Original subtitles

okay um so i wanted to start

kind of with the i want to share with

you guys what's going on in

in my driveway at the moment um this is

my my younger brother's

car it's a uh 1978 fiat spider

and my brother is uh 18 years old

now and he got this car when he turned

16 he purchased it from

our neighbors up the street and when he

purchased it it was not running

and of course now it's missing a wheel

but uh

i had to push the car down the street to

get into our driveway and so my

brother and my father have been working

on it over the past few years

and they've had it got it up and running

um you know it's very

quick fairly quickly after getting it

but they've always been

changing things and they've taken the

entire engine out of it and

replaced the transmission and and all

that um but this is just kind of recent

work and

what was interesting why i'm why i'm

bringing this up is that

they um they had the engine block

re-surfaced so they took the engine

block out and they

they sent it to a machinist in seattle

who

who re finished the engine block so now

it looks

nice and clean as you can see here so

this is the end the

i don't i'm not too familiar with all

the the lingo

um the mechanic lingo

so i might be making up some words that

don't make sense but this is the the

engine the head of the engine block

and um this these areas are where the

combustion reaction happens right the

gas and

uh oxygen mixture combusts and there's a

the spark plug comes out here

and then these areas are where the

valves open and close to allow the

oxygen or the the gas in

and um so this whole thing was uh

chemically treated and cleaned and then

resurfaced but

what i wanted to point out is that this

these engine blocks

are are typically sand casted aluminum

and you guys are familiar with sand

casting and i wanted to show you some of

the surface features of this

resurfaced engine block and

um you can see the porosity in the sand

cast

it has very high porosity just evident

just through

visually inspecting it and

what's what's also interesting is that

you know fiat

is also notorious for for having parts

break down on it and in fact uh my

father

that was talking with a a machinist who

was who's making these parts

or specializing the fiat parts and he

said yeah the engine blocks have always

been notorious for for

it's uh low quality and i think it's

pretty evident of this you know high

porosity that

these are kind of poor materials but

also why this is out right now

is that my brother was adjusting the

timing of the

of the of the car the engine

and um the timing is important because

the timing

is uh you know when the spark will will

ignite

compared to the height of the cylinder

that

the piston and also the valves when they

come out and so if you want to get it at

the right spot where

you have the spark right at the top of

the cylinder or whatever so you have the

right compression ratio i'm not i'm not

too familiar with

you know the exact uh lingo but what

happened is if the timing is off by too

much

that the head of the cylinder that

piston that's going up and down

will bump into the valve that's opening

which is this

the valve seat is right here and so what

happened is my brother

uh accidentally put the timing a little

bit too far off time

and the top of the cylinder bumped into

this valve that would open

and broke off and so now it's inside

this chamber that's

bouncing around you can see how it's

deformed the pieces of this cylinder

valve have broken off and deformed

inside while while the engine was

running

and so if we had to take the whole thing

they had to take the whole thing apart

again uh this time the engine could stay

in the car just taking the the head off

is easier

and what they did recently is they went

to a i guess a scrap yard

that had the same car and got the same

part from an

old your retired car and so here's the

same kind of part you see these are the

valve

uh parts that kind of open and close and

that's what was destroyed and so they're

going to clean this up and replace the

one that was damaged

which is too bad because it's nice and

clean already but anyways

just just a little bit of material

science uh i thought i would share

what's kind of going on in in our time

off

but that's my brother my father have

mainly been doing that

okay so let's um we're going to talk

about

a bit more about the electrochemistry i

wanted to go over some things we learned

last lecture

some things that i might have missed and

clarify some things

and we'll talk a bit more about

batteries um so last time i talked about

the standard reduction potential right

this list of potentials that these

different species will

reduce at i've i failed to emphasize

that these values are measured at

standard state

so they're empirically determined at

standard state conditions and the

standard state is at 25 degrees celsius

one atmosphere

and then also most importantly is the

concentration

of the species so in all of these uh

these cases with exception for some like

the the metals like zinc metal

zinc metal you can't have a

concentration in solution it's a solid

but like iron three plus iron two plus

the concentration of iron three plus is

equal to the concentration of iron

two plus and and for standard state i

think they keep it at one molar which is

a bit high in my opinion but

uh so these values are only accurate

when

it's at standard state and that's why

it's called the standard reduction

potential it's given by this notation

the

the e with a little knot above it that's

the standard

reduction potential so if we uh in just

looking back at this example where we

had the cell

of the iron two and three plus and the

zinc two plus and zinc metal

well this this potential that we

calculated the standard redox potential

of the reaction

it's only accurate if we say that the

concentration of iron two plus is equal

to the

concentration of iron three plus and

then the concentration of zinc two plus

is one molar

uh okay so that that's only the case and

i

kind of forgot to mention that i wanted

to go over that in more detail

you know what happens to the voltage of

this cell

if the concentration is not the same for

each species okay so what does the

voltage look like in that case

and this follows uh what's called the

nernst equation

okay so this equation relates the

voltage the redox potential

of the species related to its standard

reduction

to potential at standard state and then

how it changes concentration so minus

rtr is the gas constant t is temperature

in kelvin

n is the number of electrons involved in

the reaction so for iron two plus iron

three plus it's only one electron

like zinc two plus the zinc metal is two

electron reaction

um f is faraday's constant relates the

the amount of charge

in one mole of electrons and then q

r is the the um i want to say reaction

quotient

reaction quotient that is the ratio of

the products of the reaction

to the the uh

reactants of the reaction okay so

if at standard state the products and

the reactants are at equal concentration

and here i've plotted

the reduction potential versus the

the ratio of the products to the

reactants of that reaction of this uh

this redox reaction and you see if

they're equal so

it would be one over one or it would be

equal to one

that is the value of the standard

reduction potential and for iron

to the three oxide it's 0.77 volts

but as we increase the amount of

reactants

which would be iron in this case would

be iron three plus if we increase the

concentration of iron three plus

relative to iron two plus

the voltage of this of this redox

voltage

increases okay and the opposite is true

for if we increase the

amount of products okay this this should

be familiar with you should be familiar

with this

you've probably learned this in freshman

chemistry and even high school chemistry

this is uh a principle called

le chatelier's principle okay and that

that

says you know if you have more product

then the reaction will be favored to go

in the reverse or if you have more more

reactant it'll be favorable in the the

forward reaction

all right so all we're saying is that we

have more reactant versus product and so

there's going to be a higher a greater

thermodynamic driving force

for that reaction to proceed instead if

it had more

product than reactive okay i mean even

if

even if the amount of product is very

low

or excuse me if the amount of product is

very high there's still a positive

potential which means there's still a

thermodynamic driving force for that

reaction to proceed it's just lower than

if it was the other way around okay uh

so here's an example

of that uh i i've taken two cells

uh a solution a and solution b and they

are both made up of

iron two plus and iron three plus but

the difference is the difference in

concentration of the three plus to two

plus

so in solution a we have a greater

amount of

uh iron three plus solution b solution b

we have a greater amount of iron two

plus so you can use the nernst equation

to calculate the

the redox potential in solution a okay

so we have the standard redox potential

of iron is

0.77 and the natural log

of the concentration in this case is one

the concentration of the product iron

two plus is one in this case and the

reactant is ten okay so it has a a

redox potential that's above its

standard redux potential

and the opposite is true for solution b

as a redox potential that's lower

so if you compare these two together

there is a

potential difference between these two

cells so if you were to take the

multimeter and measure between you you'd

see a voltage and that voltage is the

difference between these two

so the cell voltage is just the

difference between the reduction

potential of

solution a and solution b so you can

always take the

the higher number minus the the smaller

number

and that will be that can indicate which

direction like electrons will flow

and which what species will be reduced

or oxidized so in this case i've used

the reduction potential but in the

previous example of the zinc

i had used oxidation potential which is

just the negative

of reduction potential all right so it

is the the

the opposite here so you have to

add uh the oxidation potential in this

case

um so the question is

is is this when will this uh reaction

stop

all right so if we if we were to short

circuit this you know

just put a wire across this then

electrons will flow from

this side the iron two plus will oxidize

it'll give

electrons and flow into this side and

reduce the iron three plus and that will

continue

until a certain point and that is of

course equilibrium of the entire system

but the question is when does it reach

equilibrium

okay so we know at equilibrium

there is no net prod uh production of

product and there's no there's no net

forward reaction there's no net reverse

reaction

okay so that means that

the potential between these two cells at

equilibrium would be

zero all right so or gibbs free energy

would be zero right there's no

uh thermodynamic driving force to go one

way or the other

and so the question is you know when at

what concentration of these two cells

will that

occur at when will the the reaction stop

uh taking place

and in this example i i should have

started with nine and one to make the

total

10 but the total concentration of iron

is actually 11 so that's kind of my

mistake but that's fine

so the total concentration of iron in

this case is 11

so when this cell has a concentration of

of 5.5 millimolar iron 3 plus and 5.5

iron 2 plus and that's the same as this

cell that's when the voltage between the

two will be zero

okay and so that's when the reaction

will stop the electrons current will

stop flowing

between the two there's no potential to

to drive

the current um so another question is

you know in this example is fairly

simple the concentrations are the same

in both

right the total amount of iron in both

cells is 11 millimolar

what if one of the cells had a much

higher concentration of iron for example

let's say solution a

had a 100 millimolar iron three plus and

one millimeter iron tubeless but

solution b was kept

the same all right then what would the

the final concentration be now before

this lecture i had prepared

the slide but unfortunately i lost it

and it didn't save

and i'm wondering we could go through it

um

but actually this lecture this morning's

lecture was kind of long so i think i'll

skip that

but uh the answer is you know for if

you have to think of um as

you take electrons away from solution b

all right for every like one millimole

of electrons you take away right you're

you're creating one millimole of iron

three plus and then the same for this

solution you're you're giving it one

millimole of electrons you're

you're creating one millimole of iron

two plus so

if this was 100 millimolars the the most

this could change in concentration at

the maximum let's say it took all the

electrons away from these irons

then the most it could be would be it

would be 9

or excuse me 10. it only has 10 electron

millimolars of electrons to give

so this would be become what 90 instead

of 100. so that would be

the maximum but it's not going to reach

that state

because remember the reduction potential

of these solutions

depends on the ratio of product to

reactant

okay so actually uh before it depletes

all those electrons that ratio is going

to create

such a a potential that

the it's going to equilibrate and turn

to zero

it's a bit it's a lot more clear when i

have that extra slide i'll i'll see if i

can upload it and

later it'll make more sense but let's

move on

so this is something i was thinking

about last night when i was making these

slides

is how how is this different

than having just iron and two plus and

three plus in solution right i mean

what's what's stopping from

stopping this iron two plus two for from

giving electrons to this iron three plus

and

reducing it right it's kind of kind of

similar to this and um

if the concentrations are different

right i i i'm embarrassed to admit i

spent

a long time thinking about this and i

was i was

it was driving me nuts thinking why why

it didn't make sense um and the reason

is

well of course in solution you you are

having

this electron transfer between iron two

blocks and iron three plus all the time

right it's it's fine it's natural to to

have some exchange

but the point is that i was missing is

that if you remove

an electron from this iron atom

and give it to this iron atom the

concentration of iron three plus and

iron two plus does not change right you

just removed it from one iron and gave

it to another and

it took me about an hour to figure that

out like oh i'm

i was a little bit embarrassed about

that anyways

so it is different this solution you

could say is at equilibrium it has

equilibrium potential

this system this system is not at

equilibrium

okay until the voltage reads zero but if

you're just looking at individual

solutions you could say yeah that

solution is at equilibrium

but the system is not um

that's you know particularly introducing

the salt bridge

uh will put the the give you this this

potential between the two electrodes uh

in a side note i

i was at a museum in uh germany this is

related it's related i was

in germany with my wife and i when i was

an undergrad and we were vacationing and

i went i think it was in dresden we went

to some science museum and there was

this really cool demo

that talked about the like corrosion

potential between different metals

or the activity between different metals

where they had a metal

like a one type of metal like aluminum

and add another type of metal

copper and they were like plates and it

had a volt meter just like in the setup

okay and

the idea was you would put your hands on

these plates

and then the voltmeter would read a

voltage okay

so how does that work it's it's very

similar

to to this setup where you have

you know say you have the copper plate

on one side the

aluminum plate on another and they're

connected to the voltmeter that we're

looking at

and then you your your own body is

making the salt bridge

to connect the two and that's when the

voltage reads so your body

is the electrolyte to for this uh to

make this

uh to measure this potential difference

and i thought it was really cool because

there's all these different metals and

you put your hands on different things

and it make the volt meter

go to different voltages anyways so

pretty cool demo you can you can try out

if you have a volt meter and see if it

works

okay so here's a method of determining

the the standard reduction potential

uh this is called potentiometric

titration

and there's other types of titrate

titration is when you when you

when you add a species to a solution

like drop by drop

and you're measuring the volume added to

the solution

okay so like there's uh you're probably

familiar with a

acid-base titration right you might have

a solution that has an indicator that

changes color

when it reaches a certain ph and then

you titrate it with an acid and you're

measuring the volume

added to that solution and you're modern

monitoring it and then all of a sudden

it hits that that

that ph and it changes color so that's a

acid-base titration

in this type of titration you have a

species

of redox active ions in solution so in

this example we have

a tin two plus in iron two plus just a

hundred percent

two plus hundred percent iron two plus

and we're titrating it with

this this ion this is um

and correct me if i'm wrong i think a

cerium

i believe cerium i could be saying that

wrong cerium four plus

um so cerium four plus it has a very

high reduction potential

1.6 volts so that means if it comes in

contact with any of these ions

it's going to easily oxidize those ions

so those ions are going to oxidize

they're going to

they're going to iron cesium four plus

is going to steal an electron from those

ions

okay so if we start

with this solution of two plus ions and

what

you do is that you measure the redox

potential

so the potential between the platinum

and the a reference electrode such as

a standard hydrogen electrode so you're

measuring the whatever reaction is

happening at the surface of this

electrode

as you titrate it okay so this this is

very much at equilibrium

you add a little bit of cerium you you

let it mix up

together and so it

goes to equilibrium and you measure the

voltage and so you take these

different points as you add the cerium

and you measure the volume of cerium and

that's

relative to the concentration of the

cerium okay and you'll get a plot like

this

and this plot very much is the nernst

equation

for each species so all i did as i

modeled the nernst equation

for 10 2 plus the 4 plus and then i also

modeled the neurons equation 2 plus the

3 plus and i just kind of added them

together

and the at the inflection point is the

standard

reduction potential and remember

standard reduction potential is when

the concentration of the product is

equal to the concentration of the

reactant

all right so at this potential and or

that this volume added

you have reduced half excuse me you have

oxidized half of the tin

ions in solution all right and then you

continue and then at this point you've

oxidized all of the tin ions

and then you start oxidizing the iron

ions and then this at the halfway point

as the standard redux

potential is a half of the iron

ions have been oxidized now one question

is uh you know when you drop the cerium

in why why is it that only the tin

oxidizes and not the iron

okay because i mean iron is also lower

in reduction potential than the cerium

so you know both of these are are fair

game to being oxidized right

but but if you were you were to measure

it you'd only see the tin

oxidized um so in reality when when we

drop this liquid in and the cerium

reacts with the the liquid it could very

well yeah it's very possible that it

does oxidize

the iron two plus the iron three plus

however

iron three plus also has a higher

reduction potential than tin four plus

okay so that means the iron three plus

is oxidizing any

tin two plus that's uh present as well

so any

iron iron two plus that gets oxidized

it goes forward and oxidizes tin two

plus and becomes reduced again

anyways okay so that's potentiometric

titration and again

the important of this type of technique

is that it's at equilibrium

all right each data point we collect is

at equilibrium

now we'll talk about cyclic voltometry

and i introduced this before

but what's different with cyclical

optometry is that we're not

we are no longer at equilibrium all

right so

in this type of test you might have a

three electrode cell like this where you

have the working electrode and that's

that's going to be the reaction that

you're you're interested in

so you're measuring the voltage of that

reaction against a reference electrode

and then you're applying a current

between that and the counter electro so

all the purpose of the counter electrode

is only only to provide electrons or

take away electrons but we're not really

interested

at what whatever redox reaction is

happening on that surface

because we're not measuring the voltage

now i said with the

we're supplying our current but i mean

really we're controlling the voltage so

this type of test

we we have we we control the voltage

and we set it so that it it linearly

increases at a certain rate and then

decreases a certain rate between

you know arbitrary points that we select

uh it's important that these voltages

are within like the

electrolyte rate range so if you're

working with water

and if you push the voltage too far then

the water or the electrolyte will start

breaking down as well and you'll get

other

things that you're not interested in um

okay so here's a

this should be a video if i click or a

um

let's see yeah this happened in the

morning too oh there we go

so this is the the test okay so we start

out

uh at a negative potential okay let's

say we're starting with iron

ferrous chloride it's a hundred percent

iron two plus

and we've set the potential to negative

four so again we're

in this test we're applying a potential

rather than

uh just measuring it at equilibrium so

it might

it's not necessarily at equal this is

below the equilibrium

potential of the iron but iron has

already been reduced completely so

you're not going to see any current yeah

the y-axis is current

going through and the x-axis is the

applied potential

so we're not at equilibrium at negative

0.4 and this is

voltage versus the standard reduction

potential so at zero volts that's just

going to be for iron

0.77 volts versus she um

so yeah so we're below the equilibrium

potential but again

all the iron has already been reduced so

it's no it can't

be reduced anymore and we sweep the

voltage

we increase the voltage and then we

start to reach the reduction potential

and that's when we see current and the

current corresponds to the reaction the

electrons are being

delivered to the iron or taken away from

the iron two plus and it's oxidizing at

that surface

okay and then we reach a maximum point

the reason why we reach a maximum limit

in this case is that the concentration

of iron two plus at the surface of this

platinum electrode

is decreasing as as we're as we're

oxidizing the iron two plus the

concentration decreases

okay and eventually we reach a limit

where there's no

longer any any iron two plus available

but there's still

there's still plenty of iron two plus in

the bulk of the

electrolyte it's just not at the surface

of the the electrode

and so what happens is we've reached a

diffusion limit

so we're we're waiting for the ions in

solution to diffuse

to the surface and then also the iron

three plus in the uh

against the surface to diffuse away from

the surface all right so that's why the

current starts to

go down even though there's plenty of

iron two plus still in the solution the

current starts to go down

because of a diffusion limit so it's a

it's a kinetic

limitation and this peak current also

can depend on you know the size

of this electrode if you have a higher

surface area of course you're going to

have more

more uh a higher current um than a

smaller like a wire a platinum wire

okay so we're continuing to push this uh

this voltage let's see if i can

go up and then we reach the the final

voltage and reverse it

okay so we've reached our end voltage

which is would be this point here and

then we start to sweep the voltage down

okay so at this point we still have

positive current

see it's above zero if we have positive

current that means that

electrons are flowing away from this

electrode they're flowing out of that

electrode we're oxidizing it so we're

still oxidizing

the iron because there's still plenty of

iron available until we reach

zero and at that point then we switch

from oxidation

to reduction okay so now the electrons

are flowing to

the platinum electrode here and reducing

the iron three plus species

and then the same thing happens that you

you have a diffusion limit of the iron

three plus available at the surface

and then it continues to go okay

so that's that's an interesting to think

about and this kind of um

the fact that you know we're at we're at

a positive potential

versus she so remember this zero it

should be really just

0.77 actually we're at 0.77 volts above

she and we

we have both you know pot we can have

positive current

or negative current and you know for the

longest time

i i forget when i learned this but i

always thought you know if you have a

battery

and you discharge the battery right

you're getting current out of the

battery and then if you want to recharge

the battery all you have to do is apply

a reverse

uh current you know if if your if your

battery discharges at three volts and

you want to recharge it you apply

negative 3 volts

and that is entirely incorrect if you do

that you're going to you're going to

blow up the battery

all right if you want to recharge a

battery let's say your lithium ion

battery discharges at 3 or 4 volts

to recharge it you actually want to

apply a higher voltage across the same

terminals you're going to

increase it to 4 volts or 5 volts okay

so it's kind of interesting to think

about

you know even though in both cases we

have positive voltage

across the electrodes it's

it's um the current can be either

positive or negative which is

kind of strange now now the that's

versus a references

of course versus the other electrode

um the potential is does change

so that's that's where it that's a bit

different

so here's another thing to think about

let's say we're running our test

and we start increasing the current and

we we hit this zero volts all right they

were at the standard state

if we were to stop the test let's say

let's say

uh we'll not necessarily stop but we we

hold the voltage at zero if we were to

hold

the voltage at zero what would happen

okay we hold the voltage at zero we

don't

we don't progress any higher or lower

what happens is the current will decay

along zero okay until it reaches zero

volts

all right and at that point we know that

zero volts the the versus the standard

reduction potential

that is when the product is equal to

the reactant okay and actually in this

case because

uh in this setup it doesn't make too

much sense actually i'll show in the

next setup

but because there's a bulk solution of

iron two plus we would probably never

reach we might not ever reach a

zero because there's still going to be

plenty of iron two plus

in the bulk that's diffusing so we'll

probably reach its current limit i

i want to correct myself there but the

next slide it makes more sense

um so yeah this type of cyclic

voltometry has this diffusion limit

okay but there's another type if you

take this electrode and you move it

around for example this is called a

spinning

disc electrode so

if you it's basically electro a platinum

electrode that's a

disc and it spins in solution and as it

spins it's creating a convection

current in the solution so you're always

delivering fresh

iron two plus to the solution and so it

no longer has that diffusion limit where

the current goes back down

but still has a current limit and this

current limit

depends on factors like the

concentration of ions

the the size of the electrode the the

speed that's spinning at

and also what's important is the

diffusion

of the the redox active

species right so you can use this type

of test

and if you know all those other

parameters you can calculate the

diffusion

coefficient of that ion in solution so

that

is kind of an interesting test to do if

you need to find diffusion coefficient

of ionic species

in uh redox ionic species in the um

the solution okay so again

these this type of cyclic voltmeter this

type of test

we're taking it away from equilibrium

and driving the reaction so in these

what i've showed is that we have a redox

active species as part of the

electrolyte it's in the solution

but batteries and other materials

the redox active solution is not you

know it's not an

aqueous ion it's actually a part of a

solid

okay so in this case the all the species

are already present at the platinum so

depending on what the material is you

don't have to worry about diffusion

of of your redox active species to the

surface

anymore it's already on the surface and

if it has high electrical conductivity

then it has no

no problem and then you would expect a

curve like this where

uh you'll have zero uh when if the

entire species is reduced

and then you start oxidizing and then

you're starting to run out of uh

the material is starting to become fully

oxidized and then at this point

c all the redox active species on the

electrode have been fully oxidized so it

should reach a

current of zero and then you can do the

opposite as well

but oftentimes in uh electrode materials

like battery materials

there's still an influence of diffusion

so you'll still see

that these peaks will shift away from

zero and because of the diffusion limit

and that's because you know if you have

a lithium ion battery

you're still relying on the lithium from

solution to diffuse

into the material and so you can you can

still deplete the concentration of

lithium at the surface

and so you're still limited by that and

not only that that's the

the solution resistance but also the

the diffusion resistance inside the

material right so you can you can

intercalate lithium or sodium right into

the surface of the material but it still

needs to diffuse throughout the material

to make room for more lithium or sodium

and so that can also limit the limit

this reaction

and so if that happens you'll see a

splitting of the peaks

okay and in general if if you increase

the scan speed right if you if we if

we're sweeping the voltage faster

you'll see a bigger split in the peaks

and also the current will also

increase but you'll see a bigger split

because diffusion is uh

more limited at faster scan speeds

okay

um oh yeah so in this example this is

where i can talk more about you know if

you were to take this uh

material and bring it you know measure

it up to this point at zero volts

uh versus its standard reduction

potential and you were to stop the test

okay what would happen well remember

even even if we have

are applying uh zero volts versus the

standard reduction potential

it's not at equilibrium even if we're

applying the standard reduction

potential it's not

equilibrium yet because we have current

we still have current if you have

current it means there's a reaction

undergoing and so that means it's not

equilibrium but over time if you hold it

at zero volts the current will

decay the current will go down and then

reach zero current and then once it has

reached zero current

then you're at equilibrium okay another

thing to think about is you know what if

we had taken this

material pushed it to zero volts and

then stop the test

and then do no longer control the

voltage if we

if we if we no longer uh apply a current

basically if we disconnect this wire you

know we push it to zero volts and we

disconnect

this wire what would happen to the

voltage the voltage would go back down

because when we had brought it to zero

volts by forcing electrons in or out of

the solution

that's not at equilibrium remember this

cyclophotometry is not at equilibrium

we're pushing it away from equilibrium

in order for the reaction to proceed

and if we had stopped it at this point

it would go back the voltage would go

back

to whatever its equilibrium voltage

would be would probably be it depends on

the number of species that had already

been reduced

would be probably somewhere and again if

we had

if we had driven it all the way to zero

held it there the current goes down to

zero

left it then at zero half the species

are reduced and half the species are

oxidized okay okay um oh yes another

question

to think about is in these three

electrode cells

i mentioned this before you know we're

taking electro

electrons away or giving electrons to

the counter electrode

so there's some sort of redox reaction

occurring at the counter electrode but

like i said we don't really care about

it

but in my research i was interested in

learning what exactly it was going on at

that

um in part of my research i was

investigating mixing

uh non-aqueous and aqueous solvents

together

and doing three electrode tests but one

of the problems was i wanted i wanted to

keep the concentration of water

in my non-aqueous solvent consistent

and but the problem is in a three

electrode test if you have your

counter electrode providing or taking

electrons

it means that something is breaking down

at that side

which meant that the water was breaking

down i wanted to keep that consistent so

i

in this experiment i just had a

completely aqueous

cell so it's all water and i had a

electrolyte

i had my material on carbon cloth

electrode

okay and what's different about this

cell is that you know typically right i

have my voltage

against the reference electrode which is

in this case with silver silver chloride

is a different type of electro reference

and the

material and then the current between

that and the counter electron

electrode but also on another channel i

was measuring the voltage

between platinum and reference which is

kind of unusual

this is the typical cv curve of my

material which again was

prussian blue and this is in water

so it's uh you have two different

species of iron and that's why you get

two

different peaks on cv because you have

they're

two different chemical chemical

environments

what's surrounding them is one is is

carbon the other one is nitrogen

so that changes the energy of the iron

so that changes the

reduction potential of the iron for each

one

and so what i did this is a little bit

confusing but i'll go through it

so on the first graph is the measuring

in blue this is the cyclic voltometry

scan all right so we're just controlling

the voltage

we're pushing it away from equilibrium

and uh

and it's sweeping it and then the red

curve

is the current of the cell okay the

current between the

carbon my my active material and the

counter electrode

the second graph is the voltage in blue

again

from the counter electrode to the

reference electrode during the same

during the same time during the test

okay and what you'll see is during high

peaks of

current when we when we we need to

supply

a lot of electrodes or receive

electrodes

you see the voltage against the counter

electrode

uh is negative and then when we switch

the current

it goes to the positive and so if you

see this plateau here i measure that

plateau

that potential at these two plateaus for

the counter electrode that corresponds

to the the

the breaking down of water the

electrolysis of water

okay so that's what is happening on this

counter electrode during our test

is that we're breaking down water and

either giving away electrons or

receiving electrons

so if we look at that in more detail so

during the

oxidation of my my material i need to

take electrons away from the material so

this is reaction a on the counter

electrode or on the counter electrode

the water is receiving electrons at this

negative potential

and breaking down into hydrogen gas and

hydroxide ions

okay and then on the reverse during

reduction of my material

i'm giving it electrons so that means

that the counter electrode needs to

supply electrons

so we have positive potential where we

break down

water into oxygen gas and hydrogen

ions and we give away electrons all

right so that that these this is uh

you know the where these electrons are

coming from

in the circuit right but again we're not

too interested in this

and as long as the cell is a large

volume

you know any changes in ph should be

rather insignificant okay unless you're

going to run the test for you know a

very long time

or if the cell size is very small then

it could influence it um

one more thing you know so i drew these

lines the lines represent the standard

reduction potential or the the reduction

potential

of uh water either reducing or

oxidizing but you notice that the actual

voltage

is either lower or higher than those

lines

corresponding to the amount of current

that is needed to supply

and so this this uh either over

potential under potential

we call it over potential and it's an

additional thermodynamic you know

driving force to

increase the the rate of the reaction i

like to

think about it in related to

how phase diagrams work for like for

example the iron carbon phase diagram

the steel phase diagram you guys are

familiar with this

you know if you're going from austenite

to um

perlite for example or austenite to

alpha ferrite

right you at high temperature the y-axis

is temperature

and you start an austenite and you lower

the temperature

and you reach that line right that line

is the equilibrium line

and then in order to transform austenite

into perlite you need to go take bring

the temperature below that line

and so that's that's called under

cooling is the amount of temperature

below that phase equilibrium they have

to bring it

to increase the thermodynamic driving

force for this transformation to

occur and you guys know if you bring it

if you quench the material if you bring

it down

faster and faster more and more that's a

higher thermodynamic driving force

for that transformation to occur the

same idea can be applied to these

voltages

is that you know this line represents

the phase

equilibrium between you know the

breaking down of water

and we need to bring the voltage lower

than that to increase the thermodynamic

driving force for this reaction to occur

so we can either supply or receive more

or less

electrons okay so that's called

overpotential

and the amount of overpotential is

related to the resistance

in the cell for example diffusion

resistance

water is kind of different because water

is everywhere in the cell but let's say

there's a different reaction happening

where you needed ions from the solution

to come to the surface

right and just like before we had

diffusion limits

you know that can add additional

resistance

and you might see this voltage drop even

lower in order to get that required

current that you need because

of the slow diffusion of ions to the

surface

that's one i one way there's other other

other things that can increase this

resistance such as the the surface area

of your electrode okay if you use a

small wire

that has very small surface area as your

counter electrode you have

you don't have too many active sites for

electron transfer

but if you use a large sheet a larger

film of platinum for example has a

larger surface area

you know you have more active redox

sites

on the surface so uh the you don't have

to have as much

over potential to create the same amount

of current

okay i i like to think of it and i'm not

sure if this is correct

although i i'm pretty sure you can be

thought of this way as ohm's law

so if if we require a certain amount of

current

but there's a certain amount of

resistance in our

cell for example the diffusion

resistance or the the

the electrical resistance then that

creates

a certain amount of over potential which

is the potential drop

in the cell uh beyond of like this

equilibrium

position okay so as i

goes up as i goes up like this red line

has a peak then the voltage drop

also goes up okay

oh okay so since we were talking about

prussian blue this would be the last

thing we talked about actually

all right um

uh i'll let me introduce a little bit

more history about

this material that i've been doing

research on

um suppression blue is the the trade

name

the chemical name is iron

hexacyanopherate

and it's a quite quite interesting

history

it was discovered in the early 1700s by

a

paint maker okay who is mixing chemicals

together to make a paint

and it was first the the synthesis was

first published in 1731

and by this guy george stahl and i

i could be wrong i thought george stahl

might have been from from

england working in berlin

and what was interesting is i found the

original

publication that includes the synthesis

of what was called at the time berlin

blue is discovered

named after the city it was made in

which at the time

berlin was part of the prussian empire

which is why it's called prussian blue

and not german blue and

you guys know have any idea what

language this was published in

so again it was i think a english guy

working in

germany and what language did he publish

this

this article in

i'll have the chat open hold on

if you guys have been asking questions i

haven't seen any of them because the

chat wasn't open

okay no no question okay well if you

can't read it

yes that's correct aaron uh the the

language

is latin and if you if you know the

the language of science during this time

period

was latin even though this guy was

english and he was in germany

uh he everything was published in latin

and so now now these days the language

of science is english

so yeah convenient for us um

that was kind of interesting so i found

this paper on like the google books and

you know how you can search through

google books like

word by word and so i i had translated

berlin blue into

latin to find exactly what page it was

on which was kind of

kind of cool i don't know what the rest

of it says but um anyway so

it's significant this material was

discovered because

it became the first synthetic

paint pigment uh before for at least for

blue i mean for blue paint pigment

before the blues were made out of

crushed up minerals which

could be uh quite expensive and this was

provided a cheaper

alternative that can also be mass

produced and so it's used in a lot of

different famous paintings a lot of

paintings it's very common paint pigment

actually

such as the starry night by vincent van

gogh

and there's a bunch of these different

paints are named after the materials

that are used to make them like a

titanium oxide white or

lead oxide white if you guys remember a

couple weeks ago i was talking about

gerocite as a material gyrocyte has a

yellow color and

it also has a paint pigment called

gerocite yellow or something like that

so kind of interesting

so this is oh let me go back here this

is the crystal structure

of uh the prussian blue

um and it has a cage-like structure

where

where you have uh iron fcc

lattice so face-centered cubic lattice

of iron

in the high spin state so these are the

corners in the face center and then also

it's

another fcc lattice of low spin iron and

the low spin iron is coordinated

uh octahedrally by six cyanide ligands

so cyanide as you probably know is a

toxic material and the reason why it's

toxic

is is similar to carbon monoxide so if

we breathe in carbon monoxide

it it binds to the iron in our in our

blood

uh the heme group and that prevents it

from delivering oxygen to different

parts of our body

and uh the same idea for cyanide is it

binds the iron in our blood if we

breathe it in

and then prevents the oxygen transport

however the iron prussian blue

is relatively non-hazardous

okay even though it contains this

cyanide ion and it's exactly the same

reason because this the bonding between

the iron the cyanide is very strong

so it's very safe to handle um

unless you unless you eat it and then

maybe

perhaps uh the it can break down in your

your stomach acid but um i'm not sure

actually actually i could be i might be

wrong actually i i think i'm wrong

because

i know for a fact that prussian blue or

at least this kind of the material is

used

as a medicine for radioactive poisoning

so for example if you ingest a

radioactive element

particularly it's just for i think

cesium a radioactive cesium

and if it's in your body you would take

this medicine prussian blue

and eat it i i guess you eat it eat it

this is not medical advice i'm not a

doctor so don't

don't be eating this stuff that you

encounter radioactive poisoning

but the idea is just like in a lithium

or sodium ion battery it has these large

cage-like structures i have another

slide here

large cage structures that can absorb uh

large monovalent and even divalent ions

okay so cesium is monovalent cesium plus

and so it's a very large ion so this

this material can absorb the radioactive

cesium from it and then i guess you

pee or poop it out layer and gets rid of

it from your body

uh but that's why how it's used as a

medicine for radio radiation poisoning

but the same idea you know the advantage

of this structure is that has these very

open cages

so there's a lot of volume interstitial

volume for large ions to be intercalated

in and out of it at very fast rates

right so the diffusion coefficient of

these ions in the material

are fairly high which is good for a

battery material

and not only that but such a large space

you can you can incorporate even larger

ions other than like lithium

for example sodium plus has good

diffusion potassium ion has been

investigated for this material potassium

ion battery that is

um i forget that also there's a there's

a research group in portland i figure

out what

university or in oregon

and they're they're investigating

ammonium

ion batteries so instead of a single

atom

it's a molecule of nh4 plus that gets

intercalated

in and out of this material and the

advantage of using

um different well one of the factors

uh changing the intercalation

ion well the size of it changes the

redox potential

of this material and the larger the ion

this has been studied that the larger

the ion

that's intercalated the higher the redox

potential which is

is good for a cathode material you want

higher potential

and the ammonium ions the largest of

them all and it has the highest redox

potential for

prussian blue if you intercalated it

and one one thing i should point out is

that you know in this diagram i i draw

the

the metal plus ion the monovalent ion

right in the center of these cages which

is is not necessarily true

um because these cages are really open

and it's just if you guys remember from

like msc 170 when you calculate the

coordination of different

uh materials you know if if the ion size

is a certain ratio to the cation size

anion the cation size it'll either be

tetrahedral octahedral

right basically you don't want to have a

small ion surrounded in a big cage

because then it's it can rattle around

it's not very energetically favorable

and so the same cases here is that this

cage is so big it's not necessarily the

most energetic

energetically favorable site is in the

middle actually there's a lot of studies

that

show simulations that the most energetic

sites might actually be you know inside

one of these squares or against one of

these squares or against one of these

corners so

it's kind of just a simplification in

this diagram

and also well i'll get into that later

there's a lot going on here

okay so like i said before uh the energy

state of

the two different irons or excuse me the

yeah the electron energies of two

different irons are different

because of what's coordinated to them so

the iron on the outside of this equation

here that's

bonded to the nitrogen of the cyanide

complex

that is in what's called the high spin

state okay so these are the d

orbitals the electron orbitals and if

you take a single

ion ion that's not surrounded by

anything

all the energy is symmetric all around

and there's nothing around it if you

take a single iron ion

all the energies are equivalent they're

all equal they're all lined up

with each other but as soon as you start

putting things around it

like ligands for example the cyanide

ligand then that's going to change the

energy states of these different

orbitals and for octahedral symmetry

it makes these this kind of deviation of

energy states

of the the d orbital electrons

okay and so depending on what is bonded

to it and also like

the bond length of the ligands that

changes the energy splitting

of these two energy levels of the d

orbitals and if that energy splitting is

large enough then you start to

start to pair up these electrons because

it's more energetically favorable

to make pairs of electrons before

distributing them to all these single

unoccupied states and this is called the

low

spin state i think i go into that a bit

more

later so this is a schematic of how the

sodium ion battery works for prussian

blue

so actually when i synthesize the

material most often it's

synthesized as the partially reduced

state

all right so it already has a sodium

inside of it and it's in the mixed

valence between three plus and two plus

for the different irons and so usually

the first step

when i uh characterize my battery is i

have to either discharge it or charge it

first

but so the charging process is when

we're taking

sodium ions out of the material and

we're also taking

electrons out remember i said we have to

maintain charged

neutrality that's the golden rule about

everything

you have to if you take an electron out

of a system you also have to take a

positive ion

out of the system so we take a sodium

out we also take an electron out

that would that oxidizes this iron two

plus iron three plus so it's empty

this is the fully charged state or fully

oxidized state

i i guess i should be using the terms

oxidized versus reduced or

instead of charged versus discharge

because

you could be saying talking about the

same thing for an anode material and

it'd be the opposite but this is a

cathode material so when we take the

ions out

it's charged and uh and then we

go through the test of discharging the

battery okay and so the

there's different potentials like i said

the first the low spin has the higher

potential

so we introduce an electron and the

sodium intercalates

and that reduces the low spin iron and

then we have another

step where we introduce a second sodium

ion

and that reduces the high spin state and

so those are the two

redox steps in this material

this is a bit more explanation about the

crystal field splitting which is the you

know the energy splitting of these d

orbitals i think i was just showing

um i don't have to go in too much detail

but showing that the chemical species

the ligand that surrounds this iron ion

affects that energy splitting and

eventually get to

a high a large enough splitting that

these electrons uh

will go into the low spin state

um you guys remember from chemistry

class

what this uh principle is called when

you're you're adding electrons to

orbitals it's um

the poly exclusion principle i think i

think that's it poly exclusion

paulie's exclusion poly exclusion

principle um where you know you're

when you add electrons to these orbitals

it's more energetically favorable to

have them unoccupied so you're just

going to add one to each orbital

but in this case for low spin right it's

more energetically favorable

to pair them before adding them to the

this empty higher energy state okay

so that when you add electron when you

pair an electron together

there's a pairing energy involved so you

know when i add this electron in here

the energy state of these orbitals

actually increases

and which is not indicated by these

diagrams but in general you know if that

that

added energy is larger than what these

energies are at

then it'll go into the high spin because

it's more energetically favorable to put

in that position

anyways this is kind of a more advanced

inorganic chemistry that

we don't need to know too much about

just kind of skim over

um oh there's more there's more i made

these slides so i like to show them

because i spent a lot of work you know

putting in the fine detail

it's important when you make

presentations that the extra detail um

like for example i put a lot of work

into these these are all handmade these

diagrams these reference electrodes so

you spend you know 30 minutes to an hour

maybe less or more

making these diagrams and then you can

use them throughout your you know your

entire career you just recycle them

right so it's

it's nice to have nice nice diagrams but

in this case i talk about how the

octahedral splitting energy which is

this this uh

this energy gap depends on different

things like the metal ion that you're

looking at so in general

uh i think you might be able to relate

this to the periodic table

but valence state is definitely one of

them so like manganese two plus versus

manganese four plus

uh that energy gap will change depending

on if the valence state

and then i think more importantly is the

ligand okay

so what is being surround what is

surrounding these ions either in liquid

or in solid

it will change that energy and so you

know

typically you're used to seeing all the

d orbitals lined up like this

right in this case there's the it's

symmetric

uh electric field there's nothing around

it coordinated to it

and then you start coordinating things

to it and then that changes the energy

okay yeah so here's carbon monoxide and

cyanide as

strong uh ligands

oh here's some more some more diagrams

uh this just talks about the

different orbitals between the the metal

ion the metal center

and the ligand and what pairs they make

and whether it's

you know if you have these pi orbitals

that make a pi bond

um right so if you have an empty pi

orbital for the ligand and then it's

accepting electrons from the

or from the metal pi uh t2g

orbitals then it's a pi acceptor and

that that correlates with the larger

energy gap

anyways don't need to uh

there's one more this is a molecular

orbital diagram of iron cyanide

uh so you have your iron ion your six

cyanides and how

how the orbitals of the cyanides pair up

with the orbitals of the

the iron to make these bonds right so

you have sigma bonds and

pi bonds and then the energy gap

and so on

oh one thing i want to bring up is uh

not it's it's loosely related

when i was when i was making these

orbital diagrams learning more about

this

these energies um you know in chemistry

like high school chemistry and freshman

chemistry we we always learn

you know when we're adding the the

energies like counting up the electrons

in these different energies

in orbitals we always learn to fill the

four

s orbitals before the 3d

orbitals and i was wondering i was

trying to figure out why that was

and because you know i actually i found

some papers

related to chemic chemistry education

saying that why we should not teach it

that way

um and the proof

that i mean 4s has higher energy than 3d

and that's the controversy

right i'm saying this is 4s has a higher

energy than 3d

and the proof is that if you had let's

say a neutral iron

atom so you have uh you have two

electrons in the 4s

and you have you know these or the six

electrons in the 3d

so you know of course yeah that makes

sense you're filling the 4s before you

filled the 3d

but which electron has the highest

energy is the question and i'm saying

the 4s has higher

energy than 3d and the proof is that if

you were to

ionize this iron let's say we're

ionizing it to iron two plus or iron one

plus

which doesn't exist which electron do

you take away

are you taking away the 3d electron

because according to what we learned in

chemistry class we would take away the

3d

electron because it has a higher energy

but i'm saying well actually 4s has a

higher length g anyways that's the proof

that

yeah you take away from the 4s orbital

when you ionize something you always

take away the highest energy electron

and it's coming from the forest and

that's a brief aside

but it's something you might want to

investigate for yourself

here's another related side note by it's

it's related to like these energy levels

in

in materials as well so we have three uh

materials here corundum ruby and

sapphire do you guys know

what what chemical equation this is

for these materials what's the chemical

formula of these materials or at least

corundum for example

and it's okay if you don't know but i

hope after today you'll know

or ruby or sapphire you know if i say

hey what's what's the

chemical equation for sapphire anyway

the chemical equation

is aluminum oxide for all three of these

materials

okay so the base material is all

aluminum oxide and it

is evident you know they all have

similar crystal

crystallographic orientations in the

crystal shapes so

they're all the same material the

difference between ruby and sapphire is

that we've added

a dopant so a dopant is a small

concentration of ions

that's substituted with the aluminum in

this material

and the dopant for ruby is chromium so

adding a little bit of chromium like

less than one percent

will make corundum red ruby

okay and then for sapphire to make it

blue it's uh

you have to add both titanium and iron

two plus

and there's some kind of exchange in

electrons when it absorbs light so

that's why you need two

um so i guess the the question is

well let me move on so here's another

example

where we compare corundum and another

mineral called barrel

okay another the beryllium aluminum

silicate

in both of these materials if you

substitute i believe it's aluminum from

barrel

that could be wrong substitute about one

percent chromium

into both of these materials it turns

the

corndom red but it turns barrel green

and that's called emerald

so the question is why how can the same

ion

that is being substituted into these two

materials

turn one red and one green okay

and uh it has to do with the

the the d orbital splitting of the

chromium which is responsible for the

absorbing the light

in the material okay just like i talked

before you know it's all about these

energy levels of these d electrons and

what is around it affects that energy

level

uh this is a tanabe sugano diagram

that's used often with these

uh kind of kind of uh light absorbent

materials

um and actually uh uh ta from the

morning group

helped explain this because i i wasn't

too familiar i made these slides a long

time ago i knew it at the time but

i i can explain it a little bit so but

i'm i'm kind of paraphrasing what he

said

so the x-axis is the ligand field energy

okay or crystal field energy you could

also say is is the

is the energy splitting of these d

electrons

in the this is the octahedral

configuration of the

d electrons so if you're at a

crystal field energy of zero what that

essentially is saying is that nothing is

surrounding

your metal ion you know it's just it's

just saying it's like this this has a

splitting energy of zero all these

energy orbitals are at the same level

there's nothing surrounding the ion but

as soon as you start surrounding

the ion then you start splitting that

those energy levels

and depending on the ligand strength and

the bond length the bond strength of the

the ions and ligands that affects the

ligand field energy

okay and then this is the energy

of light that's absorbed by the material

okay so for a chromium i found i found

these

uh this information for chromium that

the ligand field energy

of chromium in aluminum oxide is you

know 2.2 volts and then

in the barrel it's a little bit less and

that affects

the the energy level of the light

absorbed

so in aluminum oxide you're absorbing

these two like spectrums of light and

then

beryllium you're absorbing different

colors of light which means different

colors of light is transmitted

all right so this helps explain why

chromium

is responsible for both the red light

and green light in both of these

materials

and is different because the ligand

field energy and liquid field energy is

different because the local environment

surrounding the chromium atom ion

is different in these two different

materials all right

so now you kind of see how everything is

related i talked about

you know how it's related in the battery

materials

right that changes the redox potential

of the battery materials

it changes the optical properties of

these materials

there's one more i thought but i forget

anyways

and here's another chart it just relates

the dopant concentration the color

so i guess chromium oxide is green

aluminum oxide is clear but you add

chromium makes it red

um so just uh quickly go over we're

about about a bit over time that's fine

i'll just kind of talk about some of the

data from my

my publication so again this was

looking at prussian blue is looking at

the synthesis of prussian blue

now and typically the synthesis if you

add a

ferrocyanide salt such as sodium

ferrocyanide and dissolve it in water

and then add a ferric or ferrous salt

uh the pres it undergoes a precipitation

reaction

okay the the solubility product of

prussian blue

in water is very low it's like 10 to the

negative 200 it's

very very low so that means as soon as

these ions are in contact

they're going to precipitate into a

solid okay

and because of such high precipitation

or low solubility product

what ends up happening happening is you

get

very small nano particles you get very

high nucleation

and if you have high nucleation you

usually have

low growth all right because you're

taking the ions away from solution

ions allow you to uh to grow the the

crystal

and so you what ends up having you have

these really uh

rough nanoparticles um and

they're more or less not grown under

thermodynamic conditions so they don't

have

nice facets or cubic structure and

there's a lot of vacancies in water

because of the fast reaction as well

so the idea was to add a collating agent

to this reaction that would collate to

one of the iron

ions so essentially we're reducing the

activity

of the iron species in solution and that

would help slow down the precipitation

reaction

and then also adjust the ph because the

chelation strength of this

chelate is uh affected by uh

the ph uh and so what ended up happening

right you get

a different ph you get different sized

particles you get different uh

vacancy content different water content

and then more imports

most importantly you had a different uh

redox

characterization so here's another view

of it

these are the particles growing at lower

ph at higher ph the particles were

much larger but what's interesting if

you look at these particles

they're not necessarily cubic right i

mean

they're definitely faceted and if it's

faceted

it means that like i said it's grown

under thermodynamic conditions you're

you're you're when you grow a crystal

you're going to grow

you're going to expand the surfaces that

have the lowest

surface energy okay and this

in this uh crystal structure it's going

to be a cube

but like just looking at it you know you

have like a rectangular prism here you

have something with like a step

you have like an l shape here it's kind

of weird and

you can see the same thing in the

smaller one but they're just much

smaller

and so the what i had done some

literature research and it seems like

it's fairly common with the prussian

blue material

grown under thermodynamic conditions to

make a what's called a

meso crystal a mesocrystal is when you

take smaller crystals

and then they they aggregate together in

an

oriented manner and then they can fuse

together to make another bigger

single crystal out of these small nano

crystals okay

and so that that's sure that makes sense

and it's supported by literature

but the other question was you know why

why do i have small

mesocrystals versus large mesocrystals

and also saying that they're

mesocrystals of speculation i haven't i

don't have any hard evidence that they

are mesocrystals but other than

laser but the question why is this small

and why are these big

and so my my proposal my proposed

mechanism was well

uh because the the solubility product of

this material is so low and they

precipitate so fast or there's such high

nucleation

that the particles growing at low ph

have much higher nucleation

and then that results in very small

crystals and then those small crystals

together can aggregate to form smaller

mesocrystals

on the other hand the higher ph would

have low nucleation

okay and that allow for more growth

larger particles

and then that would aggregate into

larger mesocrystals

and i wanted to i wanted to prove that

at ph 3.8 you know i had some faster

growth is what i wanted to show and then

at 4.4 i had

slower growth so what i did is i took a

uv

vis spectrum of the material so these

are actually four different ph's but i

only showed two on here

uh and it shows the peak at 700

nanometers 700 nanometers is the main

absorption peak of prussian blue and so

700 nanometers is like a red

right so it's kind of almost infrared

yeah and um

so if you're absorbing red that means

you transmit all the other colors and it

ends up being blue that's why prussian

blue is blue

but what i did is i took my my material

and i put it into a cuvette and uh

i added the two components together and

started the test

right away as it was reacting and then

every five seconds or so i think it was

either five or ten seconds

10 seconds every 10 seconds i would take

a scan

or a measurement just at 700 nanometers

of the intensity

of the solution because as this material

precipitates

it's going to uh absorb more light

or it's going to absorb blue light or

excuse me it's going to absorb the red

light at 700 nanometers

so here we see a ph 3.8 as soon as i add

the two solutions together

you know it becomes saturated very

quickly

so that means that the the the

nucleation growth is very quick

but at lower higher ph it's much more

gradual which means that the growth of

the particles and nucleation is much

more slow

and that makes sense because at higher

ph the chelation strength of this

chelate is higher so it's it's more it's

inhibiting

the nucleation of this reaction so it

slows it down

and that's why you get larger growth

okay so moving on to electrochemical

properties

um so here is uh

the the capacity the discharge and

charge capacity

uh for many cycles and then also i

increase

the current rate after every 10 cycles

so remember this is galvanostatic

cycling so this is like the

the profile of each one of these dots is

like this and

this is actually the last dot of each of

each series is this profile

so if you just look at one curve this is

the discharge curve you start at high

voltage after charging

you're discharging at a constant rate

so the first section is only 100

milliamps per gram

right and you measure the voltage as it

discharges and like i said there's two

different iron species in this material

and that represent two different redox

reactions so here's one redox reaction

is a higher voltage

corresponds to low spin iron and then a

lower redox potential corresponding the

high spin iron

and then the other the other chart the

other plot

line is just the the charge profile of

the

the same material okay and

um i said before you know as we increase

the current of the battery

as we're drawing more current uh things

become

there's at there's the capacity

decreases

and the voltage decreases i remember i

said you know i was kind of relating it

to v equals ir

that there's these resistances inside

the battery

such as diffusion resistance or

electrical resistance

and as we increase the current you know

resistance more or less stays the same

but as we increase

current that means the voltage in this

case voltage

drop increases so that's why

you see this voltage as we increase

current rate the voltage drops

okay

so if we just one one idea

i mean obviously you'll see that the

capacity of the smaller particles

is larger than the capacity of the

larger particles

okay and now

i've said this before in a previous

lecture you know it's always

better to make nano materials okay

if you make something nano it means the

diffusion distance of lithium ions or

sodium ions in solution

into the materials shorter all right it

doesn't have to travel as far

and that results in less resistance for

diffusion

uh during discharge okay and that would

result in higher capacity as well

so you know right away you look at this

and you look at the different

materials and you say well you know this

has higher capacity because these are

smaller

smaller smaller particles it makes sense

right

and so if that was the case

if if capacity was kinetically limited

by the diffusion of ions and due to the

particle size

you would expect to see as we increase

the current rate

that the cur the capacity retention

of larger particles would decrease even

more

but if you compare the initial

capacities at slow rate

to the capacities at high rate between

the small particles and the large

particles so the ratio of this number to

this number basically

they're they're pretty much the same all

right which would show

which would indicate that kinetics did

not play a

major role in this material as far as

the capacities go

however if you investigate it a little

bit more in detail

but in these charts here i separate

an estimated capacity contribution

you know for each one of these lines

just the the discharge ignoring the

charge

of the low spin iron which is this

higher voltage

uh plateau here versus the high spin

iron which is the lower voltage plateau

here and so the red

is the low spin iron capacity

contribution you see it's this is a bit

smaller than this line here

and the black is the high spin and if

you compare the two

uh particle sizes together you'll see

that

as you increase the current the capacity

contribution of the

high spin iron is is more or less

constant

okay and the same for the larger

particles it's more or less constant

which would indicate that the redox

mechanism related to the high spin

iron is not dependent or

largely dependent everything is

dependent but it's not largely

dominated by kinetics of the reaction

all right

it's a we're able to supply electrons

and we're able to supply it

ions through fast diffusion uh

effectively

okay there's no big decrease in that

that level but if you look at the low

spin

capacity contribution that's where you

see a decrease

all right which is is kind of unusual

because you have one material

and there's a difference in uh

redox mechanism between the you know

this plateau and

the second plateau which is uh a bit

interesting in my

opinion and i don't have i i don't have

an explanation

for why this redox mechanism of low spin

is different

other than that perhaps um

it's not what we think it is so we

always think you know we add sodium ions

in and we're reducing iron

three plus to iron two plus and it was

the same for the both but perhaps that's

not the case for this uh

higher voltage case so i'm looking into

this

when i get back to lab this is the next

thing i'm going to look at is

investigate this redox potential at

different rates of different materials

and i speculate that actually the water

content

might have something to do with it

actually because if you if you compare a

non-aqueous

prussian blue to aqueous prussian blue

battery you

always see that this plateau is much

higher

in the aqueous state than the

non-nyquist so it makes me think that

water plays a large role in the redox

reaction of this

material even in this non-aqueous

battery because i still have water

inside my material

so anyways so the point is uh

is that well there's still a lot to

learn

and i think once i figure this out i can

publish that

next paper and i can get out of this

university and then

move on to the next big thing

i'm going to end it here it's been about

an hour and a half

any questions before we we leave

well thanks for uh sticking with me

um i believe on friday

you have um a journal review

uh due excuse me a literature literature

summary

is due on friday uh so remember

for your your review paper you need to

have at least 10

literature sources since uh 2019

but there's only five literature uh

summaries

summary assignments so if you want to be

proactive you can actually make two

summaries each assignment

but you're only you only need to submit

one to get full credit and it

is for credit so please write out uh a

summary i'm going to try to

write a summary example tonight and send

it to you guys just show you what you

know

what i'm kind of expecting but basically

the more the more you do

the easier it's going to be for when you

want to compile your your review paper

okay yeah

and then tuesday will be the first day

of lab

so we'll meet here again at the same

time uh

in that day the ta will lead most of the

lecture

and i'll be here in support um so we'll

go through

some discussion questions there's no

pre-lab so you don't need to prepare

anything

but uh you know just just be ready to

learn i guess

be let ready to learn and be sure you

you can take notes during the lecture

uh so that at by the end of the lecture

you can submit

a summary of of our lab lab

lecture so the ta will discuss uh some

some background and discussion questions

of the topic

then we will have a video of the

experiment

that tatiana has uh has

been busy putting together these videos

[Music]

and now there's an issue with

streaming the video i tried streaming

videos on

zoom and it doesn't really work because

there's like a lag so i think what we're

going to have to do is

once we get to that point you know we're

going to say okay take 20 minutes to

watch the video and they'll

regroup and you watch it on your own and

then we'll regroup and then we'll have

discussion questions

and then the ta has data you'll be given

the data

and the ta will go over a data set on

how to like plot or analyze the data

and you guys do that together so it'll

be it'll be interesting how it goes

i'm quite curious myself it'll be the

first time okay any questions and

if not we'll uh you guys can get out of

here

okay well i'll see you guys uh next week

have a good weekend everyone

try to get some sunlight but you know

stay away from other people but try to

get outdoors

do some exercise yeah

you