Fuel Cells (part 3) Flashcards
What are the 3 ways you can improve the rate of reaction in a fuel cell
- Use catalysts
- Raise the temperature
- Increase the electrode area (important). Can be done using pores.
How are the individual fuel cells connected together
In series, via a BIPOLAR PLATE which allows oxygen to be fed to the cathode and fuel gas to the anode, all while providing a low resistance path for electrons to flow between the cells. The low resistance path is desired in order to limit the voltage drop.
What is a PEM fuel cell
Proton Exchange Membrane, designed to be simple. Electrolyte is a solid polymer where protons are mobile. Run at low temperatures making them good for vehicles, but low temp also means low rate of reaction. Therefore sophisticated electrodes and a small amount of platinum catalyst is used.
Describe an alkaline fuel cell
Similar to PEM but with higher temperature (50 to 200 deg C). Low reaction rate problem is overcome using highly porous electrodes, a platinum catalyst and high pressures. Air and fuel supplies must be free from CO2.
What is a PAFC
Phosphoric Acid Fuel Cell. Often used in 200 kW systems. Porous electrodes, platinum catalysts, fairly high temperature (220 deg C). Instead of a straight hydrogen supply, natural gas (CH4, AKA methane) is reformed to produce hydrogen and carbon dioxide - this adds significantly to cost, complexity and size of the system.
What is a SOFC
Solid Oxide Fuel Cell. 600 to 1000 deg C, meaning high reaction rates happen without need for catalysts, and gases like natural gas can be used directly or reformed WITHIN the cell (no need for a separate unit to do so). High temp means they require ceramics which are expensive. The cooling system is complex and they are hard to start up. The SOFC always stays in solid state.
What is a MCFC
Molten Carbonate Fuel Cell. Like SOFC but does not stay solid, and is instead molten. Needs carbon dioxide in the air to work. High temp (650 deg C) means high reaction rate is achieved with a cheaper catalyst - nickel.
What is the BOP of a fuel cell
Balance of Plant. It refers to the parts of the system that aren’t the cell stacks (and other parts which produce the power). It means the bits that keep the conditions right such as compressors used to circulate air.
What are the 4 advantages of fuel cells (of any type)
+ Efficiency. And small systems can be just as efficient as large ones.
+ Simplicity. Few if any moving parts therefore reliable and long lifetime
+ Low emissions. Main byproduct is water (but producing the hydrogen in the first place means CO2 is released)
+ Silent operation.
What are the 2 main disadvantages of fuel cells
- Cost
- Having to use hydrogen (may not be a disadvantage in future if solar cells are used to electrolyse water to produce hydrogen)
What is Gibbs free energy
The energy available to do external work, neglecting work done by changes in pressure and volume
What does STP stand for
Standard Temperature and Pressure
What is F
It is what you press to pay respects.
lol jk it’s a constant equal to Na (Avogadro number) multiplied the charge on an electron. F = 96485 Coulombs
What does a bar over the top of a lower case letter mean in this context
It means the number is given in per mole form
What does calorific value/enthalpy of formation mean?
The heat that would be produced when burning a particular fuel/amount of fuel. Useful to compare this with the electrical energy that it would produce in a fuel cell
What is activation loss
Activation loss is the voltage loss required to drive the chemical reaction that transfers electrons to or from the electrode. This is caused by slow rate of reaction on the surface of the electrodes.
The drop is very non-linear.
There is an ‘energy hill’ you have to get over to cause the release of chemical energy through reaction.
Give 6 ways you can reduce activation over-voltage
- Increase i0
- Raise the cell temperature
- Use more effective catalysts
- Increase the roughness of the electrodes
- Increase the reactant concentration (e.g. use just oxygen instead of air)
- Increase the pressure
What are the 4 causes of energy loss in a fuel cell
- Activation loss
- Fuel crossover and internal current flow
- Ohmic losses (linear)
- Mass transport/concentration loss
What is fuel crossover and internal current flow
Is mainly a result of fuel waste passing through the electrolyte, and partly due to electron conduction through the electrolyte.
Fuel crossover is when some of the hydrogen diffuses through the electrolyte from anode to cathode.
The OCV effect of these things is usually very small but can be significant in low temp cells.
How can you reduce the internal resistance of the cell (in order to reduce ohmic losses)?
- Use electrodes with high conductivity
- Good design and appropriate choice of materials for biploar plates (or cell interconnects)
- Make the electrolyte as thin as possible
What is the mass transport/concentration loss
Result of the change in concentration of reactants at the surface of the electrodes as the fuel is used.
Can be thought of as the failure to transport enough reactant across the electrode surface.
When using a mixture, partial pressure drops. When using a pure fuel, actual pressure drops (across the plate).
May also be caused in systems where hydrogen is supplied from a reformer, because the rate of supply of hydrogen may not be able to change quickly enough to meet a change in demand.
What is a ‘charge double layer’
The build up of charge on two surfaces that are brought together. A p-n junction counts as a charge double layer
What is the ‘charge double layer effect’
The collection of charge at the ‘charge double layer’ causes a voltage, which is the ‘activation over-voltage’ (the same one mentioned elsewhere). If the current draw is higher, more charge is needed. Therefore the voltage drop is higher. The layer of charge (on or near the electrode/electrolyte interface) acts as a capacitor - e.g there is a delay between current changing and voltage changing, due to charge time. The capacitance can be very large (in the order of Farads).
What is the output voltage problem with fuel cells that is a challenge for applications such as cars
The output voltage of a fuel cell will not usually be at the level required by the system, and the output voltage will certainly not be constant. This means that a voltage regulator and/or DC-DC converter is required.
What are the ‘energy storage buffer’ problems involved with using fuel cells for cars
The traction torque for a vehicle varies considerably and rapidly, and fuel cells are best for steady loads.
1 - High current causes a large voltage drop causing low performance. Very high current is unsafe.
2 - Fuel cells cannot respond quickly to a sudden increase in current demand.
3 - If the system is sized so it is capable of providing the PEAK demand current then the rating will be much larger than the AVG power demand, and the system will be heavy and bulky.
4 - For max efficiency you should operate near the max efficiency point, which is when gas supply rate, pressure and temp are CONSTANT.
5 - Fuel cells cannot absorb any energy, so regenerative braking can’t happen without a battery.
What are flow batteries
Like conventional batteries except the electrolyte is stored in separate tanks and pumped through. The size of the external tanks is chosen to match the power demands of the application. Nowadays these are used for large systems of a few MW.
What is an RFB
Redox Flow Battery. Two liquid electrolyte tanks (anolyte connected to negative electrode, catholyte to positive) are connected to opposite ends of the main chamber, which has a PEM in the middle to keep the two electrolytes separated. H+ atoms (protons) can cross the membrane.
What is an HFB
Hyrbid Flow Battery. ONE of the electroative masses (electrolytes) is stored as a solid layer WITHIN the cell, like in a normal battery. The other one is pumped in like in an RFB.
What are the 6 advantages of flow batteries
+ Very scalable (100Wh to 10MWh)
+ Power capability is independent of available energy storage
+ Long lifetime
+ Unlimited cycle stability of installation
+ Very fast response time (minutes)
+ High round-trip efficiency
What are 3 disadvantages of RFBs
- Low power and energy densities
- Large volume needed for high power/energy
- Needs temperature control (keep between 15 and 35 deg C)
What are 6 applications for flow batteries
- Load balancing
- Localised energy storage for renewable sources for discharge when demand is low
- Peak shaving. Excess transient power requirements are provided by the battery
- Battery used as a backup for uninterruptible power supplies
- Power conversion. Battery can act as a DC-DC converter.
- Electric vehicles (portable application). Useful because rapid recharging is possible. But low energy density means low driving range.
