Anodization of Bismuth: Measuring Breakdown Voltage and

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Anodization of Bismuth: Measuring Breakdown Voltage and Optimizing an Electrolytic Cell Thomas Nagel,† Casey Mentzer,*,† and P. Mike Kivistik‡ †

Brewton Brothers, Sparks, Nevada 89434, United States Chemical and Materials Engineering Department, University of Nevada, Reno, Nevada 89557, United States

‡

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S Supporting Information *

ABSTRACT: This experiment provides students with a thorough understanding of electrolysis and the parameters that most greatly impact optimization of this process. In this laboratory exercise, students will focus on the anodization of bismuth metal to develop thin oxide films. When designing an electrolytic cell devoted to anodization, there are several major factors necessary for the optimization of the cell and these are explored in this experiment. Current density, electrolyte concentration, breakdown voltage, and anodization rate all impact development of these anodic oxide films. This exercise emphasizes the interrelationships between these critical factors and the best way to optimize an electrolytic cell. This procedure is carried out with standard chemicals and widely available technology to allow for an inexpensive and accessible laboratory experiment. KEYWORDS: Upper-Division Undergraduate, Chemical Engineering, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Applications of Chemistry, Electrochemistry, Electrolytic/Galvanic Cells/Potentials, Materials Science, Physical Properties

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They will also be introduced to the concepts of anodization in a visually stunning way through production of iridescent anodic film layers. Analysis of the experimentally derived data will also allow for practical connections to production environments. This experiment has been created with the aim of providing students with an experience in which they explore many of the real-life considerations for electrochemical systems and how to optimize some of the most important operating conditions. This laboratory exercise has also been designed to be costeffective by using inexpensive materials and commonly available equipment to provide an experiment that is accessible and easy to implement into a curriculum. The exercise has been constructed to be carried out in a typical 3 h laboratory environment by upper division chemistry or chemical engineering students with knowledge of physical and electrochemical principles.

lectrolysis is an important electrochemical unit operation utilized in many industries from the manufacture of chemicals such as chlorine and hydrogen to the production of thin films on metals to create advanced materials such as TiO2 nanotubes or porous alumina membranes.1−4 While cathodic reactions are typically of more interest in common applications of electrolysis, anodization is widely used to achieve electrical insulation, coloring pretreatment, and corrosion resistance by converting the surface layer of the metal anode into an oxide coating.5 With so much diversity in the application of this process, it becomes important for undergraduate chemistry, chemical engineering, and material science students to understand the theory behind electrolysis and the important factors that impact it. There is no shortage of experiments that seek to provide unique approaches to the area of electrolytic cells. There have been many exercises developed focusing on topics such as thin film interference,6 anodization and color treatment of aluminum,7,8 cost-effective cell construction,9,10 and corrosion;11,12 however, there is a definite lack of experiments that explore practical optimization parameters as well as the phenomena of voltage breakdown. This experiment details a thorough investigation into the various factors that impact the anodization process. Students will determine the optimal conditions for bismuth anodization by investigating the interrelationships between breakdown voltage, molar concentration of electrolyte, anodization rate, and current density. © XXXX American Chemical Society and Division of Chemical Education, Inc.

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ELECTROLYSIS SETUP The entire experiment is carried out utilizing one electrolytic cell configuration. The anode and cathode, both being 10 g cylindrical pellets of bismuth (chemicalstore, 99.99% purity), are connected to a 30 V, 3 A variable dc power supply with a Received: July 13, 2018 Revised: November 3, 2018

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DOI: 10.1021/acs.jchemed.8b00481 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

supplied with a constant current of 20 mA and the voltage rise was manually tracked at a rate of one reading per 5 s until the breakdown voltage occurred. The voltage at which this phenomenon occurs was then recorded for each trial. During electrolysis, students will observe the anode undergo a multitude of color changes. Each color represents a unique film thickness of the growing oxide layer. This demonstrates the concept of anodization not only analytically, but also visually. During this first 3.0 M trial, students will document the distinct colors that appear and record the voltage. Instructors may extend the scope of the laboratory exercise to cover additional aspects of thin film interference and iridescence as is relevant to their curriculum. After the breakdown voltage has been observed, a further 5− 10 readings were recorded after the value was identified to confirm the oscillatory behavior of the voltage. Manually tracking data removes the need for more specialized equipment such as specific computer software or X-t recorders. This reduces the cost of the experiment and makes it more accessible to teachers and students.

built-in digital voltmeter and ammeter. A list of materials is provided in the Supporting Information. The anode and cathode are then both submerged halfway in sodium hydroxide solutions of varying concentration. A multimeter with a digital display is attached in line with the circuit to provide a more precise and user-friendly interface for data acquisition (see Figures 1 and 2).

Data Analysis: Concentration and Breakdown Voltage

The voltage versus time data of each concentration was then plotted together to depict the breakdown voltage (see Figure 3). The breakdown voltage can either be identified from Figure 1. Schematic of the electrolytic cell setup with a sodium hydroxide electrolyte.

Figure 3. Arrows indicate the transition voltage on the anodization growth curves of five concentrations of sodium hydroxide electrolyte solution at constant amperage of 20 mA. Figure 2. Laboratory setup shown with dc power supply and multimeter.

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laboratory observation or may be determined graphically as the voltage at which the variation with time exhibits a sudden change, from an almost linear variation to an erratic behavior. This voltage is an important parameter to determine as it represents the point in which the oxide layer has reached its maximum obtainable thickness known as the critical thickness. This point also indicates the end of predictable and stable film generation and signals the start of the oxide layer breaking down. These breakdown voltages are indicated with arrows on Figure 3. Once the breakdown voltages were identified using Figure 3 (or noted from experimental observation), the relationship between breakdown voltage and concentration was then quantified by

IMPACT OF ELECTROLYTE CONCENTRATION ON BREAKDOWN VOLTAGE AND ANODIZATION RATE

Experimental Method

Five concentrations of sodium hydroxide were used to investigate the effect that electrolyte concentration has on the breakdown voltage. The breakdown voltage is the point identified by oscillating or erratic voltage readings. In some cases, an electrical discharge or “spark” appears at the anode, or a distinguishable popping sound may be heard.13−15 The molar concentrations used in this experiment started at 3.0 M and increased by 1.0 M steps up to a maximum concentration of 7.0 M. For each trial, the same anode and cathode were used but underwent a surface cleaning with a 4 M H2SO4 solution between successive tests to remove oxide formation and other contaminants. For each concentration tested, the cell was

Vb = ac + bc log C

(1)

where V b is the breakdown voltage, a c and b c are experimentally determined dependent variables, and C is the electrolyte concentration.13 The breakdown voltages were then B




Figure 4. Breakdown voltage as a function of the logarithm of sodium hydroxide electrolyte concentration for bismuth anodization at a constant amperage of 20 mA.

Figure 5. Plot of the anodization rate as a function of applied current for various concentrations.

Discussion

plotted as a function of log(CNaOH) to determine ac and bc through linear regression (see Figure 4).

This laboratory procedure exemplifies the phenomenon of avalanche breakdown which is the effect of impact ionization. Bismuth ions react with oxygen formed at the surface of the electrode to create a bismuth oxide thin film that is nanometers thick. As the oxide film grows, the resistance of the circuit increases which causes the voltage to increase. Once the oxide film reaches a critical thickness, and the voltage attains a certain value, the ions in solution have enough energy to impact the Bi2O3 film and dislodge an electron. This electron travels through the oxide film colliding with other species and provoking an avalanche effect of electrons releasing electrons. The successive release and movement of electrons causes electrical destruction of the oxide film. This rapid flow of electrons often causes visible sparking. This effect is heavily influenced by the concentration of the electrolyte. As the number of ions in solution increases, the probability that impact ionization will occur increases. This effectively lowers the required energy that would be needed to cause impact ionization and avalanche breakdown.13,14 This process of impact ionization is graphically represented in the example lecture provided in the Supporting Information. Upon carrying out the analysis, students should be able to recognize an inverse linear behavior between the log of the molar concentration and breakdown voltage. By analyzing this relationship and obtaining the constants ac and bc in eq 1, students will be able to predict the limitations on anodic oxide growth for any given electrolyte concentration by calculating

Data Analysis: Concentration and Anodization Rate

The anodization rate (dV/dt) for each of the five concentrations was determined by carrying out a linear regression of the voltage increases prior to breakdown. This is the region of linear growth, neglecting the region of oscillatory behavior. This process was carried out through Microsoft Excel. The slopes from this regression are a measure of the anodization rate, dV/dt (V/s). These were then plotted as a function of concentration (see Figure 5). Data Analysis: Voltage and Color Presentation

The distinct colors observed throughout the first trial were listed at the voltage they occurred. The wavelengths at which these pure colors occur were also listed. As shown in the graphical abstract, with increasing voltage the colors present in the order: orange, purple, blue, yellow, green, and pink. Reported voltages may vary slightly from group to group due to how observations and recordings are made. Following the listing of this information, students are asked to explain why the colors observed did not present in the same order as they would when looking at the electromagnetic spectrum. This relates to the trichromatic nature of human color perception coupled with constructive/destructive interference of light waves. This concept is explained in greater detail in the laboratory exercise provided in the Supporting Information. C




Figure 6. Bismuth anodization growth curves in a 4.0 M NaOH electrolyte solution at various currents.

Figure 7. Anodization rate as a function of applied current for a 4 M NaOH solution.

the voltage at which critical thickness is obtained and film destruction begins. Analysis of these trends demonstrates to students how increasing ionic strength leads to breakdown at lower voltages. The results of graphing the relationship between concentration and anodization rate allow students to observe that while increasing the concentration of an electrolyte does increase conductivity (facilitating faster anodization), there is a certain point where increasing concentration lowers the conductivity. The observed variation of ion transport is caused by the influence of the electrolyte concentration on solution viscosity and by the effect of a decrease in molecular dissociation.15 This shows students that there exists a certain concentration at which the reaction rate is maximized creating optimal conditions for anodization. When students evaluate both analyses, they will gain a greater understanding of the trade-off between finding an optimal concentration and how those concentrations would limit the maximum obtainable thickness. Instead of focusing on one single parameter, students engage in an exercise that reflects the interconnectedness of these factors. This knowledge is used to understand the major design considerations behind electrolytic cells. This is further reinforced in the postlab questions of the laboratory exercise provided in the

Supporting Information when students are asked to apply their learning in practical problems.

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RELATIONSHIP BETWEEN CURRENT DENSITY AND ANODIZATION RATE

Experimental Method

Five different applied currents were used to determine the effects that amperage has on anodization rate. For the first trial, the cell was again constructed using a 4.0 M NaOH solution. The cell was supplied a constant current of 10 mA while manually tracking the voltage rise in the fashion as previously described at a rate of one reading per 5 s for a duration of 5− 10 readings after breakdown voltage. After this trial, the H2SO4 solution from the first two experiments was used to strip the anode and cathode of any oxide layer in preparation for the next trial. This procedure was repeated for 15, 20, 25, and 30 mA. Data Analysis

The voltage versus time data for each of the five currents were then plotted. Using the data points that exhibited linear growth prior to voltage breakdown, a linear regression was used to determine the anodization rate (dV/dt) (see Figure 6). To allow for these values to be useful for further calculation, the applied current was converted into an intrinsic property by D




factor on the effects of electrical shocks of the human body. Even with the relatively low voltage range of the power supply (0−30 V), significant harm could result from improper handling of electrical equipment due to the available amperages (3 A). OSHA states that currents as low as 50 mA can cause muscular contractions resulting in an inability to let go as well as substantial pain or even respiratory arrests. Amperages of 1 A or higher can result in ventricular fibrillation and death.16 When operating the dc power supply, it is important to make sure that the power supply is off when not in use. Alligator clips must only be handled when the power is off. It is important not to exceed the current range of this experiment, which only utilizes a maximum of 30 mA, to minimize risk.

relating the current to the anode in terms of current density. This was carried out by J=

I A

(2)

where J (mA/cm2), I (mA), and A (cm2) are the current density, the current, and the surface area of the anode, respectively. This surface area was based on the dimensions supplied by the bismuth pellet manufacturer (chemicalstore, diameter, 18 mm; height, 3 mm) with any film deposition considered to be negligible. Half of the surface area of the anode was assumed to be anodized on the basis of the experimental method of only half-submerged electrodes. The anodization rates were then plotted as a function of applied current density to highlight the differences in the rate of film growth (see Figure 7).

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STUDENT PARTICIPATION AND RESPONSE The experiment was conducted in a unit operations class with senior level chemical engineering students at the University of Nevada, Reno. There were 38 students split into groups of four or five to conduct the experiment. A prelab test was given to each student to use as a baseline in tracking student comprehension and understanding of the experiment. There were 10 questions included ranging from elementary electrochemistry to specific ideas about voltage breakdown and anodization rate. After the test was completed, a lecture was given to introduce the students to the content they would be exploring in the experiment. Each group of students conducted the experiment in a 3 h lab block, analyzed the data outside of the lab, and generated lab reports. When the students turned in the final report, a postlab assessment was given to each student which was designed to be similar in content to the prelab test. In a comparison of the results from the tests, the overall scores from the prelab test to postlab test saw an increase of 47% (95% CI [30, 64]). The postlab test also contained a subjective questionnaire for students to provide input about their perception of the laboratory exercise. These findings are presented in Table 1. Overall, the test results show an increase in the students’ knowledge and understanding of electrolysis, voltage breakdown, and anodization rate. The test results also demonstrated that the average student agreed that the experience was enjoyable and improved their comprehension of the material studied.

Discussion

By investigating the effect of applied current on the anodization rate, students will gain an understanding of how increasing current density increases the reaction rate, and by what degree. With these anodization rates being expressed as a function of current density, students can extrapolate this data to solving more practical problems such as calculating the time and current required to obtain a set voltage (and therefore thickness) of an anode of any given size. This data can then be used to evaluate current density as yet another parameter in the optimization balancing act with concentration and critical thickness. Students will compare the anodization rates of various current densities with those of various concentrations allowing them an opportunity to think about different ways to increase anodization rate in an applied setting and optimize production numbers. It is also possible to extrapolate these concepts to practical applications and compare the cost of power by increasing current to the costs of raw electrolyte materials required to obtain a similar rate. Examples of these industrial related problems are provided in the postlab presented in the Supporting Information.

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SAFETY Typical safety measures around acid/base chemicals are necessary. Some hazards may be mitigated with instructors preparing stock solutions of diluted chemicals. Personal protective equipment (PPE) should be, at minimum, clothing that provides full body coverage as in the wearing of long sleeves, long pants or skirt, safety goggles, and nitrile gloves. When carrying out chemical dilution, students should ensure that acid or base is added to water. Care should be taken as these dilutions, particularly that of sulfuric acid, are exothermic and can generate fumes. Preparing these solutions should take place in a fume hood. As with many electrolysis reactions, the generation of hydrogen and oxygen does occur, so care should be taken to ensure no open flames are in the vicinity of the anodization process. While gas production is minimal, adequate ventilation should be provided. Before altering the choice of the electrolyte used, the nature of the gas liberated should be considered. Electrolyte solutions of certain chemicals could result in the generation of hazardous byproducts (e.g., hydrochloric acid electrolyte would produce chlorine gas at the anode). Caution should be exercised when operating electrical equipment. The strength of a current is the most important

Table 1. Comparison of Postlab Questionnaire Responses Average Rating,a,b N = 38

Statements for Student Response This lab helped increase my understanding of electrolysis This lab helped increase my understanding of voltage breakdown This lab helped increase my understanding of thin film interference I found this lab enjoyable I would recommend this lab to other students in my major I think this lab reflects practical engineering application I thought the lecture content was effective at preparing me for this experiment

1.1 (0.8, 1.4) 1.5 (1.1, 2) 1.1 (0.6, 1.6) 1.3 (0.6, 1.9) 0.9 (0.4, 1.4) 1.1 (0.6, 1.6) 1.1 (0.4, 1.7)

a

Students scored statements using a Likert-type scale ranging from +3 (“strongly agree”) through +1 (“agree”) and from −1 (“agree”) to −3 (“strongly disagree”). bAll ratings reported with a 95% confidence interval. E



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(12) Ivey, M. M.; Smith, E. T.; et al. Electrochemical Polishing of Silverware: A Demonstration of Voltaic and Galvanic Cells. J. Chem. Educ. 2008, 85 (1), 68−71. (13) Ikonopisov, S. Theory of Electrical Breakdown During Formation of Barrier Anodic Films. Electrochim. Acta 1977, 22 (10), 1077−1082. (14) Afshar, A.; Vaezi, M. R. Evaluation of Electrical Breakdown of Anodic Films on Titanium in Phosphate-Base Solutions. Surf. Coat. Technol. 2004, 186 (3), 398−404. (15) Prentice, G. Electrochemical Engineering Principles; PrenticeHall: Upper Sadle River, NJ, 1991; pp 6−21. (16) National Safety Council, U.S. Department of Labor OSHA. https://www.osha.gov/dte/grant_materials/fy07/sh-16610-07/01_ pg-module_1.pdf (accessed Nov 2018).

ASSOCIATED CONTENT

* Supporting Information S

Included is the , an , and a . (DOCX) (DOCX) (DOCX) (PDF) The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00481. Anodization of bismuth lab (PDF, DOCX) Anodization of bismuth lab key (PDF, DOCX) Anodization of bismuth example lecture (PDF) Anodization of bismuth materials list (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Casey Mentzer: 0000-0003-1198-4984 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the Department of Chemical and Materials Engineering, University of Nevada, Reno. The authors would like to express their gratitude to Mackenzie Parker, Caitlin Burke, and Megan Mentzer for their support in the facilitation of this work.

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REFERENCES

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Anodization of Bismuth: Measuring Breakdown Voltage and

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