Fuel Cell & Electrolysers

Question 1

Why are fuel cells different from batteries?

Answer 1

They do not store, but convert chemical energy of a fuel directly into electrical energy.
They do not have a positive and negative electrode, just an anode and cathode.

Question 2

For a fuel cell, what is inputted at the anode and cathode?

Answer 2

Anode = fuel
Cathode = oxidant

Question 3

Write the half cell reaction for oxygen in a fuel cell

Answer 3

O2 + 4H+ + 4e- <–> 2H2O

Question 4

What are the chemical products of a fuel cell?

Answer 4

CO2 and H2O

Question 5

What is the operating temperature of a SOFC?

Answer 5

600 to 1000 degrees C

Question 6

What fuels can be used in a SOFC?

Answer 6

Hydrogen, methane, alcohols

Question 7

What electrolyte is used in a SOFC?

Answer 7

Solid oxide ceramic such as YSZ.

Question 8

What are the 3 SOFC structure configurations? Draw them and give measurements.

Answer 8

1) Anode supported. 1mm-10micron-50micron
2) Electrolyte supported. 50micron-200micron-50micron
3) Cathode supported. 50micron-10micron-1mm

Question 9

Why would we want to reduce the thickness of the solid electrolyte?

Answer 9

It has the largest ohmic resistance therefore, we want to reduce this.

Question 10

Which SOFC structure configuration is the most common? Why?

Answer 10

Anode supported.

Fuel enters the anode side at 100% concentration therefore, a thicker anode will allow longer residence time for the fuel. The catalyst is also often placed in the anode due to the high temperatures, which partially converts any fuel at the anode into H2 and CO2.

Question 11

Are there any issues associated with electrode supported configurations in SOFCs?

Answer 11

Increased electrode thickness increases the length of the tortuous path and reduces the cross-sectional area available for transport. Thus, impacting the tortuosity.

Question 12

What is the negative associated with using high currents in SOFCs to overcome the mass transfer resistances in electrodes?

Answer 12

At high current and high temperature, the reactants will be used up more quickly.

Question 13

State and describe the 3 phases present in a SOFC? What transport processes occur in each?

Answer 13

1) Solid - electronically conductive to supply/remove electrons - diffusion
2) Electrolyte - conducts ions, may be solid or liquid - diffusion
3) Gas - reactants and products flow to/from electrode to allow continuous operation - convection

Question 14

What is the region of contact called in a
SOFC?

Answer 14

Triple phase boundary.

Question 15

What electrode is commonly opted for in SOFCs to increase the triple phase boundary?

Answer 15

Porous electrode

Question 16

Why is the charge-transfer smaller for a SOFC (as shown on an I-V curve)?

Answer 16

High temperatures lower the activation overpotential as the reactions happen more rapidly therefore, charge transfer is smaller.

Question 17

How do you find power density maximum on a graph? Draw the graph

Answer 17

Multiple current density and voltage. Plot current density against power density. The peak of the line is the maximum point for those operating conditions.

Question 18

True or false: A fuel cell operates at or above the maximum power density.

Answer 18

False - at or below. This is because ABOVE the maximum, both voltage (efficiency) and power density decreases.

Question 19

What is electrolysis?

Answer 19

The reverse of a fuel cell! Electrical energy is inputted to split water and/or CO2 into fuels such as H2, methane and alcohols.

Question 20

Write the equations for co-electrolysis of CO2 and H2O

Answer 20

Anode: 2O2- –> O2 + 4e-

Cathode:
CO2 + 2e- –> CO + O2-
H2O + 2e- –> H2 + O2-
CO2 + H2 <–> CO + H2O

Question 21

What is the ideal (thermoneutral) voltage of a fuel cell? (i.e. 0 current)

What letter is this denoted by?

Answer 21

E(H) = 1.2 V

Question 22

What is the heat loss in a fuel cell a result of?

Answer 22

The difference between reaction enthalpy input and the electrical power output, producing heat.

Question 23

Write the equation for the total amount of heat energy loss in fuel cell operation?

Answer 23

E(H) - V

Question 24

Why is it challenging to maintain system temperature for electrolysers?

Answer 24

There is a net consumption of heat (endothermic) when V is less than the thermoneutral voltage (E(H)). When V is above E(H), net heat is produced because entropic heat consumption is fully offset by irreversible heat production due to activation, ohmic and mass transport losses (dual-heating).

Therefore, to maintain system temperature, the electrolyser is designed to operate at or above the thermoneutral voltage.

Question 25

What is the concept of dual-heating in a electrolyser?

Answer 25

At the thermoneutral voltage, the heat energy required to drive the endothermic reaction forward is supplied by the heat energy produced from the irreversible cell resistances (activation, ohmic and mass transport). This balance is known as dual-heating as is the optimum operating conditions.

Question 26

Why is cathode-supported not a preferred arrangement?

Answer 26

At the cathode, the fuel is diluted due to the presence of 21% oxygen in atmosphere.

Question 27

Draw an I-V curve for a SOFC, label the different regions and draw another line where the electrolyte has a higher resistance

Answer 27

Steeper gradient on the second line.

Question 28

Draw the I-V graphs for a fuel cell and electrolyser showing the thermoneutral voltage.

Answer 28

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