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Fuel Cell: State-of the-Art Review and Challenges. Pradip Majumdar Professor Department of Mechanical Engineering Northern Illinois University DeKalb, IL 60115. Solid Oxide Fuel Cell. Cathode reaction:

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fuel cell state of the art review and challenges

Fuel Cell: State-of the-Art Review and Challenges

PradipMajumdar

Professor

Department of Mechanical Engineering

Northern Illinois University

DeKalb, IL 60115

solid oxide fuel cell
Solid Oxide Fuel Cell
  • Cathode reaction:
  • At the cathode, the reduction of oxygen takes place with the formation of a negatively charged oxygen ion.
  • The oxygen ion transports through the solid oxide ion conducting membrane electrolyte towards the anode
  • Anode Reaction:
  • At anode it combines with hydrogen gas producing water and electrons that travels to the cathode side through the external electrical circuit.

Overall reaction:

attractive features sofc
All solid components

Compactness

Flexible in fuel types

No electrolyte depletion

No corrosion of FC components by liquid

Reaction zone has two-phase solid-gas interface

Attractive Features SOFC
  • High temperature operation
  • No use of expensive metal

catalyst

  • High temperature provides

better system match

  • Provides high quality waste

heat for cogeneration

  • Lower activation losses and

better system match results

in higher overall conversion

efficiencies - 60%

slide4
Simpler fuel processing process

- May not need the expensive steam reforming and shift conversion

  • Allows internal reforming of hydrocarbon fuels to produce hydrogen and carbon monoxide
  • More tolerant to the presence of impurities in the reactant gases

Disadvantages

  • Higher Ohmic losses
  • Restrictions on materials for

interconnect/Bipolar plates, seals and

thermal components

doe seca goal for coal based sofc power system for distributed based on igfc
DOE-SECA goal for coal-based SOFC power system for distributed based on IGFC

SECA - Solid State Energy Conversion Alliance

nafion membrane construction
Nafion Membrane Construction
  • Starting structure is a polymer – Polyethylene
  • Modified by replacing Hydrogen with Fluorine.
  • Leads to the structure Polyterafluoroethylene

(PTFE)

  • The basic PTFE structure is Sulphonated by

adding a Sulphonic acid chain

slide7
Membrane Development Work

Additional evaluation of Nafion membrane at different temperatures.

Operation at higher temperatures for better thermal management

- up to 120 C for vehicle applications

- above 120 C for stationary application

Membrane that can operate at lower

humidity content ( <10%).

Thinner membrane

- better water distribution

- reduced resistance

sofc cell designs
SOFC Cell Designs
  • Tubular design
  • Circular design with center
  • manifold
  • Advantages

- The concept of one closed end of the tube eliminates

the need for gas seals between cells

- Can provide robust ceramic structure for the cell

  • Disadvantages

- Leads to a relatively long current path around the circumferences of the cells, resulting in higher internal resistances.

planar design
Planar Design
  • Sequential cells- integrated planar
  • Simple in design
  • Most popular
  • Flow configurations: Co-flow, counter flow or cross flow
slide11

Advantages of Planar Design

- simpler in manufacturing of the flat components

- Potential for higher power densities

Disadvantages of the Planar Design

- Necessity of sealing to avoid crossover of

reactant gasses.

- increased risk of cell fractures, particularly

during thermal cycling

planner sofc mem
Planner SOFC MEM

Planar SOFCs MEM are generally

manufactured in three different

configurations

  • Electrolyte supported cell
  • (Basic design – 1000C)
  • Anode supported cell
  • - Reduced temperature
  • - 600-800C
  • - thins electrolyte
  • Cathode supported cell
anode
Anode
  • The most common anode material is Nickel-Zirconium Cermet or a mixture of Nickel and yttria–stabilized zirconium (Ni-YSZ) with 30% Ni.
  • The nickel serves as the catalyst for anode reaction and as electron conductor.
  • The typical thickness of the anode is generally in range of 40-100 μm.
  • In recent times a thicker anode that supports a thinner ceramic electrolyte is also available.
cathode
Cathode
  • Most commonly used cathode materials are lanthanum manganite (LaMno3), strontium doped lanthanum manganite (LaSrMno3 ) or LSM,

lanthanum strontium cobalt ferrite (LaSrCoFeO).

  • These materials has good catalyst properties and good electronic conductivity
slide16

Electrolyte

  • The state-of-the-art ceramic electrolyte material is yttria stabilized zirconia (YSZ)

- YSZ has highest oxide ion conductivity than any other materials:

- lowest electronic conductivity, lowest gas permeability to prevent gas cross over losses.

- typical composition contains 8 % Yttria (y2O3) mixed with Zirconia (ZrO2) for temperature of 800-1000 C.

Yttria introduces high concentration of oxygen vacancies into zirconia crystal structure and results in a higher ion mobility.

slide17

Electrolyte

  • Current research effort is to develop materials with reduced thickness and reduce operating temperature range of the order of
  • One such new material is Ce0.9Gd0.1O1.95 (CGO) that operates at lower temperature range while maintaining a sufficient high ionic conductivity.
inter connect bipolar plate
Inter-connect/Bipolar Plate
  • Includes separated gas flow channels to supply reactant gasses as well as transfer heat and water to/from electrodes.
  • The flow field as well as the energy and mass transport in the gas channels effects

- gas concentration distribution

- current density distribution

- mass transport losses.

  • This is more critical for the operation of the fuel cell at higher current density.

MEA

Bipolar Plates

Tri-layer Membrane Electrode Assembly (MEA) with bipolar plates

interconnect or bi polar plates
Interconnect Or Bi-polar plates
  • Connects electrically and mechanically the anode of one cell to the cathode of the next.
  • Material can be ceramic or metallic or graphite or carbon composite or ceramic.
  • Plays a key role in the dissipation of heat generated within the fuel cell and in the overall thermal management of fuel cell power system

Cooling Channels

NIU design for for Fabrication and Test

Paten pending

slide20

Channel Design Options

Multiple Parallel Channel Design

Single Serpentine

Outer loop

Inner loop

Second loop

Third loop

Channel sizes

- Macro channels

- Micro channels

Channel Pattern

- Straight parallel

- multiple parallel

- Serpentine

slide21

Total pressure distribution along the channel

Velocity vectors along the channel

thermodynamic model
Thermodynamic Model

Reversible Potential:

Gibb’s Free Energy:

= -2E-05*(T3) + 0.042*(T2) + 23.313*T – 239015

nernst equation
Nernst Equation

Include effect of variation in gas concentrations

butler volmer equation for electrochemical kinetics
Butler-Volmer Equation for Electrochemical Kinetics

Thelocal current density distribution is calculated as using Butler-Volmer equation

Where, i0 = exchange current density

η = over potential

n = number of electrons

= Anodic transfer co-efficient

= Cathodic transfer co-efficient

  • The exchange current density and transfer coefficient represents the electrochemical kinetic parameters.
transport model
Transport Model

Detail components of heat and mass fluxes across the SOFC fuel cell

Heat flux

Mass flux

Membrane

Anode

Cathode

Anode Channel

Cathode Channel

MEA with bi-polar plate with flow channels

gas channels
Gas Channels

Mass and Momentum:

Navier-Stokes Equation

Energy:

Mass Concentration:

gas flow in porous electrodes
Gas flow in Porous Electrodes

Pressure driven flow through a porous

media is given by Brinkman’s equation

Effective viscosity of the gas in the porous media

gas diffusion layer electrode
Gas Diffusion Layer-Electrode

Continuity:

Momentum:

Mass Transport:

Heat Transfer:

diffusion coefficient
Diffusion Coefficient

Effective diffusion coefficient in a pore structure is given as

Alternate form given by Bruggemann equations

Binary diffusion coefficient is given based on Molecular and Knudsen diffusions

Stefan-Maxwell equation for multi-component diffusion

membrane transport equation
Membrane Transport Equation

Mass Transport

Heat Transport

water transport polymer membrane
Water TransportPolymer Membrane

Water is transported by three mechanisms:

- Electro-osmotic Drag due to the

movement of proton

- diffusion due to water concentration

difference

- Permeation due to pressure

difference

Anode

Schlogl’s Equation

Cathode

Nernst-Planck Equationto describe flux of species in the membrane pore liquid.

Takes into account of convection velocity given by electro-osmotic drag flux

activation losses
Activation losses
  • Activation losses are dominant at the low power densities due to the sluggish electro-kinematics.
  • The activation losses are directly proportional to the rate of electrochemical reaction.
  • Activation losses are dominant at the low power densities at lower temperatures.
  • For dominant cathode overpotential (PEM), a simplification of Butler-Volmer equation leads tothe activation losses as

Where Xo2 - oxygen concentration along electrode-electrolyte interface

ohmic loss
Ohmic Loss

Ohmic losses occur due to the resistance to

the ionic flow and electronic flow through the electrolyte and electrodes respectively

given as

Where is constant characterizing the electrolyte material and given as

mass transfer losses
Mass Transfer Losses

Mass transfer losses occur at high current densities due

to insufficient supply of the gases.

At the higher current densities the fuel supply may not

sufficient enough to maintain oxygen concentration at

electrode catalyst layer to a positive level to sustain the

reaction.

This is primarily affected by the gas flow field design in

terms of pressure drop and mass transfer effectiveness.

The mass transfer loss is given by

Where = limiting current density

heat generation in fuel cell
Heat Generation in Fuel cell
  • A fraction of the fuel energy is converted into heat within the fuel cell due to number of irreversibilities associated with the activation losses, mass transfer losses, and Ohmic losses for resistances to ion and electrons flows.
  • This heat energy results in a temperature distribution within the fuel cell and affects the cell’s operating conditions.
  • This waste heat has to be removed continuously in order to ensure a near isothermal operation of the fuel cell,
  • A thermal management system is essential to

maintain cell temperature and for better overall efficiency of the fuel cell power generation system.

slide37
The total heat generation due to

electrochemical reaction is given as

For single cell

Reversible Heat Generation

Irreversible Heat Generation

Irreversible Heat Generation due to Ohmic heating only

For a single cell

slide38

Hydrogen Concentration PEMFC

(a) 0.5 Amp/cm2

(b) 1.0Amp/cm2

(c)1.5Amp/cm2

oxygen concentration across the pemfc
Oxygen Concentration Across the PEMFC

(a) 0.5 Amp/cm2

Strong two-dimensional variation at the cathode-membrane interface and around the land areas.

(b) 1.0Amp/cm2

Stronger variation across the cell.

(c)1.5Amp/cm2

Variations are stronger with higher current densities.

water distribution
Water Distribution

(a) 0.5 Amp/cm2

Pickup of water is higher on cathode-side channel compared to anode-side channel.

(b) 1 Amp/cm2

As the current density increases there is a slight increase in pickup of water on anode side.

(c) 1.5 Amp/cm2

Higher accumulation in the land areas around the channels.

temperature distribution pemfc
Temperature Distribution – PEMFC

a) 0.5 Amp/cm2

Temperature increases from inlet to outlet, indicating that the gas stream is effective in removing heat.

(b) 1 Amp/cm2

Higher temperature at core and cathode side of the fuel cell.

(c ) 1.5 Amp/cm2

Increase in cell temperature at higher current density

pem fuel cell performance with stoichiometric ratios
PEM Fuel Cell Performance with Stoichiometric Ratios
  • Helps increase the limiting current density and operating current-voltage level.
  • Mass transfer losses improved significantly with higher stoichiometric ratio
hydrogen concentration sofc
Hydrogen Concentration - SOFC
  • Strong variation along the length of the channel
  • Strong two-dimensional variation at the anode-membrane interface and around the land areas.
oxygen concentration across the sofc
Oxygen Concentration Across the SOFC
  • Show similar strong variation along the length of the channel and two-dimensional variation in the cell
  • Steeper variation with increase in current density.
temperature distribution sofc
Temperature Distribution - SOFC
  • Show significantly high temperature level for high current density of 1.5 A/cm2
  • Need very effective cooling mechanism to operate around 800-1000 C
  • Restricts operation at higher current density
  • Operation of the cell for 0.5 A/cm2 operation show temp rise of 100-150 C
  • Need to use effective thermal heat management to lower temperature variation in cell
fuel cell power system1
Fuel Cell Power System
  • Includes fuel gasification, heat and water management subsystems
  • Gas clean-up includes carbon dioxide and sulphur
  • There is no practical way to store CO2 for transportation applications
  • If biodiesel is used as a fuel, SO2 removal is probably unnecessary.
transportation electric power train system
Transportation Electric Power Train System
  • In a fuel cell power locomotive, the energy from the fuel cell is

transmitted to the wheels through a electrical power train system.

  • The fuel cell

supplies the power to the motors and keeps the battery fully charged

  • The battery may provide extra power as needed and supply power needed for all auxiliary components.
  • The DC/DC converter increases the fuel cell output voltage and maintains at operating level of 650V.
  • The inverter is used to convert DC to AC for power input to AC electric motors.
  • It is possible use either a single electric motor to drive two wheels using drive shaft, gears and an axle.
fuel cell hybrid system
Fuel Cell Hybrid System
  • Heat from the SOFC can be utilized in a heat engine to increase overall performance
  • In this hybrid system the fuel cell system is configured as a topping component and the heat engine as the bottoming cycles.
  • The unspent fuel from the fuel cell, which has a limited fuel utilization, is burned in a combustion chamber.
slide52
There are number possible bottoming cycle:

- steam or organic Rankine cycle

- gas turbine cycle

- combination of the two.

  • Easiest path for the transfer of the exhaust heat is through a heat Exchanger with fuel cell operating at atmospheric conditions.
  • This is achieved using a steam cycle or with an indirectly fired gas turbine cycle.
  • Another alternative integration is achieved with a pressurized SOFC where the hot gasses are expanded directly in a gas turbine.
fuel reformers
Fuel Reformers
  • The overall goal of fuel reforming is to convert a hydrocarbon fuel into a hydrogen rich gas.

- Fuels include ethanol, methanol, diesel, bio-diesel,

natural gas, coal gas.

  • The primary conversion may be accomplished with or without a catalyst:
  • Three major types of fuel reforming processes

Need heat for endothermic reaction).

- Steam reforming (SR)

- Partial Oxidation (POX) reforming

- Auto thermal reforming (AR)

  • Secondary reforming is possible within SOFC
thermal heat management
Thermal Heat Management
  • Major thermal components include steam generator, heat exchangers for heat recovery/preheating and cooling.
  • The cooling system has to remove heat form fuel cell stack using bi-polar plates and remove heat from electric motors and inverters.
  • A part of the excess fuel from fuel cell stack can be burned in the combustor to supply necessary heat to steam reformers, steam generator and air-preheater.
acknowledgments
Acknowledgments

Thanks to following graduate students for their work:

Rajesh Boddu

Ben Summers

Uday Kumar

GauravDeshpande

Pavan Kumar

Satish Kumar

AdarshSrivastav

This work is supported by:

United States Department of Transportation - (PEMFC)

United States Department of Energy - (SOFC)

references
References
  • 1. Breiter, M. W., Electrochemical Processes in Fuel Cell, Springer-Verlag,
  • Heidelberg, 1969.
  • 2. Hamann, C. H, Hamnett, A and Vielstich, W., Electrochemistry, Wiley-VCH,
  • New York, 1998.
  • 3. Fuel Cell Technology Handbook, Edited by GregorHoogers, CRC Press,
  • 2003.
  • 4. O’hayre, R. O., S-W. Cha, W. Colella, and F. B. Prinz, Fuel Cell
  • Fundamentals, John Wiley & Sons, 2006.
  • 5. Larminie, J and A. Dicks, Fuel Cell System Explained 2nd Edition. Wiley &
  • Sons, 2003.
  • 6. Hart, A. B and G. J. Womack, Fuel cells – Theory, Chapman
  • and Hall, London, UK, 1967.
  • 7. XianguoL., Principles of Fuel Cells, Taylor & Francis, 2006.
  • 8. Hamann, C. H., A. Hamnett and W. Vielstich, Electrochemistry, 2nd Edition,
  • Wiley, 2007.
  • 9. Majumdar, P and S. Revankar, Fuel Cell: Principles, Design and
  • Analysis, CRC Press/Taylor & Francis Group, 2010 (Expected)