Fuel Cells
A _____ _____ _____ density indicates a facile reaction, while a ______ one indicates a sluggish reaction
high exchange current low
____ ____ ____ ______ is dictated by thermodynamics
ideal fuel cell performance
A simple 2D fuel cell mass transport model can be constructed to show how reactant gases are depleted in a flow channel from inlet to the outlet. In general, ______ the gas flow velocity, _________ the channel size, or __________ the diffusion layer thickness will improve the mass transport situation along the length of the flow channel
increasing decreasing decreasing
In addition to having high _____ _____, electrolytes must be stable in both:
ionic conductivity; highly reducing and highly oxidizing environments
Real fuel cell efficiency is always less than the ideal thermodynamic efficiency. Major reasons are:
irreversible kinetic losses and fuel utilizations losses
To accurately reflect the behavior of most fuel cells, an additional loss term, known as the ______ ______, must be introduced
leakage loss
Conductivity in ceramic electrolytes is controlled by defects ("mistakes") in the crystal lattice. Natural (intrinsic) defect concentrations are generally ______, so _____ (extrinsic) defect concentrations are usually introduced into the lattice on purpose via ________
low; higher; doping
____ _____ governs the supply and removal of reactants and products in a fuel cell
mass transport
Fuel cell _____ ________ includes the resistance from the electrodes, electrolyte, interconnects, and so on. However, it is usually dominated by the:
ohmic resistance; electrolyte resistance.
the _______ overvoltage is a loss that generally obeys ______ ______ ___ _________, V = iR where R is the ohmic resistance of the fuel cell
ohmic; ohms law of conduction
_________ occurs at the anode electrode. ________ occurs at the cathode electrode.
oxidation reduction
A _____ _______ is required to drive gas flow through a channel
pressure difference
The _____ ______ in a flow channel is mainly caused by _____ between fluid and the channel walls.
pressure drop; friction
G scales with __________ ________ whereas g and E do not scale with _________ __________
reaction amount
Because electrochemical reactions occur at surfaces, the rate (current) is proportional to the:
reaction surface area
An oxidation reaction __________ electrons. A reduction reaction _________ electrons.
releases consumes
Viscosity characterizes the:
resistance of a fluid to flow.
Because ________ scales with thickness, fuel cell electrolytes are made:
resistance; as thin as possible
Because ________ scales with area, area-specific fuel cell resistances (ASRs) are computed to make comparisons between:
resistance; different-size fuel cells possible (ASR = A x R)
Because _______ scales with conductivity, developing ______ ______ ________ and ________ ________ is critical
resistance; high conductivity electrode ;electrolyte material
Choice of the flow field pattern significantly affects the:
size of the mass transport losses.
In general, the _____ the ion and the ______ its charge, the higher the ________.
smaller; greater ; mobility
A simple fuel cell model can be developed by :
starting with the thermodynamic fuel cell voltage and then deducting the three major loss terms
Metals and ion conductors show vastly different:
structures and conduction mechanisms, leading to vastly different conductivities
The viscosity of a gas mixture is dependent on the __________ and ________ of the mixture
temperature composition
The exchange current density measures:
the equilibrium rate at which reactant and product species are exchanged in the absence of an activation overvoltage.
The limiting current density corresponds to:
the point where the reactant concentration falls to zero in the fuel cell catalyst layer
Total overall efficiency is given by:
the product of individual efficiencies.
Current-voltage curves show:
the voltage output of a fuel cell for a given current load
The ______ and ______ balance of a fuel cell can be obtained from input, output, and conversion fluxes of energy and mass in the fuel cell
thermal; mass
Real fuel cell efficiency is always less than the ideal _____________ efficiency.
thermodynamic
________ provides the ideal case for fuel cell performance
thermodynamics
Electrochemical reactions involve the ________ _____ _______ and occur _____ _______
transfer of electrons at surfaces
High water content leads to high conductivity. Nafion conductivity may be determined by modeling the ____ _____ in the membrane
water content
Viscosity can be thought of as a measure of the ________ _____ in the fluid.
"internal" friction
Real fuel cell performance is always less than ideal fuel cell performance due to losses. The major types of loss are:
(1) activation loss, (2) ohmic loss, and (3) concentration loss
The 3 main requirements for an effective fuel cell catalyst are:
(1) activity, (2) conductivity, and (3) stablility
There are four major ways to increase the exchange current density :
(1) increase reactant concentration, (2) increase reaction temperature, (3) decrease the activation barrier (by employing a catalyst), and (4) increase the number of reactions sites
The three major electrolyte classes employed in fuel cells are:
(1) liquid, (2) polymer, and (3) ceramic electrolytes
The four major steps in the generation of electricity in a fuel cell are:
(1) reactant transport, (2) electrochemical reaction, (3) ionic (and electronic) conduction, and (4) product removal
There are ___ major fuel cell types, differentiated by their ___________.
5 electrolyte
Fuel cells are usually operated at relatively high current densities (high activation overvoltages.) At high activation overvoltage, fuel cell kinetics can be approximated by a simplified version of the ________ ______ ________, generalized as the:
Butler-Volmer equation; Tafel equation
_________ in ceramic electrolytes is controlled by defects ("mistakes") in the crystal lattice.
Conductivity
________ refers to the transport of a species by the bulk motion of a fluid
Convection
________ refers to the transport of a species due to a ______ ______ ________.
Diffusion; gradient in concentration
_____ _____ ______ in the electrode lead to a limiting current density
Diffusive transport limitations
The Nernst equation describes how:
E varies with reactant/product activities
_________ systems must contain two coupled half reactions: an _____________ reaction and a __________ reaction.
Electrochemical oxidation reduction
The magnitude of _____ _______ ______ gives the amount of energy that is available ("free") to do electrical work.
Gibbs free energy
_____ ______ even in good electrolytes is generally four to eight orders of magnitude lower than electron conductivity in metals
Ion conductivity
____ _____ ______ _____ conduct both electrons and ions
Mixed ionic and electronic conductors (MIECs)
_________ ________ ______ ________ __________ conduct both electrons and ions. They are useful for SOFC electrodes, where simultaneous conduction of electrons and ions enables improved reactivity by:
Mixed ionic and electronic conductors (MIECs); extending three-phase boundaries into two-phase reaction zones
________ (and hence conductivity) in aqueous electrolytes is determined by:
Mobility; balance between ion acceleration under an electric field and frictional drag due to fluid viscosity
Conductivity in _______ (a polymer electrolyte) is dominated by:
Nafion ; water content
Reactant depletion affects both the _______ ______ _________ and the ______ ______ ______. Depletion leads to a similar loss in both cases.
Nernstian cell voltage kinetic reaction rate
For low-T fuel cells, ________ is commonly used as a catalyst. High-T fuel cells employ ______ or _______ catalysts.
Pt nickel or ceramic-based
_____ _____ _____ ________ tend to decrease with increasing temperature
Reversible fuel cell voltages
Convection in fuel cell flow channels is characterized by the:
Reynolds number Re, a non-dimensional parameter that characterizes the viscous behavior of the flow. Usually gas flow in fuel cells is laminar.
________ or _________ designs are the most commonly used flow field types. The provide a decent compromise between _______ ______ and ______ _____ _______.
Serpentine parallel-serpentine pressure drop water removal capability
_______ ________ _______ intrinsically includes the pressure effects on reversible cell voltage but does not fully account for the temperature effects
The Nernst equation
A fuel cell is ____
a direct electrochemical energy conversion device.
poor mass transport leads to:
a loss in fuel cell performance due to reactant depletion (or product clogging) effects
A _______ ___________ impedes the conversion of reactants to products (and vice versa)
activation barrier
The sacrificed (lost) voltage is known as:
activation overvoltage
For a H2-O2 fuel cell, the hydrogen (_______) kinetics are generally facile and produce only a ______ activation loss. In contrast the oxygen (______) kinetics are sluggish and lead to a ______ activation loss (at low temperature)
anode small cathode significant
Concentration losses are most effectively minimized by:
careful consideration of the convective transport situation in the fuel cell flow channels.
The best catalyst-electrodes are:
carefully optimized, porous, high-surface-area structures
Conductivity is determined by:
carrier concentration and carrier mobility
Due to the liquid water formation in the ________, PEMFC's require flow fields with high:
cathode; water removal capability
________ _______ in fuel cells is predominantly driven by a voltage gradient. This process is known as ____________.
charge transport; conduction
The voltage that is expended to drive ________ charge transport represents a loss to fuel cell performance. This is known as:
conductive ; the ohmic overvoltage
Resistance scales with:
conductor area (A,) thickness (L), and conductivity (sigma)
Because electrochemical reactions involve electron transfer, the _______ generated is a measure of the reaction rate
current
We use ________ ________ to normalize the effects of a system size
current density
________ ________ is more fundamental than current.
current density
The relationship between the ______ ______ _____ and the _____ _______ is exponential. It is described by the Butler-Volmer equation.
current density output; activation overvoltage
Thermodynamic fuel cell efficiency generally _________ as temperature __________. Contrast this to heat engines, for which thermodynamic efficiency generally ___________ as temperature ___________.
decreases increases increases increases
Unlike a battery, a fuel cell cannot be ________.
depleted
________ _________ can only be extracted from a spontaneous ("downhill") chemical reaction.
electrical energy
Thus the sign of G indicates whether or not ___________ ____ can be done, and the size of G indicates how much __________ ____ can be done
electrical work
Although gases in fuel cell flow channels move along the flow channel, they can also be transported between the flow channel and the _______. This is known as:
electrode; convective mass transfer coefficient, h, which may be calculated from the Sherwood number, Sh.
At a minimum, a fuel cell must contain two ____________ (an _________ and a __________) separated by an _________________.
electrodes anode & cathode electrolyte
mass transport in fuel cell _______ is typically dominated by diffusion. Mass transport in fuel cell flow ________ is typically dominated by convection
electrodes structures
A fuel cell satisfies the laws of:
energy and mass conservations
Activation overvoltage losses are minimized by maximizing the _______ _______ _______.
exchange current density
The friction is quantified by wall shear stress. Pressure drops can be determined using the _______ _______, which is dependent on the _____ ______ and channel ________
friction factor; Reynolds number; geometry
____ ___ _____ is determine by the fuel reservoir size
fuel cell capacity
____ _____ ______ can be assessed by current-voltage curves
fuel cell performance
______ _______ _______ is determined by fuel cell size
fuel cell power
A portion of the _____ _____ _______ is sacrificed to ______ _________ _________, thus increasing the rate at which reactants are converted into products and the current density generated by the reaction.
fuel cell voltage; lower activation barrier
The details of fuel cell reaction kinetics are dependent on the:
fuel, electrolyte chemistry, and operation temperature.
The ____ _____ of a fuel is given by the fuel's heat of combustion or more generally, the enthalpy of the reaction
heat potential
Not all of the ____ _____ of a fuel can be utilized to perform useful work. The work potential of the fuel is given by the :
heat potential ; Gibbs free energy