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Hydrogen Fuel Cells > Definitions > Flashcards

Definitions Flashcards

1
Q

Energy Carrier

A

substance or phenomenon that can be used to produce mechanical work, heat or to operate chemical/physical processes

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2
Q

volumetric Energy density

A

energy stored per volume

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3
Q

specific energy/gravimetric energy density

A

energy stored per mass

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4
Q

Energy Conversion

A
  • transferring the form of energy
  • often intends to make energy useable
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5
Q

Fuel Cell

A

type of energy converter that allows directly harvesting the chemical energy stored in a fuel by means of an electrochemical process

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6
Q

Polarisation curve

A

current-voltage relation
- graph: y-axis cell voltage, x-axis current density
- drawn in: theoretical reversible cell voltage & measured performance -> difference are sum of voltage losses (over potential)

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7
Q

Battery

A

electrochemical cell that allows converting chemical energy stored in its active materials to electricity & vice versa

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8
Q

Electrochemistry

A

studies reactions, which involve electrical current. The latter can either be generated by a spontaneous reaction or a reaction can be forced to proceed by applying a current
- important characteristic: not Carnot limited

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9
Q

Anode

A

electrode of an electrochemical cell through which net electric current flows & at which the predominating electrochemical reaction is an oxidation anodic current is denoted positive in the IUPAC convention

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10
Q

Cathode

A

electrode of an electrochemical cell through which net electric current flows & at which the predominating electrochemical reaction is a reduction cathodic current is denoted negative in the IUPAC convention

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11
Q

Oxidation

A
  • A -> A+ + e-
  • B- -> B + e-
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12
Q

Reduction

A
  • A+ + e- -> A
  • B + e- -> B-
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13
Q

Redox reaction

A
  • reduction-oxidation reaction
  • combination of a reduction & an oxidation, which occur simultaneously
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14
Q

Electric current I

A

rate at which electric charge is passed I = dQ/dt

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15
Q

Electric Charge

A

quantity of electricity, integral of electric current over time Q = ∫I dt = zFn

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16
Q

Electrochemical cell

A

system that consists of at least 2 electronic conductors (electrodes) in contact with an ionic conductor (electrolyte; electronic insulator)

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17
Q

Electrochemical potential

A

Total Work µi_ = Chemical Work µi + Electrical Work ziF𝜙
- Chemical Work: bringing uncharged particle into bulk of an uncharged phase
- Electrical Work: additionally accounts for effect of an electrical field on a charged particle

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18
Q

Gradient of Electrochemical potential

A

driving force for electrochemical reactions

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19
Q

Applied/measured potential

A

Difference of electric potentials between 2 electrodes of an electrochemical cell

20
Q

current normalised over electrode area

A
  • current density/A
21
Q

Fuel Cell efficiency 𝜂

A
  • electrical energy produced/heating Value of fuel (i.e. -∆H)
  • max. theoretical efficiency = -∆G/-∆H
  • specification of Heating value necessary
22
Q

Polymer (“plastic”)

A

huge molecule consisting of many repeating chemical units (“monomers”)

23
Q

Ionomer

A

Polymer composed of macromolecules in which a small but significant proportion the constitutional units has ionic or ionisable groups, or both

24
Q

Ionic groups

A

usually present in sufficient amounts to cause micro-phase separation of ionic domains from the continuous polymer phase (“ionic aggregates”)

25
Electrochemical surface ECSA
"useable" catalyst surface area
26
Hydrogen Oxidation Reaction HOR
proceeds very rapidly & is typically not considered a limitation; happens at anode
27
Oxygen Reduction Reaction ORR
- sluggish - difficult to catalyse - voltage losses due to the ORR are very significant in low T FCs - high T enhance the reaction kinetics - happens at cathode - product is water
28
ceramic
rigid material that consists of an infinite three-dimensional network of sintered crystalline grains comprising metals bonded to carbon, nitrogen or oxygen
29
cermet
composite material made from ceramic & metal
30
Reaction stoichiometry
relation of the quantities of species in a reaction
31
stoichiometric rate
exactly the amount of reactant needed for the reaction
32
stoichiometric
ratio of the reactant fed to the cell over the reactant consumed
33
Stack vs. FC System Efficiency
- BoP components consume energy -> system electrical efficiency < stack electrical efficiency - typ. ∆𝜂 = 10-20% observed
34
stack specific power
- power output per unit mass of a fuel cell stack, typically measured in W/kg - indicates efficiency & compactness of stack, crucial for applications like electric vehicles & portable power.
35
exothermic
- process that releases heat into its surroundings - has a negative enthalpy change
36
Synergy
interaction or cooperation giving rise to a whole that is greater than the simple sum of its parts
37
Radiative forcing
- happens when amount of energy that enters the Earth's atmosphere is different from the amount that leaves - net change a type of emission causes to this balance is quantified by the respective RF value in W/m^2
38
Effective Radiative Forcing ERF
adjusted RF value; typically used as a key metric
39
Global Warming Potential GWP
describes the relative potency of a greenhouse gas, taking account of how long it remains active in the atmosphere; typically calculated over 100 years & CO2 is taken as a reference (-> GWPCO2 =1)
40
Nernst Equation
Erev = Ever,0 + RT/zF ln(∏ a(Ox)^𝝂ox/∏ a(Red)^𝝂red) - Nernst Voltage doesn't apply to real system - transfer of ∆G to Wells involves losses - Umeasured deviates from Urev
41
Tafel equation
over potential V = ± a * log(I/i0) - i0 exchange current density - only valid for - high overpotentials - uniform current distributions - no side reactions (like surface passivation) - no limitations due to mass transport
42
crossover at the mebrane
- Protons H+ should crossover; also drag water (Electro-osmotic drag), has to be put back (back crossover) to anode - shouldn't cross over - Gases (O2,H2) -> Fuel loss, mixed potential, unwanted side reactions - Contaminants (CO, SO2) -> Catalyst poisoning, Membrane degradation, Reduced efficiency
43
Reasons for Mass transport losses
- Insufficient gas supply (reactants don’t reach the electrode fast enough). - Water flooding (blocks gas diffusion in PEM fuel cells). - Poor catalyst layer structure (hinders diffusion of gases). - Reduction of concentration of reactant gases
44
How to Reduce Mass transport losses
- Improve gas diffusion layer (GDL) design. - Optimize flow channels for better reactant supply. - Control water management to prevent flooding.
45
Capacitor
- passive electronic component - stores & releases electrical energy in form of an electric field - is two conductive plates separated by an insulating material (dielectric) - used for - energy storage - filtering - signal processing

Hydrogen Fuel Cells (10 decks)

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