Fuel Cells

The major types of loss are _____

(1) activation loss (2) ohmic loss (3) concentration loss.

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 (4) product removal

Power Density depends only on _____.

- Depends only upon instantaneous power output capability - Does not depend upon the amount of fuel (or oxidant) stored

Unlike a battery, a fuel cell cannot be _____. It is a "factory" that will continue to generate _____ as long as _____ is supplied.

-cannot be depleted -generate electricity -fuel is supplied

A fuel cell directly converts energy from one form (_____) into another form (_____) through _____.

-chemical energy into -electrical energy through -electrochemistry

The functions of porous electrodes in fuel cells is ___

1) conduct electrons away from or into the three-phase interface once they are formed and provide current collection and connection with either other cells or the load 2) ensure that reactant gases are equally distributed over the cell 3) ensure that reaction products are efficiently led away to the bulk gas phase

For planar-bipolar stacking, the interconnect becomes a separator plate with two functions:

1) to provide an electrical series connection between adjacent cells, specifically for flat plate cells, and 2) to provide a gas barrier that separates the fuel and oxidant of adjacent cells.

The three-phase interface is _____.

A site or area where gaseous phase reactant, solid phase electrode, and electrolyte perform the reaction.

In high-temperature FC, _____and even _____ can be internally converted to hydrogen or even directly oxidized electrochemically.

CO CH4

Fuel cell disadvantages include:

Costs. Volumetric power density. Fuel not widely available. Fuel is difficult to store. Operation temperature compatibility. Susceptibility to environmental poisons. Durability under start-stop cycling.

ΔG scales with reaction amount whereas _____ does not scale with reaction amount.

E

The Nernst equation describes how _____ varies with _____.

E (reversible thermodynamic voltage) reactant/product activities

_____ quantities such as internal energy and entropy do scale with system size.

Extrinsic quantities

Faraday's constant

F = NA × q = (6.022 × 10^23electrons∕mol) × (1.60 × 10^-19C∕electron) = 96, 485 C∕mol

Gibbs free energy equation:

G = H − TS differentiation give: dG = dH − T dS − SdT

The Gibbs free energy equations is _____.

G = U − TS + pV as a function of T & p: dG = −SdT + V dp

Not all of the heat potential of a fuel can be utilized to perform useful work. The work potential of the fuel is given by the _____.

Gibbs free energy, ΔG

The enthalpy equations is _____.

H = U + pV as a function of S and p: dH = T dS + V dp

In a fuel cell, the hydrogen combustion reaction is split into two electrochemical half reactions:

H2 ⇌ 2H+ + 2e− 1/2O2 + 2H+ + 2e− ⇌ H2O

The anode is the electrode where the _____takes place. Electrons flow _____.

HOR, Electrons flow out.

What are the 4 thermodynamic potentials?

Internal energy = U(S, V) Enthalpy = H(S, p) Gibbs free energy = G(T, p) Helmholtz free energy = F(T, V) They relate to one another by offsets of the "energy from the environment" term TS and the "expansion work" term pV

_____ quantities such as temperature and pressure do not scale with the system size.

Intrinsic quantities

What is the 1st Law of Thermodynamics?

Law of conservation of energy: energy can never be created or destroyed. d(Energy)univ = d(Energy)system + d(Energy)surroundings = 0

Energy Density must consider ____.

Must consider energy conversion equipment AND storage AND amount of fuel (and oxidant if not immediately available) in system calculations

Avogadro's number

NA = 6.022 × 10^23 electrons∕mol

The cathode is the electrode where the _____takes place. Electrons flow _____.

ORR, Electrons flow in.

The equation for charge carried by electrons is _____.

Q = nF

Total charge equation

Q = nNAq = nF

Reduction occurs at the _____ electrode. Oxidation occurs at the _____ electrode.

Red cat, an ox reduction = cathode oxidation = anode

Reversible Voltage Variation with Concentration: Nernst Equation

The Nernst equation outlines how reversible electrochemical cell voltages vary as a function of species concentration, gas pressure, and so on. This equation is the centerpiece of fuel cell thermodynamics.

What is the definitions of Gibbs Free Energy (G)?

The energy needed to create a system and make room for it minus the energy that you can get from the environment due to heat transfer. In other words, G represents the net energy cost for a system created at a constant environmental temperature T from a negligible initial volume after subtracting what the environment automatically supplied.

What is the definitions of Internal Energy (U)?

The energy needed to create a system in the absence of changes in temperature or volume.

What is the definitions of Enthalpy (H)?

The energy needed to create a system plus the work needed to make room for it (from zero volume).

Mechanical work is accomplished by _____.

The expansion of a system against a pressure. It is given by (dW)mech = pdV

SOFCs employ a _____ membrane as an electrolyte. _____ are the ionic charge carrier. Water is produced at the _____.

Thin ceramic Oxygen ions (O2-) Anode

PEMFCs employ a _____ membrane as an electrolyte. ______ are the ionic charge carrier. Water is produced at the _____.

Thin polymer Protons Cathode

The equation for electrical work to move a charge through an electrical potential difference is _____.

Welec = EQ

A fuel cell is a _________ device

a direct electrochemical energy conversion device

An electrolyte is _____.

a material that allows ions (charged atoms) to flow but not electrons.

The magnitude of ΔG gives the _____.

amount of energy that is available ("free") to do electrical work.

The battery is an energy storage device, where a fuel cell is ______.

an energy conversion device.

Fuel is fed continuously to the _____ and an oxidant (often oxygen from air) is fed continuously to the _____.

anode (negative electrode) cathode (positive electrode)

Hydrogen has a high reactivity for _____ reactions

anode reactions

The 1st Law of Thermodynamics can also be expressed as _____.

any change in the energy of a system must be fully accounted for by energy transfer to the surroundings: d(Energy)system = −d(Energy)surroundings

A reduction reaction _____ electrons.

consumes

Fuel cell performance can be assessed by _____.

current-voltage curves

Gibbs free energy equation for electrical work:

dG = −SdT + V dp − dWelec For a constant-temperature, constant-pressure process (dT, dp = 0) this reduces to dG = −dWelec Welec = −Δgrxn

internal energy that is based on the variation of two independent variables, entropy S and volume V:

dU = T dS − pdV reversible heat transfer minus mechanical work

Thermodynamic fuel cell efficiency generally _____as temperature increases. Contrast this to heat engines, for which thermodynamic efficiency generally _____ as temperature increases.

decreases increases

Fuel cells are more ____than combustion engines.

efficient and silent

Fuel cells are classified according to the choice of _____ and _____, which in turn determine the _____ and the type of ions that carry the current across the electrolyte.

electrolyte fuel electrode reactions

Spatial separation is accomplished by _____.

employing an electrolyte.

What is the formation enthalpy differences between reactants and products?

final state - initial state or products - reactants.

There are _____ major fuel cell types differentiated by _____.

five types differentiated by their electrolyte. 1. Alkaline fuel cell (AFC) 2. Polymer electrolyte membrane fuel cell (PEMFC) 3. Phosphoric acid fuel cell (PAFC) 4. Molten carbonate fuel cell (MCFC) 5. Solid-oxide fuel cell (SOFC)

Fuel cell power is determined by _____.

fuel cell size

What 2 ways can energy be transferred between a closed system and its surroundings?

heat (Q) or work (W) dU = dQ − dW the change in the internal energy of a closed system (dU) must be equal to the heat transferred to the system (dQ) minus the work done by the system (dW).

In low-temperature FC, all the fuel must be converted to _____ prior to entering the fuel cell, and the anode catalyst (mainly platinum) is strongly poisoned by _____.

hydrogen CO

Stacking involves connecting multiple unit cells in _____ via electrically conductive interconnects

in series

Stacks with tubular cells can be connected in ____ or in ____.

in series or in parallel.

Unlike batteries, fuel cells allow _____.

independent scaling between power and capacity.

The total intrinsic energy of a fuel (or of any substance) is quantified by a property known as _____.

internal energy (U)

Real fuel cell efficiency is always_____ the ideal thermodynamic efficiency. Major reasons are _____ and _____.

less than irreversible kinetic losses and fuel utilization losses

An oxidation reaction _____ electrons.

liberates

Real fuel cell performance is always less than ideal due to _____.

losses

Internal energy is associated with _____ and interactions between _____ on the atomic scale.

microscopic movement (kinetic energy) particles (chemical/potential energy)

The hydrogen reaction is an _____ reaction because electrons are being _____ by the _____.

oxidation, liberated, hydrogen oxidation reaction (HOR).

Most fuel cell geometries are ____.

planar (rectangular or circular) or tubular (either single- or double-ended and cylindrical or flattened).

Total overall efficiency is given by the _____.

product of individual efficiencies.

charge per electron

q = 1.60 × 10^-19 C∕electron

The oxygen reaction is a _____ reaction because electrons are being _____ by the _____.

reduction, consumed, oxygen reduction reaction (ORR).

Energy is released when a bond is _____. Energy is absorbed when a bond is _____.

released when formed. absorbed when broken.

Electrical energy can only be extracted from a _____.

spontaneous ("downhill") chemical reaction.

The Nernst equation intrinsically includes the pressure effects on reversible cell voltage but does not fully account for the _____.

temperature effects.

Aqueous electrolytes are limited to _____.

temperatures of about 200 °C or lower because of their high vapor pressure and rapid degradation at higher temperatures.

Energy is defined as _____.

the ability to do work. Units: joules (J), watt-hours (Wh), or kilowatt-hours (kWh) Energy = power × time

Gravimetric power density (or specific power) is _____.

the amount of power that can be supplied by a device per unit mass. Typical units are W/g or kW/kg.

Volumetric power density is _____.

the amount of power that can be supplied by a device per unit volume. Typical units are W∕cm3 or kW∕m3.

The heat potential of a fuel is _____.

the enthalpy of reaction.

Fuel cell capacity (energy capacity) is determined by _____.

the fuel reservoir size

If G represents the net energy you had to transfer to create the system, then G should also represent _____.

the maximum energy that you could ever get back out of the system. Gibbs free energy represents the exploitable energy potential, or work potential, of the system.

Entropy is determined by _____.

the number of possible microstates accessible to a system, or, the number of possible ways of configuring a system.

The power (P) delivered by a fuel cell is given by _____:

the product of current and voltage. P = iV

Power is defined as _____

the rate at which energy is expended or produced. Power represents the intensity of energy use or production. Power is a rate. Units: watt (W) = the amount of energy used or produced per second (1 W = 1 J∕s). Power = energy / time

The current (electricity) produced by a fuel cell scales with _____.

the size of the reaction area where the reactants, the electrode, and the electrolyte meet

Thermodynamics provides the _____ or _____ for fuel cell performance.

theoretical limits or ideal case

Ideal fuel cell performance is dictated by _____.

thermodynamics

Electrochemical systems must contain _____.

two coupled half reactions: an oxidation reaction and a reduction reaction.

At a minimum, a fuel cell must contain _____ separated by an _____.

two electrodes (an anode and a cathode) separated by an electrolyte.

Current-voltage curves show the ______ of a fuel cell for a given ______.

voltage output of a fuel cell for a given current load.

the sign of ΔG indicates ______, and the size of ΔG indicates _____.

whether or not electrical work can be done how much electrical work can be done

What is the relationship between Gibbs Free Energy and Reaction Spontaneity?

ΔG > 0 Nonspontaneous (energetically unfavorable) ΔG = 0 Equilibrium ΔG < 0 Spontaneous (energetically favorable)

The equations of Gibbs free energy and charged carried by electrons is _____.

Δ̂g = −nFE the Gibbs free energy sets the magnitude of the reversible voltage for an electrochemical reaction

The reversible voltage of a fuel cell, E, is related to the molar Gibbs free energy by _____.

Δ̂g = −nFE.

Planar-bipolar stacks can be further characterized according to arrangement of the gas flow:

• Cross-flow in perpendicular • Co-flow in parallel • Counter-flow in parallel and opposite directions • Serpentine flow in a zig-zag path • Spiral flow in circular cells

Balance of Plant (BoP) consist of

• Fuel preparation, except when pure fuels are used • Air supply from air compressors or blowers • Thermal management • Water management • Electric power conditioning equipment since FC stacks provide a variable DC voltage output