What is the main difference between a fuel cell and an electrolyzer?
A fuel cell will produce electricity when fuel is supplied, and the electrical circuit isclosed. An electrolyzer must be supplied with electricity to run. In principle,electrolysis is a fuel cell in reverse
What sign (plus or minus) do the cathode and anode have in a fuel cell?
A cathode is the electrode in which a reduction takes place (receives electrons). Ananode is the electrode where an oxidation takes place (emits electrons). In a fuel cell,the cathode is positively charged, while the anode is negatively charged.
Draw a diagram showing the energy change in the chemical reaction in a PEM fuelcell, with and without the catalyst. Indicate by means of arrows the activation energyand the change in Gibbs free energy of the reaction
Write down the two half-cell reactions, as well as the total reaction, for what happens in a PEM fuel cell that is fuelled by H2 and O2
If you write the chemical reaction only with whole numbers, how many electrons are involved in the reaction?
What forms of energy do you get out of fuel cells?
Electricity and heat
What does MEA stand for? Create a simplified sketch of a MEA for a PEM fuel cell,and name the individual parts (including catalyst, carbon support, electrolyte, GDL).Indicate where the three-phase boundary is. Why is this important?
MEA = membrane electrode assembly. These figures are taken from "Lecture 3 Production methods renewable"
The 3-phase boundary is where the electrochemical reaction takes place; there is contact between both reactants (hydrogen), catalyst (which transports electrons) and electrolyte (which transports H+)
Which ion conducts electricity in a Nafion electrolyte membrane?
H+ (proton)
Calculate the Gibbs free energy (G) for the formation of one mole of water in a PEM fuel cell. Assume reversible cell potential. E0red = +1,23 V, E0ox = 0,00 V.
To find ΔG, we must first calculate the cell potential.
Explain briefly how the voltage, Gibbs free energy and maximum efficiency varies with temperature. What is the state of water coming from a fuel cell?
Voltage decreases with increasing T
Gibbs free energy increases (becomes less negative=- Maximum THEORETICAL efficiency decreases
The state of water goes from gas to liquid at around 80/100 degC. Often the water is a mix of different states
What factors (overpotential) contribute to the fact that the real cell potential is always lower than the ideal potential for a fuel cell? What factors, other than over-potential,contribute to the efficiency of a PEM fuel cell being lower than theoretically possible?Write down the formula
One may have 4 types of overpotential in an electrochemical cell:
Activation losses: η-anode and η-cathode - energy loss associated with activation energy at anode and cathode. Linked to the transfer of electrons at the electrodes.
Ohmic losses: η-ohmic - energy loss associated with electrical resistance in electrodes, wires, connection points, as well as resistance to the movement ofions through the electrolyte
Concentration overpotential: η-mass (mass overpotential)- energy loss associated with transport of reactants or products to/from the electrode surface.
These overpotentials lead to a loss in voltage for the fuel cell, and reduced efficiency.
In addition, you have energy loss associated with not all fuel being used efficiently in a fuel cell (uf), and energy loss as a result of not being able to collect all the electricityproduced at the electrodes, i.e. ohmic losses outside of the cell itself (uo).
The expression of total efficiency, η, becomes
Draw a typical current-voltage curve for a PEM fuel cell. Indicate and name the three regions that determine the behavior of the fuel cell. Draw the reversible cell potential.
Explain how you put together several cells in a stack. Include concepts such as bi-polar plates, flow field, et
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