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Chemistry - Year One > Organic Energy Materials > Flashcards

Organic Energy Materials Flashcards

1
Q

What occurs as a result of pi-conjugation?

A

Thermal and Chemical Stability

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2
Q

What is Huckel’s approximation?

A

We only concern ourselves with the MO energies of the conjugated system

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3
Q

What is the number of pi-bonded MOs equivalent to?

A

The number of pi electrons

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4
Q

How do we lower the energy of molecular orbitals?

A

We increase the number of bonding interactions and decrease the number of anti-bonding interactions

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5
Q

How do we determine the net bonding interaction for a conjugated system?

A

(1) Draw all MO wavefunctions including any nodes

(2) For each wavefunction, count the number of bonding interactions (B) and the number of anti-bonding interactions (A)

(3) The net bonding of each wavefunction is equal to B - A

(4) The net bonding of the pi-system is equal to the sum of the electronically occupied interactions

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6
Q

How does increasing the pi-conjugation impact the HOMO-LUMO energy gap?

A

It generally leads to a smaller HOMO-LUMO gap

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7
Q

What do we call the band of delocalised electrons?

A

The valance band

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8
Q

What do we call the band of empty orbitals?

A

The conduction band

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9
Q

What is an exciton?

A

An electron-hole pair

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10
Q

How does the exciton binding energy relate to distance?

A

It is inversely proportional to distance

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11
Q

What are the values of the exciton binding energy of organic semiconductors compared to inorganic semiconductors?

A

They are much higher values; the electron-hole pair are tightly bound

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12
Q

How can we decrease the rate of charge recombination?

A

We can use a donor-acceptor strategy

This is where the electron is accepted by an acceptor at lower energy than the donor’s LUMO

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13
Q

How can we maximise the donor-acceptor interface in the photoactive layer?

A

We can implement bilayer structures, interdigitated structures or bulk heterojunction structures

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14
Q

What is the ionisation potential of a material?

A

It is the ability to lose electron(s) - oxidation

It is directly related to the energy of the HOMO

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15
Q

What is the electron affinity of a material?

A

It is the ability of a material to gain electron(s) - reduction

It is directly related to the energy of the LUMO

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16
Q

What is the equation to calculate the band gap (Eg)?

A

The modulus of E of LUMO - E of HOMO

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17
Q

How can we determine the optical band gap?

A

By absorption spectroscopy

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18
Q

What is a bathochromic shift?

A

A red shift (shift to increasing wavelengths)

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19
Q

What is a hypsochromic shift?

A

A blue shift (shift to decreasing wavelengths)

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20
Q

What is a hyperchromic shift?

A

An increase in intensity

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21
Q

What is a hypochromic shift?

A

A decrease in intensity

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22
Q

How do we graphically calculate the onset wavelength?

A

We draw a line of best-fit of the absorption line at the highest intensity

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23
Q

How can we calculate the optical band gap energy?

A

hc / onset wavelength

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24
Q

What shift occurs with increasing pi-conjugation?

A

A bathochromic shift occurs; this is consistent with the narrowing HOMO-LUMO band gap

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25
Q

What shift does increasing planarity lead to?

A

A bathochromic shift occurs; this is due to increased orbital overlap in planar systems

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26
Q

How does having a more regioregular backbone affect the band gap energy?

A

It leads to a smaller band gap (narrower HOMO-LUMO energies)

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27
Q

How does having a regioirregular backbone affect the band gap energy?

A

It leads to a larger bandgap and hence the weakest conductivity

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28
Q

What is an auxochrome?

A

A non-photoactive substituent that modifies the light-absorbing property of its chromophore (pi-system)

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29
Q

What effect do +M groups have on the band gap energy?

A

They lower the band gap energy, hence resulting in a bathochromic (red) shift

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30
Q

What effect do -M groups have on the band gap energy?

A

They increase the band gap energy, resulting in a hypsochromic (blue) shift

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31
Q

How can we increase the favourability of storing charge?

A

We are generally interested in stabilising the LUMO of the electron acceptor, enhancing the electron affinity

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32
Q

What happens when the applied voltage is approximately equal to the energy of the LUMO?

A

There is no driving force for the reaction to take place

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33
Q

What happens when the applied voltage is greater than the energy of the LUMO?

A

A reduction reaction can occur

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34
Q

How do we perform linear voltammetry?

A

We sweep our applied voltage in a single direction between two potentials

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35
Q

What can we learn from linear voltammetry?

A

We can determine the half potential for a given electron transfer process

It is useful for comparing the relative number of electrons transferred (n) for a redox-active molecule capable of multiple electron transfer processes

36
Q

If we sweep our voltage to more negative values, what process is occurring?

A

We are reducing our molecule

37
Q

If we sweep our voltage to more positive values, what process is occurring?

A

We are oxidising our molecule

38
Q

What is the cyclic voltammetry?

A

We sweep our voltage back and forth between two potentials, typically at the same start and end point

39
Q

Why is cyclic voltammetry more useful than linear voltammetry?

A

We can determine values for reduction and oxidation potentials

40
Q

How can we calculate half potentials from a cyclic voltammogram?

A

It is the midpoint between the reduction potential and the oxidation potential

41
Q

How can we calculate energy change from a cyclic voltammogram?

A

The modulus of the reduction potential - the oxidation potential (essentially the peak separation distance)

42
Q

How do we know if a reaction is electrochemically reversible?

A

If the energy change is approximately equal to 59 mV / n in a polar solvent, our process is electrochemically reversible

43
Q

What is the equation for the energy of the HOMO, determined by the cyclic voltammogram?

A

-n x ((Eₒₓ - E₁/₂ of Fc/Fc₊) + IP Fc, vac)

44
Q

What is the equation for the energy of the LUMO, determined by the cyclic voltammogram?

A

-n x ((E red - E₁/₂ of Fc/Fc₊) + IP Fc, vac)

45
Q

What is the height of a cyclic voltammogram representative of?

A

It is representative of the number of electrons transferred

46
Q

What effect does having an electron donor with a higher (destabilised) HOMO level have on the ionisation potential?

A

It will decrease the ionisation potential as it is easier to oxidise

47
Q

What effect does having an electron acceptor with a lower (stabilised) LUMO level have on the electron affinity?

A

The electron affinity will be higher, making it easier to reduce

48
Q

What is the Theoretical Energy (Charge) capacity (Q)?

A

The maximum amount of charge that can be accumulated (stored) per gram of active material

49
Q

How can we calculate the theoretical charge capacity (Q)?

A

Qthero = n x F / FW

50
Q

How can we increase Q?

A

We need to maximise the number of redox-active sites and decrease the FW

51
Q

What is a battery?

A

They convert chemical energy into electrical energy to power a device

52
Q

In which direction do electrons flow during battery discharge?

A

Electrons flow towards the cathode

53
Q

In what direction does the current flow?

A

It is conventionally opposite to the electron flow

54
Q

What does a cathode contain?

A

A polymeric binder

Our ‘active’ material

A liquid electrolyte

55
Q

What does an anode contain?

A

Graphite or Li metal

A liquid electrolyte

56
Q

What separates our anode and cathode?

A

A permeable membrane

57
Q

What process occurs at our anode?

A

Oxidation

58
Q

What process occurs at our cathode?

A

Reduction

59
Q

In which direction do anions flow?

A

Anions flow towards the anode

60
Q

In which direction do cations flow?

A

Cations flow towards the cathode

61
Q

What does the overall cell potential have to equal for a spontaneous reaction?

A

The overall cell potential has to be positive as the Gibbs free energy change is equal to -nFE cell

62
Q

How can we work out the equilibrium constant from the Gibbs free energy change?

A

Gibbs free energy change = -RTxln(keq)

63
Q

What is the energy capacity (Wh)?

A

How much energy a battery contains (the amount of electrical work that can be done)

64
Q

What is the power capacity (W)?

A

How quickly a battery can discharge its energy (the rate at which energy is used to do electrical work)

65
Q

What does the diagonal line in a Ragone plot equivalent to?

A

The discharge time (how long a device is expected to last when consumed at a given rate)

66
Q

What is the C-rate?

A

It is related to the discharge of a battery

67
Q

What is the equation to calculate C-rate?

A

C -rate is equal to 1/t

68
Q

What is charge (Q) equal to?

A

Current x Time

69
Q

What are battery discharge curves?

A

They show the power/voltage over time

70
Q

What information can we obtain from a battery discharge curve?

A

The nominal voltage

71
Q

What are disadvantages of lead-acid batteries?

A

Heavy and large

Brittle

Prone to leakage, gassing and sulfonation

72
Q

What are the advantages and disadvantages of NiCad batteries?

A

Good cycle life and physical durability

High cost and low power generation requires several in series

73
Q

What is the ‘memory effect’?

A

A loss in nominal capacity due to the repeated partial charging and discharging

The cell experiences a potential drop at the last point of charging

74
Q

What causes the ‘memory effect’?

A

Caused by changes in the physical characteristics of undesired active materials in the cell

75
Q

What do intercalation materials in the anode and cathode do in a Lithium ion battery?

A

They exchange Li+ ions between them during discharging (towards the cathode) and charging (to the anode)

LiCoO₂ is used as the intercalation material in a lithium-ion battery

76
Q

What is intercalation?

A

The reversible insertion of molecules or ions into materials with layered structures

77
Q

What does a coin cell battery contain?

A

Stainless steel casings

Springs

Anode

Separator

Cathode

78
Q

What is the equation for capacity retention?

A

Capacity retention = (Experimental Discharge Capacity / Theoretical Capacity) x 100

79
Q

What are the benefits of inorganic energy materials?

A

Porous materials that allow for excellent Li+ diffusion

Thermally robust and chemically resistant

Insoluble in electrolyte solvents

80
Q

What are the disadvantages of inorganic energy materials?

A

Limited and less precise capacity tuning

Expensive

Relatively heavy, toxic, brittle, non flexible

Poor solution processability

81
Q

What are the benefits of organic energy materials?

A

Precise properties tuning

More sustainably sourced

Cheaper

Solution processable

Lightweight and flexible

82
Q

What are the disadvantages of organic energy materials?

A

Poor C-rates

Low porosity

Lower thermal and chemical resistance

Soluble in organic electrolytes

Poor mechanical properties

83
Q

How can we enhance the C-rate performance?

A

We can increase molecular weight by using polymers; this increases the thermal stability

We can introduce porosity by making metal-organic hybrids

84
Q

What issues can occur in the cathode?

A

Phase changes

Dissolution

85
Q

What issues can occur in the anode?

A

SEI (solid electrode interface) and dendrite growth

Cracks and pulversiations

Chemistry - Year One (16 decks)

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