + 2e−, z = 2). As discussed in the lab manual, there are multiple forms of manganese oxide (e.g., Mn2O3, Mn3O4, MnOOH, and multiple MnO2 polymorphs), and a mixture of these is formed by electrodeposition.30 It is currently unknown what percentage of the electrodeposition results in MnO2 (for more information, see the instructor notes in the Supporting Information). For ease of calculation, students assume that only MnO2 is formed; they will challenge the validity of this assumption when explaining the poor film usage (discussed below). Students use nMnO2 to calculate the theoretical capacity (Qtheor), which describes the maximum charge that the deposited film could store assuming all MnO2 is pseudocapacitive (MnO2 + M+ + e− ⇌ MnOOM, where M+ is an electrolyte cation or proton,31 z = 1):
Figure 4. Cyclic voltammograms of (a) manganese oxide in 0.5 M Na2SO4 (cycles 1−15), (b) conventional capacitor, and (c) stainless steel in 2 mM MnSO4 (cycles 1−5). The arrows denote the changes in each CV with cycling. The star in (c) denotes a current-drop associated with diffusion limitations (described further in the text and in the instructor notes in the Supporting Information). C
DOI: 10.1021/acs.jchemed.8b00454 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
where Qi is the charge calculated between two adjacent CV potential points, and Eavg is the average of those potentials. The energy efficiency (EE) is then a ratio of delivered energy to stored energy. The CE and EE of the manganese oxide system are very high (CE ≥ 95% and EE > 85%) versus those for the stainless steel/Mn2+ system (
