Characterization of Pore Structure of Fuel Cell Components for Enhancing Performance

1. Characterization of Pore Structure of Fuel Cell Components for Enhancing Performance Dr. Akshaya Jena and Dr. Krishna Gupta Porous Materials, Inc., Ithaca, New York, USA

2. Outline • Introduction • Through pore throat diameter, distribution, gas permeability & surface area by: • Capillary Flow Porometry • Capillary Condensation Flow Porometry • Hydrophobic through and blind pore volume & distribution by: • Vacuapore • Through pore volume, diameter, distribution & liquid permeability by: • Liquid Extrusion Porosimetry • Summary and Conclusion

3. Introduction • Pore structure governs kinetics of physicochemical processes & Flows of reactants and products in fuel cells. • Quantitative measurement of pore structure is essential for Design, development and performance evaluation. • Technologies for pore structure measurement are currently being developed to characterize the complex pore structure of fuel cell components. • We will discuss several innovative techniques successfully developed and applied for evaluation of pore structure of fuel cell components.

4. Through Pore Throat Diameters, Distribution, Gas Permeability and Surface Area Importance of Such Properties

5. Through Pore Throat Diameters, Distribution, Gas Permeability and Surface Area Suitable Characterization Techniques Advanced Capillary Flow Porometry Capillary Condensation Flow Porometry

6. Advanced Capillary Flow Porometry • For wetting liquid: • Wetting Liquids fill pores spontaneously • Cannot come out spontaneously • A pressurized inert gas can displace liquid from pores provided: Work done by Gas = Increase in Interfacial Free Energy Basic Principle

7. Advanced Capillary Flow Porometry Pressure needed to displace liquid from a pore: p = 4 γcosθ / D p = differential gas pressure γ = surface tension of wetting liquid θ = contact angle of the liquid D = pore diameter • Pore diameter is defined for all pore cross-sections

8. Advanced Capillary Flow Porometry (Perimeter/Area)pore = (Perimeter/Area)cylindrical opening Pore Diameter = Diameter of Cylindrical Opening SKETCH

9. Advanced Capillary Flow Porometry • Measured differential pressure & gas flow through dry & wet sample yield pore structure

10. The Technique Advanced Flow Porometers • Accurate • Pressure transducers • Flow transducers • Regulators • Controllers • Sophisticated sample sealing mechanisms to direct flow in desired directions • Internal computers • To control sequential operations • To execute automated tests

11. The Technique Advanced Flow Porometers • Proper algorithms • To detect stable pressure and flow • To acquire data • Software • To convert acquired data to pore structure characteristics • To present data in tabular, graphical and excel formats

12. An Example: The PMI Advanced Capillary Flow Porometer

13. The PMI Advanced Capillary Flow Porometer • Features: • Sealing with uniform pressure by pneumatic piston-cylinder device • Automatic addition of measured amount of wetting liquid at appropriate time

14. The PMI Advanced Capillary Flow Porometer • Appropriate design & strategic location of transducers to minimize pressure drop in the instrument • Minimal operator involvement • Use of samples without cutting and damaging the bulk product

15. Analysis of Experimental Data Dry Flow, Wet Flow & Differential Pressure Flow rate and differential pressure measured in a solid oxide micro fuel cell component

16. Analysis of Experimental Data Through Pore Throat Diameter • Pore diameter computed from pressure to start flow = Through Pore Throat Diameter

17. Analysis of Experimental Data The Largest Through Pore Throat Diameter (Bubble Point Pore Diameter) • Computed from pressure to initiate gas through wet sample The largest pore size in a solid oxide micro fuel cell component

18. Analysis of Experimental Data The Mean Flow Through Pore Throat Diameter • 50% of flow is through pores larger than the mean flow through pore throat diameter • MFPD computed using pressure when wet flow is half of dry flow Mean flow pore diameter of a solid oxide micro fuel cell component

19. Analysis of Experimental Data The Smallest Through Pore Throat Diameter & The Pore Diameter Range • Smallest pore is computed using pressure at which wet and dry curves meet Pore diameter range measured in a solid oxide micro fuel cell component

20. Analysis of Experimental Data Flow Distribution • The flow distribution is given by the distribution function, fF fF = -d [(Fw / Fd)p × 100] / d D Fw = wet flow, Fd = dry flow Flow distribution in a membrane

21. Analysis of Experimental Data Flow Distribution • Area under distribution function in any diameter range = % flow through pores in that range

22. Analysis of Experimental Data Pore Fraction Distribution Pore Fraction Nj = the number of through pores of throat diameter Dj Fj = [1/(4 γ cosθ / pj)4] [(Fw,j / F d,j) – (Fw,j-1 / Fd,j-1)] pj = differential pressure to remove wetting liquid from pore of diameter Dj

23. Analysis of Experimental Data Pore Fraction Distribution Flow fraction distribution of a membrane

24. Analysis of Experimental Data Gas Permeability • From Darcy’s Law: F = k (A / 2μl ps) (Ts / T) (pi + po) [pi – po] F = gas flow rate in volume at STP ps= standard pressure Ts = standard temperature k = permeability A = area μ= viscosity l = thickness T = test temperature in Kelvin pi= inlet gas pressure po= outlet gas pressure

25. Analysis of Experimental Data Gas Permeability • Permeability computed from dry flow Flow rate through a dry sample

26. Analysis of Experimental Data Through Pore Surface Area • Kozeny-Carman equation relates through pore surface area to flow [Fl / p A] = {P3 / [K(1 - P)2 S2 μ]} + [Z P2 π] / [1 - P) S (2 πpρ)½] F = flow rate in volume at average pressure p (p = [pi + po / 2]), and test temperature P= porosity S = surface area per unit volume of solid ρ = density of gas at average pressure K = 5 Z = (48/13 π) Flow rate through a dry sample

27. Analysis of Experimental Data Through Pore Surface Area Change of envelope surface area with flow rate

28. Enhanced Capability • Advanced Porometers with special attachments can test samples under a variety of conditions

29. Enhanced Capability Compression & Cyclic Compression Porometry • Sample under compressive stress or cyclic compressive stress Effects of compressive stress on gas permeability of GDL

30. Enhanced Capability Controlled Thermal & Chemical Environment Porometry • Sample under desired controlled humidity and temperature The PMI Fuel Cell Porometer

31. Enhanced Capability Microflow Porometry • Samples exhibiting very low flow rates • Fuel cell components • Membranes • Dense ceramics • Tightly woven fabrics • Tiny parts • Silicon wafers • Storage materials Small flow rates through a fuel cell component measured in the microflow porometer

32. Enhanced Capability In-Plane Porometry (Directional Porometry) • In-Plane pore structure of sample or pore structure of each layer of multilayer components • Fuel cell components • Battery separators • Nonwoven filters • Felts • Paper Pore structure of each layer of a ceramic component

33. Capillary Condensation Flow Porometry Basic Principle • Capillary Condensation Flow Porometry is a recently patented novel technique Condensation of Vapor of a Wetting Liquid in Pores • Vapor at p<pocannot condense • Vapor at p<pocan condense in pores p = pressure of vapor, po = eq. vapor pressure

34. Capillary Condensation Flow Porometry Basic Principle • Free Energy Balance shows → condensation occures in pores smaller than Dc Dc = - [4 Vγl/vcosθ/ RT] / [ ln (p/po)] V = molar volume of condensed liquid R = gas constant γl/v= surface tension T= test temperature θ= contact angle Dc = pore diameter

35. Capillary Condensation Flow Porometry Basic Principle Flow of Vapor through Empty Pores • A small imposed vapor pressure gradient causes flow through empty pores greater than Dc

36. The Technique • Measured vapor pressure in equilibrium with the sample yields Dc • Measured rate of pressure change in the downstream side yields flow rate

37. An Example: The PMI Capillary Condensation Flow Porometer

38. Analysis of Experimental Data Through Pore Throat Diameter • Condensation starts at the throat of a through pore and prevents gas flow Dc = through pore throat diameter

39. Analysis of Experimental Data Change of Vapor Flow Rate • Measured Flow Rate = Flow through all pores > Dc • Molecular flow is applicable to flow through such small pores (F/AΔp)cumulative = (Ts/T) (π/12τpsl)(8RT/πM)½ [ΣDDmax Ni(Di)3] A = area of sample p = pressure drop across the sample l = sample thickness T= test temperature in K M = molecular weight, Ni= number of pores of diameter Di F= flow rate in volume at STP, ps and Ts  = average tortuosity of pores and is equal to ( L/l) where L is the length of capillary, D = pore diameter computed by adding to Dc a small correction term for thickness of adsorbed layer

40. Analysis of Experimental Data Change of Vapor Flow Rate Variation of flow rate with pore diameter Flow rate through a membrane

41. Analysis of Experimental Data Pore Distribution • Expressed in terms of distribution function, f f = - d((F/AΔp)cumulative) / dD Flow distribution in a membrane

42. Analysis of Experimental Data Number of Pores of Diameter, Di • Number of pores computed using the following relation f = (Ts/T) (π/12τpsl)(8RT/πM) ½ [3Ni(Di)2]

43. Strengths of the Technique • The diameters of pores down to a few nanometers and flow through these small pores are measured • Test pressure on the sample is almost zero • Extreme test conditions are avoided • There is no stress on the sample and structural distortion or damage to the sample is negligible

44. Strengths of the Technique • Only through nanopores are measured and blind pores are ignored unlike the gas adsorption technique • Throat diameters are measured • A wide variety of vapors can be used • Measuring technique is simple

45. Hydrophobic Through and BlindPore Volume and Distribution • Hydrophobic and hydrophilic pores are relevant for: • Water management • Transport of reactants • Reaction rates • Flow rates of reaction products

46. Vacuapore Basic Principle • Hydrophilic pores are spontaneously wetted by water • Hydrophobic pores repel water because γ (water/solid) > γ (gas/solid) • Pressure on water results in water intrusion • Intrusion volume is pore volume • Pore diameter computed from intrusion pressure Work done by water = Increase in surface free energy D = - 4 γ cosθ / p

47. The Technique • Recently patented technique • Features: • Removal of air from the pores, the sample chamber and water • Application of desired compressive stress on the sample • Optional in-plane intrusion of water

48. The Technique Vacuapore

49. Analysis of Experimental Data • Only hydrophobic through and blind pore diametersare measured. • Measured pressure yields pore diameterofhydrophobic through and blind pores. • Measured intrusion volume of water = Cumulative pore volume of hydrophobic through and blind pores.

50. Analysis of Experimental Data • Volume distribution is given as function, fv fv = - dV / d log D • Hydrophobic and hydrophilic pore distributions obtained from results of Vacuapore and Mercury Intrusion Porosimeter.