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

1. Fuel Cell: State-of the-Art Review and Challenges PradipMajumdar Professor Department of Mechanical Engineering Northern Illinois University DeKalb, IL 60115

2. 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:

3. 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%

4. 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

5. DOE-SECA goal for coal-based SOFC power system for distributed based on IGFC SECA - Solid State Energy Conversion Alliance

6. 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

7. 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

8. SOFC Components

9. 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.

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

11. 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

12. 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

13. SOFC Materials

14. 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.

15. 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

16. 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.

17. 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.

18. 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

19. 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

20. 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

21. Total pressure distribution along the channel Velocity vectors along the channel

22. Fuel Cell Simulation Model

23. Thermodynamic Model Reversible Potential: Gibb’s Free Energy: = -2E-05*(T3) + 0.042*(T2) + 23.313*T – 239015

24. Nernst Equation Include effect of variation in gas concentrations

25. 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.

26. 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

27. Gas Channels Mass and Momentum: Navier-Stokes Equation Energy: Mass Concentration:

28. 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

29. Gas Diffusion Layer-Electrode Continuity: Momentum: Mass Transport: Heat Transfer:

30. 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

31. Membrane Transport Equation Mass Transport Heat Transport

32. 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

33. 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

34. 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

35. 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

36. 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.

37. 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

38. Hydrogen Concentration PEMFC (a) 0.5 Amp/cm2 (b) 1.0Amp/cm2 (c)1.5Amp/cm2

39. 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.

40. 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.

41. 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

42. 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

43. Effect of Bi-Polar Plate Design

44. 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.

45. 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.

46. 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

47. Fuel Cell Power System

48. 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.

50. 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.