INTRODUCTION TO ELECTROCHEMICAL CELLS AND BASIC

ELECTROANALYTICAL MEASUREMENTS

Sunny Holmberg January 19th 2016

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Topics in Lecture

• Fundamentals of Electrochemistry – Two electrode systems – Reference electrodes – Three electrode systems

• Fundamental Kinetics– Types of Mass Transport

• Basic Electrochemical IV Characterization• C-MEMS IDEAS Basics

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Topics in Lecture

• Fundamentals of Electrochemistry – Two electrode systems – Reference electrodes – Three electrode systems

• Fundamental Kinetics– Types of Mass Transport

• Basic Electrochemical IV Characterization• C-MEMS IDEAS Basics

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Basic Electrochemical Cells

• Electrochemistry is the analysis of redox reactions that occurs between an electrode and a liquid medium

• The characteristics of which are measured based on Voltages and Currents

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• Voltage – the electrical potential between two distinct “nodes”– Voltage is therefore is always a measurement

between two points– Consequences of this implication is that the

minimum number of electrodes in a basic electrochemical cell must be two

• Electric potential of the solution cannot be directly discerned and only indirectly measured with electrodes.

Basic Electrochemical Cells

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• Voltage is a characteristic parameter of an electrochemical (redox) reaction. – This means that each electrochemical (redox)

potential has (redox) potential associated with it

– This redox potential could be partially interpreted as the minimum energy (condition) needed for a (redox) electrochemical reaction to occur

– This potential is also its equilibrium potential, i.e. when net current of reaction is zero

Basic Electrochemical Cells

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• Current – The number of electrons traveling between two “nodes” per unit of time– Two types of current faradaic and charging– Faradaic current – current generated by the

single electron exchange between two medium due to an electrochemical reaction

– Capacitive current – current generated by the alignment of surface charges in an attempt to offset the applied potential and reach thermodynamic equilibrium

Basic Electrochemical Cells

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• Current is a quantitative measurements. – The absolute current measured is the

superposition of all the current within the system

• Therefore careful planning of experiments is need to decouple the individual current contributing reactions (i.e. faradaic vs capacitive)

• Current Magnitudes are relatively meaningless without a baseline for comparison

Basic Electrochemical Cells

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• Capacitive Current (baseline current) – Result of charge

(ions) migrations– Ions form a

capacitive layer at interface of solution and electrodes

– These layers act to counterbalance the electrical potential

Basic Electrochemical Cells

"EDLC-simplified-principle" by Elcap - Own work. Licensed under CC0 via Commons - https://commons.wikimedia.org/wiki/File:EDLC-simplified-principle.png#/media/File:EDLC-simplified-principle.png

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• Two electrode systems:– The simplest

electrochemical setup– Capable of measuring

the voltage and current change between two electrodes in a solution

Basic Electrochemical Cells

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• Reference Electrodes– A reference point to relate all electrochemical

reactions together– Need a constant fixed potential during

electrochemical reaction that does not change during experiment (difficult)

Basic Electrochemical Cells

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• Reference Electrodes

Basic Electrochemical Cells

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• Reference Electrodes– Ag/AgCl

• +0.191 V vs SHE – Calomel

• +0.241 V vs SHE– CSE (Copper/Copper

sulfate)• +0.314 V vs SHE

Basic Electrochemical Cells

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• 3 Electrode Electrochemical Setup– Needed to measure the behavior of

electrochemical setup over a range of voltage– Voltage measured between working electrode

and Reference electrode – Voltage is applied between counter and

working electrode (using a feedback vs reference)

– Current is measured between working electrode and counter electrode

Basic Electrochemical Cells

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• 3 Electrode Electrochemical Setup

Basic Electrochemical Cells

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Topics in Lectrue

• Fundamentals of Electrochemistry – Two electrode systems – Reference electrodes – Three electrode systems

• Fundamental Kinetics– Types of Mass Transport

• Basic Electrochemical IV Characterization• C-MEMS IDEAS Basics

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Fundamental Kinetics

• Mass Transport in Solution1. Diffusion to surface2. Adsorption to surface3. Migration of reactant to reaction site4. Electrochemical reaction 5. Migration of product on surface6. Desorption of Product from Surface7. Diffusion of Product away from Surface

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Fundamental Kinetics

• Types of Mass Transport– Diffusion – transport due to

concentration gradient

– Migration – Transport due to Electric Potential gradient

– Convection – Transport due

to Pressure Gradient

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Topics in Lecture

• Fundamentals of Electrochemistry – Two electrode systems – Reference electrodes – Three electrode systems

• Fundamental Kinetics– Types of Mass Transport

• Basic Electrochemical IV Characterization• C-MEMS IDEAS Basics

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Basic Electrochemical IV Characterization

• Linear Sweep Voltammetry– Potential is sweeped linearly across a range

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Basic Electrochemical IV Characterization

• Cyclic Voltammetry– Potential is sweeped linearly across a range

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Randles-Sevcik equation:

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Topics in Lecture

• Fundamentals of Electrochemistry – Two electrode systems – Reference electrodes – Three electrode systems

• Fundamental Kinetics– Types of Mass Transport

• Basic Electrochemical IV Characterization• C-MEMS IDEAS Basics

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Redox Cycling through Inter Digitated electrodes Arrays(IDA)

• Two Working electrodes :Generator (Anode) and Collector (Cathode) electrodes in close proximity such that the adjacent regions with concentration gradients overlap

• The redox couple may redox cycle multiple times before they diffuse out into the bulk solution.

• This behavior results in an amplified signal thereby lowering the lower limit of detection (LOD) significantly (upto pM)

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•Single mode: Cyclic Potential sweep across Generator, Collector not connected.•Dual Mode: Constant reduction potential applied across collector while potential is swept linearly across generator

Redox Cycling as electrode width and gap

Redox Cycling as electrode height

Redox Cycling through Inter Digitated electrodes Arrays(IDA)

•Two Working electrodes :Generator (Anode) and Collector (Cathode) electrodes in close proximity such that the adjacent regions with concentration gradients overlap •The redox couple may redox cycle multiple times before they diffuse out into the bulk solution. •This behavior results in an amplified signal thereby lowering the lower limit of detection (LOD) significantly (upto pM).c

•Single mode: Cyclic Potential sweep across Generator, Collector not connected.•Dual Mode: Constant reduction potential applied across collector while potential is swept linearly across generator

Redox Cycling as electrode width and gap

Redox Cycling as electrode height