Part 4 - Energy

Question 1

What is dendrite formation?

Answer 1

Dendrite metal spikes are unavoidable on normal Li batteries and they grow on the anode. If they form between the anode and the cathode they can cause a short circuit.

Question 2

Why is dendrite formation bad?

Answer 2

Can lead to short circuiting

Significant reduction in performance.

Raises safety concerns and reduction in battery life

Some dendrites have grown so fast and rigid that they can pierce the membrane between the electrodes leading to spontaneous combustion.

Question 3

How can nanotechnology be applied for batteries?

Answer 3

Ultra-thin lithium metal foil for the anode, which is about 1/5 of the thickness of a traditional lithium metal anode.

Question 4

What can nanowire as Li battery electrodes offer?

Answer 4

Good strain relaxation
Large surface area, and shorter distance for Li diffusion
Interface control (better life cycle)
continuous electron transport pathway

Question 5

Why use a silicon (Si) anode over the conventional carbon anode?

Answer 5

Larger power capacity (~10x greater)… moonshot thinking.

Question 6

What are the principles of a super capacitor?

Answer 6

The capacitor accumulates charges, then discharges quickly.

There are 2 carbon sheets separated by a separator.
When the voltage is applied to positive plate, it attracts negative ions from the electrolyte.
When the voltage is applied to negative plate, it attracts positive ions from electrolyte
There is a formation of a layer of ions on both sides of the plate.
The ions are then stored near the surface of the carbons
The distance between the plates is in the order of angstroms.

Question 7

What is the equation for capacitance?

Answer 7

Capacitance = (dielectric constant of medium)(area of the plate)/(distance between the plates)

Question 8

How can nanotechnology be applied to super capacitors?

Answer 8

Required to make a super capacitor with high surface area. Methods by CNTs, porous carbon, graphene

Question 9

What are the applications of super capacitors?

Answer 9

They can charge really quickly, but also discharge quickly.
Electronic applications
Uninterruptible power (if power supply to computer is lost, it will provide back up for ~10 mins)
Buses (China&Germany)

Question 10

What are the advantages of super capacitors?

Answer 10

High rates of charge and discharge
Good reversibility
Low degradation rate over hundreds of thousands of cycles (little lowering of performance)
Low toxicity of materials - SC made from carbon, not lithium
High cycle efficiency

Question 11

What are the disadvantages of super capacitors?

Answer 11

Amount of energy stored per unit weight is 10x lower than batteries
Voltage varies with energy stored
sophisticated electric control and switching equipment is needed

Question 12

How do solar cells work ?

Answer 12

The suns energy knocks electrons loose from the atoms in the semiconductor material, causing current to flow.

Question 13

What issues do solar cells have?

Answer 13

They can only absorb a certain wavelength.
Very low efficiency (10-20%)
Cleanroom needed for manufacturing due to pure silicon, adding extra cost.

Question 14

What solutions can nano materials provide for solar cells?

Answer 14

Nanocoating can increase absorption of UV and IR
Due to multiple reflections, effective optical path is longer
Energy band gap of various layers can be made to the desired value by varying the size of the NPs
Due to the light, generated electrons and holes need to travel over a much shorter path.

Question 15

Why should we care about improving solar cells?

Answer 15

They rely on silicon as a semi conductor, which is a limited earth metal.

Question 16

What are the two main types of nanomaterial solutions for solar cells?

Answer 16

Single crystal silicon (traditional) - widespread, expensive to manufacture

Dye-sensitized - newer and less proven, inexpensive to manufacture, flexible.

Question 17

What feature of NMs would enhance the use of solar cells?

Answer 17

They exhibit a wider absorption spectrum of light.

Question 18

What are the advantages of using NMs in DSC

Answer 18

Dye Sensitized Solar Cells (DSC) have several advantages:
Increased surface area
Reduced band gap
Improved charge extraction
Improved access to holes in the semiconductor

Question 19

What are NMs being used for water splitting?

Answer 19

Vertical arrays of nanowire made from silicon and titanium oxide for the efficiency production of hydrogen through solar water splitting.
Uses NM architecture by combining semiconducting nanowire with flexible polymeric membranes

Question 20

How is Si wire absorption maximised in artificial photosynthesis?

Answer 20

Vertical nanowire arrays, with Al2O3 nanoparticles on the arrays and a silver back reflector. Good absorption even in different exposure angles.

Question 21

How do fuel cells work?

Answer 21

Main components: anode, cathode, electrolyte (membrane)
Electrolyte is only conductive for protons (doesn’t ‘t conduct electrons which are forced to take a different route, generating electricity)
Oxidation reduction reaction
Fuels can be gas or liquid (H2, methanol, ethanol etc)
oxidants can be air, oxygen etc

Question 22

What are the three categories of FC?

Answer 22

Low temperature (<100degC)
Intermediate temp (100-400 degC)
High temp (>400 degC)

Question 23

Give a disadvantage to direct methanol/ethanol fuel cells

Answer 23

Provides a low voltage, so charging e.g. phones, would be slow.

Question 24

Give advantages of using methanol FC

Answer 24

Higher energy density than H2 FCs
Easy handling and transport of fuels
Fast filling in vehicles

Question 25

Give advantage for Ethanol for FC over methanol

Answer 25

Higher specific energy compared to methanol.

More abundant fuel source

Question 26

Which category of FC is considered the future and why and what is it?

Answer 26

Intermediate temperature because it combines the best of both. Solid acid FC

Question 27

What is the disadvantages of low temperature FC?

Answer 27

Complex and expensive. Need ultra high purity H2

Question 28

What is a disadvantage for high temp FC

Answer 28

High temperatures and exotic materials.

Question 29

What is the triple phase boundary problem in FCS?

Answer 29

The contact point between catalyst/electron conductor, electrolyte and incoming oxidant is not optimised because of geometric hindrance. Triangular region. Need to make electrolyte small because platinum catalyst is already capable of nano sizing.

Question 30

How are catalyst NPs produced for high efficiency?

Answer 30

Chemical reaction
CVD
Electrochemical deposition (ECD)

Question 31

What is a popular NM synthesised electrolyte (membrane)

Answer 31

Nafion. Produced by electrospinning to make nanoFIBRE

Question 32

What is the typical assembly for Fuel cell with NM membrane?

Answer 32

Nafion (polymer solution preparation)
Electrospinning to create long nanofibres 
Pt catalyst deposition
Electrode assembly
FC stack assembly

Question 33

How is intermediate Solid Acid Fuel Cell electrolyte synthesised?

Answer 33

Electrospinning using CDP solution + clear polymer. CDP becomes super protonic at higher temperature, at low temperature is doesn’t conduct.

Question 34

What makes Solid Acid Fuel Cell electrolytes interesting?

Answer 34

They combine the best of both high temperature and low temperature fuel cell electrolytes. Interestingly, at low temperatures, they do not conduct but at higher temps (~230 degC), they undergo a phase change and conduct.