Part 4 - Energy Flashcards
What is dendrite formation?
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.
Why is dendrite formation bad?
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.
How can nanotechnology be applied for batteries?
Ultra-thin lithium metal foil for the anode, which is about 1/5 of the thickness of a traditional lithium metal anode.
What can nanowire as Li battery electrodes offer?
Good strain relaxation
Large surface area, and shorter distance for Li diffusion
Interface control (better life cycle)
continuous electron transport pathway
Why use a silicon (Si) anode over the conventional carbon anode?
Larger power capacity (~10x greater)… moonshot thinking.
What are the principles of a super capacitor?
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.
What is the equation for capacitance?
Capacitance = (dielectric constant of medium)(area of the plate)/(distance between the plates)
How can nanotechnology be applied to super capacitors?
Required to make a super capacitor with high surface area. Methods by CNTs, porous carbon, graphene
What are the applications of super capacitors?
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)
What are the advantages of super capacitors?
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
What are the disadvantages of super capacitors?
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
How do solar cells work ?
The suns energy knocks electrons loose from the atoms in the semiconductor material, causing current to flow.
What issues do solar cells have?
They can only absorb a certain wavelength.
Very low efficiency (10-20%)
Cleanroom needed for manufacturing due to pure silicon, adding extra cost.
What solutions can nano materials provide for solar cells?
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.
Why should we care about improving solar cells?
They rely on silicon as a semi conductor, which is a limited earth metal.
What are the two main types of nanomaterial solutions for solar cells?
Single crystal silicon (traditional) - widespread, expensive to manufacture
Dye-sensitized - newer and less proven, inexpensive to manufacture, flexible.
What feature of NMs would enhance the use of solar cells?
They exhibit a wider absorption spectrum of light.
What are the advantages of using NMs in DSC
Dye Sensitized Solar Cells (DSC) have several advantages:
Increased surface area
Reduced band gap
Improved charge extraction
Improved access to holes in the semiconductor
What are NMs being used for water splitting?
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
How is Si wire absorption maximised in artificial photosynthesis?
Vertical nanowire arrays, with Al2O3 nanoparticles on the arrays and a silver back reflector. Good absorption even in different exposure angles.
How do fuel cells work?
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
What are the three categories of FC?
Low temperature (<100degC) Intermediate temp (100-400 degC) High temp (>400 degC)
Give a disadvantage to direct methanol/ethanol fuel cells
Provides a low voltage, so charging e.g. phones, would be slow.
Give advantages of using methanol FC
Higher energy density than H2 FCs
Easy handling and transport of fuels
Fast filling in vehicles
Give advantage for Ethanol for FC over methanol
Higher specific energy compared to methanol.
More abundant fuel source
Which category of FC is considered the future and why and what is it?
Intermediate temperature because it combines the best of both. Solid acid FC
What is the disadvantages of low temperature FC?
Complex and expensive. Need ultra high purity H2
What is a disadvantage for high temp FC
High temperatures and exotic materials.
What is the triple phase boundary problem in FCS?
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.
How are catalyst NPs produced for high efficiency?
Chemical reaction
CVD
Electrochemical deposition (ECD)
What is a popular NM synthesised electrolyte (membrane)
Nafion. Produced by electrospinning to make nanoFIBRE
What is the typical assembly for Fuel cell with NM membrane?
Nafion (polymer solution preparation) Electrospinning to create long nanofibres Pt catalyst deposition Electrode assembly FC stack assembly
How is intermediate Solid Acid Fuel Cell electrolyte synthesised?
Electrospinning using CDP solution + clear polymer. CDP becomes super protonic at higher temperature, at low temperature is doesn’t conduct.
What makes Solid Acid Fuel Cell electrolytes interesting?
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.
