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