1. Electrochemistry for Engineers
Electrochemistry for Engineers
LECTURE 9
Lecturer: Dr. Brian Rosen
Office: 128 Wolfson
Office Hours: Sun 16:00

2. Fuel Cells

3. Important Definitions
Coulombs – Unit of CHARGE
Amperes – Unit of CURRENT [Coulombs per second]
Volts – Unit of POTENTIAL [Joules per Coulomb]
Watt – Unit of POWER [Joules per second]
Joule – Unit of ENERGY [Watts x seconds]
Watt-hour (Wh) is also a unit of energy [Watts x hours]

4. Important Definitions Pt 2
POWER DENSITY – Rate of Energy Transfer per unit volume or mass [kW/m3 or kW/kG]
ENERGY DENISTY – The amount of energy stored in a given system [kJ/m3 or kJ/kg]

5. Electrical Power Production
Chemical Energy
Nuclear Energy
Combustion
Fission
Fusion
Fuel Cells
Heat
Steam Engine
IC Engine
Mechanical Energy
Thermoelectric
Electrical Energy (current) 
Solar and Wind Energy

6. Fuel Cell vs. IC Engine

7. When to use a Fuel Cell - Temp

8. Electrochemical Systems

9. Electrochemical Systems

10. Power Density of Systems

11. Power Density Cont’d

12. Fuel Cell-Based Technologies
Stationary /
Back-up
Portable
Transportation
Direct Liquid Fuel Cells
Liquid Fuel FC
electronics
Ancillaries
Air

13. Common Types of Fuel Cells
Alkaline (AFC)
Polymer Electrolyte Membrane
(PEMFC)
Phosphoric Acid (PAFC)
Polymer Electrolyte Membrane (PEMFC)
Types of Fuel Cells
Fuel cells are classified by the type of electrolyte they use (Table 1). The different electrolytes operate at different temperatures. Low-temperature fuel cells include the alkaline fuel cell (AFC), the proton exchange membrane fuel cell (PEMFC), and the phosphoric-acid fuel cell (PAFC). All of these fuel cells use hydrogen as a fuel. This hydrogen can be extracted from natural gas, biogas, methanol, or propane by the process of reformation. The hydrogen can also be created through the electrolysis of water. High-temperature fuel cells include the molten carbonate fuel cell (MCFC) and the solid oxide fuel cell (SOFC). These fuel cells offer the advantage that they can use either natural gas or untreated coal gas as a fuel directly without the use of a reformer through a process called "Direct Internal Reforming".
The sixth type of fuel cell is the direct methanol fuel cell (DMFC). It is also a low temperature fuel cell, but it can use methanol as a fuel directly without the need of reforming. The primary limitation to the direct methanol fuel cell is that the rate of reaction on the anode is very slow. This results in a very low operating voltage output. For certain applications (e.g., portable power), the direct methanol fuel cell may be ideal.
Molten Carbonate (MCFC)
Direct Methanol (DMFC)
Direct Methanol
(DMFC)
Solid Oxide (SOFC)
Solid Oxide
(SOFC)

16. Membrane Electrode Assembly
PEM Fuel Cells
Voltage = 0.6 V
Cathode Reaction
O2 + 4H+ + 4e-  2H2O
Air
Anode
Cathode e-
Membrane Electrode Assembly
Gas Diffusion Layer H2
Bipolar plate
Load H+
Anode Reaction
2H2  4H+ + 4e-
NIST
Platinum
Catalyst
Carbon
black

17. Single Cell PEMFC

18. A small stack of about 10 cells
PEM Fuel Cell Stacks
NREL
A small stack of about 10 cells
NMSEA
3kW, 48V
fuelcellstore.com

19. Advantages of Fuel Cells Challenges Facing Fuel Cells
Pro’s and Cons of PEM FCs
Advantages of Fuel Cells
1. Higher efficiency compared to IC engines
2. Zero emissions at the point-of-use (PEM)
3. No moving parts in the stack, so quieter
Challenges Facing Fuel Cells
1. Cost (materials, labor, economy of scale)
2. Durability (membrane, catalyst)
3. Lack of H2 Infrastructure: H2 is difficult to produce, transport, and store

20. Pro’s and Con’s Pt 2 Advantages: High efficiency High energy density
Fuel flexibility
Environmental clean
Limitations:
Fuel Crossover
Transport Issues
Water Management (cathode flooding and anode dry-out)
Litster et al., Journal of Power Sources,130, 2004, 61

21. Direct Methanol Fuel Cell (DMFC) (acidic conditions)

22. Progress Over the Years!
This chart shows the dramatic increase in fuel cell investments for transportation from the governments in Canada, Europe and Japan. Primary focuses included research & development, demonstration & validation, providing market entry support, and investments in fueling infrastructure for fuel cell vehicles.
This chart shows the relative recent domination of PEM fuel cell technology, a newer type of fuel cell technology, that holds the most promise for our first fuel cell vehicles. Some will tell you that fuel cells have been around for decades. While it’s true that fuel cell technology was invented in 1839, it was a much older technology. Fuel cell technology suitable for vehicles is a relatively recent achievement.
Dramatic increase in public and private investment since 1983 as shown by the steady rise in Automotive Fuel Cell Patents

23. Reminder: Thermodynamic Potentials

24. Reminder: Products-Reactants

25. Fuel Cell Efficiency
Where..
Where..

27. Overpotential Losses in Fuel Cell

28. Total Energy Lost as Heat
Note that EH is not a real potential,
but instead a theoretical potential
assuming 100% thermodynamic
Efficiency. Therefore, EH will be higher
that the open circuit potential.

29. Power Density Curve

30. Activation Region

31. Exchange Density for H2 Oxidation+ =

32. Exchange Density’s for PEM

33. Recall: Activation Barrier

34. Current at Sacrifice of Voltage
Great catalyst
Good catalyst
Poor catalyst

35. Ohmic Region

36. Ohmic Overpotential
Charge transport between the electrodes is not a frictionless process,
therefore, there is a penalty of a loss of FC voltage
j = charge flux
σ = conductivity
j = σ (dV/dx) = σ (V/x)
V = i(L/Aσ ) = iR
ηohm = i(Relec + Rionic) ≈ iRionic
Rionic is on the order of mΩ V
V=iR=jL/σ L x

37. Direction of Voltage Drop
Anode
(-)
Cathode
(+)
Anode
(-)
Cathode
(+)
The voltage drop MUST be negative from the anode to the cathode in order to provide
the driving force for migration towards the cathode.

38. Requirements for Electrolyte
High ionic conductivity
Low electronic conductivity
High stability for oxidation and reduction
Low fuel crossover
Mechanical strength
Easily manufactured

39. Sulfonic Acid (SO3- H+) Membranes
Membranes contains
phase-separated regions
on the order of 5 nm in diameter
These swell with water uptake.

40. Endurance (Durability) Test Results for Gore Primea 56 MEA at Three Current Densities (hydrogen fuel)
Polarization Curves for 3M MEA (hydrogen fuel)

41. Laminar Flow Fuel Cells (LFFC)
Depletion
Depletion
Diffusion

42. Mass Transport Region

43. Mass Transport at PEM Anode

44. Depletion Still Occurs: Harder to Model

45. A Familiar Solution

46. Constant Flow or Stoichiometry
Constant Flow – The flow rate of fuel is constant regardless of the required current. Typically enough fuel is provided for maximum power
Constant Stoichiometry – The fuel is provided for 100% fuel utilization at a given current density (λ=1). λ = 1.5 means 50% extra fuel is provided

47. Constant Flow or Stoichiometry

48. Concentration Affects Nernst Voltage

49. Concentration Affects Nernst Voltage

50. Concentration Affects Rate of Reaction

51. High porous materials to achieve intimate contact between gas phase
Triple Phase Boundary
High porous materials to achieve intimate contact between gas phase
pores, electrically conducting catalyst and ionic conducting electrolyte
-Mechanical Strength
-Electrical conductivity
-Low corrosion
-High porosity
-Easily manufactured
-High j0

52. Compact Mixed Reactors (CMR)
A mixture of fuel and oxidant flows through a fully porous anode-electrolyte-cathode assembly. CMR cells require selective electrode catalysts (Ru/Se), the need for key enabling materials in direct methanol fuel cell development programmes.

53. Overall Analysis of Losses

54. Resistors in Series