Introduction to Cyclic Voltammetry

0:09[Music]

0:09hey folks in this video we're going to be talking about cyclic voltammetry if you are

0:14new to the subject or you're learning about for the first time in school this video is for you

0:20cyclic voltammetry or cv for short is one of if not the most popular technique

0:27used in electrochemistry and in this video we hope to make understanding cyclic voltammetry

0:33as easy as possible this video is broken up into several sections first we're going to discuss

0:39what is cyclic voltammetry what is the technique and the parameters around the technique

0:44we will then discuss how sick of voltammetry is used or applied to a three electrode system

0:50specifically with a focus on understanding what is physically happening in our system

0:56what electrochemical phenomenon are occurring and how those electrochemical phenomena manifest

1:02themselves in the data how they manifest themselves in the cyclic voltammogram

1:07time stamps are in the description below and lastly before we begin please don't forget to

1:13like comment and subscribe so what is cyclic voltammetry single voltammetry is

1:19an electroanalytical chemistry technique where a potentiostat applies a triangular potential

1:24waveform to an electrochemical system and we measure the resulting current the

1:30triangular potential waveform is made up of these potential sweeps we are sweeping the potential

1:36linearly as a function of time the measured current is also measured as a function of time

1:43but we end up plotting the current as a function of the applied potential so we have current on

1:49the y-axis and potential on the x-axis and this gives us a cyclic voltammogram the triangular

1:57potential waveform has several parameters it has an initial potential which is the start of our

2:03experiment it starts the point where we sweep the potential we then sweep it to the switching

2:09potential which defines the end of the first segment of our triangular potential waveform

2:15and then we have the final potential which then defines the second segment of our linear sweep

2:20completing the triangular waveform where we have one cycle in a cyclic voltammetry experiment

2:27and a cyclic voltammetry experiment may consist of one or multiple cycles the initial potential

2:34switching potential final potential and number of segments can all be adjusted depending on

2:39our electrochemical system in fact there's a wide variety of different potential waveforms

2:45that can be created just based on adjusting these parameters the classical triangular potential

2:52waveform isn't strictly the only waveform used in cyclic voltammetry for purposes of

2:58this video we will be sticking with the classical triangular potential waveform

3:04that being said a cyclic voltammetry experiment must consist of at least two segments if it only

3:11consisted of one segment it would be referred to as linear sweep voltammetry the final parameter

3:18that we can adjust in a cyclic voltammetry experiment is the slope of the linear sweep

3:24this is referred to as the sweep rate or the scan rate and it is a measure of how fast we can sweep

3:30the potential as a function of time so that's the technique in a nutshell but that doesn't give us a

3:38very good understanding of how the technique works let's apply cyclic voltammetry to a relatively

3:44standard three electrode electrochemical system where we have a glassy carbon working electrode

3:50a platinum wire counter electrode and a silver silver chloride reference electrode

3:55all three electrodes are submerged into an aqueous electrolyte solution the electrolyte being some

4:01kind of salt that is dissolved in the solution to maintain electrical conductivity and we add

4:07an analyte a redox active molecule in this case we'll use ferrocyanide lastly all three electrodes

4:14are connected to a potentiostat when we apply our triangular potential waveform we are sweeping the

4:20potential of the working electrode with respect to the reference electrode if you're interested in

4:27understanding how potential that works we have a video and i have a card in the upper right hand

4:32corner i also have a link to it in the description below the last section in particular describes

4:38what the potential set is doing in a three electrode system and that'll help you understand

4:43what actually is happening when we sweep the potential of the working electrode with respect

4:48to the reference electrode as we sweep the potential there are several things that are

4:53happening at the working electrode surface for example if we apply a positive bias

4:59to the working electrode surface we attract these negatively charged anions towards the surface

5:06remember that our dissolved electrolyte is a salt that's made up of positively charged cations

5:13and negatively charged anions sometimes this electrolyte is referred to as a supporting

5:18electrolyte and the anions and cations that make up the electrolyte are inert which means

5:24that they will not react and the charge will not be neutralized by the charge at the working

5:28electrode surface so our positively biased working electrode attracts the negatively charged anions

5:37towards the electrode surface where the negatively charged anions will make contact

5:41with the working electrode surface and this layer of negatively charged anions forms a plane

5:48at the working electrode which is referred to as the inner helmholtz plane

5:54well this layer of negatively charged anions attracts the positively charged solvated cations

6:02in solution these cations are solved they are surrounded by water molecules or solvent molecules

6:10these solvented cations form a loosely bound layer known as the diffuse layer that is away from the

6:18inner helmholtz plane into the bulk solution away from the working electrode the layer of solving

6:25the cations that is closest to the inner helmholtz plane is referred to as the outer helmholtz plane

6:32our charged electrode surface the inner humholds plane the outer helmholtz plane the diffuse layer

6:38all make up what is generally referred to as the electrical double layer region

6:43the electrical double layer is a fairly complex topic in electrochemistry and we

6:48paint a very simplistic picture of it we actually recommend that you take a look at a standard

6:53electrochemistry textbook such as electrochemical methods fundamentals and applications

6:59by Allen Bard and Larry Faulkner additionally the Wikipedia for the electrical double layer

7:06is quite helpful and i have a link to it in the description below the important thing about the

7:12electrical double layer is that the orientation of solvent molecules anions cations our supporting

7:20electrolyte the orientation of these molecules is different than that of the bulk solution

7:26and that the behavior of the electrical double layer is that of a capacitor recall from physics

7:33that a capacitor consists of two oppositely charged parallel plates with a dielectric in

7:39between them that prevents the charges of these two plates from combining and being neutralized so

7:46in our electrical double layer we have a charged electrode surface and we have a layer of adsorbed

7:53anions and the amazing thing is that these two layers of charged species are so close to each

8:00other and yet their charge is not neutralized the charge on the anions and the positively

8:06charged electrode surface are not neutralized if we were to take a sigma voltammogram of just

8:12the supporting electrolyte without any redox active molecule so no ferrocyanide

8:18we would get a cyclic voltammogram that looks like a rectangle that's lying down horizontally

8:25the height of the rectangle represents the charging current of our electrochemical double

8:30layer and it is equal to the capacitance times the scan rate so this blank cyclic voltammogram

8:38can actually teach us quite a bit about the capacitance of the electrochemical double layer

8:44now our cyclic voltammogram gets much more interesting when we add our redoxactive molecule

8:49ferrocyanide to our electrolyte solution as we sweep the potential the potential becomes

8:56sufficiently positive that the ferrocyanide will undergo an electron transfer with the electrode

9:02surface specifically ferrocyanide will oxidize or lose an electron to the electrode surface burrow

9:10cyanide becomes ferricyanide whenever there is an electron transfer with the electrode surface

9:18current will flow in our electrochemical cell this current is referred to as faraday current

9:24named in honor and memory of famous electrochemist Michael Faraday this is in contrast to the current

9:32mentioned earlier associated with double layer charging this charging current is referred to as

9:38non-faraday current because there is no electron transfer occurring at the electrode

9:43surface during acyclic voltammetry experiment the potentials that will measure the total current

9:50of the electrochemical system this includes both the faraday and non-faraday current

9:56so if the double layer charging and the oxidation of ferrocyanide are occurring at the same time

10:03both will contribute to the current response as we sweep the potential we will see this

10:10increase in current associated with the oxidation of ferrocyanide to ferricyanide the current will

10:16reach a peak and then the current will begin to decay the shape of this curve the shape the decay

10:25of the current is due to diffusion diffusion is the random motion of molecules it is the mode

10:33of mass transport or the way that a ferrocyanide molecule moves around in solution for an electron

10:40transfer to occur a ferrocyanide molecule must be sufficiently close to the electrode surface

10:46well that's pretty easy if you're a ferrocyanide molecule already close to the electrode surface if

10:52you're farther away from the electrode it takes longer for you to diffuse towards the electrode

10:58and that is what we observe in the cycle voltammogram we observe a peak current which is

11:03associated with the maximum number of ferrocyanide molecules oxidizing per second well eventually we

11:12start to deplete the ferrocyanide molecules near the electrode surface and we mostly have ferry

11:18cyanide near the electrode it now takes longer for a ferrocyanide molecule to diffuse towards

11:25the electrode and oxidize so we see a decay in the current we have fewer ferrocyanide molecules

11:32oxidizing per second and we observe this in the cyclic voltammogram as the current decay

11:39as we continue to sweep the potential we reach the switching potential and we start to make our way

11:45back sweeping backwards negatively at this point most of the electrode surface is fairy cyanide

11:53the oxidized form of ferrocyanide as we seek the potential backwards the potential is still

11:59sufficiently positive that it is continuing to oxidize ferrocyanide to vary cyanide however

12:06at a certain point the potential will become sufficiently negative that we will begin to reduce

12:12which is where the electrode will donate or give an electron to the ferricyanide molecule

12:19and convert it back to ferrocyanide so we see a peak current associated with the reduction

12:28of ferry cyanide to ferrocyanide but as that layer gets depleted we start to see this increase in

12:35current again and we then complete our cyclic voltammogram so the forward sweep contains

12:43the oxidation of ferrocyanide to ferricyanide and as we sweep the potential back we get the

12:50reduction of ferricyanide back to ferrocyanide and that is a cyclic voltammogram there is a lot

12:58of complicated math that describes the current and potential in a cycle voltammetry experiment

13:04however we find that going into the math doesn't really help you understand conceptually

13:09what is happening in your electrochemical system during the cycle voltammetry experiment however

13:15if you are interested in future videos where we do go into those topics in more detail

13:20please leave a comment below again don't forget to like and subscribe alright i'll see you soon

13:34you