Effect on the contact resistance of the ageing of metallic bipolar plates for PEMFC

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1 Effect on the contact resistance of the ageing of metallic bipolar plates for PEMFC Johan André a, Eric de Vito a, Ludovic Rouillon a, Hélène Plouzennec b, Jean-Pierre Petit d, Laurent Antoni c a CEA/LITEN/DTH/LEV, 17 rue des Martyrs 3854 Grenoble cedex 9 b CEA/DEN/LECA, Gif sur Yvette cedex c CEA/LITEN/DTH/LPAC, 17 rue des Martyrs 3854 Grenoble cedex 9 d LEPMI/ENSEEG/INPG 113, rue de la Piscine, Domaine Universitaire, BP 75 F St Martin d'hères Cedex ABSTRACT Increasing lifetime while maintaining performance is of major interest in the challenge for PEMFC development. One promising way when using metallic bipolar plates is to find a compromise between passivity and electrical contact resistance of the passive films formed on stainless steel. A specific test facility and testing procedure have been developed for measuring the electrical contact resistance. A parametric study has been investigated and significant differences have, for instance, been measured depending on the presence and/or the nature of the gas diffusion layer in contact with the metallic plate, on the grade and the initial surface state of the investigated 316L and 94L stainless steels. The evolution of the contact resistance is investigated after heat treatment of metallic samples to determine what in the passive film influences the contact resistance. XPS spectrometry is used to correlate electrical performance with film characteristics (thickness, composition). Ex situ ageing in a synthetic electrolyte which reproduces a cathodic PEMFC environment is being investigated. KEYWORDS : Stainless steel, PEMFC, corrosion, passivity, contact resistance, XPS Introduction Among the new technologies for energy, fuel cells and in particular PEMFC (Proton Exchange Membrane Fuel Cell) represent an attractive solution, environment friendly, to produce electrical energy, because of no release of gaseous pollutants, quick start-up, favourable output and adapted for many applications. However, some technical issues still have to be solved: increasing lifetime while maintaining performance is of major interest in the challenge for PEMFC development 1. Bipolar plate is a key component of PEMFC, with electrical, chemical, thermal, and mechanical functions 2-5. It is a current challenge to optimize its performances while maintaining the global cost : electrical contact resistance, chemical resistance to the aggressive PEMFC environment, liquid and gas tightness (water, water vapour, air, oxygen, hydrogen), thermal conductivity, mechanical resistance/density ratio maintaining easiness to shape, recyclability. Even if the design of the channels and the operating conditions strongly affect the fuel cell performance, efforts have also to be focused on finding a compromise between passivity and electrical contact resistance of the passive films formed on stainless steel bipolar plates. The properties of passive films depend not only on the steel grade, but also, on surface modifications applied to the steel before use in a fuel cell (heat treatment, polishing ) or during ageing in the fuel cell (exposure to a specific environment, polarization, ) 6. The purpose of this work was to characterize the initial ex situ performance of as received AISI 316L and AISI 94L stainless steel grades in terms of electrical contact resistance and passivity, and its evolution with exposure to PEMFC simulated conditions. Experimental 1/12

2 1/ Material AISI 316L and AISI 94L stainless steel grades were selected as the specimen materials. AISI 316L is commonly used for metallic bipolar plates but is usually covered by a protective layer as gold. while AISI 94L is usually used in more stringent conditions owing to its higher Cr and Mo content. AISI 316L and AISI 94 L specimens were 3mm diameter discs, laser cut from commercially available bright-annealed respectively.1 and.15 mm thick sheets. The diameter was chosen in order to expose a 28mm diameter disc surface in a three electrode electrochemical cell and to perform electrical tests with 25mm diameter copper plots. Before beginning the experiments, the specimens were ultrasonically degreased in a 5:5 acetone and ethanol solution then rinsed with de-ionized water. In order to reproduce direct use of industrial steels and to evaluate the performance of the steels in their as-received state (bright annealed), specimens were neither pickled nor polished. The chemical composition of the stainless steels supplied by Arcelor appears in Table 1 Table 1 : bulk elemental composition (weight %) of SS used (supplier data) C Mn Si S P Ni Cr Mo Cu N L L / Electrochemical tests Water exhausted during long-term single cell tests with graphite bipolar plates was analysed to define an electrolyte representative of a PEMFC environment 7 (Table 2). Open Circuit Voltage was followed during the ageing of the samples in these electrolytes. Is has been carried out by using a BioLogic VMP2 multi-channel potentiostat and EC-Lab software. Reference electrodes were always Hg/Hg 2 SO 4 /K 2 SO 4,sat (.65 V/SHE) in order to prevent chloride contamination, which can be detrimental 1 for corrosion resistance. Table 2 : electrolyte composition Cathodic solution F - Ca 2+ Na + Br - Cl - SO 4 2- NO3 - Concentration (mol/l) 4, , , , , , , / Electrical contact resistance measurement Electrical contact resistance measurements were carried out by a 2-point measuring device composed of a Stanford Research Systems Model SR83 DSP lock-in amplifier associated to an Instron 4465 used for the compression of the specimens. A 5V input voltage at a frequency of 1719 Hz was used. The electrical value sampling acquisition was synchronized with the mechanical one at 4 points/min. The device is detailed in Figure 1. guiding plots sample feeding wires measuring wires measuring plots Figure 1 : Contact resistance measuring device 2/12

3 The metallic sample is sandwiched between two pieces of Gas Diffusion Layer (GDL) support either Zoltek Panex 3 cloth, or Freudenberg H2315 T1A felt, or directly in contact with the copper plots (cf. Figure 2). R measured =2 R C/Cu R measured =2 R inox/cu R measured =2 R C/Cu +2 R inox/c Copper plot GDL support Metallic sample Figure 2 : Configurations used during electrical tests 4/ XPS Analysis XPS analysis was performed on a SSI-SProbe spectrometer. Spectra were acquired using a monochromatised Al Kα radiation at ev. X-ray spot size was the largest available, i.e. 25x1 µm. Detailed spectra were acquired at a pass energy of 5 ev (15 ev for survey spectra). The metallic samples were first cleaned with ethanol. In situ argon sputtering (2 kev, 1 min, Torr) was performed in the analysis chamber during 6 seconds to limit carbon contamination. Results and discussion 1/ Electrical contact resistance measurement a) Definition of the electrical contact resistance measuring procedure The first step was to ensure the reproducibility and sensitivity of the electrical contact resistance measurements. In a first approach, it seemed convenient to perform measurements without any GDL support, in order to get directly the electrical response of the metallic sample and its passive film. But as can be seen on Figure 3, it was difficult to get reproducible results. The imperfect planarity and the inhomogeneous repartition of the applied strength are responsible for these variations. 3/12

4 12 electrical contact resistance steel / GDL (milliohms.cm²) sample 1, 1st try sample 1, 2nd try sample 2, 1st try sample 2, 2nd try,5 1 1,5 2 2,5 3 stress (MPa) Figure 3 : Evolution of the electrical contact resistance 316L sample/measuring plot vs. stress, at,2kn/min, no GDL support Therefore, it seems more relevant to characterize the electrical contact resistance between stainless steel and GDL support, which is actually representative of the fuel cell assembling (Figure 4). Whatever the used teflonised GDL (felt or cloth), the reproducibility is strongly improved, in particular for pressures higher than.5 MPa which is usually the case in fuel cells. It may be noted that, for pressures higher than.5 MPa, the electrical contact resistance measured is slightly higher when using felt. This is due to the presence of teflon, electrically insulating and to the microstructure of both supports which is also different. As the measurement of evolution is improved when using higher values, and as felt is becoming an even more used GDL support for PEMFC development, the felt support has been chosen for this study. 24 electrical contact resistance steel / GDL (milliohms.cm²) steel / felt steel / cloth,5 1 1,5 2 2,5 3 stress (MPa) Figure 4 : Evolution of the electrical contact resistance of AISI 316L/GDL vs. stress, at,2kn/min using either a cloth or felt support. The contact resistance was always slightly higher during the first compression of felt (Figure 5), due to an irreversible packing of its fibres. Therefore, the first measurement performed with a pair of GDL supports was always left apart, in order to improve precision. 4/12

5 electrical contact resistance steel / GDL (milliohms.cm²) 2 1st compression 2nd compression 3rd compression 15 4th compression 5th compression 1 5,5 1 1,5 2 2,5 3 stress (MPa) Figure 5 : Evolution of the electrical contact resistance GDL felt / copper plot vs. stress, at,2kn/min for 5 successive compressions b) Electrical contact resistance for the as-received samples In the open literature, information on the electrical contact resistance of AISI 316L and AISI 94L stainless steel grades could only be found with a GDL paper support but without any information on the dispersion 8. The same tendency and ranking could be observed (Figure 6) but the measured values for AISI 94L were much higher than these from this study. 24 electrical contact resistance (milliohms.cm²) L / GDL : 1% teflonised felt 316L / GDL : 1% teflonised felt 94L / GDL Toray paper, unknown teflon content [5] 316L / GDL Toray paper, unknown teflon content [5] stress (MPa) Figure 6 : Evolution of the electrical contact resistance 316L and 94L /GDL support vs. stress, at,2kn/min with cloth (experimental), felt (experimental), and paper (bibliographic data) The electrical contact resistance at a given stress (e.g. 1MPa) is almost three times lower when using AISI 94L than with AISI 316L. Studies 9-11 attribute such a difference either to the film thickness (the greater chromium content, the lower the passive film thickness, and the lower the contact resistance), or to its composition, emphasizing the role of iron oxide 12 or some alloying elements like manganese 13 as a dopant for conduction of passive films. Therefore XPS investigations, presented below, have been carried out to contribute to a better understanding. c) Evolution of the electrical resistance on heat treated samples 5/12

6 In order to find a compromise between the conductivity and corrosion properties of passive films of stainless steels, it seems relevant to understand the influence of their composition and structure and to get an idea of the extent in which they can be modified. To enhance the difference, AISI 316L and 94L samples were heated at 2 C for 1h and 2h. F igure 7 shows the corresponding evolution of contact resistance versus compression stress. 35 electrical contact resistance (milliohms.cm²) L-2h 316L-1h 316L - as recieved 94L-2h 94L-1h 94L - as recieved stress (MPa) Figure 7 : Evolution of the electrical contact resistance 316L and 94L /felt vs. stress, at,2kn/min, as-received (bright annealed), heat treated 2 C 1h, and heat treated 2 C - 2h As expected, increasing electrical contact resistance was observed with thermal ageing. The electrical contact resistance of 316L remains higher than that of 94L, but the difference of performance between the alloys tends to reduce with time as it san be seen on Figure 8. electrical contact resistance steel / GDL (milliohms.cm²) L 94L 1 2 heat treatment duration (hours) Figure 8 : Evolution of the electrical contact resistance 316L and 94L /felt vs. heat treatment duration, at 1MPa As mentioned previously, these differences are due either to a film growth or an evolution of its chemical composition. XPS investigations have therefore been performed on these AISI 316L and 94L samples. 6/12

7 d) Evolution of the electrical resistance after ageing in a synthetic fuel cell environment AISI 316L and 94L specimens were placed in an aerated simulated cathodic electrolyte at room temperature, in order to reproduce PEMFC rest conditions. In this case, even if the relatively low temperature reduces the kinetics of electrochemical reactions, corrosion can be of great importance, because flow fluid channels may be partially filled with slightly acidic water 7;14. Figure 9 shows the evolution of contact resistance with time of immersion. It is relatively stable with time, the evolution being almost hidden by measuring errors. However, on both alloys, a slight decrease of contact resistance (about -3% of the initial value) can be observed during the first tens of hours, followed by a moderate increase after 35h. Waiting for further XPS results, we can suppose the first hours of immersion are responsible for an adsorption of various species on the film, among some may result in a doping of the oxide film, while later, the film thickens, and the contact resistance increases again. 25 electrical contact resistance (milliohms.cm²) L 316L time (h) Figure 9 : Evolution at 1 MPa of the contact resistance of 316L and 94L specimens with time of immersion in simulated PEMFC cathodic environment at room temperature The evolution of Open Circuit Voltage with time on Figure 1 also indicates a slight evolution of the samples during the first tens of hours, before reaching a relatively steady state, whose some variations may be due to oscillations of the room temperature. Nevertheless, it is difficult to interpret the initial evolution. The upper alloying element content (chromium, nickel, and molybdenum) of 94L over 316L can explain its upper OCV value (nobler alloy) but, despite the various interpretations proposed for corrosion potential 15, the mechanism involved in the initial decrease is not clear. 7/12

8 6 5 4 E (mv) / ENH L 316L time (h) Figure 1 : OCV evolution of 316L and 94L samples with time of immersion in simulated PEMFC cathodic environment at room temperature 4/ Passive film modifications XPS investigations have been performed on different AISI 316L and 94L sample to investigate the reason of these previous electrical contact resistance observations: a film growth or an evolution of its chemical composition. A typical XPS survey spectrum, recorded for an as-received AISI 94L sample, is shown on Figure 11. Alloying elements are detected as expected (Fe, Cr, Ni, Mo, O, C). Ca is also detected but its presence is probably due to surface contamination. Figure 11 : As-received AISI 94L XPS survey spectrum Detailed spectra are shown for raw 94L on Figure 12. For Fe, Cr, Ni, O and Mo. Table 3 gives the position and attribution of the various peaks determined from peak deconvolution. ox hyd In the following, M stands for M 2 O 3, M stands for M(OH) x (M = Fe, Cr, Ni). Table 3 : Position FWHM for main elements detected in studied specimens 8/12

9 Element Fe Cr Ni Position (ev) Fe: 77±.1 Cr : 574,1±.1 Ni : 853.1±.1 ox ox Fe : 79,9±.1 Cr : 576,6±.1 Ni(OH)x : 855,9±.1 Ni(sat) : 86±.1 FWHM (Full Width Half Maximum) (ev) hyd Fe :711,9±.1 Fe: 1.3±.2 ox Fe : 2.7±.3 hyd Fe : 2.9±.3 hyd Cr : 577,6 ±.1 Cr : 1.6±.2 ox Cr : 2.5±.3 hyd Cr : 2.9±.3 Ni : 1.1±.1 Ni(OH)x: 3.2±.5 Chromium spectrum Iron spectrum Nickel spectrum Oxygen spectrum Molybdenum spectrum Figure 12 : As-received AISI 94L detailed XPS spectra 9/12

10 In the following, the given values of passive film thickness have been determined using the signal of Cr metallic peak. Table 4 : Summary of main XPS results and electrical contact resistance at 1 MPa Sample Electrical contact resistance at 1 MPa (mω.cm²) Passive film thickness (nm) Cr ox / Cr (ox+hydr) (%) Fe ox / Fe (ox+hydr) (%) Peak area ratios Fe (ox) / (Fe (ox) + Cr (ox) ) (%) Fe (ox+hydr) / [(Fe (ox+hydr) + Cr (ox+hydr) )] (%) as-received 316L 21 3, L-2h 748 5, as-received 94L 7 1, L-2h 434 2, Fe (ox+hydr) : iron oxides and hydroxides Cr (ox+hydr) : chromium oxides and hydroxides Fe ox : iron oxides Cr ox : chromium oxides At initial state, the passive film is thicker on AISI 316L than on AISI 94L (respectively about 4 and 2 nm), in accordance with Davies et al. 9. This could lead that contact resistance is thickness-dependant. Indeed, resistance is related to thickness with R=ρ.l/S, where R is the resistance of the material, ρ its resistivity, l its thickness, and S the contact cross-section. However, the higher electrical resistance for the 2 h heated AISI 94L sample versus the as-received AISI 316L one (which is thicker) reveals that another dependence parameter has to be taken in account. Semi-quantitative data from XPS spectra have been obtained with peak area ratios for various species. The Fe (ox+hydr) / [(Fe (ox+hydr) + Cr (ox+hydr) )] and Fe (ox) / (Fe (ox) + Cr (ox) ) ratios increase with contact resistance, which seems in discordance with the hypothesis of Wang et al. 12. Indeed, contact resistance increases with an increase of the proportion of iron oxides. Figure 13 indicates a relation which may exist between contact resistance, thickness of the passive films, and the Fe (ox) / (Fe (ox) + Cr (ox) ) ratio. These parameters seem in quite good agreement (R²>,8), but it is hazardous to definitely conclude about the causes of the evolution of contact resistance. Composition of the passive films may have an influence on their conductivity, but the effect of the alloying elements and their form (oxide, hydroxide) needs to be clarified. XPS analysis on 1h heated specimens may allow to determine the submitted relation (1h aged 316L is approximately as resistant as 2h aged 94L). 1/12

11 1 8 passive film thickness (nm) iron oxides / chromium and iron oxides (instensity %) film thickness (nm) x [Fe ox/ (Cr ox + Fe ox)] y =,564x + 7,4 R 2 =, y =,54x + 1,4381 R 2 =, y =,364x + 64,545 R 2 =, As received 94L As received 316L 94L-2h electrical contact resistance (milliohms.cm²) 316L-2h Figure 13 : Correlation between passive film thickness, ratio oxides/hydroxides, and electrical contact resistance for as-received and thermal aged samples. Conclusions A specific test facility and testing procedure have been developed for measuring the electrical contact resistance : - a steel specimen is sandwiched between two pieces of GDL felt support prior to the compression between copper plots, - particular attention must be given to the alignment and planarity of the plots, - the first compression is always left apart. The difference of contact resistance between 316L and 94L SS was shown and analysed. The mechanisms of ageing (by heat treatment or exposure to simulated PEMFC environment) have been being studied : contact resistance of passive films depends on the steel grade and on the environment. Thickening mechanism is not sufficient to explain the increase in contact resistance of passive films, and more investigations are needed to clarify the role of the film composition. Further work will concentrate on the ageing of passive films in cathodic and anodic conditions at voltages representative of typical operating conditions, and on the resistance of the films to corrosion because passivity and contact resistance may not require the same ideal structure and composition. Acknowledgements Thanks to C. Salvi and P. Tiquet for their support in the setup of the contact resistance measuring device. 11/12

12 Reference List 1. W. Vielstich, A. Lamm, H. A. Gasteiger, Handbook of fuel cell : fuel cell fundamentals, technology, and applications part 1, R. Mosdale, in Techniques de l'ingénieur, traité Génie électrique D J. S. Cooper, Journal of Power Sources 129, (24). 4. V. Mehta and J. S. Cooper, Journal of Power Sources 114, (23). 5. Kumar, Atul and Reddy, Ramana G. PEM fuel cell bipolar plate material selection, design, and integration. Fundamentals of Advanced Materials for Energy Conversion, EDP Congress. TMS (The Minerals, Metals & Materials Society) R. C. Makkus, A. H. H. Janssen, F. A. de Bruijn, R. K. A. M. Mallant, Journal of Power Sources 86, (2). 7. Agneaux, A., Plouzennec, M. H., Antoni, L., and Granier, J.. Corrosion behaviour of stainless steel plates in PEMFC working conditions. 2 nd France Deutschland Fuel Cell Conference Fuel Cells: Materials, Engineering, Systems and Applications. Vol.2 Belfort, France (24). 8. H. Wang, M. A. Sweikart, J. A. Turner, Journal of Power Sources 115, (23). 9. D. P. Davies, P. L. Adcock, M. Turpin, S. J. Rowen, Journal of Power Sources 86, (2). 1. D. P. Davies, P. L. Adcock, M. Turpin, S. J. Rowen, Journal of Applied Electrochemistry 3, (2). 11. R. F. Silva et al., Electrochimica Acta In Press, Corrected Proof, (25). 12. H. L. Wang, G. Teeter, J. Turner, Journal of the Electrochemical Society 152, B99-B14 (25). 13. A. K. Iversen, Corrosion Science In Press, Corrected Proof, (25). 14. A. Pozio, R. F. Silva, M. De Francesco, L. Giorgi, Electrochimica Acta 48, (23). 15. V. S. Muralidharan, Bulletin of Electrochemistry 18, (22). 12/12