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PEMFC and MCFC Fundamentals
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1
Question
What does the abbreviation PEMFC stand for?
Page 2
Answer
Proton-exchange membrane fuel cell (PEMFC).
2
Question
What is the typical composition of the anode (negative electrode) in a PEMFC?
Page 2
Answer
A platinum catalyst supported on porous carbon.
3
Question
What is the primary electrochemical role of the anode in a PEMFC?
Page 2
Answer
To facilitate the hydrogen oxidation reaction (HOR).
4
Question
What is the typical composition of the cathode (positive electrode) in a PEMFC?
Page 2
Answer
A platinum catalyst supported on porous carbon.
5
Question
What is the primary electrochemical role of the cathode in a PEMFC?
Page 2
Answer
To facilitate the oxygen reduction reaction (ORR).
6
Question
What class of polymers is commonly used for the polymer electrolyte membrane (PEM) in PEMFCs?
Page 2
Answer
Perfluorosulfonic acid polymers (for example, Nafion®).
7
Question
Why are perfluorosulfonic acid polymers (like Nafion) used for PEMs in PEMFCs?
Page 2
Answer
Because they provide high proton conductivity and good chemical and mechanical properties.
8
Question
What are the distinguishing operating temperature and pressure ranges for PEMFCs mentioned in the slide?
Page 2
Answer
Lower temperature/pressure ranges, typically 50 to 100 °C.
9
Question
Why are operating temperatures above 100 °C desired for PEMFCs according to the slide?
Page 2
Answer
Because at temperatures above 100 °C the water byproduct becomes steam, making water management less critical in cell design.
10
Question
Approximately what electromotive force (EMF) does a single PEMFC cell produce when operating in air?
Page 2
Answer
About 0.7 V EMF per individual cell.
11
Question
How is a useful (higher) voltage produced from PEMFC cells given the low EMF of a single cell?
Page 2
Answer
By linking the electrodes of many individual cells in series to form a cell stack.
12
Question
What common catalyst and support combination is used at both electrodes in PEMFCs?
Page 2
Answer
A platinum catalyst on a porous carbon support is typically used at both the anode and cathode.
13
Question
What is the negative-electrode (anode) half-reaction in a proton-exchange membrane fuel cell (PEMFC)?
Page 3
Answer
\$\$H_2 \rightarrow 2H^+ + 2e^-\$\$
14
Question
What is the positive-electrode (cathode) half-reaction in a PEMFC?
Page 3
Answer
\$\$\tfrac{1}{2}O_2 + 2H^+ + 2e^- \rightarrow H_2O\$\$
15
Question
What is the overall (net) cell reaction for the PEMFC shown?
Page 3
Answer
\$\$H_2 + \tfrac{1}{2}O_2 \rightarrow H_2O\$\$
16
Question
Which species is oxidized at the negative electrode in the PEMFC and what are the oxidation products?
Page 3
Answer
Molecular hydrogen (\$H_2\$) is oxidized to protons (\$H^+\$) and electrons (\$e^-\$).
17
Question
How many electrons are transferred when one molecule of \$H_2\$ is oxidized at the anode?
Page 3
Answer
Two electrons (\$2e^-\$) are produced per \$H_2\$ molecule.
18
Question
Where do the electrons produced at the negative electrode travel, and what do they produce while traveling?
Page 3
Answer
The electrons travel through the external circuit to the positive electrode and produce an electrical current (usable power).
19
Question
How do the protons (\$H^+\$) produced at the anode reach the cathode in a PEMFC?
Page 3
Answer
Protons move through the electrolyte (the proton-exchange membrane) from the negative electrode to the positive electrode.
20
Question
What are the main products exiting the fuel cell on the cathode side according to the slide?
Page 3
Answer
Liquid water (\$H_2O\$) and heat are produced and exit on the cathode (positive electrode) side.
21
Question
What is the role of the electrolyte/proton-exchange membrane in the PEM fuel cell?
Page 3
Answer
The electrolyte conducts protons (\$H^+\$) from anode to cathode while preventing electrons from crossing internally, forcing electrons through the external circuit.
22
Question
Why is oxygen written as \$\tfrac{1}{2}O_2\$ in the cathode half-reaction instead of \$O_2\$?
Page 3
Answer
Because the stoichiometry for producing one \$H_2O\$ from one \$H_2\$ requires only a single oxygen atom, which is half of an \$O_2\$ molecule; using \$\tfrac{1}{2}O_2\$ balances atoms for the single \$H_2\$ used.
23
Question
In the diagram, which streams enter the cell and where do they enter?
Page 3
Answer
Hydrogen fuel (\$H_2\$) enters at the negative electrode (anode) side; air/oxygen (\$O_2\$) enters at the positive electrode (cathode) side.
24
Question
Explain why electrons must travel through an external circuit rather than through the electrolyte in a PEMFC.
Page 3
Answer
The electrolyte/proton-exchange membrane only conducts protons (\$H^+\$) and is electrically insulating to electrons, so electrons are forced to travel through an external circuit to reach the cathode, enabling usable electric current.
25
Question
What fuel flexibility advantage do MCFCs offer?
Page 8
Answer
MCFCs can use a variety of fuels, including natural gas and biogas, without requiring external reforming.
26
Question
What are the typical electrical efficiencies of MCFCs?
Page 8
Answer
MCFCs typically achieve electrical efficiencies in the range of 45–55%.
27
Question
How high can overall MCFC system efficiencies reach when waste heat is utilized in combined heat and power (CHP) systems?
Page 8
Answer
Overall efficiencies can reach up to 85% when waste heat is recovered and used in combined heat and power (CHP) systems.
28
Question
What is meant by 'combined heat and power (CHP)' in the context of MCFCs?
Page 8
Answer
Combined heat and power (CHP) is a system configuration where waste heat from the MCFC is captured and used for useful thermal applications, raising overall system efficiency.
29
Question
How do MCFCs contribute to carbon capture applications?
Page 8
Answer
MCFCs are naturally capable of capturing \$CO_2\$ from the fuel supply and the atmosphere, offering potential for carbon capture applications.
30
Question
At approximately what temperature do MCFCs operate?
Page 8
Answer
MCFCs operate at extremely high temperatures of about \$650\ ^\circ C\$ (roughly \$1,200\ ^\circ F\$) and above.