INTRODUCTION TO ELECTROCHEMICAL CELLS AND BASIC ELECTROANALYTICAL MEASUREMENTS Sunny Holmberg January 19 th

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 2

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 3

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 4

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. 5 Basic Electrochemical Cells

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 6 Basic Electrochemical Cells

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 7 Basic Electrochemical Cells

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 8 Basic Electrochemical Cells

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 9 Basic Electrochemical Cells "EDLC-simplified-principle" by Elcap - Own work. Licensed under CC0 via Commons - C-simplified-principle.png#/media/File:EDLC- simplified-principle.png

Two electrode systems: –The simplest electrochemical setup –Capable of measuring the voltage and current change between two electrodes in a solution 10 Basic Electrochemical Cells

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) 11 Basic Electrochemical Cells

Reference Electrodes 12 Basic Electrochemical Cells

Reference Electrodes –Ag/AgCl V vs SHE –Calomel V vs SHE –CSE (Copper/Copper sulfate) V vs SHE 13 Basic Electrochemical Cells

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 14 Basic Electrochemical Cells

3 Electrode Electrochemical Setup 15 Basic Electrochemical Cells

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 16

Fundamental Kinetics Mass Transport in Solution 1.Diffusion to surface 2.Adsorption to surface 3.Migration of reactant to reaction site 4.Electrochemical reaction 5.Migration of product on surface 6.Desorption of Product from Surface 7.Diffusion of Product away from Surface 17

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 18

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 19

Basic Electrochemical IV Characterization Linear Sweep Voltammetry –Potential is sweeped linearly across a range 20 E - t t

Basic Electrochemical IV Characterization Cyclic Voltammetry –Potential is sweeped linearly across a range 21 E - t t Randles-Sevcik equation:

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 22

23 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

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