ChE391 LabManual F15

CHE 391

CHEMICAL ENGINEERING LABORATORY 4

DEPARTMENT OF CHEMICAL ENGINEERING

Edition: Fall 2015

TABLE OF CONTENTS

INTRODUCTION .......................................................................................................................1

ACADEMIC OFFENCES UW Policy #71 .................................................................................2

LABORATORY SAFETY..........................................................................................................3

GENERAL INSTRUCTIONS ....................................................................................................4

LABORATORY REPORTS .......................................................................................................5 A. Preliminary Reports ................................................................................................................5

B. Memo reports .........................................................................................................................6 Evaluating Errors .........................................................................................................................9 Propagation of Error Equations .................................................................................................10

Error in Linear Regression Analysis .........................................................................................10 Assessing Quality of Fitted Data ...............................................................................................11 Tables and Figures.....................................................................................................................11

EXPERIMENT 1: Electrowinning of Zinc from Zinc Sulphate Solution...............................17 PRELAB REPORT QUESTIONS ............................................................................................22

REPORT QUESTIONS ............................................................................................................23

EXPERIMENT 2: Potential-Controlled Analysis of Redox Couple........................................25 PRELAB REPORT QUESTIONS: ...........................................................................................35

DATA ANALYSIS: ..................................................................................................................35

EXPERIMENT 3: Analysis of Fuel Cell Performance Curves ................................................38

PRELAB REPORT QUESTIONS: ...........................................................................................46 REPORT QUESTIONS: ...........................................................................................................46

EXPERIMENT 4: Corrosion Studies .......................................................................................48 PRELAB REPORT QUESTIONS: ...........................................................................................63 DATA ANALYSIS: ..................................................................................................................63

EXPERIMENTS 5 and 6: COMSOL Tutorials .......................................................................65

General Lab Info 1

INTRODUCTION

Progress in the field of science or technology depends not only on a clear grasp of relevant

theoretical principles, but also on the quality of experimental investigations carried out in that

field. Solutions to many complex problems of interest to chemical engineers can be approached

by a carefully designed and properly executed experimental program.

Laboratory courses allow the student to study numerous experimental arrangements, equipment

details, and measuring devices which cannot be covered adequately in a lecture course. An

appreciation is also gained of the difficulties involved in obtaining accurate data under properly

controlled conditions.

Basically, the three main course objectives of this laboratory course are:

To assist in the understanding of some basic principles of chemical engineering through actual observations of the behaviour of electrochemical systems.

To help develop skills in experimentation, data analysis and interpretation of results.

To practice in the art of writing effective engineering reports.

General Lab Info 2

ACADEMIC OFFENCES

EXCERPTS FROM THE UNIVERSITY OF WATERLOO POLICY #71

"STUDENT ACADEMIC DISCIPLINE POLICY"

Original text available at:

http://www.adm.uwaterloo.ca/infosec/Policies/policy71.htm

"A university is a community of scholars in which knowledge is generated and disseminated

through scholarship and teaching. All members of the community - faculty, students and staff

are bound to conduct themselves with honesty, integrity, fairness and concern for others. Any

action which unnecessarily impedes the scholarly activities of members of the University is an

offence punishable by appropriate disciplinary action."

Some of the academic offences outlined by the University include:

Infringing unreasonably on the work of other members of the University community

(disrupting classes or examinations; harassing, intimidating or threatening others).

Cheating on examinations, assignments, work-term reports, or any other work used to

judge student performance. Cheating includes copying from another student's work

or allowing another student to copy from one's own work, submitting another person's

work as one's own, fabrication of data, consultation with an unauthorized person

during an examination or test, and use of unauthorized aids.

Plagiarism, which is the act of presenting the ideas, words or other intellectual

property of another as one's own. The use of other people's work must be properly

acknowledged and referenced in all written material such as take-home examinations,

essays, laboratory reports, work-term reports, design projects, statistical data,

computer programs and research results.

Submitting an essay, report, or assignment when a major portion has been previously

submitted or is being submitted for another course without the express permission of

all instructors involved.

Please note that Improper Collaboration or Inappropriate Collaboration is an academic offence.

While the lab experiment is a collaborative effort, lab reports must be written independently, to

do otherwise, is an academic offence.

If unsure of what the limits are, ASK!

General Lab Info 3

LABORATORY SAFETY

It is essential that engineers develop a habit of safety in all experimental work. Dealing with

flammable corrosive liquids, spattering reagents, violent chemical reactions, or the escape of

steam can be quite dangerous in the absence of sensible protective measures or devices.

All students are required to have completed a WHMIS course. If you have any doubts about a procedure, ask the TA for assistance.

Safety goggles and lab coats must be worn inside the lab. Closed-toe shoes and long pants are required. Contact lenses must NOT be worn in lab.

Read the Material Safety Data Sheets (MSDS) prior to experiment.

All hazardous wastes must be emptied into the appropriate waste containers. Read waste labels before disposing into any receptacle. Check compatibility first.

Keep chemical bottles closed tightly when not in use.

Immediately inform your Teaching Assistant of any injuries or spills. For cuts, burns and if a chemical comes in contacts with your skin, eyes or mouth, flush immediately with

water at the sink or safety station. You must file an accident report with the department.

In the event of a serious injury, inform the TA that you wish to go to Health Services.

The TA will send someone to accompany you.

Dispose of any broken glass in the bucket labelled Broken Glass.

Clean up work area. Remove lab gloves and wash hands before leaving.

Make sure to know the location of the following:

Fire extinguisher

Safety shower

First aid kit

Fire alarm

Telephone

FOOD and BEVERAGES ARE STRICTLY PROHIBITED FROM THE LAB

General Lab Info 4

GENERAL INSTRUCTIONS

Most of the experiments require several students working together in order to manipulate the

equipment, take readings, and record the data. Accordingly, students are assigned to groups of

three.

Before commencing an experiment, all members of a group should have a thorough

understanding of:

Theoretical principles involved in the experiment.

Type and amount of data to be recorded.

Expected results.

Whenever the foregoing requirements are met, the experiment can become a true learning

experience which will immeasurably assist in understanding the underlying principles;

otherwise, the exercise becomes merely one of manipulation of equipment and the reading of

various instruments.

Every member of a lab group is expected to actively participate in the lab. The

pre-assigning of duties to each member of the group is strongly advised in order

for the experiment to be performed effectively and quickly.

PROFESSIONAL CONDUCT: Although you are going to work as a group, some marks are

allocated for individual professional conduct. Marks will be deducted for horse-play and lack of

participation in the experiment.

ATTENDANCE: Marks are reduced for latecomers to the lab. If you need to miss part of a lab

session for a co-op interview, you must inform the TA in advance to ensure that you fulfil your

lab requirements. Refer to course outline for details.

In the event of a lab missed due to an illness, contact the TA or lab instructor

within 2 days to make arrangements to make-up missed session. You must

provide the chemical engineering undergraduate secretary with a Verification of

Illness form completed by a doctor. Forms may be downloaded from:

http://uwaterloo.ca/health-services/student-medical-clinic/services/verification-illness.

General Lab Info 5

LABORATORY REPORTS

Two types of laboratory reports are required for each lab in this course. The requirements for

each type of report are outlined below:

A. Preliminary Reports

The purpose of a Preliminary Report is to prepare for the experiment. Each group prepares one

Preliminary Report for each experiment (excluding computer tutorials). The marks for these

reports count toward the course mark of each group member. This report should be submitted to

the online dropbox by noon of the day before your lab session. It will be checked to see that the

group is not pursuing some erroneous task and graded. Your TA will address any concerns at the

start of the lab session.

The report should contain the following:

1) Statement of the objective.

2) General summary of the experimental design in your own words (i.e. number of runs and range over which the experiment, equipment used, diagram of experimental set-up, etc.).

3) Equations to be used in calculation of results.

4) Possible hazards to be encountered in the experiment and safety precautions to be taken.

5) Blank datasheet with appropriate headings (bring a copy to lab).

6) Answers to Prelab Report Questions from lab manual.

SAFETY ASSESSMENT CHECKLIST When evaluating the safety hazards involved in an experiment, consider the follow hazard categories:

Chemical Hazards (e.g. exposure, toxicity, incompatibilities, waste handling, by-products formed)

Electrical shock Burn/scald potential or risk of fire Radiation exposure Pinch points or Rotating parts Cuts or Stabs potential Pressurized lines (explosions), vacuum systems (implosions) Potential Projectiles Suffocation hazard or low O2 level potential Trips/falls Overhead hazards (falling objects, low-clearance zones)

Continued on next page

General Lab Info 6

Also consider the precautions that must be taken to ensure your personal safety and that of the people working nearby.

o Proper handling of chemicals o Proper operation of equipment o Personal Protection Equipment (PPE) required o Protective devices needed on equipment (i.e. Guarding on rotating parts) o Preventative behavior (i.e. securing loose clothing and hair, not wearing contact lenses when

working with chemicals, checking integrity of PPE, etc.) o Emergency response (avoiding injuries or contamination from hazardous incidents: what to do if

there is a spill of chemical, basic first aid, etc.)

B. Memo reports

A Memo Report is a complete record of your experiment. When required, a memo report is

prepared and submitted by each group. It describes:

Why the experiment was performed.

What data were obtained, and

Your interpretation of the data

A scientific report is not merely a list of facts or observations; it must provide an

interpretation of the findings and show their significance. A good engineering

report is organized logically and the wording chosen carefully so that the reader

knows exactly what you mean and where to find the evidence that supports your

conclusions.

The Memo Report should include the following:

1) Title Page

Name of the University and Department.

Course number and name.

Experiment number and title

Group number and names all the members of the group contributing to the report.

Date experiment was performed.

To whom and the date that the report was submitted.

General Lab Info 7

2) Table of Contents

List the divisions of the report with page numbers opposite. Lengthy divisions should be

subdivided into appropriate headings.

3) Introduction

The purpose of the Introduction is to guide the reader to consider the importance of the topic

being presented. It should include:

General background information relating to the experiment.

Significance of the data obtained to Chemical Engineering (i.e. current and future applications).

Brief statement of the objective.

4) Theoretical Principles

This section should include:

Overview of the theoretical foundation of the experiment.

Include all necessary equations to be used in the calculations. Label each with a unique id. Define variables. Derivations of equations go in the Appendix. Remember to state

any assumptions associated with the derivation of the equation.

Conclude with a brief statement indicating how the objectives will be met using the theoretical principles involved.

5) Experimental

In this section:

Summarize, in your own words, what was done during the laboratory based on procedures given in the Laboratory Manual

Identify how equation(s) from theory section will be applied.

Include a labeled sketch or schematic of the experimental set-up.

State all modifications made to the procedure.

The Introduction and Theoretical Principles must be written in your own words. Do not

copy from the Lab Manual. Research your topic. Use library resources.

General Lab Info 8

6) Observations and Results

In this section, you should present your qualitative and quantitative observations. Guide the

reader through the manipulation of the data in fitting it to the theoretical model.

Qualitative Observations

a) Describe the experimental observations and, if required, refer to the appendix which contains your original measurements.

b) Report any irregularities in the experimental procedure that might explain outlying data points.

Original datasheet should go in an appendix

Quantitative Results

c) Present results which were calculated based on the data recorded during the experiment. Remember to add reference to which equation from theoretical

principles section was used. Place sample calculations in the appendix but refer to

them here.

d) Use tables or figures to clearly demonstrate trends in data (Refer to Tables and Figures section below).

e) List the estimated precision of your measurements, as well as, any other observations which may help to explain the results. Refer to the Evaluating Error

section.

f) Use significant digits. Remember to include units.

Always maintain significant figures when reporting measured or calculated values. The

number of digits used to express a value is extremely important as it indicates the precision

of the value. The last significant digit is considered to vary by 10 % unless the error is

specified. It is incorrect and misleading to report values to higher precision than possible

from the equipment used in the experiment. Refer to Chapter 11 section 6.2 in Introduction

to Professional Engineering in Canada by Andrews et. al. (2003) for more information on

determining significant figures through algebraic operations.

General Lab Info 9

Evaluating Errors

Unless one is counting individual objects, there is error inherent in all measurements. Two types

of errors occur: Systemic (or determinate) and Random errors.

Systemic errors are constant or proportional biases in a measurement occurring due to the

investigators habits or poor calibration or drift of the analytical equipment. For example, the ruler was misread by +0.5 mm each time, or the pressure gauge was zeroed when the system

pressure was actually 10 psi. Large differences between experimentally-derived results and

commonly-accepted or cited values of a parameter are likely due to the occurrence of systemic

errors. Fortunately, systemic errors may be identified and correction factors used to compensate

for them if they can be quantified. Systemic errors can also be minimized by regular

recalibration of analytical equipment and by standardizing experimental procedures.

Random errors, on the other hand, are inevitable and vary from reading to reading. It is an

estimate of these random errors which must be included when reporting any measurement or

derived experimental value. Unlike systemic errors, random errors cannot be quantified exactly

but they can be approximated. For example, you measure the length of your pen to be 16.0 cm

using a ruler with gradations to the nearest 0.5 cm. Those markings are thick and imprecise so

you might assume that you can read precisely to, say, the nearest 0.1 cm. This is the external error of this measurement. You would report the length as 16.0 0.1 cm. Depending on the size of the scale of measurement, the human eye can differentiate up to 1/10 of the interval

between markings. For digital displays, the precision is to the manufacturers specifications, if given or half of one decimal place below the last stable digit. Use your judgment in assessing

the precision of the measurements made during the experiments.

Random errors tend to offset each other so, for any given parameter the average of several

repeated measurements will be more precise than any single measurement. You can estimate

this higher precision by calculating the average value ( x ) and the standard deviation (s) of n

repeats. Your measurement would be reported as x SE where the standard error SE can be

approximated by:

n

sSE

General Lab Info 10

Propagation of Error Equations

If the precision of individual measurements are known, the overall precision of any value derived

from these measured values can be approximated using the equations shown below:

Addition and Subtraction

Multiplication and Division

Exponential In General

Where: k, n are constants, a, b, c, d, x and y are measured variables, x is the standard deviation of x which can be approximated by the standard error of a repeated measurement or the estimated

precision or external error of a single measurement.

Error in Linear Regression Analysis

Linear regression uses the Method of Least Squares to determine the line which best fits N

experimental data points. The relationship is plotted as: y = mx + b. Generally in the 290

experiments, the values of interest from a regression line are the intercept, b or the slope of line,

m. Unless the data fit perfectly to a straight (i.e. R2=1), then some error is inherent in these

regressed values and this error must be reported.

Generally in linear regression, the known value is plotted on the x-axis while the measured value

which has some error associated with it is plotted on the y-axis. Linear regression attempts to determine the error in the y-value associated with a given x-value.

To simplify the calculations, some key quantities need first be defined:

N

xxS

iixx

22

N

yyS

iiyy

22

2

2

N

SmSS

xxyyr where (xi,yi) are the individual data points.

...

...

23

22

21

321

ckbkaky

ckbkakky

2222

d

d

c

c

b

b

a

a

y

y

cd

kaby

b

bn

y

y

by n

dx

dyxy

xfy

)(

General Lab Info 11

Use the following equation to find the standard deviation of the slope, sm:

xxrm Sss /

The standard deviation of the y-intercept, sb, is given by:

2

2

1

i

i

rb

x

xN

SS

The standard deviation, sc, of any point taken from the regressed line ( = + ) based on an average of M replicate measurements, is found using:

xx

crc

Sm

yy

NMm

SS

2

211

The line over y indicates the mean (average) of all measured values of yi (where i = 1.N)

used to determine the regression line.

Assessing Quality of Fitted Data

When regressing data, it is assumed that the random deviations from the model are

independent, normally distributed, and have a constant variance. For any modeling of fitted

data, you should assess the quality of the fit of the model using the coefficient of

determination and residual plots. Outliers should be identified and justification provided if

they are removed from the evaluation of the data.

Tables and Figures

Any tables and figures must be explicitly mentioned (cited) in the text and should be inserted in the report as closely after they have been cited as possible. All figures and tables

must have titles and they must give units for all data presented. Identify tables using Roman

numerals (i.e. Table I). Tables must contain enough information to stand alone. The reader should not have to refer

to the report to understand it. An example of a table is shown below.

The experimental conditions and the measurement results are shown in Table .

General Lab Info 12

TABLE I: pH of the reaction product in the titration of 1.0 mmol/L Copper perchlorate using

0.04 wt % Ethylenediamine [en] solution (298 K).

[en] in Cu solution

mmol/L

pH

0.00 2.80

1.46 2.89

2.93 3.05

4.41 3.05

Figures should be prepared by computer. Label all figures with Arabic numerals (i.e. Figure

1, Figure 2, etc.) and a relevant title. As with tables, be sure to include all information that is

necessary for the reader to interpret or identify the image (i.e. reaction temperature, pressure,

etc.) in the title or in a caption.

For graphs:

Label each axis with a scale and title. Remember to include units.

If several sets of data being plotted, use a legend to identify the different symbols.

Depict observed data as discrete points.

Plot trendlines or fitted models using lines.

Sign and date all the figures in ink.

Figure 1:

Microbial Growth Curves (grown at 37C in Tryptose Phosphate broth

with 2.5 g/L dextrose added)

0.000

0.500

1.000

1.500

2.000

2.500

3.000

0 100 200 300 400 500 600

Incubation Time (min)

Op

tical D

en

sit

y (

AB

S)

Bacillus subtilis

Streptococcus faecalis

General Lab Info 13

7) Discussion of Results and Error Analysis

This is the most important part of the report. Many students have trouble knowing what

to discuss. A good guideline is to think of the experiment not merely as a school exercise but

as an experiment being performed as engineering research. Indicate which observations or

calculated results would be useful to other engineers who might read your report. Guide the

reader through your analysis of the observed results. Explain any discrepancies with the

theory and what may have occurred to cause the discrepancy?

Discuss the quality of your experimental data. Are there any data points which may be unreliable? State the reasons for the unreliability. Consider the purity of the reagents

used, quality of measurements made, etc.

All the equations should be presented in the theoretical principles section and

referred here in the discussion section only by equation number.

Note that you must support your ideas and this will usually require finding and

reading reference material.

Discuss your calculated results. It may be necessary to review briefly any key assumption made in the derivation of the equation used in the calculations and how they

might affect the result obtained. Mention any assumptions made in the calculations, such

as, values assigned as constants or data points that were excluded from the calculations.

Relate your results to the theory. Do your results agree with values presented in literature? Include references. Check overall error relative to error propagation results.

Are your values reasonable? If not, try to explain the course of the discrepancy.

Discuss the error analysis. Include a discussion of the individual measurement errors that contributed to the overall error in the calculated results. Which measurement error had

the largest effect on the results? Could the results be improved by taking replicate

measurements, by using different equipment, or by taking measurements under different

conditions? Is the precision in your results comparable to experiments in the literature?

Discuss the significance of your results, and any new information that has been learned.

8) Conclusions

This section should include a summary statement of the relation between the results and the

objectives of the experiment.

What are the principal results of the experiment? They can be listed in point form stating the most important ones first.

Summarize the significance of the results obtained as they relate to applications in engineering.

General Lab Info 14

9) Recommendations

Recommendations to improve the experiment should be included here.

Most important recommendation should be made first.

Make sure your recommendations are feasible from a practical point of view.

If possible, give estimates of the costs involved when making such modifications.

Approximate the % improvement in the quality of the results that could be obtained.

10) Nomenclature

Define all the symbols used in the report in alphabetical order.

11) References

For journals, give complete information, including names of all the authors, title of the

article, name of the journal, volume, page number(s) and the year of publication. For

example:

1. Reilly, P.M., B.M.E. van der Hoff, and M. Ziogas, Statistical study of the application of the Huggins equation to measure intrinsic viscocity, J. Appl. Polym. Sci., 24, 2087-2100 (1979)

For references to books give author(s), Title, edition, publisher, place and (year of publication), and the page number(s).

2. Laidler, K.J. and Meisner, J.H., "Physical Chemistry", Benjamin/Cummings Publ. Co.

Inc., Menlo Park, California (1982), p. 789.

For websites give the authors name, Title of section used, URL, (date accessed)

3. Shuzon Ohe, Distillation Computation, http://www.s-ohe.com/McCabe-Thiele.html

(accessed Jan. 19, 2005).

The abbreviation et al. should not be used in the reference list. List the names of all authors.

Do not repeat references. If the next reference refers to the same article or book write:

4. Ibid., p.238.

If referring to a reference already listed, refer to its reference number and give page number:

5. Ref. 2, p. 237.

General Lab Info 15

12) Appendices

The purpose of putting extraneous material, such as, experimental data into the appendix is to

prevent disruption of the coherence of the report by long derivations or large columns of

numbers. Materials that should be in the appendix include:

Long tables of original observations.

Lengthy derivations of equations

Sample calculations

Answers to Report questions.

Material essential for understanding the report must remain in the body of the report.

General Lab Info 16

LENGTH of MEMO REPORTS for ChE391

To reduce the time spent in writing memo reports, the following guidelines have been

established regarding the length of Memo Reports for the CHE-391 course.

Title or cover page

Table of contents 1 page

Introduction 1-2 pages

Theoretical principles 1-2 pages

Experimental 1 page

Observation of results 1-2 pages

Discussion of results and Error Analysis 2-3 pages

Conclusions and Recommendations 1 page

References page

Appendices 2+ pages

Your reports should be as straightforward and concise as possible.

Note that Figures and Tables are not included in the guidelines above.

LAB 1: Electrowinning of Zinc 17

EXPERIMENT 1: Electrowinning of Zinc from Zinc Sulphate Solution

INTRODUCTION:

Extractive Metallurgy of Zinc

The principal minerals that contain zinc are sphalerite, zinc blende (ZnS) and marmatite

(ZnFeS). Zincite (ZnO) and smithsonite (ZnCO3) are of less importance. Over 50% of zinc is

produced from mixed ZnPb ores. Canadian mines produced ~7% of the worlds zinc in 2006 (approximately 710 000 tonnes) and was ranked fifth in worldwide zinc below that of USA [774

000 tonnes] and Peru [1 233 000 tonnes] (1).

The processing of zinc sulphide ores initially involves the physical separation of the desired

mineral from waste rock to produce a concentrate. ZnS is the primary component of this

concentrate, however, minor amounts of other minerals that contain copper, cadmium, cobalt,

nickel, silver, etc are also extracted. The concentrate is treated to chemically extract and

ultimately recover zinc in a pure metallic form for subsequent fabrication. Over the last 50+ years

the predominant approach in the zinc industry has been to use hydrometallurgical methods for

chemical treatment. These methods employ aqueous chemistry to extract the desired species.

Most zinc hydrometallurgical processes involve dissolving (or leaching) metals from the

concentrate into a sulphuric acid solution. This step is usually done by one of two methods:

i) roasting (high temperature oxidation) of dry concentrate to convert ZnS to ZnO

ZnS + 3/2 O2 ZnO + SO2 (1-1)

followed by leaching out of Zn2+

in a concentrated H2SO4 solution

ZnO + 2 H+ Zn2+ + H2O (1-2)

ii) direct oxidative leaching of ZnS in a concentrated H2SO4 solution containing O2 at high

temperature and pressure

ZnS + 2 H+ + O2 Zn

2+ + S

0 + H2O (1-3)

Once leaching is complete, the solution goes through a series of purification steps to remove

other metal ions (e.g., Cu2+

, Co2+

, Ni2+

, Cd2+

) leached from the ore before being charged to the

electrowinning cells. These valuable metals are recovered but more importantly, the Zn

electrowinning process is enhanced. Zn2+

is the most difficult transition metal to reduce in

aqueous solutions. Consequently, any other transition metals present in solution will

preferentially deposit during electrowinning. For this reason, control of the solution chemistry

during zinc processing is particularly important.


Zinc Electrowinning

Zinc electrowinning is carried out in electrolytes containing concentrated ZnSO4 (typically 50-60

g/L Zn2+

) and H2SO4 (150-200 g/L H2SO4). Zinc deposition occurs at the cathode via equation

1-4 which is typically made of aluminum or titanium.

Zn2+

(aq) + 2 e- Zn(s) E0 = - 0.76 V (1-4)

Zinc is rather electronegative relative to hydrogen, however, substantial hydrogen evolution is

avoided (fortunately!) owing to the high hydrogen overpotential at a zinc surface. Still, some

hydrogen evolution does occur according to the process:

2 H+

(aq) + 2 e- H2(g) E0 = 0 V (1-5)

At the anode (usually Pb), the oxidation of H2O occurs via eqn 1-6:

H2O O2 + 2 H+ + 2 e

- (1-6)

evolving O2 gas and H+ ions.

One of the obvious aims of operators is to minimize H2 evolution since it consumes current that

would otherwise be used to produce the desired Zn metal product. Under proper conditions, the

rate of hydrogen evolution is slow and Zn deposition has relatively fast kinetics. A low working

temperature (about 35C) and high current density (about 600 A/m2) successfully keep hydrogen

evolution at a low rate.

A number of quantities are useful in characterizing the performance of electrowinning processes.

The current efficiency (CE) is defined as:

= / 100% (1-7)

where: IZn = current used for Zn deposition

I = total applied current

IZn is related to the mass of Zn deposited by Faradays Law:

= ( )/( ) (1-8)

where : n = number of electrons transferred per mol of Zn plated

F = Faraday constant (96485 C/mol or 26.8 Ah/mol)

m = mass of Zn deposited (g)

MZn = atomic mass of zinc (65.37 g/mol)

t = duration of electrolysis


A measure of the electrical energy required to produce a specific mass of deposit is given by the

specific energy consumption, SEC. It is related to the cell voltage V and the current efficiency,

CE as follows:

=

=100

=100

Specific Energy Consumption is usually expressed in units of kWh/tonne. The minimization of SEC is a prime factor in optimizing process performance.

Another quantity of importance is the Decomposition Voltage that is defined as the minimum

cell voltage required to initiate metal deposition. It is most easily determined by measuring the

cell voltage as a function of applied current (or vice versa) and extrapolating the resulting I-V

plot to I = 0. The decomposition voltage is generally close in magnitude to the cell potential (but

in contrast to cell potential, it has no theoretical significance).

Effect of Additives on Zinc Electrowinning

Electrolyte composition generally has a significant effect on the behaviour and efficiency of

metal electrowinning. It is particularly critical in Zn-electrowinning because of the sensitivity of

both Zn deposition and H2 evolution on the electrolyte composition. As with other

electrodeposition processes, small amounts of organic compounds such as glue and gelatin are

added to the plating solution. In amounts up to approx 50 ppm, these additives enhance the

morphology of the zinc deposits by improving their smoothness and compactness and reducing

their grain size. However, the trade-off for this improvement is that the organics tend to decrease

the current efficiency for Zn deposition. To counteract this, operators have learned through

experience to add other components in small amounts. One such additive is antimony (Sb) in the

+3 oxidation state.

As mentioned previously, the presence of other metal ions in the electrolyte can have a

significant effect on the effectiveness of the process. A particularly remarkable example is the

extreme sensitivity of the current efficiency for Zn deposition to the Sb(III) concentration and to

the complexity of its influence [see Table 1-1 (ref 3)]. With no other additives in solution, the

presence of Sb(III) at concentrations under 0.1 ppm has been found to decrease the current

efficiency by 50 % (4)!! Also, a significant change in the morphology of the deposits occurs. It

appears that Sb(III) is playing some catalytic role, although the details are still poorly

understood. However, when a similar amount of Sb(III) is added along with 20-50 ppm glue or

gelatin, the effect is entirely opposite. The current efficiency of the zinc deposits actually

increases over that observed without additives present. Furthermore, the adherence of the zinc

(1-9)


deposit to the aluminum substrate is reduced, facilitating its removal at the end of electrolysis.

For these reasons, operators routinely add small amounts of both glue and antimony to the

electrowinning cells but, very close control of their concentrations is essential. Note the dramatic

improvement in CE when [Sb] = 0.08 mg/L and the glue concentration is increased from zero to

60 mg/L, or even as low as 5 mg/L.

Table 1-1: The combined effect of antimony and glue on current efficiency [after Mackinnon

et al.; for full set of data, consult ref.3]

Current efficiency, CE per cent

[Sb] mg/L 0 0.04 0.08

Glue mg/L

0 91.0 85.6 29.0

5 90.1 85.5 68.4

10 88.8 86.9 75.7

30 86.8 90.2 85.8

60 85.9 90.6 91.3

Note: Re-dissolution of some Zn deposit has also been observed.

EQUIPMENT:

D.C power supply

Multimeter

Switch box

Aluminum cathodes

Lead anodes

Electrolytic cells

Drying oven

Analytical balance

Zinc Sulfate Solutions

PROCEDURE:

Following the demonstration of the operation of the electrowinning cells, continue with the

procedure outlined below. Each lab group will be assigned different applied current (0.4-0.7

A) and volume of Sb(III) solution to add (50-200 L) to each cell in Part C.

Take note of the various electrolyte solutions and information provided to you.


PART A: Effect of Zinc Concentration

1. Label and weigh each aluminum cathode. Record the weight. Measure the diameter of the electrowinning cells.

2. Assemble all four cells with spacers in between and enclose each cell with one aluminum plate and one lead plate positioned in an alternating pattern. The equipment is designed in

such a way to keep the electrode distance uniform and constant. Measure the spacing

between each anode (lead) and cathode (aluminum).

3. Tighten the assembly firmly to seal each cell against the electrodes. Check that the edges of the cells are on the plate surfaces.

4. Over the sink, fill each cell with water using the squeeze bottle. Ensure there are no leaks. If leaking, check position of plates. Otherwise, try tightening further (but not excessively as

this can damage the assembly). If leak persists, disassemble and replace cell.

5. Pour out the water and fill each cell with 50mL of the designated electrolyte solution. Warning, these solutions are prepared with 1 M sulphuric acid solution. Wear gloves. Keep

a record of the solution concentration in each cell.

6. Connect all the cells in series using the short wires with alligator clips.

7. Connect the lead electrode (anode) to the positive terminal of the D.C power supply and the aluminum electrode (cathode) to the negative terminal.

8. Connect the switch box to the electrodes. Position 1 in the switch box should be connected to cell 1, position 2 to cell 2 and so on.

9. Connect the switch box to the multimeter. Turn on the power supply and turn the voltage control knob in the clockwise direction all the way.

10. Turn the current control knob to acquire about 0.6 amperes or any other value instructed by your TA.

11. Measure the voltage across the electrodes in each cell by switching to the appropriate positions on the switch box. Also check the temperature of each cell. Record temp and

voltage of each cell every five minutes until a steady-state is reached (~30 min).

12. At the end of the experiment, switch everything off and disconnect all the wires. Discard the electrolytes in the appropriate waste container and rinse the cells with deionized water.

13. Remove all the four cathodes and place them in the oven to dry at 110C. After about 20 minutes remove the cathodes from the oven and allow them to cool to room temperature.

Reweigh each cathode and record the values.


PART B: Effect of Acidity

Repeat procedure as outlined in Part A using a different set of electrolytes in which the acidity

is varied and the zinc concentration is kept constant.

14. Record the acid and zinc concentrations in the solutions.

PART C: Effect of Additives

Follow the procedure outlined in Part A using electrolytes containing trace amounts of Sb(III)

or other additive.

15. Add 50 mL of electrolyte with the same composition of acid, Zn2+ and gelatin to each cell.

16. Using a micropipette, add an equal volume of each antimony solution to its respective cell. The TA will inform you of the volume of additive solution to use specifically for your tests

(volume added ranges from 60-140 uL).

17. Once you have completed the electrolysis and determined the change in mass of the cathodes (dry), examine the deposits relative to each other and to those from parts A and B. Note

texture and any features of each deposit. Make a sketch of what you see. Note any trends

you observe with respect to the variation in Sb(III) level.

PRELAB REPORT QUESTIONS

1. The electrowinning cells are assembled in series with equal spacing between places. What is the purpose of this configuration?

2. What is the purpose of the electrolyte in the electroplating cell?

3. What effect might the electrolyte conductivity have on the plating performance? How does solution temperature affect its conductivity?

4. The voltage drop measured across a cell includes potential differences due to energy losses. Which overpotentials might contribute to the inefficiencies in an electrowinning cell? Can

they be quantified?


REPORT QUESTIONS

1. Scott et al. (4) have obtained the following empirical expression for the conductivity of ZnSO4 H2SO4 solutions:

= 32.0 + 0.27[24]( 35) + 19.6([24] 1.12) 11.1([2+] 1.25)

where = conductivity (S/m)

T = temperature (C)

[H2SO4] = H2SO4 concentration in electrolyte (mol/L)

[Zn2+

] = Zn2+

concentration in electrolyte (mol/L)

(a) Estimate the IR drop at the beginning of each experiment in Parts A and B.

(b) Estimate the IR drop at the end of each experiment in Parts A and B assuming the current efficiency remains constant throughout electrolysis.

(c) For each case, determine the percentage of the cell voltage represented by the IR drop.

2. Looking at your results from Question 1 above, explain why the cell voltage varies with time during electrolysis?

3. The cell voltage changes more rapidly during the initial stages of electrolysis than in the later stages. Explain the behaviour observed in the initial stages.

4. How is the current efficiency in each of the experiments in Parts A and B affected by the [Zn

2+]/[H

+] ratio? Comment on the relationship between CE and this ratio. (Note: the initial

[H+] is not necessarily the same as concentration of H2SO4 initially added to the

electrolyte.)

5. Scott et al. (4) provide the following data for electrowinning of Zn in a 55 g/L Zn ; 110 g/L H2SO4 solution at 35

oC and a current density of 500 A/m

2 :

time (h) 8 24 42 56 72

CE (%) 95.9 96.2 96.0 96.2 96.2

SEC (kWh/tonne) 2861 2835 2820 2855 2855

It would appear from theory that the product of current efficiency and specific energy

consumption should be a constant. Show the relationship between the SEC and CE in your

data.


6. In a zinc electroplating process the specific rate of mass deposited in a continuous operation

is 433 g Zn/m2h . The process operates at 540 A/m2 .

(a) What is the cathodic current efficiency?

(b) If the current efficiency were improved by 10%, what would be the specific rate of mass deposition under otherwise identical conditions?

REFERENCES:

(1) http://www.zinc.org/Documents/Communications/Publications/ZincGuide2004.pdf (accessed November 24, 2009)

(2) D.J. Mackinnon, R.M. Morrison, J.E. Mouland and P.E. Warren, J.Appl.Electrochem. 20 728 736 (1990)

(3) A.R. Ault and E.J. Frazer, J. Appl. Electrochem. 18 583 589 (1988)

(4) A.C. Scott, R.M. Pitblado, G.W. Barton and A.R. Ault, J.Appl.Electrochem. 18 120 127 (1988)

LAB 2: Voltammetry 25

EXPERIMENT 2: Potential-Controlled Analysis of Redox Couple

(Reminder: Each group should bring a USB flash drive to the lab to copy your observed data)

INTRODUCTION:

In industrial processes, electrochemical cells are operated with 2 electrodes anode and cathode. Under these conditions, one can control or monitor what is happening throughout the cell as a

whole. For example, the cell voltage that can be applied or monitored includes several

components voltage drops at both electrode surfaces, ohmic drop across the electrolyte, voltage drops at contact points, etc. However, such a 2-electrode set-up is not useful to study or control

events occurring at a single electrode. For this purpose, a 3-electrode configuration is required.

As shown in Figure 2-1, a 3-electrode cell is made up of a working electrode, a reference

electrode and an auxiliary (or counter) electrode. The working electrode provides the surface on

which the reaction of interest can be studied or controlled. Most commonly, the potential drop at

the working electrode surface is controlled or monitored using an instrument called a

potentiostat.

Figure 2-1: Typical 3-electrode cell for potential-controlled experiments


The ability of the potentiostat to measure or control the potential drop at the working electrode

surface in a 3-electrode cell is due to the role of the reference electrode. During cell operation,

the potential drop between the working electrode and the reference electrode is continually

monitored. The device within a potentiostat that measures this potential drop has very high

impedance so that the circuit branch between the working and reference electrodes draws very

little current. Typically, standard reference electrodes (e.g., calomel, silver chloride, etc.) with

very stable properties are used so that the half cell potential of the reference electrode (relative to

SHE) is known and the potential of the working electrode can determined.

Although no current flows between the working and reference electrodes during operation, the

current generated by the working electrode must still flow through some part of the cell. This is

where the auxiliary (or counter) electrode comes into play. As current is generated, it flows

through the circuit branch linking the working and auxiliary electrodes. In fact, the sole purpose

of the auxiliary electrode is to complete the flow of current through the cell. It can be any

electrode as long as its electrochemical properties do not affect the behaviour of the working

electrode. In practice, it is desirable during operation for the auxiliary electrode not to produce

substances that can cause interfering reactions at the working electrode.

In this experiment, data will be obtained using different potentiodynamic electrochemical

techniques: (1) cyclic voltammetry in a stirred and unstirred solution; (2) linear sweep

voltammetry in a solution in which the working electrode is rotated at high speed and (3)

chronoamperometry. The redox reaction between the ferricyanide(III) ion Fe(CN)63-

and the

ferrocyanide(II) ion Fe(CN)64-

in electrolyte solution is:

Fe(CN)63-

(aq) + e- Fe(CN)6

4- (aq) (2-1)

In such solutions where the concentration of the reacting species (i.e., ferricyanide(III) or

hexachloroiridate(IV)) is much lower than the background electrolyte (i.e., KCl or KNO3), the

contribution of migration to the transport of the reacting species is negligible. Migration is only

significant for the transport of the background electrolyte under these conditions. (Note: Refer to

pages 5.5-5.8 of your ChE 331 course notes for more discussion of this matter).

Before describing the experimental procedure, it will be useful to provide brief background on

the three electrochemical methods to be used in these experiments.


Cyclic Voltammetry in Unstirred Solution

In Cyclic Voltammetry, the working electrode potential is varied linearly in one direction and

then reversed back in the opposite direction. The change in the electrode potential over time

follows a pattern similar to the triangular waveform shown in Figure 2-2. This process of

sweeping the potential first in one direction and then reversing in the opposite direction can be

repeated over many cycles, if desired. The current generated over the cycle is measured and

plotted versus the electrode potential on a diagram called a voltammogram. A sudden change in

current on such a plot usually signifies the occurrence of an electrode reaction.

Figure 2-2: Potential-time waveform for cyclic voltammetry

If the electrode reaction under consideration is reversible, then the reaction will proceed

cathodically, i.e.,

O + ne e- R (2-2)

during the portion of the cycle when the electrode potential is being varied in the negative

direction and the electrode potential becomes sufficiently negative. When the scan direction is

reversed and the electrode potential becomes sufficiently positive, the above reaction can reverse

itself and proceed anodically, i.e.,

R O + ne e- (2-3)

The resulting voltammogram will reflect the onset of both the cathodic and anodic reactions by

displaying cathodic and anodic current rises during the corresponding portions of the cycle. In

the situation where the solution is unstirred and the working electrode is stationary, these current

rises will appear as distinct cathodic and anodic peaks, as shown in Figure 2-3. In the example

shown, the electrode potential is scanned first in the negative direction and the electrolyte

contains only species O at the start of the experiment.


Figure 2-3: Typical voltammogram for a reversible redox process O + ne e- R in

an unstirred solution. Image courtesy of S Koshy

When the solution is quiescent, the current reaches a peak value Ip during a cycle when the

concentration of the reactant during that portion of the cycle reaches zero at the electrode surface

(i.e., reaction becomes mass transfer-limiting). For a planar electrode surface, the magnitude of

this peak current is related to the operating conditions of the scan and the conditions within the

solution by the Randles-Sevcik equation. For the general cathodic reaction (2-2), this equation is:

|| = 0.4463 (

)

1/2

1/2 1/2

where v = electrode potential sweep rate (V s-1

)

R = gas constant (8.314 J mol-1

K-1

)

T = solution temperature (K)

ne = number of electrons transferred for each reactant O consumed

A = area of working electrode surface (m2)

F = Faraday constant (96,485.31 C mol-1

)

D = diffusion coefficient of reactant O (m2 s

-1)

C = concentration of reactant O in the bulk solution (mol m-3

)

The Randles-Sevcik equation applies to the situation when the contribution of migration to the

transport of the reactant can be neglected. Since the Randles-Sevcik equation is applicable only

when the solution is unstirred, transport of the reactant is assumed to occur by diffusion alone.

(2-4)

IP


Linear Sweep Voltammetry of a Rotated Disk Electrode

In this technique, the working electrode potential is varied linearly with time in one direction

(i.e., towards more positive or more negative electrode potential) as shown in Figure 2-4. The

resulting current is measured and plotted versus the electrode potential. A rise in current

signifies the occurrence of an electrode reaction. The shape of the voltammogram is often

affected by whether the solution is being agitated during the linear sweep and the extent to which

the solution is agitated. A typical voltammogram obtained when a solution is being agitated is

shown in Figure 2-5. Typically, the linear sweep voltammogram is a sigmoidal-shaped curve,

with the current leveling off to a plateau when the electrode potential is swept to a sufficiently

negative or positive value. When the current reaches this plateau, the reaction has reached mass

transport-limiting conditions where the concentration of the reactant at the electrode surface has

reached zero. The value of the current at this plateau is called the limiting current Il (see pages

5.9-5.13 in your course notes).

The most common method of agitation is to mount the working electrode onto a rotating disk

electrode (RDE) apparatus. In this set-up, the working electrode is a small disk imbedded in an

insulator and vertically mounted in the shaft of a synchronous controllable-speed motor. During

an experiment, the shaft is rotated with constant angular velocity about an axis perpendicular to

the plane disk surface (Figure 2-6). This is a particularly popular method of agitation since it

provides well-defined hydrodynamic conditions and is one of the few agitated electrode systems

for which analytical solutions to the convective-diffusion transport equations have been obtained.

Thus, it makes analysis of experimental data relatively straightforward.

Figure 2-4: Potential-time waveform for

linear sweep voltammetry

Figure 2-5: Typical linear sweep voltammogram

obtained when solution is agitated


Figure 2-6: Rotating disk electrode assembly Images courtesy of S Koshy

Side View Bottom View


Consider the general electron transfer reaction for the conversion of species O to species R

O + nee- R (2-5)

Under conditions where transport of the reactant O in the vicinity of the working electrode

surface occurs by diffusion and convection (this corresponds to the conditions of these

experiments where the concentration of the reacting species is small compared to that of the

background electrolyte), it can be shown that the transport-limiting current Il for a reversible

reaction (such as that considered in this experiment) is given by the Levich equation:

= 0.620 2/3 1/2 1/6 (2-6)

where : = angular rotational velocity of working electrode (rad s-1)

= f60

2

f = rotational speed of working electrode (rpm)

= kinematic viscosity of solution (m2 s-1)

ne, F, A, C and D have the same meanings as in equation (2-1) above

Chronoamperometry

The chronoamperometry technique involves making a step change in the working electrode

potential from a value where no electrode reaction occurs to a value where a reaction occurs and

monitoring the resulting current as a function of time (Figure 2-7). If the working electrode is

stationary, the solution is unstirred and the concentration of the reactant is small compared to that

of the background electrolyte, then diffusion is the primary mode of transport of the reacting

species in the vicinity of the working electrode during the electrode reaction.

Figure 2-7: Chronoamperometry experiment. (A) Potential-time waveform.

(B) Typical current-time electrode response obtained when solution is unstirred.

A B


The technique usually involves stepping the electrode potential to a value where the electrode

reaction proceeds quickly enough that the concentration of the reactant at the electrode surface

almost instantaneously approaches zero (i.e., diffusion-limiting conditions). Under these

conditions, the variation of the current I with time t can be shown to follow the Cottrell equation:

I(t)=neF A C D

1/2

1/2 t1/2

where ne, F, A, C and D have been defined previously.

Deviations in the electrode response from the behaviour predicted by the Cottrell equation often

occur at long times (usually t 100 s) due to effects of natural convection within the cell. Natural convection arises from concentration differences produced by the electrode reaction in

the vicinity of the working electrode.

EQUIPMENT:

Voltalab 21 Potentiostat

CTV 101 speed control unit

ED 1101 rotator with a 2mm diameter platinum disk working electrode at the tip of the rotator.

XR 110 calomel reference electrode

XM 110 platinum auxiliary electrode

A30T970 thermostated electrochemical cell

REAGENTS:

The TA will provide a solution to be analyzed by the three electrochemical techniques during

these experiments. This solution contains the electroactive species (Fe(CN)63-

) dissolved in the

background electrolyte (0.1 - 1 M KCl). Each lab group will be given a different solution so

please take note of the composition of your solutions.

After each experiment, export and save the data to an Excel file. Transfer files to your USB

flash drive. Make note of all parameters for each experiment (i.e., scan rate, rotational speed,

potential range, working area of electrode, etc.).

(2-7)


PROCEDURE:

Detailed instructions for the operation of the equipment and proper instrument settings will be

provided by the TA.

Electrochemical Analysis Using Solution Containing Redox Couple

Cyclic voltammetry, linear sweep voltammetry and chronoamperometry will be conducted on the

solution containing Fe(CN)63-

. Take note of the exact concentration of the solution.

Allow the TA to fill the cleaned cell with the solution and install the three electrodes (working,

reference and the auxiliary) in the cell.

BEFORE EACH ANALYSIS, purge the cell with a low but, steady flow of N2 for 30 s. The

line pressure regulator for N2 is on the back-bench on opposite side of the computer.

From Voltmaster4 software (password is password):

a. Create a new sequence from FILE.

b. Then, from SEQUENCE dropdown, choose Sequence Edition and ADD appropriate voltammetry test from the list. Note Chronoamperometry is found under PULSE icon.

c. Highlight added test and press EDIT. Enter the appropriate parameters as specified in the

procedure then press Green Arrow . Create a file folder, type in filename and press ENTER to begin scan.

Part A: Cyclic Voltammetry in Unstirred Solution

1. Make sure the working electrode is not rotated during the cyclic voltammetry experiments. Wait until the solution is quiescent before beginning the scan.

The first cyclic potential scan will cover the range +0.5 V -0.5 V for 1 cycle at a scan rate of 10 mV/s. Carry out the scan.

2. Open the .CRV file that was generated and from the CURVE dropdown menu select Export Data to generate an Excel file. Alternatively, the CRV file can be imported directly to Excel

but headings/units will be missing (this data is given in SI units of volts, amperes and

seconds). NOTE, a datafile must be downloaded for each scan performed in this experiment.

3. Repeat the above cyclic potential scan for 8, 6, 5 and 4 mV/s sweep rates. Note: Turn on the rotator for a few seconds and purge with N2 after each experiment at a given sweep rate to

re-mix the solution. However, make sure that the rotator is turned off and the solution has

become quiescent before beginning the scan at the next sweep rate.


Part B: Cyclic Voltammetry in Stirred Solution

4. Set the rotation speed to 4000 rpm and repeat the scan described above in Step 1.

5. Maintaining the rotation speed at 4000 rpm, repeat using a potential scan rate of 5 mV/s

Part C: Linear Sweep Voltammetry of a Rotating Disk Electrode

6. The potential will be scanned from +0.5 V to 0.5 V at a scan rate of 10 mV/s and electrode rotation speed of 4000 rpm. Carry out the scan.

7. Repeat the above scan for each of the following electrode rotation speeds: 3200, 2100, 1600 and 900 rpm.

Part D: Chronoamperometry in Unstirred Solution

8. The first experiment will be a potential step from +0.5 V to -0.5 V in unstirred solution.

Set E1 = 500 mV, t1 = 1 sec, E2 = -500 mV, t2 = 10 sec and measure period = 0.6 sec.

Make sure the working electrode is not rotated during this analysis. Wait until the

solution has become quiescent before beginning the experiment.

9. Apply the potential step and monitor the resulting current over time.

Part E: Chronoamperometry in Stirred Solution

10. The remaining experiments will be a potential step from +0.5 V to -0.5 V in stirred solution at each of the following electrode rotation speeds: 4000, 2100, and 900 rpm. Set E1 = 500

mV, t1 = 1 sec, E2 = -500 mV, t2 = 10 sec and measure period = 0.6 sec.

When the last experiment is complete, inform the TA. Be sure to copy your datafiles from the

computer.


PRELAB REPORT QUESTIONS:

1. The reference electrode used in this lab is a Standard Calomel Electrode filled with saturated KCl. What is the standard potential for this electrode? What function does it serve in the 3-

electrode test cell?

2. Why is KCl(aq) added to the K3Fe(CN)6 solution being tested in this experiment?

3. You will be testing a solution containing less than 0.15 M K3Fe(CN)6 in 0.1 M KCl or 1 M KCl. Find an estimate for the kinematic viscosity of this solution and for the diffusion

coefficient of Fe(CN)63-

ion in KCl from literature.

4. List and define the three types of mass transport that can occur in an electrochemical cell. Which type dominates when cell is stirred?

5. The same range of potentials is proposed to be tested in the cyclic voltammetry (CV), linear sweep voltammetry (LSV) and Chronoamperometry runs. What effects will cause the

voltammagrams to differ between tests (refer to images in Introduction section)?

6. What does the area under the cathodic and anodic waves in a cyclic voltammagram represent?

DATA ANALYSIS:

Cyclic Voltammetry

1. Compare the cyclic voltammograms obtained under unstirred conditions at the different scan rates on the same plot. Based on the shape and trends observed in the voltammagrams, what

can be inferred about the redox reaction taking place (e.g. regarding kinetic rate or number of

electrons transferred or effect of scan rate, etc.)?

2. Use the cathodic peak currents from the series of cyclic voltammograms acquired at various sweep rates under unstirred conditions in the solution containing the redox couple to estimate

the diffusion coefficient of the Fe(CN)63-

ion from the Randles-Sevcik equation. Pay close

attention to units. The diffusion coefficient should be reported in units of m2

s-1

.

3. Cyclic voltammograms in the solution containing the redox couple were obtained under stirred conditions (4000 rpm) at scan rates of 10 mV/s and 5 mV/s. What effect, if any, did

the scan rate have on your voltammograms? Explain.

4. Why does the anodic current peak disappear from the cyclic voltammogram when the solution is agitated?


Chronoamperometry

1. Fit the observed chronoamperogram to the Cottrell equation. Perform a linear least square regression of the data to find the equation of the best straight line that fits the data. It is

important to keep in mind that this best straight line should pass through the origin in order

to be compatible with the Cottrell equation. To obtain the best results, you will not use all of

your transient I(t) data. At the very start of the experiment, certain effects occur that are not

accounted for by the Cottrell equation, so you may need to exclude the first two data points

corresponding to t = 0.6 s and 1.2 s.

2. Do the data points obtained near the end of the scan deviate from straight-line behaviour? Which data points might be removed from the analysis? Justify this in terms of the

phenomena occurring during the electrode reaction then make an estimate of the diffusion

coefficient of Fe(CN)63-

ion from the Cottrell equation. Pay close attention to the units.

3. Chronoamperometry experiments were conducted under stirred solutions. Each of the resulting curves obtained in the solution containing the redox couple shows that, given

sufficient time, the current reaches a constant value. Compare the constant current value

obtained to the IL value measured with the linear sweep voltammetry experiments at equal

electrode rotational speeds. Are they the same? Comment.

Linear Sweep Voltammetry of a Rotating Disk Electrode

1. Compare the linear sweep voltammograms obtained under different rotational speeds at a sweep rate of 10 mV/sec. Comment on the effect of rotation speed on the voltammograms.

2. Use the limiting currents IL obtained from the series of linear sweep voltammograms obtained in the solution containing the redox couple to estimate the diffusion coefficient of

the Fe(CN)63-

ion from the Levich equation. Note that rotation velocities should be

converted to rads-1.

General Overview of Results

1. The three voltammetric techniques used in this experiment provide varying estimates of the diffusion coefficient of ferricyanide ion. Compare each technique on the basis of providing

an accurate estimate of the diffusion coefficient of a solute. Which method(s) do you think

is(are) more reliable? Why? Provide evidence from your data to support your conclusions.

2. Consider that you are conducting a cyclic voltammetry experiment in an unstirred solution on a reversible redox couple that has slow reaction kinetics. What effect do you think this will

have on the position and the separation of the cathodic and anodic current peaks in the

resulting voltammogram?

3. Measure the areas under each wave in your cyclic voltammograms obtained at various potential scan rates in an unstirred solution. What trend is observed? Explain why the area

under the cathodic wave is generally larger than that of the corresponding anodic wave?


REFERENCES:

1. A.J. Bard and L.R. Faulkner, "Electrochemical Methods: Fundamentals and Applications, 2nd

Edn., Wiley & Sons, 2001.

2. A.J. Bard and L.R. Faulkner, "Electrochemical Methods: Fundamentals and Applications, 1st

Edn., Wiley & Sons, 1980.

3. J. Wang, "Analytical Electrochemistry", VCH, 1994.

4. R.G. Compton and C.E. Banks, "Understanding Voltammetry", Hackensack, NJ : World Scientific, 2007

LAB 3: Fuel cell 38

EXPERIMENT 3: Analysis of Fuel Cell Performance Curves

INTRODUCTION:

The global use of energy derived mostly from the combustion of fossil fuels is considered to be

the leading cause of the release of global warming compounds such as CO2 into the atmosphere.

Today, hydrogen fuel cells are considered to be one of the most promising renewable energy

technologies for the future with applications ranging from cell phones to automobiles (Thomas,

1999).

Similar to a battery, a fuel cell operates as a galvanic cell, i.e., a device that uses an

electrochemical reaction to generate electricity. Since it functions as an electrochemical cell, a

fuel cell contains the same basic components as does a battery anode, cathode and electrolyte. Unlike batteries, however, fuel cells are fed a continuous supply of reactants and therefore can

produce power for as long as the reactants are available. Fuel cells have higher theoretical

efficiencies than thermal combustion engines because they are not restricted by the Carnot cycle

and instead directly convert fuel into useable electricity. There are many different types of fuel

cells, e.g., hydrogen polymer electrolyte fuel cells (PEMFC), solid oxide fuel cells (SOFC),

direct methanol fuel cell (DMFC). This laboratory experiment is concerned with a PEMFC

which uses hydrogen and oxygen gases as reactants and contains a very thin solid polymer

membrane as the electrolyte, as shown in Figure 3-1. This membrane must be an ionic conductor

to ensure that H+ ions can move from the anode to the cathode.

Figure 3-1: Schematic of a hydrogen polymer electrolyte membrane fuel cell

LAB 3: Fuel cell 39

Electrochemistry of a Hydrogen Fuel Cell

One of the electrochemical reactions in a PEM fuel cell can be described simply as the

dissociation of hydrogen gas to hydrogen ions and electrons at the anode, as shown in (3-1):

H2 2H+ + 2e

- E

025C = 0V (3-1)

where E0 is its half-cell equilibrium electrode potential measured against a standard hydrogen

electrode. The electrons then flow through the external circuit to a load connected to the cell,

thereby providing the electrical energy to operate the load. The electrons flow through the load

and then back into the fuel cell external circuit to the cathode. Meanwhile, the H+ ions released

by reaction (3-1) are conducted through the electrolyte to the cathode. Both the electrons and

hydrogen ions recombine exothermically with oxygen on the cathode to produce heat and water

via electrochemical reaction (3-2):

2H+ + O2 + 2e

- H2O E

025C = 1.229V (3-2)

The overall reaction is given in (3-3):

H2 + O2 H2O Eo25C = 1.229V (3-3)

These reactions are made to occur on small catalyst particles (usually platinum-based) attached

to a conductive electrode backing material such as graphite or carbon fibres. This ensures that the

reactions can proceed rapidly and the active surface area for the reactions is as large as possible.

From the overall reaction, the maximum theoretical voltage that a fuel cell can generate is 1.229

V at room temperature and gas pressures of 1 atmosphere using pure gas streams. The half-cell

potentials above depend on several factors such as temperature, pressure, and the mole fractions

of the reactants in the gas streams. The relationship that relates the cell potential E to these variables is the Nernst equation:

21

0

22

2

/

OH

OH

aa

aln

nF

RTEE (3-4)

where E0 is the standard cell potential, R is the gas constant, T is the temperature in Kelvin, n is the number of electrons transferred in the reaction (i.e., 2 in this case), F is Faradays constant, and aH2O, aH2 and aO2 are the activities of water, hydrogen, and oxygen, respectively. It is

important to emphasize that the value of E determined from Eq (3-4) is the theoretical voltage generated when no current is flowing and is termed the Open Circuit Cell Voltage. In practice,

however, the highest achievable open circuit voltage is lower than the theoretical value and

usually lies between 0.95 and 1.0 V due to inefficiencies in the cell, e.g., leakage of H2 from the

anode compartment to the cathode compartment. Of course, the function of an operating fuel cell

is to generate current. When current is actually flowing, the cell voltage will be lower than the

open circuit value due to losses associated with current flow, i.e, ohmic resistance through the

electrolyte, electrodes and external connections, kinetic limitations of the electrode reactions and

LAB 3: Fuel cell 40

mass transfer limitations of reactants at the electrode surfaces.

Voltage and Power

In order to achieve voltages necessary for different fuel cell applications, many cells are

combined in series in what are called fuel cell stacks. The stack voltage VStack and average cell

voltage Vaverage are given in (3-5) and (3-6):

n

iiStack VV

1

(3-5)

n

V

V

n

ii

average

1

(3-6)

where Vi is the voltage generated by the ith

cell in a stack and n is the total number of cells in the

stack.

Stack power PStack and average cell power Paverage are given by (3-7) and (3-8):

IVIVPn

iistackStack

1

(3-7)

n

PP Stackaverage (3-8)

Fuel Cell Performance Characterization

The main measure of fuel cell performance is the polarization curve. A polarization curve is a

plot of the fuel cell current density (i.e., current through one cell per active anode or cathode area

of a cell) versus the average fuel cell voltage. The curve can be segmented into four regions

characterized by the major source of overpotential (or loss) in each of the sections, as shown in

Figure 3-2.

The first region is, in fact, a point and indicates the cell voltage when no current is flowing. This

corresponds to the open circuit potential discussed previously and is denoted as OCV in the

figure. Although the OCV should be equal to the Nernst potential, it is typically 0.1 0.2 V lower due to non-ideal conditions and mixed potentials caused by gas crossover from the anode

to the cathode.

LAB 3: Fuel cell 41

Figure 3-2: Regions of a typical PEMFC polarization curve

At low current densities, activation polarization is the dominant source of energy losses in an

operating fuel cell. Activation polarization arises due to limitations associated with the kinetics

of the electrode reactions. It becomes an important factor when the electrochemical reaction is

controlled by sluggish electrode kinetics. Of the two electrode reactions in a fuel cell, oxygen

reduction at the cathode is usually the rate limiting reaction and therefore contributes the most to

activation polarization (USDOE, 2000). Due to its relationship to reaction rates, the activation

polarization is also affected by the total active surface area of the catalyst surface. Losses of the

active catalyst surface area during operation due to phenomena such as particles falling off the

electrode backing material or the buildup of solid reaction products on the catalyst particles are

registered as activation polarization

At intermediate current densities, the major sources of energy loss come from the ohmic

resistances through the cell. These losses arise due to the resistance to ion flow through the

electrolyte membrane and the resistance to electron flow through the gas diffusion layer (GDL).

As shown in Figure 3-1, the GDL is a thin layer of carbon fibres or paper between each fuel

channel and electrode that serves to better distribute the reactant gases to the catalyst sites on

each electrode. Other factors contributing to the cell ohmic losses include contact resistances

between the different components and electronic resistance through the bipolar plate. Resistance

to proton flow through the membrane is the largest individual contributor to the ohmic losses

(USDOE, 2000).

Current Density

Ideal Voltage (1.23V)

Cathode and Anode Activation Overpotentia

l

Ohmic Overpotential

Concentration Overpotential Voltage

Current Density


Cathode and Anode Activation Overpotentia

l

Ohmic Overpotential

Vo

lta

ge

Current Density


Cathode and Anode Activation Overpotential

Ohmic Overpotential

Concentration Overpotential

OCV

LAB 3: Fuel cell 42

The final region in the polarization curve is dominated by losses due to concentration

polarization, or diffusion limited operation, at high current densities. Under such high currents,

the kinetics of the electrode reactions are so high that the transport of the reactants to the active

catalyst sites limits the rate at which the fuel cell can operate. The problems of mass transfer can

be further accentuated due to phenomena such as slow diffusion caused by nitrogen

accumulating near the cathode or the presence of water in the pores of the electrode. The term

flooding of the fuel cell is used to describe a situation when water completely fills the electrode and significantly blocks reactants from reaching reaction sites.

Stoichiometry

In order to achieve a specific current in a fuel cell, the correct amount of reactants must be

delivered to the cell. Generally stated, the volumetric flow rate of a gaseous reactant required by

the fuel cell (converted to STP conditions for convenience) for a specific current output is given

by Faradays law (3-9) as follows:

e

cSTP

nF

NVIQ (3-9)

where: Q is the volumetric flow rate (L/s) of reactant into the

stack/product out of the stack,

I is the total current (A) flowing through the stack,

Nc is the number of cells in the stack

ne is the moles of electrons transferred per mole of reactant r

consumed based on the electrode reaction stoichiometry,

VSTP is the volume an ideal gas occupies at STP (22.4 L/mol)

In general, a fuel cell is supplied with excess reactants in order to achieve the best possible

performance. The ratio of the actual inlet volumetric flow rate to the amount required from

Faradays law is typically referred to as the stoichiometry S and is given by (3-10):

Q

QF

Q

FS outin

(3-10)

where Fin and Fout are the inlet and outlet flow rates, respectively. Systems where no unreacted

gas is discharged from the cell are called dead-ended.

LAB 3: Fuel cell 43

EQUIPMENT:

A schematic of the fuel cell demonstration station is provided in Figure 3-3. The fuel cell used

in this experiment is a 5-cell stack with each cell having 10 cm2 active area. Hydrogen is

supplied via a compressed gas cylinder and air is forced through the cell by a fan. A series of

resistors are used to create a load for the fuel cell. The total resistance of the load depends on the

position of the port to which the fuel cell is connected. The range of resistances is sufficient to

generate a polarization curve that depicts the three main overpotential regions. Labview

software is used to display measurements of the fuel cell voltage, current flowing through the

circuit, H2 inlet pressure and outlet flowrate. The flowrate of H2 out of the fuel cell can be

controlled using the software.

PROCEDURE:

A polarization curve is obtained by varying the resistance of the load and measuring the current

and cell voltage output. Higher resistances lead to lower currents and vice versa. All polarization

curves in this experiment will be obtained by decreasing load resistance from high to low. Six

different configurations of the fuel cell system are to be tested. Each run will use the procedure

outline in Part A with the modification specified.

*NOTE* Minute fluctuation in voltage and current reading will occur even after the system is

allowed to stabilize. An average between the lowest and the highest value of the fluctuations

should be taken to increase the accuracy of the experimental data**

Figure 3-3: Schematic of the Fuel Cell Demonstration Station in DWE1518

Pressure Regulator

To Exhaust

Flow Controller

Solenoid Valve

P

Flowmeter

Shunt

Resistor Board

Knock Out Drum

Fuel Cell Stack

Motorized Fan

Electrical circuit

Gas Lines

LAB 3: Fuel cell 44

Part A: Fuel Cell Warm-up

NOTE: Part A will be done for you before lab session begins but procedure should be reviewed

so that you are familiar with test station operation.

1. Ensure that breaker switch on fuel cell circuit is off.

2. Open LabView and start the data collection program by pressing white at top left.

3. Turn on air fan located below fuel cell stack to its low speed setting.

4. Open the N2 and H2 cylinders and slowly increase the delivery line pressure to ~5 psig. Check that delivery line valve is open.

Check the STOP icon on bottom of Labview display. This is the safety switch that

operates a solenoid valve that controls the type of gas delivered to the fuel cell stack.

Refer to table below. Ensure that system is active with H2 supplied to the cell stack.

BUTTON

COLOUR

MODE TYPE OF GAS FLOWING TO

FUEL CELL STACK

Grey with green

light nearby SAFE N2

Burgundy (grey

light nearby) ACTIVE H2

IN THE EVENT OF A FIRE set button to grey Safe Mode to stop flow of H2 to

cell stack then inform TA and evacuate the immediate area.

5. Set H2 outflow from the fuel cell test station in Labview window to 20 mL/min.

6. Using the pressure reading display in LabView, adjust the anode pressure of the cell stack using the pressure control dial on the backboard to between 1.5 - 2 psig.

* CAUTION * DO NOT EXCEED AN ANODE PRESSURE OF 2 PSIG as

this can lead to H2 leakage from the cell stack.

Clockwise turn increases pressure. Note that when backing off pressure, system

response is slow but can be sped up by increasing outflow rate to 50 mL/min.

7. Wait for cell stack to reach OPEN CIRCUIT VOLTAGE (OCV: the voltage at zero current). It takes a few minutes for cell stack to fill with H2 and initiate reaction.

8. Apply a load on the circuit by insert the plug into resistor #8 and switching breaker to complete circuit. Let system run for ~15 minutes to warm-up cell stack.

LAB 3: Fuel cell 45

Part B: Baseline operation

Start experiment from this point.

1. Switch off circuit breaker and remove plug from resistor board. Fuel cell should now be warmed up.

2. Allow system to stabilize at its OCV for approximately 2 minutes then record the following values from the Labview display:

Current (A)

Voltage (V)

Average H2 inlet flowrate (sccpm or mL/min)

Average actual outlet flowrate and set-point (sccpm or mL/min)

Anode pressure (psig)

3. Insert the free wire from the fuel cell into resistor slot #1 (the highest resistance) and close switch to complete the circuit with a load. A decrease in voltage and a small

increase in current should be observed.

4. Adjust pressure control dial (clockwise to increase) to maintain the anode pressure recorded in Step B2.

5. Allow the readings to stabilize (approximately 1 minute) then record the values for the parameters listed in Step B2.

6. Turn off the main switch then move to next resistor slot and repeat from Step B3 until readings for all 10 resistor slots have been obtained. This baseline data will be used as

a point of comparison for subsequent runs.

7. At end, switch off the circuit, remove the resistor plug, increase the air fan to the maximum setting and allow air to flow through the cell stack for at least 1 minute

before starting the test on the next operational parameter.

Part C: Reduced pressure operation

Repeat the procedure in part B, however, reduce and maintain the anode pressure at 0.5 psi.

Part D: Dead-end anode operation

Repeat the procedure in

ChE391 LabManual F15

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