Midterm Flashcards
Reformation vs. Syngas?
Syngas has higher CH_4 and has further reformation possibilities.
*Difference between the two is that reformate has been reformed.
Why would someone propose on-board reformation of a hydrocarbon fuel?
Hydrocarbon fuel can be stored as a liquid, which means it has a higher energy density.
Why is PEMFC performance almost always better than DMFC performance?
Both of these fuel cells have Cathode Losses, but DMFC have Anode Losses too.
What is the role of the anode catalyst in a PEMFC?
Break apart Hydrogen.
What is a Raney Metal?
Heterogenous (doped) metal (not an alloy). This metal is then raised to the lowest melting temperature of the metals it has, and melts that particular metal out. This maximizes surface area of the metal.
*Doesn’t work for PEM as the sharp edges “pop” the membrane, but DMFC works as it is a liquid.
List 3 (or 4) ways of reducing activation losses:
Increase Pressure/Temperature.
Catalyst.
Change Fuel.
Increase “effective area” (i.e. Rough Surface)
What are the cathode exhaust relative humidity limites one uses for the PEMFC?
80-100%
What is Sparging, and why use it?
Sparging is pushing air flow from under water to create bubbles that bring water with them on the surface, and when it busts at the surface, it increases the humidity of the gas above.
Draw a polarization curve and point out the major loss regimes in a fuel cell. (No image, but think about it and list the important parts, and where their located.)
X axis is charge/area (mA/cm^2)
Y axis is Voltage
Ideal/Reversible (all chemical energy is converted to electricity) is the line across the top.
Activation Region is the initial area between the reversible and the rapidly declining line.
Ohmic Region is the minimally declining portion of the line.
Mass Concentration Polarization occurs after the Ohmic Region where the line steeply declines to V=0.
What does a Plate Fin Heat Exchanger look like?
Like corrugated metal sandwiched between two other plates. Gases flow along the vanes.
If there is a large heat difference between the plates, then there may be a problem of different energy in each of the vanes, so perforating or even have a large areas where the gases can be exchanged can improve on this.
What are the 3 main degradation mechanisms in catalytic systems?
Sintering
Fouling
Poisoning
What 3 (or 4) mechanisms limit the conversion in a catalytic steam reformer?
Heat Transfer
Mass Transfer
Chemical Kinetics
Catalyst Degradation
How can a single PEMFC membrane be dehydrated at one point and flooded at another point?
Electro-Osmatic Drag.
What is an MEA?
Membrane Electrode Assembly
What are the seven layers of an MEA? (What is removed if only 5 layers are used?)
1, 7) Gas Diffusion Layer
2, 6) Micropore Level (Hydrophilic)
3, 5) Electrode Catalyst Layer
4) Electrolyte
Micropore Level can be removed for 5 layer.
Water is required for the AFC cathode while it is generated at the anode. Why?
OH- is the mobile ion, and it needs H from the Water.
How can gas foul the surface of a DMFC anode?
CO_2 could block the fluid from hitting the catalyst.
What does 50% excess air mean?
It means that we’re adding 50% more air than stoichiometric equality would require.
What is the first thing to do if the voltage drops?
Purge/Flush the anode.
What’s a good target for Hydrogen Utilization?
90%
What is Turndown Ratio?
Ratio of: (Lowest Operational Power) / (Maximum Operational Power)
This can be important because if there is a long start-up time, it may be more efficient to just keep the cell running at it’s lowest operational power rather than shut it off entirely.
Utilization is INVERSELY related to turndown ratio.
What happens if the hydrogen is not absorbed in a system?
Opens the possibility of overheating.
What is the relationship of power and efficiency as shown by the general fuel cell polarization curve?
As power increases, efficiency drops.
This is the opposite of what occurs in an Internal Combustion engine. Also counter to electric motors, which has the efficiency increase as the VOLTAGE increases.
What is the max temp for a PEMFC, and why?
100 C because at that point water would evaporate.