Electrical contact resistance between stainless steel bipolar plate and carbon felt in PEFC: A comprehensive study

international journal of hydrogen energy 34 (2009) 3125–3133

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Electrical contact resistance between stainless steel bipolar plate and carbon felt in PEFC: A comprehensive study Johan Andre´a,*, Laurent Antonia, Jean-Pierre Petitb, Eric De Vitoa, Alexandre Montania a

CEA LITEN/DTH Grenoble, 17 rue des Martyrs 38054 Grenoble cedex 9, France LEPMI/ENSEEG/UMR CNRS/INPG/UJF 5631 Domaine Universitaire, BP 75, 38402 St Martin d’He`res Cedex, France

b

article info

abstract

Article history:

Bipolar plate represents a key component of Proton Exchange Membrane Fuel Cell (PEFC)

Received 1 December 2008

with several essential functions, among them the electric connection of elementary cells.

Received in revised form

Usually made of graphite, this component is studied worldwide in order to develop

28 January 2009

a commercially viable alternative: different ways have been being investigated, and to date,

Accepted 28 January 2009

despite corrosion issues, stainless steel (SS) appears as a good candidate material, but its

Available online 3 March 2009

Electrical Contact Resistance (ECR) can reach unacceptable values when exposed to PEFC environment. This paper offers a comprehensive study of the parameters acting on ECR

Keywords:

when using uncoated SS in PEFC: roughness, which influences the surface contact area

Bipolar plate

with carbon baking, bulk composition of the alloy, which influences only partly the nature

Contact resistance

of passive films, and the composition and structure of passive films, strongly modified by

Stainless steel

surface treatments and ageing conditions.

PEFC

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

XPS Ageing

1.

Introduction

Increasing energy demands of our societies are now associated to the need of reducing our greenhouse gas emissions (essentially carbon dioxide). Besides, air quality is a global concern in cities where automobile traffic has a detrimental impact. Aiming at reducing the impact of our energy needs on the environment, fuel cells represent an attractive solution, for instance answering to the intermittent behaviour of renewable energy sources (sun, wind). According to the field of use, criteria of choice for the type of fuel cell vary. For transportation application, they are particularly stringent: fuel cells associated with electric engines and hydrogen storage cope with well established technologies of thermal engines with gas tank. State-of-the-art PEFC stacks comply with the criteria of volumetric and gravimetric power densities (>1 kW/L and

>1 kW/kg), but cost and durability issues are still hindering the extensive use of fuel cells for transports [1]. To solve these concerns, many efforts are performed on Membrane Electrode Assemblies (MEA), but also on bipolar plates (BPs) which constitute the backbone of a fuel cell stack. To date, different materials for BP have been investigated. Non-porous graphite [2] is the reference material in terms of durability and performance, but its brittleness implies the realisation of bulky plates and makes it incompatible with mass production involving shaping for example by stamping (for metal foils) or injection moulding (for composites). Key characteristics of a BP material for transportation applications can be summed up as follows [3–6]: - high corrosion resistance with corrosion current at 0.1 V/SHE and H2 purge <16 mA/cm2;

* Corresponding author. E-mail address: [email protected] (J. Andre´). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.01.089

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- high corrosion resistance with corrosion current at 0.6 V/SHE and air purge <16 mA/cm2; - electrical contact resistance (ECR) <20 mU cm2 at 140 N/cm2; - does not dissolve and produce metal ions; - possess steady low ohmic resistance throughout operation; - light weight; - tightness to water, air, and hydrogen (2.106 to 104 cm3 cm2 s1); - sufficient mechanical strength and thermal conductivity ( 20 W m1 K1); - high volume cost-effective manufacturability so that the total cost of the BP should be around 1.5$/kW (DOE target); and - recyclability. Various solutions have been being studied such as composites [7–10], titanium alloys [11–13], copper alloys [14–16], aluminium alloys [17–19] or grafoil [8–10,20]. Currently precious metal plated stainless steel plates comply with performance criteria, but are not commercially viable and their durability remains linked to the corrosion resistance of base alloy. Thus cheaper coatings or surface modifications and a better understanding of protection mechanisms involved by passive films and their impact on ECR are required [21,22]. Results of this investigation were obtained on two commercial alloys (AISI 316L and 904L) commonly used for the realisation of bipolar plates in different surface states in order to understand the influence on ECR of: roughness, bulk composition of the alloy, composition and structure of passive films, surface treatments, and ageing conditions.

2.

Symbol and acronym list

BA, bright annealed BP, bipolar plate CPE, constant phase element DOE, Department of Energy ECR, electrical contact resistance EIS, electrochemical impedance spectroscopy FEG, field effect gun GDL, gas diffusion layer OCV, open circuit voltage PEFC, polymer electrolyte fuel cell Ra, arithmetic roughness Rk, corrected roughness parameter SEM, scanning electron microscope SHE, standard hydrogen electrode SS, stainless steel XPS, X-ray photoelectron spectroscopy ND, donor density VFB, flat band potential I, intensity Y, photoelectronic output (transmission factor  Scofield section) D, density l, attenuation length

d, metallic underlayer thickness M, designates the element M all, refers to an alloy compound ox, refers to an oxide compound hydr, refers to an hydroxide compound q, angle.

3.

Experimental

316L and 904L alloys were supplied in a bright annealed state (BA). Elemental bulk composition appears in Table 1. Roughness was measured with a Cotec AltiSurf 500 profilometer on 10 lengths of 5 mm each. Ra and Rk parameters were determined following ISO 13565. Bulk resistance of SS alloys was deduced from 4-probe measurements using Van Der Pauw method and a 103 A Marconi Adret current source at a 4532 mA current. Contact resistance was measured between SS specimen and H2315 T10A Freudenberg carbon felt with a 2-probe device. The geometric contact area was a disc of 25 mm diameter. SS specimen was sandwiched between two pieces of carbon felt, and compressed with two copper plots as shown in Fig. 1. Electric data were acquired with a Stanford Research Systems Model SR830 DSP lock in amplifier and a LabView program, while mechanical data were recorded thanks to an INSTRON 4465 bench from 0 to 3 MPa at a constant compression speed of 0.2 kN/min. Each value given here results from an average of three measurements. First measurement performed with a pair of GDL supports was always left apart to improve precision because of irreversible packing of carbon fibres. ECR values are given with an incertitude about 10% and were deducted from measures following Eq. (1) except for aged samples, whose one face was gold plated (Balzers Union SCD 030 Ar pressure 0.05–0.1 mbar) to reduce contact resistance to a negligible value. In this case, ECR was deducted from Eq. (2).   ECR ¼ 1=2  Rmeasured  2RC=Cu

(1)

ECR ¼ Rmeasured  2RC=Cu

(2)

Surface treatments investigated are all low-cost surface modifications referenced as follows: TA, TB, and TC. TA and TC are two kinds of chemical passivation, and TB corresponds to a kind of electropolishing. Water exhausted during 1000 h single cell tests with graphite bipolar plates was analysed to define electrolytes representative of PEFC cathode and anode environments. Their compositions are given in Table 2. Electrochemical tests were performed with a BioLogic VMP2 multi-channel potentiostat and EC-Lab software. Reference electrodes were always Hg/Hg2SO4/K2SO4,sat (0.650 V/SHE) in order to avoid chloride contamination. Ageing tests consisted of holding samples during 500 h at a fixed voltage chosen as

Table 1 – Bulk elemental composition (weight %) of SS used (supplier data). Element 316 904

C

Mn Si

S

P

Ni

Cr

Mo Cu

N2

0.027 1.39 0.39 0.0030 0.024 12.51 17.46 2.54 – 0.033 0.014 1.49 0.17 0.0003 0.020 24.91 20.05 4.18 1.45 0.082

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copper plot

CPE R1

GDL support

R2 C (ω)

metallic sample

CPE Fig. 1 – Schematic representation of the configuration used to determine contact resistance.

R (ω) representative of PEFC conditions at 60  C with hydrogen or air/oxygen. After 500 h ageing, semiconductive properties of passive films were determined through flat band potential VFB and doping density ND extraction from Mott–Schottky plots on the whole frequency range of acquisition of impedance spectra. Electrochemical Impedance Spectroscopy (EIS) was realized from 0.2 to 0.7 V/SHE with a step of 50 mV. Each spectrum was acquired twice between 0.1 Hz and 2 kHz with a sinusoidal perturbation of 14 mV peak to peak after an 8 min rest period to guaranty stationary conditions. Model used to represent the interface metal/electrolyte was inspired from McCann [23] and Iversen [24] (Fig. 2). R1 is a resistor in series with a constant phase element (CPE) while another resistor R2, is placed in parallel with this the CPE. R1 is generally mainly attributed to ohmic drop in the electrolyte, while R2 is linked to resistivity properties of the passive film (charge transfer resistance). CPE, which can be represented as the association in parallel of a resistor and a capacitor variable in frequency, can originate from surface roughness [25], a distribution of reaction rates on the electrode surface (if polycrystalline) [26], surface heterogeneities in the passive film composition or thickness [27], or current repartition (edge effects) [28]. VFB was deduced from the intersection of 1/C2 with x-axis, almost independent on frequency. ND was extracted from the whole frequency range for each sample referring to the method proposed by Antoni et al. [29]. Due to the semiconductor behaviour of passive films associated to a charge carrier density lower than in metals, a space charge layer of capacitance CSC (about some nanometers thick) is developed into the passive layer, and where almost all interfacial voltage is established. Therefore, the global differential capacitance Cd of the electrode/solution interface, which represents the actual experimental accessible data, is assimilated to CSC. Micrographs presented were realized by a FEG-SEM LEO 1530 equipped with a GEMINI column. XPS analysis was performed on a SSI-Probe spectrometer. Spectra were acquired using a monochromatic Al Ka radiation at 1486.6 eV. X-ray

Fig. 2 – Equivalent circuit used to model passive layers.

spot size was the largest available, i.e. 250  1000 mm. Detailed spectra were acquired at a pass energy of 50 eV (150 eV for survey spectra). Metallic samples were first cleaned with ethanol. In situ argon sputtering (2 keV, 1 min, 2.107 Torr) was performed in the analysis chamber during 60 s to limit carbon contamination. Parameters used for spectra decomposition are summed up in Table 3. Angular analysis were conducted under 90 and 30 to study film stratification. An iterative approach with Matlab software was conducted to study film stratification. Because of the generally accepted heterojunction p–n behaviour of passive films on SS alloys, also confirmed by our Mott–Schottky plots, and the intensity ratios of chromium and iron species under the different angle values, the hypothesis of a duplex structure of iron species (oxides, hydroxides, and/or oxy-hydroxides) superposed to chromium species was made. Only chromium and iron compounds were considered in the passive films while, following De Vito et al. [30], the formation of a metallic underlayer was considered, with a thickness d, and a composition different from the bulk, due to oxidation and dissolution processes. Did is the concentration (in mol cm3) of the element i in this layer. Iron, chromium, and nickel are considered preponderant in front of other elements. The model used requires two other strong hypothesis: different layers are supposed without interphase, and density (in mol cm3) is the same for all iron species and equal to 0.056 while equal to 0.032 for all chromium compounds. These values were estimated from a mixing law, considering a 50% molar mix of the different species. Physical equations used for the model are as following: Eqs. (3) and (4). For a compound from the outer passive layer (iron-rich layer):

Table 2 – Electrolyte composition [43]. Ion Concentration in anode solution (mol/L) (pH ¼ 3.5) Concentration in cathode solution (mol/L) (pH ¼ 3.5)

F

Ca2þ

Naþ

Br

Cl

SO2 4

5, 7.104

1, 26.105

2, 1.105

8, 1.105

2, 7.106

1, 3.106

–

4, 1.104

1, 6.105

1, 4.105

3, 2.106

2, 1.106

7, 9.106

1, 5.106

NO 3

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350

Table 3 – Parameters used for XPS spectra study.

Cr

Ni

FWHM (eV)

Position (eV)

300

Fe Fe3þ Fe2þ Feoxyhyd

10.5 10.5 10.5 10.5

GL(30)T(1.1) GL(30) GL(30) GL(30)

<1.5 <2.6 <2.7 <2.7

707–706.4 711–710 710–708.5 712.05–711.65

Cr Crox Crhyd Croxyhyd

7.6 7.6 7.6 7.6

GL(30)T(1.1) GL(30) GL(30) GL(30)

<1.5 <2.6 <2.7 <2.7

574.7–573.1 576.6–575.6 577.3–577 577–576.7

13.9 13.9

GL(30)T(1.5) GL(30)

<1.5 <2.7

853.9–852.1 857–855

Ni Niox þ Nihyd

  IMext ¼ kYMext DMext lox M sin q 1  exp 

dext lox M sin

dint lox M sin q

  exp 

c2 ¼

q

oxþhydr ICr

th

oxþhydr



IFe

dext lox M sin q



(5)

Results

4.1.

Influence of surface topography

904L BA

1200

100

800

500

220

320

1

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

2

2.1 2.2

(3)

exp

4.

150

Fig. 3 – Electrical contact resistance (at 1 MPa) vs. Rk roughness parameter obtained from polishing with different SiC papers (220–1200 numbers refer to abrasive grain mark).

#2

oxþhydr

ICr

200

Rk (µm)

Theoretical intensities corresponding to different couples of thicknesses are calculated and intensity ratios are compared to experimental values with a least-square method. The minimum of the function defined in Eq. (5) corresponds to the most probable couple of thickness values (iron compound and chromium compound layers) for the passive film. !

316L BA

0.5 0.6 0.7 0.8 0.9

(4)

" ! X Itoxþhydr Fe

250

0

For a compound of the inner passive layer (chromium-rich layer):   IMint ¼ kYMint DMint lox sin q 1  exp  M

316L 904L

50

 q

ECR (milliohms.cm2)

Fe

2400

Line shape

Preliminary measurements of bulk resistance on metallic samples showed that this parameter is not much influenced by the sample nature, and is several orders of magnitude below ECR measured by the 2-probe method. In the following, bulk resistance will be neglected with regards to ECR. One can suppose that contact area between a bipolar plate and a carbon support used as GDL may play a role on the contact resistance and may be correlated with roughness of the plate. Fig. 3 presents the evolution of ECR vs. Rk (a roughness parameter more sensitive than commonly used Ra, and detailed in ISO 13565) for as-received 316L and 904L and samples polished then passivated for one week under air. ECR is stable in a large range of roughness, with a sharp increase for Rk < 1 mm, which is in accordance with the observations of Kraytsberg et al. [31]. A polished surface with very few irregularities lowers the density of contact points between the metal and the carbon fibres, therefore increasing ECR. With the provided carbon felt, Rk of the bipolar plate material should be kept above 1.2 mm. This value may depend on the GDL structure.

As-received 316L and 904L SS samples do not exhibit the same Rk values, but both are in the stable range of ECR, which means that roughness does not totally explain the difference of electrical performance between these two alloys. Lastly, for a given roughness, a great difference of resistance is observed between polished then passivated samples and as-received ones. This indicates that structure and composition of passive films have a significant impact on ECR.

4.2.

Influence of surface treatments

Fig. 4 shows the evolution of ECR regularly observed under the pressure applied, while Table 4 summarizes the results for asreceived and treated samples. For a given surface condition (bright annealed or passivated under air), as shown in Fig. 3, 316L alloy exhibits an ECR superior to that of 904L. Bulk composition of the alloy plays a role on ECR: as-received 904L, which contains less iron, more chromium, molybdenum, and nickel than 316L offers a reduced ECR. Nevertheless, some of the tested surface modifications have a huge influence on electrical conductivity (Fig. 4) which means that passive films can be modified for a given alloy, improving electrical performance. One can notice that concerning the electropolished samples (TB), before concluding about the characteristics of the passive films, roughness should be taken into account, 600

electrical contact resistance (milliohms.cm2)

R.S.F.

316L BA 500

316L TA

Rk TC = 1.26µm

316L TB 400

316L TC

300

Rk BA = 1.21µm

200 Rk TB = 0.77µm 100

Rk TA = 1.37µm

0 0

0.5

1

1.5

2

2.5

stress (MPa)

Fig. 4 – Electrical contact resistance vs. stress for asreceived and treated 316L samples.

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international journal of hydrogen energy 34 (2009) 3125–3133

Table 4 – ECR values for 316L and 904L samples under 1 MPa. Alloy and surface state 316L 316L 316L 316L 904L 904L 904L 904L

BA TA TB TC BA TA TB TC

ECR under 1 MPa (mU cm2)

Rk (mm)

201 11 171 789 70 12 65 199

1,21 1,37 0,77 1,26 1,69 – – –

because the low Rk value (0.77 mm) is partly responsible for the relatively high ECR (Fig. 3). Fig. 5 represents SEM observations of the surface of asreceived and treated 316L samples before ageing. TA photography reveals a clear-cut surface, with a contrast comparable to an attacked sample for metallographic study. This is a clue of the presence of a very thin passive film, which could justify the good electrical performance of TA prepared samples. TB and BA samples have similar surface states, while TC induces many irregularities whose origin (pits, inclusion removal, or impurities) and impact on ECR are difficult to establish.

4.3.

Influence of ageing

4.3.1.

316L BA

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Results are summarised in Table 5. ECR is decreased with ageing whereas ageing in cathode conditions is known to increase ECR [31]. An explanation of this paradox is that the usual initial surface state reported is polished (with #600 grit SiC abrasive paper for instance [28]), whereas it is bright annealed in this study, closer to industrial conditions of use. A slight decrease of ECR is registered for the least oxidizing conditions (low voltage, air bubbling instead of oxygen bubbling). Flat band potential VFB values appear similar after ageing (as observed also for other conditions), thus this parameter will be no longer detailed elsewhere. Besides, passive films behave as n-type semiconductors which means that electrons are the dominant charge carriers. Donor densities ND are higher for samples aged in the most oxidizing conditions. However, it is difficult to correlate this phenomenon with the electrical behaviour, as it could be expected that a higher ND value should lead to a lower ECR. Particularly high ECR observed on the sample aged under 1 V/SHE may be explained in part by the morphology of the

Fig. 5 – SEM observation of surface of as-received (BA) and treated (TA, TB, TC) 316L samples observed at SEM.

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Table 5 – ECR values for 316L BA in cathodic conditions.

Table 6 – ECR values for 316L BA in anodic conditions.

Bubbling gas

Bubbling gas

Initial value

Ageing voltage mV/SHE ECR at 1 MPa (mU cm2) VFB (V/SHE) ND (0.1027 m3)

Air 400

Oxygen

800 1000 660

201

165

169

3450

– –

0.02 0.00 0.18 1.0 1.7 3.5

171

800 211

Initial value

Ageing voltage mV/SHE ECR at 1 MPa (mU cm2) ND (0.1027 m3)

201 –

Hydrogen 0

50

100

224 1.2

88 1.5

195 1.4

0.01 0.02 1.8 2.1

passive film, as shown in Fig. 6. The sample is covered by a discontinuous layer which hides the alloy microstructure, and consists of an entanglement of crystallites. Considering an intrinsic conductivity of the same order of magnitude than that of the films formed at other voltages, such a passive layer may hinder detrimentally the electronic transfer because of superior thickness and porosity [33]. Table 6 reveals that ageing under anodic conditions generally involves a reduction of ECR, as reported elsewhere [32]. This evolution is more pronounced for 50 mV/SHE than for 100 mV/SHE, because at 100 mV/SHE, SS is kept in the

passive state, whereas at lower voltages, the reduction of some oxides is possible [34].

4.3.2.

4.3.3.

Treated 316L

Ageing under air in conditions representative of a standard fuel cell operation (800 mV/SHE), only 316L TA sample shows an almost satisfying ECR (Table 8). At a higher voltage, simulating an OCV exposure (1000 mV/SHE), ECR reaches excessively high values, whatever the initial state (bright annealed or TA-passivated): the TA surface modification does not modify qualitatively the nature of the alloy surface (mixture of oxides, hydroxides, and/or oxy-hydroxides of iron and chromium). So, if above a certain voltage, surface compounds are unstable, the surface is corroded, and resulting compounds, more or less prone to dissolution, tend to accumulate at the surface, involving a sharp increase of ECR, which was also confirmed on 1 h aged samples (ECR w300 mU cm2). On the contrary, OCV periods do not have a detrimental effect on the anodic side (actual voltage remains far away from transpassivity). In standard anodic conditions of ageing, only the TA surface modification improves notably electrical conductivity (Table 9). If semiconducting properties of the sample aged at 50 mV/SHE are considered, one can notice a smaller flat band potential and a higher donor density than for other surface states. However, such a correlation is not confirmed by the exposure at 0 mV/SHE, for which ECR is similar, but semiconducting properties are very different. Donor density might be likely linked to some extent to particular defects responsible for ionic conduction, whose importance for electrical resistance is rather small at low temperature.

4.3.4.

Fig. 6 – SEM observation of surface of 316L alloy aged samples under different voltages. (a) Aged at 800 mV/SHE; and (b) aged at 1000 mV/SHE.

904L BA

Table 7 shows that the difference of performance between 904L and 316L, if consistent at the initial state, is not well established on aged samples. Cathodic polarization induces on 904L ECR values close or higher to that of 316L.

Treated 904L

The investigation of the performances of various surface treatments on 316L, and particularly the results obtained for TA samples, lead us to test the impact of TA on 904L, which is more alloyed and stated as more corrosion-resistant than 316L [35]. Table 10 shows that TA surface modification on 904L has a positive impact: ECR is always lower than for bright annealed aged samples, but performances of treated 316L appear equal or better. For the comparison of these alloys in standard ageing conditions, donor densities rank as follows: 316L TA > 904L

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international journal of hydrogen energy 34 (2009) 3125–3133

Table 7 – ECR values for 316L BA and 904L BA in anodic and cathodic conditions. Bubbling gas

Air

Alloy and ageing voltage mV/SHE ECR at 1 MPa (mU cm2) ND (0.1027 m3)

Hydrogen

904L 800

316L 800

904L 1000

316L 1000

904L 50

316L 50

904L 0

316L 0

153

169

19,974

3450

123

88

48

224

1.5

1.7

6.9

3.5

1.8

1.5

3.5

1.2

Table 8 – ECR values for treated 316L in cathodic conditions. Bubbling gas

Oxygen

Air

Alloy and ageing voltage mV/SHE

TA BA TA TB TC BA TA BA 800 800 800 800 800 800 1000 1000

ECR at 1 MPa (mU cm2) ND (0.1027 m3)

36

211

47

385

221

169 3164 3450

3.3

2.1

2.5

2.1

2.6

1.7

3.0

3.5

TA > 904L BA, which is the same as for ECR. To some extent, ECR may be linked to passive film donor densities.

4.3.5.

Impact of stamping on ECR

A strain of 10% was applied to samples before ageing in order to evaluate the impact of a stamping step in the fabrication process of the bipolar plate. Results are reported in Table 11. Whatever the conditions of ageing or the initial state, ageing on strained samples result in a strong increase of ECR. This result is of paramount importance for the evaluation of the integrity of passive film modifications, if applied before stamping bipolar plates. Passive film may be destroyed while straining the sample. The study of a polished sample repassivated under air, with a comparable roughness and in the same conditions of ageing, may confirm this hypothesis.

4.4.

Study of passive film structure

Table 12 sums up results from our model for the study of passive film stratification by XPS analysis and corresponding ECR values. Chromium content in passive films formed on 904L BA are slightly higher than on 316L BA, which may be justified by the different bulk composition. Nevertheless, surface modifications result, to different extents, in a chromium enrichment of the passive films. TA induces the highest chromium content and the lowest ECR. Ageing also results in

chromium enrichment, but impact on ECR does not appear to be correlated. Thus, on one hand, if it is not excluded that conductivity depends on the proportions of iron and chromium in the passive film, this explanation alone is not satisfying. On the other hand, the comparison of ECR on 316L TC and 316L TA samples, both exhibiting thin passive layers, shows that if the film structure is considered, links between total thickness of the passive film or part of it and ECR are still hard to establish.

5.

Discussion

These tests give some information on the possible evolution of ECR for several conditions of exposure. If we consider initial state, bright annealed state as well as TB and TC modifications are all inconvenient to reach sufficiently low ECR values. Besides, TA modification improves electrical conductivity on both 316 and 904, showing that the choice of 904 is not justified to get a proper ECR. If ageing is considered, the degradation of conductivity with exposure to cathode environment may be explained in part by an oxidation of Cr(III)-compounds to Cr(VI)-ones, which can modify greatly the film morphology and were reported as electrically insulating [36,37]. However, in a general manner, it seems difficult to correlate ECR with accessible semiconductivity parameters (flat band potential and donor density), because of the probable perturbation by ionic or structure defects, as suggested by Bou-Saleh et al. [37]. Even after studying the passive film structure, it remains impossible to give a fully satisfactory explanation of the observed differences of electrical performances. Some hypothesis used for our model may be too restrictive. For a given roughness, thickness and composition may play a role. Complementary electrical measurements on pure chromium and iron samples in different states lead us to think

Table 9 – ECR values for treated 316L in anodic conditions.

Table 10 – ECR values for treated 904L in anodic and cathodic conditions.

Bubbling gas

Bubbling gas

Hydrogen

Alloy and ageing TA 50 TB 50 TC 50 BA 50 TA 0 BA 0 voltage mV/SHE ECR at 1 MPa (mU cm2) VFB (V/SHE) ND (0.1027 m3)

28

172

192

88

31

224

0.08 2.6

0.17 2.1

0.21 1.8

0.25 1.5

0.09 4.5

0.31 1.2

Alloy and ageing voltage mV/SHE ECR at 1 MPa (mU cm2) ND (0.1027 m3)

Air

Hydrogen

904L TA 800

904L BA 800

316L TA 800

904L TA 50

904L BA 50

316L TA 50

59

153

47

29

123

28

1.8

1.5

2.5

2.3

1.8

2.6

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international journal of hydrogen energy 34 (2009) 3125–3133

Table 11 – ECR values for 10% strained 316L samples in anodic and cathodic conditions. Bubbling gas

Air

Alloy and ageing voltage mV/SHE

Hydrogen

Strained BA 800

BA 800

Strained TA 800

TA 800

Strained BA 50

BA 50

Strained TA 50

TA 50

229 2.5

169 1.7

275 2.1

47 2.5

250 2.2

88 1.5

194 2.2

28 2.6

ECR at 1 MPa (mU cm2) ND (0.1027 m3)

Table 12 – XPS spectra analysis, corresponding ECR values and results from model after 500 h ageing. Sample reference and ageing potential (mV/ SHE)

316L BA

316L TA

316L TB

316L TC

904L BA

% under 30

77

25

67

66

69

% under 90 Feoxþhyd þ Croxþhyd ECR at 1 MPa (mU cm2) Rk (mm) Calculated external layer thickness (nm) Calculated inner layer thickness (nm) Calculated total thickness (nm)

68

28

58

65

201 1,21 0.8

11 1,37 z0

171 0,77 0.3

9

0.5

9.8

0.6

Feoxþhyd Feoxþhyd þ Croxþhyd Feoxþhyd

316L BA 800 mV

316L TA 50 mV

21

31

14

0

52

9

24

17

13

789 1,26 0.2

70 1,69 0.4

88 – 0.1

169 – 0.1

28 – z0

47 – z0

1.2

0.5

2.7

10.0

2.2

2.1

0.6

1.5

0.7

3.1

10.1

2.3

2.1

0.6

that the oxidation state of the elements, unfortunately difficult to establish by XPS in the case of iron and chromium, may influence ECR. Iron could act as a dopant for chromia, as suggested by Wang et al. [38] provided its oxidation state differ from that of chromium and its stability in the conditions of test is ensured (proper pH and voltage). Then, the influence of thickness, proposed by Davies et al. [35] is conceivable considering a constant intrinsic conductivity of the passive film. Lastly, other authors such as Kapusta et al. [33] noticed that thickness plays a major role for very thin oxide layers, increasing the tunnelling distance from the underlying metal, and so reducing its probability. For films thick about some nanometers, direct tunnelling is unlikely so that thickness may be secondary compared to the passive film semiconductor properties.

6.

Conclusions

The aim of this work was to investigate the parameters influencing electrical contact resistance between a stainless steel bipolar plate and a carbon backing (GDL). Following conclusions can be drawn.  Characterizing roughness of the bipolar plate material is useful, and its topography should be adapted to the type of GDL: increasing roughness only on plate lands may improve electrical contact resistance while preventing issues such as water flooding or pressure loss.

316L BA 50 mV

316L TA 800 mV

 Bulk composition of the alloy plays a role, because it defines the quantity of alloying elements susceptible to participate to the passive film growth, but this role appears secondary. Indeed, a passivation pre-treatment can deeply modify both structure and composition of the passive layer on 316 and 904, until reducing ECR before ageing at a level comparable to graphite.  Ageing has a different effect in anode and cathode conditions, and which depends on the applied voltage. For cathode operating conditions (0.8 V/SHE), ECR becomes too high. Passive film composition changes during ageing, but its impact is not well established: oxidation state of constitutive elements may play a major role in ECR evolution. For cathode stand-by periods (1 V/SHE), chromium is oxidized to higher valence compounds, resulting in a strong ECR increase. For anode conditions, TA surface modification was shown to give satisfactory results on both alloys.  Even if charge carrier densities should influence electrical conductivity of passive layers, semiconductor parameters of passive films are uneasily linked to ECR.  Strain representative of stamping is detrimental for the passive film integrity, which means that bipolar plates should be stamped before applying any kind of surface treatment.  If still unsatisfactory to reach DOE objectives, and problematic when exposed to long stand-by periods, low-cost surface treatment TA represents a good compromise between using as-received stainless steel plates and precious metal coated plates. Compared to bright annealed state, ECR was divided by a factor 20 before ageing (about 10 mU cm2 with 10% Teflon carbon felt). After 500 h ageing simulating operating anodic conditions, ECR reached only 30 mU cm2, and a bit

international journal of hydrogen energy 34 (2009) 3125–3133

more in a cathode environment (about 50 mU cm2). This nonnegligible improvement is encouraging for further work. Looking for other alloying elements which could prevent transpassivity of chromium [39–42], such as tantalum or tungsten, may represent an attractive solution to obtain electrically conductive passive films on stainless steels.

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