Mench M.M. Fuel Cell Engines

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c05 JWPR067-Mench January 23, 2008 18:58 Char Count=

280 Transport in Fuel Cell Systems

5.15 Estimate the ratio of the time required for oxygen to

diffuse through the same length in the gas-phase and in

water.

5.16 Compare the expected diffusion coefﬁcient of oxygen

and hydrogen using Eq. (5.31).

5.17 Calculate the oxygen and hydrogen gas-phase trans-

port limiting current density in a hydrogen SOFC for the

anode and cathode sides at 900

◦

C, 1 atm pressure operation

with fully humidiﬁed gas streams and high stoichiometry.

Assume the anode and cathode catalyst layers are 50 and

100 µm thick, respectively and 50% porosity.

5.18 Calculate the oxygen and hydrogen gas-phase trans-

port limiting current density in a hydrogen PEFC for the

anode and cathode sides at 80

◦

C, 1 atm pressure operation

with fully humidiﬁed gas streams and high stoichiometry.

Assume the anode and cathode diffusion layers are 300 µm

thick, 70% porosity. Ignore the catalyst layer restriction in

this problem. If the pressure were increased to 3 atm on

both sides, what would be the effect on the limiting current

density.

5.19 Redo problem 5.18, but this time include the effects

of a 30 micron thick anode and cathode with 50% porosity.

Is the catalyst layer resistance important? Hint: Assume the

reaction occurs, on average, at the middle of the catalyst

layers.

5.20 Consider the case of a hydrogen-fed PEM fuel cell at

arbitrary P and T with a porous gas diffusion layer, with a

hydrogen mole fraction of 0.4.

(a) What is the effect of doubled total pressure on the

limiting current density?

(b) What is the effect of doubled temperature on the

limiting current density?

tion on the limiting current density?

5.21 We discussed in this chapter about the limitations of

the simpliﬁed model for determination of limiting current

density based on the gas-phase diffusivity. In particular,

besides the diffusion media, there is also diffusion resis-

tance in the catalyst layer, which is in series with the gas

diffusion layer, and the effects of a thin Naﬁon or liquid

layer on the catalyst. Assume the anode and cathode cata-

lyst layer porosity is 0.5 and thickness is 20 µm. Consider a

system with a thin layer of ﬂooding and an ionomer on the

catalyst. Consider the cathode only, at a normal operating

temperature of 80

◦

C, and a pressure of 15 psig. Assume

the average mole fraction of the oxygen in the ﬂow channel

0.12. Develop an expression for, and solve for, the mass

transfer limiting current density at the cathode. Consider

the catalyst has a 1 nm Naﬁon coating, and a 0.5 nm liquid

water coating, and the DM is 250 µm thick (porosity 0.7).

The catalyst layer is 20 µm thick, with a porosity of 0.4.

5.22 Calculate the molar ﬂow rate of hydrogen crossover

into and through a 51 µm thick Naﬁon electrolyte. The an-

ode is operating at 383 K and 1 atm pressure with a mole

fraction of 0.6 hydrogen. Determine the mass transfer lim-

iting hydrogen crossover current density from this value.

5.23 Find the Reynolds number at the inlet of a pure H

anode for a single path serpentine fuel cell ﬂow ﬁeld under

the following conditions.

= 1.5, w

square channel

= 1.0mm, T = 80

◦

C, P

inlet

= 202,560 Pa, i = 1A/cm

, A

electrode

= 50 cm

w is the channel width, and the channel has a square

x-section,µ

= 8.8 × 10

−6

kg/m·s

(a) Is it laminar or turbulent ﬂow in the anode?

(b) Will parallelizing the ﬂow channels increase or

decrease the Reynolds number?

decrease the pressure drop?

5.24 For many calculations of heat and mass transfer, we

neglect the entrance region and assume fully developed

ﬂow. At what path length L, in the ﬂow ﬁeld, will the ﬂow

in problem 5.23 part a above become fully developed? What

is the effect of the entrance length on pressure drop in the

ﬂow channel – will it be greater or less than we expect if

we neglect the entrance effects?

5.25 Make a plot of the frictional pressure drop versus

channellengthforasingle1mmx1mmchannelwith

5 m/s fully humidiﬁed air ﬂow. Assume fully developed

ﬂow. On the same plot, show the pressure drop from con-

sumption along the ﬂow path if the stoichiometry is 2.0 on

the air cathode.

5.26 Compare the ratio of pressure drop on the anode to

the cathode side for a hydrogen/air fuel cell.

5.27 Compare the ratio of entrance length on the anode to

the cathode side for a hydrogen/air fuel cell at 353 K, 1 atm

pressure.

5.28 Given a fuel cell with a 1 mm width x 0.5 mm depth

channel size and a ﬂow stoichiometry of 2.0 on the air cath-

ode side. Plot the entrance length per unit length of the

channel at 353 K, 1 atm as a function of current density.

c05 JWPR067-Mench January 23, 2008 18:58 Char Count=

References 281

5.29 Make a plot of absolute permeability versus poros-

ity for a 50 micron pore size using the Carman–Kozeny

relationship.

5.30 Make a plot of the liquid permeability in a porous me-

dia as a function of liquid saturation, with arbitrarily chosen

parameters besides saturation. On the same graph, plot the

gas-phase permeability as a function of liquid saturation.

5.31 In Example 5.14, the capillary pressure was calcu-

lated by the Leverett approach for a hydrophobic diffusion

media. In this problem, repeat Example 5.14 using the mod-

iﬁed Leverett approach given in Eq. (5.104) for a DM with

5, 10, and 20% PTFE content at a liquid saturation of 0.4.

(a) Determine the change in P

when contact angle is

changed from 110 to 150

◦

. Compare to the results

using the original Leverett approach.

(b) Determine the change in P

when the porosity is

increased to 0.9. Compare to the results using the

original Leverett approach.

5.32 Consider a 2 kWe solid oxide hydrogen–air fuel cell

operating at 0.6 V. What value of effective heat transfer

coefﬁcient h, must the surface of the fuel cell stack have

with the environment at 298 K to allow it to remain at

800

◦

C in steady state? Consider that the heat in the efﬂuent

is completely recovered in a recuperator so that none leaves

the fuel cell stack through the exhaust.

5.33 Using typical fuel cell parameters, solve for the ex-

pected ratio of conduction heat transfer through the lands

to convective heat transfer through the channel gas on the

anode and cathode sides of a PEFC at 80

◦

C. Assume Re =

200 in the anode, 2000 in the cathode, and fully humidiﬁed

gas ﬂow at 1 atm. Would changing the pressure make a

difference? Assume a channel-to-land ratio of 1:1.

5.34 Estimate the ratio of the heat dissipation by natural

convection, forced convection, and radiation from a PEFC

and SOFC stack.

5.35 Use the Wiedemann-Franz Law to relate the thermal

conductivity of a known thermal conductor copper to the

electrical conductivity of copper. Check your result versus

published values. How close is the approximation? Now

check the approximation on a known insulating solid such

as a ceramic. Is the approximation close?

5.36 Looking at Table 5.14, what can you say about the

probable relationship between PTFE content and electrical

conductivity. Does this make sense?

5.37 It has been found that the thermal conductivity of a

PTFE treated diffusion media decreases with compression.

Given that compression will decrease the effective porosity

of the diffusion media, this seems like the opposite of what

we expect. Can you explain why this happens?

Computer Problem

5.38 Update your computer simulation of PEFCs from

Chapter 4 to include ﬁlm resistances on the electrodes,

and heat generation by the various polarizations. Plot the

polarization curve along with the heat generation from the

various terms separately, and compare them for a typical

PEFC parameters.

Open-Ended Problem

5.39 Fuel cell start-up from cold conditions is a common

challenge. Considering the various different fuel cell sys-

tems and operating temperatures, there are a multitude of

engineering challenges including a damage free-start-up,

a rapid start-up, and an efﬁcient start-up. Looking at two

examples, the low temperature PEFC, and the high temper-

ature SOFC, how, in general engineering terms, would you

address these issues for these two systems? For example,

what general engineering system parameters would lead

you to a rapid start-up? What factors could cause damage?

REFERENCES

1. J. S. Newman, Electrochemical Systems, 2nd ed., Prentice Hall, Englewood Cliffs, NJ, 1991.

2. F. M. White, Fluid Mechanics, 4th ed., McGraw-Hill, New York, 1999.

3. M. Doyle and G. Rajendran, “Peﬂuorinated Membranes,” in Handbook of Fuel

Cells—Fundamentals, Technology and Applications, W. Vielstich, A. Lamm, and H. A. Gasteiger,

Eds., Wiley, New York, 2003, pp. 351–395.

4. J. S. Wainright, M. H. Litt, and R. F. Savinell, “High-Temperature Membranes,” in Handbook

of Fuel Cells—Fundamentals, Technology and Applications, W. Vielstich, A. Lamm, and H. A.

Gasteiger, Eds., Wiley, New York, 2003, pp. 436–446.

c05 JWPR067-Mench January 23, 2008 18:58 Char Count=

282 Transport in Fuel Cell Systems

5. A. Z. Weber, and J. Newman, “Transport in Polymer-Electrolyte Membranes I. Physical Model,”

J. Electrochem. Soc., Vol. 150, No. 7, pp. A1008–A1015, 2003.

6. C. Chuy, V. I. Basura, E. Simon, S. Holdcroft, J. Horsfall, and K. V. Lovell, “Electrochemi-

cal Characterization of Ethylene tetra ﬂouroethylene-g-Polystyrenesulfonic acid solid polymer

electrolytes,” J. Electrochem Soc., Vol. 147, pp. 4458–4453, 2000.

7. T. A. Zawodzinski, Jr., M. Neeman, L. O. Sillerud, and S. Gottesfeld, “Determination of Water

Diffusion Coefﬁcients in Perﬂourosulfonate Ionomeric Membranes,” Vol. 95, pp. 6040–6044,

1991.

8. M. Doyle, M. E. Lewittes, M. G. Roelofs, and S. A. Perusich, “Ionic Conductivity of Nonaque-

ous Solvent-Swollen Ionomer Membranes Based on Flourosulfonate, Flourocarboxylate, and

Sulfonate Fixed Ion Groups,” J. Phys. Chem. B., Vol. 105, pp. 9387–9394, 2001.

9. F. N. Buchi and G. G. Scherer, “Investigation of the Transversal Water Proﬁle in Naﬁon Mem-

branes in Polymer Electrolyte Fuel Cells,” J. Electrochem Soc., Vol. 148, pp. A183–A188,

2001.

10. A. Z. Weber and J. Newman, “Transport in Polymer-Electrolyte Membranes,” J. Electrochem.

Soc., Vol. 151, p. A311–A325, 2004.

11. J. T. Hinatsu, M. Mizuhata, and H. Tekenaka, “Water Uptake of Perfuorosulfonic Acid Mem-

branes from Liquid Water and Water Vapor,” J. Electrochem. Soc., Vol. 141, p. 1493–1498,

1994.

12. T. E. Springer, T. A. Zawodzinski, and S. Gottesfeld, “Polymer Electrolyte Fuel Cell Model,”

J. Electrochem. Soc., Vol. 138, pp. 2334–2342, 1991.

13. N. Minh and T. Takahashi, Science and Technology of Ceramic Fuel Cells, Elsevier, New York,

1995.

14. O. Yamamoto, “Low Temperature Electrolytes and Catalysts,” in Handbook of Fuel

Cells—Fundamentals, Technology and Applications, W. Vielstich, A. Lamm, and H. A. Gasteiger,

Eds., Wiley, New York, 2003, pp. 1002–1014.

15. C. H. Hamann, A. Hamnett, and W. Vielstich, Electrochemisty, Wiley-VCH, New York, 1998.

16. X. Li, Principles of Fuel Cells, Taylor and Francis, New York, 2006.

17. J. Larmine and A. Dicks, Fuel Cell Systems Explained, 2nd ed., Wiley, New York, 2003.

18. M. F. Mathias, J. Roth, J. Fleming, and W. Lehnert, “Diffusion Media Materials and Characteri-

zation,” in Handbook of Fuel Cells—Fundamentals, Technology and Applications, W. Vielstich,

A. Lamm, and H. A. Gasteiger, Eds., Wiley, New York, 2003, pp. 517–537.

19. E. L. Cussler, Diffusion-Mass Transfer in Fluid Systems, 2nd ed., Cambridge University Press,

New York, p. 101, 1997.

20. M. J. Moran and H. N. Shapiro, Fundamentals of Engineering Thermodynamics, 3rd ed., Wiley,

New York, 1995.

21. J. C. Slattery and R. B. Bird, “Calculation of the Diffusion Coefﬁcient of Dilute Gases and of the

Self-Diffusion Coefﬁcient of Dense Gases,” AIChE J., Vol. 4, pp. 137–142, 1958.

22. R. B. Bird, W. E. Stewart, and E. N. Lightfoot, Transport Phenomena, 2nd ed., Wiley, New York,

2002.

23. E. N. Fuller, P. D. Schettler, and J. C. Giddings, Ind. Eng. Chem., Vol. 58, p. 19, 1966.

24. H. S. Salem, and G. V. Chilingarian, “Inﬂuence of Porosity and Direction of Flow on Tortuosity

in Unconsolidated Porous Media,” Energy Sources, Vol. 22, pp. 207–213, 2000.

25. H. T. Bach, B. A. Meyer, and D. G. Tuggle, “Role of Molecular Diffusion in the Theory of Gas

Flow Through Crimped-Capillary Leaks,” J. Vacuum Sci. Technol. A: Vacuum, Surfaces, and

Films, Vol. 21, No. 3, pp. 806–813, 2003.

c05 JWPR067-Mench January 23, 2008 18:58 Char Count=

References 283

26. S. Srinivasan, A. Parthasarathy, A. J. Appleby, and C. R. Martin, “Temperature Dependence of

the Electrode Kinetics of Oxygen Reduction at the Platinum/Naﬁon

R

Interface a Microelectrode

Investigation,”J. Electrochem. Soc., Vol. 139, pp. 2530–2537, 1992.

27. Z. Ogumi, Z. Takehara, and S. Yoshizawa, “Gas Permeation in SPE Method,” J. Electrochem.

Soc., Vol. 131, pp. 769–773, 1984.

28. D. Bernardi and M. W. Verbrugge, “A Mathematical Model of the Solid Polymer-Electrolyte

Fuel,” J. Electrochem Soc., Vol. 139, pp. 2477–2491, 1992.

29. Y. A. C¸ engel and M. A. Boles, Thermodynamics, an Engineering Approach, 4th ed., McGraw-

Hill, New York, 2002.

30. D. M. Bernardi and M. W. Verbrugge, “Mathematical Model of a Gas Diffusion Electrode Bonded

to a Polymer Electrolyte,” AICHE J., Vol. 37, pp. 1151–1163, 1991.

31. C. F. Colebrook, “Turbulent Flow in Pipes, with Particular Reference to the Transition Between

the Smooth and Rough Pipe Laws,” J. Inst. Civ. Eng. Lond., Vol. 11, pp. 133–156, 1938/

1939.

32. D. P. Wilkenson and O. Venderleeden, “Serpentine Flow Field Design,” in Handbook of Fuel

Cells, Vol. 3, W. Vielstich, A. Lamm, and H. A. Gasteiger, Eds., Wiley, New York, 2003, pp.

315–324

33. Q. Dong, M. M. Mench, S. Cleghorn, and U. Beuscher, “Distributed Performance of Polymer

Electrolyte Fuel Cells under Low-Humidity Conditions,” J. Electrochem. Soc., Vol. 152, pp.

A2114–A2122, 2005.

34. G. W. Govier and K. Aziz, The Flow of C omplex Mixtures in Pipes, Van Nostrand Reinhold,

New York, 1972.

35. E. C. Kumbur, K. V. Sharp, and M. M. Mench, “Liquid Droplet Behavior and Instability in a

Polymer Electrolyte Fuel Cell Flow Channel,” J. Power Sources, Vol. 161, p. 333, 2006.

36. A. Turhan, J. J. Kowal, K. Heller, J. Brenizer, and M. M. Mench, “Diffusion Media and Interfacial

Effects on Fluid Storage and Transport in Fuel Cell Porous Media and Flow Channels,” ECS

Trans., Vol. 3, No. 1, Proton Exchange Membrane Fuel Cells,Vol.6,T.Fuller,C.Bock,S.

Cleghorn, H. Gasteiger, T. Jarvi, M. Mathias, M. Murthy, T. Nguyen, V. Ramani, E. Stuve, T.

Zawodzinski, Eds., pp. 435–444, 2006.

37. T. A. Corey, Mechanics of Immiscible Fluids in Porous Media, 1st ed., Water Resource Publica-

tions, Highlands Ranch, CO, 1994.

38. T. Young, Philos. Trans. Res. Soc., pp. 65–95, 1805.

39. J. H. Nam and M. Kaviany, “Effective Diffusivity and Water-Saturation Distribution in Single- and

Two-Layer PEMFC Diffusion Medium,” Int. J. Heat and Mass Transfer. Vol. 46, pp. 4595–4611,

2003.

40. M. Honarpour, L. Koederitz, and A. H. Harvey, Relative Permeability of Petroleum Reservoirs,

1st ed., CRC Press, Boca Raton, FL, 1986.

41. J. Bear, Dynamics of Fluids in Porous Media, American Elsevier, New York, 1972.

42. E. C. Kumbur, K. V. Sharp, and M. M. Mench, “On the Effectiveness of the Leverett Approach

to Describe Water Transport in Fuel Cell on the Diffusion Media” J. Power Sources, Vol. 172,

pp. 816–830, 2007.

43. U. Pasaogullari, C. Y. Wang, and K. S. Chen, “Two-Phase Transport in Polymer Electrolyte Fuel

Cells with Bilayer Cathode Gas Diffusion Media,” J. Electrochem. Soc., Vol. 152, pp. 1574–1582,

2005.

44. S. Litster, D. Sinton, and N. Djilali, “Ex situ Visualization Liquid Water Transport in PEM Fuel

Cell Gas Diffusion Layers,” J. Power Sources, Vol. 154, pp. 95–105, 2006.

c05 JWPR067-Mench January 23, 2008 18:58 Char Count=

284 Transport in Fuel Cell Systems

45. K. S. Udell, “Heat Transfer in Porous Media Considering Phase Change and Capillarity—the

Heat Pipe Effect,” Int. J. Heat Mass Transfer, Vol. 28, pp. 485–495, 1985.

46. M. C. Leverett, “Capillary Behavior in Porous Solids,” Am. Inst. Mining Met. Eng. Petroleum

Dev. Technol., pp. 142–152, 1994.

47. E. C. Kumbar, K. V. Sharp, and M. M. Mench, “A Validated Leverett Approach to Multiphase

Flow in Polymer Electrolyte Fuel Cell Diffusion Media: Part 3, Temperature Effect and Uniﬁed

Approach,” J. Electrochem. Soc., Vol. 154, pp. B1315–B1324, 2007.

48. E. C. Kumbar, K. V. Sharp, and M. M. Mench, “A Validated Leverett Approach to Multi-

phase Flow in Polymer Electrolyte Fuel Cell Diffusion Media: Part 1, Hydrophobicity Effect,”

J. Electrochem. Soc., Vol. 154, pp. B1295–B1304, 2007.

49. M. Khandelwal, and M. M. Mench, “Direct Measurement of Through-Plane Thermal Conductiv-

ity and Contact Resistance in Fuel Cell Materials,” J. Power Sources, Vol. 161, pp. 1106–1115,

2006.

50. F. P. Incropera and D. P. De Witt, Fundamentals of Heat and Mass Transfer, 3rd ed., Wiley, New

York, 1990.

51. W. M. Kays and M. E. Crawford, Convection Heat and Mass Transfer, McGraw-Hill, New York,

1980.

52. M. F. Modest, Radiative Heat Transfer, 2nd ed., Academic, New York, 2003.

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Polymer Electrolyte Fuel Cells

Market forces, greenery, and innovation are shaping the future

of our industry and propelling us inexorably towards hydrogen

energy. Those who don’t pursue it ...will rue it

—Frank Ingriselli, President, Texaco Technology Ventures,

April 23, 2001

Throughout this textbook, the PEFC has been emphasized because it is a likely candidate

for power replacement for portable, auxiliary, stationary, and automotive power systems.

As shown in Figure 6.1, the PEFC class of fuel cells includes the hydrogen, direct methanol,

direct alcohol, and other fuel cell systems utilizing a solid polymer electrolyte. While the

higher temperature molten carbonate and solid oxide fuel cell systems are well-suited for

steady power systems, only the low-temperature PEFC offers the rapid startup and lower

operating temperatures (20–90

◦

C) required for transient operation of portable, reserve, and

automotive power applications.

6.1 HYDROGEN PEFC

The primary advantages of the hydrogen PEFC system compared to other fuel cell systems

include the following:

1. High relative performance: H

PEFCs can reach >1.3 kW/L fuel cell power density,

>0.6 kW/L system power density, and >0.6 kW/kg mass speciﬁc power density.

2. Low-temperature operation: H

PEFCs can start and operate in subfreezing temper-

ature, although normal operating temperatures are 20–90

◦

3. Facile anode kinetics for the HOR: As a result, H

PEFCs have the lowest precious

metal loadings of all the PEFC types (typically 0.2–0.8 mg/cm

total active area;

i.e. anode plus cathode).

However, the H

PEFC has many complex technical issues that have no simple solution.

Besides issues of manufacturing, ancillary system components, control, cost, and market

acceptance not treated in this text, the main technical design issues for the H

PEFC system

include the following:

1. Water and Heat Management Due to the low-temperature operation of PEFCs,

the water generated by reaction often condenses into liquid phase, ﬂooding parts

285

Fuel Cell Engines

Matthew M. Mench

c06 JWPR067-Mench January 26, 2008 20:1 Char Count=

286 Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cell

Direct methanol fuel cell

(DMFC)

Hydrogen fuel cell

PEFC)

Direct alcohol fuel cell

(DAFC)

Figure 6.1 Subclassiﬁcations of PEFCs.

of the electrodes, DM, and ﬂow channels. At a typical operating temperature of

◦

C, the saturation pressure of water changes by almost 5% for every 1

◦

C change

at 1 atm. Therefore, the two-phase ﬂow and heat generation are inexorably linked

for all situations but very low power (<∼0.2 A/cm

), where isothermal conditions

can be assumed. The presence of liquid water or hot spots in the fuel cell can also

negatively impact lifetime durability and should be minimized.

2. Durability The performance of every fuel cell gradually degrades with time due

to a variety of phenomena. The year 2005 U.S. Department of Energy goals for

longevity of fuel cell systems are given in Table 6.1. The automotive fuel cell must

withstand load cycling and freeze–thaw environmental swings with an acceptable

level of degradation from the beginning-of-lifetime (BOL) performance over a

lifetime of 5000 h (equivalent to 150,000 miles at 30 mph). A stationary fuel cell

must withstand over 40,000 h of steady operation under vastly changing external

temperature conditions. Durability here also refers to tolerance to a variety of

environmental poisons that can harm performance.

3. Manifolding and Stack Design The ﬂow to each fuel cell within the stack should

be uniformly distributed to avoid performance degradation, which is a particularly

challenging aspect of design for a fuel cell with multiphase ﬂow.

4. Materials There is a strong interaction between the material choices, fuel cell de-

sign, durability, and water/heat management for all fuel cell components, including

bipolar plates, DM, seals, catalysts, and electrolytes.

5. Achieving Higher Efﬁciency and System Power Density As in any power source,

there is a continual need to achieve a smaller system proﬁle with better efﬁciency.

6. Dynamic Performance and Rapid Power Availability Compared to higher tem-

perature fuel cell systems, the startup time to full power of the PEFC is rapid, but

nearly immediate response from all ambient conditions is desired.

6.1.1 Components and Materials of PEFC

Recall the general components of a PEFC shown in Figure 6.2. The fuel and oxidizer ﬂow

through the ﬂow ﬁelds, diffuse through the thin (∼200–400-µm) porous diffusion media

(DM) [also known as a gas diffusion layer (GDL) or porous transport layer (PTL)] and to

the ∼5–20-µm-thick porous catalyst layers.

c06 JWPR067-Mench January 26, 2008 20:1 Char Count=

6.1 Hydrogen PEFC 287

Table 6.1 Department of Energy Goals for Fuel Cell Systems and Durability

Automotive Scale: 80 kW

(Fuel Cell System, Net) Units 2005 Status 2010 Goal 2015 Goal

Efﬁciency at rated power % 50 50 50

Transient response time (10–90% rated power) s 1.5 1 1

Cold startup time (to 50% rated power) from

−20

◦

s 120 30 30

Unassisted start

◦

C −20 −40 −40

Speciﬁc power (system) W/kg 470 650 650

Power density (system) W/L 500 650 650

Durability with load cycling h 1000 5000 5000

Cost $/kWe 110 45 30

Auxiliary Power: 3–5 kW

(Fuel Cell System, Net)

Units 2005 Status 2010 Goal 2015 Goal

Efﬁciency at rated power % 15 35 40

Speciﬁc power W/kg 50 100 100

Power density W/L 50 100 100

Durability h 100 20,000 35,000

Cost $/kWe >2000 400 400

Stationary Scale: 250 kW

(Fuel Cell System, Net) Units 2005 Status 2011 Goal

Efﬁciency at rated power % 32 40

Combined heat and power efﬁciency % 75 80

Transient response time (10–90% rated power) s <3 <3

Cold startup time (to 50% rated power) from

−20

◦

s <90 <30

Survivability (min/max ambient temperature)

◦

C −25/40 −35/40

Noise output dB >65 at 10 m >55 at 10 m

Durability <10% rated power loss h 20,000 40,000

Cost $/kWe 2,500 750

Consumer Electronics: subwatts – 50 W

(Fuel Cell System, Net) Units 2005 Status 2010 Goal

Speciﬁc power (system) W/kg 20 100

Power density (system) W/L 20 100

Energy density Wh/L 300 1000

Lifetime h >500 5000

Cost $/kWe 40 3

Based on SOFC systems, not PEFCs.

Source: From [1].

As shown in Figure 6.2, the PEFC consists of the following basic materials:

1. Solid polymer electrolyte

2. Catalyst layers (anode and cathode)

c06 JWPR067-Mench January 26, 2008 20:1 Char Count=

288 Polymer Electrolyte Fuel Cells

Figure 6.2 Schematic of typical H

PEFC and its components.

3. Diffusion media, usually including a microporous layer

4. Bipolar plates with ﬂow channels for reactants and coolant in larger stacks

Solid Polymer Electrolyte The most common solid polymer electrolytes consist of a hy-

drophobic and inert polymer backbone which is sulfonated with hydrophilic acid clusters

to provide adequate conductivity as discussed in Chapter 5. In order to ensure adequate per-

formance, some membrane hydration is required. However, excess water in the electrodes

can result in electrode ﬂooding, so that a precarious balance must be achieved. Modern

perﬂourosulfonated ionomer electrolytes for H

PEFCs are 18–25 µm thick with a practical

operating temperature limit of 120

◦

C, although PEFC operation is rarely greater than 90

◦

due to excessive humidity requirements and operational low lifetimes.

Catalyst Layer Catalyst layers (CLs) in PEFCs consist of a porous, three-dimensional

structure, with a thickness of 5–30 µm (see Figure 6.3). For supported CLs, the 2–10-nm

catalyst is physically supported on considerably larger, 45–90-nm carbon particles. As

discussed in Chapter 2, the CL is the most complex structure in the PEFC. It must have

facile transport of ions, electrons, reactants, and products with a high electrochemically

c06 JWPR067-Mench January 26, 2008 20:1 Char Count=

6.1 Hydrogen PEFC 289

Figure 6.3 Transmission electron micrograph (TEM) of 40 wt% platium/chrominum catalyst on

carbon support. (Reproduced with permission from [2].)

active surface area (ECSA), where the reactants, catalyst, proton, and electron conduction

are all available. To facilitate these requirements, the CL is highly porous (typical porosity

0.4–0.6), with a high connectivity in the carbon and catalyst particles to promote electrical

conduction. The CL typically contains a considerable fraction of ionomer (up to ∼30%

by weight) to promote ionic transport to/from the main electrolyte membrane. The CL

normally has some fraction of polytetraﬂuouroethylene (PTFE) additive as well to promote

water removal. However, like the DM, it is not totally hydrophobic because the basic

components besides PTFE are hydrophilic. Therefore, the CL consists of a mixed network

of hydrophobic and hydrophilic surfaces.

To manufacture the CL structure, several techniques are employed and summarized

brieﬂy here:

1. Physical Spray Deposition on DM or Electrolyte Surface In this method, a slurry

of catalyst, support, PTFE, solubilized polymer ionomer, and alcohol compounds (to