Battery Concepts in Physical Chemistry: Making ... - ACS Publications

Laboratory Experiment Cite This: J. Chem. Educ. 2019, 96, 1465−1471

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Battery Concepts in Physical Chemistry: Making Your Own Organic− Inorganic Battery Jacob Arnbjerg,† Amirreza Khataee,⊥ Thomas Breitenbach,‡ Jan Thøgersen,‡ Sigurd Christiansen,‡ Henriette Gavlshøj Mortensen,‡,∥ Merete Bilde,‡ Rikke Frøhlich Hougaard,§ and Anders Bentien*,⊥

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Aarhus University School of Engineering, Chemical and Biotechnology Engineering, Aarhus University, Hangøvej 2, 8200 Aarhus N, Denmark ‡ Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark § Science and Technology Learning Lab, Aarhus University, Inge Lehmanns Gade 10, 8000 Aarhus C, Denmark ∥ Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds vej 14, 8000 Aarhus C, Denmark ⊥ Department of Engineering, Aarhus University, Hangøvej 2, 8200 Aarhus N, Denmark S Supporting Information *

ABSTRACT: On the basis of recent advances in battery research and technology, we have developed a novel laboratory exercise centered on an organic−inorganic battery using the redox chemistry of the organic molecule anthraquinone-2,7-disulfonic acid disodium salt (AQDS). Although most commercially available batteries are based on inorganic redox couples, the development of batteries based on organic redox active materials has great potential for stationary energy storage. As such, the experiment described in this report exposes students to state-of-the-art battery technology, despite a rather simple experimental protocol. The exercise allows students to acquire hands-on learning and visualize central concepts of the Nernst equation, battery technology and components, half-cell reactions, charging/discharging tests, and performance analysis. Additionally, students are required to operate a range of key electronic instruments, including multimeters, power supplies, and electronic loads. This laboratory exercise is part of a third semester undergraduate course in physical chemistry and can be completed in a single laboratory session. Student feedback shows that the experimental work, coupled with a written report, significantly broadens student understanding of the electrochemistry of batteries. KEYWORDS: Second-Year Undergraduate, Physical Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Electrochemistry, Electrolytic/Galvanic Cells/Potentials, Oxidation/Reduction

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INTRODUCTION

support more conventional theoretical teaching activities in the classroom with hands-on learning in the laboratory. Furthermore, due to the continued increase of intermittent renewable electricity sources in the power grid and automotive electrification, electrical energy storage and battery technology will continue to become an even more important topic. For this reason, early introduction to batteries, and related experimental exercises, can be used as examples to explain important applied concepts in physical chemistry in undergraduate courses. As this topic naturally addresses several key aspects of general electrochemistry, a laboratory experiment focusing on battery concepts serves as a convenient platform for teaching hands-on electrochemistry. Whereas the basic underlying electrochemistry is the same for all types of batteries (rechargeable/nonrechargeable, solid/

Despite being a fundamental pillar of most undergraduate courses in physical chemistry, electrochemistry, in our experience, is often a topic which presents students with considerable challenges. This includes understanding the basic concepts and theory and relating these to technological advances and common everyday applications. One of the contributing factors to this difficulty is most likely related to students having low exposure to experimental electrochemistry and associated instrumentation, both prior to and during undergraduate studies. Several recent contributions in this Journal have described laboratory experiments focusing on aspects of electrochemistry,1−7 yet the ACS has recently identified the need for better hands-on learning about applied and quantitative electrochemistry for Bachelor’s degree programs.8 Seen from our perspective, this need is clearly echoed in the context of B.Sc. education within chemistry and chemical engineering at Danish universities. There is a need to © 2019 American Chemical Society and Division of Chemical Education, Inc.

Received: January 30, 2019 Revised: May 6, 2019 Published: May 31, 2019 1465

DOI: 10.1021/acs.jchemed.9b00090 J. Chem. Educ. 2019, 96, 1465−1471

Journal of Chemical Education

Laboratory Experiment

Cell Assembly and Experiment

liquid electrolytes), organic molecules are currently investigated as redox species for aqueous low-cost redox flow batteries (RFBs).9,10 As such, an experimental protocol centered on aqueous organic batteries will not only illustrate central learning concepts on battery functionality, but also introduce the students to state-of-the-art research and technology.

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After preparation of positive and negative electrolytes, students assemble the electrochemical cell as shown schematically in Figure 1a. The cation exchange membrane is sandwiched between rubber gaskets and carbon felt electrodes (Figure 1b,c). Bipolar graphite composite plates (8.7 cm × 6.5 cm) mechanically pressed against the carbon felts are used as current collectors. Finally, the cell is assembled with a homebuilt aluminum frame, together with a holder made of Teflon that contains the bipolar plate and carbon felt (Figure 1a). Prior to entering the lab, the students have watched short instructional videos to familiarize themselves with cell parts and assembly.11 Once filled with redox active solutions, the bipolar graphite plates are connected in a four-wire configuration with a lowcost multimeter and power supply (Figure 1d). The multimeter measures the cell voltage, while the current can be read from the power supply. The assembled setup with connected multimeter and power supply is shown in Figure 1d. Before the first charging of the cell, the voltage is both measured as well as visualized, by connection of an LED indicator where no light should be observed. Also, a drop of the ammonium bromide solution is taken out with a disposable pipet, and the color is noted.12 The cell is then charged with a constant current of 0.15 A, and cutoff voltages in the range 1.4−1.6 V have been used as a condition for a fully charged cell. Every 30 s the cell potential (ϕ) during charging/discharging and the open circuit potential (Ecell) are measured, the latter by briefly setting the current to zero followed by continued charging/discharging. Once fully charged, the LED indicator is again connected. A drop of the ammonium bromide solution is sampled and the color again noted. The cell is then fully charged again (∼1 min), in order to compensate for the discharging by the LED. Afterward, the battery is discharged with a constant current of 0.15 A by connecting it to an electronic load, while ϕ and Ecell are recorded in the same way as for the charging. During the whole charge/discharge procedure, students enter the recorded data into a plotting program. In this way, the progress of the process can be followed to indicate when the cell is charged or discharged, as evidenced by a steep rise/fall of the voltage curves. At the end of the discharging process, the LED is once more connected, and a drop of the cell solution is collected. All observations are noted by students in their report template, and the instructors can inspect data directly and stimulate inclass discussions of experimental results. Finally, the cell is disassembled and cleaned, and the waste is disposed properly. All parts of the cell are reusable once cleaned and stored in deionized water. The experimental work is carried out in small groups of preferably not more than three students. This allows for safe and efficient experimental work, while still ensuring that all students will be actively involved.

MATERIALS AND METHODS

Materials

The experiments are carried out using the cell illustrated in Figure 1. Anthraquinone-2,7-disulfonic acid disodium salt

Figure 1. (a) Fully assembled cell. The cell is contained in a holder made of Teflon and clamped together using an aluminum frame. (b) Disassembled cell showing cell components. (c) Exploded schematic view of the cell with different components. The shown end plates also contain the holders made of Teflon. (d) The electrolytic cell connected to multimeter and power supply.

(AQDS supplied from AKS Science in 80% purity) with a concentration of 0.10 M acts as redox species on the negative side of the battery, in 11−19 mL of 1.25 M ammonium bromide (Sigma-Aldrich) as a supporting electrolyte. On the positive side, 11−19 mL of 1.25 M ammonium bromide was used as both supporting electrolyte and redox couple. Prior to the experiment, the instructor pretreats the membrane and carbon felts. A cation exchange membrane, made of Nafion with a thickness of 183 μm (supplied from FuelCellStore), is employed as a separator between the two chambers of the battery. Treatment of the membrane is not mandatory, but the standard procedure is boiling of the membrane for 30 min in H2O2 (3%), followed by 30 min boiling in H2SO4 (0.5 M) and washing in boiling water (several times). Carbon felts (electrodes, GFA6 EA, 6 mm thickness cut into pieces approximately 5 cm × 5 cm) and bipolar graphite plates (current collectors, PV10) were purchased from SGL GROUP. To increase the hydrophilicity, the carbon felts are thermally treated at 500 °C in air for 3 h and soaked in the appropriate solutions before use. This is done once prior to groups of students performing experiments.

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HAZARDS Anthraquinone-2,7-disulfonic acid disodium salt (AQDS) is not a dangerous substance or mixture according to the Globally Harmonized System (GHS), and handling of NH4Br does not require special safety measures. During the charging, bromine (Br2) is formed in the redox reaction. Bromine has a high vapor pressure and is hazardous; however, the maximum concentration of bromine in the experiment is quite small (approximately 10 mmol/L), and the electrolyte volume in the 1466




Figure 2. Overview of the student workflow, involving tasks before, during, and after conducting the laboratory experiment.

Figure 3. Schematic of the AQDS-bromide liquid electrolyte battery system. Upper row: schematic of an electrolytic cell (charging). Lower row: galvanic cell (discharging).

holder made of Teflon. In order to avoid leakage, we introduced two U-shaped rubber gaskets on each side of the membrane. Another pitfall can be experienced with the carbon felts, which to some extent will disintegrate and lose cohesion when subjected to multiple experimental runs. This leads to lower volume on each side and poor reproducibility. Therefore, the instructors should monitor the state of the felts and replace them when signs of wear and/or tear occur. We have successfully performed the experiment using ammonium bromide solutions, which allows for easy and safe experimental conditions. It is also possible to use hydrobromic acid as a replacement for ammonium bromide. The acidic conditions with HBr increase cycle lifetime; however, HBr is a strong acid and should be handled and disposed of accordingly.

cell is only about 10−20 mL. Furthermore, the concentration of bromide (Br−) is at all times in great excess compared to that of bromine. This drives the complexation reaction Br2 + Br− → Br3− to the right, whereby the vapor pressure of bromine is decreased. The entire experiment, including preparation of solutions, cell assembly, and data recording, is performed in a fume hood. Students are required to wear appropriate safety goggles, lab coats, and gloves at all times.

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EXPERIMENTAL PITFALLS AND MODIFICATIONS If not treated thermally, the electrodes are not easily wetted and may introduce problems with reproducibility and stability of the experiment. Specifically, using untreated carbon felts results in high overpotentials in the battery and consequently immature charging and discharging. In addition, the disproportionation of bromine within the solution once the battery is fully charged comes into consideration. Bromine is stabilized in acidic solution while becoming increasingly unstable as the pH increases. 13 However, the amount of disproportionation products is small, and it does not affect the electrochemical reaction of bromide.14 These considerations are made explicit to the students in the manual as well as in the laboratory. After frequent use of the frames, the force exerted by the screws can induce a slight bending of the metal attached to the

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STUDENT WORKFLOW As illustrated in Figure 2, the student workflow involves mandatory tasks before, during, and after the exercise. The experimental protocol (see Supporting Information) is designed with a brief description of the related chemical theory and clear instructions for the experimental setup and data acquisition, thus making it relatively easy to adopt the experiment. It is well-documented that students learning of 1467




equation, and influences the cell potential (see Section S1 in the Supporting Information for details). Considering the half-cell reactions, the overall electrochemical reaction of the AQDS/bromide battery is written below:

higher-order cognitive skills, in relation to laboratory teaching, are more efficiently supported when the degree of openness is raised compared to traditional “verification” experiments.15 This can be achieved, e.g., by offering tasks related to planning laboratory work and making decisions about the experimental setup.16,17 Therefore, a number of mandatory prelab tasks related to description of theory, choosing experimental parameters, planning the laboratory work, and predicting the outcome of the experiment, were designed and included in the “report template” as part of the lab manual (Supporting Information). Laboratory instructors were instructed to follow up on these tasks while students performed the experiment, in order to provide feedback and stimulate discussion and higher-order thinking. The write-up after the experiments stimulates student reflection about experimental observations and data in relation to theory.

AQDS + H+ + 2Br − → AQDSH− + Br2

(3)

On the basis of reaction 3, the general Nernst equation for the cell potential is obtained as eq 4:

⊖ = Ecell

coefficients to unity), then Q cell = ⊖

SOC ≡ x AQDSH− =

The detailed electrochemical reactions of the battery are summarized in Figure 3. As mentioned earlier, ammonium bromide was used as both supporting electrolyte and redox couple on the positive side. To prevent cross-mixing of the positive and negative electrolytes, while allowing the transport of ions to maintain charge balance, a cation exchange membrane (made of Nafion) is placed between the two halfcells. Although this battery chemistry is considered for f low battery configuration,13 in the current exercise we do not pump the solutions but rather keep them stationary. The main reasons for this are safety and simplicity, and the absence of true flow conditions does not alter the learning outcome. During the charging process, and according to reaction 1, bromide is oxidized to generate bromine on the positive side: 2Br → Br2 + 2e

= + 1.1 V

The stand-

[AQDSH−]t [AQDS]t = 0

(5)

Here, [AQDS]t=0 is the initial concentration of AQDS, and xAQDSH− is the mole fraction of AQDSH−, whereby xAQDSH− is 0 and 1 when fully discharged and recharged, respectively. Since it is not feasible to measure the concentration of AQDSH− during the charging and discharging processes, the SOC is determined from the current. Using mass balances, it can be shown that Ecell =

⊖ Ecell

jij jj [AQDS]2 RT j t=0 − × lnjjjj × jj (c ⊖)2 νF jj k

(

(1 − x AQDSH−)

(1)

[Br−]t = 0 [AQDS]t = 0

2

)(

− 2x AQDSH− 2 − x AQDSH

However, on the negative side, AQDS is reduced in a single step two-electron−one-proton process as shown in reaction 2:

[Br−]t = 0 [AQDS]t = 0

y − x AQDSH− zzzz zz zz zz zz z {

)

(6)

The task of deriving this expression can be used as an additional exercise in the student report. In the experiment, the open circuit potential, Ecell, is measured by pausing the charge/ discharge. The amount of charge (Qc, subscript c to distinguish it from the reaction quotient) supplied or drawn to/from the cell is given by the temporal integral

AQDS + H+ + 2e− → AQDSH− ⊖ − ≈ − 0.2 V EAQDS/AQDSH

[Br −]2 [H+][AQDS] . [Br2][AQDSH−]c ⊖

ard concentration, c = 1 mol/L, ensures that Qcell is dimensionless. Since the concentrations change during charge/discharge, Ecell thus has a dependence of the state-ofcharge (SOC) of the battery, which we define by

Electrochemistry of AQDS/Bromide Redox Flow Battery

⊖ − E Br 2 /Br

(4)

⊖ ⊖ Here, E⊖ cell = EBr2/Br− − EAQDS/AQDSH− is the standard cell potential, and F is the Faraday constant. R the gas constant; T is the temperature, and ν = 2 is the number of electrons involved in the half-cell reactions. a refers to the thermodynamic activities of the involved reaction species. Qcell is the reaction quotient, and if it is assumed that activities can be replaced by concentrations (effectively setting all activity

RESULTS AND DISCUSSION As far as possible, nomenclature and theoretical treatment in this text follow the basic physical chemistry textbook used for the course.18 The organic−inorganic aqueous battery is based on the redox chemistry of anthraquinone-2,7-disulfonic acid disodium salt (AQDS) on the negative side and the Br2/Br¯ couple on the positive. These redox pairs are currently being considered for use in low-cost RFBs for grid scale electricity storage.10 AQDS is a stable redox active organic molecule that shows relatively high aqueous solubility both in acidic and basic solutions.9,10

−

RT ln(Q cell) νF 2 RT jijj a Br− a H+aAQDS zyzz − lnjj z νF j a Br2a AQDSH− zz k {

⊖ − Ecell = Ecell

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−

⊖ Ecell = + 1.3V

(2)

At this point, it should be noted that the standard potential of AQDS is highly pH dependent and varies between −0.300 and +0.225 V for alkaline and acidic solutions, respectively. The AQDS solution in this exercise has pH = 8, and since NH4Br is used as supporting electrolyte, there is a buffer capacity from the mixture of NH3/NH4+. Therefore, and upon considering the Pourbaix diagram of AQDS (see Supporting Information), only one proton is involved in the electrochemical reaction. This will be manifested in the Nernst

Qc =

∫0

t max

I (t ) d t

(7)

where t is the time and I is the electrical current. Also, the electrical work (w) is given by the integral over the electric potential ϕ, where the latter is being measured in the experiment 1468




Figure 4. (a) Ecell as a function of SOC upon charging. (b) Ecell as a function of SOC upon discharging. Experimental conditions: 0.10 M AQDS in 1.25 M NH4Br on the negative side and 1.25 M NH4Br on the positive side. Charging and discharging were carried out at constant current of 150 mA. Note that the higher achieved potential in student data is due to applying different cutoff voltages compared to instructor data as well as theory.

w=

∫0

Q c,max

ϕ(Q c) dQ c

student data sets (sampling from a total of 19 data sets) representing upper and lower bounds, thus illustrating the observed spread in the data obtained by students. Also, here, we see Nernstian behavior, albeit with larger deviations from theory than those seen in the instructor data sets. In their report, students are specifically asked to include the theoretical prediction for Ecell and comment on their deviations. These deviations are principally attributed to lower utilization of the capacity of AQDS than the theoretical one and upon discharging a Coulombic efficiency (significantly) less than 1. The lower utilization is related to the inaccuracy of students in weighing of AQDS in the beginning of the exercise. Here, it is also important to note that the lower Coulombic efficiencies of experimental data reflect a combined effect of oxidation of reduced AQDS through oxygen in the air and bromine crossover through the membrane to the AQDS side where it reacts with reduced AQDS. During charging and discharging of the cell, the pH values of the negative and positive electrolytes remain around 7−8 and 3−4, respectively. From the data shown in Figure 4, students calculate the battery experimental capacity (in coulombs) and compare it to the theoretical value from concentrations and volumes of the redox solutions (see Supporting Information). With respect to battery related technical concepts, students are furthermore required (as part of the postlab writeup) to plot ϕ during charging/discharging as a function of Qc (Figure 5). In this curve the charge/discharge overpotentials are clearly seen. These are a result of the internal cell resistance where the main contributions are coming from the membrane ion transport, electrode charge transfer, and mass transfer limitations. Nonetheless, the main point is that the area under the curve is the total electrical work, w, involved in the charge/discharge procedures (eq 8). To estimate w, students are asked to perform a numerical integration (using a suitable method, the choice of which they should justify) based on the experimental data. This is typically done in an associated spreadsheet calculation, where the students have to decide on a proper numerical approach (e.g., rectangular, trapezoidal etc.). Once calculated for charging and discharging, the work is used in eq 9 to calculate an overall battery efficiency. As the differences between charging and discharging in Figure 5 illustrate, this emphasizes that, practically/experimentally, batteries are characterized by energy efficiencies below 100%. Rather, application of eq 9

(8)

Finally, the energy efficiency (η E ) of the battery is thermodynamically defined as the negative ratio of work done during discharging and charging wdischarge ηE = − wcharge (9) In a single approximately 3 h experiment, students will obtain data that will allow calculations of key parameters as expressed in eqs 1−9, namely, SOC, Qc, and ηE. A more in-depth treatment of the underlying theory, including the pertinent equations, can be found in the exercise manual in the Supporting Information. Example of Charging/Discharging Data

Experimental data are collected as (i) the cell potential during charging/discharging (ϕ), (ii) the open circuit cell potential (Ecell) during charging/discharging, and (iii) the time of the individual measurements. Figures 4 and 5 show representative data for both charging and discharging, when Ecell and ϕ are plotted as a function of the SOC and capacity (Qc), respectively. In Figure 4a, data obtained by lab instructors are superimposed with the theoretical curve based on eq 6. A clear “Nernstian” behavior versus SOC is observed for instructor data sets. In addition, Figure 4a includes two

Figure 5. ϕ as a function of capacity (Qc). 1469




typically results in values ranging ηE ≈ 40−70%. This variation is most likely a result of different internal resistances that could be a result of a variation in the contact resistances between different components, with varying activity of the electrodes and differences in Coulombic efficiencies. The rest of the energy is dissipated as heat to increase the temperature of the electrolytes and the cell. Finally, students are asked to answer questions about the more qualitative aspects of the laboratory work: “Why does the color of the electrolyte change during the experiment?”, “What is the significance of the instrument reading?”, etc.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sigurd Christiansen: 0000-0002-6416-2522 Anders Bentien: 0000-0002-7204-9167

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Notes

The authors declare no competing financial interest.

CONCLUSIONS This experiment has been developed for third semester undergraduate students studying chemical engineering, chemistry, nanoscience, and medicinal chemistry. In our case, we typically have 100+ students attending the course, which also involves normal lectures and theoretical exercises. In our teaching laboratories, it is typically possible to run 6 experiments (18 students) in parallel using only one or two lab instructors. Considering the academic level of the students and the number of parallel sessions, it has been paramount to keep the complexity and cost at a reasonable level. Running/ variable costs can be kept at an acceptable level, as these reflect purchase of chemicals and replacement of membranes/ electrodes (replacement after approximately 3−6 experiments). Furthermore, the experiment is very robust, and only rarely can good data not be obtained. From a pedagogical point of view, the exercise introduces students to basic theoretical concepts from electrochemistry, but also to battery related concepts like cell potential, state-ofcharge, and energy efficiency. Part of this involves numerical integration and the use of spreadsheets, both important tools for any science student. A key emphasis is on data collection and quantification, and to this end, students use basic electrical instruments (e.g., wires, multimeters, power supplies). As an alternative to using the hand-held multimeters and power supplies, programmable battery analyzers (load/source) could be used. However, it is our opinion that some important learning objectives will be lost as many students will see the battery analyzer as a “black box”. It is our clear experience that, in particular, understanding the distinction between “current/ voltage” and “open circuit voltage/voltage measured with current” is difficult for many students. Finally, although this exercise has been developed for a third semester undergraduate course on physical chemistry, we have also developed a more advanced exercise for a graduate course in battery and membrane technology. Here we introduce flow conditions as well as more advanced thermodynamic concepts, such as the proper use of activities and activity coefficients, mass transport limitations, and reaction rates. Experimentally, we also include measurement of the polarization curve and the internal resistance, and measurement of battery capacity and energy efficiency at different charge/discharge speeds.

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drawings, student report template, and student feedback (PDF, DOCX)

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ACKNOWLEDGMENTS We thank the machine shop and electronics shop at the Department of Chemistry. Also, we thank students from the 2018 course for their in-depth feedback and high response rate.

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REFERENCES

(1) Cullen, D. M.; Pentecost, T. C. A Model Approach to the Electrochemical Cell: An Inquiry Activity. J. Chem. Educ. 2011, 88 (7), 1562−1564. (2) Hendel, S. J.; Young, E. R. Introduction to Electrochemistry and the Use of Electrochemistry to Synthesize and Evaluate Catalysts for Water Oxidation and Reduction. J. Chem. Educ. 2016, 93 (11), 1951− 1956. (3) Compton, O. C.; Egan, M.; Kanakaraj, R.; Higgins, T. B.; Nguyen, S. T. Conductivity through Polymer Electrolytes and Its Implications in Lithium-Ion Batteries: Real-World Application of Periodic Trends. J. Chem. Educ. 2012, 89 (11), 1442−1446. (4) Vidal-Iglesias, F. J.; Solla-Gullon, J.; Rodes, A.; Herrero, E.; Aldaz, A. Understanding the Nernst Equation and Other Electrochemical Concepts: An Easy Experimental Approach for Students. J. Chem. Educ. 2012, 89 (7), 936−939. (5) Jakubowska, A. A Student-Constructed Galvanic Cell for the Measurement of Cell Potentials at Different Temperatures. J. Chem. Educ. 2016, 93 (5), 915−919. (6) Brosmer, J. L.; Peters, D. G. Galvanic Cells and the Determination of Equilibrium Constants. J. Chem. Educ. 2012, 89 (6), 763−766. (7) Martin-Yerga, D.; Costa Rama, E.; Costa Garcia, A. Electrochemical Study and Determination of Electroactive Species with Screen-Printed Electrodes. J. Chem. Educ. 2016, 93 (7), 1270−1276. (8) American Chemical Society Committee on Professional Training. Undergraduate Professional Education in Chemistry: ACS Guidelines and Evaluation Procedures for Bachelor’s Degree Programs, Spring 2015. https://www.acs.org/content/dam/acsorg/about/ governance/committees/training/2015-acs-guidelines-for-bachelorsdegree-programs.pdf (accessed May 3, 2019). Page 7 refers to electrochemistry instrumentation. (9) Wedege, K.; Drazevic, E.; Konya, D.; Bentien, A. Organic Redox Species in Aqueous Flow Batteries: Redox Potentials, Chemical Stability and Solubility. Scientific Reports. 2016, 6, 39101. (10) Huskinson, B.; Marshak, M. P.; Suh, C.; Er, S.; Gerhardt, M. R.; Galvin, C. J.; Chen, X. D.; Aspuru-Guzik, A.; Gordon, R. G.; Aziz, M. J. A metal-free organic-inorganic aqueous flow battery. Nature 2014, 505, 195−198. (11) Instructional videos (both accessed May 3, 2019) on cell parts and assembly can be accessed online: https://youtu.be/cppYnEzusPI and https://youtu.be/Vx2TAQBMLUI. (12) The practical test to connect an LED was included as a proof of principle test. In all cases the LED (which requires 0.9 V operating voltage) was not lighting up when the battery was uncharged, but it was lighting up in the final state. The “drop probe” should indicate that the bromine formed is coloring the otherwise colorless ammonium bromide solution. At the discharged stage, students can

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.9b00090. Experimental protocol, pH dependence of the cell potential for AQDS, student handouts, mechanical 1470




still sometimes see a yellowish color, a point which can be discussed in the final report. It is recommended that students take a picture of the various colors developed. (13) Khataee, A.; Wedege, K.; Drazevic, E.; Bentien, A. Differential pH as a method for increasing cell potential in organic aqueous flow batteries. J. Mater. Chem. A 2017, 5, 21875−21882. (14) Michalowski, T. Calculation of pH and Potential E for Bromine Aqueous Solution. J. Chem. Educ. 1994, 71 (7), 560. (15) Domin, D. S. A content analysis of general chemistry laboratory manuals for evidence of higher order cognitive tasks. J. Chem. Educ. 1999, 76 (1), 109−120. (16) Millar, R.; Tiberghien, A.; Le Maréchal, J. Varieties of labwork: A way of profiling labwork tasks. In Teaching and Learning in the Science Laboratory; Phillos, D., Niedderer, H., Eds.; Klüwer Academic Publishers: New York, 2006; pp 9−20. (17) Nielsen, B. L.; Hougaard, R. F. Scaffolding students’ reflective dialogues in the chemistry lab: Challenging the cookbook. In Research, Practice and Collaboration in Science Education Proceedings of the ESERA 2017 Conference: Proceedings of ESERA 2017; Finlayson, O., McLoughlin, E., Erduran, S., Childs, P., Eds.; Research, Practice and Collaboration in Science Education Proceedings of the ESERA 2017 Conference; Dublin City University: Dublin, Ireland, 2018; pp 2237− 2246. (18) de Paula, J.; Atkins, P. Physical Chemistry, 10th ed.; Oxford University Press: Oxford, 2017.

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