Zentrum für BrennstoffzellenTechnik GmbH Characterization methods for bipolar plate materials Identifying low cost solutions for industrial use A common cooperation between the academic world for the industrial application Dr. Thorsten Derieth, ZBT GmbH Dr. Holger Janßen, FZ Jülich World of Energy Solutions October 6-8, 2014 Stuttgart, Germany
Starting point Request from the German industrial bipolar plate manufactures and FC-OEMs Support from the German institutes is urgently needed for many FC-issues related to BPP Electrical conductivity of BPP Thermal conductivity of BPP Corrosion issues and ageing aspects caused by BPP Mechanical properties of BPP (before and after in-cell operation) Surface questions of BPP and many more Bipolarplatten für die HT-PEM by ZBT all rights reserved. Confidential no passing on to third parties 1
Starting point - electrical conductivity measurement For a first step the determination of electrical conductivity has been agreed Many different methods/devices for measuring electrical conductivity of BPP Many different units for the conductivity No description of the measurement in technical datasheets Differences in through- and in-plane conductivity - only one value is mentioned No standards or standardizations are available Conclusion: a comparable and easy to handle measuring method for the industry, standardization should follow up Source: Wilhelm Eisenhuth GmbH by ZBT all rights reserved. Confidential no passing on to third parties 2
Round robin test on the determination of electrical conductivity Questions to be clarified before: Metal or compound plates Pressurized measurement (how much) With GDL or not Through-plane, in-plane, processing method? Sample Size and sample conductivity Treated or untreated plate surface Agreements Use of compound based specimen Measurement under pressure (use of existing eqipment) With GDL agreement: SGL 25 BC (provided by ISE) Through-plane, in-plane and injection direction is considered Sample Size 38 x 25 x 2 mm (provided by ZBT) Different conductivity levels (depending on graphite loads in polypropylene (PP) Surface untreated by ZBT all rights reserved. Confidential no passing on to third parties 3
Round robin test production of specimen Extruder Polymer Graphite Compound 70, 75 and 80 wt% graphite in PP Source: Exctricom 3 x 10 Piston injection molding machine Test specimen by ZBT all rights reserved. Confidential no passing on to third parties 4
Round robin test bipolar plates (specimen) are not 100% homogeneous Current-Flow Test specimen Source: wikimedia.org Test specimen by ZBT all rights reserved. Confidential no passing on to third parties 5
In-plane 1 In-plane 2 Round robin test in-plane and through plane variation Through plane Injection direction Current flow by ZBT all rights reserved. Confidential no passing on to third parties 6
Round robin through plane test different testing devices at each partner Source: ISE Source: ICT Source: ZBT Source: FZ Jülich by ZBT all rights reserved. Confidential no passing on to third parties 7
Round robin test through plane 45 20 8 17 2,3 5,4 One partner couldn t measure through-plane by ZBT all rights reserved. Confidential no passing on to third parties 8
Round robin in-plane test different testing devices at each partner Source: ICT Source: TU-Clausthal Source: FZ Jülich Kunststoff- Auflage Spannmodul EV Festo (pneumatisch) Kupferkontakte (vergoldet) Montageplatte Pneumatik Montage/ Isolierplatte Spannbacken Schraubstock Source: ZBT Source: ZBT Source: ISE by ZBT all rights reserved. Confidential no passing on to third parties 9
Round robin test perpendicular in-plane One partner couldn t measure in-plane perpendicular The values from the violet partner seem to be to high by ZBT all rights reserved. Confidential no passing on to third parties 10
Round robin test perpendicular in-plane X 1,43 0,74 X X Without the violet partner the max difference remains by a factor of two by ZBT all rights reserved. Confidential no passing on to third parties 11
Round robin test injection direction in-plane X X X The violet partner is significantly different from the others Others: Though different devices nearly no deviation by ZBT all rights reserved. Confidential no passing on to third parties 12
Round robin in-plane in-plane and trough plane comparison In-plane perpendicular In-plane injection direction 1.2 0.2 0.9 0.1 Through-plane 29 4.1 Significant differences between in- and through-plane values by a factor of > 20 With higher graphite load the resistivity decreases significantly by ZBT all rights reserved. Confidential no passing on to third parties 13
Conclusion for electrical conductivity Summarization Conductivity significantly depending on structure of the plate Significant differences between the different testing devices Differences depending on direction of measurement Significant differences in through-plane values Significant differences in in-plane-values perpendicular to injection direction Acceptable differences in in-plane-values along injection direction Question: Does fixing on DOE-Targets makes sense? Answer: Yes fixing targets makes sense but only when: Standardization of specimen, testing protocol and testing devices is established In-and through plane is considered All values are comparable and realistic Cheap and simple to apply measurement devices are available for everyone Source: Wang et al., 2012 by ZBT all rights reserved. Confidential no passing on to third parties 14
Corrosion workshop Fraunhofer ICT, Pfinztal Target: Evaluation of corrosion tests for basic and coated BPP materials Boundary conditions Rapid tests PEFC, HT-PEFC Planar coated and non coated samples (structured BPPs later) Ideally non-destructive (usually not possible), random test electrochemical measurements Relevance Graphitic BPPs: low, carbon corrosion in catalyst layer dominates degradation Metallic BPPs: high, surface passivation, metal dissolution, mass reduction Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering 15
Local element Challenges of Metallic BPP for HT-PEFC Corrosion Oxidant: 85 wt.% H 3 PO 4 and impact of temp. 160 C Ox: Me Me 2+ + 2e Red: 2H 3 O + + 2e H 2 + 2H 2 O E 0 = 0 V Red: O 2 + 4H 3 O + + 4e 6H 2 O E 0 = 1,23 V (at standard conditions) Stability: E 0 > 0 V (Anode) Stability: E 0 > 1.23 V (Cathode) Metallic BPP Passivation Formation of non-conductive passivation layers on metal surface (metal oxides/phosphates) Rise of interfacial contact resistance [1-2] 2e - H + Fe 2+ Poisoning of membrane and catalyst Assumption: Blocking of functional groups in membrane and/or the catalyst due to metal ion release. H + PEFC: Decrease of performance [3] HT-PEFC: To be analyzed [1] H.L. Wang, J.A. Turner, J. Power Sources, 180 (2008) 803-807 [2] K.M. Kim, K.Y. Kim, J. Power Sources, 173 (2007) 917-924 [3] X. Cheng et al., J. Power Sources,165 (2007) 739-756 H 2 Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering 16
Possible methods - without external potential Simple exposure to H 3 PO 4 Direct liquid contact of sample 85 wt.% H 3 PO 4 160 C Determination of free corrosion potential Post-mortem analysis Liquid analysis of containing dissolved metal or support species Contact resistance (time dependent) Microscopic surface analysis Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering 17
E cor / mv T / C E cor / mv T/ C Exposure Test - stability of passive layers 600 140 600 140 500 120 500 120 400 300 1.4301 1.4372 1.4404 1.4571 Temp. (1 C/min) 100 80 400 300 100 80 200 60 200 60 100 0-100 0 1 2 3 4 Time / h 40 20 0 100 0-100 1.4539 1.4876 2.4856 2.4869 Temp. (1 C/min) 0 1 2 3 4 Time / h 40 20 0 Stainless Steels: - Lowest stability of 1.4301 (Fe Cr18 Ni10) (Fe passivation instable at elevated temp.) - Increase of E cor due to Mo (1.4404), Ti (1.4571) and Mn (1.4372) Ni-based alloys: Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering - Highest E cor of 2.4869 (80 % Ni) - No significant effect of Cu, Ti, Nb, Mo at 130 C Weißbecker, Wippermann, Lehnert Electrochemical Corrosion Study of Metallic Materials in Phosphoric Acid as Bipolar Plates for HT-PEFCs, Submitted 2014 18
Exposure Test - analysis of phosphoric acid [1] Concentr. (metal ions) / mg g -1 4 3 2 1 0 42 % before after 49 % 30 % 6 % 29 % 13 % 51 % 27 % 9 % 1.4404 2.4856 Fe Cr Ni Mo Nb Exposure to 30 ml 85 wt.% H 3 PO 4 for 4 d at 160 C. Without polarization 1.4404: Fe65 Cr17 Ni12 Mo2 2.4856: Fe5 Cr22 Ni58 Mo9 Nb4 1.4404: - 4.2 mg Fe per g H 3 PO 4 (28 % of sample) (Unstable passivation based on Fe at 160 C) - High rel. dissolution ratio of Cr and Mo (49 51 %) - Moderate release of Ni (13 %) 2.4856: Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering - High content of Ni leads to passivation - Formation of metal phosphates - Decrease of rel. dissolution of metal species V. Weissbecker, U. Reimer, K. Wippermann, W. Lehnert A Comprehensive Corrosion Study On Metallic Materials for HT-PEFC Application 224th ECS Meeting in San Francisco, California (October 27-November 1, 2013) [1] Inductive coupled plasma optical emission spectrometry 19
Possible methods - with external potential Potentiodynamic polarization Continuous potential variation Corresponding corrosion current Chronoamperometry Stepwise potential variation Corrosion current as a function of time (step response) Potential influenced corrosion behavior Passivation range Active corrosion range Corrosion current, material loss Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering 20
Existing setups - potentiodynamic polarization reference electrode (Hg/HgSO 4 ) counter electrode (Ti platinated) double-walled corrosion measuring cell electrolyte (2M H 2 SO 4 ) active sample area (64 mm²) V. Weissbecker et al. 224th ECS Meeting, October, 2013 3 electrode arrangement working electrode: substrate (sample) reference electrode: RHE counter electrode: Pt mesh Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering working electrode (sample) 21
Current density Potentiodynamic polarization j / ma cm -2 1E+6 1E+5 1E+4 1E+3 1E+2 1.4301 1.4372 1.4404 1.4571 1E+1 1E+0-0,5 0,0 0,5 1,0 1,5 h / V Weißbecker et al. Electrochemical Corrosion Study of Metallic Materials in Phosphoric Acid as Bipolar Plates for HT-PEFCs, JES, submitted 2014 Stainless steels at 130 C: - E cor at 0 V - Corrosion current one order of magnitude higher than at RT - Formation of passive layers abs. tolerance: 10 na s -1 min. delay 1 s max. delay 60 s Coated materials, graphite composite: - Corrosion current 2..3 orders of magnitude lower than for uncoated steels - No formation of passive layers Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering 22
Round Robin Corrosion - in preparation Samples stainless steel amorphous metal Test conditions Temperature: 80 C Electrolyte: deionized water (< 5 µs cm -1 ), 1 mm H 2 SO 4, 0.1 ppm HF Volume: 120 ml 3 electrode arrangement well-defined and reproducible sample preparation Electrochemical characterization methods defined Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering 23
Thermal conductivity [W/mK] Next Workshop - thermal conductivity Filler material Commercial test systems are available Significantly different results Standardization necessary M.Grundler, ZBT Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering 24
Implementing steps Agreement of companies, research centers and public funding agencies about the necessity to develop standardized, cost effective tools for an industrial use Systematic evaluation of existing characterization methods for BPP Adaption of promising technologies Close cooperation of industrial and R&D partners Project initiation Project outcome: technical standards Technical and financial support from the industry required Public funding is mandatory national / international Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering 25
Workshop participants Beckhaus, Brokamp, Derieth, Koppers, Grundler Lehnert, Janßen, Weißbecker Scholta Groos Fischer, Cremers, Caglar, Richards Kirchberg, Stübler Rastedt Institute of Energy and Climate Research IEK-3: Electrochemical Process Engineering 26