Stainless Steel Alloys for Polymer Electrolyte Membrane (PEM) Fuel Cells Keegan Duff November 22, 2005
Overview: • What is a fuel cell • Subcategories of low temperature PEM FC • Basic advantages and disadvantages of fuel cell • Show slides of fuel cell • Comparison of austenitic stainless steels in PEM’s • Consideration to stainless steel for current collectors
What is a fuel cell? • A fuel cell is a electrochemical device that acts as a high efficiency electrical storage device. • Chemical energy is stored in a fuel and continually supplied to the device and chemically consumed. In the case of PEM fuel cells, hydrogen and oxygen out of the air are reacted producing electricity, water, and heat.
Low temperature (PEM) proton exchange membrane are subcategories of fuel cells 2 Source: US DOE, Office of Energy Efficiency and Renewable Energy
Fuel Cell Categories 2 Source: Renewable Energy Policy Project
SGL Carbon Group Fuel Cell Animation http://www.sglcarbon.com/sgl_t/fuelcell/#
Chemical Images: Making Membrane Electrode Assembly PEM FC
The cells do suffer from voltage degradation with time Gaskets Fail Pin hole leaks form in separator materials/ion exchange membranes Catalysts become clogged with impurities, in particular carbon monoxide, sulfur and phosphorus compounds reduce performance The ion exchange membranes like NAFION® (Dupont™) , PRIMEA® (GORE™) the industry standards have limited lives. ~1000hrs Hydration of membranes is complicated cost of machining bipolar plates Optimization of current collection Fuel Cells are not Ideal
Nafion® • Perfluorinated polymer that contains small proportions of sulfonic or carboxylic ionic functional groups • Its general chemical structure can be seen where X is either a sulfonic or carboxylic functional group and M is either a metal cation in the neutralized form or an H+ in the acid form. Figure 1. Nafion® Perfluorinated Ionomer http://www.psrc.usm.edu/mauritz/nafion.html
Power density of Fuel Cell D.P Davies et al. (journal of power sources 86(2000) 237-242
Austenitic Stainless Steel:currentdensity vs. cell potential D.P Davies et al. (journal of power sources 86(2000) 237-242
Schematic of test assembly:comparing electrical surface resistance of each material
Tin and Lead phase diagram:generation of microstructure without equilibrium cooling http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/sciviz/contracts/booncon.html, accessed on November 21, 2005
Twin Boundary in Austenitic Stainless Steel • Grain structure of austenitic stainless steel NF709, observed using light microscopy on a specimen polished and etched electrolytically using 10% oxalic acid solution in water. Many of the grains contain annealing twins. NF709 is a creep-resistant austenitic stainless steel used in the construction of highly sophisticated power generation units. • Annealing twins formed in austenite from a low-alloy steel. Austenite is unstable in such steels so it is not ordinarily possible to look at the austenite grain structure except at temperatures in excess of 900oC. This particular sample was prepared metallographically to a 1 micron finish and then heated at 1200oC in a vacuum containing only a trace of oxygen. The heat gives thermally grooves the surface to reveal the austenite grains, and the oxygen slightly oxides the surface to give an etching effect. The sample is then cooled to room temperature but the transformation of the austenite to ferrite does not influence the grooves or the oxide-etching, thus revealing the austenite grain structure. Notice the annealing twins. The chemical composition of the steel is Fe-0.16C-1.43Mn-0.33Si-0.56Cr-0.23Mo- 0.056V-0.064Al-0.062Ni wt%.
Ultrahigh Strength and High Electrical Conductivity in Copper • Research using twining in Cu alloys shows promise of manipulating the microstructure to improve mechanical properties with out significantly increasing the electrical resistance. Ultrahigh Strength and High Electrical Conductivity in Copper Lei Lu, Yongfeng Shen, Xianhua Chen, Lihua Qian, K. Lu* http://www.sciencemag.org/cgi/content/abstract/304/5669/422 Originally published in Science Express on 18 March 2004
Simulation of Dendritic Growth in Nonequlibrum Cooling • Simulation of phase field simulation of the dendritic solification of an austenitic stainless steel: • Sequence formation of δ-ferrite dendrites • nucleation and growth of austenite as the temperature decrease • austenite finally overwhelms the ferrite and becomes the leading phase to solidify http://www.msm.cam.ac.uk/phase-trans/2005/vitek.mov
In Conclusion: • Many aspects of low temperature fuel cells need optimization before they can be implemented. These are engineering and chemistry problems that can be solved. • The type of stainless steel used for the current collector effects the PEM performance. • Non equilibrium cooling results in concentration gradients and microstructure having significant effects on the corrosion of stainless steels. • Currently research does not consider how changes in microstructure of alloys effect performance in fuel cells. • Additional work is need to understand the resins for these differences. • I would like to thank Dr. Coia at PSU for allowing the use of slides of PEM fuel cell prototypes that we constructed.
References: • 1. Felten, Rick. Scanning Electron Microscopy. Stainless steel screen (image SEM used on cover page), acessed on November 19, 2005 http://www.semguy.com/gallery.html • 2 . (Had doe diagram of PEM cell, and doe comparison chart), acessed on November 20, 2005 • http://www.greenjobs.com/Public/info/industry_background.aspx?id=12 • 3. SGL Carbon Group Fuel Cell Animation, accessed on November 19, 2005 • http://www.sglcarbon.com/sgl_t/fuelcell/# • 4. Image and description of Nafion, accessed on November 21, 2005 • http://www.psrc.usm.edu/mauritz/nafion.html • 5. Davies, D.P., P.L. Adcock, M. Turpin, and S.J. Rowen. Stainless steel as a bipolar plate material for solid polymer fuel. Journal of power Sources 86(2000) 237-242 Fuel cell Research group, department of aeronautical, Automotive Engineering and Transport Studies, Loughborough Univesity, Loughbororugh, Leicestershire LE11 3TU, UK • 6. Davies, D.P., P.L. Adcock, M. Turpin, and S.J Rowen, Bipolar plate materials for solid polymer fuel cells • Fuel cell Research group, department of AAETS, loughborough university, loughborough, leicestershire, Le11 3TU, • Great Britain, journal of applied Electrochemistry 30: 101-105, 2000 • 7. Metals and Alloys, Annealing Twins,T. Sourmail, P. Opdenacker, G. Hopkin and H. K. D. H. Bhadeshia • University of Cambridge-shows twin boundrys. http://www.msm.cam.ac.uk/phase-trans/abstracts/annealing.twin.html • 8. Atlas Steels Australia http://www.azom.com/details.asp?ArticleID=1147 accessed on 20 November 2005 • 9. http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/sciviz/contracts/booncon.html, accessed on November 21, 2005
10. University of Cambridge http://www.msm.cam.ac.uk/phase-trans/abstracts/annealing.twin.html. accessed on November 17, 2005 • Lu, L., Yongfeng Shen, Xianhua Chen, Lihua Qian, and K. Lu. Ultrahigh Strength and High Electrical Conductivity in Copper. Science. March 18th 2004. http://www.sciencemag.org/cgi/content/abstract/304/5669/422 • 12. Obtained from university of Cambridge http://www.msm.cam.ac.uk/phase-trans/2005/vitek.mov , • http://www.msm.cam.ac.uk/phase-trans/2005/vitek.html,accessed on 22 November 2005 • 13. Kim, J.S. W.H.A. Peelen, K. Hemmes, R.C. Makkus. Effect of alloying elements on the contact resistance and the passivation behavior of stainless steels. Corrosion science 44(2002) 635-655 • Concludes that schottky contscs have to be considered rather than just omic resistance • 14. Schottky contacts: • http://www.ee.sc.edu/research/SiC_Research/papers/schottkycontacts.pdf