Definitions Flashcards
Energy Carrier
substance or phenomenon that can be used to produce mechanical work, heat or to operate chemical/physical processes
volumetric Energy density
energy stored per volume
specific energy/gravimetric energy density
energy stored per mass
Energy Conversion
- transferring the form of energy
- often intends to make energy useable
Fuel Cell
type of energy converter that allows directly harvesting the chemical energy stored in a fuel by means of an electrochemical process
Polarisation curve
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)
Battery
electrochemical cell that allows converting chemical energy stored in its active materials to electricity & vice versa
Electrochemistry
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
Anode
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
Cathode
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
Oxidation
- A -> A+ + e-
- B- -> B + e-
Reduction
- A+ + e- -> A
- B + e- -> B-
Redox reaction
- reduction-oxidation reaction
- combination of a reduction & an oxidation, which occur simultaneously
Electric current I
rate at which electric charge is passed I = dQ/dt
Electric Charge
quantity of electricity, integral of electric current over time Q = ∫I dt = zFn
Electrochemical cell
system that consists of at least 2 electronic conductors (electrodes) in contact with an ionic conductor (electrolyte; electronic insulator)
Electrochemical potential
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
Gradient of Electrochemical potential
driving force for electrochemical reactions
Applied/measured potential
Difference of electric potentials between 2 electrodes of an electrochemical cell
current normalised over electrode area
- current density/A
Fuel Cell efficiency 𝜂
- electrical energy produced/heating Value of fuel (i.e. -∆H)
- max. theoretical efficiency = -∆G/-∆H
- specification of Heating value necessary
Polymer (“plastic”)
huge molecule consisting of many repeating chemical units (“monomers”)
Ionomer
Polymer composed of macromolecules in which a small but significant proportion the constitutional units has ionic or ionisable groups, or both
Ionic groups
usually present in sufficient amounts to cause micro-phase separation of ionic domains from the continuous polymer phase (“ionic aggregates”)
Electrochemical surface ECSA
“useable” catalyst surface area
Hydrogen Oxidation Reaction HOR
proceeds very rapidly & is typically not considered a limitation; happens at anode
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
ceramic
rigid material that consists of an infinite three-dimensional network of sintered crystalline grains comprising metals bonded to carbon, nitrogen or oxygen
cermet
composite material made from ceramic & metal
Reaction stoichiometry
relation of the quantities of species in a reaction
stoichiometric rate
exactly the amount of reactant needed for the reaction
stoichiometric
ratio of the reactant fed to the cell over the reactant consumed
Stack vs. FC System Efficiency
- BoP components consume energy -> system electrical efficiency < stack electrical efficiency
- typ. ∆𝜂 = 10-20% observed
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.
exothermic
- process that releases heat into its surroundings
- has a negative enthalpy change
Synergy
interaction or cooperation giving rise to a whole that is greater than the simple sum of its parts
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
Effective Radiative Forcing ERF
adjusted RF value; typically used as a key metric
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)
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
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
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
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
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.
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
