Batteries

what is a primary battery

non-rechargeable battery

what is a secondary battery

rechargeable battery

when is charge flow spontaneous

when Ecell > 0 , then G<0

what happens in Li batteries during discharging

Neutral ions intercalated at the anode are oxidised and Li ions move towards the cathode and intercalate (insert) into the cathode through the electrolyte

meanwhile electrons also flow from cathode to anode through the external circuit

SPONTANIOUS

what happens in Li batteries during charging

Neutral Li ions intercalated at the cathode are oxidised and flow back into the anode through the electrolyte.

Meanwhile electrons flow through the external circuit from cathode to anode too.

NOT-SPONTANIOUS

why are li batteries reversable

Li ions can move back and fourth between anode and cathode since they intercalate at the electrodes which is reversible

what happens at the anode of Li batteries

Li intercalated ions are oxidised and released a Li+

what happens at the cathode of a Li battery

Li ions intercalate and are reduced i

what does intercalation mean

the Li enters and leaves the ion without destroying it, but still being reduced/ oxidised

where is the energy of a battery stored

in the cathode

Really energy is stored in the difference between the electrochemical gradients of both electrodes, however, the cathode limits the energy capacity:

since it determines how many electrons can be inserted at once without damaging the structure (since cathodes are usually more fragile)

what really determines voltage

the difference in potentials between the cathode and the anode.

At the anode electrons have a higher potential and are then stabilised at the anode

If the anode has high potential the electrons are "eager to leave" then the energy will be higher, since more stabilisation (loss of energy) occurs when the electron is transferred to the anode

Why are cathodes more fragile than anodes?

to maintain the electrochemical gradient! They need to be made of different materials

what are anodes usually made of

graphene or something with lots of layers to accommodate Li

what are cathodes usually made of

Metal oxides LiCoO2 type thing

The metal oxides form layers that Li intercalates into,

The discovery of using these as cathodes instead of pure lithium improved safety of the devices

why do rechargeable cathodes degrade over time

the intercalation and release of Li and Li ions over time causes expansion of the material over time ,which can lead to lattice cracking and degration.

what properties are ideal for a cathode

- a high storage capacity for electrons

- good interaction w/ Li ions (to capture and release them)

- stable

what is the electrolyte

substance allowing movement of Li ions between the between electrodes

must have:

- high conductivity

- dissolves Li

- non flammable (therefore ideally not liquid at all)

what does a more negative reduction potential mean

something is more likely to give up an electron. I always think, more negative, the less it wants a bloody electron

what properties do you want at an anode

same as cathode girlie

what does higher conjugation lead to

more stable eheheh

how can pi conjugation occur

through conjugation or through space

which properties does conjugation determine

- conductivity

-light harvesting

-light emission

-electrochemical reversibility

what does pi conjugation do to the energetics of a system

provides thermal and chemical stability.

DeltaH hydrogenation is less negative (so molecule is more stable and less reactive) than expected for a molecule if it is conjugated.

Work expected value - experimental value = stability given by conjugation.

how does conjugation decrease bandgap

it both increases the energy of the HOMO and decreases the energy of the LUMO

However despite a higher HOMO the other orbitals of a more conjugated molecule are lower in energy - so the overall energy of the molecule decreases

How does conjugation lower the LUMO

The LUMO is lowered since it will be conjugated/delocalised which stabilises a system.

how does conjugation mean a higher HOMO

conjugation makes a pi orbital more stable. However - it is not more stable than a sigma bond.

For example in butane there are only sp3 sigma bonds, but in butene there are 3 x sp2 sigma bonds and 1 pi bond which will be higher in energy.

If you extendedterm-25 ethene to become lets say octene, despite the fact overall the molecule will be lower in energy, the HOMO of octene would be higher than butene, comply because octene has more orbitals, so the pi bonding oneThe LUMO is lowered since it will be conjugated/delocalised which stabilises a system. would be of a higher order.

Overall conjugation = delocalisation = more stable. But conjugation = HOMO is a pi bond = higher energy .

how does aromaticity further stabilise a conjugated system

if you look at net in phase bonding contributions for an aromatic vs an aliphatic chain, aromatics have one more net in phase bonding contribution, so therefore are more stable

How does delocalisation destabilise a molecule

spreading out the electron density or charge, thus reducing the repulsion between electrons and decreasing the overall energy of the molecule

what happens as you approach infinite aromatic conjugation e.g graphene

you end up with so many energy levels you form continuous valence and conductive bands

how do you work out exciton binding energy, Eb

Eb = e^2 / (4pi epsilon epsilon0 R0

Or Eb = Eg - Ex

Eg = band gap

Ex = energy of photon emitted from relaxation

What does a high Eb indicate

a tightly bound and hard to excite electron

for good conductivity do you want a high or low Eb

a low Eb, you want e- and h+ separated for as long as possible

what happens when you excite an electron across Eb.g

it creates a hole, h+ in the valence band and an electron in the conductive band

why can electrons conduct easier in the conductive band

more energy and also more free space to move around that (empty) band

what happens in the main principle of photovoltaic devices

an electron is excited from a material by light, and hole travels to the anode and the electron travels to the cathode

how do we overcome the high Eb of organic semiconductors, to overcome the coulombic potential of the electron and hole to instantly recombine after excitement

using a donor acceptor model. Have the electron excited from a HOMO to the LUMO of a donor molecule, then the electron jump to a lower LUMO of an acceptor molecule

the hole in the donor and the electron on the donor can then move respectively through the donor and acceptor material to the electrodes

how can on the surface of photovoltaic devices, where electrons are excited, the donor-acceptor interactions be maximised/ electron hole recombination be minimised

by maximising the surface area of of the donor/acceptor interface. (e.g by bilayer, interdigitated or bulk heterjunction methods)

what are the only UV visible electronic orbital excitation transitions

pi -> pi*

n.b -> pi*

since these are lower energy transitions >250nm

how do we maximise the solar harvesting of a molecule

we want to make conjugated molecules with such small bandgaps that they absorb in the lower energy solar/ redish light regions

what is the ionisation potential, IP

the energy needed to excite an electron from the HOMO off to the moon

what is electron affinity, Ea

the energy needed to excite an electron from the LUMO off to the moon

what is blue shift

shifting to a lower wavelength

what is red shift

shifting to a longer wavelength

hyperchromic shift

to a higher intensity absorption

hypochromic shift

to a lower intensity absorption

how do you work out λonset

draw a tangent from the longest wavelength side of the longest wavelength absorption peak of a cluster, and where the tangent intersects the x axis is the λonset

how do you work out Ebg.optical from λonset

= hv = hc/λ

h = plank's constant ( Js)

Ebg = in eV

C = speed of light nms^-1

what is the simplified subbed in version of the equation for Ebg.opt

Ebg.opt = 1241 eVnm / λonset (nm)

how does increasing conjugation effect absorption spectra

red shift

how does planarisation effect the absorption spectra of a molecule

it increases conjugation therefore red shifts

how does regioselectivity effect the band gap of a molecule

more regioregular backbone = polarised system = encouraged conjugated = smaller bandgap

what is an auxichrome

a non-photoactive substituent that changes the light absorbing properties of a chromophore

What do electron withdrawing substituents do to the bandgap/ spectra of a molecule

they increase Eg, blue shift

However if there is strong donor-acceptor interactions it can decrease the bandgap

(they stabilise HOMO, as you take away electrons, you take away the energy levels from the HOMO = higher band gap)

what do electron donating substituents do to the bandgap/ spectra of a molecule

they decrease Eg, red shift

(they destabilise the HOMO, as more electrons = more electron energy levels = higher ones = lower band gap)

what are n-type acceptors

negative type electron acceptors

how do we enhance the electron affinity of electron acceptors

we stabilise the LUMO (making the transition from donor LUMO to acceptor LUMO even more energetically favourable)

what the **** is an FMO

frontier molecular orbital i reckon

how does electrochemistry use heterogeneous electron transfer to achieve reduction

electron transfer happens at the surface of an electrode that supplies an under/ over potential for oxidation/reduction to supply the required energy for transfer

what value of applied potential do you need for spontaneous reduction

V applied > E LUMO

and vise versa for oxidation

what are the three components of a three electrode setup

-reference electrode

-counter electrode

-working electrode (where the voltage is applied)

what happens as molecules diffuse away from an electrode

they won't react as less voltage will be applied the further it moves from the electrode

what is shown in linear voltammetry

current (A) on y axis, potential on x axis

sweep the applied voltage in a single direction between two potentials.

Used to determine the half potential, E1/2 for an electron transfer

also used to determine relative no. electrons transferred in multiple transfer processes

how can you work out relative number of electrons transferred from linear voltammetry

the intensity of the peak

For a linear voltammetry of Fc/Fc+ relative to Fc/Fc+ at what potential would E1/2 be

0 of course relative to itself, there'll be no difference/ shift

on a voltammetry a sweep towards which direction indicates which process

to negative potential = reduction

to positive potential = oxidation

what does a time against potential between two potentials plot look like for linear voltammetry

a positive gradient straight line

why is E1/2 a key value

it is always constant, them middle potential between two potentials.

Despite the fact the potentials themselves may vary experimentally

what happens in cyclic voltammetry

sweep back and fourth between two potentials

how do you calculate the E 1/2 on linear voltammetry

E1/2 = (Ered - Eoox) / 2

where E red and Eox are the peaks of the respective reduction and oxidation peaks

what does a less negative reduction potential .potential mean about a molecule

that it is harder to reduce / easier to oxidise (think of it as being negative means not wanting electrons - it wants to be oxidised, not reduced typically, an anode would have very negative reduction potential, since its being reduced)

what does a time vs potential plot look like for cyclic voltammetry between two potentials

two lines converging as you go towards more negative potential

How can you tell from cyclic voltammetry if a redox reaction is reversible

if it is reversible then delta E

otherwise known as (Ered - Eox) = about 59 n (Mv)

n = no. electrons in the process

REMEMBER THIS

NEGATIVE POTENTIAL MEANS IT DOES NOT WANT ELECTRONS

how do you determine energy of the HOMO/ LUMO from cyclic voltammetry

Ehomo/lumo = -n ( Eonset ox/red - E1/2 vs Fc/Fc+ ) + IP

IP = ionisation potential of Fc = 4.8 eV

The -E1/2 of Fc thing is to set the equation with Ferrocene as the standard/ reference electrode. For going from vs Ag/AgCl to Fc/Fc+ then -E1/2 vs Fc = 0.4

so youd do Eonset - 0.4

how do you find onset of ox/red from a cyclic voltammogram

look at notes honestly i can't explain but you read it off the graph

How can you find the LUMO from only knowing the energy of the HOMO and vise versa

using UV data to calculate the band gap energy

what happens on a molecule if there are multiple reduction/oxidation sites that are conjugated to each other

Reduction/oxidation is stepwise instead of simultaneous.

Reduction/oxidation potentials are dependant /altered by any previous reductions/oxidations in the stepwise process.

e.g after each reduction the molecule will require more energy to achieve another reduction due to already increased electron density of the system.

what will a cyclic voltammogram of a molecule that has multiple reduction/oxidation sites that are conjugates vs not conjugated look like

the conjugated one will show a peak per reduction/oxidation site, each with equal intensity.

The non conjugated molecule will only show one more intense peak for all the reductions/oxidations at the same potentials since they are simultaneous processes

how do you engineer a material to have a higher IP (easier to ionise/oxidise)

use electron donating substituents to destabilise the HOMO (if you make something more electron rich its going to be easier to oxidise)

how can you engineer something to have a lower LUMO/ become easier to reduce (have a less negative onset of reduction)

at electron withdrawing substituents

what is the limit with adding electronic effecting substituents to influence the onsets of reduction and oxidation

the effects of substituent groups aren't summative

in a cyclic voltammogram which process corresponds to the energy of which orbital

oxidation corresponds to HOMO energy

(higher HOMO = more favourable oxidation)

reduction corresponds to LUMO energy

(lower LUMO = more favourable reduction)

what is through space intramolecular electronic couplign

redox sites can communicate if close in space as orbitals close in space overlap causing conjugation/delocalisation

This leads to stepwise reduction/oxidation

how do you work out Ecell

Ecell = Eoxidation + Ereduction

how do you work out the deltaG cell

= -nFEcell

n = number of electrons transferred

F = faraday's constant

what are the SI units for V

J = V C^-1

why are organic molecules bad for being intercalation electrodes

they pi or aromatic stack too densely and don't allow for the bulk ion flow

what is the advantages of using inorganic cathode materials

- defined porous lattices for bulk ion transport

- thermally/ electrochemically stable (resistant to decomposition under high rates C rates so have longer lives)

- insoluble in electrolyte which prevents them from leaking

what are disadvantages of inorganic cathodes

- limited resource

- heavier/ toxic/ less flexible

- only so many materials to tune properties of/ design with

what are the positives of using an organic cathode

- sustainably sourced/ cheap

- light/ portable/ flexible

- structurally diverse / more isomerisable

what are the disadvantages of using an organic cathode

-poorer solid state electrochemistry C-rates

- densely packed assembly prevents bulk ion flow

- low thermal and mechanical resistance makes them more likely to leach

current

I = W/ V

(in A)

C -rate (operating rate of a battery) (in units of hours^-1)

C rate = 1 /t

or amp hours/ hours

how do you work out Qtheory (battery capacity) in (mAh g^-1) or (Ahg^-1)

Q theory = n x F / Mr

n = no.electrons stored in a molecule

F = faraday's constant

mr = molecular weight

how do you work out Amp hours

Amps x hours

or mA x hours

what is Qspecific

the specific battery capacity/ how much energy the battery can provide experimentally

what are all the ways to stabilise a LUMO to enhance the kinetic stability of an electron transfer

- extend pi conjugation

- add -M substituents

- heteroatoms

-enhance electron delocalisation through space

why does stabilising the LUMO by extending conjugation shoot your battery in the leg

as you extend the conjugation, you increase molecular weight, lowering Q overall.

However if you also add more redox sites, increasing n, this can balance out

on a discharge curve how do you find Qsp and Eted

Qsp = tangent through the x axis from where curve tails off

Ered = tangent through the y axis from the plateau of the curve

why does Qsp decrease as cycle number increases

organic batteries are unstable and degrade over time