## Blackboard System

- CH621, Electrochemistry (Graduate level)
- CH223 Quantitative Analysis
- CH102, General Chemistry
- CH101, General Chemistry

## Trends In Analytical Chem (Electrochemistry) CH 621 Section 001

## Fall 2020, Lecture

**Prerequisites**

Prerequisites for this course are general chemistry (CH101, 102). All students are assumed to have some background in half reaction potentials, standard potentials, redox reactions, and electroanalytical chemistry at the general level of undergraduate courses (e.g., quantitative analysis). Some ability in computer programming, at the minimum use of spreadsheets, like Excel (to carry out the digital simulations taught in the course) is also assumed.

**Course Description**

This course is designed to teach the fundamental aspects of electrochemistry and the application of electrochemical methods to chemical/engineering problems. Special emphasis will be given to the study of electrode reaction mechanisms and the interpretation of electrochemical results (e.g., cyclic voltammetry) for organic and inorganic systems. A rigorous consideration of voltammetric and coulometric methods and several topics of interest in electrochemistry including modified electrodes, photoelectrochemistry, scanning electrochemical microscopy and electrogenerated chemiluminescence etc will be undertaken. We will also introduce the concept of AC impedance, solar cell and fuel cell at the end of the course. There are hands-on activities (e.g., CVs, solar cell, electrolysis, fuel cell, energy transformation, rotating disc electrode) provided to enhance the learning.

**Student Learning Outcomes**

This course will help graduate students interpret various electrochemical results (e.g., cyclic voltammetry) for organic and inorganic systems. Students are expected to be capable of using digital simulation and electrochemistry theory to understand electrochemical phenomenon; secondly, students will be able to apply electrochemical methods to solve chemistry and chemical engineering problems existing at a solid-liquid interface; third, student will have a comprehensive understanding of analytical chemistry application of electrochemistry and learn how electrochemistry is used in sensors for ultrasensitive detection; Several types of devices such as fuel cell, light emitting electrochemical cell and solar cell will be introduced in order to enhance their understanding of the principles of electrochemical systems. Students are expected to have the ability to develop their own interests further based on what we will learn through the course by reading literature and conducting research in these important fields.

**Required Texts**

UA Supply Store Textbook Information

- J. Bard and L. R. Faulkner,
*Electrochemical Methods*, Second edition, John Wiley & Sons, Inc. Two copies of this book are available in Rodgers Library. The second and/or third year chemistry graduate students may have their own copies.

**Blackboard: **The UA blackboard site (https://ualearn.blackboard.com/webapps/login/) contains a variety of useful course materials, including teaching slides, reading materials and software for digital simulations.

The following references are available at Rodgers Library. These are recently purchased materials for your study and research by the course instructor.

Encyclopedia of Electrochemistry, Volume 1-10

*Thermodynamics and Electrified Interfaces; **Interfacial Kinetics and Mass Transport; **Instrumentation and Electroanalytical Chemistry; **Corrosion and Oxide Films; **Electrochemical Engineering; S**emiconductor Electrodes and Photoelectrochemistry; **Inorganic Electrochemistry; **Organic Electrochemistry; **Bioelectrochemistry; **Modified Electrodes. *

**Outline of Topics**

Topic |
---|

Introduction, Standard electrode potential, cell thermodynamics effect of concentration on electrode potential (The Nernst equation) |

Lab 1 Voltage measurement of galvanic elements
Potential/current control Electrochemical Methods I Lab 2 Electrode Potential Measurement in 3-electrode configuration (ref, counter and working) |

Double layer structure, potential/current control Electrochemical Methods |

Mass transfer |

Kinetic of electrode reactions |

Potential/current control Electrochemical Methods II
Lab 3 Cyclic Voltammetry |

Theoretical: Laplace Transform for control potential methods |

Digital simulation |

Linear Sweep and Cyclic Voltammetry
Lab 4 Rotating Disc Electrode |

Digital simulation of Linear Sweep and Cyclic Voltammetry |

Double Layer Structure and Adsorption – Modified Electrodes |

Analysis of Electrode Reaction Mechanisms |

Nanoelectrochemistry and trace analysis using electrochemical methods |

Modified electrode for electrochemical sensing |

Spectroelectrochemistry |

Introduction to electrogenerated chemiluminescence (ECL) and OLEDs |

Current status of ECL and applications
Lab 5. chemiluminescence and electroluminescence |

Microelectrode, Scanning electrochemical microscopy (SECM) using microelectrode, Application of SECM (Lab tour for SECM) |

Photoelectrochemistry
Lab 6 Alternative Renewable Energy: Solar Water Splitting for Hydrogen |

Direct water splitting at nanoelectrodes using solar energy |

Dye sensitized electrochemical solar cell |

Supercapacitors |

AC Impedance |

AC Impedance in solar cell, batteries and fuel cell
Lab. 7 Al-air battery |

Other E-chem methods (CC, CA, Stripping, LSV and other pulse techniques) |

Student presentations and papers |

Frequently Used Electrochemistry Techniques

**I. Cyclic Voltammetry **

Introduction

Cyclic voltammetry (CV) is a broadly used electrochemical technique for electrochemical mechanistic study and quantitative analysis. This technique provides accurate measurement on redox potential, mass transfer, and kinetics of (an) redox reaction(s). CV uses a linearly scanned potential of a working electrode between two potential limits E_{max} and E_{min} relative to a reference electrode while collecting the current signal drained from the working electrode to a counter electrode. Both current and electrode potential are recorded as a function of time, and then the data can be report as current as a function of potential by eliminating time because time is linear function of electrode potential

E=E_{in} +/- vt (1)

E_{in} is the initial electrode potential of a CV. It is usually the rest potential or open circuit potential (OCP) of the working electrode. Many commercial echem instrument can measure OCP of a working electrode automatically. OCP can also be estimated by the potential difference of a working electrode and a reference electrode using a voltameter. +/- sign of equation 1 is dependent on the scan direction of a CV. + is used for positive scan direction and – is used for negative scan direction. As shown in Figure 1, a CV starts scanning from its E_{in} positively toward E_{max} and reverses at E_{max} toward E_{min} at a scan rate of v. The linear scan be repeated several cycles in order to receive a stable CV.

** **

*Figure 1. Linear potential scan of CV as a function of time for constructing a CV curve E-I at a scan rate of V (mv/sec).*

CV can provide accurate measurement of reaction mechanisms, rates of oxidation and reduction processes, electrode surface area and diffusion constant etc. A CV scan generates an oxidized species during the forward scan and then probing the produced oxidized species with the reverse scan or subsequent cycles. The stability of the produced oxidized species can be detected by varying the scan rate. Unstable species can be detected by improving the scan rate. A 3-electrode configuration is used for CV collection, including a working electrode, reference electrode, and a counter electrode.

- Electrodes for CV measurement.

Working electrode is typically made of chemically inert conductive material such as platinum, Au and glassy carbon. A reference electrode, typically AgCl or calomel, is used to estimate the potential of a working electrode by measuring the potential difference of the working and the reference, and the reference electrode potential must remain stable in CV measurement. A counter electrode is employed to form a complete cycle of charge flow from a working electrode. The counter electrode’s role is essentially to ensure that current does not run through the reference electrode by providing another half reaction corresponding to the one at the working electrode.

*Figure 2. Scan rate dependence of CV and its peak current ip of a reversible reaction system.*

If the kinetic parameter electron transfer rate k_{0} is sufficiently fast for a reaction

O + ne = R (2)

And there is no homogenous reaction and surface adsorption of any reactant and product. This reaction can maintain a Nernstian equilibrium at the electrode surface and we call this kind of reaction as reversible system. It peak current, i_{p} (in amperes), for such reversible system is given by

*i _{p}*

**=(2.69**

*x*10^{5})*n*^{3/2}*AD*^{1/2}*v*^{1/2}*C*_{O }(3)where n is equal to the number of electrons gained in the reduction, A is the surface area of the working electrode in cm^{2}, D is the diffusion coefficient, v is the sweep rate, and C_{o} is the molar concentration (mole/cm^{3}) of O in the bulk solution.

The peak potential is *independent *of sweep rate and the anodic and cathodic peak potential distance of a reversible system is around 59/n mV.

**Example 1**: CV and peak current dependence on scan rate for 10 mM aqueous solution of K_{3}Fe(CN)_{6} in 0.1 M Na_{2}SO_{4 }

For irreversible system, the peak current and potential positions are affected by reaction kinetics and mechanism. For example, the oxidation peak will decrease in its current intensity if the reduced species are unstable. The peak-peak splitting is more than 59 mV if the charge transfer rate is sluggish for a redox species.

**Example 2**: CV of 10 mM acetaminophen (AAPH MW: 151.169 g/mol) (active ingredient in Tylenol) in 0.1 M Na_{2}SO_{4 . }

detailed redox reaction mechanism of AAPH is show below:

**Note: All above experimental data were taken by students from Dr. Pan’s electrochemistry class in Fall 2014. **

**II. Rotating Disc Electrode for Kinetic studies**

*Background and introduction*

CV is usually done at steady-state without agitating the solution while keeping redox species in diffusion controlled. Under such condition, the redox diffusion layer grows as a function of time and there is no linear diffusion layer to reach a limiting current. The duck shape current response in a CV curve can be explained by such nonlinear mass transfer layer growth as the electrode potential is scanned. Sophisticated digital simulation has to be taken in order to extract useful information such as diffusion constant. Many *hydrodynamic *methods, in which solution moves toward the electrode along a well-defined flow pattern, may simplify this case by providing a linear growth or a constant diffusion layer so that a limiting current region can be reached. The rotated disk electrode technique is one of the hydrodynamic methods. This technique works by rotating a working electrode at constant speed to let redox species diffuse towards the working electrode surface while potential scan is applied like regular cyclic voltammetry. *The higher the rotating speed the thinner the mass transfer layer thickness to produce a higher current density.*

*Figure 3. i-E profile of reversible, quasi-reversible, and irreversible reaction. *

Depending on the actual reversibility of a reaction, i-E curves of RDE system can be used to measure the kinetics and mass transfer parameters easily (**Figure 3**).

The current can reach a limiting current i_{L} at a large over potential region because the redox concentration beyond the diffusion layer remains constant. And the limiting current is defined by

i_{o, L}= nFAm_{o}C_{o} (1)

where Co is the bulk concentration of redox molecule O and m_{o} is the mass transfer coefficient, A is electrode surface area and n is number is charge transferred for a half reaction O + neà R. m_{o }is a function of diffusion coefficient of O and time. For a rotating disk electrode, the treatment of steady-state, mass transfer-controlled electrode reactions applies to produce

mo=0.62Do^{2/3}ω^{1/2}ν^{-1/6} (2)

Do is the diffusion coefficient (cm^{2}/s), ω is the angular velocity of the rotating disk (s^{-1}, 2πf, f is the frequency if the rotating disk or rounds per second), ν is the kinematic viscosity (ν=viscosity η/density d; for aqueous solution ν~0.010 cm^{2}/s).

Therefore the obtained limiting current for such rotating disk is given by:

i_{o, L}= nFAm_{o}C_{o}=0.62nFADo^{2/3}ω^{1/2}ν^{-1/6}Co (3)

This is the famous *Levich equation*.

the relationship between diffusion layer thickness d and mo is given by

Mo=Do/d (4)

We can calculate d by using Do/m_{o} to estimate the diffusion layer thickness in a rotating disc experiment.

As shown in **Figure4A**, the diffusion layer thickness is given by

d=1.62 Do^{1/3}ω^{-1/2}ν^{1/6 } (5)

**For Nernst reactions** where charge transfer rate are infinite large, only mass transfer plays a role.

By taking the current-potential data shown in **Figure 4B**, we can measure E_{1/2} which is given by

E_{1/2}=E^{0}O_{/R }+ RT/nFln(D_{R}/Do)^{2/3 } (6)

Figure 4. Concentration profile of redox O under condition of a rotating disc control with limiting current.

**For totally irreversible one step, one electron reaction**,

i=FAk_{f}Co(x=0)=FAk_{0}exp[-αf(E-E_{0}’)] Co(x=0) (7)

K_{0} is the standard rate constant for charge transfer as we discussed in echem kinetics previously. α is the charge transfer coefficient. Co(x=0) is the surface concentration of O.

Co(x=0)=Co* (1-i/i_{L,O}) (8)

Where i_{L,O }is the limiting current for reduction of O given by equation 1 and 3.

The current at a rotating disc is given by

1/i=1/i_{k }+ 1/i_{L,O }(9)

i_{k} is defined by i_{k}=FAK_{f}C_{O}*

the current potential profile is shown in **Figure 5**, which can be used to calculate k0 by fitting the high over potential region where the kinetics play a major role as shown by the linear function of i_{k}. The low over-potential region produces a diffusion layer region whose current slop is proportional to rotation square root of angular velocity ω.

For more generation quasi-reversible system, the current potential curve of a RDE experiment is given by

(10)

Figure 5. i-E curve of a RDE for a totally irreversible system.

**Example 3**. RDE data taken at different rotation speeds for 10 mM aqueous solution of K_{3}Fe(CN)_{6} in 0.1 M Na_{2}SO_{4 }

**Note: All above experimental data were taken by students from Dr. Pan’s electrochemistry class in Fall 2014. **

* References: Electrochemical methods, fundamental and applications, 2nd edition, Allen J. Bard and Larry R. Faulkner. John Willey & Sons, Inc. 2001. *